phd thesis huhang

High-κ Dielectric MIM Capacitors

for Silicon RF and Analog Applications

HU HANG

(M. Sc., Jilin University)

A thesis submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy

Electrical and Computer Engineering Department National University of Singapore

Singapore

December, 2003

Abstract

ABSTRACT

Metal-insulator-metal (MIM) capacitors in silicon integrated circuits have

attracted great attention due to their high conductive electrodes and low parasitic

capacitance. The conventional MIM capacitors using SiO2 and Si3N4 usually provide

low capacitance density, which is far from the requirement predicted by ITRS

roadmap. Therefore, to adopt high-κ materials is an unavoidable choice to improve the

overall electrical performance by using physically thicker dielectric films.

In this thesis, a thorough research has been done for high-κ MIM capacitors

using HfO2 based dielectrics for the first time. Various fabrication methods such as

pulsed-laser deposition, sputtering, and atomic-layer-deposition have been employed

to prepare high-κ dielectrics, and different dielectric structures like laminate, stack,

sandwich, etc, have also been explored as well.

Extensive electrical characterization was conducted to evaluate HfO2 based

high-κ MIM capacitors. DC properties in terms of leakage, voltage coefficients,

reliability etc, have been analyzed which are strongly correlated to the preparation

methods and material properties. In addition, well behaved RF characteristics of these

dielectrics have been demonstrated showing the almost invariable dielectric constants

of HfO2 based dielectrics in RF regime. As a result, all the experimental results justify

the suitability of HfO2 based dielectrics for MIM capacitors application.

Mechanisms with regard to the electronic conduction in high-κ dielectrics,

voltage coefficients of capacitance (VCCs) dependency, oxide degradation etc., have

been discussed and clarified. A good understanding of process-structure-property

I

Abstract

correlation is thus been achieved for high-κ dielectrics fabrication in back-end of line

process, and the information obtained in this thesis is paramount for the operation of

MIM capacitor devices.

Finally, a free carrier injection model has been employed to understand VCCs’

mechanism of MIM capacitors. The results reveal that, the thickness (t) dependence of

quadratic VCCs is an intrinsic problem due to electrical field enhancement in the

scaled dielectric film, which exhibits a relation of (n~2). Besides, the frequency

dependence of VCCs, and the stress modified VCCs could also been well interpreted

using this model.

nt−∝α

II

Table of Contents

III

TABLE OF CONTENTS

Page No.

CHPATER 1

INTRODUCTION OF HIGH-Κ MIM TECHNOLOGY

1.1. Capacitors in Si technology………………………...…………………………..1

1.2. Review of the literature………………………………………………………...4

1.2.1. Motivation of metal-insulator-metal (MIM) technology……………………….4

1.2.2. Current status of MIM technology……………………………………………..5

1.2.3. High-κ dielectrics for MIM capacitors application…………………………….7

1.2.4. Challenges and unsolved problems…………………………………………...12

1.3. Contribution of this thesis…………………………………………………….12

1.4. Thesis outline………………….……………………………………………….13

References…………………………………………………………………………….15

CHAPTER 2

HFO2 MIM CAPACITORS BY PULSED-LASER DEPOSITION (PLD)

2.1. Introduction………………..………………………………………………….22

2.2. Experiments…………………………………………………………………...24

2.3. Results and discussion………………………………………………………25

2.3.1. Physical characterization of PLD processed HfO2…………………………………………..25

2.3.2. Electrical characterization of HfO2 MIM capacitor………………………….35

2.4. Limitations of PLD for thin film fabrication………………………………….43

Table of Contents

IV

2.5. Conclusion……………………………………………………………….……46

References…………………………………………………………………………….48

CHAPTER 3

CHARACTERIZATION OF HFO2 MIM CAPACITORS FOR RF

APPLICATION

3.1. Introduction…………………………………………………………………...53

3.2. Experiments…………………………………………………………………...54

3.2.1. RF MIM capacitor fabrication………………………………………………54

3.2.2. S-parameters for RF characterization…………………………………………57

3.3. Results and discussion………………………………………………………58

3.3.1. RF characterization……………………………………………………………58

3.3.2. DC and low frequency measurements……...…………………………………63

3.4. Conclusion…………………………………………………………………….70

References…………………………………………………………………………….72

CHAPTER 4

HFALOX MIM CAPACITORS BY ATOMIC-LAYER-DEPOSITION (ALD)

4.1. Introduction…………………………………………………………………...76

4.1.1. ALD method for thin films fabrication………………………………………76

4.1.2. Characteristics of ALD processed HfO2 and Al2O3…………………………..77

4.2. Experiments…………………………………………………………………...80

4.3. Electrical characterization of HfO2-Al2O3 laminated MIM capacitors………81

4.3.1. RF characteristics of laminated MIM capacitors….…………………………82

4.3.2. Leakage and breakdown characteristics of laminated MIM capacitors………84

Table of Contents

V

4.3.3. VCCs dependence and reliability of laminated MIM capacitors……………93

4.4. Effects of dielectric structures on the electrical properties………………….100

4.5. Conclusion…………………………………………………………………...105

Reference…………………………………………………………………………….106

CHPATER 5

UNDERSTANDING VOLTAGE COEFFICIENTS OF HIGH-Κ MIM

CAPACITORS

5.1. Introduction………………………………………………………………….112

5.2. Theory………………………………………………………………………..113

5.3. Results and discussion……………………………………………………….115

5.3.1. Thickness dependence of VCCs for HfO2 MIM capacitor…………………..115

5.3.2. Frequency dependence of VCCs…………………………………………..123

5.3.3. Electrical stress modified VCCs……………………………………………..125

5.3.4. Prediction of VCCs…………………………………………………………..126

5.4. Conclusion………………………………………………………………...…129

References………………………………………………………………………….130

CHAPTER 6

Summary and future works………………………………………………………134

6.1. Summary……………………………………………………………………..134

6.2. Future works…………………………………………………………………135

List of Figures

VI

LIST OF FIGURES

Figure 1.1 Dielectric constant κ versus band gap for oxides…………………..….8

Figure 2.1 Experimental configuration of pulsed-laser deposition system in this

work…………………………………………………………………...23

Figure 2.2 XRD patterns of HfO2 thin films deposited on Si(100) substrates at

various substrate temperatures………………………………………..26

Figure 2.3 Deposition rates of HfO2 thin films deposited on Si substrates at various

substrate temperatures………………………………………………...27

Figure 2.4 Three dimensional AFM images of HfO2 thin films deposited on Si

substrates at various substrate temperatures of (a). 25, (b). 200, (c). 300,

and (d). 500oC respectively…………………………………………...28

Figure 2.5 Spectral dependence of refractive indexes of HfO2 films deposited at (a)

various substrate temperatures (oxygen pressure: 50 mTorr) and (b)

various deposition pressures (all deposited at room temperature)……32

Figure 2.6 Spectral dependence of extinction coefficients of HfO2 films deposited

at (a) various substrate temperatures (oxygen pressure: 50 mTorr) and

(b) various deposition pressures (all deposited at room temperature)...34

Figure 2.7 TEM photos of 56 nm HfO2 MIM capacitor fabricated at 200oC…….36

Figure 2.8 Current-voltage characteristic of HfO2 MIM capacitors prepared at 200,

300, and 400oC respectively…………………………………………..37

Figure 2.9 Capacitance versus frequency at zero bias for HfO2 MIM capacitors

prepared at 200, 300, and 400oC respectively………………………...38

List of Figures

VII

Figure 2.10 Normalized capacitance of HfO2 MIM capacitors prepared at (a) 200, (b)

300, and (c) 400oC as a function of voltage applied at a frequency of 1

kHz, 10 kHz, 100 kHz, and 1 MHz respectively………………………39

Figure 2.11 Normalized capacitance of HfO2 MIM capacitor prepared at 200oC as a

function of temperature…………………...…………………………..42

Figure 2.12 SEM top views of HfO2 film surfaces prepared with the laser fluence of

(a) 4.0 and (b) 7.0 J/cm2 respectively (fabricated at room

temperature)…………………………………………………………...44

Figure 3.1 Major fabrication steps and schematic top views of RF HfO2 MIM

capacitor and open dummy structure………………………………….56

Figure 3.2 The definition of S-parameters for a two-port network……………….57

Figure 3.3 The equivalent circuit model for capacitor simulation at RF regime…59

Figure 3.4 The measured and simulated S-parameters for (a) HfO-1 and (b) HfO-2.

(Simulation and parameter extractions were done by ICCAP.)………60

Figure 3.5 High frequency response of PVD HfO2 MIM capacitors from 50 MHz

to 20 GHz for HfO-1 and HfO-2……………………………………..62

Figure 3.6 The frequency dependence of capacitance density for PVD HfO2 MIM

capacitors HfO-1 and HfO-2…………………………………………62

Figure 3.7 Stress induced leakage currents (SILCs) characteristics of (a) HfO-1

and (b) HfO-2 under the constant voltage stress at 1.5 V…………….64

Figure 3.8 Stress time dependence of (a) the quadratic voltage coefficients and (b)

the linear voltage coefficients for HfO-1 under the constant voltage

stress at 1.5 V…………………………………………………………66

List of Figures

VIII

Figure 3.9 Stress time dependence of (a) the quadratic voltage coefficients and (b)

the linear voltage coefficients for HfO-2 under the constant voltage

stress at 1.5 V…………………………………………………………68

Figure 3.10 The equivalent circuit for HfO2 MIM capacitors after stress. The added

branch stands for the generated trapped states in MIM capacitor after

stress…………………………………………………………………..69

Figure 4.1 The growth rates dependence on deposition cycles for ALD processed

(a) HfO2 and (b) Al2O3………………………………………………79

Figure 4.2 TEM cross section of 13 nm HfO2-Al2O3 laminated dielectric……….81

Figure 4.3 Measured and simulated S-parameters for (a) 13 nm, (b) 31 nm and (c)

43 nm laminated MIM capacitors……………………………………83

Figure 4.4 The capacitance density dependence on frequency for laminate

capacitors with three thicknesses, the inset shows high frequency

response of laminate MIM capacitors from 50 MHz to 20 GHz……...84

Figure 4.5 J-V characteristics of 13, 31 and 43 nm laminated capacitors measured

at 125oC.……………………………..……………………………….85

Figure 4.6 J-V characteristics of 13 nm laminated MIM capacitor as a function of

temperature……………………………...…………………………….85

Figure 4.7 Conduction mechanisms for the 13 nm laminated MIM capacitor: (a)

Poole-Frenkel mechanism occurring at high electric field, exhibiting a

shift to lower electric field with increasing the temperature, (b)

Schottky emission fitting at low electric field……………..………….87

Figure 4.8 The characteristics of leakage current versus stress time under 4V stress

for the 13 nm laminated MIM capacitor. Square and round symbols

List of Figures

IX

represent the 1st stress and the 2nd stress after an interruption of 10

hours, respectively…………………………………...……………….90

Figure 4.9 I-V measurements showing the hysteresis loop of 13 nm laminated

MIM capacitor……………………………………......………………90

Figure 4.10 (a) The typical breakdown characteristics of 13 nm laminate under

different constant voltage stress; (b) the cumulative probability

dependence on breakdown voltage for the laminated MIM capacitors

with different thicknesses…..…………………………………………92

Figure 4.11 (a) The voltage-dependent normalized capacitance (∆C/C0) at 1 MHz

for 13, 31 and 43 nm laminated capacitors, fitted by a second order

polynomial equation; and (b) the corresponding plot of ∆C/C0 versus

electric field (E)…………………………...…………………………..94

Figure 4.12 Frequency dependences of α for 13, 31 and 43 nm laminated capacitors,

showing a linear fitting in log-log scale……………...……………….95

Figure 4.13 Thickness dependence of quadratic VCC (α) for laminated MIM

capacitors.……………………………………………………………..95

Figure 4.14 Temperature dependences of α and β at 100 kHz for 13, 31 and 43 nm

laminated capacitors……………………………………………..…..96

Figure 4.15 The dependence of α/α0 on stress time at 10 kHz, 100 kHz and 1 MHz.

The inset shows stress time dependence of β/β0 at the same frequencies.

α0 and β0 represent the data before voltage stress (β0 is of negative

sign.), α and β denote the data after different time stress…………….97

Figure 4.16 (a) Cumulative TDDB curves under various constant voltages stress for

13 nm laminated MIM capacitor measured at room temperature, (b)

List of Figures

X

lifetime projection of 13 nm laminated MIM capacitor, using 50%

failure time as the criteria………………..……...……...……………99

Figure 4.17 Illustrations of five different HfO2-Al2O3 material structures for

electrical characteristics comparison………..……………………….101

Figure 4.18 Typical J-V characteristics for MIM capacitors with different dielectric

structures at 125oC. The inset shows the corresponding breakdown

characteristics obtained at the same temperature. (Device area: 10-4

µm2)………………………..……………………………………….101

Figure 4.19 TEM photos of (a) 13 nm HfO2-Al2O3 laminate, (b) 10 nm HfO2 and

(3) 30 nm HfO2 films, illustrating the amorphous structure of laminate

film and improved crystallinity with the increase of HfO2

thicknesses…………………………………………………………...102

Figure 4.20 Evolution of C0 at zero DC bias with stress time, illustrating the highest

stability for the laminated capacitor compared to other dielectric

structures…………………….……………………………………….104

Figure 5.1 Schottky plot of 30 nm HfO2 MIM capacitor. The inset shows the

typical J-V curve……………………………………………………..116

Figure 5.2 Measured and simulated normalized capacitance as a function of

voltage. n0 and µ are extracted by fitting the measured data.……..…117

Figure 5.3 Carrier concentration pre-factor (n0) dependence on thickness……...118

Figure 5.4 Simulated normalized capacitance as a function of voltage for different

thickness of 20, 30, 40, 50, and 60 nm…………..…………………..119

Figure 5.5 The simulated VCCs as a function of thickness with and without taking

account of the change of pre-factor (n0) with thickness……….…….120

List of Figures

XI

Figure 5.6 Linear voltage coefficients versus the capacitance density for HfO2

based high-κ dielectrics at 100 kHz……………...…………………..121

Figure 5.7 Normalized capacitance versus DC bias measured at 100 kHz, showing

good symmetrical CV characteristics (small linear coefficients β) for

sandwich and laminate structures……………………………………122

Figure 5.8 The measured VCCs for 30 nm HfO2 MIM capacitor together with the

extracted carrier mobility at frequencies of 10k, 100k, 500k, 1

MHz……………………………………………………………….…124

Figure 5.9 Simulated normalized capacitance as a function of voltage for 30 nm

HfO2 MIM capacitors at frequencies of 10k, 100k, 500k, and

1MHz………………………………………………………………...124

Figure 5.10 (a) The simulated VCCs of HfO2 MIM capacitors as a function of

thickness with different carrier concentration pre-factor (n0), and (b) the

simulated VCCs as a function of thickness with different carrier

mobility in dielectric film……………………………...…………….127

List of Tables

XII

LIST OF TABLES

Table 1.1 Mixed-signal capacitor technology requirements ― Short-term…...….2

Table 1.2 Mixed-signal capacitor technology requirements ― Long-term...…….2

Table 1.3 Integration of MIM capacitors into Al BEOL ― Current status…..…6

Table 1.4 Integration of MIM capacitors into Cu BEOL ― Current status…........6

Table 2.1 The ratios of SIMS intensities O/Hf for HfO2 thin films prepared at

various substrate temperatures (oxygen pressure: 50 mTorr)……...…30

Table 2.2 The ratios of SIMS intensities O/Hf for HfO2 thin films prepared at

various oxygen pressures (All samples are prepared at room

temperature 25oC.)………………………………………………….....30

Table 2.3 Voltage linearity coefficients as a function of frequency for HfO2 MIM

capacitors prepared at 200, 300, and 400oC respectively……………..41

Table 4.1 ALD process conditions for the deposition of HfO2 and Al2O3……....77

Table 4.2 Variations of VCCs and leakage current under different condition

(frequency: 100 kHz; stress voltage: 4V; area: 1×10-4 cm-2)…………98

Table 4.3 Comparison of various high capacitance density MIM capacitors using

high-κ dielectrics (year 2002-2003)…………………………………100

Table 5.1 Different structural HfO2-Al2O3 high-κ MIM capacitors prepared by

ALD method……………………………………………..………..…122

Acknowledgments

XIII

ACKNOWLEDGMENTS

I would like to take this opportunity to express my gratitude to all the people

who make it possible to complete this thesis work.

First and foremost, I would like to give my great thanks to my principle

supervisor, Dr. Zhu Chunxiang, who provides me with an interesting project, constant

direction, valuable advice, and most of all, for providing me with opportunity. He has

my tremendous appreciation and respect.

I am deeply indebted to my co-supervisors, Dr. Lu Yongfeng currently in

University of Nebraska Lincoln and Dr. Subhash Chander Rustagi from Institute of

Microelectronics, Singapore, for being a constant source of help and advice; I truly

appreciate the time, support and encouragement they have given me during the course

of my PhD study.

I owe most thanks to Prof. Li Ming-Fu, A/P Cho Byung Jin, A/P Yoo Won

Jong, Prof. Albert Chin from National Chiao Tung University, Taiwan, and Prof. Lee-

Dim Kwong from UT at Austin, for their always available helping hands, many great

conversations. My special thanks go to Dr. Ding Shi-Jin, I feel privileged to have had

the opportunity to work with him during my time in the PhD program, lots of

collaboration work and fruitful discussions contribute to my thesis development.

I would like to thank my peers with Silicon Nano Device Laboratory in

alphabetical sequence: Chen Jingde, Chen Jinghao, Chen Xiaoyu, Joo Moon Sig, Kim

Sun Jung, Loh Wei Yip, Park Chang Seo, Poon Chyiu Hyia, Debora, Ren Chi, Tan

Kian Ming, Wang Yingqian, Wu Nan, Yang Tian, Yeo Chia Ching, Yu Xiongfei, and

Acknowledgments

XIV

Zhang Qingchun. I have benefited the collaboration work with them, and their

friendship makes my stay in NUS more enjoyable.

Last, and certainly the most, I would like to thank my parents for their love and

support. I can never forget their inspiration and encouragement during my education

years, their constant love and support made the long hours and frustrations bearable.

Chapter 1 Introduction of High-κ MIM Technology

1

Chapter 1

Introduction of High-κ MIM Technology

1.1. Capacitors in Si technology

Basic passive devices including capacitor, inductor, and resistor are

indispensable elements in Si integrated circuits (ICs). Intuitively, passive devices can

only consume or store energy, where active device like metal-oxide-semiconductor

field effect transistor (MOSFET) can also provide amplification. A precise definition

of passive elements was given by Desoer et. al [1]. Given a one-port with port voltage

)(tv and port current )(ti , the one-port is said to be passive if

∫ ≥+t

ttdttitv

0

0)(')'()'( 0ε (1.1)

where )( 0tε is the energy stored by the one-port at time 0t . Similarly, with the aid of

the scattering matrix usually used for high frequency measurement, one could also

deduce that the definition of passivity implies

10 ≤≤ ijS (1.2) [2].

Compared to active devices such as MOSFET in the ultra large scale integrated

circuit (ULSI) technology, passive devices played a relatively minor role. However,

the recent advances in wired and wireless communication trigger demands for high

quality passive devices for radio frequency (RF) and mixed signal applications, and


2

therefore spawned a revival interest in passive devices. A good introduction of passive

component technology could be found elsewhere given by R. K. Ulrich [3].

Among the basic passive devices, capacitor is one of the essential elements,

which may find its wide applications in RF circuits for oscillators and phase-shift

networks, in various configurations of analog ICs such as the converters and filters,

and decoupling capacitance in microprocessor units (MPUs), and so on.

Table 1.1 Mixed-signal capacitor technology requirements ― Short-term [4]

Year of Production 2001 2002 2003 2004 2005 2006 2007 Density (fF/µm2) 2 3 3 3 4 4 4 Q (1/KQ2•/µm2•GHz) 200 300 300 300 450 450 450

Voltage linearity (ppm/V2) 100 100 100 100 100 100 100

Leakage (fA[pF•V]) 7 7 7 7 7 7 7

Analog capacitor

3 σ Matching (%•µm2) 4.5 3 3 3 2.5 2.5 2.5 Density (fF/µm2) 7 7.5 8 9 10 11 12 Q (1/KQ2•/µm2•GHz) 22 25 25 29 30 30 30 RF bypass

capacitor Voltage linearity (ppm/V) 1000 1000 1000 1000 1000 1000 1000

Table 1.2 Mixed-signal capacitor technology requirements ― Long-term [4]

Year of Production 2010 2013 2016

Density (fF/µm2) 7 10 15 Q (1/KQ2•/µm2•GHz) 700 1000 1500

Voltage linearity (ppm/V2) 100 100 100

Leakage (fA[pF•V]) 7 7 7

Analog capacitor

3 σ Matching (%•µm2) 2 1.5 1 Density (fF/µm2) 17 20 23 Q (1/KQ2•/µm2•GHz) 35 40 40 RF bypass

capacitor Voltage linearity (ppm/V) 1000 1000 1000

Manufacturable Solutions Exist, and Are Being Optimized

Manufacturable Solutions are Known

Manufacturable Solutions are Not Known

Based on the international technology roadmap for semiconductors (ITRS

roadmap) [4], the main requirements and specifications for capacitors are summarized


3

in Table 1.1 and Table 1.2, where aggressive projections have been extent to year 2016

with ever increasing performance requirements.

According to Table 1.1 and 1.2, capacitors are categorized into analog and RF

bypass capacitors by ITRS roadmap, and it is believed that the requirements of analog

capacitor are more difficult to be achieved compared to RF bypass capacitor. Here, we

may detail generally the above technique specifications as follows:

1. Capacitance density

One of the main issues for capacitors is to increase the capacitance per unit

area in order to improve the integration level and reduce the system cost.

2. Leakage current

The requirement of low leakage is obvious. However, it may be relaxed to

some extent at very high clock frequency [4].

3. Quality (Q) factor

Q factor is a measure for parasitic effects including the distributed resistance

and inductance, which could be computed by )(/)( CapCap ZrealZimagQ = [5]

4. Voltage coefficients of capacitance (VCCs)

VCC can be approximated by C(V) = C0 (αV2+βV+1) [6],

where C0 is the capacitance at zero volt and α, β are the quadratic and linear

voltage coefficients of the capacitance respectively.

5. Temperature coefficients of capacitance (TCCs)

TCC can be usually defined as: CppmdTdC

TTCC o/106

= [3, 7]

6. Compatibility to back end of line (BEOL) integration


4

The capacitors’ fabrication needs to be compatible to existing ULSI backend

technology. Thus, high quality dielectric must be formed at a very low

temperature of ~400oC limited by backend process.

1.2. Review of the literature

1.2.1. Motivation of metal-insulator-metal (MIM) technology

Traditionally, metal-insulator-silicon (MIS) [8, 9] structure has been used in Si

ICs. However, this structure was replaced by polysilicon-oxide-polysilicon (double-

poly) capacitor since the electrical performance of double-poly structure was superior

in terms of small VCCs and stray capacitance [10], where the capacitors’ precision is

paramount for those of applications such as A/D converter. Accordingly, double-poly

structure was established as a mature analog component. In addition, the improved

capacitor structures like metal-ploy have also been reported [11, 12].

Though the polysilicon structure could be tailored in many ways to yield good

electrical properties making it suitable for many analog applications, it suffered from

limited RF capability in multi-GHz range [13]. The limitations in the quality factor are

primarily due to the large resistive loss from the electrodes, and the parasitic

capacitance because of the proximity to the lossy silicon substrate [14]. Therefore,

metal-insulator-metal (MIM) structures have been proposed as the next generation

capacitor structure due to their high conductive electrodes and low parasitic

capacitances. In addition, the intrinsic depletion free MIM structures would provide

better voltage linearity property [15].


5

Except the applications in Si ULSI circuits, MIM capacitor is also a key

element in GaAs based monolithic microwave integrated circuits (MMICs) [16, 17]. In

addition, the above mentioned problems are also anticipated by dynamic random

access memories (DRAM) that use MIS structure as the charge storage cell. Therefore,

advanced DRAM cell with MIM structures have also been studied [18, 19, 20]. It is

possible to implement MIM for DRAM application beyond the 90 nm node in 2004,

according to ITRS roadmap [4]. In this work, our focus is on high-κ MIM capacitors

integrated into BEOL process for Si RF and analog applications, which is much

different with the requirements of MIM capacitors in MMICs and DRAM cells in

terms of materials, structures, process flow, and other aspects.

1.2.2. Current status of MIM technology

As an emerging technology, MIM capacitors draw great attentions among

semiconductor industry companies in the very recent years. Based on the literature

survey, we summarize the reported MIM capacitors technology from several major

semiconductor companies, which are presented in Table 1.3 and Table 1.4 for Al and

Cu BEOL integration respectively. As can be seen, SiO2 and Si3N4 are usually chosen

as the dielectric materials for MIM capacitors fabrication in the current technology

node. In comparison, Si3N4 has a higher dielectric constant (κ) of 7 compared to SiO2

(~3.9), which usually provides relatively higher capacitance density than SiO2 MIM

capacitors. In addition, Si3N4 could also be used to serve as a good Cu diffusion barrier

[21], therefore eliminating Cu barrier metal stacks usually required in Cu BEOL

process [21, 22]. However, the frequency dependence of capacitance and voltage

linearity for Si3N4 capacitors may degrade the capacitors’ accuracy, which was


6

proposed to be originated from bulk traps in nitride films [23]. Low temperature

deposited Si3N4 was reported to show higher relaxation recovery voltage than oxide

[24]. When compared to LPCVD SiO2, the breakdown field strength of Si3N4 is lower,

and both its voltage and temperature coefficients are usually higher. Therefore,

schemes such as nitrous oxide plasma treatment [25], silicon oxynitride [26], SiO2-

Si3N4 stacks [27] have also been explored to combine the merits of SiO2 and Si3N4.

Though SiO2 and Si3N4 MIM capacitors with excellent electrical performance have

been successfully demonstrated in Al and Cu BEOL process; the capacitance density is

still low, usually ≤ 2 fF/µm2.

Table 1.3 Integration of MIM capacitors into Al BEOL ― Current status

Conexant system [15] IBM [28] Toshiba [29]

Dielectric PECVD Nitride (30~60 nm)

Single/multi layers (SiO2/Si3N4, 50-125 nm)

(Ta2O5, 50 nm) (Si3N4, 50 nm)

Bottom electrode Ti/TiN/AlCu/Ti/TiN TiN/AlCu/TiN WSi2

C (fF/µm2) 1.0-1.9 for 30-60 nm thick film 0.44-1.40 4.36

1.01 Leakage (A/cm2) ~1E-10 A/mm2 for 50

nm Ta2O5

Remark Life time: 106 and 103 years for 60 and 30 nm films.Q >80 at 2 GHz

T50> 10-7 hours, phase improved by 2× compared

with MOS

Good leakage property obtained after 300oC

furnace annealing

Table 1.4 Integration of MIM capacitors into Cu BEOL ― Current status

Lucent [21] IBM [22] Motorola [30] TSMC [31] Dielectric Si3N4 (30 nm) SiO2 (48 nm) Si3N4 (40 nm) Si3N4

Bottom plate Direct on Cu A conductive metal stack Sputtering-TaN

C (fF/µm2) 0.72 1.6 1

VCC (ppm/V) 150 TTC= -40 ppm/oC <15 VCC= 60 ppm/V TCC= 50 ppm/oC

Leakage (A/cm2) 10-10 at 5V 10-6 A at 30 V 10-10 at 5V

Leakage on temperature Weak (25-200oC) Weak (25-125oC)

EBD (MV/cm) 10 10 9-10

TDDB T50>1000 pwr-on-hrs Beyong 10 years

Remarks Si3N4 as both dielectric and

diffusion barrier

Leakage sensitivity to Tox

Q2GHz= 30-200 at 3-0.1 pf

Q=100 (2.4 GHz) and 40 (5.3 GHz)

at 1.1 pf


7

Furthermore, it was noted that these of SiO2 and Si3N4 works are focused on

the integration of MIM capacitors, and the process related issues have thus been well

addressed. Planar structures were usually implemented for MIM capacitors integrated

in BEOL process, and positioning the capacitors beneath the final metal level could

further minimize the loss to the substrate. At or below 0.18 µm technology, Cu

metallization is used instead of Al metallization due to copper’s low resistivity and

feasibility of thick and fine pattern through damascene process [4]. However the

introduction of Cu interconnects will create unique challenges for fabricating high

reliability MIM capacitors, such as surface roughness of Cu on the reliability of MIM

capacitors [21], the proper choice of Cu barrier metal stack [22], and the compatibility

of capacitor dielectric with inter-level dielectric [22], etc.

1.2.3. High-κ dielectrics for MIM capacitors application

As described above, SiO2 and Si3N4 are dielectrics that are commonly used in

conventional MIM capacitors [6-31]. Although these SiO2 and Si3N4 MIM capacitors

could provide excellent electrical properties, their capacitance densities are limited due

to their low dielectric constants (κ~3.9 for SiO2, κ~7 for Si3N4). This is far from the

requirement on capacitance density projected by the 2002 ITRS roadmap [4].

Further reduction in dielectric thicknesses of SiO2 and Si3N4 can increase the

capacitance density, but it may offset leakage current, breakdown voltage, and voltage

linearity property [22, 29]. For instance, it was reported that a 30-nm-thick SiO2 MIM

capacitor has a voltage linearity of ~ 20 ppm/V2 [27]. From the 1/t2 (t: thickness)

dependence [32], the voltage linearity of 14-nm-thick SiO2 MIM capacitor is supposed


8

to reach an upper limit of 100 ppm/V2 according to ITRS roadmap [4]. However, the

capacitance density of 2.5 fF/µm2 is low.

Figure 1.1: Dielectric constant κ versus band gap for oxides [34].

Therefore, the adoption of high-κ materials is imperative to meet the

requirements of MIM capacitors in Si RF and analog IC applications. This is because

of the fact that using physical thicker high-κ dielectric films may potentially improve

the overall electrical performance. In the search to find suitable high-κ dielectrics,

Figure 1.1 presents a compilation of a few potential high-κ dielectric candidates

indicating the relationship of dielectric constant versus band gap. This provides a

simple criterion of selecting suitable high-κ materials as the dielectrics for MIM

capacitors. It is important to note that the general band gap reduction with the increase

of κ value for dielectrics is a limitation that must be considered when selecting a


9

suitable high-κ material for MIM capacitor application [33, 34]. The decrease in band

gap is usually coupled with the reduction of breakdown voltage for the dielectric

materials [35].

Among various high-κ candidates for MIM capacitors application, Ta2O5,

Al2O3, and HfO2 high-κ dielectrics are of great interests among researchers due to their

relatively good materials properties and industry’s familiarity.

Ta2O5 based high-κ dielectrics have drawn a great attention, which may be

inspired by memory capacitor applications and the resultant semiconductor

manufacturing tool infrastructure [18, 36]. T. Yoshitomi et. al demonstrated pure

Ta2O5 MIM capacitors by reactive sputtering [29]. With an O2 annealing at 300oC, the

Ta2O5 MIM capacitor exhibit superior electrical performance when compared to its

Si3N4 counterpart at the same equivalent oxide thickness (EOT) in terms of leakage

property. T. Ishikawa et. al integrated Ta2O5 MIM capacitors into Cu BEOL process,

and insertion of thin Al2O3 layer between Ta2O5. The bottom electrode was designed to

improve the interface quality and the resulting electrical performance [37]. Y. L. Tu et.

al achieved a very good voltage linearity (25 ppm/V2 and 13 ppm/V) for 4 fF/µm2

Ta2O5 MIM capacitor when compared to TaOxNy, HfO2, Al2O3 and Ta2O5/Al2O3

stacks MIM capacitors in their work [38]. However, the electrical properties of those

Ta2O5 MIM capacitors have been reported to be strongly dependent on the fabrication

methods and the following thermal treatments [36, 38]. Therefore, a good

understanding of process-structure-property correlation is of great importance before

selecting Ta2O5 thin films for MIM capacitors application.

Compared with other high-κ candidates, Al2O3 has a moderate dielectric

constant of ~9, making it to be only a short term solution for industry’ need. However,

the low oxygen diffusivity of Al2O3 [34, 39] may improve the interface quality by


10

reducing the chemical reaction with metal electrode. A large band gap of 8.9 eV is also

beneficial for the improvement of leakage and breakdown characteristics. For MIM

capacitors application, Al2O3 based high-κ materials including pure Al2O3 [39], Ti

doped Al2O3 [39], and Ta doped Al2O3 [40, 41] have been investigated using an

evaporation/oxidation method, and a high capacitance density of 17 fF/µm2 has been

achieved for AlTaOx MIM capacitor [41]. In particular, the RF performance of high-κ

MIM capacitors have been studied for Al2O3 based dielectrics up to 20 GHz. A

mathematical method was recently proposed for the computation of VCCs in RF

regime for Al2O3 based dielectrics [40]. However, the low thermal budget in the

fabrication of those Al2O3 based high-κ materials is probably responsible for their

marginal electrical performance.

HfO2 has the advantages of high dielectric constant (~25), high heat of

formation (271 Kcal/mol), and large band gap (5.68 eV), etc. [34]. Most important of

all, HfO2 based high-κ materials are well established as the next generation gate

dielectric in MOSFETs [42] and DRAM [20]. HfO2 MIM capacitor was first reported

using a pulsed-laser deposition (PLD) method [43]. Following that, other fabrication

techniques including PVD [44], atomic-layer deposition (ALD) [45] have also been

demonstrated. These techniques are more favourable for the mass production

compared to PLD method. In addition, materials engineering of HfO2 dielectric such

as Tb doping [44], Al alloying [45], and novel structures of HfO2-Al2O3 laminate [46]

and stacks [47] have been further explored to improve the leakage and voltage linearity

properties of HfO2 MIM capacitors. In summary, compared to the reported Ta2O5 and

Al2O3 MIM capacitors, HfO2 based high-κ MIM capacitors exhibited nearly the best

overall electrical properties, indicating that they are very promising for the next

generation MIM capacitors application.


11

It was also noticed that some novel oxide systems have been explored for MIM

capacitors application, such as Zr-Sn-Ti oxide with a high dielectric constant of 62

[32], Nb stabilized Ta2O5 [48], etc. However, the final justification of these oxide

systems for MIM capacitors application needs more study, especially on their

capacitance characteristics.

Finally, it is worthwhile to point out that there are very few systematic reports

on high-κ MIM capacitors’ reliability and mechanism study. The possible reason is

that high-κ MIM capacitor is an immerging technology. This is in sharp contrast with

the extensive and thorough investigation of high-κ materials for gate oxides

applications. The reliability assessments for high-κ MIM capacitors could be found for

HfO2 based high-κ dielectrics in [46, 47] and for Ta2O5 in [49]. Among them, HfO2-

Al2O3 laminate and pure Ta2O5 MIM capacitors both show promising reliability

characteristics for 10-year lifetime under optimized process conditions [46, 49].

The conduction mechanism in dielectric insulating films is another subject with

extensive theoretical and experimental investigation. A good understanding of

electronic transport phenomena is paramount for the operation and leakage

improvement of practical devices. To be specific, Poole-Frenkel effect [44, 46, 50],

Shottky emission [46, 50], trap assisted tunnelling [50] are found to be prevalent in the

leakage components in thick high-κ films used in MIM capacitors. Other major issues

related to MIM capacitors, such as VCCs dependency [27, 28, 32, 37-41, 43-46], the

dielectric relaxation [24, 51, 52], etc. have also been frequently observed. These

phenomena are worthwhile for further investigations considering the poor knowledge

thus far.


12

1.2.4. Challenges and unsolved problems

Though the implementation of MIM structure and high-κ dielectrics appears to

be straightforward, putting high-κ MIM capacitors into practical use may be

problematic until a clear understanding and thorough research have been done. The

main challenges and problems for high-κ materials to be used in MIM capacitors are as

follows:

1. Suitable high-κ systems for MIM capacitors need to be identified.

2. The effects of fabrication techniques, structures, thermal treatment on high-κ

MIM capacitors need to be studied.

3. Knowledge on electrical characteristics of high-κ MIM capacitors such as DC

characteristics, RF performance, reliability, etc must be obtained.

4. The mechanisms with regard to high-κ MIM capacitors, such as conduction,

voltage linearity dependency (parameters including thickness, frequency, and

electrical stress), dielectric relaxation, etc must be understood.

1.3. Contribution of this thesis

The followings are the research contribution of this project:

1. A thorough research has been done for HfO2 based high-κ dielectrics using

various fabrication methods for the first time. The electrical performance has

been demonstrated to be superior to SiO2 and Si3N4 counterparts in many

aspects.

2. The RF characteristics of HfO2 based MIM capacitors have been reported for

the first time. The good capacitance-frequency dependency indicate the

usefulness of HfO2 based dielectrics for Si RF and mixed signal applications.


13

3. We firstly observed that the voltage linearity of high-κ HfO2 based MIM

capacitors could be modified after the electrical stress, and a possible

explanation was given with the aid of a phenomenological circuit model.

4. For the first time, a free carrier injection model has been employed to

understand voltage coefficients of capacitance (VCCs) of MIM capacitors. The

thickness (t) dependence of quadratic VCCs, which exhibits a relation of

nt−∝α (n~2), is an intrinsic property due to electrical field enhancement in the

scaled dielectric film.

1. 4. Thesis outline

In Chapter 2, HfO2 high-κ dielectrics prepared by PLD have been employed for

MIM capacitors application with systematic material characterization, and excellent

electrical properties had been demonstrated as well.

In Chapter 3, RF characteristics of HfO2 MIM capacitors were investigated. In

addition, thickness dependence of stress induced leakage currents and the evolution of

voltage linearity with stress time for HfO2 MIM capacitors had been discussed with

the help of proposed circuit model.

In Chapter 4, high performance HfO2-Al2O3 laminate MIM capacitors have

been developed using ALD method. The conduction mechanism, voltage linearity

dependency, reliability issue have been investigated for these laminate films. In

addition, the laminate structure has also been compared with other material structures

such as stack, sandwich, etc.


14

In Chapter 5, the free carrier injection model was employed successfully to

explain VCCs dependence on dielectric thickness, frequency, and electrical stress for

high-κ MIM capacitors. A unified understanding of VCC is achieved for the first time.

Finally, Chapter 6 concludes with suggestions for future work.


15

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21

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Chapter 2 HfO2 MIM Capacitors by Pulsed-Laser Deposition (PLD)

22

Chapter 2

HfO2 MIM Capacitors by Pulsed-Laser Deposition (PLD)

2.1. Introduction

Although remained little known until the late 1980’s, pulsed-laser deposition

(PLD) has emerged as a simple, reliable and fast technique that offers a great

experimental versatility. The setup of PLD system used in this work is shown in Fig.

2.1. A pulsed laser is used as the light source to ablate a target, and the material

ablated from the target is condensed on a substrate and forms a thin film.

Over the past few years, PLD is increasingly being used to prepare a wide

variety of materials in thin film forms [1, 2]. Its low start-up cost, laser energy source

independent of the deposition system, the stoichiometric removal of constituent

species from the target during ablation, and the relatively small number of control

parameters appeal more and more attention. More importantly, the PLD technique is

well known for the quality of the layers grown at relatively lower substrate

temperatures than other thin film deposition methods [1, 2]. This is desirable for MIM

capacitors integrated into back-end of line (BEOL) process where a low thermal

budget (~400oC) is needed.


23

Figure 2.1: Experimental configuration of pulsed-laser deposition system in this work.

For successful integration of MIM capacitors, extremely reliable and high

quality HfO2 thin films are needed. Several thin film growth techniques such as

atomic-layer-deposition (ALD) [3, 4], evaporation with ion-assisted deposition (IAD)

[5], sol-gel [6], sputtering [7, 8], in-situ rapid thermal CVD (RTCVD) [9, 10], and

metallorganic chemical vapor deposition (MOCVD) [11, 12] have been employed to

fabricate good quality HfO2 thin films. The properties of HfO2 thin films have been

reported to be strongly dependent on the fabrication method. Therefore, an

understanding of process-structure-property correlation is of great importance to

exploit HfO2 for thin film devices application.

In this chapter, HfO2 thin films were fabricated using PLD at various substrate

temperatures and oxygen pressures, in order to study physical properties dependence

on process parameters. In addition, excellent electrical performance of HfO2 MIM


24

capacitor had been demonstrated for the first time [13, 14], suggesting the great

potential of HfO2 for MIM capacitors application.

2.2. Experiments

The deposition was accomplished in a stainless-steel vacuum chamber of the

PLD system that was evacuated by a turbo-molecular pump to 1×10-5 Torr, the oxygen

pressure was measured by a convectron vacuum gauge (1 mTorr-1000 Torr) and a

single gauge TM (< 1 mTorr) respectively, and it was controlled and adjusted by a gate

valve. The purity of Hf target is 99.5%; it was placed on the target holder that rotated

constantly by an external motor to prevent surface craters. The target was irradiated by

a focused KrF excimer laser beam (λ = 248 nm, τ = 30 ns) at an incidence angle of 45o

with a repetition rate of 5-10 Hz. The laser fluence was set between 3.0 and 7.0 J/cm2.

In this experiment, for physical properties characterization, silicon substrates

were used for HfO2 thin films deposition at various substrate temperatures ranging

from room temperature (25oC) to 750oC and at various oxygen pressures varying from

10 to 200 mTorr. For electrical and dielectrical measurement, MIM capacitors with

HfO2 high-κ dielectric films were fabricated on a layer of 500 nm SiO2 deposited on

silicon substrate. The SiO2 is for isolation purpose. A layer of Ta film (~1000 Å) was

then deposited ex situ by sputtering on the SiO2 layer as the bottom electrode.

Following that, high-κ HfO2 dielectric films were deposited in an oxygen ambient of

50 mTorr at 200, 300, and 400oC. It is noted that the deposition temperature was kept

at or below 400oC for MIM capacitor fabrication in order to meet the thermal budget

requirement of BEOL process. Finally, Al was deposited by evaporation and patterned

as top electrode by wet etching with the device area being 2.54×104 µm2.


25

The crystal structures of HfO2 thin films deposited on Si substrates were

investigated by X-ray diffraction (Phillips PW 1729 X-Ray diffractometer, Cu Kα

radiation, λ = 0.5418 nm). The surface morphology was obtained using an Au-to-

Probe CP atomic force microscope (AFM) in the contact mode. The film thicknesses

were measured by an Alpha-step 500 surface profiler (Tencor Instruments) and further

confirmed by cross-sectional transmission electron microscopy (TEM) analyses.

Scanning electron microscopy (SEM) was also used to inspect particulates generation

on the surfaces of HfO2 films. Time of Flight Secondary Ion Mass Spectroscopy IV

(TOF-SIMS IV) was employed to characterize the change of the relative stoichiometry

of the HfO2 films prepared at various deposition conditions. The optical constants, i.e.

refractive index and extinction coefficient, of HfO2 thin films were evaluated by a

variable angle spectroscopic ellipsometer (VASE). For electrical measurements, the

leakage current was measured using an HP4155A parameter analyzer, and the

capacitance was characterized using an HP4284A precision LCR meter at frequencies

varied from 500 Hz to 1 MHz.

2.3. Results and discussion

2.3.1. Physical characterization of PLD processed HfO2

HfO2 is a material forming several polymorphs, although pure HfO2 tends to

appear in the monoclinic phase at room temperature and atmospheric pressure,

orthorhombic and tetragonal phases could be formed at high pressures and/or high

temperatures [15, 16], which is generally thought to be metastable if present at room

temperature and atmospheric pressure. Figure 2.2 shows the XRD patterns of HfO2


26

thin films deposited at various temperatures with oxygen pressure of 50 mTorr. For

comparison, spectrum (a) of the bare Si substrate is shown. Spectra (b) and (c) show

no apparent diffraction peaks of HfO2, indicating that the films deposited at 200oC or

below are amorphous. Spectrum (d) shows some weak diffraction peaks, which are

identified from monoclinic phase of HfO2. Spectrum (e) exhibits intense diffraction

peaks indicating the improved crystallinity at high temperature; and no tetragonal or

orthorhombic phase was found to be present. In addition, different orientations of

crystallites in the thin film are observed compared to HfO2 prepared at 300oC, which

suggest a polycrystalline structure change at a temperature above 300oC.

30 35 40 45 500

1000

2000

3000

4000

(211)(020)

(122)(221)

(220)(202)(112)(002)

(200)(111) HfO2 peaks

Si peaks e. HfO2 (500oC)

d. HfO2 (300oC)

c. HfO2 (200oC)

b. HfO2 (25oC)

a. Si substrate

Inte

nsity

(A.U

.)

2 θ, deg

Figure 2.2: XRD patterns of HfO2 thin films deposited on Si(100) substrates at various

substrate temperatures.

The substrate temperature also influences the surface mobility of ablated

species and therefore affects the deposition rates of the deposited films. Figure 2.3


27

shows the deposition rate as a function of the substrate temperature with different laser

fluence. It is found that the deposition rates are of the order of 10−2 nm per pulse, and

the highest deposition rate was at 25oC. As the substrate temperature increases, the

sticking coefficient of Hf-bearing species drops, this may result in a decrease in the

deposition rate. In addition, the as-deposited films become more densified with the

increase of substrate temperature, which is believed to be another factor affecting the

deposition rates. The similar trend could also be found elsewhere [17, 18]. In Fig.2.3,

it is also found that a higher deposition rate can be achieved by using a higher laser

energy density, due to a higher ablation rate on the target.

Surface morphology is another important physical property which may affect

the electrical properties of dielectric thin films. Figure 2.4 shows the three-dimensional

AFM images of HfO2 samples used for XRD measurement in Fig. 2.2, and it is in

2

3

4

5

6

7Laser Energy Density 4.0 J/cm2

7.0 J/cm2

Dep

ositi

on R

ate

(10-2

nm

)/Pul

se

10 Hz, 60 min50 mTorr

Substrate Temperature (oC)50030020025

Figure 2.3: Deposition rates of HfO2 thin films deposited on Si substrates at various

substrate temperatures


28

Figure 2.4: Three dimensional AFM images of HfO2 thin films deposited on Si

substrates at various substrate temperatures of (a). 25, (b). 200, (c). 300, and (d).

500oC respectively.

(a) (b)

(d) (c)


29

agreement with XRD experimental result. In Fig. 2.4(a) and Fig. 2.4(b), the film

deposited at or below 200oC shows no obvious crystallization. In contrast, the film

deposited at 500oC exhibits an apparent polycrystalline structure. In addition, it is

further shown by AFM measurement that the average surface roughness of HfO2 thin

films changes with substrate temperature, with approximately 7.8, 2.7, and 3.2, and 4.6

Å for HfO2 prepared at 25, 200, 300, and 500oC, respectively. It is found that HfO2

thin film prepared at 200oC can effectively improve the surface roughness. In

amorphous phase, it is believed that thin film prepared at 200oC could enhance the

surface diffusion during deposition and thus minimize the microstructure, and

resulting in a smoother surface. However, when HfO2 was prepared at or above the

crystallization temperature of 300oC, the film surface again became rough which is

believed to be due to crystallization induced large grain size. Grain formation at or

above 300oC and the evolution of surface roughness of HfO2, confirmed by XRD and

AFM measurements, may affect the electrical properties, as will be shown later.

In this work, in order to characterize the change of the stoichiometry of HfO2

films, we used secondary ion mass spectrometry (SIMS) with time-of-flight (TOF)

detection of secondary ions for analysis. Depth profiles were obtained using dual beam

technique with 25 kV Ga+ primary ions for analysis and 3 kV Ar+ ions for sputtering.

A low energy electron gun was used to compensate static electric charge on the oxide.

To avoid surface and interface transients, the signals of secondary ions used for

characterization of the film were measured only in the range of depth, where the

signals are constant. The mass spectrum of HfO2 film consists of O+, Hf+ and various

HfxOy+ clusters. While intensities of the clusters depend on composition, ionization

and fragmentation probabilities, the intensities of the elements depend on composition

and ionization yield only. Thus we take the ratio 16O/180Hf as a measure of relative


30

composition of the elements. Though the ionization yield also depends on composition

(so called matrix effect [19]), by taking the ratio of the signals, we were able to

minimize this effect. Hence higher O/Hf ratio corresponds to higher oxygen content in

the sample, and equal ratio for different samples indicates equal composition.

Table 2.1: The ratios of SIMS intensities O/Hf for HfO2 thin films prepared at various

substrate temperatures (oxygen pressure: 50 mTorr).

Temperatures (oC) O/Hf

25 0.18

200 0.17

300 0.18

500 0.21

750 0.18

Table 2.2: The ratios of SIMS intensities O/Hf for HfO2 thin films prepared at various

oxygen pressures (All samples are prepared at room temperature 25oC.).

Pressure (mTorr) O/Hf

10 0.10

50 0.18

200 0.21

Table 2.1 presents the O/Hf ratio for HfO2 films deposited at various substrate

temperatures. This ratio remains almost constant indicating that the deposition


31

temperature up to 750oC has little effect on the stoichiometry of HfO2 films. However,

the fluctuations of O/Hf ratio in the whole temperature range can be attributed most

likely to the change of surface roughness of the films, which is evidenced from Fig.

2.3. Table 2.2 shows the O/Hf ratio for HfO2 films deposited at room temperature and

various oxygen pressures. At the pressure 50 mTorr and above, the composition of the

oxide remains essentially the same. However at lower pressure, the content of oxygen

falls significantly; at 10 mTorr, it drops down to a half of 50 mTorr. It is thus

concluded that deposition pressure affects the stoichiometry of the as-deposited thin

films whereas deposition temperature does not.

In order to further study the effects of deposition temperature and oxygen

pressure on the physical properties of HfO2 thin films, the optical constants, i.e.

refractive index and extinction coefficient, of HfO2 thin films deposited at various

parameters were determined with a variable angle spectroscopic ellipsometer, and

compared with that of the bulk material HfO2. Figure 2.5(a) shows refractive indexes

of HfO2 films deposited at various substrate temperatures. It can be seen that, with the

increase of deposition temperatures, the refractive indexes increase steadily, and the

curves of spectral dependence of the refractive indexes at 300 and 500oC are nearly

overlapped with each other. It is noticed that when the deposition temperature

increases to 200oC, the refractive index at 600 nm is 1.98, which is close to the bulk

refractive index 2.08 at 600 nm [20]. For the film prepared at 300 and 500oC, the

refractive indexes at 600 nm are 2.11 and 2.12 respectively, which are almost same as

that of bulk material. From the above SIMS measurement, it is concluded that the

change of deposition temperatures has little effect on the stoichiometry of HfO2 thin

film prepared by PLD, which means that the change of refractive index could not be


32

Figure 2.5: Spectral dependence of refractive indexes of HfO2 films deposited at (a)

various substrate temperatures (oxygen pressure: 50 mTorr) and (b) various deposition

pressures (all deposited at room temperature).

200 400 600 800 1000 1200 1400 1600 18001.6

1.7

1.8

1.9

2.0

2.1

2.2

2.3

25 oC 200 oC 300 oC 500 oC

Ref

ract

ive

inde

x

Wavelength (nm)

200 400 600 800 1000 1200 1400 1600 18001.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

2.1

10 mTorr 50 mTorr 200 mTorr

Ref

ract

ive

inde

x

Wavelength (nm)

(a)

(b)


33

due to the variation of oxygen concentration of HfO2 thin films prepared at various

deposition temperatures. However, it is believed that usually for the films deposited at

lower temperature, the as-deposited film has a lower packing density because of a

loose arrangement. A higher deposition temperature increases the mobility of the film

atoms or molecules and therefore favours the formation of more closely packed thin

films, which leads to a higher refractive index.

Figure 2.5(b) shows the refractive indexes of HfO2 films deposited at various

oxygen pressures. Since all the samples were prepared at room temperature, the

influence of deposition temperature on the packing densities of thin films deposited at

various oxygen pressures is assumed to be the same. The change of refractive indexes

at various deposition pressures is mainly attributed by the change of oxygen

concentration in HfO2 thin films from SIMS measurements. It can be seen from Fig.

2.5(b) that, for the film deposited at a low pressure of 10 mTorr, the refractive index is

1.92 at 600 nm, the highest among all the films prepared at various oxygen pressures.

However, such a high refractive index may be due to oxygen vacancies formed during

film preparation [21]. As for the film deposited at a high pressure of 200 mTorr, the

refractive index is only 1.41 at 600 nm. This could be attributed to the incorporation of

excessive oxygen during deposition, which may create voids and absorb water vapor

upon venting the vacuum chamber. Also the reduction of the kinetic energy of the

ablated Hf-bearing species, which undergo more collisions with the background O2

molecules before reaching the growing film surface, will result in a little lower density

film [22]. All the above effects are believed to be responsible for the reduction of the

refractive index of HfO2 thin film deposited at high pressure.

In addition, Figures 2.6(a) and 2.6(b) show the extinction coefficient (K factor)

dependence of wavelength for HfO2 thin films prepared at various deposition


34

Figure 2.6: Spectral dependence of extinction coefficients of HfO2 films deposited at

(a) various substrate temperatures (oxygen pressure: 50 mTorr) and (b) various

deposition pressures (all deposited at room temperature).

200 400 600 800 1000 1200 1400 1600 18001E-12

1E-10

1E-8

1E-6

1E-4

0.01

25oC 200oC 300oC 500oC

Extin

ctio

n co

effic

ient

K

Wavelength (nm)

200 400 600 800 1000 1200 1400 1600 18001E-5

1E-4

1E-3

0.01

0.1

10 mTorr 50 mTorr 200 mTorr

Extin

ctio

n Fa

ctor

K

Wavelength (nm)

(a)

(b)


35

conditions. As shown in Fig. 2.6(a), the extinction factors (K values) decrease for the

samples prepared from room temperature to 300oC. The K value for the sample

prepared at 300oC is quite low in the visible region (380-770 nm) and near infrared

region (770-1400 nm), which indicate that a good quality HfO2 film is obtained in

terms of optical properties. As for the sample deposited at 500oC, due to the measured

wavelength limitation of the instrument, no obvious absorption could be detected in

the ultraviolet region, and the K values are well above those of all other samples in

visible and near infrared regions after simulation. Figure 2.6(b) shows the extinction

factor dependence of wavelength of HfO2 thin films deposited at various oxygen

pressures. It is shown that the extinction factor of the film deposited at low pressure 10

mTorr is well above those of the films deposited at relative high pressures (50 and 200

mTorr), which could be due to large amount of defects in the film, such as oxygen

vacancies [23].

2.3.2. Electrical characterization of HfO2 MIM capacitor

Following the physical properties characterization, HfO2 MIM capacitors are

fabricated at 200, 300, and 400oC in oxygen ambient of 50 mTorr, with the thickness

of 56, 57, and 54 nm for each deposition temperature, to evaluate the electrical

properties. Figure 2.7 presented the TEM photo of 56 nm HfO2 MIM structure

prepared at 200oC, and there is no evidence of interfacial layers formation as

demonstrated by TEM showing an in-depth homogeneous dielectric material. Here it

needs to be mentioned that, we keep the maximum temperature at or below 400oC for

compatibility with the BEOL process. Among various requirements for analog MIM

capacitor, leakage characteristic is a critical issue for MIM capacitor application.


36

Figure 2.8 shows the current-voltage (I-V) characteristics of three samples of HfO2

MIM capacitors. The HfO2 thin film prepared at 200oC shows the lowest leakage

current of 2×10-9 A/cm2 at 3 V. The HfO2 samples deposited

Figure 2.7: TEM photos of 56 nm HfO2 MIM capacitor fabricated at 200oC

at 300oC, however, show the worst leakage property. This is believed to be due to the

formation of grain boundaries after crystallization (Figure 2.2). At an even higher

substrate deposition temperature of 400oC, the leakage property improves though it is

still inferior to the samples prepared at 200oC. This may be due to possible oxidation

of the bottom electrode (Ta) before HfO2 thin films deposition [24]. In addition, as

evidenced from Fig. 2.2, for HfO2 films prepared above 300oC, the crystallinity is

improved and the crystallographic orientation is changed. This implies a change of the

polycrystalline HfO2 structure, and it may alter the electrical properties to some extent.

From previous discussion, it is desirable to prepare HfO2 thin film at a high

temperature such as 300oC in terms of optical properties [25]. However, in order to

obtain good insulating properties, it is desirable to keep the HfO2 thin film to be in a


37

glassy (amorphous) phase, since grain boundaries are detrimental to serve as leakage

paths. In addition, increased surface roughness of HfO2 after crystallization could be

interpreted as an image force which lowers the barrier height for electron transport.

This appears to be another factor which may degrade the leakage property [26].

0 1 2 3 4 5

10-10

10-9

10-8

10-7

10-6

10-5

Le

akag

e cu

rren

t (A

/cm

2 )

DC bias on top electrode (V)

200oC 300oC 400oC

TaHfO2

Al dotGND

Vg +

Figure 2.8: Current-voltage characteristic of HfO2 MIM capacitors prepared at 200,

300, and 400oC respectively.

Figure 2.9 shows the dielectric constants (κ) and dissipation factors ( δtan ) of

HfO2 thin films as a function of the deposition temperature. For HfO2 thin film

deposited at 200oC, the dielectric constant is less dependent on frequency and remains

relatively constant in the whole frequency range (500 kHz-1 MHz). In addition, a low

dissipation factor (below 0.02) is observed at up to 1 MHz. This suggests that the HfO2

MIM capacitor prepared at 200oC has very low dielectric loss with a capacitance

density of 3 fF/µm2. However, for HfO2 films prepared at substrate temperatures of


38

300 and 400oC, the dielectric constants decrease at high frequencies with an obvious

increase in the dissipation factor. This may be related to the higher leakage current

observed.

0

5

10

15

20

25

30

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

D

iele

ctric

con

stan

t (κ)

D

issi

patio

n fa

ctor

10k 1M100k1kFrequency (Hz)

200oC 300oC 400oC

Figure 2.9: Capacitance versus frequency at zero bias for HfO2 MIM capacitors

prepared at 200, 300, and 400oC respectively.

As already known, capacitor voltage linearity is another very important

parameter for MIM capacitors in silicon analog circuit applications. Figure 2.10 shows

the normalized capacitance of the MIM capacitor prepared at 200, 300, and 400oC with

applied voltage at different frequencies. At each measured frequency, the normalized

capacitance of the MIM capacitor can be fitted by

dC/C0=(C(V)–C0)/C0 = αV2 + βV


39

-1 0 1 2 3 4 5 6-2000

0

2000

4000

6000

8000

10000-2000

02000400060008000

1000012000-1000

0

1000

2000

3000

4000

5000

400oC deposition

300oC deposition

200oC deposition

(c)

(b)

1 kHz 10 kHz 100 kHz 1MHz

∆C

/C0 [

ppm

]

DC bias on top electrode (V)

(a)

Figure 2.10: Normalized capacitance of HfO2 MIM capacitors prepared at (a) 200,

(b) 300, and (c) 400oC as a function of voltage applied at a frequency of 1 kHz, 10

kHz, 100 kHz, and 1 MHz respectively.


40

where, C0 is the zero biased capacitance at each frequency, α and β are the quadratic

and linear voltage coefficient, respectively. The capacitances were measured with the

voltage changed from 0 V to 5 V at 1 kHz, 10 kHz, 100 kHz, and 1 MHz respectively.

The α and β values obtained are shown in Table 2.3. The results indicate that the

voltage coefficients of capacitance (VCCs) strongly depend on the frequency and

increase with decreasing the frequency (Figure 2.10 and Table 2.3). This dispersive

behaviour of MIM capacitors with HfO2 dielectric is temporarily attributed to the

existence of traps within high-κ films. Different traps will induce charges with

different time constants and strongly modulate capacitor charges at certain frequencies.

When the frequency is decreased, the induced charges will be easier to follow the ac

signal, and therefore result in higher VCCs. For the mechanism of the frequency

dependence of voltage coefficients, we will further discuss this in Chapter 5. From

Table 2.3, it is noticed that, HfO2 MIM capacitor prepared at 200oC shows both low

quadratic and linear voltage coefficients, which are comparable to MIM capacitor with

Si3N4 [27]. For the HfO2 thin films prepared at 300 and 400oC, the capacitance

variation ∆C/Co turns out to be larger than that of HfO2 thin films deposited at 200oC,

due to their poorer leakage property. Thus, we conclude that the relative capacitance

variation at a certain voltage is correlated to the insulating properties of HfO2 thin

films prepared at various deposition conditions. This kind of behaviour and relation

has also been observed in Ta2O5 MIM capacitors [28].

For comparison, it is also noted that, HfO2 samples prepared at 300 and 400oC

have linear voltage coefficients that are much larger than those prepared at 200oC.

However, their quadratic coefficients are smaller when compared to those amorphous

HfO2 prepared at 200oC. When we consider the voltage linearity requirements for

analog MIM capacitor, the decrease of quadratic coefficients is significant. Since


41

according to the 2001 ITRS roadmap [29], quadratic voltage coefficients are more

critical for the dynamic range of analog capacitor. On the other hand, linear voltage

coefficients can be cancelled out by differential techniques. However, the correlation

between materials properties and the variation of α and β values remains unclear.

Table 2.3: Voltage linearity coefficients as a function of frequency for HfO2 MIM

capacitors prepared at 200, 300, and 400oC respectively.

200oC Deposition 300oC Deposition 400oC Deposition

Frequency α

(ppm/V2)

β

(ppm/V)

α

(ppm/V2)

β

(ppm/V)

α

(ppm/V2)

β

(ppm/V)

1 kHz 134.3 176.6 -20.2 2189.7 -53.1 1797.9

10 kHz 89.0 181.9 31.9 1255.9 -22.4 802.7

100 kHz 64.9 156.5 -10.5 1048.9 3.2 377.4

1 MHz 44.3 125.2 32.8 461.6 8.3 207.6

Following VCCs’ characterization, temperature coefficients of capacitance

(TCCs) were evaluated for 200oC prepared HfO2 MIM capacitors, which could be

defined as:

CppmdTdC

TTCC o/106

=

It should be noted that the capacitance variation with temperature is mainly caused by

the temperature dependence of the dielectric properties. The capacitance change due to

thermal expansion of capacitor area is believed to have negligible effect [30].


42

Figure 2.11 shows the normalized capacitance of the MIM capacitor fabricated

at 200oC as a function of temperatures at a frequency of 10 kHz, 100 kHz, and 1 MHz,

respectively. The results show that the capacitance increases with the increase of the

temperature. The temperature coefficients of capacitance (TCCs) are 196 ppm/oC, 196

ppm/oC, and 147 ppm/oC for the frequency at 10 kHz, 100 kHz, and 1 MHz,

respectively. The TCC at 1 MHz is the lowest for HfO2 MIM capacitor. Compared to

the TCCs of 127 ppm/oC at 1 MHz and 187 ppm/oC at 1 kHz for Si3N4 MIM capacitor

[27], HfO2 MIM capacitor shows comparable TCCs.

0 25 50 75 100 125 150

0

5000

10000

15000

20000

25000

30000

35000 TCC = 147 ppm/oC (1 MHz) TCC = 196 ppm/oC (100 kHz) TCC = 196 ppm/oC (10 kHz)

Nor

mal

ized

cap

acita

nce

[ppm

]

Temperature (oC)

Figure 2.11: Normalized capacitance of HfO2 MIM capacitor prepared at 200oC as a

function of temperature.

Considering the leakage characteristics and the capacitance performance at

various deposition temperatures, it is desirable to select the optimum substrate

temperature of 200oC for HfO2 thin film deposition using PLD. In addition, recent


43

progress in Cu and low-κ interconnect technology requires much lower temperature

than 400°C which is traditionally accepted for BEOL process. The maximum allowed

temperature for low-κ dielectric decreases with the decrease of κ value [31]. To get a

κ value of around 2, we need to keep the maximum process temperature as low as

200°C [31]. Furthermore, Cu diffusion will be much reduced at a lower temperature.

Comparing to 400°C, Cu diffusivity in low-κ dielectric at 200°C is 4 orders lower [32].

In summary, according to the ITRS roadmap [29], the typical requirements of

analog capacitor are capacitance density of 3 fF/µm2, voltage linearity of 100 ppm/V2,

and leakage current of 7 fA/[PF·V] in the current technology node. For the 56 nm

HfO2 MIM capacitor prepared at 200oC, the obtained parameters are comparable with

an even lower leakage of ~2.8 fA/[PF·V], which can meet the strict specifications of

analog capacitor.

2.4. Limitations of PLD for thin film fabrication

Even though PLD has been proved to be an effective method for the fabrication

of high quality films, like HfO2 as a dielectric in MIM capacitors, some intrinsic

drawbacks severely limited PLD’s application for thin film deposition in the current Si

ULSI technology. Among them, particulate splashing and non-uniformity are of the

most serious concerns which we like to point out as follows:

1. Particulate splashing

Particulate splashing is an intrinsic problem of PLD method for many thin film

fabrications [1, 2], which is particularly problematic for semiconductor films when


44

(a)

(b)

Figure 2.12: SEM top views of HfO2 film surfaces prepared with the laser fluence of (a)

4.0 and (b) 7.0 J/cm2 respectively (fabricated at room temperature).


45

considering small feature sized electronic devices. In the PLD processing of HfO2

films, particulates were observed to be present on the surface of HfO2 thin films.

Figure 2.12(a) and Figure 2.12(b) illustrated the top views of PLD processed HfO2

films with the laser fluence of 4.0 and 7.0 J/cm2 respectively. The feature sizes of the

particulates were in the sub-micron regime. As expected, the number of generated

particulates decreases with laser fluence. Therefore, in our work, we keep the laser

energy low to minimize the particulates density on the surfaces of HfO2 films.

However, it would be very difficult in most cases to completely avoid the particular

splashing during the PLD processing of thin films using the conventional

configuration of PLD system as shown in Fig. 2.1.

With regard to the particulate occurrence, the following mechanisms were

believed to be responsible, like subsurface boiling, the shock wave recoil pressure

induced liquid layer expulsion, and splashing can be one or a combination of those

mechanisms. To avoid splashing, numerous attempts have been made, such as the

manipulation of plume and target, and mechanical filter, etc. However we do not

intend to further investigate the particulates splashing on HfO2 films in this work.

2. Non-uniformity for thin film deposition

Another major drawback is the lack of uniformity over the substrate area due to

the narrow angular distribution of the plume [1, 2], which makes the PLD method

unfeasible for the current Si wafer processing. In practice, several methods have been

employed to alleviate this problem, including the rastering of the laser, the substrate

rotation, and so on. In our work, we used small pieces of Si wafers with the area being

~1×1 cm2.

In a word, even though PLD is a good tool for materials study and research

purpose, it remains to be difficult to find its wide acceptance in industry for thin film


46

fabrications. In the following chapters, sputtering and atomic-layer deposition (ALD)

methods will be used to further explore HfO2 based high-κ dielectrics for MIM

capacitors application which are free from the drawbacks mentioned above.

2.5. Conclusion

In this chapter, HfO2 thin films have been prepared by PLD at various

deposition conditions. For the physical properties characterization, the influence of the

substrate temperature and pressure on film properties, including crystallinity, surface

morphology, relative stoichiometry, and optical properties, were investigated. It can be

concluded that substrate temperature has little effect on the stoichiometry, whereas

deposition pressure plays an important role in determining the ratio of Hf and O. The

result also shows that the optical properties of HfO2 thin films have a strong

dependence on both the deposition temperature and pressure. Thus, the appropriate

choice of substrate temperature and deposition pressure in PLD processing of HfO2

thin films is of great importance.

Based on the physical characterization, MIM capacitors using HfO2 dielectric

were fabricated. The electrical properties of HfO2 MIM capacitors were evaluated as a

function of deposition temperature. It is also shown to have strong correlation to the

material properties. For the first time, a high performance MIM capacitor using high-κ

HfO2 dielectric was demonstrated with a low thermal budget of 200oC. It showed an

overall high electrical performance such as a high capacitance density of ~3.0 fF/µm2,

a low leakage current of 2x10-9 A/cm2 at 3 V, low voltage coefficients of capacitance,

and good frequency dispersion property. Compared with Si3N4 MIM capacitors

currently used in the industry, the capacitance density is high. In summary, all these


47

indicate that HfO2 is a very good high-κ dielectric used in MIM capacitors for Si

analog circuit applications.


48

References:

[1] D. B. Chrisey and G. K. Hubler, Pulsed Laser Deposition of Thin Films (Wiley,

New York, 1994)

[2] E. Fogarassy and S. Lazare, Laser ablation of electronic materials: basic

mechanisms and applications (Amsterdam: North-Holland, 1992)

[3] M. Y. Ho, H. Gong, G. D. Wilk, B.W. Busch, M. L. Green, P. M. Voyles, D. A.

Muller, M. Bude, W. H. Lin, A. See, M. E. Loomans, S. K. Lahiri, and P. I.

Raisanen, “Morphology and crystallization kinetics in HfO2 thin films grown by

atomic layer deposition,” J. Appl. Phys., Vol. 93, No. 10, pp. 1477-1481, 2003.

[4] J. Aarik, A. Aidla, H. Mändar, T. Uustare, K. Kukli, and M. Schuisky, “Phase

transformations in hafnium dioxide thin films grown by atomic layer deposition at

high temperatures,” Applied Surface Science, Vol. 173, pp. 15-21, 2001.

[5] M, Gilo and N, Croitoru, “Study of HfO2 films prepared by ion-assisted deposition

using a gridless end-hall ion source,” Thin Solid Films, Vol. 350, pp. 203-208,

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[6] T. Nishide, S. Honda, M. Matsuura, and M. Ide, “Surface, structural and optical

properties of sol-gel derived HfO2 films,” Thin Solid Films, Vol. 371, 61-65, 2000.

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Krishnan, H. -J Cho, A. Shahriar, and J. C. Lee, “Fabrication of high quality ultra-

thin HfO2 gate dielectric MOSFETs using deuterium anneal,” in Proc. of IEDM,

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49

[9] S. J. Lee, H. F. Luan, W. P. Bai, C. H. Lee, T. S. Jeon, Y. Senzaki, D. Roberts, and

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on Electron Devices, Vol. 50, pp. 2088-2094, 2003.

[13] H. Hu, C. Zhu, Y. F. Lu, M. F. Li, B. J. Cho, and W. K. Choi, “A High

Performance MIM Capacitor Using HfO2 Dielectrics,” IEEE Electron Device

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[14] H. Hu, C Zhu, Y. F. Lu, Y. H. Wu, T. Liew, M. F. Li, B.J. Cho, W. K. Choi, and

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50

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“Spectroscopic ellipsometry characterization of high-κ dielectric HfO2 thin films

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52

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2187, 1999.

Chapter 3 Characterization of HfO2 MIM capacitors for RF Application

53

Chapter 3

Characterization of HfO2 MIM capacitors for RF Application

3.1. Introduction

As known in Chapter 1, MIM capacitor is spawned by wireless communication

revolution. For instance, the mobile phone works at 0.9 and 1.9 GHz, Bluetooth at 2.4

and direct satellite TV operation at 10 GHz. With continuing improvement of Si

CMOS technology, low-cost Si circuits could operate well in the 1-10 GHz range or

even higher frequency [1, 2]. As one of the passive elements, MIM capacitors

integrated into Si ICs need to provide well behaved and predictable capacitance

characteristics in RF regime.

SiO2 and Si3N4 are traditionally employed for Si RF applications with well

behaved capacitance characteristics in the RF regime [3-6]. According to the ITRS

roadmap, a high capacitance density of ≥ 9 fF/µm2 will be needed at or beyond year

2004 [7] for RF bypass capacitors. This will necessitate the dielectric thicknesses of

4.3 and 7.8 nm for SiO2 and Si3N4, respectively. However, such thin SiO2 or Si3N4

layers can not be put into practical use due to the anticipated poor electrical

performance as detailed in Chapter 1. Though multiple-layer MIM capacitors intended

to increase the capacitance density have been reported using the conventional

dielectrics of SiO2 and Si3N4 [8, 9], it is not favourable in terms of process complexity

and cost. Therefore, selecting high-κ dielectric will benefit for RF MIM capacitors


54

application to a great extent. However, if high-κ materials were to be considered for

the replacement of SiO2 and Si3N4, their RF performances have to be evaluated. With

regard to high-κ HfO2 dielectric, no RF performance of HfO2 MIM capacitors has been

reported so far. In particular, the dielectric constant dependence on frequency in RF

regime is paramount because preserving high-κ even at high frequency is a

fundamental requirement. In this chapter, we investigated the RF characteristics of

HfO2 MIM capacitors prepared by physical vapor deposition (PVD) [10]. Compared to

pulsed-laser deposition (PLD) technique [11], PVD is much superior in terms of

particulate splashing and film non-uniformity [12]. PVD also has the advantage of

high throughput which is beneficial for mass production. Recently, PVD has been

successfully employed for fabrication of high-κ dielectrics such as HfO2 [13] and

Ta2O5 [14] for high performance MIM capacitors application in back-end line (BEOL)

integration.

In addition, our results show that HfO2 MIM capacitors with various

thicknesses exhibit different stress induced leakage currents (SILCs) characteristics are

functions of the stress time under constant stress voltage. The variation of voltage

coefficients of capacitance (VCCs) with stress time has also been investigated under

prolonged electrical stress. Generation of traps under the stress has been proposed as a

plausible mechanism for explanation of SILCs and stress related variation of VCCs.


55

3.2. Experiments

3.2.1. RF MIM capacitor fabrication

The MIM capacitors with RF test structures were fabricated on standard p-type

Si substrates with a resistivity of 4-8 Ω·cm. Before defining the bottom electrode of the

HfO2 MIM capacitors, 500 nm SiO2 was deposited on silicon substrate for isolation

purpose. In order for RF characterization, coplanar transmission lines were fabricated

on SiO2 to serve as the top and bottom electrodes of MIM capacitors. High-κ HfO2

dielectric films were deposited by reactive sputtering on 0.6-µm-thick patterned

bottom Ta transmission line at room temperature in a gas mixture of Ar (27 sccm) and

O2 (3 sccm). The chamber pressure was maintained at 5 mtorr during sputtering, and a

DC power of 250 W was applied to the Hf target. Two HfO2 samples were fabricated

with the physical thicknesses of 22 and 47 nm, measured by ellipsometer; and they are

denoted as HfO-1 and HfO-2, respectively. Finally, a layer of 0.6-µm-thick Al was

deposited to define both the top electrode and the transmission line. MIM capacitors

with different area were prepared. Figure 3.1 illustrates major fabrication steps and

schematic top views of RF HfO2 MIM capacitor together with open dummy structure,

where the open dummy device was used to de-embed the parasitic from the bond-pads

and transmission lines [15, 16].

The leakage current was measured using an HP4155B semiconductor

parameter analyzer, and the capacitance voltage characteristics were acquired with the

help of an HP4284A precision LCR meter with frequencies varying from 10 kHz to 1

MHz. On-wafer scattering (S) parameters were measured using HP 8510C network

analyzer with the GGB's air coplanar probes (ACP) in ground-signal-ground (GSG)


56

Figure 3.1: Major fabrication steps and schematic top views of RF HfO2 MIM

capacitor and open dummy structure.

Mask 1: Transmission line patterning after bottom electrode deposition

Mask 2: Contact hole etching after high-κ HfO2 deposition

Mask 3: RF MIM structures patterning after top electrode deposition

RF capacitor structure Open dummy structure

Transmission lines


57

configuration for RF characterization, and a precise calibration procedure including

open, short, through, 50 Ω load has been implemented using impedance standard

substrate before devices measurement.

3.2.2. S-parameters for RF characterization

For RF characterization and circuit modelling of HfO2 MIM capacitors, S

parameter is essential for the practical system characterization in RF regime, since

measuring most other parameters like ABCD, Y, Z etc. calls for the input and output of

the device to be successively opened and short circuited. This can be hard to do,

especially at RF frequencies where lead inductance and capacitance make short and

open circuits difficult to obtain. In addition, short or open circuits will induce

oscillations which could make the measurement invalid and even destroy the device

[17, 18].

Figure 3.2: The definition of S-parameters for a two-port network

The wave functions used to define S-parameters for a two-port network are

presented in Fig. 3.2, where αn and bn (n = 1, 2) are the normalized incident and


58

reflected power waves used in S-parameter definitions shown in (3.1) and (3.2), and

0Z is the characteristic line impedance.

The S-parameters could then be defined as:

⎪⎭

⎪⎬⎫

⎪⎩

⎪⎨⎧

⎥⎥⎦

⎤

⎢⎢⎣

⎡=

⎪⎭

⎪⎬⎫

⎪⎩

⎪⎨⎧

2

1

2221

1211

2

1a

a

SS

SS

b

b (3.3)

0

011 ZZ

ZZS

in

inin +

−=Γ= (3.4)

Simply speaking, S-parameters can be directly related to the reflection coefficients

( inΓ ) at the input and output of the two-port network as defined in (3.4). As a result,

the measurement of S11 and S21 could be achieved by matching the line impedance 0Z

at port 2 as shown in Fig. 3.2 through a corresponding load impedance ZL = 0Z , where

0Z is 50 Ω in our measurement system.


3.3.1. RF characterization

To investigate the capacitance characteristics of HfO2 MIM capacitors in RF

regime, we use a π network based equivalent circuit model for the MIM capacitor as

shown in Fig. 3.3. The Rp and C describe the basic electrical model of high-κ HfO2

MIM capacitors, where Rp is proposed to have its origin in the dielectric loss of the

high-κ material and Rs, Ls1 and Ls2 represent the parasitic resistance and inductance

)()2/1(

)()2/1(

00

00

nnn

nnn

IZVZb

IZVZa

−×=

+×= (3.1)

(3.2)


59

from the coplanar transmission lines used for RF measurements. The elements (Cox1,

R1, C1 and Cox2, R2, C2) in the shunt branches in Fig. 3.3 represent the coupling of the

top and bottom electrodes to ground through SiO2 and Si substrate. Standard

procedures were used to de-embed the parasitic from the probe-pads [19]. One can

refer to [17, 18] for S parameters’ conversion into Y parameters.

[SDUT/dummy] ⇒ [YDUT/dummy] (3.5)

Ycapacitor = YDUT-Ydummy (3.6)

Here SDUT (YDUT) denote the S-(Y-) parameters of the MIM capacitors along with the

probe pads and transmission lines while Sdummy(Ydummy) denote the S-(Y-) parameters

of probe pads and transmission lines alone. It is important to point out here that shunt

elements (Cox1, R1, C1 and Cox2, R2, C2) and series elements Rs, Ls1 and Ls2 are used to

account for the transmission lines. The shunt elements are de-embedded as described

above and the part of circuits within the dotted enclosure in Fig. 3.3 has been used to

simulate the S-parameters of the MIM capacitor.

Figure 3.3: The equivalent circuit model for capacitor simulation at RF regime.

The de-embedded and simulated S-parameters were computed by Agilent

Technologies’ ICCAP [20] modelling software using SPICE3 as the circuit simulator.


60

Figure 3.4: The measured and simulated S-parameters for (a) HfO-1 and (b) HfO-2.

(Simulation and parameter extractions were done by ICCAP.)

(a)

(b)


61

Fig. 3.4(a) and 3.4(b) show the measured and simulated two-ports S-parameters (S11

and S21) for HfO-1 and HfO-2 from 50 MHz to 20 GHz. As can be seen, the measured

and simulated data over the entire frequency range from 50 MHz to 20 GHz are in

excellent agreement. This suggests that the equivalent circuit model shown in Fig. 3.3

is suitable and reliable for the capacitor modelling and parameters extraction.

With regards to the lumped equivalent circuit model for capacitor simulation, it

is noticed that other similar approaches have been used. There appears to be slightly

difference when considering the capacitor geometry and physical topology [21, 22]. In

addition, Ls1+Ls2 is ~110 pH, Rs is ~12 Ω, and Rp is in the range of ~10 MΩ for HfO-1

and HfO-2. Large value of Rs can be mainly ascribed to the relatively highly resistive

Ta transmission line and thin metal layers. The use of high conductivity metals like Cu

and/or increasing the metal layer thickness will reduce Rs and hence higher quality

factor. In comparison, the high value of Rp indicates the good insulating property of

the HfO2 RF MIM capacitors.

The overall effective capacitance (including the impedance due to Ls1, Ls2 and

Rs) versus frequency for HfO-1 and HfO-2 is as presented in Fig. 3.5. This was

calculated from the 1-port admittance (Y11) using equations (3.5) and (3.6):

jC

RYZeff

effin *1/1 11 ω

−+== (3.5)

)((1

ineff Zimag

C∗

−=ω

(3.6)

It is noticed that the transition from capacitive to inductive behaviours are due to the

above mentioned parasitic inductances. These are believed to be associated with the

interconnect and are extrinsic to the MIM structure. The self-resonance frequency

including the transmission line parasitic for HfO-1 is 9.4 GHz in comparison with

HfO-2 positioned at 14.3 GHz. This is mainly due to the smaller capacitance value of


62

0.0 5.0G 10.0G 15.0G 20.0G-200.0p

0.0

200.0p

400.0p

600.0p

Sing

le e

fftiv

e ca

paci

tanc

e

Frequency

HfO-1 HfO-2

Figure 3.5: High frequency response of PVD HfO2 MIM capacitors from 50 MHz to

20 GHz for HfO-1 and HfO-2.

1k 100k 10M 1G 100G0

2

4

6

8

10

12

Cap

acito

r den

sity

(fF/µm

2 )

Frequency (Hz)

HfO-1 HfO-2

Low frequency ≤ 1MHz RF analysis up to 20 GHz

Figure 3.6: The frequency dependence of capacitance density for PVD HfO2 MIM

capacitors HfO-1 and HfO-2.


63

HfO-2 [23]. The useful operating frequency range for MIM capacitor could be further

extended by minimizing the parasitic from the interconnect.

In order to further study the capacitance-frequency characteristics of HfO2

MIM capacitors, we have plotted the capacitance measured at low frequency (10 kHz-

1MHz) and the extracted capacitance from RF measurements in Fig. 3.6. It is

straightforward to conclude from Fig. 3.6 that the PVD HfO2 shows good capacitance-

frequency dependency for MIM capacitors, which is in agreement with the conclusions

of Dong et al. who investigated HfO2 thin film for RF-MOSFET applications [24]. In

addition, HfO2 MIM capacitors with different thicknesses show high capacitance

densities of 7.3 and 3.5 fF/µm2 for HfO-1 and HfO-2 respectively. The dielectric

constant was calculated to be approximately 18.

It is worthwhile to point out that PVD high-κ HfO2 could still preserve its high-

κ value in RF regime (up to 20 GHz) for MIM capacitor applications within the

thermal budget limitation of the back-end integration. In contrast, the dielectrics

constants of some high-κ materials like TiSiOx and AlTiOx have been reported to have

strong frequency dependence with the dielectric constant dropping as the frequency

increase [15, 25]. Therefore, those types of capacitors are unsuitable for MIM

capacitors application from the circuit design perspective. In addition, no area

dependency was found for these two HfO2 MIM capacitors. A small device area

(25×25 µm2) were selected for characterization in this work, since RF circuit

applications in GHz range would need typical capacitance values of a few pico-farads

only.


64

Figure 3.7: Stress induced leakage currents (SILCs) characteristics of (a) HfO-1 and (b)

HfO-2 under the constant voltage stress at 1.5 V

.

0.0 0.5 1.0 1.5 2.010-9

10-8

10-7

10-6

10-5

10-4

10-3

Voltage (V)

J (A

/cm

2 )

Fresh 2000 s @1.5 V 4000 s @1.5 V 6000 s @1.5 V

0.0 0.5 1.0 1.5 2.0

10-9

10-8

10-7

10-6

10-5

10-4

10-3 Fresh 2000 s @1.5 V 4000 s @1.5 V 6000 s @1.5 V

J (A

/cm

2 )

Voltage (V)

(a)

(b)


65

3.3.2. DC and low frequency measurements

Figures.3.7(a) and 3.7(b) show the J-V characteristics of HfO-1 and HfO-2 at

room temperature (~25oC) before and after the stress, respectively. As shown in Fig.

3.7(a), HfO2 MIM capacitor with thin film (HfO-1) shows large SILCs, the

observation is similar to what has been found in SiO2 system. The large SILCs are

believed to be due to the neutral traps generation during the stress [26], since the

generated trap sites under stress will act as “stepping stones” for tunnelling carriers

and give rise to excess leakage. Comparing with measurement of fresh capacitors, the

leakage current density increased by nearly one order of magnitude after 4000 s stress

at 1.5 V. With the stress time increased up to 6000 s, the SILC nearly saturated, which

is believed to be due to the saturation of created trap sites under certain stress

conditions [27]. In contrast, the leakage of HfO-2 showed minimal increase for the

same amount of stress up to 6000 s. The different characteristics of SILCs for HfO2

MIM capacitors may be due to the smaller amount of traps generated during the

prolonged stress for thick oxide with a lower electric field [28]. However, for high

capacitance density capacitor (like bypass RF capacitor), thinner dielectric film

becomes imperative. However, excessive leakage is undesirable under prolonged bias

condition. This becomes a limiting factor for the scaling of dielectric films [29]. In

order to improve the leakage property of PVD HfO2 dielectric, the methods including

novel plasma treatment [30, 31], and new structures like HfO2-Al2O3 laminate [32]

may need to be implemented. However, the challenge is to achieve high quality

dielectric films while complying with the thermal budget of back-end-of line

integration (≤ 400oC) and maintaining the high throughput.


66

Figure 3.8: Stress time dependence of (a) the quadratic voltage coefficients and (b) the

linear voltage coefficients for HfO-1 under the constant voltage stress at 1.5 V.

0 1000 2000 3000 4000 5000 60001500

1800

2100

2400 100 kHz 500 kHz 1 MHz

Stress time (s) @ 1.5 V

Qua

drat

ic c

oeffi

cien

ts (p

pm/V

2 )

0 1000 2000 3000 4000 5000 6000900

1200

1500

1800

2100

Line

ar c

oeffi

cien

t (pp

m/V

)


100 kHz 500 kHz 1 MHz

(a)

(b)


67

The voltage linearity property of HfO2 MIM capacitors under stress has also

been investigated and the evolution of voltage linearity has been studied. The

capacitance-voltage characteristics of the MIM capacitors were measured from –2 V to

2 V at 100 kHz, 500 kHz, and 1 MHz before and after the stress. At each frequency,

the measured capacitance of the MIM capacitor can be fitted by

C(V) = C0 (αV2+βV+1) (3.7)

where, C0 is the zero biased capacitance at each frequency, V is the bias voltage; α and

β are the quadratic and linear voltage coefficients which have different implications for

analog and RF MIM capacitor applications according to ITRS roadmap [7]. Figs.

3.8(a) and 3.8(b) depict both the quadratic and the linear coefficients as a function of

stress time for HfO-1. It is interesting to note that both α and β values decrease with

stress time at 1.5 V for each measured frequency, which is contrary to the observed

SILCs in Fig. 7(a) showing that the SILCs increase with the stress time. Figures.3.8(a)

and 3.8(b) present the voltage coefficients of HfO-2. It could be also seen that both α

and β coefficients decrease with the stress time. However, thick HfO2 film showed less

stress dependence in comparison with HfO-1, which is in accordance with SILCs

characteristics of HfO-2. In addition, the reduction of linear coefficient seems slightly

more obvious than that of quadratic coefficient under stress for HfO-1 and HfO-2.

It needs to be mentioned here that the mechanism for stress-time evolution of

VCCs is under investigation and is still not clear. However, the reduction of voltage

coefficients after stress should not be construed to suggest the improvement of the film

quality. Rather, the film quality degrades under stress as indicated by SILCs. Further,

it seems plausible that the different characteristics of SILCs and evolution of voltage

linearity under stress have the same physical origin. For the CV characteristics of HfO2

MIM capacitors, which could be fitted by the polynomial equation, the distortion of


68

Figure 3.9: Stress time dependence of (a) the quadratic voltage coefficients and (b) the

linear voltage coefficients for HfO-2 under the constant voltage stress at 1.5 V.

0 1000 2000 3000 4000 5000 6000200

300

400

500

600

700

800 100 kHz 500 kHz 1 MHz

Qua

drat

ic c

oeffi

cien

ts (p

pm/V

2 )


0 1000 2000 3000 4000 5000 6000100

200

300

400

500

600 100 kHz 500 kHz 1 MHz

Line

ar c

oeffi

cien

t (pp

m/V

)


(a)

(b)


69

CV curve for the device after stress was reflected by the reduction of quadratic

coefficients. The flattening-out of the characteristic (CV curve bent downward after

stress.) is similar to the stretch-out of the CV curve in MOS device when interface

states are present [33]. The variation of linear coefficients is manifested in the parallel

shift of CV curve towards the positive side of the voltage axis akin to a positive shift

of flat band voltage due to the negative charges seen in the MOS capacitors [33]. This

suggests the plausibility of the trap generation under stress explaining both the SILCs

and the evolution of VCCs after application of stress.

Figure 3.10: The equivalent circuit for HfO2 MIM capacitors after stress. The added

branch stands for the generated trapped states in MIM capacitor after stress.


70

An equivalent circuit shown in Fig. 3.10 may help to develop a

phenomenological model for the variation of quadratic and linear coefficients after

stress, where C and Rp have the same physical meanings as described in Fig.3.3 for the

fresh device. After stress, the additional branch comprising C1 and R1 was introduced,

where C1 and R1 account for the traps generated after the DC stress in the dielectric,

with energy levels distributed within the oxide band gap. From this circuit diagram in

Fig. 3.10, C1 in the added branch equals to Q1/V1, where Q1 stands for the charges

stored in the added branch and V1 is the voltage on C1. It may be surmised that if Q1 is

of negative sign, it could account for the flattening-out of the CV curves after stress

corresponding to a net charge loss in the dielectric. The voltage dependence of C1 and

R1 can thus effectively model both linear and quadratic coefficients.

Though more works are needed for a clear physical understanding, it appears

plausible that the generated traps after stress in high-κ dielectric are responsible for the

reduction of the quadratic and linear coefficients. It may be of interest to note that even

though the oxide quality deteriorates during stress, the reduced voltage coefficients

seem beneficial from the perspective of MIM capacitors applications. In addition, it

may also be noticed that the frequency dependence of voltage linearity (Fig.3.8 and

Fig.3.9) is almost unchanged after the stress, even though both quadratic and linear

coefficients improve at higher frequency.

3.4. Conclusion

In this chapter, we investigated the electrical characteristics of high-κ PVD

HfO2 MIM capacitors from IF (10 kHz) to RF (20 GHz) frequency range for the first

time. High-κ HfO2 dielectric has been demonstrated to exhibit good capacitance-


71

frequency dependency, i.e. the dielectric constant of HfO2 remains almost constant in

the entire frequency range up to 20 GHz. This suggests that HfO2 is a good high-κ

candidate for RF MIM capacitors applications. In addition, HfO2 MIM capacitor with

thin dielectric film showed large SILCs compared to thick HfO2 film, which is

believed to be due to the reduced amount of traps generated in thick oxide film on

account of the smaller electric field in the thicker film. The evolution of voltage

linearity under stress reveal that both the quadratic and the linear coefficient reduce in

magnitude with stress time, and correlate to the variation of SILCs in HfO2 MIM

capacitors. It has been proposed that the reduced voltage coefficients originate from

the generated traps in high-κ HfO2 films after the electrical stress.


72

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[8] N. P. Kim, K. L. Coates, C.-P. Chien, and M. H. Tanielian, “Thin firm passive

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Rustagi, M. B. Yu, A. Chin, and D. -L. Kwong, “Investigation of PVD HfO2 MIM

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[11] H. Hu, C. Zhu, Y. F. Lu, M. F. Li, B. J. Cho, and W. K. Choi, “A high

performance MIM capacitor using HfO2 dielectrics,” IEEE Electron Device Lett.,

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[12] D. B. Chrisey and G. K. Hubler, Pulsed Laser Deposition of Thin Films (Wiley,

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[13] S. J. Kim, B. J. Cho, M. F. Li, C. Zhu, A. Chin, and D. -L. Kwong, “HfO2 and


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[14] T. Yoshitomi, Y. Ebuchi, H. Kimijima, T. Ohguro, E. Morifuji, H. S. Momose, K.

Kasai, K. Ishimaru, F. Matsuoka, Y. Katsumata, M. Kinugawa, and H. Iwai,

“High performance MIM capacitor for RF BiCMOS/CMOS LSIs,” in Proc. of

BCTM, pp. 133-136, 1999.

[15] S. B. Chen, J. H. Chou, A. Chin, J. C. Hsieh, and J. Liu, “RF MIM capacitors

using high-κ Al2O3 and AlTiOx dielectrics,” in Proc. of IEEE MTT-S Intl.

Microwave Symp., pp. 201-204, 2002.

[16] C. H. Huang, M.Y. Yang, A. Chin, C. X. Zhu, M. F. Li, and D. L. Kwong, “High

density RF MIM capacitors using high-κ AlTaOx dielectrics,” in Proc. of IEEE

MTT-S International Microwave Symp., pp. 507-510, 2003.


74

[17] D. M. Pozar, Microwave Engineering (John Wiley & Sons, 1998)

[18] Thomas H. Lee, The design of CMOS radio-frequency integrated circuits

(Cambridge press, 1998)

[19] P.J.van Wijnen, H. R. Claessen, and E. A. Wolsheimer, “A new straightforward

calibration and correction procedure for ‘On Wafer’ high frequency S-parameters

measurement (45MHz-18GHz),” in Proc. of BCTM, pp. 70-73, 1987.

[20] IC-CAP manual, Hewlett Packard, 1998.

[21] C. Geng, K. W. Chew, K. S. Yeo, M. A. Do, J. Ma, C. T. Chua, and K. Shao,

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[22] S. –S. Song, S. –W. Lee, J. Gil, and H. Shin, “A simple wide-band MIM capacitor

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of SSDM, pp. 438-439, 2003.

[23] J.–H. Lee, D.–H. Kim, Y.–S. Park, M.-K. Sohn, and K.-S. Seo, “DC and RF

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345–347, 1999.

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Kavalieros, Anand Murthy, Brian Roberds, Pat Stokley and Robert Chau, “High

frequency response of 100 nm integrated CMOS transistors with high-κ gate

dielectrics,” in Proc. of IEDM, pp. 231-234, 2001.

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dielectric thin films grown by pulsed-laser deposition,” Appl. Phys. Lett., Vol. 80,

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of the stress time dependence of SILC,” in Proc. IEEE Intl. Reliability Physics

Symp., pp. 396–399, 1999.

[28] E. F. Runnion, S. M. Gladstone, IV, R. S. Scott, Jr., Member, D. J. Dumin, L. Lie,

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Silicon Oxide,” IEEE Trans. Electron Devices, Vol. 44, p. 993-1001, 1997.

[29] H. Satake and A. Toriumi, “Common origin for stress-induced leakage current

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No. 11, pp. 1308-1310, 1998.

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1996.

[33] E H Nicollian and J R Brews, MOS (Metal Oxide Semiconductor Physics and

Technology (Wiley, New York, 1982)

Chapter 4 HfAlOx MIM Capacitors by Atomic-Layer-Deposition (ALD)

76

Chapter 4

HfAlOx MIM Capacitors by Atomic-Layer-Deposition (ALD)

4.1. Introduction

4.1.1. ALD method for thin films fabrication

The concept of atomic-layer-deposition (ALD) was first introduced by T.

Suntola, et. al in 1977 as a variant of physical vapor deposition named as atomic layer

epitaxy (ALE) [1]. This technique is limited to very few material systems and required

stringent process conditions due to the PVD implementation of ALD. Nowadays, the

implementation of basic ALD concepts using chemically reactive molecular precursors

is capable of preparing insulating dielectric films, metal electrodes and interconnect

barriers as well [2].

The foundation of ALD is the self-limiting characteristics of the process steps

in which a chemical vapor reacts with a surface until a monolayer has been

chemisorbed. The reaction then stops, so the process is called “self-limiting”. A

second vapor then reacts with this surface in a second self-limiting reaction depositing

a second of atoms onto the films. The second reaction must return the surface to a state

in which it is ready to react with the first reactant. The cycle of reactions can then be

repeated to build up a compound atomic layer by atomic layer.


77

Due to the self-limiting nature of ALD allowing for atomic scale control, it has

the promises of unprecedented control of film thickness, uniformity, quality and

materials properties. In particular, ALD has the exclusive advantage of manipulating

the physical and electronic properties of thin films by changing their composition,

which is practiced by layering nanolaminate stacks of films as well as by deposition of

mixed component alloys. Though ALD has been criticized for its low deposition rates,

some novel schemes have recently been proposed to address this concern [3, 4].

Currently, the silicon semiconductor industry is devoting a large amount of

research to ALD, as it is considered to be a likely future method for the production of

thin dielectric films. ALD is already used to grow gate oxides [4, 5] and DRAM

dielectrics [6, 7]. In this chapter, ALD was chosen as an alternative method to fabricate

HfO2-Al2O3 dielectrics using various dielectric structures.

4.1.2. Characteristics of ALD processed HfO2 and Al2O3

In this work, the precursors for HfO2 deposition are HfCl4 and H2O, and those

for Al2O3 are Al(CH3)3 (TMA) and H2O. N2 was used as both carrier gas and purge gas.

Table 4.1 summarizes ALD process conditions for pure HfO2 and Al2O3 films growth

Table 4.1: ALD process conditions for the deposition of HfO2 and Al2O3

Pressure HfCl4 pulse HfCl4 purge H2O pulse H2O purge Temperature[mtorr] [ms] [ms] [ms] [ms] [oC]

HfO2 deposition

125 2000 6000 400 3000 320 Pressure TMA pulse TMA purge H2O pulse H2O purge Temperature[mtorr] [ms] [ms] [ms] [ms] [oC] Al2O3

deposition 200 200 1500 500 3000 340


78

using GENUS LYNX2 system. The deposition cycle is broken up into a sequence of

discrete process steps, and the low thermal budget (~300oC) of ALD method is

compatible with back-end of line (BEOL) integration for MIM capacitor fabrication.

In comparison, it was noticed that HfCl4 necessitate much longer pulse and purge time

compared to TMA.

Figures 4.1(a) and 4.1(b) present the growth rates as a function of growth

cycles for HfO2 and Al2O3. The film thickness was measured by ellipsometry and the

underlayer thickness was deducted using recipes calibrated by SiO2. Those films were

grown on 6-inch Si (100) substrates, and the films uniformity on wafer is less than 2%.

It is straightforward to note that the linear growth rates dependence on the growth

cycle is characteristic of ALD processed HfO2 and Al2O3 deposition. The intercepts of

-1.8 and 0.1 is observed for zero HfO2 and Al2O3 thicknesses respectively, where the

relatively large negative intercept implies that the growth of HfO2 must be nonlinear

for the initial several cycles, which is in agreement with [8]. In comparison, HfO2 has

a slightly lower deposition rate of 0.6 Å/cycle than that for Al2O3 at 0.76 Å/cycle. It is

believed that the high sticking coefficient of HfCl4 precursor due to its strong ionic

bonding is responsible for long purge/pulse time and the low growth rate of HfO2 [6].

Recently, it is reported that using the liquid precursors such as Hf(NEtMe)4 for HfO2

growth could alleviate this problem [6].

In this chapter, the motivation of adding Al2O3 into HfO2 is that, Al2O3 is an

high-κ material with the merits of high energy band gap of ~8.9, middle dielectric

constant (9~10), immunity to oxygen diffusion, etc [9]. In practice, Al alloyed TiO2,

Ta2O5, and HfO2 MIM capacitors have been demonstrated to provide the overall better

electrical performance than using each dielectric alone [10-12]. Besides, HfO2-Al2O3

laminate has been studied as DRAM dielectrics [6]. In the following section,


79

Figure 4.1: The growth rates dependence on deposition cycles for ALD processed (a)

HfO2 and (b) Al2O3.

(a)

(b)


80

we will focus on the investigation of HfO2-Al2O3 laminated dielectric. In addition, the

electrical characteristics of HfO2-Al2O3 MIM capacitors using different material

structures will be studied and compared.

4.2. Experiments

The MIM capacitors were fabricated on SiO2 deposited on Si substrate. The

bottom electrode of Ta/TaN was formed by sputtering, where Ta was used to reduce

the parasitic resistance of the electrode and TaN was acted as an oxidation-resistant

barrier layer [13]. After that, the laminated dielectrics with alternate Al2O3 (1 nm) and

HfO2 (5 nm) layers were deposited using ALD technique, and the beginning and end

layers were 1 nm Al2O3, respectively. The insertion of 1 nm Al2O3 as the contacting

layer to the top and bottom electrodes is to improve the metal/dielectric interface

quality [14]. Three thicknesses of laminated dielectrics (i.e. 13, 31 and 43 nm) were

deposited for electrical evaluation. Figure 4.2 illustrates the fabricated laminate

structure and the TEM photo of 13 nm laminated film. TaN was then sputtered as the

top electrode, followed by the post deposition annealing in N2 at 420oC for 30 min.

Finally, a photolithography step and dry etching were used to define the MIM

capacitors. In consideration of RF characterization for laminate dielectrics, the

coplanar transmission lines were also fabricated, which also served as the top and

bottom electrodes; and Al was used as the contact pads after TaN top electrode

formation. For more details with regard to the RF structure fabrication, one can refer to

Chapter 3. The maximum temperature in the device fabrication was 420ºC, which is

compatible with Si back-end of line process. In addition, HfO2-Al2O3 dielectrics using


81

different material structures have been fabricated for electrical evaluation and

comparison using the same fabrication procedures as described above.

For electrical characterization, capacitances from 10 kHz to 1 MHz were

measured using a HP4284A precision LCR meter. For RF characterization, the

scattering (S) parameters were measured on wafer using HP 8510C network analyzer

with the GGB's air coplanar probes (ACP) for ground-signal-ground (GSG)

configuration. The measured S-parameters were de-embedded using the same

procedures as described in Chapter 3. Leakage currents were measured using a

HP4155B semiconductor parameter analyzer. To study dependence of voltage

coefficients of capacitance (VCCs) on constant voltage stress (CVS), C-V

characteristics were measured with an interruption of stress after different stress time.

The breakdown of the dielectric was characterized by monitoring the change of

leakage current during CVS.

Figure 4.2: TEM cross section of 13 nm HfO2-Al2O3 laminated dielectric.


82

4.3. Electrical characterization of HfO2-Al2O3 laminated MIM capacitors

4.3.1. RF Characteristics of laminated MIM capacitors

Using the method and equivalent circuit model described in Chapter 3, the

measured two-port S parameters (S11 and S21) after de-embedding the shunt elements

are obtained for the laminate MIM capacitors with three thicknesses. In comparison,

the simulated two-port S parameters are also obtained using the aforementioned

equivalent circuit. The respective two-port S parameters are depicted in Figs. 4.3(a, b,

c) for all three laminated capacitors. Again, it can be found that the measured and

simulated data over the entire frequency range from 50 MHz to 20 GHz are in very

good agreement, suggesting the equivalent circuit model is suitable and reliable for the

capacitor modeling and parameters extraction.

In addition, the transition from capacitive to inductive behavior was evidenced

from the inset of Fig. 4.4, which are believed to be associated with the interconnect

and are extrinsic to the MIM structure. The resonance frequencies are 5.5, 7.7, 9.3

GHz for the 13, 31, 43 nm laminate MIM capacitors, respectively. The different

resonance points result from the different capacitance values are due to different

dielectric thicknesses. Figure 4.4 again presents the capacitance densities measured at

low frequency (10 kHz ~ 1MHz) and those extracted in RF regime. It is revealed that

all of the laminate MIM capacitors can offer nearly constant capacitance densities of

4.3, 6.1 and 12.8 fF/µm2 from 10 kHz to 20 GHz for 43, 31 and 13 nm thick laminates

respectively. The calculated dielectric constant of the laminate is around 19. This

indicates that the incorporation of a small quantity of Al2O3 can still preserve a high

enough dielectric constant. Considering the 13 nm laminated MIM capacitor,


83

(a)

(b)

(c)

Figure 4.3: Measured and simulated S-parameters for (a) 13 nm, (b) 31 nm and (c) 43

nm laminated MIM capacitors


84

the capacitance density of 12.8 fF/µm2 can fulfill the requirement of RF capacitor up

to year 2007 according to ITRS roadmap [15]. In addition, the well-behaved and

predictable RF characteristics could also be observed for HfO2-Al2O3 dielectrics using

other material structures like stack and sandwich. Based on all the experimental RF

data, it is concluded that pure HfO2 and HfO2-Al2O3 dielectrics are good materials for

RF MIM capacitor applications.

4.3.2. Leakage and breakdown characteristics of laminated MIM capacitors

Figure 4.5 shows the dependence of leakage current density (J) on biasing

voltage at 125oC for MIM capacitors with different thicknesses of laminate.

1k 100k 10M 1G 100G0

4

8

12

16

20

24

28

0.0 5.0G 10.0G 15.0G 20.0G-300p

-200p

-100p

0

100p

200p

Sing

le e

nded

effe

ctiv

e ca

paci

tanc

e

Frequency (GHz)

13 nm 31 nm 43 nm

Capacitive

Inductive

RF analysis up to 20 GHz

Cap

acita

nce

dens

ity (f

F/µm

2 )

Frequency (Hz)

13 nm 31 nm 43 nm

Low frequency ≤ 1 MHz

Figure 4.4: The capacitance density dependence on frequency for laminate capacitors

with three thicknesses, the inset shows high frequency response of laminate MIM

capacitors from 50 MHz to 20 GHz.


85

-4 -2 0 2 410-10

10-8

10-6

10-4

10-2

Leak

age

curr

ent (

A/c

m2 )

Voltage (V)

13 nm 31 nm 43 nm

measured at 125oC

area: 100µm*100µm

Figure 4.5: J-V characteristics of 13, 31 and 43 nm laminated capacitors measured at

125oC.

-4 -2 0 2 410-10

10-8

10-6

10-4

Leak

age

Cur

rent

(A/c

m2 )

Voltage (V)

22 oC 50 oC 75 oC 100 oC 125 oC

13 nm laminate

Figure 4.6: J-V characteristics of 13 nm laminated MIM capacitor as a function of

temperature.


86

It is noticed from Fig. 4.5 that the leakage current density decreases with the increase

of the laminate thickness at the same voltage. It can be found that the 13 nm HfO2-

Al2O3 laminated MIM capacitor can provide much smaller leakage current than the

reported HfO2 [16] and Tb-doped HfO2 [17] MIM capacitors while maintaining similar

capacitance density. Such a small leakage current density for the laminated MIM

capacitor is attributed to the incorporation of 1 nm Al2O3 layers. This exploits the

merits of large band gap of Al2O3 which helps to reduce leakage current and slow

oxygen diffusion through Al-O matrix resulting in improved interface properties [9,

10]. The intermediate amorphous Al2O3 layers inhibit the continuous crystal growth of

HfO2. Therefore, this eliminates the grain boundary channels extending from one

electrode to the other, and further contributes to the reduction in conductivity [18].

Figure 4.6 presents the J-V curves for the 13 nm laminated MIM capacitor

measured at several different temperatures. It is found that the leakage current density

at low voltages exhibits weak temperature dependence in comparison with that at high

voltages, i.e. compared to the leakage current at 50oC, the leakage currents at 125oC

increase by 1.6 and 24.8 times at 1 and 3V, respectively. In addition, the J-V

characteristics exhibit two distinct regions. One is the low bias region (typically at <

2V), where the leakage current increases slowly with the applied voltage; the other is

the high bias region (> 2V), where the leakage current increases quickly with the

applied voltage. These phenomena reflect different current transport mechanisms in

the laminated films.

Before we proceed to discuss the electron conduction mechanism in the

laminated dielectric, we should introduce two major conduction mechanisms

frequently observed in high-κ dielectric films, i.e. Schottky emission and Poole-

Frenkel (P-F) emission [19-21]. This can be categorized into electrode limited and


87

400 800 1200 1600

-20

-18

-16

400 800 1200 1600-21

-20

-19

-18

-17

-16

LnJ

E1/2 (V/cm)1/2

β=0.00167

22 oC

LnJ

E1/2 (V/cm)1/2

22oC

50oC

75oC

100oC

125oC

1200 1400 1600 1800 2000-34

-32

-30

-28

-26

-24

-22

Ln (J

/E)

E1/2 (V/cm)1/2

22oC S=0.0167

50oC S=0.0179

75oC S=0.0178

100oC S=0.0183

125oC S=0.0182

Poole-Fren

kel e

mission

(a)

(b)

Figure 4.7: Conduction mechanisms for the 13 nm laminated MIM capacitor: (a)

Poole-Frenkel mechanism occurring at high electric field, exhibiting a shift to lower

electric field with increasing the temperature, (b) Schottky emission fitting at low

electric field.


88

bulk limited processes. For Schottky effect, the electrode limited current obeys the

Richardson-Schottky law and can be expressed as follows:

)2

exp()exp(* 2

KTE

KTe

TAJ Sb βφ−= (4.1)

where 0φ is the barrier height, T is the temperature, and sβ is the Schottky slope:

2/1

0*

3

)4

(επ

βKe

s = (4.2)

Here K*=n2, is the optical dielectric constant, n is the refractive index. If we plot

sd βlog vs. E1/2, the resulting slope will be:

ss

KTe

KTdEJd β

β 434.0loglog2/1 == (4.3)

As for P-F emission, it is due to the field-enhanced thermal excitation of trapped

electrons, which could be regarded as the bulk analog of the Schottky emission. The

effect yields a field-dependent conductivity of the form:

)exp()exp(2/1

0 KTE

KTE PFb βφ

σσ −= (4.4)

where 0σ is a constant. This equation can also be written in the form:

)exp()exp(2/1

0 KTE

KTEJ PFb βφ

σ −= (4.5)

When plot log(J/E) vs. E1/2, the resulting slope will be:

PFPF

KTe

KTdEJd β

β 434.0loglog2/1 == (4.6)

where 2/1

0*

3

)(επ

βKe

PF = (4.7)

is the P-F coefficient. Note that SPF ββ 2= . By analysis of the J-V curves, the

conduction mechanism in the dielectric could be determined, and we find that both


89

Schottky emission and P-F are involved in the electronic conduction process in the

laminate films.

To further verify the possible effect shown in Fig. 4.6, Figure 4.7 (a) shows the

dependence of ln(J/E) on E1/2 at high bias for each temperature, together with extracted

slope (S) by linear fitting. As a result, we can deduce that the refractive index of the

laminate is close to 1.8 in the case of 22oC, which is in good agreement with the

refractive index (~2.0) of ALD HfO2, suggesting that P-F emission is satisfactorily

followed at high electric field. Moreover, it is noticed that the transition to P-F

emission shifts from 2.4 to 1.6MV/cm with increasing the temperature from 22oC to

125oC.

In comparison, the J-V curves at low bias could only be well fitted with the

Schottky emission taken into consideration. Figure 4.7(b) shows ln(J) dependence on

E1/2, and the inset of Fig. 4.7(b) presents the resultant slope by a linear fitting for J-V

curve measured at 22oC. However, the calculated refractive index is about 9, which is

much larger than that of HfO2. Therefore, it is considered that the Schottky emission is

not a unique conduction mechanism at low bias. Recently, it has been reported that

trap-assisted-tunneling (TAT) current at low voltage (field) for Ta2O5 MIM capacitors

is characterized by weak temperature dependence, weaker electric-field-dependence

than P-F current, and a very prominent “knee” feature due to a significant change of J-

V slope in the case of the transition from TAT to P-F emission [22, 23]. The

aforementioned features can also be observed in Fig. 4.6 in the case of HfO2-Al2O3

laminated films, suggesting that the leakage current at low voltages probably includes

the TAT component as well.

Figure 4.8 shows the evolution of leakage current during constant voltage


90

Figure 4.8: The characteristics of leakage current versus stress time under 4V stress

for the 13 nm laminated MIM capacitor. Square and round symbols represent the 1st

stress and the 2nd stress after an interruption of 10 hours, respectively.

10 100 1000

1.2x10-5

1.6x10-5

2x10-5

J (A

/cm

2 )

Stress time (sec)

Area=1E-4 cm2As-stressed

Remeasured after 10hurs

Stressed @4V

-4 -3 -2 -1 0 1 2 3 410-9

10-8

10-7

10-6

10-5

10-4

Leak

age

curr

ent (

A/c

m2 )

Voltage (V)

0 to 4 4 to -4 -4 to 4 4 to -4 -4 to 4

Figure 4.9: I-V measurements showing the hysteresis loop of 13 nm laminated MIM

capacitor.


91

stress for 13 nm laminate. It can be found that the leakage current reduces swiftly at

the beginning of stress, followed by a gentle decrease with stress time. For fresh

capacitors, the leakage current density decreases by about 38% with increasing stress

time to 1500 sec under 4V, suggesting the electron charge trapping. Subsequently, the

stress voltage is removed and the device is kept at zero-bias for 10 hours, and then the

leakage current density is re-measured under the same voltage stress. As shown in Fig.

4.8, the leakage current density at the starting point is remarkably recovered after 10

hours interruption, i.e. to 94% of the original. This indicates that electron charge

trapping process occurs mostly at the dielectric/electrode interface, not at the bulk of

the dielectric [24].

To measure the current dependence on voltage bias and time, we varied the

applied voltage from the negative extreme to the positive extreme and back, and

recorded the corresponding currents (see Figure 4.9). It is noticed that IV curves

exhibit a good symmetry for positive and negative polarities, which is ascribed to a

symmetrical structure of laminated MIM capacitor different from MOS capacitor with

SiO2/Al2O3 gate stacks [25]. Furthermore, the laminated MIM capacitor exhibits very

small hysteresis for both polarities. Here it also needs to mention that CV curves of

MIM capacitors exhibit negligible hysteresis, which is simply due to its symmetrical

metal-dielectric-metal structures. Compared with that of MOS capacitors using doped

Si as the substrate, different types of charges trapping and defect generation can occur

depending on the voltage sweep polarity [25], this could possibly lead to an obvious

hysteresis behavior.

In addition, breakdown characteristics of the laminate MIM capacitors are

demonstrated in Figs. 4.10(a, b). The typical evolutions of leakage current versus

stress time under different CVS are presented in Fig. 4.10(a) for the 13 nm laminate.


92

100 101 102 10310-4

10-1

102J

(A/c

m2 )

Stress Time (sec)

stressed @ 6V stressed @ 5.8V stressed @ 5.5V

5 10 15 20 25 30 35 400

20

40

60

80

100

Cum

ulat

ive

prob

abili

ty (%

)

Breakdown voltage (V)

13 nm 33 nm 43 nm

Area = 2.5x10-5 cm2

(a)

(b)

Figure 4.10: (a) The typical breakdown characteristics of 13 nm laminate under

different constant voltage stress; (b) the cumulative probability dependence on

breakdown voltage for the laminated MIM capacitors with different thicknesses.


93

When the stress time surpasses a critical point, the leakage current exhibits a sudden

rise, which is indicative of hard breakdown characteristic. The cumulative probability

plot of breakdown voltage is presented in Fig. 4.10(b). In the case of 50% probability

of failure, the breakdown voltages corresponding to 13, 31 and 43 nm laminate MIM

capacitors are equal to 7.6, 17.8 and 25.6 V respectively, which corresponds to the

breakdown field of ~ 6 MV/cm.

4.3.3. VCCs dependence and reliability of laminated MIM capacitors

Voltage coefficients of capacitance (VCCs) were again analyzed by fitting the

measured data with the second order polynomial equation of

)1()( 20 ++= VVCVC βα , where C0 is the zero-biased capacitance, α and β represent

the quadratic and linear voltage coefficients of capacitance, respectively. Figure 4.11(a)

shows bias-dependent normalized capacitance (∆C/C0) fitted by the above-mentioned

equation and the resulting VCCs as well. Obviously, α decreases with increasing the

laminate thickness. In the case of the 13 nm laminated MIM capacitor, β is equal to

211ppm/V at 1MHz, which can easily meet RF capacitor requirement (1000 ppm/V)

[15]. Moreover, the electric field (E) dependence of ∆C/C0 is also presented in Fig.

4.11(b), showing a similar dependence of ∆C/C0 on E regardless of the dielectric

thickness. On the other hand, the effect of the applied frequency on α is also

demonstrated in Fig. 4.12. It can be noticed that logarithmic α (logα) decreases

linearly with a logarithmic increase in frequency, while maintaining a similar slope in

spite of the laminate thickness, i.e. α decreases by about 30% when the applied


94

-2 -1 0 1 2

0.000

0.005

0.010

0.015

0.020

∆C

/C0

Electric field (MV/cm)

13nm 31nm 43nm

voltage scan: -4V~+4Vmeasurement frequency: 1MHz

Figure 4.11: (a) The voltage-dependent normalized capacitance (∆C/C0) at 1 MHz for

13, 31 and 43 nm laminated capacitors, fitted by a second order polynomial equation;

and (b) the corresponding plot of ∆C/C0 versus electric field (E).

-2 -1 0 1 2

0.000

0.005

0.010

0.015

∆

C/C

o 13nm α = 1990ppm/V2; β = 211ppm/V

Voltage (V)

measured @1MHz

31nm α = 405ppm/V2; β = -95ppm/V 43nm α =207ppm/V2; β = -65ppm/V

(a)

(b)


95

10k 100k 1M100

1000

10000

α (p

pm/V

2 )

Frequency (kHz)

13nm 31nm 43nm

10 20 30 40 50 60102

103

104


α (p

pm/V

2 )

Thickness (nm)

Figure 4.12: Frequency dependences of α for 13, 31 and 43 nm laminated capacitors,

showing a linear fitting in log-log scale.

Figure 4.13: Thickness dependence of quadratic VCC (α) for laminated MIM

capacitors.


96

frequency increases from 10 kHz to 1 MHz. Furthermore, we plot α versus thickness

as well as capacitance density, as shown in Fig. 4.13. It is found that α decreases

linearly with increasing the laminate thickness in the log-log scale, exhibiting a similar

slope despite of the operating frequency. However, β does not exhibit the

aforementioned frequency and thickness dependences. The foregoing results reveal

that α of laminated capacitors is affected by the operating frequency and dielectric

thickness as well. The frequency dependence of α can be explained as the change of

relaxation time with different carrier mobility in insulator. The thickness dependence

of α is an intrinsic property due to electric field polarization [25]. We will further

discuss these phenomena in Chapter 5.

For temperature coefficient of capacitance (TCC), we have measured C-V

characteristics of the laminated MIM capacitors at 100 kHz as a function of

temperature. It is observed that the TCCs are 182, 196, 199 ppm/oC for 13, 31, and 43

30 60 90 120

100

200

300

400

30 60 90 120102

103

Y =2.12-3.38E-4 X

Y =2.37-10.7E-4 X

Y =2.59-22.8E-4 X

|β| (

ppm

/V)

Temperature (oC)

13nm 31nm 43nm

(b)

Y =2.34+9.32E-4 X

Y =2.62+9.62E-4 X

Y =3.30+10.4E-4 X

α (p

pm/V

2 )

Temperature (oC)

(a)

Figure 4.14: Temperature dependences of α and β at 100 kHz for 13, 31 and 43 nm

laminated capacitors.


97

nm laminated capacitor, and it is nearly independent high-κ laminate thicknesses.

Meanwhile, temperature dependence of VCCs are also analyzed, as presented in

Figs.4.14(a, b). It is indicated that the logα increases linearly as a function of the

temperature, and the slope becomes smaller with increasing the laminate thickness.

This suggests that the temperature has a bigger influence on the thinner laminate. On

the other hand, logarithmic absolute value of β (log|β|) reduces with the temperature.

Similarly, the variation of β with temperature is more remarkable for thinner laminated

films.

In the case of 13 nm laminate, the VCCs dependent on CVS have been

investigated. Figure 4.15 shows stress time dependence of normalized VCCs (α/α0 and

Figure 4.15: The dependence of α/α0 on stress time at 10 kHz, 100 kHz and 1 MHz.

The inset shows stress time dependence of β/β0 at the same frequencies. α0 and β0

represent the data before voltage stress (β0 is of negative sign.), α and β denote the

data after different time stress.

0 1000 2000 3000 40000.88

0.90

0.92

0.94

0.96

0.98

1.00

0 1000 2000 3000 4000

1.0

1.1

1.2

1.3

β/β 0

Stress Time (sec)

α/α

0

Stress Time (sec)


Stressed @ 4V


98

β/β0) under constant voltage stress and different frequencies. Noticeably, α decreases

and β increases with a rise in stress time at each frequency, and the variation of α and

|β| values corresponds to the “flatten-out” and the positive shift along the voltage axis

of CV curves, respectively. The trend observed for the change of α and β with stress is

as the same as what we observed for pure HfO2 in Chapter 3, and could be explained

using the same method. Additionally, it is noted that the variations of α and β at the

beginning of stress are rapid, which agrees with the variation of leakage current under

constant voltage stress (see Figure 4.8). Table 4.2 summarizes variations of VCCs and

leakage current density under different measuring conditions. In comparison with the

pre-stressed device, the resulting VCCs and leakage current density reduce after

voltage stressing for 1500 sec. After interruption of voltage stress for 1 hour, the

device is re-measured, and the resultant VCCs and leakage current density nearly

recover to the pre-stress level, even though slightly lower than those of the fresh

device. The consistent variations of VCCs and leakage current density reveal that the

VCCs may correlate with the leakage current. This is significant for interpreting the

origin of VCCs despite incomprehension so far. It is likely related to the trapping and

detrapping in the dielectric under and after stress.

Table 4.2: Variations of VCCs and leakage current under different condition

(frequency: 100 kHz; stress voltage: 4V; area: 1×10-4 cm-2)

Parameters α (ppm/V2) β (ppm/V) J (ppmA/cm2)

Pre-stress 2212 -285 20.60

After 1500 sec stress 2132 -338 13.97

After 1hr stress interruption 2194 -306 19.63


99

0.0 1.3 2.6 3.9 5.2 6.5 7.8

0 1 2 3 4 5 6

100

102

104

106

108

1010

E (MV/cm)

Tim

e to

50%

Fai

lure

(sec

)

Voltage (V)

10 years Area=1x10-4cm2

Figure 4.16: (a) Cumulative TDDB curves under various constant voltages stress for

13 nm laminated MIM capacitor measured at room temperature, (b) lifetime projection

of 13 nm laminated MIM capacitor, using 50% failure time as the criteria.

0

20

40

60

80

100

101 102 103

Time (sec)

Cum

ulat

ive

Failu

re (%

) Stressed @-6V Stressed @-5.8V Stressed @-5.5V

Area=1E-4cm2

(a)

(b)


100

Finally, the lifetime of the 13 nm laminated MIM capacitor has been accessed

using 1×10-4 cm2 size devices at room temperature. Time to breakdown characteristics

of the 13 nm laminated MIM capacitors were evaluated under different constant

voltage stress, as shown in Fig. 16(a). The projection of the operating voltage for a 10-

year lifetime is 3.3 V by taking the T50 as the failure criterion, as illustrated in Fig.

16(b). It indicates that the HfO2-Al2O3 laminate MIM capacitors are of promising

reliability for practical applications. Finally, Table 4.3 compares our results on HfO2-

Al2O3 laminated MIM capacitors with other high capacitance density MIM capacitors

reported recently [2-4], [6], it is shown that the laminated MIM capacitors exhibit

nearly the best electrical performance as well as promising device reliability,

suggesting it a good candidate for next generation MIM capacitors application.

Table 4.3: Comparison of various high capacitance density MIM capacitors using

high-κ dielectrics (year 2002-2003)

Reference [13] [16] [27] [28] This work [29]

Dielectric Ta2O5 (PVD)

Tb-doped HfO2 (PVD)

Ta2O5 (CVD)

HfO2 (ALD)

HfO2-Al2O3 laminate (ALD)

Capacitance density (fF/µm2) 9.2 13.3 9 13 12.8

Leakage (A/cm2) 2×[email protected] 1×10-7@2V — 5.7×10-7@2V 7.45×10-9@2V

β (ppm/V) α (ppm/V2)

2060 3580

332 2667

2050 475

607 853

211 1990

TCC (ppm/oC) ~200 123 — — 182

4.4. Effects of dielectric structures on the electrical properties

As evidenced from Table 4.3, it was believed that the good quality high-κ films

prepared by ALD method and the incorporation of Al2O3 are two main factors

contributing to the superior electrical properties of laminated films. In reality, various

dielectric structures have been employed for MIM capacitors application including


101

Figure 4.17: Illustrations of five different HfO2-Al2O3 material structures for electrical

characteristics comparison.

-4 -3 -2 -1 0 1 2 3 410-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

0

20

40

60

80

100

3.0 3.5 4.0 4.5 5.0 5.5Breakdown voltage (V)

Cum

ulat

ive

prob

abili

ty (%

)

H A/H H/A A/H/A L

Leak

age

curr

ent (

A/c

m2 )

Voltage (V)

H A/H H/A A/H/A L

Figure 4.18: Typical J-V characteristics for MIM capacitors with different dielectric

structures at 125oC. The inset shows the corresponding breakdown characteristics

obtained at the same temperature. (Device area: 10-4 µm2)


102

stacks, sandwich, laminate, and so on [14, 27, 29]. However, little study has been

conducted to compare the electrical characteristics of different structural MIM

capacitors. In this work, it was found that the proper selection of material structure

also plays an important role for HfO2-Al2O3 high-κ systems. As depicted in Fig. 4.17,

five structures were used for comparison in this work, i.e. pure HfO2, two stacks

structures, sandwich, and laminate. For fair comparison, the nominal thicknesses of all

the five samples were kept the same of 13 nm.

Figure 4.18 presents the typical J-V curves of all the five samples obtained at

125oC. At low voltage level, stacks and laminated MIM capacitors exhibit the

advantage of small leakage current density in comparison with sandwich and pure

Figure 4.19: TEM photos of (a) 13 nm HfO2-Al2O3 laminate, (b) 10 nm HfO2 and (3)

30 nm HfO2 films, illustrating the amorphous structure of laminated film and

improved crystallinity with the increase of HfO2 thicknesses

(a)

(b)

(c)


103

HfO2 counterparts. As for stack structures, the relatively thick Al2O3 layer (3 nm) in

direct contact with the electrode contributes to the small leakage current. In

comparison, 1.5 nm Al2O3 in the sandwich structure may not be thick enough to

effectively block the oxygen diffusion during HfO2 growth and therefore worsen the

interface quality. With regard to the laminated structure, the alternate insertion of 1 nm

Al2O3 between 5 nm HfO2 inhibit the grain growth extending from bottom electrode to

top electrode. A similar concept of using multiple annealing for HfO2 treatment to

offset the grain boundary has also been reported elsewhere [30].

To confirm this, TEM photos of laminated (13 nm) and pure HfO2 (10 nm and

30 nm) films are illustrated in Figs. 4.19(a, b, c). As seen from Fig. 4.19(a) (also

shown in Fig 4.2), HfO2 layer in the laminate film is amorphous. In comparison, some

crystallites could be observed in the case of 10 nm HfO2 illustrated in Fig. 4.19(b), and

the crystallinity improves when the dielectric thickness increases to 30 nm.

Considering the temperature for HfO2 growth by ALD is ~340oC in this work, the

crystallization of thick HfO2 is expected and in agreement with the conclusion

obtained in Chapter 2. Besides, the TEM results suggest that the crystallization of thin

film is closely dependent on the dielectric thickness. As the film thickness increases,

the growth of polycrystalline structure dominates, and thinner dielectric film probably

has relatively higher crystallization temperature [31]. Therefore, the alternate insertion

of Al2O3 can effectively suppress the crystallization of HfO2 and inhibit the grain

growth. The oxygen diffusion could be reduced as well due to the alternate Al2O3

layers. To conclude, laminate dielectric exhibit the unique advantage in terms of

material structure which could be further evidenced from the breakdown

characteristics of all five samples as illustrated in the inset of Fig. 4.18 with the similar


104

trend as the J-V characteristics. The laminated film is of the highest breakdown

strength of ~5.0 V at 125oC by taking 50% probability of failure as the criteria.

In addition, the evolution of AC capacitance with stress time has been

evaluated for different structural MIM capacitors as shown in Fig. 4.20. It was noted

that, with increase of stress time, the capacitance values of all five samples increase

steadily showing the similar trend. This phenomenon was recently reported for SiO2

MIM capacitor [32], and it was proposed to be due to the dipole generation which is

directly correlated to the amounts of trapped charges in the dielectric films [32]. In

comparison, the laminated MIM capacitor shows the smallest capacitance variation,

which is probably attributed to slight charge trapping in the laminated film as

discussed before. When considering that the drift of capacitance value is detrimental to

the accuracy of circuit operation, the laminated MIM capacitor exhibits the highest

Figure 4.20: Evolution of C0 at zero DC bias with stress time, illustrating the highest

stability for the laminated capacitor compared to other dielectric structures.

0 10 20 30 40 50 60

0.0000

0.0005

0.0010

0.0015

(C-C

0)/C

0@0V

Stress time (min)

CVS@4V A/H H/A A/H/A L


105

capacitance stability when subject to electrical stress. In addition, the variation of α

and β under stress was also found to be the smallest in the case of laminated MIM

capacitor (not shown). Though it is conceptually clear that the laminated dielectric is

in possession of many unique advantages, further electrical characterization and

material analyses are needed for better understanding. In Chapter 5, we will further

present the CV characteristics of various structural HfO2-Al2O3 MIM capacitors,

showing the advantage of symmetrical material structure.

4.5. Conclusion

In summary, MIM capacitors using atomic layer deposited HfO2-Al2O3

laminate are fabricated and characterized. The laminated capacitor can offer high

capacitance density (12.8 fF/µm2) up to 20 GHz, low leakage current (4.9×10-8 A/cm2

at 125oC), breakdown field of ~6 MV/cm, small linear VCC and TCC values. In

addition, the HfO2-Al2O3 laminate exhibits Poole-Frenkel emission at high electric

field, and the leakage current density at low electric field likely results from both

Schottky emission and TAT contributions. A decrease and recovery of the leakage

current density under and after CVS are observed in the laminated film, which is

ascribed to trapping and detraping in the dielectric. It is also found that α may have a

logarithmic relationship with dielectric thickness and frequency as well. In addition,

the VCCs also depend on CVS, which is attributed to electron charge trapping thereby

modifying the electric field profile within the MIM capacitor. Finally, different

dielectric structures including laminate, stack, and sandwich are compared, and the

HfO2-Al2O3 laminated MIM capacitor exhibits the advantages in terms of leakage,


106

reliability, and capacitance stability owing to its unique merits in terms of material

structure.


107

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Chapter 5 Understanding Voltage Coefficients of High-κ MIM Capacitors

112

Chapter 5

Understanding Voltage Coefficients of High-κ MIM Capacitors

5.1. Introduction

In the previous chapters, we thoroughly investigate the electrical characteristics

of HfO2 based high-κ dielectrics; the results indicate those dielectrics are very

promising for both RF and analog MIM capacitors application in the future technology

node. Among the technical specifications of MIM capacitors prescribed by ITRS

roadmap [1], voltage coefficients of capacitance (VCCs) are of great importance,

however the parameters dependence of VCCs such as thickness [2-6], frequency [5, 7-

9], etc has been less understood up to now for high-κ MIM capacitors. Among the

parameters affecting the VCCs, the dielectric thickness was widely documented to

have a strong non-linear relationship with the quadratic VCCs (α). This will increase

rapidly with the scaling of high-κ dielectric thickness. Therefore, it poses a great

limitation to meet the strict voltage linearity specifications especially for analog MIM

capacitors when high-κ dielectrics were scaled to achieve higher capacitance density.

However, the so-called “thickness effect” has only been observed experimentally, no

theoretical analysis has been done to clarify this phenomena to our best knowledge. In

this chapter, for the first time, we present a unified understanding of the quadratic

VCCs dependence on thickness based on the free carrier injection model using HfO2

MIM capacitors [10]. It is revealed that the thickness (t) dependence of the quadratic


113

VCCs exhibits a relation of nt−∝α (n~2), which is an intrinsic problem due to

electrons injection under the enhanced electrical field in the scaled dielectric film. The

obtained information is paramount for high-κ dielectric MIM capacitors application in

every aspect and is very useful for the design purpose. In addition, this model has been

extended to interpret the frequency dependence of the quadratic VCCs, and the stress

modified quadratic VCCs. It is proposed that the quadratic VCCs’ variation could be

attributed to the change of relaxation time, which was modulated through the change

of carrier mobility and/or concentration in the dielectric films. In a word, this model

could also be applied to other high-κ dielectric MIM capacitors, predicting their

voltage linearity characteristics and help to establish the process-property relationship.

5.2. Theory

To be more specific, the VCCs are differentiated into the quadratic (α) and

linear (β) coefficients, which could be obtained by fitting the C-V curves using a

polynomial equation (C(V) = C0(αV2+βV+1)) [11]. Among them, the quadratic VCCs

are more critical for analog capacitors according to ITRS roadmap [1]. Especially, it

was reported extensively that the quadratic VCCs have a nonlinear dependence on

thickness, namely “thickness effect”. In this work, we mainly discuss the quadratic

VCCs’ mechanism; and to be simple, the VCCs were referred to the quadratic VCCs

unless otherwise specified.

When considering the C-V characteristics of MIM capacitors, it is known that

the carriers’ transport is responsible for the capacitance variation [12]. Based on this

understanding, the free carrier injection model was employed in this work, which

correlates the dielectric property change (capacitance variation) to the injected carriers


114

(electrons). According to Coelho [13, 14], the complex dielectric constant is given by

AAj

jj)tanh(

1'''*+

+=−=

ωτ

ωτεεεε 5.1),

where ω is the frequency of 2πf , ε is dielectric constant defined as the sum of the

optical part ( opε ) and the low frequency (non-optical) part, and τ is the relaxation

time of the free carriers which is given by

0qnµετ = (5.2).

A in (1) is given by

)2

(1 tD

jAτωτ+

= (5.3),

where D in (5.3) was described as

qkTD /µ= (5.4).

From the above equations, ε’ numerically computed by (5.1) could finally be

correlated to the measured capacitance value by

t

C 'ε= (5.5),

where t is the dielectric thickness. According to this model, the charge excess in the

insulator could follow the alternating (AC) signal with a relaxation time of τ that

depends on the mobility of carrier, carrier density, and dielectric constant; and a higher

relaxation time means that the carriers are more difficult to follow the alternating

signal.

As noted from (5.2), n0 is defined as the uniform free-electron concentration at

the equilibrium. However, when the capacitor was subject to DC bias, the free carrier

concentration in the dielectric is modified accordingly due to the electron injection


115

from the cathode. Therefore, n which stands for the modified free-carrier concentration

under DC bias is intended to replace n0 when simulating the C-V characteristics of

MIM capacitors. This can be obtained by taking account of the conduction mechanism

in the dielectric film by analysis of J-V characteristics according to the various

proposed theoretical models [15, 16].


5.3.1. Thickness dependence of VCCs for HfO2 MIM capacitor

Figure 5.1 showed the typical J-V characteristic of 30 nm HfO2 MIM capacitor

prepared by atomic-layer-deposition (ALD) with Ta serving as the electrodes; the

detailed fabrication procedures of HfO2 MIM capacitors could be found elsewhere [5].

In order to identify the conduction mechanisms, the J-V curve was fitted by possible

electronic conduction models; including Schottky emission, Poole-Frenkel conduction,

space charge limited conduction, tunneling, hopping, etc. As a result, the J-V curve

shown in the inset of Fig. 5.1 could be well fitted by the Schottky conduction

mechanism as evidenced in Fig. 5.1, indicating the electrode limited current transport

in 30 nm HfO2 MIM capacitor. For the Schottky emission conduction, the leakage

current density as a function of the electric field, could be expressed by

)2

exp()exp(* 2

KTE

KTe

TAJ Sb βφ−= (5.6),

where ∗A is the Richardson constant, bφ is the potential barrier height, βs is given by

0

3

επεβ

ops

e= 5.7).


116

Here, εop is the optical part of the dielectric constant given by

2rop n=ε (5.8),

while rn is the refractive index of the dielectric.

0 200 400 600 800 1000 1200 1400 1600-20

-18

-16

-14

-12

-10

0 1 2 3 4 5 610-9

10-8

10-7

10-6

10-5

10-4

Cur

rent

den

sity

(A/c

m2 )

Voltage (V)

Ln(J

)

Sqrt(E)

30 nm HfO2 MIM Capacitor

Figure 5.1: Schottky plot of 30 nm HfO2 MIM capacitor. The inset shows the typical J-

V curve.

In consideration of the Schottky dominated leakage current transport in ALD

HfO2 dielectric film, the free electrons’ concentration as a function of electrical field

(or voltage) could be given by

)2

exp()exp(0 KTE

KTenn Sb βφ

−= (5.9).

Accordingly, the capacitance variation could be modeled as a function of DC bias.

Figure 5.2 presented both the measured and simulated C-V data using the above free


117

carrier injection model, and all the fitting parameters have also been stated as well.

Among them, a barrier height bφ of 1.28 eV was used [17, 18].

-6 -4 -2 0 2 4 6

0

2000

4000

6000

8000

10000

12000

14000

Nor

mal

lized

cap

acita

nce

(ppm

)

Voltage (V)

Measured CV (30 nm, 1 MHz) Calculated CV (30 nm, 1 MHz)

T=300Kφb=1.28 eV

βS=3.2x10-24 J/(V/m)1/2

n0=1.636x1014 cm-3

µ=6.6x10-5 cm2/vs

Figure 5.2: Measured and simulated normalized capacitance as a function of voltage.

n0 and µ are extracted by fitting the measured data.

Beside, βs is of 3.2×10-24 J/(V/cm)1/2, and rn is 2.0 extracting from the Schokkty

emission fitting, which is similar with the value of 1.97 obtained by ellipsometer.

Finally, the best fitting corresponded to the fitting parameters of n0 = 1.636×1014 cm-3

and µ = 6.6×10-5 cm2/V*s, respectively. The good agreement between the measured

and simulated data suggested the free carrier injection model could predict the C-V

characteristics of MIM capacitors quite well. However, the small discrepancy is

reflected by the non-zero linear coefficient for the measured C-V curve, which

corresponded to the parallel shift of the C-V curve along the voltage axis.


118

20 30 40 50 60

1.0

1.5

2.0

2.5

3.0

3.5

Car

rier c

onc.

pre

-fact

or (x

1014

cm

-3)

Thickness (nm)

qµn0(V/t)=A*T2exp(-qφb/kT)

so, n0/t ~ constant

Figure 5.3: Carrier concentration pre-factor (n0) dependence on thickness.

Before we proceeded to discuss the thickness dependence of the VCCs, the

change of n0 (the pre-factor in (5.9)) with dielectric thickness had to be taken into

account. Considering the fact that only the AC signal (V0 = 25 mV) is applied on the

dielectric with zero DC bias, it is reasonable to believe that the thermal movement is

dominant over the electronic diffusion according to [12, 18]. Therefore, the current

density is given by

)exp(200 kT

eTAEne bφµ −∗= (10),

where E0 = V0/t is the electrical field of the applied AC signal. Figure 5.3 shows the

linear dependence of carrier concentration pre-factor n0 on thickness. As a result, the

simulated normalized capacitances (∆C/C0) as a function of voltage with different


119

-6 -4 -2 0 2 4 6-5000

0

5000

10000

15000

20000

25000

30000

35000

Nor

mal

lized

cap

acita

nce

(ppm

)

Voltage (V)

normallized capacitance decreases as thickness increased from 20 to 60 nm by step of 10 nm

Figure 5.4: Simulated normalized capacitance as a function of voltage for different

thickness of 20, 30, 40, 50, and 60 nm.

thicknesses is shown in Fig. 5.4 after taking the change of pre-factor n0 with thickness

into account. It was noticed that the C-V curves bent more upwards with the decrease

of the dielectric thickness, suggesting a fast increase of VCCs. In comparison, Figure

5.5 presents the VCCs dependence on thickness in log-log scale with and without

considering the change of pre-factor n0. A nearly linear thickness dependence of VCCs

(in log-log scale) was revealed which may be valid with certain range of thicknesses,

indicating a relation of nt−∝α . Further, after taking the effect of thickness on the pre-

factor into account, the VCCs changes more gently with the thickness. Besides, the

exponential factor n is 2.1 after extraction, which is consistent with the report on PVD

HfO2 where n is 1.93 [6]. In addition, R. B. van Dover et al also proposed that VCCs

was inversely proportional to the square of the dielectric thickness (n=2) based on their


120

experimental results on Ta2O5 MIM capacitor [2]. The recent data on HfO2-Al2O3

laminate MIM capacitors also supported our finding with n of 1.97 [19]. According to

the above discussion, the implication of nt−∝α (n~2) is significant that the exponential

increases of VCCs are unavoidable when the dielectric thickness was scaled to achieve

higher capacitance density. The requirement for achieving high capacitance density

and preserving small VCCs appears to be a fundamental problem for high-κ MIM

capacitors.

20 30 40 50 60 70101

102

103

104

Qua

drat

ic V

CC

(ppm

/V2 )

Thickness (nm)

Without accounting thickness depedence of carrier concentration pre-factor

Accounting thickness depedence of carrier concentration pre-factor

Figure 5.5: The simulated VCCs as a function of thickness with and without taking

account of the change of pre-factor (n0) with thickness.

Furthermore, in the modeling of thickness effect on the C-V characteristics, it

relied on the leakage characteristics of HfO2 MIM capacitor. It was implicitly assumed

in this work that the conduction mechanism was dominated by Schottky emission


121

which was valid within certain thicknesses and voltage ranges. With regard to other

possible conduction mechanisms, Poole-Frenkel conduction mechanism had been

widely observed for high-κ dielectrics [12, 19-22], which could be considered as the

internal Schottky emission of the trapped electrons from the defects levels to the

conduction band of the dielectric [14, 15]. Thus, the similar conclusion is anticipated

for the thickness dependence of VCCs.

With regard to the linear VCCs, the available literature data for high-κ MIM

capacitors doesn’t indicate that it has a direct relationship with dielectric thickness (or

capacitance density). To be clear, we have summarized the linear voltage coefficients

versus the capacitance density (or dielectric thickness) for HfO2 based MIM capacitors

in Fig. 5.6, where the weak linear VCCs dependence on the dielectric thickness could

be observed. It was concluded that the linear VCCs could be affected by many

Figure 5.6: Linear voltage coefficients versus the capacitance density for HfO2 based

high-κ dielectrics at 100 kHz.

0 2 4 6 8 10 12 14 16101

102

103

104

105

ALD HfO2 [5] PLD HfO2 [7] HfO2-Al2O3 laminate [19] HfO2-Al2O3 composite [23] PVD HfO2 [6, 24] Tb doped HfO2 [24]Li

near

coe

ffici

ents

β (p

pm/V

)

Capacitance density (fF/µm2)


122

Table 5.1: Different structural HfO2-Al2O3 high-κ MIM capacitors prepared by ALD

method

Samples Dielectric structures Thickness (nm) Experimental

Hf-Al-1 Pure HfO2 13

Hf-Al-2 Al2O3/HfO2 3/10

Hf-Al-3 HfO2/Al2O3 10/3

Hf-Al-4 Al2O3/HfO2/Al2O3 1.5/10/1.5

Hf-Al-5 Al2O3/·····HfO2·····/Al2O3 1/5/1/5/1

Deposition technique: ALD

Top electrode: Ta/TaN

Bottom electrode: TaN/Ta

-3 -2 -1 0 1 2 31.000

1.005

1.010

1.015

1.020

C/C

0

Voltage (V)

Hf-Al-1 Hf-Al-2 Hf-Al-3 Hf-Al-4 Hf-Al-5

Figure 5.7: Normalized capacitance versus DC bias measured at 100 kHz, showing

good symmetrical CV characteristics (small linear coefficients β) for sandwich and

laminate structures.


123

factors such as fabrication techniques, electrodes in contact, dielectric structures, the

bulk property of the dielectric film, and so on. Therefore, chances exist for the

optimization of the linear voltage coefficients for MIM capacitors. In our work, we

further investigate the effects of different HfO2-Al2O3 dielectric structures on the linear

VCCs with the sample details summarized in Table 5.1. Accordingly, Figure 5.7

presents the CV curves for various HfO2-Al2O3 dielectric structures measured at 100

kHz. It is noted that sandwich and laminate structures both show very good

symmetrical CV characteristics suggesting the small linear coefficients in comparison

with pure HfO2 and stacks MIM capacitors. As known in chapter 4, insertion of Al2O3

layer in contact with bottom electrode improves the interface quality by reducing the

reaction between dielectric and bottom electrode in ALD deposition. As a result,

sandwich and laminate structures exhibit the great advantages of small linear VCCs

which are probably attributed to their symmetrical material structures [25] as well as

good interface properties. In comparison, the quadratic VCCs for all the structures are

relatively the same at ~1500 ppm/V2 measured at 1 MHz. This is expected since the

nominal thicknesses of all the samples are the same at 13 nm. However, the detailed

mechanism of linear VCCs dependency still need more study and under development.

5.3.2. Frequency dependence of VCCs

From the model itself, the VCCs are nearly independent of frequency from

numerically computation when keeping the remaining fitting parameters unchanged.

However, the frequency dependence of VCCs has been observed for many high-κ

dielectric materials, and the VCCs virtually decreased with frequency, which was


124

10k 100k 1M101

102

103

10-5

10-4

10-3

Qua

drat

ic V

CC

(ppm

/V2 )

30 nm HfO2 MIM Capacitor

Measured quadratic VCC at different frequency

Fitted carrier mobility based on the measured quadratic VCC

Frequency (Hz)

Car

rier m

obili

ty (c

m2 /v

s)

Figure 5.8: The measured VCCs for 30 nm HfO2 MIM capacitor together with the

extracted carrier mobility at frequencies of 10k, 100k, 500k, 1 MHz.

-6 -4 -2 0 2 4 6

0

10000

20000

30000

40000

Nor

mal

lized

cap

acitn

ace

(ppm

)

Voltage (V)

Normallized capacitance decreases as the frequency increasing from 10 kHz, 100 kHz, 500 kHz, and 1MHz

Figure 5.9: Simulated normalized capacitance as a function of voltage for 30 nm

HfO2 MIM capacitors at frequencies of 10k, 100k, 500k, and 1MHz.


125

simply attributed to the traps with the different time constants in the dielectric [8, 26].

In this work, according to the free carrier injection model, we noticed that the

frequency dependence of VCCs could be interpreted when considering the change of

the carriers’ mobility at different frequencies. As shown in Fig. 5.8, the measured

quadratic VCCs for 30 nm HfO2 at different operating frequencies was shown;

together with the extracted carrier mobility at various operating frequency. It was

found that the reduced VCCs corresponded to the decreased free carrier mobility in the

dielectric film. Further, the simulated normalized capacitances as a function of voltage

at different frequencies are shown in Fig.5.9. Recently, it was reported by C. H. Huang

et al that VCCs of high-κ AlTaOx MIM capacitor continued to decrease from IF to RF

frequency regime using a newly developed mathematical method [9].

According to Coelho [14], the free carrier’s mobility (µ) accounts for all the

inelastic collisions between electrons and the lattice and/or defects in the dielectrics.

As a result, the electrons moving from the cathode to anode will become much more

affected with the increase of frequency, causing the carrier mobility to decrease with

frequency; therefore the capacitance variation become weak with DC bias. According

to (5.2), the decreased carrier mobility will lead to higher relaxation time which was

generally used to interpret the frequency dependence of VCCs for high-κ MIM

capacitors.

5.3.3. Electrical stress modified VCCs

In Chapter 3, we observed that the quadratic VCCs will reduce monotonously

with stress time for PVD HfO2, and a physical circuit model was employed to explain


126

the underlying mechanism. Similarly, the same trend could be found in HfO2-Al2O3

laminate MIM capacitors [19].

Based on the free carrier model, we are able to further interpret these

phenomena. As we know, the relaxation time (τ) could be regarded as the approach of

free carrier distribution to the steady state. In other words, certain amounts of

relatively immobile species may result in much longer relaxation time. Considering the

traps generation in the dielectric film after electrical stress (see Figure 3.7 in Chapter

3), the injected electrons could be captured in the trap sites to become immobilized.

Therefore, an increase of average relaxation time was anticipated, and it could be

explained as the carrier mobility change to some extent in the case of the traps

generation [27]. As a result, the capacitance will change more slowly with bias due to

traps generation after stress. The very similar case as seen in the distortion (stretch-out)

of C-V curve in MOS device could also been found when the interface states are

present [28]. With the continuously increased stress time, more electrons trapping

probably renders the VCCs decrease monotonously. In addition, the different amounts

of the reduced VCCs may correlate to the different amounts of traps created in oxide

films.

5.3.4. Prediction of VCCs

According to the above discussion, it is concluded that the effect of dielectric

thickness, frequency, and stress on the VCCs could be either due to the change of free

carrier mobility, and/or carrier concentration in high-κ dielectric film. In conclusion,

Figure 5.10(a) and 5.10(b) summarize the VCCs of HfO2 MIM capacitors as a function

of thickness, using different pre-factor n0 and carrier mobility values. The results


127

Figure 5.10: (a) The simulated VCCs of HfO2 MIM capacitors as a function of

thickness with different carrier concentration pre-factor (n0), and (b) the simulated

VCCs as a function of thickness with different carrier mobility in dielectric film.

20 30 40 50 60100

101

102

103

104

105

n0=1.636x1014 cm-3

n0=1.636x1015 cm-3

Qua

drat

ic V

CC

(ppm

/V2 )

Thickness (nm)

n0=1.636x1015 cm-3

Capacitance density=5fF/µm2

Quadratic VCC=100 ppm/V2

20 30 40 50 60100

101

102

103

104

µ=6.6x10-5 cm2/vs

µ=1.1x10-5 cm2/vs

Qua

drat

ic V

CC

(ppm

/V2 )

Thickness (nm)

µ=1.1x10-4 cm2/vs

Capacitance density=5fF/µm2

Quadratic VCC=100 ppm/V2

(a)

(b)


128

showed that the VCCs will drop monotonously with thickness when the pre-factor and

carrier mobility values are pre-defined. In addition, the VCCs drop by nearly one order

of magnitude when the pre-factor or the carrier mobility decreases by the same order at

certain thickness. For a specific instance, in order to meet the specifications for analog

MIM capacitor beyond 2007 (VCCs ≤ 100 ppm/V2, capacitance density of 5 fF/µm2),

the hatched areas in Figs. 5.10(a) and 5.10(b) are the regions where the capacitance

density and VCCs meet the ITRS roadmap requirements [1].

Comparing with the dielectric thickness, the carrier mobility and pre-factor n0

are mainly affected and determined by certain process conditions for high-κ dielectrics

fabrication. Thus, the effective way to reduce VCCs is to increase the dielectric

thickness. However, it severely limits the scaling of high-κ dielectric for improving

capacitance density of MIM capacitors. In order to meet the continuously increased

capacitance density requirement as well as small VCCs, dielectrics with much higher

dielectric constants than HfO2 needs to be researched and implemented in order for

future applications. Besides, it is believed that carrier mobility and pre-factor n0 are

strongly affected by the fabrication methods and process conditions [3, 12, 29-31],

which directly correlates to the quality of the high-κ materials. In particular, Y. L. Tu

observed that the deposition temperature could affect the VCCs of Ta2O5 MIM

capacitor to some extent [3]. However, low thermal budget in back-end line integration

severely limits the improvement of high-κ dielectrics quality, which impedes the

successfully wide implementation of high-κ dielectrics for MIM capacitors application.

Recently, several plasma treatment schemes prove to be effective for the leakage

property improvement for some high-κ films at low temperature compatible with

BEOL process [30, 31], which may also be promising for the improvement of C-V

characteristics for high-κ MIM capacitors.


129

5.4. Conclusion

In this chapter, a free carrier injection model has been established successfully

to explore the VCCs’ mechanism for high-κ MIM capacitors using HfO2 as the

dielectric. For the first time, it has been revealed that the thickness dependence of

VCCs, which exhibits a relation of nt−∝α (n~2), is an intrinsic problem due to the

electrical field increment when the dielectric film is scaled down. This finding is of

great implication for high-κ MIM capacitors application in many aspects and is also

useful for the design purpose. In addition, the frequency dependences of VCCs, and

the stress modified VCCs have also been discussed. For both cases, the C-V curves

bent downwards with the increase of frequency and/or under electrical stress, i.e. the

VCCs will decrease with frequency and/or after electrical stress. Based on the free

carrier model, it is proposed that the VCCs variations are attributed to the change of

relaxation time, which is in the origin of carrier mobility and/or pre-factor n0 change in

the dielectric film. In conclusion, this model could be also easily applied to other high-

κ MIM capacitors predicting the C-V characteristics, and help to understand the

process-property correlationship of high-κ dielectrics.


130

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[23] H. Hu, C. Zhu, X. Yu, A. Chin, M. F. Li, B. J. Cho, and D. -L. Kwong, “MIM



[24] S. J. Kim, B. J. Cho, M. F. Li, C. Zhu, A. Chin, and D. -L. Kwong, “HfO2 and



[25] S. Van Huylenbroeck, S. Decoutere, R. Venegas, S. Jenei, and G. Winderickx,

“Investigation of PECVD dielectrics for nondispersive metal-insulator-metal

capacitors,” IEEE Electron Device Lett., Vol. 23, No. 4, pp. 191-193, 2002.


and B. El-Kareh, “Analog characteristics of metal-insulator-metal capacitors


133

using PECVD nitride dielectrics,” IEEE Electron Device Letters, Vol. 22, No. 5,

pp. 230-232, 2001.

[27] B. Gross, “Dose rate dependence of carrier mobility,” Solid State Commun., Vol.

15, pp. 1655-1657, 1974.

[28] E H Nicollian and J R Brews, MOS (Metal Oxide Semiconductor) Physics and

technology (Wiley, 1982)

[29] H. Hu, C. Zhu, X. F. Lu, Y. H. Wu, T. Liew, M. F. Li, B. J. Cho, W. K. Choi and

N. Yakovlev, “Physical and electrical characterization of HfO2 metal-insulator-

metal capacitors for Si analog circuit applications,” J. Appl. Phy., Vol. 94, pp.

551-557, 2003.

[30] G. B. Alers, R. M. Fleming, Y. H. Wong, B. Dennis and A. Pinczuk, “Nitrogen

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[31] S. J. Chang, J. S. Lee, J. F. Chen, S. C. Sun, C. H. Liu, U. H. Liaw and B. R.

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Chapter 6 Summary and Future Works

134

Chapter 6

Summary and Future Works

6.1. Summary

In this thesis, HfO2 based high-κ MIM capacitors have been thoroughly

investigated. These have been accomplished by preparing HfO2 based high-κ films

using various preparation methods such as pulsed-laser deposition (PLD), sputtering,

atomic-layer-deposition (ALD) and studying the material and electrical properties of

these high-κ films. The important findings and conclusions obtained in the course of

the studies can be summarized as the following:

1) HfO2 high-κ dielectrics prepared by PLD have been fabricated for MIM capacitors

application for the first time with superior electrical performance; a good

understanding of process-structure-property correlation for HfO2 film processing

has been achieved.

2) RF characteristics of HfO2 based high-κ MIM capacitors were investigated; the

results indicate that HfO2 based high-κ materials exhibit good capacitance-

frequency dependency, i.e. the dielectric constant of HfO2 remains almost constant

in the entire frequency range up to 20 GHz. In addition, thickness dependence of

stress induced leakage currents and voltage linearity for HfO2 MIM capacitors

have been discussed with the help of an equivalent circuit model, which are

proposed to be in the origin of traps generation in the dielectric films.


135

3) ALD prepared high-κ HfO2-Al2O3 MIM capacitors using laminate, stack and

sandwich structures were characterized. The electronic conduction mechanism,

voltage coefficients of capacitance (VCCs) dependence, etc have been investigated

in detail. In comparison with stack and sandwich structures, the laminate structure

is superior in terms of electrical performance. In addition, ALD technique shows

the advantages of film quality, uniformity, and the ease of dielectric structure

engineering over PLD and PVD methods.

4) Finally, a free carrier injection model has been established successfully to explore

the VCCs’ mechanism for high-κ MIM capacitors using HfO2 as the dielectric. It

has been revealed that the thickness dependence of VCCs, which exhibits a

relation of nt−∝α (n~2), is an intrinsic problem due to the electrical field increment

when the dielectric film is scaled down. In addition, the frequency dependences of

VCCs, and the stress modified VCCs have been also discussed using the free

carrier model. It is proposed that the VCCs variations are attributed to the change

of relaxation time, which is in the origin of carrier mobility and carrier

concentration change in the dielectric film.

6.2. Future works

There are a few aspects the author thinks would be worth for further

investigation:

1) HfO2 films processing

In this work, HfO2 films are in their as-deposited states without any post-

deposition treatment, traditional furnace annealing in this work was found to be

less effective for the quality improvement of thick HfO2 films which might be due


136

to the allowable thermal budget in back-end of line (BEOL) integration. Therefore,

it is worthwhile to develop some novel thermal treatment schemes, such as multi-

deposition and multi-annealing method, high density plasma annealing process,

and so on.

2) Dielectrics properties engineering

The reported VCCs and temperature coefficient of capacitance (TCCs) for high-κ

dielectrics are much larger than those of SiO2 and Si3N4. Considering the positive

VCC and TCC values of most high-κ dielectrics, it would be nice to search

alternative dielectrics in possession of negative VCCs and TCCs values, and

incorporating those dielectrics into current high-κ systems to achieve low TCCs

and VCCs values.

3) VCCs mechanism study

In this work, our current results indicate that the VCCs dependence on many

parameters such as thickness, frequency, and electrical stress is mainly a bulk

effect. However, it is believed that the metal/dielectric interface may also play a

role affecting the VCCs. The effects of the interface properties on the VCCs needs

more study.

In addition, the extent of capacitance variation is shown to be dependent on the

bias polarity, how could this by affected by each leakage component (like Schottky

emission, Poole-Frenkel conduction and so on.) need further investigation.

4) Integration of HfO2 based dielectrics

In the current technology node, HfO2 based dielectrics needs to be put into Cu/low-

κ BEOL process for their real use, the process related issues like the proper choice

of metal stacks, the compatibility with low-κ material, the effects of plasma


137

etching on high-κ dielectrics, and so on, are useful topics for integration of HfO2

based materials.

List of Publications

138

LIST OF PUBLICATIONS

1. H. Hu, C. Zhu, Y. F. Lu, M. F. Li, B. J. Cho, and W. K. Choi, “A high

performance MIM capacitor using HfO2 dielectrics,” IEEE Electron Device Lett.,

Vol. 23, pp. 514-516, 2002.

2. H. Hu, C Zhu, Y. F. Lu, Y. H. Wu, T. Liew, M. F. Li, B.J. Cho, W. K. Choi, and

N. Yakovlev, “Physical and electrical characterization of HfO2 metal-insulator-

metal capacitors for Si analog circuit applications,” J. Appl. Phys., Vol. 94, No. 1,

pp. 551-557, 2003.

3. H. Hu, C. Zhu, X. Yu, A. Chin, M. F. Li, B. J. Cho, and D. -L. Kwong, “MIM



4. X. Yu, C. Zhu, H. Hu, A. Chin, M. F. Li, B. J. Cho, D. -L. Kwong, F. D. Foo, and

M. B. Yu, “A high density MIM capacitor (13 fF/µm2) using ALD HfO2

dielectrics,” IEEE Electron Device Lett., Vol. 24, pp. 63-65, 2003.

5. H. Hu, C. Zhu, Y. F. Lu, J. N. Zeng, Y. H. Wu, T. Liew, M. F. Li, and W. K. Choi,

“Material and electrical characterization of HfO2 films for MIM capacitors

application,” Mat. Res. Soc. Symp. Proc. Vol. 766, E.3.3.1, 2003.

6. X. Yu, C. Zhu, H. Hu, A. Chin, M. F. Li, B. J. Cho, D. -L. Kwong, P. D. Foo, and

M. B. Yu, “MIM capacitors with HfO2 and HfAlOx for Si RF and analog

applications,” Mat. Res. Soc. Symp. Proc. Vol. 766, E.5.9.1, 2003.

7. H. Hu, S. –J. Ding, H. F. Lim, C. Zhu, M. F. Li, S. J. Kim, X. Yu, J. H. Chen, Y. F.

Yong, B. J. Cho, D. S. H. Chan, Subhash C Rustagi, M. B Yu, C. H Tung, A. Y.

Du, D. My, P. D. Foo, A. Chin, and D.-L. Kwong, “High performance ALD HfO2-

List of Publications

139

Al2O3 laminate MIM capacitors for RF and mixed signal IC applications,” in Proc.

of IEDM, pp. 379-382, 2003.

8. C. Zhu, H. Hu, X Yu, S. J. Kim, A. Chin, M. F. Li, B. J. Cho, and D. -L. Kwong,

“Voltage and temperature dependence of capacitance of high-κ HfO2 MIM

capacitors: A unified understanding and prediction,” in Proc. of IEDM, pp. 879-

882, 2003.

9. H. Hu, S. –J. Ding, C. Zhu, Y. F. Lu, M. F. Li, B. J. Cho, and D. S. H. Chan, S. C.

Rustagi, M. B. Yu, A. Chin, and D. -L. Kwong, “Investigation of PVD HfO2 MIM

capacitors for Si RF and mixed signal ICs application,” International

Semiconductor Device Research Symposium (ISDRS), pp. 328-329, 2003.

10. S. –J. Ding, H. Hu, S. J. Kim, H. F. Lim, C. Zhu, M. F. Li, B. J. Cho, D. S. H.

Chan, S. C. Rustagi, M. B. Yu, A. Chin, D. –L. Kwong, “High performance MIM

capacitor using ALD high-κ HfO2-Al2O3 laminate dielectrics,” IEEE Electron

Device Lett., Vol. 24, pp. 730-732, 2003.

11. S. –J. Ding, H. Hu, H. F. Lim, S. J. Kim, X. F. Yu, C. Zhu, M. F. Li, B. J. Cho, D.

S. H. Chan, S. C. Rustagi, M. B. Yu, A. Chin, D. –L. Kwong, “DC, RF, and

reliability characteristics of atomic layer deposited HfO2-Al2O3 laminate MIM

capacitors for Si RF IC applications,” IEEE Trans. on Electron Devices Vol. 51,

pp. 886-894, 2004.

12. S. –J. Ding, H. Hu,, C. Zhu, M. F. Li, S. J. Kim, B. J. Cho, S. H. Chan, M. B. Yu,

A. Y. Du, A. Chin, D. –L. Kwong, “Evidence and understanding of atomic-layer-

deposited HfO2-Al2O3 laminate MIM capacitors outperforming sandwich

counterparts,” accepted by IEEE Electron Device Lett.

phd thesis huhang

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