backside fabrication, sensor application and...

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BACKSIDE FABRICATION, SENSOR APPLICATION AND RELIABILITY STUDY OF COMPOUND SEMICONDUCTOR TRANSISTORS

By

KE-HUNG CHEN

A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT

OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA

2010

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© 2010 Ke-Hung Chen

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To my family, who always love and support me

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ACKNOWLEDGMENTS

Foremost, I would like to express my sincere gratitude to my advisor Professor

Fan Ren for the continuous support of my PhD study and research, for his immense

knowledge, world first class research environment and enthusiasm. He supplies

research resource much better than any group I know in Taiwan. His guidance helped

me in all the time of research and writing of this thesis. I also thank my committee

members, Professor Jenshan Lin, Professor Kirk Ziegler and Professor Stephen

Pearton for their contribution to my proposal presentation and dissertation defense.

I thank Dr. Brent Gila, Mrs. Wen-Hsing Wu, Dr. Byoung Sam Kang, Dr. Travis

Anderson, Dr. Lii-Cherng Leu, Dr. Yu-Lin Wang, Byung-Hwan Chu, Erica Douglas,

Chien-Fong Lo and Sheng-Chung Hung for giving me lots of suggestions and assisting

to my research.

Finally, and most importantly, I express my thanks and gratitude to my parents and

brothers in Taiwan, who always give me warm love and support me. I would like to

thank my dear wife, I-Fen Wang, who has shared my ups and downs for past ten years.

She is my best friend and I love her forever.

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TABLE OF CONTENTS page

ACKNOWLEDGMENTS.................................................................................................. 4

LIST OF TABLES............................................................................................................ 7

LIST OF FIGURES.......................................................................................................... 8

LIST OF ABBREVIATIONS........................................................................................... 11

ABSTRACT ................................................................................................................... 16

CHAPTER

1 LITERATURE REVIEW AND MOTIVATION........................................................... 19

1.1 InAlAs/InGaAs MHEMTs Degradation Study .................................................... 19 1.2 AlGaN/GaN High Electron Mobility Transistor .................................................. 23 1.3 Field-Effect Transistor Based Semiconductor Sensors..................................... 26 1.4 Laser Technology ............................................................................................. 28 1.5 Laser Technology For Glass Application .......................................................... 31 1.6 Study Outline .................................................................................................... 33

2 DEGRADATION OF 150 NM MUSHROOM GATE INALAS/INGAAS MHEMTS DURING DC STRESSING AND THERMAL STORAGE......................................... 38

2.1 Background....................................................................................................... 38 2.2 Experiment........................................................................................................ 39 2.3 Results And Discussion .................................................................................... 40

3 C-ERBB-2 SENSING USING ALGAN/GAN HIGH ELECTRON MOBILITY TRANSISTORS FOR BREAST CANCER DETECTION......................................... 55

3.1 Background....................................................................................................... 55 3.2 c-erbB-2 Antigen Detection Using AlGaN/GaN High Electron Mobility

Transistors........................................................................................................... 56

4 LOW HG (II) ION CONCENTRATION ELECTRICAL DETECTION WITH ALGAN/GAN HIGH ELECTRON MOBILITY TRANSISTORS ................................ 63

4.1 Background....................................................................................................... 63 4.2 Hg (II) Metal Ion Detection Using AlGaN/GaN High Electron Mobility

Transistors........................................................................................................... 64

5 CU-PLATED THROUGH-WAFER VIAS FOR ALGAN/GAN HIGH ELECTRON MOBILITY TRANSISTORS ON SI.......................................................................... 71

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6 UV EXCIMER LASER DRILLED HIGH ASPECT RATIO SUBMICRON VIA HOLE...................................................................................................................... 81


7 193 NM EXCIMER LASER DRILLING OF GLASS SLICES: DEPENDENCE OF DRILLING RATE AND VIA HOLE SHAPE ON THE DIAMETER OF THE VIA HOLE...................................................................................................................... 89


8 SUMMARY AND FUTURE WORK ....................................................................... 104

LIST OF REFERENSCES........................................................................................... 107

BIOGRAPHICAL SKETCH.......................................................................................... 118

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LIST OF TABLES

Table page 1-1 Comparison of laser technologies showing performance parameter ranges of

excimer lasers versus solid state laser .............................................................. 34

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LIST OF FIGURES

Figure page 1-1 Schematic drawing of the crystal structure of Wurtzite Ga-face and N-face

GaN. ................................................................................................................... 34

1-2 Bandgaps of the most important elemental and binary cubic semiconductors versus their lattice constant at 300K .................................................................. 35

1-3 Band diagram of normal AlGaN/GaN heterostructure. ....................................... 36

1-4 Different laser process techniques . ................................................................... 36

1-5 (Top left) A schematic of a flat trench created by using a square mask. (Top right) A schematic of an inclined grove created by using a diamond shape mask. (Bottom left) A schematic of a rounded trench created by using a circular mask. (Bottom right) A schematic of a stepped grove created by using a cross mask ............................................................................................ 37

2-1 Cross-section schematic of InAlAs/InGaAs MHEMT. ......................................... 44

2-2 Top view of OM image for InAlAs/InGaAs MHEMT device with 2 fingers of 75um gate width (top) and TLM pattern (bottom). .............................................. 45

2-3 The drain current density (IDS) and gate voltage (VGS) of virgin MHEMT as function of drain voltage (VDS) (top) and microwave characteristics of the InAlAs/InGaAs MHEMT device at room temperature (bottom). .......................... 46

2-4 The drain current (IDS) and gate voltage (VG) of the InAlAs/InGaAs MHEMT after thermal stress at 250 oC for 36hrs as function of drain voltage (VDS) (top) and the drain current (IDS) and gate voltage (VG) of the InAlAs/InGaAs MHEMT after DC stress at 2.7 V for 36 hrs (bottom).......................................... 47

2-5 Data from a TLM pattern stored at 250 C for 48 hours (top) resistance v.s. gap distance (middle) sheet resistance (bottom) specific contact resistivity....... 48

2-6 Data from a TLM pattern on a MHEMT stressed with a DC stress of 27 mA at 165 C (top) resistance v.s. gap distance (middle) sheet resistance (bottom) specific contact resistivity. .................................................................................. 49

2-7 Low magnification cross-section view TEM image of InAlAs/InGaAs MHEMT after stored at 250 C for 48 hours (top) Low magnification cross-section view TEM image of InAlAs/InGaAs MHEMT after DC stress for 36hrs (bottom)......... 50

2-8 The higher magnification TEM images of Ohmic contact at the edge of source and drain contact (top). The higher magnification TEM images of ohmic contact at the edge of the contact (bottom).............................................. 51

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2-9 The gate current of InAlAs/InGaAs MHEMT device for unstressed, thermal stress (250 oC, 36 hours) and DC stress (VDS 2.7 V, IDS 167 mA/mm, 36 hours). ................................................................................................................ 52

2-10 Low magnification cross-section view TEM image (top) and the higher magnification TEM image (bottom) of the mushroom gate after 165 oC, VDS 3 V, JDS 300 mA/mm for 36 hr. .............................................................................. 53

2-11 EDS elemental analysis of the mushroom gate after 165 oC, VDS 3V, JDS 300 mA/mm for 36 hr................................................................................................. 54

3-1 (Top) Plan view photomicrograph of a completed device with a 5-nm Au film in the gate region. (Bottom) Schematic of AlGaN/GaN HEMT. The Au-coated gate area was functionalized with c-erbB-2 antibody/antigen on thioglycolic acid..................................................................................................................... 59

3-2 I-V characteristics of AlGaN/GaN HEMT sensor before and after exposure to 0.25 μg/ml c-erbB-2 antigen. .............................................................................. 60

3-3 Drain current of an AlGaN/GaN HEMT over time for c-erbB-2 antigen from 0.25 μg/ml to 17 μg/ml. ....................................................................................... 61

3-4 Change of drain current versus different concentrations from 0.25 μg/ml to 17 μg/ml of c-erbB-2 antigen. .................................................................................. 62

4-1 Plain view photomicrograph of a completed device with a 5 nm Au film in the gate region (top). A schematic of AlGaN/GaN HEMT. The Au-coated gate area was functionalized with thioglycolic acid (bottom). ..................................... 68

4-2 Photographs of contact angle of water drop on the surface of bare Au (left) and thioglycolic acid functionalized Au (right). .................................................... 69

4-3 Time dependence of the drain current for a HEMT sensor exposed to different concentrations of Hg2+ ion solution. ...................................................... 69

4-4 The difference of drain current for the HEMT sensor exposed to different Hg2+ ion concentration to the DI water................................................................ 70

5-1 Schematic of via in AlGaN/GaN HEMT on Si wafer............................................ 77

5-2 Cross-sectional SEM of dry etched via in AlGaN/GaN HEMT on Si wafer. ........ 77

5-3 Cross-sectional SEMs of dielectric/metal stack on field (top) or sidewall (center) and Cu/Ti/SiO2 stack at high magnification (bottom) ............................. 78

5-4 Schematic of plating sequence for Cu. ............................................................... 79

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5-5 Cross-sectional SEM of Cu-plated via after mechanical polishing. The diameter of the openings of the vias is 50 μm. ................................................... 79

5-6 Defect type as a function of seed aging time. ..................................................... 80

5-7 Optical plan view micrograph of front-side of plated via wafer showing HEMTs and contact pads. .................................................................................. 80

6-1 (Top) Cross sectional micrographic image of a via hole drilled through a Si substrate with a diameter of 90 and 45 m for the entrance and exit hole. (bottom) Top view SEM images of a 5 m diameter via hole drilled on a Si substrate with two different drilling times; 30 sec for the image on the left and 40 sec for the image on the right. ....................................................................... 86

6-2 (Top) Side view of a series of 160 m diameter via holes drilled on the edge of a glass with different numbers of repetitive laser pulses. (Bottom) Side view of a series of 80 m diameter via holes drilled on the edge of a glass substrate with different numbers of repetitive laser pulses. ................................ 87

6-3 A micrographic side view image of via holes drilled from both sides of the glass substrate. The diameters of the entrance holes are 80 and 10 m for the top and bottom via hole, respectively. .......................................................... 88

7-1 Schematic of the excimer laser drilling system. .................................................. 96

7-2 Drilling rate of the glass as a function of the via hole diameter........................... 97

7-3 Side view images of drilled holes with the diameters of the entrance holes being 120, 80, 40, and 5 m respectively. .......................................................... 98

7-4 (Top) Time dependent drilling rate of the 120 m diameter via hole. (Bottom) Side view images of the 120 m diameter via holes drilled for different times.... 99



7-7 (Top) Time dependent drilling rate of the 5 m diameter via hole. (Bottom) Side view images of the 5 m diameter via holes drilled for different times...... 102

7-8 Cross-sectional SEM of glass slips with 5 µm entrance diameter drilled with 2 min drilling time................................................................................................. 103

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LIST OF ABBREVIATIONS

2DEG two dimensional electron gas

AAS atomic absorption spectroscopy

AES auger electron spectroscopy

Al aluminium

Al2O3 aluminium oxide

AlGaAs aluminium gallium arsenide

AlGaN aluminium gallium nitride

AlInAs aluminium indium Arsenide

AlN aluminium nitride

Ar argon

ArF argon fluoride

Au gold

AuGe gold Germanium

BCB benzocyclobutene

CaF2 calcium fluoride

CCD charge coupled device

CCTV closed-circuit television

Cl2 chlorine

CMP chemical mechanical planarization

CNT carbon nanotube

CNTFET carbon nanotube field effect transistor

CTE coefficient of thermal expansion

Cu copper

DC direct current

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DI de-ionized

DNA deoxyribonucleic acid

DPSS diode pumped solid state

DRIE deep Reactive Ion Etching

EDS energy-dispersive X-ray spectroscopy

ELISA enzyme-linked immunsorbent assay

EP electroplating

F2 fluorine

FET field effect transistor

FIB focused ion beam

FLICE femtosecond laser irradiation followed by chemical etching

fmax maximum frequency of oscillation

fT unity current gain

fs femtosecond

FWHM full width at half maximum

Ga gallium

GaAs gallium arsenide

GaN gallium nitride

gm max maximum transconductance

H2 hydrogen

HAZ heat-affected zone

HCl hydrochloric acid

He helium

HEMTs high electronic mobility transistors

Hg mercury

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HgCl2 mercury chloride

HTOL high temperature operating life test

ICP inductively coupled plasma

ICP-MS inductively coupled plasma-mass spectroscopy

ICs integrated circuits

Ids source-drain current

IGFET insulated-gate field effect transistor

IGs source-gate current

In indium

InAlAs indium aluminum arsenide

InGaAs indium gallium arsenide

InP indium phosphide

I-V current-voltage

IR Infrared radiation

ISE ion selective electrode

ISFET ion-sensitive field effect transistor

I-V current-voltage

KCl potassium chloride

KOH potassium hydroxide

KrF krypton fluoride

KSP solubility product

LD laser diodes

LED light-emitting diodes

LIBWE laser induced backside wet etching

LOD limit of detection

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MBE molecular beam epitaxy

MgF2 magnesium fluoride

MHEMT metamorphic high electronic mobility transistor

MMICs microwave monolithic integrated circuits

Mo molybdenum

MOCVD metal organic chemical vapor deposition

MODFET modulation doped FET

MOSFET metal oxide semiconductor field effect transistor

MTTF mean team to failure

N2 nitrogen

Nd neodymium

Ne neon

NH4OH ammonium hydroxide

Ni nickel

ns nanosecond

PBS phosphate-buffered saline

PECVD plasma-Enhanced Chemical Vapor Deposition

PMMA polymethyl methacrylate

ppb parts per billion

ppm parts per million

ps picosecond

Pt platinum

PVD physical vapor deposition

Rc contact resistance

Rd drain resistance

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RF radio frequency

Rs sheet resistance or source resistance

Rt transfer resistance

RTA rapid thermal annealing

SDHT selectively doped heterojunction transistor

SEM scanning electron micrograph

Si silicon

SiC silicon carbide

SiO2 silicon oxide

SiNx silicon nitride

Ta2O5 tantalum pentoxide

TE thermionic emission

TEGFET two-dimensional electron gas FET

TEM transmission electron microscopy

Ti titanium

TLM transmission line measurement

UV ultraviolet

Vds source-drain voltage

VUV vacuum ultraviolet

WSix tungsten silicide

XeCl xenon chloride

XeF xenon fluoride

XPS X-ray photoelectron spectroscopy

YAG yttrium aluminium garnet

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Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy

BACKSIDE FABRICATION, SENSOR APPLICATION AND RELIABILITY STUDY OF

COMPOUND SEMICONDUCTOR TRANSISTORS

By

Ke-Hung Chen

May 2010

Chair: Fan Ren Major: Chemical Engineering

Reliability studies of InAlAs/InGaAs metamorphic high electron mobility transistors

(MHEMTs) grown on GaAs substrates for high frequency/power applications are

reported. The MHEMTs were stressed at a drain voltage of 3 V for 36 hrs, as well as

undergoing a thermal storage test at 250 ˚C for 48 hrs. The drain current density of the

MHEMTs at zero gate bias dropped about 12.5 % after either the thermal storage test or

DC stress. The gate leakage current of the MHET devices with thermal storage was

much higher than that of devices after DC stress. In the latter case, significant gate

sinking was observed by transmission electron microscopy. The main degradation

mechanism during thermal storage was reaction of the Ohmic contact with the

underlying semiconductor.

AlGaN/GaN high electron mobility transistors (HEMTs) were used to detect c-

erbB-2 antigen, an important biomarker for breast cancer early detection. The Au gated

region of the HEMT was functionalized with thioglycolic acid and then used to

immobilize the c-erbB-2 antibodies. The source-drain current (Ids) showed a clear

dependence on the c–erbB-2 antigen concentration in phosphate-buffered saline (PBS)

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solution. The limit of detection (LOD) was 0.25 μg/ml, which is less than the c-erbB-2

antigen concentration present in both healthy people and people with breast cancer.

This approach showed the promise of early stage breast cancer screening and pre-

clinical disease diagnosis by rapid, noninvasive and portable electronic biological

sensors based on AlGaN/GaN HEMT technology.

Thioglycolic acid functionalized Au-gated AlGaN/GaN based HEMTs were also

used to detect mercury (II) ions. The source-drain current of the HEMT sensors

monotonically decreased with the mercury (II) ion concentration from 1.5×10−8 to 4×10−8

M. The source-drain current reached equilibrium around 15–20 sec after the

concentrated Hg ion solution was added to the gate region of the HEMT sensors. The

effectiveness of the thioglycolic acid functionalization was evaluated with a surface

contact angle study. The results suggested that portable, fast response, and wireless-

based heavy metal ion detectors can be realized with AlGaN/GaN HEMT-based

sensors.

The Cu filled backside via holes were used to improve the electrical performance

and improve heat dissipation of the AlGaN/GaN HEMTs. The 70 μm deep through-wafer

backside via holes with a diameter of 50 μm were etched by deep Si reactive ion

etching system on the backside of AlGaN/GaN HEMTs. The pulsed Cu electroplating

process was used to fill up the etched vias with Cu. Mechanical polishing was then

performed to planarize the Cu layer. This approach is attractive for increasing the

effective thermal conductivity of the composite substrate for high power device

applications.

The effect of the via hole diameter on the laser drilling rate of glass as well as the

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shape of the drilled via holes were investigated. The via holes with a diameter of 120

μm showed a 7.5°–9°angled, tapered side wall, and the drilling rate was relatively

constant at around 17 μm/s. For the smaller via holes with diameters ranging from 30 to

80 μm, significantly different results were obtained due to laser reflection from the

tapered side wall of the via hole leading to the drilling rate being slightly increased and

via hole becoming conical in shape. For the smallest via holes, with an entrance

diameter of 10 μm, the drilling resulted in a very high aspect ratio, funnel shaped via

hole with a significantly reduced drilling rate.

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CHAPTER 1 LITERATURE REVIEW AND MOTIVATION

1.1 InAlAs/InGaAs MHEMTs Degradation Study

Silicon (Si) based devices dominate electronics devices and are extensively used

in the integrated circuits (ICs) and data storage industry based on several important

factors: excellent dielectrics (SiO2 and SiNx), good mobility, transport properties, low

surface recombination velocity (~10 cm/s), low cost, mechanical hardness, good

stability, and huge experience base [1].

However, the III-V semiconductors attract attention due to their superior properties

compared to Si in some aspects. Gallium Arsenide (GaAs) has higher electron mobility

and is a direct bandgap material that can be used in photonics. Wide energy bandgap

Aluminium Gallium Arsenide (AlXGa1-XAs) is lattice matched with GaAs and can be

grown on GaAs to form HEMT structures. HEMT is also called modulation doped field

effect transistor (MODFET), two-dimensional electron gas FET (TEGFET) and

selectively doped heterojunction transistor (SDHT). HEMT structure is similar with metal

oxide semiconductor FET (MOSFET) and both have structure including gate, source,

drain and substrate.

The lattice mismatch existing in the heterojunction material will cause traps and

greatly reduce device performance. An extremely thin material with optimal lattice

constant can be used to allow larger bandgap difference for heterostructure devices,

which is called a pseudomorphic HEMT (pHEMT). Metamorphic HEMT (MHEMT), with

a buffer layer between different lattice constant materials, is an advanced HEMT

structure compared to pHEMT. InAlAs buffer layer has a graded indium concentration

so that it can match the lattice constant of both the GaAs substrate and the InGaAs

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channel layer using the MHEMT approach. The metamorphic buffer layer is

accommodated for the lattice mismatch between the GaAs substrate and the HEMT

layers. The larger conduction band discontinuity between the InAlAs and InGaAs layers

allows a more effective charge transfer into the channel layer and improves carrier

confinement [1-10].

InAlAs/InGaAs/InP MHEMTs have numerous performance advantages over the

more commonly used GaAs pHEMTs due to the high velocity and carrier density.

However, the size of InP substrates is limited for 4 inch and InP substrates are very

brittle. Therefore, GaAs substrate has become a much more promising substrate for

InAlAs/InGaAs MHEMTs due to numerous advantages: wafers up to 6-inch, lower cost,

less fragility/brittleness, more mature backside processing, the ability to tailor the lattice

constant by varying the indium content and compatibility with smaller via holes for

compact chip size [1-3, 5, 8, 10-14].

The good stability is crucial for InAlAs/InGaAs MHEMTs to compete with other

technology, so its reliability is extensively studied. The failure criterion for MHEMTs is

usually defined as a 10% degradation of maximum transconductance (gm max) or a 20%

reduction of the maximum source-drain current (Ids). Normally, high temperature

operating life test (HTOL), DC biased accelerated stress test, and environmental test

are used to investigate degradation mechanisms. HTOL is to store device in high

temperature ambient (up to 250 oC) without bias. DC stress was bias the device at fixed

source-drain voltage (VDS, 1~3V) at different temperatures (150~250 oC) to get the

median time to failure (MTTF) information. MTTF can be obtained at a specified channel

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temperature (e.g. 125 oC) by performing a least-square linear fit of median life time

versus inverse channel temperature [3-9, 11-13].

The main degradation mechanisms include gate sinking, ohmic contact

degradation, hot electron induced degradation and buffer crystalline defects. The gate

sinking is defined as the diffusion of gate metal into the underlying semiconductor. This

decreases the distance between the metal-semiconductor interface and the active

channel. Different gate metal schemes have already been employed for InAlAs/InGaAs

MHEMTs including Ti/Au, Ti/Pt/Au, Pt/Ti/Pt/Au…etc. Pt/Ti/Pt/Au gate performs better

than Ti/Pt/Au gate in the following aspects: longer life time, less gm max degradation, less

drain resistance increase, less source resistance increase, and less threshold voltage

positive shift [3-7, 9-10, 13-14].

Ohmic contact degradation is caused by thermally activated metal-metal and

metal-semiconductor interdiffusion during annealing and causes contact resistance,

source resistance and drain resistance to increase. Various Ohmic metal schemes have

already been reported including AuGe/Ni, AuGe/Pt, AuGe/Ni/Ti/Pt/Au, AuGe/Pt/Ti/Pt/Au,

AuGe/Ni/Au, Ti/AuGe/Au, and non-alloyed Ohmic contacts (i.e. Mo/Au). The Ohmic

contact degradation was caused by the diffusion of Au through the Ti and Pt barriers. By

using WSix as an Ohmic contact metal, the diffusion of In into the contact is suppressed

and Ohmic contact will get better stability. The non-annealed Ohmic-recess approach

was also used to demonstrate better reliability performance. The Ohmic contact

degradation is main degradation mechanism for HTOL since the pinch-off voltage (Vto)

is unchanged after thermal storage. The pinch-off voltage shifted more positively under

DC bias stress than HTOL [5-7, 9-10, 12-14].

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Hot electron degradation is caused by the electric field between the gate and drain

and accelerates the carriers in the channel. The high electric field can supply hot

electrons enough energy to overcome the conduction band discontinuity. The hot carrier

degradation will cause the drain resistance increase faster than the source resistance

and decrease source-drain current, and maximum transconductance. The shorter gate

length and the increase of the indium content will induce hot electron degradation due to

the higher electrical field and smaller channel bandgap. The smaller gate to channel

distance or the smaller recess length will lead to a higher electrical field on the drain

side and cause the enhancement MHEMTs degrades faster than depletion MHEMTs.

The electron-hole pair is separated by the electric field, trapping the holes close to the

gate contact and leading to a negative shift of the threshold voltage. The surface traps

in the SiNx passivation layer between gate and drain will degrade power gain and

increase drain resistance [4-7, 9, 11-12].

Ambient hydrogen degrades InP and GaAs MHEMTs with Ti/Pt/Au as the gate

electrode. The Pt gate metal layers can split hydrogen (H2) into hydrogen atom. Ti

inside the gate metal stack forms Ti-H with hydrogen atom and causes an expansion of

Ti, which then results in stress applied to the semiconductor under the gate metal. The

total gate metal–semiconductor interface potential drifts as more hydrogen is absorbed

and alters the HEMT pinch off voltage over time. Hydrogen has little impact on the

maximum drain current or transconductance [8].

MHEMTs with BCB passivation showed lower degradation than ones with SiNx

passivation. The increase in the parasitic capacitances and intrinsic capacitances

induced in the passivation process was smaller in BCB compared to in SiNx because of

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BCB’s lower dielectric constant. The trapping effect of induced surface states and

degradation of the RF performance are also smaller with BCB passivation [10].

Currently, the InAlAs/InGaAs MHEMTs require a burn-in step to improve the

device stability and reduce the device performance deterioration during usage. By

investigating the degradation behavior of InAlAs/InGaAs MEHT and identifying its failure

mechanisms, we can further improve device performance and reduce its fabrication cost.

1.2 AlGaN/GaN High Electron Mobility Transistor

GaN can have either a Wurtzite or Zincblende crystal structure, though it normally

has a Wurtzite crystal structure with a hexagonal Bravis lattice and four atoms per unit

cell (lattice constant a0=3.189 Å, c0=5.185 Å and u=0.376). For Ga-face structure or Ga-

polarity structure, the crystallization direction follows [0001] direction and the top layer is

Ga. In other words, Ga on the top position of the 0001 bilayer corresponds to the

[0001] polarity. If the top layer is an N atom layer, then the structure is called N-face

structure or N-polarity structure as shown in Figure 1-1. AlGaN/GaN heterostructures

grown on AlGaN nucleation layer by MOCVD are always found to have a Ga face. GaN

(bandgap 3.4 eV) is a wide bandgap material and can form ternary or quaternary

compound with InN (bandgap 0.65 eV) and AlN (bandgap 6.2 eV) as shown in Figure 1-

2. GaN is a direct bandgap material and its bandgap can be adjusted to cover visible

light range making it an ideal candidate for light-emitting diode (LED) and laser diode

(LD) material [16-20].

AlGaAs/ GaAs and AlGaN/ GaN are most commonly used materials for HEMT.

When two semiconductor material contact with each other, the free electrons will diffuse

from the wide bandgap of AlGaN to the narrow bandgap of GaN near the interface. The

band will bend due to the fixed Fermi energy and will form a quantum well close to the

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interface on the narrow energy band side. A potential barrier confines the electrons in a

triangular shaped quantum well and form 2DEG as shown in Figure 1-3. The carriers

are limited in the potential barrier and are only allowed to move in two dimension

spaces instead of three dimensions. The bandgap difference between AlGaN and GaN

form two dimensional electron gas (2DEG) in the heterojunction interface without

intentional doping. The mobility will increase since the electron can move quickly

without colliding with any impurities or dopant ions [16-18, 21-26].

The piezoelectric polarization will be induced by the strain due to the lattice

mismatch between AlGaN and GaN and is more than five times larger as compared to

AlGaAs/GaAs structures. The piezoelectric polarization increases with increase of strain.

The bonds between group III elements (Al, Ga, and In) and group V element (N) are not

only covalent but also ionic, so spontaneous polarization (polarization at zero strain) will

also be induced. The total polarization is the sum of piezoelectric polarization and

spontaneous polarization. The total polarization will increase under tensile strain and

decrease under compressive strain. The sheet carrier concentration of 2DEG will

increase with the increase of polarization. The sheet carrier concentration for

AlGaN/GaN structure can be as high as 1013 cm-2 and is much higher than

AlGaAs/GaAs structure, also making AlGaN/GaN HEMTs more promising for high

power applications. Besides electron mobility, GaN devices have the following

advantages compared to Si based device: high breakdown voltage, high switching

frequency, low power losses, high output power density, high operating voltage, and

high input impedance [16-18, 20-22, 24, 27].

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GaN is a material with outstanding mechanical properties and chemically stability,

making it extremely suitable for operation in chemically harsh environments. The high

saturation velocity and high temperature (i.e. 500 C) operating characteristics allow

GaN to be used in high speed, high power, high frequency and high temperature

applications [1-2, 19-20].

Present DC and RF performance records for GaN transistor are fT 190 GHz, fmax

241 GHz, maximum current handling 30 A, maximum drain current density 1.6 A/mm,

peak transconductance 424 mS/mm, blocking voltage 8,300 V, breakdown electric field

strength 6 MV/cm, breakdown voltage 1,650V, Gain 22 dB at 26 GHz, output power 110

W at 60 V, power density 40 W/mm at 4 GHz, and can deliver power in the millimeter-

wave range (60, 76 and 94 GHz) [19, 24, 27, 28].

Several methods can be used to boost GaN transistor performance. Catalytic CVD

process is used to form the SiNx passivation layer because it does not damage the GaN

surface. The gate length is reduced to 30 nm, which increases the HEMT’s speed.

Multiple fingers will reduce gate resistance and a T-shaped gate decreases gate-to-

drain capacitance. Double heterojunction structure can mitigate short channel effects

and result in better substrate isolation. Higher aluminum content in the channel layers

will produce higher breakdown voltages. A field plate can redistribute the electric field in

the gate-drain region, reduce the peak electric field strength and increase the

breakdown voltage. Highly doped cap layers can be added to the epi structure to reduce

source resistance. The non-alloyed Ohmic contacts can allow the reduction of the gate-

drain spacing, thus further lowering the access resistance [24, 27].

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Even though AlGaN/GaN HEMTs can be used in high temperature conditions, the

poor heat dissipation by substrate (i.e. sapphire) reduces HEMTs device performance.

The high thermal conductivity Cu formed in the etched vias can provide low-inductance

grounding and improve heat transfer characteristics for AlGaN/GaN HEMTs used in

high power device applications.

1.3 Field-Effect Transistor Based Semiconductor Sensors

Field-effect transistor (FET) type sensors have made great progress recently in

metal ion detection and biomaterial detection. The electric field controls the flow of

source drain current (source to drain) by affecting the conductive channel and applied

gate voltage.

The main concept of the FET based semiconductor sensors is to correlate the

target concentration with the change induced by electric field. The ideal FET based

sensors should be very sensitive to the change caused by the analyte of interest at or

nearby the gate surface with molecular receptors or ion-selective membranes in a short

time. The analyte of interest will induce the chemical or electrical change at the gate

surface and modulate the current in the channel of the FET. FET based sensors can

directly translate the analyte-surface interaction into a readable signal, without the need

for elaborate optical components.

The insulated-gate field-effect transistor (IGFET) has a structure similar to a

MOSFET and the gate is electrically isolated from the source and drain. The ion-

sensitive FET (ISFET) is similar to IGFET, but in the ISFET, the metal gate is replaced

by ion-selective membrane, electrolyte and reference electrode. The ISFET is

extensively used as pH sensor or biosensors with SiO2, Si3N4, Al2O3, Ta2O5 and other

dielectric as its gate. The various biomolecules such as DNA, proteins, enzymes, and

27

cells can be monitored and detected by numerous ways including nucleic acid

hybridizations, protein-protein interactions, antigen-antibody binding, and enzyme-

substrate reactions [29-32].

Carbon nanotubes (CNTs), 1D-conductive polymer nanomaterials and nanowires,

are used as conductive channels in FETs, which are notable candidates for highly

sensitive label-free biosensors owing to their unique geometries with a high surface-to-

volume ratio. CNTFETs are expected to have a high sensitivity for biomolecules

detection because the proteins are much larger than the diameter of the CNT channels

(1-2 nm). CNTFET can also be used as mercury ions detection sensors with the

detection limit of 10 nM [33-37].

AlGaN/GaN HEMTs can be operated at high power, elevated temperature

conditions. AlGaN/GaN system is more radiation-hard than the conventional

AlGaAs/GaAs heterostructure due to the higher displacement energies in the nitrides.

AlGaN/GaN HEMTs not only have the above-mentioned advantages but also have the

following characteristics: low power consumption, low quantity of sensing materials, fast

response, long lifetime, portable size and technology, and wireless feasibility. These

advantages make AlGaN/GaN HEMTs a relatively ideal candidate option as sensors [1,

2, 38].

AlGaN/GaN HEMTs can be used to detect breast cancer bio-marker. The source-

drain current can be transformed to the concentration of the breast cancer biomarker.

So AlGaN/GaN HEMTs sensor can be used as screening method to decide whether it is

necessary to do further diagnoses by checking the biomarker concentration. Compared

28

to the traditional method (mammography), HEMT biosensors can reduce cost and

prevent exposure of invasive radiation to patients.

Not only in biosensing field, AlGaN/GaN HEMTs can also be used to detect metal

ion concentration. The mercury (II) ion is toxic and can cause serious problems for the

ecosystem as well as to humans. So how to detect mercury (II) ion concentration is

important topic. Based on similar idea, the source-drain current can be transformed to

mercury (II) ion concentration. So AlGaN/GaN HEMTs can be used as mercury (II) ion

detection sensor and bring multiple advantages compared to traditional spectroscopic or

electrochemical measurement methods. Spectroscopic methods include atomic

absorption spectroscopy, Auger-electron spectroscopy, and inductively coupled plasma

mass spectrometry. Electrochemical methods include ion selective electrodes and

polarography. All of these methods exhibit the same disadvantages which are

expensive, time consuming and not practical for real-time detection.

1.4 Laser Technology

Laser processing is emerging as an alternative to conventional micro-machining

methods, such as wet etching and dry etching, due to several advantages. It has more

flexibility to produce feature sizes down to micron scale as well as the ability to produce

various shapes. It has the ability to selectively remove material by adjusting the applied

fluence between two material’s threshold fluence. Unlike photolithography processes, it

does not require a flat substrate. Laser processing allows one to skip several

processing steps including spin-coating, developing, and stripping compared to

lithography and etching. It does not require the use of vacuum equipment compared to

X-ray, electron and ion beam process, and it is a contactless process [39-42].

29

Excimer lasers and diode pumped solid state (DPSS) lasers are the most common

lasers used in semiconductor field applications. They have different wavelengths and

can produce intense ultraviolet light pulses. This allows different materials including

ceramics, semiconductors and polymers, to be processed for a specific application need.

Solid state lasers exhibit good beam quality, produce very high repetition rates and

have short pulses down to the picosecond (ps) or femtosecond (fs) range [43, 44].

Excimer lasers have minimal heat-affected zone (HAZ) and surface contamination

because the ablation mechanism is a cold process. While DPSS lasers typically rely on

heating to ablate the material, the excimer lasers have high peak power and small

interaction volume resulting in high-energy material ablation with little heat transfer to

the surrounding material. Excimer lasers produce a much larger beam than solid state

lasers, allowing for relatively large processing area. Excimer lasers generate intense

and short ultraviolet pulses directly without the complex converting schemes while

DPSS requires a series of nonlinear optics crystals to alter the wavelength. The power

output for excimer lasers is higher than DPSS power output because the power output

will substantially drop for the wavelength conversion for DPSS. Excimer lasers have

much smaller beam divergence and significant capability to initiate photochemical

reactions. Excimer lasers provide better reproducibility of results, as well as higher

precision and quality [40-41, 43-46].

The excimer laser’s gain medium is composed of inert and reactive gases. The

gases include helium, neon, krypton and fluorine (for 157, 193 and 248 nm systems)

and xenon (for 308 and 351 nm systems). A dimer molecule is produced by an

electrically stimulated reaction and produces light in the ultraviolet range. The main

30

performance characteristics of the excimer and solid state lasers are summarized in

Table 1-1 [41, 47].

Over the years, four drilling processes have been developed, including single-

short, percussion, trepanning and helical drilling as shown in Figure 1-4 [48]. The laser

systems can drill shapes such as circles, squares, rectangles, lines and 3D complex

patterns. The lines or patterns with a desired curvature are fabricated by employing

specially designed masks and stage movement. Ideally, the drilled depth is proportional

to the laser drilled irradiation time. For example, if one burst pulse will etch 1 unit of

material, then the trench edge depth would be 1 unit and the trench center depth would

be D unit by using the D unit diameter circle mask. So, even when using the same

square mask, the drilled angle will cause different shapes of trenches. If the drilled

direction is 45 degree then the drilled trench would be similar to that drilled by a

diamond mask with 0 degree drilled direction. The cross-section view for a square mask

with 0 degree drilled direction is rectangular in shape while a 45 degree drilled direction

with the same square mask will result in a triangular shape. Figure 1-5 shows four

different shapes of grooves/ trenches created by using specially designed masks. A

stepped groove can be created by using a cross mask. When the sample stage moves

in the direction along the center line of the cross in the mask, the center groove receives

a higher dose of the laser light as compared to the areas under the two wings of the

cross. Therefore the drilled depth of the center groove will be deeper and a stepped

groove can be produced. Similarly, by using a diamond shape mask, an inclined trench

can be created. By adjusting the lengths of the two side of the diamond shape mask,

31

different slopes of the inclined trenches can be achieved. The curvature of the rounded

trenches can also be changed with an elliptical mask.

1.5 Laser Technology For Glass Application

Various lasers including excimer lasers (157 nm, 193 nm, 248 nm, 308 nm) [46,

49-64], DPSS lasers (355 nm, 1064 nm) [49, 65], and Ti:Sapphire laser (800 nm) [42,

66-68] were used to drill holes, micro vias, channels and 3D patterns to investigate the

ablation rate, morphology, and impact of fluence, repetition rate, irradiation time, and

pulse width on drilling behavior.

F2 laser induces strong absorption and defines the smallest features (~100 nm) of

any laser process. The rectangular channels with near-vertical walls and flat bottoms, V

shape channel and smooth near-vertical walls with flat bottom can be formed with

suitable fluence [49-53].

ArF laser produces good quality of entrance holes, rectangular cavity, the drilled

structures with high aspect ratios, smooth and steep sidewalls, and high ablation rates

which are better than those drilled with KrF laser. KrF laser with ns and fs pulse widths

were used to study the morphology of drilled holes [53-59].

Laser fluence is a very important parameter in laser micro-machining of glass. In

bulk glass, the drilling will spontaneously stop at a certain depth if the laser fluence

drops below a specific limit. A drilled-through hole can be made once the fluence

exceeds a certain minimum value. The smaller holes require higher fluence in order to

drill through the same thickness of glass. The increase of laser fluence will cause a

rougher surface due to higher reactivity of the drilled material. The effect of the laser

fluence on the drilling rate is relatively small once the fluence above some point. The

excess of energy will be absorbed by ejected material and result in stronger laser

32

plasma. The entrance diameter of a drilled hole is weakly dependent on the incident

fluence. The increase of irradiation time will result in an increase in the size of the exit

hole until it reaches a saturated size. The increase of irradiation time will also generate

more debris and recast due to a denser plasma cloud. The plasma cloud will absorb

part of beam energy, reduce the ablation rate and cause melting and vaporization

around the drilled area. Compared to 1064 nm IR laser, the 355 nm UV laser with

higher fluences will generate longer cracks in the drilled channel. At fixed fluence, UV

light drills faster than IR light and the drilling threshold is also lower at 355 nm compared

with 1064 nm [46, 53, 54, 59, 63-65].

Laser induced backside wet etching (LIBWE) normally use KrF, XeCl and XeF

laser to etch UV-transparent materials including fused silica, quartz, Pyrex, sapphire,

CaF2 and MgF2. The idea of LIBWE is based on the deposition of laser energy on the

sample surface during the ablation of an absorption solution. LIBWE has few pre-/ post-

treatments for a target substrate, large area irradiation, much lower required fluence,

very small roughness, and no debris and cracks compared to directly drilling method [51,

53, 58, 66-70]. Unlike the absorption solution used in LIBWE, the water in contact with

the rear surface of the glass can be used to remove debris from the hole and reduce the

effects of redeposition and blocking. The high aspect ratio drilled hole can be achieved

by focusing on the rear surface of the sample [71-73].

The fs Laser Irradiation followed by Chemical Etching (FLICE) technique can be

used to form high aspect ratio cylindrical microchannels with moderate fluence. The

focus is located hundred μm below the surface of the glass during the drilling [74, 75].

33

The laser drilled structure can be used for various applications (e.g. microfluidic

devices, microfilters, nanopores, electrical through-connections, biophotonic chip and so

on). A 193nm wavelength light is well coupled with glass and makes it possible to study

how to drill a high aspect ratio of the small via holes as well as study the drilling rate and

shape of the via holes as a function of the entrance diameter of the via hole on glass,

which has not yet been reported by another group.

1.6 Study Outline

The main objective of this research is to focus on two sections: III-V

semiconductor electronic device (Chapter 2, 3, 4 and 5) and excimer laser application

study (Chapter 6, and 7). Chapter 2 illustrates the degradation mechanisms of

InAlAs/InGaAs MHEMTs devices. Chapter 3 describes the details of fabrication and

sensitivity measurements of the breast cancer biomarker, c-erbB-2 antigen, using

AlGaN/GaN HEMTs with c-erbB-2 antibody coated gates. Chapter 4 presents detection

of mercury ions using AlGaN/GaN HEMTs through a surface coating of carboxyl groups

on the gate region. This is an extension of the previous work done by Dr. Hung-Ta

Wang. Chapter 5 shows how to form and planarize Cu in through vias in AlGaN/GaN

HEMT devices by electroplating and mechanical polishing. Chapter 6 demonstrates the

ability to drill high aspect ratio vias in glass drilled by ArF excimer laser. Chapter 7

studies the effect of the via hole diameter on the laser drilling rate of glass as well as the

drilled shape. Chapter 8 summarizes above works and suggests future studies.

34

Table 1-1. Comparison of laser technologies showing performance parameter ranges of excimer lasers versus solid state laser [41, 44].

Laser Technologies Parameter Excimer Laser Photon

Energy DPSS Laser

Available wavelengths 157 nm 193 nm 248 nm 308 nm 351 nm

7.90 eV 6.42 eV 5.00 eV 4.66 eV 4.02 eV 3.53 eV 3.53 eV 2.33 eV 1.17 eV

266 nm 355 nm 532 nm 1064 nm

Power Range Up to 540 W 60 W Energy Range Up to 1,100 mJ 0.3 mJ Repetition Rate Variable 1 to 600 Hz Variable 1 to 300 KHz Pulse Length, FWHM 10 to 20 ns fs to 100 ns Beam Profile Homogenous flat-top Near-Gaussian Shot-to-shot stability 0.5 to 1.0 %, rms 5 to 10 %, rms Max. Energy Density 200 J/cm2 2,500 J/cm2 Max. Peak Power Density 1,000 MW/cm2 200 GW/cm2

Figure 1-1. Schematic drawing of the crystal structure of Wurtzite Ga-face and N-face

GaN [16].

35

Figure 1-2. Bandgaps of the most important elemental and binary cubic

semiconductors versus their lattice constant at 300K [76].

36

Figure 1-3. Band diagram of normal AlGaN/GaN heterostructure [77].

Figure 1-4. Different laser process techniques [48].

37

Figure 1-5. (Top left) A schematic of a flat trench created by using a square mask. (Top right) A schematic of an inclined grove created by using a diamond shape mask. (Bottom left) A schematic of a rounded trench created by using a circular mask. (Bottom right) A schematic of a stepped grove created by using a cross mask [41].

38

CHAPTER 2 DEGRADATION OF 150 NM MUSHROOM GATE INALAS/INGAAS MHEMTS DURING

DC STRESSING AND THERMAL STORAGE

2.1 Background

Reliability studies of both GaAs and InP-based HEMTs have identified numerous

degradation mechanisms, including contact problems (especially gate sinking), surface

states that contribute to gate lag, hot carrier-induced impact ionization at the gate edge

or avalanche breakdown in the semiconductor, mechanical stress due to hydrogen

absorption into Ti metallization, fluorine contamination, and corrosion (mainly related to

Al oxidation) [3, 6, 8, 78-93]. MHEMT technology has been developed using

metamorphic buffer layers to grow InAlAs/InGaAs on larger diameter GaAs substrates

to overcome the limitations of InP substrates as described in section 1.1. The

commercial applications of MHEMTs are predominantly in low noise mm-wave

amplifiers for radio communications, automotive collision avoidance radar and high bit-

rate fiber systems [93] .The choice of whether they can be used in place of InP-based

HEMTs depends upon their DC/RF performance and the chip cost requirements. A

number of studies have shown that MHEMTs can exhibit similar reliability to InP

HEMTs, with over 106 hour mean-time–to-failure at 125 ˚C [6, 8].

However, the InAlAs/InGaAs MHEMTs require a burn-in step to improve the

device stability. Current device designs tend to suffer from device degradation, and a

costly burn-in process is typically performed to make the device more stable and

eliminate the early degradation when the devices are placed in service. The transistors

are generally biased at certain gate and drain voltages for 24-60 hours before sending

the devices to customers. During the burn-in process, the drain current decreases and

Ohmic contact resistance increases with time, and level off around 36 hours.

39

Minimizing the burn-in time or eliminating the burn-in step is highly desirable to reduce

the device fabrication cost. In order to effectively identify the failure mechanisms, both a

high temperature storage test and DC stress were used in this study. Besides MHEMT

devices themselves, transmission line method (TLM) patterns were also used to isolate

the gate effect on the device degradation.

2.2 Experiment

The MHEMTs were obtained from a commercial vendor. A schematic of the device

structure used is shown in the cross-section of Figure 2-1. The MHEMTs were

fabricated on lattice matched InGaAs/InAlAs HEMT structures. The epitaxial layer

structures including InGaAs, InAlAs, InGaAs, and InAlAs were grown on 6-inch semi-

insulating GaAs wafer by molecular beam epitaxy (MBE) and used for cap layer, spacer

layer, channel layer and buffer layer. The conductive InGaAs layer was used to form

better Ohmic contact with Ohmic metal. However, the conductive InGaAs layer will

influence the gate operation. So after form the Ohmic metal, the photo lithography

process will be used to open the recess opening. The wet etching process will be used

to etch off the InGaAs layer, this etching process is selectively etch InGaAs than InAlAs,

which means that the InAlAs layer of the HEMT structure acts as an etch stop. The

buffer layer is used to isolate defects from the substrate and create a smooth surface for

following growth of the active layers of the transistor. The 150 nm gate length

mushroom shaped Ti/Pt-based Schottky gate was formed with 1.2 μm spacing between

both gate/drain and gate/source. The final metal layers are formed and provided for

interconnection. The passivation layer is SiNx formed by PECVD. Mesa etch is

employed for device isolation. The two finger design with gate width 75 μm is shown in

the optical micrograph of Figure 2-2 (top). The top layer InGaAs/InAlAs does not

40

undergo the gate recess process for TLM sample. The TLM patterns also present on

the device chip employed 45 x 70 μm pads with gaps of 3, 6, 9, 12 and 15 μm, as

shown in the optical micrograph at the bottom of Figure 2-2.

Typical DC and RF characteristics of the MHEMTs prior to stressing are shown in

Figure 2-3. The maximum drain-source current density was 270 mA/mm, with a gate

current in the hundreds of nA range. The unity current gain, fT, was 94GHz while the

maximum frequency of oscillation, fmax, was 124 GHz.

The devices were stressed in one of two ways. Some of the MHEMTs were biased

at a source-drain voltage of 3 V for 36 hours at 165 oC. Other devices were given a

thermal storage test in an oven at 250 ˚C for 36 hours. The DC characteristics were

measured before and after both kinds of stressing using an Agilent 4156C parameter

analyzer. Some of the devices were also examined by cross-sectional TEM to look for

reactions of the contacts with the underlying semiconductor. EDS elemental analysis

was performed to obtain the elemental profiles near the reacted contacts.

2.3 Results And Discussion

Figure 2-4 shows the drain current (IDS) of the InAlAs/InGaAs MHEMT after both

thermal storage at 250 oC for 36 hrs as function of drain voltage (VDS) (top) and the

drain current (IDS) of the InAlAs/InGaAs MHEMT after DC stress at 3 V for 36 hrs

(bottom). Both forms of stress lead to a reduction in drain current density of ~12.5%

compared to the unstressed devices (Figure 2-3). Thus, after the burn-in process, the

MHEMTs suffer from higher parasitic resistance.

To further investigate the origin of the degradation in drain-source current, the

sheet resistance and specific contact resistance of the devices were obtained from TLM

data as a function of thermal storage time and a function of constant bias voltage stress

41

time. Using the TLMs to examine the increase of the parasitic resistance isolated the

effect of the gate sinking on the degradation of drain-source current from the effect of

Ohmic metal contact degradation. As shown in Figure 2-5, the total resistance of TLMs

increases significantly with time in the first 12 hours of thermal storage at 250 C, while

the specific contact resistivity increases much more than sheet resistance and shows

the contact between Ohmic metal and semiconductor dominated the degradation during

the thermal storage. Similar data for the constant current stressed devices is shown in

Figure 2-6. In this case, the sheet resistance increased around 18%, while specific

contact resistance was reduced by 40%. The device resistance increase was dominated

by changes in the sheet resistance instead of contact resistance.

TEM cross-sections of a degraded thermal storage HEMT and a constant current

stressed HEMT are illustrated in Figure 2-7 (top) and Figure 2-7 (bottom), respectively.

Both samples showed metal spikes with the Ohmic metal diffusing into epitaxial layer,

which were formed during the high temperature Ohmic annealing. For the constant

current stressed sample, the density of the spikes was higher around the edge of the

source Ohmic contact pad and drain Ohmic contact pads, as illustrated in the Figure 2-8

(top). Interestingly enough, the region of the high density spikes in the TEM picture

matched the estimated transfer length of the TLM measurement as shown in top plot in

Figure 2-6. Thus the formation of the high density spikes can result from the current

induced electro-migration.

The drain current density of the commercial power MHEMTs fabricated with the

structure described here is around 0.5 A/mm to 1 A/mm. In the fabrication, the final

metal often has a thickness of 4-6 m, while the Ohmic metal contact has a thickness of

42

less than 0.3 m. In these devices, the Ohmic metal is alloyed with the semiconductor

and the resistance of the alloyed metal is also larger than the unalloyed metal stack.

The 4-6 m thick final metal does not exhibit a problem with a current density of 0.5

A/mm to 1 A/mm. However, as shown by the TEM image of Figure 2-8, the metal

thickness of the region at the edge of the Ohmic contact pad is too thin to sustain the

current density to avoid electromigration. For example, the Ohmic metal thickness for in

the device shown in Figure 2-8 is roughly 250 nm. During the burn-in process, 20 mA

was used to stress a device with gate width of 75 m (20 mA/75 m = 266 mA/mm).

Therefore, the current density is given as 20mA/ (75 m × 0.25 m) = 1 × 105 A/cm2,

which is the current density through the Ohmic pad. Such high current density of

current flowing across the thin Ohmic metal then across the metal semiconductor

interface into the semiconductor causes the Ohmic metal diffusion during the burn-in

process. This caused the electromigration-induced voids and the formation of additional

metal spikes at the edge of the Ohmic metal contact pads, as shown in Figure 2-8, the

source contact pad (left) and drain contact pad (right) after performing the constant

current stressed HEMT.

The gate ideality factor of the unstressed HEMT extracted from the gate current-

voltage (I-V) characteristics was ~1.6, indicative of both recombination as well as

thermionic emission electron transport mechanisms being present (Figure 2-9). The

gate characteristics of the thermal and DC stressed HEMTs showed significant

degradation and gate current increased several order in both forward and reverse bias

conditions. Figure 2-10 (top) shows the low magnification cross-section view TEM

image of a Pt/Ti/Pt/Au mushroom gate after 165 oC, VDS 3 V, JDS 300 mA/mm for 36 hr.

43

A higher magnification TEM image is illustrated in Figure 2-10 (bottom); the white area

in the gate is Ti layer and the dark area represents the Pt layer. Apparently, the bottom

Pt of the Pt/Ti/Pt/Au mushroom gate diffused into the InAlAs gate contact layer. EDS

elemental analysis was used to analyze the Pt diffusion depth the gate region. As

shown in Figure 2-11, around 5-10 nm of Pt diffused into InAlAs layer.

44

Figure 2-1. Cross-section schematic of InAlAs/InGaAs MHEMT.

InGaAs

InAlAs

Source DrainGate

InGaAs

InAlAs Metamorphic buffer

GaAs Substrate

45

Figure 2-2. Top view of OM image for InAlAs/InGaAs MHEMT device with 2 fingers of

75um gate width (top) and TLM pattern (bottom).

75μm

46

0.0 0.3 0.6 0.9 1.2 1.50

100

200

300 VG= 0 V

Vstep

= -0.1V

I DS(m

A/m

m)

VDS

(V)

1 10 1000

10

20

30

40

50

Gai

n (d

B)

Vds= 1.5 V, VG= 0 V

fT= 94.1 GHz

fMax

= 124 GHz

Frequency (GHz)

Figure 2-3. The drain current density (IDS) and gate voltage (VGS) of virgin MHEMT as function of drain voltage (VDS) (top) and microwave characteristics of the InAlAs/InGaAs MHEMT device at room temperature (bottom).

47

Figure 2-4. The drain current (IDS) and gate voltage (VG) of the InAlAs/InGaAs MHEMT

after thermal stress at 250 oC for 36hrs as function of drain voltage (VDS) (top) and the drain current (IDS) and gate voltage (VG) of the InAlAs/InGaAs MHEMT after DC stress at 2.7 V for 36 hrs (bottom).

48

-5 0 5 10 15 200

20

40

60

80

100

120

LT

Res

ista

nce

(o

hm)

Gap (m)

before stress

after 250oC 12hr

after 250oC 24hr

after 250oC 36hr

after 250oC 48hr

-10 0 10 20 30 40 500

50

100

150

200

250

300

she

et r

esis

tanc

e (o

hm/s

q)

Stress time at 250oC(hours)

0 10 20 30 40 501E-7

1E-6

1E-5

1E-4

spec

ific

co

nta

ct r

esis

tan

ce(o

hm

-cm

2 )

Stress time at 250oC(hours)

Figure 2-5. Data from a TLM pattern stored at 250 C for 48 hours (top) resistance v.s. gap distance (middle) sheet resistance (bottom) specific contact resistivity.

49

-5 0 5 10 15 200

20

40

60

80

100

120

LT

Gap (m)R

esis

tanc

e (

ohm

)

before stress

after 165oC 12hr at 27mA



after 1650C 48hr at 27mA

-10 0 10 20 30 40 500

50

100

150

200

250

300

shee

t re

sist

ance

(oh

m/s

q)

Stress time at 165oC with 27 mA (hours)

0 10 20 30 40 501E-7

1E-6

1E-5

1E-4

spec

ific

cont

act

resi

stan

ce(o

hm

-cm

2 )

Stress time at 165oC with 27 mA (hours)

Figure 2-6. Data from a TLM pattern on a MHEMT stressed with a DC stress of 27 mA at 165 C (top) resistance v.s. gap distance (middle) sheet resistance (bottom) specific contact resistivity.

50

Figure 2-7. Low magnification cross-section view TEM image of InAlAs/InGaAs

MHEMT after stored at 250 C for 48 hours (top) Low magnification cross-section view TEM image of InAlAs/InGaAs MHEMT after DC stress for 36hrs (bottom).

51

Figure 2-8. The higher magnification TEM images of ohmic contact at the edge of

source and drain contact (top). The higher magnification TEM images of Ohmic contact at the edge of the contact (bottom).

52

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.61E-12

1E-10

1E-8

1E-6

1E-4

0.01

1

100

Unstress DC stress Thermal stress

VGS

(V)

-IG

S (

A)

1E-12

1E-10

1E-8

1E-6

1E-4

0.01

1

100IG

S (A)

Figure 2-9. The gate current of InAlAs/InGaAs MHEMT device for unstressed, thermal stress (250 oC, 36 hours) and DC stress (VDS 2.7 V, IDS 167 mA/mm, 36 hours).

53

Figure 2-10. Low magnification cross-section view TEM image (top) and the higher magnification TEM image (bottom) of the mushroom gate after 165 oC, VDS 3 V, JDS 300 mA/mm for 36 hr.

54

0 10 20 30 40 50 600

5

10

15

20

Inte

nsi

ty (

A.U

)

Distance (nm)

% (Titanium ) % (Arsenic ) % (Platinum) % (Indium )

Figure 2-11. EDS elemental analysis of the mushroom gate after 165 oC, VDS 3 V, JDS 300 mA/mm for 36 hr.

55

CHAPTER 3 C-ERBB-2 SENSING USING ALGAN/GAN HIGH ELECTRON MOBILITY

TRANSISTORS FOR BREAST CANCER DETECTION

3.1 Background

Currently, the overwhelming majority of patients are screened for breast cancer by

mammography. This procedure involves a high cost to the patient and is invasive

(radiation), which limits the frequency of screening. Work by Michaelson et al [98]

indicates a 96% survival rate if patients can be screened every three months. Thus,

mortality in breast cancer patients can be reduced by increasing the frequency of

screening. However, this is not presently feasible due to the lack of cheap and reliable

technologies that can noninvasively screen breast cancer.

There is recent evidence to suggest that salivary testing for biomarkers of breast

cancer may be used in conjunction with mammography [99-108]. Saliva-based

diagnoses for the protein c-erbB-2 have tremendous prognostic potential [106, 109].

Soluble fragments of the c-erbB-2 oncoprotein and the cancer antigen 15-3 were found

to be significantly higher in the saliva of women who had breast cancer than in those

patients with benign tumors [107]. Other studies have shown that epidermal growth

factor is a promising marker in saliva for breast cancer detection [109, 110]. These initial

studies indicate that the saliva test is both sensitive and reliable and can be potentially

useful in initial detection and follow-up screening for breast cancer. However, to fully

realize the potential of salivary biomarkers, technologies are needed to enable facile,

sensitive, and specific detection of breast cancer. AlGaN/GaN HEMTs have shown

promise for biosensing applications [111-114], since they include a high electron sheet

carrier concentration channel induced by piezoelectric polarization of the strained

AlGaN layer as discussed in section 1.2 [111-122]. There are positive counter charges

56

at the HEMT surface layer induced by the electrons located at the AlGaN/GaN interface.

Any slight changes in the ambient can affect the surface charge of the HEMT, thus

changing the electron concentration in the channel at AlGaN/GaN interface.

3.2 c-erbB-2 Antigen Detection Using AlGaN/GaN High Electron Mobility Transistors

The HEMT structures consisted of a 3 m thick undoped GaN buffer, 30 Å thick

Al0.3Ga0.7N spacer, 220 Å thick Si-doped Al0.3Ga0.7N cap layer. The epi-layers were

grown by MOCVD on thick GaN buffers produced on Si substrates. Mesa isolation was

performed with an Inductively Coupled Plasma (ICP) etching with Cl2/Ar based

discharges at –90 V DC self-bias, ICP power of 300 W at 2 MHz and a process

pressure of 5 mTorr. 10 × 50 µm2 Ohmic contacts separated with gaps of 5 µm

consisted of e-beam deposited Ti/Al/Pt/Au patterned by lift-off and annealed at 850 ºC,

45 sec under flowing N2. 400-nm-thick 4% Polymethyl Methacrylate (PMMA) was used

to encapsulate the source/drain regions, with only the gate region open to allow the

liquid solutions to cross the surface. The source-drain current-voltage characteristics

were measured at 25 °C using an Agilent 4156C parameter analyzer with the gate

region exposed.

A plan view photomicrograph of a completed device is shown in Figure 3-1 (top).

The Au surface was functionalized with a specific bi-functional molecule, thioglycolic

acid. We anchored a self-assembled monolayer of thioglycolic acid, HSCH2COOH, an

organic compound and containing both a thiol (mercaptan) and a carboxylic acid

functional group on the Au surface in the gate area through strong interaction between

gold and the thiol-group of the thioglycolic acid. The devices were first placed in the

ozone/UV chamber and then submerged in 1 mM aqueous solution of thioglycolic acid

57

at room temperature. This resulted in binding of the thioglycolic acid to the Au surface in

the gate area with the COOH groups available for further chemical linking of other

functional groups. XPS and electrical measurements confirming a high surface

coverage and Au-S bonding formation on the GaN surface have been previously

published [122]. The device was incubated in a PBS solution of 500 μg/ml c-erbB-2

monoclonal antibody for 18 hours before real time measurement of c-erbB-2 antigen.

Figure 3-1(bottom) shows a schematic device cross sectional view with thioglycolic acid

followed by c-erbB-2 antibody coating.

After incubation with a PBS buffered solution containing c-erbB-2 antibody at a

concentration of 1 μg/ml, the device surface was thoroughly rinsed off with deionized

water and dried by a nitrogen blower. The source and drain current from the HEMT

were measured before and after the sensor was exposed to 0.25 µg/ml of c-erbB-2

antigen at a constant drain bias voltage of 500 mV, as shown in Figure 3-2. Any slight

changes in the ambient of the HEMT affect the surface charges on the AlGaN/GaN.

These changes in the surface charge are transduced into a change in the concentration

of the 2DEG in the AlGaN/GaN HEMTs, leading to the slight decrease in the

conductance for the device after exposure to c-erbB-2 antigen.

Figure 3-3 shows real time c-erbB-2 antigen detection in PBS buffer solution using

the source and drain current change with constant bias of 500 mV. No current change

can be seen with the addition of buffer solution around 50 seconds, showing the

specificity and stability of the device. In clear contrast, the current change showed a

rapid response in less than 5 seconds when target 0.25 µg/ml c-erbB-2 antigen was

added to the surface. The abrupt current change due to the exposure of c-erbB-2

58

antigen in a buffer solution was stabilized after the c-erbB-2 antigen thoroughly diffused

into the buffer solution. Three different concentrations (from 0.25 µg/ml to 16.7 µg/ml) of

the exposed target c-erbB-2 antigen in a buffer solution were detected. The experiment

at each concentration was repeated five times to calculate the standard deviation of

source-drain current response. The limit of detection of this device was 0.25 µg/ml c-

erbB-2 antigen in PBS buffer solution. The source-drain current change was nonlinearly

proportional to c-erbB-2 antigen concentration, as shown in Figure 3-4. Between each

test, the device was rinsed with a wash buffer of 10 μM, pH 6.0 phosphate buffer

solution containing 10 μM KCl to strip the antibody from the antigen.

Clinically relevant concentrations of the c-erbB-2 antigen in the saliva and serum

of normal patients are 4-6 μg/ml and 60-90 μg/ml respectively. For breast cancer

patients, the c-erbB-2 antigen concentrations in the saliva and serum are 9-13 μg/ml

and 140-210 μg/ml, respectively [106]. Our detection limit suggests that HEMTs can be

easily used for detection of clinically relevant concentrations of biomarkers. Similar

methods can be used for detecting other important disease biomarkers and a compact

disease diagnosis array can be realized for multiplex disease analysis.

59

Figure 3-1. (Top) Plan view photomicrograph of a completed device with a 5-nm Au film

in the gate region. (Bottom) Schematic of AlGaN/GaN HEMT. The Au-coated gate area was functionalized with c-erbB-2 antibody/antigen on thioglycolic acid.

60

Figure 3-2. I-V characteristics of AlGaN/GaN HEMT sensor before and after exposure to

0.25 μg/ml c-erbB-2 antigen.

61

Figure 3-3. Drain current of an AlGaN/GaN HEMT over time for c-erbB-2 antigen from

0.25 μg/ml to 17 μg/ml.

62

Figure 3-4. Change of drain current versus different concentrations from 0.25 μg/ml to

17 μg/ml of c-erbB-2 antigen.

63

CHAPTER 4 LOW HG (II) ION CONCENTRATION ELECTRICAL DETECTION WITH ALGAN/GAN

HIGH ELECTRON MOBILITY TRANSISTORS

4.1 Background

Mercury is one of the extremely toxic metals and its compounds produce

irreversible neurological damage to human health [123]. Due to its toxic effects, the

standard limit of mercury in drinking water is 0.001 mg/g and while that in industrial

waste water is 0.01mg/g [124]. Hence, it is important to develop methods to detect

mercury ions in contaminated waste water to innocuous levels. Sensitive detection of

mercury (II) (Hg2+) ions is essential because its toxicity has long been recognized as a

chronic environmental problem [123-128]. Traditionally, there are several approaches

can be used to detect Hg2+ concentration including spectroscopic (AAS, AES, or ICP-

MS) or electrochemical (ISE, or polarography) methods. However, these methods have

shortcomings in practical use owing to high cost, and large size which impedes on-site

detection. There is a need for hand-held portable devices that can detect heavy metal

ions with high sensitivity [129-139].

AlGaN/GaN HEMTs are an attractive alternative as Hg ion sensor, since they

include a high electron sheet carrier concentration channel induced by piezoelectric

polarization of the strained AlGaN layer. A variety of gas, chemical and health-related

sensors based on HEMT technology have been demonstrated with proper surface

functionalization on the gate area of the HEMTs. Such HEMTs have been used to

detect carbon dioxide, hydrogen, chloride ion, prostate-specific antigen (PSA), kidney

injury molecule, DNA, and glucose [117, 121, 122, 140-143]. Because HEMTs operate

over a broad range of temperatures and form the basis of next-generation microwave

communication systems, so an integrated sensor/wireless chip is feasible. Bare Au-

64

gated and thioglycolic acid functionalized Au-gated HEMTs were used to detect mercury

(II) ions before [144, 145]. Fast detection of less than 5 seconds was achieved for

thioglycolic acid functionalized sensors. This is the shortest response time ever reported

for mercury detection. Thioglycolic acid functionalized Au-gated AlGaN/GaN HEMT

based sensors showed 2.5 times larger response than bare Au gated- based sensors.

The sensors were able to detect mercury(II) ion concentration as low as 10−7 M. The

sensors showed an excellent sensing selectivity of more than 100 for detecting mercury

ions over sodium or magnesium ions [142, 143]. However, in many applications, even

lower detection sensitivities are required [144, 145].

4.2 Hg (II) Metal Ion Detection Using AlGaN/GaN High Electron Mobility Transistors

The HEMT structures consisted of a 2 μm thick undoped GaN buffer and 250 Å

thick undoped Al0.28Ga0.72N cap layer. The epi-layers were grown by molecular beam

epitaxy system on 2 inch sapphire substrates at SVT associates. The sheet carrier

density and mobility of the HEMT sample were 1.1 × 1013 cm−2 and 1,600 cm2/(V s),

respectively. Mesa Isolation, Ohmic metal deposition, gate metal deposition and

functionalization with thioglycolic acid, and PMMA formation were discussed in section

3.2. A plain view photograph of a fabricated sensor array is shown in Figure 4-1 (top).

Hg2+ ion solutions ranged from 10−7 to 10−10 M were prepared by dissolving HgCl2 in DI

water with pH 2 controlled by adding HCl in DI water. The Ksp of Hg(OH)2 =3×10−26 and

it is necessary to control to lower pH value to achieve low concentration Hg2+ ion

solutions.

The source–drain current–voltage characteristics were measured at 25 °C using

an Agilent 4156C parameter analyzer with the thioglycolic acid functionalized Au-gated

65

region exposed to different concentrations of Hg2+ solutions. Ac measurements with

modulated 500 mV at 11 Hz were performed to prevent side electrochemical reactions

A schematic cross-section of the device with Hg2+ ions bound to thioglycolic acid

functionalized on the gold gate region and plan view photomicrograph of a completed

device is shown in Figure 4-1 (bottom). A self-assembled monolayer of thioglycolic acid

molecule was adsorbed onto the gold gate due to strong interaction between gold and

the thiol-group. The extra thioglycolic acid molecules were rinsed off with DI water. An

increase in the hydrophilicity of the treated surface by thioglycolic acid functionalization

was confirmed by contact angle measurement which showed a change in contact angle

from 58.4 to 16.2 after the surface treatment, as shown in Figure 4-2.

Figure 4-3 shows the time dependence of the drain current for a HEMT sensor

with 20 μm × 50 μm of gate sensing area exposed to different concentrations of Hg2+

ion solution. 15 μl of DI water was enough to cover the entire area of the gate area. No

drain current changes were observed another 5 μl of DI water added to the 15μl of DI

water (at 50 seconds). This measurement ruled out effect of change in mass of the

buffer solution on the signal. In order to expose the gate area of the sensor to target

Hg2+ ion concentrations of 1×10−9 and 1.5×10−8 M, 5 μl of 5×10−9 M Hg2+ ion

concentration was added into the 20 μl of DI water already on the gate area at 150

seconds and followed with additional 5 μl of 6×10−8 M Hg2+ ion solution at around 200

seconds. Although the sensor was not sensitive enough to detect the Hg2+ ion

concentration of 10−9 M (Figure 4- 3, at 150 seconds), the current showed a rapid

response (drain current dip) when 5 μl of 6×10−8 M Hg2+ ion solution added to reach a

target concentration of 1.5×10−8 M Hg2+ ions. This drain current dip was due to the gate

66

sensing area instantly exposed to higher Hg2+ ion concentration solution than the target

1.5×10−8 M Hg2+ ion concentration and the drain current took nearly 20 seconds to

stabilize. The 20 seconds of lag time was interpreted for the Hg2+ ions to uniformly

diffuse through the entire liquid drop covered on the gate sensing area. As illustrated in

Figure 4-3, larger drain current dip was observed for higher Hg2+ ion concentration

solution added to the solution already on the sensor. When sensing higher Hg2+ ion

concentration, in order to reach the target Hg2+ ion concentration, such as 3.37×10−8 M,

5 μl of Hg2+ ion solution with much higher concentration, 3×10−7 M, than the target one

was needed to be used. This was due to the gate sensing area covered by 20 μl of DI

water plus several injections 5 μl of lower ion concentration solutions. This drain current

dip actually showed our sensor extremely sensitive to the Hg2+ ion solution and the

sensor can detect the instantaneous Hg2+ ion concentration right above the gate

sensing area. It is possible to eliminate the drain current dip by employing the

microfluidic device to produce sharp transition of Hg2+ ion concentration. The

thiolglycolic acid functionalized HEMT sensor can also be repeatably used to detect

Hg2+ ion solution and the results was previously published [142].

The drain current reduction is due to the chelating with Hg2+ ions of the carboxylic

acid functional groups of the self-assembled monolayer of thioglycolic acid molecules

on the gold surface. The charges of trapped Hg2+ ion in the R–COO−(Hg2+)−OOC–R

chelates was hypothesized to change the polarity of the thioglycolic acid molecules.

This can induce negative counter charges on the gate surface of the HEMT sensor,

resulting in a reduction in drain current. Upon exposure the HEMT sensor to the higher

concentration of Hg2+ ion solution the drain current reduced further. The degree of drain

67

current dip was more serious and it took long time for the sensor to reach equilibrium.

The difference of drain current for the HEMT sensor exposed to different Hg2+ ion

concentration to the DI water is also illustrated in Figure 4-4. The drain current of each

Hg2+ ion concentration was repeated five times. As shown in Figure 4-3 and Figure 4-4,

the Hg2+ ion concentration detection limit of the thioglycolic acid functionalized

AlGaN/GaN HEMT sensor is 1.5×10−8 M. This is about an order of magnitude lower

than previously published [142, 143].

68

Figure 4-1. Plain view photomicrograph of a completed device with a 5 nm Au film in the gate region (top). A schematic of AlGaN/GaN HEMT. The Au-coated gate area was functionalized with thioglycolic acid (bottom).

69

Figure 4-2. Photographs of contact angle of water drop on the surface of bare Au (left)

and thioglycolic acid functionalized Au (right).

Figure 4-3. Time dependence of the drain current for a HEMT sensor exposed to

different concentrations of Hg2+ ion solution.

70

Figure 4-4. The difference of drain current for the HEMT sensor exposed to different Hg2+ ion concentration to the DI water.

.

71

CHAPTER 5 CU-PLATED THROUGH-WAFER VIAS FOR ALGAN/GAN HIGH ELECTRON

MOBILITY TRANSISTORS ON SI

5.1 Background

Through-wafer, backside vias are common elements in compound semiconductor

power device technologies for providing a low-inductance grounding and improving heat

transfer characteristics [146, 147]. The inductance generated by the long Au wire

between the bond pads and package can be minimized by full backside Ohmic contact

through vias. Furthermore, a thermally- and electrically- grounded drain does not

require an expensive and complicated air-bridge fabrication process and, thus, can

simplify the fabrication process and increase the device reliability as air-bridge

structures regularly deform at high temperature [148, 149]. These were developed for

GaAs microwave monolithic integrated circuits (MMICs) but more recently have been

used with AlGaN/GaN HEMTs for high frequency power amplifier technology. These

AlGaN/GaN heterostructures are usually grown on SiC due to a smaller lattice

mismatch and higher thermal conductivity compared to those grown on sapphire and

this leads to a significant reduction in device operating temperature at high power levels

[150-153]. Vias can be realized by either dry etching or laser drilling of the substrate

[154-156].

There is also significant interest in use of Si substrates because of lower costs and

capability for scaling to larger wafer diameters [157-164]. Based on the reliability

comparisons between GaN/Si and GaN/SiC, it appears that GaN/Si offers similar

reliability combined with the cost model of GaAs technology. Specifically, the reduced

cost of the materials (economical substrates), same tooling factors as silicon industry

(back side processing, die attach technology), and scalable processing for large

72

diameter wafers allow for economical manufacture of MMICs that require high levels of

uniformity across large substrates to allow proper circuit design of both active and

passive components.

For power amplifier applications, improving ability to extract heat and the allowed

thermal budget of operation is very critical. Thermal simulations employing a 3-D finite

element analysis show that the Si substrate is an effective heat sink relative to sapphire,

but device temperatures are still higher than on SiC substrates [163, 164]. This model is

based on the unsteady state energy balance equation using rectangular coordinators (x-

, y- and z-axes),

where T is the temperature, is time, is density, Cp is the heat capacity, k is

thermal conductivity (W/cm K) and PD is the source of power, i.e., the rate of internal

power generation. The latter can be determined from the product of bias voltage and

drain current through the HEMT, divided by the HEMT layer volume. The primary

modes of heat transfer are conduction through the layers and convection at the free

surfaces.

One approach for increasing heat transfer is to use a high thermal conductivity

metal such as Cu in vias specifically designed for minimizing device operating

temperatures at high power levels. This does not necessarily have to provide through

wafer electrical connection but rather are employed to enhance the effective thermal

conductivity of the substrate. This approach includes deep Reactive Ion Etching (DRIE)

followed by deposition of the insulation layer, barrier layer and seed layer. Vias were

then electroplated with Cu and the metal planarized by mechanical and chemical polish

Pz

T

y

T

x

Tk)

τ

T(ρC D2

2

2

2

2

2

P

73

processes. This approach is shown to produce via arrays with smooth sidewall and via

base, conformal coverage of the seed layers lining the via and void-free electroplating

(EP) of the vias.

5.2 Experiment

The HEMT on Si wafers were grown by MOCVD with conventional precursors in a

cold-wall, rotating disc reactor designed from flow dynamic simulations. The growth

process was nucleated with an AlN layer to avoid unwanted Ga-Si interactions. The

epitaxial stack then consisted of a proprietary AlGaN transition layer [165, 166], ~800

nm GaN buffer layer, and 16 nm unintentionally doped Al0.26Ga0.74N barrier layer. The

nominal growth temperature for the GaN buffer and AlGaN barrier layers was 1030 ºC.

HEMT fabrication has been published previously [157, chapter 2 and 3] but in brief

summary, began with Ti/Al/Ni/Au Ohmic metallization and RTA in flowing N2 at

approximately 825 ºC. Contact resistance, specific contact resistivity, and specific on-

resistance were 0.45 Ω mm, 510-6 Ω m2, and 2.2 Ω mm, respectively. Immediately

following Ohmic anneal, the wafers were passivated with SiNx in a PECVD chamber

maintained at a base plate temperature of 300 ºC. Inter-device isolation was

accomplished by using multiple energy N+ implantation to produce significant lattice

damage throughout the thickness of the GaN buffer layer. The ion implantation step

maintains a planar geometry in the fabricated device and reduces parasitic leakage

paths that may exist in passivated, mesa-isolated HFETs [167]. Schottky contacts were

formed by selectively removing the SiNx passivation layer and subsequent deposition of

0.7 μm Ni/Au gates. Large periphery devices were air bridged for source interconnection

using standard Au electroplating techniques to a thickness of ~3 μm.

74

Backside vias in the Si were formed by the conventional DRIE process that uses

pulsed etch/deposition cycles to prevent sidewall undercut. A schematic of the structure

is shown in Figure 5-1 and a SEM image of the cross section of an etched via is shown

in Figure 5-2. Seed layer formation consisted of deposition of low temperature SiO2 by

PECVD to avoid problems with wafer-carrier separation. The thickness was 0.7-1 μm.

This layer provides electrical isolation. The seed layers were deposited for the Cu

plating, namely, Ti (500-650 nm) and Cu (1.5-1.85 μm), both of which were deposited

by sputtering at room temperature. The sidewall layer thickness is about 33% of the top

layer thickness as shown in Figure 5-3, i.e., SiO2 was 0.84 μm on the field, but 0.29 μm

on the sidewall, the Ti was 0.52 μm on the field and 0.17 μm on the sidewall and the Cu

was 1.54 μm on the field and 0.50 μm on the sidewall. Cu was plated into the via and

mechanically polished to planarize. The schematic plating sequence is shown in Figure

5-4.


The wafers were plated with Cu for various times to understand both the time

required and how the via fill proceeded as a function of Cu thickness. It was found that

complete coverage was not obtained until at least 5 hours of plating. The average

current density during the plating was 6.7 A cm-2, leading to an average plating rate of

9.3-12.5 μm/h into the actual via, with the high end occurring in the initial stages. We

then performed a mechanical polish in standard Cu polishing solution to planarize the

Cu and cleaved the wafers to get cross-section views of the vias. As seen in the optical

micrographs in Figure 5-5, the vias with Cu were successfully filled and achieved good

planarization of the remaining Cu film. Approximately 15 % of the Cu plugs showed the

presence of voids that filled typically 15 %- 20 % of the volume of the via. The reasons

75

are currently investigated for the void formation, including the effect of Cu seed aging

time and plating rate. Another issue is whether there is an effect on the Cu seed layer of

the time prior to the CMP being carried out. In separate experiments on Cu seed layers,

various types of defects formed on the electroplated Cu as a function of time after the

deposition of the Cu seed layer, leading us to realize an evaluation of the films using

surface analytical techniques to obtain a fundamental understanding of barrier/seed and

electroplated copper film behavior as a function of time and treatment was necessary.

Other researchers successfully used laser removal of copper oxide from copper, while a

wet cleaning mixture of 1:200 NH4OH has also been found to be useful in removing

copper oxide films [168]. The known copper oxide thickness films were deposited and

then treated them to determine which treatment is most effective and the rate of surface

contamination removal. The results can be summarized as follows:

(i)The time delay between physical vapor deposition (PVD) copper seed layer

deposition and copper electroplating was observed to influence copper electroplating

film defects.

(ii) The total defect count increased with delay time between electroplating and

seed layer deposition.

The defects consist of both embedded defects and voids. Both types are shown in

Figure 5-6, where it is seen that the voids are the ones that increase with aging time.

Only the latter is a problem because the embedded defects are removed at the

chemical mechanical planarization (CMP) stage. Embedded defects are induced by EP

process, removed by CMP, and composed of copper. Voids leave metal interconnect

lines void of copper material, prevents current carrying capability of the metal

76

interconnect and are hypothesized to be due to a lack of wetting during electroplating.

All four applied treatments decreased post electroplating defects caused by seed aging.

These were Cu oxide reduction, single wafer clean, electrolyte rinse and reverse

plating. The time delay between PVD Cu seed layer deposition and Cu electroplating

increased the density of copper electroplating film defects. A decrease in wettability was

shown by an increase in contact angle from 40° to 63° over a fourteen days period. The

larger contact angle indicates hydrophobic behavior while the increase in hydrophobicity

suggests a decrease in wettability. This means that the copper is less likely to be

adequately wetted by the plating electrolyte as time delay increases. The copper seed

reflectance decreases with delay time and is similar to a wafer with 30 Å copper oxide

layer. Hydrogen reduction treatment results in a 42°contact angle as compared to other

treatments with a 48°-50° contact angle. The seed surface decreases in reflectivity over

a fourteen day aging period. All treatments tried increase the reflectivity of the Cu film,

suggesting they all remove the Cu oxide.

Figure 5-7 shows the front side of the HEMT-on-Si wafer after Cu plating and

planarization. The HEMT devices and contact pads are not visibly damaged by the via

fill/planarization process.

77

Figure 5-1. Schematic of via in AlGaN/GaN HEMT on Si wafer.

Figure 5-2. Cross-sectional SEM of dry etched via in AlGaN/GaN HEMT on Si wafer.

78

Figure 5-3. Cross-sectional SEMs of dielectric/metal stack on field (top) or sidewall

(center) and Cu/Ti/SiO2 stack at high magnification (bottom).

79

Figure 5-4. Schematic of plating sequence for Cu.

Figure 5-5. Cross-sectional SEM of Cu-plated via after mechanical polishing. The

diameter of the openings of the vias is 50 μm.

80

Figure 5-6. Defect type as a function of seed aging time.

Figure 5-7. Optical plan view micrograph of front-side of plated via wafer showing

HEMTs and contact pads.

81

CHAPTER 6 UV EXCIMER LASER DRILLED HIGH ASPECT RATIO SUBMICRON VIA HOLE

6.1 Background

Laser drilling has been used for creating holes with high aspect ratios in various

types of materials such as semiconductors, metals, plastic, and different types of

ceramics [56, 64, 156, 169, 170]. The ability to control the location, time, and duration of

the energy deposition process as well as the high machining rate make this laser drilling

process more appealing than other conventional techniques, such as lithography in

conjunction with wet chemical or plasma based dry etching [54, 171-174] .The majority

of the through-substrate via hole fabrication work has focused on diameters in the mm

and µm range. There are many potential applications such as microfluidic arrays,

microfilters and nanoporous arrays where there is a need for a technology to fabricate

submicron sized via holes.

In this chapter, the fabrication of via holes in both Si and glass substrates with 300

nm diameter entrance holes was reported by using an UV excimer laser. The effects of

both the diameter of the entrance hole and the number of repetitive laser pulses on the

formation of the submicron via holes were studied.

6.2 Experiment

4 inch silicon wafers and cover glass slips (22 mm × 22 mm, Fisher Premium

Cover Glass, Fisher Scientific, Inc.) were used in this work and the samples were drilled

with a JPSA™ IX-260 ArF excimer laser system. A convex/convex doubler objective

lens with a meniscus corrector was used to correct the spherical aberration and the

focal length of the doubler was 10 cm. A multiple hole metal mask with openings varied

from 5 μm to 200 μm was focused on the process sample surface with a

82

demagnification of 12.5. The laser pulse duration was fixed at 25 ns and the repetition

rate was varied from 1 Hz up to 100 Hz in this study. The output pulse energy was 200

mJ. The average power measured at the maximum repetition rate was 12 Watt. The

energy density (fluence) of the focused processing beam was in the range 2-4 J/cm2.

The sample stage was designed to accommodate 6 inch wafers. The resulting vias

were examined by both optical microscopy and SEM.


Figure 6-1 (top) shows a photo of a cleaved 400 m thick Si wafer with a laser

drilled via hole. The diameter of the entrance and exit holes are 90 and 45 m,

respectively, and the taper angle from top to bottom of the via holes was estimated

around 3.2 to 3.5 based on the diameter of the entrance and exit hole. Although the

laser pulse can be considered to transmit its energy in a single time unit, the laser

drilling process consists of three steps. Initially, the drilled material absorbs the laser

energy, and the top layer of the drilling material is melted. The molten materials

continue to absorb energy, which then leads to vaporization of the drilled material. The

resulted saturated vapor pressure (recoil pressure) generated by the sudden expansion

of the vaporization of the molten material applies a force on the molten material and

laterally pushes the molten material out of the via holes along the side of the holes [171].

After expelling the molten material, the surface of solid metal on the bottom of the via

holes is then exposed to the laser light again and starts to absorb laser energy. This

process is repeated many times, leading to drilling of the material exposed to the laser

beam. The time scale of the melting process and the process of pushing out the molten

material by the recoil pressure are much faster than the system’s laser pulse repetition

83

rate of 100 Hz used in this experiment. Consequently, a higher drilling rate was

achieved with higher laser pulse repetition rate to avoid the cooling down of the drilled

material. During the drilling process, the rates of expelling, cooling, and re-solidification

of the molten material on the sidewalls of the via holes and the deviation from the focal

plane for the deep drilling determined the tapered angle of via hole. Figure 6-1 (bottom)

shows the top SEM images of a 5 m diameter via hole drilled on the Si wafer with two

drilling times using a 62.5 m diameter circular mask. The drilling times were 30 and 45

seconds for the via hole shown on the bottom left and right in Figure 6-1, respectively.

The via hole after 30 seconds accumulated drilling time had a depth of around 5 micron.

Although the via hole with 45 seconds of accumulated drilling time had a similar depth

and shape as the via hole drilled for 30 seconds, there was an additional tiny via hole

with a diameter around 300 nm visible inside the original via hole. So far, there have

been no reports on such second via hole formation inside a larger diameter via during

the laser drilling process using a single drilling process. It was difficult to find out the

exact depth of the 300 nanometer size via hole even using focused ion beam (FIB)

sectioning since small variations during the FIB process would miss the tip of the via

hole. In order to examine the exact drilled depth and the shape of the 300 nanometer

size via hole, a same sized via hole was drilled on the edge of glass slices. Then the

depth of the drilled via holes were examined from the side of the glass slices with an

optical microscope.

In order to find out the formation mechanism of this tiny via hole inside the normal

larger via hole, circular via holes with two larger diameters and a different number of

laser pulses on the edge of the glass slice were drilled. The top row of pictures in Figure

84

6-2 show the cross-sectional view of the 160 m diameter via holes drilled on the glass

with 70, 80, and 100 pulses. The depth of the via holes increase with the number of the

laser pulses applied, and the bottom of via hole was flat in all cases. For the 80 m

diameter via hole, a similar via hole shape with a flat bottom as the larger via hole (160

m) was obtained for 70 laser pulses. When a higher number of laser pulses were

employed in the 80 m via holes, an additional smaller via hole formed in the center of

the big via hole. This additional tiny via hole formation to the laser light was attributed by

reflecting from the side walls of the via hole and refocusing at the center of the bottom

surface in the larger via hole. This additional dose of the reflected laser light enhanced

the drilling rate in the center of the larger hole. For the shallow and larger via hole, the

most of incoming laser light directly exposed to the bottom of the via hole. The portion of

the reflected laser light from the tapered via hole side walls was smaller and not

focused. Therefore, the bottom of the drilled via kept flat. As the entrance diameter of

the via hole became smaller and the depth of the via hole became larger, the portion of

the incoming laser light on the tapered side wall became larger as compared to the

laser light directly exposed on the bottom of the via hole. Once the reflected laser light

was focused on the bottom of via hole, the drilling rate significantly increased at the

bottom surface due to the additional dose of the reflected laser light. When the tiny hole

was formed, the laser light was trapped inside the tiny via hole, which then behaved as

a wave-guide for the laser light. However, the drilled material was difficult to vaporize in

this geometry and the possibility of expelling the molten material by the recoil pressure

in such a small via hole was even smaller. Thus the drilling rate suddenly decreased

significantly. Figure 6-3 shows a picture of via holes drilled on both sides of a glass

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slide. The top and bottom via hole had an entrance diameter of 80 m and 10 m,

respectively. The diameter at the tip of the bottom via can be adjusted by changing the

drilling time, since the drilling rate of the tiny hole is very slow and controllable. Since

the UV excimer laser system can drill arrays of via holes at the same time, this

technique can be readily used to make microfilters or nanopores.

86

Figure 6-1. (Top) Cross sectional micrographic image of a via hole drilled through a Si

substrate with a diameter of 90 and 45 m for the entrance and exit hole. (bottom) Top view SEM images of a 5 m diameter via hole drilled on a Si substrate with two different drilling times; 30 sec for the image on the left and 40 sec for the image on the right.

87

Figure 6-2. (Top) Side view of a series of 160 m diameter via holes drilled on the edge

of a glass with different numbers of repetitive laser pulses. (Bottom) Side view of a series of 80 m diameter via holes drilled on the edge of a glass substrate with different numbers of repetitive laser pulses.

88

Figure 6-3. A micrographic side view image of via holes drilled from both sides of the

glass substrate. The diameters of the entrance holes are 80 and 10 m for the top and bottom via hole, respectively.

5 m

89

CHAPTER 7 193 NM EXCIMER LASER DRILLING OF GLASS SLICES: DEPENDENCE OF

DRILLING RATE AND VIA HOLE SHAPE ON THE DIAMETER OF THE VIA HOLE

7.1 Background

Glass has attractive properties including high optical transparency in a wide

wavelength range, low fluorescence, chemical inertness, benign surface, dimensional

stability, as well as being electrically and thermally insulating. Glass also has a similar

CTE to that of Si so it can be used to reduce the thermomechanical stresses in flip-chip

assemblies. These characteristics make glass a suitable substrate for Si-based

electronics package applications. Glass is a brittle material and laser micromachining

can be an option once the glass is too thin. Pyrex glass has anodic bondability to Si and

can be used in the field of microsystems technology. There are several important glass

material received most attention including borosilicate glass, Pyrex glass, fused Quartz,

fused silica, and soda-lime glass [42, 49, 53-56, 60, 63, 175, 176].

Traditionally, there are several methods commonly used to pattern glasses

including ultra-sonic drilling and etching. The ultra-sonic drilling process has limitations

in forming small diameter via holes in the range of 300 μm. Wet etching and dry etching

requires photolithographic masking of the material surface prior to etching. Moreover,

wet etching is characterized by an isotropic etch behavior, tends to under-etch the mask

and does not provide a good feature control. Dry etching exhibits an excellent profile

control but has difficulty in producing small holes with high aspect ratio due to mask

erosion and low etch rate. Thus these methods are not appropriate to realize surface

structures, which required both small holes and high aspect ratio [53-56, 59, 177-180].

Laser drilling has several benefits over the ultra-sonic drilling and etching methods

for engineering of via holes on glass substrates as described in section 1.4. The lasers

90

with visible or near-visible wavelengths are not suitable for micromachining glass due to

their weak interaction with glass. The ablation rate using a shorter wavelengths laser is

much greater than that at longer wavelengths. The ablation threshold also increases

with an increase of wavelength due to lower material absorption [64, 181].

Effective laser ablation requires a sufficiently high absorption coefficient of the

material to be ablated. Transparent glasses are difficult to process with most lasers

because they poorly interact with visible or IR wavelengths. ArF excimer lasers are

preferred to be used for the laser ablation of glass because of sufficiently high

absorption at 193 nm. The short wavelength with higher photon energy than obtained in

the IR range also makes it easier to remove the target material. The photon energy at

193 nm is higher than any other solid state lasers and also can break the chemical

bonding of most drilled materials. The larger beam size of the excimer laser can also be

used to realize simultaneous drilling of multiple holes, which can further save process

time and cost [49, 58, 63].

In this chapter, the drilling rate and the shape of the via hole in glass substrates as

a function of the diameter of the drilled holes was reported with a 193 nm excimer laser

drilling system. The effects of the laser light reflected off the side wall of the drilled hole

on the shape and drilling rate of the drilled holes were investigated as well.

7.2 Experiment

Corning 0211 cover glass slips were used in this work. The laser system and the

related parameters are covered in section 6.2. The fluence of the processing beam in

focus was in the range of 3-6 J/cm2. A schematic diagram of the experimental set-up is

shown in Figure 7-1. Along the laser beam path, there were turning mirrors, an up

collimator and a dichroic beam splitter installed on kinematic mounts, a metal mask, and

91

a convex/convex doubler objective lens.

In the beam delivery system, a computer-controlled closed-circuit television

(CCTV) was installed to provide a viewing image of the drilled surface; the CCTV can

be automatically adjusted parfocally and coaxially to the focused laser beam spot. The

CCTV consisted of a multi-element objective lens with coarse/fine focus barrel, a

kinematically mounted mirror, a CCD high resolution camera with lens and corrective

optics, and a 15" monitor with electronic crosshair.

The sample stage was designed to accommodate 6 inch wafers. Two He–Ne

lasers were used to guide the high-accuracy air-bearing x-y linear-motor sample stage,

and the minimum stage movement was 0.1 μm/step. The stage had an accuracy of +/- 3

m over a full range of motion with a stage velocity of 6-8 inches per second; the stage

position can be programmed with a resolution of +/- 1 um and stage movement

repeatability of +/- 1 μm. In order to examine the depth and the shape of the via holes,

the via holes were drilled along the edge of glass slices. The images of the via holes

were taken from the side of the glass slices with an optical microscope.


Theoretically, the shape of the via holes should be independent of the diameter of

the via holes until the point where the diameter is comparable to the wavelength of the

laser source. The drilling rate and the shapes of the via holes were highly dependent on

the opening size of the via hole. Figure 7-2 shows the drilling rate of the glass as a

function of the via hole diameter. The drilling rate was very slow in the range of 0.1

µm/sec for the via holes with diameter less than 10 µm. The drilling rate quickly

increased to 19 µm/shot for the via holes with diameter of 40 µm, and then slightly

92

decreased to 17 µm/shot for larger via holes.

As shown in Figure 7-3, the shape of the via holes was highly dependent on the

diameter of the entrance hole. The via holes with the entrance diameter of 120 µm

showed a tapered side wall with a 7.5-9 degree angle after 10 second of laser drilling.

The via holes with diameter of 80 µm also showed a 7.5-9 degree tapered side wall

after 10 second of laser drilling; however, there was an additional smaller via hole

observed at the bottom of the big via hole. The via hole with the diameter of 40 µm of

the entrance hole showed a cone shape via hole with a similar 5-6 degree tapered

angle. The via hole with diameter of 5 µm of the entrance hole exhibited a very high

aspect ratio funnel shape.

In order to confirm the relationship between the drilling rate and the shape of the

via hole on the diameter of the entrance hole, the drilling rate and shape of the via hole

as the function of drilling time have been examined. As illustrated in Figure 7-4 (top), the

drilling rate for the via hole with a diameter of 120 µm stayed fairly constant. Figure 7-4

(bottom) shows the side view images of the via holes for different drilling times; the

tapered angle of the via holes remained almost the same. The only difference between

the via holes at different drilling stages was that the via holes became deeper as the

drilling time increased.

The drilling rates and the side view images of the via hole with a 80 µm diameter

at different drilling times are shown in Figure 7-5. The drilling rates were very similar in

the beginning, and then the drilling rates increased to a higher level after 3 second of

drilling. As illustrated in Figure 7-5 the tapered angle of the via holes stayed the same in

the beginning. A smaller via hole formed at the bottom of the original via hole after 3

93

second of drilling, as shown in Figure 7-5 (bottom). This rate increase and smaller via

hole formation at 3 second of drilling were due to the additional laser light that was

reflected from the tapered side wall of the via hole and refocused at the center of the via

hole bottom.

The laser light that reached the bottom of the via hole consisted of both the laser

light directly exposed upon the bottom of the via hole and the laser light reflected off the

tapered sidewall to the bottom of the via hole, as marked in white lines in Figure 7-5

(bottom). At the start of the drilling, the via hole was shallow and the portion of the

reflected laser light reaching the bottom of the via hole was minimal as compared to the

laser light directly exposed to the bottom of the via hole. Once the via hole became

deeper, the portion of the reflected laser light became a significant addition to the laser

light directly exposed upon the bottom of the via hole; the aggregate dose of the laser

light increased, which, subsequently, caused an increase in the drilling rate. When the

via hole became deeper and some of the reflected laser light became focused upon the

center of the via hole, an additional smaller via hole began to form at the bottom of the

big via hole.

Figure 7-6 shows the drilling rates and the side view images of the via hole with a

40 µm diameter at different drilling times. The drilling rate was higher than that of a 80

or 120 µm via hole. Unlike the 80 or 120 µm via hole, a conical via was formed

throughout the entire drilling time. This was due to the reflected laser light dominating

the drilling process. The laser light that was reflected from the sidewall of the via hole

and focused on the bottom of the via hole, produced both the highest drilling rate and

conical shape of the via hole.

94

Figure 7-7 shows the drilling rate and the cross sectional images of the same via

hole at different stages during the drilling. The drilling rates were significantly slower

than those of the larger via holes discussed previously, and the drilling rates also

decreased exponentially. Figure 7-7 (bottom) illustrates optical images of the via holes

drilled with a range of drilling times ranging from 1 to 10 minutes. The initial drilling

formed the top part, a funnel shape, of the via holes, which had a higher drilling rate.

However, the depth of the top part of the via holes barely changed throughout the rest

of drilling time. Once the top part of the via holes formed, the incoming laser light

reflected off from the sidewall the via hole, refocused on the center of the bottom of the

via hole and drilled a smaller high aspect-ratio via hole. The diameter of the bottom part

of the via holes was less than 0.5 µm and the aspect-ratio of this smaller via hole was

>200. The high aspect-ratio via hole hindered the ability of the drilled material to be

vaporized or expelled despite the high recoil pressure; this in turn, slowed down the

drilling rate dramatically.

To further confirm the dimension of the smaller via, a top view SEM image of 5 µm

via hole was taken, as shown in Figure 7-8. It clearly shows the additional tiny via hole

with a diameter around 300-400 nm visible inside the 5 µm via hole. Once this high

aspect-ratio via hole formed, it served as a wave guide to the laser light propagation

inside a circular waveguide. The diameter of this via hole was only 1.5 to 2 times the

wavelength of the laser light used. The TE01 mode, which is fundamental mode,

normally appears in a small waveguide, with a circular symmetry with maximum

intensity in the center should dominate in the light propagation within the via hole. As

the via hole was developing, it further squeezed the light down the waveguide and the

95

intensity profile became sharper as indicated by the tapered angle of the via hole.

96

Figure 7-1. Schematic of the excimer laser drilling system.

97

Figure 7-2. Drilling rate of the glass as a function of the via hole diameter

98

Figure 7-3. Side view images of drilled holes with the diameters of the entrance holes being 120, 80, 40, and 5 m respectively.

99

Figure 7-4. (Top) Time dependent drilling rate of the 120 m diameter via hole.

(Bottom) Side view images of the 120 m diameter via holes drilled for different times.

100

Figure 7-5. (Top) Time dependent drilling rate of the 80 m diameter via hole. (Bottom)

Side view images of the 80 m diameter via holes drilled for different times.

101



102



103

Figure 7-8. Cross-sectional SEM of glass slips with 5 µm entrance diameter drilled with

2 min drilling time.

104

CHAPTER 8 SUMMARY AND FUTURE WORK

In order to address the needs for eliminating the burn-in processes and improving

device reliability, the mechanisms of InAlAs/InGaAs MHEMT degradation were

investigated. The degradation mechanism of MHEMTs under DC stress at a drain

voltage of 2.7 V and current density of 167 mA/mm as well as thermal stress was

examined. After DC stress, the Pt in the Pt/Ti/Pt/Au mushroom gate stack diffused into

the InAlAs layer, which was confirmed with TEM images and EDS analysis. The DC

stress at a drain voltage of 2.7 V at 165 oC led to drain current degradation and gate

sinking. The gate leakage current after thermal storage (oven storage at 250 oC for 36

hrs) increased much larger than that from DC stress. The Rc deterioration, metal spike

formation and Ohmic metal diffusion were observed during the thermal storage.

In order to alleviate Ohmic contact degradation, non-annealed Ohmic process can

be investigated since non-annealed Ohmic-recess approach showed improved MHEMT

DC and RF performance [9]. Passivation material should also be studied since the

MHEMTs with BCB passivation showed lower degradation than ones with Si3N4

passivation [10].

Through a chemical surface modification, the c-erbB-2 antibody was immobilized

in the Au-gated region of an AlGaN/GaN HEMT structure for the detection of c-erbB-2

antigen and a limit of detection of ~ 0.25 µg/ml was achieved. This electronic detection

of biomolecules was a significant step towards a compact sensor chip, which can be

integrated with a commercial available hand-held wireless transmitter to realize a

portable, fast response and high sensitivity breast cancer detector.

105

In the current study, the c-erbB-2 antigen was dissolved in PBS and human trial

should be conducted. Besides the c-erbB-2 antigen, there are three other biomarkers

commercially available [107, 109, 110]. The integrated sensors employing these four

different antigens can be used to detect the breast cancer on the same time and

improve the detection accuracy.

TGA functionalized AlGaN/GaN HEMTs showed an excellent sensitivity to

detection Hg2+ ions. A detection limit of 1.5×10−8 M of the Hg2+ ion was achieved. It is

also important to study the assembly and package of sensor devices. For example, the

sensor chips can be inserted into microfluidic devices. By integrating the sensor chip

with a wireless transceiver, we can make a wireless and portable device for on-site

measurement.

A backside via hole process using Cu plugs was developed for AlGaN/GaN

HEMTs grown on Si substrates to improve the heat dissipation. The diameter of the via

hole was 50 µm and at least 80 µm thick of Cu was required to be electroplated to fill

the via. The excess electroplated Cu can be polished away for planarization. The device

DC and RF performance improvement should be verified after forming the Cu-plated

through-wafer vias for AlGaN/GaN HEMTs on Si.

Laser processing was shown as a promising alternative to replace conventional

micro-machining methods, such as wet etching and dry etching, for microfluidic device

and backside via forming application. The effect of the via hole diameter on the drilling

rates and the shapes of the drilled via holes was investigated by glass due to its well

coupled with irradiation of ArF excimer laser. An additional smaller via hole at the

bottom of the big via was observed due to the refocused aggregate light from reflected

106

tapered sidewall. The drilling via hole shape (e.g. submicron via hole with high aspect

ratio) and associated process parameters (e.g. irradiation time, mask size) were studied

and allowed us to drill desired pattern. Other patterns of masks, e.g. square and

rectangle, can be used to compare the dependence of drilling rate and via hole shape

with circle mask. By obtaining required process recipe, e.g. fluence, irradiation time,

Rep. Rate, for different patterns of masks, the complex desired pattern with vias,

channels and submicron holes can be formed with ArF based UV excimer laser.

107

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BIOGRAPHICAL SKETCH

Ke-Hung Chen grew up in Kaohsiung, Taiwan. He received his bachelor’s degree

at the National Taiwan University in 2001 and his master’s degree at National Tsing

Hua University in 2003. He joined MEGIC Corporation from July 2003 to January 2006.

He received the basic military training during October~December in 2003. He worked

for DuPont Taiwan from February 2006 to July 2007. He began his graduate studies at

the University of Florida in August 2007 and joined Professor Fan Ren’s semiconductor

material and device research group to pursue a doctorate degree. He graduated in the

May of 2010 after spending three years being educated in chemical engineering and

material science.

backside fabrication, sensor application and...

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