inductively coupled plasma etching of inp · 1 introduction 2 background and review 2.1 basic...
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INDUCTIVELY COUPLED PLASMA ETCHING OF InP
Hsin-Yi Chen
A thesis submitted in conformity with the requirements
for the degree of Master of Applied Science
Graduate Department of Metallurgy and Materials Science
University of Toronto
O Copyright by Hsin-Yi Chen 2000
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INDUCTIVE COUPLED PLASMA ETCHING OF InP
Hsin-Yi Chen
Master of Applied Science 2000
Department of Metallurgy and Materiais Science
University ot Toronto
ABSTRACT OF THESIS
lnductively coupled plasma (ICP) etching is a promising low-pressure high-densrty
process for pattern transfer required during microelectronic and opto-electronic
fabrication. In this work. an ICP system has been successfully constructed for the
purpose of etching InP. a highly attractive material for applications in optical
communication and high-speed integrated circuits. Different types of gas mixtures
including CHJ/H2, CHJHdAr, CHdH&, H$N2 and HdAr were used as plasma
precursors. The influence of gas composition, RF power, total flow rate and pressure on
etch rate, etch profile and surface rnorphology (roughening and stoichiometry) was
studied. CHdH2-based plasmas provided an anisotropic etching proceçs with hig h
selectivity. Surface roughening and phosphorous-depletion were yielded on etched
surfaces due to an imbalance in removal of In and P. ICP etching of InP using H$NÎ
was dernonstrated for the first time. Mirrorlike etched surfaces were obtained. A
cornmon occurrence of overcut was found on mesa sidewalls, believed to be due to Si02
masks erosion.
To my f amily .
iii
I would like to express my gratitude to my supervisor, Prof. Harty E. Ruda, and also
Dr. Alvaro Zapata for al1 the suggestions and guidance that they have provided to me
over the pass years. Their assistance and encouragement helped me overcorne
obstacles I encountered in my rasearch.
I would like to thank Kate Zhao for assistance with patteming, Dr. Slava Dudnik for
assistance with AFM, Fred Neub and Sal boccia for assistance with SEM, and Dr. Rana
Sodhi for measurement of XPS. I am grateful to Dr. Carlos Fernandes for his
suggestions, which have contributed a great deal to this thesis.
I would like to acknowledge Department of Engineering Physics in McMaster University.
Department of Electrical Engineering and Department of Chemistty in University of
Toronto for access to irleir facilities.
TABLE OF CONTENTS
Absttact
Acknowledgements
Table of Contents
List of Tables
List of Figures
List of Symbols and Abbreviations
1 INTRODUCTION
2 BACKGROUND AND REVIEW
2.1 Basic Concepts of Plasmas
2.1.1 DefinitionofPlasma
2.1 -2 Overview of Three Comrnon Plasma Sources
2.2 Plasma Etching
2.2.1 Introduction to Plasma Etching
2.2.2 Evaluation of Plasma Etching
2.2.3 Plasma Etching of Specific Materials
2.3 Review of MethaneIHydrogen-Based Plasma Etching of InP
First Demonstration
Optirnization of Etching
Investigation of Surface and Sidewall Damage
Spectrometric Analysis of Plasmas
Etching Mechanisms
Device Fabrication
ii
iv
v
viii
ix
xi
INDUCTIVELY COUPLED PLASMA SYSTEM &
CHARACTERlZATlON TECHNIQUES
3.1 lnductively Coupled Plasma System
3.1 .l Vacuum Chamber
3.1.2 Power Supply
3.1.3 Pressure Control
3.1.4 Gas Supply
3.1.5 Residual Gas Analyzer
3.2 Characterization Techniques
3.2.1 Profilometry
3.2.2 Scanning Electron Microscopy
3.2.3 Atornic Force Microscopy
3.2.4 X-ray Photoelectron Spectroscopy
4 INDUCTIVELY COUPLED PLASMA ETCHING OF InP
4.1 Experimental
4.1 -1 Sample Preparation : Cleaning and Patterning
4.1.2 lnductively Coupled Plasma Etching
4.1.3 Sample Characterization
4.2 Influence of Gas Composition on Etching
4.2.1 Etch Rate
4.2.2 Etch Profile
4.2.3 Surface Roughness and Elemental Analysis
4.3 Influence of Power on Etching
4.3.1 Etch Rate
4.3.2 Etch Profile
4.3.3 Surface Roughness and Elemental Analysis
4.4 Influence of Total Flow Rate on Etching
4.4.1 Etch Rate
4.4.2 Etch Profile
4.4.3 Surface Roug hness and Elernental Analys~s
4.5 Influence of Pressure on Etching
4.5.1 Etch Rate
4.5.2 Etch Profile
4.5.3 Surface Roughness and Elemental Analysis
4.6 Mass Spectrometric Identification of Etch Products
4.6.1 Experirnental
4.6.2 Reçu tts and Discussion
5 CONCLUSIONS
APPENDlX
A. Appearance Potential Mass Spectrometry
B. Reactions in Methane Plasmas
C. RF Electronics : lmpedance Matching
D. Repeatability and Error Bars
REFERENCES
vii
LIST OF TABLES
Table 1.1
Table 1.2
Table 2.1
Table 2.2
Table 3.1
Table 4.1
Table 4.2
Table 4.3
Table 4.4
Table B.l
Table B.2
Table 8.3
Table B.4
Lists of elemental and cornpound semiconductors.
Physical and electrical properties of InP.
Principle collisions among plasma particles.
Plasma chemistry used for etching of different rnaterials.
Various techniques for sample characterization.
Standard cleaning procedures used for InP.
Etch rate of InP etched with various CHs/H2/Ar ratio.
Etch rate of InP etched with various flow rates of CH4, H2 and N2.
Likely peaks in mass spectra for etch product identification.
Electron impact reaction in CH4 plasmas.
Neutral-neutral reactions in CH4 plasmas.
Ion-rnolecule reactions in CH4 plasmas.
Surface reactions between CH, plasmas and chamber walls.
Figure 2.1
Figure 2.2
Figure 2.3
Figure 2.4
Figure 2.5
Figure 2.6
Figure 3.1
Figure 3.2
Figure 3.3
Figure 3.4
Figure 3.5
Figure 4.1
Figure 4.2
Figure 4.3
Figure 4.4
Figure 4.5
Figure 4.6
Figure 4.7
Figure 4.8
Figure 4.9
figure 4.1 0
Figure 4.1 1
Figure 4.1 2
Schernatic representations CCP reactors.
Schernatic representations of ECR reactors.
Schematic representations of ICP reactors.
Etching mechanisms.
Common etch profiles.
Three-layer-model representation of etched surface.
3D mode1 of ICP system.
Block diagram of 1CP system.
Substrate holder assembling.
Profilometric surface scanning.
Determination of sidewall angle.
Mas king procedu re.
Etch rate as a function of CH4 concentration in CHsIH2.
Etch rate as a function of CH4 concentration in CHdH2/N2.
Etch rate as a function of H$N2 ratio in CHJ/H2/N2.
Etch rate as a function of N2 concentration in HdN2.
Sidewall angle as a function of CH, concentration in CH JH2.
Sidewall profile etched in CHs/H2.
Sidewall angle as a function of N2 concentration in HÊ/N2.
Reduction of Si02 during etching.
Illustration of reduction of SiO2 during etching.
Surface morphology etched in CH&.
Roughness and Plln ratio as a function of CH4 concentration in CH4/H2 57
ix
Figure 4.1 3
Figure 4.1 4
Figure 4.15
figure 4.1 6
Figure 4.1 7
Figure 4.18
Figure 4.1 9
Figure 4.20
Figure 4.21
Figure 4.22
Figure 4.23
Figure 4.24
Figure 4.25
Figure 4.26
Figure 4.27
Figure 4.28
Figure 4.29
Figure 4.30
Figure 4.31
Figure A.l
Figure C.1
Different rough topography.
Roughness and Plln ratio as a function of Np concentration in H2/N2.
AFM topography.
XPS spectra.
Etch rate as a function of RF power in CH&.
Etch rate as a function of RF power in H$N,.
Two-step profile etched with high RF power.
Sidewall profile etched in H2/N2.
ln-rich droplets on etched surface.
Roughness and Plln ratio as a function of power in H$N2.
Etch rate as a function of total flow rate in CHs/H2.
Sidewall profile etched in CHJ/H2.
Roughness as a function of total flow rate in CHs/H2.
Etch rate as a function of pressure in CHdH2.
Etch rate as a function of pressure in H2/N2.
Sidewall profile etched in CH JH2.
Roughness and Plln ratio as a function of pressure in CH JH2.
Roughness and PJln ratio as a function of pressure in H2/N2.
m/e 31 -34 mass spectra of CHJ/H2 plasma.
QMS CH; output signal as a function of electron beam energy.
Representation of circuit matching.
LIST OF ABBREVIATIONS AND SYMBOLS
area; constant
molecular fraction: capacitance
molecular diameter
etch depth of InP
etch depth of Si02
density
binding energy; electron beam energy
electron energy at spectrometer
Planck's constant
electron beam current
current
QMS out signal
inductance
mass
mass to charge ratio
molecular weig ht
plasma density; CH3 denstty
ideality factor; gmole
electron density
ion densrty
number of molecule pr unit volume or produced per minute
Avogadro's number
coordinates
coordinates
pressure
gas constant; resistance
average roug hness
reduction in width of Si02 mask
time
temperature
thickness of Si02 mask after etching
thickness of Si02 mask before etching
electron tempe rature
ion temperature
volume; voltage
reactance
passivation depth
impedance
degree of ionization
sloping angle of InP
sloping angle of SiOz
mean free path
frequency
ionization cross-section
Schottky barrier height
work function of spectrometer
angular frequency
Attemating Current
xii
AES
AFM
APD
APMS
BH
BHF
CCP
CR
C-v
DBR
DC
DFB
DI
ECR
ECU
FP
HEMT
ICP
1-v
LMMS
LTE
MESFET
MFC
OElC
OES
Auger Electron Spectrometry
Atomic Force Microscopy
Avalanche Photodiode
Appearance Potential Mass Spectrometty
Buried Heterostructure
auiiereti Hydroiiuoric Acid
Capacitively Coupled Plasma
Cathode Ray Display
Capacitance-Voltage
Distributed Bragg Refledor
Direct Cunent
Distributed Feedback
Deionized
Electron Cyclotron Resonance
Electronic Control Unit
Fabry-Perot
High Electron Mo bility Transistor
lnductively Coupled Plasma
Current-Voltage
Laser Microprobe Mass Spectrometry
Local Themiodynamic Equilibrium
Metal-Serniconductor Filed Effect Transistor
Mass Flow Controller
Opto-Electronic lntegrated Circuit
Optical Emission Spectrometry
xiii
PECVD
PL
PR
QMS
RF
RGA
RIE
rms
SEM
SlMS
TEM
Tl MS
UV
XPS
Plasma Enhanced Chernical Vapor Evaporation
Photoluminescence
Photoresisi
Quadrupole Mass Spectrometry
Radio Çrequency
Residual Gas Anaiyzer
Reactive Ion Etching
root-mean-square
Scanning Electron Microscopy
Secondary Ion Mass Spectrometry
Transmission Electron Microscopy
Threshold lonization Mass Spectrometry
Ultra Violet
%Ray P hotoelectron Spectrometry
Chapter 1
Introduction
Semiconductors are a group of materials having electrical conductivities intermediate
between conducton and insulators. generally from 1 to 10' (R-m)" [ i . t ] (Table 1 A).
Electrical and optical properties of these materials can be modulated by varying impurity
content, thermal excitation (temperature) and optical excitation. Silicon and germanium
found in Group IVA of the periodic table are the two elernental semiconductors. In
addition, compounds of Group HIA and Group VA, Group IIB and Group VIA, as well as
Group IVA also display semiconducting behavior. They are refened tu as compound
semiconductors. It is undeniable that semiconductors play an indispensable role in
modern electronics and optoelectronics~ Together with rnetals and dielectric materials,
they can be made into a variety of integrated circuits and devices.
Table 1.1 List of elemental and compound semiconductors [t .2).
Binary III-V Binary Il-VI Elemental IV Compounds
Compounds Cornpounds
Si Sic AI P ZnS
Ge SiGe Al As ZnSe
AlSb ZnTe
Gap CdS
GaAs CdSe
GaSb CdTe
InP
lnAs
InSb
Most developments in plasma processing over the past 20 years have been driven by
the semiconductor industry, where plasmas are involved in a number of processes such
as thin film deposition, etching, ion implantation and surface modification (oxidation,
hydrogenation, nitridation, passivation, cleaning , ashing ). In particular, plasma etching
has drawn much attention since it is the most assured technique to date to remove
materials selectively and yet directionally. One of the key steps during fabrication of
semiconductor integrated circuits and devices is pattern transfer, which usually requires
standard lithography followed by etching. Wet (chernical) etching has been used
extensively because it does not require sophisticated apparatus or skilled personnel. and
still yields high etch rates and excellent selectivity. However, when wet etching
progresses downward into the sample, it also proceeds laterally at a comparable rate.
This type of etching, which proceeds at similar rates in al1 directions. is called isotropic
etching. When the feature spacing (e.g. between trenches) is large compared to the
thickness of etched layer, undercut (i.e. etching inward to the mask edge) does not
matter signlicantly. However, this no longer holds true with sub-micrometer patterns,
where the reduction in feature sizes and spacing makes undercut strictly unacceptable.
Anisotropy is therefore regarded as important a criterion as selectivity and etch rate.
Wet etching, which is typically isotropic, is clearly inapplicable to the fabrication of high
density integrated circuits and srnall dimension devices.
To overcome this barrier. developing anisotropic etching is the only solution. Fortunately
plasmas have been used successfully to achieve that. Plasma etching possesses both
chemical and physical characteristics The chemical aspect enables selective etching as
in wet etching. and the physical aspect generates directional etch profiles. By correctly
selecting plasma chemistry and carefully tuning etching parameters. plasma etching can
be a very promising technique for the fine pattern transfer required during processing of
semiconductors for microelectronic and opto-electronic applications. For example, many
opto-electronic integrated circuits (OEICs) dernand the fabrication of lasers with other
components on the same chip. One of the technical problems encountered is the
developrnent of inside mirror facets for Fabry-Perot (FP) laser diodes. Plasma etching
has been demonstrated to generate mirror facets having tolerable surface roughness. so
that laser performance is not seriously compromised p.31. Another valuable use of
plasma etching conceming lasers is mesa formation. It is essential for distributed
feedback (DFB) and distributed Bragg reflector (DBR) lasers as well as for certain types
of FP laser diodes. Plasma etching also contributes to the production of other vital
components for optical communication. such as waveguides, switches, modulators and
photodetectors. For microelectronic application, plasma etching is often used to define
small gates and isolated mesas for transistors such as short-channel metal-
semiconductor field effect transistors (MESFETs) and high electron rnobility transistors
(HEMTs)- The permitteci width of etched grooves or mesas reduces gradually as plasma
etching technology become more mature [t.4-1.q. It is well believed that application of
plasma etching will extend prevalently to nanoscale structures (e.g., quantum dots,
photonic crystals, p-cavity) in the future [i.s].
The recent evolution of information technology has been phenomenal, as evidenced by
the inconceivable expansion of the Internet and wireless telecommunication. Indium
phosphide (InP) and its related ternary or quaternary cornpounds (InGaP, InAsP,
InGaAsP) are considered highly attractive materials for a variety of devices utilized in
optical communication systems and high-speed integrated circuits. The bandgap
energies of the InP group are suitable for light emitters and receivers in the long-
wavelength (0.9-1.6pm) reg ion. It also has high saturation velocity of electrons, which is
essential to the active channel in high-power and high-speed electronic devices.
Moreover. the InP group has high thermal conductivity and high threshold of optical
catastrophic degradation. which make it very encouraging for the fabrication of reliable
devices. Table 1.2 Iists some important physical and electrical properties of InP.
Table 1.2 Physical and electrical properties of InP (Ail values at 300 K).
Property Value Re f
formula weight 145.795 -- [1.71
lattice zincblende [1.21
lattice constant (A) 5.87 V.21
density (g/cm3) 4.79 -21
rnelting point ("C) 1070 [1-21
electrical resistivity (R cm) 0.008 D.1)
bandgap energy (eV) 1.35 V.81
bandgap wavelength (pm) 0.92 -81
electron mobiiity (cmZ/V-s) 4000 [1.21
hole mobility (cm2/V-s) 1 O0 V.21
transition direct [1-21
electron affinity (eV) 4.40 [t -7
work function (eV) 4.65 [1 JI photoelectric threshold (eV) 5.69 (1 -II
refractive index 3.5 [1 .a1
relative dielectn'c constant 12.4 Il -2)
thermal conductivity (Wlcm K) 0 -7 U-71
Despite the presence of a more advanced technology based on silicon and gallium
arsenide (GaAs), it is expected that InP-based devices will continue to develop and their
significance will surpass GaAs-based devices in soma unique areas such as optical
communication 11.61. The main problern prohibiting InP-based devices from prevalence in
the commercial market is their relatively high fabrication cost for lack of larger-diameter
inexpensive InP wafers. One approach to lower the cost is to minimize the wasted areas
on InP substrates by reducing feature size and spacing and increasing process yield. It
is without any doubt that efficient and satisfactory plasma etching must be established
for InP in order to do so.
In this thesis, an inductively coupled plasma (ICP) system was designed and
constructed for the purpose of etching InP. Methanelhydrogen-based plasmas (CHs/H2,
CHJIHdAr, CHs/H2/N2) as well as hydrogednitrogen (H2/N2) plasmas were used as
plasma precursors. To the author's knowledge. etching of InP with H$N2 chemistry had
not been studied before. The dependence of etching on variable experimental
parameters including composition and total flow rate of gas precursors, radio-frequency
(RF) power and etching pressure was investigated. Profilometry, scanning electron
moicroscopy (SEM), atomic force microscopy (AFM) and x-ray photoelectron
spectroscopy (XPS) were employed to characterize the etching results. Also, a residual
gas analyzer (RGA) was demonstrated to be able to identify phosphines as phosphorous
etch products.
Basic concepts of plasmas and plasma etching are described in Chapter 2. In addition,
literature review of CHJH2-based plasma etching of InP is discussed as well.
Description of the ICP system and characterization techniques is given in Chapter 3.
Chapter 4 presents experimental procedures. resulb and discussion regarding ICP
etching of InP using different chemistries and mass spectrometric identification of etch
products. Finaily, Chapter 5 concludes the thesis and outlines future works.
Chapter 2
Background and Review
2.1 Basic Concepts of Plasmas
2.1 .1 Definition of Plasmas
A plasma can be defined as a partially ionized quasi-neutral gas containing some or al1
of the following : electrons. positive ions, negative ions, atoms and molecules. The
plasma is oenerated and maintained via collisions producing a certain distribution of
ionic and neutral species. Some collisions which can occur among electrons. ions and
neutral species are listed in Table 2.1.
Table 2.1 Principle collisions arnong plasma particles
Electrons
lonization
Excitation
Penning lonization
Elastic Scattering
Dissociation
Dissociative lonization
Dissociative attachment
Recombination - --- -
-. Charge Exchange
Elastic Scattering
lonization
Excitation
Recornbination
Dissociation
Chernical Reaction
Taking into consideration the energy of particles constituting it. plasma is often refened
to as the fourth state of matter, separate from solid, liquid and gas states. After all. a
plasma is created by adding energy to a gas. The densities and energies of neutral and
charged particles are the fundamental parameters that characterize the plasma.
Temperature in the thermodynarnic equilibrium sense is a measure of the average
energy of free particles. Although therrnodynamic equilibrium is rarely achieved in the
entire plasma, especially in weakly ionized plasmas, temperature is still frequently used
when referring to the average energies of electrons (Te), ions (n, etc. It is custornary to
express temperature in the electron volt which is equivalent to a temperature of 11,600K.
Since plasmas are assumed to be macroscopically quasi-neutral and to have few
negative ions. the densities of electrons ne and of positive ions nt are usually equal.
n,=nt=n. where n is designated the plasma density.
Another important parameter related to the density is the degree of ionization a. It
specifies the fraction of the particles ionized in the gaseous phase. a is nearly unity for
fully ionized plasmas. and far less than unity for weakly ionized plasmas. There is a
great range of plasma densities and electron temperatures for both man-made
(laboratory) and natural (space) plasmas. The plasmas of interest in this study are
process plasmas, also known as low-pressure plasma, non-LTE (local thermodynamic
equilibrium) plasma, cold plasma and glow discharge. Process plasmas usually have
plasma densities between 10' and 1 oi3 cmJ, and average electron energies between 1
and 10 eV (IO' -los K). The degree of ionization is typically IO* -10" for process
plasmas [2.1-2.31.
Plasmas have both physical and chemical effects, which are very important in micro-
electronic processing. The physical effects of plasmas are due to bornbardment
between energetic charged particles and the target (samples. substrates). Electrons
and ions are accelerated throughout the plasma region. By the time they arrive at the
target, these particles may have sufficient energy to cause physical effects at the
surface, such as sputtering and stress generation. Inelastic collisions within the plasma
can produce reactive species efficiently from rather stzble precursor gases. The
chemical effects of plasmas thus result from the chemicai activity of reactive radicais and
ions. In most cases of plasma procesçing, both physical and chernical effects take
place. The relative importance of both effects depends on many processing parameters
including pressure, gas flow, applied power, reactor geornetry, substrate bias and
temperature. ionhdical species and so on.
2.1.2 Overview of Three Common Plasma Sources
Capacitively coupled plasma (CCP), inductively coupled plasma (ICP) and electron
cyclotron resonance plasma (ECR) are the three most popular sources for plasma
processing in electronics and opto-electronics. CCP source particularly has been used
widely in thin film deposition and reactive ion etching due to its simplicity [r.t8-1.20]. An
idealized planar-geometry configuration is shown in Figure 2.l(a). It consists of a
vacuum chamber containing two parallel planar electrodes driven by radio-frequency
(RF) power. Applied RF power generates electric fields which promote collisions
between particles of the precursor gas, resulting in ionization and plasma formation
between two electrodes. This type of reactor is sometimes called an RF diode. Barrel
reactors (coaxial geometry) shown in Figure 24b) are also popular. especially in
industry where mass production is favored. Although CCP source is fairly well
understood and easy to construct, it usually has relatively low plasma densities ranging
from 10' to 10" cme3. Further increase of the plasma density will elevate the ion energy
simultaneously.
Figure 2.1 Schemaüc representations of idealized (a) planar-geometry
CCP reactor, and (b) banel (coaxial-geometry) CCP reactor.
During plasma processing of semiconductor devices, ions with high energies can cause
severe damage to the sample surface. Therefore, a high density plasma source
combined with controllable low ion energy becomes crucial and favorable for
manufacturing nanoscale devices which have low tolerance for any kinds of defects.
Both ICP and ECR sources meet this requirernent. The general belief is that ECR
source has even higher plasma densities than ICP source, but ICP source is easier to
scale up and more economical in terms of power consumption and apparatus
cost [2.2-2-51.
In ECR source, microwave radiation excites a right hand circularly polarized wave
propagating along an axial magnetic field. supplied by either a current-driven coi1 or
permanent magnets, to a resonance zone where the wave is absorbed and the plasma
is produced. Several configurations have been developed for ECR source [m. 1.211.
Figure 2.2(a) illustrates the multipolar, tuned cavity design. In this type of ECR source. a
variable length microwave launching probe enters the resonant cavity at the side
introducing microwave power from the magnetron/waveguide assembly. The wave then
penetrates through a dielectric (usually quartz) window into a plasma confined area. and
propagates along a rnagnetic field caused by permanent magnets. The resonant cavity
is tuned by adjusting the sliding short and the launching probe. Figure 2.2(b) shows
another prototype of €CR source where microwave is injected directly through a
dielectric window into the plasma chamber and a magnetic field is generated by coils. In
this type of arrangement, coils are sometirnes replaced by permanent magnets.
. WAGNET COILS
Figure 2.2 Schematic representations of (a) multipolar, tune cavity ECR reactor,
and (b) high profile, electromagnetic ECR reactor.
Plasma in an ICP source is produced and sustained by application of RF power to either
a non-resonant or resonant coil. RF power is coupled to the plasma by ohmic
dissipation of induced RF currents caused by an oscillating magnetic field. Two coi1
configurations, cylindrical and planar, are shown in Figure 2.3(a) and (b) for non-
resonant inductively coupled plasmas. In cylindrical geometry, a coi1 is simply wound
around a dielectric tube, usually made of quartz or Pyrex glass. In planar geometry, an
"electric stovetopn shape coi1 is placed on the top of a processing chamber. Multipole
permanent rnagnets can be used around the processing chamber circumference to
enhance radical plasma uniformity in both situations.
I
>
: magnet +
external antenna -
RF Bias - RF Bias
Figure 2.3 Schematic representations of ICP reactors (a) non-resonant cylindrical geornetry,
(b) non-resonant planar geometry, (c) helical resonator, (d) helicon reactor.
Resonant inductively coupled plasmas are more complicated than non-resonant
versions. They also have two different configurations, the helical resonator and the
helicon reactor, shown in Figure 2.3(c) and (d). The helical resonator is composed of a
coaxial coi1 wrapped around a dielectric tube, and a grounded coaxial metal cylinder
surrounding the coi1 and the tube. The coi1 length is tuned to be an integer of quarter
wavelength of the operating frequency. The helicon reactor consists of an extemal
antenna (two loops diametrically opposed) placed outside the dielectric tube and an
electromagnet that generates a DC magnetic field in the processing chamber. The non-
resonant cylindrical inductively coupled plasma source has been chosen for this study.
Details of the experimental arrangement are given in Chapter 3.
2.2 Plasma Etching
2.2.1 introduction to Plasma Etching
In the early stage of semiconductor processing, wet etching was the only rnethod used
to remove material. Wet etching using corrosive acids or bases is usually associated
with isotropy and high etch rates, which imply a less controllable process. As
dimensions of rnicroelectronic and opto-electronic devices shrink down gradually each
year, plasma etching (also called dry etching) becomes preferable in transferring fine
patterns since it can be controlled much more speclically. Another advantage of plasma
etching over wet etching is that the vacuum chamber used for plasma etching can be
easily integrated with other processing chambers. Therefore the chance of
contamination due to contact with air is significantly reduced between processes. The
tem plasma etching signifies those etching processes participating in a low-pressure
plasma environment. Spunering (Figure 2.4(a)) and purely chernical etching (Figure
2.4(b)) are two extreme mechanisms in plasma etching. Sputtering is the ejection of
atoms from surfaces due to energetic ion bombardment. In a low-pressure plasma
environment, ions are accelerated and given energy and momentum by the electric
andior magnetic fields. For sputtering to take place, ions must have energies above a
material-dependent threshold which is detenined mainly by the surface binding energy
and the masses of the target. In addition. a fairly low-pressure environment is also
required so that the ejected atoms have small chance to collide with other molecules or
ions and re-adsorb onto the surface. In purely chemical etching, reactive species in the
plasma remove surface atorns by foming volatile products. The process is a sequence
of adsorption of reactive species ont0 the surface, formation of volatile products and
desorption of volatile products from the surface. Generally speaking, sputtering is highly
anisotropic but has low etch rates and poor selectivity. On the other hand, purely
chemical etching has high etch rates and good selectivity but is isotropic.
voialile Neutra1 producis -
Volatile , products Neutral ._ ;.
layer
Figure 2.4 Etching mechanisms (a) sputtering, (b) purely chemical etching,
(c) accelerated ion-assisted etching, (ci) sidewall-protected ion-assisted etching.
Most cases of plasma etching involve both sputtering and chernical etching [22-23.
252-71. In general, both mechanisms occur simultaneously and it is difficuit to distinguish
which is the more dominant. Figure 2.4(c) and (d) show two distinguishable classes of
plasma etching which combine physical and chemical effects. Both of them are capable
of anisotropy. In accelerated ion-assisted etching (Figure 2.4(c)), energetic ions disrupt
a relatively unreactive substrate surface, introducing damages (defects, dislocations,
dangling bonds) to the lattice. The net effect is to transfomi the sample near-surface
region into a more reactive state, allowing chemical reactions to occur more easily. The
vertical surfaces are left nearly unrnodified because of the directionality of the ion flux.
In sidewall-protected ion-assisted etching (Figure 2.4(d)), the sidewall is protected by a
barder layer to block chemical attack of radicals while horizontal surfaces are kept clear
of protective layers by ion bombardment. Two theories have been suggested to explain
the formation of this sidewall protective Iayer [MI . The first one is SC-called blocking, in
which plasma species form a passive barrier-layer over feature sidewalls, and the
second is recombinant mechanism which theorizes that the radicals absorb on sidewalls
and selectively deactivate the impinging etchant flux. The cornplex nature of plasma
etching provides flexibility to the engineer and optimizes the etching process. By
properly selecting the etchants and carefully tuning the etching parameters, anisotropic
plasma etching with high selectivity and controllable etch rates is achievable. Etching
parameters inciude gas flow rate, gas composition. total pressure, partial pressure of
different gases. radio-frequency or microwave power. substrate bias. substrate
temperature, etc,
The iiterature uses many ternis to describe plasma etching, and sometimes it can be
inconsistent and quite confusing. Since it is dinicult to recognize different etching
mechanisrns, there is a tendency now to describe etching processes by the apparatus
being used. For example, reactive ion etching (RIE), inductively coupled plasma (or
inductive plasma)) etching and electron cyclotron resonance etching represent etching in
CCP, ICP and ECR reactors respectively.
2.2.2 Evaluation of Plasma Etching
Plasma etching is evaluated by criteria such as etch rate, selectivity, anisotropy and
surface morphology. The etch rate must be sufficiently high to maintain high processing,
but it also cannot be too high, otherwïse it raises the difficulty to control etching precisely
for fine structures. By definition, anything that is not isotropie (non-directional) is
anisotropic. However. in the case of plasma etching, anisotropy generally suggests a
very directional etching which has a large verticailhorizontal etch ratio. Highly
anisotropic etching results in a vertical sidewall profile. For rnost applications a vertical
profile is often preferred, though sometimes tilted sidewall with a specific angle is
required in certain situations. Etching selectivity refers to the relative etch rates of two
different materials. A good choice of etchants should have a large etch rate ratio of
target material to masks, photoresists and underlying materials. High selectivity can
reduce the thickness of masking layers necessary and ease the control of etching
process. Plasma etching sornetimes degrades the surface morphology of samples by
inducing chemical damage (e.g . contamination, polymerization, amorphization) and
physical damage (e.g. atomic displacement in the lanice). Both chemical and physical
damages result in rough surfaces. Uniformity, another criterion, is added to whole wafer
procesçing. Uniformrty refers to the eveness of etching across the wafer. In other
words, it means etching endpoint is reached simultaneously across the entire area. This
is an important factor in reducing rnaterial wastage.
2.2.3 Plasma Etching of Specific Materials
Most of the materials used in electronic and opto-electronic devices can be etched by
plasmas. as listed in Table 2.2 (2.2-23. 2.7. Silicon is commonly etched in halogen-
containing plasmas where atomic fluorine (F) and chlorine (CI) convert silicon into
volatile SiF. and SiCl,. Silicon dioxide (SiO2) and silicon nitride (Si3N4) are usually
etched in fluorocarbons, such as CF4, C2F6 and C3F8. Metals used in semiconductor
devices can be divided into three groups. Aluminum (Al) and chromium (Cr) are etched
by chlorine-containing gas mixtures (e.g. CI,, BCI,, CCI,, SiCl,), and molybdenum (Mo),
tungsten (W). titanium (Ti). niobium (Nb) and tantalum (Ta) are etched by fluorine-
containing gas mixtures (e.g. NF3, CF4, SF6). As to etching gold (Au),
chlorofluorocarbons (e.g. CC12F2, CCIF3) are used. Photoresist is simply an extended
hydrocarbon network, so it can be easily stripped off by oxygen plasma. The reaction
products include carbon monoxide, carbon dioxide and water vapor. Ill-V and II-VI
compound semiconducton are etched either by chlorine-containing plasmas (e.g. Cl2,
BC13, CC[,, SiCl,, CC13F. CC12F2) or by alkanehydrogen-based plasmas (CH,, C2ii6,
C3HB). Argon (Ar), oxygen (O2) and nitrogen (N2) are quite often added to
alkanelhydrog en- based and halogen-containing gases to improve etching processes.
Table 2.2 Plasma chemistry used for etching of different materials
Material Plasmas
Si CFA, CF2CI2, CF3CI, SFs, C2CIF5,
SiFA, NF3, CC&, C2F6
Si02 CF,, C2F6, C$B, CHF3
Si3Na CFA, C2F5, C3F8, CHF3
photoresist 0 2
AI BCI3, CCIJ, SiCt,, CI2
Cr CI2, CCIJ
Mo, Nb. Ta, Ti, W CF,, SF6, NF3
AU C2C12Fj, Cl2, CCIF3
Ill-V or Il-VI BCI3, CC14, SiClAl CC13F, CC12F2,
Cl2, CHdH2, CzHdH2, C3H&
Like other Ill-V and Il41 compound semiconductors, InP is usually etched in either
chlorine-based or alkanelhydrogen-based plasmas. Most of the early studies used
precursors containing chlorine (CI*), chlorides @Cl3, CC14, SiCl4) andfor
chlorofluorocarbons (CC13F. CCI2F2) tn.8-2.191. Due to the low volatility of indium
chlorides, it is necessary to increase substrate temperature (2 200°C) to prevent severe
surface roughening. In addition, halogens are well known for their corrosive, toxic
nature, which makes them difFicult to handle. Although chlorofluorocarbons do not have
this problem. usage of them are strictly controlled by the environmental legislation
because they can cause depletion of the ozone layer. Alkanelhydrogen-based
chemistry (CH4. C2Hs, C3H8 with Ha) represents an alternative for plasma etching of InP.
They are found to give smoother surfaces at room temperature and have higher
selectivrty with respect to standard masking materials than chlorine-based plasmas. A
disadvantage of using alkanelhydrogen-based plasmas is polymer deposition on the
walls of the vacuum chamber, and periodic cleaning is therefore required. Review of
etching of InP in CHdH2-based plasmas is discussed next.
2.3 Etching of InP in CHdH2-Based Plasmas
2.3.1 First Demonstration
Methane/hydrogen (CHdH,) mixtures were first employed by Niggergrugge et al. as an
alternative etchant for plasma etching of InP in a conventional planar diode system C.ZO].
The outcomi was quite satisfying in tems of itch rate, so!octivity, sidom!! profite and
surface morphology. An outstanding feature found during their study based on the fact
that resistant films were deposited on masking rnaterials, silicon dioxide (SQ) and
photoresist, while InP was etched. As a result, an infintely high selectivity over masking
materials can be achieved in CHJH2 etching whereas CI-containing plasmas etch
photoresist, SiOz and SilNl at signlicant rates. It was later confirmed by other
researchers that CHdH2-based plasmas deposit amorphous C-H polymer films on al1
materials other than I l l 4 or Il-VI semiconductors [2+211. However, when CH4
concentration in the precursor exceeded 65 volO/o, etching of InP was inhibited because
polymer began to grow on InP as well. (AH concentrations mentioned in this thesis are
volume concentrations.)
Niggergrugge's novel process for etching InP using non-toxic, non-corrosive CH*/H2
plasmas soon caught people's attention. Experiments were performed in CCP p.3-1.4.
2.21-2.431, ICP [2.34-2.35. 2-44], ECR [i .S. 2-28. 2-30. 2-37. 2.45-2-56] and other reactors p.n-2.581,
covering areas such as optimization of etching, investigation of surface and sidewail
plasma-induced damage, spectrometric analysis of CHJH2-based plasmas. proposal for
etching mechanisms and fabrication of electronic and opto-electronic devices.
2.3.2 Optimization of Etching
The process of etching has to be calibrated and optimized for each system because
plasma (or etching) parameten affect plasma properties very differently from one reactor
to another. Parameters which are varied nomially include composition of precursors, RF
or microwave power, total gas flow rate, etching pressure, substrate bias and substrate
temperature. On top of CH4/H2 [I .3-1.5.220-2.21.2.23-2.27.2.31-2.33.2.36.2.~-2.a. 2.50-2.51.2.581,
Ar p.28. 2.30. 2-35, 2-37, 2.44-2.50. 2-52, 2.54-2.56], o2 j2.341, Na (2.44. 2.521 and chlorides (PC13, Ci2)
p.28, zs6-2.5q have been added to the mixtures. A shared observation in al1 kinds of
plasma chemistry is polymer deposlion on InP at high CH, concentrations. Optimal CH4
concentration has been found to lie in the range of 5-25%. The power. the total flow
rate and the bias used by researchers spread over a wide range. This is due to a
significant difference in the geornetry of reactors (e.g. size of vacuum chamber and
electrode). RIE, ICP etching and ECR etching typically ran at pressures behveen
10-1 20 mTorr, 1-1 5 mTorr and 1-5 mTorr respectively. InP substrates were usuaIly
cooled by water or helium to maintain room-temperature unless temperature
dependence was studied deliberately [2.26.2.54,2.56].
The etch rate was often determined by stylus profilometry or weight loss measurement.
Scanning electron microscopy (SEM) was the most frequently used technique for
revealing etch profiles and surface roughness. Other techniques such as transmission
electron microscopy (TEM) p.3a1 and atomic force microscopy (AFM) p.35. 2.37. 2-44, 2.51.
2.551 were also applied. Barreled sidewall, undercut and overcut (Le. sidewall profiles
sloping outward from the mask edge) are phenomena that were seen in SEM images.
They are illustrated schematically in Figure 2.5.
Figure 2.5 Common etch profiles : (a) barreled sidewall,
(b) overcut (sloping sidewall) and (c) undercut.
2.3.3 Investigation of Surface and Sidewafl Damage
Surface chemistry of InP etched in CHs/H2-based plasmas had been investigated by
techniques such as Auger electron spectroscopy (AES) [m. ~34-~35.249.254-25q, x-ray
photoelectron spectroscopy (XPS) p.25, 2-38, 241. 2461, secondary ion mass spectrometry
(SIMS) p.23. 2.31, 2.341, and laser microprobe mass spectrometry (LMMS) p.231.
Phosphorous depletion, carbon contamination and arnorphization were discovered
during examination. The major modification in the surface composition is phosphorous
depletion, due to an irnbalance between the etch rates of indium and phosphorous.
There was evidence shown that carbon tends to attach to indium in the surface
layer p.411. Those carbon-indium species are most likely the precursors of volatile
organo-indium etch products. Plasma-surface interaction changed the top layer of InP
from single-ctystaIline to amorphous. XPS analysis showed that a lower degree of
surface amorphization is correlated to the irnprovement of the stoichiometry.
Feurprier et al. p.sa. 2.411 proposed a three-layer etching model as represented in
Figure 2.6. This mode1 took into account modifications and damages stated above. The
superficial layer is amorphous. P depleted, and consists of In-ln-C (or In-ln-P), (In)P-H.
In-C and C-C species. The second damaged layer is considered as amorphous but
stoichiometric in composition. The third layer is the InP bulk substrate. Polyrner films
deposited on masks were analyzed elementally as well. As expected, they were highly
cross-linked hydrocarbon structures. A slight amount of indium and phosphorous were
found to be incorporated into the polymer.
InP BuIk Substrate
Figure 2.6 Representation of the etched surface using a three-iayer model.
Chernical and physical defects introduced to the InP surface dunng plasma etching lead
to degradation of electrical and optical properties. It is important to know the degree and
the depth of damage since they affect device performance substantially. Electrical
damage was evaluated through capacitance-voltage (C-V) and current-voltage (1-V)
measurements [223.~~5-~26.228.2.40,2.~2,2.45.246.246.2.~2.2.4q.
Many studies had shown that atomic hydrogen (H) modifies carrier concentrations in
Ill-V semiconductors. In the case of InP etched in CHS/Hrbased plasmas, reactive H is
capable of diffusing into p-lnP and passivating dopants, resulting in reduction of acceptor
concentrations l2.23, 2-25. 226. 246. 2.581. lnterestingly passivation does not occur in n-lnP
p.231. Although carrier concentration is related to resistivity. it is usually derived from C-V
measurernent. Carrier profiling (carrier concentration vs. depth) can be obtained from a
C-V curve [ i . ~ ] . Normally passivation depth (Xs) of p-lnP is about 0.2-0.4 ym, but it
appears to increase with power (RF or microwave), CH4 concentration, etch duration and
substrate ternperature. Annealing at 350-400°C for 1 minute has been proved to
recover the carrier concentration to nearly the bulk value p.23. 2.251. Passivation of
acceptors can be beneficial for electrical isolation applications where high resistivity is
preferred. On the other hand. it can also be detrimental to performance of active
devices. Optimizing etching parameters coupled with heat treatment makes it possible
to generate appropriate carrier concentration for each specific application. Near-surface
electrical damage alço reflects on the Schottky bamer height ($*) and the ideality factor
(n). 60th can be obtained from forward 1-V measurement p.611. Comparing és and n
between etched and controlled (unetched) InP reveals the degree of electrical damage.
Photoluminescence (PL) is a non-destructive technique in characteriring optical damage
induced by plasma etching. Lattice defects tend to behave as non-radiative
recombination centers which decrease the optical emission from the etched InP.
Therefore. damage can be evaluated by reduction of the total band to band PL intensity.
The low-temperature measurement yields more accurate spectroscopic information than
the room-temperature measurement by avoiding thermally-activated non-radiative
recombination and thermal line broadening. Results show that the depth of optical
damage is within several nanometers, which is shallower than that of electrical damage
[228.233-2.34, 245-2.46. 2.48.252].
2.3.4 Spectrometric Analysis of Plasmas
Analyzing gas phase chemistry by quadrupole mass spectrometer (QMS) is an excellent
way to investigate the chemical reactions between InP and CH.& plasmas. Methyl
radicals (CH3) are presumed responsible for both etching of In and polymer deposition.
In order to confim this assumption. CH3 flux density waç measured by Appearance
Potential Mass Spectrornetry (APMS) p.37-238. 2.40. 2.59-1.60]. The results were found to
be in good agreement with the etch rate measurement (Le. the more CH3 available in the
plasma. the higher the etch rate of InP). A detailed description of APMS is given in
Appendix A. Another function of QMS is identification of volatile etch products p24.2.41.
2.501. Phosphines (PH,) and organo-indium (In(CH3).) were recognized as primary
contributors that carried P and In away from the InP surface. CH3PH2 is suspected to
serve as a minor etch product of P. Optical emission spectroscopy (OES) was used as
an alternative for monitoring plasma species (2.38.2.51 1.
2.3.5 Etching Mechanisms
In the case of CHJH2 plasma etching of InP. the etching mechanisms should be
approached from both chemical and physical aspects. First. CH3 and H radicals are
considered responsible for chemical etching of In and P respectively. In(CH3)3 and PH3
are regarded as the primary volatile etch products, which has been confimed by QMS
and OES analysis. In the plasma region, CH3 and H can be readily generated from CH4
because the dissociation potential is only 4.6 eV for CH, + e -, CH, + H + e. The other
component in the precursor, Ha, not only serves as an alternative source of H for etching
P but also as a dilutant for CHs plasmas. It is inferred that the presence of Hp in CH4
plasmas can reduce the amount of gas phase polymerization of heavy hydrocarbon
compounds.
The physical aspect of the process is the consequence of ion bombardment. Ions
accelerated by the electromagnetic field stimulate etching in the vertical direction via the
cleaning and damaging mechanisms p.241. In the cleaning mechanism. energetic ions
enhance removal of In(CH& and PH3. ln the damaging mechanism, ions generate a
surface that is more reactive with CH3 and H by introducing damages. Due to the
directionality of accelerated ions. vertical surfaces are impacted by ion bom bardment
only as a result of gas phase scattering or particle reflection from horizontal surfaces.
High ratios of vertical to lateral etch rate obtained from experirnents suggest that the
physical mechanism caused by ion bombardment plays a very important role during the
etching process. Furthemore. the flux density of C2H5+. which is the major ion species
in CH4 plasmas. was found to increase when increasing the percentage of H, in the
precursor by Feurpier et a/. [2.39]. Given that CH4 plasmas cannot etch InP without
including a certain amount of H2 in the precursor, it c m be concluded that some level of
sputtering action is required to counteract hydrocarbon polymerization on InP surfaces.
The overall mechanism is best described as accelerated ion-assisted chemical etching.
Ions may gather an huge amount of energy under some circurnstances. e.g. appiying
excessive powers. When ions possess energies high enough to break bonds at the InP
surface. they becorne capable of physically etching In(CH3). and PH. (x=0.1.2.3) via
sputtering further than simply assisting chemical etching.
The rnechanisrns discussed above can be summarized into the following reactions :
Gas phase reactions
Formation of chemical etchants (radicals)
CH4+e-- iCH3+H +e'
H 2 + e A + H + H + e '
Formation of physical assisting agents or etchants (ions)
CH4 + e' + CH: + 2e-
CH4 + e' + CH3' + H + 2e'
CH4 + CH3' + C2HJ + H2
Surface reactions
Chernical removal
CH3 + In + In(CH3)
CH3 + In(CH3) -, III(CH~)~
CH3 + In(CH& -+ I~I(CH~)~
H + P + P H
H + PH + PH2
H + PH2 4 PH3 T
Physical removal
ions + In(CH3), - In(CH3], (x=0,1.2.3)
ions + PH, -+ PH, (x=0.1.2,3)
(t : volatile products)
Detailed gas phase reactions in CH4 plasmas are given in Appendix B.
In general. etching of InP in CH JHdAr or CH JH2/N2 plasmas shares very similar
mechanisrns with etching in CHJIH2 plasmas. Ar' is known for its efficient sputtering
capability. By adding Ar. the physical aspect of the etching process. including ion-
assisting mechanism and physical etching via sputtering, is drastically enhanced.
Besides, addition of Ar to CHJH2 precursors enables easy ignition and high stability of
plasmas at low chamber pressures. N2 plasmas also have sputtering function but are
less efficient compared to Ar plasmas. N; and N+ are dominant ion species in Nt
plasmas. Adding Na to CH& precursors has çignificant chernical effects on the
distribution of hydrocarbon species due to the existence of highly reactive N
radicals p.621. One of thern is suppression of polymer deposition. For polymer
deposition to happen. free radicals have to be present at the surface for the nucleation of
a polymer seed, and then unsaturated hydrocarbons (or higher order radicals) have to
be supplied for the growth of a film p.631. N can eliminate hydrocarbon radical precursors
by tuming them into nitriles. Another effect of N is redudion of H concentration by
forming arnmonia (NH3).
Ar piasmas
Ar + e + Ar* + 2e-
N2 plasmas
N2 + e + N2+ + 2e'
N2+N2++N2+N'+N
Ci-i4'N2 piasmas
N + CH +HCN
4N + 3CH2 + 3CHN + NH3
2N + C2H2 4 2HCN
4N + 3C2H4 I, 3C2H3N + NH3
H2/NP plasmas
N + H i N H
NH+H+NH2
NH2 + H NH3
2.3.6 Device Fabrication
The ultimate objective of researching etching of InP in CHs/Hrbased plasmas is to
facilitate the fabrication of InP-based fine structures and devices for applications in
optical communication systems or electronic integrated circuits. Roberts et al. [t -261
demonstrated the construction of single mode InP/GalnAs multiple quantum well rib
waveguides by RIE in a CH& plasma. The result showed propagation loss as low as
1.4 dBIcm. It proved that the dry etching process did not necessarily induce serious
optical damage. One of the most important applications of plasma etching is mesa
formation during the fabrication of buried heterostructure (BH) lasers. Kjebon et al.
reported the first InP-InGaAsP Fabry-Perot BH quantum well laser operating at 1.55 pm
utilizing RIE in CH JHB [i.3]. ECR source is considered to be ideally suitable for devices
which require high-aspect ratio etch profiles and have low tolerance of damage. Pearton
et al. employed a high-density, low-energy ECR source to increase anisotropy and
decrease damage [is. 2-30]. They applied this ECR etching in CHdHdAr to the fabrication
of a varieiy of structures and devices including etched-mesa BH InP-InGaAsP lasers,
through-lnP wafer via holes for electrical connection. whispenng-gallery mode InP-
InGaAsP microdisk lasers and InP-based heterojunction bipolar transistors. The first
InPflnGaAs avalanche photodiode (APD) using CHJ/H2 RIE was demonstrated by
Park et al. [2.31]. A very low dark current, less than 1 nA at 90°io of breakdown voltage,
was attained, Plasma technology has been proved to be capable of etching nanoscale
structures. Gratings with linewidths down to 35 nm and periods of 70 nm have been
achieved in InP and InGaAdlnP heterostructures by Adesida et al. [i.q.
Chapter 3
lnductively Coupled Plasma System
& Characterization Techniques
3.1 lnductively Coupled Plasma System
The project began with design and construction of an inductively coupled plasma systern
for etching. Unlike CCP and ECR plasma. ICP has been less frequently adopted by
researchers. However, there now appears to be a growing interest in it p.17-2.18.2.34-2-35,
2.44, 3.1-3.41. ECR plasma etching, with its high plasma density, has been proven more
superior than RIE. ICP is an alternative source for achieving high plasma density at low
pressures. In addition, it does not require high apparatus cost and power consumption
during operation as in the case of ECR plasma etching. ICP is also easier to scale up
than ECR. Due to these reasons, ICP is more suitable for use in industrial mass
production, despite the fact that ECR has even higher plasma density than ICP.
Figure 3.1 and 3.2 represent a 30 mode1 and a block diagram of the custom-made
apparatus. This ICP reactor is non-resonant with cylindrical geometry. The system can
be divided into five elements including the vacuum chamber. power supply. pressure
control, gas supply and residual gas analyzer (RGA).
Convectron Gauge Right-Angle Valve Pyrex & Coil MFCs
Figure 3.1 ICP system : 3D model.
28
3.1.1 Vacuum Chamber
The ICP tube. Le. the dielectric part of the vacuum chamber, was made of Pyrex glass
with a diameter of 10 cm and a length of approximately 25 cm. It can withstand
temperatures up to 300°C. The rest of the vacuum chamber was assembled with
ConFlat stainless steel vacuum components to provide ports for pumps, gauges, sarnple
holder and residual gas anaIyzer. The stainless steel ConFlat components were joined
by a graded glass seal to the Pyrex tube. During plasma production, the surface
temperature of the chamber tended to be elevated. A cooling fan was used to avoid
excessive surface temperatures.
An important criterion for deterrnining the dimensions of the ICP tube is the mean free
path À. i.e. the distance travelled by a particle between two successive collisions. The
diameter of Pyrex tube has to be at least twice A at plasma ignition pressure, otherwise
collisions cannot take place. Also, the relative size of the ICP tube compared to the size
of samples is expected to affect unifomity of etching across the entire sample surface.
The mean free path, h (cm), can be calculated from [3.5]:
d rnolecular diameter(cm) N number of molecules per unit volume (cm")
N is directly proportional to pressure in the chamber at constant pressure :
where
Therefore,
P pressure (mTorr) R gas constant (6.23656~10' m~orr-cm3/gmole-K) T temperature (assuming 300 K) No Avogadro 's nurnber (6.0221 69x1 oZ3)
Given that d is around 3x1 o9 cm in average for air (3.61 and ignition pressure is 7 mTon.
A = 7 x I O - l 5
= 1.1 1 (cm) (3 x IO-')* x 7
Substrates were fixed to a sample holder by placing a 1-mm-thick mask with 8x8-mm2
window on top, as shown in Figure 3.3, and the holder was placed inside the vacuum
chamber perpendicularly to the central axis of the inductive coil.
Mask
Figure 3.3 Side view of the entire substrate holder assembling and top view of the mask.
3.1.2 Power Supply
The power source was an ENI Model OEM-6 RF Power Generator, providing a
maximum continuous power output of 650 W into a 50a impedance at 13.56 MHz. RF
power was inductiveiy coupled to a 20-turn helical painted copper coi1 wound around the
ICP (Pyrex) tube. Between the OEM-6 and the coi1 existed a matching network
(modified L-type, RFPP Model AMN-2001E) accompanied by its power supply (RFPP
Model AMN-PS-2.A). The function of the matching network is to minimize the reflected
power. By tuning the LOAD and TUNE capacitors of AMN-2001 E, the impedance of the
source (RF generator) and of the load (inductive coi!) can be matched, and the power
absorbed by the plasma can reach the maximum value (Appendix C). Nevertheless,
when the inductive coi1 was attached for the first time, the number of coils had to be
adjusted carefully so that the impedance was within the working range of the matching
network. RF was noticed to interfere with near-by electronic equipment. When the RF
power was turned on, pressure and gas flow rates fluctuated between a large amplitude
instead of settling at preset points. In an attempt to block this unwanted RF interference,
a Faraday Shield was placed around the ICP chamber, and the cables were shielded by
aluminum foils.
3-1.3 Pressure Control
Two separate pumping units were adopted to achieve differential purnping for the main
vacuum chamber and the RGA. The vacuum chamber was evacuated first by the
mechanical pump (Edwards E2MS). Once the pressure dropped below 5 rnTorr, a
turbomolecular pump (Pfeiffer Balzers TPU 170) was then turned on to obtain higher
vacuum. The pressure inside the chamber was measured by a convectron gauge
(Granville-Phillips Series 275) and a Bayard-Alpert nude ion gauge (Granville-Phillips).
The pressure range of the convectron gauge and of the ion gauge are 1 x 10~-990 Torr
and 1 x 10'-l 10" Torr respectively. A 307 Vacuum Gauge Controller (Granville-
Phillips) senred as the controller and the display for both gauges. A butterfly exhaust
valve (MKS Type 253A) was installed between the turbo pump and the vacuum chamber
to Vary the pumping speed. By utilizing a Baratron Exhaust Valve Controller (MKS Type
252A), it was possible to control the pressure between 10" and 5 x 1 oJ Torr. The
Baratron Controller adjusted the position of the butterfly valve according to the desired
pressure set by the author and the analog signal (current pressure) received from the
pressure gauge controller. If the current pressure in the chamber was higher than the
desired pressure, the Baratron controller would open the valve further to increase the
pumping speed, and vice versa. Another combination of mechanical pump (Alcatel
2004A) and turbornolecular pump (Pfeiffer Balzers TPU 060) was used to achieve the
base pressure of 5 x 10" Torr in the residual gas analyzer. The pressure was monitored
by a full range gauge (Balzers) and kept constant for every spectrurn scanning by tuning
a right-angle valve manually.
3.1.4 Gas Supply
Ultra-high-purity (UHP) methane. hydrogen, nlrogen and argon supplied by Matheson
Gas Product Inc. were used for feedstock. Mass-FloB controllers (MFC) (MKS Type
1179A) accurately controlled the flow rates and allow a maximum flow rate ûf 14.4 sccm
for CH4 and 20 sccm for the other gases. The MFCs were connected to a 4 Channel
Readout (MKS Type 247C). displaving the flow rates. The gases were premixed before
being introduced to the vacuum chamber. A gate valve was placed in between MFCs
and the ICP tube.
3.1.5 Residual Gas Analyzer
Residual Gas Analyzer (RGA-300. Stanford Research System) is a compact mass
spectrometer consisting of a quadrupole probe and an electronics control unit (ECU),
which mounts directly on the probe's flange and contains al1 the necessary electronics
for operation. An associated Windows software package was used for data acquisition
and analysis as well as probe control. The total probe equipment consists of ionizer.
quadrupole mass filter and ion detectors (Faraday cups and continuous dynode electron
multiplier). The mass to charge ratio (rn/e) range is 1 to 300. Both ionic and neutral
species effusing from the bulk plasma region were introduced to the RGA through a
0.15-mm-diam orifice. but only neutral species could be ionized and detected by RGA
because of the negatively biased repeller grid and the positively biased anode grid. The
distance between samples and the orifice is about 2 cm. and the distance between the
orifice and the ionizer is about 20 cm.
3.2 Characterization Techniques
Several techniques were used to characterite etching results, as listed in Table 3.1.
Table 3.1 Various techniques for sample characterization.
Profilometry SEM AFM XPS
Etch Rate
Etch Profile
Roughness
Elemental Analysis
3.2.1 Profilometry
A stylus profilorneter, Alpha-Step 200 from Tencor Instruments, was routinely employed
to record the surface profile of etched InP samples. Alpha-Sep can scan the surface at
various rates in the range of 0.04-5 um/sec for a maximum scan distance of 1 cm. The
(standard) vertical and horizontal resolutions are 5 nm and 12.5 Pm radius respectively.
For this study, the scan rate and scan distance were selected as 0.2 pm/sec and 400 um
respectively. The etch rate was obtained by dividing the etch depth from the profile by
the etching duration. Figure 3.4 shows a typical surface scanning across an etched
trench. To measure surface textures, Alpha-Step automatically calculated average
roughness. However, due to the relatively low resolution, Alpha-Step was only able to
determine the surface roughness of some samples. The lowest measurable average
roughness is 5 nm.
Figure 3.4 Surface profiling by Alpha-Step.
3.2.2 Scanning Electron Microscopy
Cross-sectional examination using a scanning electron microscope (SEM) is considered
the most popular method to observe the etch profile of InP samples because SEM offers
much higher magnification than optical (or light) rnicroscopy and unlike transmission
electron microscopy (TEM), SEM is nondestructive and does not require special sample
preparation. SEM can also provide additional depth verification and visual assessrnent
of surface roughness. Images in SEM are created by scanning samples with a focused
electron beam commonly generated from a tungsten or lathanum hexaboride (LaBs)
filament. As the beam interacts with the sample. secondary electrons are emitted at
each beam location and subsequently detected by an electron collecter. The signais are
amplified to control the brightness of a cathode ray display tube (CRT) scanned
synchronically with the sample beam scan in the SEM. A correlation is therefore
established between each point on the display and each point on the sample. A Hitachi
S-4500 SEM was employed in this study. An electron accelerated voltage as low as
1 kV was used to eliminate surface charging and the working distance was typically
3-5 mm. Sidewall angles were detenined as follow (Figure 3.5):
1. locate half height point on the sidewall (Point M)
2. draw a best-fit line passing Point M (Line V)
3. draw a line parallel to the etched surface (Line H)
4. calculate the angle between Line V and Line H (sidewall angle)
Pant M Y
Figure 3.5 Determination of sidewall angle.
3.2.3 Atomic Force Microscopy
An atomic force microscope (AFM), Slover P47-SPM-MDT from NT-MDT, operating in
air was employed for rnorphological studies of the etched surface. An AFM functions by
scanning across the sample surface with a sharp tip mounted on a microfabricated
cantilever. lnteratomic forces between atoms on the surface and those on the tip cause
deflection of the cantilever, and the motion of the cantilever is sensed by a segmented,
position sensitive photodetector. Keeping the signal and therefore the cantilever
deflection constant by adjusting the sample height through a feedback loop, gives the
sample topography in al1 three dimensions. An AFM can operate in several different
modes such as contact, noncontact and tapping mode. As far as InP samples in this
study are concerned, tapping mode was found to be most suitable since it caused
minimal damage to the surface and yet yielded reasonable resolution. A Slover AFM
scan consisted of 51 2x51 2 sample points with a scan rate of 1024 points per second.
Once surface topography and phase graph of a selective area were obtained, and root-
mean-square (mis) roughness, average roughness (Ra) and roughness distribution were
calculated for comparison between different samples.
mis roughness
3.2.4 X-ray Photoelectron Spectroscopy
Surface elemental analysis of InP samples was perfomed by a Leybold MAX 200 X-ray
photoelectron spectroscope (XPS) systern equipped with both monochromatic and non-
monochromatic Al K, X-ray sources, operated at 15 kV and 30 mA. Duting XPS
analysis, X-rays eject photoelectrons from the top 7-10 nm of the sample. The
measured energy of the emitted electron at the spectrometer Ew is re!ated to the binding
energy Eb by
where hv energy of X-rays 9 electron charge
work function of the spectrometer
Since binding energies of elements are known, peaks in the spectrum can be identified
and the relative intensity of each element can also be derived from the peak area.
Chapter 4
lnductively Coupled Plasma Etching of InP
4.1 Experimental
4.1.1 Sample Preparation : Cleaning and Patterning
(100) oriented undoped InP wafers (2" in diameter, 1 mm in thickness, supplied by
Sumitorno) were cleaned with organic and acid solutions to eliminate any contamination
or oxido layer. Tho st+i?darcl cloaning procedures 2re listed in Table 4.1. First
trichloroethylene and methanol were used to remove grease. Because organic solvents
left a film made of hydrogen, carbon and oxygen on the surface, InP was then treated in
an ultra violet ozone cleaning oven to remove hydrogen and carbon. After that, InP was
etched in buffered hydrofluoric acid (BHF) to detach oxygen from the surface.
Table 4.1 Standard cleaning procedures used for InP.
Step Description Time
1 clean with trichloroethylene at 90°C 10 min
2 clean with (fresh) trichloroethylene at 90°C 10 min
3 clean with rnethanol at 90°C
4 dean with (fresh) methanol at 90°C
5 rinse with deionized (DI) water
6 blowdry with filtered N2
7 clean in an ultra violet ozone oven
8 Etch with HF : Dl water (1:lO in volume)
9 rinse with DI water
10 blowdry with filtered N2
10 min
10 min
5 min
10 min
30 sec
5 min
To investigate the etch rate and the etch profile, InP wafers were partially covered by
masking materials prior to etching. The choice of masking materials must meet several
requirements. It should define patterns precisely, should not be etched substantially
during the process and should be easily removable after the process. Three masking
materials, gold (Au), photoresist (PR) and silicon dioxide (Sioz), were investigated in the
initial phases of the study. Au layers were deposited on InP by an evaporator and
patterns were created by placing a mechanical mask between substrates and the Au
source. Au masks produced in this manner were not densely packed films. but more like
gold powder laying loosely on the surface. Thus, Au masks could be easily sputtered
away by ion bombardment. An even more serious problem occurred with pattern
definition. The use of mechanical masks could not generate distinct patterns. This
setback made examination of etch profiles extremely difficult. In order to attain well-
defined patterns, recourse was made to photolithography .
SiOz and PR were selected to be patterned by photolithography. For Si02 masks, the
process began with a Si02 layer, approxirnately 180 nm thick, being deposited on the
InP substrate by plasma-enhanced chernical vapor deposition (PECVD), as illustrated in
Figure 4.1. Then a layer of positive PR, a light-sensitive polymeric material, was coated
ont0 the Si02 by a spinner. The thickness of PR was approximately 860 nm. Next the
substrate was exposed to an ultra-violet light through a quartz mask irnprinted with the
desired pattern (narrow stripes). The UV light broke the polymer bonds in the exposed
areas (for positive resist). The pattern was then developed in an organic solvent which
washed away only the exposed resist but left the rest unaffected. After that, the sample
was put into a solution containing BHF that etched Si02 but not the PR. Once the
pattern was transferred down onto the Si02 layer, PR was stripped by acetone, leaving
patterned SiOa masks on the InP substrate. PR masks were generated by following
steps 2-4 in Figure 4.1 .
Experiments performed with CH JH2-based plasmas showed no etching effect on both
Si02 and PR. Instead, carbon-based polymer films were deposited on these masks
during the etching process. A significantly greater amount of polymer deposition was
observed when using PR. PR masks and polymers on top had to be removed by dry
etching in O2 plasmas. In the other case, SiOa masks could be wet-etched by BHF
directly without O2 plasma etching beforehand. Since it was necessary to remove masks
before the etch depth measurement by Alpha-Step, SiOa was considered more suitable
than PR for CH JH2-based plasma etching. Si02 masks were then tested against HdN2
plasmas. Without CH4 present in the plasma, Si02 could no longer have polymer films
as protective layers. PR masks were too soft to withstand ion bombardment coming from
H$N2 plasmas. Nevertheless, the etch rate of InP was found to be approximately 10
times higher than that of Si02 in H2/N2 plasmas.
Figure 4.1 Si02 masking procedure.
4.1.2 Inductive Coupled Plasma Etching
After cleaning and patteming, substrates were cut in such way that they had almost the
same surface area and Si02 rnask covering portion in order to avoid uncertainty arising
from the loading effect. (According to the loading effect, the etch rate varies invenely
with the substrate area.) First the chamber had to be evacuated by the pumps for
approxirnately 3 hours to minimize the amount of impurity that would later exist in the
plasma. Once the pressure dropped below 5x10" Ton, gases were fed into the
vacuum chamber, and the pressure was adjusted to the preferred etching pressure.
Fve different etching precurson, CH4/H2, CHdHdAr. CH*/HÊ/N2, HdN2 and HdAr were
investigated in this study. The system was kept under fixed conditions (certain gas flow
rates and chamber pressure) for at least one hour to ensure that equilibrium was
reached between gas inlet and pumping outlet. Afterward the plasma was ignited and
sustained by applying RF power to the inductive coil, and the reflected power was
reduced to almost zero by tuning the two capacitors of the matching network. The
etching was continued for an hour each time.
4.1.3 Sample Characterization
Every plasma-etched InP sample was cleaved into hatf before undergoing any
characterization. A diamond-tip pencil was used to scribe a small defect at the edge of a
sample. and by applying a pressure on the defect. the sample would break into two
pieces with cleaved cross-section. One haif of the sample was wet-etched by BHF for
60-1 00 seconds to remove the SiOa masks and polymers, and then cleaned in methanol
for 30 minutes (cleaned sample). The other haif was kept as it was (masked sample).
Etch depth was rneasured by Alpha-Step at at least five points for each cleaned sample,
and the etch rate was obtained by dividing the average etch depth by the etching
duration (rnostiy one hour). From the measurement with the Alpha-Step, the etch depth
was found to be smaller at the out part of the sarnple. This phenomenon was believed
to result from the geometry of the vacuum chamber and the sample holder. As seen in
Figure 3.1, the plasma traveled from the ICP tube (10 cm in diam.) through a narrow
tube (3.8 cm in diam.) and then re-entered a wide space before reaching the sample.
The sudden change in cross-section was expected to affect considerably plasma
uniformity. In addition, samples were fixed to the holder by placing a 1-mm-thick mask
with 8x&mrn2 window on top, as shown in Figure 3.3. The edge of the open window
caused "shadows" on samples, and hence, the outer part of a sample was etched to a
lesser degree than the central part. Due to these effects. only an area within 4x4 mm2 in
the center of samples was used for etch rate examination.
Cleaved cross-section of both cleaned and masked samples was examined by SEM so
as to observe any undercut, overcut or barreling phenornena. Images of tilted etched
surfaces were also taken for evaluation of surface roughening. However, in order to
measure the surface roughness more precisely and be able to compare with the results
reported by other researchers, AFM was used to scan cleaned samples and calculate
the mis roughness. XPS analysis was performed over an area of 2x2 mm2 of masked
samples to determine the etched surface stoichiometry and contamination. Because
Si02 rnasks are much thicker than the X-ray penetration depth (7-10 nm), it is
reasonable to assume In and P signals appearing in the spectra are contributed entirely
by the etched surface.
Issues concerning repeatability and error bars in the following graphs are addressed in
Appendix D.
4.2 Influence of Gas Composition on Etching
4.2.1 Etch Rate
Of al1 the variable experimental parameten. composition of gas feedstock was found to
have the most significant influence on the etching outcome, Different combinations of
gases were used to etch InP during the study induding CH JH,, CHJHgAr, CHdH$N2,
H2/N2 and HdAr.
ln Figure 4.2, etch rate of InP is plotted against CH4 concentration in CH4/H2 mixtures,
Le. the flow rate of CH, divided by the total flow rate of CH4 and H2. It can be noticed
from the chart that even pure Hp plasma etched InP at a rate of 7 nm/min. By raising the
CH4 concentration in precursors, the etch rate increased continuously to a maximum of
35 nmimin at 22% CH4. However, InP surfaces began to undergo polymer deposition
rather than etching when CH4 concentration exceeded 25%.
Methane Concentration (volao)
Figure 4.2 InP etch rate as a function of CH, concentration in CHdH2 plasmas. The RF power,
the total flow rate and the pressure were 100 W. 7.5 sccm and 8 mTorr respectively.
Activities at InP surfaces exposed to CHs/H2 plasmas can be described as a cornpetition
between InP etching and polymer deposition. Gas composition of feedstock obviously
controlled the rates of both processes. In pure H2 plasmas. H radicals were abundantly
generated to etch P via chemical volatilization. but without the presence of CH4, In could
not be chemically etched like P. Instead, one can only expect In to be removed
physicaily by ion bombardment. Nonzero etch rates in pure HZ plasmas were previously
reported by several other researchers (2.24. 2.31. 2.381. Yet the problem of polymer
deposition did not happen in the case of etching with pure Ha plasmas since there was
no hydrocarbon in the gas phase. The etching mechanism of In gradually transformed
frorn physical etching to accelerated ion-assisted chemical etching with increasing CH4
concentration in precursors. The transformation increased the etch rate as expected.
However, the mechanism of polymer deposition was also enhanced simultaneously with
an increase in CH, concentration. At a certain point, InP surfaces started to become
oversaturated with hydrocarbons and also became lack of adequate ion bombardment
for assisting desorption of In(CH& As a result, polymer got a chance to build up on InP
surfaces. A similar trend of etch rate versus CH4 concentration was found in the
literature [2.20.2.24.2.381, but the CH4 concentrations at the maximum point (maximum etch
rate) and the transition point (InP etching to polymer deposition) were different.
Transition happened at a lower CH, concentration in this study because no bias was
applied to the substrate to enhance ion bombardment during etching. Therefore, the
etch rate can be expected to be increased by incorporating a substrate bias element into
the ICP system.
Ar and N2 were added to CH JH2 mixtures with the intention of improving surface
morphology by enhancing the ion-assisting mechanism for In removal. Unfortunately,
CH*/H2/Ar plasmas in fact generated rougher etched surfaces than CHdH2 plasmas.
Discussion of surface morphology will be given later in this section. The following
information regarding etch rate was attained from the experirnents. Gas compositions
and results are listed in Table 4.2.
Table 4.2 Etch rate for InP etched with various CHs/H2/Ar ratio.
CH4 : H2 : Ar (vol?&) Result
1 4 0 : M : 12 deposition of polymer
2 35:s: IO deposition of polymer
3 20 :70 : 10 etch rate 13.5 (nm/min)
4 27 : 53 : 20 etch rate 9.8 (nrn/min)
5 10:60:30 etching through mask
6 17:57:26 etching through rnask
When there was not enough Ar in precunors to dilute CH, (samples 1 and 2), polymer
deposition overwhelmed InP etching at surfaces as in the case of etching with CHJH,
plasmas. When using high Ar-concentration plasmas (samples 5 and 6), 100-nm thick
SiO2 masks were removed completely due to the high sputtering yield of Ar. Even their
underlying InP surfaces were etched by plasmas as well. This incident was detected
du ring SEM examination. In their micrographs, the horizontal surfaces of etched mesas
(surfaces which used to be covered by Si04 appeared unusually rough as opposed to
the smooth topography observed with other samples. For this reason, etch depths could
not be determined by measunng the difference in height of covered or uncovered InP
surfaces. When etching with gas compositions wlh medium Ar concentration (Samples
3 and 4) CH4/HdAr plasmas were found, in general. to produce lower etch rates than
CH& plasmas. The results suggested that the presence of Ar in plasmas may reduce
the degree of dissociation of CH4 and Ha, and shift the etching mechanisms closer to
physical etching from ion-assisted chernical etching . 5CHdl 7H2/8Ar (sarne as Sample
6) was found by Pearton et ai. to be the optimal precursor for InP etching in their ECR
system [2.49.2.511 alfhough CH JHdAr appeared to be unsuitable for the ICP system used
in this study.
Extensive efforts had been made to reveal the influence of CHJHdN2 composition on
InP etching. The etch rate resulting from each condition is given in Table 4.3, and trends
summarized from data in Table 4.3 are shown in Figure 4.3 and Figure 4.4. The etch
rate was apparently reduced by including N2 in the feedstock. similar to the effect of Ar
addition. No etching dependence on the CH jH$N2 composition was found in the
literature. Sendra et al. p.521 and Carlstrom et al. p.441 used fixed CH JH$N2 ratios of
1 O:60:3O and l7:6l:Z respectively throughout their articles.
Table 4.3 Etch rate of InP etched with various flow rates of CH4, H2 and N2.
CH4 Hz N2 Total CH4 : HI! : N2 Etch Rate
( S C C ~ ) (sccm) (sccm) (sccm) (voIOO) (nrn/min)
The plot in Figure 4.3 characterizes the etch rate dependence on CH4 concentration of
CH JHdN2 mixtures. In these experiments, the total gas flow rate and the ratio of H2/N2
were kept constant at 7.5 sccm and 311 respedively. The trend is noticed to correspond
to that in Figure 4.2, which represents the etch rate dependence on CH4 concentration of
CHJH2 precursors. By making a cornparison between the two charts, one can find that
the CH4 concentrations at the maximum point and the transition point were shifted from
22% to 40% and from 25% to 50?/0 when replacing ?6 of Hp in the precursors with Ne.
The result showed that N2 indeed was a polymer deposition inhibitor in CH4-based
plasmas as suggested by Keller et al. p.621 and also implied that N2 serves as a better
dilutant for CH, plasmas than HZ.
Methane Concentration (volOo)
figure 4.3 InP etch rate as a function of CHJ concentration in CH JHdN2 plasmas. The Hf12
ratio, the RF power, the total flow rate and the pressure were 3, 100 W, 7.5 sccrn and 7 mTorr
The next step was to establish the dependence of etch rate on HJN2 ratio (relative
volume concentrations of H2 and N2) in the precursors. InP was treated with CH JHdN2
plasmas having different HdN2 ratios but the same CH4 concentration (40°h), and a total
gas flow rate of 7.5 sccm at al1 times. The etch rate result is plotted against HdN, ratio
in Figure 4.4. With increasing Hfl, ratio, the etch rate was found to increase initialiy,
but then started to decrease after the ratio exceeds 4. The main function of H2 was to
into a hydrocarbon network on top of the InP surface. The result depends on whether
organo-indiums escape or stay attached to the surface. In atoms whose adjacent P
atoms have been removed have weaker bonds between them and the surface than
those atoms whose P neighbors are still in place. Therefore, they can desorb from the
surface more easily. The theory can be suppotted by the fact that the sputtering yield of
In metal is indeed much higher than that of tnP.
In addition to CHa-containing precurson, HdN2 mixtures were applied to etching of InP in
the ICP reactor. N2 concentration was varied between 0-50°/0 of the total flow rate 7.5
sccm and the etching pressure and the RF power were maintained constant at 7 mTorr
and 100 W respectively. The etch rate is plotted against N2 concentration in Figure 4.5.
By adding 10% Na to pure H, plasma. the etch rate dropped from 7 nmfmin to
4.6 nm/min, but then appeared to reach an approximate value (-4 nrn/min) when N2
concentration was in the range of 20-509'0.
O 1 O 20 30 40 50 60
Nitrogen Concentration (vol%)
Figure 4.5 InP etch rate as a function of N2 concentration in H2/N2 plasmas. The RF power, the
total flow rate and the pressure were 100 W, 7.5 sccm and 7 mTorr respectively.
Without CH4 present in precursors, CH3 is no longer available for chemical etching of In.
Nevertheless. H, the chemical etchant of P, can still be generated from dissociation of
Hz. Once P is removed by H, In left behind is believed to be so soft that it can be
sputtered away from the InP surface by N2+ and N'. The etching mechanism is therefore
half chemical and haff physical : first P is etched chemically by foning volatile PH3 with
H, and then In is removed physically by ion bombardment by N2+ and N'. This kind of
mechanism is sometimes referred to as reactive sputtering. However, it should be made
clear that sputter agents N$ and N' are not chemical reactive to InP in any way. H
concentration is reduced by adding N2 to Hî plasmas due to formation of NH3.
InP was also etched with a plasma consisting of 30% Ar and 7096 HP. The resulting
etch rate was 5.4 nm. which was higher than H2(7O0/~)/N2(30%) plasma but still lower 0
than pure H2 plasma. Since Ar does not react chemically with H, the decrease in etch
rate infers that the presence of Ar lowers H concentration in the plasmas via reduction of
H2 dissociation.
4.2.2 Etch Profile
Barreled sidewalls combined with mask undercuting and sloping sidewalls (overcut)
were observed on InP samples etched with CHJH2 plasmas. ln pure H2 plasma, the
sidewalls of etch rnesas inclined with an angle of 65" and showed no sign of barreling.
Increasing CH, concentration in the feedstock enhanced verticality to 87" for 2296 CH4.
but baneling of sidewalls emerged after CH, exceeded 20%. The sidewall angle is
plotted against CH, concentration in Figure 4.6, and Figure 4.7 shows a SEM
micrograph of 20% CH4.
Barreling of sidewalls and masking undercutting can be improved by applying additional
RF substrate bias (2.20. 2-24]. Substrate bias enhances ion energy in the direction
perpendicular to the surface, and therefore, verticai/lateral etch rate ratio is expected to
increase.
O 5 10 15 20 25
Methane Concentration (voloa)
Figure 4.6 Sidewall angle of InP as a function of CH4 concentration in CHS/H2 plasmas. The RF
power, the total flow rate and the pressure were 100 W, 7.5 sccm and 8 mTorr respectively.
Figure 4.7 An SEM micrograph of InP etched wfai C H d 2 plasma. CH4 concentration, the RF
power, the total flow rate and the pressure were 20%, 100 W, 7.5 sccm and 8 mTorr respectively.
For InP mesas etched w l h CH JH$N2 plasmas. the sidewall profile was found to
correlate to etch rate rather than gas composition. This conclusion was based on the
SEM results. As mentioned previously in sample characterization, etch rate was not
untorm over the entire surface of one sample. It decreased gradually from the center of
the sample towards the outside. The etch profile appean to change accordingly. (This
situation does not occur to InP etched with CH JH2 plasmas.) In general. when the etch
rate was lower than -5 nrn/min, rarnps but not steps were seen between etched surfaces
and protected surfaces. Verticality was improved with increase in etch rate. By comparing sidewalls etched in CH4/H2 and CH JH21N2 plasmas, addition of Np was found
to elirninate barreling but worsen sloping of sidewalls.
Though quite sloping sidewalls were observed on ail InP samples etched with HSN2
plasmas. no barreling or mask undercutting was induced and sidewalls form sharp
angles with horizontal surfaces. The sidewall angle versus N2 concentration is given in
Figure 4.8. When N2 was included in the precursor. verticality was optimized at 40% N2
in the precursor under the conditions that the charnber pressure. the RF power and the
total gas flow rate were kept constant at 7 mTorr, 100 W and 7.5 sccm respectively.
O 10 20 30 40 50 60
Nitrogen Concentration (volas)
Figure 4.8 Sidewall angle of InP etched mesa as a function of N2 concentration in H a 2
plasmas. The RF power, the total fIow rate and the pressure were 100 W. 7.5 sccrn and 7 mTon
res pectivel y.
Sloping of sidewalls can occur via chemical or physical mechanism. Aithough chemical
etching is nonnally isotropic. resulting classic mask undercutting type of profile
(Figure 2.5). examples of chemical etching which is crystaIlographically selective have
been demonstrated for Ill-V compounds p.11. Cystallographically selective etching
meanç that the macroscopic etch rate of different crystallographic facets of one material
in the same etchant can differ significantly. For instance, the relative rate at which Ill-V
crystal planes etch in halogen (CI2, Br2) is (1 1 1 )B>(l 00)>(110)>(111 )A 14.2-4-31. The
subscription A and !3 denote group Ill-rich and group V-rich (1 11) planes respectively.
As a result, sloping sidewalls can be obtained by chemical etching under kinetic control,
diffusion or intermediate control. (Please refer to p.41 for advanced discussion.) Sloping
sidewalls can also happen in physical etching (sputtering). If the mask itself has a
sloping profile, the edge moves laterally du ring etching, exposing more substrate surface
to the etchant and resulting in a sloping profile.
There are two ways to determine whether the chemical or physical mechanism causes
the sloping profile. One is by examining if there is masking undercutting since sloping
sidewalls are accompanied by undercut in chemical etching but not in physical etching.
The other way is by calculating the sloping angle to see if it corresponds to any
crystallographic planes.
In the case of InP etching in H,/N, plasma, no mask undercutting was seen and the
sloping angle was not 54.41°, which corresponded to (1 1 l),, facet giving (100) InP
surface. Therefore, sloping of the sidewalls was concluded to be due to the poor etch
profile of Si02 masks. Because wet chemical etching in %HF was used to transfer
pattern from photoresist down ont0 Si02, sidewalls of Si02 stripes were extremely
sloping due to severe undercut. When exposing the samples to H$N2 plasmas. Si02
masks were etched by N2+ and N+ physically at the same time of InP etching, and their
widths were reduced. As a result, InP gained the sloping profile. Figure 4.9(a) and (b)
show SEM micrographs of an InP substrate patterned with SiOa mask before etching
and after etching in HSN2 plasma. Reduction in the thickness of Si02 masks can be
seen.
Figure 4.9 SEM micrographs of InP (a) masked with Si02 before etching and (b) etched with
H& plasma. Si02 Mask was still in place-
The mask sloping effect is calculated as below (Figure 4.1 0) :
The thickness of Si02 before etching Tb = 180 nm
The sloping angle of Si02 masks es,Z = 4"
The etch depth of InP d,,, = 300 nrn
The etch depth of Si02 dSn2 = 30 nm
The thickness of SiO2 after etching Ta =150 nrn
Reduction in the width of Si02 masks R,,_ = 180~ot4~-150cot4" = 429 nm
The sloping angle of etched InP O,,, = tan-' (3001429) = 35'
Figure 4.10 Reduction of Si02 masks in thickness and width duting etching.
The sloping angle depends on the etching selectivity over InP and the mask, and
therefore verticality of sidewalls should be increased by improving rnasks. Mask
erosion can be diminished by using more sputtering-resistant masking materials such as
Si3N4 AISO, sloping of mask profile can be elirninated by using plasma etching to
transfer pattern from photoresist to Si02 or Si3N4.
Summarizing chemical etching tends to generate barreling or undercut, and physical
etching tends to generate sloping (overcut). Therefore, if the chemical aspect of the
etching mechanism were more significant than the physical aspect. the sidewall profile
would appear more vertical but barreling may occur. On the other hand, if the physical
aspect of the etching mechanism were more significant, there is no barreling but sloping
is more severe.
4.2.3 Surface Morphology and Elemental Analysis
The most important cause for deterioration of InP surfaces etched by CH4/H2 plasmas is
irnbalance in etch rates of In and P. It is inevitable given that not only diffusion and
reactivity of H are higher than CH3 but also volatility of PH, is greater than In(CH&
When the imbalance is too significant, In atoms tend to coalesce and form In-rich
droplets, leading to roughening and severe P-depletion on InP etched surfaces
(Figure 4.1 1). The same conclusion was reached by Etrillard et al. from their work in
ICP etching of InP using CHJH2/02 mixtures [ma]. In addition. before In(CH3).
compounds desorb from the surface, they can act as micromasks blocking etching
propagation locally.
Figure 4.11 InP etched in C H a 2 plasma, Si02 mask was still in place.
The rms roughness and Pfln ratio are plotted against CHI concentration in Figure 4.12.
By increasing CHI concentration in CH JHp, imbalance in etch rates of In and P was
improved because more CH3 radicals were provided. According ly. surface roug hening
and P-depletion were reduced.
Figure 4.12 Rms
Methane Concentration (volO;o)
roughness and Plln ratio of InP etched surface as a function of CH4
concentration in CHJ/H2 plasmas. The RF power, the total flow rate and the pressure were 100
W, 7.5 sccrn and 8 mTorr respectively. represents roughness and O represents Plln ratio.
Different kinds of rough topography were seen on InP surfaces etched with CHJ/H2 or
CHdHdAr plasmas, as shown in Figure 4.13. CH JH2 plasmas generated rounded
features on etched surfaces. It is consistent with the ln-coalescence mechanism. As for
InP surfaces which were etched in CHdHdAr plasmas, pointed features were observed
instead. The same type of pointed topography has been seen on InP surfaces sputtered
by pure Ar plasmas pal. This suggested that surface degradation of InP etched in
CHdHdAr plasmas was due to ion bombardment by Ari. Topography of InP etched in
CHdHdN, plasmas appeared to be more close to the CH& kind, meaning the role N2+
and N' play was more close to assisting desorption of etch products than sputten'ng.
Figure 4.13 SEM micrographs of InP surfaces etched with
(a) CH** plasma and (b) CH.,/HJAr plasma.
Morphology of InP surfaces etched with HdN2 plasmas was much better than those
etched with CH4-based plasmas. The mis roughness obtained from AFM decreased
dramatically from 30 nm for pure Ha plasma to 3.1 nm for only 10% NP, as show in
Figure 4.1 4. When NÎ concentration was in the range of 30-50%. the roughness was
reduced to less than 1 nrn. Figure 4.1 5 shows AFM 3D images of InP before etching,
etched in CHJ/H2 and etched in H2/N2 plasma. InP etching in HdN2 plasmas can be
optimized to generate a srnooth surface almost like unetched substrate. Also, the
surface composition of InP samples etched with HdN2 plasmas was closer stoichiometric
than those etched with CH*/H2-based plasmas based on the elemental analysis of XPS
(Figure 4.14). The roughness cuwe and the P/ln ratio curve show opposite tendency,
suggesting that surface degradation of InP etched in HdNl plasmas was caused by
preferential loss of P rather than ion bombardment. Addition of N2 balanced the removal
rates of In and P by suppressing H concentration. Consequently, surface roughening
and P-depletion were improved.
Nitrogen Concentration (volOo)
Figure 4.14 Rms roughness and P/ln ratio of InP etched surface as a function of N2
concentration in Hf12 plasmas. The RF power, the total flow rate and the pressure were 100 W,
7.5 sccm and 7 mTorr respectivefy. represents roughness and O represents P/ln ratio.
(a) mis roughness 0.1 79 nm
(b) rrns roughness 17.9 nm
(c) mis roughness 0.213 nm
Figure 4.15 AFM topographies : (a) control, (b) etched in CH&i2 and
(c) etched in H a 2 (x,y : pm and z : nm)
Figure 4.1 6 shows XPS spectra (C 1 s, N 1 s, In 2d5, P 2p) frorn inP samples etched in
CH4/H2 or HdN2 plasmas. Control sample results are added for cornparison.
Binding Energy (eV)
Binding Energy (eV)
456 .rJ8 440
Binding Energy (eV)
Binding Energy (eV)
Figure 4.16 XPS spectra form InP sarnples. solid line : control,
dot line : etched in CHJ/H2, dash line : etched in H2/N2.
4.3 Influence of RF Power on Etching
4.3.1 Etch Rate
Different RF powers were applied to the inductive coi1 with the purpose of studying its
influence on etching of InP using CHJ/H2 or H$N2 plasmas. First, a series of etchings
was perfomed with powers in the range of 50-200 W for CHJH, plasmas containing
2 2 O / 0 CH4. The total gas flow rate and the etching pressure were kept constant at
7.5 sccm and 7 mTorr. As seen in Figure 4.17. RF power changed inP etch rate
profoundly. It appeared that a minimum power between 50 and 75 W was required to
initiate etching. After that. the etch rate increased linearly in two different slopes. The
increase in the etch rate became faster when the power was above 150 W.
RF Power (W)
Figure 4.17 InP etch rate as a function of RF power in CHdH2 plasmas. The CH4 concentration,
the total ftow rate and the pressure were 22%, 7.5 sccm and 7 mTorr respectively. (1) and (II)
represent low and high power region respectively.
Higher RF power resulted in higher degree of ionization, providing more CH3 and H for
chernical etching of In and P. Also, because increasing the RF power increased the ion
energy, the etch rate of InP was very likely to grow due to enhancernent of the ion
assisting mechanism. When RF powers were greater than 150 W. highly energetic ions
were expected to etch InP physically. As a result, increase in etch rate became faster in
high-power (>150 W) region because now InP was not only etched by CH3 and H
chemically but physically by energetic ions as well.
The conclusion that the etch rate increases with increasing RF power in RIE and ICP
etching or microwave power in €CR etching was obtained by many researchers for InP
etching in CHJ/H2 or CHdH2/Ar plasmas [2.20.2.24.2.28.249.251.2.55.2.581.
As shown in Figure 4.18. increasing RF power was also found to increase the etch rate
of InP using HdN2 plasmas. The trend is understandable since both of the etching
mechanisms of In and P can be enhanced by increasing power. For In. which was
removed physically via sputtering. higher powers resulted in more energetic ions, and for
P. which was etched chemically via volatilization. higher powers generated more H
radicals.
RF Power (W)
Figure 4.18 InP etch rate as a function of RF power in Hf12 plasmas. The N2 concentration, the
total flow rate and the pressure were 40°h, 7.5 çccm and 7 mTorr respectively-
4.3.2 Etch Profile
The previous set of InP samples etched with various RF powers (75-200 W) using
CHJ/H2 plasmas was examined by SEM. When the power applied to the coi1 was as low
as 75 W, there was no etched mesa observed on the surface even though the Alpha-
Step rneasured an etch depth of 540 nm. Instead of having a clear step between the
surface covered by Si02 rnasks and the surface exposed to CH& plasmas, the etch
depth slowly increased from zero at the mask edge to 540 nm at a distance of 5 pm
away frorn the mask edge. (It should be noted that the gap between two SiOz stripes is
100 pm.) The sloping angles of sidewalls etched with 100, 125 and 150 W were 80°,
76" and 72" respectively. It appeared to decrease with increasing RF power. As
regards to the two samples etched at higher powers. 175 and 200 W. similar two-step
sidewalls were seen on SEM micrographs (Figure 4.19). The two-step sidewalls
consisted of a more vertical sidewall adjacent to the mask edge with an angle of 80' and
a more horizontal ramp with an angle of only IO0.
Figure 4.19 Two-step sidewall profiIe of InP etched in the CH,& plasma. The CHo
concentration, the RF power, the total flow rate and the pressure were 22O/0, 200 W, 7.5 sccm and
7 mTorr respectively.
Verticality of sidewalls of InP mesas etched with HJN2 plasmas was optimized at an RF
power of 100 W (Figure 4.20). The slope decreased with both increase or decrease
power. As mentioned previously in Section 4.2. the sloping angle of InP sidewalls
etched in HdN2 was mainly detemined by the etching selectivity. Therefore. the results
suggested that the difference in the etch rates of InP and SiOa was greatest at 100 W.
Figure 4.20 The sidewall profile of InP etched in the H2/N2 plasma. The N2 concentration, the
RF power, the total flow rate and the pressure were 40%. 100 W, 7.5 sccm and 7 mTorr
respectively.
4.3.3 Surface Morphology and Elemental Analysis
Deterioration of InP surfaces etched with CH JH2 plasmas was clearly intensified by
increasing RF power. Virtual evaluation of SEM micrographs and numerical calcuiation
of particle size by AFM software demonstrated that increase in roughness was due to
increase in sizes of In-rich droplets formed on the etched surfaces. It has been
discussed earlier that morphology of etched surfaces depends mainly on the extent of
the imbalance between the removal of In and P. The creation of H radicals was more
efficient than that of CH3 radicals, leading to a faster removal of P relative to In p.81.
Besides CH3 and H radical flux, ion energy was another factor that controls this
imbalance. When very high RF powers were applied to the inductive coil, ions were
accelerated by strong electromagnetic fields. The ions were assumed to possess a
large amount of energy by the time they arrived at the InP surface. This caused both
positive and negative effects on surface quality. On the one hand, high-energy ions can
assist desorption of l r ~ ( C i i ~ ) ~ or sputter In(CH3), (x=0,1,2,3) from the surface better than
low-energy ions. On the other hand, a significant amount of energy can be transferred
from highly energetic ions to the surface during bombardment, and In atoms can absorb
enough energy to coalesce into big droplets. Sarnple surfaces could actually reach very
high temperatures during etchingç with high RF powers since the sample holder was not
cooled by any means. Only this mechanism can offer an explanation for oversize In
droplets seen on the InP surface etched at 175 or 200 W (Figure 4.21 ).
Figure 4.21 In-rich droplets on InP etched in the CH& plasma. The CH, concentration, the RF
power, the total flow rate and the pressure were 22% 200 W, 7.5 sccrn and 7 rnTorr respectively.
InP samples etched with H2(60%)/N,(40%) plasmas at 50. 75. 100 or 125 W were
examined by AFM and XPS. The resulting rms roughness and Plln ratio are given in
Figure 4.22.
RF Power (W)
figure 4.22 Rms roughness and Plln ratio of InP etched surface as a function of RF power in
H$N2 plasmas. The N2 concentration. the total flow rate and the pressure were 409'0, 7.5 sccm
and 7 mTorr respectively. represents roughness and O represents P/ln ratio.
4.4 Influence of Total Flow Rate on Etching
4.4.1 Etch Rate
The appropriate feedstock flow rate is different for every system. It depends on the
pumping speed and the dimensions of the vacuum chamber. A set of experiments was
devoted to characterizing the influence of total gas flow rate on etching. A flow rate
range of 3-10 sccm was studied. In order to eliminate any possible effect caused by
etching parameters other than total flow rate, CHdH2 ratio. RF power and pressure were
kept as 2278. 7 mTon and 150 W. The dependence of the InP etch rate on the total
flow rate of CH4 and Ha is given in Figure 4.23. The InP etch rate was sensitive to the
total gas flow rate. Initially, the etch rate increased with increasing the total flow rate up
to a certain level (around 5-6 sccm), and then it began to decrease from there.
Eventually etching came to a complete stop at 10 sccm, and instead a thin layer of
hydrocarbon was deposited on both the InP surface and Si02 masks.
Total Flow Rate (sccm)
Figure 4.23 InP etch rate as a function of total flow rate in CHJH2 plasmas. The CHJ
concentration, the RF power and the pressure were 22O/0. 150 W and 7 mTorr respectively.
Hayes et ai. 12.241 and Pearton et al. [z.za] both reported a nonlinear increase of etch rate
with increasing the total gas flow rate. By increasing the total gas flow rate, the nurnber
of CH3 and H radicals per unit time reaching the InP surface was increased. or in other
words, the residence time was shortened. The shorter the residence time, the faster the
gas phase near the InP surface can be refreshed by replacing etch products with new
etchants. However, if the total flow rate was increased continuously, sooner or laterthe
surface would be oversaturated by CH,, and the probability of forming an hydrocarbon
network on the surface was significantly enhanced. The other plasma factor associated
with the total flow rate is the average ion energy. Since the RF power applied to the
plasma was fixed. the average ion energy decreased with increasing the total flow rate.
The higher the ion energy, the more efficiently ions can assist etching via the cleaning
and damage mechanism. As a result, increasing the total gas flow rate showed both
positive and negative effect on the InP etch rate. In the low-flow rate region where the
gas phase was far from saturation of CH,, the residence time factor dominated and the
etch rate was found to increase with increasing total flow rate. In the high-flow rate
region where the gas phase was getting close to saturation with CH,, the average ion
energy factor began to dorninate and the trend reversed.
4.4.2 Etch Profile
InP etched mesas had similar sidewall profiles when the total flow rates were 3, 4, 7.5
and 8.5 sccm. The sidewall angles were approximately 75c (Figure 4.24). InP samples
etched with a total flow rate of 5 or 6.5 sccm had profiles of two-step sidewalls as in
high power condition described before, suggesting that the Mo-step sidewalls were
accompanied with high etch rates.
Figure 4.24 The sidewall profile of InP etched in the CH& plasma. The CH4 concentration, the
RF power, the total flow rate and the pressure were 22%, 150 W, 4 sccm and 7 mTorr
respectively.
4.4.3 Surface Morphology
Due to use of high RF power (150W), this whole set of InP samples appeared to be on
the rough side. From the discussion in the previous section. the surface roughness can
be improved if a lower power is utilized. Nevertheless. the trend of total flow rate versus
roughness in Figure 4.25 should be more or less similar. Based on AFM
characterization. the roughness was found to increase initially and then decrease by
increasing the total flow rate. The form of the curve is similar to the curve of the etch
rate in Figure 4.23.
Total Flow Rate (sccm)
Figure 4.25 Rms roughness of InP etched surface as a function of total flow rate in CH&
plasmas. The CH4 concentration, the RF power and the pressure were 22%. 100 W and 8 rnTorr
4.5 Influence of Pressure on Etching
4.5.1 Etch Rate
In addition to gas composition. RF power and total flow rate, the dependence of InP
etching with CH4/H2 plasmas on operating pressure in the range of 5-12 mTorr was
investigated as we!L From profilome!ric rneasurornon?. ?ho etch rate rias found to De
less sensitive to the etching pressure in this range for CH,JH2 plasmas. As shown in
Figure 4.26, only a slight increase in the etch rate was observed by raising the pressure
from 5 to 12 mTorr.
Pressure (mforr)
Figure 4.26 InP etch rate as a function of pressure in CHJ/H2 plasmas. The CHJ concentration.
the RF power and the total flow rate were 2296, 100 W and 7.5 sccm respectively.
Hayes et al. and Werking et ai. both found that the etch rate increased with increasing
pressure. However, the increasing slopes were very different. The etch rate reported by
Hayes increased only 20% when increasing the pressure 50 mTon to 125 mTorr (2.241,
but the etch rate reported by Werking increased almost 3 times when increasing the
pressure from 10 mTorr to 40 mTorr p2q. The opposite dependence was obtained by
Pearton et al. In their work, the etch rate decreased from 12.5 nm/min at 1 mTorr to 5.6
nmhin at 20 mTorr for RIE and from 18 nmfrnin at 1 mTorr to 10.2 nm/min for ECR
etc hing (2.281.
Low pressure means decreasing particles density, larger mean free path and fewer
collisions. The total number of gas phase species in the chamber was higher at the high
pressure condition. Accordingly the amount of CH3 and H available for chemical etching
can be assumed to be increased by elevating the pressure. However, the etch rate was
also govemed by ion bombardment. Since the mean free path is inversely proportional
to pressure, the time which ions can travel and be accelerated by the electromagnetic
field before colliding with other plasma particles is prolonged by decreasing the pressure
in the chamber, which rneans that ion energies are expected to be higher when the
pressure is lower. Judging from the trend in Figure 4.26. one can say that the etch rate
was controlled by radical flux and ion energy approximately equally over this pressure
range.
Compared to CH JH2 plasmas, the operating pressure altered the etch rate of InP much
more significantly in the case of HSN, plasmas, as seen in Figure 4.27. In these
experiments, the flow rates of H2 and N p were 4.50 and 3.00 sccm respectivety, and the
RF power applied to the inductive coi1 was 100 W. The results showed that the etch rate
decreased almost Iinearly with increasing pressure. When the pressure was as high as
10 mTorr, the etch rate of InP reduced to only 1.5 nm/min. As mentioned previously, In
removal is rate limiting in this process, and it is strictly dominated by sputtering
efficiency. By increasing the pressure, ion energies were lowered due to a shortening in
the mean free path, so it became more difficult to remove In from the surface.
Pressure (mTorr)
Figure 4.27 InP etch rate as a function of pressure in H$N2 plasmas. The N3 concentration, the
RF power and the total flow rate were 40°/', 1 00 W and 7.5 sccm respectively.
4.5.2 Etch Profile
The sidewall profile of the InP mesas etched with CH JH2 plasmas. which contained 20%
CH4, had a total flow rate of 6 sccm and were sustained by a RF power of 100 W was
not affected appreciably by the pressure in the range of 5-12 mTorr. The sloping angles
were within 84'-86". and barreling of sidewalls were seen on al1 sarnples (Figure 4.28).
With regards to etching wit h H,(6O0/~)/N2(4O0/~) plasmas. verticality of sidewalls was
optimized when the operating pressure is 7 mTorr (Figure 4.20). The dope decreased
with increasing or decreasing the pressure.
Figure 4.28 The sidewall profile of InP etched in the plasma. The CH4 concentration, the
RF power, the total flow rate and the pressure were 229'0, 100 W, 7.5 sccm and 8 mTorr
respectivety.
4.5.3 Surface Morphology and Elemental Analysis
The pressure in the range of 5-12 mTorr had quite an impact upon surface morphology
of InP samples treated with CHJHp plasmas although it seemed to have little effect on
their etch rate and etch profile. Dependence of nns roughness and Plln ratio on the
etching pressure is given in Figure 4.29. It shows that surface roughening and P-
depletion were improved by etching at a higher pressure. This was most likely due to a
lowering in ion energy when increasing the pressure in the chamber.
InP samples etched with H$N2 plasmas at 5,7 or 10 mTorr were exarnined by AFM and
XPS. Rms roughness and Plln ratio are show in Figure 4.30. Similar to the conclusion
from the study of etching dependence on RF power, roughening and P-depletion
seemed to increase with increase in sloping angle of etched sidewalls.
Pressure (rnTorr)
Figure 4.29 Rrns roughness and P/ln ratio of InP etched surface as a function of pressure in
CH3/H2 plasmas. The CH, concentration, the RF power and the total flow rate were 22%, 100 W
and 7.5 sccm respectively. represents roughness and O represents P/ln ratio.
Pressure (mTorr)
Figure 4.30 Rms roughness and Plln ratio of InP etched surface as a function of pressure in
HdN2 plasmas. The Np concentration, the RF power and the total flow rate were 40%. 100 W and
7.5 sccm respectively. represents roughness and O represents Plln ratio.
4.6 Mass Spectrometric Identification of Etch Products
Mass spectrometric identtication of the etch products was carried out by RGA. Mass
spectra were recorded from the CHs/H2 plasma (1.5 sccm of CH,, 6 sccm of Hz, 7 mTorr,
125 W) with (sçlid h e ) and witithoü: (clash line) InP in the iëactoi. The pressure in the
RGA chamber was kept at 2x10' Torr. Attention was paid to likely peaks listed in
Table 4.4,
Table 4.4 Likely peaks of etch products.
In Etch Products P Etch Products
ion mie ion rn/e
In' 115 P ' 31
(w343)' 130 PH' 32
Hln(CH3)' 131 PH; 33
I~"~(CH&' 143 PH; 34
I ~ ' " ( C H ~ ) ~ + 145 PHjc 35
((wH3)3+ 160 PCH2+ 45
PCHC 46
HPCH3' 47
H2PCH3' 48
P(C2H~)' 60
P(CH3)2+ 6 1
HP(CH3); 62
P(CH3); 76
4.6.2 Results and Discussion
Figure 4.31 shows the spectra in the m/e range of 31-35 obtained from the CHJHl
plasma with (solid line) and without (dash line) in the chamber. Data recorded without
InP in the chamber serves as the background level. Peaks appeared above background
at rn/e 31. 32, 33 and 34 corresponding to P', PH', PH;, PH; and PH, respectively. In
an attempt to identify In etch products, mass spectra were recorded in the m/e range of
1 12-1 17, 128-1 33, 142-1 47 and 157-1 62 to cover possible peaks. However, no
signals were detectabie above background.
One of the reasons that make detection of etch products difficult is their relatively low
concentration. The concentration of In or P etch products in the chamber during etching
was estimated as followed.
Figure 4.31 m/e 31-34 mass spectra of CHdH2 plasma with (solid line) and without (dash line)
InP in the reactor.
number of gas molecules (CH4+H2) entering the chamber per minute, N, :
N,RT ldeal Gas Equation pv = nRT = - No
P pressure (assurning 1 atm) V volume (7.5 cm3, since the total flow rate is 7.5 %cm) n grnole of CHd+Hâ R gas constant (0.08206 atm-UgmoteK) T temperature (assuming 273 K)
No Avogadro 's number (6.0221 69x1 0 9
therefore, PVN,
N, =- RT
nurnber of In or P produced per minute, NI, = N, = NlnP:
mN, VDN, dlnpADN, --= NlnP = - - M M M
where
therefore.
rn mass of InP etched by the plasma M molecutar weight of InP (1 45.794) V volume of InP etched by the plasma 0 density of InP (4.79 g/cm3}
d,,, etch depth (1 5 nm. since the etch rate is 15 nrnfminl A area of InP exposed to the plasma (0.35 cm2)
15 x 1 0 - ~ Y 0.35 x 4.79 x 6.02217 x loZ3 NlnP = = 1.03874 x 10li (min-')
145.794
If ignoring ionization of the plasma and various pumping speeds for different molecules
first, molecule fractions of In or P etch products in the chamber, C,, = C, = Cinp:
Since Cl, represents the total concentration of al1 In and In-containing compounds and
Cp represents the total concentration of al1 P and P-containing compounds, the
concentration of each peak. e.g. In(CH3); or PH;. should be lower than 0.06%. Also, if
ionization of the plasma were taken into account. the percentage is expected to drop
because the total nurnber of species in the plasma, ha, is higher than N,. Another
factor that should be considered is the various pumping speeds for different molecules.
The nature of the pump is faster removal of heavy species than Iight species. In
general, In and P etch products have larger molecular weights than CH JH2 plasma
particles, and therefore C,, and C, is supposed to be even lower.
So far, C,, and C, have been assumed equal, Le. Ci, = Cp = Clnp In fact, judging from
both the literature review and XPS surface elemental analysis performed in the study,
CHJE, plasmas appear to etch P faster than In. In addition, organo-indium has an
higher reaction rate than phosphine. At a pressure of 7 mTon. the mean free path is
approximately 7 mm, which is smaller than the distance between the InP surface and the
orifice. Therefore, organo-indium is likely to react with the plasma and be incorporated
into polymers on SiOa masks or chamber walls. The Auger electron spectra (AES) data
repotted by Hayes et al. p.281 indeed showed that In and lower level of P existed in the
polyrners on Si02 masks. Due to these two reasons. low etch rate and high reaction
rate, In etch products were not detected by RGA.
Given that RGA is able to record signals of P products well above background. it is
promising to apply this rnass spectrometric etch product identification technique in
endpoint detection when the process involves multilayer etching.
Chapter 5
Conclusions
An inductively coupled plasma system has been successfully constnicted for the
purpose of etching InP. Different chemistries including CHdH2, CHdHdAr, CHJHdN2
H$N2 and HdAr were used as plasma precursors. The influence of gas composition, RF
power, total flow rate and pressure on etch rate. etch profile and surface morphology
was studied.
In CH JH2 plasmas. etching was optimized at 22% CH,. When CH4 concentration
exceeded 25%. polymer began to be deposted on InP due to oversaturation of
hydrocarbons at the surface. A minimum RF power was required to initiate etching. but
the etched surface deteriorated severely when a power greater than 125 W was applied.
The optimum total flow rate for the reactor was found between 5-8 çccrn. Pressure
variation over the range of 5-12 mTorr did not affect etch rate or etch profile
substantially, but surface roughening and stoichiometry were improved by increasing the
pressure.
Addition of Ar to CHJ/H2 precursors was found to decrease etch rate and increase
surface roughness. The results suggest that the presence of Ar in plasmas may reduce
the degree of dissociation of CH4 and HL>, and shift the etching mechanisms closer to
physical etching from ion-assisted chernical etching. Cornpared with CH*/H2 plasmas,
CHJ/H2/N2 plasmas generated a smoother etched surface because N radicals improved
the imbalance between removal of In and P. However, etch rate and verticality were
decreased by addition of N2.
HdN, plasmas were used to etch InP for the first time. They have advantages such as :
1 ) easy to handle due to the non-toxic, non-corrosive, nonflammable nature
2) economical in terms of the cost of feedstock
3) require less maintenance for the vacuum chamber (chlorine-containing
plasmas will corrode the charnber. and alkane-containing plasmas will
deposit polymer on the chamber wall)
The etched surface was found to be very smooth (mis roughness c 0.3 nm) and
stoichiometric. Although sidewall profiles appear sloping, it is believed that vertica1.W
can be improved by replacing the masking materials or applying substrate bias.
PH, was detected during mass spectrometric analysis of plasmas by RGA. It shows the
promising application of RGA in endpoint detection for etching of heterostructures.
Below are some suggestions for future work regarding this study:
improving the ICP reactor by applying additional RF bias and water cooling to
the substrate holder
changing the masking material to Si3N,, and using plasma etching for pattern
transfer from photoresist to Si3N4 so that Si3N4 can have an anisotropic profile
characterizing electrical and optical damages on plasma-etched InP surfaces
measuring plasma properties of the ICP reactor by means of optical emission
spectrometry and electBc probe measurements
extending approach to GaAs
Appendix
A. Appearanœ Potential Mass Spectrometry
Since neutral CH3 radicals have to be ionized fint in order to be detected by the mass
spectrometer, the main problem cornes in distinguishing CH,' ions generated by direct
ionization of CH3 radicals (CH3 + CH3+ ; 9.8 eV) from those generated by dissociative
ionization of CH4 molecules (CH, + CH3' ; 14.3 eV). It should be noted that the
threshcfd cf dired ionization Gr the ionkation poteztial Is 4.5 eV lower :han the thieshdd
of dissociative ionization or the appearance potential. When the OMS operates at
electron energies between the two thresholds, only direct ionization of CH3 will take
place. By taking advantage of this energy difference, it is possible to separate the two
ionization processes. This method is referred to as appearance potential mass
spectrometry (APMS) or threshold ionization mass spectrometry (TIMS). Other than
coming from the plasma. CH3 radicals also arise from pyrolysis of molecules on the hot
filament of the ionizer and from background due to impurities in the OMS. This explains
why CH,' signals appear in "plasma-off'' spectra even when the electron energy is below
appParance potential. The net CH3 density in the plasma can be derived from the
preliminary data through a certain mathematical procedure. Figure A.l shows the typical
QMS output signal 1, vs. Electron beam energy Eb for both "plasma-off" and "plasma-onw
conditions. The hump visible in the chart is attributed to the electron impact ionization of
CH3 radicals originating from the plasma. Since the QMS output signal is proportional to
CH3 density n and the ionization cross section a(Eb), it can be expressed as
1, = ACT(Eb)lbn
where lb is the electron beam cunent. The proportionality constant A depends on the
sensibility of the QMS and the vacuum conductance of the orifice between the bulk
plasma region and the QMS. The product of A and lb can be denved from the "plasma-
of f signals where n is replaced by N (CH, molecular density) and a(Eb) by the
dissociative ionization cross section [AI]. Plasma-off 1, is replotted in a linear scale for
Ep14 eV. The best fitting for the known energy dependence of the dissociative cross
section yields the product Ad,. Plasma-on 1, is replotted on a Iinear scale for Ebc14 eV
and are best fit by using this value of A-lb and the partial ionization cross section for
CD3 + CD; [~.2]. It is assumed that the ionization cross section of CH3 is almost the
same as C 4 and that the radical temperature is equal to the CH4 temperature (-400K).
This procedure enables the evaluation of CH3 density n from the QMS output signal 1,.
Figure A.1 CH3' count rates as a function of E, : (a) plasma-on and (b) plasma-off p.601.
B. Reactions in Methane Plasmas
Reactions in methane plasmas are divided into four groups: electron impact reactions.
neutral-neutral reactions, ion-molecule reactions, and surface reactions between
plasmas and walls of reactors [B.I -B.~] .
Table B.l Electron impact reactions - . .. -.
Reaction Reaction
Table B.2 Neutral-neutral reactions
Reaction Reaction
CH + H2 + CH3
C2H6 + CH3 -+ CH4 + CzHs
CnHs + H + C2H5 + H2
C2H6' -+ CH3 + CH3
C2H6' 4 C2H4 + H2
C2H5 + H 4 CH3 + CH3
C2H5 + H + C2H4 + H2
C2H5' -I C2Hj + H
CzHj +H 4 C2H5
C2H4 +H 4 C2H3 + H2
C2H4 +CH3 4 C2H3 + CH.:
C2H4' 3 C2H2 + H2
C2H2 +C2H2 + C4H3 + H
C2H3 + H + C2H2 + H2
C2H2 + H - C2H3
C2H2 -t H + C2H + H2
H + H + H 2
Table 8.3 Ion-rnolecule reactions
Reaction Reaction
CH5' + C2Hb 3 C2HS+ + H2 + CH4 C2HL + CH4 + CZH3+ + CH3
CH4' + CH4 + CHS' + CH3 C2H2+ + CH4 -+ C3H; + Hz
C H l + H 2 + C H g + H C2H2+ + CH4 -+ C3H5+ + H
CH3' + CH* + C2HS+ + Hz C2H3' + CH4 + C3Hs' + Hz
CH3+ + CH4 + CHd' + CH3 C2H3+ + C2Hs + C2Hc + GH:,
CH; + CH4 + CH3' + CH3 C2H3' + C2H2 + COHg
CH; + CH4 -+ C2H2* + 2H2 C2H; + C2Hd + C3H; + CH3
CH2' + H2 3 CH3+ + H
CH2' + CH4 + C2Hac + H2
CH2+ + CH4 + C2H5+ + H
CH; + CH4 + C 2 H j + H + H2
CH' + CH4 + C2H2+ + H2 + H
CH' + CH4 -+ C2H3+ + H2
CH' + CH4 + C2H4+ + H
CH' + Hz 4 CH2+ + H
C' + CH4 + C2H2' + H2
C' + CH4 4 C2H< + H
Table 8.4 Surface reactions between plasma particles and chamber walls
Reaction Reaction
The notation (s) indicated an adsorbed species
C. RF Electronics : lmpedance Matching [c.q
Impedance, in an AC circuit. plays a rote similar to that of resistance in a DC circuit. The
irnpedance Z of the circuit is defined as the ratio of the phasor representing the
sinusoidal voltage across it to the phasor representing the sinusoidal current flowing
through it. The real and imaginary parts of impedance are called resistance R and
reactance X, respectively. lnductor and capacitor are basic elements in an AC circuit
along with resistor. The voltage across an inductor is L dlldt, where L is the inductance
of the inductor. If the current flowing through is l,eJ"' , the corresponding voltage is
(jd)l,eid . The impedance of this inductor is therefore joL. The current into a capacitor
is C M d , where C is capacitance of the capacitor. If the voltage across a capacitor is
v,ei". the current is (j&)~,e~"<. The impedance of this capacitor is then l/joC. The
total impedance for elements in series is the sum of their separate impedances. The
inverse of the total impedance for elements in parallel is the sum of the inverse of
individual impedance. Matching normally means using a non-resistive network between
a RF (or any other AC) source and a load in order to maximize the power transferred to
the load. ln DC circuit shown in Figure C. 1 (a), maximum power is transferred when the
load resistance is equal to the source resistance. It is verified as follows.
v Current passing through the load is IL =-
Rs + R L
Voltage across the load is
Power of the load is
PL reaches maximum when dP, R: -R: -= VZ = O , i.e. when R, = 4 dRL (RS +RJ4
Figure C.l (b) shows the idealized AC circuit of the power generator, matching network
and the coi1 used in this study. The function of the matching network here is to offset the
reactance of the load and equalize the resistance of the source and the load by adjusting
the value of two parallel capacitors.
- - - - - - - - - - - - - - - - - - - -
SOURCE LOAD SOURCE MATCHING LO A O
Figure C.l Representation of circuit matching in (a) DC circuit, (b) AC circuit.
D. Repeatability and Significance of Error Bars
Error bars in the etch rate and sidewall angle figures represent within-sample error
rather than sample-to-sarnple error. Only a fraction of etching experiments were
repeated because the entire process (cleaning, patterning, etching and charaderization)
is very time-consuming and InP wafers are expensive. For those etching conditions
which were repeated, the difference in etch rate resulted from the identical etching
parameters is generally less than 20%. As mentioned previously. etch depth was
measured by Alpha-Step at a minimum of five points for each sample within an area of
4x4 mm2 in the center, and the average etch rate and the enor bars were obtained from
those measurements. Therefore, error bars in the etch rate graphs represent uniformity
within samples. During cross-sectional examination of SEM. more than one etch profile
(etched step) were recorded, and the average sidewall angle and the error bars were
determined according to the method introduced in Section 3.2.2. No error bar was
shown in the mis roughness figures since the difference in mis roughness was smaller
than the diameter of the black dots in the figures. The reason for such small difference
in roughness within samples is the relatively large AFM scan area (10x10 Rms
roughness obtained by AFM depends on scan area significantly. For the same sample,
if a large area was scanneci. the rms roughness would usually be higher but the variance
within samples would be less compared to a small scan area. Therefore, al1 the rms
roughness appeared in this thesis was resulted by scanning a same area.
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