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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