Chen F.F., Chang J.P. Lecture notes on Principles of plasma processing (Kluwer, 2002)(249s)_PPl_.pdf

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Lecture Notes on

PRINCIPLES OF

PLASMA PROCESSING

Francis F. Chen

Electrical Engineering Department

Jane P. Chang

Chemical Engineering Department

University of California, Los Angeles

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Plenum/Kluwer Publishers

2002

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Preface

v

P RINCIPLES OF P LASMA P ROCESSING

PREFACE

We want to make clear at the outset what this book

is NOT. It is not a polished, comprehensive textbook on

plasma processing, such as that by Lieberman and

Lichtenberg. Rather, it is an informal set of lecture notes

written for a nine-week course offered every two years at

UCLA. It is intended for seniors and graduate students,

especially chemical engineers, who have had no previous

exposure to plasma physics. A broad range of topics is

covered, but only a few can be discussed in enough depth

to give students a glimpse of forefront research. Since

plasmas seem strange to most chemical engineers,

plasma concepts are introduced as painlessly as possible.

Detailed proofs are omitted, and only the essential ele-

ments of plasma physics are given. One of these is the

concept of sheaths and quasineutrality. Sheaths are

dominant in plasma Ðreactors,Ñ and it is important to de-

velop a physical feel for their behavior.

Good textbooks do exist. Two of these, to which

we make page references in these notes for those who

want to dig deeper, are the following:

M.A. Lieberman and A.J. Lichtenberg, Principles of Plasma Dis-

charges and Materials Processing (John Wiley, New York,

1994).

F.F. Chen, Introduction to Plasma Physics and Controlled Fusion ,

Vol. 1, 2 nd ed. (Plenum Press, 1984).

In addition, more topics and more detail are available in

unpublished notes from short courses offered by the

American Vacuum Society or the Symposium on Plasma

and Process Induced Damage. Lecture notes by such

specialists as Prof. H.H. Sawin of M.I.T. are more com-

prehensive. Our aim here is to be comprehensible..

The lectures on plasma physics (Part A) and on

plasma chemistry (Part B) are interleaved in class meet-

ings but for convenience are printed consecutively here,

since they were written by different authors. We have

tried to keep the notation the same, though physicists and

chemists do tend to express the same formula in different

ways. There are no doubt a few mistakes; after all, these

are just notes. As for the diagrams, we have given the

source wherever possible. Some have been handed down

from antiquity. If any of these are yours, please let us

know, and we will be glad to give due credit. The dia-

grams are rather small in printed form. The CD which

Reference books used in this course

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vi

accompanies the text has color figures that can be ex-

panded for viewing on a computer monitor. There are

also sample homework problems and exam questions

there.

Why study plasma processing? Because we canÓt

get along without computer chips and mobile phones

these days. About half the steps in making a semicon-

ductor circuit require a plasma, and plasma machines ac-

count for most of the equipment cost in a Ðfab.Ñ Design-

ers, engineers, and technicians need to know how a

plasma behaves. These machines have to be absolutely

reliable, because many millions of transistors have to be

etched properly on each chip. It is amazing that this can

be done at all; improvements will certainly require more

plasma expertise. High-temperature plasmas have been

studied for decades in connection with controlled fusion;

that is, the production of electric power by creating

miniature suns on the earth. The low-temperature plas-

mas used in manufacturing are more complicated be-

cause they are not fully ionized; there are neutral atoms

and many collisions. For many years, plasma sources

were developed by trial and error, there being little un-

derstanding of how these devices worked. With the vast

store of knowledge built up by the fusion effort, the

situation is changing. Partially ionized, radiofrequency

plasmas are being better understood, particularly with the

use of computer simulation. Low-temperature plasma

physics is becoming a real science. This is the new

frontier. We hope you will join in the exploration of it.

A small section of a memory chip.

Francis F. Chen

Jane P. Chang

Los Angeles, 2002

Straight holes like these can be etched

only with plasmas

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Table of Contents

i

TABLE OF CONTENTS

P REFACE v

Plasma Physics

Plasma Chemistry

P ART B1: O VERVIEW OF P LASMA P ROCESSING

IN M ICROELECTRONICS F ABRICATION

P ART Al: I NTRODUCTION TO P LASMA S CIENCE

I. What is a plasma? 1

II. Plasma fundamentals 3

1. Quasineutrality and Debye length

2. Plasma frequency and acoustic velocity

3. Larmor radius and cyclotron frequency

4. E × B drift

5. Sheaths and presheaths

I. Plasma processing 99

II. Applications in Microelectronics 100

P ART B2: K INETIC T HEORY AND C OLLISIONS

I. Kinetic theory 103

II. Practical gas kinetic models and

macroscopic properties 109

1. Maxwell-Boltzmann distribution (MBD)

2. A simplified gas model (SGM)

3. Energy content

4. Collision rate between molecules

5. Mean free path

6. Flux of gas particles on a surface

7. Gas pressure

8. Transport properties

9. Gas flow

III. Collision dynamics 119

1. Collision cross sections

2. Energy transfer

3. Inelastic collisions

P ART A2: I NTRODUCTION TO G AS D ISCHARGES

III. Gas discharge fundamentals 11

1. Collision cross section and mean free

path

2. Ionization and excitation cross sections

3. Coulomb collisions; resistivity

4. Transition between neutral- and ion-

dominated electron collisions

5. Mobility, diffusion, ambipolar diffusion

6. Magnetic field effects; magnetic buckets

Cross section data 21

P ART A3: P LASMA S OURCES I

IV. Introduction to plasma sources 25

1. Desirable characteristics of plasma

processing sources

2. Elements of a plasma source

P ART B3: A TOMIC C OLLISIONS AND S PECTRA

I. Atomic energy levels 125

II. Atomic collisions 126

1. Excitation processes

2. Relaxation and recombination processes

III. Elastic collisions 129

1. Coulomb collisions

2. Polarization scattering

IV. Inelastic collisions 130

1. Constraints on electronic transitions

2. Identification of atomic spectra

3. A simplified model

P ART A4: P LASMA S OURCES II

V. RIE discharges 31

1. Debye sheath

2. Child-Langmuir sheath

3. Applying a DC bias

4. Applying an RF bias

5. Displacement current

6. Ion dynamics in the sheath

7. Other effects in RIE reactors

8. Disadvantages of RIE reactors

9. Modified RIE devices

ii

Table of Contents

P ART A5: P LASMA S OURCES III

VI. ECR sources 47

VII. Inductively coupled plasmas (ICPs) 49

1. Overview of ICPs

2. Normal skin depth

3. Anomalous skin depth

4. Ionization energy

5. Transformer coupled plasmas (TCPs)

6. Matching circuits

7. Electrostatic chucks (ESCs)

P ART B4: M OLECULAR C OLLISIONS AND

S PECTRA

I. Molecular energy levels 137

1. Electronic energy level

2. Vibrational energy level

3. Rotational energy level

II. Selection rule for optical emission of

molecules 139

III. Electron collisions with molecules 140

1. Frank-Condon principle

2. Dissociation

3. Dissociative ionization

4. Dissociative recombination

5. Dissociative electron attachment

6. Electron impact detachment

7. Vibrational and rotational excitation

IV. Heavy particle collisions 142

V. Gas phase kinetics 143

P ART A6: P LASMA S OURCES IV

VIII. Helicon wave sources and HDPs 61

1. Dispersion relation

2. Wave patterns and antennas

3. Mode jumping

4. Modified skin depth

5. Trivelpiece-Gould modes

6. Examples of helicon measurements

7. Commercial helicon sources

P ART B5: P LASMA D IAGNOSTICS

I. Optical emission spectroscopy 151

1. Optical emission

2. Spectroscopy

3. Actinometry

4. Advantages/disadvantages

5. Application: end-point detection

II. Laser induced fluorescence 161

III. Laser interferometry 162

IV. Full-wafer interferometry 163

V. Mass spectrometry 164

IX. Discharge equilibrium 69

1. Particle balance

2. Energy balance

3. Electron temperature

4. Ion temperature

P ART A7: P LASMA D IAGNOSTICS

X. Introduction 75

XI. Remote diagnostics 75

1. Optical spectroscopy

2. Microwave interferometry

3. Laser Induced Fluorescence (LIF)

XII. Langmuir probes 79

1. Construction and circuit

2. The electron characteristic

3. Electron saturation

4. Space potential

5. Ion saturation current 83

6. Distribution functions 90

7. RF compensation

8. Double probes and hot probes

P ART B6: P LASMA S URFACE K INETICS

I. Plasma chemistry 167

II. Surface reactions 167

1. Spontaneous surface etching

2. Spontaneous deposition

3. Ion sputtering kinetics

4. Ion-enhanced chemical etching

III. Loading 177

IV. Selectivity 178

V. Detailed reaction modeling 179

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