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PREFERENTIAL SPUTTERING,

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PREFERENTIAL SPUTTERING EFFECTS

IN Nb~05."

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By.

OASARAO I{RISHNA MURTI, M. Se.

...

A *1'heiUs

Submitted to the SO~~:~ of Grad~ate~u~ies" ,

in Partial Fulfilment of tn~ Requirem~ts

.for the Degree

Dopto~ of'Phi~osophy

..

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DOCTOR OF PHILOSOPHY (1975)(Materials Science)

McMASTER UNIVERSItyHamilton, Ontario

I I,

TITLE: Preferential Sputtering Effects in Nb20S

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Ion bombardment has been used as a technique for the sput-

tering of metals for several- years1 very few studies have, how­

ever, been made on alloys and ~ompounds, especially studies where

~he target itself was analyzed after 'ion bombardment. In this

" ABSTRACT

...

.thesis results are presented concerning the c~n~es in targets

of Nb20 S and related systems due to -ion bombardment. The targets

used were in the form of'anodic thin films and sintered pellets.. + +Bombardments were carried out using Kr and. 02 ions with energies

of 2 to 3S keV and at 4o~es v~rying from 1013 'to 1017 ions/cm2•

The structural changes induced by the bombardment were studied .

o with an electron microscope and the cha~ges in electrical conduc-

tivity were stuftied with a four point probe.

The results indicate that an altered 'layer is formed which.

has the stoichiometry ~f NbO when Nb20Starg~ts are bombarded

'with 35 ke; ions ~t_ high doses (~ 1 x 1017 ions/cm2>. A technique

based on the changes in the chemical reactivity of the altered~ .

lay~r as compar~d to the substrate is used to estimate' the thickness. ,

of the a~tered lay~r. That on Nb20S varies.from 18 nm at very

low doses (SX1013 io~s/cm2) t9 23 nm at high doses (lxl017 ions/cm2).

The con4uctivity of the altered ~a~er formed at high doses is about

4xio~ slm, ~n i~o~ease QY a 'factor-of iX108, An energy threshold, "

for oonductivity changes was found using an oxygen :Lon beam,.. ., ~ r ...

whereas 10: ion bombardment reveals conductivity changes at all,y ... .

Oxygen loss from bombarded Nb20S can be considered to• f

have threQ alternative origins based on a vaporiaation model, an~ ~

internal displacement model and a preferential sputtering model.,

The results favour the preferential sputtering of oxygen as the

likely mechanism to explain the altered layer in the ta~get. A

modified'sputtering mode~ explains the energy thr~Bhold effect

f'fI.'

observed when a~ oxygen beam is used. \

Several applications of the techniques and results pre-

sented here have been considered. Th~y include, among ~thers,

. _ t-~ating of minerals, limitations of surface analytical techniques,

and preliminary results for Ta~OS'

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ACI<NOt1LEDG~1ENTS

I am indebted to Dr. R. Kelly for his continuous

gUidance, encouragement and friendship throughout the course

of this work. I wish to express my sincere gratitude to the

members of my supervisery committee, Drs. J. D. Embury,

C.J.L. tock and Professor C. M. Sargent for their interest

and encouragement.

Thanks are due to Dr •• P. Sig,mund (U~iversity of

Copenhagen) for supplying Fig. 2-3 and Dr. 'K. B. Winterbon (AECL!.Chalk River) for supplying the data on range Prrameters in

Nb20S (Table II-I). The technical a~sistance of F. Smith and.D. Hodgson is greatly appreoiated. I wish to thank Mrs~ H.

Rennelly for her skill and patience in typing this thesis.

This work was supported by grants from 'the Defence

Research Board ~f Canada, the National Research Council of

Can.ada, and the Geological Survey of Canada- .to Dr. R. Kelly.

I thank the National ~esearch Council of Canada for the award

of ~ P.I.E.R. fellowship and McMaster University for the

award.of a teaching assistantship.

Finally; I am deeply grateful to my.wife, Brinda, for\

her patience and constant suppor~ throughout the course of

'-this work.

TABLE OF CONTENTS

CHAPTER 1 INTRODUCTION 1

1.1 PARTICLE SOLID INT~RACTIONS 1

1.2 APPLICATIONS OF SPUTTERING 4"

1.3 MODIFICATIONS OF THE SPUTTERED SURFACE 8

1.4 WHY STUDY Nb20S? 12....

CHAPTER 2 A REVIEW OF iON IMPACT THEORY 15

2.1 INTRODUCTION 15

2.2 ENERGY LOSS MECHANISMS 18

2.3 LSS THEORY.

2..4 APPLICATION OF LSS THEORY

2.5 WSS THEORYCo

2.6 RANGE AND D~GE PROFILES

20

24

26

29

2. 7~' fl'HEQRY OF ROt, FLU IT AND KISTEMAKER 30

2.8 SIGMUND'S THEORY OF SPUTTERING 31

2.9 ENERGY DISSIPATION, IN COMPOUND TARGETS 36

CHAPTER 3 SXPERlMENTAL TECHNIQUES . 41•

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.3.1 '. SAMPLE PREP-MAT.ION

3.1.1 Anodic Thin Films of Nb20S', .

3.1.2 Sintered Pellets

3.2 ION, BOMBARD~T

(3.3 ELECTRICAL CONDUCTIVITY MEASUREl<1ENTS .

41

41

42

43

46-

46

47

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3.4 ELECTRON MICROSCOPY

3.4.1 Introduction

3.4.2 Transmission Mode

3.4.3 Reflection Mode

Page

57

'57

58

·61

3.4.4 ~nalysis of Electron Diffraction Patterns 63

CHAPTER 4 EXPERIMENTAL RESULTS: STRUCTURAL CHANGES 67

4.1 INTRODUCTION 67

4.2 STRUCTURE OF AMORPHOUS MATERIALS 68

4.3 THIN FILMS OF Nb20S 70

4.j.l Structure of as Prepared Films 70

4.3.2 Effect of Ion Dose

4.4

4.5

CHAPTER 5

5.1

5.2

-5.3

5.4

5.5

CHAPTER 6

6.1

6.2

SINTERED Nb20S PELLETS

SUMMARY

EXPER!MENTAL RESULTS: CONDUCTIVITY'CHANGES

EFFECT OF' ION DOSEI

EFFECT OF ION ENERGY

ANNEALING OF CONDUCTIVITY

EFFECT OF ION CURRENT

SUMMARY AND CONCLUSIONS

gxpERIMEN'I'AL RESULTS: 'l'HICKNESS OFAL'l'EREO LAYER

INTRODUCTION

THICKNESS OF AL'l'E~ LAYER 1\'1' LOW TOMED;IUM DOSES

73

77

18

82

82

84

86

88

91

93

t-, \IIi I

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Page.i

6.4 IMPLICATIONS OF THICKNESS OF NbO LAYER 10~

6.S SUMMARY AND CONCLUSIONS 103

CHAPTER 7 DISCUSSION OF EXPERIMENTAL RESULTS 104

7.1 INTRODUCTION 104

7.2 MECHANISMS FOR OXYGEN LOSS lOS

7.2.1 Vaporization Model 105,7.2.2 Internal Displacement Model 105

7.2.3 Preferential Sputtering Model 107

7.3 SPUTTERING MODEL APPLIED TO AN OXYGEN 110BEAM

7.4 PREFERENTIAL SPUTTERING COMBINED WITH 117DIFFUSION

-----..7.4.1 Diffusion Model 117

7.4.2 Thickness 'of Altered Layer 122

7.5 SUMMARY 122

CHAPTER 8 APPLICATIONS 124

8.1 CHANGES IN AND CONTROL OF CONDUCTIVITY 124

8.2 PREPARATIQN OF LOwER VALENCE OXIDES 124

8.3 REDUCTION OF OXIDES - MICROWAVE DISCHARGE 125

8.4 MINERALS ON THE MOON AND IN THE EARTH! S 126INTERIOR

. 8.5 DATING OF MINERALS 127r'r "

8.6 PREFE~NTIAL SPUTTEtdNG EFFECTS IN SUR- 131'. FACE ANALYSIS

. •It 8.7 PREFERENTIAL SPUTTERING OF CERMET FILMS 135

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8.8 ION BOMBARDMENT EFFECTS WITH Ta20S

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135

8.8.2

8.8.3

8.8,.4

CHAPTER 9

APPENDIX I

APPENDIX II

REFERENCES

Introduction ..Experimental Results - Conductivity 136Changes

Experimental Results - Structural 136Changes

Conclusions 138

SUMMARY 141

RANGE PARAMETERS FOR 35 keV 8SKr IONS 144

INCIDENT ON NbC....

RANGE PARAMETERS FOR + IONS INCIDENT 148keV 02ON Nb20 S

r 149

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Images for the case of the four point probeson a thin slice with a non conductingsubs'trate

Ray diagram showing the bright field modeof operation

FIGURENO.

1-1

1-2

1-3 .

1";'4

2-1 (a)

(b)

2-3

3-1

3-2

3-3

3-4

3-5

3-6

LIST OF ILLUSTRATIONS

TITLE

The simple focusing process

A focused collision sequence

Spatial distributton of the sputteringprocess

Distribution of sputtering yield

Definition of ion range

Ion range distribution

Theoretical nuclear and electronicstopping power curves

Displacement efficiencies for the systemNb-O

Radio frequency ion accelerator

Rectangularly shaped section of thin filmfor conductivity measurement

Schematic of conductivity measurementtechnique

Schematic of four point probe

PAGE

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5

10

10

16

.16

23

39

44

48

49

<,.. 51

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59

I1

3-7

4-1

4-2

, W'

Ray diagram showing the diffraction 'mode, of operation

Transmission electron microscopy of asprepared anodic Nb2~S films

Transmission electron microscopy of Nb20~

films bombarded to a dose of.SX1015~ ion$/cm2

60

72

FIGURENO.

4-3

4-4

TITLE " PAGE '

Transmission electron microscopy of Nb20S·; ~ 75films bombarded to a dose of 5xl016 ions/cm2

Reflection electron diffraction patterns of 79sintered Nb20S pellets before 'and afterbombardment

sheet conduc-•

sheet conductivity

5-1

.f~5-2

5-3

5-4

6-1

Effect of ion dose on the sheet conductivityof Nb20

Spellets

Effect of ion energy on the sheet conduc­tivity of Nb20S pellets

Effect of annealings on theof bombarded Nb20S pellets

Effect of ion current on thetivity of Nb205 pellets

Activity versus time of exposure to fluoboricacid saturated with ammonium chloridg forNb2?5 pellets bombarded with 35 keV 5Kr tovar~Ous doses

83

90

95

6-2 Activity versus time of exposure to a solution 96consisting of HF, NH03 and H20 for Nb205pellets bombarded with 35 keV 85Kr tovarious doses

6-3

6-4

7-1

7-2

7-3

Igtegral concentration profiles for 35 keV8 Kr ions incident on a) Nb20S and b) NhO

Dissolution of altered layers formed onN~205 ~ellets bombarded with 35 keV Kr+ and02 :Lons ,

+Dissolution curve for 5 keV 02 ion bombardedI with Nb20S pellets

Integral concentration profile for 10 keV8SKr ions incidene on Nb20S

Effect of ion energy on ~puttering yield

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98

101 '

113

114

116

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FIGURENO.

8-1

8-2

8-3

8-4

8-S

8-6

TITLE

Reflection electro~ diffraction patternsfor columbite - before and after bombardment

Dissolution ~urves for columbite bombardedwith 3S keV SKr ions to various doses

Q

X-ray photoelectron spectra of Mo03 exposedto Ar+ and O~ ions

Dependence of the photon yield from Al ~nthe oxygen pressure

Effect of ion dose on the s~eet conductivityof bombaraed Ta20S pellets

Transmission electron microscopy of anodicTa20S films/-before and after bombardment

PAGE

129

130

132

134

137 .

139

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TABLENO.

1-1

2-1

3-1

3-2

LIST OF TABLES

Examples of bombardment induced stoichio­metry changes

Displacement efficiencies for the systemNb-o

Conductivity measuremen~ correction factOrfor finite thickness

Conductivity m~asurement - correction forfinite thickness

PAGE

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40

53

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3-3 Depth of penetration of'electrons. Reflection 64electron diffraction

4-1 Diffraction analysis of N020S films following 76bombardment with Sxl016 ions/cm2

4-2 Diffraction analysis ~; N020S p~llets following 80bombardment with 2xlO ionslem

6-1

6-2

7-1

7-'2

11-1

Altered layer thicknesses for 35 keV Kr bom­bardment of N~OS pellets

Comparison of Ra/<x> for various materials~

Heats of atomization appropriate t~ the sysystem Nb-O

Oxygen ion bombardment of Nb20S - criticalbombardment energies to cause reduction

Range parameters for oi ions incident onNb20S

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97

99

1,08

115

148

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CHAPT~ 1

INTRODUCTION

1.1 PARTICLE SOLID INTERACTIONS

When a solid is ~ombarded with heavy ions, many phenomena

can arise depending largely on the kinetic energy of the incident

ions. At very low kinetic 'energies (~5 eV) the interaction is

essentially ~onfined to the outermost atomic layer of the target

material. The incident ion may be reflected or may come into.~ .

thermal equilibrium with the surface and subsequently be adsorbed.or evaporated. The potential energy of the bombarding species .

plays an important role at these energies since it is responsible~

for electronic transitions which ca~ give rise to the ejection

of secondary electrons. In the case of compound m~terials, low

energy ion bombardment can lead to the breaking or rearranging of

chemical bonds. This leads to desorption, chemical reactions,

and polymerization, with similar effects being induced also b~

bombarding with electrons or photons.

If the in:ident ions hav~a higher energy ,

theyenter'atarget and gradually lose their energy through a

series of collisions with the target atoms. Whenever the energy ~

transferred to the target atoms exceeds a threshold energy of rough-I

1y 4 AHs ·(where AHs is the he~t of s~blimation of" the t~rget

material), the atoms involved in the collision are removed ~

SUfficient distance from their originaf sit~s that they remain