THE SPUTTERING OF NON-METALS UNDER SLOW MULTIPLY CHARGED IONS

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THE SPUTTERING OF NON-METALS UNDER SLOW MULTIPLY CHARGED IONS

I. Bitensky, E. Parilis

To cite this version:

I. Bitensky, E. Parilis. THE SPUTTERING OF NON-METALS UNDER SLOW MULTI- PLY CHARGED IONS. Journal de Physique Colloques, 1989, 50 (C2), pp.C2-227-C2-230.

�10.1051/jphyscol:1989236�. �jpa-00229434�

THE SPUTTERING OF NON-METALS UNDER SLOW MULTIPLY CHARGED IONS

I . S . B I T E N S K Y and E . S . P A R I L I S

Arifov Institute of Electronics, Tashkent 700143, U.S.S.R.

~6sumQ

-

^I1 s t a g i t de l'analyse th60rique de l a p u l v & r i s a t i o n des SW

faces non-mQtallique dfi "ltexplosion Coulomb" sous l e bombardement avec l e s i o n s l e n t multiple chargke. I1 a Qt& montri. que l ' e f f e t e s t sensible l t 6 n e r g i e des i o n s , l a temperature e t l a conductivit6 d t Qchantillon, diminuand avec l e u r augmentation.

Abstract

-

The t h e o r e t i c a l a n a l y s i s of s p u t t e r i n g of non-metals under slow multiply charged i o n s v i a "Coulomb explosion" i s c a r r i e d out. It was shown t h a t t h e e f f e c t i s s e n s i t i v e t o i o n energy, temperature and conductivity of a s o l i d and decreases with t h e i r increase.

l

-

INTRODUCTION

The i n t e r a c t i o n of multiply charged i o n s with a s o l i d surface i s of i n t e r e s t both f o r atomic and plasma physica.

I n ref./?/ it has been shown that the i o n emission and sputtering o f non-me- t a l s under slow multiply charged i o n s should s i g n i f i c a n t l y exceed t h e corre- sponding e f f e c t s f o r s i n g l e charged i o n s due t o "Coulomb explosion" of a po- s i t i v e l y charged microscopic domain which r e s u l t s from step-by-step Auger n e u t r a l i z a t i o n of t h e ion/2/and e j e c t i o n of several Auger electrons from l a t - t i c e atoms. Indeed, i n t h e experiments /3-6/ it has been found t h a t t h e se- condary i o n emission from s i l i c o n a s opposed t o a metal increases with t h e ion charge increase /3/.

I n ref./7/ on t h e b a s i s of "Coulomb explosion" mechanism t h e s p u t t e r i n g of a non-metal under slow multiply charged i o n s has been calculated and t h e resu- l t s have been compared with t h e a v a i l a b l e experiments

/5/

by t h a t time.

I n some recent experiments /8,9/ t h e increase of t h e secondary i o n emission with t h e i o n charge increase has been confirmed. A s f o r She t o t a l s p u t t e r i n g y i e l d t h e experimental data were contradictory. I n ref./9/ t h e s p u t t e r i n g of S i under A r z + (z = l

-

^{9 )} with t h e energy E = 20 keV was independent on ion charge z but i n r e f . / l 0 / t h e sputtering f

09

d i f f e r e n t non-metals ( s i l i - con, g l a s s ) under A r with E = 10 keV exceeded %he s p u t t e r i n g under ArO atoms with t h e same energy by a f a c t o r 1.5

-

2.6. A t the same t i m e t h e sput- t e r i n g y i e l d f o r Au d i d not depend on i o n charge

.

2

-

SPUTTEFUNG BY "COULOMB EXPLOSION"

Under low energy bombardment the n e u t r a l i z a t i o n of a multiply charged ion occurs near t h e surface and %he formed charged domain i s a semisphere with radius R /7/. It could be calculated from t h e energy balance equation. The t o t a l n e 8 t r a l i z a t i o n energy W of z-charged p r o j e c t i l e i s shared between t h e Coulomb repulsion energy W" and t h e k i n e t i c energy of (N -2) Auger elec- t r o n s E ₍ N being t h e nmberCof i o n s appeared a t Auger neu8ralization of 2-charg&d ioa). I n ref./2/ it has been assumed t h a t N c zz.

The nmber of i o n s appearing per u n i t time i n t h e charged domain i s NZ /C where

T.

i s t h e e f f e c t i v e time of . t o t a l Auger n e u t r a l i z a t i o n of z-charge&

ion. ~ i m b l t a n e o u s l ~ , during t h e time 'iT they a r e n e u t r a l i z e d by t h e conduc- t i n g electrons. For an i n s u l a t o r

T

>?ei and f o r a metal T <<'Ci. A s a re-

sult t h e charged domain t o t h e mome%t

Ti

c o n s i s t s of N ions?

N

=

NZ;(

Ze/Ti>

[I -

^{exp (}

^{-Zi/ Ce} ) I

⁽¹

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989236

C2-228 JOURNAL DE PHYSIQUE

The e l e c t r o s t a t i c energy of an unif oxmly charged hemisphere W =0. 327'n2

e Z f l

/a€? e n t e r s i n t o t h e ener balance equation used t o determing N and, the-' r e f o r e , R. s i n c e N = ( 2 / 3 ? ~ n :R :

W,

-

e e

y

⁼ 0.55 ( ~ T n ) l / ~ e2 N ~ / ~ / x

+

^{(N z}

-

^z)

^-

^Ek (2) where n i s t h e number of atoms p e r u n i t volume, e i s e l e c t r o n charge, M? i s d i e l e c t r i c constant, 9 i s t h e work function. For m e t a l s N = O and t h e e l e c t - ron emission y i e l d X --N

--

(W

-

^{z e lp)} i n accordance with ref,/2/, I n c a s e of non-metals ( C be'C ) ?he Coulomb energy of t h e charged domain sho- u l d be t a k e n i n t o acco6nt bifth i n secondary i o n and e l e c t r o n emission. For high z, when t h e f i r s t t e r n i n t h e r i g h t hand s i d e of eq.(2) dominates t h e n N and t h e r e f o r e

X -

W 3/S

.

The Auger n e u t r a l i z a t i o n of a f a s t primary i o n o ~ c u r s p a r t i a l l y i n s l d e %he s o l i d along t h e p a t h v i 2 . ( v. i s t h e i o n ve- l o c i t y ) . The shape of t h e charged domain i s changed.~o&ever,'as i n ref./7/

t h e i o n y i e l d K i s determined by t h e volume i n s i d e t h e d i s r u p t i o n s u r f a c e which i s s semisphere with r a d i u s R reduced by a q u a n t i t y C2depending on ne- u t r a l i z a t i o n time 2,

K = 0 . 4 g T n 2 + ( R ( 3 ⁾

where

z+

^{i s} i o n i z a t i o n degree, Q i s given by

Q = ( 2 M E ~ ) " ~ / ( v e2 n r e ) ( 4 )

H e r e M i s t h e atomic mass of t h e s o l i d , Eb i s t h e s u r f a c e binding energy.The q u a n t i t y a equals t h e e f f e c t i v e t h i c k n e s s of a l a y e r from which t h e p a r t i c - l e s can not be e j e c t e d during time 2 .

The r a d i u s R(v.=

a)=

R

,

R being d e t g m i n e d from eq.(2),while R decreases w i t h vi increahe accor8ing0to

R = R. 2 /( R.

+

0.37 v i z i ) (5)

The eq.(3) d i s p l a y s a t h r e s h o l d of i o n emission from non-metals under multi- p l y charged i o n s : f o r R = 9 t h e y i e l d K = 0. For a given s o l i d R decrea- s e s w i t h both i o n charge and primary energy i n c r e a s e , Besides, from eq.(3) it follows t h a t t h e diagram

versus

^R should be a s t r a i g h t l i n e and one can determine %he value CLby t h e segment c u t o f f by t h e l i n e from t h e axis R.

3

-

COMPARISON WITH EXPERINIEN'PS

I n f i g . ? t h e dependence of K ' / ~ on R i s shown f o r d i f f e r e n t t a r g e t s and d i f - f e r e n t primary i o n s with energies: I

-

^{E = 0.1}

-

1 keV /4/ and e x t r a p o l a t e d t o E = 0

/5/,

I1

-

^{E = 7}

-

^{8 keV}

^{/5/, P11} -

^{E =} 20 keV /g/. The y i e l d K was g a l c u l a t e d by suberacting t h e Sigmund casca8e s p u t t e r i n g c o e f f i c i e n t from t h e t o t a l y i e l d .

The following values of parameters were used: f o r LiF

-

² 0.125 i-:g=2.4, e

y =

6 eV, l&=e$; BY, T . = 2

.

1 0 ; y s 8 f o r S i

-

ⁿ = 0.05

,

^{= 4,} p = 4 eV, E = -2- ='1.5 10- S. A s it i s seen from f i g . 1 t h e p o ~ n t s a r e wel?f a l i g n e d alohg t h e s t r a i g h t l i n e s K v s R

,

i . e . eq.(3) s a t i s f a c - t o r i l y d e s c r i b e s t e exp$rimental d a t a with t h e following f i t i n g paramet2rs:

f o r Lip

- ^{a =}

^0.7

k ^,

^{= 1.7} 10-z1 f o r , s i />/

- ^{a =}

^2.2

1 ^{, ?+=}

^{9 1 0 - 8}

f o x S i /g/

-

^{Q =} 3.4 A and p+= 2.9 10-

A n e s t i m a t i o n of n e u t r a l i z a t i o n time f o r a p o s i t i v e z

-

charged d e f e c t i n a s o l i d h a s been obtained i n r e f . / l ? / :

h 2 2 2 -1/4 -5/12

L e = 0.25 ( 2 d z l e k T/m

1

_nc (6

h e r e m i s t h e e l e c t r o n mass, k i s Boltzman constant, T is t h e temperature, n i s t h e c u r r e n t c a r r i e r concentration.

AE

follows from eq, ( 4 ) and (5)

a*

T ''9 nvf2, t h e r e f o r e w i t h T and n increa- se, t h e l i n e s K " ~ ( R ) should be s h i f t e d toehe r i g h t . Indeed, i n t h e eftperi- ments /5,9/ some samples w i t h d i f f e r e n t c o n c e n t r a t i o n n , of boron

,

^which

i s t h e source of c u r r e n t c a r r i e r s

,

have been used: S i wafer with n, = 10"

-

10'" atom/cm3 corresponding t o s p e c i f i c r e s i s t a n c e 25-2.5 kR/cm i n r e f ,/9/ and a high r e s i s t a n t s i l i c o n (50 k R/cm) w i t h n, =

5

10'9 atom/cm I n ref./5/. Perhaps, it e x p l a i n s t h e d i f f e r e n c e i n CL and

7+

f o r both experi-

Fig.1

-

^{IC1I3 vs R f o r : I -ArZ+/4/, Fig.2}

-

Electron emission y i e l d metal i o n s / 5 / ~ L i F ; 1 1 - ~ 1 ~ ~ p o l y c ~ s - vs W

-

^{z e}

^g

^{f o r A l z ' and}

^{V L +}

^{i o n s}

t a l S i

/5/

; 111

-

^ArZ'-, s i n g l e cry- on L ~ F and S i /6/. S o l i d curves:

s t a l S i /g/. The i o n charges z a r e theory.

designated by numbers.

The e l e c t r o n emission y i e l d i s given by

where p i s t h e p r o b a b i l i t y of e l e c t r o n escape. I n ref./'/2/ it has been foun- ded t h a t t h e dependence K(W ) d e v i a t e s from a l i n e a r one f o r A r g f imping- ing. t h e p o l y c r y s t a l W ( E ='20 keV). It was explained by t h e emission of one f a s t Auger e l e c t r o n af? f i l l i n g t h e deep i n n e r s h e l l vacancy, L e . t h e assumption /2/ of equal energy s t e p Auger t r a n s i t i o n s was v i o l a t e d , The Mon- t e Carlo c a l c u l a t i o n /13/ of t h e cascade Auger n e u t r a l i z a t i o n of K r Z + o n me- t a l surface confirmed t h e d e v i a t i o n of

2C

(W ) from a l i n e a r dependence f o r high z. I n t h e case of i n s u l a t o r t h i s d e v i a k o n i s caused a l s o by t h e trans- f e r of a p a r t of i o n energy t o e l e c t r o s t a t i c energy of t h e charged domain, I n ref./6/ t h e mentioned dependence indeed d e v i a t e s from l i n e a r one and a s it i s seen from f i g o 2 s a t i s f a c t o r i l y agrees with eq,(7) f o r t h e same values of parameters a s t h e used ones f o r c a l c u l a t i o n of t h e i o n yield.

4

-

DISCUSSION

The comparison of t h e theory w i t h t h e experiment shows t h a t t h e Coulomb ex- plosion model describes r i g h t l y t h e r e g u l a r i t i e s of t h e dependence of i o n emission from non-metals on both i o n charge and i t s v e l o c i t y , A s f o r t h e in- dependence of t o t a l s p u t t e r i n g y i e l d S on i o n charge /g/, it should be taken i n t o account t h a t we have assumed t h e a d d i t i v i t y of "Coulomb explosion" and c o l l i s i o n a l cascade contributions i n t o s p u t t e r i n g , t h e former g i v e s S

-

K/qf

0.6, t h e second S = 'l .7 f o r x = 9, E = 20 keV. T h i s ratioC8&ni t h a t i n t h e charged d 0 m a f 8 ~ h e i o n s begin t o mo$e not only due -t;o Coulomb repulsion,diminishing i t s e f f e c t , The dependence of K on z /9/ can be expla- ined by a r a e h e r high p r o b a b i l i t y f o r t h e e j e c t e d i o n s t o preserve t h e i r charge because they leave t h e domain which i s not n e u t r a l i z e d during t h e ti- me Te

=

1 0 - ' ~ s . It i s c l e a r t h a t f o r recording t h e proper Coulomb explosion s p u t t e r i n g an experiment f o r lower energy should be c a r r i e d out. Even a t 'l0 keV A 1 S i bombardment S z 1 /14/ while according t o eq.(3) SCou1ZL"3, i. e. i n this case t h e ~ o u l o m ~ ~ ~ ~ ~ l o s i o n s p u t t e r i n g p r e v a i l s ,

I n t h e experiment /9/ a comparative measurement of t h e secondary i o n energy d i s t r i b u t i o n under Ar 9t and A r * bombardment of S i s i n g l e c r y s t a l has drawn t h e authors t o t h e conclusion t h a t t h e enhancement of i o n y i e l d occurs only due t o t h e change of e l e c t r o n s t a t e of t h e s p u t t e r e d p a r t i c l e s while t h e me- chanism of s p u t t e r i n g remains e s s e n t i a l l y of cascade type.But a c a r e f u l exa-

C2-230 JOURNAL DE PHYSIQUE

inination of t h e s e energy d i s t r i b u t i o n s shows t h a t they a r e n o t s i m i l a r : whi- l e t h e t o t a l y i e l d r a t i o K /K,

z 3

t h e r a t i o f o r d i f f e r e n t p a r t s of t h e spe- ctrum has d i f f e r e n t values? For low secondary i o n energy i t equals 4 but i n t h e high energy p a r t of t h e spectrum this r a t i o decreases t o 2. I f t h e en- hancement of i o n y i e l d f o r z= 9 would r e s u l t from p a r t i c l e s with some i n n e r

s h e l l vacancies, t h e n t h e s e i o n s should enhance j u s t t h e h i g h energy p a r t of t h e spectrum, while t h e low energy i o n s a r e t y p i c a l r a t h e r f o r Coulomb repu- l s i o n , Therefore it seems t h a t t h e Coulomb explosion s p u t t e r i n g does cont- r i b u t e i n t o t h e secondary i o n emission i n experiment /9/.

5 -

CONCLUSION

Thus t h e t h e o r e t i c a l a n a l y s i s has shown t h a t some h i g h l y charged i o n s with low energy (small v . and S ) bombarding an i n s u l a t o r o r semiconductor with small c o n d u c t i h t y atCB8G temperature ( l a r g e

T

) should be used f o r re- cording t h e proper Coulomb explosion s p u t t e r i n g , It 6ould be i n t e r e s t i n g t o c a r r y o u t a comparative i n v e s t i g a t i o n of s p u t t e r i n g and i o n emission depen- ding on t h e mentioned parameters.

The bombardment of t h i c k biomolecule l a y e r s by slow h i g h l y charged i o n s (up t o b a r e n u v l e i ) , e.g. Kr 30* o r Xe50+, t h e Auger n e u t r a l i z a t i o n of which l e - ads t o emission of about qo2 e l e c t r o n s /13/ and accumulation of p o s i t i v e i o n s of t h e same q u a n t i t y , should r e s u l t i n a Coulomb explosion. I t would be i n t e r e s t i n g t o c l e a r up experimentally whether it causes desoxption of t h e i n t a c t biomolecules o r only t h e i r fragmentation and s p u t t e r i n g .

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,

Pmc.IX 1ntern.Conf. on Phenoniena i n Ionized Gases, Bucharest, 1969, p.q4,,Atomic c o l l i s i o n s i n S o l i d s . Ed. Palmer, N-H Publ. C., 1970, p.324.

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,

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S o v i e t Conf. on Emission E l e c t r o n i c s . Kiev, USSR, 1987, p.151,

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,

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