AMORPHOUS SILICON NITRIDE THIN FILMS PERFORMED IN TWO PECVD EXPERIMENTAL DEVICES

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AMORPHOUS SILICON NITRIDE THIN FILMS PERFORMED IN TWO PECVD EXPERIMENTAL

DEVICES

Jean-Louis Jauberteau, M. Baraton, Mm Gerbier, P. Quintard, J. Desmaison, J. Aubreton, A. Catherinot

To cite this version:

Jean-Louis Jauberteau, M. Baraton, Mm Gerbier, P. Quintard, J. Desmaison, et al.. AMORPHOUS SILICON NITRIDE THIN FILMS PERFORMED IN TWO PECVD EXPERIMENTAL DEVICES.

Journal de Physique Colloques, 1989, 50 (C5), pp.C5-657-C5-664. �10.1051/jphyscol:1989576�. �jpa-

00229609�

AMORPHOUS SILICON NITRIDE THIN FILMS PERFORMED IN TWO PECVD EXPERIMENTAL DEVICES

J.L. JAUBERTEAU, M.I. BARATON, MM. GERBIER, P. QUINTARD, J. DESMAISON, J. AUBRETON and A. CATHERINOT

Laboratoire de Cdramiques Nouvelles, CNRS UA-320, Universitd de Limoges, 123, avenue Albert Thomas, F-87060 Limoges Cedex, France

RCsurnC : L e s f i l m s d e n i t r u r e d e silicium sont rkalisCs par deux p r o c i d i s d e P E C V Z diffCrents.

D a n s l e p r e m i e r , c e s f i l m s sont formCs d a n s une d e c h a r g e c o u r a n t continu d a n s un p l a s m a d ' a r g o n - s i l a n e - a z o t e a l o r s q u e d a n s l e second dispositif, u n e espirce a c t i v e d ' a z o t e est c r e k e s e l e c t i v e m e n t d a n s une post-dkcharge d ' a z o t e et r6agit a v e c l e silane d a n s u n e z o n e d e r k a c t i o n oG e s t positionn6 l e s u b s t r a t .

C a n s l e s deux c a s , l e s f i l m s r k a l i s e s s o n t sous-stoechiomCtriques e n a z o t e . L e u r s c a r a c t k r i s - t i q u e s : morphologie, s t r u c t u r e e t c o m p o s i t i o n sont k t u d i i e s e t c o m p a r k e s .

Abstract : Silicon n i t r i d e t h i n f i l m s a r e o b t a i n e d using t w o d i f f e r e n t PECVD devices.

In t h e f i r s t one, t h e s e t h i n f i l m s a r e c r e a t e d i n a D C d i s c h a r g e i n a n argon-silane-nitrogen g a s m i x t u r e w h e r e a s i n t h e second, n i t r o g e n a c t i v e s p e c i e s a r e s e l e c t i v e l y p r o d u c e d i n a flowing n i t r o g e n post-discharge and t h e n r e a c t w i t h s i l a n e in a r e a c t i o n region w h e r e i s positioned t h e s u b s t r a t e .

In both c a s e s , t h e o b t a i n e d f i l m s a r e s u b s t o i c h i o m e t r i c i n nitrogen. Their c h a r a c t e r i s t i c s : morphology, s t r u c t u r e a n d c o m p o s i t i o n a r e studied a n d c o m p a r e d .

I. INTRODUCTION

Silicon n i t r i d e a n d silicon o x i d e t h i n f i l m s a r e widely used i n t h e e l e c t r o n i c i n d u s t r y a s d i e l e c t r i c t h i n l a y e r s ( t h i c k n e s s ^1. 2000

A)

o r f o r t h e passivation of m i c r o - i n t e g r a t e d c o m p o u n d s / l / 121.

P l a s m a e n h a n c e d c h e m i c a l v a p o r deposition p r o c e s s e s (PECVD processes), a r e known t o allow d e p o s i t i o n of silicon c o m p o u n d s a t q u i t e low t e m p e r a t u r e , avoiding t h e r m a l d a m a g e s t o t h e s e m i c o n d u c t o r s u b s t r a t e s . H o w e v e r in t h e s e "low t e m p e r a t u r e " processes, t h e s u b s t r a t e a n d t h e film a r e g e n e r a l l y s u b m i t t e d t o a c o n t i n u o u s born b a r d m e n t e f f e c t by e n e r g e t i c p a r t i c l e s (mainly ions). I t i s well known t h a t e l e c t r i c a l and m e c h a n i c a l p r o p e r t i e s of d e p o s i t e d l a y e r s a r e s t r o n g l y d e p e n d e n t o n composition, morphology a n d s t r u c t u r e a n d t h e r e f o r e o n d e p o s i t i o n conditions. In o r d e r t o d e t e r m i n e t h e role of b o m b a r d m e n t by e n e r g e t i c p a r t i c l e s we r e p o r t i n t h i s p a p e r t h e c h a r a c t e r i s t i c s of f i l m s o b t a i n e d with and w i t h o u t b o m b a r d m e n t in t w o d i f f e r e n t PECVD devices.

T h e s e t w o e x p e r i m e n t a l d e v i c e s a r e p r e s e n t e d and film c h a r a c t e r i s t i c s , c o m p a r e d .

2. EXPERIMENTAL DEVICES AND FILM DIAGNOSIS

2.1. PECVD reactors

T h e f i r s t e x p e r i m e n t a l s e t up i s shown in f i g u r e 1. A D C discharge i n argon-silane-nitrogen g a s m i x t u r e i s c r e a t e d b e t w e e n t w o p a r a l l e l f l a t aluminium e l e c t r o d e s ( i n t e r - e l e c t r o d e d i s t a n c e = 17 m m , e l e c t r o d e d i a m e t e r s = 70 m m ) within a n inox cylindrical vessel ( d i a m e t e r = 400 m m , h e i g h t =

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989576

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400 mm). G a s m i x t u r e (Ar-5% SiH4-N2) i s i n j e c t e d through t h e upper e l e c t r o d e (anode) and t h e s u b s t r a t e s (monocrystalline silicon wafers) a r e positioned o n t h e lower e l e c t r o d e (cathode). G a s f l o w r i t e s a r e controlled a n d r e g u l a t e d using m a s s f l o w m e t e r s (TYLAN), and a b s o l u t e g a s pressure i s m e a s u r e d by a c a p a c i t a n c e m a n o m e t e r (MKS-BARATZON). Typical working conditions a r e : A t o t a l p r e s s u r e P T = 0,4 Torr, a discharge c u r r e n t Id = 1 3 m A , a discharge voltage Vd = 400 volts a t t h e beginning o f t h e e x p e r i m e n t a t i o n a n d a b o u t 500 volts a f t e r o n e hour of working d u e t o t h e insulator silicon nitride deposition o n t h e t w o electrodes.

T h e second e x p e r i m e n t a l d e v i c e i s shown i n f i g u r e 2. A DC d i s c h a r g e i s c r e a t e d i n a nitrogen flow b e t w e e n t w o e l e c t r o d e s E l a n d E2, 20 c m d i s t a n t f r o m o n e another, upstream of t h e Ar-5%

SiH i n j e c t o r exit. "Active" nitrogen species, c r e a t e d within t h e G C d i s c h a r g e a n d flowing i n t h e 4

post-discharge tube, r e a c t with SiH4 molecules, leading t o SiH4 dissociation a n d r e a c t i v e species.

A specific study of e l e m e n t a r y r e a c t i v e processes, b e t w e e n "active" nitrogen s p e c i e s and SiH4 i s p r e s e n t e d elsewhere.

T h e silicon w a f e r s a r e positioned at r i g h t a n g l e relatively t o t h e post-discharge t u b e a x i s and typical working conditions a r e : A DC d i s c h a r g e c u r r e n t i n t e n s i t y I = 20 m A , a discharge voltage V = 850 volts and a nitrogen g a s pressure P = 0,8 Torr. The g a s velocity v a r i e s b e t w e e n 10 a n d 20 m / s within t h e post-discharge t u b e and t h e Ar-5% SiH4 flow r a t e i s k e p t c o n s t a n t and equal t o 30 c m j/m n.

mass f lowmeters

THR 1000 Pump

Fig.1 : Experimental set up of t h e f i r s t d e v i c e

Fig. 2 : E x p e r i m e n t a l s e t up of t h e second d e v i c e

2.2. Film c h a r a c t e r i z a t i o n s

FT-IR analysis of t h e deposited silicon nitride f i l m s a r e p e r f o r m e d using a NICOLET 5DX-B s p e c t r o m e t e r . F o r t h e s e analysis, s u b s t r a t e w a f e r s m u s t b e t r a n s p a r e n t i n t h e i n f r a r e d region of i n t e r e s t (3300cm-I - 400 c m - 1 ) a n d f o r t h i s r e a s o n t h e s u b s t r a t e s a r e generally rnonocrystalline

s i l i c o n w a f e r s . FT-IF! s i l i c o n w a f e r s p e c t r a e x h i b i t o n l y t w o n a r r o w a b s o r p t i o n b a n d s , a t 6 1 0 cm-' and a t 1107 cm-' respectively assigned t o a t w o phonon-absorption p r o c e s s i n t h e silicon n e t w o r k a n d t o t h e s t r e t c h i n g v i b r a t i o n of t h e S i - 0 b o n d s a t t h e s u b s t r a t e s u r f a c e .

thickness m e a s u r e m e n t s .

R e m a r k : m a n y r e s u l t s a r e p r e s e n t e d versus t h e R f a c t o r defined a s t h e r a t i o of t h e nitrogen flow o n t h e silane flow : R = flow N2/flow SiH4 = flow N2/ ( 0,05 X flow (Ar 5% SiH4)).

3. RESULTS AND DISCUSSION

3.1. Film elaborated in the DC discharge

The typical e x p e r i m e n t a l conditions a r e Vd = 400 volts, Id = . l 3 m A , P r = 0,4 Torr. Total f l o w r a t e

= 20 c m 3 / m n , t h e growth r a t e i s found t o b e a b o u t 200 A/mn. Deposition of t h e silicon nitride f i l m s a r e p e r f o r m e d without a n y auxiliary heating of t h e substrate, consequently t h e s u b s t r a t e t e m p e r a - t u r e is only due t o ionic bombardment and s t a n d s a t a b o u t 80°C during t h e deposition process.

As shown i n f i g u r e 3 t h e Auger e l e c t r o n energy s p e c t r a of t h e deposited f i l m s exhibit four m a i n e l e c t r o n s groups. These four groups do not depend o n t h e g a s m i x t u r e composition within t h e r e a c t o r and a r e observed at t h e following e l e c t r o n s energy v a l u e s :

- 6 5 , l e v and 82,7 e v : This group i s ascribed t o t h e Si(LVV) t r a n s i t i o n of a t o m i c silicon i n Si3N4 131, /4/

- 276 e v : t h i s group c h a r a c t e r i z e s a t o m i c c a r b o n i m p u r i t i e s i n t h e m a t e r i a l

- 387,2 e v : t h i s o n e is ascribed t o t h e N(KLL) t r a n s i t i o n of a t o m i c nitrogen i n Si 3 4 N (observed a t 386 e v /3/ a n d 382 e v /4/)

- 514, 2 e v : a s c r i b e d t o t h e G(KLL) t r a n s i t i o n of a t o m i c oxygen i n S i 0 2 /5/

.

T h e s e four a t o m i c e l e m e n t s Si, C, N, 0 a r e also observed by RBS, SIMS, ESCA and IR analysis and a t o m i c hydrogen i s d e t e c t e d by SIMS a n d IR.

Nevertheless, t h e s e values m u s t b e t a k e n cautiously. Silicon nitride being a n insulator m a t e r i a l , c h a r g e e f f e c t s o n s a m p l e s u r f a c e c a n strongly d i s t u r b e t h e e n e r g y s p e c t r u m of t h e secondary e l e c t r o n s and t h e r e b y t h e y c a n drastically c h a n g e t h e m e a s u r e d values of t h e binding e n e r g i e s of t h e electrons.

KINETIC ENERGY - aV

Fi6. 3 : Typical AES s p e c t r u m

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RBS analysis r e s u l t s point o u t t h a t a r g o n e l e m e n t i s included within t h e s e f i l m s ( t h e a t o m i c r a t i o Ar/Si = 2,5 %)

.

This observation m a y be ascribed t o t h e b o m b a r d m e n t by ~ r + ions leading t o inclusion of a r g o n i n t h e film network i n i n t e r s t i t i e l positions.

ESCA and SIMS analysis s p e c t r a show homogeneous nitrogen, silicon and hydrogen profil concen- trations, w h a t e v e r t h e g a s phase m i x t u r e i s (R = 5 o r R = 30). Generally, f i l m s a r e mainly oxidized a t t h e s u r f a c e and a t t h e film-substrate i n t e r f a c e b e c a u s e of t h e oxidation of t h e s u b s t r a t e before t h e experimentation. The film s u r f a c e oxidation a p p e a r s when t h e sample i s l e f t i n t h e a t m o s p h e r i c environment a f t e r e l a b o r a t i o n a n d during a significant while (2 months) b e f o r e c h a r a c t e r i z a t i o n . The oxidation i n t h e film bulk i s ascribed t o t h e e f k c t o n SiH4 o r SiHx r a d i c a l s of residual oxygen while t h e experimentation, i n s p i t e of t h e secondary vacuum (10 -6 Torr) m a i n t a i n e d b e t w e e n e a c h

deposition experiment. In t h e s a m e way, c a r b o n i m p u r i t i e s i n t h e f i l m s a r e ascribed t o t h e e f f e c t of residual hydrocarbides (pump oil) i n t h e vessel.

A f t e r this f i r s t p r e s e n t a t i o n of t h e m a t e r i a l i n a g e n e r a l a s p e c t , w e a r e going t o p r e s e n t a specific study of t h e composition variation of t h e deposited f i l m s when t h e nitrogen c o n c e n t r a t i o n v a r i e s i n t h e g a s mixture. This study has b e e n mainly p e r f o r m e d using Fourier T r a n s f o r m e d Infrared analysis a n d r e s u l t s a r e c o r r e l a t e d with AES analysis.

A typical FT-IR s p e c t r u m i s shown in figure 4. We observe t h e broad and i n t e n s e Si-N s t r e t c h i n g absorption band (840 cm-'),, t h e Si-N bending (480 cm-'), and Si-H s t r e t c h i n g (2100 cm-') ,vibrations. T h e Si-H bending m o d e i s m a s k e d by t h e Si-N absorption band. T h e N-H s t r e t c h i n g absorption band o c c u r s a t about 3330 cm-' a n d t h e corresponding bending m o d e a t 1150 cm-'. The w e a k shouldet' a t 1 0 5 0 c m - ' i s a s s i g n e d t o t h e s t r e t c h i n g m o d e of t h e S i - 0 bonds.

WAVENUMBER

-

C M - 1

Fig. 4 : Typical FT-IR s p e c t r u m (film p e r f i r m e d i n t h e f i r s t device)

Pt ~ 0 . 4 torr

.

^-

_{. .}

^1.

_{1 NH}

0 54 R

Fig. 5 : Evolution of t h e t w o r a t i o s SiN/NH a n d SiN/SiH v e r s u s R

e v o l u t i o n of t h e c o n c e n t r a t i o n r a t i o Si-N/N-H a n d Si-N/Si-H when, t h e niti-ogen c o n c e n t r a t i o n v a r i e s i n t h e g a s m i x t u r e ( R variation). R e s u l t s a r e shown f i g u r e 5. T h e r e l a t i v e c o n c e n t r a t i o n of Si-N bonds and N-H i n c r e a s e s w i t h t h e R v a l u e s w h e r e a s Si-H bond c o n c e n t r a t i o n d e c r e a s e s . F o r R l a r g e r t h a n R = 30, t h e c u r v e s p r e s e n t a n a s s y m p t o t i c behavior and t h e film c o m p o s i t i o n r e m a i n s nea;ly c o n s t a n t . A s i m i l a r e v o l u t i o n i s observed f o r N/Si a t o m i c r a t i o g i v e n by t h e AES a n a l y s i s results. A s shown i n f i g u r e 6 , t h e a t o m i c r a t i o i n c r e a s e s w i t h t h e R v a l u e up t o N/Si = 1. F o r l a r g e r R, N/Si r a t i o r e m a i n s nearly c o n s t a n t . Thus, t h e s e silicon n i t r i d e t h i n f i l m s a r e a l w a y s s u b s t o i c h i o m e t r i c i n n i t r o g e n w h a t e v e r t h e g a s m i x t u r e c o m p o s i t i o n i s

.

This r e s u l t q u i t e a g r e e s w i t h t h e o b s e r v a t i o n s of Lucovsky 161, t h e s e silicon n i t r i d e f i l m s o b t a i n e d by PECVD p r o c e s s e s , con- t a i n until 30% ( a t o m i c ) of hydrogen. Under t h e s e conditions, m a n y Si a t o m s i n t h e m a t e r i a l a r e bound t o H a t o m s i n s t e a d of t o N a t o m s only.

Thus, silicon n i t r i d e t h i n f i l m s p e r f o r m e d i n t h i s f i r s t PECVD d e v i c e a r e a l w a y s amorphous, h y d r o g e n a t e d a n d s u b s t o i c h i o m e t r i c i n nitrogen. T h e s e r e s u l t s point o u t t h e s t r o n g c o r r e l a t i o n b e t w e e n t h e g a s m i x t u r e and t h e film compositions. T h e f i g u r e 7 r e p r e s e n t s t h e c h a n g e of m a t e r i a l c o m p o s i t i o n w i t h t h e c o m p o s i t i o n of t h e u s e d g a s m i x t u r e . F o r t h e R v a l u e s b e l o w R o (20 \( R o ,( 301, t h e m a t e r i a l c o m p o s i t i o n c h a n g e s w h e n R i n c r e a s e s . T h e Si-N a n d N-H bonds c o n c e n t r a t i o n s i n c r e a s e w h e r e a s t h e Si-H c o n c e n t r a t i o n d e c r e a s e s . F o r R a b o v e Ro, t h e film c o m p o s i t i o n r e m a i n s

Fig. 6 : E v o l u t i o n of t h e a t o m i c r a t i o N/Si v e r s u s R

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constant. However, f o r l a r g e R values, f i l m s a r e strongly d a m a g e d with s u r f a c e c r a c k s and inclusions. This l a s t point m a y b e explained by t h e f a c t t h a t rich nitrogen thin f i l m s do not exhibit a n "Island s t r u c t u r e " c h a r a c t e r i z e d by t h e growth of c o l u m n s perpendicularly t o t h e surface. These s t r u c t u r e s a r e known t o p r e v e n t t h e c r a c k s from propagating a t t h e film surface. A similar s t r u c t u r e has never b e e n observed e v e n by MET analysis i n our rich nitrogen s a m p l e s ( R = 20) w h e r e a s i t h a s been evidenced i n amorphous silicon t h i n f i l m s 171.

Fig. 7 : M o d i f i c a t i o n of t h e m a t e r i a l c o m p o s i t i o n a s a f u n c t i o n of t h e u s e d g a s m i x t u r e

3.2. E l a b o r a t i o n i n t h e flowing post-discharge d e v i c e

In t h e l a s t device, silane dissociation mainly r e s u l t s o f e l e c t r o n i c collisions 181, /9/ w h e r e a s i n t h i s new device, silane dissociation i s only due t o r e a c t i o n b e t w e e n a c t i v e nitrogen s p e c i e s and silane m o l e c u l e s 191. The r e a c t i v e processes leading to t h e silicon nitride deposition i n t h e s e t w o e x p e r i m e n t a l devices has also been studied and t h e r e s u l t s will b e r e p o r t e d later.

In t h e following, w e p r e s e n t t h e c h a r a c t e r i s t i c s of t h e thin f i l m s obtained i n t h i s new d e v i c e and a comparison w i t h previous results i s given.

A t f i r s t sight, f i l m s a r e smooth, without any c r a c k o r inclusion.

The g r o w t h r a t e strongly depends o n t h e working conditions and m o r e particularly o n t h e nitrogen g a s pressure i n t h e upstream D C discharge. In typical working conditions, nitrogen p r e s s u r e = 0,8 Torr, A r - 5% SiH4 f l o w = 30 cm3/mn, D C discharge c u r r e n t intensity = 20 mA, voltage = 8 5 0 volts, a n d t o t a l flow r a t e = 10 t o 2 0 m/s, t h e growth r a t e i s a b o u t 20 A / m n (10 t i m e s less t h a n i n t h e f i r s t device).

ESCA analysis i n d i c a t e s a nitrogen c o n c e n t r a t i o n lower i n t h e s e f i l m s t h a n i n t h e f i l m s o b t a i n e d i n t h e f i r s t device. F i l m s a r e obviously m o r e oxidized b e c a u s e of t h e l a r g e d i s t a n c e b e t w e e n s u b s t r a t e a n d r e a c t i v e region (10 c m i n s t e a d of l e s s t h a n I cm i n t h e f i r s t device), and t h e c a r b o n i m p u r i t i e s i n t h e r e a c t o r a r e mainly p r e s e n t a t t h e film surface. The secondary e l e c t r o n intensity r a t i o of Ic/lSi = 1,l a t t h e s u r f a c e a n d 0,04 f o r a 5 0 A d e p t h i n a s a m p l e o b t a i n e d i n typical working conditions. The binding e n e r g i e s of t h e secondary e l e c t r o n s correspond t o ESi = 105.8 e v t o 104.6 e v a n d t o EN = 400.9 e v to 400.4 ev. T h e s e values a r e similar t o t h o s e previously observed f o r t h e s e t w o elements. Si and N neighbouring a t o m s a r e t h e s a m e i n both cases.

C h e m i c a l bonds i n t h e films, observed by FT-IR analysis, a p p e a r t o be strongly d e p e n d e n t o n nitrogen pressure. For a low nitrogen pressure, O,4 Torr, FT-IK s p e c t r a f e a t u r e t h e S i - 0 and Si-N s t r e t c h i n g absorption bands but neither t h e Si-H nor N-M o n e s

.

These t w o bands a r e only observed f o r a

F i g u r e S shows a FT-IR s p e c t r u m of a film obtained f o r a nitrogen pressure P N 2 = 0,8 Torr, i n typical working conditions. We o b s e r v e t h e broad and i n t e n s e N-H s t r e t c h i n g absorption band ( 3347 cm-').

This absorption f r e q u e n c y is higher t h a n t h e o n e observed for s a m p l e s p e r f o r m e d i n t h e f i r s t d e v i c e (3337 t o 3330 cm-'). According t o Lucovsky 1101, t h e shift m a y be a t t r i b u t e d t o t h e p r e s e n c e of NH2 groups i n t h e films. The e x i s t e n c e of t h e s e groups a r e proved by t h e NH2 scissoring m o d e t h e absorption frequency of which i s d e t e c t e d a t 1557 cm-'. The Si-H s t r e t c h i n g absorption band (2190 cm-') i s observed a t t h e s a m e frequdncy i n both films. Nevertheless, t h e FT-IR s p e c t r u m (figure 8) f e a t u r e s a shoulder a t 2270 cm-' which i s assigned t o various S i n x ( X = 1,2,3) groups.

The Si-N s t r e t c h i n g absorption band a p p e a r s a t 9 3 0 cm-' i n s t e a d of 8 8 7 cm-' f o r f i l m s p e r f o r m e d i n t h e f i r s t d e v i c e (R = '20). This shift m a y b e due t o t h e p r e s e n c e of a n S i - 0 s t r e t c h i n g absorption band (1050 c m - 1 ) overlaping t h e broad Si-N band.

WAVENUMBER - C M - l

Fig. S : Typical FT-IR s p e c t r u m (film p e r f o r m e d i n t h e second device)

4. CONCLUSION

In t h i s paper, w e have c o m p a r e d silicon nitride t h i n f i l m s obtained i n t w o d i f f e r e n t PECVD reactors.

T h e f i r s t o n e i s a D C d i s c h a r g e i n a SiH4, NZ, Ar g a s m i x t u r e , w h e r e t h e silane i s mainly dissociated by e l e c t r o n i c collisions. F i l m s have homogeneous thickness and composition and t h e i r c h a r a c t e r i s t i c s a r e strongly d e p e n d e n t o n t h e g a s mixture.

In t h e second device, silane dissociation i s only d u e t o r e a c t i o n s with a c t i v e nitrogen s p e c i e s c r e a t e d upstream t h e r e a c t i o n region. Film composition and thickness a r e strongly dependent o n t h e nitrogen g a s pressure.

C5-664 JOURNAL DE PHYSIQUE

Both f i l m s a r e s u b s t o i c h i o m e t r i c i n n i t r o g e n a n d t h e i r s t r u c t u r e p r e s e n t s t h e s a m e c h e m i c a l bonds.

Nevertheless, f o r t h e f i l m s e l a b o r a t e d i n t h e second d e v i c e t h e FT-IR analysis i n d i c a t e t h e p r e s e n c e of NH2 o r polyhydride groups, SiHx, which a r e not o b s e r v e d i n t h e f i l m s o b t a i n e d in t h e f i r s t device.

This c o m p o s i t i o n d i f f e r e n c e m a y b e a s c r i b e d t o t h e e f f e c t of s u r f a c e ionic b o m b a r d m e n t i n t h e f i r s t d e v i c e 191. Indeed, w e a k c h e m i c a l bonds such a s N-H o r Si-H and m o r e o v e r polyhydride g r o u p s SiHx

(X= 2,3) o r N H g r o u p s o b s e r v e d i n t h e s e f i l m s e l a b o r a t e d i n t h e flowing post-discharge a r e probably ₂ b r o k e n b y i o n i c b o m b a r d m e n t i n t h e f i r s t d e v i c e , l e a d i n g t o s p u t t e r i n g of H a t o m s .

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