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Fundamental mechanisms in silane plasma decompositions and amorphous silicon deposition
Journal of Non-Crystalline Solids 59 & 60 (1983) 649-658 North-Holland Publishing Company
649
FUNDAMENTAL MECHANISMS IN SILANE PLASMA DECOMPOSITIONS AND AMORPHOUS SILICON DEPOSITION J.P.M. SCHMITT Equipe Synth@se de Couches Minces pour l'Energ@tique, L.P.N.H.E., Ecole Polytechnique, 91128 PALAISEAU Cedex, France. The mechanisms r e s u l t i n g in amorphous s i l i c o n deposition from silane glow discharges are c l a s s i f i e d in primary processes, (electron impact on s i l a n e ) , and secondary processes such as ion-molecule and radical-molecule reactions. The low pressure l i m i t of a multipole DC discharge is shown to allow the measurement of various cross-sections associated with the primary processes, in p a r t i c u l a r spontaneous emission. The results from plasma probing with ion mass spectrometry, laser induced fluorescence and other techniques are described in relevance to the problem of the thin f i l m growth. 1. INTRODUCTION As f o r a l l other thin f i l m s , and as f o r a l l preparation techniques, hydrogenated amorphous s i l i c o n (a-Si:H) deposited by silane glow discharge is found to have s t r u c t u r a l , chemical and e l e c t r o n i c properties very dependant on the depos i t i o n conditions. Not only the q u a l i t y of the semiconductor varies when in a given deposition chamber the parameters such as temperature, pressure or power are modified, but one may assume that the scattering among the results from the various teams preparing a-Si:H is to be explained by the differences in the preparation condition. As a consequence, there is a need to understand silane glow discharges in order to deduce from the external parameters of the system a complete description on a microscopic scale of the p a r t i c l e s reaching or leaving the surface. Then, hopefully, i t w i l l be possible to c o r r e l a t e the properties of the f i l m to the chemical and the energetic states of the species impinging onto the subs~ra~e. Such a complex task has led, w i t h i n the l a s t 8 years to a growing a c t i v i t y i-c~ involving diagnostic and modeling of silane discharge~ The ambition of the present paper is to scan through the up to date knowledge of the mechanisms occuring in a silane discharge and to point out at the many dark areas which remain w i t h i n the plasma black box. After s e t t i n g the orders of magnitude f o r the main parameters the most important plasma mechanisms are reviewed. The measurements of some e l e c t r o n - s i l a n e cross-sections in a low pressure m u l t i p o l e discharge are reported. The e f f e c t of secondary reactions and surface reactions are described along with the l a t e s t results concerning the various species responsible f o r the a-Si:H f i l m growth. Further research lines are discussed in the concluding remarks. 2. A FEW ORDERS OF MAGNITUDE In a l l deposition plasmas the major component is made of neutral stable gases including the injected gases (SiH4, B2H6, PH3...), the c a r r i e r gases (Ar, He, H2... ), the by-product gases (H2, Si2 H6... ) and possibly unwanted gases (degasing, l e a k . . . ) . Even at f a i r l y large deposition rate (powerfull discharges) the proportion of unstable molecular fragments ( r a d i c a l s ) remains r e l a t i v e l y small with respect to stable molecule (below 1%) ; the ionized f r a c t i o n is even lower (below 0.1%). As a consequence a reactive species w i l l experience mostly c o l l i sions with the background gases and/or the w a l l s . Hence, except f o r very specif i c cases, reactions between excited species such as e l e c t r o n - i o n , electronr a d i c a l , i o n - r a d i c a l , r a d i c a l - r a d i c a l can be neglected. The gas temperature 0022-3093/83/0000 0000/$03.00 ©1983 North-Holland/Physical Society of Japan
J,P.M. Schmitt / Silane plasma decompositions
650
remains close to the wall temperature. The bulk of the electron population has a temperature of the order of 0.1-I eV but the high energy t a i l is generally non thermal and must extend above 12 eV in order to provide enough ionization for sv}tain~ng the discharge. The electron and ion densities5, 63 vary from 108 to 10I~ cm-~ depending on the discharge intensity. The Debyelength, typical scale for the plasma Coulomb screening of a potential disturbance remains below imm. 3. PRIMARY PROCESSES In all standard glow discharges the driving electric power is fed into the electron kinetic energy. The primary process responsible for the discharge rea c t i v i t y is the inelastic collision between an energetic electron and a molecule from the background gas. Examples for the possible consequence of electron impact on silane are given here e + SiH4,._ + dissociative ionization19,30-32 \-"~SiH 3 + H + 2e (I) neutral dissociation \SiH2 + H+ H+ e (2) dissociative excitation \SiH* + H~ + H + e + hv (3) pair formation 31 ~ \SiH~ + H~ + e (4) electron dissociative attachmentoJ ~SiH5 + H2 (5) rovibrational excitation34 Si ~ + e (6) 4. ELECTRON IMPACT CROSSSECTION MEASUREMENTS The measurementswere performed in a D.C. multipole discharge35-40. Its specific features are i ) i t s a b i l i t y to work at low pressure (from 0.1 mTorr), i i ) below 0.5 mTorr a quasi monoenergetic distribution for the fast electrons, i i i ) a partial confinement of the charged species by the multipolar magnetic walls. The multipole is shown in f i g . l . In the low pressure regime, due to the
permanent
ISZ\-H[]---/_(
i==
ine cusp
magnets
heating resistance
I
FIGURE 1 Multipole device with its electrical diagnostics.
F-5
i :=
low temperature of the bulk electron, the primary processes are entirgly generated by the primary electrons, the density and the energy of which can be independently selected by tuning the filament emissivity and the discharge bias. The consequence of the primary processes were measured : total dissociation (reactions of type (1) to (5)) deduced from the silane partial pressure depletion 40, ionization (reactions of type (1) and (4)) was related to the ion flow onto the walls ~u, photoemission (reaction type (3)) is analysed by spectrometry 44 of the discharge l i g h t . All effects were found to be s t r i c t l y proportion-
/
J.P.M. Schmitt Silane plasma decompositions
651
al to the discharge c u r r e n t and the pressure (below 0.5 mTorr), confirming the electron-molecule primary process model. The measurements were made q u a n t i t a t i v e by c a l i b r a t i o n with already known gases such as H2 and CH441,42. cm 2
TOTAl DISSI)CIATON
10-~5 8
~
4i
lO-t~
8
f
r
j
r
gro r~d sta e S i H / \
FIGURE 2
4
Cross sections f o r
2
e l e c t r o n impact on s i l a n e .
10-I~ 8
/
4
!He..__
2
10-1e 8 4 2 i0-19 0
/ I0
/
/
//~~~-
20
30
~ $i 451Po $iH+ A11"I
40
50
60
70 eV
The r e s u l t s f o r s i l a n e are summarized in f i g . 2 . The region near or below the i o n i z a t i o n threshold were inaccessible to m u l t i p o l e . Hence the threshold part of the i o n i z a t i o n curve was taken from Turban e t a l . 19, the emission curve was extended with a crossed beam system45 and the d i s s o c i a t i o n curve was e x t r a p o l a ted toward the U.V. absorption threshold 43. One point is given f o r the d i s s o c i a t i o n o f silan~ with formation of a ground s t a ~ SiH r a d i c a l , i t was measured by laser induced fluorescence in the m u l t i p o l e ~ . The branching r a t i o between the various mechanisms f o r 70 eV e l e c t r o n impact induced d i s s o c i a t i o n of SiH 4 is i l l u s t r a t e d in f i g . 3 . I t appears t h a t i o n i z a t i o n and neutral d i s s o c i a t i o n are of comparable importance. SiH is a minor component among the created r a d i c a l s (14%) and only 2% o f the SiH are created on the emissive e x i t e d s t a t e . I f l i g h t emission gives some i n f o r m a t i o n on the plasma primary processes 44 i t is a minor e f f e c t in the discharge. Among the created
FIGURE 3 P r o b a b i l i t y manifold f o r the s i l a n e d i s s o c i a t i v e channels due to a 70eV e l e c t r o n impact.
J.P.M. Schmitt / Silane plasma decompositions
652
radicals the r e l a t i v e importances of Si, SiH 2 and SiH~ are unknown, however f o r fast electrons (40eV and more) there is some t h e o r e t i c a l i n d i c a t i o n 46-47 that SiH 3 and SiH 2 should dominate and be of comparable importance. For lower energy electrons, i t is obvious from f i g . 2 that neutral d i s s o c i a t i o n becomes much l a r g e r than i o n i z a t i o n . As pointed out by Turban et a l . °, in glow discharges where the active electrons are expected to be in the 8-12 eV range the primary processes should mostly generate r a d i c a l s . 5. SURFACE PROCESSES The d e t a i l s of the surface mechanisms are f a r to be understood. Globally hydrogenated s i l i c o n compounds condensate into the amorphous s i l i c o n matrix and l i b e r a t e some excess hydrogen. The reverse process (etching) can take place 4849 in presence of a large excess of atomic hydrogen. 5.1. Sticking p r o b a b i l i t y One major issue in modeling the discharge is the p r o b a b i l i t y y for a potent i a l l y reactive species to leave the surface without incorporating. A device f o r measuring y is shown in f i g . 4 . The substrate is covered by a polished perforated f o i l , the distance a should remain smaller than the mean free path. The r a t i o between the f i l m thickness on the back of the grid and on the substrate is equal to an average y f o r the species responsible f o r deposition. In a m u l t i pole device (below 4 mTorr) the s t i c k i n g p r o b a b i l i t y ( l - y ) was measured to be l a r g e r than 70%. Obtaining the same information in 0 . 1 - I Torr glow discharges w i l l require handling of a very small device (a < 50 ~).
_ PL sM
FIGURE 4 Principle of the measurement of the sticking probability,
-1 $ I I $ $
FIGURE 5 Principle of the measurement of the ion flow (a) and the ion k i n e t i c energy (b).
5.2 lon flow, ion energy Positive ion surface r e a c t i v i t y can be very d i f f e r e n t from radicals due to the i o n i z a t i o n potential energy and because ion can reach the substrate with a large k i n e t i c energy° I . Measuring the ion flow, the ion energy and the ionic composition is r e l a t i v e l y easy. As shown in f i g . 5 the substrate can be made of a g r i d , behind which a col]Rctor is biased negatively enough to repel a l l electrons and c o l l e c t the ions TM. In the e l e c t r o s t a t i c analyser a second grid allows by scanning the voltage Vg an analysis of the ion k i n e t i c energy d i s t r i b u t i o n . I d e a l l y in both systems tile electrode spacing should remain smaller than the Debye length and the mean free path and l a r g e r than the grid mesh size. I n t e r Bretinq the ion flow in terms of s i l i c o n mass flow requires mass spectrometry - i 0 , 16, 22, 52, 53. Care should be taken to c a l i b r a t e the system mass response54, 55. The ion k i n e t i c energy was measured in multipole and demonstrated to be well c o n t r o l l e d by substrate bias 50, s i m i l a r work is to be done in silane glow d i s charges where large k i n e t i c energy due to s e l f bias and broad d i s t r i b u t i o n due to c o l l i s i o n s in the sheath are to be expected 56. Despite the very large
J.P.M. Schmitt / Silane plasma decompositions
653
polymerized ions observed the average number o f s i l i c o n atoms deposited per p o s i t i v e charge seems8 - I 0 to remain below 2. F i n a l l y , the r e l a t i v e amount o f s i l i c o n reaching the surface as ion must be very small 3 in a l l discharges except in m u l t i p o l e l O where i t can reach 80%. 6. SECONDARY PROCESSES 6.1. D i f f u s i o n time Before reaching the w a l l s the r e a c t i v e species must d i ~ u s e out. The r a d i c a l d i f f u s i o n r a t e s can be estimated from the molecular sizes J~ and the ion ambipof a r d i f f u s i o n r a t e s from the gases p o l a r i s a b i l i t y 6 0 . I f y is supposed to be zero a l l d i f f u s i o n times can be w r i t t e n f o r an Hydrogen s i l a n e m i x t u r e TD = ~b + R2(~ (H2) + B(SiN4) ) where ~ and c a l , gap i f l i m i t . Note exceedingly
(7)
~ are given in t a b l e I , and R is the chamber size (radius i f s p h e r i p l a n a r ) . The b a l l i s t i c time Tb accounts f o r the non c o l l i s i o n a l t h a t f o r n e u t r a l s i l i c o n c l u s t e r s the residence time can become large.
Table I : C o e f f i c i e n t s f o r the d i f f u s i o n time a t 500°K The e l e c t r o n temperature should be expressed in eV. H
SiH n
SiNH m
n =0-3 (lO-21s/cm)
1.3
3.9
(lO-21s/cm)
1.7
13
N
H+
SIH n "+
SiNH ~
5
2.5 N I0 N
0.3 Te 0.5 Te
3.2
2.2 Te
Te
6.2 Secondary r e a c t i o n s While d i f f u s i n g o u t , a r e a c t i v e species can experienc~ various secondary r e a c t i o n s . Some o f the r e a c t i o n rates were measured 2~, b~-b4, and a majorant o f the ion r e a c t i o n r a t e s can be deduced from the Langevin cross s e c t i o n 6 0 ; they are given i n ' t a b l e I I . The r e a c t i o n time ZK is given in pure s i l a n e by ~1 Table I I
= k (SIN4).
: Some secondary r e a c t i o n r a t e s Enthalpy Kcal/mole
Reactions
N(~QO°~) I 10-1Ccm~s - I
H + SiH 4
÷
SiH 3 + H2
(8)
- 11
5
SiH 2 + H2
÷
SiH 4
(9)
- 50
0.008
SiH 2 + SiH 4
+
Si2H 6
(10)
- 49
2.3
Si2H 5 SiH + SiH 4 SiH~ + SiH 4
+ ~
SiNH ~ + SiH 4 +
- 36 (II)
Si2H 3 + H2 SiH~ + SiH 3
(12)
SiN+lB,_2 + H2
(13)
3.3 7 - 20
~ I000 850 <
A r e a c t i v e species w i l l r e a c t before reaching the w a l l s i f T D ~ T. . Both times were c a l c u l a t e d f o r a 17cm diameter spherical chamber and p u r e K s i l a n e , they are p l o t t e d in f i g . 6 . I t appears t h a t i f ions have a f a i r l y s h o r t residence
J.P.M. Schmitt / Silane plasma decompositions
654
ms
'
t
.
i
•
,
,
l
,
i
•
,
10FIGURE 6
,,, ~.:,'y
Diffusion time (solid lines) and reaction time for various species. The crossing point, where gas phase reaction takes over, are encircled. The small circles are SiH l i f e time as measured by laser induced fluorescence28 and the small dot line is the calculated SiH total lifetime.
,
0.1
0.01. i
0
i~
10
,
i
i
2"0
30
i
40
i
50
i
mTorr
time due to the discharge build-in electric f i e l d , they react even faster and this is i l l u s t r a t e d by the complex ion spectra observed in most glow discharges 8, 9, 52, 53. SiH and H react in silane above 25 and 60 mTorr respectively, these thresholds should s h i f t upward for smaller chambers. Atomic hydrogen is a by-product of several primary and secondary reactions. Most of i t w i l l subsequently give birth via reaction (8) to SiH3 which does not react with silane 58. As a consequence, in many systems, SiHR is expected to be the dominant radical, a point which recently found two experYmental confirmations25, 29. However in silane depleted discharge Si may be dominant18. On the other hand, as pointed out by Hailer 27, the ion reaction rate is so large that the quantity of silane consumed via ionic'processes may become important despite the small ionized fraction. 6.3. Long residence times The clusters SiNHX because of their slow diffusion and the negative ions SiNH~ because they are trapped in the plasma53 have such long residence time that they can eventually encounter another reactive species. They can then build up to a f a i r l y large size and possibly end up as powder a desastrous phenomena for the film. 7. CONCLUSION IN FORMOF A TENTATIVE RESEARCH PROGRAM Daring to be speculative, I w i l l l i s t here the species which, to my point of view, can be recognized to be important to the film growth and final properties: - The radical SiH3 because i t is probably the main silicon carrier. - The clusters because, even i f they are a minority, they may incorporate very differently in the film. - Ions because they bombardthe surface. - Atomic Hydrogen because, by selective etching, i t influences the f i l m structure. - Impurities, often ignored for lack of knowledge, they may be vital to the electronic properties.
J.P.M Schmitt / Silane plasma decompositions
655
Even i f this l i s t is correct, the r e l a t i v e influence of these components with respect to the f i l m structure is s t i l l to be understood. This w i l l require more systematic analysis of more samples grown in more c a r e f u l l y controlled conditions. The plasma chemists should build a r e l i a b l e model for silane discharges based on measurements of the radical content. The impurity control should be drastic and the plasma related transport of impurities and dopants should be studied. F i n a l l y the l i n k between the plasma phase and the f i n a l structure of the grown semiconductor thin f i l m is the surface. There is a great need of informations on the surface processes 64-65 which lead to the condensation of complex flow of radical and ions into a solid amorphous phase. ACKNOWLEDGMENT I would l i k e to thank a l l my colleagues at Ecole Polytechnique who helped me in a l l the steps of this work. REFERENCES i) G. Nolet, J. Electrochem. Soc. 122 (1975) 1030. 2) S. Vepreck, Pure and Appl. Chem. 48 (1976) 163. 3) G. Turban and Y. Catherine, Thin Solid Films 35 (1976) 179. 4) G. Turban and Y. Catherine, Thin Solid Films 48 (1979) 57. 5) Y. Catherine and G. Turban, Thin Solid Films 60 (1979) 147. 6) J. Perrin and E. Delafosse, J. Phys. D:Appl. Physi. 13 (1980) 759. 7) P. Kocian, M.J. Mayor and S. Bourquard, in Proceedings of the 14th Int.Symp. on Plasma Chem. eds. S. Vepreck and J. Hertz (University of Zurich, 1979)663. 8) I. Haller, Appl. Phys. Lett. 37 (1980) 282. 9) H.A. Weakliem, AIAA 13th Fluid and Plasma Dynamics Conference AIAA (1980) 1327. IO)B. Drevillon , J. Huc, A. Lloret, J. Perrin, G. de Rosny and J.P.M. Schmitt, Appl. Phys. Lett. 37 (1980) 177. l l ) A . Matsuda, L. Nakagawa, K.Tanaka, M. Matsumura, S. Yamasaki, M. Okushi and S. lizima, J. Non Cryst. Solids 35-36 (1980) 183. 12)M. Tanigushi, M. Hirose, T. Hamasaki and Y. Osaka, Appl. Phys. Lett. 37 (1980) 787. 13)R.W. G r i f f i t h , F.J. Kampas, P.E. Vamier and M.D. Hirsh, J. Non Cryst. Solids 35-36 (1980) 391. 14)M.A. Paesler, T. Okumura and W.Paul, J. Vac. Sci. Technol.17 (1980) 1332. 15)F.J. Kampas and R.W. G r i f f i t h , J. Appl. Phys. 52 (1981) 1285. 16)J.J. Wagner and S. Vepreck, Plasma Chem. and Plasma Process 2 (1982) 95. 17)J.C. Knights, J.P.M. Schmitt, J. Perrin and G. Guelachvili, J.Chem. Phys. 76 (1982) 3414. 18)K. Tashibana, H. Tadukaro, H. Harima and Y. Urano, J.Phys.D: Appl. Phys. 15 (1982) 177. 19)G. Turban, Y. Catherine and B. Grolleau, Plasma Chem. and Plasma Process. 2 (1982) 61. 20)T.Hamasaki, H. Kurata, M. Hirose and Y. Osaka, Appl. Phys.Lett. 37 (1980) 1084. 21)M. Hirose, T. Hamasaki, Y. Mishima, H. Kurata and Y. Osaka in Tetrahedrally bQnded amorphous semiconductors, eds. R.A.Street, D.K. Biegelsen and
656
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J.C. Knights (American Institute of Physics, New York 1981) 10. 22)A. Matsuda and K. Tanaka, Thin Solid Films 92 (1982) 171. 23)C.A. Joseph Jr., A. Gascarden and D.R. Pond, Spectroscopic measurements in silane reactor plasmas, in Proceedings of LASERS'82 (New Orleans 1982). 24)N. Hata, A. Matsuda, K. Tanaka, K. Kajiyama, N. Moro and K. Sajiki, Jap.J. Appl. Phys. 22 (1983) LI. 25)R. Robertson, D. H i l l s , H. Chatham and A. Gallagher, to be published in Appl. Phys. Lett. 26)G. de Rosny, E.R. Mosburg J r . , J.R Abelson, G. Devaud and R.C. Kerns, J. Appl. Phys. 54 (1983) 2272. 27)I. Haller, J. Vac. Sci. Technol. to be published. 28)J.P.M. Schmitt, P. Gressier, M. Krishnan, G. de Rosny and J. Perrin, to be published in Chem. Phys. 29) P.A. Longeway, R.D. Estes and H.A. Weakliem (1983) submitted to J.Phys.Chem. 30)F.E. Saafeld and H.J. Svec, Inorg. Chem. 2 (1963) 46. 31)P. Potzinger and F.W. Lampe, J. Phys. Chem. 73 (1969) 3912. 32)J.Do Morrison and J.C. Traeger, Int. J. Mass Spect. and lon Phys. 11 (1973) 289. \ 33)Von H. Ebinghauss, K.Krauss, W. MUller-Duysing and H. Neuert, Z. Naturforsh. 19a (1964) 732. 34 W.J. Pollock, Trans. Faraday Soc. 64 (1968) 2919. 35 R. Limpaecherand K.R. Mac Kenzie, Rev. Sci. Instr. 44 (1973) 726. 36 N. Hershkowitz, J.R. Smith and J.R. de Kock, Plasma Phys. 21 (1979) 823. 37 K.N. Leung, N. Hershkowitz and K.R. Mac Kenzie, Phys. Fluids 19 (1976) 1045. 38 A. Truc, T.D. Mantei and D. Gresillon, Rev. Phys. Appl. 12 (1977) 1143. 39 A. Lang and N. Hershkowitz, J. Appl. Phys. 49 (1978) 4707. 40 J. Perrin, J.P.M. Schmitt, G. de Rosny, B. Drevillon, J. Huc and A. Lloret, Chem. Phys. 73 (1982) 383. 41)H.F. Winters, J. Chem. Phys. 63 (1975) 3462. 42)J.F.M. Aarts, C.I.M. Beenakkerand F.J. de Heer, Physica 53 (1971) 32. 43)Y. Harada, J.N. Murell and H.H. Sheena, Chem. Phys. Lett. I (1968) 595. 44)J. Perrin and J.P.M. Schmitt, Chemical Physics 67 (1982) 167. 45)J. Perrin and J.F.M. Aarts, to be published in Chem. Phys. 46)D.E. Donohue, J.A. Schiavoneand R.S. Freund, J. Chem.Phys. 67 (1977) 769. 47)C.E. Melton and P.S. Rudolph, J. Chem. Phys. 47 (1967) 1171. 48)A.P. Webb and S. Vepreck, Chem. Phys. Lett. 62 (1979) 173. 49)G. Turban, Y. Catherine and B. Grolleau, Thin Solid Films 77 (1981) 287. 50)B. Drevillon, J Perrin, J.M. Siefert, J. Huc, A. Lloret, G. de Rosny and J.P.M. Schmitt, Appl. Phys. Lett. 42 (1983) 801. 51)J.W. Coburn and E. Kay, J. Appl. Phys. 43 (1972) 4965. 52)G. Turban, Y. Catherine and B. Grolleau, Thin Solid Films 67 (1980) 309. 53)A. Lloret, J. Perrin, G. de Rosny and J.P.M. Schmitt, to be published.
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54)W.E. Austin, A.E. Holine and J. M. Leck, in "Quadrupole Mass Spectrometry", ed. P.M. Dawson (Elsevier, Amsterdam, 1976) chap.6. 55)R.L. Inglebert and J.F. Hennequin, Revue Phys. Appl. 15 (1980) 1489. 56)W.D. Davis and T.A. Vanderslice, Phys. Rev. 131 (1963) 219. 57)S. Dushman, Scientific Foundations of Vacuum Technique (Wiley & Sons, New York, 1972). 58)E.R. Austin and F.W. Lampe, J.Phys. Chem. 81 (1977) 1134. 59)P. John and J.H. Purnell, J. Chem. Soc. Faraday Trans.l 69 (1973) 1455. 60)E.W. McDaniel, Collision Phenomena in Ionized Gases (Wiley, New York, 1964). 61)J,M.S. Henis, G.W. Stewart, M.K. Tripodi and P.P. Gaspar, J.Chem. Phys. 57 (1972) 389. 62)FoW. Lampe, lon-Molecule Reactions in silane and Organosilane systems in "Interactions between ions and molecules", ed. P. Ausloss (Plenium,New York 1975). 63)E.R. Mosburg Jr., R.C. Kerns, J.R. Abelson, to be published . 64)A. Matsuda, M. Matsumura, N. Nakagawa, S. Yamasaki and K. Tanaka, J. Physique 42 (1981) C4-687. 65)F.j. Kampas, J. Appl. Phys. 53 (1982) 6408.
649
FUNDAMENTAL MECHANISMS IN SILANE PLASMA DECOMPOSITIONS AND AMORPHOUS SILICON DEPOSITION J.P.M. SCHMITT Equipe Synth@se de Couches Minces pour l'Energ@tique, L.P.N.H.E., Ecole Polytechnique, 91128 PALAISEAU Cedex, France. The mechanisms r e s u l t i n g in amorphous s i l i c o n deposition from silane glow discharges are c l a s s i f i e d in primary processes, (electron impact on s i l a n e ) , and secondary processes such as ion-molecule and radical-molecule reactions. The low pressure l i m i t of a multipole DC discharge is shown to allow the measurement of various cross-sections associated with the primary processes, in p a r t i c u l a r spontaneous emission. The results from plasma probing with ion mass spectrometry, laser induced fluorescence and other techniques are described in relevance to the problem of the thin f i l m growth. 1. INTRODUCTION As f o r a l l other thin f i l m s , and as f o r a l l preparation techniques, hydrogenated amorphous s i l i c o n (a-Si:H) deposited by silane glow discharge is found to have s t r u c t u r a l , chemical and e l e c t r o n i c properties very dependant on the depos i t i o n conditions. Not only the q u a l i t y of the semiconductor varies when in a given deposition chamber the parameters such as temperature, pressure or power are modified, but one may assume that the scattering among the results from the various teams preparing a-Si:H is to be explained by the differences in the preparation condition. As a consequence, there is a need to understand silane glow discharges in order to deduce from the external parameters of the system a complete description on a microscopic scale of the p a r t i c l e s reaching or leaving the surface. Then, hopefully, i t w i l l be possible to c o r r e l a t e the properties of the f i l m to the chemical and the energetic states of the species impinging onto the subs~ra~e. Such a complex task has led, w i t h i n the l a s t 8 years to a growing a c t i v i t y i-c~ involving diagnostic and modeling of silane discharge~ The ambition of the present paper is to scan through the up to date knowledge of the mechanisms occuring in a silane discharge and to point out at the many dark areas which remain w i t h i n the plasma black box. After s e t t i n g the orders of magnitude f o r the main parameters the most important plasma mechanisms are reviewed. The measurements of some e l e c t r o n - s i l a n e cross-sections in a low pressure m u l t i p o l e discharge are reported. The e f f e c t of secondary reactions and surface reactions are described along with the l a t e s t results concerning the various species responsible f o r the a-Si:H f i l m growth. Further research lines are discussed in the concluding remarks. 2. A FEW ORDERS OF MAGNITUDE In a l l deposition plasmas the major component is made of neutral stable gases including the injected gases (SiH4, B2H6, PH3...), the c a r r i e r gases (Ar, He, H2... ), the by-product gases (H2, Si2 H6... ) and possibly unwanted gases (degasing, l e a k . . . ) . Even at f a i r l y large deposition rate (powerfull discharges) the proportion of unstable molecular fragments ( r a d i c a l s ) remains r e l a t i v e l y small with respect to stable molecule (below 1%) ; the ionized f r a c t i o n is even lower (below 0.1%). As a consequence a reactive species w i l l experience mostly c o l l i sions with the background gases and/or the w a l l s . Hence, except f o r very specif i c cases, reactions between excited species such as e l e c t r o n - i o n , electronr a d i c a l , i o n - r a d i c a l , r a d i c a l - r a d i c a l can be neglected. The gas temperature 0022-3093/83/0000 0000/$03.00 ©1983 North-Holland/Physical Society of Japan
J,P.M. Schmitt / Silane plasma decompositions
650
remains close to the wall temperature. The bulk of the electron population has a temperature of the order of 0.1-I eV but the high energy t a i l is generally non thermal and must extend above 12 eV in order to provide enough ionization for sv}tain~ng the discharge. The electron and ion densities5, 63 vary from 108 to 10I~ cm-~ depending on the discharge intensity. The Debyelength, typical scale for the plasma Coulomb screening of a potential disturbance remains below imm. 3. PRIMARY PROCESSES In all standard glow discharges the driving electric power is fed into the electron kinetic energy. The primary process responsible for the discharge rea c t i v i t y is the inelastic collision between an energetic electron and a molecule from the background gas. Examples for the possible consequence of electron impact on silane are given here e + SiH4,._ + dissociative ionization19,30-32 \-"~SiH 3 + H + 2e (I) neutral dissociation \SiH2 + H+ H+ e (2) dissociative excitation \SiH* + H~ + H + e + hv (3) pair formation 31 ~ \SiH~ + H~ + e (4) electron dissociative attachmentoJ ~SiH5 + H2 (5) rovibrational excitation34 Si ~ + e (6) 4. ELECTRON IMPACT CROSSSECTION MEASUREMENTS The measurementswere performed in a D.C. multipole discharge35-40. Its specific features are i ) i t s a b i l i t y to work at low pressure (from 0.1 mTorr), i i ) below 0.5 mTorr a quasi monoenergetic distribution for the fast electrons, i i i ) a partial confinement of the charged species by the multipolar magnetic walls. The multipole is shown in f i g . l . In the low pressure regime, due to the
permanent
ISZ\-H[]---/_(
i==
ine cusp
magnets
heating resistance
I
FIGURE 1 Multipole device with its electrical diagnostics.
F-5
i :=
low temperature of the bulk electron, the primary processes are entirgly generated by the primary electrons, the density and the energy of which can be independently selected by tuning the filament emissivity and the discharge bias. The consequence of the primary processes were measured : total dissociation (reactions of type (1) to (5)) deduced from the silane partial pressure depletion 40, ionization (reactions of type (1) and (4)) was related to the ion flow onto the walls ~u, photoemission (reaction type (3)) is analysed by spectrometry 44 of the discharge l i g h t . All effects were found to be s t r i c t l y proportion-
/
J.P.M. Schmitt Silane plasma decompositions
651
al to the discharge c u r r e n t and the pressure (below 0.5 mTorr), confirming the electron-molecule primary process model. The measurements were made q u a n t i t a t i v e by c a l i b r a t i o n with already known gases such as H2 and CH441,42. cm 2
TOTAl DISSI)CIATON
10-~5 8
~
4i
lO-t~
8
f
r
j
r
gro r~d sta e S i H / \
FIGURE 2
4
Cross sections f o r
2
e l e c t r o n impact on s i l a n e .
10-I~ 8
/
4
!He..__
2
10-1e 8 4 2 i0-19 0
/ I0
/
/
//~~~-
20
30
~ $i 451Po $iH+ A11"I
40
50
60
70 eV
The r e s u l t s f o r s i l a n e are summarized in f i g . 2 . The region near or below the i o n i z a t i o n threshold were inaccessible to m u l t i p o l e . Hence the threshold part of the i o n i z a t i o n curve was taken from Turban e t a l . 19, the emission curve was extended with a crossed beam system45 and the d i s s o c i a t i o n curve was e x t r a p o l a ted toward the U.V. absorption threshold 43. One point is given f o r the d i s s o c i a t i o n o f silan~ with formation of a ground s t a ~ SiH r a d i c a l , i t was measured by laser induced fluorescence in the m u l t i p o l e ~ . The branching r a t i o between the various mechanisms f o r 70 eV e l e c t r o n impact induced d i s s o c i a t i o n of SiH 4 is i l l u s t r a t e d in f i g . 3 . I t appears t h a t i o n i z a t i o n and neutral d i s s o c i a t i o n are of comparable importance. SiH is a minor component among the created r a d i c a l s (14%) and only 2% o f the SiH are created on the emissive e x i t e d s t a t e . I f l i g h t emission gives some i n f o r m a t i o n on the plasma primary processes 44 i t is a minor e f f e c t in the discharge. Among the created
FIGURE 3 P r o b a b i l i t y manifold f o r the s i l a n e d i s s o c i a t i v e channels due to a 70eV e l e c t r o n impact.
J.P.M. Schmitt / Silane plasma decompositions
652
radicals the r e l a t i v e importances of Si, SiH 2 and SiH~ are unknown, however f o r fast electrons (40eV and more) there is some t h e o r e t i c a l i n d i c a t i o n 46-47 that SiH 3 and SiH 2 should dominate and be of comparable importance. For lower energy electrons, i t is obvious from f i g . 2 that neutral d i s s o c i a t i o n becomes much l a r g e r than i o n i z a t i o n . As pointed out by Turban et a l . °, in glow discharges where the active electrons are expected to be in the 8-12 eV range the primary processes should mostly generate r a d i c a l s . 5. SURFACE PROCESSES The d e t a i l s of the surface mechanisms are f a r to be understood. Globally hydrogenated s i l i c o n compounds condensate into the amorphous s i l i c o n matrix and l i b e r a t e some excess hydrogen. The reverse process (etching) can take place 4849 in presence of a large excess of atomic hydrogen. 5.1. Sticking p r o b a b i l i t y One major issue in modeling the discharge is the p r o b a b i l i t y y for a potent i a l l y reactive species to leave the surface without incorporating. A device f o r measuring y is shown in f i g . 4 . The substrate is covered by a polished perforated f o i l , the distance a should remain smaller than the mean free path. The r a t i o between the f i l m thickness on the back of the grid and on the substrate is equal to an average y f o r the species responsible f o r deposition. In a m u l t i pole device (below 4 mTorr) the s t i c k i n g p r o b a b i l i t y ( l - y ) was measured to be l a r g e r than 70%. Obtaining the same information in 0 . 1 - I Torr glow discharges w i l l require handling of a very small device (a < 50 ~).
_ PL sM
FIGURE 4 Principle of the measurement of the sticking probability,
-1 $ I I $ $
FIGURE 5 Principle of the measurement of the ion flow (a) and the ion k i n e t i c energy (b).
5.2 lon flow, ion energy Positive ion surface r e a c t i v i t y can be very d i f f e r e n t from radicals due to the i o n i z a t i o n potential energy and because ion can reach the substrate with a large k i n e t i c energy° I . Measuring the ion flow, the ion energy and the ionic composition is r e l a t i v e l y easy. As shown in f i g . 5 the substrate can be made of a g r i d , behind which a col]Rctor is biased negatively enough to repel a l l electrons and c o l l e c t the ions TM. In the e l e c t r o s t a t i c analyser a second grid allows by scanning the voltage Vg an analysis of the ion k i n e t i c energy d i s t r i b u t i o n . I d e a l l y in both systems tile electrode spacing should remain smaller than the Debye length and the mean free path and l a r g e r than the grid mesh size. I n t e r Bretinq the ion flow in terms of s i l i c o n mass flow requires mass spectrometry - i 0 , 16, 22, 52, 53. Care should be taken to c a l i b r a t e the system mass response54, 55. The ion k i n e t i c energy was measured in multipole and demonstrated to be well c o n t r o l l e d by substrate bias 50, s i m i l a r work is to be done in silane glow d i s charges where large k i n e t i c energy due to s e l f bias and broad d i s t r i b u t i o n due to c o l l i s i o n s in the sheath are to be expected 56. Despite the very large
J.P.M. Schmitt / Silane plasma decompositions
653
polymerized ions observed the average number o f s i l i c o n atoms deposited per p o s i t i v e charge seems8 - I 0 to remain below 2. F i n a l l y , the r e l a t i v e amount o f s i l i c o n reaching the surface as ion must be very small 3 in a l l discharges except in m u l t i p o l e l O where i t can reach 80%. 6. SECONDARY PROCESSES 6.1. D i f f u s i o n time Before reaching the w a l l s the r e a c t i v e species must d i ~ u s e out. The r a d i c a l d i f f u s i o n r a t e s can be estimated from the molecular sizes J~ and the ion ambipof a r d i f f u s i o n r a t e s from the gases p o l a r i s a b i l i t y 6 0 . I f y is supposed to be zero a l l d i f f u s i o n times can be w r i t t e n f o r an Hydrogen s i l a n e m i x t u r e TD = ~b + R2(~ (H2) + B(SiN4) ) where ~ and c a l , gap i f l i m i t . Note exceedingly
(7)
~ are given in t a b l e I , and R is the chamber size (radius i f s p h e r i p l a n a r ) . The b a l l i s t i c time Tb accounts f o r the non c o l l i s i o n a l t h a t f o r n e u t r a l s i l i c o n c l u s t e r s the residence time can become large.
Table I : C o e f f i c i e n t s f o r the d i f f u s i o n time a t 500°K The e l e c t r o n temperature should be expressed in eV. H
SiH n
SiNH m
n =0-3 (lO-21s/cm)
1.3
3.9
(lO-21s/cm)
1.7
13
N
H+
SIH n "+
SiNH ~
5
2.5 N I0 N
0.3 Te 0.5 Te
3.2
2.2 Te
Te
6.2 Secondary r e a c t i o n s While d i f f u s i n g o u t , a r e a c t i v e species can experienc~ various secondary r e a c t i o n s . Some o f the r e a c t i o n rates were measured 2~, b~-b4, and a majorant o f the ion r e a c t i o n r a t e s can be deduced from the Langevin cross s e c t i o n 6 0 ; they are given i n ' t a b l e I I . The r e a c t i o n time ZK is given in pure s i l a n e by ~1 Table I I
= k (SIN4).
: Some secondary r e a c t i o n r a t e s Enthalpy Kcal/mole
Reactions
N(~QO°~) I 10-1Ccm~s - I
H + SiH 4
÷
SiH 3 + H2
(8)
- 11
5
SiH 2 + H2
÷
SiH 4
(9)
- 50
0.008
SiH 2 + SiH 4
+
Si2H 6
(10)
- 49
2.3
Si2H 5 SiH + SiH 4 SiH~ + SiH 4
+ ~
SiNH ~ + SiH 4 +
- 36 (II)
Si2H 3 + H2 SiH~ + SiH 3
(12)
SiN+lB,_2 + H2
(13)
3.3 7 - 20
~ I000 850 <
A r e a c t i v e species w i l l r e a c t before reaching the w a l l s i f T D ~ T. . Both times were c a l c u l a t e d f o r a 17cm diameter spherical chamber and p u r e K s i l a n e , they are p l o t t e d in f i g . 6 . I t appears t h a t i f ions have a f a i r l y s h o r t residence
J.P.M. Schmitt / Silane plasma decompositions
654
ms
'
t
.
i
•
,
,
l
,
i
•
,
10FIGURE 6
,,, ~.:,'y
Diffusion time (solid lines) and reaction time for various species. The crossing point, where gas phase reaction takes over, are encircled. The small circles are SiH l i f e time as measured by laser induced fluorescence28 and the small dot line is the calculated SiH total lifetime.
,
0.1
0.01. i
0
i~
10
,
i
i
2"0
30
i
40
i
50
i
mTorr
time due to the discharge build-in electric f i e l d , they react even faster and this is i l l u s t r a t e d by the complex ion spectra observed in most glow discharges 8, 9, 52, 53. SiH and H react in silane above 25 and 60 mTorr respectively, these thresholds should s h i f t upward for smaller chambers. Atomic hydrogen is a by-product of several primary and secondary reactions. Most of i t w i l l subsequently give birth via reaction (8) to SiH3 which does not react with silane 58. As a consequence, in many systems, SiHR is expected to be the dominant radical, a point which recently found two experYmental confirmations25, 29. However in silane depleted discharge Si may be dominant18. On the other hand, as pointed out by Hailer 27, the ion reaction rate is so large that the quantity of silane consumed via ionic'processes may become important despite the small ionized fraction. 6.3. Long residence times The clusters SiNHX because of their slow diffusion and the negative ions SiNH~ because they are trapped in the plasma53 have such long residence time that they can eventually encounter another reactive species. They can then build up to a f a i r l y large size and possibly end up as powder a desastrous phenomena for the film. 7. CONCLUSION IN FORMOF A TENTATIVE RESEARCH PROGRAM Daring to be speculative, I w i l l l i s t here the species which, to my point of view, can be recognized to be important to the film growth and final properties: - The radical SiH3 because i t is probably the main silicon carrier. - The clusters because, even i f they are a minority, they may incorporate very differently in the film. - Ions because they bombardthe surface. - Atomic Hydrogen because, by selective etching, i t influences the f i l m structure. - Impurities, often ignored for lack of knowledge, they may be vital to the electronic properties.
J.P.M Schmitt / Silane plasma decompositions
655
Even i f this l i s t is correct, the r e l a t i v e influence of these components with respect to the f i l m structure is s t i l l to be understood. This w i l l require more systematic analysis of more samples grown in more c a r e f u l l y controlled conditions. The plasma chemists should build a r e l i a b l e model for silane discharges based on measurements of the radical content. The impurity control should be drastic and the plasma related transport of impurities and dopants should be studied. F i n a l l y the l i n k between the plasma phase and the f i n a l structure of the grown semiconductor thin f i l m is the surface. There is a great need of informations on the surface processes 64-65 which lead to the condensation of complex flow of radical and ions into a solid amorphous phase. ACKNOWLEDGMENT I would l i k e to thank a l l my colleagues at Ecole Polytechnique who helped me in a l l the steps of this work. REFERENCES i) G. Nolet, J. Electrochem. Soc. 122 (1975) 1030. 2) S. Vepreck, Pure and Appl. Chem. 48 (1976) 163. 3) G. Turban and Y. Catherine, Thin Solid Films 35 (1976) 179. 4) G. Turban and Y. Catherine, Thin Solid Films 48 (1979) 57. 5) Y. Catherine and G. Turban, Thin Solid Films 60 (1979) 147. 6) J. Perrin and E. Delafosse, J. Phys. D:Appl. Physi. 13 (1980) 759. 7) P. Kocian, M.J. Mayor and S. Bourquard, in Proceedings of the 14th Int.Symp. on Plasma Chem. eds. S. Vepreck and J. Hertz (University of Zurich, 1979)663. 8) I. Haller, Appl. Phys. Lett. 37 (1980) 282. 9) H.A. Weakliem, AIAA 13th Fluid and Plasma Dynamics Conference AIAA (1980) 1327. IO)B. Drevillon , J. Huc, A. Lloret, J. Perrin, G. de Rosny and J.P.M. Schmitt, Appl. Phys. Lett. 37 (1980) 177. l l ) A . Matsuda, L. Nakagawa, K.Tanaka, M. Matsumura, S. Yamasaki, M. Okushi and S. lizima, J. Non Cryst. Solids 35-36 (1980) 183. 12)M. Tanigushi, M. Hirose, T. Hamasaki and Y. Osaka, Appl. Phys. Lett. 37 (1980) 787. 13)R.W. G r i f f i t h , F.J. Kampas, P.E. Vamier and M.D. Hirsh, J. Non Cryst. Solids 35-36 (1980) 391. 14)M.A. Paesler, T. Okumura and W.Paul, J. Vac. Sci. Technol.17 (1980) 1332. 15)F.J. Kampas and R.W. G r i f f i t h , J. Appl. Phys. 52 (1981) 1285. 16)J.J. Wagner and S. Vepreck, Plasma Chem. and Plasma Process 2 (1982) 95. 17)J.C. Knights, J.P.M. Schmitt, J. Perrin and G. Guelachvili, J.Chem. Phys. 76 (1982) 3414. 18)K. Tashibana, H. Tadukaro, H. Harima and Y. Urano, J.Phys.D: Appl. Phys. 15 (1982) 177. 19)G. Turban, Y. Catherine and B. Grolleau, Plasma Chem. and Plasma Process. 2 (1982) 61. 20)T.Hamasaki, H. Kurata, M. Hirose and Y. Osaka, Appl. Phys.Lett. 37 (1980) 1084. 21)M. Hirose, T. Hamasaki, Y. Mishima, H. Kurata and Y. Osaka in Tetrahedrally bQnded amorphous semiconductors, eds. R.A.Street, D.K. Biegelsen and
656
J.P.M. Schmitt / Silane plasma decompositions
J.C. Knights (American Institute of Physics, New York 1981) 10. 22)A. Matsuda and K. Tanaka, Thin Solid Films 92 (1982) 171. 23)C.A. Joseph Jr., A. Gascarden and D.R. Pond, Spectroscopic measurements in silane reactor plasmas, in Proceedings of LASERS'82 (New Orleans 1982). 24)N. Hata, A. Matsuda, K. Tanaka, K. Kajiyama, N. Moro and K. Sajiki, Jap.J. Appl. Phys. 22 (1983) LI. 25)R. Robertson, D. H i l l s , H. Chatham and A. Gallagher, to be published in Appl. Phys. Lett. 26)G. de Rosny, E.R. Mosburg J r . , J.R Abelson, G. Devaud and R.C. Kerns, J. Appl. Phys. 54 (1983) 2272. 27)I. Haller, J. Vac. Sci. Technol. to be published. 28)J.P.M. Schmitt, P. Gressier, M. Krishnan, G. de Rosny and J. Perrin, to be published in Chem. Phys. 29) P.A. Longeway, R.D. Estes and H.A. Weakliem (1983) submitted to J.Phys.Chem. 30)F.E. Saafeld and H.J. Svec, Inorg. Chem. 2 (1963) 46. 31)P. Potzinger and F.W. Lampe, J. Phys. Chem. 73 (1969) 3912. 32)J.Do Morrison and J.C. Traeger, Int. J. Mass Spect. and lon Phys. 11 (1973) 289. \ 33)Von H. Ebinghauss, K.Krauss, W. MUller-Duysing and H. Neuert, Z. Naturforsh. 19a (1964) 732. 34 W.J. Pollock, Trans. Faraday Soc. 64 (1968) 2919. 35 R. Limpaecherand K.R. Mac Kenzie, Rev. Sci. Instr. 44 (1973) 726. 36 N. Hershkowitz, J.R. Smith and J.R. de Kock, Plasma Phys. 21 (1979) 823. 37 K.N. Leung, N. Hershkowitz and K.R. Mac Kenzie, Phys. Fluids 19 (1976) 1045. 38 A. Truc, T.D. Mantei and D. Gresillon, Rev. Phys. Appl. 12 (1977) 1143. 39 A. Lang and N. Hershkowitz, J. Appl. Phys. 49 (1978) 4707. 40 J. Perrin, J.P.M. Schmitt, G. de Rosny, B. Drevillon, J. Huc and A. Lloret, Chem. Phys. 73 (1982) 383. 41)H.F. Winters, J. Chem. Phys. 63 (1975) 3462. 42)J.F.M. Aarts, C.I.M. Beenakkerand F.J. de Heer, Physica 53 (1971) 32. 43)Y. Harada, J.N. Murell and H.H. Sheena, Chem. Phys. Lett. I (1968) 595. 44)J. Perrin and J.P.M. Schmitt, Chemical Physics 67 (1982) 167. 45)J. Perrin and J.F.M. Aarts, to be published in Chem. Phys. 46)D.E. Donohue, J.A. Schiavoneand R.S. Freund, J. Chem.Phys. 67 (1977) 769. 47)C.E. Melton and P.S. Rudolph, J. Chem. Phys. 47 (1967) 1171. 48)A.P. Webb and S. Vepreck, Chem. Phys. Lett. 62 (1979) 173. 49)G. Turban, Y. Catherine and B. Grolleau, Thin Solid Films 77 (1981) 287. 50)B. Drevillon, J Perrin, J.M. Siefert, J. Huc, A. Lloret, G. de Rosny and J.P.M. Schmitt, Appl. Phys. Lett. 42 (1983) 801. 51)J.W. Coburn and E. Kay, J. Appl. Phys. 43 (1972) 4965. 52)G. Turban, Y. Catherine and B. Grolleau, Thin Solid Films 67 (1980) 309. 53)A. Lloret, J. Perrin, G. de Rosny and J.P.M. Schmitt, to be published.
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657
54)W.E. Austin, A.E. Holine and J. M. Leck, in "Quadrupole Mass Spectrometry", ed. P.M. Dawson (Elsevier, Amsterdam, 1976) chap.6. 55)R.L. Inglebert and J.F. Hennequin, Revue Phys. Appl. 15 (1980) 1489. 56)W.D. Davis and T.A. Vanderslice, Phys. Rev. 131 (1963) 219. 57)S. Dushman, Scientific Foundations of Vacuum Technique (Wiley & Sons, New York, 1972). 58)E.R. Austin and F.W. Lampe, J.Phys. Chem. 81 (1977) 1134. 59)P. John and J.H. Purnell, J. Chem. Soc. Faraday Trans.l 69 (1973) 1455. 60)E.W. McDaniel, Collision Phenomena in Ionized Gases (Wiley, New York, 1964). 61)J,M.S. Henis, G.W. Stewart, M.K. Tripodi and P.P. Gaspar, J.Chem. Phys. 57 (1972) 389. 62)FoW. Lampe, lon-Molecule Reactions in silane and Organosilane systems in "Interactions between ions and molecules", ed. P. Ausloss (Plenium,New York 1975). 63)E.R. Mosburg Jr., R.C. Kerns, J.R. Abelson, to be published . 64)A. Matsuda, M. Matsumura, N. Nakagawa, S. Yamasaki and K. Tanaka, J. Physique 42 (1981) C4-687. 65)F.j. Kampas, J. Appl. Phys. 53 (1982) 6408.