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Mass spectrometric investigation of the thermal decomposition of various organosilicon compounds in SixC1-x(H) chemical vapour deposition
end Applied &-rolysk, 4 (1982) 59-72 Elsetier Scientific Publiig Company, Amsterdam - Printed in 1.e
Journal of Andytied
59
Sc*9aerLnds
_UW SPECI’ROME’I’RIC IXVESTIGXlXOX OF THE THERXAL DECOMPOSITIOX OF VARIOUS ORGASOSILICOX COXPOL3~S S&C,,(H) CHEMICAL VAPOC? DEPOSITIOS
J-P. GERXFLT
IS
and G. COSSTASF
+. R. XORXSCHO
Lcborctoim de Chimie Minircle dt Cristc!!ochimie. E-22 263. Cris~c?!ockkk. R&+eiizftP et Pro?action des Mc?&+azu. E__._S_C_T_. IIS route de _Vsbosrx. 3IG7i To~!ozsc Cefa ,Fnnce)
P- MAZEROLLES
and G- !+IXSUEL
Lcboruto~re de+ Orgznon:i?tJiiques. _Ycrbonne. 31062 Toulouse Cedex
L’n~rc.-si:6 PczES=Jstizr. EZ-4 524
i IS ror;ie dc
fFrsce~
(Received SOT-ember 9th, 19Sl;accepZed
Janszq- 12vk. 1952)
Thermal decompcsition studies 03 various organometallIc molecazk co~?ti~IILg silicon have &en pe,cforxned to obtain amorphous silicon azd &icon csrbXe cos~gs by cbeaiml repour deposition_ In parziculzr, some 0rgacome~AIic comgcnxds ccntai~i~g wea2 silicox--carbun bonds were employed. The therzz.xddecomposi;io=l of four =rolea!es was investigated: two cyclic molecules, sila-5_spkof4-4 ~~or~-2.‘;CIene {IA) zr,d d%e;hyl-l.Ty-diene (I& a& txo nOlqCk mok’z!esF :ez?apXQaq&s~axe sila-%piro[4-4]nona-2. (II,) and tetraerhy-nykkne (II&_ F%-eobsem?d that the therrzai &compo&io~ of II* proceeds via the formation of IA_ The ~apour-phase analysis was taxied oxt by mzss qxctromefry azd decomposkk~ sc5emes are proposed. The influence of vzrious esperimenul pas.mete,rs 03 tie amo;rrr*s of by-products of decompodtiaa was in\-ertigared_1: is imporar IO =o-cc ia padcAr that the temperature of decomposition is a function of the st.nckre of +ke compoz2~&, eqecklly with respect KO the ptition of the me-ky-‘l ~Ssizuueot (for cydic rrro!ccti!es~or rhe acerylenic bond in the Q- or $position relative EO the &con a:om {for zo?-+ydic molecules)_
During the two past decades, the use of organometalk compounds as starting materials for the chemical rapour deposition (C_V_D_) of 1horgan.k cornpour& has been widely developed. It is well known that silicon de\;ices are made induskially from Si& or SiCL and also that 2 great number of experiments have been carried out with compounds containing 53-S bonds (in which X is I, Br, Cl, F, H, N, ek). On the 0th~ hand, whew oxganosilicon compounds are used in C.V_D_ processes, their decomposition leads to the formation of silicon carbide only at high temperatures 11 f _
60
We have undertaken a programme to e\tiuati the use of organometallic
compounds at low temperatures in order to determine the detailed mechanism of decomposition of new organosilicon molecules. Two hinds of mokuks were synthesized: Type I: cyclic molecules whose thermal decomposition can proceed according to the following schemes:
(‘\ -I _ ,s .\ ..: ‘. ,,’
s
n
-2
‘J’
,
IA
Llc
Sila-55piro[4.4]nona-2,7diene -.-.p
=-c
.---
2
-+
-_
_
__I
I.
. .-
/ ‘-c .
5
:
I3 Dimeth~L2,7+ihG~piro
[4.-LJnona-2,7diene
Type Ii: non-cyclic molecules:
I&, Tetrapropargylsilane Si(CXH);
Hc.‘W:
+ 2 Hz -
Si: + 4 HeCH
IIB Tetraethynylsilane Such molecules have some interesting characteristics:
(a) the silicon atom is surrounded by four carbon atoms and the Si-C bonds are appreciably the same in each molecule; (b) for cyclic molecules the constraints induced by the ring may destabilize the Si--C bonds, whereas for non-cyclic molecules the presence of the acetvlenic bonds has the same effect; ic) the presumed decomposition by-products, xiz., butadiene, methylbutadiene, propyne and acetylene, are thermal& stable, so it can be expected that the coating will not be contaminated by carbon at low temperature.
ESPERIMESTAL
The preparation and Aaramtion of IA, IB, II,, and IIn have been reporied pre\iously [2-4]. Compounds I AI 1s and IIA are liquids whereas
61 IIB is crystalline, and they are easily purified by distilktion and sublimation, respectively. They are not very sensitive to air and can be handled in the laboratory atmosphere, except for IIS, which has a high \a_pourpressure and sublimes at room temperature. It is also highly explosive and may detonate violently upon physical contact.
Appamtus
and procedure
The apparatus used to study the thermal decomwsition of the four molecules, a classical C.V.D. apparatus, is presented in Fig. 1. The thermal decomposition of the fust three molecules was performed in a helium atmosphere whereas the decomposition of IIP -;;es carried out under helium at reduced pressure (34 mbar). The liquid molecules (IA, Ia and II,) are contdmd in a thennostated bubbler between 40 and 60°C according to the starting product This ternperature regulates the partial vapour pressure of the molecules in the carrier gas, purified helium in this instance. It is possible to introduce another gas such as hydrogen. The gaseous phase passes through a silica reactor tube (SO mm O.D.) where the decomposition occurs on the substrates heated by a high-&equency generatir io between 330 and ‘XO’C, according to tie molecule. The solid molecule IIB is sublimed in helium at recuced pressure (34
mbar) by heating a tube to 100°C with a smali furnace on the gas Iine (B). The thermal decomposition occurs on& on the substrates that are, according to the experiments, sintered aiumina plates or monocrystalline plates (AI=O,, Si, Ge). Some comments must be made about the thermal exchange between the substrates and the gaseous phase. The substrates and the susceptor(graphite) are heated only by the high-tiquenc~- generator, whereas the gaseous phase is heated by a temperature gradient between the substrates and the siIica reactor, The decomposition by-products are coikcted in a trap cooled to liquid nitrogen temperature, then anaiysed by mass spectrometry. _-lnaiyticaiprocedure -4nai~se~ were carried out in a Bakers QMS 311 quadrupole mass spectrometer with an iontition voltage of 70 eV. The trap is isolated from the apparatus by means of the v&e C and the carrier gas is removed by pumping. The temperature of the trap is raised slow1~ to ambient and enters the mass spectrometer in the gaseous phase. From this time, mass spectra are recorded continuousiy. The first gas entering the mass spectrometer is that which has the lowest boiling point, for e-sample, ethylene (-1_03_7’C(1 atm) or acetylene (-M.O=C/l atm). Increasing to room temperature, gases having higher boiIing are anai~sed. By comparison with standard mass spectra compiled by Cor_w and Massoz { 51, it is possible to identify the different species present in : he trap and to tiablkh their variation as a function of the temperature of the Iatter. A second esperiment is necessary, using the same experimental conditions, to obtain a mass spectrum of the total contents of the trap when it is Wg to ambient temperature_ Knowing the nature of the gas phase and the intensity of each peak in the total spectrum, it is possible to evaluate the percentage of every product in the gaseous phase. These vaIues are given with an accuracy of about 10%. For us, this deviation is not important because we oni:- study the relative \xriations of each product present in the gaseous phase and these are adequate for estabhshing the decomposition reaction.
RESCLTS
XSD DISCL-SSIOS
Before studying the gaseous phase, the mass specka of the starting molecuk were recorded in order to obtain reference spectra. Moreover, some indications of the stabihty of these compounds can be obtained from these mass spectra (Fig. 2)_ For clariv, the isotopic peaks (29Si, ‘OSi) have been omitted. Jhs
spectra of cyclic moZecuZes For IA, according to SaIomon [6] there are two strong peaks with m/z 82
and mP 136 assigned to a siIylene ion,
r\ t 7
- ,which results from the loss of
63
A/)
a butadiene molecule and to the molecular ion peak 1 i/V
-
There are
also peaks at m/z 54,39,27 and 53 due to butadiene. For IB, a similar f&gmentation occurs. As for I,, the specirum shows the molecular ion w~_/y=
at m/z 164, but the most ;Jnporknt peak cor-
VW responds to the silylene ion y.
_
‘v’
64 Fig. 2 :coztinaxd). --
-_
?- -
_?
-
_. .*
.c
?,r?:
J:
--. I_.
d
at rn:z 96. .There is also the characteristic fragmentation of methylbutadiene at n?:‘r67,53,68 and 39. _Ykssspectra of non-c>clic
molecules
The mass spxtnzm of IIB was obtained by Davidsohn and Henry [7]_ 0~ mass spectrum Tsin good agreement with theirs, with a mokdar ion at r;& 128 and the stepwise loss of +ZCH fkagments at mfz 103,77,53 and 26. The mass spectrum of II_* has some analogies with that of II,; the molecular ion at m/z lS4 wit5 weak intensity is a notable feature. The peaks of
65
m/z lower than 184 can be explained by loss of XH:-CSH and -H fragments. Extrusion of -H fkagments gives a cluster of low-intensi&. peaks of m/z 154-160. This phenomenon again occurs since we note clusters of peaks at m[z 128-132,101-107 and 77-81, always induced by successive loss of -H or CzHz hgments. Succesive eMon of
Fii. 3. Mass wectral axslssis of the principal eshausz gws formed in the decomposition of 1~ at different temperatures 0, CH:=CH-CH=CH:; I, CH==CH:; =, C,H+
66
4.
/’
j
b3 Si
\
i 1 i
7’
C
t
CSH6
Fig_ -L Decozzlposition scheme of 1~ versus coating tempeatun.
the ions at m/z 103 and 82 come from the loss of ethylene and butadiene molecules, respectively_ Gaseous phase from the decompositibn of IS The masimum percentage of methyl-2-butadiene and propadiene is obtained at 65O’C. When the temperat-= increases, the percentage of these two compounds decreases and they are replaced by ethylene and acetylene. The lariation of the percentages of the different by-products of decomposition versus temperature and the decomposition scheme are shown in Figs. 5 and 6, respectively, -The thermal decomposition and fragmentation of In under an eleckon beam in the mass spectrometer source displays some analogies with IA. The prese::ce of methyl-2-bukliece, propene and ethylene in the gaseous phase appears to be due to the stelxise loss of ethylene (mlz 136), propene (m/z 122) and methyl-2-butadiene (m/z 96) in the mass spectrum. Moreover, the difference in the decomp%ition temperatures of IA and Ia (560 and 6OO”C, respecti~-ely) can be explained by the presence of the methyl groups which ha\= an inductor donor effect which stabilizes the ring. Guseous phase from the decomposition o,CIIA Unlike the decomposition scheme proposed in the Intiuction, the percentage of propyne rem2b.s low (generally 10% with a maximum of 19% at 38C”C). However, large amounts of acetylene and ethylene are formed,
67
JL-
:-3
-
Fig. 5.3Gss spectral analyti of the principal eshmst gzses form& in the decomposido~ of 1~ at different temperatures_ 1. CH:==CH=; C, CH:=QCH3)-_CH=CH:;*.CH2=C=CH2_
probably owing to cleavage of the C-C bond of the propaxgyl group, The sum of the percentages of these two compounds is mslxjmal at 460°C (77%) and decreses at temperatures between 480 and 600% (ca- 55%) (Fig. T). As these products are lost, IA is identified in-the gas phase- Al of the characteristic peaks (m/z 136, 82, lOS, 54) of this molecule are present. This phenomenon is surprising, and an attempt at an esplanation can only be given with caution. We can postulate the hypothesis that a cefain rscom-
Pig- 6. Decomposition sdmxnt
of 1~ nmxs coating ttnperature.
bination occurs, inside the stagnant Iaxer (i-e-, near the substrate), between radical species according to the scheme in Fig. 8. The organic moiety shown in the scheme w-as identified in the mass spectrum of II,, (m,:2 range 12S--132) and we can aSfllme that &is species may be formed after ekctron impact as weIl as after thermoIysis_ This hypothesis is supported by the work of ScSaden [S-lo], who observed the same phenomenon with aromatic carbonyl compounds and phenols. The biradicaIs -.
-
-‘\
_-
/
/b\
:- . -_-. _:-. :“f
\.
1-s _
-
_
_-:_-: I
-
.
b
-
I
:
:.I_
I..
-1
. _- .:-_:-
-
I-
-
I-.
1 .
:_
. I
-1
I-.
.
I_
of IIA versus coating temperature in z helium+@roge~
69
Fig. 9. Mass spectral analysis of the principal exbmsr gases formed ir, ae ckoqxx~tio~ of IIB, =. CHrCH:--CH3. CHr(CH:):-CH3 aF_d CH3-(CH2)3--CH3; I, CH:=CH: a-?d CH=CH_
formed are vey reactiveand we can assume that intramokukr recombination occurs, forming the ringcontracted product. This inkzmediate compound appears between 540 and 56O’C, which comnds to the mkimum decomposition temperature of IA. On the other hand, the presence of b-a@diene, in the same amount as I,, (14%) in the gaseous phase, is also detectedThe mechankm of decomposition observed is then the same as that for I,. Gcseous phase fivm the decomposition of& As can be seen from Fig, 9, two kinds of decomposition b:=products are present in the gaseous phase. The first consis& of unsaturati molecules with two caxbon atoms coming from i;he cleatzge of the Si-C bond and
Fig. 10. Decompcasition s&eme of 11~ ve_sus coatbg remperataare atmosphav @+IJsH~ - O-5%) under reduced pressure (34 nbar)_
21 a helium-hybogerr
which do not undergo molecular recombination. This is the case for ethylene and acetylene, whose total percentage passes through a maximum, at 440°C (7-J%), and then declines rapidly to about 40% and remains constant throughout the remainder of the tempmture range. The second kind of by-product is formed by saturated compounds such as propane, butane, pentane. Therefore, the decomposition scheme is as proposed in Fig. 10. In this instance also we observe some analogies between the thermal decompodrion and the mass spectrum of the starting molecule. It is interesting to note the influence of the position of the acetylenic bond with respect to silicon. Uith II* and IIB we mainly identify molecules with two carbon atoms (ethylene, acetylene), probably owing to the position of the acetylenic bond which regulates the cleawge of the C< or Si-C bonds by an inductor attractor effect. Co_z&tion
betu-enngcseorrsphcse composition and deposited solids
.-kcording to the results, we conclude that thermal decomposition is connected to the structures of the molecules. In aU C.V.D. reactions, there is a gaseous phase and a solid phase, which is the coating. To understand the siz~ow correlation between the gaseous phase and the coating, it is nm to invrxtigate the latter. We therefore carried out quantitative analysis by S-ray photoelectron spectroscopy (XPS). The resuhs for all of the deposits have been reported previously 1111 and are summarized in Table 1. The results show that pure amorphous silicon is nerer obtained (the best re&t is S2 atom-72 of silicon for IIB at 3SO’C). However, it is easy to estab-
113
3scl 460 560 620
32 7.5 60 35
IS 23 40 63
il
L
Sk
6h
&?
732
155
‘t
Fig- ll_
Tempe_,r~ture dependence bttaeea the atotic xrceatage of Dixon in zhe costings and the respective percentages of butadiene and methyl-%bucx?ier?e sfrer rbermd
decomposition of IA and Is_
lish a correlation between the Herent percentages of silicon and carbon shown in Table 1 and the amox.-- of every decomposition by-product- found in our gaseous phase analysis. Ths different bond clea\zges that-occur in the molecules induce a great \xriety of coating compo~!tions. For the cyclic molecules IA and IB, the n~~ximum percentage of atomic silicon is obtained when there is a masimum of butadiene ar,d methyl-2butadiene, respectively, in the gas phase (Fig. ll)_
Fig. 12. \‘a&tion cf aro_mic percentage of dicon temperawn for II&
axxi decompod:ion,
~~~_xx?xs
vers1zs
For non-cyclic molecules, the hypothesis of the influence of the position of the acetylenic bond relative to the silicon atom is in good agreement with the amount of silicon and carbon embedded in the coatings. For II*, where the acety-lenic bond induces the clealxge of the C--C bond, there are smaU amounts of silicon (ca. 25 atom-%), whereas for IIn, where the cleabzge occurs between the s&on and carbon atoms, a high percentage of sikon (S2 atom-51 is ob’%ined. Moreover, jhe increase in the atomic percentage of &icon for II, in the temperature range ISO-ZQO’C is probably due to the formation then the decomposition of the intermediate IA (Fig. 12).
In C.V.D. reactions, gas-phase analysis provides information on the decomposition mechanismisof four organosilicon compounds which are good precursors of amorphous silicon and silicon carbide deposits [ll]. It is necem to correlate the results of *he mass spectrometry of the gas phase and the _SPS results of the solid phase, in order to explain the decomposition mechanisms. Mass spectrometric results, which permit the identification of the different species in the gas phase and their respectire amounts, zuv in good agreement with those obtained by XPS. Sloreowr, for these types of silicon derivatives, the mass spectrum of each starting molecule shows, according to the bond energies, the preferential fzgmentation that we find again in thermal decomposition.
REPZXESCES i J. ScXic:l:ir~. -, D. Tercmna, Jag.. 50 (19i1)
Powder lL!c.a!l. kt., 12 j19s-O) 141. S. Hatrs, T. Araki, T. Ceki, T. Okzzaki and T. Suzuki, Bull. Chem. Sot. X%:5.
3 L.Q. l!ir.h, J.C. Bi!liorc and P. Ckdiot, C. R. Aed. Sci. Ser. C, 251 (1960) i30. 4 X1.. Komarov and 0-G. Yarosh. Zh. O.%xh. Khim., 3i (1967) 264. ,..d 8. Mw.so:, Index de Speares de Masse, Presses Cnirersitzires de Fraxe, 5 .A. cornu qFbris, 1965.
6 R.G. S~.o.?-,o~, J. Org. Chem., 39 (1974) 3602. I\‘. DxZsohn ad XC. Hem, J. Orgazzonetall. Chem., 5 ( 1966) 29. G. Scksdea, Adran. 1k.s Specrrom.. 7B (1976) 1340. G. Schaden. J~srus LieSTgs Xna. Chem., 4 (19X) 559. G. S&mticza. Z. S~~~dozsch.
B: Anozg.
cira,
w.
Chem.,
33B
(1976)
663.
D Mazerolla, G. .\hn;lel and J.J. Ekhtrdr, J.P. Gc.c;ruk, R. Moran&o. G. Corsunt. -. k JAI Blocher, Jr., G.E. \.uiUard and G. Wahl (Editors), Proc. 6th Int. Conf. Chemiol \.apor Deposition. Chsntiliy. Paris. September 15-18, 19S1, l-o!. 61-7, The Ekezcxkemica! Society, Per.r.icgron, SJ. p. 338.
Journal of Andytied
59
Sc*9aerLnds
_UW SPECI’ROME’I’RIC IXVESTIGXlXOX OF THE THERXAL DECOMPOSITIOX OF VARIOUS ORGASOSILICOX COXPOL3~S S&C,,(H) CHEMICAL VAPOC? DEPOSITIOS
J-P. GERXFLT
IS
and G. COSSTASF
+. R. XORXSCHO
Lcborctoim de Chimie Minircle dt Cristc!!ochimie. E-22 263. Cris~c?!ockkk. R&+eiizftP et Pro?action des Mc?&+azu. E__._S_C_T_. IIS route de _Vsbosrx. 3IG7i To~!ozsc Cefa ,Fnnce)
P- MAZEROLLES
and G- !+IXSUEL
Lcboruto~re de+ Orgznon:i?tJiiques. _Ycrbonne. 31062 Toulouse Cedex
L’n~rc.-si:6 PczES=Jstizr. EZ-4 524
i IS ror;ie dc
fFrsce~
(Received SOT-ember 9th, 19Sl;accepZed
Janszq- 12vk. 1952)
Thermal decompcsition studies 03 various organometallIc molecazk co~?ti~IILg silicon have &en pe,cforxned to obtain amorphous silicon azd &icon csrbXe cos~gs by cbeaiml repour deposition_ In parziculzr, some 0rgacome~AIic comgcnxds ccntai~i~g wea2 silicox--carbun bonds were employed. The therzz.xddecomposi;io=l of four =rolea!es was investigated: two cyclic molecules, sila-5_spkof4-4 ~~or~-2.‘;CIene {IA) zr,d d%e;hyl-l.Ty-diene (I& a& txo nOlqCk mok’z!esF :ez?apXQaq&s~axe sila-%piro[4-4]nona-2. (II,) and tetraerhy-nykkne (II&_ F%-eobsem?d that the therrzai &compo&io~ of II* proceeds via the formation of IA_ The ~apour-phase analysis was taxied oxt by mzss qxctromefry azd decomposkk~ sc5emes are proposed. The influence of vzrious esperimenul pas.mete,rs 03 tie amo;rrr*s of by-products of decompodtiaa was in\-ertigared_1: is imporar IO =o-cc ia padcAr that the temperature of decomposition is a function of the st.nckre of +ke compoz2~&, eqecklly with respect KO the ptition of the me-ky-‘l ~Ssizuueot (for cydic rrro!ccti!es~or rhe acerylenic bond in the Q- or $position relative EO the &con a:om {for zo?-+ydic molecules)_
During the two past decades, the use of organometalk compounds as starting materials for the chemical rapour deposition (C_V_D_) of 1horgan.k cornpour& has been widely developed. It is well known that silicon de\;ices are made induskially from Si& or SiCL and also that 2 great number of experiments have been carried out with compounds containing 53-S bonds (in which X is I, Br, Cl, F, H, N, ek). On the 0th~ hand, whew oxganosilicon compounds are used in C.V_D_ processes, their decomposition leads to the formation of silicon carbide only at high temperatures 11 f _
60
We have undertaken a programme to e\tiuati the use of organometallic
compounds at low temperatures in order to determine the detailed mechanism of decomposition of new organosilicon molecules. Two hinds of mokuks were synthesized: Type I: cyclic molecules whose thermal decomposition can proceed according to the following schemes:
(‘\ -I _ ,s .\ ..: ‘. ,,’
s
n
-2
‘J’
,
IA
Llc
Sila-55piro[4.4]nona-2,7diene -.-.p
=-c
.---
2
-+
-_
_
__I
I.
. .-
/ ‘-c .
5
:
I3 Dimeth~L2,7+ihG~piro
[4.-LJnona-2,7diene
Type Ii: non-cyclic molecules:
I&, Tetrapropargylsilane Si(CXH);
Hc.‘W:
+ 2 Hz -
Si: + 4 HeCH
IIB Tetraethynylsilane Such molecules have some interesting characteristics:
(a) the silicon atom is surrounded by four carbon atoms and the Si-C bonds are appreciably the same in each molecule; (b) for cyclic molecules the constraints induced by the ring may destabilize the Si--C bonds, whereas for non-cyclic molecules the presence of the acetvlenic bonds has the same effect; ic) the presumed decomposition by-products, xiz., butadiene, methylbutadiene, propyne and acetylene, are thermal& stable, so it can be expected that the coating will not be contaminated by carbon at low temperature.
ESPERIMESTAL
The preparation and Aaramtion of IA, IB, II,, and IIn have been reporied pre\iously [2-4]. Compounds I AI 1s and IIA are liquids whereas
61 IIB is crystalline, and they are easily purified by distilktion and sublimation, respectively. They are not very sensitive to air and can be handled in the laboratory atmosphere, except for IIS, which has a high \a_pourpressure and sublimes at room temperature. It is also highly explosive and may detonate violently upon physical contact.
Appamtus
and procedure
The apparatus used to study the thermal decomwsition of the four molecules, a classical C.V.D. apparatus, is presented in Fig. 1. The thermal decomposition of the fust three molecules was performed in a helium atmosphere whereas the decomposition of IIP -;;es carried out under helium at reduced pressure (34 mbar). The liquid molecules (IA, Ia and II,) are contdmd in a thennostated bubbler between 40 and 60°C according to the starting product This ternperature regulates the partial vapour pressure of the molecules in the carrier gas, purified helium in this instance. It is possible to introduce another gas such as hydrogen. The gaseous phase passes through a silica reactor tube (SO mm O.D.) where the decomposition occurs on the substrates heated by a high-&equency generatir io between 330 and ‘XO’C, according to tie molecule. The solid molecule IIB is sublimed in helium at recuced pressure (34
mbar) by heating a tube to 100°C with a smali furnace on the gas Iine (B). The thermal decomposition occurs on& on the substrates that are, according to the experiments, sintered aiumina plates or monocrystalline plates (AI=O,, Si, Ge). Some comments must be made about the thermal exchange between the substrates and the gaseous phase. The substrates and the susceptor(graphite) are heated only by the high-tiquenc~- generator, whereas the gaseous phase is heated by a temperature gradient between the substrates and the siIica reactor, The decomposition by-products are coikcted in a trap cooled to liquid nitrogen temperature, then anaiysed by mass spectrometry. _-lnaiyticaiprocedure -4nai~se~ were carried out in a Bakers QMS 311 quadrupole mass spectrometer with an iontition voltage of 70 eV. The trap is isolated from the apparatus by means of the v&e C and the carrier gas is removed by pumping. The temperature of the trap is raised slow1~ to ambient and enters the mass spectrometer in the gaseous phase. From this time, mass spectra are recorded continuousiy. The first gas entering the mass spectrometer is that which has the lowest boiling point, for e-sample, ethylene (-1_03_7’C(1 atm) or acetylene (-M.O=C/l atm). Increasing to room temperature, gases having higher boiIing are anai~sed. By comparison with standard mass spectra compiled by Cor_w and Massoz { 51, it is possible to identify the different species present in : he trap and to tiablkh their variation as a function of the temperature of the Iatter. A second esperiment is necessary, using the same experimental conditions, to obtain a mass spectrum of the total contents of the trap when it is Wg to ambient temperature_ Knowing the nature of the gas phase and the intensity of each peak in the total spectrum, it is possible to evaluate the percentage of every product in the gaseous phase. These vaIues are given with an accuracy of about 10%. For us, this deviation is not important because we oni:- study the relative \xriations of each product present in the gaseous phase and these are adequate for estabhshing the decomposition reaction.
RESCLTS
XSD DISCL-SSIOS
Before studying the gaseous phase, the mass specka of the starting molecuk were recorded in order to obtain reference spectra. Moreover, some indications of the stabihty of these compounds can be obtained from these mass spectra (Fig. 2)_ For clariv, the isotopic peaks (29Si, ‘OSi) have been omitted. Jhs
spectra of cyclic moZecuZes For IA, according to SaIomon [6] there are two strong peaks with m/z 82
and mP 136 assigned to a siIylene ion,
r\ t 7
- ,which results from the loss of
63
A/)
a butadiene molecule and to the molecular ion peak 1 i/V
-
There are
also peaks at m/z 54,39,27 and 53 due to butadiene. For IB, a similar f&gmentation occurs. As for I,, the specirum shows the molecular ion w~_/y=
at m/z 164, but the most ;Jnporknt peak cor-
VW responds to the silylene ion y.
_
‘v’
64 Fig. 2 :coztinaxd). --
-_
?- -
_?
-
_. .*
.c
?,r?:
J:
--. I_.
d
at rn:z 96. .There is also the characteristic fragmentation of methylbutadiene at n?:‘r67,53,68 and 39. _Ykssspectra of non-c>clic
molecules
The mass spxtnzm of IIB was obtained by Davidsohn and Henry [7]_ 0~ mass spectrum Tsin good agreement with theirs, with a mokdar ion at r;& 128 and the stepwise loss of +ZCH fkagments at mfz 103,77,53 and 26. The mass spectrum of II_* has some analogies with that of II,; the molecular ion at m/z lS4 wit5 weak intensity is a notable feature. The peaks of
65
m/z lower than 184 can be explained by loss of XH:-CSH and -H fragments. Extrusion of -H fkagments gives a cluster of low-intensi&. peaks of m/z 154-160. This phenomenon again occurs since we note clusters of peaks at m[z 128-132,101-107 and 77-81, always induced by successive loss of -H or CzHz hgments. Succesive eMon of
Fii. 3. Mass wectral axslssis of the principal eshausz gws formed in the decomposition of 1~ at different temperatures 0, CH:=CH-CH=CH:; I, CH==CH:; =, C,H+
66
4.
/’
j
b3 Si
\
i 1 i
7’
C
t
CSH6
Fig_ -L Decozzlposition scheme of 1~ versus coating tempeatun.
the ions at m/z 103 and 82 come from the loss of ethylene and butadiene molecules, respectively_ Gaseous phase from the decompositibn of IS The masimum percentage of methyl-2-butadiene and propadiene is obtained at 65O’C. When the temperat-= increases, the percentage of these two compounds decreases and they are replaced by ethylene and acetylene. The lariation of the percentages of the different by-products of decomposition versus temperature and the decomposition scheme are shown in Figs. 5 and 6, respectively, -The thermal decomposition and fragmentation of In under an eleckon beam in the mass spectrometer source displays some analogies with IA. The prese::ce of methyl-2-bukliece, propene and ethylene in the gaseous phase appears to be due to the stelxise loss of ethylene (mlz 136), propene (m/z 122) and methyl-2-butadiene (m/z 96) in the mass spectrum. Moreover, the difference in the decomp%ition temperatures of IA and Ia (560 and 6OO”C, respecti~-ely) can be explained by the presence of the methyl groups which ha\= an inductor donor effect which stabilizes the ring. Guseous phase from the decomposition o,CIIA Unlike the decomposition scheme proposed in the Intiuction, the percentage of propyne rem2b.s low (generally 10% with a maximum of 19% at 38C”C). However, large amounts of acetylene and ethylene are formed,
67
JL-
:-3
-
Fig. 5.3Gss spectral analyti of the principal eshmst gzses form& in the decomposido~ of 1~ at different temperatures_ 1. CH:==CH=; C, CH:=QCH3)-_CH=CH:;*.CH2=C=CH2_
probably owing to cleavage of the C-C bond of the propaxgyl group, The sum of the percentages of these two compounds is mslxjmal at 460°C (77%) and decreses at temperatures between 480 and 600% (ca- 55%) (Fig. T). As these products are lost, IA is identified in-the gas phase- Al of the characteristic peaks (m/z 136, 82, lOS, 54) of this molecule are present. This phenomenon is surprising, and an attempt at an esplanation can only be given with caution. We can postulate the hypothesis that a cefain rscom-
Pig- 6. Decomposition sdmxnt
of 1~ nmxs coating ttnperature.
bination occurs, inside the stagnant Iaxer (i-e-, near the substrate), between radical species according to the scheme in Fig. 8. The organic moiety shown in the scheme w-as identified in the mass spectrum of II,, (m,:2 range 12S--132) and we can aSfllme that &is species may be formed after ekctron impact as weIl as after thermoIysis_ This hypothesis is supported by the work of ScSaden [S-lo], who observed the same phenomenon with aromatic carbonyl compounds and phenols. The biradicaIs -.
-
-‘\
_-
/
/b\
:- . -_-. _:-. :“f
\.
1-s _
-
_
_-:_-: I
-
.
b
-
I
:
:.I_
I..
-1
. _- .:-_:-
-
I-
-
I-.
1 .
:_
. I
-1
I-.
.
I_
of IIA versus coating temperature in z helium+@roge~
69
Fig. 9. Mass spectral analysis of the principal exbmsr gases formed ir, ae ckoqxx~tio~ of IIB, =. CHrCH:--CH3. CHr(CH:):-CH3 aF_d CH3-(CH2)3--CH3; I, CH:=CH: a-?d CH=CH_
formed are vey reactiveand we can assume that intramokukr recombination occurs, forming the ringcontracted product. This inkzmediate compound appears between 540 and 56O’C, which comnds to the mkimum decomposition temperature of IA. On the other hand, the presence of b-a@diene, in the same amount as I,, (14%) in the gaseous phase, is also detectedThe mechankm of decomposition observed is then the same as that for I,. Gcseous phase fivm the decomposition of& As can be seen from Fig, 9, two kinds of decomposition b:=products are present in the gaseous phase. The first consis& of unsaturati molecules with two caxbon atoms coming from i;he cleatzge of the Si-C bond and
Fig. 10. Decompcasition s&eme of 11~ ve_sus coatbg remperataare atmosphav @+IJsH~ - O-5%) under reduced pressure (34 nbar)_
21 a helium-hybogerr
which do not undergo molecular recombination. This is the case for ethylene and acetylene, whose total percentage passes through a maximum, at 440°C (7-J%), and then declines rapidly to about 40% and remains constant throughout the remainder of the tempmture range. The second kind of by-product is formed by saturated compounds such as propane, butane, pentane. Therefore, the decomposition scheme is as proposed in Fig. 10. In this instance also we observe some analogies between the thermal decompodrion and the mass spectrum of the starting molecule. It is interesting to note the influence of the position of the acetylenic bond with respect to silicon. Uith II* and IIB we mainly identify molecules with two carbon atoms (ethylene, acetylene), probably owing to the position of the acetylenic bond which regulates the cleawge of the C< or Si-C bonds by an inductor attractor effect. Co_z&tion
betu-enngcseorrsphcse composition and deposited solids
.-kcording to the results, we conclude that thermal decomposition is connected to the structures of the molecules. In aU C.V.D. reactions, there is a gaseous phase and a solid phase, which is the coating. To understand the siz~ow correlation between the gaseous phase and the coating, it is nm to invrxtigate the latter. We therefore carried out quantitative analysis by S-ray photoelectron spectroscopy (XPS). The resuhs for all of the deposits have been reported previously 1111 and are summarized in Table 1. The results show that pure amorphous silicon is nerer obtained (the best re&t is S2 atom-72 of silicon for IIB at 3SO’C). However, it is easy to estab-
113
3scl 460 560 620
32 7.5 60 35
IS 23 40 63
il
L
Sk
6h
&?
732
155
‘t
Fig- ll_
Tempe_,r~ture dependence bttaeea the atotic xrceatage of Dixon in zhe costings and the respective percentages of butadiene and methyl-%bucx?ier?e sfrer rbermd
decomposition of IA and Is_
lish a correlation between the Herent percentages of silicon and carbon shown in Table 1 and the amox.-- of every decomposition by-product- found in our gaseous phase analysis. Ths different bond clea\zges that-occur in the molecules induce a great \xriety of coating compo~!tions. For the cyclic molecules IA and IB, the n~~ximum percentage of atomic silicon is obtained when there is a masimum of butadiene ar,d methyl-2butadiene, respectively, in the gas phase (Fig. ll)_
Fig. 12. \‘a&tion cf aro_mic percentage of dicon temperawn for II&
axxi decompod:ion,
~~~_xx?xs
vers1zs
For non-cyclic molecules, the hypothesis of the influence of the position of the acetylenic bond relative to the silicon atom is in good agreement with the amount of silicon and carbon embedded in the coatings. For II*, where the acety-lenic bond induces the clealxge of the C--C bond, there are smaU amounts of silicon (ca. 25 atom-%), whereas for IIn, where the cleabzge occurs between the s&on and carbon atoms, a high percentage of sikon (S2 atom-51 is ob’%ined. Moreover, jhe increase in the atomic percentage of &icon for II, in the temperature range ISO-ZQO’C is probably due to the formation then the decomposition of the intermediate IA (Fig. 12).
In C.V.D. reactions, gas-phase analysis provides information on the decomposition mechanismisof four organosilicon compounds which are good precursors of amorphous silicon and silicon carbide deposits [ll]. It is necem to correlate the results of *he mass spectrometry of the gas phase and the _SPS results of the solid phase, in order to explain the decomposition mechanisms. Mass spectrometric results, which permit the identification of the different species in the gas phase and their respectire amounts, zuv in good agreement with those obtained by XPS. Sloreowr, for these types of silicon derivatives, the mass spectrum of each starting molecule shows, according to the bond energies, the preferential fzgmentation that we find again in thermal decomposition.
REPZXESCES i J. ScXic:l:ir~. -, D. Tercmna, Jag.. 50 (19i1)
Powder lL!c.a!l. kt., 12 j19s-O) 141. S. Hatrs, T. Araki, T. Ceki, T. Okzzaki and T. Suzuki, Bull. Chem. Sot. X%:5.
3 L.Q. l!ir.h, J.C. Bi!liorc and P. Ckdiot, C. R. Aed. Sci. Ser. C, 251 (1960) i30. 4 X1.. Komarov and 0-G. Yarosh. Zh. O.%xh. Khim., 3i (1967) 264. ,..d 8. Mw.so:, Index de Speares de Masse, Presses Cnirersitzires de Fraxe, 5 .A. cornu qFbris, 1965.
6 R.G. S~.o.?-,o~, J. Org. Chem., 39 (1974) 3602. I\‘. DxZsohn ad XC. Hem, J. Orgazzonetall. Chem., 5 ( 1966) 29. G. Scksdea, Adran. 1k.s Specrrom.. 7B (1976) 1340. G. Schaden. J~srus LieSTgs Xna. Chem., 4 (19X) 559. G. S&mticza. Z. S~~~dozsch.
B: Anozg.
cira,
w.
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33B
(1976)
663.
D Mazerolla, G. .\hn;lel and J.J. Ekhtrdr, J.P. Gc.c;ruk, R. Moran&o. G. Corsunt. -. k JAI Blocher, Jr., G.E. \.uiUard and G. Wahl (Editors), Proc. 6th Int. Conf. Chemiol \.apor Deposition. Chsntiliy. Paris. September 15-18, 19S1, l-o!. 61-7, The Ekezcxkemica! Society, Per.r.icgron, SJ. p. 338.