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CO2 laser-induced thermal chemical vapour deposition of polymers
Journal of Analytical and Applied Pyrolysis 30 (1994) 73-90
CO, laser-induced
JOURNALo( ANALYTICALand APPLIED PYROLYSIS
thermal chemical vapour deposition of polymers Josef Pola
Institute of Chemical Process Fundamentals, Czech Academy of Sciences, 165 02 Prague, Czech Republic
Received 25 January 1994; accepted in final form 15 March 1994
Abstract Chemical reactions in the gas phase which are induced by CO* lasers and lead to the formation of organic or organometallic polymers are reviewed. The reactions involve the thermolysis of heterocycloalkanes, alkoxygermanes and acyclic organosilanes, the explosive decomposition of fluoromethylsilanes, and thermal chemistry in mixtures of silane with organic compounds. Attention is given to the mechanism of the gas-phase reactions as well as to the structure of the polymers. Keywords:
CO, laser; Chemical
vapour
deposition;
Polymers;
Pyrolysis;
Thermal
1. Introduction
Interest in polymerization in the gas phase has recently increased owing to the use of cloud chamber [l-3], exciplex laser radiation [4,5], particle beam [6], and plasma [7,8] techniques which are well suited to use in this field. Another type of polymerization reaction in the gas phase can be induced by radiation from infrared lasers. This process involves thermal generation of very reactive species which undergo various reactions, finally resulting in the formation of high-molecular substances. The overall reaction mechanisms in the gas phase are often complex. The reactive species can be produced by laser-powered homogeneous pyrolysis (LPHP) of a single precursor through photosensitization by sulphur hexafluoride [9-121, or by thermal chemistry in binary mixtures initiated via infrared multiphoton excitation and decomposition (IRMPD) [ 13,141 of one component. Both LPHP and IRMPD are solely restricted to the gas phase; laser-beam 01652370/94/$07.000
1994 - Elsevier
SSDZ 0165-2370(94)00804-A
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Anal. Appl. Pyrolysis 30 (1994) 73-90
heating occurs in a limited gas-phase volume and it enables reactor walls to remain at ambient temperature. In this way, heterogeneous contributions to thermal reactions, which are so often observed in conventional pyrolyses, are totally avoided [ 11,121, and polymer particles, formed in the gas phase and deposited on cold substrate housed on reactor walls, do not experience consecutive chemical changes feasible at high temperatures. There is enormous interest in various oligomers and polymers with special properties for a wide variety of applications (see, for example, Refs. [ 15]-[ 171). This review shows that the IR laser technique, applied to volatile compounds at pressures of up to several kPa, can be very effective for preparation of novel organic and organometallic polymeric coatings. It is not our intention to provide the reader with a detailed survey of the physical features involved in laser-induced thermal processes (excitation and energy transfer phenomena, temperature profiles, convection currents, etc.) in the gas phase. These have been dealt with elsewhere (see, for example, Refs. [9],[ lo], [ 181 and [ 191). We classify laser-induced thermal reactions, which result in the deposition of polymers, according to their mode of initiation and according to the major route leading to formation of polymers. We suggest that the techniques and polymeric materials produced could find application in microelectronics, film sensors and, if chemically modified, as precursors for other materials.
2. Inducement of thermal processes Both IRMPD and LPHP applied to compounds described in this review are thermal processes. They take place from vibrationally excited ground states, which makes them different from UV/VIS laser-induced photolytical chemical vapour deposition which occurs from electronically excited states. 2.1. IRMPD
[18,19]
Multiphoton vibrational excitation of absorbing molecules into a quasicontinuum absorption region is followed by energy absorption by the quasicontinuum until the dissociation channel(s) are reached. Although the reaction mechanism and products of IRMPD can in some instances be identical to those obtained in conventional pyrolysis, IRMPD is regarded as having some differences from purely thermal processes. Examples of chemical vapour deposition involving this initiation which are given below require collisions and they can be induced by pulsed TEA CO, laser radiation at fluences lower than 10 J cm-2 at pressures higher than 10 Torr. 2.2. LPHP [9- 1I]
The thermal decomposition of a laser-radiation, non-absorbing precursor into reactive species polymerizing in the gas phase is induced through collisions between the precursor and photosensitizer. The photosensitizer is a chemically inert, ther-
J. Pola
75
/ J. Anal. Appl. Pyrolysis 30 (1994) 73-90
mally stable compound which is a very good absorber of laser radiation and which has low thermal conductivity and very rapid intramolecular (vibrational-vibrational and vibrational-rotational/translational) conversion. The fast collisional energytransfer agents used are mostly SF, and SiF,. The laser heating of the precursor is an instant event; it can be accomplished within lop6 s at pressures of several-several tens Torr. The hot zone, in which most of thermal reactions take place, is characterized by temperature profiles, and the highest temperatures occur near the entrance of the laser beam into the reactor and alongside the beam passing through the gaseous mixture. Maximum temperatures can be up to 1500 K. Although LPHP can be induced by both continuous wave (cw) and pulses lasers, the examples gathered in this article include only the use of cw COZ laser, with output typically around 10 W.
3. Thermolysis of heterocycloalkanes The thermal decomposition of heterocycloalkanes has been achieved [ 9- 1l] by strong absorption of continuous wave (cw) CO, laser radiation in very inert SF6 which is used as an energy-conveying agent. Mixtures of SF, and a single precursor of polymer at pressures of several kPa are irradiated with laser output in the range of lo-20 W. The progress of the precursor decomposition, and that of the polymer formation, is enhanced at higher laser outputs and pressures which cause an increase in effective temperature [9] (usually 600- 1000 K) and can also lead to changes in reaction mechanism and consequently in the properties of the produced polymers. 3.1. LPHP
of silacyclobutanes
LPHP of silacyclobutanes Rz SiCH, CH2 C& (R is H, CH3, H2 C=CH, HC=C, Cl and (CH,),) is a very efficient route for the generation of very reactive silenes R,Si=CH2 and for polymerization of these short-lived species to completely saturated polycarbosilanes [20-231. It is worth noting that conventional (hot wall) pyrolysis (CP) of the same compounds [24,25] favours silene cyclodimerization (Scheme l), and that different final products observed in LPHP are obviously caused by the elimination of surface effects and by a small hot zone, wherein temperature gradients and high concentrations of silenes facilitate silene polymerization.
LPHP ,-.
polymer
\ ,SI-CH2
-
‘_
- w4 -L CP Scheme
1.
’ Si A /v’
Si’
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J. Pola 1 J. Anal. Appl. Pyrolysis
30 (1994) 73-90
A further intriguing feature of this laser thermolysis is revealed in the case of silacyclobutanes with unsaturated substituents on the silicon: l-methyl-l-vinyl-lsilacyclobutane and bis( 1-ethynyl) -1-silacyclobutane were decomposed and polymerized into completely saturated polycarbosilanes [21,22]. We assumed [22] that transient silaisoprene produced from the former and the most (ever observed) unsaturated diethynylsilene generated from the latter, polymerize by a sequence of steps shown in Schemes 2 and 3. The efficient and complete polymerization of the multiply unsaturated silenes is surprising in view of the very poor thermal polymerization of their carbon analogues, e.g. isoprene and vinylacetylene [26], vinylsilanes [27] and ethynylsilanes [28]. This great difference in the ability of the H,C=M(CH,)-CH=CH, and HC=CM=CH, moieties (M is C, Si) to undergo thermal polymerization suggests that new types of polymers can be produced from highly unsaturated hydrocarbons possessing C=C or CkC bonds which are adjacent to a Si=CH, bond.
1
N @4/
’
Si=CH
2
-ii-CH2-
-
----+
I
-k-CH2I -C-
C/ HC-
l -CH-
‘Si-CH,// C
Scheme
2.
1,4-polymerhioIl n H2C=Si(CHj)-CH=CH2
-
*
W;W”-CH2hr
-(CH2-Si(CH;)-CH-CH2hScheme
3.
1
J. Pola 1 J. Anal. Appl. Pyrolysis
4
CI$i=CH2
-
-(C12Si-CH&-
HCI CISZCH
-
30 (1994) 73-90
-
copolymer
HCl CI+I=CH~
_
Cl$SiCH;
Scheme 4.
The LPHP of silacyclobutanes can be performed as slow or fast (explosive) processes. The former require an irradiation time of several minutes and the latter, observed at higher pressures of SF6, can be carried out to completion with irradiation times within 1 s [21]. The deposits show mostly very good adhesion to metals and glass and can be used as precursors to silicon carbide films. Silacyclobutanes with halogens on the silicon are laser-decomposed into reactive polycarbosilanes. This has been proved with the decomposition of 1, l-dichloro-lsilacyclobutane which can be described [29] in Scheme 4. The deposited, very adhesive, polymeric materials are composed mostly of --(C1,SiCH2)- units and they arise mainly via polymerization of dichlorosilene. There are two reasons for this assumption: firstly 2 + 2 cycloreversion of the parent dichlorosilacyclobutane is favoured over dehydrochlorination, and secondly, silyne can add HCl and form dichlorosilene. The occurrence of chlorosilyne has been proved [29] by scavenging experiments using DCl, which represents the first (though indirect) chemical evidence of a member of a family of compounds having a triple bond between the Si and C atoms. The deposited material rapidly reacts with vapours of alcohols, trifluoroacetic acid and water, liberates hydrogen chloride and changes its structure and morphology as evidenced by IR spectra and SEM analysis. The reactions taking place are depicted in Scheme 5. Until now, poly[( methylchlorosilene)methylene] [ 301 and polysilapropylene [ 311 had been the only members of new reactive organosilicon polymers which could be used as precursors for various novel polymeric structures. The LPHP of dichlorosilacyclobutane resulting in chemical vapour deposition of reactive polycarbosilane materials could be of interest in the preparation of other agglomerates or thin films with reactive functional groups which can be further modified by subsequent reactions.
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J. Pola 1 J. Anal. Appl. Pyrolysis
+
-Si-Cl
A
30 (1994) 73-90
+
+
t
HOX
-S&OX
+ HCI
X = CH3, CF3CH2, CF3C0, H
(LIl.III) X=H
0 + Hz0
I
Scheme
vinyl
acetate
5.
(CH3)2yy-_CP2 .
(CH;)+=CH~
I I
CHF=CH
I I \
CO$H;
1
I
CO$H; Scheme
6
LPHP of l,l-dimethyl-1-silacyclobutane has a different course and yields mostly the cyclodimer (CH,),‘SiCH,(CH,),SiC’H, [32]. If carried out in the presence of common unsaturated monomers, such as vinyl acetate, ally1 methyl ether, acrolein or methyl acrylate, the intermediary dimethylsilene is trapped by these compounds, and this results in deposition of viscous and solid organosilicon oligomers and polymers which are soluble in organic solvents. These macromolecules possess molecular weights in the range of 0.2 x 103-34 x lo3 and can be assumed to be formed by copolymerization of dimethylsilene with monomers and/or products of monomer decompositions. As an illustration of this, the copolymerization course with vinyl acetate is given in Scheme 6. 3.2. LPHP of silacyclohexane It was of interest to investigate the thermal behaviour of silacyclohexanes which, in contrast to silacyclobutanes, do not slowly polymerize at ambient temperature
J. Pola 1 J. Anal. Appl. Pyrolysis 30 (1994) 73-90
79
.H$XH2 siH2
\
B
SiH2
-
H2Si=CH2
/ - C2H4
*H2C
Scheme
C Si.H~ - w-J.4
- C2H4 7.
Csir+
C2Hq + H2B=CH2 and
-
pober
C3Hg + H2Si:
I
1
.
CH3(CH2)3CH=SiH2 , CH3(CH2)jSiH=CHz and CH~=CH(CH&$H~(CH~)3,H
CH2=CH(CH2).#42(CH2)3,H
(x=0-3)
CHz=CHSii3
-
CHz=Si(CH3)H
(x=0-3)
CH3(H)C=SiH2 Scheme
8.
and can be stored for prolonged periods of time. Laser irradiation of silacyclohexane-SF,-Ar mixtures results [33] in the formation of gaseous hydrocarbons (ethene along with some methane, ethyne, propene and butadiene) and silicon-containing compounds (silacyclobutane, methylsilane, dimethylsilane and trimethylsilane), the latter being formed in much lower quantities. The deposition of polycarbosilane materials which incorporate most of the silicon from the parent compound is also observed. The material balance indicates that cleavage of silacyclohexane producing two ethene molecules (Scheme 7) does not play a major role; this pathway provides silacyclobutane, which can further decompose into ethene, propene and a polymer [34,35]. Other reactions judged to occur are isomerization of silacyclobutane into silenes and alkenylsilanes, 1,2-H shift of the silenes to silylenes, and retroene reaction of alkenylsilanes (Scheme 8). Both silenes and silylenes are known to undergo gas-phase polymerization, and this provides plausible explanation for the insufficiency of silicon in the volatile products. LPHP of silacyclohexane-dz, deuterium-labelled at the silicon, confirmed that all the hydrocarbons are deuterated, and revealed that the deposit contains both Si-H and Si-D bonds, which implies the involvement of radical reactions (abstraction of D of the parent and a 1,2-D shift in the silylmethyl D-Si-CH; radical to give ‘Si-CH,D [ 361). The presence of the d,-isotopomers of the hydrocarbons as well as those of the methylsilylhydrides suggests the occurrence of another H-D scrambling mechanism similar to that invoked (but left unspecified) for the thermolysis of 1,l -dideutero-1 -silacyclobutane [ 351. The efficient deposition of polycarbosilane material reflects the availability of many pathways contributing to
80
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30 (1994) 73-90
the formation of macromolecules. A discontinuous structure of the deposit, revealed by SEM analysis, support the majority of the polymerization (agglomerization) taking place in the gas phase. 3.3. LPHP
of azetidine
Similarly to the previous LPHP of silacyclobutanes, the cw CO2 laser SF,-photosensitized decomposition of azetidine (AZ) also differs markedly from conventional pyrolysis. The latter [37] affords mostly diazetidinylmethane by a mechanism assumed in Scheme 9, while the former [38] is a source of a new type of polymer, polymethanimine, and it progresses according to Scheme 10. In structure V, ccand p indicate H, CH2NH,, or CH,N(CH,N=),.
C
AZ
AZ
[CH?=NHj -
NH -
NCH2NH2 -_)
C
- c2Hz
[
C
Nl2CH2 + NH3
Scheme 9.
C
NH
-
[CH?=NHj-
4CH+Hh,,- @/or
- c2Hq
N
+332-N),,-
I CH2 I
YN\13 V Scheme
10.
The polymethanimine deposit, the structure of which has been inferred from NMR analysis as mostly branched polymer V with only minor contribution of a linear polymer IV, forms adhesive films and its average molecular weight M, is approx. 100,000 with a low-molecular part of the distribution starting from above 10,000. SEM analysis reveals that the polymethaneimine film has a compact structure. The principal reasons for the apparently different reaction pathways under LPHP and CP are similar as those mentioned in the case of the LPHP of silacyclobutanes: (1) heterogeneously catalysed contributions are avoided; (2) less volatile reaction products are condensed (or deposited) on the cold reactor walls where they cannot be further pyrolysed; and (3) the generation of high concentrations of methanimine CH,=NH molecules in the hot reaction zone, where the prevalence of this species increases the importance of its recombination (polymerization) and makes first-order reactions in it (the reaction of CH,=NH with AZ) less probable.
J. Pola 1 J. Anal. Appl. Pyrolysis
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81
4. Thermolysis of alkoxygermanes Considerable interest has been attached to the possibility of producing polymeric organogermanium oxides, analogous to silicones, using hydrolysis of dihalo- and dialkoxygermanes (see, for example, refs. 39 and 40). It was soon recognized, however, that the hydrolytic preparation of these organooxogermanium polymers cannot be achieved efficiently owing to the formation of low molecular weight, water-soluble oligomers [ 39,41,42]. LPHP and IRMPD of alkoxygermanes provide a clue to this problem. It has been found that both processes applied to tetramethoxygermane and trimethylethoxygermane can be used for chemical vapour deposition of reactive organooxogermanium polymers, the properties of which differ depending on the mode of alkoxygermane decomposition. 4.1. Tetramethoxygermane IRMPD of tetramethoxygermane (TMG) [43] is enhanced with the increase in TMG.pressure, which indicates that it proceeds by a collisional mechanism, and yields a solid brown layer together with gaseous methanol, dimethoxymethane and an unidentified C5H,202 compound. The major reactions occurring in the gas phase were assumed to be cleavage of the weaker GeO bond, the abstraction of hydrogen by the methoxy radical, and a cleavage of the trimethoxygermoxymethyl radical into CH,O and (CH30),Ge’ radical (Scheme 11). These steps are supported by the occurrence of methanol and by the reported [44] reactions between methanol and formaldehyde, giving the observed dimethoxymethane. The chemistry taking place during the SF,-photosensitized LPHP of TMG is, owing to the higher temperatures in the hot zone in this process, apparently much more complicated, and it results, apart from the deposition of a brown material, in the formation of methane, carbon monoxide, and dimethoxymethane. Different chemical processes occurring in the gas phase during IRMPD and LPHP are reflected in the differing compositions of deposited materials: ESCA analysis revealed the stoichiometry Ge,,OC,., 0,,9 for the former, and Ge,,0C1.90,.,_,,5 for the latter. The films obtained by IRMPD are transparent at wavelengths larger than 750 nm, showing an absorption edge near 600 nm, while those obtained by LPHP show an absorption edge near 1000 nm. Both kinds of films show a non-uniform deposition formed by almost spherical particles of
tw (CH30)3GeOCH3
-(CH30)3Ge.
+ CH30
-
(CH30)3GeOCH2 - CH3OH I
f - CH20 Scheme
11.
82
J. Pola / J. Anal. Appl. Pyrolysis 30 (1994) 73-90
OH
0CH3 I +-GMf
Hz0
I +a+
- CH3OH
9
Scheme 12.
yC2H5
(CH3COhO
OC(OJCH3
average diameter of 0.2 pm, which are polycrystalline materials. The deposits produced by IRMPD possess extremely good adhesion to various materials. The most interesting property of the deposited films is their reactivity towards ambient air. We assumed that the polymeric layers contain CH,-OGe groups which react with air moisture (Scheme 12). The resulting polymers with Ge-OH bonds can be expected to be unstable, owing to their possible dehydration to corresponding oxides. These chemical reactions do not, however, result in a damage (decay) of the polymeric surface. 4.2. Trimethylethoxygermane The laser-induced thermal chemistry of trimethylethoxygermane (TMEG) and the properties of the deposited material [45] are also affected by the mode of laser-induced decomposition. IRMPD yields methane, ethene, ethyne and acetaldehyde together with digermoxane (H,CH,Ge),O, while LPHP also yields carbon monoxide, trimethylgermane and tetramethylgermane. The stoichiometry of the deposit differs with the irradiation mode in the range Gel C,,,_1.800,1_0.8 and contains reactive Ge-OC2H5 groups which, upon exposure to air or acetanhydride vapour, undergo the reactions depicted in Scheme 13. Adhesion of these reactive organooxogermanium coatings to glass and metals is weaker than in the case of deposits produced from TMG.
5. Thermolysis
of acyclic organosilanes
The thermal behaviour of potential single organosilane precursors and its correlation with deposit properties can provide useful information for the design of
J. Pola / J. Anal. Appl. Pyrolysis
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83
thermal chemical vapour deposition processes and considerable effort is now being given to the examination of conventional pyrolytic processes for production of Si/C materials and silicon. There are several studies of IR laser-induced thermolysis of organosilanes, which report the formation of sinterable powders due to direct absorption of high-power cw or TEA CO, laser radiation in organosilanes. The interaction of laser radiation with tetramethylsilane and hexamethyldisilane at temperatures around 3000 K yields particles of silicon carbide enveloped by polycarbosilane [44]. Laser treatment of hexamethyldisilazane or hexamethyldisiloxane has been utilised for the production of refractory Si/C/N and Si/C/O powders [46-491 formed via intermediary polymers. The course of the thermal decomposition of alkoxysilanes and poly(dimethylsiloxanes) has not been thoroughly studied and some examinations deal only with the gaseous products [50,51]. Direct absorption of CO, laser radiation in hexamethyldisiloxane (HMDS) [ 5 l] yields methane and proves that cleavage of the Si-C bonds is an important contribution to the thermal decomposition. LPHP in the presence of SF6 of tetramethoxysilane (TMS), methyltrimethoxysilane (MTMS) and HMDS [52] yields mostly hydrocarbons (TMS: CH4, C2H2, CO; MTMS: CO, (CH,),SiH, CH,, HMDS, (CH,OCH,),, Si(CH,)XH, (X is CH,, CH,O); HMDSO: CH,, C,H,, CzH4, C2H6) and allows most of the silicon of the parent to be contained in a high-molecular-weight deposit. It can be presumed that TMS and MTMS undergo cleavage of both CH,-O and Si-0 bonds, and that the primary steps of the decomposition can be as described for TMS in Scheme 14. A similar decomposition course is also probable with HMDS, where the -OSi(CH,); and -OSi(CH,),CH; radicals can recombine and form structures with Si-CH,-Si links. LPHP of 3-pyridinyl( trimethyl) silane (PTMS) and 3-pyridinyl( triethoxy) silane (PTES) has been used [ 531 to deposit organosilicon films in which the 3-pyridinyl group remains partly preserved and attached to silicon in the polymer framework. Deposits contain all the silicon from the parent compounds and consist of a low and a high molecular weight component. Production of these tihns is highly desirable since linear end-blocked and crosslinked siloxane homopolymers and copolymers with N-functionality have been shown [54-561 to possess very promising catalytic properties, particularly in acyl transfer reactions.
(CH30)dSi
I
A
(CH30)3SiO*
+
CH;-
CH;*
1-w
(CH30)$WCHy
_
(CH3O)$i* - CH?O
-CH4 Scheme
14.
84
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Anal. Appl. Pyrolysis
30 (1994) 73-90
6. Explosive decomposition of fluoromethylsilanes IRMPD of (fluoromethyl)silanes H,CFSiH, , HCF, SiH,, CF, SiH, and (CF,),SiH,, which provides Si/C/F/H-containing deposits, has been induced by irradiation with a single CO, laser pulse at pressures of 0.1-6.7 kPa [57,58]. The single-pulse irradiation results in an explosive reaction when the pressure exceeds a certain limit, which depends on the laser fluence and the wavelength. The gaseous decomposition products are hydrocarbons (CH, and C,H2) and silanes rich in fluorine (SiF, and HSiF,), which indicate an efficient gas-phase reduction of the strong C-F bond. The relative amounts of the gas-phase products as well as the composition of the deposited materials depend significantly on both the parent molecule and its pressure. Organosilicon polymers were produced together with graphitic and amorphous carbon, silicon carbide and hydrogenated amorphous silicon. The interesting gas-phase chemistry was assumed to be initiated by a dyotropic rearrangement of HCF,SiH3, and by a sequence of carbene elimination and insertion reactions with the other fluoromethylsilanes (Scheme 15), and ensued by reactions with the intermediates silylenes, vinylsilanes, fluorovinylsilanes and ethynylfluorosilanes. The initial steps have an analogy in conventional thermal decompositions [ 59,601. The produced deposits [61] contain polymeric components which possess a significant Si-C framework content, incorporating some C-H and Si-H bonds. These polymers are extremely sensitive to the atmosphere and they very efficiently trap oxygen or molecules of water to produce Si-0 bonds. The high oxygen content found in those deposits which are rich in silicon confirms that silicon provides a route for oxygen incorporation. This unsaturated nature of the polymeric deposit is worthy of further study, especially from the point of view of the chemical reactions of the polymer with reactive molecules. The deposited layers do not adhere strongly to the surfaces of the substrates, but their adhesion to metals can be dramatically increased upon a long (several months) exposure to atmosphere. Nd:YAG laser spallation experiments carried out with the deposits [61] indicated that the profound adhesion can be correlated with a strong interaction between metal and carbon. Such IRMPDs utilising other similar compounds can be suitable for deposition of reactive particles or layers which might be deliberately modified afterwards by chemical reactions. Another practical application can be found in their use as photoconductive materials [62].
ASi/ ‘\F ’ \
C
F;C-SiH3 +
\c.z.bi/ /
‘\
‘.F,’
[:CFz + FSiH3] -
I’ \
L/L s. /
-/
F
HCF#iHzF
Scheme 15.
-:CHF
A
+ F2SiK2
J. Pola 1 J. Anal. Appl. Pyrolysis
30 (1994) 73-90
85
7. Thermal chemistry in mixtures of silane with organic compounds IR laser-induced bimolecular reactions have been investigated much less frequently than their unimolecular counterparts. They can be induced by single-wavelength laser radiation of both reactants when the absorption bands of the reactants overlap, or by simultaneous irradiation of the reactants using single (simultaneous or delayed) pulses at different wavelengths which match the absorption of the reactants located in different regions of the IR spectrum. Chemical reactions induced by a single laser, with an irradiating wavelength absorbed by only one of two different reactants, can occur by three different mechanisms. The first is a primary dissociation of the absorbing species and subsequent reactions of the fragments which result. The second is a photosensitized process, and the third is a mode induced through reactive collisions between both energized reactants. Several reactions in which formation of polymeric deposits certainly plays a role have been studied, as in the treatment of silane with ammonia or some hydrocarbons for the synthesis of silicon carbide and nitride [63], IRMPD of silane in the presence of phosphine, germane, hydrogen chloride, methyl chloride and nitric oxide [62], reactions in mixtures of silane and hexafluorobenzene [64], and reactions in mixtures of silane with perhaloethenes [65]. However, in none of these studies were the properties of the solid products discussed. The following are examinations in which both the gas-phase chemistry and the properties of the deposited products were investigated. 7.1. Decomposition of silane in the presence of oxygenated olejins IRMPD of silane is a sequence of the dissociation of highly vibrationally excited SiH, molecules into silylene SiH, and hydrogen H,, and insertion of SiH2 into the Si-H bonds, which yields higher silanes Si, H, + 2 (n = 2-5) and solid hydrogenated silicon (SiH,), [66-681. In the presence of methyl methacrylate (MMA), IRMPD of silane at total pressures of 3.9-5.3 kPa yields gaseous methane, ethyne, butenes and carbon monoxide along with a solid polymer composition which is consistent with the poly( dimethylsiloxane) -[( CH,),SiO], - structure [ 691. With a high MMA:SiH, ratio and laser pulse energy, elemental silicon was also evidenced, but this form is absent with a broad range of the ratios using low energy pulses. With less than one hundred pulses, the thickness of the polymer layer reaches 5-8 pm, and agglomerates bonded together have a size of 10 x 3 pm’. The major chemical route taking place in the irradiated system is judged to be addition of silylene across the C=C bond of MMA and a reorganization of the adduct into the siloxane structure (Scheme 16). The absence of higher silanes in this reaction implies that (1) SiH, reaction with MMA is much faster than its insertion into the Si-H bond, and (2) SiH2 addition to the double bond of olefins may be accelerated by a carbalkoxy group in the a-position to this double bond. A similar solid deposit has also been found to be produced upon irradiation of SiH,-MMA mixture with cw CO, laser [70].
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J. Pola /J.
H2C=C(CH#O+H3
Anal. Appl. Pyrolysis
30 (1994) 73-90
2si\-YH3
+ :%I+:! -H2c-c-=0
-co POWS
I
HpSi(OCH$C(CH+CH2
* Scheme
16.
IRMPD of silane in the presence of other oxygenated olefins [71], such as methyl acrylate (MA), acrolein (AC), ally1 methyl ether (AME), ally1 acetate (AA), methyl vinyl ether (MVE), and vinyl acetate (VA), carried out at pressures of 2.3-3.5 kPa, is more complicated. These irradiated mixtures yield various gaseous compounds and solid deposits. The latter incorporate all the silicon of reacted silane, consist of different siloxane units, and possess some Si-H and c--O bonds. It was assumed that silylene adds to both the parent olefin and compounds formed by thermal decomposition of these olefins. The absorptivity of the v,,(SiOSi) of the solid deposits, taken as a criterion of the olefine capability to form siloxane, revealed the order MA > AC = AME > AA > MVE = VA, which seems to indicate that olefins with oxygen bonded directly to the unsaturated carbon bond (as in H2C=CH-OCH,) are the least reactive. ESCA analysis of the deposits indicated that their stoichiometry is dependent on the parent oxygenated olefin, the pulse laser energy, and also on the location of the substrate, and that it varies in the range of SiC0.7-1.301.3-1.8. The films consist of regions of different morphology, confirming that organosilicon powder formation occurs in the gas phase. All these reactions proved to be a good technique for deposition of siloxane coatings. 7.2. Chemistry in mixtures of silane with fluorinated compounds TEA CO, laser irradiation of SiH,-hexatluoroacetone (HFA) mixtures at total pressures of 1.3-5.3 kPa results in the formation of various gaseous carbonaceous (CF,H, CH,, C,H,, C,H*F,, COF,, CO, C,F, and C,F,) and silicon-containing (SiF,, SiF,H) products, and deposition of microstructures of carbon, C/F/O and Si/C/O/F polymeric materials [72]. Complete depletion of the parent reactants has been observed after a single pulse for an SiH,:HFA ratio (I) ranging between 0.7 and 1.4. The relative amounts of both gaseous and solid products differ according to this ratio. The more hydrogenated volatile products are preferred at lower HFA contents, whereas poly- and perfluorinated products dominate at higher HFA contents. The solid material obtained with r = 0.7-5.0 contains silicon, carbon, oxygen, and fluorine, the content of carbon significantly increasing with lower r. The deposit stoichiometry can be varied depending on the irradiated conditions in the range Si,C,.,_,.,0,.,_,.,F,_,,,-C,00.1~0.7F0.1_1.5. The depletion of both parent reactants is initiated by an efficient absorption of the laser radiation in silane, and
J. Pola 1 J. Anal. Appl. Pyrolysis 30 (1994) 73-90
81
by the SiH,-photosensitized decomposition of HFA. Further reactions in the system are those of the products of HFA decomposition with silane of which the major ones are H-abstraction by CF, radical, and reactions of :CF, H, _ n carbenes with SiH, [59]. The observation of 100% conversion of some reaction mixtures in one pulse proves a thermally propagated explosion. The complexity of the probable reactions taking place in the irradiated mixtures, however, makes it difficult to determine whether the explosion is a result of branching reaction chains, or whether it stems from a high exothermicity of the reaction steps involved. These results not only provide a gas-phase alternative to efficient reduction of the strong C-F bonds [64,73,74], but they reveal that the technique can be useful for deposition of fluorinated polymers which can have potential as special coatings [75] or lubricants [75,76]. Similar features are also shown upon irradiation of a mixture of silane with trifluoroacetic acid (TFA) [77]. The chemistry in the gas phase involves reactions of silane with TFA and with products of TFA decomposition [78], and the deposits are, depending on the irradiation conditions, rich in silicon or carbon. Chemical reactions induced by single- and two-wavelength CO2 laser irradiation in a mixture of silane and chlorodifluoromethane (0.5 kPa each) [79] are initiated by the reaction between :CF, and SiH, and yield volatile carbonaceous products with C-H bonds (C2H4, C2H2, CH*=CF,, CHF=CH,, CH,=C=CH,, CH,F,, CH,C=CH) and silicon compounds (SiHF, and SiF,), together with a solid deposit consisting of silicon carbide and Si/C/H/F polymer. The formation of this solid material might be explained by the recombination of transient difluorocarbene and silylene, but the assumed inducement of the gas-phase chemistry via reactive collisions between :CF, and silane is in agreement with the major reactions assumed in Scheme 17. The deposited polymer is extremely sensitive to atmosphere and quickly incorporates oxygen. The superficial layers consist of fluffy agglomerates with the size of
:CF?* + SiIQ
-
-
H-CF+HJ*
I
HCFvHB: -
-Hz
SE * -2HF II F$=SiH~
I
polymerization/dehydofluorination
I simF/H polymer A Scheme
17.
88
J. Pola 1 J. Anal. Appl. Pyrolysis 30 (1994) 73-90
around 20 pm. Both their stoichiometry, Si,C1,,F0.2_0.301,2_,,4, and the IR spectral evidence for C-Si-O-X (X is H, Si) moieties is in corroboration with the occurrence of naked unsaturated carbon and/or silicon centres which undergo very fast reaction with oxygen and/or water. These reactions are much faster than the similar aging process observed with plasma-deposited silicon-containing films [7].
8. Conclusions
This review has shown that organic, organosilicon and organogermanium polymers can be deposited from the gas phase using radiation from CO* lasers. These laser-induced chemical vapour depositions provide an alternative to processes initiated by plasma and by UV laser radiation. The CO, laser-induced chemical vapour deposition involves thermal reactions in the gas phase and is, in this regard, different from UV-radiation- or plasma-induced processes. It is initiated in a limited volume of the gas phase at high temperatures and allows polymeric particles formed in the gas phase to be deposited on a cold substrate. In this way, deposition of various polymeric materials on thermally unstable surfaces can be achieved. Polymerization occurring in the gas phase results in the gas-phase formation of agglomerates which show different adhesion to substrates depending on the irradiation conditions, gas-phase chemical reactions and properties of the substrate. IR laser-induced thermal chemical vapour deposition can be suitable for modification of membranes used for separation of gases, preparation of thin films with sensor or catalytic properties and for deposition of lowtemperature precursors for silicon carbide. Other interesting materials which can be produced are %/C/F/H materials with photovoltaic properties, special coatings, lubricants and also reactive polymeric materials which can be deliberately modified by chemical reactions after deposition. The IR laser chemical deposition of polymers in the gas phase is clearly a very useful part of laser chemical vapour deposition processes [SO], so much studied nowadays due to their potential for many novel materials.
Acknowledgements
The results reported from our laboratory of Laser Chemistry of the Institute of Chemical Process Fundamentals in Prague have been achieved in collaboration with the Institute of Petrochemical Synthesis in Moscow (Professor L.E. Guselnikov), the Department of Inorganic Chemistry of Bergische Universitat in Wuppertal (Professor H. Burger), the Instituto de Optica “Daza de Valdes” in Madrid (Professor P.F. Gonzalez-Diaz) and the Laser Department of Institute of Physics and Technology of Radiation Devices (Professor R. Alexandrescu). The author would like to express his appreciation to all colleagues whose names are cited in the references.
J. Pola 1 J. Anal. Appl. Pyrolysis 30 (1994) 73-90
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[541 I551 [561 [571 [581 [591 [601 [611
CO, laser-induced
JOURNALo( ANALYTICALand APPLIED PYROLYSIS
thermal chemical vapour deposition of polymers Josef Pola
Institute of Chemical Process Fundamentals, Czech Academy of Sciences, 165 02 Prague, Czech Republic
Received 25 January 1994; accepted in final form 15 March 1994
Abstract Chemical reactions in the gas phase which are induced by CO* lasers and lead to the formation of organic or organometallic polymers are reviewed. The reactions involve the thermolysis of heterocycloalkanes, alkoxygermanes and acyclic organosilanes, the explosive decomposition of fluoromethylsilanes, and thermal chemistry in mixtures of silane with organic compounds. Attention is given to the mechanism of the gas-phase reactions as well as to the structure of the polymers. Keywords:
CO, laser; Chemical
vapour
deposition;
Polymers;
Pyrolysis;
Thermal
1. Introduction
Interest in polymerization in the gas phase has recently increased owing to the use of cloud chamber [l-3], exciplex laser radiation [4,5], particle beam [6], and plasma [7,8] techniques which are well suited to use in this field. Another type of polymerization reaction in the gas phase can be induced by radiation from infrared lasers. This process involves thermal generation of very reactive species which undergo various reactions, finally resulting in the formation of high-molecular substances. The overall reaction mechanisms in the gas phase are often complex. The reactive species can be produced by laser-powered homogeneous pyrolysis (LPHP) of a single precursor through photosensitization by sulphur hexafluoride [9-121, or by thermal chemistry in binary mixtures initiated via infrared multiphoton excitation and decomposition (IRMPD) [ 13,141 of one component. Both LPHP and IRMPD are solely restricted to the gas phase; laser-beam 01652370/94/$07.000
1994 - Elsevier
SSDZ 0165-2370(94)00804-A
Science B.V. All rights reserved
J. Pola /J.
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Anal. Appl. Pyrolysis 30 (1994) 73-90
heating occurs in a limited gas-phase volume and it enables reactor walls to remain at ambient temperature. In this way, heterogeneous contributions to thermal reactions, which are so often observed in conventional pyrolyses, are totally avoided [ 11,121, and polymer particles, formed in the gas phase and deposited on cold substrate housed on reactor walls, do not experience consecutive chemical changes feasible at high temperatures. There is enormous interest in various oligomers and polymers with special properties for a wide variety of applications (see, for example, Refs. [ 15]-[ 171). This review shows that the IR laser technique, applied to volatile compounds at pressures of up to several kPa, can be very effective for preparation of novel organic and organometallic polymeric coatings. It is not our intention to provide the reader with a detailed survey of the physical features involved in laser-induced thermal processes (excitation and energy transfer phenomena, temperature profiles, convection currents, etc.) in the gas phase. These have been dealt with elsewhere (see, for example, Refs. [9],[ lo], [ 181 and [ 191). We classify laser-induced thermal reactions, which result in the deposition of polymers, according to their mode of initiation and according to the major route leading to formation of polymers. We suggest that the techniques and polymeric materials produced could find application in microelectronics, film sensors and, if chemically modified, as precursors for other materials.
2. Inducement of thermal processes Both IRMPD and LPHP applied to compounds described in this review are thermal processes. They take place from vibrationally excited ground states, which makes them different from UV/VIS laser-induced photolytical chemical vapour deposition which occurs from electronically excited states. 2.1. IRMPD
[18,19]
Multiphoton vibrational excitation of absorbing molecules into a quasicontinuum absorption region is followed by energy absorption by the quasicontinuum until the dissociation channel(s) are reached. Although the reaction mechanism and products of IRMPD can in some instances be identical to those obtained in conventional pyrolysis, IRMPD is regarded as having some differences from purely thermal processes. Examples of chemical vapour deposition involving this initiation which are given below require collisions and they can be induced by pulsed TEA CO, laser radiation at fluences lower than 10 J cm-2 at pressures higher than 10 Torr. 2.2. LPHP [9- 1I]
The thermal decomposition of a laser-radiation, non-absorbing precursor into reactive species polymerizing in the gas phase is induced through collisions between the precursor and photosensitizer. The photosensitizer is a chemically inert, ther-
J. Pola
75
/ J. Anal. Appl. Pyrolysis 30 (1994) 73-90
mally stable compound which is a very good absorber of laser radiation and which has low thermal conductivity and very rapid intramolecular (vibrational-vibrational and vibrational-rotational/translational) conversion. The fast collisional energytransfer agents used are mostly SF, and SiF,. The laser heating of the precursor is an instant event; it can be accomplished within lop6 s at pressures of several-several tens Torr. The hot zone, in which most of thermal reactions take place, is characterized by temperature profiles, and the highest temperatures occur near the entrance of the laser beam into the reactor and alongside the beam passing through the gaseous mixture. Maximum temperatures can be up to 1500 K. Although LPHP can be induced by both continuous wave (cw) and pulses lasers, the examples gathered in this article include only the use of cw COZ laser, with output typically around 10 W.
3. Thermolysis of heterocycloalkanes The thermal decomposition of heterocycloalkanes has been achieved [ 9- 1l] by strong absorption of continuous wave (cw) CO, laser radiation in very inert SF6 which is used as an energy-conveying agent. Mixtures of SF, and a single precursor of polymer at pressures of several kPa are irradiated with laser output in the range of lo-20 W. The progress of the precursor decomposition, and that of the polymer formation, is enhanced at higher laser outputs and pressures which cause an increase in effective temperature [9] (usually 600- 1000 K) and can also lead to changes in reaction mechanism and consequently in the properties of the produced polymers. 3.1. LPHP
of silacyclobutanes
LPHP of silacyclobutanes Rz SiCH, CH2 C& (R is H, CH3, H2 C=CH, HC=C, Cl and (CH,),) is a very efficient route for the generation of very reactive silenes R,Si=CH2 and for polymerization of these short-lived species to completely saturated polycarbosilanes [20-231. It is worth noting that conventional (hot wall) pyrolysis (CP) of the same compounds [24,25] favours silene cyclodimerization (Scheme l), and that different final products observed in LPHP are obviously caused by the elimination of surface effects and by a small hot zone, wherein temperature gradients and high concentrations of silenes facilitate silene polymerization.
LPHP ,-.
polymer
\ ,SI-CH2
-
‘_
- w4 -L CP Scheme
1.
’ Si A /v’
Si’
76
J. Pola 1 J. Anal. Appl. Pyrolysis
30 (1994) 73-90
A further intriguing feature of this laser thermolysis is revealed in the case of silacyclobutanes with unsaturated substituents on the silicon: l-methyl-l-vinyl-lsilacyclobutane and bis( 1-ethynyl) -1-silacyclobutane were decomposed and polymerized into completely saturated polycarbosilanes [21,22]. We assumed [22] that transient silaisoprene produced from the former and the most (ever observed) unsaturated diethynylsilene generated from the latter, polymerize by a sequence of steps shown in Schemes 2 and 3. The efficient and complete polymerization of the multiply unsaturated silenes is surprising in view of the very poor thermal polymerization of their carbon analogues, e.g. isoprene and vinylacetylene [26], vinylsilanes [27] and ethynylsilanes [28]. This great difference in the ability of the H,C=M(CH,)-CH=CH, and HC=CM=CH, moieties (M is C, Si) to undergo thermal polymerization suggests that new types of polymers can be produced from highly unsaturated hydrocarbons possessing C=C or CkC bonds which are adjacent to a Si=CH, bond.
1
N @4/
’
Si=CH
2
-ii-CH2-
-
----+
I
-k-CH2I -C-
C/ HC-
l -CH-
‘Si-CH,// C
Scheme
2.
1,4-polymerhioIl n H2C=Si(CHj)-CH=CH2
-
*
W;W”-CH2hr
-(CH2-Si(CH;)-CH-CH2hScheme
3.
1
J. Pola 1 J. Anal. Appl. Pyrolysis
4
CI$i=CH2
-
-(C12Si-CH&-
HCI CISZCH
-
30 (1994) 73-90
-
copolymer
HCl CI+I=CH~
_
Cl$SiCH;
Scheme 4.
The LPHP of silacyclobutanes can be performed as slow or fast (explosive) processes. The former require an irradiation time of several minutes and the latter, observed at higher pressures of SF6, can be carried out to completion with irradiation times within 1 s [21]. The deposits show mostly very good adhesion to metals and glass and can be used as precursors to silicon carbide films. Silacyclobutanes with halogens on the silicon are laser-decomposed into reactive polycarbosilanes. This has been proved with the decomposition of 1, l-dichloro-lsilacyclobutane which can be described [29] in Scheme 4. The deposited, very adhesive, polymeric materials are composed mostly of --(C1,SiCH2)- units and they arise mainly via polymerization of dichlorosilene. There are two reasons for this assumption: firstly 2 + 2 cycloreversion of the parent dichlorosilacyclobutane is favoured over dehydrochlorination, and secondly, silyne can add HCl and form dichlorosilene. The occurrence of chlorosilyne has been proved [29] by scavenging experiments using DCl, which represents the first (though indirect) chemical evidence of a member of a family of compounds having a triple bond between the Si and C atoms. The deposited material rapidly reacts with vapours of alcohols, trifluoroacetic acid and water, liberates hydrogen chloride and changes its structure and morphology as evidenced by IR spectra and SEM analysis. The reactions taking place are depicted in Scheme 5. Until now, poly[( methylchlorosilene)methylene] [ 301 and polysilapropylene [ 311 had been the only members of new reactive organosilicon polymers which could be used as precursors for various novel polymeric structures. The LPHP of dichlorosilacyclobutane resulting in chemical vapour deposition of reactive polycarbosilane materials could be of interest in the preparation of other agglomerates or thin films with reactive functional groups which can be further modified by subsequent reactions.
78
J. Pola 1 J. Anal. Appl. Pyrolysis
+
-Si-Cl
A
30 (1994) 73-90
+
+
t
HOX
-S&OX
+ HCI
X = CH3, CF3CH2, CF3C0, H
(LIl.III) X=H
0 + Hz0
I
Scheme
vinyl
acetate
5.
(CH3)2yy-_CP2 .
(CH;)+=CH~
I I
CHF=CH
I I \
CO$H;
1
I
CO$H; Scheme
6
LPHP of l,l-dimethyl-1-silacyclobutane has a different course and yields mostly the cyclodimer (CH,),‘SiCH,(CH,),SiC’H, [32]. If carried out in the presence of common unsaturated monomers, such as vinyl acetate, ally1 methyl ether, acrolein or methyl acrylate, the intermediary dimethylsilene is trapped by these compounds, and this results in deposition of viscous and solid organosilicon oligomers and polymers which are soluble in organic solvents. These macromolecules possess molecular weights in the range of 0.2 x 103-34 x lo3 and can be assumed to be formed by copolymerization of dimethylsilene with monomers and/or products of monomer decompositions. As an illustration of this, the copolymerization course with vinyl acetate is given in Scheme 6. 3.2. LPHP of silacyclohexane It was of interest to investigate the thermal behaviour of silacyclohexanes which, in contrast to silacyclobutanes, do not slowly polymerize at ambient temperature
J. Pola 1 J. Anal. Appl. Pyrolysis 30 (1994) 73-90
79
.H$XH2 siH2
\
B
SiH2
-
H2Si=CH2
/ - C2H4
*H2C
Scheme
C Si.H~ - w-J.4
- C2H4 7.
Csir+
C2Hq + H2B=CH2 and
-
pober
C3Hg + H2Si:
I
1
.
CH3(CH2)3CH=SiH2 , CH3(CH2)jSiH=CHz and CH~=CH(CH&$H~(CH~)3,H
CH2=CH(CH2).#42(CH2)3,H
(x=0-3)
CHz=CHSii3
-
CHz=Si(CH3)H
(x=0-3)
CH3(H)C=SiH2 Scheme
8.
and can be stored for prolonged periods of time. Laser irradiation of silacyclohexane-SF,-Ar mixtures results [33] in the formation of gaseous hydrocarbons (ethene along with some methane, ethyne, propene and butadiene) and silicon-containing compounds (silacyclobutane, methylsilane, dimethylsilane and trimethylsilane), the latter being formed in much lower quantities. The deposition of polycarbosilane materials which incorporate most of the silicon from the parent compound is also observed. The material balance indicates that cleavage of silacyclohexane producing two ethene molecules (Scheme 7) does not play a major role; this pathway provides silacyclobutane, which can further decompose into ethene, propene and a polymer [34,35]. Other reactions judged to occur are isomerization of silacyclobutane into silenes and alkenylsilanes, 1,2-H shift of the silenes to silylenes, and retroene reaction of alkenylsilanes (Scheme 8). Both silenes and silylenes are known to undergo gas-phase polymerization, and this provides plausible explanation for the insufficiency of silicon in the volatile products. LPHP of silacyclohexane-dz, deuterium-labelled at the silicon, confirmed that all the hydrocarbons are deuterated, and revealed that the deposit contains both Si-H and Si-D bonds, which implies the involvement of radical reactions (abstraction of D of the parent and a 1,2-D shift in the silylmethyl D-Si-CH; radical to give ‘Si-CH,D [ 361). The presence of the d,-isotopomers of the hydrocarbons as well as those of the methylsilylhydrides suggests the occurrence of another H-D scrambling mechanism similar to that invoked (but left unspecified) for the thermolysis of 1,l -dideutero-1 -silacyclobutane [ 351. The efficient deposition of polycarbosilane material reflects the availability of many pathways contributing to
80
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the formation of macromolecules. A discontinuous structure of the deposit, revealed by SEM analysis, support the majority of the polymerization (agglomerization) taking place in the gas phase. 3.3. LPHP
of azetidine
Similarly to the previous LPHP of silacyclobutanes, the cw CO2 laser SF,-photosensitized decomposition of azetidine (AZ) also differs markedly from conventional pyrolysis. The latter [37] affords mostly diazetidinylmethane by a mechanism assumed in Scheme 9, while the former [38] is a source of a new type of polymer, polymethanimine, and it progresses according to Scheme 10. In structure V, ccand p indicate H, CH2NH,, or CH,N(CH,N=),.
C
AZ
AZ
[CH?=NHj -
NH -
NCH2NH2 -_)
C
- c2Hz
[
C
Nl2CH2 + NH3
Scheme 9.
C
NH
-
[CH?=NHj-
4CH+Hh,,- @/or
- c2Hq
N
+332-N),,-
I CH2 I
YN\13 V Scheme
10.
The polymethanimine deposit, the structure of which has been inferred from NMR analysis as mostly branched polymer V with only minor contribution of a linear polymer IV, forms adhesive films and its average molecular weight M, is approx. 100,000 with a low-molecular part of the distribution starting from above 10,000. SEM analysis reveals that the polymethaneimine film has a compact structure. The principal reasons for the apparently different reaction pathways under LPHP and CP are similar as those mentioned in the case of the LPHP of silacyclobutanes: (1) heterogeneously catalysed contributions are avoided; (2) less volatile reaction products are condensed (or deposited) on the cold reactor walls where they cannot be further pyrolysed; and (3) the generation of high concentrations of methanimine CH,=NH molecules in the hot reaction zone, where the prevalence of this species increases the importance of its recombination (polymerization) and makes first-order reactions in it (the reaction of CH,=NH with AZ) less probable.
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4. Thermolysis of alkoxygermanes Considerable interest has been attached to the possibility of producing polymeric organogermanium oxides, analogous to silicones, using hydrolysis of dihalo- and dialkoxygermanes (see, for example, refs. 39 and 40). It was soon recognized, however, that the hydrolytic preparation of these organooxogermanium polymers cannot be achieved efficiently owing to the formation of low molecular weight, water-soluble oligomers [ 39,41,42]. LPHP and IRMPD of alkoxygermanes provide a clue to this problem. It has been found that both processes applied to tetramethoxygermane and trimethylethoxygermane can be used for chemical vapour deposition of reactive organooxogermanium polymers, the properties of which differ depending on the mode of alkoxygermane decomposition. 4.1. Tetramethoxygermane IRMPD of tetramethoxygermane (TMG) [43] is enhanced with the increase in TMG.pressure, which indicates that it proceeds by a collisional mechanism, and yields a solid brown layer together with gaseous methanol, dimethoxymethane and an unidentified C5H,202 compound. The major reactions occurring in the gas phase were assumed to be cleavage of the weaker GeO bond, the abstraction of hydrogen by the methoxy radical, and a cleavage of the trimethoxygermoxymethyl radical into CH,O and (CH30),Ge’ radical (Scheme 11). These steps are supported by the occurrence of methanol and by the reported [44] reactions between methanol and formaldehyde, giving the observed dimethoxymethane. The chemistry taking place during the SF,-photosensitized LPHP of TMG is, owing to the higher temperatures in the hot zone in this process, apparently much more complicated, and it results, apart from the deposition of a brown material, in the formation of methane, carbon monoxide, and dimethoxymethane. Different chemical processes occurring in the gas phase during IRMPD and LPHP are reflected in the differing compositions of deposited materials: ESCA analysis revealed the stoichiometry Ge,,OC,., 0,,9 for the former, and Ge,,0C1.90,.,_,,5 for the latter. The films obtained by IRMPD are transparent at wavelengths larger than 750 nm, showing an absorption edge near 600 nm, while those obtained by LPHP show an absorption edge near 1000 nm. Both kinds of films show a non-uniform deposition formed by almost spherical particles of
tw (CH30)3GeOCH3
-(CH30)3Ge.
+ CH30
-
(CH30)3GeOCH2 - CH3OH I
f - CH20 Scheme
11.
82
J. Pola / J. Anal. Appl. Pyrolysis 30 (1994) 73-90
OH
0CH3 I +-GMf
Hz0
I +a+
- CH3OH
9
Scheme 12.
yC2H5
(CH3COhO
OC(OJCH3
average diameter of 0.2 pm, which are polycrystalline materials. The deposits produced by IRMPD possess extremely good adhesion to various materials. The most interesting property of the deposited films is their reactivity towards ambient air. We assumed that the polymeric layers contain CH,-OGe groups which react with air moisture (Scheme 12). The resulting polymers with Ge-OH bonds can be expected to be unstable, owing to their possible dehydration to corresponding oxides. These chemical reactions do not, however, result in a damage (decay) of the polymeric surface. 4.2. Trimethylethoxygermane The laser-induced thermal chemistry of trimethylethoxygermane (TMEG) and the properties of the deposited material [45] are also affected by the mode of laser-induced decomposition. IRMPD yields methane, ethene, ethyne and acetaldehyde together with digermoxane (H,CH,Ge),O, while LPHP also yields carbon monoxide, trimethylgermane and tetramethylgermane. The stoichiometry of the deposit differs with the irradiation mode in the range Gel C,,,_1.800,1_0.8 and contains reactive Ge-OC2H5 groups which, upon exposure to air or acetanhydride vapour, undergo the reactions depicted in Scheme 13. Adhesion of these reactive organooxogermanium coatings to glass and metals is weaker than in the case of deposits produced from TMG.
5. Thermolysis
of acyclic organosilanes
The thermal behaviour of potential single organosilane precursors and its correlation with deposit properties can provide useful information for the design of
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thermal chemical vapour deposition processes and considerable effort is now being given to the examination of conventional pyrolytic processes for production of Si/C materials and silicon. There are several studies of IR laser-induced thermolysis of organosilanes, which report the formation of sinterable powders due to direct absorption of high-power cw or TEA CO, laser radiation in organosilanes. The interaction of laser radiation with tetramethylsilane and hexamethyldisilane at temperatures around 3000 K yields particles of silicon carbide enveloped by polycarbosilane [44]. Laser treatment of hexamethyldisilazane or hexamethyldisiloxane has been utilised for the production of refractory Si/C/N and Si/C/O powders [46-491 formed via intermediary polymers. The course of the thermal decomposition of alkoxysilanes and poly(dimethylsiloxanes) has not been thoroughly studied and some examinations deal only with the gaseous products [50,51]. Direct absorption of CO, laser radiation in hexamethyldisiloxane (HMDS) [ 5 l] yields methane and proves that cleavage of the Si-C bonds is an important contribution to the thermal decomposition. LPHP in the presence of SF6 of tetramethoxysilane (TMS), methyltrimethoxysilane (MTMS) and HMDS [52] yields mostly hydrocarbons (TMS: CH4, C2H2, CO; MTMS: CO, (CH,),SiH, CH,, HMDS, (CH,OCH,),, Si(CH,)XH, (X is CH,, CH,O); HMDSO: CH,, C,H,, CzH4, C2H6) and allows most of the silicon of the parent to be contained in a high-molecular-weight deposit. It can be presumed that TMS and MTMS undergo cleavage of both CH,-O and Si-0 bonds, and that the primary steps of the decomposition can be as described for TMS in Scheme 14. A similar decomposition course is also probable with HMDS, where the -OSi(CH,); and -OSi(CH,),CH; radicals can recombine and form structures with Si-CH,-Si links. LPHP of 3-pyridinyl( trimethyl) silane (PTMS) and 3-pyridinyl( triethoxy) silane (PTES) has been used [ 531 to deposit organosilicon films in which the 3-pyridinyl group remains partly preserved and attached to silicon in the polymer framework. Deposits contain all the silicon from the parent compounds and consist of a low and a high molecular weight component. Production of these tihns is highly desirable since linear end-blocked and crosslinked siloxane homopolymers and copolymers with N-functionality have been shown [54-561 to possess very promising catalytic properties, particularly in acyl transfer reactions.
(CH30)dSi
I
A
(CH30)3SiO*
+
CH;-
CH;*
1-w
(CH30)$WCHy
_
(CH3O)$i* - CH?O
-CH4 Scheme
14.
84
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6. Explosive decomposition of fluoromethylsilanes IRMPD of (fluoromethyl)silanes H,CFSiH, , HCF, SiH,, CF, SiH, and (CF,),SiH,, which provides Si/C/F/H-containing deposits, has been induced by irradiation with a single CO, laser pulse at pressures of 0.1-6.7 kPa [57,58]. The single-pulse irradiation results in an explosive reaction when the pressure exceeds a certain limit, which depends on the laser fluence and the wavelength. The gaseous decomposition products are hydrocarbons (CH, and C,H2) and silanes rich in fluorine (SiF, and HSiF,), which indicate an efficient gas-phase reduction of the strong C-F bond. The relative amounts of the gas-phase products as well as the composition of the deposited materials depend significantly on both the parent molecule and its pressure. Organosilicon polymers were produced together with graphitic and amorphous carbon, silicon carbide and hydrogenated amorphous silicon. The interesting gas-phase chemistry was assumed to be initiated by a dyotropic rearrangement of HCF,SiH3, and by a sequence of carbene elimination and insertion reactions with the other fluoromethylsilanes (Scheme 15), and ensued by reactions with the intermediates silylenes, vinylsilanes, fluorovinylsilanes and ethynylfluorosilanes. The initial steps have an analogy in conventional thermal decompositions [ 59,601. The produced deposits [61] contain polymeric components which possess a significant Si-C framework content, incorporating some C-H and Si-H bonds. These polymers are extremely sensitive to the atmosphere and they very efficiently trap oxygen or molecules of water to produce Si-0 bonds. The high oxygen content found in those deposits which are rich in silicon confirms that silicon provides a route for oxygen incorporation. This unsaturated nature of the polymeric deposit is worthy of further study, especially from the point of view of the chemical reactions of the polymer with reactive molecules. The deposited layers do not adhere strongly to the surfaces of the substrates, but their adhesion to metals can be dramatically increased upon a long (several months) exposure to atmosphere. Nd:YAG laser spallation experiments carried out with the deposits [61] indicated that the profound adhesion can be correlated with a strong interaction between metal and carbon. Such IRMPDs utilising other similar compounds can be suitable for deposition of reactive particles or layers which might be deliberately modified afterwards by chemical reactions. Another practical application can be found in their use as photoconductive materials [62].
ASi/ ‘\F ’ \
C
F;C-SiH3 +
\c.z.bi/ /
‘\
‘.F,’
[:CFz + FSiH3] -
I’ \
L/L s. /
-/
F
HCF#iHzF
Scheme 15.
-:CHF
A
+ F2SiK2
J. Pola 1 J. Anal. Appl. Pyrolysis
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85
7. Thermal chemistry in mixtures of silane with organic compounds IR laser-induced bimolecular reactions have been investigated much less frequently than their unimolecular counterparts. They can be induced by single-wavelength laser radiation of both reactants when the absorption bands of the reactants overlap, or by simultaneous irradiation of the reactants using single (simultaneous or delayed) pulses at different wavelengths which match the absorption of the reactants located in different regions of the IR spectrum. Chemical reactions induced by a single laser, with an irradiating wavelength absorbed by only one of two different reactants, can occur by three different mechanisms. The first is a primary dissociation of the absorbing species and subsequent reactions of the fragments which result. The second is a photosensitized process, and the third is a mode induced through reactive collisions between both energized reactants. Several reactions in which formation of polymeric deposits certainly plays a role have been studied, as in the treatment of silane with ammonia or some hydrocarbons for the synthesis of silicon carbide and nitride [63], IRMPD of silane in the presence of phosphine, germane, hydrogen chloride, methyl chloride and nitric oxide [62], reactions in mixtures of silane and hexafluorobenzene [64], and reactions in mixtures of silane with perhaloethenes [65]. However, in none of these studies were the properties of the solid products discussed. The following are examinations in which both the gas-phase chemistry and the properties of the deposited products were investigated. 7.1. Decomposition of silane in the presence of oxygenated olejins IRMPD of silane is a sequence of the dissociation of highly vibrationally excited SiH, molecules into silylene SiH, and hydrogen H,, and insertion of SiH2 into the Si-H bonds, which yields higher silanes Si, H, + 2 (n = 2-5) and solid hydrogenated silicon (SiH,), [66-681. In the presence of methyl methacrylate (MMA), IRMPD of silane at total pressures of 3.9-5.3 kPa yields gaseous methane, ethyne, butenes and carbon monoxide along with a solid polymer composition which is consistent with the poly( dimethylsiloxane) -[( CH,),SiO], - structure [ 691. With a high MMA:SiH, ratio and laser pulse energy, elemental silicon was also evidenced, but this form is absent with a broad range of the ratios using low energy pulses. With less than one hundred pulses, the thickness of the polymer layer reaches 5-8 pm, and agglomerates bonded together have a size of 10 x 3 pm’. The major chemical route taking place in the irradiated system is judged to be addition of silylene across the C=C bond of MMA and a reorganization of the adduct into the siloxane structure (Scheme 16). The absence of higher silanes in this reaction implies that (1) SiH, reaction with MMA is much faster than its insertion into the Si-H bond, and (2) SiH2 addition to the double bond of olefins may be accelerated by a carbalkoxy group in the a-position to this double bond. A similar solid deposit has also been found to be produced upon irradiation of SiH,-MMA mixture with cw CO, laser [70].
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H2C=C(CH#O+H3
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2si\-YH3
+ :%I+:! -H2c-c-=0
-co POWS
I
HpSi(OCH$C(CH+CH2
* Scheme
16.
IRMPD of silane in the presence of other oxygenated olefins [71], such as methyl acrylate (MA), acrolein (AC), ally1 methyl ether (AME), ally1 acetate (AA), methyl vinyl ether (MVE), and vinyl acetate (VA), carried out at pressures of 2.3-3.5 kPa, is more complicated. These irradiated mixtures yield various gaseous compounds and solid deposits. The latter incorporate all the silicon of reacted silane, consist of different siloxane units, and possess some Si-H and c--O bonds. It was assumed that silylene adds to both the parent olefin and compounds formed by thermal decomposition of these olefins. The absorptivity of the v,,(SiOSi) of the solid deposits, taken as a criterion of the olefine capability to form siloxane, revealed the order MA > AC = AME > AA > MVE = VA, which seems to indicate that olefins with oxygen bonded directly to the unsaturated carbon bond (as in H2C=CH-OCH,) are the least reactive. ESCA analysis of the deposits indicated that their stoichiometry is dependent on the parent oxygenated olefin, the pulse laser energy, and also on the location of the substrate, and that it varies in the range of SiC0.7-1.301.3-1.8. The films consist of regions of different morphology, confirming that organosilicon powder formation occurs in the gas phase. All these reactions proved to be a good technique for deposition of siloxane coatings. 7.2. Chemistry in mixtures of silane with fluorinated compounds TEA CO, laser irradiation of SiH,-hexatluoroacetone (HFA) mixtures at total pressures of 1.3-5.3 kPa results in the formation of various gaseous carbonaceous (CF,H, CH,, C,H,, C,H*F,, COF,, CO, C,F, and C,F,) and silicon-containing (SiF,, SiF,H) products, and deposition of microstructures of carbon, C/F/O and Si/C/O/F polymeric materials [72]. Complete depletion of the parent reactants has been observed after a single pulse for an SiH,:HFA ratio (I) ranging between 0.7 and 1.4. The relative amounts of both gaseous and solid products differ according to this ratio. The more hydrogenated volatile products are preferred at lower HFA contents, whereas poly- and perfluorinated products dominate at higher HFA contents. The solid material obtained with r = 0.7-5.0 contains silicon, carbon, oxygen, and fluorine, the content of carbon significantly increasing with lower r. The deposit stoichiometry can be varied depending on the irradiated conditions in the range Si,C,.,_,.,0,.,_,.,F,_,,,-C,00.1~0.7F0.1_1.5. The depletion of both parent reactants is initiated by an efficient absorption of the laser radiation in silane, and
J. Pola 1 J. Anal. Appl. Pyrolysis 30 (1994) 73-90
81
by the SiH,-photosensitized decomposition of HFA. Further reactions in the system are those of the products of HFA decomposition with silane of which the major ones are H-abstraction by CF, radical, and reactions of :CF, H, _ n carbenes with SiH, [59]. The observation of 100% conversion of some reaction mixtures in one pulse proves a thermally propagated explosion. The complexity of the probable reactions taking place in the irradiated mixtures, however, makes it difficult to determine whether the explosion is a result of branching reaction chains, or whether it stems from a high exothermicity of the reaction steps involved. These results not only provide a gas-phase alternative to efficient reduction of the strong C-F bonds [64,73,74], but they reveal that the technique can be useful for deposition of fluorinated polymers which can have potential as special coatings [75] or lubricants [75,76]. Similar features are also shown upon irradiation of a mixture of silane with trifluoroacetic acid (TFA) [77]. The chemistry in the gas phase involves reactions of silane with TFA and with products of TFA decomposition [78], and the deposits are, depending on the irradiation conditions, rich in silicon or carbon. Chemical reactions induced by single- and two-wavelength CO2 laser irradiation in a mixture of silane and chlorodifluoromethane (0.5 kPa each) [79] are initiated by the reaction between :CF, and SiH, and yield volatile carbonaceous products with C-H bonds (C2H4, C2H2, CH*=CF,, CHF=CH,, CH,=C=CH,, CH,F,, CH,C=CH) and silicon compounds (SiHF, and SiF,), together with a solid deposit consisting of silicon carbide and Si/C/H/F polymer. The formation of this solid material might be explained by the recombination of transient difluorocarbene and silylene, but the assumed inducement of the gas-phase chemistry via reactive collisions between :CF, and silane is in agreement with the major reactions assumed in Scheme 17. The deposited polymer is extremely sensitive to atmosphere and quickly incorporates oxygen. The superficial layers consist of fluffy agglomerates with the size of
:CF?* + SiIQ
-
-
H-CF+HJ*
I
HCFvHB: -
-Hz
SE * -2HF II F$=SiH~
I
polymerization/dehydofluorination
I simF/H polymer A Scheme
17.
88
J. Pola 1 J. Anal. Appl. Pyrolysis 30 (1994) 73-90
around 20 pm. Both their stoichiometry, Si,C1,,F0.2_0.301,2_,,4, and the IR spectral evidence for C-Si-O-X (X is H, Si) moieties is in corroboration with the occurrence of naked unsaturated carbon and/or silicon centres which undergo very fast reaction with oxygen and/or water. These reactions are much faster than the similar aging process observed with plasma-deposited silicon-containing films [7].
8. Conclusions
This review has shown that organic, organosilicon and organogermanium polymers can be deposited from the gas phase using radiation from CO* lasers. These laser-induced chemical vapour depositions provide an alternative to processes initiated by plasma and by UV laser radiation. The CO, laser-induced chemical vapour deposition involves thermal reactions in the gas phase and is, in this regard, different from UV-radiation- or plasma-induced processes. It is initiated in a limited volume of the gas phase at high temperatures and allows polymeric particles formed in the gas phase to be deposited on a cold substrate. In this way, deposition of various polymeric materials on thermally unstable surfaces can be achieved. Polymerization occurring in the gas phase results in the gas-phase formation of agglomerates which show different adhesion to substrates depending on the irradiation conditions, gas-phase chemical reactions and properties of the substrate. IR laser-induced thermal chemical vapour deposition can be suitable for modification of membranes used for separation of gases, preparation of thin films with sensor or catalytic properties and for deposition of lowtemperature precursors for silicon carbide. Other interesting materials which can be produced are %/C/F/H materials with photovoltaic properties, special coatings, lubricants and also reactive polymeric materials which can be deliberately modified by chemical reactions after deposition. The IR laser chemical deposition of polymers in the gas phase is clearly a very useful part of laser chemical vapour deposition processes [SO], so much studied nowadays due to their potential for many novel materials.
Acknowledgements
The results reported from our laboratory of Laser Chemistry of the Institute of Chemical Process Fundamentals in Prague have been achieved in collaboration with the Institute of Petrochemical Synthesis in Moscow (Professor L.E. Guselnikov), the Department of Inorganic Chemistry of Bergische Universitat in Wuppertal (Professor H. Burger), the Instituto de Optica “Daza de Valdes” in Madrid (Professor P.F. Gonzalez-Diaz) and the Laser Department of Institute of Physics and Technology of Radiation Devices (Professor R. Alexandrescu). The author would like to express his appreciation to all colleagues whose names are cited in the references.
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