Trisilane

The partial pressures of the stable neutral molecules in the discharge (silane, hydrogen, disilane, trisilane) can be measured by a quadrupole mass spectrometer (QMS). The QMS usually is mounted in a differentially pumped chamber, which is connected to the reactor via a small extraction port [286]. In the ASTER system a QMS is mounted on the reactor that is used for intrinsic material deposition. The QMS background pressure (after proper bake-out) is between 10 and 10 mbar. The controllable diameter in the extraction port is adjusted so that during discharge operation the background pressure never exceeds 10"" mbar. 

In the deposition of a-Si H the dissociation of the process gas silane leads to the formation of hydrogen, disilane, trisilane, higher-order silanes, and a solid film. Silane is depleted. From the consumption of silane, one may estimate the deposition rate of the solid film. The difference in the silane partial pressure between... 

FIG. 32. The partial pressures of disilane and trisilane. The dashed line is an extrapolation of the disilane partial pressure in the o -regime. (From E. A. G. Hamers, Ph.D. Thesis, Universiteit Utrecht. Utrecht, the Netherlands, 1998.)... 

The partial pressures of disilane and trisilane are shown in Figure 32, for the same process conditions as in Figure 31. Both partial pressures increase as a function of pressure. Around the a-y transition at 30 Pa the disilane partial pressure increases faster with increasing pressure, as can be seen from the deviation from the extrapolated dashed line. The disilane partial pressure amounts to about 1 % of the total pressure, and the trisilane partial pressure is more than an order of magnitude lower. Apparently, in the y -regime the production of di- and trisilane is enhanced. 

Mass-spectrometric research on silane decomposition kinetics has been performed for flowing [298, 302-306] and static discharges [197, 307]. In a dc discharge of silane it is found that the reaction rate for the depletion of silane is a linear function of the dc current in the discharge, which allows one to determine a first-order reaction mechanism in electron density and temperature [302, 304]. For an RF discharge, similar results are found [303, 305]. Also, the depletion and production rates were found to be temperature-dependent [306]. Further, the depletion of silane and the production of disilane and trisilane are found to depend on the dwell time in the reactor [298]. The increase of di- and trisilane concentration at short dwell times (<0.5 s) corresponds to the decrease of silane concentration. At long dwell times, the decomposition of di- and trisilane produces... 

The increase in the deposition rate rj (Fig. 63d) corresponds to the increase in the ion flux (Fig. 63c) the fraction of arriving ions per deposited atom, / ,, is constant at about 0.25. Such observations have also been reported by Heintze and Zedlitz [236], who furthermore suggested that the deposition rate may well be controlled by tbe ion flux. The kinetic ion energy per deposited atom, max, is also constant and amounts to about 5 eV. As was shown in Section 1.6.2.3, the material quality as reflected in the refractive index 2 eV (Fig. 63e) and the microstructure parameter R (Fig. 63f) is good 2 cv is around 4.25, and R is low (<0.1). The depletion of the silane stays constant at a value of 4.0 0.4 seem in this frequency range. The partial pressures of silane, hydrogen, disilane (1.3 x 10 - mbar), and trisilane (2 x 10 mbar) in the plasma are also independent of frequency. Similar... 

The method employed for the first synthesis of stable disilene l,1 the photolysis of linear trisilanes, has been the most widely used synthetic method for disilenes. However, photolysis of cyclic trisilanes and dehalo-genation of dihalosilanes and 1,2-dihalodisilanes are also good routes in some cases (Table I). 

The disilene synthesis by the photolysis of linear trisilanes proceeds via initial formation of a silylene followed by its dimerization (Eq. 1). Disilene 1 has now become a common organometallic reagent. A detailed synthetic procedure employing photolysis of the corresponding linear trisilane is described in Inorganic Syntheses (Eq. 2).6... 

Disilene 2 having a bulkier substituent, 2,4,6-triisopropylphenyl, was obtained similarly.7 When trisilanes bearing two different substituents on the central silicon atoms are used, a mixture of E- and Z-isomers of disilenes is obtained (Eq. 3).8-10 Attempts to synthesize disilenes with other substituents using trisilane Tip(R)Si(SiMe3)2 (R = H, F, Cl, or 1-pyrroyl) were unsuccessful.10... 

In the photolysis of trisilanes RR Si(SiMe3)2 (R = R = Mes R = Mes, R = Tip R = R = Tip), the rate of disilene formation was predicted to increase in this order because the Si—Si—Si bond angles decrease in the same order, as revealed by X-ray structural analyses, thus facilitating the formation of Me3SiSiMe3. Although the predicted trend in the rates of photolysis was found, the differences were small.12... 

The first synthesis of a disilene by photolysis of the corresponding cyclic trisilane was reported by Masamune et al. in 1982.14 This method has been adapted for the synthesis of a variety of stable and marginally stable disilenes (Eq. 6).15-20 More recently, Kira et al. synthesized the first example of a stable tetrakis(trialkylsilyl)disilene 22 by this method.21... 

Tris(trimethylsilyl)silane (1,1,1,3,3,3-Hexamethyl-2-(trimethylsilyl)trisilane) [1873-77-4] C9H28Si4... 

See Other OXOSALTS OF NITROGENOUS BASES, PLATINUM COMPOUNDS 4579. Trisilane... 

Si29Si-Coupling Constants of Bromo- and Iododisilanes and -trisilanes XnSi2H6 n and XnSi3H8 n (X = Br,I)... 

All disilanes and trisilanes were synthesized with methods described in the literature [6]. 29Si NMR spectra were recorded with a BRUKER MSL 300 spectrometer, using solutions of the silanes in C6D6 (app. 50 %). 29Si29Si coupling constants were measured with the standard INADEQUATE pulse sequence. 

Table 1 29Si chemical shifts [ppm, TMS] and SiSi coupling constants [Hz] of bromo- and iododisilanes and -trisilanes... 

In the absence of light or a catalyst, reactions of 2 proceed by cleavage of one Si-Si bond and lead, depending on the reactants, to either acyclic trisilanes or ring-enlarged products. 

We think two mechanisms take place at the same ime. Besides the P -elimination mechanism, which has been described above as a kind of disproportionation, an a-elimination like the Harrod mechanism seems also to take place. From the experimental results, the polymerization of trisilanes or tetrasilanes yields preferably the dimeric species, hexa- or octa-silanes respectively. In addition, oligosilanes with odd numbers of silicon atoms were formed. We do not yet understand why these hexa-and octasilanes were formed in the iso-forms. 

Correlation of the SiSi bond lengths in disilane derivatives X3Si-SiX3 with X = CH3, C(CH3)3 and Si(CH3)3 together with those of sterically overcrowded cyclic [7a] and linear [7b] trisilane derivatives versus their Pauling bond orders [5a,6b,7], expectedly, produces a linear regression (Fig. 4). 

Figure 4. Correlation of SiSi bond lengths in di- and trisilanes versus Pauling bond orders, lg(PBO) = [d(l) - d(x)]/60 (framed PBO values).

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