Amorphous silicon carbon nitride (a-SiCN:H) films were synthesized using vapor transport-chemical vapor deposition technique. Poly(dimethylsilane) was used as a single source for both Si and C. NH3 gas diluted in Ar is used as a source for nitrogen. The composition and bonding states are uniquely characterized with respect to NH3/Ar ratio by Fourier transform infrared spectroscopy (FTIR) and x-ray photoelectron spectroscopy (XPS). Spectral deconvolution is used to extract the individual components of the FTIR and XPS spectra. For instance, the FTIR spectra show a remarkable drop in the intensity of SiC vibration accompanied by the formation of further bonds including SiN, CN, CN, CN, and NH with increasing NH3/Ar ratio. Moreover, the XPS spectra show the existence of different chemical bonds in the a-SiCN:H films such as SiC, SiN, CN, CN, and CC. Both FTIR and XPS data demonstrate that the chemical bonding in the amorphous matrix is more complicated than a collection of single SiC SiN, or SiH bonds.
Poly(methylsiladiazane) (PMSDZ) is synthesized by the copolymerization of MeHSiCl2 with H2NNH2 in 1:14 molar ratio. The structure of the polymer is shown by NMR and IR spectroscopies to consist of tetraazadisilacyclohexane rings bridged by MeHSi groups. The polymer gives a mixture of silicon nitride and silicon carbonitride when heated above 1100°C under an inert atmosphere. Ceramic yields are increased relative to those obtained with the native polymer (67%) by cross‐linking with a dimethyltitanocene catalyst, or by thermal curing at 150°C (ceramic yields of 76% and 81%, respectively). The pyrolytic conversion of PMSDZ to ceramic products is studied by characterizing the solid residues by using FT‐IR and solid‐state 29Si MAS NMR spectroscopies, elemental analysis, and X‐ray powder diffraction. Thermal curing of PMSDZ at 150°C results mostly in increased cyclization of the reaction of bridging MeHSi moieties. At 300°C N‐N bond cleavage occurs rapidly with the formation of a polysilazane structure, in which the silicon is mosly coordinated as MeHSi(N)2 and MeSi(N)3 fragments. A mechanism involving the formation and reaction of silylaminyl radicals is proposed to account for these structural changes. Some methylene insertion occurs at 500°C, to form MeSi(CH2)(N)2 units. This step is critical for the incorporation of carbon into the ceramic product. At the same time Si(N)4 fragments are also generated. Between 700° and 900°C a high concentration of persistent free radicals is formed. Amorphous silicon carbonitride (SiNxCy) and Si3N4 are produced at 1100°C. Even after heating to 1500°C most of the product is still amorphous, only a small amount of crystalline α‐Si3N4 being detected by XRD.
Thin layers of a-Si3N4 were synthesized by the pyrolysis of thin films of poly(methylsilane)
(PMS) and poly(dimethylsilane) (PDMS) spin-coated on silicon single-crystal wafers and via
deposition of the volatile species resulting from the thermal cracking of the bulk precursor
in the presence of ammonia. The process was monitored by FT-IR spectroscopy. The reaction
between NH3 and PMS begins at 200 °C with the slow production of a slightly cross-linked
product involving Si3N knots. Extensive amination of PMS occurs on pyrolysis at 300 °C,
under 5−10 Torr NH3 overpressure. The product exhibits IR bands characteristic of both a
silazane and an aminosilane species, which are presumably formed by Si−H and N−H
heterodehydrocoupling. Between 200 and 450 °C, this cross dehydrocoupling reaction
competes very effectively with the Kumada rearrangement. Significant loss of carbon occurs
from the resulting poly(carbosilazane) between 500 and 600 °C. Prolonged curing under NH3
at 300 °C, to remove all Si−H groups and to give a densely cross-linked polysilazane,
suppresses the Kumada rearrangement, and negligible carbon loss occurs on raising the
pyrolysis temperature to 700 °C. Although the main product is still a-SI3N4, there is an
increased amount of residual carbon.
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