Application of laser ionization and electron ionization to the deposition chemistry in the hot-wire chemical vapor deposition process with silane-ammonia gas mixtures

Hot-wire chemical vapour deposition (HWCVD) is a new and promising technique for the production of device quality silicon nitride (SixN y) thin films. Mixture of silane (SiH4) and ammonia (NH 3) are the most common source gases used in the HWCVD growth of Si xNy films. Gas-phase reactions between the primary hot-wire decomposition products and source gas molecules are believed to play a critical role in determining the identity and quantity of the film-growth precursors, which in turn dictate the properties of the resulting thin films. To date, however, a comprehensive understanding of the HWCVD chemistry of SiH 4/NH3 gas mixtures has not been presented. In this work, 118 nm (10.5 eV) vacuum ultraviolet (VUV) laser single photon ionization (SPI), and laser-induced electron ionization (LIEI), coupled with time-of-flight mass spectrometry (TOF-MS) were used to study the gas-phase reaction products of SiH4, NH3, and the interplay of these components in mixtures of SiH4 and NH3. A heated tungsten filament was used as the catalyzer. It was found that SiH4 decomposes on the filament, and subsequent gas-phase reactions result in the production of H2, Si2H6, and Si3H8. NH3 decomposition and secondary reactions produce primarily H 2 and N2. The interplay of the two components in the mixtures is characterized by the suppression of NH3 decomposition in the presence of SiH4; as the relative amount of SiH4 in the mixture increases, the degree of suppression also increases, though the suppression is less pronounced at high filament temperatures. SiH4 decomposition is unaffected by the presence of NH3 in the mixture. The identity of secondary gas-phase reaction products in the mixtures depends strongly on the relative amounts of SiH4 and NH3 in the mixture. When NH3 content is low, production of H2, Si2H6 and Si3H8 dominates, whereas at high NH3 content, the production of aminosilanes (particularly Si(NH2)3 and Si(NH2)4) becomes the primary reaction pathway.;A comparison of the ionization sources employed in this work reveals that SPI provides excellent signal strengths and resolution, but is limited to studying only those species with ionization potentials (IP) lower than the photon energy (10.5 eV). LIEI provides an excellent complementary technique, as the high energy ionization method is well suited for studying high IP species. The combination of these two techniques developed in this work produces mass spectra containing weak peaks due to both SPI and LIEI processes. A pure LIEI generator was constructed and applied to the SiH4/NH3 system in an effort to develop a better understanding of the LIEI process. Electrons produced by a thermionic process were used to cause electron impact ionization of the sample. This ionization, coupled with delayed ion extraction, produced EI-type mass spectra with excellent mass resolution and high sensitivity. The chemistry of the SiH4/NH3 system was confirmed using this ionization source.