Development of a new laser induced electron impact ionization source for studying the hot-wire chemical vapor deposition chemistry of silane–ammonia mixtures

Abstract

A newly-developed laser induced electron impact ionization (LIEI) method is presented. In this method, electrons are generated by thermionic emission when directing a low-power, pulsed UV laser beam to a stainless steel rod exterior to the ion optics of a time-of-flight mass spectrometer. The electrons are collimated into a beam almost parallel to the ion optics by the electric field between the rod and a copper mesh placed in the vicinity and the utilization of a delayed ion extraction pulse. It has been demonstrated that EI-type mass spectra can be obtained with a mass resolution that is comparable to that using laser ionization. The detection sensitivity is very high, with a detection limit of < 32 attomole for NH₃. The gas-phase chemistry of SiH₄/NH₃ mixtures in the hot-wire chemical vapor deposition (HWCVD) process was examined using the new LIEI source. It is found that this method is well suited for detecting stable species with high ionization potentials, such as H₂ and N₂, which characterizes the NH₃decomposition chemistry in a HWCVD reactor. The suppression of NH₃ chemistry by the presence of SiH₄ in the mixture is demonstrated, as is the temperature dependence of the suppression extent.