Modeling and diagnostics of silicon nitride deposition from 254-nm Hg photosensitization of SiH4-NH3 mixtures: Luminescence of HgNH3 excimer and laser-induced fluorescence of NH2(Ã 2A1)

Abstract

Silicon nitride thin films are obtained from the Hg(6³P₁) ‐photosensitized decomposition of SiH₄ and NH₃ using the 254‐nm radiation of a low‐pressure Hg discharge lamp. Several diagnostics are associated to characterize the physicochemical mechanisms and measure the densities of reactive species or reaction products. The photon flux is obtained by radiometry, the Hg density is determined by absorption of the 254‐nm radiation, and the SiH₄ gas depletion is measured by mass spectrometry. The luminescence at 340 nm of HgNH₃ excimers is used as a probe of the metastable Hg(6³P₀) density and the NH₂ radical is detected by laser‐induced fluorescence (LIF) using the 597.8‐nm line to excite NH₂(òA₁) Σ(0,9,0) vibronic states. The interpretation of LIF results in terms of NH₂ density takes into account collisional transfer and quenching of NH₂(òA₁) revealed by nonexponential time decay of the fluorescence. In addition, the silicon nitride film deposition rate is monitored and the N/Si stoichiometry is determined by x‐ray photoelectron spectroscopy. All these data are used to validate a one‐dimensional spatially resolved modeling of the photochemistry. The model involves radiative transfer of the 254‐nm radiation from the lamp, excitation and quenching of Hg(6³P₁) and Hg(6³P₀) by photoabsorption and reactive collisions, dissociation of NH₃ and SiH₄ into H, NH₂, and SiH₃ radicals, gas‐phase diffusion and secondary reactions of these radicals, and eventually surface reactions of SiH₃ and NH₂ on the growing silicon nitride film surface, in terms of sticking and recombination probabilities. It is shown that SiH₄ diluted in NH₃ is essentially dissociated by reactive collisions with Hg(6³P₀) metastables produced by collisional deexcitation of Hg(6³P₁) with NH₃. Moreover, within the assumptions and the precision of the model, we derive from LIF data that the rate constant for the reaction of NH₂ with SiH₄ is lower or equal to 10⁻¹³ cm³ s⁻¹, in agreement with a recent determination. It is also shown that the radical‐radical reactions between SiH₃, NH₂, and H are the main cause for the limitation of radical gas‐phase density and deposition rate.