Stability of hydrogen in silicon nitride films deposited by low-pressure and plasma enhanced chemical vapor deposition techniques

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Abstract
Hydrogen concentration depth profiles in silicon nitride films deposited by low-pressure chemical vapor deposition (LPCVD) and plasma enhanced chemical vapor deposition (PECVD) techniques were studied. Quantitative hydrogen profiling was carried out using the resonant nuclear reaction 15N+1H→12C+4He+γ ray. Hydrogen concentration in as-deposited LPCVD silicon nitride films was ∼2.5×1021 atoms/cm3 and was stable even after a furnace anneal at 450 °C in 3% H2/Ar for 30 min or a rapid thermal anneal at 1000 °C in oxygen for 30 s. These nitride films thus appear to be good hydrogen diffusion barriers. In contrast, hydrogen concentration in as-deposited PECVD silicon nitride films was ∼1.75×1022 atoms/cm3 and dropped to ∼7.5×1021 atoms/cm3 after rapid thermal annealing at 1000 °C in oxygen for 30 s. During high-temperature anneals, the hydrogen diffused from PECVD silicon nitride film into the underlying SiO2 layer. A comparison of the hydrogen concentration in these deposited oxides under the nitrides with those previously reported for as-deposited, but uncovered, SiO2 films points out that the observed threshold voltage shifts in nitride covered metal–oxide semiconductor capacitors are related to the loss of hydrogen from the silicon oxide during vacuum processing for nitride deposition and to the subsequent diffusion in SiO2 from the PECVD nitride films.

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