In-situ monitoring of DC saddle-field plasma-assisted deposition of amorphous silicon and silicon nitride

The excitation processes in a DC glow discharge ignited using a saddle-field electrode configuration have been examined using optical emission spectroscopy (OES) for discharges established in SiH4, and in Ar/SiH4 and N2/SiH4 gas mixtures. The emission intensities of excited gas-phase species are correlated with plasma probe mass and energy spectral analysis of the resulting reactive radicals impinging onto the substrate holder. Discharges ignited in SiH4 exhibit strong Si optical emission lines relative to the SiH lines, reflecting extensive gas-phase decomposition of the starting SiH4. The corresponding mass spectra of positively charged radicals exhibit a dominant peak at 28 amu that is associated with Si⁺. The resulting deposition rate of a-Si:H scales linearly with the flow rate of SiH4. The addition of argon to the glow discharge in SiH4 assists the gas-phase dissociation of the SiH4 as indicated by higher partial pressure at 28 and 29 amu, corresponding to the enhanced formation of Si and SiH. Moreover, gas-phase interactions with excited argon result in greater excitation of the background H2, leading to a higher concentration of atomic hydrogen in the discharge. Ionized atomic hydrogen dominates the discharge current at higher Ar to SiH4 gas flow ratios. OES spectra of DC saddle-field discharges in N2/SiH4 gas mixtures indicate strong activation of N 2⁺ and good dissociation of the SiH4 over a wide range of flow ratios, facilitating the preparation of Si:N:H films with stoichiometric ranging from N/Si = 0 to 1.8.