The Structure of Plasma-Deposited Silicon Nitride Films Determined by Infrared Spectroscopy

Plasma-deposited silicon nitride, a-SiN:H, has been deposited at low ammonia-to-silane gas ratios. The nitrogen-to-silicon ratio in the film is proportional to the NH sub 3 /SiH sub 4 flow ratio in the reactor. The Si-H peak in the infrared spectrum of the a-SiN:H shifts to lower frequency as the N/Si of the film decreases. We use the random bonding model to calculate the average electronegativity that a Si-H bond experiences for a particular N/Si ratio in the film. The measured Si-H frequency correlates with the calculated electronegativities and agrees with a similar correlation of Si-H obtained for molecules. For N/Si less than 0.27 we observe an additional peak at 650 cm sup -1 that also appears in the spectrum of plasma-deposited amorphous Si and is the Si-H wagging vibration. The random bonding model predicts amorphous Si to be the predominant constituent for N/Si less than 0.27, in agreement with the infrared data.