Films of Si<sub>3</sub>N<sub>4</sub> on Si substrates were prepared by both sputtering and low pressure chemical vapor deposition. Infrared transmission measurements of the strong 800-900 cm<sup>−1</sup> Si-N absorption show large (up to 50 cm<sup>−1</sup>) shifts as a function of film thickness in the range ~0.1 to 1.2 μm. The agreement between experimental spectra and simulated spectra calculated from parallel layer optical theory indicates that these shifts can be attributed primarily to optical dispersion distortion effects. However, there is some discrepancy between experiment and theory at the extreme ends of the thickness range for the sputtered films which suggests small intrinsic structural changes of these films may arise as a function of deposition thickness. These results point out the importance of considering optical effects in the interpretation of thin film spectra.
W. R. Knolle and D. L. Allara, "Infrared Spectroscopic Characterization of Silicon Nitride Films—Optical Dispersion Induced Frequency Shifts," Appl. Spectrosc. 40, 1046-1049 (1986)