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Should the refractive index at 633 nm be used to characterize silicon nitride films?

The refractive index at 633 nm is often used to characterize silicon nitride films. Besides providing information about the reflection at this particular wavelength, it is frequently used to indicate additional information regarding the film's absorption and even regarding its surface passivation quality. In this study, we compare nine different silicon nitride films, all with a similar refractive index at 633 nm (2.09±0.01). We demonstrate that these films exhibit very different electrical, chemical and optical properties despite their similar refractive index values. As a result of this investigation, we have developed industrial low-absorption silicon nitride films that provide excellent surface passivation, with saturation current density of 7 fA/cm2 on both n- and p-type wafers. This surface passivation quality is equal to that obtained by industrial silicon-rich silicon nitride films. All the films developed in this study were fabricated using industrial equipment and are thermally stable.