Reflection of infrared radiation from lamellar gratings on a silicon wafer

Infrared reflectance spectra in the spectral range from 1.5 to 14 μm of a smooth-surface silicon sample cut from a Si wafer and grating structures fabricated on the same wafer have been measured and analyzed theoretically. The structural dimensions of the gratings are in the range of the radiation wavelengths. Two types of experiments have been conducted. In the first experiment we investigated specular reflectance. The results show that the reflectance spectra depend on the mounting type: planar- or conical-diffraction mounting, and on the side of the sample which is irradiated. The differences become very pronounced for wavelengths larger than the grating period. For the reflection spectra in the case of conical-diffraction mounting, reasonably good agreement between experimental spectra and theoretical zero-order efficiency spectra is obtained. In the case of planar-diffraction mounting an agreement between theory and experiment is achieved only if we assume an admixture of the higher diffraction orders. In the second experiment we studied the angular diffraction pattern from the different gratings in planar-diffraction mounting. We found that it depends strongly on the radiation incidence angle, the radiation wavelength and grating parameters. It is observed and confirmed by simulation that the reflection dip at about 9 μm due to vibrational transition in complex Si-interstitial O₂ is enhanced by the grating structure.