High efficiency diffraction gratings in As–S layers

Abstract

Photoinduced changes in the dissolution rate of amorphous As–S layers have been studied with the aim of identifying an etchant which can be used to produce deep (>1 μm) grating structures for use as diffractive optical elements in the infrared. A nonaqueous solvent based on triethylamine was found to yield an adequate differential etch rate and reduced problems with layer adhesion during the etching process. Grating structures ∼4 μm deep were made in As 33S 67 layers with this etchant, the layers being exposed through a binary mask containing an equal lines and spaces grating pattern. The grating profiles were observed by scanning electron microscope and showed that the expected rectangular profile could be achieved by controlling the etching time. Over-etching resulted in under-cutting of the grating lines. The spectral dependence of diffraction efficiency has also been measured. The highest 1st order diffraction efficiency in the near infrared for these gratings was about 26%. This is less than the maximum theoretical efficiency (40.5%) for a rectangular profile, possibly because of deviations from the ideal mark/space ratio (0.5) or from a perfectly rectangular profile, or because of the surface quality of the etched material.