Development of laser holographic interference system for optical grating fabrication

Periodic corrugations in the form of gratings are an important component in integrated optical devices. In most cases, the period of the corrugations required is of the order of a micron or less and hence conventional photolithography could not be applied. In this paper, we discuss the development of a holographic interference exposure system based on a He-Cd laser of wavelength 325nm. The laser light is kept as one beam as it tranverses through the various optics. Upon incidence on the exposure assembly, the wavefront reflected from a mirror and that incident directly onto the photoresist-coated sample will interfere to generate the required interference pattern on the sample. Gratings with periods of 320nm to 1.69nm have been fabricated on positive photoresist-coated silicon samples. The periods were analysed by both Atomic Force Microscopy and optical diffraction measurements. The periodic patterns have also been transferred onto glass substrates via etching. The measured grating periods are in good agreement with the designed values.