Abstract

A new method to fabricate the gratings used for integrated optical circuits is described. The method combines optical projection lithography, coherent illumination, and spatial filtering. Only the two first orders diffracted by the mask grating are allowed to pass through the lens. This produces a grating pattern in the image plane that has a high contrast (near 100%), a large depth of focus (aproximately ±13 μm), and a period half of what would have been obtained in normal imaging. Thus with a 5× reduction lens a 5-μm period grating on the mask produces a 0.5-μm period image on the wafer. Gratings of different periods, sizes, locations, orientations, and configurations (chirped, phase shifted, etc.) can all be produced on the same chip with a single exposure. Using a deep UV lens with 248-nm KrF excimer laser for illumination, we printed a high-quality 0.5-μm period grating in an oxide layer on 10- × 10-mm silicon chips. The extended depth of focus was observed. This method is primarily useful for patterning gratings. The method can be extended to fabricate phase-shifted gratings of a 0.23-μm period for DFB semiconductor lasers. This technique opens up the possibility of high-volume production of IOC chips with Bragg filters using standard IC fabrication facilities.

© 1988 Optical Society of America