Spatial frequency doubling lithography (SFDL) of periodic structures for integrated optical circuit technology

A novel technique for fabricating gratings useful for integrated optical circuits (IOCs) is described. The technique combines optical projection lithography, partially coherent illumination, and spatial filtering. With this technique, only the first two 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, and a period half of what would have been obtained in normal imaging. 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, the authors printed high-quality 0.5-μm period gratings in an oxide layer on 10 mm×10 mm silicon chips. Extended depth of focus was observed. This technique opens up the possibility of high-volume production of IOC chips with Bragg filters using standard IC fabrication facilities