Multiscale fabrication of integrated photonic chips by electron beam lithography

The readily accessible commercial electron beam lithography (EBL) has high-accuracy and mask-free characteristics which enable fast exploration of novel on-chip devices. However, current EBL technique would be challenging to solve the dilemma between high accuracy and large writing field. Here we report an effective recipe to fabricate such multiscale photonic devices. It is realized by improving the standard procedure of stitching small writing fields with alignment markers. The key is the small patterns stitching and exposure alignment process. We divide the large design structure into several small patterns and take pictures of their corresponding alignment markers by the EBL instrument itself with exactly the same parameters used in the subsequent e-beam exposure. As such, the exposure alignment errors caused by calibration procedures are completely eliminated. We precisely write the divided patterns to desired locations by their surrounding markers and finally achieve gapless and precise stitching within the whole photonic circuit. The protocol is demonstrated by a Mach-Zehnder Interferometer (MZI) structure on a 200nm thick Si₃N₄ chip, in which nano-scale grating coupler have been clearly developed. Compared with traditional EBL technique, the connection accuracy of a waveguide between adjacent writing fields has been significantly improved to be less than 10 nm even without a laser interferometric stage. Moreover, due to the stitching mechanism, the maximum chip size for exposure becomes limitless and could reach up to the entire wafer. Our technique greatly expands the fabrication size of EBL while maintaining its high resolution and opens more opportunities to the development of integrated photonic circuits.