High precision grating of waveguide combiner with scanning overlapped phase interference lithography

We achieved the manufacture of diffraction gratings with line edge roughness of less than 3% at 3σ/ave by utilizing a scanning overlapped phase interference lithography (SOPHIL) system. The SOPHIL method consists of two steps. As the first step, a spot fringe pattern generated by two-beam interference is scanned direction along the grid on the wafer. As the second step, the spot is moved just the integer multiple length of its fringe period, then the spot is exposed again over the 1st fringe. In addition, a 2D-EPE waveguide combiner was fabricated using SOPHIL system. Measured MTF was 0.5 at 0.6 cycles/ milliradian.