Simulation verification for the cause of overlapped interference fringes in homogeneity measurement of quartz substrate

Photomask manufacturing is an essential cornerstone of the semiconductor industry. Photomask substrate made of quartz is a hot topic in photomask manufacturing today, and the high requirements of photomask on quartz substrate make its optical homogeneity index extremely important as well. When measuring the optical homogeneity of a quartz substrate using a four-step absolute measurement method based on the Fizeau phase shift interference principle, the phenomenon of overlapping interference fringes was discovered. The cause was analyzed and the structure of measurement was simulated by Zemax. The simulation results identify the non-parallelity of the front and rear surfaces of the quartz substrate as the main effect.