Direct phase measurement in phase-shift masks with a differential heterodyne interferometer

Lasertec has developed a novel phase-shift measurement system 1PM11, which uses a differential heterodyne interferometer. In 1PM11, phase information is converted into a low- frequency heterodyne beat that is easily measurable by an electric current. Two-frequency laser beams which are oscillated from a He-Ne laser (632.8 nm) tube are used. When a 40X objective lens being used, the beam is 2.3 micrometers in diameter and the distance of the two beams is adjustable from 3.0 to 6.0 micrometers . The performance of 1PM11 for three types of phase-shift masks: (1) the shifter of etched quartz, (2) spin-on-glass shifter on etch- stop/quartz, and (3) the attenuating shifter, is reported.