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

The ability of the revised version of a phase-shift measuring system 1PM11, developed by Lasertec, is evaluated. When a newly-equipped 60× objective lens being used, the beam is 1.6 μm in diameter and the distance of the two beam spots is adjustable from 2.9 to 5.1 μm. These features are suitable for measurement of photomasks for 64 M bit dynamic random access memories. The stability is found to be improved from 0.5-0.8 degrees (3σ) to 0.1-0.3 degrees. The repeatability is found to be 0.1-0.4 degrees (3σ) for etched-quartz shifters, and 0.2-0.8 degrees for attenuating bilayered shifters. The above values are also satisfactory. This improvement can be attributed to the revision of its modulation subsystem. Three types of phase shift masks with 1) a shifter of etched quartz, 2) a spin-on-glass shifter on etchstop/quartz, and 3) an attenuating, monolayered shifter, are measured. The agreement between measurement and calculation is reasonable for all cases. A possibility of using 1PM11 for the process control of masks with attenuated, bilayered shifters is demonstrated