WORKPIECE CHARGING IN ELECTRON-BEAM LITHOGRAPHY

A major contribution to total overlay error can be pattern placement imprecision due to charging of the workpiece in electron beam lithography for mask manufacture. A first-generation, worst-case model is presented which indicates that an electron can experience quite a large placement error for a modest workpiece surface potential (100 nM/V). This model also predicts that the amount of error is proportional to the working distance. A novel method which measures the surface potential, to within 50 mV, is also presented. Results indicate that when exposed with 10 kV electrons the surface potential of 3000 angstrom PMMA on silicon is 1.5 V while that of 4000 angstrom SAL-601 on chrome on quartz is 0.5 V. The discharging time for both samples was found to be of the same order as the write time for a typical mask.