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 Å PMMA on silicon is 1.5 V while that of 4000 Å 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.