0.25 MU-M ELECTRON-BEAM DIRECT WRITING TECHNIQUES FOR 256 MBIT DYNAMIC RANDOM-ACCESS MEMORY FABRICATION

This paper describes improved 0.25 mum electron beam (EB) direct writing techniques for 256 Mbit dynamic random access memory (DRAM) fabrication. In particular, three techniques were each improved and optimized: (1) an EB writing system technique for improving overlay accuracy, (2) a resist process technique for fabricating reliable fine patterns, and (3) a pattern data preparation technique for correcting proximity effect and reducing data conversion time. The overlay accuracy for the EB direct writing layer to the mark detection layer was improved to under 0.075 mum (\xBAR\ + 3sigma), which is sufficient for the required alignment tolerance. The resist system was optimized for each EB direct writing layer considering a deposited energy distribution, which was calculated by the Monte Carlo method. To reduce data conversion time (central processing unit (CPU) time), a vector processing technique and a 1-dimensional caiculation method applied to proximity effect correction were developed, and a drastic reduction to about 30-60 min was achieved. Utilizing these iechniques, a 256 Mbit DRAM having a feature size of 0.25 mum was successfully fabricated.