0.15μm electron beam direct writing for Gbit dynamic random access memory fabrication

This paper describes 0.15 mu m electron beam (EB) direct writing techniques for Gbit dynamic random access memory (DRAM) fabrication. In order to use EB direct writing for reliable fine pattern fabrication on the 0.15 mu m level, an EB direct writing system technique, a resist process technique, a cell projection (CP) mask preparation technique, which is indispensable for improving the writing throughput, and a data preparation technique with proximity effect correction must be improved respectively and combined successfully. The proximity effect correction for all fine patterns in a full-scale DRAM chip is especially important for achieving a CD accuracy of less than 0.02 mu m, which is required for device fabrication and margin. For obtaining the reliable shot stitching accuracy between CP and variably shaped (VS) EB writings, we adopted the cross-correlation method, which was used to decide the size and center position of the CP shot. A single-layer resist system without an over-coated conducting layer was used for reliable device fabrication. In addition, for improving the CD accuracy for all 0.15 mu m designed patterns in a full-scale chip, we developed a data partition process suitable for CP mask pattern data and an improved 1-dimenshinal(1-D) calculation method for proximity effect correction. Utilizing these techniques, the full-scale 4Gbit DRAM, which was designed with 0.15 mu m minimum feature size, was fabricated successfully with 0.05 mu m (\mean\ + 3 sigma) overlay accuracy, 0.02 mu m (\mean + 3 sigma) stitching accuracy, and less than 0.02 mu m (3 sigma) CD accuracy, all of which were sufficient for the required device fabrication.