Electron beam direct writing technology applied to 512 kbit ROM with 1 µm geometry

A 512 kbit read-only memory (ROM) to store Chinese ideographs has been fabricated using variable-shaped electron beam and dry-etching lithography. 1.0-µm minimum line width was used to delineate device area spacings smaller than those obtained with conventional design rules using photoimaging techniques. SiO2, Si3N4, and polysilicon etchings were accomplished by reactive sputter techniques with CF4+ H2and CCl3F gases using negative electron beam resist PGMA and positive resist AZ-2400. Al etching was carried out by plasma with CCl4gas using negative electron beam resist NER-1. The alignment marks detectability and their locating accuracy were improved by properly using the basis arithmetic operations, subtraction and summation, in backscatter signal processings. 6.6 mm × 8.9 mm chip-by-chip alignment yielded about 0.2-µm level-to-level registration accuracy. Memory cell size and chip size are 5.2 µm × 8.4 µm and 6.6 mm × 8.9 mm, respectively; access time and power dissipation are 400 ns and 800 mW, respectively.