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. SiO
2
, Si
3
N
4
, and polysilicon etchings were accomplished by reactive sputter techniques with CF
4
+ H
2
and CCl
3
F gases using negative electron beam resist PGMA and positive resist AZ-2400. Al etching was carried out by plasma with CCl
4
gas 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.