FABRICATION OF A 1 MBIT DYNAMIC RANDOM-ACCESS MEMORY WITH 4 LEVELS USING X-RAY-LITHOGRAPHY

A 1 Mbit dynamic random access memory (DRAM) was recently fabricated with all critical levels exposed using synchrotron x rays. Lithography for the critical levels (isolation, transfer gate, contacts, and metal) was performed at the Brookhaven National Laboratory-National Synchrotron Light Source (vacuum ultraviolet ring) on an IBM beamline using a Karl Suss XRS-200 x-ray stepper. The remaining levels were exposed with standard step-and-repeat optical tools. Most nonlithographic processing was performed in a mature 1 Mbit production line. Bit yields of up to 50% were found on several 256 K input-output (I/O) segments. An optical control lot processed through equivalent nonstandard steps produced a comparable yield. Considering the extensive off-line processing required for this experiment, this yield exceeded expectations.