X-RAY MASK FABRICATION USING ADVANCED OPTICAL LITHOGRAPHY

A viable method of x-ray mask fabrication using advanced optical lithography is proposed and the results of a feasibility study using conventional optical lithography is presented. This method reduces the proximity effect compared to electron-beam lithography for x-ray masks. Thus, it achieves excellent critical dimension control and offers high productivity and low cost with a single layer resist process. This technology was applied to fabricating x-ray masks with two 64 Mb dynamic random access memory (DRAM) circuit dies. The 64 Mb DRAM pattern on a five time reticle was replicated onto a 1-mum-thick positive resist coated on an extremely flat x-ray mask substrate using a conventional i-line stepper. The resist pattern was directly transferred to an x-ray absorber Ta film using low-wafer-temperature electron cyclotron resonance plasma etching with SF6 gas. The resulting 0.75-mum-thick Ta pattern with vertical sidewalls was fabricated with 20 nm (3sigma) critical dimension control. This simple process makes x-ray masks with low defect density.