High-accuracy X-ray masks with sub-half-micron 1M-DRAM chips

X-ray masks with four sub-half-micron 1M-DRAM chips have been fabricated for four lithography levels. A four-level mask-to-mask overlay accuracy of higher than 0.13 μm (3σ) was obtained. In these masks, the distance between chip boundaries and window edges was about 4 mm. This large margin is responsible for the large position shifts. By reducing the distance to 1 mm, position shifts at the chip boundary were reduced below 0.03 μm even for X-ray masks with a 22-by-22-mm window and 4-by-4 1M-DRAM chips within a 20-by-20-mm device area.