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Application of phase-shifting mask to DRAM cell capacitor fabrication

We have investigated the application of phase-shifting mask (PSM) or DRAM cell capacitor fabrication. Narrow spaces formed by PSM enables us to attain a high DRAM cell capacitance. Four types of phase-shifting masks are tested: a rim-type mask, transparent type mask (I) (or `shifter-shutter' type), Levenson type mask and transparent type mask (II) (or `shifter-edge' type). The improvement in resolution and depth-of-focus (DOF) by the former two types of PSMs is small, although we can continue to use conventional positive resist with these PSMs. The latter two PSMs provide a significant increase in resolution and DOF. Since the transparent type (II) mask has the difficulty in mask fabrication, we select a Levenson type PSM for the fabrication of the DRAM cell capacitor. Though 0.28 micrometers spaces with the DOF of 1.5 micrometers can be formed on a bare silicon substrate by the Levenson type PSM, the printed patterns on an actual device substrate are deformed by the reflection from the substrate. Dyed negative resist is used to reduce the effects of the reflection, the patterns of 0.28 micrometers spaces with 0.6 micrometers DOF on the actual substrate can be successfully printed. We confirm the effectiveness of the Levenson type phase-shifting mask combined with a dyed negative resist for the DRAM cell capacitor fabrication.