Ta/SiN-Structure X-Ray Masks for Sub-Half-Micron LSIs

Abstract

In the subtractive X-ray mask fabrication process, suppressing distortions introduced in the bulk-Si etching step is of the utmost importance. Influences of absorber and membrane stresses on mask distortions were investigated in order to obtain good position accuracy. The required stress conditions in order to suppress absorber- and membrane-stress-induced distortions are 8 dyn/cm² for Ta, and 8 dyn/cm² for SiN with a thickness of 2 µm. A five-level mask-to-mask overlay accuracy of <0.085 µm (3σ) was obtained by applying these conditions to actual device mask fabrication. Distortions induced in the bulk-Si etching step were drastically suppressed below 0.058 µm (3σ) in these masks. Absorber patterns with linewidths down to 0.14 µm were successfully fabricated. Pattern position shifts occurring in the frame-mounting step are dominant in our X-ray mask fabrication process.