High flatness mask for step and repeat x-ray lithography

Abstract

A flat and high contrast mask for step and repeat x‐ray lithography has been developed. The mask has a 1‐μm‐thick, 20×20 to 25×25 mm, plasma chemical vapor deposition (PCVD) SiNx membrane, on which x‐ray absorber gold patterns are fabricated by selective electropolating. Stress in the SiNx membrane was reduced to about 1×108 dyn/cm2 by optimizing the plasma CVD conditions. Also, stress‐free lustrous gold pattern was prepared by optimizing the plating conditions. Furthermore, a stress compensating structure has been employed, which utilized a thermal SiO2 film for compensating a tensile stress in the SiNx membrane. Thus, warpage in the SiNx membrane was reduced to less than 1 μm, regardless of the gold pattern density or of the membrane size. The effective x‐ray mask contrast was also much improved by coating the x‐ray absorber pattern with another 0.1‐μm‐thick SiNx film. The SiNx film traps most of the undesirable secondary electrons, which are ejected from the gold pattern during the x‐ray exposure process. By using the developed x‐ray mask, high aspect ratio half‐micron patterns were replicated in the practical x‐ray resist, such as negative CPMS (JSR‐MES‐E) resist or positive NPR (RE‐5000P) resist, with high pattern definition.