Subfield distortion of an EPL stencil mask

Abstract

EPL (Electron Projection Lithography) is one of viable candidates for the NGL (Next Generation Lithography) used for the 65nm technology node and below. EPL uses the membrane mask with grillage structure to reinforce the membrane area. The NGL poses stringent requirements to mask accuracy. The most severe problem of membrane mask is IP (Image Placement) accuracy. In this study, distortion characteristics of an EPL stencil mask are analyzed by FEM (Finite Element Method). Its dependence on the grillage structure and the distribution of pattern density is presented. The distortion of the wet-etched wide grillage is negligible compared to the required IP accuracy. However, the distortion of the dry-etched narrow grillage cannot be negligible, which exceeds 5nm. The accumulation of the distortion will deteriorate the IP accuracy. The non-uniform distribution causes much larger distortion in membrane than the uniform one, which is quite unacceptable for the 65nm technology node and below. Improvement of the IP accuracy by the correction using an EPL optical system is also evaluated. The EPL optical system can reduce the distortion caused by the uniform distribution. However, the large distortion caused by the non-uniform distribution cannot be corrected. EPL requires its own design rules regulating the distribution of pattern density from the view point of IP accuracy.