New mask format for low energy electron beam proximity projection lithography

In order to solve the various problems associated with a LEEPL mask as originally demonstrated in the form of single-membrane diamond mask, we propose a new mask format termed COSMOS (complementary stencil mask on strut-supports). The COSMOS has small-area membranes with strut reinforcement and is somewhat similar to the masks used for other types of electron projection lithography (EPL). However, the exposure strategy is completely different from the other EPLs; a complete pattern image can be transcribed by overlaying complementary portions of a mask pattern via multiple exposures. The inter-membrane and intra-membrane distortions of image placement have been computed by the finite element method (FEM) simulation. It is concluded that the global distortion induced by the inversion of gravity can be corrected for by mask writing, and the intra-membrane distortion, induced by both the gravitational flexure of a membrane and the pattern density distribution, can be neglected with the membrane intrinsic stress of approximately 5 MPa.