This article describes the fabrication of eight-inch continuous membrane masks with a 15-nm-thick support membrane for electron projection lithography (EPL). In order to develop an extremely thin support membrane with a tensional stress, two techniques were applied ; one is a low pressure deposition process to improve the membrane bulk density, and the other is Si addition during the conventional carbon membrane deposition for improving the atomic distance mismatch between substrate and deposited film, and also to address the membrane volume strain cause by the rise in the tensional stress under the low pressure deposition condition. By using Si-additional techniques an extremely thin membrane with a tensional stress was formed under the conditions of less than 0.5 Pa chamber pressure.. Long-term membrane stress stability of Si-added extremely thin membranes were particularly improved. It was less than 3 MPa in elapsed times of 400 h. The zero-loss electron transmittance for the fabricated 15-nm-thick membrane mask was measured to be 70.4%. Due to the development of an extremely thin membrane with high zero-loss electron transmittance of more than 50%, our high-performance membrane masks are superior to the complementary stencil masks in terms of exposure throughput.(c) 2005.