Silicon Membrane Mask Blanks for X-Ray and Ion Projection Lithography

Abstract

In the subquarter-micron range, X-ray lithography and demagnifying ion projection are promising printing techniques. For both methods special designed masks are needed which have to fulfil strong requirements including flatness, stabiltiy, defect density, transparency, and surface properties. In this paper we would like to demonstrate that silicon membrane based mask blanks are well suited to meet most of these demands. Furthermore, silicon as membrane material offers the advantage to make use of the experience in semiconductor process technology. A fabrication sequence has been developed, based on silicon epitaxial growing, clean-room compatible wet etching and anodic bonding techniques. Highly boron doped silicon layers with germanium as counter-dopant offer the possibility of stress engineering. An etching process on the base of KOH/IPA is applied in a clean-room environment to decrease the particle density during etching. A very comfortable bonding process for fixing silicon membranes on glass ring carriers was optimized and yieds super flat mask blanks. The application of this process and the results of the blank characterization are presented and discussed.