Graphite-based x-ray masks for deep and ultradeep x-ray lithography

Abstract

The cost-effective fabrication of high-aspect-ratio microstructures using x rays largely depends on the availability and quality of x-ray masks. The architecture of a mask is mainly determined by the photon energy of the synchrotron source, the x-ray flux, and the thickness of the resist. Typically, the mask membrane is made from a low-atomic-number material and can either be a frame-supported, several microns thin membrane (carbon, silicon carbide, silicon nitride, or silicon) or a bulk material (beryllium) with a thickness of up to 1 mm. The absorber pattern is formed from high atomic number materials such as gold, tungsten, or tantalum, and the final pattern geometry can be defined either with additive (electroplating) or subtractive (etching, milling) processes. One approach that is designed to reduce cost and turn-around time is the fabrication of x-ray masks using graphite sheet stock for the mask membrane. Rigid graphite offers unique properties, such as moderate x-ray transmission, relatively low cost, and the ability to be used with either subtractive or additive processes. This article will report details on the lithographic fabrication of graphite x-ray masks using intermediate x-ray masks for pattern transfer. First experimental results will be presented for synchrotron x-ray exposures performed at the Center for Advanced Microstructures and Devices synchrotron in Baton Rouge and the Advanced Photon Source at Argonne National Laboratory.