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

The cost-effective fabrication of high-aspect-ratio microstructures using x rays largely depends on the availability and quality of x-ray masks. The a rchitecture of a mask is mainly determined by the photon energy of the sync hrotron 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 carbi de, silicon nitride, or silicon) or a bulk material (beryllium) with a thic kness 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 geome try can be defined either with additive (electroplating) or subtractive (et ching, milling) processes. One approach that is designed to reduce cost and turn-around time is the-fabrication of x-ray masks using graphite sheet st ock for the mask membrane. Rigid graphite offers unique properties, such as moderate x-ray transmission, relatively low cost, and the ability to be us ed with either subtractive or additive processes. This article will report details on the lithographic fabrication of graphite x-ray masks using inter mediate x-ray masks for pattern transfer. First experimental results will b e presented for Synchrotron x-ray exposures performed at the Center for Adv anced Microstructures and Devices synchrotron in Baton Rouge and the Advanc ed Photon Source at Argonne National Laboratory. (C) 1998 American Vacuum S ociety. [S0734-211X(98)09606-1].