Fabrication Process for Ultra High Aspect Ratio Polysilazane-Derived MEMS Tsali Cross, Li-Anne Liew, Victor M. Bright , Martin L. Dunn, John W. Daily, and Rishi Raj High-Temperature Materials Laboratory Center for Advanced Manufacturing and Packaging of Microwave, Optical and Digital Electronics Department of Mechanical Engineering, ECME 150, Campus Box 427, University of Colorado at Boulder Boulder, CO, USA 80309-0427 Phone (303) 735-2651, Fax (303) 492-3498, email: crosst@ucsub.colorado.edu ABSTRACT We present a new process for fabricating polysilazane- derived MEMS components with ultra high aspect ratios. The width-to-height ratio of actual structures fabricated at this time is (~20:1), but shows promising results to achieve aspect ratios of 50:1. Polysilazane-derived materials are a group of polymers and ceramics that can be functionalized to have a wide range of material properties such as electronic, magnetic, dielectric, and optical. The fabrication process is based on contact lithography of a liquid photopolymer precursor, poly urea methyl vinyl silazane, PUMVS (Kion, Corp.), with photoinitiator 2,2 dimethoxy, 2-acetophenone, DMPA (Aldrich) for polysilazane. Contact lithography of aqueous photopolymers presents a substantial improvement in resolution, flatness of structures, and aspect ratios compared to microcasting, and proximity printing for polysilazane-derived MEMS. In the future, this fabrication technique may be extended beyond polysilazane-derived materials to a wide variety of aqueous photopolymerizable sol-gels, preceramics, and photopolymers. INTRODUCTION A variety of fabrication techniques for potentially high (height-to-width) aspect ratio microcomponents of different materials have been developed. Some of these techniques include but are not limited to: deep reactive ion etching [1], ion diffusion in glass, photosensibilization of glass, excimer laser ablation and melting, diffractive techniques, proton irradiation of poly(methyl methacrylate) (PMMA), synchrotron irradiation [2], micro-molding techniques, embossing [3], direct photolithography of inorganic-organic sol-gels [4], and ink jet printing [5]. However, key deterrents to using these fabrication processes for polysilazane-derived materials were availability, complexity, cost, and performance. Polysilazane-derived materials are a group of polymers and ceramics that can be functionalized to have a wide range of material properties such as electronic, magnetic, dielectric, and optical [6, 7, 8, 9]. Polysilazane, the crosslinked form of poly urea methyl vinyl silazane (PUMVS, Kion Corp.), is a novel hybrid organic/inorganic polymeric glass that is highly transparent in the infrared to ultraviolet [9] Polysilazane microcomponents have potential applications in optical MEMS, and microfluidics [10]. Polysilazane may also be converted into a black-colored, novel Silicon Carbon Nitride ceramic, which has potential use in high temperature and corrosive environment applications for MEMS [6]. Our previous work focused on the fabrication of such polysilazane, and polysilazane-derived silicon carbon-nitride (SiCN) ceramic MEMS by micro-casting [10,11]. Some disadvantages to this fabrication process are its ability to generate completely flat structures, its resolution, and number of processing steps [10]. We therefore present a cost-effective fabrication technique that shows substantial improvement to microcasting’s flatness, resolution, and processing steps. In this paper we will describe the process developed to improve fabrication of polysilazane MEMS structures over microcasting, and discuss some key design issues. FABRICATION The process is shown in Figure 1. First, a negative glass photomask is generated from the CAD layout of the device. The emulsion side of the mask is then coated with a thin layer of Teflon (a). For this, 1% liquid Teflon AF solution (from Dupont Corp) is spun onto the mask and subjected to a series of heat treatments in an oven, during which the solvent in the Teflon solution evaporates, leaving behind a thin conformal coating on the mask. Next, a sacrificial layer (for example SiO 2 ) is deposited on a silicon wafer (b). The liquid photopolymer precursor solution is then dispensed at an arbitrary thickness onto the wafer (c) Our precursor solution for polysilazane is a mixture of the liquid photopolymer precursor, PUMVS, with photoinitiator 2,2 dimethoxy, 2- acetophenone, DMPA (Aldrich). The mask is then placed on the wafer with the Teflon-coated side in contact with the PUMVS (d). The mask is supported by spacers that keep the mask at a predetermined distance from the wafer, thereby setting the thickness of the PUMVS structures. The system is exposed to UV light. The dark areas in the mask prevent the UV from penetrating the glass through to the PUMVS, thus the PUMVS solidifies in regions corresponding to the clear sections of the mask (e). Following photopolymerization, the still-liquid PUMVS is removed from the wafer by a high-speed spin-rinsing with a solvent such as acetone or hexane (f). Solid polymer structures thus remain on the wafer (g). The sacrifical 0-7803-7185-2/02/$10.00 ©2002 IEEE 172