Lift-off Methods for MEMS Devices
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Lift-off Methods for MEMS Devices Shih-Chia Chang and Jeffrey M. Kempisty Delphi Research Laboratories 51786 Shelby PKWY Shelby Twp, MI 48315-1786 ABSTRACT Five different methods were explored for the formation of the lift-off mold. In the first method, a tri-level resist scheme was used to generate a lift-off mold. The lift-off mold has a vertical wall slope. In the second method, an aluminum/photoresist double layer was used as the mold material. An overhang structure is obtained by undercutting the underlying aluminum layer. In the third method, a composite layer of two different photoresists (AZ 1811 and LOR 10A) was used to form the lift-off mold. With the fast dissolution rate of the underlying LOR 10A photoresist an overhanging mask structure was obtained. Since regular photoresists were used as part of the mold material in these three lift-off methods, they are suitable for thin film materials with thickness of ≤2 µm and processing temperatures ≤100°C. For thicker device film materials and higher processing temperature (~200°C), methods 4 and 5 using negative resists, Futurrex NR73000PY (~3 µm) and SU-8 (~50 µm), respectively, were experimented with to form the lift-off mask. The fabrication processes used in these two methods were relatively simple and a negative wall slope was readily obtained. While Futurrex photoresist is easily stripped by the resist remover provided by the manufacturer, there is no effective chemical solution available for the removal of SU-8. We found that plasma etching with a mixture of O2 and C2F6 can be used for the removal of SU-8. INTRODUCTION There are two primary techniques used for the patterning of thin film materials in a microfabricated device: either subtractive or additive.[1-3] In subtractive methods, the thin film material is deposited on the substrate followed by patterning and etching to form the thin film device element. In additive methods (commonly called lift-off methods) the opposite sense device pattern is defined photolithographically in a layer of photoresist. The thin film material is then deposited on both the photoresist and the exposed substrate. The thin film on top of the photoresist is removed by dissolving the photoresist layer in an appropriate liquid, leaving behind the material on the substrate. For conventional ICs, the most commonly used thin film materials are aluminum (Al), aluminum/silicon (Si)/copper (Cu) alloy, polycrystalline silicon, silicon dioxide and silicon nitride. The delineation of these materials is usually done by a deposition/etching (subtractive) method due to the fact that the etching processes for these thin film materials are well established. For certain devices (most notably MEMS) however, thin film materials that are hard to etch may be used for device materials due to functional requirements and/or processing compatibility considerations. For examples, thin film platinum (Pt), due to its high temperature coefficient of resistance and chemical inertness to the chemical solutions, used in bulk micromachining (ethylene diami
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