Lift-off Methods for MEMS Devices

Abstract

AbstractFive 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 NR7-3000PY (∼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.