Optimization of Thick Photoresist for Uniform Thickness in RF MEMS Applications

Micro-electromechanical systems (MEMS)-based devices comprise suspended structures. A sacrificial layer is required to realize these structures. Usually, metal and dielectric materials are used as sacrificial layer materials. Metallic sacrificial material has a selectivity problem, whereas dielectric materials require higher deposition temperatures. Hence, in the present paper, photoresist (PR) material is used as the sacrificial layer. PR requires low-temperature processing and has no selectivity issues. Hence, photoresists are preferred as sacrificial layer materials. In the CMOS fabrication process, thin photoresists such as S1818, S1813, and AZ1505 are used. In these photoresists, the processing wafers are placed in a vertical position during patterning, which occupies less space. The recipes of such photoresists are well optimized for a uniform thickness across the wafer. However, special thick photoresists are used as a sacrificial layer for MEMS devices. These thick photoresists reflow in the vertical position and lead to non-uniform thickness; consequently, the yield is decreased. This paper has optimized a special thick photoresist (HiPR 6517) for RF MEMS SPST switches for better yield and uniformity. The measured pull-in voltage and the switch's resonant frequency are 13.00 V and 12.32 kHz, respectively. The fabricated switch offers isolation and insertion loss better than − 35 dB and − 1 dB at 24.5 GHz, respectively.