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The lateral actuated switch requires an isolation structure to provide mechanical coupling and electrical isolation between the actuator and the contacts. This isolation structure usually imposes extra difficulty on the fabrication process. In previous reports, we demonstrated a thermal actuated lateral switch, where the nitride isolation structure was a weak point, leading to reliability problems. In this paper, we developed a modified switch utilizing undoped polysilicon as the isolation material. The undoped-polysilicon isolation structure requires only one extra step of sheltered implantation, and it provides robust mechanical connection. A 20-mum-long undoped-polysilicon isolation structure has a current leakage of less than 2 nA under a 15-V operation voltage. The proposed switch works under a 12-V driving voltage with 60-mW input power. The time response is measured to be 130 mus, and a maximum operation frequency of 4.5 kHz is reached. An ON-state insertion loss of -0.41 dB at 20 GHz and an OFF-state isolation of -20 dB at 20 GHz have been achieved on the normal low-resistivity silicon substrate. The undoped-polysilicon isolation method can be used in other surface-micromachined lateral switches as well.