Improving off-state capacitance of SOI-CMOS RF switches: how good are air microcavities?

This paper investigates the reduction of the off-state capacitance (COFF) of SOI-CMOS RF switches induced by the introduction of air microcavity in the back-end interconnection network. A detailed methodology combining electromagnetic and semiconductor transport simulations is used to separately evaluate the respective contributions of the interconnects and junction capacitances. A baseline switch and an optimized version of the same are studied to evaluate their ability to take advantage of air microcavities. Simulations show a reduction of 73 fF/mm regardless of the switch structure considered, resulting in a 24.6% and 30.6% improvement in COFF for baseline and optimized switches, respectively, bringing the optimized version to a 60fs record Ron × Coff. This concept was experimentally implemented using a partial etch process that resulted in a reduction of 21.7 fF/mm, i.e., 7.2%. Finally, the implementation of a more isotropic and selective etching process using HF in vapor phase is shown to approach the optimal configuration of air microcavities.