Post-enabled precision flip-chip assembly for variable MEMS capacitor

Abstract

A variable capacitor array with a high Q-factor and a high tuning ratio is demonstrated. A novel "post-enabled" flip-chip assembly allows precise multiple digital capacitance levels within one device. This capacitor array is realized by a hybrid integration of a MEMS device with RF circuits on an alumina substrate. The MEMS is prefabricated using a commercially available foundry process and is initially suspended using mechanical tethers on a silicon substrate, which is removed during the flip-chip assembly. The precise gap between the MEMS and the RF circuit is controlled using posts. Each post is designed by a stack of structural layers available in the commercial foundry process. We measured a Q-factor above 200 at 1 GHz, a capacitance ratio of 4.7:1, and tuning range of 171 MHz in a resonator circuit. More importantly, we achieved a digital capacitance level and negligible warpage due to the excellent gap control following the flip-chip assembly.