A fabrication process based on structural layer formation using Au–Au thermocompression bonding for RF MEMS capacitive switches and their performance

Abstract

This paper presents a radio frequency micro-electro-mechanical-systems (RF MEMS) fabrication process based on a stacked structural layer and Au-Au thermocompression bonding, and reports on the performance of a sample RF MEMS switch design implemented with this process. The structural layer consists of 0.1 mm SiO 2 /0.2 mm Si x N y /1 mm Cr-Au layers with a tensile stress less than 50 MPa deposited on a silicon handle wafer. The stacked layer is bonded to a base wafer where the transmission lines and the isolation dielectric of the capacitive switch are patterned. The process flow does not include a sacrificial layer; a recess etched in the base wafer provides the air gap instead. The switches are released by thinning and complete etching of the silicon handle wafer by deep reactive ion etching (DRIE) and tetramethylammonium hydroxide (TMAH) solution, respectively. Millimeter-wave measurements of the fabricated RF MEMS switches demonstrate satisfactory up-state performance with the worst-case return and insertion losses of 13.7 and 0.38 dB, respectively; but the limited isolation at the down-state indicates a systematic problem with these first-generation devices. Optical profile inspections and retrospective electromechanical analyses not only confirm those measurement results; but also identify the problem as the curling of the MEMS bridges along their width, which can be alleviated in the later fabrication runs through proper mechanical design.