Resettable circuit breaker cantilevers use silicon dioxide (SiO2) for temper ature compensation. The compressive stress in the SiO2 causes the cantilever to bow upwards 400 μm after release. In the DC-DC voltage converter, the gap between the electrodes is 1.4 μm. After release, the cantilevers are stuck down. A short-loop experiment was conducted to better understand the cantilever behavior by omitting the contact metal and SiO2 layers from test cantilevers.
Silicon-only cantilevers were fabricated with the existing mask set. The silicon-on-insulator (SOI) wafer (cantilever wafer) was patterned with the release and the metal3 (bond metal) masks. The release mask defines the handle silicon area that is etched to allow the cantilever to move. The metal3 layer defines the bonding area to a double-sided polished (DSP) wafer. The DSP wafer, or substrate wafer, was patterned with the indent (3.1 μm deep) and metal1 (bond metal).
The SOI and DSP wafers were bonded together using Au-Au thermal compression bonding at 360 oC with a force of 800N for 45 minutes. Following the bonding, the handle silicon was etched using deep reactive ion etching (DRIE). To release the cantilevers, the buried oxide on the SOI wafer was etched using reactive ion etch (RIE). The figure shows a scanning electron microscope (SEM) image and an optical profilometer image of the MEMS switch for a DC-DC voltage converter. The smaller cantilevers (<1000 μm) have a slight downward bow that is due to thermal effects and permanent deformation of the metals during bonding, or a thermal and charging effect from the reactive ion etch. The larger cantilevers bow enough to cause contact with the bottom electrode. The optical profilometer data suggest that the smaller cantilevers have an acceptable gap of 2-3 μm between the cantilever and the bottom electrode. Using these data, a Rev B full wafer fabrication process was developed.
For the Rev B process, the indent is increased by 2 μm to 5 μm. For the resettable circuit breaker, tensile deposited silicon nitride is used for thermal compensation in place of compressive thermally grown SiO2. The tensile stress of the silicon nitride film causes the cantilever to bow downward or come in contact with the bottom electrode, closing the circuit for the MEMS circuit breaker. For the DC-DC voltage converter, thinner (200 nm) SiO2 is used as contact isolation material at each cantilever tip. This balances the tensile stress from metal with compressive stress from oxide, keeping the cantilever from touching the bottom electrode. The fabrication of Rev B wafers is currently in progress.
This work was done by Susana Stillwell, Sunny Kedia, Weidong Wang, Shinzo Onishi, and Scott Samson of SRI International for the Office of Naval Research. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Electronics/Computers category. ONR-0016
This Brief includes a Technical Support Package (TSP).
MEMS Resettable Circuit Breaker and Switch for DC-DC Voltage Converters
(reference ONR-0016) is currently available for download from the TSP library.
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