Design and Optimization of Power Supplies for Wireless Integrated Microsystems.

Abstract

In this work, we developed a novel power supply for the WIMS-ERC intraocular sensor (WIMS-IOS), an autonomous and implantable system. This device is representative of a broad class of microscale devices, whose full implementation in environmental and medical systems will require significantly smaller power supplies; presently, battery systems represent 85% mass and 50% volume of typical devices. Strategies using both commercial and specially developed devices, using a variety of electrochemistries have been used. The smallest of the batteries reported to date, are thin-film lithium (Li) cells, using a chemical vapor deposition (CVD) or a pulsed laser deposition (PLD) approach. Thin film batteries fabricated with these techniques have achieved electrodes thicknesses less than 5µm (unpackaged), capacities of ~100µAh/cm2 and footprints of 1cm2. However, clean-room fabrication and high power laser equipment needed for ceramic materials entail high cost (~$300/Wh), and the elevated processing temperatures (500-720ºC) and use of chemicals and etchants, make them incompatible with CMOS materials. Finally, the intrinsically high power (3.5-4.2V) of lithium chemistry complicates integration with low-voltage MEMS, since it necessitates voltage regulation. In our study we deposited thin film electrodes using physical vapor deposition (PVD), a low temperature (270-500ºC) purely physical process in a vacuum ~10-7Torr. Our underlying hypothesis was that this technique would reduce intrinsic losses because of the high resulting precision, while allowing integration with chips because of more benign processing conditions to MEMS. Our specific objectives were to: 1) analyze commercial systems for the WIMS-IOS; 2) create batteries from commercial active materials; and finally 3) create test and integrate novel batteries. Commercial Zn/Ag batteries were selected using a previously developed system analyzer (POWER algorithm). Active materials from the same commercial systems were packaged in ceramics and tested in three cells. Six novel cells (cathode/anode) were deposited on MEMS platforms (glass substrates). The last of these was integrated with the WIMS-IOS microchip and tested. Deposited batteries had suitable voltages (1.55) for MEMS, thicknesses of ~25µm (unpackaged) and footprints of ~2mm2, with capacities ~0.1mAh/cm2, and processing conditions compatible with MEMS materials. They also offer potentially lower cost than existing systems.