A fully integrated microbattery for an implantable microelectromechanical system

Abstract

The Wireless Integrated Microsystems Engineering Research Center’s Intraocular Sensor (WIMS-ERC IOS) was studied as a model system for an integrated, autonomous implantable device. In the present study, we had four objectives: (1) select and designing an optimized power supply for the WIMS-IOS; (2) develop a fabrication technique allowing small scale, low-cost, and integrable fabrication for CMOS systems, and experimentally demonstrate a microscopic power source; (3) map capacity and lifetime of several fabricated microbatteries; (4) determine the effects of miniaturization on capacity, lifetime and device architecture.

Physical vapor deposition (PVD) was used to deposit thin layers (≤1 μm) of metal sequentially onto glass substrates (SiO₂, as used in the device). To map the influence of size over cell capacity and cycle life, we fabricated and tested five stand-alone cells using a Solartron^® 1470E battery tester and a Maccor^® 4000 series tester. A sixth battery was fabricated to investigate the effects of system integration, variable discharge rate and size reduction simultaneously. The highest experimental capacity among the larger cells O(cm²) was 100 μAh, achieved by IOS-C-1 at 250 μA (1.4 C) discharge. Among O(mm²) cells, IOS-M-1 achieved the highest capacity (2.75 μAh, ∼76% of theoretical) at 2.5 μA discharge (0.7 C rate).

Introduction

The reduction in size and improvement in capability of microsystems is presently limited by the specific and gravimetric properties, and overall sizes, of on-board power supplies [1]. This is most readily seen in autonomous devices used in environmental [2] biological [1], [3] and medical [4], [5], [6] applications (Table 1), which rely mainly on batteries for power. Indeed, power supplies often comprise up to ten times the mass of the other elements of the system, combined.

The Wireless Integrated Microsystems Engineering Research Center’s Intraocular Sensor (WIMS-ERC IOS) was studied here as a model system for an integrated, autonomous implantable device. This device is based on the Phoenix Processor [7], [8], an ultra low power consumption platform (∼30 pW) used in several WIMS microsystems. Its small scale (∼0.4 mm³ in volume, with a footprint of ∼2 mm², Fig. 1) is required, to allow implantation in the eye to monitor intraocular pressure during treatment of glaucoma, for up to 2 years. At present (Table 2), there is no commercial power supply available which fits within the 2 mm² device area, and simultaneously meets the lifetime and implantability requirements of the device.

Despite their excellent volumetric energy (Table 2), existing thin-film batteries require surface areas of O(cm²) to power typical MEMS devices, for more than 1 day. With specific capacities of 100–300 μAh cm⁻², limited by the maximum thickness of thin-film electrodes construction (<5 μm), these batteries have been unable to meet the required lifetime of MEMS sensors and actuators. Moreover, typical thin-film processing conditions involve high temperatures (500–900 °C) and masking and etchant materials that are incompatible with chip materials and packages. To date, none has been implemented or integrated as power supply in a MEMS of the scale described here; instead, novel devices of similar scale have been demonstrated using macroscopic, unintegrated power sources (Table 1).

Possibly the most significant development in autonomous MEMS in the coming decades, for new applications, will be the achievement of fully integrated and optimized power supplies, realized cost-effectively, that are also capable to achieve energy densities (∼27 mWh cm⁻² [9]) required to operate MEMS. Adopting manufacturing techniques that can be performed outside the clean room [10], which are also compatible with CMOS fabrication techniques, have the potential to reduce processing steps. Thus, in the present study, we had four objectives:

• Select and designing an optimized power supply for the WIMS-IOS;

• develop a fabrication technique allowing small scale, low-cost, and integrable fabrication for CMOS systems, and experimentally demonstrate a microscopic power source;

• map capacity and lifetime of several fabricated microbatteries;

• determine the effects of miniaturization on capacity, lifetime and device architecture.

Our methodology builds on our prior efforts in the area, including optimization of power supplies [11], [12], manufacture of novel microbatteries [13], [10] and electrode optimization [14], [15].