Temperature Stabilization for MEMS Devices
By Seri Lee, Ph.D., Nextreme Thermal Solutions
Advances in thin-film thermoelectrics enable novel temperature stabilization for applications in MEMS packaging. As the electronics industry moves towards smaller packaging form factors and higher heat densities, non-uniform thermal conditions can directly affect the performance and reliability of MEMS devices.

Designers of MEMS-based products not only need to worry about the environmental stresses but also the thermal stresses devices may experience as a part of the fabrication or packaging process. For this reason and due to their typically low thermal mass, MEMS devices, when in operation, must have a tightly controlled thermal management system.

MEMS devices tend to be sensitive, fragile, and difficult to fabricate and package. As such, they can be affected by thermal excursions during the manufacturing process or during operation. Minimizing these temperature excursions and package-induced stresses will lead to a more reliable, tightly controlled device.

Thermal management systems comprise either passive or active devices. Because MEMS devices tend to be temperature sensitive, passive thermal management schemes offer minimal control for these types of devices. Active thermal management systems such as water-based cooling with mechanical pumps tend to be bulky, unreliable, and more difficult to implement. From an electromechanical point of view, bulk and now thin-film thermoelectric coolers (TECs) offer an attractive option for active control with enhanced reliability.

TECs have been used to temperature stabilize these very sensitive devices and allow for more well-defined performance characteristics. Thin-film based TECs offer an additional level of control due to their exceptionally low thermal mass which is oftentimes on the order of the mass of the device itself as opposed to traditional TECs that may be 100× more massive. These thin-film TECs have millisecond response times for rapid cooling and heating to maintain precise temperature control. They are known to have higher heat pumping capability than standard bulk TECs, but for applications in thermal stabilization, the superior switching speed and size of the devices may ultimately prove to be their most valuable assets. Recent measurements have shown that due to their smaller footprint and weight, these thin-film TECs translate into a more efficient use of energy and more precise temperature control.

The following are examples of the use of thin-film TECs for temperature stabilization of MEMS-based devices.

Piezoresistive sensors
MEMS-based sensors may make use of piezoresistance to measure pressure, acceleration, strain, or force by converting changes in the lattice spacing to an electrical signal. These piezoresistive sensors have proven to be versatile tools for the measurement of various processes. They are used for quality assurance, process control, and process development in many different industries.

It has been shown that even moderate fluctuations of temperature within 200°C can significantly alter the voltage reading from a piezoresistive sensor. Therefore, a reliable interpretation of data from piezoresistive sensors requires an engineer to account for the thermal effects on the sensor and the substrate material. Stabilizing the temperature of the device in the package not only reduces the sensor drift but improves the signal-to-noise ratio that in turn impacts the resolution of the sensor. Embedded thin-film thermoelectric devices can maintain the sensor at a fixed temperature for optimal performance.

Inertial Sensors
MEMS accelerometers and gyroscopes are increasingly used in the automotive industry to detect yaw, which can be used to deploy a roll over bar, trigger dynamic stability control, or deploy an airbag in collisions. For such applications, integration of temperature stabilization into the package of these small, low-cost sensors can enable certain requirements for sensitivity, working range, or response time and can enable a larger temperature range as often occurs in the car.

Optical Switching>
Optical switching enables signals in optical fibers or integrated optical circuits to be selectively switched from one circuit to another. Systems that perform this function by physically switching light are often referred to as "photonic" switches. One MEMS-based approach for photonic switches involves the use of infrared laser and arrays of micro-mirrors that can deflect an optical signal to the appropriate receiver.

Incident infrared laser radiation can be absorbed by the component that in turn can lead to localized heating. The resulting thermal expansion and deformation creates undesirable mechanical and optical performance degradation. Temperature stabilization for this system is crucial to providing a solution that mitigates these concerns.

Bolometers
A bolometer is a device for measuring the energy of incident electromagnetic radiation. A microbolometer is a specific type of bolometer used as a detector in a thermal camera. It consists of an "absorber" connected to a heatsink through an insulating link. The result is that any radiation absorbed by the absorber raises its temperature above that of the heatsink—the higher the energy absorbed, the higher the temperature will be.

Bolometers are sensitive to the energy left inside the absorber. They are very slow to respond and to reset (i.e., return to thermal equilibrium with the environment). Embedding thermoelectric devices within the detector package can introduce new temperature control capabilities that can also address thermal non-uniformity across the device that often results in optical noise, and accelerates localized thermal and electrical failures.

Conclusion
Thin-film thermoelectrics offer the promise for improved temperature stabilization and increased product lifetimes due to their ability to be integrated into a variety of packaging at the MEMS level. Thin-film TECs are just now emerging as a viable product alternative. They offer many positive attributes that can be incorporated into product packages across a broad range of markets for precise temperature control.

SERI LEE, PH.D. CTO, may be contacted at Nextreme Thermal Solutions, 3908 Patriot Dr. Suite 140, Durham, NC 27703 919/497-7313; E-mail: slee@nextreme.com