Are MEMS bolometers the next big thing?

By Steve Breit, Vice President Engineering

I often feel that Coventor is in the crow’s nest for spotting trends in the MEMS industry because our customers use design and simulation software early in the product development cycle to evaluate and optimize new concepts. Through evaluation and support requests from our worldwide customer base, we get some visibility on the types of MEMS our customers are working on. We’re obligated, of course, to keep the details of customer requests confidential. But, when multiple customers start asking questions about a particular type of MEMS that previously hasn’t seen much activity, we begin to suspect a new trend is developing. Such is the case for MEMS bolometers, or microbolometers.

Our flagship MEMS software suite, CoventorWare® has included specific capabilities for simulating microbolometers since I joined Coventor in the year 2000, such as static and transient analysis of coupled electro-thermal-mechanical effects. Yet, I can recall only a smattering of customer requests concerning microbolometers in the intervening years. In the past year, however, we’ve received a spate of evaluation and support requests from around the world. What’s going on?
Microbolometers sense infrared radiation by detecting the temperature increase of a thermally isolated sensing element that absorbs the incident radiation. A 2D array of microbolometers, known as a focal plane array, can therefore be used as a thermal image sensor. According to an informative article in LaserFocusWorld, microbolometers have been under development for more than two decades, and in production for almost that long. The early devices were developed for military night vision systems and other classified uses, and very costly. Of course microfabrication know-how has increased greatly and costs have dropped dramatically over the intervening period. So, like other electronics, the performance of microbolometer arrays has been increasing and the costs have been coming down. The size of individual microbolometers, or pixels, has decreased from 50um to 25um to 17um and the array sizes have increased from 320 x 240 to 1024 x 768 and beyond. These advances have resulted in thermal image sensors that have higher resolution and higher frame rates. And lower cost…to the point where they are becoming viable for consumer applications like home security systems and wildlife observation.
But why might we be seeing an increase of requests for simulating microbolometers? Here again, I can look at our experience with other types of MEMS. In the early days, engineers relied on analytic formulae and hand calculations to come up with designs. Fabrication was relatively inexpensive, or so it seemed, and the engineers spent most of their time figuring out how to build and test their devices. Also, there was very little competitive pressure on time-to-market or cost. Now, with the prospect of volume markets for thermal imagers, the spread of microfabrication know-how, and worldwide competition, there is a lot more competitive pressure. The low hanging fruit has already been plucked in terms of design and fabrication techniques. Further improvements involve subtleties that cannot be captured with simplified equations, or quickly sorted out through a series of build-and-test cycles. That’s where simulation software like CoventorWare and SEMulator3D can really add value. To be fair, the simulation software and computing power have come a long way during this period too, to the point where it’s now possible to do very accurate simulations of microbolometers on garden variety desktop systems. An inflection point in the thermal imaging market resulting in more competitive pressures could well explain the recent upturn in interest in our microbolometer simulation capabilities.
microbolometer

Electro-thermal-mechanical simulation of a microbolometer showing mechanical deformation due to fabrication-induced pre-stress in thin films and temperature change, from a new tutorial in the upcoming CoventorWare 2014 release. Microbolometer design is courtesy of the National Nanofabrication Center (NNFC), Korea.

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