MEMS Infrared Emitters

Xiang Zheng Tu

Infrared thermal emitters can be approximated as black body radiation, which is the type of electromagnetic radiation. The radiation has a specific spectrum and intensity that depends only on the temperature of the emitters. An emitter at room temperature appears black, as most of the energy it radiates is infra-red and cannot be perceived by the human eye. When it becomes a little hotter, it appears dull red. As its temperature increases further it eventually becomes blue-white.

POSIFA Microsystems Company has developed a new generation of MEMS infrared emitters that form its hot-heater based thermal flow sensors and thermal conductivity sensors. Proprietary advanced porous silicon technology combined with silicon processing result in the highest performance MEMS infrared emitters.

The MEMS infrared emitter consists of a resistive thin film platinum based heater which is positioned on a free standing thin-film stack membrane, and allows the heater to operate continuously and reliably at a higher temperature. The freestanding thin-film stack membrane thermally and electrically isolates the heater from the silicon substrate, and reduces the power consumption of the heater. In addition, the membrane has low thermal mass so that the heater is easy to modulate. The rise time of the heater is as low as 5 ms, indicating the frequency of driving pulse voltage can be up to 100 Hz.

As shown in the above figure, when the MEMS infrared emitter is operated at 300 k, the produced infrared spectrum range can be from 2 to 20 µm. This is mid-infrared spectral region containing strong characteristic vibrational transitions of many important molecules as well as two atmospheric transmission windows of 3-5 μm and 8-13 μm, which makes it crucial for applications in spectroscopy, materials processing, chemical and biomolecular sensing, security and industry.

The MEMS infrared emitters have many industrial applications including:

• Medical (CO₂ / other gases monitoring, breath /vapor analyzing),
• Military / Law Enforcement,
• Automotive / Transportation (breath alcohol testing / exhaust monitoring)
• Aerospace (calibration systems, image sensing),
• HVAC (demand controlled ventilation, refrigerant monitoring), and
• Safety / Industrial / Environment Control (combustion gas analyzers, gas detection, air pollution). 

In the above figure the key components of an infrared greenhouse gases measurement system are infrared emitter, measurement chamber, interference filter, and infrared detector. Infrared radiation is directed from the infrared emitter through the measured greenhouse gases to the infrared detector. An interference filter located in front of the detector prevents wavelengths other than that specific to the measured gases from passing through to the detector. POSIFA Microsystems Company can provide not only infrared emitters, but also interference filters and infrared detectors. The interference filter is made of multi porous silicon layers and the infrared detectors are thermopile type.