Starting in the mid-1980s scientists at Honeywell Research Center under a classified
development contract began work on a new class of infrared sensors. Until this time infrared sensors were photon-sensing devices that converted
infrared energy or photons into electrons. Photon detectors typically operate at very cold temperatures and rely on costly cryogenic
refrigeration units to achieve –200 C operating temperatures. While new photon type infrared sensors were being developed that operated at
elevated temperatures that solid-state thermal electric coolers could achieve, those devices only achieved spectral response in the
short and mid wavelength regions. The US military determined that for most night vision applications the preferred wavelength region was
7-14 microns
Scanning Electron Microscope Image of microbolometer array elements
Sensing temperature using a Bolometer is a well-understood technique and is the basis of a radical new design that relies
on a new silicon processing called MEMS or micro-electro-mechanical-systems. Starting with a silicon wafer, CMOS multiplexer
circuitry is fabricated by a commercial silicon foundry. The infrared detector foundry then deposits successive layers of
silicon nitride and silicon dioxide to fabricate a bridge structure only 50 microns across. A layer of amorphous silicon or
vanadium oxide is used to form a thin film resistor over the surface of the bridge deck. Integral thin legs are released from
the successive layers of SiO2, which provides structural support of the detector and electrical contact between the CMOS
multiplexer and a thin film resistor deposited on the bridge deck. Once all the layers necessary to build the complete pixel
structure have been deposited a gas is used to dissolve away the sacrificial materials and a freestanding suspended bridge
is left standing. The wafer with individual detector die is tested and good parts are diced and packaged into a vacuum
package.
It is difficult to image just how little mass each of these detector elements really have.
As the detector absorbs photon energy, its surface heats up proportional to the incident radiation falling on it. A voltage or current is
passed across the detector and the signal sampled. The device is so sensitive that a temperature change of only 50mk on an object provides enough
signal change to create an image. The signals are processes by the integrated circuit readout and an image is created through signal processing.
The performance of a microbolometer is determined by 5 design considerations:
Readout noise. CMOS devices exhibit some level of noise and maybe a significant contributing factor
to device performance.
The thermal coefficient of resistance (TCR), which is a measure of the change in resistance as a
function of temperature change.
Thermal isolation of the active device from the substrate.
The vacuum level in the sealed detector package. This eliminates the convective transfer of energy between pixels.
Detector time constant. Even though the detector must be thermally isolated it must be able to transfer some energy to the
substrate so that the device doesn’t heat up due to constant radiation impinging on it. Most designs try to optimize the time constant to provide
high sensitivity and integration time of approximately 10 milliseconds.
Key Features :
Monolithic device – Detectors are built directly on a CMOS wafer.
