The SWIR sensor boom

MICHAEL WHEELER, EDITOR-IN-CHIEF MICHAEL.WHEELER@PHOTONICS.COM

Systems designers have gravitated to shortwave IR (SWIR) for a host of inspection tasks in recent years — namely, in cases when materials appear similar in visible light, such as discerning between water and other liquids, or identifying where hard-to-spot moisture accumulates, such as in bruising on produce.

In addition to their usefulness in inspection, SWIR imaging systems incorporate many of the same components that visible imaging systems do, such as conventional glass optics. SWIR sensors also follow the same drive modes and pixel structure as standard CMOS sensors.

But there is one drawback, and it’s not small: cost.

Indium gallium arsenide (InGaAs) sensors sell for hundreds of dollars. Efforts are underway to develop alternatives. According to IDTechEx’s Matthew Dyson — author of “Sensor Innovations Drive Expansion into New Markets” — one alternative involves extending the sensitivity of silicon photodetectors beyond the usual 1000 nm by increasing their thickness and structuring their surface. Conventional fabrication techniques can be used, though such advancements are likely best suited to applications in the lower end of the SWIR spectral region.

Another approach involves using a hybrid structure that comprises quantum dots (QDs) mounted onto a CMOS readout integrated circuit. QDs, as Dyson points out, have highly tunable absorption spectra that can be controlled by changing their diameter, enabling light up 2000 nm to be absorbed. QD-on-CMOS hybrid detectors capable of imaging up to 1700 nm are commercially available today. STMicroelectronics announced details of its QD SWIR global-shutter sensor with a 1.62-µm pixel pitch and a quantum efficiency of 60% at 1400 nm.

“STMicro’s entry into the SWIR market is significant, because, unlike existing players, it’s a larger company that will be able to supply significant volumes of sensors with high reliability, making them viable for integration into industrial, consumer, and automotive devices,” Dyson said in a follow-up correspondence.

Looking forward, both the prospect of new players emerging — including those capable of high-volume manufacturing — and continued emphasis on addressing technological challenges bode well for increased adoption in the years ahead.
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