The widespread adoption of machine vision makes image sensors}} that can detect light outside the visible spectrum increasingly useful, since the aim is to augment human sight rather than duplicate it. For example, imaging in the short-wave infra-red (SWIR) spectral region, which spans from 1000 to 2000 nm, offers multiple benefits that can supplement conventional cameras.
Motivation for SWIR imaging
Arguably the most compelling attribute of imaging in the SWIR region is the reduced optical scattering of longer wavelengths of light - a phenomenon that makes sunsets red since moisture droplets scatter the blue light during the light's longer path through the atmosphere. Reduced scattering means that SWIR cameras on vehicles and drones can see through fog and dust clouds, greatly improving visibility and hence safety.
Another benefit of SWIR imaging is distinguishing visually similar materials, which may have similar absorption (and thus reflection) spectra in the visible region but significant differences in the SWIR region. This ability is highly valuable for applications such as quality control in industrial processes, where it enables unwanted items such as rocks and metals to be spotted in food production for example, and sorting recycling.
Other benefits include thermal imaging for items with temperatures between 200 and 500 C, and the ability to see through materials that are opaque to visible light but transparent to SWIR. Silicon is a great example of this, with SWIR imaging used to check the quality of wafer attachment.
The problem with InGaAs
If SWIR imaging offers all the outlined advantages, why is it not more common? The answer is that at present short-wave infra-red (SWIR, 1000-2000 nm) imaging is dominated by expensive InGaAs (Indium Gallium Arsenide) sensors that can absorb light up to around 1800 nm. These can cost upwards of $10,000 due to the expense of producing the InGaAs layer via vapor deposition, low manufacturing yields, and limited pixel density that increases material consumption for a given resolution relative to standard silicon photodetectors.
Emerging SWIR image sensor technologies
The combination of commercially desirable attributes and an expensive incumbent technology creates a clear opportunity for a disruptive, low-cost alternative. As such, multiple competing approaches for SWIR imaging are being developed.
One approach is to extend the sensitivity of silicon photodetectors beyond the usual 1000 nm by increasing their thickness and structuring the surface. Since this 'extended silicon' approach can utilize existing CMOS fabrication technologies, it is likely to be a durable and relatively low-cost alternative that is highly applicable to autonomous vehicles and ADAS systems. However, since it is based on silicon this technology is best suited to detecting light towards the lower end of the SWIR spectral region.
Another method, which will likely be more expensive to produce than 'extended silicon' but capable of imaging at longer wavelengths, is using a hybrid structure that comprises quantum dots mounted onto a CMOS read-out integrated circuit (ROIC). Quantum dots have highly tunable absorption spectra that can be controlled by changing their diameter and can absorb light up to 2000 nm. This makes them especially promising for industrial imaging applications and potentially hyperspectral imaging.
The accompanying graphic compares the technological and commercial readiness level of the competing SWIR imaging technologies.
Technological and commercial readiness of emerging SWIR imaging technologies. Source: IDTechEx - "Emerging Image Sensor Technologies 2021-2031: Applications and Markets".
The prospects for the new SWIR imaging technologies outlined above are analyzed in detail in IDTechEx's report "Emerging Image Sensor Technologies 2021-2031: Applications and Markets". The report also covers multiple other emerging image sensor technologies, including OPD-on-CMOS, thin-film photodetectors, event-based vision, hyperspectral imaging, flexible x-ray sensors, and wavefront imaging.
Also included are highly granular 10-year market forecasts by revenue and volume (split by technology and application), along with multiple application case studies and technological/ commercial readiness assessments. The report also includes many company profiles based on interviews with early-stage and established companies, providing a clear view of the competitive landscape. Further details and downloadable sample pages can be found at www.IDTechEx.com/imagesensor.