TECH FOCUS: IMAGE SENSORS


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IEDM paper is inspecting structures through silicon using lens-free imaging, a type of microscopy where the image is reconstructed computationally. Here, there is an advantage of having a large field of view and sub- wavelength pixels in the shortwave infrared. ‘Te new sensor is an enabler, because right now lens-free imaging for SWIR is quite difficult,’ Malinowski explained. A bit further off, the SWIR


sensors could potentially be used onboard cars – although automotive has strict requirements in terms of reliability – and also in consumer devices. Te cost would need to drop even further, but one potential application is in VR glasses for eye tracking at 1,500nm. To reduce the price point


further, the sensor would have to be manufactured on 300mm wafers, which would allow pixels to be scaled down even more and additional functionality added. Imec is also considering


making a multispectral sensor, to incorporate different absorbers for different pixels or pixel regions. Malinowski said that the


priority at the moment, however, is using just one type of absorber for a straight SWIR sensor.


Optimising on 200mm wafers Peak absorption in the 1,450nm sensor detailed in the IEDM paper has a full width at half maximum of around 100nm. Te detector has a sensitivity profile of 60 to 70 per cent sensitivity in the visible spectrum, which drops to 10 per cent between the visible and the peak in SWIR, with the


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peak optimised to a maximum of around 50 per cent EQE. ‘It’s a pretty broadband


detector,’ Malinowski said. ‘If you want to use it with a light source of a certain wavelength this is where you want to optimise the peak. If you want to capture as much light as possible, all the wavelengths between the visible and the peak are usable.’ At the moment Imec is


working at room temperature, but there are tests to see the effects of operating at higher or lower temperatures.


‘Tere’s quite a lot of movement disrupting the SWIR market’


‘We keep on optimising


the stack and working with material suppliers to explore new materials,’ Malinowski said. ‘Going lead-free is a big topic.’ At the moment, Imec’s baseline technology is lead sulphide quantum dots, but some companies have a strict lead-free policy. Indium arsenide is another potential material, he said. ‘Tis is not a technology that


will substitute or take away market from traditional SWIR imagers,’ Malinowski added. ‘Tis is filling the gap between the silicon image sensors and the InGaAs or III-V for applications that can’t afford SWIR, or that need higher resolution than that available in flip-chip sensors. ‘I see this technology as a new thing, rather than a competitor. InGaAs will always be preferable for high-end SWIR imaging, where you need the highest efficiency.’


Gpixel has announced the first sensor in a new family of CMOS line scan imagers supporting true charge-domain time delay integration (TDI). GLT5009BSI is a backside illuminated, TDI image sensor with 5µm pixels and 9,072 pixel horizontal resolution. The sensor has two photosensitive bands, 256 stages and 32 stages respectively, enabling a high dynamic range imaging mode. The sensor’s 5µm pixel


provides a full well capacity of 16ke-


and noise of 8e-


which delivers 66dB dynamic range. The GLT sensor family employs scientific CMOS BSI technology to achieve excellent sensitivity from the ultraviolet to the near infrared.


Read out of the image data is achieved through 86 pairs of sub-LVDS channels at a combined maximum data rate of 72.58Gb/s. This output architecture supports line rates up to 600kHz using 10-bit single band mode, and 300kHz using 12-bit single band mode. The length of the


photosensitive area is 45.36mm and the sensor is assembled in a 269- pin µPGA package. The sensor integrates several features to ease camera integration, including an internal sequencer, channel


Progress is being made in this area from other companies too, notably SWIR Vision Systems in the US, with its Acuros quantum dot SWIR sensors and cameras, and Finnish firm Emberion, which is working on integrating graphene and other nanomaterials onto CMOS integrated circuits for broadband detectors. In addition, Spanish research institute ICFO last year spun out Qurv Technologies to


multiplexing, and selectable scan direction. The GLT family’s


combination of high speed and sensitivity from the UV to the NIR, is ideal for applications such as flat panel display inspection, printed circuit board inspection, wafer inspection, digital pathology and fluorescence imaging. Other recent image sensors from Gpixel include: GMax2518, a global shutter, 18 megapixel sensor with correlated double sampling and an optical format of 1 inch; GSprint4521, a 21 megapixel APS-sized sensor designed with 4.5μm charge domain global shutter pixels; and the 103 megapixel GMax32103 medium-sized sensor with a 3.2μm global shutter pixel. Elsewhere, foundry Tower


Semiconductor and Opix, a supplier of time-of-flight sensors, have developed a ToF technology platform for 3D imaging and face recognition. The technology is implemented in a high- end image sensor product that is being integrated into a 3D camera module in partnership with a provider of imaging systems for mobile applications. The product will serve a variety of markets such as mobile, AR/VR, retail, robotics, automation and industrial inspection.


develop quantum dot image sensors operating from the visible to the shortwave infrared. ‘Tere’s quite a lot of


movement disrupting the SWIR market, which has for decades been made up of traditional players,’ Malinowski said. ‘We are at an interesting point in time, and we hear from integrators and application companies that they want to have SWIR. I’m optimistic for the future.’ O


FEBRUARY/MARCH 2021 IMAGING AND MACHINE VISION EUROPE 29


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