SWIR IMAGING
on SWIRSaving
Matthew Dale finds that new sensor technologies based on quantum dots are set to bring down the cost of shortwave infrared imaging
I
n October, Belgian research institute Imec unveiled a prototype thin-film monolithic image sensor capable of
capturing light in both the near infrared (NIR) and shortwave infrared (SWIR) wavelengths. Te sensor consists of a thin-film
photodetector pixel stack based on low- bandgap quantum dot materials, deposited directly on top of an electronic readout. Quantum dot sensor technology – Imec’s
sensor is the latest release in this area, but US-based SWIR Vision Systems is now selling its Acuros CQD VIS-SWIR cameras based on colloidal quantum dots – has the potential to lower the price of SWIR imaging substantially. Pawel Malinowski, Imec’s thin-film
imagers programme manager, explained: ‘Te advantage of using quantum dots in sensor fabrication is that unlike other
processes, you don’t need to grow any material on wafers. Quantum dots can instead be coated onto wafers and then processed through a semiconductor fab in mass-production processes, so this is the big differentiator.’ Currently, the SWIR imaging market is
dominated by sensors made with indium gallium arsenide (InGaAs), a semiconductor material grown epitaxially on indium phosphide substrates. In order to form electrical connections between each photodiode and the corresponding silicon readout integrated circuit, the two devices are joined, one detector at a time, by metal- to-metal bonding. Tis process, known as hybridisation, is time-consuming, expensive and also imposes limitations on the sensors’ pixel size, pixel spacing and resolution. Because of this, InGaAs cameras are mostly VGA
10 IMAGING AND MACHINE VISION EUROPE DECEMBER 2019/JANUARY 2020
The sensor of SWIR Vision Systems’ Acuros CQD 1920 camera
resolution, with only a few exceptions. In addition, because of the cost of their fabrication, these cameras are often considered to be too expensive for most machine vision applications. Te new SWIR imaging solutions based on quantum dots, on the other hand, can be fabricated monolithically on silicon readout wafers, and can therefore take advantage of the scale and cost structure of the silicon integrated circuit industry. Quantum dot SWIR sensors are also able to reach higher resolutions, up to the multi-megapixel level. Tey therefore have the potential to move SWIR imaging from being a specialised niche into broad commercial markets.
Nanocrystal clear imaging Quantum dots are nanocrystals that, depending on their size, offer different light absorption properties. For example, particles approximately 3nm in size absorb at 940nm, while particles around 5.5nm in size absorb at 1,450nm. Te pixel stacks of the new sensor can be tuned to target a spectrum from visible light all the way up to 2µm wavelength. ‘Right now there isn’t much of a SWIR
imaging market, because there is such a high [price] threshold for acquiring a SWIR camera,’ said Malinowski. ‘In a lot of machine vision applications people are
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