TECH FOCUS: IMAGE SENSORS


SWIR cost cut: Imec achieves 1.82µm pixels


Greg Blackman speaks to Imec’s Paweł Malinowski about the institute’s new quantum dot SWIR sensor


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ate last year Imec presented a shortwave infrared quantum dot


image sensor with a pixel pitch of 1.82µm at the IEEE International Electron Devices Meeting (IEDM). Te work is the latest in


several advances in SWIR image sensor technology, which have been gathering pace where, previously, progress in III-V detectors had ground to a halt. A year earlier at the previous IEDM, Sony announced SWIR sensors with a 5µm pixel pitch, achieved by connecting the InGaAs photodiode to the readout circuit via copper-to- copper bonding. Tis meant that, finally, the limitation of double-digit pixel size found in traditional hybrid bonded InGaAs sensors was broken, offering a path to higher resolution SWIR sensors. Te Imec sensor is based on thin-film quantum dots, which are integrated monolithically on top of the readout circuit. Tis means it doesn’t have the limitations of flip-chip bonding or interconnects, so the pixel can be very small – 1.82µm is the smallest pixel Imec has made with this particular technology node of 130nm in a three-transistor (3T) structure. ‘We want to have small pixels,


which is good for image quality but also for squeezing more pixels in the same footprint,’ explained Paweł Malinowski,


program manager for pixel innovations at Imec. ‘Tis translates to lower cost per pixel.’ Malinowski said the


sensor could one day be manufactured in the €10 to €100 range, once larger volumes are possible – current InGaAs sensors can cost a few thousand euros. He said that at this early stage €100 or €200 is adequate for most customers. Quantum dots can be


engineered to have absorption at 2µm and above. Te absorption peak can be tuned to certain wavelengths by the way the thin-film layers are constructed. In the IEDM paper, Imec focuses on sensing at 1,450nm, a useful detection wavelength for some applications, especially if water content is a desired metric. A quirk of designing image


sensors with quantum dots, Malinowski noted, is that the quantum dot absorber layers, at 300 or 400nm thick, are significantly thinner than the wavelength of light they are used to detect, 1,450nm. ‘From a physics point of view, it’s quite amazing,’ he said. Imec has reached around


50 per cent external quantum efficiency (EQE) on test structures, while on ROIC samples EQE is currently 10 to 20 per cent. InGaAs reaches


28 IMAGING AND MACHINE VISION EUROPE FEBRUARY/MARCH 2021


Imec TFPD chip


up to 80 to 90 per cent EQE, so quantum dot sensors won’t compete where high quantum efficiency is needed, but for a lot of applications, 50 per cent EQE is acceptable, Malinowski said.


Complete package and new applications Te announcement is the result of several things coming together: the optimisation of the photodiode for the infrared range; designing a circuit that can read out the signals from such a thin-film photodiode – typically these materials have higher noise or higher dark current, so the photodiode has to cope with that; and all the necessary integration, the interconnects, and how the sensor is encapsulated and packaged. Imec is now working on


transferring the process to its 200mm fab and making the devices on full wafers. Once that is available, throughputs will be higher and the device can be optimised for uniformity and reliability. Malinowski said that the first


applications might be for areas in industrial vision requiring higher resolution in SWIR, for instance inspecting solar cells or silicon chips. Higher resolution means the field of view can be increased. Smart agriculture is another


area that he highlighted. He said that agricultural robots don’t use SWIR sensors at the moment because they can cost more than the robot itself, so a less expensive SWIR imager might open the door to more widespread use of SWIR in agriculture. Te example given in the


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