Wednesday, December 09, 2020

Imec Presents Thin Film SWIR Sensor with 1.82um Pixel Pitch

Imec unveils a prototype high-resolution SWIR image sensor with record small pixel pitch of 1.82 µm. It is based on a thin-film photodetector that is monolithically integrated on a custom Si-CMOS readout circuit. A fab-compatible process flow paves the way to high-throughput, wafer-level manufacturing.  The presented technology largely exceeds the capabilities of today’s InGaAs-based SWIR imagers in terms of pixel pitch and resolution, with disruptive cost and form factor potential. New applications are enabled even in cost-sensitive domains, such as in industrial machine vision, smart agriculture, automotive, surveillance, life sciences and consumer electronics.

Sensing in SWIR band (with wavelengths from ~1.4 um to above 2 um) offers advantages over the visible  and NIR bands for some applications. SWIR image sensors can, for example, see through smoke or fog, or even through silicon – which is especially relevant for inspection and industrial machine vision applications. To date, SWIR image sensors are produced through a hybrid technology, in which a III-V-based photodetector (usually InGaAs-based) is flip-chip bonded to a silicon readout circuit. These sensors can be made extremely sensitive, but the technology is quite expensive for mass manufacturing and limited in size of pixel and number of pixels – hindering its adoption in markets for which cost, resolution and/or form factor are crucial.

Imec introduces an alternative solution. The photodetector pixel stack implements a thin absorber layer such as 5.5 nm PbS quantum dots – corresponding to peak absorption at 1400 nm wavelength. The peak absorption wavelength can be tuned by adjusting the nanocrystal size and is extendable to wavelengths even above 2000 nm. At the peak SWIR wavelength, an external quantum efficiency (EQE) of 18% is obtained (and can be upgraded towards 50% with further improvements). The photodetector stack is monolithically integrated with a custom ROIC in 130 nm process. In this readout circuit, the 3-transistor pixel design was optimized for the scaling of pixel size in the accessible 130 nm technology node, resulting in record small pitch of 1.82 µm for the prototype SWIR imager.

Pawel Malinowski, imec’s thin-film imagers program manager: “With our compact, high resolution SWIR image sensor technology, we offer our customers a path to affordable low-volume manufacturing within imec’s 200 mm facility. These image sensors can be deployed in industrial machine vision (e.g. photovoltaic panel monitoring), smart agriculture (e.g. inspection and sorting), automotive, surveillance, life sciences (e.g. lens-free imaging) and many more. Due to their small form factor, they can potentially be integrated in small cameras, such as in smartphones or AR/VR glasses – with eye-safe SWIR light sources. Some of exciting future developments include increasing of the EQE (which currently is already at 50% in SWIR on test samples), reducing the sensor noise and introducing multispectral arrays with customized patterning approach.


Imec kindly provided me with a deck of slides presented at a webinar devoted to the new sensor announcement:

9 comments:

  1. What is the status of this technology regarding ROHS regulation ?

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    1. Hi Pierre,
      if you look at the photoactive (absorber) layer which is only around 300 nm thick, the Pb content for the image sensor chip is below 0.1%. If you look at the directive 2011/65/EU, there were exemptions for several specific cases. Additionally, this year, Laser Components (DE) took the initiative for RoHS exemption (https://www.lasercomponents.com/de-en/news/laser-components-takes-the-initiative/). At the same time, some companies have a strict Pb-free policy, which makes the research into Pb-free alternatives very timely and needed. There has been some good progress on more environmentally friendly QDs, so this is definitely a very interesting (and hopeful!) R&D topic.
      Greetings / Pawel

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  2. can anyone clarify what does it mean by pixel pitch in context of quantum dots.
    As in paper I have read that the size of CQD is few nanometers and still in resolution and pixel pitch it is written 640x512 with 15um. I am a bit confused.

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    1. in order to detect the photons you still need photodiodes, right? these are in 15um pitch. the quantum dots just do some magic so that the >1100nm photons can be detected by the si photodiode (with a bandgap energy equivalent to 1100nm...). What the sensor is able to resolve is limited by the 15um pitch of the underlaying photodiodes.

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    2. So can these photodiode size be reduced to say 2-5um pitch and have the same responsivity curve to different wavelength in SWIR domain as that of 15um pixel pitch.

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    3. if you read the article above, you'll see that it talks about 1.82um pitch. The CQD principle has a lower QE than lets say InGaAs, the charts above show this as well. For many applications the properties of this principle will be good enough and the advantage of cmos'ish scaleablility will be favored over QE. In fact I think we are close to a real SWIR mass product based on CQD. ST for example published work in this area. I would not be surprised if some product is already around and we just have to wait for the teardown of some of the latest smartphones to have it discovered...

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    4. this is what I meant: https://www.yolegroup.com/event/webcasts/the-appeal-of-swir-imaging-technologies-for-mass-markets-webcast/ . I would not be surprised if this SWIR under display kind of thing based on CQD sensors are already in mass products or at latest appear in the 2023/24 generations...

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    5. There is one more recent announcement (again by Yole...): https://www.yolegroup.com/product/report/biometrics-for-consumer-2022-/
      They claim "SWIR under-display face recognition" will soon create a huge market for SWIR imagers.

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