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Imec releases prototype SWIR sensor with 1.82µm pixel pitch

Imec has announced a shortwave infrared image sensor with a pixel pitch of 1.82µm.

The sensor is based on a thin-film photodetector monolithically integrated on a custom Si-CMOS readout circuit.

The 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, according to Imec.

It follows on from Sony releasing two SWIR sensors earlier in the year with pixel pitches of 5µm, smaller than traditional InGaAs sensors that are restricted to around 10µm.

Imec's sensors are built in a fab-compatible process flow, which paves the way to high-throughput, wafer-level manufacturing. It will present its results at the 2020 IEEE International Electron Devices Meeting in session 16 on 15 December.

To date, SWIR image sensors are produced using 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 form factor are crucial.

Imec's alternative solution enables sub-2µm pixel pitch, by monolithically integrating a thin-film photodetector stack with a Si-CMOS readout circuit. The photodetector pixel stack implements a thin absorber layer such as 5.5nm PbS quantum dots – corresponding to peak absorption at 1,400nm wavelength.

The peak absorption wavelength can be tuned by adjusting the nanocrystal size and is extendable to wavelengths even above 2,000nm.

At the peak SWIR wavelength, an external quantum efficiency (EQE) of 18 per cent is obtained – Imec said this can be upgraded towards 50 per cent with further improvements.

The photodetector stack is monolithically integrated with a custom readout circuit, processed in 130nm CMOS technology. In this readout circuit, the three-transistor pixel design was optimised for the scaling of pixel size in the accessible 130nm technology node, resulting in record small pitch of 1.82µm for the prototype SWIR imager.

Pawel Malinowski, Imec’s thin-film imagers programme manager, said: 'With our compact, high resolution SWIR image sensor technology, we offer our customers a path to affordable low-volume manufacturing within Imec’s 200mm facility.'

Malinowski said the image sensors can be deployed in industrial machine vision, for photovoltaic panel monitoring, for example; smart agriculture in inspection and sorting; automotive; surveillance; and life sciences, such as lens-free imaging. They could also potentially be integrated in smartphone cameras or AR/VR glasses, with eye-safe SWIR light sources.

He added that some future developments Imec is working on include increasing the EQE, which currently is already at 50 per cent in SWIR on test samples; reducing the sensor noise; and introducing multispectral arrays with customised patterning.

The prototype SWIR image sensor was developed in Imec’s Pixel Technology Explore research programme. In this activity, Imec collaborates with material companies, image sensor companies, equipment suppliers and technology integrators to develop accessible innovative and customised CMOS imaging technologies.


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