LEUVEN (Belgium), 10 December 2020 — Imec, a world-leading research and innovation hub in nanoelectronics and digital technologies, presents a
prototype high-resolution short-wave-infrared (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. Imec
will present these results at IEDM Conference 2020 in session 16.5.
Sensing in the short-wavelength infrared (SWIR) range (with wavelengths
from around 1400 nm to above 2000 nm) offers advantages over the visible
(VIS) and near-infrared (NIR) range 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 that enables record small sub-2
µm 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 5.5 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 readout
circuit, processed in 130 nm CMOS technology. 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.”
The prototype SWIR image sensor was developed in imec’s Pixel Technology
Explore research program. In this activity, imec collaborates with
material companies, image sensor companies, equipment suppliers and
technology integrators to develop accessible innovative and customized
CMOS imaging technologies.
SWIR images for 3 different pixel pitches. Highest resolution images
could be captured with the smallest (1.82 µm) pixel pitch.
The thin-film photodetector was monolithically integrated on a custom
Si-CMOS readout circuit.
