Si based image sensors have proliferated in recent decades as their reduction in cost and increase in performance reached extraordinary levels. This talk will highlight advancements made in Si-compatible materials for imaging (2D and 3D) at wavelengths extending into the longwave infrared (LWIR). Monolithic integration of low dark current Ge photodetectors on Si extends the sensitivity of integrated sensors to about 1.6 μm. Recently, researchers have grown high quality epitaxial GeSn alloys on Ge where the cutoff wavelength approaches 5.2 μm. Using these new materials, it will be possible to create high performance midwave infrared (MWIR) and LWIR image sensors monolithically integrated on Si.
Visible-band cameras using silicon imagers
provide excellent video under daylight
conditions, but become blind at night. The
night sky provides illumination from 1-2 μm
which cannot be detected with a silicon sensor.
Adding short-wave infrared detectors to a
CMOS imager would enable a camera which
can be used day or night.
A germanium-enhanced CMOS imager
(TriWave®) has been developed with
broadband sensitivity from 0.4 μm to 1.6 μm.
A 744 x 576 format imager with 10 μm pixel
pitch provides a large field of view without
incurring a size and weight penalty in the
optics. The small pixel size is achieved by
integrating a germanium photodetector into a
mainstream CMOS process. A sensitive
analog signal chain provides a noise floor of 5
electrons. The imagers are hermetically
packaged with a thermo-electric cooler in a
windowed metal package 5 cm3 in volume. A
compact (<650 cm3) camera core has been
designed around the imager. Camera
functions implemented include correlated
double sampling, dark frame subtraction and
non-uniformity corrections.
In field tests, videos recorded with different
filters in daylight show useful fog and haze
penetration over long distances. Under clear
moonless conditions, short-wave infrared
(SWIR) images recorded with TriWave make
visible individuals that cannot be seen in
videos recorded simultaneously using an
EMCCD. Band-filtered videos confirm that
the night-sky illumination is dominated by
wavelengths above 1200 nm.
NoblePeak Vision has developed monolithic visible to short-wave infrared (SWIR) imaging
arrays. An innovative growth technique is used to integrate germanium islands with the silicon
transistors and metal layers of a CMOS process. Imaging arrays of 128x128 pixels at a 10 μm
pitch were designed and fabricated, with the silicon photodiodes of a conventional CMOS
imager replaced by germanium photodiodes. Broadband response from 400 nm to 1650 nm has
been measured. Imaging die have been packaged with a Peltier cooler and built into a camera
evaluation kit.
The rapid thermal chemical vapor deposition of heavily boron-doped Ge layers on silicon substrates is characterized and optimized for the purpose of ultrashallow junction applications. Incorporation of a very high concentration of boron in the Ge layer is observed with a moderate flow rate (2 - 20 sccm) of 1% B2H6 in hydrogen. The surface coverage of the B:Ge layer depends strongly on the B2H6 flow rate, favoring higher content of boron for better coverage. The substrate temperature during deposition also shows a strong effect on the film morphology with 550 degree(s)C yielding the most uniform surface.
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