Aerospace & Defense Technology - June 2024 - 14

(Image: Icebird72)
Image Sensors and Cameras
Based on Colloidal Quantum
Dots for Defense Applications
C
olloidal quantum dot (CQD) -based image sensors
accomplish several advantageous and unique features
such as tailorable wavelength response via selection of
dot size and material system, monolithic integration by
processing directly onto the CMOS wafer surface thus avoiding
hybridization cost and complexity, and scalability in pixel
dimensions and density, whereby the limitations are primarily
set by the resolution of the underlying CMOS process. Major
efforts and successes have been reported in recent years by several
academic groups and industrial entities in the research and
development of CQD image sensors and their monolithic integration
on CMOS platforms. To achieve uniform and defect-free
imagers performing at high speed with low read-out noise, the
entire system from ROIC front-end to wafer surface planarization
to pixel stack structure, must be optimized as a whole; this
is the approach Emberion has pursued with our in-house ROIC
design and end-to-end fabrication process development.
This article reports on the image sensor technology and the
VS20 camera product developed at Emberion. The article does
this by describing the image sensor design comprising the sensor
stack based on a p-i-n photodiode implementation with
PbS QDs sandwiched between thin-film hole transport (HTL)
and electron transport (ETL) layers as well as the CMOS ROIC
architecture and pixel front-end interfacing the photodiode
stack. Further on, the functionalities of the camera are
described at hardware (HW) and software (SW) level including
a brief introduction of the image processing pipeline (IPP)
implementation. Next, we outline the image sensor fabrication
process involving wafer planarization steps and photosensor
stack processing including CQD deposition and ligand
exchange steps.
Image Sensor and Camera Design
The VS20 camera has been entirely custom-designed
in-house at Emberion including the image sensor IC, sensor
material stack design, sensor package, camera circuitry and
mechanics, as well as the camera FW/SW including the
image processing pipeline (IPP) and user-interface (UI). This
way, it has been possible to optimize the interface between
the CQD-based pixel stack and read-out circuitry and maintain
full design versatility from imager to camera design.
Quantum dots provide a means to engineer device structures
The Emberion VS20 camera. (Image: Emberion)
14
exhibiting unique optical and electronic properties beneficial
to numerous practical applications. With material dimensions
reduced to around 1 to 20 nm in diameter, the effective bandgap
of the semiconductor becomes size dependent resulting from
quantum confinement. Here, we utilize PbS quantum dots as
the optical absorbing material. At a nominal diameter of 8 nm,
the excitonic absorption peak resides at ca. 1,800 nm and the
resulting device stack exhibits photoresponsivity from visible to
SWIR with a cut-off wavelength at ca. 2,000 nm. Similarly to
many literature reports, we employ bulky surface ligands to sterically
stabilize the QDs as a colloidal dispersion during
wet-deposition by spin-coating. A sequence of chemical treatments
is subsequently carried out to interchange the long-chain
insulating molecules for shorter and conductive ligands facilitating
the QDs to self-organize into a densely packed matrix
with favorable coupling and charge transport characteristics.
mobilityengineeringtech.com
Aerospace & Defense Technology, June 2024
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Aerospace & Defense Technology - June 2024

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