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Ultraviolet CMOS Technology Opens a Spectrum of Possibility
Photonics Spectra
Apr 2025Supported by advancements in sensor components and materials, ultraviolet CMOS technology enables numerous applications — and streamlines others.FRANÇOIS YAYA OXFORD INSTRUMENTS, FIRST LIGHT IMAGING
The ultraviolet (UV) range
of the electromagnetic
spectrum has historically
been split into three bands.
These three bands — UVA
(315 to 400 nm); UVB (280
to 315 nm); and UVC (100 to 280 nm) —
corresponded to barium-silex, bariumsilex-
Pyrex, and Pyrex filters. All three
filter types are widely available.
A butterfly is imaged under ultraviolet
(left) and visible light. Ultraviolet imaging
provides insights that enable the approach’s
use in a range of applications —
from environmental to industrial. Courtesy of Oxford Instruments — First Light Imaging.
As some of the shortest and most energetic
wavelengths in the spectrum, UV
light at any wavelength exhibits distinct
characteristics, which are harnessed
across various domains (Figure 1). For example,
UVA is valuable for fluorescence
imaging, and UVB light can be used for
medical treatments, such as psoriasis
therapies. The highly energetic UVC band
is commonly used for sterilization.
Figure 1. A graphical representation of UV
CMOS applications by wavelength, with
specific examples, as grouped by the distinct
UV spectral bands. Courtesy of Oxford Instruments — First Light Imaging.
Accordingly, UV-emitting sources are
already used in a wide range of applications
in both industry and R&D. Now, the
prospect of using UV-sensitive sensors to
record data over time and store the data
for subsequent analysis is increasing in
interest.
However, most CMOS sensors for
visible light imaging are fabricated from
silicon. This material exhibits low transmission
efficiency in the UV range.
To optimize transmittance, all elements
within the camera’s optical path, including
the microlenses integrated into the
CMOS sensor and the protective window,
must be meticulously engineered and
treated for UV (Figure 2a,b). The shorter
UV wavelengths, and significantly lower
penetration depth compared with visible
light, mean that photons are less prone to
absorption by the photodiodes within the
sensor. Also, the quantum efficiency is
typically lower than for visible light.
Figure 2. Design architectures of back-side- and
front-side-illuminated UV CMOS image sensors
(a, b). A gallium nitride (GaN)-based CMOS
image sensor, isolating material layers (c). Courtesy of Oxford Instruments — First Light Imaging.
Manufacturers have worked to
overcome this constraint through the
development of image sensors capable
of accommodating UV cameras and UV
imaging more broadly. In the early days
of working to meet this challenge, CCD
sensors became more sought after due
to their higher sensitivity and reduced
noise. CCD sensors transfer the generated
charges from an array of photodiodes
after photons hit pixels, leading to lower
noise. However, CCD sensors come with
limitations and known artifacts, such as
blooming.
During the last few decades, CMOS
technology has undergone significant
improvements. Owing to advancements
in lithography, these gains are most
noticeable in the sensors’ ability to handle
noise. The resulting image quality has
approached levels comparable to that
which CCD sensors can deliver. In addition
to image quality, CMOS sensors, now
operating in or for the UV range, offer an
effective alternative to their CCD counterparts.
Among other benefits, they are
cost-effective, consume ~100× less power
than CCDs, and offer higher operational
speeds
1.
Technical and material innovations
Adapting CMOS technology for the UV
range requires developers to consider the
high energy of UV photons and the corresponding
effects of this characteristic
on sensor design. With UV photons, penetration
depth — or the distance at which
light intensity decreases as it moves
through the sensor — is low.
CMOS manufacturers have explored
various approaches to address the low
penetration depth. In 2011, Sony released
its first back-side-illuminated CMOS
sensor. Compared with the front-side-illuminated
sensors, which represented the
original approach to building sensors, the
photosensitive matrix in the Sony design
was positioned closer to the surface. This
enabled better light collection, including
for the shorter UV wavelengths (Figure
2a).
In a similar vein, fabricators have
turned to thinning silicon substrates to
enhance UV photon collection. Further,
certain designs apply antireflective coating
to maximize the transmission of photons;
indeed, UV light interacts strongly
with the sensors’ surface, which makes
it more prone to noise. Algorithms have
been implemented in several recent cases
to mitigate this. The effect of correlated
double sampling, for example, reduces
noise by subtracting the background noise
on the pixel level. The latest iteration of
CMOS sensors features an on-chip digital
signal processing mechanism that reduces
noise more rapidly, making these sensors
ideal for fast imaging applications.
UV-sensitive sensors are available on
the market, and research is ongoing to
improve their durability and quantum
efficiency.
As these innovations have taken
off, various materials have been under
investigation. Materials that have a large
bandgap due to the high energy of UV
photons and effectively transmit UV light
have gained preference for UV imaging
and sensing.
Photodetectors made of various materials
are currently under development as
well, though progress must be made
before multi-pixel and CMOS integration
is achieved. Among the most popular of
these options, nitride alloys — particularly
of gallium, aluminum, and indium-aluminum
— offer large bandgaps. At the
same time, oxides, such as zinc and/or
magnesium-zinc, also respond well to UV
light, but in some applications, customers
may need to select a specific range in the
UV. For instance, ternary zinc germanium
oxide responds well to UVC but not UVA
or UVB.
For high-energy imaging applications
(imaging alpha particles, neutrons,
gamma rays, and more) diamond and
silicon carbide (SiC)-based sensors
are strong candidates due to their wide
bandgap and resistance in harsh environments
2.
Operating in or under UV light
is challenging due to its high energy,
and UV CMOS sensors, which are usually
under prolonged UV radiation, can
undergo reduced performance over time,
or experience irreversible damage, in the
worst cases. Although the fabrication of
multi-pixel sensors combining CMOS
technology with large bandgap materials
is still complex and costly, sensors based
on SiC are emerging for such applications.
Applications: Industrial and R&D
UV CMOS technology currently enables
applications that are firmly established —
though, in many cases, still improving
— as well as those in basic and applied
research. Among the most notable applications,
UV CMOS sensors help to
inspect damaged high-tension electric
cables by detecting corona discharges
that ionize the air and emit UV photons.
UV light also enables the discernment of
different types of plastics. And most body
fluids strongly absorb in the UV, making
UV sensors a powerful resource in disciplines
such as forensics.
As much as technological advancements
have positioned UV CMOS to aid
numerous imaging applications, recent
improvements in manufacturing are likely
to lead to many more advancements.
There is an increasing demand in the
industrial sector for automation systems
to handle repetitive and tedious tasks for
high-throughput productions. Humanbased
quality checks can be subjective, as
well as time-intensive, whereas automated
systems support unbiased, reliable, and
contactless methods.
Under this umbrella of inspection
tasks, nondestructive testing refers to
techniques that check properties of a
material without contact. This method is
essential to industries such as automotive,
aerospace, and electronics.
UV CMOS sensors are excellent candidates
for nondestructive testing deployments
due to their ease of integration,
versatility, and facilitation of high-speed,
high-resolution imaging. By combining
rapid detection with machine vision
algorithms, these systems enable efficient
materials sorting, automatic inspection,
process control, and countless other possibilities.
In the semiconductor industry, electronic
components are miniaturized to
unprecedented sizes. Identifying fine
cracks, defects, and dust contamination
on wafers is crucial, because these factors
often determine whether a device will
function as intended. UV CMOS technology
excels in providing detailed and fast
imaging, though UV CMOS technology
cannot match the detail of electron-based
microscopies, due to the diffraction limit
of light’s shorter wavelengths, which
reveal minute details in the blink of an
eye. But, electron-based microscopies can
complement UV CMOS to provide deeper
insights into specific wafer areas.
UV CMOS in the field
The application benefits of UV CMOS
can apply to areas outside of industrial
settings. In agriculture, UV CMOS sensors
mounted on drones have the potential
to identify fruit flowers for artificial
pollination, which can provide a path to
improved yields. In surveillance applications,
cameras similar to the infrared
imagers already in use could be used to
detect hazardous or biological agents.
And with the growing interest in artificial
intelligence, large volumes of images
generated by high-speed recording CMOS
sensors can be used to train models that
could ultimately be used to assist humans,
resulting in rapid analysis and decisionmaking
in real time.
In astrophysics, the potential of UV
light has always garnered high interest.
But this has remained underexplored,
mainly for technological bottlenecks. It
is also worth noting that absorption and
diffusion of UV light by the ozone layer
makes ground-based observations complex,
and these necessitate the deployment
of instruments in outer space. UV CMOS
sensors, which are more cost-effective
than CCD options, consume less power
and can more effectively withstand harsh
environments, such as UV radiations.
With recent advancements in UV
CMOS technology, astrophysicists are
aiming to uncover critical information
about high-energy processes, such as stellar
formation, hot sources, and interstellar
media. This technology allows scientists
to trace stellar nurseries and gain a better
understanding of the early stages of
star formation. Additionally, UV CMOS
sensors can be mounted on a satellite and
seize the atmosphere composition of exoplanets.
Gases that have strong absorption
in the UV spectrum, such as helium,
hydrogen, and even ozone, can indicate
whether life-forms are present.
For example, UV CMOS technology
was integrated on a satellite called the
Ultraviolet Transient Astronomy Satellite
(ULTRASAT). This wide-field Schmidt
telescope in orbit aims to deliver a
comprehensive understanding of the
high-energy transient universe. It refers
to high-energy events that take place in a
brief timescale in the cosmos. ULTRASAT
excels in the near-ultraviolet range
from 230 to 290 nm. Its observations are
expected to contribute to multiple fields,
including the study of supernovae and
neutron stars, galaxies, and gravitational
waves.
Scheduled for launch in 2027, ULTRASAT
uses UV CMOS technology to
capture high-resolution images in real
time. Deutsches Elektronen-Synchrotron
(DESY) developed the camera, which
features a focal plane array with four
independent 45- × 45-sq-mm back-sideilluminated
CMOS sensors and a pixel
size of 9.5 × 9.5 μm. To address the low
penetration depth of UV, DESY designed
a sensor with a proper antireflective coating,
called T2. This coating has a high
quantum efficiency in the near-ultraviolet
range for the target application.
Gas spectroscopy
On Earth, gas detection, particularly of
noxious gases, is also a significant focus.
Scientists in the last few decades have
gained a heightened awareness regarding
global warming, often highlighting
ozone. This gas protects the planet from
harmful radiation, and it exhibits a strong
absorption in the deep-UV (<300 nm).
On a larger scale, numerous pernicious
gases in ambient quantities pollute
the atmosphere and have negative health
effects when they enter the respiratory
tract. Therefore, monitoring and quantifying
these gases is crucial to minimizing
risks. Nitrogen dioxide, i.e., from car
exhaust, thermal power plants, and steel
manufacturing; sulfur dioxide (SO
2), from
fossil fuels; and benzene, from residential
wood-heating, are gases that each absorb
in UV and are therefore detected and
quantified using UV CMOS sensors.
Looking ahead
Despite significant advancements in
UV CMOS technology, certain challenges
remain, limiting the full potential
of these sensors for some applications.
The main issue: UV CMOS is relatively
low in terms of sensitivity and quantum
efficiency compared with visible light
sensors. Although thinner material layers
enhance UV transmission, there is still
room for improvement. Overcoming
these obstacles and improving the sensor
fabrication processes will expand the
scope of UV CMOS sensors and enhance
their overall efficiency, durability, and
performance in harsh environments.
It is fortunate, in this sense, that CMOS
technology has been around for decades
and continues to benefit from a wellestablished
road map, even if only for the
visible and infrared ranges. UV CMOS
opens its own spectrum of possibilities
— but it does not exist in a vacuum. With
continuous improvements, UV CMOS
sensors are pushing limits and signaling a
new era for the CMOS industry.
Meet the author
François Yaya, Ph.D., is a product manager at
Oxford Instruments — First Light Imaging,
specializing in high-performance cameras for
scientific research and industrial applications.
He collaborates with scientists from a variety
of research areas for solutions development;
email:
[email protected].
References
1. M. Bigas et al. (2006). Review of CMOS
image sensors.
Microelectronics J, Vol. 37,
No. 5, pp. 433-451.
2. N.G. Wright et al. (2008). Prospects for SiC
electronics and sensors.
Mater Today,
Vol. 11, No. 1-2, pp. 16-21.
UV CMOS: Application in Action
In an application example, attempts were
made to detect sulfur dioxide (SO
2)
emission out of a refinery’s chimney. A
CB1 UV camera from Oxford Instruments
— First Light Imaging, with an
f/2.8 100-mm UV sensitive objective
(CERCO) was used for the test. This
objective is apochromatic and demonstrates
excellent transmission between
240 and 900 nm. A UV (310-nm ± 5-nm)
bandpass filter was used to select the
wavelength where the absorption of UV
by SO2 is optimal.
It is well known that the processing of
crude oil involves combustion, and the
sulfur contained inside the oil is rejected
by the chimney after oxidation as SO
2.
While emission is not detected with visible
light, results enabled the observation
(black) of the presence of SO
2 (Figure 3).
This was possible due to the 310-nm
(± 5-nm) bandpass filter used in the test.
This setup allows for imaging of emitted
gases from a certain distance, ~10 s of m.
Figure 3. A refinery chimney emitting sulfur
dioxide (SO2) imaged in ultraviolet (top)
and visible light (bottom). Courtesy of Oxford Instruments — First Light Imaging.