UV lamps are used to cure coatings and adhesives in many industrial
manufacturing processes. And special sensors are used to measure the
intensity of the UV light applied to these surfaces. But because these
sensors age too quickly, they can only be used to record intermittent
measurements. Fraunhofer researchers have developed a new generation of
sensors capable of continuously monitoring UV intensity. These devices
will be presented for the fi rst time at the Sensor + Test trade show in
Nuremberg, from May 14 to 16 (Hall 12, Booth 537).
“UV exposure” is a term that tends to ring alarm bells, as most people
associate it with unpleasant consequences such as sunburn and the risk
of skin cancer. But ultraviolet (UV) light can also be benefi cial, or
indeed essential: the human body needs it to produce vitamin D.
Industry, too, makes use of UV light, for example to cure adhesives or
the coatings applied to food packaging, and also to disinfect water. On
the other hand, surfaces can be damaged if they are exposed to too much
UV light, and poorly regulated UV lamps also waste energy and generate
excessive amounts of ozone. UV sensors are therefore used to optimize
light intensity.
Usually these sensors are made of silicon or silicon carbide. The
problem with silicon sensors is that they only deliver useful results if
visible light is excluded from the measurement by external fi lters.
Unfortunately, the fi lters used are very expensive and not particularly
resistant to ultraviolet light. So to reduce ageing, measurements can
only be taken intermittently, as snapshots. Silicon carbide sensors have
the advantage of being able to withstand longer exposure to UV light,
but they only operate in a narrow spectral band. In the majority of
industrial curing processes, it is the longer wavelengths that are of
interest – precisely the area in which these sensors are least accurate.
Researchers at the Fraunhofer Institute for Applied Solid State Physics
IAF in Freiburg have now developed a new UV sensor in collaboration with
colleagues at the Fraunhofer Institutes for Manufacturing Technology
and Advanced Materials IFAM, for Optronics, System Technologies and
Image Exploitation IOSB, for Silicon Technology ISIT and for Physical
Measurement Techniques IPM.
“Our sensor is based on aluminum gallium nitride technology and can withstand continuous exposure to UV light without damage,” says IAF project manager
Dr. Susanne Kopta.
“This enables it to be used not only for intermittent snapshots but also for permanent inline monitoring.”
A sapphire wafer serves as the substrate for the sensors. The
researchers apply epitaxial growth to deposit layers of the active
material onto the substrate, in other words the layers have a
crystalline structure.
Sensor for high UV intensities
The particular strength of this novel sensor is its suitability for
applications involving very high UV intensities – and for tasks that
require the monitoring of specifi c spectral ranges. This is due to the
fact that the detectors can be set to operate in two different ways. The
fi rst option is to defi ne a maximum wavelength threshold. In this
case the sensor detects all UV light emitted at wavelengths below the
set limit. The alternative is to defi ne two wavelength thresholds, thus
“cutting out” certain parts of the spectrum.
“The narrowest range we have been able to achieve is a separation of 20 nanometers,” reports
Kopta.
This makes it possible to manufacture one sensor for UV-A, another for
UV-B, and a third for UV-C. But how do the researchers set the
wavelengths to be detected by the sensor?
Kopta replies:
“We do this by varying the ratio of gallium to aluminum in one of the aluminum gallium nitride layers.”
Defining this ratio is one of the challenges that the researchers are
working on at present. Another challenge is growing the aluminum gallium
nitride crystal – the heart of the sensor – in such a way that it is
free of structural defects and impurities. Failure to do so would result
in unreliable measurements because different areas of the sensor would
absorb light at different wavelengths.
“The hardest part is dealing
with the wide range of parameters that affect the manufacture of thin
crystal fi lms, which demands a great deal of experience,” explains
Kopta.
A few demonstration models have already been produced. In the next stage
of the project, the researchers aim to optimize crystal growth and
obtain more sharply defi ned wavelength limits. They are also
investigating the component durability, with very encouraging results so
far.
“Initial tests have confirmed that the sensors are capable of
operating for 1000 hours under high UV exposure without suffering any
damage,” reports
Kopta.
UV sensors as team players
The UV sensors are not only excellent “solo artists”; they are also
great team players.By placing more than 100 detectors side by side in a
strip, you obtain a UV camera. This device can be used to monitor plasma
deposition processes, such as those employed to coat solar cells with
an antirefl ective fi lm. The sensor strip can also serve as a
spectrometer. In this case, the UV light is fi rst passed through a
diffraction grating which splits the light into its various spectral
components, like the colors of a rainbow. Each individual sensor detects
a specifi c wavelength and provides information on the intensity of
light at that wavelength. This would be a good way of conducting ageing
tests on the mercury lamps commonly used for water disinfection or UV
curing. Does the lamp still emit light of the desired intensity
throughout the entire spectrum, or are certain wavelengths weaker than
they ought to be?
The researchers will be presenting the novel UV sensors for different
wavelengths at the Sensor + Test trade show in Nuremberg from May 14 to
16 (Hall 12, Booth 537)
