Continuous measurement of UV light made possible
There are many industrial manufacturing processes that require the use of UV lamps to cure coatings or adhesives. Its use is particularly prominent in the UK's food industry where it is commonly used to cure the adhesive around packaging containers and to disinfect water.
These processes often require the precise measurement of the UV light on a surface. Special sensors are normally used but these tend to quickly age under constant exposure to the UV light, and deteriorate in performance as a result. To combat this problem they are normally used intermittently, taking short snapshots to monitor UV levels.
Often these sensors are made of silicon, which can only deliver useful results if visible light is blocked out while measurements are taken. This is usually done using external filters that are both expensive and not particularly resistant to UV.
Silicon carbide sensors on the other hand have the advantage of being able to withstand longer exposure to UV light, but they usually only operate within a narrow spectral band. In the majority of industrial curing processes it is the longer wavelengths that are of interest, precisely the area that silicon carbide sensors are least accurate.
As a result, researchers at Germany's Fraunhofer Institute have been considering the problem and have been able to develop a new generation of sensors that they say will be capable of continuously monitoring UV light.
"Our sensor is based on aluminium gallium nitride technology and can withstand continuous exposure to UV light without damage," says 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 and researchers have been able to apply epitaxial growth to deposit layers of the active material onto the substrate. In other words, the layers have a crystalline structure.
The detectors can be set to operate in two different ways, the first is to define a maximum wavelength threshold where the sensor detects all UV light emitted at wavelengths below the set limit. The alternative is to define two wavelength thresholds, blocking out certain parts of the spectrum. The narrowest range researchers have been able to achieve is a separation of just 20nm. The researchers set the wavelengths to be detected by the sensor by varying the ratio of gallium to aluminium in one of the aluminium gallium nitride layers.
A key challenge has been the growing of aluminium gallium nitride crystals in such a way that they are free of structural defects and impurities, which would result in unreliable measurements as different areas of the sensor absorb different wavelengths.
The UV sensors can work individually or can be made into an array by placing more than 100 detectors side by side in a strip. This produces a 'UV camera' that can monitor plasma deposition processes such as those employed to coat solar cells with an anti-reflective film.
The sensor strip can also serve as a spectrometer where the UV light is first passed through a diffraction grating to split the light into its various spectral components. Each individual sensor detects a specific wavelength and provides information on the intensity of light at that wavelength. This allows for aging tests of mercury lamps commonly used for water disinfection or UV checking if lamps are still emitting light at the desired intensity throughout the entire spectrum.
The sensing technology is now entering a period of refinement before it is rolled out to wider industrial applications. The next stage of the project is to optimise crystal growth and obtain more defined wavelength limits.
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