news digest ♦ Novel Devices
Now, Fraunhofer researchers have developed a new generation of sensors capable of continuously monitoring UV intensity.
“UV exposure” is a term that tends to ring alarm bells, as most people associate it with sunburn and the risk of skin cancer. But ultraviolet (UV) light can also be beneficial, 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 optimise light intensity.
Usually these sensors are made of silicon or SiC. The problem with silicon sensors is that they only deliver useful results if visible light is excluded from the measurement by external filters. Unfortunately, the filters used are very expensive and not particularly resistant to ultraviolet light. So to reduce ageing, measurements can only be taken intermittently, as snapshots.
SiC 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 aluminium gallium nitride technology and can withstand continuous exposure to UV light without damage,” says IAF project manager 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.
Processed AlGaN-based UV sensors on a sapphire wafer (© Fraunhofer IAF)
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 specific spectral ranges. This is due to the fact that the detectors can be set to operate in two different ways.
The first option is to define a maximum wavelength threshold. In this case the sensor detects all UV light emitted at wavelengths below the set limit. The alternative is to define 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 nanometres,” 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 aluminium in one of the AlGaN layers.”
Defining this ratio is one of the challenges that the researchers are working on at present. Another challenge is growing the AlGaN 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 films, 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 optimise crystal growth and obtain more sharply defined wavelength limits. They are
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www.compoundsemiconductor.net June 2013
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