Low-pressure plasmas are familiar to anyone who has ever held a fluorescent light tube in their hand. The lamps contain a gas under very low pressure. By applying a voltage, the electrons are greatly accelerated. When they collide with gas particles, charged ions or particularly reactive neutral particles, so-called radicals, can be generated. “These can be used to remove material from surfaces and thus create miniaturised circuits on microchips,” explains Dr Roland Friedl from the working group for experimental plasma physics at the University of Augsburg.

VUV can corrode surfaces

Low-pressure plasmas also produce particularly high-energy ultraviolet radiation, known as VUV (vacuum UV). Sometimes this is desirable. VUV radiation is used to disinfect the surfaces of medical implants, for example. But it can also trigger unwanted effects like corroding exposed surfaces, causing harmful chemical reactions.

“The aim is therefore to keep the damage caused by VUV radiation as low as possible and to enhance its desirable effects,” emphasises Friedl, who completed his doctorate in Prof. Dr Ursel Fantz’s working group in experimental plasma physics. “This can be achieved by selective modification of the pressure, temperature, or composition of the plasma.” In this way, it is often possible to optimise the intensity and wavelength distribution of the VUV radiation. But to do this, you first have to measure both. “This is not as trivial as it sounds,” explains Friedl. “You often have to use large and expensive spectrometers, which require complex calibration.”

Device simplifies the optimisation of low-pressure plasmas

The researchers have now developed a portable and inexpensive device that does not have these disadvantages. It consists of a sensor that is sensitive to UV light, a so-called photodiode. During the measurement process, different filters are placed in front of this diode, each of which only allows certain parts of the UV spectrum to pass through. “This allows us to determine which wavelengths occur in the VUV radiation,” says Dr Caecilia Fröhler-Bachus, who significantly advanced the project through her doctorate under Friedl’s supervision. But the team also wanted to know how intense the VUV radiation was in the different wavelength ranges. “To do this, we calibrated our detector on a commercially available VUV spectrometer, which had already been meticulously calibrated,” explains Fröhler-Bachus. As a result, the new device can precisely quantify VUV radiation over a broad spectrum, which was not possible with small and inexpensive detectors until now.

The new device can be used, for example, to optimise the various parameters in the generation of plasmas so that the resulting VUV radiation causes as little damage as possible without losing the plasma's other process-enhancing properties. The project was funded by the German Research Foundation (DFG). “During the project, we comprehensively analysed the possibilities and limits of this method,” says Friedl. The detector works so well that it is already being used by various research laboratories in Germany, which has made the Augsburg working group noticeably proud.

Publication:

R. Friedl et al 2023 Meas. Sci. Technol. 34 055501; DOI 10.1088/1361-6501/acab23

Funded:
Supported by the German Research Foundation (DFG) - FR 3881/1-1