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Plasmas allow a wide range of material modifications in order to optimize material properties for a specific purpose including etching processes in the semiconductor industry, generating functional layers via plasma enhanced chemical vapour deposition or sterilizing medical instruments and human skin, respectively. These examples illustrate the great number of plasma treatment processes which all use the different plasma species. Heavy particles (i.e. atoms, molecules, ions or radicals), electrons as well as photons produce a collective flux onto the material. The resulting effect depends both on the particular absolute fluxes and the energy of the impinging particle species. Photons cover a wide energy range from the infrared with energies below 1.8 eV to the vacuum ultraviolet region with a photon energy above 6.2 eV. Due to the high photon energy, VUV/UV photons might have a significant influence on the material. While the supply gas roughly sets the photons’ spectral distribution, the discharge’s operating parameters (e.g. pressure, input power) have an influence on the absolute fluxes as well as on the photon-to-ion flux ratios. Depending on the specific application, the photon-to-ion flux ratio might affect the process performance beneficially or adversely and can serve as tuning knob to optimize or to tailor a specific plasma setup. For this purpose, an energy resolved quantification of VUV/UV photon fluxes with respect to external operating parameters is essential. However, this claim implies great challenges regarding absolute in-situ wavelength resolved measurements down to the vacuum ultraviolet range for which large and expensive VUV spectrometers are commonly used. Due to their size, these instruments cannot be easily transported or applied at different setups. For the required absolute intensity calibration, radiation produced by electron storage rings serves as typical primary standard source below 116 nm which is associated with high efforts. To overcome these difficulties regarding VUV spectrometers, the recent years have shown a growing interest in transferrable VUV detectors. Wavelength resolution can be achieved by inserting spectral filters between the plasma and the detector. In this work, a portable diagnostic tool based on a VUV silicon diode and a set of bandpass and edge filters is developed. Its unique feature is the direct absolute calibration against an absolutely intensity calibrated VUV spectrometer. This calibration is performed both individually for each filter and specifically for a variety of supply gases. It directly includes the wavelength dependency of the filter transmission and the diode’s sensitivity as well as the viewing volume of the device. The absolute intensity calibration of the VUV spectrometer is performed in-house. It extends the standard method based on a deuterium arc lamp and branching ratios in nitrogen by applying a high current hollow cathode. Absolute scaling against an absolutely intensity calibrated optical spectrometer using a helium discharge leads to an absolute intensity calibration in the wavelength range between 46 nm and 300 nm. In order to compile appropriate filter sets for the VUV diode system, emission ranges of gases that are typically applied for process plasmas are investigated in pressure and power scans at the experimental setup PlanICE. These gases include the pure gases argon, hydrogen, nitrogen and oxygen as well as mixtures thereof in a pressure range between 0.3 Pa and 10 Pa. Emission ranges with relevant photon fluxes are identified with respect to the ion flux. The latter is determined using Langmuir probe measurements and an energy resolved mass spectrometer. Having selected appropriate filter sets, the diode system is absolutely calibrated in each gas and for each corresponding filter individually against the VUV spectrometer. Furthermore, a detailed characterization of the diode system is carried out regarding, inter alia, different performance aspects (e.g. reproducibility, linearity) and aging effects due to high energetic VUV radiation. It is complemented by extensive benchmark measurements at PlanICE using the VUV spectrometer and the optical spectrometer as reference. The diode system’s applicability as a portable, flexible and reliable VUV diagnostic tool is demonstrated at three different plasma experiments—the in-house laboratory setup ACCesS, the low pressure sterilization reactor PlasmaDecon at the Ruhr-Universität Bochum and the ion source of the Batman Upgrade test stand at the Max-Planck-Institut für Plasmaphysik (IPP, Garching). |
