Setup of a UHV System for the Production of Tungsten TES

Abstract

Humanity has concerned itself with the question of the basic elements of nature over thousands of years. The technological evolution of the last hundred years improved our understanding of the particle nature of matter. However, this is only true for particles, that reveal themselves to us by participating at electromagnetic interactions, or, like neutrinos, are closely related to an electromagnetically interacting partner. Large scale gravitation studies concluded, that these known forms of matter only account for about a sixth of the overall gravitationally interacting matte in the universe. Due to its invisibility to electromagnetic particles, the missing mass is referred to as dark matter. While gravitation still remains inaccessible to us on particle scales, technical evolution opened up the field of particle studies based on the weak interaction force, which is a promising field for dark matter studies, since reasonable suspicion exists, that dark matter is also weakly interacting.

Low interaction cross sections and small signals are making particle detection via weak interaction challenging and require sophisticated technology. The CRESST experiment (Cryogenic Rare Event Search with Superconducting Thermometers) and SuperCDMS (Super Cryogenic Dark Matter Search) are aiming to directly detect dark matter particles and investigate their properties by taking advantage of TES (transition edge sensors) to measure the small signals. These TES’s are manufactured from thin tungsten films, that are thermally coupled to absorber crystals. To also meet the detection requirements for low interaction rates, a large numbers of detectors, and thus a large numbers of TES are needed.

In order to achieve the required sensitivity, tuning the superconductive properties of the tungsten films is essential. The transition temperature defines the sensitivity range of the TES and the slope of the transition edge their sensitivity. In particular the low superconductive transition of about 15 mK in bulk material makes tungsten the favored material in cryogenic particle detection. However, it is challenging to grow thin tungsten films with such a low transition. The quantum effect of superconductivity is very sensitive to even small changes in the material properties, resulting from varying deposition conditions. Therefore, it is crucial to investigate and understand the influences of differing deposition conditions in order to grow TES of reproducible properties. In thin films especially crystal structure, grain size, impurities and film stress are known to influence superconductivity.

In the framework of this thesis, a production facility for thin tungsten films, that is capable of mass production has successfully been set up. It proved to be capable of producing films with transition temperatures of 15 mK, meeting the requirements of the CRESST experiment. However, due to a lack of reliable transition curve measurements, influences of the deposition conditions on the superconductive properties and their reproducibility could not be investigated. Instead several analysis techniques for investigations of the crystal phase, impurity inclusions, surface morphology, electrical resistance and film stress have been performed. These techniques provide knowledge about film properties, that are known to affect superconductivity. Especially the influence of the substrate temperature during deposition on these film properties were studied. The results of the analysis techniques are specific to the deposition system that has been set up for this work and might differ quantitatively for other deposition systems.