Investigation of mechanical losses of thin silicon flexures at low temperatures

Studies of the mechanical loss of silicon flexures in a temperature region from 5 to 300 K are presented, where the flexures have been prepared by different fabrication techniques of interest for the construction of suspension elements of future interferometric gravitational wave detectors. A lowest mechanical loss of 3 × 10−8 was observed for a 130 μm thick flexure at around 10 K. While the mechanical loss follows the thermo-elastic predictions down to 50 K, at lower temperatures the observed loss is found to be a function of surface roughness. This surface loss is of interest for all applications using silicon-based oscillators at low temperatures. The extraction of a surface loss parameter using results from our measurements and those of other authors is presented and the relevance for future gravitational wave detector suspensions is discussed. A surface loss parameter αs = 0.5 pm was obtained. This reveals that the surface loss of silicon is significantly lower than the surface loss of fused silica. (paper)