Uncertainty Improvements of the NIST Electronic Kilogram

Tungsten thin films can form in one of two crystal structures: alpha (bcc), with a superconducting transition temperature (T/sub c/) of 15 mK, and beta (A15), with a T/sub c/ between 1 and 4 K. Films with intermediate T/sub c/s are composed of both alpha and beta phases. We have investigated how to tune the film T/sub c/ in order to obtain certain values (T/sub c/ /spl sim/ 100 mK) suitable for the fabrication of photon number resolving transition-edge sensor (TES) and arrays of TES detectors for astronomical and quantum information applications. Variation of deposition conditions, and also the choice of the underlayer/coating for equal deposition conditions, affect the T/sub c/s of tungsten films. We have used x-ray diffraction to determine the structure of tungsten thin films and film stress. The results indicates that the film stress state depends on the underlying substrate and coating. To understand the variation of T/sub c/ values and to allow precise tuning of these values, we have investigated substrates and coatings for tungsten film multilayer stacks and determined tungsten film stress by x-ray diffraction at both room temperature and 8 K.