Tc tuning of tungsten transition edge sensors using iron implantation

We have developed a technique for precisely tuning the transition temperature of superconducting tungsten (W) films used in Quasiparticle trap-assisted Electrothermal-feedback Transition edge sensors (QETs). We have demonstrated our technique using several 350 Å-thick W films with “as-deposited” transition temperatures in the range of ∼90–150 mK. The accuracy of our tuning process is excellent, and Tc suppressions of prescribed amounts ranging from a few mK to 100 mK have been demonstrated. Our implantation method is highly reproducible and does not result in broadened transitions of the superconducting films. Our results represent an important breakthrough in the optimization of detectors used in the Cold Dark Matter Search (CDMS) experiments, where we prefer W phonon sensors with Tc ∼65 mK. In this paper, we describe in detail our Tc tuning technique and we present results from characterization experiments performed with the Tc - adjusted films.