Growth of alpha-beta phase W thin films over steep Al topography in a confocal sputtering machine

We report on thin-film processing improvements in the fabrication of superconducting quasiparticle-trap-assisted electrothermal-feedback transition-edge sensors (QETs) used in the design of Cryogenic Dark Matter Search (CDMS) detectors. The work was performed as part of a detector upgrade project that included optimization of a new confocal sputtering system and development of etch recipes compatible with patterning 40 nm-thick, mixed-phase W films deposited on 300-600 nm-thick, patterned Al. We found that our standard exothermic Al wet etch recipes provided inadequate W/Al interfaces and led to poor device performance. We developed a modified Al wet-etch recipe that effectively mitigates geometrical step-coverage limitations while maintaining our existing device design. Data presented here include SEM and FIB images of films and device interfaces obtained with the new Al etch method. We also introduce a method for quantitatively measuring the energy collection efficiency through these interfaces.