Development of Thick Electroplated Bismuth Absorbers for Large Collection Area Hard X-ray Transition Edge Sensors

Transition edge sensors (TES) offer some of the highest resolutions for solid-state X-ray spectrometers. We are developing TES detectors for use at hard X-ray synchrotron light sources for energy ranges up to 20 keV. Because TES resolving power scales inversely with the square root of heat capacity, it is important to have an absorber with both a small heat capacity and a large X-ray stopping power. We are developing electroplated bismuth (Bi) absorbers to meet these criteria. Although Bi has a smaller X-ray absorption at 20 keV than gold, the specific heat is up to two orders of magnitude smaller, allowing for much larger collection area (up to 1 mm
²
without significantly increasing the total device specific heat. However, due to its low thermal conductivity, Bi absorbers may have longer thermalization times. Also, some evaporated Bi absorbers may produce spectra with low-energy tails that will hinder X-ray line shape analysis and increase minimum detectability limits of trace metals for X-ray fluorescence microscopy. We examine the impact of plating current density, agitation, film thickness, and seed layer thickness on the grain size, residual resistance ratio, and uniformity of Bi absorbers. Additionally, we discuss processing considerations important for successful electroplating.