Electroplating Deposition of Bismuth Absorbers for X-ray Superconducting Transition Edge Sensors

Abstract

An absorber with a high absorbing efficiency is crucial for X-ray transition edge sensors (TESs) to realize high quantum efficiency and the best energy resolution. Semimetal Bismuth (Bi) has shown greater superiority than gold (Au) as the absorber due to the low specific heat capacity, which is two orders of magnitude smaller. The electroplating process of Bi films is investigated. The Bi grains show a polycrystalline rhombohedral structure, and the X-ray diffraction (XRD) patterns show a typical crystal orientation of (012). The average grain size becomes larger as the electroplating current density and the thickness increase, and the orientation of Bi grains changes as the temperature increases. The residual resistance ratio (RRR) (R300 K/R4.2 K) is 1.37 for the Bi film (862 nm) deposited with 9 mA/cm2 at 40 °C for 2 min. The absorptivity of the 5 μm thick Bi films is 40.3% and 30.7% for 10 keV and 15.6 keV X-ray radiation respectively, which shows that Bi films are a good candidate as the absorber of X-ray TESs.

Electrodeposition and analysis of thick bismuth films

“…Many parameters profoundly affect Bi electroplating, making proper conditions paramount to obtaining a film with homogenous surface coverage and good adhesion. In addition to Bi(NO 3 ) 3 , we added tartaric acid and glycerol as chelating agents in the same manner as Chen et al 15 to help moderate film growth 10 and stabilize Bi 3+ ion. Plating in this solution is highly sensitive to solution pH, with the best coating obtained at a pH range of 0.01–0.1, resulting in a robust film, while higher pH values produce a film with poor adhesion, able to be wiped away easily by hand.…”

“…Electrodeposition is particularly attractive, being amenable to mild temperature and pressure conditions on irregularly shaped substrates of a wide range of sizes, with great control over resulting surface morphology 10 . Previous studies have demonstrated electrodeposition of Bi, generally obtaining nanometer 14 to single micron 1 , 15 thicknesses. For some practical applications (particularly radiation shielding), thicker, robust films are desirable 16 .…”