Quantum Efficiency Study and Reflectivity Enhancement of Au/Bi Absorbers

Abstract

X-ray absorbers of the X-ray Integral Field Unit (X-IFU) microcalorimeters are required to provide high quantum efficiency (QE) for incident X-rays and high reflectivity to longer wavelength radiation. The thickness of the electroplated Au and Bi layers of the absorber is tuned to provide the desired pixel heat capacity and the QE. To calculate the QE precisely, in addition to filling factor, we have included the effects of surface roughness, edge profile of the absorbers and the effects of the different angles of incidence of the incoming X-rays from the X-IFU optic. Based on this analysis, it is found that thickness of the Bi layer needs to be adjusted by 4.3% to achieve the X-IFU QE requirements. To enhance the absorber’s rejection of low-energy radiation, a second thin layer of Au is sputter-deposited on top of the Bi layer. Optical measurements in the wavelength range 0.3–20 μm show a significant increase in reflectivity compared to a bare Bi layer.