Sub-micron ultra thin SiC free standing membranes for soft X-rays beam monitoring

X-ray beam position monitors (XBPMs) are essential diagnostics in modern synchrotron light sources, where they must operate with minimal beam disturbance, excellent linearity and stability under intense photon fluxes. For soft X-ray beamlines, the limited photon penetration depth demands highly transparent detectors. Single-crystal CVD diamond devices, the current standard for many XBPM applications, have been thinned down to a few micrometres to improve transmission, but fabrication challenges and residual absorption below 2 keV remain. In this work, we characterize a submicrometric freestanding 4H-SiC membrane for the first time using a soft X-ray source, proving such a device to be effective as a beam monitoring element. The membrane, fabricated using a doping-selective electrochemical etching process, has a nominal thickness of 220 nm and a 2 mm × 2 mm active area, with Al electrodes deposited on both sides. Measurements were carried out at the Pollux beamline (SLS) and the Metrologie beamline (SOLEIL) using photon energies of 900 eV and 1000 eV, respectively. Raster-scan transmission mapping demonstrated good and uniform transparency, while current–voltage and beam-induced current measurements yielded the charge collection efficiency across the device. These results demonstrate that ultra-thin 4H-SiC membranes combine exceptional transparency, mechanical integrity, and electrical performance, making them a promising alternative to diamond for non-invasive beam diagnostics in soft X-ray synchrotron beamlines