Radiation Hardness and Charge Collection Efficiency of Lithium Irradiated Thin Silicon Diodes

Due to their low depletion voltage, even after high particle fluences, improved tracking precision and momentum resolution, and reduced material budget, thin substrates are one of the possible choices to provide radiation hard detectors for future high energy physics experiments. In the framework of the CERN RD50 Collaboration, we have developed PIN diode detectors on membranes obtained by locally thinning the silicon substrate by means of TMAH etching from the wafer backside. Diodes of different shapes and sizes have been fabricated on 50-µm and 100-µm thick membranes and tested, showing a low leakage current (of 300 nA/cm3) and a very low depletion voltage (in the order of 1 V for the 50 µm membrane) before irradiation. Radiation damage tests have been performed with 58 MeV lithium (Li) ions up to the fluence of 1014 Li/cm2 in order to determine the depletion voltage and leakage current density increase after irradiation. Charge collection efficiency tests carried out with a beta-particle source have been performed on both nonirradiated devices and samples irradiated up to 1.8x1013 Li/cm2. Results reported here confirm the advantages of thinned diodes with respect to standard 300-µm thick devices in terms of low depletion voltage and high charge collection efficiency.