Analysis of the radiation hardness and charge collection efficiency of thinned 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 p-n diode detectors on membranes obtained by locally thinning the silicon substrate by means of TMAH etching from the wafer back-side. Diodes of different shapes and sizes have been fabricated on $50-\mu\rm{m}$ and $100-\mu\rm{m}$ thick membranes and tested, showing a low leakage current (about 300 nA/cm$^3$) and, as expected, a very low depletion voltage (in the order of 1 V for the $50-\mu\rm{m}$ membrane). Radiation damage tests have been performed with 58-MeV Li ions at the SIRAD Irradiation Facility of the INFN National Laboratory of Legnaro, Italy, up to a fluence of $1.83 \times 10^{13} \ \rm{Li}/cm^2$. Moreover, charge collection efficiency tests performed at INFN Firenze with a $\beta$ particle source have been performed on both non-irradiated and irradiated samples. Results here reported confirm the advantages of thinned diodes with respect to standard ones in terms of low depletion voltage and charge collection efficiency even after the highest ion fluence.