Fabrication, Simulation and Performance of Ultra-Thin Silicon Detectors-Development of a Detection System for Living Cell Irradiation

Sammanfattning: For the development of a Single–Ion Hit Facility (SIHF) for living cells irradiation at the Lund Ion Beam Analysis facility (LIBAF), newly developed pre-cell detectors have been investigated. These detectors can determine the number of ions reaching the sample. Central to the thesis is their fabrication and characterization. Two different detectors ’ Design A and Design B’ were fabricated with thicknesses down to 3.5 μm, and different physical and structural characteristics using silicon micro-fabrication technology. The main electrical characteristics of the detectors were an extremely low leakage current less than 12 nA and a low full depletion voltage at a reverse bias less than 1.5 V. The Silvaco software was used to simulate the structure and electrical characteristics of the fabricated detectors. By this software, the doping level for p+- and n+ type layers was found as a function of diffusion time and temperature. Also, the I-V characteristic of the simulated device was predicted to be <6 nA. Using the MeV proton microprobe facility at the LIBAF, the detectors were tested using a ΔE-E configuration. The results showed that the detection efficiency for protons was >91% for Design B detectors, and >77% for Design A detectors. Furthermore, the efficiency of the detectors of Design B reached 98% by cooling them to below 2°C. Design B detectors were thus shown to have better performance than Design A detectors for counting the passage of MeV protons. The radiation tolerance of these detectors was evaluated using a 2.55 MeV proton microbeam. The radiation tolerance of the detectors that have thicknesses less than or equal to 10 μm was found to be 2 ×1013 cm-2, and for detectors thicker than 10 μm to be 2 ×1012 cm-2. In an experiment irradiating living cells, the new pre-cell detector was used to count 2.4 MeV protons for two different doses (405 and 83 mGy). The preliminary results of this study showed that the viable fraction was lower at the higher dose than at the lower dose.