# Silicon 3D Microdosimeters for Advanced Quality Assurance in Particle Therapy
## 3 Silicon on Insulator (SOI) Detector for Microdosimetry
### TCAD Simulations of 3D Mushroom SOI Microdosimeters  
Modern numerical simulation tools, such as Technology Computer Aided Design (TCAD), offer considerable support in the design and characterisation of novel radiation detectors. A detailed numerical model of the devices can be created to simulate the operation of the sensors to be fabricated. Typical simulations include Current-Voltage (I-V) curves, Capacitance-Voltage (C-V) curves and transient simulations in response to impinging radiation. The tools chosen for this TCAD simulation study were the SYNOPSYS TCAD tools. Process and device simulations were carried out at SINTEF. The simulation tools were tuned with the known SINTEF process parameters to obtain models as close as possible to the final devices. Numerical models were created in 2D for both the Trenched-3D and Trenched-planar sensors for SV thickness of 10 \(\upmu\)m (Figure 6). These structures represent a radial slice of a single SV and can be used to perform simulations in cylindrical coordinates (the centre of the SV is located at x = 25 \(\upmu\)m). The main difference between the two structures is that in Figure 6a, empty regions are present at x = 11 \(\upmu\)m and x = 25 \(\upmu\)m to simulate the lack of polysilicon filling in the Trenched-3D approach. It can also be observed that in Figure 6b the n+ electrode is planar. Electrical simulations are performed by scanning the bias voltage on the p\({}^{+}\) electrode from 0 V to \(-\)100 V while keeping the n\({}^{+}\) electrode grounded. Breakdown voltage was found to be at biases higher than 60 V, which is more than sufficient to achieve over-depletion. The simulations showed that, for a fully depleted single SV, the expected current was on the order of 50A, and the expected capacitance was on the order of 9 fC. These values can be used to obtain an estimate of what to expect when multiple SVs are connected in an array to cover larger areas while still providing an excellent signal-to-noise ratio. The scaling is typically very reliable for dark currents, while extra capacitive components must be considered for large arrays due to the long metal interconnections between the SVs. The simulator can also provide 2D plots of electrostatic potential and electric field distributions. For example, the equipotential drift lines at a bias of \(-\)20 V are shown for both structures in Figure 6. It can be observed that, for the Trenched-3D structure (Figure 6a), the  
Figure 6: 2D structures used for numerical simulations of (**a**) the Trenched-3D and (**b**) Trenched-planar sensors. The figures also show the drift lines based on the simulated electric field. lines were mostly horizontal, while this was not the case for the Trenched-planar structure (Figure 6b). While this does not drastically affect the operation of both approaches, it was found that higher electric fields can be achieved in the Trenched-3D structure, potentially resulting in faster charge collection. The operation of the numerical structures under irradiation was tested using the radiation generation models available in the SYNOPSYS Device. The simulator allows a particle track to be defined and offers both a heavy-ion and an alpha particle generation model. For this study, an alpha particle of energy 5.5 MeV was scanned radially through the SV, and transient simulations were performed at different bias voltages to estimate the charge collection efficiency. It was found that bias voltage had very little impact on charge collection because of the small size of the SV. This suggested that these devices can be operated in passive mode. The charge collection profiles for the Trenched-3D and Trenched-planar approaches at a bias of 20 V are compared in Figure 7 where 100% charge collection corresponds to 1.5 MeV energy deposition. Charge collection outside the SV (0 < x < 10 \(\upmu\)m) was only observed in the Trenched-planar structure.