Development of radiation hard silicon detectors for the CBM Silicon Tracking System using Simulation Approach

The very intense radiation environment of the planned Compressed Baryonic Matter (CBM) experiment at the international research center FAIR makes radiation hardness a very important issue for the Silicon Tracking System (STS). The STS will consist of eight stations of double sided strip detectors at a distance between 25 cm to 100 cm downstream of the target. It is expected that the total integrated fluence will reach 1 × 10 cm 1 MeV neutrons equivalent which is more than expected at LHC at CERN. The major macroscopic effect of radiation damage in determining the viability of long-term operation of silicon sensors is the change in the effective charge carrier concentration (Neff), leading to type inversion. For the safe operation over full CBM life time, detectors are required to sustain very high voltage operation, well exceeding the bias voltage needed to fully deplete the heavily irradiated sensors. Thus, the main effort in the development of silicon sensors is concentrated on a design that avoids p-n junction breakdown at operational biases. Simulations are carried out to study the effect of change in Neff on the breakdown performance of the device using device simulation package PISCES [1]. The simulation grid is shown in Fig. 1. Detailed calculations with the Hamburg Model [2] have allowed the parameterization of these effects to simulate the operation scenario of silicon detectors over full CBM life time. Also, the impact of various crucial geometrical parameters like device depth (WN ), width of back N layer (WN), strip width (W) and strip pitch (P) on the guard ring equipped structure after typeinversion has been studied in detail. It is clear from Fig. 2 that the peak electric field occurs at the junction curvature which is responsible for the premature breakdown of the device. Also process simulations have been performed using SUPREM-4 [3] for studying the annealing behaviour of Boron implanted in silicon. Fig. 3 shows the impact of annealing parameters on the electrical characteristics of the device. We plan to procure 3-dimensional simulation packages from Synopsis [4] through EuroPractice Software Service of which GSI is a member so that we can accurately simulate double sided strip detectors.