Advancements in modeling conformally doped X-MSND radiation imagers

Abstract

A novel bimodal radiation imager (X-ray and thermal neutron) was designed using a fin–trench microstructured silicon diode backfilled with 6LiF, then bonded to a Timepix pixelated imaging ASIC (X-MSNDs). A simulation scheme had been devised which combined finite element analysis and discrete charge carrier transport to simulate the imaging characteristics of such arbitrarily doped 3D silicon Timepix sensors. However, the precise dopant profile of the deeply etched silicon surface that forms the X-MSND diode junction is difficult to measure with conventional optical or chemical staining means. A set of boundary conditions was instead derived from simple particle transport simulations and the constant-surface-concentration silicon diffusion model. Finite element analysis of the microstructured devices using COMSOL Multiphysics and this derived boundary condition revealed that the fin region of the sensors was generally un-depleted under the maximum 300V reverse bias, and the calculated electric field along the thickness of the sensor in the microstructured region is negligible. Charge carrier transport simulations using Allpix2 predicted charge carrier motion in the fin region is dominated by slow diffusion along the fin dimension, resulting in mean charge drift times of 1000ns to collect 95% of charge carriers. Neutron transport simulations indicated that single neutron detection events resulted in highly elongated pixel clusters, typically 1-pixel wide by 6-pixels along the fin dimension. Measured data collected from KSU fabricated X-MSND imagers matched these simulation results.