The collection zone is one of the critical regions in a SDD. Here the electron cloud is forced to move from the middle plane of the detector toward the anode array in the n-side. The forcing electric field is applied by properly biasing the last few cathodes in the proximity of the anodes. It is very important to simulate precisely the potential distribution because of the various constrains we have to satisfy. First we must avoid a trapping of the signal electrons under the oxide when approaching the surface. Second we have to minimize the non-linearity of the drift speed associated with the transversal movement towards the n-side of the detector. Third we have to guarantee a good potential separation between anodes and perimeter to avoid inefficiencies of the electron collection.
The figure below represents the cross section used for the simulation of the collection zone of the ALICE-1B detector. Crucial point for a good simulation is the length of the adopted structure. Indeed, the boundary conditions must not alter the result in the region of interest. From the following picture we note that the length of the region is greater than 2.5mm against a wafer thickness of 0.3mm. This assures a correct solution in the central region.
Clearly, the longer is the structure the greater are the number of mesh points involved. This implies an increasing of both the computational time and the allocated memory.
The figure below shows a 3D representation of the potential distribution obtained solving both the Poisson's equation and the carrier continuity equations.
This kind of representation is very useful because it gives a qualitative idea of the behaviour of the structure. For a detailed analysis it is better to adopt a 2-D map. The picture below shows an overlay of the potential map and the trajectory of eight electrons placed at various positions. It is worthwhile noting that the "pull-up" region is very short (200um), minimising the systematic error on the drift time. Such a kind of collection minimizes also the risk of electron trapping under the oxide because the trajectories are kept far from the surface up to the anode.
In order to evaluate the potential separation between anodes and n+bulk contact the figure below shows a close up of the anode region with the colour palette ranging from 0 to -10V. The potential barrier between anodes and n+bulk contact is about 6V.