Frontemare di Trieste
Istituto Nazionale di Fisica Nucleare
Sezione di Trieste

ALICE-D1 and D1B prototypes

ALICE-D1

As in the previous prototype, the device is produced on NTD 5-inch silicon wafers with a resistivity of 3kOhmcm and a thickness of 300um. The detector maintains a bi-directional structure, the drifting occurs from the detector centre towards two arrays of anodes. It has an hexagonal shape to allow for the minimization of the overlapping of adjacent SDDs in the ladder assembly. The figures below show respectively a sketch and a picture of the detector.

When working at a field of 600V/cm, the drift speed of the charge cloud will be about 8 um/ns with a maximal drift time of about 4.3us. The drift cathodes have a pitch of 120um and are 70um wide, while the cathode metalization exceeds the p+ implant by 5um at both edges, to form a symmetrical field plate. The guard cathodes have a pitch of 32 um. There is one guard every two drift cathodes, so at a given bias the voltage difference between adjacent guards is twice that between adjacent drift cathodes. There are separate integrated high-voltage dividers to bias independently the drift and guard cathodes. Thereby. possible distortions of the potential distribution on the guard cathodes, due to breakdown or punch-through, will not be directly transferred to the drift region.

The drifting charge is collected by 256 anodes with a pitch of 294um. The figure below shows a close up on the collection zone. It consists of eight cathodes biased independently from the high voltage divider. The first, around the anodes, has been called "grid" cathode, and has the function of separating the anodes. Meanwhile three "pull-up" cathodes opposite to the anodes force the drifting charge towards the anode array. Four outermost cathodes (on both faces of the detector) serve to ensure electrical separation of anodes from the n+ ring at the edge of the detector. The design of this complex region has been carried out following the criteria reported in the paragraph "simulations of the collection zone".

As in the previous prototype, we implemented "point-like" MOS charge injectors. Each half-detector is provided with two arrays of injectors at distances of 17.6mm and 35mm from the anodes. The injector pitch is 2.35mm (one every eight anodes).

ALICE-D1B

ALICE-D1 detector needed an upgrade because of its high sensitivity to the environmental humidity. Indeed, the moisture causes, if no solutions are adopted to attenuate its influence, a premature hole flow between consecutive guard cathodes at different potentials. This phenomenon can be considered as a parasitic resistor inserted in parallel to the implanted resistor of the divider, thus altering the linear potential distribution on the drift cathodes. To overcome this problem we implemented in the ALICE-D1B design a cathode metallisation with "punch-through safe" field-plates. The figure below shows the geometry of the guard structure implemented in the ALICE-D1B design (b), and in the precedent design ALICE-D1 (a).

Adopting this simple solution we have been able to delay the onset of the current flow out of the working conditions.
For a detailed description of this item please see the "Surface effects" paragraph.

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