Silicon Pad Detector Design

The pad detectors are designed in 30-degree wedges and consist of 252 pads, arranged in 12 columns x 21 rows. The individual pads vary in area from 2mm x 2mm at the inner radius of the pad detector to 4.5 mm x 4.5 mm at the outer radius of the pad detector. The height of the detector from the inner to the outer radius is 72 mm. In a Au + Au central collision, the average occupancy of the detector could reach up to 50%.

Figure Fig.3

Similar to the strip detectors, the pad detectors are single sided, ac-coupled detectors. The dielectric oxide is 200 nm thick. The biasing of the detector is through polysilicon resistors. The bias resistors are distributed between alternate rows of pads, within a 300 um wide gap, connecting to the pads that are located above and below the gap. The gap between pads in adjacent columns is 200 um. There are two guard rings on the detector. The inner guard ring is placed close to the pads, inside of the bias line. The outer guard ring is placed between the bias line and the cut-edge of the detector

The novel part in the silicon pad detector design is the use of double metal technology. The silicon detectors designed in the past were usually single metal ones, which require a lot of effort to connect the output cables to the detectors. The double metal pad detector incorporates the signal output cable directly on the detector itself. This design has the advantages of avoiding the extra assembly step associated with the single metal design. To minimize the disadvantage of possible additional cross talk induced by the double metal design, we have made the silicon dioxide layer that separates the two metal layers as thick as possble (~ 4 um thick). We use the detectors manufactured by Micro Semiconductor Ltd for both strip and pad detectors.


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