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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