Double-sided Silicon Detector, DSSD

We use S6936 DSSDs fabricated by Hamamatsu Photonics (HPK). These detectors were originally developed for the DELPHI micro-vertex detector [19]. Each DSSD consists of 1280 sense strips and 640 readout pads on opposite sides. The sense strips are biased via 25 M$\Omega$ polysilicon bias resistors. The $z$-strip pitch is 42 $\mu$m and the $\phi$-strip pitch is 25 $\mu$m. The size of the active region is $53.5 \times 32.0$ mm$^2$ on the $z$-side and $54.5 \times 32.0$ mm$^2$ on the $\phi$-side. The overall DSSD size is $57.5 \times 33.5$ mm$^2$. In total 102 DSSDs are used. For the $z$-coordinate ($z$ is the approximate beam direction) measurement, a double-metal structure running parallel to $z$ is employed to route the signals from orthogonal $z$-sense strips to the ends of the detector. Adjacent strips are connected to a single readout trace on the second metal layer which gives an effective strip pitch of 84 $\mu$m. The ohmic side is chosen to be on the $z$-side and a $p$-stop structure is employed to isolate the sense strips. A relatively large thermal noise (600 e$^-$) is observed due to the common-$p$-stop design. On the $\phi$ side, only every other sense-strip is connected to a readout channel. Charge collected by the floating strips in between is read from adjacent strips by means of capacitive charge division.