Silicon Sensors and Ladders

The three main components of the silicon tracker each use different silicon sensor designs and layouts. Because it will be mounted closest to the beam, Layer 00 will consist of single-sided AC coupled p-in-n silicon with a guard structure designed to minimize leakage currents. This configuration is intended to improve radiation resistance[10]. Two widths of sensors (8.4 mm and 14.6 mm) will be interleaved in a 12-sided pattern that is physically mounted on and supported by the beam pipe. Each sensor will have a mechanical length of 78.4 mm and will be bonded to one other sensor to form pairs that are electrically 15.7 cm in active length. These sensors have an implant pitch of 25 $\mu$m, implant widths of 8 $\mu$m, and a readout pitch of 50 $\mu$m achieved by reading out alternate strips. Fine-pitch kapton cables carry signals from each of the six pairs of Layer 00 sensors to hybrids mounted at the ends of the array.

The next five layers of the tracker, which comprise the SVX II portion of the design, consist of wire-bonded pairs of double-sided detectors with readout electronics in the form of hybrids that are mounted directly to the silicon surface at each end of each four-sensor mechanical ladder assembly. The length of each ladder is 29 cm, with each consisting electrically of two half-ladders that are read out independently. A perspective view of one end of one of these assemblies is shown in Figure 7.

Figure 7: One end of a 2-chip-wide version of the SVX II ladder.

Both 90-degree and small-angle stereo sensors are used in the SVX II, in the pattern $(90, 90, -1.2, 90, +1.2)$ degrees for the n-strips from the innermost to outermost SVX II layers. All SVX II sensors are AC coupled, 300 $\mu$m thick, and biased using polysilicon resistors. The p-strips on the non-stereo side run in the axial direction of the detector and are used to measure the azimuthal angle $\phi$ in the experiment. These strips are spaced in $r\phi$ by approximately 60 to 65 microns, depending on layer, and have implant widths of 14 to 15 microns. The stereo n-strips of the SVX II are spaced by $(141, 125.5, 60, 141, 65)$ microns, and have implant widths of 20 microns for the $90^{\circ}$ strips and 15 microns for the small-angle stereo layers. The $90^{\circ}$ layers have an additional layer of insulator and readout strips in the ``double-metal'' configuration[15]; these strips carry the $z$ signals to the SVX3 chips with a pitch that ranges from 58 to 60 microns depending on layer. The $90^{\circ}$-stereo sensors are manufactured by Hamamatsu Photonics. The small-angle stereo SVX II sensors are manufactured by Micron Semiconductor.

Common p-stops with widths of 21 $\mu$m along with individual p-stops of width 15 $\mu$m are used in the $90^{\circ}$-stereo layers. In the small-angle stereo SVX II layers, individual p-stops are not used, and the common p-stops have widths of 28 to 30 microns. Type inversion at high radiation doses might render these p-stops ineffective, but the n+ implants will remain isolated. Other radiation-induced effects will in practice limit the operating life of the sensors, such as the increased voltage necessary to achieve depletion, the resulting increases in current and dissipated power, and degradation of interstrip resistance with radiation[14].

Present estimates indicate that the innermost SVX II sensors will degrade beyond usable levels after the first 2 to 3 years of operation, corresponding to several $\times 10^{13}$ n/cm$^2$[14]. Layer 00, which is single-sided and thus can operate acceptably even when not fully depleted, should be able to withstand the higher radiation doses that it will encounter at its small inner radius. Along with the remaining layers of the SVX II and ISL, this should preserve functional $r\phi$ tracking and at least some stereo capability to as much as 5 fb$^{-1}$ of accumulated Tevatron data[10].

Figure 8: A perspective view of the ISL ladder design.

The ISL portion of the tracker utilizes larger-pitch double sided sensors made in both 4" (Hamamatsu) and 6" (Micron) technology[16]. These sensors are also AC coupled, with polysilicon biasing and common p stops. A fixed strip pitch of 112 $\mu$m is used on both the axial and 1.2-degree stereo sides. Pitch adapters are used to bring the signals from the strips to the more closely spaced inputs of the SVX3D chips. The stereo strips are on the n side for the Micron sensors and on the p side for the Hamamatsu sensors.

The ISL ladders are composed of six sensors, arranged as half-ladders of three sensors each. This arrangement is shown in Figure 8. Because of the larger strip pitch, positioning tolerances for placement of these ladders are easier to achieve in the ISL. Taking advantage of the increased amount of space compared to the SVX II, these ladders overlap each other in $z$ and are read out at each ladder end with double-sided hybrids that extend beyond the silicon. The innovative design of the carbon-fiber ladder support allows wirebonding to be done after the ladder has been assembled. This arrangement simplifies ladder construction and permits microbonds to be repaired even after assembly.

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