3D silicon strip detectors
Introduction
The Super Large Hadron Collider (sLHC) is expected to deliver roughly 10 times the luminosity of the Large Hadron Collider (LHC). In addition to the higher occupancy in the detectors, the luminosity increase will translate into a corresponding increase of the radiation dose with which the existing tracking systems cannot cope. Improved tracking detectors need to be developed for the sLHC. In particular, a new generation of radiation hard silicon detectors will be required for the innermost tracking layers.
The dominant radiation induced effects are an ever-growing depletion voltage and increased trapping of charges in the silicon bulk. The 3D-design [1], where electrodes extend into the silicon bulk, is resistant to these effects by decoupling the depletion voltage and collection distance from the detector thickness. The Single Type Column (STC) design is a simplification of the original 3D-design with electrodes of a single doping type only [2]. The detectors described in this article have deep n-type columns etched in thick p-bulk material with a -back side implant. Rows of columns are joined together to form strips as in a planar design. This n-in-p 3D-detector will not type-invert due to the p-type silicon bulk, and collect electrons at the columns, which is advantageous to counteract trapping as electron mobility is significantly larger than hole mobility.
Section snippets
3D detectors and modules
Our detectors are 3D-STC detectors with 64 strips of different lengths (either 1.8 or 0.8 cm), fabricated by FBK-irst [3]. Using custom-made pitch adapters and ultrasonic wire bonding, the detectors are connected to input channels of front-end electronics originally designed for LHC trackers. On most modules, we use the binary front-end ASICs and hybrids made for the ATLAS SCT endcap [4]. The module fabricated for the beam test described in Section 5 used the analogue Beetle ASIC from the LHCb
Laser test results
In our test systems in the lab, we generate charge in the detector in two ways. One method uses electrons from a -source, and will be described in Section 4. The other uses a pulsed IR-laser system with a wavelength of 982 nm, resulting in a penetration depth of around . The laser delivers pulses of less than 2 ns duration into a spot size on the sensor. In the laser system, motorized –-stages allow spatial signal measurements with a few accuracy [7]. With this IR-laser, position
Source test results
A second test set-up, based on an -source and two thin scintillators, was employed to make measurements of the absolute CCE [9]. The key differences of the -source system w.r.t. the IR-laser are the homogenous distribution of the generated charge along the path of the electron as opposed to the exponentially decaying laser intensity, and the amount of generated charge which follows a Landau distribution. Fig. 3 shows the signal measured in a p-type 3D-STC sensor with p-spray isolation as
Beam test results
We also tested two 3D-STC detectors with high-energy minimum ionizing particles (MIPs). The MIPs were charged pions with an energy of 180 GeV generated by the CERN SPS. The beam test used a beam telescope with four planes of SSDs with pitch to define reference tracks. A scintillator-based trigger system was used to select events with a pion passing through the telescope. The time of arrival of the pion relative to the 40 MHz system clock was measured with a TDC to allow time-resolved
Conclusions and outlook
3D SSDs in the STC design were successfully connected to and read out with LHC front-end electronics. The results demonstrate that 3D strip detectors can be operated like normal planar strip detectors. Noise measurements show that the noise per unit length (which is dominated by the detector capacitance) is about a factor three larger than for planar detectors of the same dimensions. The increased noise is clearly attributed to the extra capacitance from the columnar electrodes, but poses no
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