Semiconductor Tracker Research Articles

Acceptance tests and criteria of the ATLAS transition radiation tracker

The Transition Radiation Tracker (TRT) sits at the outermost part of the ATLAS Inner Detector, encasing the Pixel Detector and the Semi-Conductor Tracker (SCT). The TRT combines charged particle track reconstruction with electron identification capability. This is achieved by layers of xenon-filled straw tubes with periodic radiator foils or fibers providing TR photon emission. The design and choice of materials have been optimized to cope with the harsh operating conditions at the LHC, which are expected to lead to an accumulated radiation dose of 10 Mrad and a neutron fluence of up to 2middot10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sup> n/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2 </sup> after ten years of operation. The TRT comprises a barrel containing 52 000 axial straws and two end-cap parts with 320 000 radial straws. The total of 420 000 electronic channels (two channels per barrel straw) allows continuous tracking with many projective measurements (more than 30 straw hits per track). The assembly of the barrel modules in the US has recently been completed, while the end-cap wheel construction in Russia has reached the 50% mark. After testing at the production sites and shipment to CERN, all modules and wheels undergo a series of quality and conformity measurements. These acceptance tests survey dimensions, wire tension, gas-tightness, high-voltage stability and gas-gain uniformity along each individual straw. This paper gives details on the acceptance criteria and measurement methods. An overview of the most important results obtained to-date is also given

The ATLAS Semiconductor Tracker – design and status of construction

The ATLAS Semiconductor Tracker (SCT) is one part of the tracking system of ATLAS – general-purpose detector for 14 TeV Large Hadron Collider under construction at CERN. It consists of about 4000 silicon microstrip detector modules, which compose 4 barrel layers and 9 endcap disks on each side. Design and construction of this large area (61 m 2 ) silicon detector present several challenges (radiation hardness, thermal and electrical stability, mechanical precision, etc.). Basic principles of SCT design, performance and status of construction will be shown.

Performance of P-type micro-strip detectors after irradiation to 7.5/spl times/10/sup 15/ p cm/sup 2/

Exploiting the advantages of reading out segmented silicon from the n-side, we have produced test detectors with LHC pitch but 1-cm-long strips which even after proton irradiation at the CERN PS (24 GeV protons) to 7.5/spl times/10/sup 15/ cm/sup -2/ show signal to noise greater than 8:1 using LHC speed electronics. This dose exceeds by a factor of 2 that required for a replacement of the ATLAS semiconductor tracker to cope with an upgrade of the LHC to a Super-LHC with ten times greater luminosity. These detectors were processed on p-type starting material of resistivity /spl sim/2 k/spl Omega/cm and, unlike n-in-n designs, only required single-sided processing. Such technology should therefore provide a relatively inexpensive route to replacing the central tracking at both ATLAS and CMS for Super-LHC (SLHC). The shorter strip length is required to limit the noise. Even at these extreme doses 30% of the nonirradiated signal is seen. This 7000 e/sup -/ signal (in 280-/spl mu/m-thick sensors) is very competitive with the postirradiation performance of other, more exotic detector options . The hit density expected at a SLHC would anyway require a scaling down of the sense element length (for a given pitch) to retain an occupancy of less than 1% in the layers of the central tracker. We therefore propose such a short-strip design as a possible low cost and easily implemented route to achieving the requirements for very high luminosity tracking at an upgraded LHC.

The detector control system for the ATLAS semiconductor tracker assembly phase

The ATLAS Semiconductor tracker (SCT) consists of 4088 silicon microstrip modules, with a total of 6.3 million readout channels. These are arranged into 4 concentric barrel layers and 2 endcaps of 9 disks each. The coherent and safe operation of the SCT during commissioning and subsequent operation is an essential task of the Detector Control System (DCS). The main building blocks of the SCT DCS, the cooling system, the power supplies and the environmental system, are described. First results from DCS testing are presented.

Design and performance of the ABCD3TA ASIC for readout of silicon strip detectors in the ATLAS semiconductor tracker

The ABCD3TA is a 128-channel ASIC with binary architecture for the readout of silicon strip particle detectors in the Semiconductor Tracker of the ATLAS experiment at the Large Hadron Collider (LHC). The chip comprises fast front-end and amplitude discriminator circuits using bipolar devices, a binary pipeline for first level trigger latency, a second level derandomising buffer and data compression circuitry based on CMOS devices. It has been designed and fabricated in a BiCMOS radiation resistant process. Extensive testing of the ABCD3TA chips assembled into detector modules show that the design meets the specifications and maintains the required performance after irradiation up to a total ionising dose of 10 Mrad and a 1-MeV neutron equivalent fluence of 2×10 14 n/cm 2, corresponding to 10 years of operation of the LHC at its design luminosity. Wafer screening and quality assurance procedures have been developed and implemented in large volume production to ensure that the chips assembled into modules meet the rigorous acceptance criteria.

The control and monitoring system for the ATLAS semi-conductor tracker

The ATLAS Semi-Conductor Tracker (SCT) will be made of 4088 silicon modules mounted on four barrel supports and eighteen end cap disks. At present module production has started in different sites all around the world. The assembly of the modules into the support structures is taking place at three different macro-assembly sites. To allow for the safety operation of the detector a Detector Control System (DCS) is being developed at CERN that will be close to the final ATLAS configuration. The DCS will control and monitor the power supply provided to the modules, the cooling system and the environmental variables. This paper will give an overview of the system required to operate and readout large-scale silicon detectors.

Moisture sensitivity of AC-coupled silicon strip sensors

A study is presented on the I– V behaviour of a part of the AC-coupled silicon strip sensors used in the ATLAS semiconductor tracker (SCT) at the LHC. For these sensors the metal strips are narrower than the implants while for the majority of the SCT sensors the strips are wider than the implants (fieldplate layout). The I– V curves of these sensors show an early I– V breakdown in a dry environment that did not occur at the I– V test by the manufacturer done under normal laboratory conditions. The breakdown behaviour was not observed at the sensors with the fieldplate layout. In this paper the dependence of the breakdown on the environmental moisture level will be described and a possible explanation will be given that is based on the detector layout. It will also be shown that the breakdown behaviour does not cause operational problems for the SCT.

Production of endcap modules for the ATLAS semiconductor tracker

The ATLAS SemiConductor Tracker (SCT) will consist of a barrel and two endcap parts equipped with modules made from single-sided p-on-n silicon microstrip detectors. The detectors are mounted back-to-back, with a 40 mrad stereo angle, on a support structure, the spine, and readout in binary mode by custom made ASICs. For the modules of the endcap part of the SCT, the detectors are glued to a graphite support structure and the ASICs are mounted on a double-sided hybrid produced from a six layer copper/kapton flex. From mid-2003 the mass production of 1976 SCT end-cap modules has been started at various production sites. I will report on the experiences and problems during the production at the Max-Planck Institute for Physics (MPI). This includes mechanical and electrical issues as well as results on quality assurance.

Diagnostic analysis of silicon strips detector readout in the ATLAS Semi-Conductor Tracker module production

The ATLAS Semi-Conductor Tracker (SCT) Collaboration is currently in the production phase of fabricating and testing silicon strips modules for the ATLAS detector at the Large Hadron Collider being built at the CERN laboratory in Geneva, Switzerland. A small but relevant percentage of ICs developed a new set of defects after being mounted on hybrids that were not detected in the wafer screening. To minimize IC replacement and outright module failure, analysis methods were developed to study IC problems during the production of SCT modules. These analyses included studying wafer and hybrid data correlations to finely tune the selection of ICs and tests to utilize the ability to adjust front-end parameters of the IC in order to reduce the rejection and replacement rate of fabricated components. This paper will discuss a few examples of the problems encountered during the production of SCT hybrids and modules in the area of ICs performance, and will demonstrate the value of the flexibility built into the ABCD3T chip.

The ATLAS semiconductor tracker (SCT)

The ATLAS detector (CERN/LHCC/94-43 (1994)) is designed to study a wide range of physics at the CERN Large Hadron Collider (LHC) at luminosities up to 10 34 cm −2 s −1 with a bunch-crossing rate of 40 MHz. The Semiconductor Tracker (SCT) forms a key component of the Inner Detector (vol. 1, ATLAS TDR 4, CERN/LHCC 97–16 (1997); vol. 2, ATLAS TDR 5, CERN/LHCC 97-17 (1997)) which is situated inside a 2 T solenoid field. The ATLAS Semiconductor Tracker (SCT) utilises 4088 silicon modules with binary readout mounted on carbon fibre composite structures arranged in the forms of barrels in the central region and discs in the forward region. The construction of the SCT is now well advanced. The design of the SCT modules, services and support structures will be briefly outlined. A description of the various stages in the construction process will be presented with examples of the performance achieved and the main difficulties encountered. Finally, the current status of the construction is reviewed.

The read-out and control system For the ATLAS SemiConductor Tracker

The SemiConductor Tracker (SCT) in the ATLAS experiment has entered the stage of system assembly. Around 35% of the 4088 silicon modules are already produced, tested and will soon be mounted on the four barrel cylinders and 18 end-cap disks which make up the SCT. A new Data Acquisition System (DAQ) will provide binary readout, via front-end ASICs, of 16,000 silicon wafers and 6.3 million read-out channels using optical links. A new Detector Control System (DCS) will control up to 500 V bias voltage and the 30 kW low voltage power to the modules, as well as monitor the C 3 F 8 evaporative cooling system, humidity and temperatures. Recently, several macro-assembly sites have mounted modules on both end-cap and barrel prototype structures and gained first experience with system-operation of the SCT. This presentation will give an overview of the full system required to operate and read-out a large-scale silicon detector. A description of both off-detector systems, DAQ and DCS, and their interactions will be presented, as well as the macro-assembly status.

Beam splash effects on ATLAS silicon microstrip detectors evaluated using 1-w Nd : YAG laser

On an incident of accelerator beam loss, the tracking detector located close to the beam line is subjected to receive intensive radiation in a short period. We used a 1-W focused Nd : YAG laser and simulated the effects on the ATLAS microstrip detector. The laser corresponds to intensity of up to 1 × 10 9 mips / pulse with a pulse width of about 10 ns. We observed breaks on Al strips on extreme conditions, depending on the laser intensity and bias voltage applied to the silicon sensor. The break can be interpreted as the oxide breakdown due to a large voltage locally created across the oxide by the intensive signal charges. The robustness of the Semiconductor Tracker (SCT) module including readout ASICs is also evaluated.

Test of ATLAS SCT barrel modules with Nd:YAG laser

As a part of the quality assurance procedure of the ATLAS SCT (Semiconductor Tracker) barrel modules, the response of the microstrip detector is measured by injecting focused Nd:YAG laser at each strip. The test is sensitive to sensor originated problems and a cross check to the results obtained from the electrical tests performed with the test pulse system implemented in the readout ASICs. Combining these results with the probing results obtained by the silicon sensor manufacturer, we verified the reliabilities of these tests and classified the type of defects for overall performance evaluation of the modules.

ATLAS pixel detector: Analysis of test beam data

The ATLAS (A Toroidal LHC AppratuS) at the future LHC (Large Hadron Collider) at CERN (European Center for Particle Physics) will use silicon pixel detectors for the three innermost layers of the semiconductor tracker. Their aim is to provide precise position of particles passing through the detector.

Radiation tests of photodiodes for the ATLAS SCT and PIXEL opto-links

In previous research, epitaxial Si PIN photodiodes produced by Centronic which will be used in the ATLAS semiconductor tracker have been irradiated with 1 MeV neutrons and 24 GeV protons with fluences up to an equivalent of 10 15 1 MeV neutrons [1,2]. In this work 30 MeV proton beams were used to irradiate Centronic and Truelight epitaxial Si PIN diodes with accumulated fluences of up to 2.1×10 14–30 MeV p cm −2, an equivalent of 5.7×10 14 cm −2 1 MeV neutrons, to reach the pixel radiation environment. The responsivity was measured with different levels of fluence in order to study the responsivity behaviour of two different types of photodiodes. The responsivity behaviour of these two photodiodes was similar: a linear degradation at large fluences, >10 14 30 MeV p cm −2, but with different slopes. The response of the Centronic PIN diode showed a degradation to 73% after a proton fluence of 10 13 p cm −2 of 30 MeV and a linear degradation at a rate of −0.32% per 10 13 p cm −2 for fluences larger than 10 13 30 MeV p cm −2. The Truelight PIN degraded to 84% after a fluence of 10 13 p cm −2, with a linear degradation rate of –1.8% per 10 13 p cm −2. Annealing of both photodiodes with a bias voltage of –10 V for 180 h showed a 7% recovery for the Truelight PIN diode and a 2% recovery for the Centronic PIN diode after irradiation fluences of 2.1×10 14 p cm −2. A gamma source was also used to irradiate 6 Truelight PIN diodes with a total dose larger than 600 kGy; no obvious responsivity change was observed in this case.

Beam tests of ATLAS SCT silicon strip detector modules

The design and technology of the silicon strip detector modules for the Semiconductor Tracker (SCT) of the ATLAS experiment have been finalised in the last several years. Integral to this process has been the measurement and verification of the tracking performance of the different module types in test beams at the CERN SPS and the KEK PS. Tests have been performed to explore the module performance under various operating conditions including detector bias voltage, magnetic field, incidence angle, and state of irradiation up to 3 × 10 14 protons per square centimetre. A particular emphasis has been the understanding of the operational consequences of the binary readout scheme.

Design and test of a prototype silicon detector module for ATLAS Semiconductor Tracker endcaps

The ATLAS Semiconductor Tracker (SCT) will be a central part of the tracking system of the ATLAS experiment. The SCT consists of four concentric barrels of silicon detectors as well as two silicon endcap detectors formed by nine disks each. The layout of the forward silicon detector module presented in this paper is based on the approved layout of the silicon detectors of the SCT, their geometry and arrangement in disks, but uses otherwise components identical to the barrel modules of the SCT. The module layout is optimized for excellent thermal management and electrical performance, while keeping the assembly simple and adequate for a large scale module production. This paper summarizes the design and layout of the module and present results of a limited prototype production, which has been extensively tested in the laboratory and testbeam. The module design was not finally adopted for series production because a dedicated forward hybrid layout was pursued.

Frequency scanning interferometry in ATLAS: remote, multiple, simultaneous and precise distance measurements in a hostile environment

ATLAS is the largest particle detector under construction at CERN Geneva. Frequency scanning interferometry (FSI), also known as absolute distance interferometry, will be used to monitor shape changes of the SCT (semiconductor tracker), a particle tracker in the inaccessible, high radiation environment at the centre of ATLAS. Geodetic grids with several hundred fibre-coupled interferometers (30 mm to 1.5 m long) will be measured simultaneously. These lengths will be measured by tuning two lasers and comparing the resulting phase shifts in grid line interferometers (GLIs) with phase shifts in a reference interferometer. The novel inexpensive GLI design uses diverging beams to reduce sensitivity to misalignment, albeit with weaker signals. One micrometre precision length measurements of grid lines will allow 10 µm precision tracker shape corrections to be fed into ATLAS particle tracking analysis. The technique was demonstrated by measuring a 400 mm interferometer to better than 400 nm and a 1195 mm interferometer to better than 250 nm. Precise measurements were possible, even with poor quality signals, using numerical analysis of thousands of intensity samples. Errors due to drifts in interferometer length were substantially reduced using two lasers tuned in opposite directions and the precision was further improved by linking measurements made at widely separated laser frequencies.

Double peak electric field distortion in heavily irradiated silicon strip detectors

Non-uniform distribution of the electric field outlined as double peak distortion (DPD) is considered for heavily irradiated silicon strip detectors, which were developed for the CERN-ATLAS semiconductor tracker. DPD originates from the non-uniform accumulation of electrons and holes from the bulk generated current that are captured by radiation induced defects: deep acceptors and donors with mid-gap energy levels. This corresponds to the formation of the low electric field region in the detector central part that consequently will delay charge collection. The electric field distributions at different reverse biases, fluences and detector operational temperatures are calculated using a one-dimensional Poisson equation as it was done earlier for pad detectors. It has been shown that due to the electric field focusing at the strips the DPD effect is more pronounced for strip detectors as compared to that in pad detectors. The double peak electric field distribution is evinced experimentally in current pulse response shape when the charge is collected along the strip axis.

The off-detector opto-electronics for the optical links of the ATLAS Semiconductor Tracker and Pixel detector

The off-detector part of the optical links for the ATLAS SCT and Pixel detectors is described. The VCSELs and p–i–n diodes used and the associated ASICs are described. A novel array packaging technique is explained and an analysis of the performance of the arrays and the overall system performance is given. The proposed procedure for the set-up of the optical links in ATLAS is described.