The ATLAS

S

emi

C

onductor

T

racker Marko Mikuž, University of Ljubljana & Jožef Stefan Institute, Ljubljana, Slovenia on behalf of the ATLAS SCT Collaboration Abstract

The ATLAS SemiConductor Tracker (SCT) is presented. About 16000 silicon micro-strip sensors with a total active surface of over 60 m 2 and with 6.3 million read-out channels are built into 4088 modules arranged into four barrel layers and nine disks covering each of the forward regions up to pseudo-rapidity of 2.5. Challenges are imposed by the hostile radiation environment with particle fluences up to 2x10 14 cm -2 1 MeV neutron NIEL equivalent and 100 kGy TID, the 25 ns LHC bunch crossing time and the need for a hermetic lightweight tracker. The solution adopted is carefully designed strip detectors operated at -7 o C, biased up to 500 V and read out by binary rad-hard fast BiCMOS electronics. A zero-CTE carbon fibre structure provides mechanical support. 30 kW of power are supplied on aluminium/Kapton tapes and cooled by C 3 F 8 evaporative cooling. Data and commands are transferred by optical links. Prototypes of detector modules have been built, irradiated to the maximum expected fluence and successfully tested. The detector is in full production now. This will be followed by integration starting in 2004 and installation in 2006 to match the LHC start-up in 2007.

Requirements

 provide precision space-points for robust particle tracking at intermediate inner detector radii  back to back 40 mrad stereo angle 80 µm pitch strip sensors provide 16 µm x 500 µm resolution      4 barrel layers & 9 disks per end-cap hermetically cover solid angle up to η < 2.5

> 99 % single-plane efficiency for MIP’s detection stiff zero-CTE lightweight carbon fibre structure provides precision support for detector modules tight module building tolerances down to 5 µm  frequency scanning interferometry on-line alignment system survive 10 years in LHC environment with up to 2x10 14 cm -2 NIEL and 100 kGy ionizing dose  operation at -7ºC limits reverse bias current and suppresses reverse annealing   detector reverse bias up to 500 V ensures full depletion and efficient charge collection detector and module irradiation to full dose as part of standard quality control

Modules

 building blocks  2 pairs of daisy-chained sensors glued to high thermal conductivity TPG substrate   flexible circuit Cu/Kapton hybrid with 6 ASIC’s per side laminated on carbon-carbon substrate o barrel: wrap around over detector surface o end-cap: at detector edge, flex wrapped & laminated on substrate, see N44-4 by C. Ketterer glass pitch adapters for bonding from ASIC to detector silicon sensors cooling points   barrel module assembly  four production clusters: Japan, UK, US, Scandinavia, with 400-800 modules to produce per site  ~ 750 modules produced up to date out of 2112 needed   extensive mechanical & electrical QA very good quality: on average < 1 dead channel out of 1512 end-cap module assembly  seven production clusters with distributed assembly / QA   several clusters qualified for production problems with start-up due to delays in component delivery

Layout

 barrel: two daisy chained silicon sensors per module side with strips ~ in z-direction, tilted by 11º to the barrel, arranged by 12 on staves along z Barrel # staves # modules 1 2 32 40 384 480 3 4 Total 48 56 576 672 2112 modules

End-cap

 end-cap: three rings (inner, middle, outer) per fully populated disk, strips in r-direction, two sensors/side on outer & middle, one on inner Disk 1 2 3 4 5 6 7 8 9 Rings M,O I,M,O I,M,O I,M,O I,M,O I,M,O M,O M,O O # modules 92 132 132 132 132 132 92 92 52 Total 2 x 988 = 1976 modules

Detectors

 single-sided AC-coupled p + -n detectors with 768 strips processed on 285 µm thick high-resistivity 4” wafers        > 99 % good strips spec, tested at manufacturer leakage current specs before and after full dose irradiations 6 detector types: 1 square – barrel, 5 wedge – end-cap ~ 20000 detectors procured from Hamamatsu (~ 85 %) and CiS (~ 15 %), all detectors in hand detector QA on  every detector: visual inspection,    sample: excellent detector quality: 99.9 % good strips samples per batch irradiated to full dose I-V S/N-V C-V , full strip test, on all detectors I with β-source on sample I-V stability

Barrel End-cap

Log scale !

QA: Detector current @ 350 V & number of strip defects SCT system-test: barrel (↑) & end-cap (↓)

Performance

 binary – single bit digital – R/O electronics  in experiment provides hit/no-hit information only    figure of merit: MIP’s efficiency & noise occupancy vs. threshold specification: > 99 % efficiency & < 5 x10 -4 noise occupancy (NO) diagnostics: threshold scan and calibration charge injection: cumulative distributions – S-curves S-curve: 50 % point gives the gain, width the noise, both are extracted from erfc fit   bench & system-test performance figures see N28-6 by R. Bates  non-irradiated modules o gain ~ 55 mV/fC o noise ~ 1500 e ENC o noise occupancy @ 1 fC: ~ 10 -5  irradiated to 3 x 10 14 o gain ~ 30 mV/fC o noise ~ 1900-2100 e o operational threshold for 5 x 10 -4 NO: 1.0 – 1.2 fC test beam performance p/cm 2 > full dose in 10 years Schematic of SCT set-up in SPS H8 test beam operational range operational range SCT set-up in SPS H8 test beam in May 2003

ASIC’s

  S/N-V after irradiation to 3x10 14 p/cm 2 I-V ABCD – 128 R/O channels, bi-polar front-end & CMOS back-end, produced in biCMOS rad-hard DMILL process at ATMEL front-end with ~ 20 ns shaping, 50 ns double-pulse resolution, ~ 50 mV/fC gain     discriminator with 8-bit programmable threshold and 4-bit per-channel adjustment in 4 selectable ranges 132 cell deep binary 40 MHz pipeline for L1 trigger latency, 24 cell derandomizing buffer storing 8 events  R/O of compressed binary data via 40 MHz optical link radiation hardness  tested with X-rays, protons, pions and neutrons   procurement  order placed under CERN-ATMEL frame contract with 26 % guaranteed yield  acceptance testing at wafer level performed at CERN, UCSC and RAL   meets specifications after full dose anomalous gain degradation observed with thermal neutrons – see N20-4 by I.Mandić yield problems in recent ATMEL runs ~ 85 % perfect chips in hand, CERN negotiating with ATMEL @ -18ºC after irradiation Two SCT barrels with module mounting parts (↑), end-cap cylinder with one out of nine disks(↓) Test beam efficiency & noise occupancy for non-irradiated (←) and irradiated (→) module

Services & structures

   R/O, control and power  2 R/O & 1 clock/command fibre per module  1 power supply channel with cable/tape (17 leads) per module cooling  ~ 30 kW of power, C 3 F 8 evaporative cooling @ ~ -20ºC  active, thermally neutral thermal enclosure support structures  carbon fibre barrels & cylinders with disks  lots of small parts: inserts, brackets… (~40000 for barrel only)

Integration & schedule

 modules mounted on barrels @ Oxford & KEK      barrels integrated & commissioned @ CERN modules mounted on disks and assembled into cylinders @ Liverpool & NIKHEF final end-cap commissioning @ CERN ATLAS integration schedule calls for  SCT barrel available in December 2004  SCT end-caps available in very tight schedule to meet !

March & May 2005