Trakcing systems with Silicon with special reference to ATLAS-SCT

1. Some generalities about tracking Special requirements in LHC environments About silicon About the ATLAS-SCT Trakcing systems with Siliconwith special reference to ATLAS-SCT

2. Some general considerations Tracking measures particle 3-momenta Particle track Sagitta, s Lever arm, L r Interaction point Precision of sagitta measurement: (N position measurements)

3. Requirements for good resolution • Lever arm as long as possible (large plarge detector) • Measurement of sagitta as precise as possible • Magnetic field as large as possible

4. A ‘typical’ event Granularity is essential

5. Requirements to LHC tracker • FAST (40MHz) • Excludes ‘standard’ Drift Chambers due to large drift times • Spacial resolution a few tens of microns • High granularity • Radiation hard • Must minimize material • Drift Chambers would be optimal from this point of view • Silicon is a good choice

6. Does very precise tracking give very precise momentum estimates? • Not necessarily due to Multiple (coulomb) scattering • Direction change due to a concentrated scatterer: x/X0 is the amount of material traversed in units of radiation lengths

7. Example: Atlas SCT, 3% X0 /layer, and pixel measurements inside (probably also about 3% X0per layer).

8. Displacement of tracks in different tracker layers as function of momentum due to MSC. Excellent resolution never harms, but is sometimes useless….

9. The silicon strips of the ATLAS-SCT has a pitch of 80 µm • …. So position resolution is 23 µm per hit…. • Standard deviation of a flat distribution =width/(√12)

10. Basics of Silicon detectors • ‘Simple’ p-n junctions • Reverse biased • Junctions can be segmented into strips ~300um thickness From L.G. Johansen, Thesis (Bergen)

11. A model for the diode: Constant charge density in the depletion region of the n-bulk, and heavily doped p-side Nd is the donor concentration

12. Setting a reverse potential across the diode depletes it to a depth given by (about):

13. How to measure the depletion depth? • Charge stored in bulk: (charge density x volume (Al)) • Capacitance:

14. The formula works! 20 detectors from Hamamatsu (L.G. Johansen, thesis)

15. Charge collection • Typical detector thickness is 300 µm. • Bethe-Bloch equation + Landau fluctuations gives a most probable energy loss of about 80keV • To create a free electron-hole pair you need 3.6eV • Signal is about 22000 electrons ( 3.5 fC)

16. Energy loss distributions for 2 GeV electrons,pions and protons, broader than Landau. (Bak et al , NPB288:681,1987)

17. The ATLAS SCT barrel detector • Pitch 80 µm (resolution 23 µm) • 768 strips per detector • Thickness 285 µm • Size 6.36x6.40 cm2 • Should biassed to 350V • p strips on n material

18. Read-out electronics • Must have low noise • Noise scales with capacitance  Size limitations • microstrip detectors: inter-strip capacitances dominate. • Must be compact ASICs • For ATLAS: Must operate at 25 Mhz • For ATLAS: Must be radiation tolerant

19. Signal from electrons from a Ru-106 source (mostly minimum ionising particles). Fluctuations are dominated by Landau fluctuations in the deposited energy. Spectrum collected with fast analog electronics Chip SCT128A (from B.Pommersche, thesis, Bergen) N/bin Signal/noise

20. Charge collection vs bias, 25 ns collection time Difference in signal could be explained by differences in detector thickness From B. Pommeresche, Cand. Scient thesis, (U of Bergen) We must over-deplete the detectors to collect all the charge in time

21. What do we look for to assess the quality of a detector? • Depletion voltage • Inter-strip capacitances • Radiation hardness • will require biassing to high volts • Leakage currents must under control at high volts

22. The leakage current nightmare • Current through the bulk: No problem • The Problem: Currents on detector surface, around corners and who knows from where..…. • High currents into the readout destroys the electronics, to avoid it we take the following measures: • Capacitively (AC) coupled aluminium readout strips • Guard ring structures around active detector area (connect to ground to suck out current)

23. The nightmare (part II) • Large currents result in high power dissipation and heating of the detector system. • Must be able to control the current, if not fully understood, it should at least be stable with time! • Must test all detectors and detector modules for leakage current.

24. Careful detector design is required!

25. A detail of the detector for ATLAS-SCT (picture from L.G. Johansen, thesis)

26. Leakage currents for some SCT detectors (From L.G.Johansen, thesis (Bergen))

27. The detector modules must be radiation hard • All components tested in a proton beam to a fluence of 3 x 1014 protons/cm2 • This is 50% more than expected for ten years of LHC operation

28. A few words on radiation damage • The two most important effects are: 1: Crystal defects are created in such a way that the effective doping gets more p-like with fluence (dose). Vdep decreases Type inversion Vdep increases 2: Leakage current increases  increase in noise 3: Depletion from ‘below’ n+ doping of back side preserves diode junction

29. Development of leakage current with time L.G. Johansen, thesis

30. Signal/noise of an irradiated detector Plot from L.G. Johansen (of course….)

31. Leakage current doubles for a temperature increase of 7 degrees • ATLAS-SCT will be operated at about -10 degrees C • Detector modules to be in thermal contact with cooling agent.

32. ATLAS-SCT readout electronics • Digital readout (hit/no hit) • Pros and cons.of digital electronics • Rad. Hard. • 128 readout channels per chip.

33. Schematic of the ABCD3T chip

34. The ALTAS SCT module

35. ATLAS-SCT barrel module • 4 detectors • 1 baseboard (Patented TPG solution) • Must be thermally conductive • Hybrid with 12 chips, wraps around the sensor-baseboard. • Strips are bonded together in pairs, to form 12 cm long strips. • About 3000 wire bonds per module

36. A drawing of the module

37. Production of about 2000 modules at 4 university ‘clusters’ around the world • Necessary for efficient use of small resources at each university. • Production clusters: • Japan • US • UK • Scandinavia

38. Equipment needed or developed • Cleanrooms • Tools for precision mounting (motorized jigs etc) • Microscopes • Metrology equipment (‘smart microscope’) • Bonding machines • Setups for electric tests

39. Module production • Detector testing • Glueing of detectors to baseboard (5 um precision) • Testing (IV), metrology • Hybrid testing and glueing • Bonding • Testing

40. To mount to 5 micron precision is not trivial!

41. Noise occupancy must be under control!

42. Some module IV curves (nightmare, part III)

43. But, in the end, the project seems to have been successful • Yield factor OK (above 85 %) • Module mounting on barrels in Oxford • Transfer to CERN OK • Cosmic tests OK • Now, the barrel is in the ATLAS pit to be cabled and tested further……

46. Conclusions • Silicon tracking is very attractive in HEP • But not at all trivial to make…. • Very cost and manpower intensive…