Techniques for High Performance Solid State Detectors

1. Solid State Detectors- 3 T. Bowcock

2. 1 Position Sensors 2 Principles of Operation of Solid State Detectors 3 Techniques for High Performance Operation 4 Environmental Design 5 Measurement of time 6 New Detector Technologies Schedule

3. Techniques for High Performance Operation • Strip Detectors • Design and Fabrication Issues • What to avoid!

4. Review... • In the p-strip in n-bulk (“p-in-n”) detectors • Vdep=100V • Energy to create electron hole pair is • 3.6eV ( not 1.1eV-why? ) • Average energy lost/mm • 39keV (108eh/ mm) -V Al Si + -

5. Drift • Electric field in Depleted region linear • 300mm detector • at 100V E=3.0keV/cm • Diffusion/Drift by multiple collisions • Takes 7ns for e’s, 20ns for holes Higher diffusion at low temps!

6. Ballistic Deficit Charge lost is known as the ballistic deficit Collection time

7. Strip Pitch and Readout Pitch and resolution • Select it: d Single strip has d/12 d/10

8. Choosing the Pitch • Why not make it infinitely small • transverse diffusion • 10-20 microns • construction • readout electronics! • Readout pitch • not necessarily the same as diode pitch (cost$$$) 75mm readout (25mm diode)

9. Intermediate Strips • Work by capacitive coupling • induced current/charge is that seen by the electrons and holes (not a post-facto charge sharing!) • Why no broader strips ? • Interstrip capacitance <1pF Need field map!

10. Intermediate Strips? • Loose signal • An option if • limited by resources • little noise in electronics (slow e’s) • Optimal choice is • readout each strip • pitch and width evaluated by FEA • pitch between 20 microns and 100 microns

11. Performance 50 mm with intermediate strip 25mm readout

12. Resolution • Test your resolution • series of particles of known position • testbeam telescope • cosmic telescope • longwavelength laser

13. Checking Resolution Optical fiber • Tests • laser • problems? • transparancy • cosmics • slower • testbeam • expensive • labour intensive Focus to 5 mm 1064nm Si transparent

14. Two Track Resolution • Reconstruction position as a function of proximity of one track to another

15. Occupancy • Best to reduce occupancy • 1% considered the benchmark • 10% too high • Reduce the length of strips • usually about 6cm • reduce to 1cm for example

16. AC Coupling Revisited • e=0.34pF/cm • 200nm oxide • 10pF/cm • Greater than Interstrip capacitance • Electronics at ground!

17. Double Sided • Needs AC coupling! • Correlation of signals • Strips can run opposite directions • 2D style r/o -V - - - - + + + + 0V

18. Double Sided Detector • Would like electronics at one end • Can get correlated measurement (E) giving x/y measurement • Reduces fakes • Punchthrough

19. Double Metal • Add another routing layer • more processing via • Expense can double • Built in stresses in SiO2 can warp Si wafer badly

20. Double Metal Can also use to route on single sided detectors

21. Strips

22. Example of Double Metal Detectors • LHCb prototypes

23. Bond Pads • Structure you will often see Typically 80 by 200 microns

24. n-strip detectors • We can make single sided n-strip detectors (note depletion!)

25. Field Plates • MOS structures

26. p-stops • Individual p-stops

27. Operating Voltage • High (overvoltage is desirable) • 250V • reduced ballistic deficit • BUT • introduces very high field regions? • Avalanche will set in if field exceeds 30V/m

28. Analysis of structure

29. Electric Field Sample field map

30. Guard Rings • Reduce fields at edge

31. Micro-Discharges • Discharges may be seen as in increase in the noise with voltage

32. Si Choices • Resistivity • n-type • p-strips • n-strips • double sided • p-type • Crystal orientation

33. Benchmark measures • Charge Collection Efficiency • Partial Depletion • Ballistic Deficit

34. Fabrication • Control of all steps critical • Of special interest • resistor values • implantation • oxide quality for breakdown • quality of lithography

35. Quality Assurance • Job of the physicist is to measure all the key parameters of the detectors • IV and CV • interstrip capacitance • resistor values • lightspot response

36. Readout Chain

37. F/E Electronics • Binary vs Analog • Amplifier Characteristics • rise time and falltime • undershoot • Digital Performance • pipeline & logic • Noise

38. Hybrid Design

39. Noise • Hybrid is often a source of noise • bad grounding for electronics • bad grounding for supplies to detector • sensor,analog and digital all connected • The detector, f/e electronics and the hybrid should be regarded as one unit or MODULE

40. Module and Mounting

41. Material Budget • Ideally should be as low as possible • avoid high mass materials • gold • Good detector about 1% of a radiation length

42. Example: DELPHI barrel

43. Offline Analysis • Can give improvement in resolution w L R Only true if charge uniform and if the width of the cluster matches the strip width In general we have a Gaussian distribution of width determined by the diffusion coefficient (for normal incidence) d x

44. Offline • Corrections for the angle of the track and the known (measured) charge sharing can give great improvement • 20 to 30% in the case of 25 microns pitch detectors • Good software must accompany good hardware • Removal of deltas

45. 7 things to avoid • Picking the wrong technology • Picking the wrong manufacturer($) • Not enough Quality Control • Bad design limiting operation • Noise in system • Treating sensor and hybrid separately • Bad analysis

46. Summary • We have all the elements now to think about real detectors in real environments • design issues • noise problems • See how we design a detector for LHCb