Detectors for particles and radiation Advanced course for Master students

1. Detectors for particles and radiation Advanced course for Master students Spring semester 2010 S7139 5 ECTS points Tuesday 10:15 to 12:00 - Lectures Tuesday 16:15 to 17:00 - Exercises

2. Detectors for particles and radiation

3. Particle-matter interactions: review

4. Particle-matter interactions: review

5. Particle-matter interactions: review

6. - Why use Semiconductor Detectors ?- How are Silicon Detectors made and how do they work ?- Some types of practical design- Radiation Damage in Silicon Detectors- Outlook: Radiation tolerant detectors - References Semiconductor Solid State Detectors

7. Why semiconductors?

8. Why semiconductors?

9. Semiconductor Detectors

10. Semiconductor Detectors

11. Choice of material

12. Semiconductors in periodic table

13. Energy levels in semiconductor

14. Conductivity

15. Semiconductors General

16. Semiconductors General

17. Semiconductors: Silicon

18. Semiconductor Detectors

19. Particle energy loss in Silicon

20. Signal formation in Silicon

21. Silicon doping

22. Doping: p-type Silicon- add elements from IIIrd groupacceptors (B,..) • - holes are the majority carriers • Doping: n-type Silicon • - add elements from Vth group donors (P, As,..) • - electrons are majority carriers e.g. Phosphorus Si Si Si P Si Si Si Si Si • Resistivity - carrier concentrations n, p - carrier mobility mn, mp Doping and resistivity

23. P-N junction

24. P-N junction

25. P-N junction in forward bias

26. P-N junction in reverse bias

27. Depth of the depletion region

28. Depth of the depletion region

29. Poisson’s equation Positive space charge, Neff =[P](ionized Phosphorus atoms) neutral bulk(no electric field) Reverse biased abrupt p+ n junction Electrical charge density Electrical field strength Electron potential energy Full charge collection only for VB>Vdep ! depletion voltage effective space charge density

30. Poisson’s equation w = depletion depthd = detector thicknessU = voltage Neff = effective doping concentration Calculation of depletion voltage (diode) with depletion voltage effective space charge density

31. P-N junction: overview

32. Charge transport

33. Drift velocity in Silicon

34. Charge diffusion in Silicon

35. Energy resolution: Fano factor

36. Fano factor: derivation

37. Fano factor: derivation

38. Silicon detectors: manufacturing

39. Manufacturing of Si monocristals

40. Manufacturing of Si monocristals

41. Float Zone processUsing a single Si crystal seed, melt the vertically oriented rod onto the seed using RF power and “pull” the monocrystalline ingot • Monocrystalline Ingot grind into round shape make the flat or a notch • Produce a polysilicon rodMelt verypure sand (SiO2) together with coke (~1800°C)  Grind the “metallurgical grade silicon” (98% Si) and expose it to hydrochloric gasTrichlorsilane boils at 31.7°C and can thus be distilled and purified Deposit silicon in a Chemical Vapour Deposition process Cast silicon into a polycrystalline silicon rod Poly silicon rod • Wafer productionSlice the ingot into wafers of 300-500 mm (diamond saw) lapping of wafers etching of wafers polishing of wafers Single crystal silicon How to make a Float Zone Silicon wafer?

42. Manufacturing of Si monocristals

43. A "simple“ production sequence (schematic) Polished n-type silicon wafer (typical  ~ 1-10 Kcm )  Oxidation (800-1200°C)  Photolithograpy (coat with photo resist; align mask, expose to UV light, develop photoresist); Etching of oxide  Doping with boron and phosphorus by implantation (or by diffusion) Annealing to cure radiation damage and activate dopants - p+ n junction on front side - n n+ ohmic contact on back side •  Aluminize surface (e.g. by evaporation) •  Pattern metal for diode contacts Silicon Sensor Production

44. Collected Charge for a Minimum Ionizing Particle (MIP) • Mean energy loss dE/dx (Si) = 3.88 MeV/cm 116 keV for 300m thickness • Most probable energy loss≈ 0.7 mean  81 keV • 3.6 eV to create an e-h pair  72 e-h / m (mean)  108 e-h / m (most probable) • Most probable charge (300 m)≈ 22500 e ≈ 3.6 fC Most probable charge ≈ 0.7 mean Mean charge The Charge Signal

45. Charge Readout

46. Landau distribution has a low energy tail - becomes even lower by noise broadening • Noise sources:(ENC = Equivalent Noise Charge) - Capacitance - Leakage Current - Thermal Noise(bias resistor) Signal to noise ratio (S/N) Noise Signal • Good hits selected by requiring NADC > noise tail If cut too high  efficiency loss If cut too low  noise occupancy • Figure of Merit: Signal-to-Noise Ratio S/N • Typical values >10-15, people get nervous below 10. Radiation damage severely degrades the S/N. Cut (threshold)

47. Charge Collection time  Drift velocity of charge carriers v ≈E, so drift time, td = d/v = d/E Typical values: d=300 m, E= 2.5 kV/cm, with e= 1350 cm2 / V·s and h= 450 cm2 / V·s  td(e)= 9ns , td(h)= 27ns • Diffusion •  Diffusion of charge “cloud” caused by scattering of drifting charge carriers, radius of distribution after time td: with diffusion constant •  Same radius for e and h since td  1/ • Typical charge radius:  ≈ 6m, could exploit this to get better position resolution due to charge sharing between adjacent strips (using centroid finding), but need to keep drift times long (low field). Charge Collection time and diffusion

48. Monolithic detectors • readout electronics directly within sensor material (same epi layer) • charge collected at n-well / p-epi diode • thermal diffusion of free charge • reflection at potential barriers between areas with different doping concentration • no depletion voltage applied potential formed by different doping concentrations only MAPS – Monolithic Active Pixel Sensors 15mm • no connections needed to electronics (e.g. no bumps) • very small sizes achievable

49. ~1mm + + + ~300 mm • FET integrated on high resistivity bulk, bulk sideward depleted • electrons collected in potential minimum at internal gate - transistor current modulated by collected charge - charge removed by reset mechanism (clear) • switch on/off by (external) top gate to read out DEPFET - DEP(leted)F(ield)E(ffect)T(ransistor) • amplification of charge at the position of collection  no transfer loss • full bulk sensitivity, bulk can be thinned down to 50 mm if needed • non structured entrance window (backside) • very low imput capacitance  very low noise

50. Limitations of Silicon: radiation damage