1. Charge collection in irradiated pixel sensors V. Chiochia a, C.Amsler a, D.Bortoletto c, L.Cremaldi d, S.Cucciarelli e, A.Dorokhov a,b*, C.Hörmann a,b, M.Konecki e, D.Kotlinski b, K.Prokofiev a,b, C.Regenfus a, T.Rohe b, D.Sanders d, S.Son c, T.Speer a, D.Kim f, M.Swartz f a Physik Institut der Universität Zürich-Irchel, 8057, Zürich, Switzerland b Paul Scherrer Institut, 5232, Villingen PSI, Switzerland c Purdue University, Task G, West Lafayette, IN 47907, USA d Department of Physics and Astronomy, Mississippi State University, MS 39762, USA e Institut für Physik der Universität Basel, Basel, Switzerland f Johns Hopkins University, Baltimore, MD, USA * Now at: Institut de Recherches Subatomiques, F67037 Strasbourg, France Beam test measurements and simulation

2. V. Chiochia – Charge collection in irradiated pixel sensors 10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005 2Outline 1.The CMS pixel detector and data reconstruction 2.Analysis ingredients: beam test data and detector simulation 3.Physical modelling of radiation damage: a)Models with a constant effective doping concentration b)EVL models (V.Eremin, E.Verbitskaya, Z.Li) c)Advanced double junction models (V.C., M.Swartz) 4.Conclusions

3. V. Chiochia – Charge collection in irradiated pixel sensors 10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005 3 The CMS pixel detector 3-d tracking with 66 million channels Barrel layers at radii = 4.3cm, 7.2cm and 11.0cm Pixel cell size: 100x150 µm 2 Fluence 3(1)x10 14 n eq /cm 2 year, inner layer for high(low) luminosity Modules are unit cells of the system (1% of X 0 ) 704 barrel modules / 96 barrel half modules / 672 endcap modules ~15k front end chips and ~1m 2 of silicon

4. V. Chiochia – Charge collection in irradiated pixel sensors 10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005 4 The LHC radiation environment Fluence per year at full luminosity 4 cm layer  3x10 14 n/cm 2 /yr Fluence decreases quadratically with the radius Pixel detectors = 4-15 cm mostly pion irradiation Strip detectors = 20-110 cm mostly neutron irradiation  pp inelastic = 80 mb L = 10 34 cm -2 s -1  pp inelastic = 80 mb L = 10 34 cm -2 s -1 What is the sensors response after few years of operation?

5. V. Chiochia – Charge collection in irradiated pixel sensors 10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005 5 Impact on reconstruction Sensor irradiation Charge Carriers Trapping Asymmetric pixel clusters Example: Long clusters along the z-coordinate at high  Variation of the electric field profile Lorentz deflection Example: Non-linear charge sharing in the r-  plane

6. V. Chiochia – Charge collection in irradiated pixel sensors 10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005 6 Prototype sensors for beam tests p-spray design with biasing grid and punch through structures (CiS, Germany)p-spray design with biasing grid and punch through structures (CiS, Germany) 125x125  m 2 cell size125x125  m 2 cell size 22x32 pixel matrix, 285 μm thick DOFZ wafer, n-in-n type22x32 pixel matrix, 285 μm thick DOFZ wafer, n-in-n type Samples irradiated with 21 GeV protons at the CERN PS facilitySamples irradiated with 21 GeV protons at the CERN PS facility Fluences:  eq =(0.47,2.0,5.9)x10 14 n eq /cm 2Fluences:  eq =(0.47,2.0,5.9)x10 14 n eq /cm 2 Annealed for three days at 30º CAnnealed for three days at 30º C Bump bonded at room temperature to non irradiated front-end chips with non zero- suppressed readout, stored at -20ºCBump bonded at room temperature to non irradiated front-end chips with non zero- suppressed readout, stored at -20ºC 125  m 2

7. V. Chiochia – Charge collection in irradiated pixel sensors 10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005 7 2004 Beam test setup CERN Prevessin site H2 area 3T Helmoltz magnet beam: 150 GeV  B field pixel sensor support Silicon strip beam telescope: 50 μm readout pitch,~1 μm resolution Cooling circuit T =-30 ºC or -10ºC

8. V. Chiochia – Charge collection in irradiated pixel sensors 10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005 8 Charge collection measurement Charge collection was studied with the cluster profiles in a row of pixels illuminated by a 15º beam and no magnetic field ½ year LHC low luminosity 2 years LHC low luminosity n + sidep-side charge trapping charge trapping 2 years LHC high luminosity Temperature = -25 ºC and -10ºC  eq = (0, 0.5, 2, 6)x10 14 n/cm 2

9. V. Chiochia – Charge collection in irradiated pixel sensors 10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005 9 Detector simulation Charge deposit Charge transport Trapping Trapping times from literature Electronic response + data formatting ROC+FED response ROOT Analysis Double traps models (DESSIS) 3-D Electric field mesh ISE TCAD 9.0 PIXELAV

10. V. Chiochia – Charge collection in irradiated pixel sensors 10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005 10 The classic picture After irradiation the sensor bulk becomes more acceptor-like The effective doping concentration is constant (and negative) across the sensor thickness The p-n junction moves to the pixel implants side Based on C-V measurements! after type inversion N eff <0 -

11. V. Chiochia – Charge collection in irradiated pixel sensors 10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005 11 Models with constant N eff A model based on a type-inverted device with constant N eff across the bulk does not describe the measured charge collection profiles  = 6x10 14 n/cm 2

12. V. Chiochia – Charge collection in irradiated pixel sensors 10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005 12 Two traps models E Conduction E Valence Electron traps Hole traps Eremin-Verbitskaya-Li Model (EVL) N A and N D are fixed to TCT measurements 1.12 eV donor E V +0.48 eV acceptorE C -0.525 eV Given these parameters the charge carriers dynamics is governed by the Shockley-Read-Hall statistics

13. V. Chiochia – Charge collection in irradiated pixel sensors 10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005 13 The double peak electric field n+p junction np+ junction -HV a) Current density b) Carrier concentration c) Effective doping concentration d) Electric field p-like n-like

14. V. Chiochia – Charge collection in irradiated pixel sensors 10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005 14 EVL models 100% observed leakage current  =1.5x10 -15 cm 2 30% observed leakage current  =0.5x10 -15 cm 2 The EVL model based on double traps can produce large tails but description of the data is still unsatisfactory  1 =6x10 14 n/cm 2

15. V. Chiochia – Charge collection in irradiated pixel sensors 10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005 15 Advanced EVL models The recipe: 1.Relax the assumption on the cross sections 2.Let the parameters (N A, N D,  A/D e,  A/D h ) vary 3.Keep the traps energy levels (E A, E D ) to the EVL values Constraints to the model: 1.Charge collection profiles (at different V bias and  eq ) 2.Trapping rates 3.Generated leakage current  e/h from literature  eq known within 10%  e/h from literature  eq known within 10%

16. V. Chiochia – Charge collection in irradiated pixel sensors 10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005 16 Best fit:  1 =6x10 14 n/cm 2  Data --- Simulation  Data --- Simulation  1 =6x10 14 n/cm 2 N A /N D =0.40  h /  e =0.25  1 =6x10 14 n/cm 2 N A /N D =0.40  h /  e =0.25

17. V. Chiochia – Charge collection in irradiated pixel sensors 10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005 17 Temperature dependence Charge collection profiles depend on temperature T-dependent recombination in TCAD and T-dependent variables in PIXELAV (  e/h,  e/h, v e/h ) The model can predict the variation of charge collection due to the temperature change T=-10ºC T=-25ºC  1 =6x10 14 n/cm 2

18. V. Chiochia – Charge collection in irradiated pixel sensors 10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005 18 Scaling to lower fluences (1)  2 =2x10 14 n/cm 2 N A /N D =0.68   h /   e =0.25  D h /  D e =1.00  2 =2x10 14 n/cm 2 N A /N D =0.68   h /   e =0.25  D h /  D e =1.00 Preserve linear scaling of  e/h and of the current with  eq Not shown: Linear scaling of trap densities does not describe the data! !

19. V. Chiochia – Charge collection in irradiated pixel sensors 10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005 19 Scaling to lower fluences (2) Near the ‘type-invesion’ point: the double peak structure is still visible in the data! Profiles are not described by thermodynamically ionized acceptors alone At these low bias voltages the drift times are comparable to the preamp shaping time (simulation may be not reliable)  3 =0.5x10 14 n/cm 2 N A /N D =0.75   h /   e =0.25  D h /  D e =1.00  3 =0.5x10 14 n/cm 2 N A /N D =0.75   h /   e =0.25  D h /  D e =1.00

20. V. Chiochia – Charge collection in irradiated pixel sensors 10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005 20 Scaling summary Donors concentration increases faster than acceptors N A /N D increases for decreasing fluences Electric field peak at the p+ backplane increases with irradiation

21. V. Chiochia – Charge collection in irradiated pixel sensors 10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005 21 Lorentz angle vs depth Lorentz angle and electric field extracted from the test beam measurements The Lorentz angle is not constant across the sensor thickness Lorentz angleElectric field

22. V. Chiochia – Charge collection in irradiated pixel sensors 10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005 22 Conclusions (1) 1.A simulation based on a constant effective doping (or “type inverted”) across the sensor bulk does not describe the measured charge collection profiles 2.A effective model based on two defect levels can be tuned to describe the observed charge collection profiles 3.Trapping of the leakage current produces an electric field profile with two maxima at the detector implants. Is it time to leave the classical notion of ‘partial depletion’? 4.The model can: account for the expected leakage current and, within the uncertainties, for free carriers trapping predict the temperature dependence of charge collection

23. V. Chiochia – Charge collection in irradiated pixel sensors 10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005 23 Conclusions (2) 5.In reality the chemistry of Si defects is more complicated and there are several trap states 6.The levels in this model have no physical reality and have to be considered as an ‘effective sum’ of multiple charged states 7.The simulation is a very nice tool for predicting the behavior of our pixel sensors during the operation in CMS. The hit reconstruction algorithms need to be fine tuned to cope with radiation effects

24. V. Chiochia – Charge collection in irradiated pixel sensors 10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005 24References V. Eremin, E. Verbitskaya, and Z. Li, “The origin of double peak electric field distribution in heavily irradiated silicon detectors”, Nucl. Instr. Meth. A476, pp. 556- 564 (2002)V. Eremin, E. Verbitskaya, and Z. Li, “The origin of double peak electric field distribution in heavily irradiated silicon detectors”, Nucl. Instr. Meth. A476, pp. 556- 564 (2002) M.Swartz, “CMS Pixel simulations”, Nucl.Instr.Meth. A511, 88 (2003)M.Swartz, “CMS Pixel simulations”, Nucl.Instr.Meth. A511, 88 (2003) V.Chiochia, M.Swartz et al., “Simulation of Heavily Irradiated Silicon Pixel Sensors and Comparison with Test Beam Measurements”, accepted for publication on IEEE Trans.Nucl.Sci., eprint:physics/0411143V.Chiochia, M.Swartz et al., “Simulation of Heavily Irradiated Silicon Pixel Sensors and Comparison with Test Beam Measurements”, accepted for publication on IEEE Trans.Nucl.Sci., eprint:physics/0411143 A.Dorokhov et al.,A.Dorokhov et al., ISE TCAD 9.0: http://www.synopsys.com/products/acmgr/ise/dessis_ds.html

25. V. Chiochia – Charge collection in irradiated pixel sensors 10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005 25 Backup slides

26. V. Chiochia – Charge collection in irradiated pixel sensors 10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005 26 CMS pixel sensor design n+ p+ n- bulk Si 3 N 4 passivation Si 3 N 4 Al nitride Indium-Bump p+ pspray Al nitride + LTO Gold Nickel Titanium punch-through biasing Bump-bond contact n+/Al opening cross section metal line Vendor: CiS, Erfurt - www.cismst.de

27. V. Chiochia – Charge collection in irradiated pixel sensors 10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005 27 Test beam setup Magnetic field = 3 T or pixel sensor Four modules of silicon strip detectors Beam telescope resolution ~ 1  m Sensors enclosed in a water cooled box (down to -30ºC) No zero suppression, unirradiated readout chip Setup placed in a 3T Helmoltz magnet  PIN diode trigger 

28. V. Chiochia – Charge collection in irradiated pixel sensors 10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005 28 ISE TCAD simulation

29. V. Chiochia – Charge collection in irradiated pixel sensors 10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005 29 PIXELAV simulation

30. V. Chiochia – Charge collection in irradiated pixel sensors 10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005 30 SRH statistics

31. V. Chiochia – Charge collection in irradiated pixel sensors 10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005 31 SRH generation current

32. V. Chiochia – Charge collection in irradiated pixel sensors 10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005 32 Lorentz angle vs bias ‘Effective’ Lorentz angle as function of the bias voltage Strong dependence with the bias voltage (electric field) Weak dependence on irradiation This is a simplified picture!! Magnetic field = 4 T