CMS Pixel Simulation Vincenzo Chiochia Quarter pixel: Electrostatic potential Vincenzo Chiochia Physik Institut der Universität Zürich-Irchel CH-8057 Zürich (Switzerland) PIXEL 2005 Workshop Chuzenji Lake, Nikko (Japan) November 7-11, 2005

Outline The CMS pixel detector Radiation damage and impact on reconstruction Beam test data and detailed sensor simulation Physical modelling of radiation damage: Models with a constant space charge density Double-trap models Fluence and temperature dependence V. Chiochia – CMS Pixel Simulation – Vertex 2005 - Chuzenji Lake, Nikko, Japan , November 7-11, 2005

The CMS pixel detector 3-d tracking with about 66 million channels Barrel layers at radii = 4.3cm, 7.2cm and 11.0cm Pixel cell size = 100x150 µm2 704 barrel modules, 96 barrel half modules, 672 endcap modules ~15k front-end chips and ~1m2 of silicon ~1 m 0.3 m V. Chiochia – CMS Pixel Simulation – Vertex 2005 - Chuzenji Lake, Nikko, Japan , November 7-11, 2005

The LHC radiation environment sppinelastic = 80 mb L = 1034 cm-2s-1 4 cm layer F=3x1014 n/cm2/yr Fluence decreases quadratically with the radius Pixel detectors = 4-15 cm mostly pion irradiation Strip detectors = 20-110 cm mostly neutron irradiation What is the sensors response after the first years of operation? Fluence per year at full luminosity V. Chiochia – CMS Pixel Simulation – Vertex 2005 - Chuzenji Lake, Nikko, Japan , November 7-11, 2005

Radiation effects r-f plane r-z plane  B field no B field charge width charge width after irradiation charge charge after irradiation E E  B field no B field Irradiation modifies the electric field profile: varying Lorentz deflection Irradiation causes charge carrier trapping V. Chiochia – CMS Pixel Simulation – Vertex 2005 - Chuzenji Lake, Nikko, Japan , November 7-11, 2005

Prototype sensors for beam tests n-in-n type with moderated p-spray isolation, biasing grid and punch through structures (producer: CiS, Germany) 285 μm thick <111> DOFZ wafer 125x125 mm2 cell size, 22x32 pixel matrix Samples irradiated with 21 GeV protons at the CERN PS facility Fluences: Feq=(0.5, 2.0, 5.9)x1014 neq/cm2 Annealed for three days at +30º C Bump bonded at room temperature to non irradiated front-end chips with non zero-suppressed readout, stored at -20ºC 125 mm2 V. Chiochia – CMS Pixel Simulation – Vertex 2005 - Chuzenji Lake, Nikko, Japan , November 7-11, 2005

Beam test setup Silicon strip beam telescope: 50 μm readout pitch,~1 μm resolution Cooling circuit T =-30 ºC or -10ºC CERN Prevessin site H2 area 3T Helmoltz magnet beam: 150 GeV p B field pixel sensor support Data collected at CERN in 2003-2004 V. Chiochia – CMS Pixel Simulation – Vertex 2005 - Chuzenji Lake, Nikko, Japan , November 7-11, 2005

2 years LHC low luminosity 2 years LHC high luminosity Charge collection measurement Charge collection was measured using cluster profiles in a row of pixels illuminated by a 15º beam and no magnetic field Feq = 5×1013 n/cm2 2 years LHC low luminosity n+side p-side 2 years LHC high luminosity AGEING Feq = 2×1014 n/cm2 Feq = 6×1014 n/cm2 ½ year LHC low luminosity V. Chiochia – CMS Pixel Simulation – Vertex 2005 - Chuzenji Lake, Nikko, Japan , November 7-11, 2005

Detector simulation PIXELAV ISE TCAD 9.0 Double traps models (DESSIS) 3-D Electric field mesh ISE TCAD 9.0 Charge deposit Charge transport Trapping Trapping times from literature Electronic response + data formatting ROC+ADC response ROOT Analysis PIXELAV M.Swartz, Nucl.Instr. Meth. A511, 88 (2003); V.Chiochia, M.Swartz et al., IEEE Trans.Nucl.Sci. 52-4, p.1067 (2005). V. Chiochia – CMS Pixel Simulation – Vertex 2005 - Chuzenji Lake, Nikko, Japan , November 7-11, 2005

The classic picture Neff=ND-NA<0 Based on C-V measurements! after type inversion After irradiation the sensor bulk becomes more acceptor-like The space charge density is constant and negative across the sensor thickness The p-n junction moves to the pixel implants side Sensors may be operated in “partial depletion” Neff=ND-NA<0 Based on C-V measurements! - …is all this really true? V. Chiochia – CMS Pixel Simulation – Vertex 2005 - Chuzenji Lake, Nikko, Japan , November 7-11, 2005

Models with constant Neff F = 6x1014 n/cm2 A model based on a type-inverted device with constant space charge density across the bulk does not describe the measured charge collection profiles V. Chiochia – CMS Pixel Simulation – Vertex 2005 - Chuzenji Lake, Nikko, Japan , November 7-11, 2005

(Shockley-Read-Hall statistics): Two traps models EConduction acceptor EC-0.525 eV Electron traps 1.12 eV donor EV+0.48 eV Hole traps EValence EA/D = trap energy level fixed NA/D = trap densities extracted from fit se/h = trapping cross sections extracted from fit Model parameters (Shockley-Read-Hall statistics): V. Chiochia – CMS Pixel Simulation – Vertex 2005 - Chuzenji Lake, Nikko, Japan , November 7-11, 2005

The double peak electric field a) Current density c) Space charge density n+p junction np+ -HV p-like n-like There are P-N junctions at both sides of the detector detector depth detector depth b) Carrier concentration d) Electric field V. Chiochia – CMS Pixel Simulation – Vertex 2005 - Chuzenji Lake, Nikko, Japan , November 7-11, 2005

Model constraints The two-trap model is constrained by: Comparison with the measured charge collection profiles Signal trapping rates varied within uncertainties Measured dark current Typical fit iteration: (8-12h TCAD) + (8-16h PIXELAV)xVbias + ROOT analysis V. Chiochia – CMS Pixel Simulation – Vertex 2005 - Chuzenji Lake, Nikko, Japan , November 7-11, 2005

Fit results Electric field and space charge density profiles Data --- Simulation F1=6x1014 n/cm2 NA/ND=0.40 sh/se=0.25 V. Chiochia – CMS Pixel Simulation – Vertex 2005 - Chuzenji Lake, Nikko, Japan , November 7-11, 2005

Scaling to lower fluences F3=0.5x1014 n/cm2 NA/ND=0.75 sAh/sAe=0.25 sDh/sDe=1.00 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) V. Chiochia – CMS Pixel Simulation – Vertex 2005 - Chuzenji Lake, Nikko, Japan , November 7-11, 2005

Space charge density Space charge density uniform before irradiation Current conservation and non uniform carrier velocities produce a non linear space charge density after irradiation The electric field peak at the p+ backplane increases with irradiation n-type p-type sensor depth (mm) V.Chiochia, M.Swartz,et al., physics/0506228 V. Chiochia – CMS Pixel Simulation – Vertex 2005 - Chuzenji Lake, Nikko, Japan , November 7-11, 2005

Temperature dependence F2=2x1014 n/cm2 T=-25º C Comparison with data collected at lower temperature T=-25º C. Use temperature dependent variables: recombination in TCAD variables in PIXELAV (m, D, G) The double-trap model is predictive! F1=6x1014 n/cm2 T=-25º C V.Chiochia, M.Swartz,et al., physics/0510040 V. Chiochia – CMS Pixel Simulation – Vertex 2005 - Chuzenji Lake, Nikko, Japan , November 7-11, 2005

Lorentz deflection tan() linear in the carrier mobility (E): Switching on the magnetic field tan() linear in the carrier mobility (E): LHC startup 2 years LHC low luminosity 2 years LHC high luminosity The Lorentz angle can vary a factor of 3 after heavy irradiation: This introduces strong non-linearity in charge sharing V. Chiochia – CMS Pixel Simulation – Vertex 2005 - Chuzenji Lake, Nikko, Japan , November 7-11, 2005

Conclusions and plans After heavy irradiation trapping of the leakage current produces electric field profiles with two maxima at the detector implants. The space charge density across the sensor is not uniform. What is the meaning of Vdep, depletion depth and type inversion? Measurements reflecting the electric field profile (e.g. TCT, CCE, long clusters etc.) are preferable to C-V characterization to understand radiation damage in running detectors A physical model based on two defect levels can describe the charge collection profiles measured with irradiated pixel sensors in the whole range of irradiation fluences relevant to LHC operation Our model is an effective theory: e.g. in reality there are several trap levels in the silicon band gap after irradiation. However, it is suited for applications related to silicon detector operation at LHC. We are currently using the PIXELAV simulation to develop hit reconstruction algorithms optimized for irradiated pixel sensors. V. Chiochia – CMS Pixel Simulation – Vertex 2005 - Chuzenji Lake, Nikko, Japan , November 7-11, 2005

References PIXELAV simulation: Double-trap model: M.Swartz, “CMS Pixel simulations”, Nucl.Instr.Meth. A511, 88 (2003) Double-trap model: V.Chiochia, M.Swartz et al., “Simulation of Heavily Irradiated Silicon Pixel Sensors and Comparison with Test Beam Measurements”, IEEE Trans.Nucl.Sci. 52-4, p.1067 (2005), eprint:physics/0411143 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) Model fluence and temperature dependence: V.Chiochia, M.Swartz et al., “A double junction model of irradiated pixel sensors for LHC”, submitted to Nucl. Instr. Meth., eprint:physics/0506228 V.Chiochia, M.Swartz et al., “Observation, modeling, and temperature dependence of doubly-peaked electric fields in silicon pixel sensors”, submitted to Nucl. Instr. Meth., eprint:physics/0510040 V. Chiochia – CMS Pixel Simulation – Vertex 2005 - Chuzenji Lake, Nikko, Japan , November 7-11, 2005

Backup slides

EVL models F1=6x1014 n/cm2 100% observed leakage current s=1.5x10-15 cm2 30% observed leakage current s=0.5x10-15 cm2 The EVL model based on double traps can produce large tails but description of the data is still unsatisfactory V. Chiochia – CMS Pixel Simulation – Vertex 2005 - Chuzenji Lake, Nikko, Japan , November 7-11, 2005

E-field profiles Constant space charge density Double trap model V. Chiochia – CMS Pixel Simulation – Vertex 2005 - Chuzenji Lake, Nikko, Japan , November 7-11, 2005

Scaling to lower fluences Preserve linear scaling of Ge/h and of the current with Feq F2=2x1014 n/cm2 NA/ND=0.68 sAh/sAe=0.25 sDh/sDe=1.00 ! Not shown: Linear scaling of trap densities does not describe the data! V. Chiochia – CMS Pixel Simulation – Vertex 2005 - Chuzenji Lake, Nikko, Japan , November 7-11, 2005

Temperature dependence F1=6x1014 n/cm2 T=-10ºC T=-25ºC Charge collection profiles depend on temperature T-dependent recombination in TCAD and T-dependent variables in PIXELAV (me/h, Ge/h, ve/h) The model can predict the variation of charge collection due to the temperature change V. Chiochia – CMS Pixel Simulation – Vertex 2005 - Chuzenji Lake, Nikko, Japan , November 7-11, 2005

Lorentz deflection Lorentz angle Electric field V. Chiochia – CMS Pixel Simulation – Vertex 2005 - Chuzenji Lake, Nikko, Japan , November 7-11, 2005

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 V. Chiochia – CMS Pixel Simulation – Vertex 2005 - Chuzenji Lake, Nikko, Japan , November 7-11, 2005

ISE TCAD simulation V. Chiochia – CMS Pixel Simulation – Vertex 2005 - Chuzenji Lake, Nikko, Japan , November 7-11, 2005

PIXELAV simulation V. Chiochia – CMS Pixel Simulation – Vertex 2005 - Chuzenji Lake, Nikko, Japan , November 7-11, 2005

SRH statistics V. Chiochia – CMS Pixel Simulation – Vertex 2005 - Chuzenji Lake, Nikko, Japan , November 7-11, 2005

SRH generation current V. Chiochia – CMS Pixel Simulation – Vertex 2005 - Chuzenji Lake, Nikko, Japan , November 7-11, 2005

Test beam setup  Magnetic field = 3 T pixel sensor or 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 V. Chiochia – CMS Pixel Simulation – Vertex 2005 - Chuzenji Lake, Nikko, Japan , November 7-11, 2005