Vertex 2004, Villa Vigoni Menaggio – Como, 13 – 18 September 2004 A. Dorokhov et al. 1 Pixel sensors under heavy irradiation A. Dorokhov a, b,*, C. Amsler a, D. Bortoletto c, V. Chiochia a, L. Cremaldi d, S. Cucciarelli e, 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 Vertex 2004 Villa Vigoni Menaggio – Como 13 – 18 September 2004 * Corresponding author, address: Slides are available at 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

Vertex 2004, Villa Vigoni Menaggio – Como, 13 – 18 September 2004 A. Dorokhov et al. 2 Contents  Introduction: pixel detectors  Silicon sensor design for CMS  Test setup, data reconstruction, measurement technique  Lorentz angle measurements  Signal and noise, charge collection efficiency measurements  A new method to measure electric field in heavily irradiated pixel sensor  Summary and outlook

Vertex 2004, Villa Vigoni Menaggio – Como, 13 – 18 September 2004 A. Dorokhov et al. 3 Introduction: pixel detectors Used to provide a precise (~12um CMS Pixel) position for track seeds, primary and secondary vertex reconstruction, particles tagging… operate very close (~4cm CMS Pixel) to the particles interaction point at high luminosity at LHC highly segmented (CMS pixel size 100x150  m (  Z)) 56cm high irradiation dose ~ 3x10 14 N eq /cm 2 per year at full luminosity - change properties during the operation time large number of channels (60 millions CMS Pixel) has to be readout very fast – zero suppressed (only amplitude above a threshold) readout need high signal-to-noise ratio, charge sharing (due to the Lorentz drift in magnetic field) proper design and study the detector properties exposed to heavy irradiation

Vertex 2004, Villa Vigoni Menaggio – Como, 13 – 18 September 2004 A. Dorokhov et al. 4 Silicon sensor designs p-spray p-stop

Vertex 2004, Villa Vigoni Menaggio – Como, 13 – 18 September 2004 A. Dorokhov et al. 5 Beam, Magnetic field Test setup  beam telescope planes, trigger PIN diode and pixel installed between two Helmholtz coils, maximum magnetic field 3T  beam telescope position resolution ~1  m  pixel frame can be tilted, pixel is cooled by the Peltier cooler  full analogue pixel amplitudes readout (no zero suppression)  magnetic field parallel/perpendicular to the pions beam  data collected in June’03, September’03 and August’04 at CERN SPS H2 area Trigger PIN diode (3x6 mm) Pixel Array (22x32) of 125  m x 125  m one pixel size X4,Y4 plane (25  m) X3,Y3 plane (25  m) X2,Y2 plane (25  m) X1,Y plane (2  m)

Vertex 2004, Villa Vigoni Menaggio – Como, 13 – 18 September 2004 A. Dorokhov et al. 6 Data reconstruction  analog amplitudes correction (pedestals, common mode noise subtraction)  event pre-selection using geometry and simple event topology  reconstruction beam entry points  alignment of the pixel with beam telescope After the reconstruction pixel amplitudes (in ADC counts) and beam entry points (in the pixel system coordinates) predicted by the telescope are available for the analysis.

Vertex 2004, Villa Vigoni Menaggio – Como, 13 – 18 September 2004 A. Dorokhov et al. 7 S/H is here Pixel amplitude calibration/cross-calibration By injecting of electrons with calibration pulse to non-irradiated pixel readout chip and comparing the resulting ADC counts with the signal from pions the sample and hold delay time can be measured. Each pixel for each sample is calibrated with the calibration pulse. The readout chip is linear up to about 1 MIP signal.

Vertex 2004, Villa Vigoni Menaggio – Como, 13 – 18 September 2004 A. Dorokhov et al. 8 Measurement of signal-to-noise, charge collection efficiency : beam is perpendicular to the pixel plane Charge collected by single pixel in dependence of particle entry point for non- irradiated and irradiated sensors for both designs.  charge in the centre of pixel decreases after irradiation by ~ 30%, what is design independent property of silicon  the integrated charge depends on the design features

Vertex 2004, Villa Vigoni Menaggio – Como, 13 – 18 September 2004 A. Dorokhov et al. 9 Particles detection efficiency : beam is perpendicular to the pixel plane  better particle detection efficiency(~98%) than p-stop at threshold ~ 5 sigma noise  the inefficiency is less bias voltage dependant  the inefficiency is not increasing very fast with threshold – more safe From particle detection efficiency point, p-spray design has the following advantages over p-stop:

Vertex 2004, Villa Vigoni Menaggio – Como, 13 – 18 September 2004 A. Dorokhov et al. 10 Signal and noise of single pixel p-spray design: signal-to-noise ratio for non-irradiated is ~70, for the heavily irradiated at 8*10 14 n eq /cm 2 S/N ~45 p-stop design: signal-to-noise ratio for non-irradiated is ~30, for the heavily irradiated ~40 – the reason is larger charge spread between pixels for non-irradiated device

Vertex 2004, Villa Vigoni Menaggio – Como, 13 – 18 September 2004 A. Dorokhov et al. 11 Measurement of Lorentz angle and charge cluster profiles : grazing angle technique tan(  L )  tan(  )/sin(  ) Along particle track,  m  the beam enters the pixel at a small angle   the charge drifts in the direction of the Lorentz angle  L  the charge collected by the pixels is deflected by the angle 

Vertex 2004, Villa Vigoni Menaggio – Como, 13 – 18 September 2004 A. Dorokhov et al. 12 Lorentz angle measurements: grazing angle technique, averaged over depth Lorentz angle Averaged Lorentz angle depends mainly on the bias voltage. Since heavily irradiated sensors have to operate at large bias, the Lorentz angle decreases. At 4T magnetic field, after 10*10 14 n eq /cm 2 the Lorentz angle is ~8 degrees.

Vertex 2004, Villa Vigoni Menaggio – Como, 13 – 18 September 2004 A. Dorokhov et al. 13 Charge collection efficiency: beam enters the pixel at  =15 0 p-spray again reveal better charge collection for inclined tracks even after irradiation at 10*10 14 n eq /cm 2

Vertex 2004, Villa Vigoni Menaggio – Como, 13 – 18 September 2004 A. Dorokhov et al. 14 Charge collection efficiency: beam enters the pixel at  =15 0, bias dependency for 0.5*10 14 n eq /cm 2 As it was expected for not heavily irradiated detector the depletion voltage is very small. For 0.5*10 14 n eq /cm 2 irradiation dose at bias ~100V already above 95% charge is collected.

Vertex 2004, Villa Vigoni Menaggio – Como, 13 – 18 September 2004 A. Dorokhov et al. 15 Charge cluster profiles: beam enters the pixel at  =15 0, bias dependency For about 1 year full luminosity at CMS Pixel the irradiation dose is 2*10 14 n eq /cm 2 the bias voltage has to be increased up to 300V. It should be noted, that charge is collected even at low bias voltage from both sides of the detector.

Vertex 2004, Villa Vigoni Menaggio – Como, 13 – 18 September 2004 A. Dorokhov et al. 16 Charge cluster profiles: beam enters the pixel at  =15 0, bias dependency For about first 4 years of LHC at CMS Pixel the irradiation dose is 6*10 14 n eq /cm 2. Again charge is collected even at low bias voltage from both sides of the detector – the assumption that depletion starts from pixel side only is not valid.

Vertex 2004, Villa Vigoni Menaggio – Como, 13 – 18 September 2004 A. Dorokhov et al. 17 Heavily irradiated detectors – a method of electric field measurement There are models predicting non-linear electric filed in heavily irradiated silicon: the main idea that the thermally generated electrons and holes, while drift towards the electrodes, are trapped by the trapping centers (induced by the irradiation damage) in the silicon bulk and form non-uniform space-charge region. The concentration of traps can be measured with standard Deep Level Transient Spectroscopy (DLTS) or Thermally Stimulated Current (TSC) technique. The electric field than can be calculated from Poisson’s equation. Motivation: as it was seen from profiles of charge cluster for the heavily irradiated sensors, the depletion doesn’t start from one side of the silicon bulk, which means that linear electric field approximation and, hence, constant effective dopant concentration assumption doesn’t correctly describe the physics in the heavily irradiated silicon detector. Another direct method of electric field measurement in heavily irradiated silicon pixel sensors is proposed. It is based on the precise measurement of the Lorentz deflection in the sensor bulk. The Lorentz deflection is related to the mobility and the electric field can be calculated, using mobility dependence on electric field formula.

Vertex 2004, Villa Vigoni Menaggio – Como, 13 – 18 September 2004 A. Dorokhov et al. 18 Lorentz angle in dependence on the depth Charge asymmetry, p-spray,Vb=150V The Lorentz deflection in the silicon bulk can be calculated from asymmetry distribution. Asymmetry fitted with Erf function in each vertical slice. From each fitted slice one can determine position where the asymmetry is equal to zero. The position where asymmetry is equal to zero gives the Lorentz deflection in magnetic field in dependence on the depth

Vertex 2004, Villa Vigoni Menaggio – Como, 13 – 18 September 2004 A. Dorokhov et al. 19 Electric field measurement: Mobility extracted from Lorentz deflection tan(  L )(depth)  H  depth)  tan(  L )( depth) determined from Lorentz deflection

Vertex 2004, Villa Vigoni Menaggio – Como, 13 – 18 September 2004 A. Dorokhov et al. 20 Electric field measurement from known mobility   0 [1+(E/Ec)  ] –1/  Where Ec  for electrons and holes are known functions of temperature, one of the standard parametrization used. One can see that for heavily irradiated sensors the electric field is very non- uniform and has U-like shape – that means charge collection starts from both sides Region with geometrical distortion

Vertex 2004, Villa Vigoni Menaggio – Como, 13 – 18 September 2004 A. Dorokhov et al. 21 From measured electric field to simulation of induced charge Very simple simulation to validate measurement of electric filed: flat drift field, but magnitude is taken from measurements. Using measured trapping constants and Ramo theorem one can find induced charge. No diffusion, no delta-rays, no energy deposition spread (Landau) are taken into account. The effective potential is calculated, assuming no gaps between pixels.

Vertex 2004, Villa Vigoni Menaggio – Como, 13 – 18 September 2004 A. Dorokhov et al. 22 From measured electric field to simulation of induced charge The results of the simulation: beam enters the pixel at  =15 0, electrons and holes contribution separately. Important to see, that the charge cluster is mainly formed by the electrons.

Vertex 2004, Villa Vigoni Menaggio – Como, 13 – 18 September 2004 A. Dorokhov et al. 23 From measured electric field to simulation of induced charge The charge cluster simulated at zero magnetic field gives very good agreement with the measured data – all the features of the cluster shape are very similar to the measured ones.

Vertex 2004, Villa Vigoni Menaggio – Como, 13 – 18 September 2004 A. Dorokhov et al. 24 From measured electric field to simulation of induced charge The simulation using measured electric filed gives also good agreement with the measured data in the presence of magnetic field – therefore with the measured electric field it is possible to describe correctly the physical processes in the pixel sensor.

Vertex 2004, Villa Vigoni Menaggio – Como, 13 – 18 September 2004 A. Dorokhov et al. 25 Summary and outlook The sensors reveal sufficient particle detection efficiency(>98%) even after exposed to high irradiation doses (~10 15 n eq /cm -2 ) and at high bias voltage (up to 600V) The Lorentz angle is up to 26 0 (4T) for non-irradiated (Vbias=100V) and (4T) for the irradiated n eq /cm -2 (Vbias=600V) devices Signal-to-noise ratio decreases from ~70 to ~40 after irradiation, charge collection reaches 60% A method of electric field measurement in heavily irradiated silicon pixel sensor is proposed and validated with simple simulation