Jens Janssen Diploma Thesis - Prof. Dr. Norbert Wermes ...

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6.2 Unirradiated Planar SensorsThe ATLAS sensor consists of a 256 µm thick oxygenated n-bulk. On the readoutside, there are n + -type pixel implants separated by moderated p-sprays. Ap + -type implant covers almost the entire backside forms a p-n junction. Thedepletion starts to grow from the backside before type inversion. After irradiationinduced type inversion of the bulk the depletion starts to grow from thesegmented (or pixel) side and the behavior is similar to unirradiated n + -on-pdetectors. To protect the pixels side of high voltage 16 guard rings are locatedon the back side [30].Figure 6.1 shows the layout of a single pixel cell of the WR13 and W11sensor. The oxygenated substrate is of 300 µm thickness. The sensors are ofn + -on-p-type and the n + -type pixel implants are separated by p-sprays. TheWR13 has moderated and the W11 has homogeneous p-spays. In contrast tothe ATLAS sensor the 19 guard rings are on the readout side. To reduce therisk of a spark between sensor and readout chip, the surface is coated by a thinlayer of Benzocyclobutene (BCB).All three sensors are tuned at 20°C and at a reverse bias voltage of 150 V.The results of the tuning process and the tuning parameters are listed in Appendix5.5.Due to the mistakenly rotated sensor, not all bump bonds are connectedproperly. Therefore the W11 detector shows a higher noise in rows 0–3. Thepixel of the uppermost two rows (158–159, ganged and inter-ganged pixels) arenot connected properly. The pixel 10/112 6 shows a higher threshold but thisdoes not a ect the measurements so it stays unmasked.All three detectors show a comparable tuning performance. A thresholddispersion of approximately 22 e ≠ was measure for every device. The noise fornormal pixels is in case of the n-on-p devices of approximately 190 e ≠ and forthe ATLAS device 180 e ≠ . The results of the ToT Verification Scans show alsocomparable performance. A ToT dispersion of about 0.6 ToT was possible (atan injected charge of 20, 000 e ≠ ) with all three detectors. The ToT dispersionstrongly depends on the preamplifier feedback current. The IFDAC and TrimIFtuning parameters were set manually to find the optimum value.The ATLAS n-on-n device needs a minimum reverse-bias voltage of 60 Vto work properly. The depletion zone grows from the backside and below thatvoltage the pixels cells are shorted (ohmic n + -n contact). The pixel cells becomeisolated once the depletion voltage is exceeded (depletion zone reached the pixelside).Figure 6.1: Structure of a pixel cell of a CiS n-on-p sensor. The metal layer isshown in pink, the n-type implantation in green, the opening in the nitride andoxide in blue and the opening in the passivation layer in red. The bulk is ofp-type [32].6 In the following column and row are denoted as column/row.66
Figure 6.2: Leakage currents of the planar devices at 20°C.6.2.1 Results of the Bias ScanLeakage Current Figure 6.2 shows the leakage current of the planar pixelsensors. At 250 V the leakage current is below 0.04 µA for the ATLAS device.At the same bias voltage, the W11 and WR13 devices have a 15 (0.55 µA) and18 (0.69 µA) times higher leakage current.Threshold and Noise Figure 6.3 shows the mean noise for the normal, long,ganged and interganged pixels depending on the reverse bias voltage. In caseof the W11 device the ganged and interganged cannot be plotted due to thereasons stated above. The noise drops constantly with increasing the reversebias voltage. For voltages below the depletion voltage the noise increases rapidlydue to the increased detector capacitance. At 250 V the mean noise for normalpixels is 167 e ≠ for the ATLAS device, 186 e ≠ for the W11 device and 183 e ≠for the W11 device. Both n-on-p devices behave very much the same. Clearlyvisible is a step in the noise for the long pixels for both n-on-p devices which ismost likely a geometric e ect of the electric field in this region of the sensor.6.2.2 Results of the Source ScanFigure 6.4 shows the most probable value (MPV) for di erent pixel clustersizes depending on the reverse-bias voltage. The ATLAS device almost reachesmaximum charge collection close to the depletion voltage (60 V). Both n-onpdevices are fully depleted at approximately 50 V. Below depletion voltagethe charge collection decreases rapidly. Above depletion voltage the chargecollection stays almost constant.Table 6.2 summarizes the results of the Source Scans. The maximum possibleoverall charge collection e ciency is between 76% and 81% whereas the CiS n-on-p devices show a slightly better performance.For the MPV an error of 10% is estimated (not drawn), due to the uncertaintiesof the ToT Calibration (see section 5.9.3). Another source of uncertaintyarises from the fact the charge below the threshold of pixels in a cluster cannotbe detected. In case of charge sharing between adjacent pixel cells it can happen67
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Universität BonnPhysikalisches Ins
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Chapter 1IntroductionThe ATLAS expe
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Chapter 2The LHC Experiment2.1 The
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Figure 2.3: Cut-away view of the in
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The electromagnetic calorimeter use
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Chapter 3Silicon Pixel Detectors3.1
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Figure 3.2: Layout and interconnect
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Jens Janssen Diploma Thesis - Prof. Dr. Norbert Wermes ...

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