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

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6.3.1 Results of the Bias ScanLeakage Current Figure 6.7 shows the result of the leakage current measurementsfor both FBK devices. The leakage current is measured at three di erenttemperatures: ≠10°C, ≠20°C and ≠30°C. The current limit is set to 100 µA toprotect the sensors from damage.The 2EM6 detector shows an early single-pixel breakdown (see figure 6.6).This pixel su ers from higher leakage current and increased noise. Pixels inthe vicinity of that pixel are also a ected by the breakdown and show an alsoincreased noise .Single-pixel breakdowns have also been observed on other 3D devices but isnot yet understood [49].Noise Figure 6.8 shows the mean noise for the normal, long, ganged and intergangedpixels depending on the reverse bias voltage. The noise was measured at≠10°C. In case of the 2EM6, a step in the noisecurve at 60 V was measured. Thestep has also been observed on other FBK 3D devices but is not yet understood.MonLeak mod 0 chip 0Row1401201008060402000 2 4 6 8 10 12 14 16Column(a)5045403530252015105Noise mod 0 chip 0Row1401201008060402000 2 4 6 8 10 12 14 16Column(b)500450400350300250200150Figure 6.6: Single-pixel breakdown of the 2EM6 detector at 106 V. (a) MonleakScan: The circle denoted the point of origin of the breakdown. (b) The pixelsin the vicinity are also a ected. They su er from increased noise.72
Sensor 2EM6 4EM9Calculated MPV [e ≠ ] 15111 ± 805 15111 ± 805MeasuredqMPV [e ≠ ] 10070 10540E ciency 66.6% 69.8%1PixelE ciency 64.2% 67.4%2PixelE ciency 73.9% 77.0%3PixelE ciency 88.4% 99.8%Table 6.3: Average charge collection e ciency and cluster size dependent chargecollection e ciency of 3D devices: 2EM6 at 105 V and 4EM9 at65 V. The mostprobable value of electron-hole pairs is calculated according to equation 3.3.The error on the calculated MPV is calculated by assuming an error of 10 µmon the bulk thickness. The measured most probable value includes all clustersizes.6.3.2 Results of the Source ScanFigure 6.9 shows the results of the Source Scan for the FBK 3D devices at ≠30°C.This temperature was chosen to reduce disturbing e ects of the leakage current(fluctuations at higher temperatures). It is evident that the MPV stronglydepends on reverse bias voltage and pixel cluster size. Even at higher voltagesthe MPV is not constant and still tends to increase for higher voltages. Theupper voltage is limited by the breakdown of the sensors which appears at lowerbias voltages than suggested in figure 6.7. The breakdown (2EM6 at 106 V,4EM9 at 66 V) is characterized by an abrupt increase of noise hit rate that isdistributed over the whole pixel matrix. The noise hits tend to have a lower ToT(below MPV) but the large amount of noise hits make the analysis impossible.The unexpected shape of the 3-pixel and 2-pixel at lower bias voltages is ane ect of low statistics. There the MPV of the “Langau” distribution tends toshift to higher values.Table 6.3 shows the charge collection e ciency of the irradiated FBK 3Ddevices. The overall charge collection e ciency is 67% and 70% for the 2EM6and 3EM9 device respectively. Again, a bigger pixel cluster size leads to highercharge collection e ciency as stated in section 6.2.2 for planar devices. But thedi erence between 3-pixel cluster size and all other cluster sizes is more evident:in case of the 4EM9 almost full charge collection e ciency can be achieved.But one has to take into account that the 3D modules were annealed afterirradiation. The annealing time can be approximated from 5 to 7 days at roomtemperature. The modules take advantage of the decreased charge trapping.73
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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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(a) (b)Figure 3.4: On top view of t
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Jens Janssen Diploma Thesis - Prof. Dr. Norbert Wermes ...

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