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

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Figure 3.14: A 3D n-in-p sensor (left) in comparison to a planar n-on-n sensor(right) after type inversion. In case of a 3D sensor, the charge, which is generatedby a traversing ionizing particle, is collected by a much shorter length. Theresult is a faster signals and a much lower depletion voltage. The dashed linesshow how the depletion region grows from the n + -implants, when a reverse biasvoltage is applied [28].Planar Sensors The planar technology is the most commonly used and preferredtechnology for vertex and tracking detectors. The electrical and mechanicalproperties of planar sensors are very well understood and the manufacturingis relatively low cost and has high yield. Compared to the other sensor technologies,planar sensors require the lowest operating temperature and a highbias voltage. This is because of its geometry: The electrodes are located on topand bottom of the bulk. For full charge collection, the depletion zone has tobe extended throughout the whole bulk (¥ 250 µm). According to formula 3.6and Ramo’s Theorem (formula 3.8) a high voltage has to be applied and chargecarriers have to travel long distances to generate an adequate signal.If one assumes negative charge collection at the readout electrode (due tolimitations of the CSA), two di erent approaches for the sensor design are possible(see also figure 3.15):n-on-n This type is used in the current Atlas pixel detector. Before type inversionthe depletion region starts growing from the high voltage backsideelectrode (p + -type) into the n-bulk. The potential drop towards the cuttingedge is ensured by a multi guard ring 17 structure. For full chargecollection, unirradiated n-on-n sensors have to be operated fully depletedor over-depleted. P-stop or p-spray implant are needed to isolate the n + -pixel implants from each other due to electron accumulation beneath theSi-SiO 2 junction. After type inversion (now “p”-bulk) the depletion zonestarts growing from the n-pixel implants. To be able to collect chargenegative carriers the detector does not need to be operated fully depletedafter type inversion.n-on-p In contrast to the n-on-n type sensors, the p-bulk of the n-on-p typesensors does not change its type. Independent of the fluence, the depletionzone always starts to grow from the pixel side. Therefore n-on-p-type sensorssu ce to having only a homogeneous p + -implant on the high voltage17 Guard rings are biased by a punchthrough mechanism. A punchthrough current flowsthrough the high field region at the cutting edge of the sensor, and creates a potential dropthrough the di erent guard rings [30].30
p-on-n typen-on-n typeguard rings p + -type pixel implantsn + -type pixel implantsbefore type inversionn-on-p typen + -type pixel implantsguard ringsdepletedn-substratedepleteddepletedp-substraten + -type backside guard rings p + -type backside p + -type backsideafter type inversiondepleted“p”-substratedepleted(no type inversion)Figure 3.15: Comparison between basic planar sensor concepts: p-on-n, n-onnand n-on-p type from left to right, respectively. Both, the p-on-n and then-on-n type, have initially an n-bulk. After a certain fluence (several times10 12 n eq /cm ≠2 ), the bulk becomes p-like. The p-on-n type sensor is the onlysensor collecting holes at the pixel implants. It is mentioned for the sake ofcompleteness and will not be addressed any further [29].backside. Having structured implants (namely n + -pixel and p-stop/sprayimplants) only on one side makes this sensor cost-e ective [31]. In addition,the sensor can easily be thinned down. This reduces the materialbudget and improves tracking performance. A major drawback are possibledischarges at the cutting edge of the sensor into the readout chip[32]. Although the sensor is biased from the back side the potential drophappens at the front side. A multi guard ring structure on the pixel sideensures the potential drop towards the pixel implants.3D Sensors 3D sensors are the most advanced technology concerning radiationhard silicon detectors. It also requires most advanced process technologywhich is responsible for the higher costs compared to planar sensors. Cylindricalelectrodes are implemented perpendicular to the sensor surface. Implantingthe electrodes makes this sensor cost-intensive. 3D sensors require the lowestbias voltage by preserving the full bulk thickness and are operable at intermediatetemperatures. This is because of the short distance between the electrodes(Æ 100 µm). Due to the geometry of electrodes, the major drawback is the higherdetector capacitance which leads to a higher noise and thus reduces S/N ratio.Another drawback is the reduced hit detection probability and charge collectione ciency for normal incidence. This only a ects ionizing particles traversingthe sensor electrodes. The detector recover full e ciency under a certain incidentangle. Another possibility is the implementation of the electrodes under acertain angle into the bulk.Two di erent design concepts are proposed for the IBL upgrade [27]: Singlesided(“full-3D”) with active edges and double-sided 3D sensors. The activeedge technology allows a minimum of dead area at the edge of the sensor (a31
Page 1: Universität BonnPhysikalisches Ins
Page 7 and 8: Chapter 1IntroductionThe ATLAS expe
Page 9 and 10: Chapter 2The LHC Experiment2.1 The
Page 11 and 12: Figure 2.3: Cut-away view of the in
Page 13 and 14: The electromagnetic calorimeter use
Page 15 and 16: Chapter 3Silicon Pixel Detectors3.1
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Page 28 and 29: Stopping power [MeV cm 2 /g]100101L
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Appendix BFPGA ConfigurationRegiste
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[13] Peric, I. et al.: The FEI3 rea
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[37] Backhaus, M. et al.: Developme
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Jens Janssen Diploma Thesis - Prof. Dr. Norbert Wermes ...

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