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

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The first point particularly concerns irradiated sensors: the mobility µ and thelifetime · 11 of free charge carriers is reduced and the result is a smaller signalcharge.3.4.2 Radiation Damage in SiliconHigh energy particles, which traverse the sensor material, not only transferenergy to the valence electrons crystal lattice atoms, but also to the crystallattice itself. The impact can be severe, so that atoms are displaced from theirlattice sites. The displacement leads to point defects such as interstitials (atomsin between lattice sites) or vacancies (empty lattice sites). If the impact is strongenough, defect clusters (conglomerates) may occur. Also possible are directnuclear interactions, which lead to nuclear transmutation, and thus possibly toactivation.Such externally generated defects disturb the thermodynamic equilibrium.Most of these defects are mobile and attempt to attain thermodynamic equilibrium,e. g. point defects can annihilate or di use out of the surface. But it canbe shown that also stable defects are formed: defects interact with each otherand form new type of defects which can become immobile. Some of the defectsare intentional such as dopants and oxygen but they can form a complex withunwanted defects, thereby losing their former functionality. This will result in achange of macroscopic (i. e. electrical) properties of the crystal. Especially thecapturing dopants, e. g. phosphorus and boron for silicon, lead to the so-callede ect of type inversion 12 : Initially n-typed silicon becomes e ectively 13 intrinsicafter a fluence of a few times 14 10 12 n eq /cm 2 (NIEL 15 ) [22]. Above this point,it becomes e ectively p-type (see figure 3.13).Many defects in the crystal, as stated above, diminish over time and henceall negative e ects, which a ects the sensor characteristics, will degrade in time.The speed of this process, which is called beneficial annealing, depends stronglyon the temperature. In case of silicon, this process slows down in time. At somepoint it reverses and a increase of the e ective doping change take place. Thereason are former inactive defects which after a time become electrically active.This disadvantageous e ect is called reverse annealing. The whole process canbe slowed down by cooling down the sensor material.The annealing and the following degradation of the detector can be controlledby the temperature at which the detector is kept during the maintenanceperiods.Oxygen is used to radiation-harden silicon (oxygenated silicon). Oxygen hasbeen found capturing silicon vacancies which leads to a decreased post-radiationdepletion voltage [26].To conclude, defects will have the following main consequences [22]:11 The charge collection distance (CCD) is the product of µ and · and is commonly used tocharacterize detector materials.12 Applies to low-doped bulk silicon as the doping concentration is several orders of magnitudeless than in the electrodes.13 The di erence in the concentration of donors and acceptors determines the type.14 Depends on the doping concentration.15 NIEL stand for non-ionizing energy loss and is a quantity which describes the incidentparticle dependent rate of energy loss due to atomic displacements. The product of theNIEL and the particle fluence (time integrated flux) gives the displacement damage energydeposition per unit mass of material [24].28
d = 300µmU dep[V]n-typetype inversion“p”-type│N eff│[10 11 cm -3 ]Φ eq[10 12 cm -2 ]| depend-Figure 3.13: Absolute value of the e ective doping concentration |N eing on the fluence eq (neutrons, 1MeV neutron equivalent) [25].• They act as recombination–generation centers (recombination and generationof electron-hole pairs); in the depletion zone this leads to an increasedreverse-bias (or leakage) current (shot noise, thermal runaway).• They act as trapping centers; this leads to a reduced charge collectiondistance and hence smaller signal charge (loss of charge).• They can change the charge density in the space-charge region; higherdepletion (or bias) voltage is needed or under depletion may occur.The first two points have consequences for the S/N ratio. The increase ofthe reverse-bias current is the most noticeable change. The volume-generatedleakage current I vol increases linearly 16 with the fluence „ [22]:—I vol = –V „ , (3.10)where V denotes the detector volume and – has to be determined. If the fluenceis scaled by the non-ionizing energy loss, then the increase of the leakage currentis independent of the particle type.3.4.3 Sensor Types and TechnologiesFor a detector, an e cient charge collection is necessary to meet the requirementsof space, time and energy resolution and overall e ciency. All of thesepoints depend largely on the properties of the sensor. In addition the sensorhas to meet geometrical constraints on size and thickness while sustaining amassive amount of radiation damage. Concerning the IBL upgrade, there arethree sensor technologies which may fulfill the requirements [27]: planar, 3Dand diamond sensors. Each technology is demanding for di erent operating andenvironmental variables such as reverse-bias voltage and operating temperature.The first two sensor concepts (see figure 3.14) will be explained in the following.16 Above type inversion, the rise is stronger (see figure 3.13).29
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 AFE-I3 StandardConfigurati
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Appendix BFPGA ConfigurationRegiste
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Appendix CTuning ResultsThis page i
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Figure C.1: Tuning of the 3631-303
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8642Figure C.3: Tuning of the WR13
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8642Figure C.5: Tuning of the 4EM9
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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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