Radiation tolerant sensors for the ATLAS pixel detector

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330 R. Wunstorf / Nuclear Instruments and Methods in Physics Research A466 (2001) 327–334 As crystal defects are responsible for the radiation induced changes ofthe sensor parameters, the approach ofthe ROSE collaboration was to alter the defect kinetics by controlled introduction of individual impurities in the silicon crystal [13,14]. After the first step of getting the necessary processing developed and silicon detectors produced, the ROSE collaboration tested them under irradiation with high energy neutrons and charged hadrons. While almost no difference in the radiation induced parameters were found under neutron irradiation, much improvement was observed under irradiation with charged hadrons for silicon with high oxygen content [15]. The improvement concerns only the radiation induced change ofthe doping concentrations, while the current and charge collection parameters are the same. The different responses ofoxygenated silicon to neutrons and charge hadrons indicates that the improvement due to point defects, may be caused by an introduction ofdonor-like defects compensating part ofthe ‘unwanted’ acceptors, which would therefore be cluster related. Although the defect kinetics of oxygen in silicon is not fully understood and it might be a different mechanism, it is interesting that already, several years ago, oxygen was thought to be a candidate for improvement, when it was observed that close to the surface, where the oxygen diffuses in during a normal oxidation process, the silicon does not convert to p-type behavior, even though the bulk has been converted [16]. Several tests with different oxygenation processes at different vendors and different starting materials have confirmed the improvement of silicon in the following three aspects [15,17]: * reduction ofthe stable damage, * reduction ofthe amount ofreverse annealing, and * deceleration ofthe reverse annealing. Fig. 3 shows, for example, the improved behavior ofthe effective doping measured directly after irradiation, due to the reduction ofthe stable Fig. 3. Comparison ofoxygen enriched silicon (DOFZ-silicon) with standard silicon in the radiation induced change ofthe effective doping concentration measured after successive irradiation steps with 24 GeV protons. The DOFZ-silicon for the ROSE diodes used here were simultaneous oxygen diffused with the second prototype ATLAS sensor wafers.
R. Wunstorf / Nuclear Instruments and Methods in Physics Research A466 (2001) 327–334 331 damage by oxygen diffused silicon (DOFZ-silicon). The ROSE test diodes used here, are ofthe same material as used for the second ATLAS pixel sensor prototype and have undergone an identical oxidation process, ofa 24 h diffusion under nitrogen atmosphere at CiS, Erfurt. The expected improvement using oxygenated silicon, as shown in Fig. 4, has been calculated using the damage parameters evaluated by the ROSE collaboration [15]. These figures include the prediction calculation for the ‘standard’ warm-up scenario (3 days 208C and 14 days 178C) [1] and also calculations for longer warm-up time (30 days resp. 60 days 208C) which will be possible with the use ofoxygenated silicon. The sensors for the Blayer, for example, would survive not only the expected 3 years at lower luminosity, but could work the full ATLAS operation time of 10 years. The radiation is largely dominated by pions, leading to this impressive advantage ofusing oxygenated silicon for the ATLAS pixel detector. Fig. 4. Damage projections for the B-layer (a) and the first layer (b) ofthe ATLAS pixel detector. 4. Prototyping Before ordering the final sensor wafers, which will go into the ATLAS detector, the ATLAS pixel collaboration had two prototype series. The prototype sensors were important for the development of the sensors as well as for testing front-end electronic chips and other module aspects [1]. The pixel detector module utilizes high density interconnect technologies such as bump bonding and MCMD, which are using further photolithography processing steps. These processing steps cannot be done on diced parts but require full wafers. Therefore, the sensor prototyping was already done with full wafer layouts given to the vendors as GDS-II file ready for the production of masks. The main sensor issues under investigation in the first prototype had been the different isolation techniques and different designs ofthe pixel cell itself. As shown in Fig. 2, the wafer includes two full size tiles, and several small single chips fitting with different design variation for both isolation techniques. These prototype sensors have been studied thoroughly by the ATLAS pixel collaboration [11,18] and resulted in the optimal choice for the radiation tolerant sensor design [19]. Moderated p-spray has been shown to provide superior current characteristics without any excess current, very high breakdown voltages above 1000 V and low noise [18,20]. Extensive studies ofsingle chip sensors bonded to prototype ATLAS pixel electronic chips have been performed in the H8 test beam at CERN [21]. Even after 10 15 neq/cm 2 the noise occupancy was below 10 7 measured with low thresholds in the testbeam and allow e.g. a direct measurement ofthe depletion depth with the inclined particle tracks as shown in Fig. 5. The optimal pixel cell design was the so-called ‘smallgap’ design, which showed in the test beam a signal efficiency of99.1%, a flat top of22 mm by single hits and with double hits 5 mm space resolution. The analog charge measured across the pixel plane using the time-over-threshold information is shown in Fig. 6. Only the area ofa few mm 2 around the small bias dot shows a charge loss of 10%, which is still well above the threshold. These good results left only two sensor issues to the second prototype run: the yield oflarge tiles
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Radiation tolerant sensors for the ATLAS pixel detector

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