Radiation tolerant sensors for the ATLAS pixel detector

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332 R. Wunstorf / Nuclear Instruments and Methods in Physics Research A466 (2001) 327–334 Fig. 5. Depletion depth measurement ofan unirradiated and an irradiated ATLAS pixel sensor. Fig. 6. Analog charge measurement across two adjacent pixel cells. and the use ofoxygenated silicon. In the first prototype, the yield ofthe 16 times smaller single chips was much higher than that ofthe full size tiles. The reason for this could be randomly distributed defects, like scratches introduced during the different processing and handling steps or a particular risky design feature. The most challenging design feature is the small bias dot which requires an alignment accuracy better than 2 mm. This question was investigated with the second prototype, where three large tiles were implemented in the wafer layout with different design options ofthe bias dot, a small dot, a large dot, and a bias grid without integrated dots. The resulting yield statistic showed for two different vendors no yield loss due to the small dot design. Therefore, this design (see Fig. 7) was taken for the production, which will have the optimum charge collection. Both vendors, CiS (Erfurt) and ITC (Trento), produced halfofthe second prototype wafers with oxygen diffusion of24 h at 11508C to allow a comparison in the sensor performance. As already shown before (Fig. 3) the material will be more radiation tolerant to charged hadrons. Irradiations ofoxygenated and not oxygenated second prototype sensors have been performed up to 10 15 neq/ cm 2 with 55 MeV protons at LBNL, Berkely, and with 24 GeV protons at CERN, Geneva, and are currently under investigation. The I2V characteristics show for both cases only the known increase in the bulk generation current without any indication ofbreakdown up to 1000 V. Irradiated single chips have been bump bonded to FE-chips and were successfully operated in the CERN testbeam. As expected, the oxygenated silicon shows full depletion and full charge collection at much lower bias voltages. These pixel sensors can be operated without any problems even after high irradiations and at the same time allow the direct measurement ofthe depletion depth (Fig. 5) and the measurement ofthe analog signal. Therefore, the pixel sensors allow for the first time the study ofthe dependence ofthe depletion depth and the charge collection from the applied voltage after different fluences, especially after type conversion. 5. Production The ATLAS pixel detector will need totally more than 2000 modules for the outer two layers
and the 5 disks in each forward direction. The modules for the B-layer are needed about 2 years later allowing a separate production and the use of the MCMD-technology, which will be optimized according to the special B-layer requirements, e.g. 300 mm long cells. One important option is to have equal-sized pixel cells all over the sensor without elongated and ganged pixel as the cell geometry is not dictated by the read-out chip and also the implementation ofbricked pixels would easily be possible without any additional cross-talk caused by routing in one metal layer. These will improve the spatial resolution in this important layer for the impact parameter measurement and the b-tagging. Therefore, the starting sensor wafer production will be for tiles for the outer modules which will be built with a flex hybrid. The wafer layout has been finalized according to the results ofthe prototype studies. The design ofthe pixel cells is shown in Fig. 7, with the bias-grid structure in the middle and the passivation opening for bump bonding on the other side ofeach pixel. Besides three tiles for the ATLAS pixel detector modules, the wafer will contain six single chip devices and a number oftest structures for quality control. According to the Quality Assurance plan, part ofthe control measurement will have already been performed by the vendors, but these measurements and additional tests will be performed by the ATLAS institutes. 6. Conclusions R. Wunstorf / Nuclear Instruments and Methods in Physics Research A466 (2001) 327–334 333 With the knowledge ofthe radiation effects in silicon detectors as a good starting point, the Fig. 7. Design ofthe pixel sensor cell for the ATLAS pixel detector. ATLAS pixel collaboration began in 1996 to develop silicon pixel sensors capable ofsurviving the harsh radiation environment at the LHC. To reach this challenging goal, a dual-track strategy was followed. Besides the studies concerning the sensor design and prototyping within the ATLAS pixel collaboration, the investigations ofthe radiation hardness ofsilicon were done within the ROSE collaboration. The result is an n + –in–n pixel sensor with moderated p-spray using oxygen diffused silicon. Indispensable to the quality control during production is the implemented bias grid, which enables measurements ofthe I2V characteristic with two contacts and prevents problems due to missing bumps during operation. Acknowledgements These important results for the ATLAS Experiment could not have been achieved without the considerable engagement ofthe members ofthe ATLAS Pixel Collaboration and the ROSE collaboration and their cooperation. Therefore, the acknowledgement goes to all who participated in the sensor design, the sensor measurements, the irradiations, the testbeam runs and the logistics, sending the sensors on time to the different places. References [1] ATLAS Collaboration, ATLAS Pixel Detector Technical Design Report, CERN/LHCC/98-13, Geneva, 1998. [2] ATLAS Collaboration, ATLAS Inner Detector Technical Design Report, CERN/LHCC/97-16 and CERN/LHCC/ 97-17, Geneva, 1997.
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Radiation tolerant sensors for the ATLAS pixel detector

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