Diborane Electrode Response in 3D Silicon Sensors for the CMS and ATLAS Experiments
Unusually high leakage currents have been measured in test wafers produced by the manufacturer SINTEF containing 3D pixel silicon sensor chips designed for the ATLAS (A Toroidal LHC ApparatuS) and CMS (Compact Muon Solenoid) experiments. Previous data has shown the CMS chips as having a lower leakage current after processing than ATLAS chips. Some theories behind the cause of the leakage currents include the dicing process and the usage of copper in bump bonding, and with differences in packaging and handling between the ATLAS and CMS chips causing the disparity between the two. Data taken at SLAC from a SINTEF wafer with electrodes doped with diborane and filled with polysilicon, before dicing, and with indium bumps added contradicts this past data, as ATLAS chips showed a lower leakage current than CMS chips. It also argues against copper in bump bonding and the dicing process as main causes of leakage current as neither were involved on this wafer. However, they still display an extremely high leakage current, with the source mostly unknown. The SINTEF wafer shows completely different behavior than the others, as the FEI3s actually performed better than the CMS chips. Therefore this data argues against the differences in packaging and handling or the intrinsic geometry of the two as a cause in the disparity between the leakage currents of the chips. Even though the leakage current in the FEI3s overall is lower, the current is still significant enough to cause problems. As this wafer was not diced, nor had it any copper added for bump bonding, this data argues against the dicing and bump bonding as causes for leakage current. To compliment this information, more data will be taken on the efficiency of the individual electrodes of the ATLAS and CMS chips on this wafer. The electrodes will be shot perpendicularly with a laser to test the efficiency across the width of the electrode. A mask with pinholes has been made to focus the laser to a beam smaller than the width of an electrode in order to properly scan it. This will provide more information on whether something in the electrodes, such as the polysilicon filling, is contributing to the leakage current or if there is another cause to be found. It will also reveal whether the diborane doping method and the new polysilicon filling has increased the electrode efficiency as expected. Thus, the cause of these leakage currents on the wafers from SINTEF has yet to be definitively found.
- Research Organization:
- SLAC National Accelerator Lab., Menlo Park, CA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC02-76SF00515
- OSTI ID:
- 1017226
- Report Number(s):
- SLAC-TN-11-003; TRN: US1103336
- Country of Publication:
- United States
- Language:
- English
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