Speaker
Description
ATLAS ITk Collaboration
We describe the extensive quality assurance programme undertaken during the production of the ATLAS ITk strip sensors. Quality Assurance (QA) is the systematic process of preventing defects during production, rather than identifying them after the fact. It uses well defined testing procedures to monitor the production process, ensuring consistent process control and minimizing variability. ATLAS QA is done on test structures fabricated at the periphery of the same wafers used to produce the actual sensors, thus ensuring that these structures are a good representation of the sensor quality. The test structures consist of test chips, which include strips, diodes, MOS capacitors, and special structures for monitoring of parameters such as bias resistor values, interstrip resistance, coupling capacitance and flat band voltage. Additionally, miniature sensors with the same design as the main sensor, but with reduced area, allow measurement of the charge generated during irradiation with a beta source.
Test structure semiconductor parameters are measured on both unirradiated samples and samples subjected to irradiation. From every batch, at least one miniature sensor and one diode are irradiated with protons or neutrons to monitor bulk damage, and one test chip and one diode are irradiated with protons or gammas for ionization damage. The test chips are irradiated using gammas and reactor neutrons to the equivalent dose (660 kGy) and fluence $ (1.6 \times 10^{15} neq/cm^2)$ expected in the high luminosity-LHC after 1.5 times its lifetime of 14 years. Miniature sensors are irradiated to the same neutron fluence as the test chips, or the equivalent proton fluence. Monitor diodes are irradiated alternately to the maximum fluence to measure the degradation of the leakage current, and to one third this value to observe the full depletion voltage within the operational range of the measurement setups. Unirradiated test structures are measured on a probe station at room temperature. Irradiated test structures are wire bonded to a PCB to provide connectivity, and measured at -21C and 4% relative humidity.
All the measurements obtained from the tests are uploaded to a database and analysed using python scripts that provide validation, parameter extraction, statistics, trends and parameter correlations.
After four years of sensor production, the QA process has shown reliable performance across multiple test sites. Both tests and irradiations remained fully operational throughout production. A few deviations from the acceptance criteria, such as low flat band voltage, high full depletion voltage, and variations in the magnitude and uniformity of the p-stop doping have been identified and timely reported back to the vendor. However, most of the evaluated parameters have remained between the defined specifications. Out of the total ordered strip sensors, over 80% (572 batches) are tested and accepted as-is. Six batches are rejected, including four due to QA issues related to low p-stop doping.
