Quality assurance for the radiation hard ATLAS pixel sensors Contents: - Why is systematic QA necessary? - Procedures and design features - Measurements before irradiation - Measurements after irradiation Pixel 2000, June 2000, Genova QA for rad. hard ATLAS pixel sensors, J.M. Klaiber-Lodewigs - Univ. Dortmund
Introduction Why systematic quality assurance for the ATLAS pixel sensor? • large number of detector parts (2228 modules fitted with one sensor tile and 16 front-end chips each) • parts not easily accessible after assembly (central position, cooling and radiation) • every bad pixel degrades performance • ≈ 1.4·10 8 pixel channels in total 2 QA for rad. hard ATLAS pixel sensors, J.M. Klaiber-Lodewigs - Univ. Dortmund Pixel 2000, June 2000, Genova
Important steps for QA Setting technical specifications Designing according to specifications Identifying relevant qualities Defining reliable measurement procedures Calibrating involved test sites Archiving measurement data in data base 3 QA for rad. hard ATLAS pixel sensors, J.M. Klaiber-Lodewigs - Univ. Dortmund Pixel 2000, June 2000, Genova
Pixel design requirements • pixel size 50µm x 400µm 50 µm pitch 12µm diameter bump connection • total active area 2.3m 2 (2228 modules) high yield testability • 10 years operation fault tolerance • harsh radiation environment up to 10 15 cm -2 (1 MeV neutron eq.) radiation hard technology and design 4 QA for rad. hard ATLAS pixel sensors, J.M. Klaiber-Lodewigs - Univ. Dortmund Pixel 2000, June 2000, Genova
Isolation techniques Optimizing performance before and after type-inversion and ensuring testability p-stop p-spray moderated p-spray For p-spray testing of worst case of breakdown before irradiation 5 QA for rad. hard ATLAS pixel sensors, J.M. Klaiber-Lodewigs - Univ. Dortmund Pixel 2000, June 2000, Genova
Testability I-V tests on test pixels using punch-through 1.00E+02 1.00E+05 defective pixel matrix with defective pixels 1.00E+01 1.00E+04 1.00E+00 1.00E+03 current [nA] current [nA] 1.00E-01 1.00E+02 good pixel 1.00E-02 1.00E+01 good pixel matrix 1.00E-03 1.00E+00 1.00E-04 1.00E-01 0 20 40 60 80 100 120 140 160 180 200 0 20 40 60 80 100 120 140 160 180 200 bias voltage [V] bias voltage [V] current through current through single pixel punch-through array Leakage current indicative for quality of every pixel 6 QA for rad. hard ATLAS pixel sensors, J.M. Klaiber-Lodewigs - Univ. Dortmund Pixel 2000, June 2000, Genova
Testability I-V tests before bonding using bias grid • Punch-through effect across a bias grid allows testing of all pixels using only two probes on wafer p-side ground p-side bias guard ring punch-through 7 QA for rad. hard ATLAS pixel sensors, J.M. Klaiber-Lodewigs - Univ. Dortmund Pixel 2000, June 2000, Genova
Leakage current Tile classification by pixel quality • I-V measurements of leakage current show pixel quality • breakdown voltage indicates type of defect • tile classification possible 8 QA for rad. hard ATLAS pixel sensors, J.M. Klaiber-Lodewigs - Univ. Dortmund Pixel 2000, June 2000, Genova
Leakage current Yield analysis based on I-V curves 9 QA for rad. hard ATLAS pixel sensors, J.M. Klaiber-Lodewigs - Univ. Dortmund Pixel 2000, June 2000, Genova
Sensor depletion Diagnostic measurement by guard ring diode capacitance p + Defined • Test diodes on production wafer n-Bulk area for well defined capacitance n + measurements • Full depletion visible by levelling out of C vs. V -1/2 curve (suppression of possible constant stray capacitances) 10 QA for rad. hard ATLAS pixel sensors, J.M. Klaiber-Lodewigs - Univ. Dortmund Pixel 2000, June 2000, Genova
Oxide characteristics Diagnostic measurements on MOS and GCD • oxide breakdown and capacitance measured in I-V and C-V curves on MOS pads • interface generation current measured on gate controlled diodes as I-V curve around flat-band case with identical gate and diode voltage • current step indicates charge density on interface 11 QA for rad. hard ATLAS pixel sensors, J.M. Klaiber-Lodewigs - Univ. Dortmund Pixel 2000, June 2000, Genova
P-spray dose Method of measurement • source - drain V gate Drain V drain = 0.1V measurement on MOSFET Source I on depleted bulk n + n + • p-spray inverts at higher n-Bulk p-spray gate voltage: threshold V th p + • source-drain current rises backside voltage V b rapidly at V th 12 QA for rad. hard ATLAS pixel sensors, J.M. Klaiber-Lodewigs - Univ. Dortmund Pixel 2000, June 2000, Genova
P-spray dose Analysis • after V th is identified p-spray dose is given by over depleted under depleted φ p-spray = C ox ·(V th -V fb ) / e using flat-band voltage and oxide capacitance from MOS C-V 13 QA for rad. hard ATLAS pixel sensors, J.M. Klaiber-Lodewigs - Univ. Dortmund Pixel 2000, June 2000, Genova
Radiation hardness tests Bulk damage testing • after irrad. with 3.1 ⋅ 10 14 cm -2 • depletion measurement on diode neutron equivalent protons • after irrad. with 10 15 cm -2 neutron • I-V measurements on mini chip equivalent protons (design fluence) and diode (small structures) Surface damage testing • interface generation current • after irrad. with 500 kGray low measurement on GCD energetic electrons (design dose) • p-spray measurement on MOSFET 14 QA for rad. hard ATLAS pixel sensors, J.M. Klaiber-Lodewigs - Univ. Dortmund Pixel 2000, June 2000, Genova
Testing responsibilities vendor ATLAS institutes • providing process data • checking process data • testing pixel quality on sensor • testing pixel quality on sensors tiles on wafer level tiles, single chips and mini chips • performing diagnostic tests on • performing all diagnostic tests wafer level for depletion, oxide on wafer level and on diced test quality and capacitance, and p- structures spray dose • measuring irradiated structures 15 QA for rad. hard ATLAS pixel sensors, J.M. Klaiber-Lodewigs - Univ. Dortmund Pixel 2000, June 2000, Genova
Summary Progress of quality assurance process • technical specifications completed and approved • sensor design completed and approved • quality test for every pixel on sensor defined and demonstrated • diagnostic tests for relevant qualities defined and demonstrated • cross calibration of test sites in progress • data base for test results under construction 16 QA for rad. hard ATLAS pixel sensors, J.M. Klaiber-Lodewigs - Univ. Dortmund Pixel 2000, June 2000, Genova
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