128 September, 2005 Silicon Sensor for the CMS Tracker The Silicon Sensors for the Inner Tracker of...
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28 September, 2005 Silicon Sensor for the CMS Tracker
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The Silicon Sensors for the Inner Tracker of CMS
• CMS Tracker and it‘s Silicon Strip Sensors
• Radiation Hard Design
• Quality Assurance
• QA Results
Thomas BergauerMarko DragicevicManfred Krammer
28 September, 2005 Silicon Sensor for the CMS Tracker
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CMS: The Silicon Tracker
The Tracker:206 m² of Silicon24.244 individual Sensors15.148 Modules9.316.352 Channels
4 Detector Systems:• Silicon Tracker (particle tracking)
• ECAL (e/m particle energy)
• HCAL (hadron energy)
• Muon System (muon momentum, trigger)
The CMS ExperimentTotal weight : 12500 tOverall Diameter : 15 m Overall length : 21.5 mMagnetic Field : 4 T
28 September, 2005 Silicon Sensor for the CMS Tracker
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A Silicon Strip Detector:Basic Principle
• Ionizing particle creates charge carriers along track• Intrinsic number of free charge carriers inside semiconductor
much higher A depletion layer must be created by applying a reverse bias
voltage to make the bulk sensitive
• Reverse bias voltage between backplane and p+ implant• n+ backplane to ensure proper contact to HV• Signal on p+ implant is coupled capacitively to the Al readout strips
28 September, 2005 Silicon Sensor for the CMS Tracker
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A Silicon Strip Detector: Basic Principle
• capacitively coupled Al readout strips to protect readout electronics from
strip current
• p+ implants connected to bias ring via polysilicon resistors to decouple strips from each other
• guard ring protects strips from edge effects
• metal overhang to reduce local field spikes both improves HV stability of the
sensor
The structures in a little more detail
28 September, 2005 Silicon Sensor for the CMS Tracker
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The CMS Silicon Detector
Edge of a typical CMS Silicon Strip Detector
Strip Pitch80-170μm
28 September, 2005 Silicon Sensor for the CMS Tracker
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Radiation Hard Design in the CMS Tracker
Reduction of dangling bondsLess surface charges-> Empirically validated by the CMS Sensor Group for Hamamatsu Sensors
Overhang reduces field spikes near edge of strips
• <100> type silicon instead of <111>
• Metal overhang for Al readout strips
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Radiation Hard Design in the CMS Tracker
• Effective doping concentration changes with irradiation (removal of donors and creation of acceptors)
• n-type silicon gets p-type due to radiation defects in the bulk (type inversion)
• Low resistivity (n+) substrate material
2dNV effFD Higher VFD at the beginning – lower VFD necessary at end of LHC lifetime
28 September, 2005 Silicon Sensor for the CMS Tracker
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Quality Assurance: QTC
Sensor Samples are completely characterised in 3h.
2 Global Measurements:• IV- Curve (Dark current, Breakthrough)
• CV- Curve (Depletion voltage, Capacitance)
4 Strip Parameters:• Strip leakage current • Poly-silicon resistance• Coupling capacitance• Dielectric current
Quality Test Center
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Quality Assurance: QTC
Strip MeasurementsGlobal Sensor Measurements
QTC Measurements
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Quality Assurance: PQC
• Special structures on the cutoff of the sensor wafer
• Behaviour of the test structures allows drawing a conclusion on the performance of a complete sensor batch
• Custom built setup able to perform characterization in about 35 min.
Process Quality Control
28 September, 2005 Silicon Sensor for the CMS Tracker
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Quality Assurance: PQC
Except for the loading procedure – completely automatised
11 Measurements;• Coupling Capacitance
• Interstrip Capacitance• Dielectric Breakdown• Depletion Voltage/Resistivity• Flatband Voltage• Interstrip Resistance• Dark Current and Breakthrough• Surface Current• Sheet Resistances (Al, p+, poly SI)
28 September, 2005 Silicon Sensor for the CMS Tracker
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PQC Setup
28 September, 2005 Silicon Sensor for the CMS Tracker
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Status of Qualification
Some remarks:• 28.584 Sensor have been ordered from 2 suppliers
(Hamamatsu HPK and ST Microelectronics STM)• 25.555 Sensors have been delivered and are within
Specs.So 90% of the sensor production has been
delivered to the Sensor Group!• 8673 sensor and 4144 test structures have been
measured by 7 centers.• The summary presented can be regarded as almost
final.
28 September, 2005 Silicon Sensor for the CMS Tracker
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QTC ResultsH
amam
atsu
, Jap
an
1252
Sen
sors
ST
Mic
roel
ectr
onic
s
2572
Sen
sors
Number of Bad Stripsmax: < 1% of 512/768 Channels
Leakage Current at 450Vmax: < 10µA
Full Depletion Voltagemax: 100 V – 335 V
Data from accepted and fully characterised sensor only!
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QTC Results: Example of an identified problem
• Limit: Rint > 1GΩ to have a good separation of adjacent strips
• Since production week 27/03 STM suffered from low values
• Due to the long production pipeline, a significant amount of ~1000 sensors were affected
• These sensors will not be used for CMS Tracker!
Interstrip Resistance
Many more similar problems where encountered and solved by the senor group and the two supplying companies!
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Conclusion
• The final delivery is scheduled for the end of November,
• but 90% of the required sensors have already been delivered and accepted.
• Of these, the number of useable channels is (on the silicon level) a magnificent
99,71%99,71%• Thanks to the effort of the CMS Tracker sensor
collaboration and the 2 supplying companies, almost perfect silicon strip sensors will be used for the CMS Tracker.
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…and the final picture?
A partially completed Layer of the Inner Barrel with a close look at the sensors. Sensors are mounted on so called modules which already
incorporate some read-out electronics
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Backup/Deleted
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QTC ResultsH
amam
atsu
Jap
an
1252
Sen
sors
ST
Mic
roel
ectr
onic
s
2572
Sen
sors
Number of Bad Strips Leakage Current at 450VFull Depletion Voltage
Data from accepted and fully characterised sensor only!
28 September, 2005 Silicon Sensor for the CMS Tracker
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Quality Assurance
• Complex Logistics involving more than 10 institutes in Europe
• Accompanied by a sophisticated Oracle database storing all measurements performed on most Tracker related components
•Quality Test Centers characterise sensor production by tests on sensor samples and special test structure samples
•Irradiation qualification done less frequently during advanced sensor production phase