The Silicon Sensors for the Inner Tracker of CMS

1. 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 Bergauer Marko Dragicevic Manfred Krammer Silicon Sensor for the CMS Tracker

2. CMS: The Silicon Tracker The CMS Experiment Total weight : 12500 t Overall Diameter : 15 m Overall length : 21.5 m Magnetic Field : 4 T The Tracker: 206 m² of Silicon 24.244 individual Sensors 15.148 Modules 9.316.352 Channels • 4 Detector Systems: • Silicon Tracker(particle tracking) • ECAL(e/m particle energy) • HCAL(hadron energy) • Muon System(muon momentum, trigger) Silicon Sensor for the CMS Tracker

3. 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 Silicon Sensor for the CMS Tracker

4. 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 Silicon Sensor for the CMS Tracker

5. The CMS Silicon Detector Strip Pitch 80-170μm Edge of a typical CMS Silicon Strip Detector Silicon Sensor for the CMS Tracker

6. Radiation Hard Design in the CMS Tracker • <100> type silicon instead of <111> Reduction of dangling bonds Less surface charges -> Empirically validated by the CMS Sensor Group for Hamamatsu Sensors • Metal overhang for Al readout strips Overhang reduces field spikes near edge of strips Silicon Sensor for the CMS Tracker

7. Radiation Hard Design in the CMS Tracker • Low resistivity (n+) substrate material • 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)  Higher VFD at the beginning – lower VFD necessary at end of LHC lifetime Silicon Sensor for the CMS Tracker

8. Quality Assurance: QTC Quality Test Center • 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 Silicon Sensor for the CMS Tracker

9. Quality Assurance: QTC QTC Measurements Strip Measurements Global Sensor Measurements Silicon Sensor for the CMS Tracker

10. Quality Assurance: PQC Process Quality Control • 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. Silicon Sensor for the CMS Tracker

11. 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) Silicon Sensor for the CMS Tracker

12. PQC Setup Silicon Sensor for the CMS Tracker

13. Status of Qualification Some remarks: • 28.584Sensor 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. Silicon Sensor for the CMS Tracker

14. QTC Results Full Depletion Voltage max: 100 V – 335 V Number of Bad Strips max: < 1% of 512/768 Channels Leakage Current at 450V max: < 10µA Hamamatsu, Japan 1252 Sensors ST Microelectronics 2572 Sensors Data from accepted and fully characterised sensor only! Silicon Sensor for the CMS Tracker

15. QTC Results: Example of an identified problem Interstrip Resistance • 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! Many more similar problems where encountered and solved by the senor group and the two supplying companies! Silicon Sensor for the CMS Tracker

16. 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% • 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. Silicon Sensor for the CMS Tracker

17. …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 Silicon Sensor for the CMS Tracker

18. Backup/Deleted Silicon Sensor for the CMS Tracker

19. QTC Results Full Depletion Voltage Number of Bad Strips Leakage Current at 450V Hamamatsu Japan 1252 Sensors ST Microelectronics 2572 Sensors Data from accepted and fully characterised sensor only! Silicon Sensor for the CMS Tracker

20. 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 Silicon Sensor for the CMS Tracker