128 september, 2005 silicon sensor for the cms tracker the silicon sensors for the inner tracker of...

28 September, 2005 Silicon Sensor for the CMS Tracker

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 BergauerMarko DragicevicManfred Krammer


2

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


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


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

Marko Dragicevic


5

The CMS Silicon Detector

Edge of a typical CMS Silicon Strip Detector

Strip Pitch80-170μm


6

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


7

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


8

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


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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)


12

PQC Setup


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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.


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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!


15

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!


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

128 september, 2005 silicon sensor for the cms tracker the silicon sensors for the inner tracker of...

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