30th March 2010

IoP HEPP/APP Annual Conference 2010 UCL

Charge Collection Annealing in ATLAS

SCT Silicon Sensors

Craig Wiglesworth

The ATLAS Experiment and the SCT

Craig Wiglesworth IoP HEPP/APP Annual Conference UCL March 2010 1

The ATLAS semiconductor tracker (SCT) is a silicon

microstrip detector.

Provides discrete space points for tracking and

vertexing.

Exposure to high levels of radiation damages sensors.

Subsequent changes in sensor properties continue to change after irradiation.

Important to understand SCT performance over full 10 year lifetime of ATLAS.

Evolution of Sensor Properties After Irradiation

Evolution of VDEP shows dependence on both temperature and time

spent after irradiation.

Require full depletion of sensor in order to optimise CCE.

ILEAK Shows strong temperature dependence but decreases with time.

(Typical behaviour for a silicon microstrip sensor).

Sensor leakage current (ILEAK), depletion voltage (VDEP) and charge collection efficiency (CCE) are all affected by irradiation.

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Need to avoid the sensors being warm (> 0 oC) for long periods of time!

Low temperatures will suppress ILEAK but risk of thermal runaway (ILEAK not discussed further).

In the ATLAS Inner Detector Technical Design Report (1997) VDEP and ILEAK were predicted for SCT sensors.

Assumed LHC luminosity profile:

3 years @ 1033 cm-2s-1

7 years @ 1034 cm-2s-1

(~ 1.4x1014 neq cm-2)

ATLAS maintenance scenarios (days@temp yr-1):

Standard Access Procedure (SAP)

Many of the inputs to these calculations

have since changed.

Evaluating the Evolution of VDEP and ILEAK in the SCT

In addition, the radiation damage model itself has evolved.

Predictions re-evaluated with new radiation damage model

and updated inputs.

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Re-Evaluating the Evolution of VDEP and ILEAK in the SCT

Paul Dervan, Joost Vossebeld, Tim Jones (Liverpool), Taka Kondo (KEK), Graham Beck (QMUL), Georg Viehhauser (Oxford), Steve McMahon (RAL), Koichi Nagai (Brookhaven),

Kirill Egorov (Indiana), Richard Bates, Alexander Bitadze (Glasgow).

An updated LHC luminosity profile now exists.

Maintenance/shutdown time and cooling temperatures

reviewed in line with:

Achievable coolant temperatures.

Insertable B-Layer installation.

Possibly longer maintenance.

VDEP predictions suggest 450 V (max for SCT) is sufficient for

at least 10 years operation.

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Hamburg model is now believed to be the best model available to predict VDEP.

However, large differences observed between the predictions of the TDR model and the Hamburg model.

Origin is in reverse annealing contribution NY to the predicted change in effective doping concentration NEFF:

Comparison of Hamburg Model and TDR Model

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2

2EFF

DEP

NedV tTNtTNNtTN ACEFF ,,,,,,

TDR model parameterised reverse annealing as a second order process.

Hamburg model parameterises reverse annealing as a modified first order process.

Need high fluence + long annealing data to compare to predictions of both models.

Sensor performance traditionally studied by determining VDEP from CV measurements.

In Liverpool the focus has been on measuring the annealing of CCE.Much data available for n-side readout sensors. Less detailed information for p-in-n sensors.

Manufacturer HPK

Wafer Tech. FZ

Structure p-in-n

Size 1 cm x 1 cm

Thickness 285 µm

Programme of Accelerated Annealing Measurements

New programme of accelerated annealing measurements on ATLAS mini sensors.

Pair of sensors irradiated with neutrons at Ljublijana (V. Cindro et al) to 2x1014 neq cm-2

(new prediction for SCT = 1.6x1014 neq cm-2).

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One sensor used for CCE measurements and one sensor used for VDEP measurements.Both sensors annealed together at same temperature for same length of time.

All following plots by…. A. Affolder, H. Brown, G. Casse, P. Dervan,

J. Vossebeld, C. Wiglesworth (Liverpool)

Charge Collection Measurements

90Sr fast electron source used to generate signal.Readout triggered by scintillator.

Charge collection measured using analogue electronics chip (SCT128) clocked at LHC

speed (40 MHz clock, 25 ns shaping time).

System calibrated to most probable value of MIP energy loss in non-irradiated 300 m thick sensor (~ 23000 e).

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Experimental Setup and Analysis Procedure

Depletion Voltage Measurements

VDEP Determined by standard method of measuring the VBIAS at which 1/C2 saturates.

Both sets of measurements performed in freezer at temperature of ~25 oC with N2 flush.

@ 1 kHz

Experimental Setup and Analysis Procedure

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Strip Number

Strip Number

Noi

se (

AD

Cs)

Clu

ster

Fre

quen

cy

Clu

ster

Fre

quen

cy

Cluster Charge (ADCs)

(1 ADC ~ 22 e)

A landau convoluted with a gaussian is fitted to the resulting distribution of the collected charge.

Most probable value is recorded.

Hot channels are masked out

in analysis

Channels with low hit frequency are masked out

Measure charge on strip (+ neighbours)

with largest S/N ratio

Smooth fall off after ~ 100 days @ 20 oC

Results of Charge Collection Measurements

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SCT Default threshold 1 fC (~ 6.2 ke)

No sharp drop in the collected charge

Results of Charge Collection Measurements

SCT Default threshold 1 fC (~ 6.2 ke)

Still collecting significant charge after 1000 days @ 20 oC

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Results of Charge Collection Measurements

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Above ~ 500 V collected charge remains almost constant in time

Largest increase in charge collection ratio @ 300 V

Comparison of Measured VDEP and Charge Collected

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Comparison of Predicted VDEP and Measured VDEP

Data shows a slower annealing effect than the

two models

Data looks closer to Hamburg model

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Conclusions and Future Plans

A programme for measuring the annealing of CCE in p-in-n microstrip sensors has started.

No sharp drop in collected signal when sensor becomes under-depleted.

Slow drop in collected signal with annealing time.

Significant charge collected even when VBIAS is well below the VDEP predicted by the models.

For ATLAS SCT this means operational range defined by VDEP prediction is pessimistic.

Next….

Repeat measurements on pair of ATLAS mini sensors irradiated to 3x1014 neq cm-2.

Make more detailed comparisons between results and predictions of the two models. (+ Perhaps an attempt at an updated model).

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