Analysis of Edge and Surface TCTs for Irradiated 3D Silicon Strip Detectors

Graeme Stewarta, R. Batesa, C. Corralb, M. Fantobab, G. Krambergerc, G. Pellegrinib, M. Milovanovicb

a: SUPA, School of Physics and Astronomy, University of Glasgow, Glasgow, UKb: Centro Nacional de Microelectrónica, Campus Universidad Autónoma de Barcelona, Spain c: J. Stefan Institute, Jamova cesta 39, 1000 Ljubljana, Slovenia

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Contents

• Introduction– TCT Measurements– 3D Detectors

• TCT Results– Non-Irradiated Top and Edge TCTs– Irradiated Top TCTs– Annealing Effects

• Conclusions

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Introduction

• Transient Current Techniques (TCTs) provide a method for investigating electric fields in silicon detectors.

• In a TCT measurement, a short, IR laser pulse is incident on a particular line through the detector.

• Current data is collected giving information on the charge and velocity of carriers in 3D devices.

• This can be repeated at many points across a detector’s surface to map the electric field.

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• Columns etched from opposite sides of substrate and don't pass through full thickness

• All fabrication done at CNM

• Distance between columns is 80 μm, with a 25 μm wide Aluminium strip connecting n-type columns.

• Substrate is 245 μm thick.

• 11 strips were bonded up but with readout only from the central strip.

3D Detector Design

IR Photon

Inter-column depletion at ~2V

Full, under-columndepletion at 40V

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Detector

Co

ole

d s

up

po

rt

y table

Laser

Laser driver

detector HV

Peltier controller

The whole system is completely computer controlled

z tablex table

1 GHz oscilloscope

cooling pipes

2 fast current amplifiers (2.5 GHz)

trigger line

Cu block

The system is set in dry air atmosphereCooling to -20oC

Bias T

100 ps pulse200 Hz repetition=1064 nm

TCT setup

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Top and Edge TCTs

Advantages of TCTs:

• Position of e-h generation can be controlled by moving tables

• The amount of injected e-h pairs can be controlled by tuning the laser power

• Not charge but induced current is measured – a lot more information is obtained

FWHM ~8 μm

Top TCTλ = 1064 nm

Edge TCTλ = 1064 nm

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Drawbacks of TCTs

Edge TCT:• Applicable only for strip/pixel detectors if 1064 nm laser is used (light must penetrate guard ring region)• Only the position perpendicular to strips can be used due to widening of the beam! Beam is “tuned” for a

particular strip • Light injection side has to be polished to have a good focus – depth resolution• It is not possible to study charge sharing due to illumination of all strips

Top TCT:• Cannot illuminate under Al strips.

FWHM ~8 μm

Top TCTλ = 1064 nm

Edge TCTλ = 1064 nm

Top and Edge TCTs

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Example Waveform (Top Illumination)

Rise time of first peak gives velocity profile

Integration of peaks gives charge collected

N-type column

P-type column

Charge Deposition

First Rise: ElectronsMove towards CollectionColumn

First Fall: HolesMove into Region ofLower Space Charge

Second Rise: ElectronsMove to Very High SpaceCharge Region

Second Fall: ElectronsCollected at Column

Third Rise: HolesApproach Column

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Non-Irradiated Top TCT

Map is charge collected in 20 ns after laser pulse.

Readout n-type Electrodes

Non-readout n-type Electrodes p-type Electrodes

Laser scans across surface

Unit CellCharge Collected

[Arb. Units]

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Non-Irradiated Top TCT

62 V

Charge Collected[Arb. Units]

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Non-Irradiated Top TCT – Charge Collection

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Non-Irradiated Top TCT – Velocity Maps

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Non-Irradiated Top TCT – Velocity Maps (80 V)

Velocity[Arb. Units]

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Charge Collected[Arb. Units]

Laser scans across edge

Non-Irradiated Edge TCT

P-type Electrodes

N-type Electrodes

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• Full depletion of inter-column region by 4 V

• Depletion of the region beneath the electron collecting n-type columns beginning by 4 V

• Column ends not fully depleted by 20 V

Non-Irradiated Edge TCT - Charge Collection

0 V 2 V 4 V

6 V 8 V 10 V

20 V

Charge Collected[Arb. Units]

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• Non-uniform velocity profile across the device

• Velocity increases past lateral depletion voltage of 4 V

• Edges of detector show low velocities, even at 20 V

0 V 2 V 4 V

6 V 8 V 10 V

Non-Irradiated Edge TCT - Velocity Profiles

20 V

Velocity[Arb. Units]

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Irradiation and Annealing

• Sample irradiated in Ljubljana facilities.• Irradiation fluence was 5x1015 1 MeV nequ cm-2.

• Sample always annealed in the setup with the Peltier element• constant sample temperature: -20 oC• stable position/laser • sample temperature stabilized to less than 1°C

• Annealing at 60°C for a cumulative time of 600 minutes.• After each annealing step, voltage scans from

0V up to 400V were performed

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Irradiated Top TCT

100 V

400 V

Charge Collected[Arb. Units]

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Irradiated Top TCT - Charge Collection

Charge Collected[Arb. Units]

20 V 40 V 60 V

80 V 120 V 160 V

200 V 300 V 400 V

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Irradiated Top TCT - Velocity Profile

20 V 40 V 60 V

80 V 120 V 160 V

200 V 300 V 400 V

Velocity[Arb. Units]

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Annealing Effects

• End of beneficial annealing at around 80 mins.

• After 100 minutes, we have a longer term reverse annealing NC

NC0

gC eq

NYNA

1 10 100 1000 10000annealing time at 60oC [min]

0

2

4

6

8

10

N

eff [

1011

cm-3

][M.Moll, PhD thesis 1999, Uni Hamburg]

• Significant annealing beyond beneficial annealing leads to a decrease in the interstrip resistance.

• Eventually, the strips short together.

Resistance vs Annealing time, shown by C. Fleta at 15 RD50, June 2010.

[M. Moll, PhD thesis 1999, Uni. Hamburg]

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400V bias

Charge Collected[Arb. Units]

20 minutes 40 minutes

100 minutes 300 minutes

Post-Annealed Irradiated Top TCT - Charge Collected

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Post-Annealed Irradiated Top TCT - Velocity Profiles

Velocity[Arb. Units]

20 minutes 40 minutes

100 minutes 300 minutes

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Conclusions

• Edge and top TCTs provide a new method to probe 3D devices.– Velocity information can be collected.

• In a non-irradiated device, the velocity continues increasing after full charge collection is achieved.

• Velocity and charge collection is greater below n-type columns than p-type columns at same bias voltage.

• Irradiation and subsequent annealing alters the collection of electrons and holes.– Charge Trapping suppresses hole signal

– After annealing, charge multiplication effects at 400 V

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Future Work

• Edge TCT scan of non-irradiated device up to saturated velocity (80 V)

• Edge TCT of irradiated device

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Backup Slides

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Leakage Current

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Annealing Effects – 20 mins0V - 400V in steps of 50V

0V 50V 100V

150V 200V 250V

300V 350V 400V

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Annealing Effects – 40 mins0V - 400V in steps of 50V

0V 50V 100V

150V 200V 250V

300V 350V 400V

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Annealing Effects – 100 mins0V - 400V in steps of 50V

0V 50V 100V

150V 200V 250V

300V 350V 400V

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Annealing Effects – 300 mins0V - 400V in steps of 50V

0V 50V 100V

150V 200V 250V

300V 350V 400V

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Annealing Effects – 600 mins100V - 300V in steps of 100V

100V

200V

300V