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
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