1. G. Casse, University of Liverpool and M. Moll, CERN (RD50)
A. Dierlamm, KIT; D. Eckstein, DESY; A. Junkes, University of Hamburg; T. Rohe, PSI (CMS) D. Muenstermann, University of Geneva (ATLAS)
Pion induced change of material parameters in Silicon
PSI,
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10. Juni 2018
10. Juni 2018

2. Introduction
All LHC experiments use silicon detectors for tracking. The size of the trackers exceeds previous projects by an order of magnitude
CDF (Tevatron, largest pre LHC silicon tracker):
Barrel only: layer00 + SVX-II (5 layers) + ISL (1.5 layers), total < 10m2
ATLAS:
Pixel: 3 barrel layers, 2×3 disks, total ~1.7m2
Strip: 4 barrel layers, 2×9 disks, total ~40m2
CMS:
Pixel: 3 barrel layers, 2×2 disks, total ~1.1m2
Strip: 4 inner + 6 outer barrel layers, 2×3 inner + 2×9 outer disks, total ~200m2
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3. Motivation for pion irradiation
LHC (2009)
Planned for ~500/fb within 10years
F(r=4cm) ~ 3×1015 Neqcm-2
Long and intensive R&D phase for all LHC experiments
Maximum fluence specified so far is
0.6-1×1015 Neqcm-2 (present experiments)
1.5-5×1015 Neqcm-2 phase I upgrades (CMS) and IBL (ATLAS)
Radiation field is dominated by pions for
r < 20 cm (ATLAS)
r < 60 cm (CMS)
Proton and neutron irradiation facilities available
pE1 is the only pion beam in the world suitable for irradiation
HL-LHC (2022 ?)
Planned for ~2500/fb within ~5years
F(r=4cm) ~ 1×1016 Neqcm-2
New detector concept needed (scope of RD50)
New developments in the recent years
Other materials: Epitaxial, mCz, p-type sensors, diamond
Other device types: optimisation of device thickness, “n-in-p”, 3D, ...
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4. Charge collection [A. Affolder at 13th RD50 workshop]
The signal height is the “figure” of merit for the operation of pixel detectors (threshold presetly installed ROCs ~3000 electr., next generation ~1500 electrons)
First results on very highly irradiated indicate that such levels of radiation hardness might be reachable
Pion data cover < 5% of the total range
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5. Radiation damage in Silicon
10 MeV p
24 GeV p
1 MeV n
Surface damage
Mainly by ionisation in the covering layers
Built up of positive surface charge
Danger of breakdown close to n-side electrodes
 careful choice of n-side isolation
Crystal damage by displacement
Leakage current increase proportional to F F ~ ?? (depending much on T)
power load (cooling), power
Preamplifier (if DC coupled)
Change of internal electric field F > a few 1014 Neq/cm2 (L > ~100/fb)
Bias voltage has to be
Charge is focused  spatial resolution degrades
Complicated annealing behaviour
Reduced signal (trapping) F > ~1015 Neq/cm2 (L > ~250/fb):
Possibly charge amplification > 1kV  RD50
High voltage is presently limited by connectors, cables and power supplies
[Mika Huhtinen NIMA 491(2002) 194]
generation  leakage current Levels close to midgap most effective Ec
Electrons
Holes Ev
charged defects  Neff , Vdep e.g. donors in upper and acceptors in lower half of band gap Ec
Donor +
Acceptor - Ev
Trapping (e and h)  CCE shallow defects do not contribute at room temperature due to fast detrapping Ec Ev
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6. NIEL How to compare different types of radiation?
Assume the damaging parameters scale with deposited energy, not used for the (reversible) process of ionization, the so called NIEL
Use 1MeV neutrons as standard (most present in the outer tracker regions)
Calculate “hardness factor” for each type of irradiation
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7. Dependence on particle type
Leakage current
calculate damage constant a
Increase of leakage current scales with the hardness factors calculated from NIEL hypothesis
Leakage current is nowadays used to measure the hardness factors
DLTS (deep level transient spectroscopy)
Measure capacitance of a diode as function of the temperature
Data scaled with a
Level E4 is correlated to leakage current (and damage clusters)
Concentration scales with a
Other defect levels do not scale
→ Other properties (internal electric field, trapping) need a more detailed understanding
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8. Built up of space charge (NIEL violation)
TSC (Thermally stimulated current):
n-type epitaxial material (Oxygen rich)
Shallow donor (positive space charge) at E(30K) produced with protons much more efficiently than with neutrons
This donor overcompensates the deep acceptors responsible for “type inversion” in proton irradiated samples
Macroscopic behaviour differs
Also the case for different energies of protons
What about pions?
[A. Junkes, PhD thesis Uni Hamburg, 2011]
[A. Junkes, priv. comm 2013]
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9. Summary of known defects
Some identified defects
Trapping: Indications that E205a and H152K are important (further work needed)
Converging on consistent set of defects observed after p, p, n, g and e irradiation.
Defect introduction rates are depending on particle type and particle energy and (for some) on material!
positive charge (higher introduction after proton than after neutron irradiation, oxygen dependent)
positive charge (higher introduction after proton irradiation than after neutron irradiation)
Phosphorus: shallow dopant (positive charge)
Leakage current E4/E5: V3 (?)
leakage current & neg. charge current after  irrad, V2O (?)
Reverse annealing (negative charge)
Boron: shallow dopant (negative charge)
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10. Simulation to predict device performance
Field in irradiated sensors becomes very complex
[Chiochia et al., IEEE Trans. Nucl. Sci. Vol. 52(4), 2005, p ]
Field in irradiated sensors becomes very complex
In order to be able to make predictions using device simulation programs a strong simplification to “effective” levels has to be made
Measured defects → TCAD input
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11. Simulation of trapping
TCAD simulations can reproduce TCT data, leakage current, depletion voltage and (partly) charge trapping of irradiated sensors with one parameter set!
Example: Input parameter set tuned to match TCT measurements (R.Eber, Uni.Karlsruhe)
Same set of data used to simulate CCE measurements taken in a CMS test beam
[R.Eber, RD50 Workshop, June 2013]
Simulation predicts leakage current correctly (not shown)
Simulation predicts CCE for proton and neutron irradiated samples of different thickness within 20%
Simulations start to get predictive power; still the phase space of input parameters is huge and input (defect) parameters have to be tuned and adopted to measured defect parameters.
Up to now, no data for pions
[T.Peltola, RD50 Workshop, June 2013]
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12. Previous irradiations
In the beginnings
Test of n-type silicon with different oxygen content
Before 2000 also: GaAs, MSGCs, .....
Aim is to offer an irradiation time slot every 3 years
Last irradiations 2007 and 2010
Irradiation was mainly used for charge collection measurements in strip and pixel detectors
“New” detector types (e.g. single sided n-in-p) seem good candidates for upgrade of LHC detectors [Affolder et. al presented at the TIPP09]
Generate “mixed irradiations” (irradiate samples with neutrons afterwards to have the realistic particle composition as later in the Experiment) [NIM A 612 (2010) ]
Pixels for signal height measurements [NIM A 612 (2010) ], [NIM A 650 (2011) ]
Trapping and annealing studies [2011 JINST 6 P11008]
Modelling of Electric fields (publication not yet released)
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13. Plans for 2014 Plan/Question
How can pion damage be related to protons/neutron of different energies?
Main activities
Diodes (different materials mCz, epi, …)
defect determination with spectroscopic methods (DLTS, TSC)
Modelling of internal electric field (TCT)
Segmented detectors (strips and pixels)
Test with “real” particles (b-source tests, test beams)
Required beam time
Assume that about 20% of the final fluence is necessary for meaningful extrapolation
Intensity ~2×109 p/s focused on ~1cm2 1015Neq/cm2/week ~ 2× 1015Neq/cm2 in 2 weeks
About 20 samples can be mounted in the “focus”
Another ~40 in front and behind the focus for smaller fluences
Irradiation procedure
Announce irradiation at web page of the CERN irradiation service
Collect requests (normally overbooked)
Select proposals according to scientific quality (with help of experts from RD50)
So far many (oral) requests for epi-Silicon, mCz n- and p-type silicon and diamond
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14. Required resources
pE1-area has to be arranged with the shielding (redesigned “Gabathuler-hut”)
no steering magnet in the area
beam dump has to be provided (to prevent activation of the concrete blocks)
Equipment presently in WAKA:
Ionisation chamber for on-line dosimetry
XYZ-stage (provided by CERN)
Wire chambers for beam tuning
Spectrometer for off-line dosimetry (will be provided by CERN)
In the beginning help of the Hallendienst for calibration welcomed
Beam line support will be provided by T. Rohe (and occasionally by K. Deiters and D. Reggiani)
Irradiation will be run by
Maurice Glaser and Federico Ravotti (CERN)
PhD students for participating institutes
All visiting scientists are sufficiently supported by their home institutes/RD50
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15. Study fundamental properties of silicon Defect formation in Silicon
Conclusions
Joined request by all key players in the silicon tracker community (ATLAS/CMS/RD50)
Study fundamental properties of silicon
Defect formation in Silicon
Description of pion damage by simulation and with other particla
Important for the R&D strategy of LHC experiments
PE1 is the only place in the world providing a sufficient pion flux
Requested beam time (1 month every 3 years) and resources are very modest
PSI,
PSI,
10. Juni 2018
10. Juni 2018