Michael Moll ( CERN

1
CERN PH-DT2 Scientific Tea meeting 13.10.2006

Radiation Tolerant Silicon Detectors

• Michael Moll ( CERN PH-DT2-SD)

Outline

• What is a silicon detector? How does it work?
• What is radiation damage? What are the
  problems?
• Radiation damage in future experiments
  Super-LHC (LHCb Upgrade)
• The CERN RD50 collaboration
• Strategies to obtain more radiation tolerant
  detectors
• Some examples how to obtain radiation tolerant
  detectors
• Material Engineering
• Device Engineering
• Summary

2
Silicon Detector Working principle

• Take a piece of high resistivity silicon and
  produce two electrodes (not so easy !)
• Apply a voltage in order to create an internal
  electric field (some hundred volts over
  the 0.3mm thick device)
• Traversing charged particles will produce
  electron-hole pairs
• The moving electrons and holes will create a
  signal in the electric cicuit

3
Silicon Strip Detector

• Segmentation of the p layer into strips (Diode
  Strip Detector) and connection of strips to
  individual read-out channels gives spatial
  information

typical thickness 300mm (150?m - 500?m used)

• using n-type silicon with a ?resistivity of
  ? 2 KWcm (ND 2.2.1012cm-3) results
  in a depletion voltage 150 V

• Resolution ? depends on the pitch p (distance
  from strip to strip)
• - e.g. detection of charge in binary way
  (threshold discrimination) and
  using center of strip as measured coordinate
  results in
• typical pitch values are 20 mm 150 mm ? 50 mm
  pitch results in 14.4 mm resolution

4
Example The ATLAS module
5
LHCb VELO Silicon sensor details
R-measuring sensor (45 degree circular segments)

• 300 mm thick sensors
• n-on-n, DOFZ wafers
• 42 mm radius
• AC coupled, double metal
• 2048 strips / sensor
• Pitch from 40 to 100 mm
• Produced by Micron Semiconductor

42 mm
8 mm
F-measuring sensor (radial strips with a stereo
angle)
Martin van Beuzekom, STD6, September 2006
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LHCb-VELO - Module construction
Beetle

• 4 layer kapton circuit
• Heat transport with TPG
• Readout with 16 Beetle chips
• 128 channels, 25 ns shaping time,
• analog pipeline
• 0.25 mm CMOS
• no performance loss up to 40 Mrad
• Yield gt 80

Kapton hybrid
Carbon fibre
Thermal Pyrolytic Graphite (TPG)
Martin van Beuzekom, STD6, September 2006
7
Motivation for RD on Radiation Tolerant
Detectors Super - LHC

• LHC upgrade ?LHC (2007), L 1034cm-2s-1
  f(r4cm) 31015cm-2
• ?Super-LHC (2015 ?), L 1035cm-2s-1
  f(r4cm)
  1.61016cm-2
• LHC (Replacement of components) e.g. - LHCb Velo
  detectors (2010) - ATLAS Pixel B-layer
  (2012)
• Linear collider experiments (generic RD)Deep
  understanding of radiation damage will be
  fruitful for linear collider experiments where
  high doses of e, g will play a significant role.

? 5
8
Overview Radiation Damage in Silicon Sensors

• Two general types of radiation damage to the
  detector materials
• ? Bulk (Crystal) damage due to Non Ionizing
  Energy Loss (NIEL) - displacement
  damage, built up of crystal defects
• Change of effective doping concentration (higher
  depletion voltage,
  
  under- depletion)
• Increase of leakage current (increase of shot
  noise, thermal runaway)
• Increase of charge carrier trapping (loss of
  charge)
• ? Surface damage due to Ionizing Energy Loss
  (IEL) - accumulation of positive in the
  oxide (SiO2) and the Si/SiO2 interface
  affects interstrip capacitance (noise
  factor), breakdown behavior,
• Impact on detector performance and Charge
  Collection Efficiency (depending on detector
  type and geometry and readout electronics!)Signa
  l/noise ratio is the quantity to watch
• ? Sensors can fail from radiation
  damage !

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The charge signal

• Collected Charge for a Minimum Ionizing
  Particle (MIP)
• Mean energy loss dE/dx (Si) 3.88 MeV/cm ?
  116 keV for 300?m thickness
• Most probable energy loss 0.7 ?mean
  ? 81 keV
• 3.6 eV to create an e-h pair ? 72 e-h /
  ?m (mean) ? 108 e-h / ?m (most
  probable)
• Most probable charge (300 ?m) 22500 e
  3.6 fC

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Signal to Noise ratio

• Landau distribution has a low energy tail -
  becomes even lower by noise broadening
• Noise sources (ENC Equivalent Noise Charge)
  - Capacitance -
  Leakage Current - Thermal
  Noise (bias resistor)

• Good hits selected by requiring NADC gt noise tail
  If cut too high ? efficiency
  loss If cut too low ? noise
  occupancy
• Figure of Merit Signal-to-Noise Ratio S/N
• Typical values gt10-15, people get nervous below
  10. Radiation damage severely degrades
  the S/N.

11
The CERN RD50 Collaboration http//www.cern.ch/rd
50
RD50 Development of Radiation Hard Semiconductor
Devices for High Luminosity Colliders

• Collaboration formed in November 2001
• Experiment approved as RD50 by CERN in June 2002
• Main objective

Development of ultra-radiation hard semiconductor
detectors for the luminosity upgrade of the LHC
to 1035 cm-2s-1 (Super-LHC). Challenges -
Radiation hardness up to 1016 cm-2 required
- Fast signal collection (Going
from 25ns to 10 ns bunch crossing ?) - Low mass
(reducing multiple scattering close to
interaction point) - Cost effectiveness (big
surfaces have to be covered with detectors!)

• Presently 261 members from 52 institutes

Belarus (Minsk), Belgium (Louvain), Canada
(Montreal), Czech Republic (Prague (3x)),
Finland (Helsinki, Lappeenranta), Germany
(Berlin, Dortmund, Erfurt, Freiburg, Hamburg,
Karlsruhe), Israel (Tel Aviv), Italy (Bari,
Bologna, Florence, Padova, Perugia, Pisa, Trento,
Turin), Lithuania (Vilnius), The Netherlands
(Amsterdam), Norway (Oslo (2x)), Poland (Warsaw
(2x)), Romania (Bucharest (2x)), Russia (Moscow),
St.Petersburg), Slovenia (Ljubljana), Spain
(Barcelona, Valencia), Switzerland (CERN, PSI),
Ukraine (Kiev), United Kingdom (Exeter, Glasgow,
Lancaster, Liverpool, Sheffield, University of
Surrey), USA (Fermilab, Purdue University,
Rochester University, SCIPP Santa Cruz, Syracuse
University, BNL, University of New Mexico)
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Approaches to develop radiation harder solid
state tracking detectors

• Defect Engineering of SiliconDeliberate
  incorporation of impurities or defects into the
  silicon bulk to improve radiation tolerance of
  detectors
• Needs Profound understanding of radiation damage
• microscopic defects, macroscopic parameters
• dependence on particle type and energy
• defect formation kinetics and annealing
• Examples
• Oxygen rich Silicon (DOFZ, Cz, MCZ, EPI)
• Oxygen dimer hydrogen enriched Si
• Pre-irradiated Si
• Influence of processing technology
• New Materials
• Silicon Carbide (SiC), Gallium Nitride (GaN)
• Diamond (CERN RD42 Collaboration)
• Amorphous silicon
• Device Engineering (New Detector Designs)
• p-type silicon detectors (n-in-p)
• thin detectors, epitaxial detectors
• 3D detectors and Semi 3D detectors, Stripixels
• Cost effective detectors

• Scientific strategies
• Material engineering
• Device engineering
• Change of detectoroperational conditions

CERN-RD39Cryogenic Tracking Detectorsoperation
at 100-200K to reduce charge loss
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Silicon Materials under Investigation by RD50
Material Symbol ? (?cm) Oi (cm-3)
Standard FZ (n- and p-type) FZ 17?10 3 lt 5?1016
Diffusion oxygenated FZ (n- and p-type) DOFZ 17?10 3 12?1017
Magnetic Czochralski Si, Okmetic, Finland (n- and p-type) MCz 1?10 3 5?1017
Czochralski Si, Sumitomo, Japan (n-type) Cz 1?10 3 8-9?1017
Epitaxial layers on Cz-substrates, ITME, Poland (n- and p-type) EPI 50 - 100 lt 1?1017

• DOFZ silicon
• Enriched with oxygen on wafer level,
  inhomogeneous distribution of oxygen
• CZ silicon
• high Oi (oxygen) and O2i (oxygen dimer)
  concentration (homogeneous)
• formation of shallow Thermal Donors possible
• Epi silicon
• high Oi , O2i content due to out-diffusion from
  the CZ substrate (inhomogeneous)
• thin layers high doping possible (low starting
  resistivity)

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Standard FZ, DOFZ, Cz and MCz Silicon
24 GeV/c proton irradiation

• Standard FZ silicon
• type inversion at 2?1013 p/cm2
• strong Neff increase at high fluence
• Oxygenated FZ (DOFZ)
• type inversion at 2?1013 p/cm2
• reduced Neff increase at high fluence

• CZ silicon and MCZ silicon
• no type inversion in the overall fluence range
  (verified by TCT measurements) (verified for CZ
  silicon by TCT measurements, preliminary result
  for MCZ silicon) ? donor generation
  overcompensates acceptor generation in high
  fluence range
• Common to all materials (after hadron
  irradiation)
• reverse current increase
• increase of trapping (electrons and holes) within
  20

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EPI Devices Irradiation experiments
G.Lindström et al.,10th European Symposium on
Semiconductor Detectors, 12-16 June 2005
G.Kramberger et al., Hamburg RD50 Workshop,
August 2006

• Epitaxial silicon
• Layer thickness 25, 50, 75 ?m (resistivity 50
  ?cm) 150 ?m (resistivity 400 ?cm)
• Oxygen O ? 9?1016cm-3 Oxygen dimers
  (detected via IO2-defect formation)

?105V (25mm)
?230V (50mm)
?320V (75mm)

• Only little change in depletion voltage
• No type inversion up to 1016 p/cm2 and 1016
  n/cm2?high electric field will stay at front
  electrode!?reverse annealing will decreases
  depletion voltage!
• Explanation introduction of shallow donors is
  bigger than
  generation of deep acceptors

• CCE (Sr90 source, 25ns shaping)? 6400 e (150
  mm 2x1015 n/cm-2) ? 3300 e (75mm 8x1015
  n/cm-2) ? 2300 e (50mm 8x1015 n/cm-2)

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Device engineeringp-in-n versus n-in-p detectors
n-type silicon after high fluences
p-type silicon after high fluences
non-p
pon-n

• p-on-n silicon, under-depleted
• Charge spread degraded resolution
• Charge loss reduced CCE

• n-on-p silicon, under-depleted
• Limited loss in CCE
• Less degradation with under-depletion
• Collect electrons (fast)

Be careful, this is a very schematic
explanation,reality is more complex !
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n-in-p microstrip detectors
n-in-p - no type inversion, high electric field
stays on structured side - collection
of electrons

• n-in-p microstrip detectors (280mm) on p-type FZ
  silicon
• Detectors read-out with 40MHz

CCE 6500 e (30) after 7.5 1015 p cm-2 at 900V
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3D detector - concepts
Introduced by S.I. Parker et al., NIMA 395
(1997) 328

• 3D electrodes - narrow columns along detector
  thickness, - diameter 10mm, distance 50 -
  100mm
• Lateral depletion - lower depletion voltage
  needed - thicker detectors possible - fast
  signal - radiation hard

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3D detector - concepts
Introduced by S.I. Parker et al., NIMA 395
(1997) 328

• 3D electrodes - narrow columns along detector
  thickness, - diameter 10mm, distance 50 -
  100mm
• Lateral depletion - lower depletion voltage
  needed - thicker detectors possible - fast
  signal - radiation hard

n-columns
p-columns
wafer surface
n-type substrate

• Simplified 3D architecture
• n columns in p-type substrate, p backplane
• operation similar to standard 3D detector
• Simplified process
• hole etching and doping only done once
• no wafer bonding technology needed

• Simulations performed
• Fabrication
• IRST(Italy), CNM Barcelona

metal strip
hole
C. Piemonte et al., NIM A541 (2005) 441
hole
Hole depth 120-150mm Hole diameter 10mm
C.Piemonte et al., STD06, September 2006

• First CCE tests under way

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Conclusion

• New Materials like SiC and GaN have been
  characterized (not shown in this talk) . ?
  ? CCE tests show that these materials are not
  radiation harder than silicon ? Silicon
  (operated at e.g. -30C) seems presently to be
  the best choice
• At fluences up to 1015cm-2 (Outer layers of SLHC
  detector) the depletion voltage change and the
  large area to be covered is major problem
• MCZ silicon detectors could be a cost-effective
  radiation hard solution
• p-type (FZ and MCZ) silicon microstrip detectors
  show good results CCE ? 6500 e Feq
  4?1015 cm-2, 300mm, collection of electrons,
  no reverse annealing observed in CCE
  measurement!
• At the fluence of 1016cm-2 (Innermost layer of a
  SLHC detector) the active thickness of any
  silicon material is significantly reduced due to
  trapping. New options
• Thin/EPI detectors drawback radiation hard
  electronics for low signals needed
  e.g. 3300e at Feq
  8x1015cm-2, 75mm EPI,
  . thicker layers (150 mm presently
  under test)
• 3D detectors drawback very difficult
  technology
  .. steady progress within RD50

Further information http//cern.ch/rd50/