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
RD50 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
Michael Moll – PH-DT2 – Scientific Tea, 11 October 2006
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RD50
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
pitch
typical thickness: 300mm
(150mm - 500mm 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)
p
and using center of strip as measured coordinate results in:  
12
 typical pitch values are 20 mm– 150 mm  50 mm pitch results in 14.4 mm resolution
Michael Moll – PH-DT2 – Scientific Tea, 11 October 2006
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RD50
Example – The ATLAS module
Michael Moll – PH-DT2 – Scientific Tea, 11 October 2006
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RD50 LHCb – VELO: Silicon sensor details







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
R-measuring sensor
(45 degree circular segments)
42 mm
8 mm
F-measuring sensor
(radial strips with a stereo angle)
[Martin van Beuzekom, STD6, September 2006]
Michael Moll – PH-DT2 – Scientific Tea, 11 October 2006
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RD50 LHCb-VELO - Module construction
Beetle
Kapton hybrid
Carbon fibre
Thermal Pyrolytic Graphite (TPG)
[Martin van Beuzekom, STD6, September 2006]
• 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 > 80 %
Michael Moll – PH-DT2 – Scientific Tea, 11 October 2006
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RD50
Motivation for R&D on
Radiation Tolerant Detectors: Super - LHC
• LHC upgrade
SUPER - LHC (5 years, 2500 fb-1)
LHC (2007), L = 1034cm-2s-1
Ministrip (?)
Pixel (?)
16
10
f(r=4cm) ~ 3·1015cm-2
500 fb-1
5
Super-LHC (2015 ?), L = 1035cm-2s-1
5 years
f(r=4cm) ~ 1.6·1016cm-2
2500 fb-1
• LHC (Replacement of components)
e.g. - LHCb Velo detectors (~2010)
- ATLAS Pixel B-layer (~2012)
Macropixel (?)
5
total fluence Feq
Feq [cm-2]
10 years
1015
5
neutrons Feq
pions Feq
1014
5
ATLAS SCT - barrel
(microstrip detectors)
ATLAS Pixel
13
10
0
10
other charged
hadrons Feq
20
30
40
50
60
[M.Moll, simplified, scaled from ATLAS TDR]
r [cm]
• Linear collider experiments (generic R&D)
Deep understanding of radiation damage will be fruitful for linear collider experiments where
high doses of e, g will play a significant role.
Michael Moll – PH-DT2 – Scientific Tea, 11 October 2006
-7 / 20-
RD50
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 –
I.
Change of effective doping concentration (higher depletion voltage,
under- depletion)
II. Increase of leakage current (increase of shot noise, thermal runaway)
III. 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!)
Signal/noise ratio is the quantity to watch
 Sensors can fail from radiation damage !
Michael Moll – PH-DT2 – Scientific Tea, 11 October 2006
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RD50
The charge signal
Collected Charge for a Minimum Ionizing Particle (MIP)
Most probable charge ≈ 0.7 mean
 Mean energy loss
dE/dx (Si) = 3.88 MeV/cm
 116 keV for 300mm thickness
Mean charge
500
400
Counts
 Most probable energy loss
≈ 0.7 mean
 81 keV
600
300
 3.6 eV to create an e-h pair
 72 e-h / mm (mean)
200
 108 e-h / mm (most probable)
100
 Most probable charge (300 mm)
[M.Moll]
0
≈ 22500 e
≈ 3.6 fC
noise
0
10
20
30
40
50
Signal [1000 electrons]
60
70
80
Cut (threshold)
Michael Moll – PH-DT2 – Scientific Tea, 11 October 2006
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RD50
Signal to Noise ratio
Landau distribution has a low energy tail
- becomes even lower by noise broadening
What is signal and
what is noise?
Noise sources: (ENC = Equivalent Noise Charge)
1200
- Capacitance
ENC  Cd
1000
9.3 x 1015 p/cm2
p-type MCZ silicon
5x5 mm2 pad
90
Sr - source
- Leakage Current
R
800
Counts
- Thermal Noise
(bias resistor) ENC  k BT
more noise
ENC  I
1.1 x 1015 p/cm2
600
400
non irradiated
200
[M.Moll]
Good hits selected by requiring NADC > noise tail
If cut too high  efficiency loss
If cut too low  noise occupancy
0
0
10
20
30
40
50
Signal [1000 electrons]
60
70
80
less signal
Figure of Merit: Signal-to-Noise Ratio S/N
Typical values >10-15, people get nervous below 10.
Radiation damage severely degrades the S/N.
Michael Moll – PH-DT2 – Scientific Tea, 11 October 2006
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RD50
The CERN RD50 Collaboration
http://www.cern.ch/rd50
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)
Michael Moll – PH-DT2 – Scientific Tea, 11 October 2006
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RD50
Approaches to develop
radiation harder solid state tracking detectors
 Defect Engineering of Silicon
Deliberate incorporation of impurities or defects into the
silicon bulk to improve radiation tolerance of detectors
Scientific strategies:
I.
Material engineering
II. Device engineering
III. Change of detector
operational conditions
CERN-RD39
“Cryogenic Tracking Detectors”
operation at 100-200K
to reduce charge loss
 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
 Monolithic devices
Michael Moll – PH-DT2 – Scientific Tea, 11 October 2006
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RD50
Silicon Materials under Investigation by RD50
Material
Standard FZ (n- and p-type)
Diffusion oxygenated FZ (n- and p-type)
Magnetic Czochralski Si, Okmetic, Finland
(n- and p-type)
Czochralski Si, Sumitomo, Japan (n-type)
Epitaxial layers on Cz-substrates,
ITME, Poland (n- and p-type)
Symbol
 (Wcm)
[Oi] (cm-3)
FZ
1–710 3
< 51016
DOFZ
1–710 3
~ 1–21017
MCz
~ 110 3
~ 51017
Cz
~ 110 3
~ 8-91017
EPI
50 - 100
< 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)
Michael Moll – PH-DT2 – Scientific Tea, 11 October 2006
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RD50
Standard FZ, DOFZ, Cz and MCz Silicon
800
600
• 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
Vdep [V]
 Standard FZ silicon
10
8
400
6
4
200
Neff [1012 cm-3]
24 GeV/c proton irradiation
12
CZ <100>, TD killed
MCZ <100>, Helsinki
STFZ <111>
DOFZ <111>, 72 h 11500C
2
0
0
2
4
6
8
10
0
proton fluence [1014 cm-2]
 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%
Michael Moll – PH-DT2 – Scientific Tea, 11 October 2006
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EPI Devices – Irradiation experiments
RD50
 Epitaxial silicon
G.Lindström et al.,10th European Symposium on Semiconductor Detectors, 12-16 June 2005
G.Kramberger et al., Hamburg RD50 Workshop, August 2006
 Layer thickness: 25, 50, 75 mm (resistivity: ~ 50 Wcm); 150 mm (resistivity: ~ 400 Wcm)
 Oxygen: [O]  91016cm-3; Oxygen dimers (detected via IO2-defect formation)
12000
105V
(25mm)
Neff(t0) [cm-3]
2.1014
25 mm, 80 oC
50 mm, 80 oC
75 mm, 80 oC
230V
(50mm)
1014
0
0
320V
(75mm)
2.1015
4.1015 6.1015
Feq [cm-2]
8.1015
150 mm - neutron irradiated
75 mm - proton irradiated
75 mm - neutron irradiated
50 mm - neutron irradiated
50 mm - proton irradiated
10000
Signal [e]
23 GeV protons
1016
 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
8000
6000
4000
2000
[Data: G.Kramberger et al., Hamburg RD50 Workshop, August 2006]
0
0
[M.Moll]
20 40 60 80 100
Feq [1014 cm-2]
 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)
Michael Moll – PH-DT2 – Scientific Tea, 11 October 2006
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RD50
Device engineering
p-in-n versus n-in-p detectors
p-type silicon after high fluences:
n-type silicon after high fluences:
p+on-n
n+on-p
p-on-n silicon, under-depleted:
n-on-p silicon, under-depleted:
• Charge spread – degraded resolution
•Limited loss in CCE
• Charge loss – reduced CCE
•Less degradation with under-depletion
•Collect electrons (fast)
Be careful, this is a very schematic explanation,
reality is more complex !
Michael Moll – PH-DT2 – Scientific Tea, 11 October 2006
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RD50
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 time [days at 20oC]
500
1000
1500
2000
Detectors read-out with 40MHz
20 0
25
24 GeV/c p irradiation
20
CCE ~ 6500 e (30%)
after 7.5 1015 p cm-2 at
900V
15
10
5
[Data: G.Casse et al., NIMA535(2004) 362]
0
0
[M.Moll]
2
4
6
8
10
fluence [1015cm-2]
CCE (103 electrons)
CCE (103 electrons)


18
16
14
12
10
8
6
4
2
0
2500
800 V
1.1 x 1015cm-2
500 V
3.5 x 1015cm-2 (500 V)
7.5 x 1015cm-2 (700 V)
[Data: G.Casse et al., to be published in NIMA]
M.Moll
0
100
200
300
400
o
time at 80 C[min]
500
 no reverse annealing visible in the CCE measurement !
e.g. for 7.5  1015 p/cm2 increase of Vdep from
Vdep~ 2800V to Vdep > 12000V is expected !
Michael Moll – PH-DT2 – Scientific Tea, 11 October 2006
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RD50
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
ionizing particle
carriers collected
at the same time
wafer surface
n-type substrate
Michael Moll – PH-DT2 – Scientific Tea, 11 October 2006
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RD50
3D detector - concepts
 “3D” electrodes: - narrow columns along detector thickness,
- diameter: 10mm, distance: 50 - 100mm
Introduced by: S.I. Parker et
al., NIMA 395 (1997) 328
n-columns
 Lateral depletion: - lower depletion voltage needed
- thicker detectors possible
- fast signal
- radiation hard
p-columns
wafer surface
n-type substrate
 Simplified 3D architecture
 n+ columns in p-type substrate, p+ backplane
 operation similar to standard 3D detector
 Simulations performed
 Fabrication:
 IRST(Italy), CNM Barcelona
metal strip
hole
[C. Piemonte et al.,
NIM A541 (2005) 441]
hole
 Simplified process
 hole etching and doping only done once
 no wafer bonding technology needed
Hole depth 120-150mm
Hole diameter ~10mm
C.Piemonte et al., STD06, September 2006
 First CCE tests under way
Michael Moll – PH-DT2 – Scientific Tea, 11 October 2006
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RD50

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. -30°C) 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/
Michael Moll – PH-DT2 – Scientific Tea, 11 October 2006
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