radiation tolerant silicon detectors michael moll ( cern – ph-dt2-sd) cern – ph-dt2 –...
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Radiation Tolerant Silicon Detectors
Michael Moll Michael Moll ( CERN – PH-DT2-SD)( CERN – PH-DT2-SD)
CERN – PH-DT2 – Scientific Tea meeting 13.10.2006
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
Outline
Michael Moll – PH-DT2 – Scientific Tea, 11 October 2006 -2 / 20-
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 -3 / 20-
RD50
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 m– 150 m 50 m pitch results in 14.4 m resolution
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: 300m (150m - 500m used)
using n-type silicon with a resistivity of
= 2 Kcm (ND ~2.2.1012cm-3)
results in a depletion voltage ~ 150 V
12
p
Michael Moll – PH-DT2 – Scientific Tea, 11 October 2006 -4 / 20-
RD50 Example – The ATLAS module
Michael Moll – PH-DT2 – Scientific Tea, 11 October 2006 -5 / 20-
RD50 LHCb – VELO: Silicon sensor details
42 mm8 mm
300 m thick sensors n-on-n, DOFZ wafers 42 mm radius AC coupled, double metal 2048 strips / sensor Pitch from 40 to 100 m Produced by Micron Semiconductor
-measuring sensor(radial strips with a stereo angle)
R-measuring sensor(45 degree circular segments)
[Martin van Beuzekom, STD6, September 2006]
Michael Moll – PH-DT2 – Scientific Tea, 11 October 2006 -6 / 20-
RD50 LHCb-VELO - Module construction
Kapton hybrid
Carbon fibre
Thermal Pyrolytic Graphite (TPG)
• 4 layer kapton circuit• Heat transport with TPG• Readout with 16 Beetle chips
• 128 channels, 25 ns shaping time, analog pipeline• 0.25 m CMOS• no performance loss up to 40 Mrad• Yield > 80 %
Beetle
[Martin van Beuzekom, STD6, September 2006]
Michael Moll – PH-DT2 – Scientific Tea, 11 October 2006 -7 / 20-
RD50 Motivation for R&D on Radiation Tolerant Detectors: Super - LHC
• LHC upgrade LHC (2007), L = 1034cm-2s-1
(r=4cm) ~ 3·1015cm-2
Super-LHC (2015 ?), L = 1035cm-2s-1
(r=4cm) ~ 1.6·1016cm-2
• LHC (Replacement of components) e.g. - LHCb Velo detectors (~2010) - ATLAS Pixel B-layer (~2012)
• Linear collider experiments (generic R&D)Deep understanding of radiation damage will be fruitful for linear collider experiments where high doses of e, will play a significant role.
5 years
2500 fb-1
10 years
500 fb-1
5
0 10 20 30 40 50 60
r [cm]
1013
5
1014
5
1015
5
1016
eq
[cm
-2]
total fluence eqtotal fluence eq
neutrons eq
pions eq
other charged
SUPER - LHC (5 years, 2500 fb-1)
hadrons eqATLAS SCT - barrelATLAS Pixel
Pixel (?) Ministrip (?)
Macropixel (?)
(microstrip detectors)
[M.Moll, simplified, scaled from ATLAS TDR]
Michael Moll – PH-DT2 – Scientific Tea, 11 October 2006 -8 / 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 -9 / 20-
RD50
0 10 20 30 40 50 60 70 80Signal [1000 electrons]
0
100
200
300
400
500
600
Cou
nts
[M.Moll][M.Moll]
The charge signal
Collected Charge for a Minimum Ionizing Particle (MIP)
Mean energy loss dE/dx (Si) = 3.88 MeV/cm 116 keV for 300m 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
Mean charge
Most probable charge ≈ 0.7 mean
noise Cut (threshold)
Michael Moll – PH-DT2 – Scientific Tea, 11 October 2006 -10 / 20-
RD50
0 10 20 30 40 50 60 70 80Signal [1000 electrons]
0
200
400
600
800
1000
1200
Cou
nts
non irradiatednon irradiated
1.1 x 1015 p/cm21.1 x 1015 p/cm2
9.3 x 1015 p/cm29.3 x 1015 p/cm2
p-type MCZ silicon p-type MCZ silicon 5x5 mm2 pad5x5 mm2 pad
90Sr - source90Sr - source
[M.Moll][M.Moll]
Signal to Noise ratio
Good hits selected by requiring NADC > 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 >10-15, people get nervous below 10. Radiation damage severely degrades the S/N.
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)
dCENC
IENC
RTkENC B
less signal
more n
oise
What is signal and what is noise?
Michael Moll – PH-DT2 – Scientific Tea, 11 October 2006 -11 / 20-
RD50 The CERN RD50 Collaboration http://www.cern.ch/rd50
Collaboration formed in November 2001 Experiment approved as RD50 by CERN in June 2002 Main objective:
Presently 261 members from 52 institutes
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!)
RD50: Development of Radiation Hard Semiconductor Devices for High Luminosity Colliders
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 -12 / 20-
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
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
Scientific strategies:
I. Material engineering
II. Device engineering
III. Change of detectoroperational conditions
CERN-RD39“Cryogenic Tracking Detectors”
operation at 100-200K
to reduce charge loss
Michael Moll – PH-DT2 – Scientific Tea, 11 October 2006 -13 / 20-
RD50 Silicon Materials under Investigation by RD50
Material Symbol (cm) [Oi] (cm-3)
Standard FZ (n- and p-type) FZ 1–710 3 < 51016
Diffusion oxygenated FZ (n- and p-type) DOFZ 1–710 3 ~ 1–21017
Magnetic Czochralski Si, Okmetic, Finland
(n- and p-type)
MCz ~ 110 3 ~ 51017
Czochralski Si, Sumitomo, Japan (n-type) Cz ~ 110 3 ~ 8-91017
Epitaxial layers on Cz-substrates, ITME, Poland (n- and p-type)
EPI 50 - 100 < 11017
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 -14 / 20-
RD50 Standard FZ, DOFZ, Cz and MCz Silicon
24 GeV/c proton irradiation
0 2 4 6 8 10proton fluence [1014 cm-2]
0
200
400
600
800
Vde
p [V
]0
2
4
6
8
10
12
Nef
f [10
12 c
m-3
]
CZ <100>, TD killedCZ <100>, TD killedMCZ <100>, HelsinkiMCZ <100>, HelsinkiSTFZ <111>STFZ <111>DOFZ <111>, 72 h 11500CDOFZ <111>, 72 h 11500C Standard FZ silicon
• type inversion at ~ 21013 p/cm2
• strong Neff increase at high fluence
Oxygenated FZ (DOFZ)• type inversion at ~ 21013 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%
Michael Moll – PH-DT2 – Scientific Tea, 11 October 2006 -15 / 20-
RD50 Epitaxial silicon
Layer thickness: 25, 50, 75 m (resistivity: ~ 50 cm); 150 m (resistivity: ~ 400 cm) Oxygen: [O] 91016cm-3; Oxygen dimers (detected via IO2-defect formation)
EPI Devices – Irradiation experiments
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
G.Lindström et al.,10th European Symposium on Semiconductor Detectors, 12-16 June 2005 G.Kramberger et al., Hamburg RD50 Workshop, August 2006
0 2.1015 4.1015 6.1015 8.1015 1016
eq [cm-2]
0
1014
2.1014
Nef
f(t0)
[cm
-3]
25 m, 80 oC25 m, 80 oC
50 m, 80 oC50 m, 80 oC
75 m, 80 oC75 m, 80 oC
23 GeV protons23 GeV protons
320V (75m)
105V (25m)
230V (50m)
CCE (Sr90 source, 25ns shaping): 6400 e (150 m; 2x1015 n/cm-2) 3300 e (75m; 8x1015 n/cm-2) 2300 e (50m; 8x1015 n/cm-2)
0 20 40 60 80 100eq [1014 cm-2]
0
2000
4000
6000
8000
10000
12000
Sign
al [
e]
150 m - neutron irradiated 75 m - proton irradiated 75 m - neutron irradiated 50 m - neutron irradiated 50 m - proton irradiated
[M.Moll]
[Data: G.Kramberger et al., Hamburg RD50 Workshop, August 2006]
Michael Moll – PH-DT2 – Scientific Tea, 11 October 2006 -16 / 20-
RD50
p-on-n silicon, under-depleted:
• Charge spread – degraded resolution
• Charge loss – reduced CCE
p+on-n
Device engineeringp-in-n versus n-in-p detectors
n-on-p silicon, under-depleted:
•Limited loss in CCE
•Less degradation with under-depletion
•Collect electrons (fast)
n+on-p
n-type silicon after high fluences: p-type silicon after high fluences:
Be careful, this is a very schematic explanation,reality is more complex !
Michael Moll – PH-DT2 – Scientific Tea, 11 October 2006 -17 / 20-
RD50
0 2 4 6 8 10fluence [1015cm-2]
0
5
10
15
20
25
CC
E (
103 e
lect
rons
)
24 GeV/c p irradiation24 GeV/c p irradiation
[M.Moll][M.Moll]
[Data: G.Casse et al., NIMA535(2004) 362][Data: G.Casse et al., NIMA535(2004) 362]
n-in-p microstrip detectors
n-in-p microstrip detectors (280m) on p-type FZ silicon Detectors read-out with 40MHz
CCE ~ 6500 e (30%) after 7.5 1015 p cm-2 at
900V
n-in-p: - no type inversion, high electric field stays on structured side - collection of electrons
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 !
0 100 200 300 400 500time at 80oC[min]
0 500 1000 1500 2000 2500time [days at 20oC]
02468
101214161820
CC
E (
103 e
lect
rons
)
800 V800 V
1.1 x 1015cm-2 1.1 x 1015cm-2 500 V500 V
3.5 x 1015cm-2 (500 V)3.5 x 1015cm-2 (500 V)
7.5 x 1015cm-2 (700 V)7.5 x 1015cm-2 (700 V)
M.MollM.Moll
[Data: G.Casse et al., to be published in NIMA][Data: G.Casse et al., to be published in NIMA]
Michael Moll – PH-DT2 – Scientific Tea, 11 October 2006 -18 / 20-
RD50 “3D” electrodes: - narrow columns along detector thickness,
- diameter: 10m, distance: 50 - 100m Lateral depletion: - lower depletion voltage needed
- thicker detectors possible- fast signal- radiation hard
3D detector - concepts
n-columns p-columnswafer surface
n-type substrate
ionizing particle
carriers collectedat the same time
Introduced by: S.I. Parker et al., NIMA 395 (1997) 328
Michael Moll – PH-DT2 – Scientific Tea, 11 October 2006 -19 / 20-
RD50 “3D” electrodes: - narrow columns along detector thickness,
- diameter: 10m, distance: 50 - 100m Lateral depletion: - lower depletion voltage needed
- thicker detectors possible- fast signal- radiation hard
3D detector - concepts
n-columns p-columns wafer surface
n-type substrate
Introduced by: S.I. Parker et al., NIMA 395 (1997) 328
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
[C. Piemonte et al., NIM A541 (2005) 441]
hole
hole metal strip
C.Piemonte et al., STD06, September 2006
Hole depth 120-150mHole diameter ~10m
First CCE tests under way
Michael Moll – PH-DT2 – Scientific Tea, 11 October 2006 -20 / 20-
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; eq
= 41015 cm-2, 300m, 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 eq 8x1015cm-2, 75m EPI, …. thicker layers (150 m presently under test)
3D detectors : drawback: very difficult technology ….. steady progress within RD50
Further information: http://cern.ch/rd50/