Page 1
Jaap Velthuis (University of Bristol) 1
Radiation damage in silicon sensors• Topics:
– Damage due to protons and neutrons• Radiation induced defects• Annealing• Donor removal (type
inversion)• Alternatives to p-n
– Damage due to electrons, photons,…
In case of any questions:
[email protected]
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Jaap Velthuis (University of Bristol) 2
Quick review• Semiconductor detectors are used close
to primary vertex to – Limit occupancy and reduce ambiguities– Give very precise space point
• Need trick to remove free charge carriers– Use high band gap semiconductor– Cool to cryogenic temperatures– Build p-n junction and deplete detector
kT
ENNpnn GVCi 2
exp
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Jaap Velthuis (University of Bristol) 3
Quick review (II)
• Energy loss described by Behte-Bloch equation– Minimum ionizing particle– Energy loss (=signal) is Landau distributed
• Particles scatter in matter, so need to have thin detectors
• MIP yields 8900 e-h pairs per 100 m Si
• If pitch ~ charge cloud, charge is shared. Need lots of strips.– Trick intermediate strip using C-charge sharing,
but non-linear charge sharing
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Jaap Velthuis (University of Bristol) 4
Charged … matter• Bethe-Bloch describes
average energy loss• Collisions stochastic nature,
hence energy loss is distribution instead of number.
• First calculated for thin layers was Landau. Hence energy loss is Landau distributed.
x
ex
xL2
1exp)(0
is most probable value
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Jaap Velthuis (University of Bristol) 5
-electrons• Some of generated carriers
have so much kinetic energy that they will free more charge carriers– Lots of signal– Do not know where hit was
• Responsible for tails signal distribution
• Number of ’s dependent on material: EG high-> less
-+ - -
-
++ +
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Jaap Velthuis (University of Bristol) 6
Charge collection• If pitch > charge cloud all charge
collected on 1 strip
• In this case analog signal value not importantchose digital or binary readout
• To do better need to share charge over more strips need pitch20m for 300 m thick sensor
• Problem: connecting all strips to readout channel yields too many strips
12
11
0
21
0
22 dxxxdxxx
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Jaap Velthuis (University of Bristol) 7
Strip pitch & Analogue vs binary
• Can improve position reconstruction by using neighbour signal. Simplest: CoG
• But the further away, the more effect on . So S/N neighbour limits resolution.
• Now choice:– use many strips to get
enough S/N on neighbour and use analogue readout
– Live with limitations, spread out strips and readout binary
i
iirecon S
xSx
2
2
22
reconix xxS
N
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Jaap Velthuis (University of Bristol) 8
Strip pitch & Analogue vs binary
• If analogue, need fancy chip that consumes lots of power to process signal. – Need: shaper, pipeline, ADC
& Storage of pulse heights– Advantages: high precision,
good understanding noise– Disadvantages: high power,
lots of processing, loads of infrastructure
• ATLAS chose binary. 50m pitch thus 17m precision and easy read out
Onl
y th
is n
eede
dfo
r bi
nary
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Jaap Velthuis (University of Bristol) 9
sLHC radiation dose• 5 year radiation dose
close to beam pipe ~1016 neq/cm2
– too high for state-of-the-art standard silicon sensors
• Most radiation hard material: diamond– High bandgap– Displacement energy
43eV (13-20 for Si)
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Jaap Velthuis (University of Bristol) 10
Radiation with protons/neutrons
• Silicon crystal is organised in a Face-Centred-Cubic lattice (or diamond lattice)
• Remember: impurities yield lattice sites with different number of valence e- doping
• Energetic radiation knocks atoms out of lattice: similar thing
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Jaap Velthuis (University of Bristol) 11
Radiation with protons/neutrons
• Energy needed to displace atom from lattice=15eV
• Damage energy dependent– ~<2keVisolated point defect– 2-12keVdefect cluster– ~>12keVmany defect clusters
• This damage is called Non-Ionizing Energy Loss (NIEL)– Results scaled to 1MeV neutrons
• Electrons and photons don’t make defects!
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Jaap Velthuis (University of Bristol) 12
Radiation … neutrons
• Displacement changes band structure– Levels in middle of band gap arise– Can capture electrons/holes
(trapping)– Can donate electrons/holes– Can increase leakage current by
two-step transitions from valence to conduction band
– Can act as recombination centre
• Role dependent on neighbours and present impurities
Conduction band
+ + + + + + +
Valence band
- - - - - -
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Jaap Velthuis (University of Bristol) 13
Annealing• Annealing is process in which crystal
recovers from radiation damage– Atoms occupy vacancies– Defect complexes change in different
complexes
• New defect complexes can also worsen damage (reverse annealing)
• Processes highly temperature dependent• “Not very well understood”
– Depends on which (unintentional) impurities present
– Which defects are formed– Alchemy!
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Jaap Velthuis (University of Bristol) 14
Radiation damage: Leakage current
• I = Volume• Material
independent– linked to defect
clusters
• Annealing material independent
• Scales with NIEL• Temp dependence
1011 1012 1013 1014 1015
eq [cm-2]
10-6
10-5
10-4
10-3
10-2
10-1
I /
V
[A/c
m3 ]
n-type FZ - 7 to 25 Kcmn-type FZ - 7 to 25 Kcmn-type FZ - 7 Kcmn-type FZ - 7 Kcmn-type FZ - 4 Kcmn-type FZ - 4 Kcmn-type FZ - 3 Kcmn-type FZ - 3 Kcm
n-type FZ - 780 cmn-type FZ - 780 cmn-type FZ - 410 cmn-type FZ - 410 cmn-type FZ - 130 cmn-type FZ - 130 cmn-type FZ - 110 cmn-type FZ - 110 cmn-type CZ - 140 cmn-type CZ - 140 cm
p-type EPI - 2 and 4 Kcmp-type EPI - 2 and 4 Kcm
p-type EPI - 380 cmp-type EPI - 380 cm
kT
ETTI g
2exp2 = 3.99 0.03 x 10-17Acm-1
after 80minutes annealing at 60C
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Jaap Velthuis (University of Bristol) 15
Type inversion• Dopants may be
captured into defect complexes.
• Donor removal and acceptor generation– type inversion: n p– depletion width
grows from n+ contact
• Increase in full depletion voltage
biasbi
DA
DAd VV
NqN
NNx
2
0 0.5 1 1.5 2eq [1014cm-2]
1
2
3
4
5
|Nef
f| [1
012 c
m-3
]
50
100
150
200
250
300
Vde
p [V
] (
300
m)
1.8 Kcm Wacker 1.8 Kcm Wacker 2.6 Kcm Polovodice2.6 Kcm Polovodice3.1 Kcm Wacker 3.1 Kcm Wacker 4.2 Kcm Topsil 4.2 Kcm Topsil
Neutron irradiationNeutron irradiation
cNN effeff exp0
= 0.025cm-1 measured afterbeneficial anneal
P-strips in p-bulk
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Jaap Velthuis (University of Bristol) 16
Partially depleted detectors
• Undepleted region like high-ohmic resistor
• If detector partially depleted – from strip side
only charge in depleted region contributes smaller signal, similar spatial resolution
– from backplane carriers travel towards
strips, but don’t reach it signal spread over many strips poor spatial resolution
undepleted
undepleted
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Jaap Velthuis (University of Bristol) 17
Example: NA60 type inversion
• After type inversion Vdep increases with dose smaller radii
• Lowering Vbias leaves larger area undepleted– Depletion from
backside layer almost dead
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Jaap Velthuis (University of Bristol) 18
Example: NA60 type inversion
• After type inversion Vdep increases with dose smaller radii
• Lowering Vbias leaves larger area undepleted– Depletion from
backside layer almost dead
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Jaap Velthuis (University of Bristol) 19
Example: NA60 type inversion
• After type inversion Vdep increases with dose smaller radii
• Lowering Vbias leaves larger area undepleted– Depletion from
backside layer almost dead
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Jaap Velthuis (University of Bristol) 20
Example: NA60 type inversion
• After type inversion Vdep increases with dose smaller radii
• Lowering Vbias leaves larger area undepleted– Depletion from
backside layer almost dead
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Jaap Velthuis (University of Bristol) 21
Example: NA60 type inversion
• After type inversion Vdep increases with dose smaller radii
• Lowering Vbias leaves larger area undepleted– Depletion from
backside layer almost dead
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Jaap Velthuis (University of Bristol) 22
Example: NA60 type inversion
• After type inversion Vdep increases with dose smaller radii
• Lowering Vbias leaves larger area undepleted– Depletion from
backside layer almost dead
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Jaap Velthuis (University of Bristol) 23
Example: NA60 type inversion
• After type inversion Vdep increases with dose smaller radii
• Lowering Vbias leaves larger area undepleted– Depletion from
backside layer almost dead
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Jaap Velthuis (University of Bristol) 24
Thermal runaway• Problem with donor removal:
– Need higher voltages to deplete detector
– Higher voltage higher leakage current
– Higher leakage current more power dissipated in detector
– More power heating– heating more leakage current
• “Solution”: cool detectors– ATLAS will operate at –7oC
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Jaap Velthuis (University of Bristol) 25
Solutions radiation damage• Start with n+ strips in n-type detector
– After inversion substrate p type depletion now from strip side (LHC-b)
– Build p-type detectors with n-strips• Different crystal orientation
– Less dangling bonds at Si-SiO2 interface
• Material Engineering• Operate very cold • Use different material (e.g. CVD
diamond)• 3D-structures
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Jaap Velthuis (University of Bristol) 26
N+-on-n detectors• Standard: p-strips on n-bulk• Problem: n-bulk becomes p-type
– pn-junction moves from strip-bulk to
bias contact-bulk interface– Only good spatial resolution when
fully depleted
• Solution: make n+-strips in n-bulk– After radiation: n+-strips and p-bulk– Disadvantages:
• strips not well isolated before radiation. Need p-strips (or spray) between n-strips.
• Need guard rings at bottom (expensive)
n+-strips N-type bulkbecomes p-typeAfter radiation
n+ biascontact
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Jaap Velthuis (University of Bristol) 27
P-type detectors• P-type bulk with n-
type strips– Collect electrons
instead of holes• Electron mobility ~3>
hole mobility less trapping
– Depletion starts from strip side
• Even at partial depletion good spatial resolution
– No need for guard rings on backside cheaper than n+-on-n
1E15cm-2 10 yearsDose for ATLAS strips
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Jaap Velthuis (University of Bristol) 28
Material engineering
• Diffusing oxygen suppresses V2O formation – V + O VO
– V + VO V2O
– V2O reverse annealing
• Still alchemy…
St = 0.0154
[O] = 0.0044 0.0053
[C] = 0.0437
0
1E+12
2E+12
3E+12
4E+12
5E+12
6E+12
7E+12
8E+12
9E+12
1E+13
0 1E+14 2E+14 3E+14 4E+14 5E+14
Proton fluence (24 GeV/c ) [cm-2]
|Nef
f| [c
m-3
]
0
100
200
300
400
500
VF
D f
or 3
00
m t
hic
k d
etec
tor
[V]
Standard (P51)O-diffusion 24 hours (P52)O-diffusion 48 hours (P54)O-diffusion 72 hours (P56)Carbon-enriched (P503)
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Jaap Velthuis (University of Bristol) 29
Lazarus Effect• Remember:
• Radiation induces (more) traps. Capture and emission very temperature dependent.
• Cooling makes sure traps stay filled no trapping
• Can actually operate forward biased• Downside: need to keep detector in
cryostat
kT
ENNpnn GVCi 2
exp
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Jaap Velthuis (University of Bristol) 30
CVD Diamond• Remember:
• Limit background charge by using large bandgap material– Si: EG=1.12 eV 1.5E10 free carriers/cm3
– Diamond: EG=5.47 eV 6E-28 free carriers/cm3 no need for pn-junction
– Diamond lattice very strong very radiation hard
• Note: large EG only few eh pairs produced (3600 vs 8900 per 100 m), but also lower noise
kT
ENNpnn GVCi 2
exp
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Jaap Velthuis (University of Bristol) 31
Diamond growth• Trap- and recombination
centers limit charge collection
• Trapped charge at grain boundaries builds up a polarization field superimposed on the biasing field
• Substrate and growth side must be ground and polished for good quality
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Jaap Velthuis (University of Bristol) 32
ATLAS CVD diamond pixel module• Sensor:
– Active area: 61x16.5mm2
– Thickness 800µm– Pixel size 400(600)x50µm2
– 46k pixels• ATLAS frontend chip FE-I3
– 0.25µm IBM– Radiation tolerant >50MRad– Designed for Si sensors– Binary/low res. analog readout
• Noise same as bare FE-chip– Noise ≈137e-
– Threshold ≈1454e-
– Threshold spread ≈25e-
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Jaap Velthuis (University of Bristol) 33
CVD diamond radiation hardness
• Still S/N≈18-25 after 1.8x1016 p/cm2 (~500 Mrad) depending on field
• No problem operating in sLHC conditions– Noise limited by
electronics, NOT by sensor capacitance or leakage current
E=2V/µm
E=1V/µm
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Jaap Velthuis (University of Bristol) 34
Single crystal diamond
• Largest single crystal diamond 14x14mm– No grain boundaries → no trapping
• Produced 400 µm thick single chip sensor using ATLAS FE chip
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Jaap Velthuis (University of Bristol) 35
Diamond signal spectrum
S/N≈99N=136.8 e-
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Jaap Velthuis (University of Bristol) 36
CVD diamond as radiation monitor
• Babar uses diamond radiation monitors for almost 2 years
• Response is fast and material doesn’t die
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Jaap Velthuis (University of Bristol) 37
Czochralski silicon (Cz)
Czochralski Growth
• Pull Si-crystal from a Si-melt contained
in a silica crucible while rotating.• Silica crucible is dissolving oxygen into
the melt high concentration of O in CZ
• Material used by IC industry (cheap) • Recent developments (~2 years)
made • CZ available in sufficiently high purity
(resistivity) to allow for use as particle detector.
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Jaap Velthuis (University of Bristol) 38
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
Czochralski silicon (Cz)• 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 donor generation overcompensates acceptor generation in high fluence range
24 GeV/c proton irradiation
Page 39
Jaap Velthuis (University of Bristol) 39
3D detectors
• Problems standard sensors: – After radiation high voltage needed to fully
deplete high currents high noise & thermal runaway
– Need guard ring structures lot of wasted space
• Make “strips” vertical inside bulk
3D standard
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Jaap Velthuis (University of Bristol) 40
3D detectors
• Sideways depletion: smaller distance between electrode and strips lower depletion voltage
• Sideways charge collection – Edgeless (dead edge < 5 m)– Still use full 300 m thickness– Rapid charge collection (~2 ns)– Radiation hardness
3D standard
P=50, 100 or 200 m
Physicaledge
Page 41
Jaap Velthuis (University of Bristol) 41
3D detectors• S/N source test=13 with 121 m
thick detector
Page 42
Jaap Velthuis (University of Bristol) 42
3D detectors• Efficiency loss underneath electrodes
– P-type loss 66%– N-type loss 43%
• Signal: 10-90% <5µm !– Standard sensors need 5-6mm
• Used for TOTEM
electrodes
Page 43
Jaap Velthuis (University of Bristol) 43
Summary radiation hardness
• Radiation damage in sensors mainly bulk damage– Atoms knocked out of their lattice
position extra levels in band gap • Effectively donor removal (type inversion)• High leakage currents
– High noise– Thermal runaway
• Problems to get full depletion
Page 44
Jaap Velthuis (University of Bristol) 44
Summary radiation hardness (II)
• Solutions:– n+-on-n or even better n-on-p detectors– Material engineering (oxygenated Si/CZ)– Cool
• ATLAS at –7oC• cryogenic temperatures (Lazarus effect)
– Use different materials like diamond– Use different detector type like 3D