Dipartimento di Fisica e INFN, Ferrara, Italy
Oct. 21, 2005
Radiation Damage Effects in Silicon Recent Developments, Models and Scenarios
Si
Smithsonian Magazine
P. Riedler/CERN
Overview
•
•
•
•
•
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Silicon detectors in HEP experiments
Radiation environment in LHC experiments
Radiation effects in sensors and ASICs
Macroscopic and microscopic effects in sensors
New developments
Summary
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Silicon Detectors in HEP Experiments - Overview
Silicon Strip Detectors
Amplifier
Al strip
• Each strip is connected to one readout channel
p+
+ +
- ++-
• First prototypes: ~ 1980
• Different “flavours”, e.g.:
- N-in-n detectors
n bulk
n+
Al
particle
- Double sided detectors
- Floating intermediate strips
SiO2/Si3N4
• Material budget: ~ sensor thickness
• Not ideal for high track density environment
3D track points-> require 2 layers, information
can be ambiguous (ghost hits)
Strip
• Usually used for large surfaces
©ATLAS SCT
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Silicon Detectors in HEP Experiments - Overview
Silicon Pixel Detectors
•
Each 2D pixel is connected to one readout cell
•
First prototypes: ~ 1990
•
Currently applied technology: hybrid pixel detectors
•
Different “flavours”, e.g.:
•
Material budget: ~sensor + ASIC
•
Unambiguous 3D track information, ideal for high track
density environment
•
Usually not too large areas, large number of r.o.
channels
(R.O. ASICs are connected to a sensor via flip chip bonding)
–
–
–
N-in-n
Bias grids
Recent developments: monolithic detectors
ALICE SPD
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Silicon Detectors in HEP Experiments
…an incomplete list:
W98, NA57, ALEPH, DELPHI, L3, OPAL, NA60, CDF, D0, PHOBOS, PHENIX,
ALICE, ATLAS, CMS, LHCb, PANDA, CMB, P326, ……
Silicon detectors were/are used in practically all HEP experiments to provide
precise tracking information.
In collision experiments the high density track region is usually covered by
pixel detectors followed at larger radii by strip detectors.
In the last generation of HEP detectors radiation damage defects become
an important factor (higher luminosity, e.g. LHC).
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Silicon Detectors in LHC Experiments
ALICE
ATLAS
CMS
LHCb
Strips
4.9m2
64m2
210m2
14.3m2
Drift
1.3m2
Pixels
0.2m2
2m2
1m2
0.02m2
6.3 x 106
9.6 x 106
1 x 106
80 x 106
33 x 106
1 x 106
Number of Channels
Strips
2.6 x 106
Drift
1.3 x 105
Pixels
9.8 x 106
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Radiation Environment in LHC Experiments
ATLAS SCT - © H. Pernegger
• Large Si-tracking systems
• Projected operation: 10 yrs
• High radiation levels
Detectors must be operational over the
full lifetime of the experiment!
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Radiation Environment in LHC Experiments
Detailed simulations are necessary to understand the radiation levels and
particle spectra in the detector (DTUJET, FLUKA, …).
Ionization Effects
Non-Ionizing Effects
TID (Total Ionizing Dose)
e.g. by charged hadrons, gammas, ….
NIEL (Non Ionizing Energy Loss)
displacement of a Si-atom, …
Unit: rad
1 MeV neutrons/ cm2 “eq. fluence”
All experiments add a safety factor to their radiation level estimates. Safety factors
are usually between 1.5 to 2 (sensors) but can vary between 1-10 (COTS).
(see talk M. Huhtinen, CMS COTS workshop, Ohio, 1999)
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Radiation Environment in LHC Experiments
TID/Gy/yr
ATLAS
NIEL/cm2/yr
Simulations show high radiation levels over 10 years
Up to 50 Mrad and 1.5 1015 1 MeV neq/cm2
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Radiation Environment in LHC Experiments
ATLAS Pixels
ATLAS Strips
CMS Pixels
CMS Strips
ALICE Pixel
LHCb VELO
TID
Fluence
50 Mrad
7.9 Mrad
~24Mrad
7.5Mrad
250krad
-
1.5 x 1015
2 x 1014
~6 x 1014 *
1.6 x 1014
3 x 1012
1.3 x 1014/year**
1MeV n eq. [cm-2] @ 10 years
All values including safety factors.
*Set as limit, inner layer reaches this value after ~2 years
**inner part of detector (inhomogeneous irradiation )
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Radiation Environment in LHC Experiments
Several R&D working groups were formed to study the effects of
radiation damage in the silicon detectors at LHC:
e.g.
RD29 (DMILL radiation hard readout electronics): 1992-1998
RD48 (radiation hard silicon detectors): 1996-2000
http://rd48.web.cern.ch/RD48/
RD49 (RADTOL radiation tolerant ICs for LHC): 1997-2000
http://rd49.web.cern.ch/RD49/
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Radiation Effects in Sensors and ASICs
Surface Damage
Bulk Damage
ASICs
Sensors
Full bulk is sensitive
to passing charged
particles
Sensitive components
are located close to
the surface
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Radiation Effects in ASICs
Cumulative Effects:
 TID (total ionising dose)
 Displacement Damage (NIEL)
Single Event Effects:
SE Gate rupture (permanent)
SE Upset (reversible)
TID (rad):
Ionization in the SiO2
layer and SiO2-Si interface
Transistor level leakage
and threshold voltage shift
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Radiation Effects in ASICs
Ionization in the SiO2 layer:
• Fixed positive oxide charge
• Accumulation of electrons at the interface
• Additional interface states are created at the SiO2-Si border
The trapped positive oxide charges will cause electrons from the bulk to
accumulate near the border region (electron accumulation layer).
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Radiation Effects in ASICs
Using a 0.25µm CMOS process reduces th-shift.
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Radiation Effects in ASICs
Enclosed geometrie to avoid leakage
Leakage path
S
Gate
D
Gate
Standard Geometry
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D
S
Enclosed gate (S-D leakage)
Guard ring (leakage between devices)
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Radiation Effects in ASICs
LHC experiments:
ALICE Pixel
ALICE Strips
ALICE Drift
ATLAS Strips
ATLAS Pixel
CMS Pixel
CMS Strips
LHCb VELO
LHCb Tracker
Technology
Chip
TID (10yrs)
0.25µm CMOS
0.25µm CMOS
0.25µm CMOS
DMILL
0.25µm CMOS
0.25µm CMOS
0.25µm CMOS
0.25µm CMOS
0.25µm CMOS
ALICE1
HAL25
PASCAL
ABCD
FE-D25
PSI
APV25
Beetle
Beetle
250 krad*
7.9 Mrad
50 Mrad
24 Mrad
7.5 Mrad
*tested up to 12 Mrad
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Radiation Effects in ASICs
Total of 0.25µm chips required (ordered through CERN): <2500 wafers
F.Faccio, FEE 2003, Snowmass
At LHC most experiments use the 0.25µm CMOS process and radiation
hardening design features (enclosed gates, guard rings, triplication, …).
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Radiation Effects in Sensors
Bulk Damage
Surface Damage
Displacement of an Si Atom
and creation of a vacancy and
an interstitial
• Creation of positive charges
in the oxide and additional
interface states
• Electron accumulation layer
• Point like defects (g, electrons)
• Cluster Defects (hadrons, ions)
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Radiation Effects in Sensors- Macroscopic Effects
Bulk Damage (NIEL)
Surface Damage (TID)
• Change of effective doping concentration
- Increase of depletion voltage
- Underdepleted operation
• Increase of leakage current
- higher shot noise
- thermal runaway
• Increased charge trapping
- loss of signal
• Increase of inter-strip
capacitance (strips!)
• Pin-holes (strips!)
The defects also evolve after irradiation - ANNEALING!
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Radiation Effects in Sensors- Microscopic Effects
Bulk Damage=>Damage to the silicon crystal: Displacement of lattice atoms
Vacancy
+
Interstitial
V
EK>25 eV
I
“point defects”, mobile in silicon,
can react with impurities (O,C,..)
Schematic
Simulation
[Van Lint 1980]
[M.Huhtinen 2001]
I
V
80 nm
I
V
Distribution of vacancies
created by a 50 keV Si-ion
in silicon (typical recoil
energy for 1 MeV
neutrons):
M. Moll/CERN
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Radiation Effects in Sensors- Microscopic Effects
Defects can be electrically active (levels in the band gap)
- capture and release electrons and holes from conduction and valence band
can be charged
can be generation/recombination centers
can be trapping centers
generation recombination
Donor levels +++
trapping
-
Conduction band
compensation
Band gap
Acceptor levels
Valence band
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Radiation Effects in Sensors- Microscopic Effects
TSC (thermally stimulated current) and DLTS (deep level transient
spectroscopy) measurements to identify the defect levels after irraditiation.
[I.Pintilie, RESMDD, Oct.2004]
E.g.:
I–defect: deep acceptor level at
EC-0.54eV
good candidate for the V2O
defect)
 negative charge
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Macroscopic Effects - Leakage Current
During Irradiation
Linear increase of leakage current
with fluence:
DIvol=a fne
After irradiation
Current anneals with time
~stable value after 3 weeks at RT
(a=4-6 x 10-17 A/cm)
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Macroscopic Effects - Leakage Current
Example:
200µm thick ALICE ladder, 71mm x 13mm
after 4.3E14 n/cm2 at RT Ileak will have increased by: 1.59mA (172µA/cm2
But: I  exp(-Eg/2kT)
Cool the sensors during
operation to keep the leakage
current at an acceptable level:
ATLAS Strips: -7°C
CMS Pixel: -8°C
P. Riedler PhD-thesis
(Measures to control humidity required)
Other measures to counteract the leakage current increase due to
irradiation: thin sensors (but: compromise with signal!)
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Macroscopic Effects - Depletion Voltage
Change of doping concentration (Neff) => change in depletion voltage Vfd
Vfd=
e Neff d2
2es
During Irradiation
Donors become more and more
compensated with increasing
fluence
=> Material seems to change from
n-type to p-type (type inversion)
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Macroscopic Effects - Depletion Voltage
Before type inversion:
Depletion region grows from the p-n junction
side:
 For p-in-n detectors: from the p+ implant
depletion
Type-Inversion:
n-type bulk starts to behave like p-type bulk
 depletion from the backside of the diode
n+
If the detector is under-depleted:
 Charge spread
 Charge loss
After type inversion:
depletion
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V
P. Riedler - "Radiation Effects in Silicon"
V
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Macroscopic Effects - Depletion Voltage
After type inversion Vdep increases with fluence:
During Irradiation
If depletion voltage is too high:
 High leakage current (runaway)
 Leakage current breakdown
 detector cannot be fully depleted
 under depleted operation causes:
- Charge spread
- Charge loss
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Macroscopic Effects - Depletion Voltage
After irradiation (annealing)
2 Phases:
• Beneficial Annealing (short term)
• Reverse Annealing (long term)
Strong temperature dependence:
time constant depends on temperature:
~ 500 years
(-10°C)
~ 500 days
( 20°C)
~ 21 hours
( 60°C)
Cool sensors also after irradiation!
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Macroscopic Effects - Trapping
The Charge Collection Efficiency (CCE) is reduced by trapping.
Trapping is characterized by an effective trapping time eff for electrons and holes:


1
Qe ,h (t )  Q0 e ,h exp  
t
 

 eff e,h 
1
 eff e,h
 N defects
Increase of inverse trapping time (1/) with fluence:
M. Moll/CERN
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Macroscopic Effects - Trapping
Annealing
Different annealing for electrons
and holes
=> electronics!
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Radiation Effects - Sensors and Fast Electronics
If the charge collection time is in the order of the shaping time of the
amplifier (e.g. with low depletion voltage) output signal amplitude will be lost
due to ballistic deficit.
P. Riedler/PhD thesis
P. Riedler/PhD thesis
Operation at sufficient over-depletion is recommended.
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Radiation Effects in Sensors - New Developments
What can be done to make Si-sensors more radiation hard?
I.
Change Operating Conditions
II.
Material Engineering
III. Device Engineering
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Operating Conditions
Cooling during Irradiation
Cooling after Irradiation
+ Lower currents
+ Slow down voltage annealing
- Humidity
- Mechanical stability (TEC!)
- Slows down current annealing
+ Slows down voltage annealing
- Difficulties for access and repair
Cooling implies also the development of a cooling system which is in agreement
with the material budget constraints.
Example: ALICE pixels: X0 < 1% per layer (including everything!)
=> 2-phase cooling system uses PHYNOC tubes with 40 micron wall thickness
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Material Engineering
• Better understand the influence of the different defect levels.
• Find a model to describe the relation between microscopic defects
and all macroscopic effects.
• Study e.g. influence of oxygen and oxygen-dimers in wafers.
• Try to compensate by introducing new materials.
- Oxygen enriched silicon
- CZ grown silicon
- Epitaxial silicon
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Material Engineering
Example: Czochralski Silicon
• 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.
Polycrystalline
Monocrystalline
FZ Silicon
CZ Silicon
M. Moll/CERN
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Material Engineering
800
• 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
Vdep [V]
600
10
8
400
6
4
200
2
0
0
• CZ silicon and MCZ silicon
12
CZ <100>, TD killed
MCZ <100>, Helsinki
STFZ <111>
DOFZ <111>, 72 h 11500C
Neff [1012 cm-3]
24 GeV/c proton irradiation
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
But: current increase and trapping remains!
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M. Moll/CERN
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Device Engineering
After irradiation:
N-in-n detectors
n+
Junction before irradiation is on the sensor back side
after type inversion on the strip/pixel side
depletion
Under depleted operation is
possible!
P-in-n (1014 neq)
Vfd
N-in-n
(1014
neq)
•Higher production price
•Lower yield
•Require isolation between
pixels/strips
Vfd
P. Allport et al., NIMA 1999
ATLAS pixel, CMS pixel, LHCb VELO
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Device Engineering
3D-sensors
Electrodes inside the bulk
Lateral depletion (Vdep<10V)
First tests indicate RH up to 1E15 n/cm2 at 20°C.
Connections via bump bonding?
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C. Da’Via, VERTEX 2003:
40
Device Engineering
Influence of the processing:
e.g. prototype studies of ATLAS strip detectors:
same irradiation and annealing scenario for detectors from different producers
P. Riedler/PhD thesis
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P. Riedler/PhD thesis
41
Adopted Solutions for LHC
P-in-n
ALICE
ATLAS
CMS
LHCb
x
x (stips)
x (stips)
x (tracker, HPD)
N-in-n
x (VELO)
N-in-n ox
x (pixels)
x (pixels)
x (pixels)
x
Electronics
0.25µm
DMILL
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x
x
x (strips)
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Summary
• Silicon detectors are used in all modern HEP experiments.
• The high fluences and doses expected for LHC experiments and
future projects require radiation hard readout electronics and
sensors.
• Several R&D groups have worked on a better understanding of the
radiation damage in silicon, both for sensors and ASICs.
• The adopted solutions for LHC experiments allow an operation of
the Si-detectors up to 50Mrad and 1-2 1015 neq/cm2.
• Research in this field is continued in view of detector upgrades
and new projects.
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