Transcript ppt - UCSB HEP

Silicon Detectors
– How They Work
Rainer Wallny
Silicon Detector Workshop at UCSB
May 11th, 2006
05/11/2006
Slides ruthlessly stolen from:
Paula Collins, CERN
Alan Honma, CERN
Christian Joram, CERN
Michael Moll, CERN
Steve Worm,
CDF Silicon Workshop
atRAL
UCSB
1
Outline
o
o
Why Silicon ?
Semiconductor Basics
– Band-gap, PN junction
– Silicon strip detectors
o
Some Technicalities
- Wafer Production
- Wire Bonding
o
Radiation Damage
– Effect on Vd
– Effect on Leakage Currents
o
05/11/2006
Conclusions
CDF Silicon Workshop at UCSB
2
Tracking Chambers with Solid Media
o
Ionization chamber medium could be gas, liquid, or solid
– Some technologies (ie. bubble chambers) not applicable in collider
environments
o
Gas
Liquid
Solid
Density
Low
Moderate
High
Atomic number
Low
Moderate
Moderate
Ionization Energy
Moderate
Moderate
Low
Signal Speed
Moderate
Moderate
Fast
High-precision tracking advantages with solid media
– Easily ionized, relatively large amount of charge
– Locally high density means less charge spreading
– Fast readout possible
“Solid-state detectors require high-technology devices built by specialists
and appear as black boxes with unchangeable characteristics.”
-Tom Ferbel, 1987
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3
Why Silicon?
o
Electrical properties are good
– Forms a native oxide with excellent electrical properties
– Ionization energy is small enough for easy ionization, yet large enough to maintain
a low dark current
o
Mechanical properties are good
– Easily patterned and read out at small dimensions
– Can be operated in air and at room temperature
– Can assemble into complex geometries
o
Availability and experience
– Significant industrial experience and commercial applications
– Readily available at your nearest beach
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4
The Idea is Not Quite New …
Semiconductors used since 50’s for energy
measurement in nuclear physics
o Precision position measurements up until 70’s
done with emulsions or bubble chambers
-> limited rates, no triggering
o Traditional gas detectors: limited to 50-100 μm
o First silicon usage for precision position
measurement: NA11 at CERN, 1981
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Pioneering Silicon Strip Detectors
E706 (FNAL 1987)
NA11 (CERN 1981)
sensor
o 24x36 mm2 active area
o 8 layers of silicon
o 1m2 readout electronics!
fan out to readout electronics
o 50x50 mm2 active area
Silicon sensor and readout electronics technology closely coupled
with electronics miniaturization (transistors, ICs, ASICs …) silicon quickly took off …
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Contemporary Silicon Modules
CDF SVX IIa half-ladder: two silicon
sensors with readout electronics
(SVX3b analog readout chip)
mounted on first sensor
ATLAS SCT barrel module: four
silicon sensors with center-tapped
readout electronics (ABCD binary
readout chip)
Silicon sensor and readout chip development intimately related
BUT will concentrate on silicon only here …
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7
Large ‘Contemporary’ Silicon Systems
DELPHI (1996)
~ 1.8m2 silicon area
175 000 readout channels
CMS Silicon Tracker (~2007)
~12,000 modules
~ 223 m2 silicon area
CDF SVX IIa (2001-)
~25,000 silicon wafers
~ 11m2 silicon area
~ 10M readout channels
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~ 750 000 readout channels
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New Production Paradigm
evolution
: DELPHI (888 detectors, 8 geometries)
CDF (8000 sensors, 8 geometries)
CMS (25000 sensors, 15 geometries)
Each sensor treated individually,
nurtured into life in many hours
of careful handling
Systematic assembly
line production with
decent QA systems
P. Collins,
Warwick 2006
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‘Moore’s Law’ for Silicon Detector Systems
Moore’s Law: Exponential growth of sensitive area and number
of electronic channels with time
(from Computer Science: doubling of IC integration capacity every 18 months)
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Large Silicon Detector Systems ….
LEP
Tevatron
Whoops…
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CDF Silicon Workshop at UCSB
LHC
P.Collins, ICHEP 2002
11
The Basics ……
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Semiconductor Basics – Band Gap
o
In a gas, electron energy levels are
discrete. In a solid, energy levels split
and form a nearly-continuous band.
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o
If the gap is large, the solid is an insulator. If
there is no gap, it is a conductor. A
semiconductor results when the gap is small.
o
For silicon, the band gap is 1.1 eV, but it takes
3.6 eV to ionize an atom. The rest of the energy
goes to phonon exitations (heat).
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Semiconductor Basics – Principle of Operation
- Basic motivation: charged particle
position measurement
- Use ionization signal (dE/dx) left
behind by charged particle passage
E
conductance band
e
Ef
+ __
+
+ __
+
h
valence band
- In a semiconductor, ionization produces electron hole pairs
- Electric fields drift electrons and holes to oppositely electrodes
BUT:
- In pure intrinsic (undoped) silicon, many more free charge
carriers than those produced by a charged particle.
300 m
1 cm
1 cm
Have 4.5x108 free charge carriers; only 3.2x104 produced by MIP
- Electron –hole pairs quickly re-combine …
Need to deplete free charge carriers and separate e-holes ‘quickly’!
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Doping Silicon
E
Ef
E
CB
e
CB
Ef
VB
h
VB
n-type:
• In an n-type semiconductor, negative
charge carriers (electrons) are obtained
by adding impurities of donor ions (eg.
Phosphorus (type V))
• Donors introduce energy levels close
to conduction band thus almost fully
ionized => Fermi Level near CB
p-type:
• In a p-type semiconductor, positive charge
carriers (holes) are obtained by adding impurities
of acceptor ions (eg. Boron (type III))
Electrons are the majority carriers.
Holes are the majority carriers.
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• Acceptors introduce energy levels close to
valence band thus ‘absorb’ electrons fromVB,
creating holes => Fermi Level near VB.
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The pn-Junction
Exploit the properties of a p-n junction (diode) to collect ionization charges
p
+
+
+
+
+
+
+
+
+

+ –
–
+– + ++
+

–
 –
–


–
–
–
+
n
When brought together to form a junction, a gradient of electron and hole densities results in a
diffuse migration of majority carriers across the junction. Migration leaves a region of net
charge of opposite sign on each side, called the depletion region (depleted of charge carriers).
Electric field set up prevents further migration of carriers resulting in potential difference V bi
Another way to look at it:
E
Fermi-Levels need to be adjusted
so thus energy bands get
distorted => potential Vbi
Ef
E
CB
e
E
CB
p
n
CB
e.V
Ef
VB
h
VB
Ef
VB
Funky cartoon from Brazil: http://www.agostinhorosa.com.br/artigos/transistor-6.html
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pn - Junction
o
p-type and n-type doped silicon
forms a region that is depleted of
free charge carriers
p
+
+
o
o
++ +
+
+
+
+
+– + +
The depleted region contains a
non-zero fixed charge and an
electric field. In the depletion
zone, electron – hole pairs won’t
recombine but rather drift along
field lines
Dopant
concentration
Artificially increasing this
depleted region by applying a
reversed bias voltage allow
charge collection from a larger
volume
Electric
field
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n
–


–
+

–

 –
–
–


–
–
+
Space charge
density
Carrier
density
Electric
potential
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How to Build a Silicon Detector
If we make the p-n junction at the surface of a silicon wafer with the
bulk being n-type (you could also do it the opposite way), we then need
to extend the depletion region throughout the n bulk to get maximum
charge collection by applying a reverse bias voltage.
p
p
p
n
–
h+ e-
+
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Properties of the Depletion Zone
– Depletion width is a function of the bulk
resistivity , charge carrier mobility and
the magnitude of reverse bias voltage Vb:
w = 2Vb
–
Vb
+
Depletion zone
w
d
undepleted zone
where  = 1/ q N for doped material where N is the doping concentration
and q is the charge of the electron and  is the carrier mobility (v= E)
– The bias voltage needed to completely deplete a device of
thickness d is called the depletion voltage, Vd
Vd = d2 /(2)
– Need a higher voltage to fully deplete a low resistivity material.
– A higher voltage is needed for a p-type bulk since the carrier
mobility of holes is lower than for electrons (450 vs 1350 cm2/ V·s)
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Properties of the Depletion Zone (cont’d)
– One normally measures the depletion behavior (finds the
depletion voltage) by measuring the capacitance versus
reverse bias voltage. The capacitance is simply the parallel
plate capacity of the depletion zone.
C=A
 / 2Vb
capacitance vs voltage
1/C2 vs voltage
Vd
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Leakage Current
- Two main sources of (unwanted) current
flow in reversed-biased diode:
– Diffusion current, charge generated in
undepleted zone adjacent to depletion
zone diffuses into depletion zone
(otherwise would quickly recombine)
negligible in a fully depleted device
– Generation current Jg, charge generated in
depletion zone by defects/contaminants
Jg  exp(-b/kT)
Exponential dependence on temperature due
to thermal dependence of e-h pair creation by
defects in bulk. Rate is determined by nature
and concentration of defects.
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Bias Resistor and AC Coupling
– Need to isolate strips from each other
and collect/measure charge on each
strip => high impedance bias
connection (resistor or equivalent)
– Usually want to AC (capacitatively)
couple input amplifier to avoid large
DC input from leakage current.
– Both of these structures are often
integrated directly on the silicon
–
sensor. Bias resistors via deposition of
doped polycrystalline silicon, and
+
capacitors via metal readout lines over
the implants but separated by an
insulating dielectric layer (SiO2 , Si3N4).
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h+ e-
22
The Charge Signal
• Collected charge usually given for
Minimum Ionizing Particle (MIP)
Most probable charge ≈ 0.7 x mean
Mean charge
dE/dx)Si = 3.88 MeV/cm, for 300 m thick
= 116 keV
This is mean loss, for silicon detectors use
most probable loss (0.7 mean) = 81 keV
3.6eV needed to make e-h pair
Collected charge 22500 e (=3.6 fC)
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But There Is Noizzzzzz …..
Landau distribution has significant low
energy tail which becomes even lower
with noise broadening.
noise distribution
Landau distribution
with noise
Noise sources:
o Capacitance
ENC ~ Cd
o Leakage Current ENC ~ √ I
o Thermal Noise
ENC
~ √( kT/R)
One usually has low occupancy in silicon sensors most channels have no
signal. Don’t want noise to produce fake hits so need to cut high above noise tail
to define good hits. But if too high you lose efficiency for real signals.
Figure of Merit: Signal-to-Noise Ratio S/N.
Typical Values ~ 10-15, people get nervous below 10. Radiation Damage can
degrade the S/N. Thus S/N determines detector lifetime in radiation environment.
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Charge Collection and Diffusion
– Drift velocity of charge carriers v = E, so drift time,
td = d/v = d/E
– Typical values: d=300 m, E= 2.5kV/cm, e= 1350;h= 450 cm2 / V·s,
gives: td(e)= 9ns , td(h)= 27ns
– Diffusion of charge “cloud” caused by scattering of drifting charge
carriers, radius of distribution after time td:
=
2D td , where D is the diffusion constant, D = kT/q
– Typical charge radius: ≈ 6 m
– Charge Radius determines ‘Charge
Sharing’, i.e. deposition of charge on
several strips.
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Double Sided Detectors
Why not get a 2nd coordinate by measuring position of
the (electron) charge collected on the opposite face?
BUT:
Unlike the face with the p-strips, nothing prevents
horizontal charge spread on back face. n-strips alone
are not sufficient to isolate the charge because of an
electron accumulation layer produced by the positively
charged SiO2 layer on the surface.
SOLUTION:
• Put p-strips in between the n-strips.
OR
• Put “field plates” (metal over oxide) over the n-strips
and apply a potential to repel the electrons.
p+
p+
n-bulk


n n
n
n
n-bulk
n n n
p+
CDF Silicon Workshop at UCSB
p+
n-bulk
n n n
+
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p+
+
+
26
Guard Rings and Avalanche Breakdown
We have treated the silicon strip device as having infinite area, but it has
edges. What happens at the edges?
Single guard ring structure
– Voltage drop between biasing ring and edge, top
edge at backplane voltage.
– Typically n-type implants put around edge of the
device and a proper distance maintained between p
bias ring and edge ring.
– Usually one or more “guard” rings (left floating) to
assure continuous potential drop over this region.
– Defects or oxide charge build-up in this region could
lead to additional leakage current contributions
– If one increases the bias voltage, eventually the field
is high enough to initiate avalanche multiplication.
This usually occurs around 30V/m (compared to a
typical operating field of <1V/m). Local defects and
inhomogeneities could result in fields approaching
the breakdown point.
– .
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Some Technicalities ……
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Material: Float Zone Silicon (FZ)
Float Zone process
Mono-crystalline Ingot
 Using a single Si crystal seed, melt
the vertically oriented rod onto the
seed using RF power and “pull” the
single crystal ingot
Poly silicon rod
Wafer production
 Slicing, lapping, etching, polishing
RF Heating coil
Single crystal silicon
Oxygen enrichment (DOFZ)
 Oxidation of wafer at high temperatures
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Czochralski silicon (Cz) & Epitaxial silicon (EPI)
Czochralski silicon
o
o
o
o
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)
Czochralski Growth
Recent developments (~2 years) made CZ
available in sufficiently high purity (resistivity) to allow for use as
particle detector.
Epitaxial silicon
o
o
o
o
o
o
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Chemical-Vapor Deposition (CVD) of Silicon
CZ silicon substrate used  in-diffusion of oxygen
growth rate about 1m/min
excellent homogeneity of resistivity
up to 150 m thick layers produced
price depending on thickness of epi-layer but not
extending ~ 3 x price of FZ wafer
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Wafer Processing (1)
1)
n-Si
SiO2
2)
3)
Start with n-doped silicon wafer,
 ≈ 1-10 kcm
Oxidation at 800 – 1200 0C
Photolithography (= mask align + photo-resist layer + developing) followed
by etching to make windows in oxide
UV light
etch
mask
Photo-resist
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Wafer Processing (2)
B
4)
Doping by ion implantation (or by diffusion)
As
5)
p+
p+
Annealing (healing of crystal lattice) at 600 0C
n+
Al
6)
Photolithography followed by Al metallization
over implanted strips and over backplane usually by
evaporation.
Simple DC-coupled silicon strip detector
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Bringing It All Together
• Connectivity technology: some of the possibilities
– High density interconnects (HDI):industry standard and custom cables,
usually flexible kapton/copper with miniature connectors.
– Soldering still standard for surface mount components, packaged chips
and some cables. Conductive adhesives are often a viable low
temperature alternative, especially for delicate substrates.
– Wire bonding: the standard method for connecting sensors to each
other and to the front-end chips. Usually employed for all connections of
the front-end chips and bare die ASICs. A “mature” technology (has
been around for about 40 years).
OPAL (LEP) module
~200 wire bonds
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4 x 640 wire bonds
CDF Silicon Workshop at UCSB
Total ~2700 wire bonds
33
Wire Bonding
• Uses ultrasonic power to vibrate needle-like
tool on top of wire. Friction welds wire to
metallized substrate underneath.
• Can easily handle 80m pitch in a single row
and 40m in two staggered rows (typical FE
chip input pitch is 44m).
Electron micrograph of bond “foot”
• Generally use 25m diameter aluminium wire
and bond to aluminium pads (chips) or gold
pads (hybrid substrates).
• Heavily used in industry (PC processors) but
not with such thin wire or small pitch.
Microscope view of wire bonds
connecting sensor to fan-out circuit
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Radiation Damage …
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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!)
Sensors can fail from radiation damage by virtue of…
–
–
–
Noise too high to effectively operate
Depletion voltage too high to deplete
Loss of inter-strip isolation (charge spreading)
 Signal/Noise Ratio is the quantity to watch !
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Run I Experience: SVX’ Signal-to-Noise
Radiation Damage limits the ultimate
lifetime of the Detector
-Need S/N >8 to perform online b-tagging with SVT
-Need S/N >5 for offline b-tagging
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Surface Damage
Metal (Al)
o
Surface damage generation over time:
– Ionizing radiation creates electron/hole pairs in the
SiO2
– Many recombine, electrons migrate quickly away
– Holes slowly migrate to Si/SiO2 interface.
Hole mobility is much lower than for electrons
(20 cm2/Vs vs. 2x105 cm2/Vs)
– Some holes ‘stick’ in the boundary layer
o
Oxide (SiO2)
Interface (SiOx)
- + + -+ -+
- - +
-+ +- +- +-
Semiconductor (Si)
After electron
transport:
+ + + ++
+
+
+ ++ +- +
Surface damage results in
–
–
–
–
Increased interface trapped charge (see picture)
After transport
Increase in fixed oxide charges
of the holes:
+ + + + +
Surface generation centers
Electron accumulation under the oxide interface can alter
the depletion voltage (depends on oxide quality and sensor geometry)
– In silicon strip sensors, surface damage effects (oxide charge) saturate
at a few hundred kRad
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Bulk Damage
o
– Results in silicon interstitial, vacancy, and
typically a large disordered region
– 1 MeV neutron transfers 60-70 keV to
recoiling silicon atom, which in turn
displaces ~1000 additional atoms
o
O
Vacancy
Disordered
region
Interstitial
Defects can recombine or migrate through
the lattice to form more complex and stable
defects
– Annealing can be beneficial, but…
– Defects can be stable or unstable
o
Vacancy/Oxygen
Center
Bulk damage is mainly from hadrons
displacing primary lattice atoms (for E > 25
eV)
Displacement damage is directly related to
the non-ionizing energy loss (NIEL) of the
interaction
– Varies by incident particle type and energy
– Normalize fluence to 1 MeV n-equivalent
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C
Carbon-Carbon
Pair
C
Carbon
Interstitial
C
Di-vacancy
Phosphorous
dopant
P
CDF Silicon Workshop at UCSB
Carbon-Oxygen
pair
O
C
39
Microscopic defects
Damage to the silicon crystal: Displacement of lattice atoms
o
SiS
particle
EK>25 eV
V
I
EK > 5 keV
80 nm
I
V
“point defects”, mobile in silicon,
can react with impurities (O,C,..)
point defects and clusters of defects
Distribution of vacancies
created by a 50 keV Si-ion
in silicon (typical recoil
energy for 1 MeV neutrons):
I
V
Vacancy
+
Interstitial
Schematic
[Van Lint 1980]
Simulation
[M.Huhtinen 2001]
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
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Impact of Defects on Detector properties
Shockley-Read-Hall statistics
(standard theory)
charged defects
 Neff , Vdep
e.g. donors in upper
and acceptors in
lower half of band
gap
Trapping (e and h)
generation
 CCE
 leakage current
shallow defects do not
Levels close to
contribute at room
midgap
temperature due to fast
most effective
Inter-center charge
transfer model
(inside clusters only)
enhanced generation
 leakage current
 space charge
detrapping
Impact on detector properties can be calculated if all defect parameters are known:
n,p : cross sections
E : ionization energy
Nt : concentration
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Radiation Damage: Effect on Neff
Change of Depletion Voltage Vdep (Neff)
…. with time (annealing):
103
1000
500
102
 600 V
type inversion
100
50
10
5
101
1014cm-2
1
10-1
10
"p-type"
n-type
[M.Moll: Data: R. Wunstorf, PhD thesis 1992, Uni Hamburg]
10
0
10
1
10
2
eq [ 1012 cm-2 ]
10
3
0
 Neff [1011cm-3]
5000
10
| Neff | [ 1011 cm-3 ]
Udep [V] (d = 300m)
…. with particle fluence:
10-1
• “Type inversion”: Neff changes from positive to
negative (Space Charge Sign Inversion)
before inversion
p+
n+
p+
n+
8
6
NY
NA
4
NC
gC eq
2
NC0
[M.Moll, PhD thesis 1999, Uni Hamburg]
0
1
10
100
1000 10000
annealing time at 60oC [min]
• Short term: “Beneficial annealing”
• Long term: “Reverse annealing”
- time constant depends on temperature:
~ 500 years (-10°C)
~ 500 days ( 20°C)
~ 21 hours ( 60°C)
- Consequence: Detectors must be cooled
even when the experiment is not running!
after inversion
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Depletion Voltage: Death of SVX Layer 0
Depletion Voltage (V)
Steve Worm, Vertex 2003
Central Prediction
+1σ Prediction
–1σ Prediction
Data & Extrapolation
300
200
100
0
0
2
4
6
8
Integrated Luminosity (fb–1)
SVXII L0 lifetime prediction based on Hamburg Model (M.Moll)
- Will SVXII L0 survive Run II ? -> Antonio’s Talk
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Radiation Damage – Leakage Current
Change of Leakage Current (after hadron irradiation)
I / V [A/cm3]
10
10-2
10-3
10
10-5
80 min 60C
12
10
n-type FZ - 780 cm
n-type FZ - 410 cm
n-type FZ - 130 cm
n-type FZ - 110 cm
n-type CZ - 140 cm
p-type EPI - 380 cm
13
10
14
eq [cm ]
-2
10
10
 is constant over several orders of fluence
and independent of impurity concentration in Si
 can be used for fluence measurement
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5
4
3
3
2
2
.
1
17
-3
oxygen enriched silicon [O] = 2 10 cm
parameterisation for standard silicon
1
[M.Moll PhD Thesis]
10
100
1000
o
10000
annealing time at 60 C [minutes]
[M.Moll PhD Thesis]
Leakage current
per unit volume
and particle fluence
80 min 60C
4
0
15
6
5
1
Damage parameter  (slope in figure)
I
α
V   eq
o
6
n-type FZ - 7 to 25 Kcm
n-type FZ - 7 Kcm
n-type FZ - 4 Kcm
n-type FZ - 3 Kcm
p-type EPI - 2 and 4 Kcm
-4
10-6 11
10
o
…. with time (annealing):
…. with particle fluence:
(t) [10-17 A/cm]
-1
o
Leakage current decreasing in time
(depending on temperature)
o
Strong temperature dependence
 E

I  exp   g

2
k
T
B 
Consequence: 
Cool detectors during operation!
Example: I(-10°C) ~1/16 I(20°C)
CDF Silicon Workshop at UCSB
44
Recent Bias Current (Re-) Analysis
L0
P.Dong et. al, upcoming CDF note
05/11/2006
CDF Silicon Workshop at UCSB
45
Radiation Damage – Trapping
Deterioration of Charge Collection Efficiency (CCE) by trapping
Trapping is characterized by an effective trapping time eff for electrons and holes:
where
Inverse trapping time 1/ [ns-1]
Increase of inverse trapping time (1/) with fluence
0.5
24 GeV/c proton irradiation
0.4
data for electrons
data for holes
0.3
0.2
0.1
[M.Moll; Data: O.Krasel, PhD thesis 2004, Uni Dortmund]
0
0
2.1014 4.1014 6.1014 8.1014
1015
particle fluence - eq [cm-2]
05/11/2006
1
 eff e,h
 N defects
….. and change with time (annealing):
Inverse trapping time 1/ [ns-1]


1

Qe,h (t )  Q0 e,h exp 
t
  eff e,h 


0.25
24 GeV/c proton irradiation
eq = 4.5.1014 cm-2
0.2
0.15
data for holes
data for electrons
0.1
[M.Moll; Data: O.Krasel, PhD thesis 2004, Uni Dortmund]
5 101
5 102
5 103
annealing time at 60oC [min]
CDF Silicon Workshop at UCSB
46
Decrease of Charge Collection Efficiency
o
Two basic mechanisms reduce collectable charge:
– trapping of electrons and holes  (depending on drift and shaping time !)
– under-depletion
 (depending on detector design and geometry !)
o
Example: ATLAS microstrip detectors + fast electronics (25ns)
o
p-in-n : oxygenated versus standard FZ
- beta source
- 20% charge loss after 5x1014 p/cm2 (23 GeV)
Laser (1064nm) measurements
max collected charge (overdepletion)
80
60
collected at depletion voltage
40
oxygenated
standard
20
n-in-n versus p-in-n
- same material, ~ same fluence
- over-depletion needed
CCE (arb. units)
Q/Q0 [%]
100
o
1.00
0.80
0.60
05/11/2006
1
0.20
2
3
4
p [1014 cm-2]
5
p-in-n
0.40
M.Moll [Data: P.Allport et all, NIMA 501 (2003) 146]
0
0
n-in-n
n-in-n (7.1014 23 GeV p/cm2)
p-in-n (6.1014 23 GeV p/cm2)
[M.Moll: Data: P.Allport et al. NIMA 513 (2003) 84]
0
CDF Silicon Workshop at UCSB
100 200 300 400 500 600
bias [volts]
47
Oxygenation Benefits
• oxygenation increases radiation hardness
Michael Moll, IWORID Glasgow 2004
• sometimes, standard FZ exhibits similar radiation hardness - reasons unclear
• Concentrate R&D on CZ and EPI silicon
05/11/2006
CDF Silicon Workshop at UCSB
48
Summary
• Silicon strip detectors built on simple pn junction principle
have become a ‘mature’ technology over 25 years.
• Provide reliable tracking in high density/high rate environment
• Widespread use thanks to cost drop and advances in microelectronic
industry
• Silicon Radiation hardness to a few 1015 p/cm2
- radiation hardness frontier > 1016 p/cm2
(SLHC inner pixel layer)
- CZ, EPI, new materials/structures?
•
• Silicon People are fun to work with, outgoing
and (usually) in a good mood (eh …)
Have fun in California - You deserve it!
05/11/2006
CDF Silicon Workshop at UCSB
49
Backup
05/11/2006
CDF Silicon Workshop at UCSB
50
Sensor Materials: Diamond, SiC and GaN
Property
Eg [eV]
Ebreakdown [V/cm]
e [cm2/Vs]
h [cm2/Vs]
vsat [cm/s]
Z
r
e-h energy [eV]
Density [g/cm3]
Displacem. [eV]
Diamond
5.5
107
1800
1200
2.2·107
6
5.7
13
3.515
43
GaN
3.39
4·106
1000
30
31/7
9.6
8.9
6.15
15
4H SiC
3.26
2.2·106
800
115
2·107
14/6
9.7
7.6-8.4
3.22
25
Si
1.12
3·105
1450
450
0.8·107
14
11.9
3.6
2.33
13-20
o

o

o
R&D on diamond detectors:
RD42 – Collaboration
http://cern.ch/rd42/
05/11/2006

Wide bandgap (3.3eV)
lower leakage current
than silicon
Signal:
Diamond 36 e/m
SiC
51 e/m
Si
89 e/m
more charge than
diamond
Higher displacement
threshold than silicon
radiation harder than
silicon (?)
Recent review: P.J.Sellin and J.Vaitkus on behalf of RD50 “New
materials for radiation hard semiconductor detectors”, submitted to NIMA
CDF Silicon Workshop at UCSB
51
Microscopic defects
Damage to the silicon crystal: Displacement of lattice atoms
o
SiS
particle
EK>25 eV
V
I
EK > 5 keV
80 nm
I
V
“point defects”, mobile in silicon,
can react with impurities (O,C,..)
point defects and clusters of defects
Distribution of vacancies
created by a 50 keV Si-ion
in silicon (typical recoil
energy for 1 MeV neutrons):
I
V
Vacancy
+
Interstitial
Schematic
[Van Lint 1980]
Simulation
[M.Huhtinen 2001]
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
05/11/2006
CDF Silicon Workshop at UCSB
52
Radiation Damage in Silicon
Close proximity to the interaction region means the sensors are subject to
high doses of radiation
o
Two general types of radiation damage
– “Bulk” damage due to physical impact within the crystal
– “Surface” damage in the oxide or Si/SiO2 interface
o
Cumulative effects
–
–
–
–
o
Increased leakage current (increased Shot noise)
Silicon bulk type inversion (n-type to p-type)
Increased depletion voltage
Increased capacitance
Sensors can fail from radiation damage by virtue of…
– Noise too high to effectively operate
– Depletion voltage too high to deplete
– Loss of inter-strip isolation (charge spreading)
o
Ratio of signal/noise is the important quantity to watch
05/11/2006
CDF Silicon Workshop at UCSB
53
Bulk Damage – Depletion Voltage
7
standard FZ
400
neutrons
pions
protons
5
oxygen rich FZ
neutrons
pions
protons
4
3
300
200
2
Vdep [V] (300m)
|Neff| [1012 cm-3]
6
Depletion voltage is often parameterized
in three parts (Hamburg model):
Neff(T,t,) = NA + NC + NY
o
1
0.5
1
1.5
2
2.5
3
3.5
o
eq [10 cm ]
14
-2
Figure 9: Dependence of Neff on the accumulated 1 MeV neutron equivalent
fluence for standard and oxygen enriched FZ silicon irradiated with reactor
neutrons (Ljubljana), 23 GeV protons (CERN PS) and 192 MeV pions (PSI).
10
 Neff [1011cm-3]
8
6
NY, = gY eq
NA = ga eq
4
NC
S
NA = eq iga,iexp(-ka,i(T)t)
– Reduces NY (beneficial)
– Time constant is a few days at 20 C
100
0
Short term annealing (NA)
o
Stable component (Nc)
Nc = Nc0(1-exp(-ceq))+gceq
– Does not anneal (does not depend on time
or temperature)
– Partial donor removal (exponential or limited
exponential)
– Creation of acceptor sites (linear)
Long term reverse annealing (NY)
NY = NY,∞[1-1/(1+ NY,∞kY(T)t)], NY,∞= gYeq
NC0
– Strong temperature dependence
0
1
10
100
1000
10000
– 1 year at T=20 C is the same as <1 day at
o
annealing time at 60 C [min]
T=60 C or ~100 years at T= -7 C (ATLAS)
Fig.13: Annealing behaviour of the radiation induced change in the
– Can be significant long term; must cool Si
effective doping concentration N at 60C.
05/11/2006
CDF Silicon Workshop at UCSB
54
gC eq
2
eff
Bulk Damage – Leakage Current
o
Defects created by bulk damage provide intermediate states within the band
gap
– intermediate states act as ‘stepping stones’ of thermal generation of electron/hole
pairs
– Some of these states anneal away; the bulk current reduces with time (and
temperature) after irradiation
o
Annealing function (t)
– Parameterized by the sum of several exponentials iexp(-t/i)
– Full annealing (for the example below) reached after ~1 year at 20ºC
– At low temperatures, annealing effectively stops
05/11/2006
CDF Silicon Workshop at UCSB
55
Bulk Damage Effects (Simple View)
o
Before Irradiation:
Leakage Current:
I (t)V
Small area:
small current
– Current depends on (t) (annealing
function), V (volume), and  (fluence).
– Annealing reduces the current
– Independent of particle type
o
P
Depletion Voltage:
Vdep = q|Neff|d2/20
– Depends on effective dopant concentration
(Neff = Ndonors – Nacceptors), sensor thickness
(d), permitivity (0).
– Depletion voltage is often parameterized in
three parts:
• Short term annealing (Na)
• A stable component (Nc)
• Long term reverse annealing (NY)
05/11/2006
N
Small difference:
small depletion V
After Irradiation:
Large area:
large current
P
CDF Silicon Workshop at UCSB
N
Large difference:
Large depletion V
56
Bulk Damage – Leakage Results
o
Measured values of (t)
– Typically one quotes measured values of (t) after complete annealing at T=20ºC:
∞ = (t=∞)
– Some typical ‘world averages’ for ∞ are
• 2.2 x 10-17 A/cm3 for protons, pions
• 2.9 x 10-17 A/cm3 for neutrons
– Recent results show (t=80min,T=60ºC) = 4.0 x 10-17 A/cm3 for all types of silicon,
levels of impurities, and incident particle types (NIM A426 (1999)86).
10-2
10-3
n-type FZ - 780 cm
n-type FZ - 410 cm
n-type FZ - 130 cm
n-type FZ - 110 cm
n-type CZ - 140 cm
p-type EPI - 380 cm
10-4
10-5
10-6 11
10
05/11/2006
n-type FZ - 7 to 25 Kcm
n-type FZ - 7 Kcm
n-type FZ - 4 Kcm
n-type FZ - 3 Kcm
p-type EPI - 2 and 4 Kcm
1012
1013
eq [cm-2]
1014
6.10-17
6.10-17
4.10-17
4.10-17
(t)
I / V [A/cm3]
10-1
1015
Fig. 7.: Fluence dependence of leakage current for detectors
produced by various process technologies from different silicon
materials. The current was measured after a heat treatment for
80 min at 60C [14].
2.10-17
2.10-17
oxygen enriched silicon [O] = 2.1017 cm-3
parameterisation for standard silicon
0
100
5 101
5 102
5 103
5 104
0
5 105
annealing time at 60oC [minutes]
Fig.8: Current related damage rate  as function of cumulated
annealing time at 60C. Comparison between data obtained for
oxygen diffused silicon and parameterisation given in Ref. [14].
CDF Silicon Workshop at UCSB
57
Silicon Detectors
– How They Work
Rainer Wallny
Silicon Detector Workshop at UCSB
May 11th, 2006
05/11/2006
Slides ruthlessly stolen from:
Paula Collins, CERN
Alan Honma, CERN
Christian Joram, CERN
Michael Moll, CERN
Steve Worm,
CDF Silicon Workshop
atRAL
UCSB
58