Transcript Slides - Indico

Silicon Detectors
XII ICFA School on Instrumentation
Bogotá, Nov. 25th – Dec. 6th, 2013
Part 1
Manfred Krammer
Institute of High Energy Physics, Vienna, Austria
Silicon Detectors
Content
1 Introduction
2 Basics
3 Detector Structures
4 Performance Parameter
5 Radiation Damage
6 Silicon Detectors in the LHC Experiments
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1 Introduction
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1 Introduction
What is a silicon detector?
NOT a gas detector!
anode wire
ionising particle
cathode
Charged particles ionise the gas. Electrons and positive ions drift to the
electrodes. Large electric field close to the wire causes secundary ionisation
and signal multiplication.
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1 Introduction
What is a Silicon Detector?
NOT a scintillator!
Scintillators: Cristalls, Plastics, Liquids, Gases
Penetrating charged particles excite atomic or molecular levels. When decaying
photons are emitted.
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1 Introduction
What is a Silicon Detector?
A semiconductor detector!
Also called a solid state detector.
Through going charged particles create electron hole pairs. These charges
drift to the electrodes. The drift generates a signal.
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1 Introduction
Where are semiconductor detector used?
Semiconductor detectors are used for:
• Nuclear Physics
Energy measurement of charged particles (MeV range),
gamma spectroscopy (precise determination of
photon energy)
• Particle Physics:
Tracking or vertex detectors, precise determination of
particle tracks and decay vertices
• Satellite Experiments
Tracking detectors
• Industrial Applications
Security, Medicine, Biology,...
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1 Introduction
History of semiconductor detectors
1951: First detector was a germanium-pn-diode (McKay).
1960: p-i-n semiconductor detectors for - und -spectroscopie.
(E.M. Pell)
1960ies: Semiconductor detectors from germanium but also silicon are
more and more important for the energy measurement in nuclear physics.
1980: First silicon surface barrier micro strip detector (E. Heijne)
1983: First use of a planar silicon strip detector in a fix target experiment NA11 at CERN. (J. Kemmer)
1980ies and after: micro structured silicon detectors gain rapid
importance for tracking detectors in high energy physics experiments!
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1 Introduction
Advantages of semiconductor detectors
Semiconductor detectors have a high density
large energy loss in a short distance
Diffusion effect is smaller than in gas detectors resulting in achievable
position resolution of less than 10 µm
Low ionisation energy (few eV per e-hole pair) compared to gas detectors
(20-40 eV per e-ion pair) or scintillators (400-1000 eV to create a photon).
Large experience in industry with micro chip technology (silicon).
Ô Rapid development of detectors, reduction of cost
 Easy integration with readout electronics due to identical materials
used (silicon)
 Self supporting structure
 High intrinsic radiation hardness
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1 Introduction
Disadvantages of semiconductor detectors
 No internal amplifications (with the exception of APDs)
 High cost per surface
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2 Basics
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2.1 Materials
Elemental semiconductors
Germanium: Used in nuclear physics, due to small band gap
(0.66 eV) needs cooling (usually done with liquid nitrogen at 77 K)
Silicon: Standard material for vertex and tracking detectors in high
energy physics, can be operated at room temperature, synergies with
micro electronics industry.
Diamond (CVD or single crystal): Large band gap, requires no
depletion zone, very radiation hard, drawback is a low signal and high
cost!
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2.1 Materials
Compound semiconductors
Compound semiconductors consist of two (binary semiconductors) or
more atomic element. Depending on the column in the periodic system
of elements one differentiates between IV-IV- (e.g. SiGe, SiC), III-V-,
und II-VI compounds
important III-V compounds:
– GaAs: Faster and probably more radiation resistant than Si.
Drawback is less experience in industry and cost.
– GaP, GaSb, InP, InAs, InSb, InAlP
important II-VI compounds:
– CdTe: High atomic numbers (48+52) hence very efficient to detect
photons.
– ZnS, ZnSe, ZnTe, CdS, CdSe, Cd1-xZnxTe, Cd1-xZnxSe
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2.2 Material Properties
Silicon, germanium und diamond are group IV elements. The crystal structure
is the diamond lattice, consisting of 2 interpenetrating sublattices shifted by
one quarter along the diagonal of the cube. Each atom is surrounded by four
equidistant neighbours.
Diamond lattice
Zincblende lattice
Most III-V semiconductors (e.g. GaAs) have a zincblende lattice. This lattice is
similar to the diamond lattice, except that each sublattice consists of one
element.
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Source: S.M. Sze, Semiconductor Devices , J. Wiley & Sons, 1985
Crystal structure of semiconductors
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2.2 Material Properties
Bond model of semiconductors
Example of column IV elemental semiconductor (2dim projection) :
T=0K
T >0K
Valence electron
Conduction electron
Each atom has 4 closest neighbors, the 4 electrons in the outer shell are
shared and form covalent bonds.
At low temperature all electrons are bound
At higher temperature thermal vibrations break some of the bonds
cause conductivity (electron conduction)
The remaining open bonds attract other econduction)
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free e-
The “holes” change position (hole
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2.2 Material Properties
Energy bands: isolator–semiconductor–metal
In an isolated atom the electrons have only discrete energy levels. In solid state
material the atomic levels merge to energy bands. In metals the conduction and
the valence band overlap, whereas in isolators and semiconductors these levels
are separated by an energy gap (band gap). In isolators this gap is large.
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2.2 Material Properties
Fermi distribution, fermi level
Fermi distribution ƒ(E) describes the probability that an electronic state with
energy E is occupied by an electron.
1
f (E) =
1+ e
E -E F
kT
The fermi level EF is the energy at which the probability of occupation is 50%. For
metals EF is in the conduction band, for semiconductors and isolators EF is in the
band gap
Fermi distribution function for
different temperautures
T4 > T3 > T2 > T1 > T0 = 0 K
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2.2 Material Properties
Intrinsic carrier concentration
 Due to the small band gap in semiconductors electrons already occupy
the conduction band at room temperature.
 Electrons from the conduction band may recombine with holes.
 A thermal equilibrium is reached between excitation and recombination
Charged carrier concentartion ne = nh = ni
This is called intrinsic carrier concentration:
In ultrapure silicon the intrinsic carrier concentration is 1.45·1010 cm-3.
With approximately 1022 Atoms/cm3, about 1 in 1012 silicon atoms is ionised.
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2.2 Material Properties
Drift velocity and mobility
Drift velocity
For electrons:
and for holes:
Mobility
For electrons:
and for holes:
e

mn , mp
n , p
…
…
…
…
electron charge
external electric field
effective mass of e- and holes
mean free time between collisions
for e- and holes
Source: S.M. Sze, Semiconductor Devices , J. Wiley & Sons, 1985
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2.2 Material Properties
Resistivity
Resistivity of semiconductor material:
ne , nh
n , p
… Charge carrier density for electrons and holes
… Mobility for electrons and holes
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2.2 Material Properties
Resistivity – example silicon
In silicon the mobilities are in good approximation constant below an
electric field of ≈ 1 kV/cm.
At T = 300 K:
µn(Si, 300 K) ≈ 1450 cm2/Vs
µp(Si, 300 K) ≈ 450 cm2/Vs
The charge carrier concentration in pure silicon (i.e. intrinsic Si) for
T = 300 K is:
ne = nh ≈ 1.45 · 1010 cm-3
This yields an intrinsic resistivity of:
 ≈ 230 kcm
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2.2 Material Properties
Comparison of different semiconductor materials – 1
Si
Ge
GaAs
GaP
CdTe
Diamond*
Atomic number Z
14
32
31+33
31+15
48+52
6
Atomic mass A (amu)
28.086
72.61
69.72+74.92
Lattice constant a (Å)
5.431
5.646
5.653
5.451
6.482
3.567
Density  (g/cm3)
2.328
5.326
5.32
4.13
5.86
3.52
Eg (eV) bei 300 K
1.11
0.66
1.42
2.26
1.44
5.47–5.6
Eg (eV) bei 0 K
1.17
0.74
1.52
2.34
1.56
≈6
rel. permittivity r =/0
11.9
16.0
12.8
11.1
10.9
5.7
Melting point (°C)
1415
938
1237
1477
1040
3527
eff. e–-mass (mn /me)
0.98, 0.19
1.64, 0.08
0.067
0.82
0.11
0.2
eff. hole mass+ (mh /me)
0.16
0.044
0.082
0.14
0.35
0.25
69.72+30.97 112.4+127.6
12.011
*usually considered an isolator
Material
Source: http://www.ioffe.rssi.ru/SVA/NSM/Semicond/ ; S.M.Sze, Physics of Semicon. Devices , J. Wiley & Sons, 1981,
J. Singh, Electronic & Optoelectronic Properties of Semiconductor Structures, Cambridge University Press, 2003
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2.2 Material Properties
Comparison of different semiconductor materials – 2
Si
Ge
GaAs
GaP
eff. density of states in
conduction band nCB (cm-3)
3 · 1019
1 · 1019
4.7 · 1017
2 · 1019
≈ 1020
eff. Density of states in
valence band nVB (cm-3)
1 · 1019
6 · 1018
7 · 1018
2 · 1019
≈ 1019
Electron mobility µe
bei 300 K (cm2/Vs)
~1450
3900
8500
< 300
1050
1800
Hole mobility µh
bei 300 K (cm2/Vs)
~450
1900
400
< 150
100
1200
instrins. charge carrier
density at 300 K (cm-3)
1.45 · 1010
2.4 · 1013
2 · 106
2
instrins. resistivity at 300 K
(cm)
2.3· 105
47
≈ 108
Breakdown field (V/cm)
3 · 105
≈ 105
4 · 105
≈ 106
Mean E to create an e–h+
pair (eV), 300 K
3.62
2.9
4.2
≈7
*usually considered an isolator
CdTe
Diamond*
Material
≈ 10-27
≈ 109
≥ 1042
3 · 107
4.43
13.25
Source: http://www.ioffe.rssi.ru/SVA/NSM/Semicond/ ; S.M.Sze, Physics of Semicon. Devices , J. Wiley & Sons, 1981,
J. Singh, Electronic & Optoelectronic Properties of Semiconductor Structures, Cambridge University Press, 2003
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2.3 Constructing a Detector
The ideal semiconductor detector
One of the most important parameter of a detector is the signal to noise ratio
(SNR). A good detector should have a large SNR. However this leads to
contradictory requirements:
Large signal
low ionisation energy
small band gap
Low noise
very few intrinsic charge carriers
large band gap
An optimal material has a band gap which is small enough to create a large
number of e-h+ pairs through ionisation, but large enough to have an almost
empty conduction band at room temperature, i.e. about Eg ≈ 6 eV.
Such a material exist, it is Diamond. However even artificial diamonds (e.g. CVD
diamonds) are too expensive for large area detectors.
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2.3 Constructing a Detector
Estimate SNR in an intrinsic silicon detector
How about Silicon? Let’s make a simple calculation:
Mean ionization energy I0 = 3.62 eV,
Mean energy loss per flight path (mip*) dE/dx = 3.87 MeV/cm,
Intrinsic charge carrier density at T = 300 K ni = 1.45 · 1010 cm-3.
Assuming a detector with a thickness of d = 300 µm and an area of A = 1 cm2.
Signal (mip*) in such a detector:
dE dx × d 3.87 ×10 6 eV cm× 0.03cm
=
» 3.2 ×10 4 e-h +-pairs
I0
3.62eV
Intrinsic charge carrier in the same volume (T = 300 K):
ni d A =1.45 ×1010 cm-3 × 0.03cm×1cm2 » 4.35 ×108 e-h+-pairs
Number of thermal created e–h+-pairs are four orders of magnitude larger
than signal!!!
Have to remove the charge carrier!
Depletion zone in reverse biased pn junctions
* minimum ionising particle
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2.4 Doping
pn junction needs doped materials
A pn junction consists of n and p doped substrates:
Doping is the replacement of a small number of atoms in the lattice
by atoms of neighboring columns from the atomic table (with one
valence electron more or less compared to the basic material).
Typical doping concentrations for Si detectors are ≈1012 atoms/cm3
(1014 und 1018 atoms/cm3 for CMOS elements).
These doping atoms create energy levels within the band gap and
therefore alter the conductivity.
An undoped semiconductor is called an intrinsic semiconductor
A doped semiconductor is called an extrinsic semiconductor.
In an intrinsic semiconductor for each conduction electron there
exists the corresponding hole. In extrinsic semiconductors there is a
surplus of electrons or holes.
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2.4 Doping
Bond model: n-doping in Si
Doping with an element 5 atom (e.g. P, As, Sb). The 5th valence electrons is
weakly bound.
The doping atom is called donor
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The released conduction electron
leaves a positively charged ion
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2.4 Doping
Bond model: n-doping in Si
The energy level of the donor is just below the edge of the conduction band. At
room temperature most electrons are raised to the conduction band.
The fermi level EF moves up.
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2.4 Doping
Bond model: p-doping in Si
Doping with an element 3 atom (e.g. B, Al, Ga, In). One valence bond
remains open. This open bond attracts electrons from the neighbor atoms.
The doping atom is called acceptor.
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The acceptor atom in the lattice is
negatively charged.
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2.4 Doping
Bond model: p-doping in Si
The energy level of the acceptor is just above the edge of the valence band. At
room temperature most levels are occupied by electrons leaving holes in the
valence band.
The fermi level EF moves down.
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2.4 Doping
Donor and acceptor levels in Si und GaAs
Measured ionization energies
for doping atoms in Si and
GaAs.
Levels above band gap middle
are donators and are measured
from the edge of the
conduction band (exceptions
denoted D).
Levels below band gap middle
are acceptors and are
measured from the edge of the
valence band (exceptions
denoted A).
Source: S.M. Sze, Semiconductor Devices , J. Wiley & Sons, 1985
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2.4 Doping
Temperatur dependance of the carrier concentration
At low temperatures the thermal
energy is not sufficient to ionize all
donors. Some e- are frozen at the
donor level.
Electron density as a function of temperature
for a Si sample with a donor concentration of
1015 cm–3:
Source: S.M. Sze, Semiconductor Devices ,
J. Wiley & Sons, 1985
As the temperature increases all
donors become ionized (“extrinsic
region”).
At even higher temperature
(kT ≈ Eg) the intrinsic carrier
concentration becomes
comparable to the donor
concentration. Beyond this point
the semiconductor becomes
intrinsic.
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2.5 The p-n Junction
Creating a p-n junction
At the interface of an n-type and p-type semiconductor the difference in the fermi
levels cause diffusion of surplus carries to the other material until thermal equilibrium
is reached. At this point the fermi level is equal. The remaining ions create a space
charge and an electric field stopping further diffusion.
The stable space charge region is free of charge carries and is called the depletion
zone.
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2.5 The p-n Junction
Electrical characteristics
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2.5 The p-n Junction
Operation with forward bias
p-n junction with forward bias
Applying an external voltage V with the anode
to p and the cathode to n e- and holes are
refilled to the depletion zone. The depletion
zone becomes narrower.
The potential barrier becomes smaller by eV
and diffusion across the junction becomes
easier. The current across the junction
increases significantly.
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2.5 The p-n Junction
Operation with reverse bias
p-n junction with reverse bias
Applying an external voltage V with the cathode
to p and the anode to n e- and holes are pulled
out of the depletion zone. The depletion zone
becomes larger.
The potential barrier becomes higher by eV and
diffusion across the junction is suppressed. The
current across the junction is very small
“leakage current”.
That’s the way we operate our semiconductor detector!
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2.5 The p-n Junction
Width of the depletion zone
Example of a typical p+-n junction in a silicon detector:
Effective doping concentration Na = 1015 cm–3 in p+ region and Nd = 1012 cm–3 in
n bulk.
Without external voltage:
Wp = 0.02 µm
Wn = 23 µm
Applying a reverse bias voltage of 100 V:
Wp = 0.4 µm
Wn = 363 µm
Width of depletion zone in n bulk:
with
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p+n junction
V …
 …
 …
Neff …
External voltage
specific resistivity
mobility of majority charge carriers
effective doping concentration
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2.5 The p-n Junction
Current-voltage characteristics
Typical current-voltage of a p-n junction (diode): exponential current increase
in forward bias, small saturation in reverse bias.
Ideal diode equation:
I0 … reverse saturation current
S.M. Sze, Semiconductor Devices , J. Wiley & Sons, 1985
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2.6 Detector Characteristics
Leakage Current
A silicon detector is operated with reverse bias, hence reverse saturation
current is relevant (leakage current). This current is dominated by thermally
generated e-h+ pair. Due to the applied electric field they cannot recombine and
are separated. The drift of the e- and h+ to the electrodes causes the leakage
current.
Measured detector leakage current, CMS
strip detector (measurement at room
temperature):
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2.6 Detector Characteristics
Depletion Voltage
The depletion voltage is the minimum voltage at which the bulk of the sensor is
fully depleted. The operating voltage is usually chosen to be slightly higher
(overdepletion).
reverse bias voltage V [V]
High resistivity material (i.e. low doping) requires low depletion voltage.
Depletion voltage as a function of the
material resistivity for two different
detector thicknesses (300 µm, 500 µm).
resistivity  [kOhm cm]
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2.6 Detector Characteristics
Capacitance of a detector
For a typical Si p-n junction (Na >> Nd >> ni) the detector capacitance is given
as:
 … specific resistivity of the bulk
e0er
 … mobility of majority charge carrier
C=
×A
V … bias voltage
2mr V
A … detector surface
Measured detector capacitance as a
function of the bias voltage, CMS strip
detector:
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2.7 Manufacturing Si Detectors
Wafer production
Properties of Si bulk required for detectors:
• Diameter: 6 inches (soon 8?)
• Lattice orientation <111> or <100>
• Resistivity 1–10 kΩcm
Therefore, float-zone technique for ingot
production is used*
– technique moves a liquid zone
through the mater
• Result: single-crystal ingot
*More recently wafers from other production
processes are tried, e.g. Magnetic Czochralski.
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2.7 Manufacturing Si Detectors
Planar process - 1
1. Starting Point: single-crystal n-doped
wafer (ND ≈ 1–5·1012 cm-3)
2. Surface passivation by SiO2-layer
(approx. 200 nm thick). E.g. growing by
(dry) thermal oxidation at 1030 °C.
3. Window opening using
photolithography technique with
etching, e.g. for strips
4. Doping using either
• Thermal diffusion (furnace)
• Ion implantation
- p+-strip: Boron, 15 keV,
NA ≈ 5·1016 cm-2
- Ohmic backplane: Arsenic,
30 keV, ND ≈ 5·1015 cm-2
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2.7 Manufacturing Si Detectors
Planar process - 2
5. After ion implantation: Curing of
damage via thermal annealing at
approx. 600°C, (activation of dopant
atoms by incorporation into silicon
lattice)
6. Metallization of front side: sputtering
or CVD
7. Removing of excess metal by
photolitography: etching of noncovered areas
8. Full-area metallization of backplane
with annealing at approx. 450°C for
better adherence between metal and
silicon
9. Last step: wafer dicing (cutting)
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2.7 Manufacturing Si Detectors
Lithography
exposure
mask
photoresist
SiO2
developing
etching
Photoresist
removal
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2.7 Manufacturing Si Detectors
Sensor mask design
•
Design tools like in commercial chip
industry
– ICStation from Mentor Graphics
– Cadence
•
Design is not drawn but actually
“programmed”
– using simple programming
language (C like)
•
Therefore, it is easy to change any
parameter and re-create the full
sensor within minutes
– e.g. width of strips
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Polysilicon pieces
2.6 Manufacturing Si Detectors
Single crystal
Silicon wafers with different diameter
Wafers in a package box
Electronic parts
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