Download - Introduction to Silicon Detectors
Introduction to Silicon Detectors
G.VillaniSTFC Rutherford Appleton Laboratory
Particle Physics Department
Outlook
• Introduction to physics of Si and detection– Si electronic properties, transport mechanisms, detection
• Examples of detectors– Strips, CMOS,CCD,MOS
• Radiation damage• Conclusions
2
Introduction
The Si detection chain
3
Sensing/Charge creation
Charge transportand collection
ConversionSignal
processingData TXE
Si physical properties
Si device properties Si device topologies properties
almost all the boxes of the detection chain process based upon Silicon
The crystalline structure is diamond cubic (FCC), 8 atoms/cell with lattice spacing of 5.43 A ~ 5x1022 cm-3
* In electronic industry all crystallographic forms are used (Single crystal, Polysilicon, α-Si)
The key to success of Si is related to its abundance and oxide SiO2, an excellent insulator (BV ~ 107 V/cm).
* Micro crystals but the flexible bond angles make SiO2 effectively an amorphous: its conductivity varies considerably (charge transport in SiO2 via polaron hopping between non-bonding oxygen 2p orbitals)
Silicon properties
After Oxygen, Silicon is the 2nd most abundant elementin Earth’s crust (>25% in mass)
Si
1.48A
4
Silicon electrical properties
5
* The appearance of Band Gap, separating CB and VB
* The 6 CB minima are not located at the center of 1st Brillouin zone, INDIRECT GAP
CB VB-H VB-L
1st Brillouin zone of Diamond lattice
CB
VB
Silicon Band structureThe electronic band structure can be obtained within the independent electron approximation (normally 1 electron SE in periodic potential neglecting electron interactions) in terms of Bloch functions
kEerurEUT nrjk
knkn ,,~ a wave associated with free motion of electrons modulated by the periodic solution u n,k. The energy E is periodic in k so is specified just within the 1st unit WS cell of the reciprocal lattice (the Brillouin zone).
Silicon electrical properties
The detailed band structure is complicated: usually quasi-equilibrium simplifications are sufficient to study the charge transport.Assuming that the carriers reside near an extremum, the dispersion relationship E(k) is almost parabolic:
*0
*0
*0
22 1
2 m
p
m
kkEv
m
kkE k
FV
dt
pd
dt
tdkr
* Under the assumptions of small variation of the electric field, the carrier dynamics resembles that one of a free particle, with appropriate simplifications.* The effective mass approximation takes into account the periodic potential of the crystal by introducing an effective carrier mass ( averaged over different longitudinal and transverse masses). The lower the mass, the higher mobility (µ 1/m*)
* Similar approach used to calculate the E(k) for phonons.
oEE
mEg
m
kkE
32
2/3*0
*0
22
2
2
2
6
(3D)
Silicon electrical propertiesThe carrier density is calculated from:
• The density of states g(E), which depends on dimension;• The distribution function F(E);
Only partly filled bands can contribute to conduction: carrier density in CB and VB.At equilibrium the carrier density is obtained by integrating the product:
The density of states gD(E) depends on the dimension
ii
kTEvEcVCDD pneNdEEFEgn /
/
kTEE
EFFexp1
1
3
2
1
0In intrinsic Si a creation of e in CB leaves behind a hole in VB, that can be treated as an e with positive charge and mobility of the band where it resides
CB
VB
Fermi level: energy level @ 50% occupancy
7
KTeNNnpn
kTE
VCi
g300@10
20/2
Conduction of Si intrinsic @ T = 300K:
σ = q(μn +μp) ni = 3.04x10-6mho-cm ->329kOhm-cm
By adding atoms of dopants, which require little energy to ionize( ~10’s mEV, so thermal energies @ ambient temp is enough) we can change by many odg the carrier concentration.
Doping concentration: 1012 to 1018 cm-3
In crystalline Si ~ 5*1022atomscm-3
In equilibrium and for non degenerate casethe relationship between carrier concentration and E is the same as in the intrinsic case:
32
17
20/2
1010:..
300@10
D
iDD
kTE
VCi
N
nppNpnNge
KTeNNnpn g
8
Silicon electrical properties
Charge transport
Charge transport:The charge transport description in semiconductors relies on semi-classical BTE (continuity equation in 6D phase space)
k
k
k
coll
krk
tkrfkEV
trW
tkrfkvV
qtrJ
tkrfV
trn
tkrSt
tkrftkrf
FtkrfkE
t
tkrf
,,1
,
,,,
,,1
,
,,,,
,,,,1,,
The distribution function f(r,k) can be approximated near equilibrium:
equilibrium Near equilibrium
k0
fcoll
ff
t
tkrf
0,,
Q conservation
P conservation
E conservation
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Charge transport
Under (many) simplifying assumptions the 1st moment of BTE gives the DD model(The semiconductor equations):
AD
pp
nn
ppp
nnn
NNnpV
UJqt
p
UJqt
n
pqDrEqpJ
nqDrEqnJ
1
1
Transport of charge is a combination of drift and diffusion mechanism
* DD expresses momentum conservation: it becomes invalid when sharp variation in energyof carriers occur (due to F for example: deep submicron devices)When feature size is 0.’sμm the DD model becomes invalid: higher momentum required
Diffusion termDrift term
10
Detection principles
A: Ionization: by imparting energy to break a bond, electrons are lifted from VB to CB then made available to conduction ( ionization chambers, microstrip, hybrid pixels, CCD, MAPS…)
Bethe formula for stopping power gives the rate of energy loss/unit length for charged particles through matter
Photon interaction zEoeIzI
α
MIP
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Detection
nmI
vR
cmRdx
dEn
i
110
10311 315
2
zEoeIzI
A MIP forms an ionization trail of radius R when traversing Si, creating ~ 80e-/μmLow injection regime:the generated charge is too small to affect the internal electric field
MIP charge density
cmL
cmEm
h
7
15
1015.0
102
The associated wavelength is much smaller than mean free path:Each charge is independent from each other;Carrier dynamics does not need QM
meze
h
Pzn in
/106.5 6
Photoelectric charge density
An optical power of -60dBm (= 1nW) of 1keV photons generates ~ 6*106e-/μm
High injection regime:Plasma effects The internal electric field can be affected by the generated charge
12
Detection
B: Excitation: Charge or lattice (acoustic or optical phonons) some IR detectors, bolometer
Dispersion relation for phonons in SiPhonon excitation energy ~ 10 meV : much lower threshold
60meV
Ec
EF
EVEF
~10’s meV
Eigenvalues separation in quantized structures ~ 10’s meV
SiSiO2Poly Si
13
Signal conversion: The pn junction
Homojunction: consider two pieces of same semiconductor materials with different doping levels:In equilibrium, the Fermi level equalizes throughout the structureThe thermal diffusion of charge across the junction leaves justthe ionized dopants : an electric potential, and a field F, develops across the junction
t
t
Vpp
Vnn
epppqDrEqp
ennnqDrEqn
0
0
0
0
In equilibrium J = 0: using DD model
ASCE (Abrupt Space Charge Edge) approximation:A ‘positive’ voltage increases (exponentially) the charge concentration: high direct current.A ‘negative’ voltage decreases it (down to leakage): the current reduces and at the same time widens the depleted region.Unidirectionality of current characteristics
Near the interface, the carrier concentration exponentially drops: a depletion region (empty of free charge) is formed.
0
14
Signal conversion: The pn junction
The electric field F in the depletion region of the junction is sustained by the ionized dopants. When charge is generated is swept across by the field
PN junction signal converter: A capacitor with a strong F across
A device with a large depleted region W can be used to efficiently collect radiation generated charge ( Solid state ionization chamber)
da
dab
NN
NN
q
VW
2W
To achieve large W high field region:• Low doping (high resistivity) Silicon is
needed• Large voltages
Conversion: Q to V // Q to I
15
Detectors examples
Strip detectors
Scientific applications
Monolithic Active Pixel Sensors (MAPS)
Imaging, consumer applications
Charge Coupled Devices (CCD)
Imaging, scientific and consumer applications
MOS detector
scientific applications
RAL PPD has (is) actively involved with all these detector technologies
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Detectors examples
Use of Si Strip detectors
Almost all HEP experiments use Si detectors:The high density track region usually covered by pixel detectors; by strip at larger radius (cost reason)
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Detectors examples
ATLAS SCT
4 barrel layers,2 x 9 forward disks
4088 double sided modules
Total Silicon surface 61.1m²
Total 6.3 M channels
Power consumption ~ 50kW
Events rate: 40MHz
Put stave pics of AUG!
768 Strip Sensors
RO
module
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Detectors examples
Array of long silicon diodes on a high resistivity silicon substrateA strong F in the high resistivity Si region helps collect charge efficiently (drift).The transversal diffusion of charge implies a spread of signal over neighbouring strips
The high resistivity Si is not usually used in mainstream semiconductor industry:
Hybrid solution: detectors connected (wire/bumpbonded) to the readout electronic (RO)
P++
N+ (high res)
Vbias ~100’sV
F
Strip detectors 768 Strip Sensors
RO electronic
Power supply
80μm
30
0μ
m
Wires
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Detectors examples
High events rate require fast signal collection:Estimate of charge collection time in strip detector:
z
zo
z
z
dzF
Wz
Fdz
zvzt
0011
111
For a detector thickness of 300um and overdepleted Vb = 50V and 10kohm resistivity
tcoll(e)≈ 12ns tcoll(h)≈ 35ns
The fast collection time helps the radiation hardness:The radiation damage to sensors is a crucial issue in modern HEP experiments
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Detectors examples
RO electronic
3T ( 3MOS) MAPS structure
2D array of ~106 pixelsMonolithic solution:Detector and readout integrated onto the same substrate
≈10’s m
RO
electronic
MAPS detectors
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Detectors examples
MAPS detectors
The charge generated in the thin active region moves by diffusion mainly:‘Long’ collection timeSmall signal
Different implants arrangements for charge collection optimizationCircuit topologies for low noise
N++ (low res)
P++ (low res)
P+ (low-med res)
Vbias ~V’s
Mechanical substrate100’s μm
Active region‘s μm
Electronics0.’s μm
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Detectors examples
Charge collection time (s) in MAPS vs. perpendicular MIP hit
10-7
TPAC 1 pixel size 50x50um2Chip size ~1cm2
Total pixels 28k>8Meg Transistors
n
collnn D
ltUnD
t
n 22
Example of MAPS detectors:
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Detectors examples
Once the charge has been generated, it accumulates in the potential well, under the capacitor.The control circuitry shifts the accumulated charge to the end of the row, to the input of a charge amplifier. The sensor is fabricated in a optimized, dedicated process and the RO on a separate chip. Superior imaging quality but less integration and speed.
Nobel Prize 2009 for Physics toinventors Boyle and Smith
CCD detectors
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Detectors examples
In-situ Storage Image Sensor: ISIS
CCD in CMOS process 0.18μmCharge collection under a PG then stored under a 20 pixels storage CCD
RG RD OD RSEL
Column transistor
On-
chip
logi
c
On
-ch
ip s
witc
he
s
Global Photogate and Transfer gate
ROW 1: CCD clocks
ROW 2: CCD clocks
ROW 3: CCD clocks
ROW 1: RSEL
Global RG, RD, OD
Imaging pixel
5 mm
80 mm
55Fe g source Mn(Ka)
Mn(Kb)
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Signal conversion: The unipolar MOS device
By applying a voltage to the G with respect to the Substratean electric field develops across the SiO2: a charge channel is formed between Source and Drain. The Ids characteristics depends on the Vgs applied.
The CMOS process refers to the minimum feature sizeachievable i.e. the channel length)Currently 45nm: the modelling of the characteristics of the device of this size is non-trivial: • Quantization effects at the boundary;• QM tunnelling across the gate;• Hot carriers near the D/S junction;• …
Metal Oxide Semiconductor device are unipolar devices based on voltage modulation of charge.The control gate is physically separated by the active region where the charge moves by a thin (nm) layer of SiO2.
P++
N++
SiO2NMOS
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LET in SiO2 for different particles
0 0 1
)()(1
!!),,(
m n nml
lmKA
oox l
K
m
AeeKrTFY
kT
qr
r
rA
SiO
c
o
c
24
,2
kT
rqFK oox
Generation rate in SiO2 vs. electric field
The SiO2 is a very good insulator: a strong electric field can be applied to it and the charge generated in SiO2 by ionizing radiation efficiently collected
However SiO2 is a polar material: the recombinationprocesses are stronger than in Si. Furthermore, holeTransport is non Gaussian (low ‘mobility’) and traps form near Si interface.
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Signal conversion: The unipolar MOS device
Addition of a Floating Gate (FG): the electrical characteristics of the device are controlled by the chargestored in the FG. The electric field in the SiO2 due to the FG drifts charge towards/away from it.The discharge of the FG alters the device electrical characteristics
FGSiO2
FGSiO2
Floating Gate
Control Gate
Conversion: Q to I
Pre-rad
Post-rad
Reprog
Ids(A)
Chip #1 100Gy
<∆Vth > 0.6152
Std dev 0.00598
Radiation sensitivity
The MOS structure easily allows
excitation based radiation detection
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SiO2
Signal conversion: The unipolar MOS device
Radiation damage
NIEL/cm2/yrTID/Gy/yr ATLAS
Total Ionizing Dose (rad = 0.01Gy) Non Ionizing energy Loss (1MeV neutrons/cm2 fluence)
In HEP and space applications the detectors are exposed to high level of radiation:LHC: 10’s Mrad (100kGy) over 10years of operation
N.B.: 1 rad/cm3 Si ~1013e/h pairs
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Radiation environment in LHC experiment
TID Fluence 1MeV n eq. [cm-2] @ 10 years
ATLAS Pixels 50 Mrad 1.5 x 1015
ATLAS Strips 7.9 Mrad 2 x 1014
CMS Pixels ~24Mrad ~6 x 1014 CMS Strips 7.5 Mrad 1.6 x 1014
ALICE Pixel 250 krad 3 x 1012
LHCb VELO - 1.3 x 1014/year
All values including safety factors.
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Radiation damage
Microscopic effects: Bulk damage to Silicon : Displacement of lattice atoms (~ Kinetic Energy Released)
Atoms scattered by incoming particles leave behind vacancies or atoms in interstitial positions (Frenkel pairs).Low energy particle ~ point defectsHigh energy particles ~ cluster defects
V
I
Vacancy + InterstitialEK>25 eV
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Radiation damage
Energydeposition
Atoms displacement
alteredLattice
periodicity
Band gap Spurious
states
The appearance of spurious band gap states affects the electro/optical characteristics of the device:
• Thermal generation of carriers (increased leakage current @ same T)• Reduced recombination time ( quicker charge loss , reduced signal)• Charge trapping• Scattering• Type conversion
AlteredElectrical
characteristics
Conduction band
Valence band
Band gap
+++Donor levels
Acceptor levels
generation recombination
-compensation
trapping
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Radiation damage
Detrimental Macroscopic effects:• Noise increases because of increase leakage current• Charge Collection Efficiency (CCE) is reduced by trapping• Depletion voltage increases because of type inversion
1015 1MeV n-eq.
tQtQ
heeffhehe
,,0,
1exp)(
defectsheeff
N,
1
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Radiation damage
To increase the Radiation Hardness of Sensors:• Operating conditions (cooler – lower leakage)• Material engineering ( OFZ - Diamond detectors)• Device engineering (n in n – 3D detectors)
Electrodes in the bulk – lateral collectionThe device achieve full depletion• Low depletion voltage• short collection time• claim reduction in signal 33% after 8.8X1015 1Mevn• difficult to manufacture
• 3D DDTC similar to 3D but easier to manufacture; alsobetter mechanical strength.
* Radiation damage affects also the RO electronics, but modern process can address the problem efficiently( guard rings, sub micron devices)
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Radiation damage
Addendum - Detector systems
The ATLAS SCT (semiconductor tracker) detector. The thick red cables on show feed the detector with half of its power – adding more will take up even more space
HEP experiments: large detector systemsChallenging engineering issues
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ALICE ATLAS CMS LHCb
Strips 4.9m2 64m2 210m2 14.3m2
Drift 1.3m2
Pixels 0.2m2 2m2 1m2 0.02m2
Number of Channels
Strips 2.6 x 106
6.3 x 106
9.6 x 106
1 x 106
Drift 1.3 x 105
Pixels 9.8 x 106
80 x 106
33 x 106
1 x 106
Addendum - Detector systems
A serial powering or DC2DC approach can increase efficiency in power distribution compared to a parallel approach
SP
DC2DC
Alternative powering schemes:
ATLAS SCT Barrel 3 at CERN. Half of the 384 cables are visible; the rest enters the other end of the detector.
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The field of semiconductor detectors encompasses different scientific and technology fields: solid state physics, nuclear and particle physics, electrical engineering, …
Some of the issues relevant to radiation detectors:
• Radiation hardness • Topologies optimization (power reduction, noise reduction)• Development of new detection techniques based on novel and well established
semiconductor material: ( phonon-based detectors, compounds, low dimensional)• Integration with electronics (monolithic solution to achieve more compactness
and reduce cost),3D structures
Conclusions
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Backup - Detector systems
=
At pixel level, power consumption could be optimized by using a non linear approach:
The positive feedback structure is biased near threshold (variable)
A small signal triggers the structure
Power reduction at detector level
I
Backup - Detection
Intrinsic resolution of Si and Ge based detectors
The variance in signal charge σi associated to the ionization process is related to the phonon excitation
1
i
i
i
pn
i
oi EE
EE
High resolution requires smaller band gap (εi ), direct or small phonon excitation energy
Fano factor ~0.1 in Si
II
Backup -Detection
The indirect BG of Si requires higher energy for charge excitation, because energy and momentum must be conserved (Phonon-assisted pair creation/recombination)
In Si an average of 3.6 eV is required for pair creationPut values of photon momentum typ.
Ph: DQ~107 m-
1
DQ~1010 m-1
/p a
III
Backup
Quantization effects due to band bending in Si-SiO2 interface: excitation based detection
Si-polySi-sub
SiO2
Q-effects
IV
Backup - The bipolar transistor device
A bipolar transistor can be thought of as a two diode system, connected in anti series;• One is forward biased;• The other is reverse biased
The bipolar transistor can be (and it is) used as a high gain detector
Main limitations arising from speed: the minority carriers diffuse through the base ( relatively low speed)
V