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Introduction to Silicon Detectors
E.G.VillaniSTFC Rutherford Appleton Laboratory
Particle Physics Department
1
Outlook
• Introduction to physics of Si and detection
• Examples of detectors• Radiation damage• Systems of detectors• 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
Detector physics:Silicon physical and electrical propertiesDetection principlesTransport mechanismsConversion
Detector examples:Strips MAPS
Detector system issues:Detection efficiencyPower
all the boxes of the detection chain process based upon Silicon
The crystalline structure is Diamond Cubic (FCC), with lattice spacing of 5.43 A
• Polysilicon consists of small Si crystals randomly oriented; in α-Si there is no long range order.
Apart from its abundance, the key to success of Si is related to its 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 element in 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
The electronic band structure can be obtained by solving 1 electron SE in periodic potential neglecting electron interactions : Bloch functions
kEerurEUT nrjk
knkn ,,~ a wave associated with free motion of electrons modulated by the periodic solution u n,k. The dispersion relationship E(k) is periodic in k so is specified just within 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 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. 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
Silicon electrical properties The carrier density is calculated from:
• The density of states g(E)• 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
KTcmeNNnpn
kTEVCi
g 300@10 320/2
Silicon electrical properties
From carrier density: Conduction of Si intrinsic @ T = 300K:
σ = q(μn +μp) ni = 3.04x10-6mho-cm = 329 kOhm-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 orders the carrier concentration.
Doping concentration: 1012 to 1018 cm-3
In crystalline Si ~ 5*1022atoms cm-3
The relationship between carrier concentration and E is the same as in the intrinsic case:
332
17 1010:.. cmN
nppNpnNge
D
iDD
8
Charge transport
The charge transport description 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,,
Q conservation
P conservation
E conservation
9
Higher moments of BTE correspond to more accurate transport descriptions
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
DD expresses momentum conservation: it becomes invalid when sharp variation in energyof carriers occur (due to F for example: deep submicron devices, which require higher moments or alternative models)
Even in low injection regime, a small F renders the drift term >> diffusion term
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. Most exploited concept ( ionization chambers, microstrip, hybrid pixels, CCD, MAPS…)
B: Excitation: Charge or lattice (acoustic or optical phonons) some IR detectors, bolometer
Bethe-Bloch formula for stopping power gives the rate of <energy loss>/unit length for charged particles through matter
Photon interaction zEoeIzI
α
MIP ~ = 3
11
Detection
In Si an average of 3.6 eV is required for pair creation (partly going into phonons) The MPV of e- generated by MIP is ~77/µm, vs. an average of 110/µm
The indirect BG of Si requires higher energy for charge excitation, because energy and momentum must be conserved (Phonon-assisted pair creation/recombination)
Ph: DQ~107 m-
1
DQ~1010 m-1
/p a
12
Detection
nmI
vR
cmRdx
dEn
i
110
10311 315
2
zEoeIzI
A MIP forms an ionization trail of radius R when traversing Si, creating ~ 77 e-/μm
MIP charge density
cmL
cmEm
h
7
15
1015.0
102
Low injection regime:The generated charge is too small to affect the internal electric field
Carrier dynamics does not need QM
mezeh
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
13
Detection
B: Excitation:
Dispersion relation for phonons in SiPhonon excitation energy ~ 10 meV : much lower threshold Can be used for sensitive bolometers
60meV
Ec
EF
EVEF
~10’s meV
Eigenvalues separation in quantized structures ~ 10’s meV
SiSiO2Poly Si
14
Half summary
Under conditions of • quasi equilibrium• non small feature size• low injection…The behaviour of detectors can be drastically simplified
Sensing/Charge creation
Charge transportand collection
ConversionE
E(k):
Free particle, effective mass
Charge transport:
Drift- diffusion model
Q generation:
<q>/L
15
Detectors examples
Strip / pixel detectors
HEP, Scientific applications
Monolithic Active Pixel Sensors (MAPS)
Consumer and scientific applications
16
Charge Coupled Devices (CCD)Imaging, scientific and consumer applications
PN junction
RAL PPD has (is) actively involved with all these detector technologies
Signal conversion: The pn junctionHomojunction: two pieces of same semiconductor materials with different doping levels:
• In equilibrium, the Fermi level equalizes throughout the structure
• thermal diffusion of charge across the junction leaves just a depleted region, with the ionized dopants : an electric potential Φ, and a field F, develops
• the charge concentration depends exponentially on Φ:
• by applying a small voltage to decrease the barrier, the charge increases exponentially
• by applying a voltage to increase the barrier, the depleted region
Increases
Unidirectionality of current characteristics
t
t
Vpp
Vnn
epppqDrEqp
ennnqDrEqn
0
0
0
0In equilibrium J = 00
17
F
Signal conversion: The pn junction
• when charge is generated in the depleted region is swept across by the electric field sustained by the ionized dopants and the biasing.
PN junction as detector and signal converter: capacitor with a F across
• A device with a large depleted region can be used to efficiently collect radiation generated charge (Solid state ionization chamber)
da
dab
NN
NN
q
VW
2
W
To achieve large W high field region:• Low doping (high resistivity)• Large biasing voltages
18
Depletion width
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, mostly by drift.
The high resistivity Si is not usually used in mainstream semiconductor industry:Hybrid solution: detector (high resistivity) connected (wire/bump-bonded) to the readout electronic (low resistivity)
(high res)
Vbias ~100’sV
F
ATLAS Strip detectors80μm
≈ 3
00
μm
19
Q transversal diffusion
Detectors examples
ATLAS SCT
4 single-sided p-on-n sensors
2x2 sensors daisy chained
Stereo angle 40 mrad
Strip pitch 80 µm
768 channels/side
Binary RO
Bias Voltage up to 500 V
Operating temperature -2 C
Space point resolution: rφ 17µm / z 50 µm
Power consumption: 5.6 W (initially ) to 10 W ( after 10y)
Rad-hard up to 2x10^14 1-MeV n eqv/cm2
20
768 Strip Sensors12 RO ASIC
ATLAS SCT RO ASIC (ABCD3TA)
128 channels/ASIC
RAD-HARD DMILL technology
ASICS glued to hybrid
40 MHz Ck
Detectors examples
ATLAS SCT
61 m2 of Si
6.3 * 10^ 6 channels
50 kW total power consumption
1 barrel- 4 layers- 4088 modules
2 end caps- 9 disks/each- 988 modules
2 T solenoidal field
21
Detectors examples
22
ATLAS Tracker overview
Pixel: (n+ on n) 1.8 m2, 80M channels
SCT: 6.3M 61 m2 channels
TRT: 0.4M channels
Detectors examples
23
Barrel insertion in the ATLAS Cavern
ATLAS SCT Red cables : power cables
Detectors examples
24
Detectors in operation
25
Detectors in operation
26
Detectors examples
RO electronic
3T ( 3MOS) MAPS structure
≈1’s m
RO
electronic
MAPS detectors
27
2 D array of pixels
Monolithic solution: detector and readout integrated onto the same substrate
Detectors examples
MAPS detectors
The charge generated in the thin active region moves by diffusion mainly:‘Long’ collection timeSmall signalLow radiation hardness
Complex circuit topologies allow DSP on pixels 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
28
Detectors examples
Charge collection time (s) in MAPS vs. perpendicular MIP hit
10-7
TPAC 1 pixel size 50x50 µm2
Chip size ~1cm2
Total pixels 28k>8Meg Transistors
n
collnn D
ltUnD
t
n 22
Example of MAPS detectors:
29
Detectors examplesExample of charge collection in MAPS: simulated MIP vs.1064nm laser 2x2um 5ns pulse
30
Detectors exampleProposed use of MAPS (50x50 μm2) sensors in SuperB Vertex Tracker
DNW MAPS proposed should improve collection efficiency and speed of response
31
Radiation damageIn HEP and space applications the detectors are exposed to high level of radiation: LHC: 10’s Mrad (100kGy) over 10years of operationN.B.: 1 rad/cm3 Si ~1013e/h pairs Lethal dose: 500 rad Total Body Irradiation
32
Radiation damageRadiation 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 7.5Mrad 1.6 x 1014
ALICE Pixel 250krad 3 x 1012
LHCb VELO - 1.3 x 1014/year
All values including safety factors.
33
Radiation damageMicroscopic effects: Bulk damage to Silicon : Displacement of lattice atoms
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
34
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)• 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
35
Radiation damage
Macroscopic effects: Charge Collection Efficiency (CCE) is reduced by trapping Noise increases because of increase leakage current Depletion voltage increases because of type inversion
1015 1MeV n-eq.
tQtQ
heeffhehe
,,0,
1exp)(
defectsheeff
N,
1
36
Radiation damage To increase the Radiation Hardness of Sensors:• Operating conditions (cooler – lower leakage)• Material engineering ( OFZ - Diamond detectors)• Device engineering (n in n/p – 3D detectors)
• Electrodes in the bulk – lateral collection reduces the drift distance
• Lower depletion voltage – less power consumption• difficult to manufacture
• 3D DDTC similar to 3D but easier to manufacture; alsoBetter mechanical strength.
n+
p+F
37
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 systems
A serial powering or DC2DC approach can increase efficiency in power distribution compared to a parallel approach
Alternative powering schemes:
SP
DC2DC
38
Detector systems
Low power solutions are crucial for future HEP experiments:
RO consists of several block, each consumes power:
10 – 1000 μW continuousEnergy deposited by a particle: 0.1 – 10’s fJNoise occupancy 1% : hit pixel fires/100secRequired energy/deposited energy >> 1010 !!!Extremely huge energy inefficiency
HEP presents challenging engineering and technical issues: their solution may have important consequences in fields outside PP (biomedical, telecommunications…)
39
The field of semiconductor radiation detectors encompasses a number of scientific and technology fields:solid state physics, nuclear and particle physics, electrical engineering, …
Some of the specific issues relevant to radiation detectors:
Development of new detection techniques, based on novel or common semiconductor material: ( phonon-based detectors, quantum detectors, compounds, low dimensional)
Integration with electronics (monolithic solution to achieve more compactness and reduce cost) and/or 3D structures
Topologies optimization (power reduction, noise reduction)
Radiation hardness
Conclusions
40
Backup slides
Quantization effects due to band bending in Si-SiO2 interface: excitation based detection
Si-polySi-sub
SiO2
Q-effects
I
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)
II
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
III