jaap velthuis (university of bristol)1 silicon detectors in hep introduction semi-conductor physics...
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Jaap Velthuis (University of Bristol) 1
Silicon detectors in HEP
• Introduction• Semi-conductor physics• Real Si detectors• Radiation damage in Si• Radiation hard sensors• Novel devices/State-of-
the-Art In case of any questions:
Bluffing your way intoparticle physics detectors
Jaap Velthuis (University of Bristol) 2
Introduction• Particle Physics is more than hunting
for Higgs and CP violation• Need to make very advanced detector
systems • Forefront of
– Engineering (stiff light weight support structures, cooling, tunnel building)
– High speed and radiation hard electronics– Computing (web, grid, online)– Accelerators (e.g. cancer therapy,
diffraction)– Imaging sensors (e.g. nth generation light
source, medical imaging)
Jaap Velthuis (University of Bristol) 3
Introduction• Why semi-conductor
devices• P-N junction• Particle traversing
matter– Scattering– Signal generation
• Summary• Baseline detector
Jaap Velthuis (University of Bristol) 4
Why semi-conductor devices
Jaap Velthuis (University of Bristol) 5
Standard experiment
Jaap Velthuis (University of Bristol) 6
The “Onion” peeled…• Fundamental parameters:
– Charge– Momentum– Decay products– Life time– Decay vertex– Mass – Spin– Energy
• Need very precise tracking close to primary vertex. Then follow track to calorimeter and measure energy.
Very precise
tracking
tracking
Electro-
magnetic
calorimeter
hadroniccalorim
eter
Muon
chamber
OutwardTrack density drops
Jaap Velthuis (University of Bristol) 7
Tracking• Track described by 5 parameters• Modern tracking uses “Kalman
Filter”– Start with “proto” track– Add new point– Update 2 – Decide to in- or exclude point based on
2
• Modern Vertexing– Use tracks with errors– Add them to vertex– Calculate 2 etc
• So to do good tracking and vertexing, need detectors with small error and little “deflection”
R-
Pri
mary
vert
ex
Secon
dary
vert
ex
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Wire chambers
• Traditionally tracking in wire chambers
Jaap Velthuis (University of Bristol) 9
Wire chambers
• Problem in wire chambers:– Wires long– Many hits per wire for wires close to primary vertex
(high occupancy)– Leads to ambiguities in track fitting
• Solution: very short wires! solid state
Jaap Velthuis (University of Bristol) 10
Charged particle traversing matter
• Energy loss described by Bethe-Bloch equation:
Z
C
I
Wvm
A
ZzcrmN
dx
dE eeeav 22
2ln4 2
2max
22
2
222
Some constant
Atomic number/mass absorber
Electric charge incident particle
Mean excitation energy
Maximum kinetic energy which can beimparted to a free electron in a single collision
2
2
222
max
21
2
M
m
M
m
cmW
ee
e
Z
C
I
Wvm
A
ZzcrmN
dx
dE eeeav 22
2ln4 2
2max
22
2
222
Jaap Velthuis (University of Bristol) 11
Charged…matter• dE/dx different for different particles due to
different M and • Is used to identify different particles
Jaap Velthuis (University of Bristol) 12
Charged … matterRelevant for detectors
dE/d
x [M
eV c
m2 /
g]
• Energy loss wildly varying function,– MINIMUM IONIZING PARTICLE (4)
Jaap Velthuis (University of Bristol) 13
Charged … matter• Bethe-Bloch describes
average energy loss• Collisions stochastic
nature, hence energy loss is distribution instead of number.
• First calculated for thin layers was Landau. Hence energy loss is Landau distributed.
• Signal proportional to energy loss
x
ex
xL2
1exp)(0
is most probable value
Jaap Velthuis (University of Bristol) 14
Multiple scattering• During passage through matter Coulomb
scattering on nucleideviation from original track
• Deflection distribution Gaussian with width 0
• More dense material, more scattering, shorter X0
000 /ln038.01/6.13
XxXxzcp
MeV
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Detector trade off
• Thick detectors (in X0) lots of energy lost lots of signal generated
• But loads of scattering bad for tracking
• Loads of -electrons more signal but not right direction
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Silicon trivia• Silicon was discovered by Jöns
Jacob Berzelius in 1824• Name from “silicis” (Latin for flint)• With 25.7% second most abundant
element in earth’s crust• First crystalline silicon produced by
Deville in 1824
Jaap Velthuis (University of Bristol) 17
Why solid state detectors
• Small band gap – low energy required for e-h pair (3.6 eV in
Si ~30 eV gas)– Many e-h pairs per unit length (80/m in
Si)
• High density – Large energy loss per unit length
• Can make thin detectors with high signal
– Small range for -electrons • Very good spatial resolution
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Why solid state detectors
• Electron and hole mobility very high– Fast charge collection (~10 ns)
• Excellent rigidity – Self-supporting structures
• Possibility of creating fixed space charge by doping
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Intrinsic semi-conductors
• Single non-interacting atom has set of well-defined energy levels
• When forming crystals, levels undergo minor shifts resulting in bands
• Probability for e- to occupy state given by Fermi-Dirac function
kTEE
EFFexp1
1)(
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Intrinsic semi-conductors
• Density of states
• Density of free electrons n given by product Fn(E) and density of states
kinkinkinkin dEEh
mdEEN 2/1
2/3
2
24)(
kT
EE
VkT
EE
CkT
EEn
VFFCFC
eNpeNeh
kTmn
2/3
2
22
Jaap Velthuis (University of Bristol) 21
Intrinsic semi-conductors• Typically for intrinsic Si carrier density at
300K ~1010 cm-3 suppose strip 20m wide, 10 cm long, sensor 0.3 mm thick S/N=410-
3
• Re-writing concentrations:
– Concentrations highly dependent on T and material
• So three solutions:– Use high EG material– Cool device down (ni at 77K ~10-20)– Remove mobile carriers
kT
ENNpnn G
VCi 2exp
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“Trick”: doping
• By introducing atoms with different number of valence e- can change number of free carriers– E.g. P, As: 5 valence e-; donor (n-type)– E.g. Al, B: 3 valence e-; acceptor (p-type)– Activation energies
~0.04eV<<EG=1.12eV
Intrinsic N-type P-type
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PN-junction• Holes in p-type
recombine with e- in n-type, creating zone without mobile carriers (depletion)
• Depleted silicon ideal for detector. Same signal, but no background!
• Note:– Holes move towards p-type– Electrons move towards n-
type
Jaap Velthuis (University of Bristol) 24
PN-junction• Can express as function Vbias:
Vjunc
– Depletion width
– Cjunc
2ln
2
)(
i
DAbi
pnnDbiasbijunc
n
NN
q
kTV with
xxxqN
VVV
biasbi
DA
DAd VV
NqN
NNx
2
biasbiDA
DA
VVNN
NqNAC
1
2
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PN-junction• By biasing detector, the depletion width
can be extended over entire thickness of detector (full depletion).
• Important: PN junction itself is located at interface between p-strips and n-bulk. Depletion region grows from PN junction towards n-type bias contact.
• Typical values for full depletion 10-100 V before irradiation.
biasbi
DA
DAd VV
NqN
NNx
2
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Depletion voltage
• Bias voltage very important:– Creating large depletion zone
•Signal proportional depletion thickness•Depletion zone also reduces
background
– Isolating strips from each other– Separating e-h pairs
• Depletion voltage obtained from C-V curve
Jaap Velthuis (University of Bristol) 27
C-V curve• Re-write C(Vbias)
relation
• Plotting 1/C2 vs Vbias yields:– depletion voltage– Doping
concentration (for asymmetric doping)
biasbi
DA
DA
biasbiDA
DA VVNqN
NN
ACVVNN
NqNAC
2111
2 22
Depletion voltage
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I-V curves• Measure I-V to check long
term stability of sensors and maximum Vbias
• Note current NOT zero (leakage current)
• If Vbias too large, get high currents (breakdown)– Zener breakdown
• Tunnelling from occupied state in p side valence band to n side conduction band
– Avalanche breakdown• Carriers from leakage current
get so much kinetic energy that due to collisions new free carriers are generated
IV Scan
020040060080010001200140016001800
0 100 200 300 400 500Bias voltage (V)
Lea
kag
e cu
rren
t at
20C
(n
A)
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Signal generation• Lost energy converted into free carriers• Energy needed to generate 1 e-h pair in Si is 3.6 eV
• Results in 8900 e-h pairs per 100 m Si for a MIP• Charge cloud Gaussian with 10m• E-h pairs might recombine, need (strong) field to prevent
this signal loss
RamankinGpair rEEEE 2
Carriers carrykinetic energy3/5EG
Energy transferredto lattice r10 ERaman0.165 eV Taken from
http://britneyspears.ac
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Baseline detector• Need many diodes (here p-strips to
n-bulk)• Need reverse bias to
– Deplete entire sensor– Separate e-h
• Need to readout signals from p-strips
• Design issues:– Thick large signal– Thin less scattering– Thin lower depletion voltage– Short strips less ambiguities– Strips close very precise
measurement impact position– Strips far apart less electronics
hence less expensive
Occupancy: fraction of strips that has been hit
Jaap Velthuis (University of Bristol) 31
Real detectors• Real sensors have
much more features:– Backplane contacts– Guard rings– Bias resistors– P-strips– Al readout strips– Coupling capacitors– …
• Typical scale:– Sensors 6x6 cm– Pitch ~100 m– 512 Al strips
Jaap Velthuis (University of Bristol) 32
Charge collection• Determined by
– Spatial distribution of generated charge
– Field strength• Accelerates carriers in field
direction• Determines time charge is
moving• Separation of e-h pairs
– “Horizontal” movement through diffusion
– Hall effect
Jaap Velthuis (University of Bristol) 33
Charge collection• If pitch > charge cloud all charge
collected on 1 strip
• In this case analog signal value not importantchose digital or binary readout
• To do better need to share charge over more strips need pitch20m for 300 m thick sensor
• Problem: connecting all strips to readout channel yields too many strips
12
11
0
21
0
22 dxxxdxxx
Jaap Velthuis (University of Bristol) 34
Summary• Semiconductor detectors are used close
to primary vertex to – Limit occupancy and reduce ambiguities– Give very precise space point
• Energy loss described by Behte-Bloch equation– Minimum ionizing particle– Energy loss (=signal) is Landau distributed
• Particles scatter in matter, so need to have thin detectors
• MIP yields 8900 e-h pairs per 100 m Si
Jaap Velthuis (University of Bristol) 35
Summary (II)
• Need trick to remove free charge carriers– Use high band gap semiconductor– Cool to cryogenic temperatures– Build p-n junction and deplete detector
• If pitch ~ charge cloud, charge is shared. Need lots of strips.– Trick intermediate strip using C-charge
sharing, but non-linear charge sharing