Introduction to Silicon Detectors

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Introduction to Silicon Detectors
G.VillaniSTFC Rutherford Appleton
LaboratoryParticle Physics Department
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Outlook

• Introduction to physics of Si and detection
• Si electronic properties, transport mechanisms,
  detection
• Examples of detectors
• Strips, CMOS,CCD,MOS
• Radiation damage
• Conclusions

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Introduction
The Si detection chain
Sensing/ Charge creation
Charge transport and collection
Conversion
Signal processing
Data TX
E
Si physical properties
Si device properties
Si device topologies properties
almost all the boxes of the detection chain
process based upon Silicon
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Silicon properties
After Oxygen, Silicon is the 2nd most abundant
element in Earths crust (gt25 in mass)
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, a-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)
Si
1.48A
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Silicon electrical properties
Silicon Band structure The 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
a wave associated with free motion of electrons
modulated by the periodic solution un,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).
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
VB
CB
1st Brillouin zone of Diamond lattice
CB
VB-H
VB-L
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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
(3D)
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.
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Silicon electrical properties

• The 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

CB
VB
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Fermi level energy level _at_ 50 occupancy
In 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
0
The density of states gD(E) depends on the
dimension
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Silicon electrical properties
Conduction of Si intrinsic _at_ T 300K s q(µn
µp) ni 3.04x10-6mho-cm -gt329kOhm-cm By adding
atoms of dopants, which require little energy to
ionize( 10s mEV, so thermal energies _at_ ambient
temp is enough) we can change by many odg the
carrier concentration. Doping concentration
1012 to 1018 cm-3 In crystalline Si
51022atomscm-3 In equilibrium and for non
degenerate case the relationship between carrier
concentration and E is the same as in the
intrinsic case
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Charge transport
Charge transport The charge transport
description in semiconductors relies on
semi-classical BTE (continuity equation in 6D
phase space)
Q conservation
P conservation
E conservation
The distribution function f(r,k) can be
approximated near equilibrium
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Charge transport
Under (many) simplifying assumptions the 1st
moment of BTE gives the DD model (The
semiconductor equations)
Drift term
Diffusion term
Transport of charge is a combination of drift and
diffusion mechanism DD expresses momentum
conservation it becomes invalid when sharp
variation in energy of carriers occur (due to F
for example deep submicron devices) When feature
size is 0.sµm the DD model becomes invalid
higher momentum required
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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)
a
MIP
Photon interaction
Bethe formula for stopping power gives the rate
of energy loss/unit length for charged particles
through matter
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Detection
MIP charge density
Photoelectric charge density
An optical power of -60dBm ( 1nW) of 1keV
photons generates 6106e-/µm High injection
regime Plasma effects The internal electric
field can be affected by the generated charge
A MIP forms an ionization trail of radius R when
traversing Si, creating 80e-/µm Low injection
regime the generated charge is too small to
affect the internal electric field
The associated wavelength is much smaller than
mean free path Each charge is independent from
each other Carrier dynamics does not need QM
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Detection
B Excitation Charge or lattice (acoustic or
optical phonons) some IR detectors, bolometer
60meV
Si
SiO2
Poly Si
Ec
EF
10s meV
EF
EV
Eigenvalues separation in quantized structures
10s meV
Dispersion relation for phonons in Si Phonon
excitation energy 10 meV much lower threshold
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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 structure The thermal diffusion of
charge across the junction leaves just the
ionized dopants an electric potential, and a
field F, develops across the junction
In equilibrium J 0 using DD model
Near the interface, the carrier concentration
exponentially drops a depletion region (empty of
free charge) is formed.
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
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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)
W

• To achieve large W high field region
• Low doping (high resistivity) Silicon is needed
• Large voltages

Conversion Q to V // Q to I
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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
module
768 Strip Sensors
RO
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!
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Detectors examples
Strip detectors
768 Strip Sensors
80µm
P
N (high res)
300µm
F
Wires
RO electronic
Power supply
Vbias 100sV
Array of long silicon diodes on a high
resistivity silicon substrate A 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)
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Detectors examples
High events rate require fast signal
collection Estimate of charge collection time in
strip detector
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
MAPS detectors
10s mm
RO electronic
RO electronic
3T ( 3MOS) MAPS structure
2D array of 106 pixels Monolithic
solution Detector and readout integrated onto
the same substrate
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Detectors examples
MAPS detectors
Vbias Vs
N (low res)
Electronics 0.s µm
P (low-med res)
Active region s µm
P (low res)
Mechanical substrate 100s µm
The charge generated in the thin active region
moves by diffusion mainly Long collection
time Small signal Different implants
arrangements for charge collection
optimization Circuit topologies for low noise
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Detectors examples
Example of MAPS detectors
TPAC 1 pixel size 50x50um2 Chip size 1cm2 Total
pixels 28k gt8Meg Transistors
Charge collection time (s) in MAPS vs.
perpendicular MIP hit
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Detectors examples
CCD detectors
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 to inventors Boyle and
Smith
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Detectors examples
In-situ Storage Image Sensor ISIS
55Fe g source
Mn(Ka)
Mn(Kb)
CCD in CMOS process 0.18µm Charge collection
under a PG then stored under a 20 pixels storage
CCD
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Signal conversion The unipolar MOS device
NMOS
SiO2
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.
N
P

• By applying a voltage to the G with respect to
  the Substrate
• an 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
  size
• achievable 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

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Signal conversion The unipolar MOS device
LET in SiO2 for different particles
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
recombination processes are stronger than in Si.
Furthermore, hole Transport is non Gaussian (low
mobility) and traps form near Si interface.
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Signal conversion The unipolar MOS device
Floating Gate
Control Gate
SiO2
Addition of a Floating Gate (FG) the electrical
characteristics of the device are controlled by
the charge stored 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
Radiation sensitivity
Chip 1 100Gy
lt?Vth gt 0.6152
Std dev 0.00598
The MOS structure easily allows excitation based
radiation detection
Conversion Q to I
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Radiation damage
In HEP and space applications the detectors are
exposed to high level of radiation LHC 10s
Mrad (100kGy) over 10years of operation N.B. 1
rad/cm3 Si 1013e/h pairs
Total Ionizing Dose (rad 0.01Gy) Non Ionizing
energy Loss (1MeV neutrons/cm2 fluence)
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Radiation damage
Radiation environment in LHC experiment
TID Fluence 1MeV n eq. cm-2 _at_ 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 defects High energy particles cluster
defects
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Radiation damage
Energy deposition
Atoms displacement
altered Lattice periodicity
Band gap Spurious states
Altered Electrical characteristics
Conduction band
trapping
recombination
generation
Donor levels

Band gap
Acceptor levels
Valence band

• The appearance of spurious band gap states
  affects the electro/optical characteristics of
  the device
• Thermal generation of carriers (increased
  leakage current _at_ same T)
• Reduced recombination time ( quicker charge loss
  , reduced signal)
• Charge trapping
• Scattering
• Type conversion

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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.
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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 collection
• The 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
  also
• better 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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Addendum - Detector systems
HEP experiments large detector
systems Challenging engineering issues
ALICE ATLAS CMS LHCb
Strips 4.9m2 64m2 210m2 14.3m2
Drift 1.3m2
Pixels 0.2m2 2m2 1m2 0.02m2
Number of Channels Number of Channels Number of Channels Number of Channels 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
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
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Addendum - Detector systems
Alternative powering schemes
SP
ATLAS SCT Barrel 3 at CERN. Half of the 384
cables are visible the rest enters the other end
of the detector.
DC2DC
A serial powering or DC2DC approach can increase
efficiency in power distribution compared to a
parallel approach
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Conclusions

• 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

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Backup - Detector systems
Power reduction at detector level

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
I
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Backup - Detection
The variance in signal charge si associated to
the ionization process is related to the phonon
excitation
Fano factor 0.1 in Si
High resolution requires smaller band gap (ei ),
direct or small phonon excitation energy
Intrinsic resolution of Si and Ge based detectors
II
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Backup -Detection
Ph DQ107 m-1
DQ1010 m-1
p/a
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 creation Put values of
photon momentum typ.
III
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Backup
Quantization effects due to band bending in
Si-SiO2 interface excitation based detection
SiO2
Si-poly
Si-sub
Q-effects
IV
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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