Transcript pptx

Introduction to Silicon Detectors
E.G.Villani
STFC Rutherford Appleton Laboratory
Particle Physics Department
1
Outlook
•
•
•
•
•
2
Introduction to physics of Si and detection
Examples of detectors
Radiation damage
Systems of detectors
Conclusions
Introduction
The Si detection chain
E
Sensing/
Charge creation
Charge transport
and collection
Si physical properties
Conversion
Si device properties
Signal
processing
Data TX
Si device topologies properties
all the boxes of the detection chain process based upon Silicon
Detector examples:
Silicon physical and electrical properties Strips
Detection principles
MAPS
Detector physics:
Transport mechanisms
Conversion
3
Detector system issues:
Detection efficiency
Power
Silicon properties
 After Oxygen, Silicon is the 2nd most abundant
element in Earth’s crust (>25% in mass)
 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.
Si
1.48A
4
 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 electrical properties
The electronic band structure can be obtained by solving 1 electron SE in periodic potential
neglecting electron interactions : Bloch functions
T  U   E   n,k r   un,k r e jkr  En k 
~ 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.
VB
 The appearance of Band Gap, separating CB
and VB
CB
 The 6 CB minima are not located at the center
of 1st Brillouin zone, Indirect Gap
1st Brillouin zone of Diamond
lattice
CB
5
VB-H
VB-L
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:
E k  
2 k
2
2m0*
2m 
 g E  
* 3/ 2
0
2 3
2 
E  Eo
2
2 k
1
k  p
E k  
 v   k E k   *  *
*
2m0

m0 m0

dk t 
d  p

   rV  F
dt
dt
 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.
6
Silicon electrical properties
 The carrier density is calculated from:
• The density of states g(E)
• The distribution function F(E);
CB
Only partly filled bands can contribute to conduction: carrier
density in CB and VB.
VB
At equilibrium the carrier density is obtained by integrating
the product:
3
nD   g D E F E dE  NC /V e  Ec Ev / kT  ni  pi
F E  
2
1
pn  ni2  N C NV e
1
 E  EF 
1  exp

 kT 
 
 E g / kT
 10 20 cm 3 @ T  300 K
*Fermi level: energy level @ 50% occupancy
0
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
The density of states gD(E) depends on the dimension
7
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:
e.g. : N D  10
8
17
ni2
 pn  pN D  p 
 10 3 cm 3
ND
Charge transport
The charge transport description relies on semi-classical BTE (continuity equation in 6D
phase space)



f r , k , t 1
F
f r , k , t
  k E k   r f r , k , t     k f r , k , t  
t


t
 
 f r , k , t 
Q conservation
 
q
vk  f r , k , t 
V k
P conservation
 
1
E k  f r , k , t 
V k
E conservation
n r, t 
1
V
J r, t  
W r, t 

 S r , k , t 
coll
k
 Higher moments of BTE correspond to more accurate transport descriptions
9
Charge transport
Under (many) simplifying assumptions the 1st moment of BTE gives the DD model
(The semiconductor equations):
J n  qn n E r   qDnn
Drift term
Diffusion term
J p  qp p E r   qDp p
n 1
   Jn Un
t q
p
1
    J p U p
t
q
  V   p  n  N D  N A 
 DD expresses momentum conservation: it becomes invalid when sharp variation in energy
of 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
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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. Most exploited concept ( ionization chambers, microstrip, hybrid
pixels, CCD, MAPS…)
B: Excitation: Charge or lattice (acoustic or optical phonons) some IR detectors, bolometer
α
MIP ~  = 3
Photon interaction
Bethe-Bloch formula for stopping power gives the rate of <energy loss>/unit
length for charged particles through matter
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I z   I o e   E z
Detection
Ph: DQ~107 m-1
DQ~1010 m-1
/a
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)
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Detection
MIP charge density
n
I z   I o e   E z
dE 1 1

 3 1015 cm 3
2
dx  i   R
R
 v
I
 110nm
A MIP forms an ionization trail of radius R
when traversing Si, creating ~ 77 e-/μm
h
 1015 cm
2m E
L  0.5  1107 cm

 Low injection regime:
The generated charge is too small to affect the internal electric field
 Carrier dynamics does not need QM
13
Photoelectric charge density
z  

n
Pin
   z
  e
 5.6 106 e  / m
h
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
Detection
B: Excitation:
60meV
Poly Si
SiO2
Si
Ec
EF
~10’s meV
EV
EF
Eigenvalues separation in quantized structures ~ 10’s meV
Dispersion relation for phonons in Si
Phonon excitation energy ~ 10 meV : much lower threshold
 Can be used for sensitive bolometers
14
Half summary
Under conditions of
• quasi equilibrium
• non small feature size
• low injection
…
The behaviour of detectors can be drastically simplified
E
Sensing/
Charge creation
Charge transport
and collection
Q generation:
E(k):
<q>/L
Free particle, effective
mass
Charge transport:
Drift- diffusion model
15
Conversion
Detectors examples
Strip / pixel detectors
HEP, Scientific applications
PN junction
Monolithic Active Pixel Sensors (MAPS)
Consumer and scientific applications
Charge Coupled Devices (CCD)
Imaging, scientific and consumer applications
RAL PPD has (is) actively involved with all these detector technologies
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Signal conversion: The pn junction
Homojunction: two pieces of same semiconductor materials
with different doping levels:
• In equilibrium, the Fermi level equalizes throughout the structure
F
• 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:
 0
In equilibrium J = 0
0  qnn E r   qDnn  n  n0e
D
Vt
0  qp p E r   qDpp  p  p0e

D
Vt
• 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
17
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)
W
Depletion width
W
2Vb N a  N d
q Na  Nd
To achieve large W high field region:
• Low doping (high resistivity)
• Large biasing voltages
18
Detectors examples
ATLAS Strip detectors
(high res)
F
≈ 300μm
80μm
Q transversal diffusion
Vbias ~100’sV
Array of long silicon diodes on a high resistivity silicon substrate
A 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)
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Detectors examples
ATLAS SCT RO ASIC (ABCD3TA)
12 RO ASIC
768 Strip Sensors
128 channels/ASIC
RAD-HARD DMILL technology
ASICS glued to hybrid
40 MHz Ck
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
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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
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Detectors examples
ATLAS Tracker overview
Pixel: (n+ on n) 1.8 m2, 80M channels
SCT: 6.3M 61 m2 channels
TRT: 0.4M channels
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Detectors examples
Barrel insertion in the ATLAS Cavern
ATLAS SCT Red cables : power cables
23
Detectors examples
24
Detectors in operation
25
Detectors in operation
26
Detectors examples
MAPS detectors
≈1’s m
RO electronic
RO electronic
3T ( 3MOS) MAPS structure
2 D array of pixels
 Monolithic solution: detector and readout
integrated onto the same substrate
27
Detectors examples
MAPS detectors
Vbias ~V’s
N++ (low res)
Electronics
0.’s μm
Active region
P+ (low-med res)
‘s μm
P++ (low res)
Mechanical substrate
100’s μm
 The charge generated in the thin active region moves by diffusion mainly:
‘Long’ collection time
Small signal
Low radiation hardness
Complex circuit topologies allow DSP on pixels for low noise
28
Detectors examples
Example of MAPS detectors:
10-7
TPAC 1 pixel size 50x50 µm2
Chip size ~1cm2
Total pixels 28k
>8Meg Transistors
n
l2
2
 Dn n  U n  tcoll 
t
Dn
Charge collection time (s) in MAPS vs. perpendicular MIP hit
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Detectors examples
Example of charge collection in MAPS: simulated MIP vs.1064nm laser 2x2um 5ns pulse
30
Detectors example
Proposed use of MAPS (50x50 μm2) sensors in SuperB Vertex Tracker
DNW MAPS proposed should
improve collection efficiency and
speed of response
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Radiation damage
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
 Lethal dose: 500 rad Total Body Irradiation
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Radiation damage
Radiation environment in LHC experiment
ATLAS Pixels
ATLAS Strips
CMS Pixels
CMS Strips
ALICE Pixel
LHCb VELO
TID
Fluence
50 Mrad
7.9 Mrad
~24Mrad
7.5Mrad
250krad
-
1.5 x 1015
2 x 1014
~6 x 1014
1.6 x 1014
3 x 1012
1.3 x 1014/year
All values including safety factors.
33
1MeV n eq. [cm-2] @ 10 years
Radiation damage
Microscopic effects: Bulk damage to Silicon :
Displacement of lattice atoms
V
EK>25 eV
I
Vacancy
+
Interstitial
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
34
Radiation damage
Energy
deposition
Atoms
displacement
altered
Lattice
periodicity
generation recombination
Donor levels +++
Band gap
Spurious
states
trapping
-
Altered
Electrical
characteristics
Conduction band
compensation
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)
• Reduced recombination time ( quicker charge loss , reduced signal)
• Charge trapping
• Scattering
• Type conversion
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.
1
 eff e,h
36
 N defects


1

Qe,h (t )  Q0 e,h exp 
t
  eff e,h 


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; also
Better mechanical strength.
n+
F
37
p+
Detector systems
HEP experiments: large detector systems
Alternative powering schemes:
SP
DC2DC
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
38
A serial powering or DC2DC approach can increase efficiency in
power distribution compared to a parallel approach
Detector systems
Low power solutions are crucial for future HEP experiments:
RO consists of several block, each consumes power:
10 – 1000 μW continuous
Energy deposited by a particle: 0.1 – 10’s fJ
Noise occupancy 1% : hit pixel fires/100sec
Required 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
Conclusions
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
40
Backup slides
Quantization effects due to band bending in Si-SiO2 interface: excitation based detection
SiO2
Si-sub
Si-poly
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
The variance in signal charge σi associated to the ionization process is related to the phonon excitation
i 
Eo
i
E pn   i

  1
Ei  Ei 
Fano factor ~0.1 in Si
High resolution requires smaller band gap (εi ),
direct or small phonon excitation energy
Intrinsic resolution of Si and Ge based detectors
III