1 Introduction to Silicon Detectors Marc Weber, Rutherford Appleton Laboratory RAL Graduate Lectures, October 2008 •Where are silicon detectors used? • How do they work? • Why silicon? • Electronics for silicon detectors • Silicon detectors for the ATLAS experiment • Radiation-hardness • Future
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Introduction to Silicon Detectors Marc Weber, Rutherford Appleton Laboratory
Introduction to Silicon Detectors Marc Weber, Rutherford Appleton Laboratory. Where are silicon detectors used? How do they work? Why silicon? Electronics for silicon detectors Silicon detectors for the ATLAS experiment Radiation-hardness Future. RAL Graduate Lectures, October 2008. - PowerPoint PPT Presentation
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Introduction to Silicon Detectors Marc Weber, Rutherford Appleton Laboratory
RAL Graduate Lectures, October 2008
•Where are silicon detectors used?
• How do they work?
• Why silicon?
• Electronics for silicon detectors
• Silicon detectors for the ATLAS experiment
• Radiation-hardness
• Future
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Where are silicon detectors used?
in your digital Cameras to detect visible light
A basic 10 Megapixel camera is less than $150 …
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in particle physics experiments to detect charged particles
Example: ATLAS Semiconductor Tracker (SCT); 4088 modules; 6 million channels
1 billion collisions/sec
Up to 1000 tracks
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in astrophysics satellites to detect X-rays
Example: EPIC p-n CCD of XMM Newton New picture of a supernova observed
in 185 AD by Chinese astronomers
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in astrophysics satellites to detect gamma rays
11,500 sensors350 trays18 towers
~106 channels83 m2 Si surface
INFN, Pisa
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Silicon detectors are used at many other places
• in astrophysics satellites and telescopes to detect visible and infrared light, X ray and gamma rays
• in synchrotrons to detect X-ray and synchrotron radiation
• in nuclear physics to measure the energy of gamma rays
• in heavy ion and particle physics experiments to detect charged particles
• in medical imaging
• in homeland security applications
What makes silicon detectors so popular and powerful?
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1. Incident particle deposits energy in detector medium positive and negative charge pairs
(amount of charge can vary wildly from ~100 – 100 M e, typical is 24,000 e = 4 fC)
2. Charges move in electrical field electrical current in external circuit
Most semiconductor detectors are ionization chambers
How to chose the detection medium ?
Operation principle ionization chamber
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Desirable properties of ionization chambers
Always desirable: signal should be big; signal collection should be fast
for particle energy measurements: particle should be fully absorbed
high density; high atomic number Z; thick detector
Example: Liquid Argon
for particle position measurements: particle should not be scattered
3. Charge trappingthe most dangerous effect at high fluences
collect electrons rather than holes
reduce drift distances
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Strong candidate for inner layer: 3D pixels• 3D pixel proposed by Sherwood Parker in 1985
• vertical electrodes; lateral drift; shorter drift times; much smaller depletion voltage
• Difficulty was non-standard via process; meanwhile much progress in hole etching; many groups; simplified designs
see talk of Sabina R. (ITC-irst)
3D planar
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0
20
40
60
80
100
0 5 1015 1 1016 1.5 1016 2 1016
Sig
nal
eff
icie
ncy
[%
]
Fluence [n/cm2]
0 8 1015 1.6 1016 2.4 1016 3.2 1016
Fluence [p/cm2]
3D silicon C. DaVia et a. March 06
Diamond W. Adam et al. NIMA 565 (2006) 278-283
n-on-p strips P. Allport et al.IEEE TNS 52 (2005) 1903
n-on-n pixels CMS T. Rohe et al. NIMA 552(2005)232-238
C. Da Via'/ Aug.06
Signal loss vs. fluence see C. da Via’s talk at STD6 “Hiroshima” conference
3D pixels perform by far the best
Large Hadron Collider: the world’s most powerful accelerator
7 TeV protons vs. 7 TeV protons; 27 km circumference
7 x the energy and 100 x the luminosity of the Tevatron
ATLAS detector
ATLAS detector
• Huge multi-purpose detector; 46 m long; diameter 22 m; weight 7000 t
• Tracking system much smaller; 7 m long; diameter 2.3 m; 2 T field
ATLAS Silicon Tracker
17 thousand silicon sensors (60 m2 )
6 M silicon strips (80 m x 12.8 cm)
80 M pixels (50 m x 400 m)40 MHz event rate; > 50 kW power
2 m 5.6 m
1 m
1.6 m
What’s charged particle tracking ?
1. Measure (many) space points/hits of charged particles
2. Sort out the mess and reconstruct particle tracks
Difficulty is:
- not to get confused
- achieve track position
resolution of 5-10 m
…it’s not easy !Up to 1000 tracks
1 billion collisions/sec
Status as of October 2006
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How does it look in real life ? SCT Detector• 4 barrel layers at 30, 37, 45, 52 cm radius and 9 discs (each end)
• 60 m2 of silicon; 6 M strips; typical power consumption 50 kW
• Precision carbon fiber support cylinder carries modules, cables, optical fiber, and cooling tubes
• Evaporative cooling system based on C3F8 (same for pixel detector)
Barrel 6 at CERN
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Why tracking at LHC is tough ?• Too many particles in too short a time
- 1000 particles / bunch collision
- too short: collisions every 25 ns
• Too short need fast detectors and electronics; power!
• Too many particles
- need high resolution detectors with millions of channels
- detectors suffer from radiation damage
to date this requires silicon detectors
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Example
Need many channels to resolve multi-track patterns
Expect 30-60 M strips and >100 M pixels
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Extreme radiation levels !• Radiation levels vary from 1 to 50 MRad in tracker volume
- less radiation at larger radii; more close to beam pipe
- more radiation in forward regions
• Fluences vary from to 1013 to 1015 particles/cm2
• Vicious circle: need silicon sensors for resolution and radiation hardness cooling (sensors and electronics) more material even more secondary particles etc.
Don’t win a beauty contest in this environment, but detectors are still very good !
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Extreme radiation levels !Plots show radiation dose and fluence per high luminosity LHC year for
ATLAS (assuming 107 s of collisions; source: ATL-Gen-2005-001)
“Uniform thermal neutron gas” Put your cell phone into ATLAS ! It stops working after 1 s to 1 min.
• Neutrons are everywhere and cannot easily be suppressed
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The Boring masks the Interesting HZZ ee + minimum bias events (MH= 300 GeV)
LHC in 2008 ?? : 1032 cm-2s-1 LHC first years: 1033 cm-2s-1
LHC: 1034 cm-2s-1 SLHC: 1035 cm-2s-1
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Why are silicon detectors so popular ?
• Start from a large signal
good resolution; big enough for electronics• Signal formation is fast
• Radiation-hardness
• SiO2 is a good dielectric
• Ride on technological progress of Microelectronics industry
extreme control over impurities; very small feature size; packaging technology
• Scientist and engineers developed many new concepts over the last two decades
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Technologies come and go
Random examples are
• Bubble chamber
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Technologies come and go
Steam engines
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Silicon detectors are not yet going!Future detectors are being designed and will be
• Larger: 200-2000 m2 • More channels: Giga pixels• Thinner: 20 m• Less noise• Better resolution
Your next digital camera will be better and cheaper as well
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Appendix
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Missing Material for future talksThis would require >2h talk rather than 1h
• What is doping• What is a pn junction• Show HS fig 1.14; 2.19; 2.20• Say what a MIP is• Cover photon and x-ray stuff • More semiconductor fundamentals poisson equation, limits
of approximation• What is a FET• Explain radiation damage for FETs• Wafer processing and wire-bonding etc.
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• Semiconductor band structure ’ energy gap• Asymmetric diode junction: example p(+) into contact with n (Na>>Nd)• Space charge region formed by diffusion of free charges, can be increased with "reverse bias“
Silicon Detectors
p+ nV=0 VRB>0
W
bias reverse applied V) 0.8(~ potentialin built
101e
1 material n type ofy resistivit
11.9 mobility,electron
5.02 :idthjunction w
0
RBBI
D
RBBIRBBI
VV
cmkN
VVmVVW
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Point Resolution
• Discrete sensing elements (binary response, hit or no hit), on a pitch p, measuring a coordinate x
• Discrete sensing elements (analog response with signal to noise ratio S/N) on a pitch p, where f is a factor depending on pitch, threshold, cluster width
12
px
)(~S
Nfpx
p
x
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Position Sensitive StructureSingle sided configurationPixel readout
for good resolution on angles ( and ) and intercepts (d, z0 )Precision track point measurementsMaximize separation between planes for good resolution on interceptsMinimize extrapolation - first point close to interaction
Vertex Resolution
x1 x2
y1
y2
a
53
x
xya
yyy
xx
81
2
errors with points, measured 2,1
planest measuremen 2,1
for good resolution on angles ( and ) and intercepts (d, z0 )Precision track point measurementsMaximize separation between planes for good resolution on interceptsMinimize extrapolation - first point close to interactionMaterial inside 1st layer should be at minimum radius (multiple scattering)
Vertex Resolution: Material
x1 x2
y1y2
a
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Signal Processing Issues
• Signal: expressed as input charge, typically 25,000 pairs (4 fC)
• Leakage current: blocked DC by oxide, but RB CB >>TM
– Before (after) radiation damage ~ 1nA(1ma)
– AC component is seen by pre-amp
• Noise fluctuations ~ Gaussian N – Leakage Current– Preamp “input noise charge”, white
noise, decreases with pre-amp current, increases with faster risetime where a,b are constants and CD is the detector capacitance
– Bias resistor: source of thermal noise
MLEAKN TI
DN bCa
BIASN R
1
V bias
R bias
integrator shaper
t rise TM
V out
n+
np+
oxideCb
G
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pre-amp leakage
(ignore scale, just an example)
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Silicon Collider Detectors• 1st generation: LEP and CDF vertexers, L=1029
– 2-4 layers, single sided DC coupled silicon or early double sided, ~50K channels– Charge integration + S/H, analog readout, 3 m radsoft CMOS and NMOS– Rad soft components (~25 KRad)
• 2nd generation: LEP and CDF , L=1030 (~100 KRad)– AC coupled detectors, improved double sided structures– Rad hard components, 1.8 m radhard CMOS– Early pixel implementations
• 3rd generation: CDF2a, D0, and B factories , L=1031 (few MRads, 1012-1013/cm2)– Early examples of trackers– Complex double sided constructions, ~500K channels– On chip storage pipelines, ADC’s, digital readout, 0.8 m radhard CMOS
• 4th generation: ATLAS, CMS trackers, CDF2b , L=1032-34 (~10 MRads, 1014-1015/cm2)– Large scale systems (5-10M channels), uniform designs, mass construction methods– Return to single sided detectors (radiation hardness and HV operation: SSC/LHC R&D)– New IC processes (Maxim, DMILL, 0.25 m), fast front ends, deep pipelines– Engineered, large pixel systems for vertexing
• 5th generation: New trackers for L=1035 (~100 MRad, 1014-1016/cm2)– Very large scale systems, simplifications– New rad hard sensor structures and materials– Lower mass supports and services– Increased azimuthal AND longitudinal segmentation, pixel structures move to larger radii– Further evolution of IC (0.13,0.09 m, heterostructures…) technology
First Cosmic Particle Tracks (Summer 2006)
Excellent build precision! Resolution will improve after alignment and for higher momentum tracks