Post on 05-Jan-2016
VCI2010
Photonic Crystals: A Novel Approach to Enhance the Light Output of Scintillation Based
Detectors
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Arno KNAPITSCHa, Etiennette AUFFRAYa, Paul LECOQa
a PH-CMX,CERN, Geneva, Switzerland
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Outline:
– Introduction• Scintillating Crystals• Motivation• Photonic Crystals
– Simulations• Monte Carlo• A Frequency- Domain Eigenmode Solver• Results
– PhC Fabrication• Sputter Deposition• Electron Beam Lithography• Reactive Ion Etching• Results
– Conclusion
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Scintillating Crystals
• What is scintillation?– Emission of light due to an ionizing
event
• What kind of scintillators are there?– Intrinsic (BWO, BGO) or extrinsic(LYSO:Ce, LuAG:Ce) scintillators– Organic, inorganic, liquid-, plastic, gaseous
• Common scintillators in HEP and medical imaging– LYSO:Ce(Lu2-xYxSiO5), BGO(Bi4Ge3O12), LuYAP:Ce(LuxY1-xAlO3), LuAG:Ce(Lu3Al5O7)
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ionizing radiation
Light emission
Scintillating Crystal
Cerium-doped Lutetium-Yttrium Aluminum Perovskite
Cerium-doped Lutetium Yttrium Orthosilicate
Bismuth germinate Cerium-doped Lutetium Aluminum Garnet
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Motivation: Application
Field of application of heavy inorganic scintillators
– High energy physics (HEP), medical imaging (e.g. PET), spectroscopy
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Principle of PET [2]CMS (Compact Muon Solenoid) at the LHC [1]
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Motivation: Increase Nr. of detected Photons
Main factors governing energy- and time resolution:
• Detected number of photoelectrons Npe
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Main limiting factor for the light collection efficiency :
•Total reflection due to a mismatch of the refractive index of crystal and detector
Snell’s Law:
Efficiency:
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Motivation: Photonic Crystal
How can a photonic crystal help to overcome those limits?
• Light extraction due to a periodic grating of the interface:
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incident light
reflected light
extracted modes
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Photonic Crystals (PhCs)
Photonic crystal basics:
• Periodic arrangement of two materials with different index of refraction, in one-, two-, or three dimensions
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1D 2D 3D
[3] J. D. Joannopoulos, Photonic crystals – Molding the flow of light, 2008
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How does the Photonic Crystal Work?
Diffracted modes interfere constructively in the PhC- grating and are therefore able to escape the Crystal
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Plain crystal- air interface: (EM – fieldplot [5] ) Crystal- air interface with PhC grating:
Plane Waveθ>θc
Total Reflection at the interface since Extracted Mode
(~60% Transmission)θ>θc
crystal air crystal air
(0% Transmission)
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Outline:
– Introduction• Scintillating Crystals• Motivation• Photonic Crystals
– Simulations• Monte Carlo• A Frequency- Domain Eigenmode Solver• Results
– PhC Fabrication• Sputter Deposition• Electron Beam Lithography• Reactive Ion Etching• Results
– Conclusion
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Simulation: A two Step Approach
1. Look at the angular distribution at the crystal- detector interface with a Monte- Carlo simulation tool (LITRANI [5])
2. Take the light distribution from the Monte-Carlo program and simulate the light extraction of a scintillator- PhC- air interface with an eigenmode expansion software (CAMFR [4])
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1. 2.
θc
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Optimize the PhC Design
• PhC crystal parameters:– Lattice constant: a– Hole diameter: D– Hole depth: d
• Optimize the parameters for maximal light transmission over all angles:
• Parameters in case of LYSO:– a = 340nm– D = 200nm– d = 300nm
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x
z
y
Scintillator
ITO
Si3N4
a
hole depth: d
hole diameter: D
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Light Gain
Light Gain when comparing to an unstructured Crystal:
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Crystal Type: LYSO
Crystal measurements: 1.3x2.6x8mm
Wrapping: Tyvek
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Results of different Crystals:
Crystal LYSO LuYAP BGO LuAG
Light gain 2.08 2.1 2.11 1.92
Angular distribution of the extracted
light
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Outline:– Introduction
• Scintillating Crystals• Motivation• Photonic Crystals
– Simulations• Monte Carlo• A Frequency- Domain Eigenmode Solver• Results
– PhC Fabrication• Sputter Deposition• Electron Beam Lithography• Reactive Ion Etching• Results
– Conclusion
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PhC Fabrication
Nano Lithography
• PhC is produced in cooperation with the INL (Institut des Nanotechnologies de Lyon)
• Three step approach:1. Deposition of a pattern transfer
material2. Patterning of the resist using a scanning
electron Microscope3. Pattern transfer using reactive ion
etching (RIE)
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Reactive ion etching reactor
Scanning electron Microscope
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Sputter Deposition
Sputtering of two different Materials:
1. ~70nm of ITO (Indium Tin Oxide)
2. ~300nm of Si3N4 (Silicon Nitride)
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x
z
y
Scintillator
ITO70nm
x
z
y
Scintillator
ITO
Si3N4
70nm
300nm
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Electron Beam Patterning
1. Deposit of an resist material (PMMA) by spin coating
2. Writing the PhC pattern into the resist with a scanning electron microscope (SEM)
3. Removing the exposed areas on the resist with an chemical solvate
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x
z
yScintillator
ITOSi3N4
PMMA
x
z
yScintillator
ITOSi3N4
PMMA Resist
x
z
yScintillator
ITOSi3N4
PMMA Resist
Electron beam
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Reactive Ion Etching (RIE)
1. Chemically reactive plasma removes Si3N4 not covered by the resist
2. Change the composition of the reactive plasma to remove the resist (PMMA) without etching the Si3N4
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x
z
y
Scintillator
ITOSi3N4
a
Hole depth: 300nm
hole diameter: 200nm
x
z
y
Scintillator
ITOSi3N4
Ion Bombardment
PMMA Resist
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PhC Results
Scanning Electron Images:
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a = 340nm
D = 200nm
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Outline:
– Introduction• Scintillating Crystals• Motivation• Photonic Crystals
– Simulations• Monte Carlo• A Frequency domain Eigenmode Solver• Results
– PhC Fabrication• Sputter Deposition• Electron Beam Lithography• Reactive Ion Etching• Results
– Conclusion• Outlook• Acknowledgement
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Conclusion
– Simulations show an light yield enhancement between 80% and 120%
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1. 90% and 110%
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Conclusion
2. The PhC- production process has been adapted to the requirements of the crystal
– Due to the ITO layer we have good electrical connectivity from the Si3N4 to the surrounding
– The RIE parameters were adapted to the required etching depth without having anisotropic effects on the pattern
– Lattice parameters of the PhC could be verified
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Outlook
• Optical Characterization of the PhC– Light Yield Measurements– Angular Distribution
• Compare the measurement- results to the simulations and classify possible deviations
• Use the knowledge obtained by the measurements to further optimize the PhC pattern of the next samples
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Acknowledgments
Many thanks to the staff of the INL – Lyon, especially to J.-L. Leclercq and C. Seassal for their support and advice during my stays in Lyon.
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References
[1] http://public.web.cern.ch/public/en/LHc/CMS-en.html [2] http://www.nature.com/nrc/journal/v4/n6/box/nrc1368_BX1.html [3] J. D. Joannopoulos,
Photonic crystals – Molding the flow of light, 2008[4] Photonic crystal LEDs - designing light extraction, C. Wiesmann, 2009[5] CAMFR, (CAvity Modelling FRamework), http://camfr.sourceforge.net[6] LITRANI, http://gentit.home.cern.ch/gentit/litrani/