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

ITOSi3N4

70nm300nm

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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

yScintillatorITOSi3N4

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

yScintillator

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/