fabrication of photonic crystal structures on light emitting diodes by nanoimprint lithography

EE235 Class Presentation on Nanoimprint Lithography (Spring 2007)

Fabrication of photonic crystal structures on light eFabrication of photonic crystal structures on light emitting diodes by nanoimprint lithographymitting diodes by nanoimprint lithography

Authors: Sang Hoon Kim, Ki-Dong Lee, Ja-Yeon Kim, Min-Ki KAuthors: Sang Hoon Kim, Ki-Dong Lee, Ja-Yeon Kim, Min-Ki Kwon and Seong-Ju Parkwon and Seong-Ju Park

Presented by Darsen Lu (3/19/2007)Presented by Darsen Lu (3/19/2007)

EE235 Class Presentation on Nanoimprint Lithography (Spring 2007) UC Berkeley - 2

OutlineOutline

IntroductionIntroduction Some background InformationSome background Information

Mold FabricationMold Fabrication How to fabricate the mold for nanoimprint lithographyHow to fabricate the mold for nanoimprint lithography

PCLED Device FabricationPCLED Device Fabrication The fabrication of a photonic crystal light emitting diode The fabrication of a photonic crystal light emitting diode

(PCLED) using nanoimprint(PCLED) using nanoimprint

ResultsResults LED Performance Enhancement due to Photonic Crystal SLED Performance Enhancement due to Photonic Crystal S

tructuretructure


IntroductionIntroduction

Some background InformationSome background Information


Light Emitting Diode (LED)Light Emitting Diode (LED)

LEDLED Definition: a semiconductor device that emits incoherent narrow-spDefinition: a semiconductor device that emits incoherent narrow-sp

ectrum light when electrically biased in the forward directionectrum light when electrically biased in the forward direction

LED v.s. Incandescent (Edison’s lightbulb) and Flourescent BulLED v.s. Incandescent (Edison’s lightbulb) and Flourescent Bulbsbs

Much longer life span (10Much longer life span (1055 - 10 - 1066 hrs v.s. 10 hrs v.s. 1033 / 10 / 1044 hrs) hrs) Suitable for applications that are subject to frequent on-off cyclingSuitable for applications that are subject to frequent on-off cycling Efficiency: better than incandescent but currently worse than floureEfficiency: better than incandescent but currently worse than floure

scent bulbsscent bulbs

Courtesy of Wikipedia http://en.wikipedia.org/wiki/LED

Source: US Department of Energy http://www.netl.doe.gov/ssl/faqs.htm


LED EfficiencyLED Efficiency

Internal Quantum Efficiency (Internal Quantum Efficiency (ηη intint)) Definition: Definition: ratio of the number of electrons floratio of the number of electrons flo

wing in the external circuit to the number of pwing in the external circuit to the number of photons produced hotons produced withinwithin the device the device

Has been improved up to 80%Has been improved up to 80%

External Quantum EfficiencyExternal Quantum Efficiency Definition: The percentage of photons that caDefinition: The percentage of photons that ca

n be extracted to the ambient.n be extracted to the ambient. Typically 1% ~ 10%Typically 1% ~ 10% Limiting factor of LED efficiencyLimiting factor of LED efficiency Improvement techniques: dome-shaped packImprovement techniques: dome-shaped pack

age, textured surface, age, textured surface, photonic crystalphotonic crystal, …, …

Source: Lecture Note of “Optoelectronic Devices” (by Sheng-fu Horng, Dept. of Electrical Engrg, NTHU, Hsinchu, Taiwan)


Photonic CrystalPhotonic Crystal

Photonic CrystalPhotonic Crystal Definition: Periodic optical nanostructures that are desigDefinition: Periodic optical nanostructures that are desig

ned to affect the propagation of EM waves.ned to affect the propagation of EM waves. The periodic structure creates a “photonic bandgap.” The periodic structure creates a “photonic bandgap.”

No light with frequency within the gap can propagate.No light with frequency within the gap can propagate.

Source: SPIE Photonics West by Steven G. Johnson http://ab-initio.mit.edu/photons/tutorial/;


Photonic Crystal for enhancing the Photonic Crystal for enhancing the external quantum efficiency of an external quantum efficiency of an

LEDLED

Part of the extrinsic loss in LEDPart of the extrinsic loss in LED LED: a thin slab serves as a waveguideLED: a thin slab serves as a waveguide At some frequencies, spontaneously emitted light At some frequencies, spontaneously emitted light

can be coupled in to the waveguide can be coupled in to the waveguide Efficiency Loss Efficiency Loss

Photonic Crystal + LEDPhotonic Crystal + LED The “optical bandgap” prevents spontaneously The “optical bandgap” prevents spontaneously

emitted light from coupling into the waveguide, temitted light from coupling into the waveguide, therefore enhancing the efficiency of the device.herefore enhancing the efficiency of the device.

PC Structure DesignPC Structure Design Simulators are available: Ex: R-soft Simulators are available: Ex: R-soft

Source: Shanhui Fan, Pierre R. Villeneuve, and J. D. Joannopoulos, “High Extraction Efficiency of Spontaneous Emission from Slabs of PhotonicCrystals”


Mold FabricationMold Fabrication

How to fabricate the mold for nanoimprint How to fabricate the mold for nanoimprint lithographylithography


Mold Fabrication Process (1)Mold Fabrication Process (1)

1.1. Start with a silicon wafer.Start with a silicon wafer.Grow/Deposit layers: SiOGrow/Deposit layers: SiO22, Cr, Phot, Cr, Photoresist (PR)oresist (PR)

2.2. Pattern the PR twice using a Pattern the PR twice using a Laser Interference Lithography Laser Interference Lithography (LIL)(LIL)

3.3. Develop the PR.Develop the PR.Use a thermal treatment method Use a thermal treatment method to alter the pillar shape and to alter the pillar shape and increase its diameterincrease its diameter

SiO2

Cr

PR


Mold Fabrication Process (2)Mold Fabrication Process (2)

4.4. Use RIE to etch the Cr.Use RIE to etch the Cr.Then use the Cr as a mask and Then use the Cr as a mask and etch SiOetch SiO22 with RIE with RIE

(This allows a thin PR layer)(This allows a thin PR layer)SiO2

Cr

PR

5.5. Remove the Cr maskRemove the Cr maskCoat Anti-sticking layer by Coat Anti-sticking layer by vacuum evaporationvacuum evaporation


Thermal Treatment MethodThermal Treatment Method

Procedure:Procedure: Heat the wafer with PR at 120C for 5 minutesHeat the wafer with PR at 120C for 5 minutes

Purpose:Purpose: Increase the pillar diameter (110nm to 150nm)Increase the pillar diameter (110nm to 150nm) Control the shape of the photoresist pattern (circular)Control the shape of the photoresist pattern (circular)


Thermal Treatment Method (2)Thermal Treatment Method (2)

120°C is found to be the optimum temperature120°C is found to be the optimum temperature Temperature too low Temperature too low no significant tampering no significant tampering

effectseffects Temperature too high Temperature too high PR becomes too thin and PR becomes too thin and

lithography quality becomes poorlithography quality becomes poor


PCLED Device PCLED Device FabricationFabrication

The fabrication of a photonic crystal layer uThe fabrication of a photonic crystal layer using nanoimprintsing nanoimprint


LED Device FabricationLED Device Fabrication

1.1. Prepare an LED substrate Prepare an LED substrate sample. Deposit Cr and PR.sample. Deposit Cr and PR.

2.2. Nanoimprint Process (50bar, 1Nanoimprint Process (50bar, 145C)45C)

3.3. Remove the residual area Remove the residual area using Ousing O22 plasma plasma

4.4. Etch the Cr and p-GaN regionEtch the Cr and p-GaN region


ResultsResults

LED Performance Enhancement due LED Performance Enhancement due to Photonic Crystal Structureto Photonic Crystal Structure


Device Performance Device Performance EnhancementEnhancement

LED intensity as a function of etch depth LED intensity as a function of etch depth (With PC)(With PC)

LED intensity as a function of etch depth LED intensity as a function of etch depth (Without PC)(Without PC)

With Photonic Crystal Structure:9x Enhancement

Control Group:4x Enhancement

(Due to “penetrating”)


SEM PicturesSEM Pictures

(a) FESEM Image of the LED sample after removal of the resid(a) FESEM Image of the LED sample after removal of the residual layer and Cr Patterningual layer and Cr Patterning

(b) Final PC strctures on a p-GaN layer(b) Final PC strctures on a p-GaN layer The diameter of the hole increases after each patterningThe diameter of the hole increases after each patterning

FESEM: Field Emission Scanning Electron Microscope


SummarySummary

The energy efficiency of an LED can be improved by enhancinThe energy efficiency of an LED can be improved by enhancing the external quantum efficiency.g the external quantum efficiency.

Photonic Crystal is one of the possible mechanisms to enhanPhotonic Crystal is one of the possible mechanisms to enhance the external quantum efficiencyce the external quantum efficiency

A mold is fabricated using laser interference lithography (LIL) A mold is fabricated using laser interference lithography (LIL) and thermal treatment methodand thermal treatment method

A Photonic Crystal LED is fabricated using nanoimprint lithogA Photonic Crystal LED is fabricated using nanoimprint lithographyraphy

The light intensity of the LED is significantly improved due to The light intensity of the LED is significantly improved due to the photonic crystal structurethe photonic crystal structure


The End The End

Thank youThank you

Questions?Questions?

fabrication of photonic crystal structures on light emitting diodes by nanoimprint lithography

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photonic crystal light

photonic bandgap

emitted light

orgwikiled source

semiconductor device

johnson http

rsoft source

pr layersio2crprremove