EE235 Class Presentation on Nanoimprint Lithography (Spring 2007) Fabrication of photonic crystal structures on light emitting diodes by nanoimprint lithography Authors: Sang Hoon Kim, Ki-Dong Lee, Ja-Yeon Kim, Min-Ki Kwon and Seong-Ju Park Presented by Darsen Lu (3/19/2007)

EE235 Class Presentation on Nanoimprint Lithography (Spring 2007) UC Berkeley - 2 Outline Introduction Introduction Some background Information Some background Information Mold Fabrication Mold Fabrication How to fabricate the mold for nanoimprint lithography How to fabricate the mold for nanoimprint lithography PCLED Device Fabrication PCLED Device Fabrication The fabrication of a photonic crystal light emitting diode (PCLED) using nanoimprint The fabrication of a photonic crystal light emitting diode (PCLED) using nanoimprint Results Results LED Performance Enhancement due to Photonic Crystal Structure LED Performance Enhancement due to Photonic Crystal Structure

EE235 Class Presentation on Nanoimprint Lithography (Spring 2007) Introduction Some background Information

EE235 Class Presentation on Nanoimprint Lithography (Spring 2007) UC Berkeley - 4 Light Emitting Diode (LED) LED LED Definition: a semiconductor device that emits incoherent narrow- spectrum light when electrically biased in the forward direction Definition: a semiconductor device that emits incoherent narrow- spectrum light when electrically biased in the forward direction LED v.s. Incandescent (Edison’s lightbulb) and Flourescent Bulbs LED v.s. Incandescent (Edison’s lightbulb) and Flourescent Bulbs Much longer life span ( hrs v.s / 10 4 hrs) Much longer life span ( hrs v.s / 10 4 hrs) Suitable for applications that are subject to frequent on-off cycling Suitable for applications that are subject to frequent on-off cycling Efficiency: better than incandescent but currently worse than flourescent bulbs Efficiency: better than incandescent but currently worse than flourescent bulbs Courtesy of Wikipedia Source: US Department of Energy

EE235 Class Presentation on Nanoimprint Lithography (Spring 2007) UC Berkeley - 5 LED Efficiency Internal Quantum Efficiency ( η int ) Internal Quantum Efficiency ( η int ) Definition: ratio of the number of electrons flowing in the external circuit to the number of photons produced within the device Definition: ratio of the number of electrons flowing in the external circuit to the number of photons produced within the device Has been improved up to 80% Has been improved up to 80% External Quantum Efficiency External Quantum Efficiency Definition: The percentage of photons that can be extracted to the ambient. Definition: The percentage of photons that can be extracted to the ambient. Typically 1% ~ 10% Typically 1% ~ 10% Limiting factor of LED efficiency Limiting factor of LED efficiency Improvement techniques: dome-shaped package, textured surface, photonic crystal, … Improvement techniques: dome-shaped package, textured surface, photonic crystal, … Source: Lecture Note of “Optoelectronic Devices” (by Sheng-fu Horng, Dept. of Electrical Engrg, NTHU, Hsinchu, Taiwan)

EE235 Class Presentation on Nanoimprint Lithography (Spring 2007) UC Berkeley - 6 Photonic Crystal Photonic Crystal Photonic Crystal Definition: Periodic optical nanostructures that are designed to affect the propagation of EM waves. Definition: Periodic optical nanostructures that are designed to affect the propagation of EM waves. The periodic structure creates a “photonic bandgap.” No light with frequency within the gap can propagate. The periodic structure creates a “photonic bandgap.” No light with frequency within the gap can propagate. Source: SPIE Photonics West by Steven G. Johnson

EE235 Class Presentation on Nanoimprint Lithography (Spring 2007) UC Berkeley - 7 Photonic Crystal for enhancing the external quantum efficiency of an LED Part of the extrinsic loss in LED Part of the extrinsic loss in LED LED: a thin slab serves as a waveguide LED: a thin slab serves as a waveguide At some frequencies, spontaneously emitted light can be coupled in to the waveguide  Efficiency Loss At some frequencies, spontaneously emitted light can be coupled in to the waveguide  Efficiency Loss Photonic Crystal + LED Photonic Crystal + LED The “optical bandgap” prevents spontaneously emitted light from coupling into the waveguide, therefore enhancing the efficiency of the device. The “optical bandgap” prevents spontaneously emitted light from coupling into the waveguide, therefore enhancing the efficiency of the device. PC Structure Design PC 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”

EE235 Class Presentation on Nanoimprint Lithography (Spring 2007) Mold Fabrication How to fabricate the mold for nanoimprint lithography

EE235 Class Presentation on Nanoimprint Lithography (Spring 2007) UC Berkeley - 9 Mold Fabrication Process (1) 1.Start with a silicon wafer. Grow/Deposit layers: SiO 2, Cr, Photoresist (PR) 2.Pattern the PR twice using a Laser Interference Lithography (LIL) 3.Develop the PR. Use a thermal treatment method to alter the pillar shape and increase its diameter SiO 2 Cr PR

EE235 Class Presentation on Nanoimprint Lithography (Spring 2007) UC Berkeley - 10 Mold Fabrication Process (2) 4.Use RIE to etch the Cr. Then use the Cr as a mask and etch SiO 2 with RIE (This allows a thin PR layer) SiO 2 Cr PR 5.Remove the Cr mask Coat Anti-sticking layer by vacuum evaporation

EE235 Class Presentation on Nanoimprint Lithography (Spring 2007) UC Berkeley - 11 Thermal Treatment Method Procedure: Procedure: Heat the wafer with PR at 120C for 5 minutes Heat 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)

EE235 Class Presentation on Nanoimprint Lithography (Spring 2007) UC Berkeley - 12 Thermal Treatment Method (2) 120°C is found to be the optimum temperature 120°C is found to be the optimum temperature Temperature too low  no significant tampering effects Temperature too low  no significant tampering effects Temperature too high  PR becomes too thin and lithography quality becomes poor Temperature too high  PR becomes too thin and lithography quality becomes poor

EE235 Class Presentation on Nanoimprint Lithography (Spring 2007) PCLED Device Fabrication The fabrication of a photonic crystal layer using nanoimprint

EE235 Class Presentation on Nanoimprint Lithography (Spring 2007) UC Berkeley - 14 LED Device Fabrication 1.Prepare an LED substrate sample. Deposit Cr and PR. 2.Nanoimprint Process (50bar, 145C) 3.Remove the residual area using O 2 plasma 4.Etch the Cr and p-GaN region

EE235 Class Presentation on Nanoimprint Lithography (Spring 2007) Results LED Performance Enhancement due to Photonic Crystal Structure

EE235 Class Presentation on Nanoimprint Lithography (Spring 2007) UC Berkeley - 16 Device Performance Enhancement LED intensity as a function of etch depth (With PC) LED intensity as a function of etch depth (With PC) LED intensity as a function of etch depth (Without PC) LED intensity as a function of etch depth (Without PC) With Photonic Crystal Structure: 9x Enhancement Control Group: 4x Enhancement (Due to “penetrating”)

EE235 Class Presentation on Nanoimprint Lithography (Spring 2007) UC Berkeley - 17 SEM Pictures (a) FESEM Image of the LED sample after removal of the residual layer and Cr Patterning (a) FESEM Image of the LED sample after removal of the residual 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 patterning The diameter of the hole increases after each patterning FESEM: Field Emission Scanning Electron Microscope

EE235 Class Presentation on Nanoimprint Lithography (Spring 2007) UC Berkeley - 18 Summary The energy efficiency of an LED can be improved by enhancing the external quantum efficiency. The energy efficiency of an LED can be improved by enhancing the external quantum efficiency. Photonic Crystal is one of the possible mechanisms to enhance the external quantum efficiency Photonic Crystal is one of the possible mechanisms to enhance the external quantum efficiency A mold is fabricated using laser interference lithography (LIL) and thermal treatment method A mold is fabricated using laser interference lithography (LIL) and thermal treatment method A Photonic Crystal LED is fabricated using nanoimprint lithography A Photonic Crystal LED is fabricated using nanoimprint lithography The light intensity of the LED is significantly improved due to the photonic crystal structure The light intensity of the LED is significantly improved due to the photonic crystal structure

EE235 Class Presentation on Nanoimprint Lithography (Spring 2007) UC Berkeley - 19 The End Thank you Thank you Questions? Questions?