3. **Reducing the thickness of the buffer layer**

A thick buffer layer is typically necessaryto smooth out the PSSpatterned surface prior to the epitaxy process. If the size of the PSS structures is in nanoscales, the required buffer layer is thin and may be achieved by the side growth of the epitaxial layer, streamlining the LED growth process and increasing the process yield.

Huang et al. [58] created polymer nanostructures on the sapphire substrate using the thermal pressing technique. The imprint material also acted as the resist layer for the ICP etching process. A concave NPSS pattern was obtained with a periodicity of 450 nm, a diameter of 240 nm, and a depth of 165 nm. SEM measurements showed that the GaN did not fulfill the the NPSS holescompletely, causing a hole array at the GaN/NPSS interface. Due to the difference in the refraction indices between these holes and the surrounding matrix, the light originating from the MQW underwent scattering and multiple reflections at the GaN/NPSS interface. The light was bounced back to the GaN/Air interface, increasing the likelihood of light escaping the LED. Under a 20 mA electrical current injection, the NPSS was found to enhance the light extraction efficiency by as much as 1.33 times that ofconventional LEDs. Huang et. al. [59] used a similar thermal pressing technique combined with ICP etching to create a convex NPSS with a periodicity of 750 nm and a diameter of 450 nm. The height of the NPSS structures was 182 nm. Under a 20 mA electrical excitation, the 300×300 μm2 NPSS had 35% more light-extraction efficiency compared toconventional LEDs. If the quasi-photonic crystal effect wereconsidered, the efficiency could be further increased to48%. C.C. Kao et al. [60] used a similar process for creating NPSS to study the dependance of the light extraction on the aspect ratios of the NPSS. Their experiment showed that efficiency was higher for higher aspect ratios. The efficiency was increased from 11% to 27% for aspect ratios from 2.00 to 2.50, respectively.

Results from the micron scale PSS structures shows that increasing the aspect ratio of the PSS can also increase the light-extraction efficiency. However, the height of the ICP-etched NPSS is directly related to the size of the nanoimprinted polymer structures. Due to the high mechanical strength and the soundchemical stability, the etching selective ratio between the sapphire and the polymer resist is small, resulting in a less than desired etching depth on the sapphire substrate. Figure 13 shows the NPSS structures etched by the sulfuric acid and the phosphoric acid at high temperatures. The periodicity is 1.25 nm and the depth is 340

Improving the Light-Emitting Efficiency of GaN LEDs Using Nanoimprint Lithography 191

Nanotechnology is typically defined as the fabrication of structures under 100 nm. The fabrication of structures between 100 nm~999 nm is referred to as sub-micron technology. However, much literature related to LEDs referring to the sub-micron process as nanotechnology exists. Therefore, this study referred to the nanoimprint as the fabrication of several hundred nanometer structures via the imprint technique, nonetheless indicating that the few hundred-nanometer fabrication process is applicable to the industry. This scale is suitable for the development of the nanoimprint technique because the limitation of optical lithography does not affect it, and the technique is low cost compared to the expensive advanced projection lithography and the electron beam lithography. The LED chip has a 0~10 μm bow and the epitaxy is usually processed under a temperature near 1000C. Because the coefficients of thermal expansion of the substrate and the GaN epitaxial layer are different, the epitaxy process is likely to cause the deformation of the chip. Moreover, due to (a) the cleanness issues, (b) surface defects of the epitaxial layer, and (c) the emphasis on the height of the electrode after the chip process, the imprint process of the entire wafer is

This article presents the nanoimprinting technique integrated with the conventional LED fabrication process. Due to its low cost, simplicity, easy integration, and the enhancement of the optical efficiency, nanoimprinting has a widespread application in various industries. The future business aspect of nanoimprint technology is dependent on the development of

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GaN-based flip-chip light-emitting diodes by adopting geometric sapphire shaping

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extraction efficiency of GaN-based light-emitting diodes via surface roughening,"

light-emitting diodes with surface texture and/or patterned substrate", Optics

rendered difficult. Therefore, the use of a flexible soft mold is crucial.

manufacturing equipment suitable for LEDs fabrication.

**5. Conclusion** 

**6. References** 

1020, 2006.

nm. A thin layer of silicon oxide is used as the etching resist at high temperatures. If the etching time is long, the PSS pattern transforms intocylindrical rods with amaximum height of 700 nm (Figure 14). With the ICP etching process using anNi thin film as the resist layer, an even higher cylindrical rod can be obtained due to the higher etching selective ratio between the sapphire and the Ni, as shown in Figure 15. [61].

Fig. 13. AFM images of concave NPSS formed using wet etching

Fig. 14. SEM images of convex NPSS formed using wet etching

Fig. 15. SEM images of dry etched cylinder NPSS

Hsieh et al. [62] created NPSS via a thermal nanoimprint technique, usingNi as the resist layer. A anti-adhesive layer (1H, 1H, 2H, and 2H-perfluorooctyltrichlorosilane) was coated on the mold, followed by a thin Ni layer. A layer of PMMA was sprayed onto the substrate. The Ni layer on the protrudent mold was peel-off before being transferred to the PMMA, using thermal pressing. NPSS (~0.4 μm depth) was then created via ICP etching. The diameters were 0.4, 0.6, 0.8, 1.0, 2.0, and 3.0 μm, with the corresponding EL intensity of 128.4, 120.2, 109.2, 102.5, 91.6, 90.3, and 69.5 mcd, respectively. In comparison to the unpatterned substrate, the efficiency of the NPSS (0.4 μm) LED was increased by 84.7%.
