**3. Discussions of the nanoimprint techniques**

Based on the aforementioned analysis, the light-emitting efficiency of the LED can be improved by fabricating micro- or nanostructures inside or on the surface of the chip/substrate. The small dimension patterns can be fabricated via nanoimprint lithography. The advantages of nanoimprint lithography are fast production and the low cost, which satisfy industry production requirements. In addition, the method simplifies the complex optical lithography, freeing it from the diffraction limit. The nanoimprint lithography was regarded as one of the ten emerging technologies in 20003 with the potential to change the world.

Improving the Light-Emitting Efficiency of GaN LEDs Using Nanoimprint Lithography 179

link of the material. After detachment of the mold, dry etching is used for cleaning the residual layer to complete the process. The UV-curing nanoimprint lithography is realized under room temperature and at a low pressure, and therefore, has the advantage of fast production and of being a simple process. However, its disadvantage is that the mold or

In the research of soft nanoimprint lithography [19], a soft polymer such as polydimethylsiloxane (PDMS) is used as the mold to duplicate the pattern of hard mold. Acting like a stamp wetted with ink, PDMS mold is wetted with an ink made of, for instance, the self-assembly monomer (SAM). Alternatively, the substrate could also be sprayed with the imprint material. By compressing the PDMS soft mold and the substrate, the pattern of the mold is imprinted to the substrate. The soft nanoimprint lithography, similar to the UV-curing nanoimprint lithography, has the advantages of fast production, simple process, and low equipment cost. The key benefit of the soft nanoimprint lithography is its flexible mold, which makes it suitable for the non-planar imprint process, thereby having wider application. The disadvantage is that the mechanical strength of the mold is relatively weak compared to that of the others, resulting in easy wearing of the imprint patterns or the molds. To improve the imprint technique, a reversal imprint is developed. As shown in Figure 5, the aforementioned imprint techniques place the imprinting materials on the substrate surface. Conversely, the reversal imprint technique places the imprinting material on the surface of the mold before the entire material is pressed for transferring to the substrate. This procedure is similar to "planting" structures on the substrate, and the force required for the reversal imprint is comparably smaller. A 3D structure can be fabricated by stacking the structures layer-by-layer using the reversal imprint technique [20]. A successful reversal imprint requires the imprinting material to be hydrophilic to the substrate and hydrophobic to the mold, thereby enabling completion of the demolding process. Figure 5 shows the PTFE placed on the mold and the substrate preprocessed via HMDS. The attractive/repulsive surface of the materials can be analyzed by measuring the

Various nanoimprinting techniques have been developed to improve the abovementioned imprint processes. Among them, the roller imprint technique, originating from the roller press of the plastic sheets, is applied for nanoimprint. For example, a roller with nanostructures on its surface is rotated on a substrate composed of the thermoplastic polymer. When the temperature is set above the glass transition temperature, the thermoplastic polymer is in conjunction with the structures on the roller [21]. The nanostructure on the roller can be fabricated by the electroplating nickel mold technique,

substrate material must be UV-transparent (e.g., quartz, aluminum oxide).

contact angle.

Fig. 5. Schematic diagram of the reversal imprint

Nanoimprint lithography can be distinguished in three types: hot embossing nanoimprint lithography (HE-NIL), UV-curing nanoimprint lithography (UV-NIL), and soft imprint lithography (SIL). A schematic diagram of the three imprint types is shown in Figure 4:

Fig. 4. Schematics of the hotembossing nanoimprint lithography, the UV-curing nanoimprint lithography, and the soft nanoimprint lithography (from left to right, respectively)

Steven Y. Chou from Princeton University developed the first hot embossing nanoimprint lithography in 1995 [17]. The electron beam lithography and etching were used to fabricate a template pattern on a silicon mold. Under a temperature of 200 C and a pressure of 1900 psi, the silicon mold was pressed into a thermoplastic polymer called poly(methyl methacrylate) (PMMA). When the temperature was above its glass transform temperature (Tg), the PMMA became rubber-like and started to fill the pattern on the silicon mold. When the temperature dropped below Tg, the PMMA transformed into a high mechanical strength and glass-like material. Once the mold was separated from the sample, dry etching was used for cleaning the PMMA residues to complete the process. A disadvantage of the hot embossing nanoimprint lithography is that the nanostructures are fabricated under high temperature and high-pressure environments, and hence, its structure is prone to changes in temperature. Another disadvantage is the limitation of the application due to the requirement of long thermal cycle during the process.

The UV-curing nanoimprint lithography [18] uses UV-light to solidify the imprint material. A highly UV transparent and hard quartz is selected as the mold material. Quartz mold is pressed into the imprinting material to transfer the pattern into the silicon substrate at room temperature. The imprinting material is then irradiated with UV light, resulting in a cross-

Nanoimprint lithography can be distinguished in three types: hot embossing nanoimprint lithography (HE-NIL), UV-curing nanoimprint lithography (UV-NIL), and soft imprint lithography (SIL). A schematic diagram of the three imprint types is shown in Figure 4:

Fig. 4. Schematics of the hotembossing nanoimprint lithography, the UV-curing nanoimprint lithography, and the soft nanoimprint lithography (from left to right,

requirement of long thermal cycle during the process.

Steven Y. Chou from Princeton University developed the first hot embossing nanoimprint lithography in 1995 [17]. The electron beam lithography and etching were used to fabricate a template pattern on a silicon mold. Under a temperature of 200 C and a pressure of 1900 psi, the silicon mold was pressed into a thermoplastic polymer called poly(methyl methacrylate) (PMMA). When the temperature was above its glass transform temperature (Tg), the PMMA became rubber-like and started to fill the pattern on the silicon mold. When the temperature dropped below Tg, the PMMA transformed into a high mechanical strength and glass-like material. Once the mold was separated from the sample, dry etching was used for cleaning the PMMA residues to complete the process. A disadvantage of the hot embossing nanoimprint lithography is that the nanostructures are fabricated under high temperature and high-pressure environments, and hence, its structure is prone to changes in temperature. Another disadvantage is the limitation of the application due to the

The UV-curing nanoimprint lithography [18] uses UV-light to solidify the imprint material. A highly UV transparent and hard quartz is selected as the mold material. Quartz mold is pressed into the imprinting material to transfer the pattern into the silicon substrate at room temperature. The imprinting material is then irradiated with UV light, resulting in a cross-

respectively)

link of the material. After detachment of the mold, dry etching is used for cleaning the residual layer to complete the process. The UV-curing nanoimprint lithography is realized under room temperature and at a low pressure, and therefore, has the advantage of fast production and of being a simple process. However, its disadvantage is that the mold or substrate material must be UV-transparent (e.g., quartz, aluminum oxide).

In the research of soft nanoimprint lithography [19], a soft polymer such as polydimethylsiloxane (PDMS) is used as the mold to duplicate the pattern of hard mold. Acting like a stamp wetted with ink, PDMS mold is wetted with an ink made of, for instance, the self-assembly monomer (SAM). Alternatively, the substrate could also be sprayed with the imprint material. By compressing the PDMS soft mold and the substrate, the pattern of the mold is imprinted to the substrate. The soft nanoimprint lithography, similar to the UV-curing nanoimprint lithography, has the advantages of fast production, simple process, and low equipment cost. The key benefit of the soft nanoimprint lithography is its flexible mold, which makes it suitable for the non-planar imprint process, thereby having wider application. The disadvantage is that the mechanical strength of the mold is relatively weak compared to that of the others, resulting in easy wearing of the imprint patterns or the molds. To improve the imprint technique, a reversal imprint is developed. As shown in Figure 5, the aforementioned imprint techniques place the imprinting materials on the substrate surface. Conversely, the reversal imprint technique places the imprinting material on the surface of the mold before the entire material is pressed for transferring to the substrate. This procedure is similar to "planting" structures on the substrate, and the force required for the reversal imprint is comparably smaller. A 3D structure can be fabricated by stacking the structures layer-by-layer using the reversal imprint technique [20]. A successful reversal imprint requires the imprinting material to be hydrophilic to the substrate and hydrophobic to the mold, thereby enabling completion of the demolding process. Figure 5 shows the PTFE placed on the mold and the substrate preprocessed via HMDS. The attractive/repulsive surface of the materials can be analyzed by measuring the contact angle.

Fig. 5. Schematic diagram of the reversal imprint

Various nanoimprinting techniques have been developed to improve the abovementioned imprint processes. Among them, the roller imprint technique, originating from the roller press of the plastic sheets, is applied for nanoimprint. For example, a roller with nanostructures on its surface is rotated on a substrate composed of the thermoplastic polymer. When the temperature is set above the glass transition temperature, the thermoplastic polymer is in conjunction with the structures on the roller [21]. The nanostructure on the roller can be fabricated by the electroplating nickel mold technique,

Improving the Light-Emitting Efficiency of GaN LEDs Using Nanoimprint Lithography 181

an LED with size 350×350μm2showeda 12% improvement in the light-emitting efficiency with an input current of 20 mA. Huang et al. [27] fabricated p-GaN nanostructures on the surface of a 1×1 mm2 high-power green LED chip. An SiO2 thin film was first deposited onto p-GaN, and a silicon mold was used for the hot embossing imprint process. Reactive ion etching (RIE) was used for cleaning the residues of the imprinting material and SiO2, to complete the transferring of the structure using SiO2 as the resist. A p-GaN nanostructure of 75 nm depth was later produced viaICP etching. The residual SiO2 was removed using a buffer oxidation etchant (BOE). Finally, an ITO thin film was deposited onto p-GaN. Due to high scattering effect, the light-emitting efficiency of the nanostructured LED was 48% higher than that of conventional LEDs with an input current of 350 mA. Zhou et al. [28] used the soft UV-nanoimprint lithography to fabricate nanostructures onto a 1×1 mm2 p-GaN high-efficiency LED chip. A porous membrane mold with a nanohole approximately100 nm in size was first fabricated using theanodic aluminum oxide (AAO) method. A two-inch PDMS soft mold was replicated for UV-nanoimprint process. The process followed was similar to that of Huang et al. [27]. The light-emitting efficiency of the unpackaged nanostructured LED chip was 10.9% higher than that of conventional LEDs with an input current of 350 mA. The enhancement of efficiency would be 48% when the

Other than the imprint method that imprints structures onto the LED surface via etching, Lee et al. [29] fabricated the microstructure directly onto the ITO. Figure 7 is the SEM image of the one-dimensional and two-dimensional imprint structures. The imprinting material is not a conventional polymer, but is a spin on glass (SOG), which is similar to SiO2. The chemical and

mechanical properties of this material are superior when compared to the polymer.

Fig. 7. SEM images of the one-dimensional and the two-dimensional imprint structures

Figure 8 is the schematic diagram that combines the imprint and the conventional LED fabrication process. The mesa etching (to expose the n-GaN) was first achieved usingICP. The electrode patterns were defined and deposited with p-type and n-type electrodes to fabricate conventional LED chips. After the hot embossing imprint process, the imprinting material on the electrodes was removed. Finally, the electrical and optical properties of the

LED chip is packaged.

which applies nickel film on the roller [22, 23]. If the substrate is flexible, structures can be reproduced massively via continuous roll-to-roll process. This method is suitable for the thermoplastic polymer films. The diffuser film used for current backlights of TFT-LCD panel can be fabricated via roll-to-roll process transferring of the microstructure to the PET film by roller imprint. The roller imprint, in practice, can adapt a process similar to the aforementioned imprint process. That is, PDMS soft mold is applied over the surface of the quartz roller, thereby enabling the continuous imprinting process, as shown in Figure 6. [24]. Various light sources can be used to harden the imprint material. For example, the hot embossing imprint can be heated up rapidly using an infrared light or an infrared laser [25]. The UV-hardening can be achieved via the UV light or UV laser. Using the laser has the benefit of using optical lenses to focus the beam to the ideal imprint area.

Continuous imprinting process is demonstrated using the roller imprint method. Instead of imprinting the entire surface at once, the roller and the substrate are imprinted progressively, rendering the roller imprint useful for the fabrication of structures on a large surface.
