**4.3 The Effects of NPSS**

188 Recent Advances in Nanofabrication Techniques and Applications

Cheng et al. [46] applied the photonic crystal [1x1 mm2] to the green power chip and combined the omnidirectional reflector at the back of the chip. The overall efficiency

Nao et al. [47] attached a photonic crystal in a triangular lattice (diameter=100~250 nm, height=120 nm) to the flip chip between the sapphire and the GaN, allowing the diffractive coupling of photons to enter the air from the back of the sapphire substrate. The far-field pattern was collimated, and a lobe was observed near the horizontal direction. The lobe was

Huang et al. [48] applied quasi-photonic crystals to both n- and p-GaN layers in a vertical

Byeon et al. [49] created a hexagonal array of holes with a diameter of 250 to 380 nm and 600 to 900 nm of pitch as the photonic crystal layer on the ITO of a green LED, using soft imprint technology (to prevent damages on the GaN LED). The light emitting efficiency improved

Khokhar et al. [50] compared e-beam lithography and nanoimprinting technology concerning the creation of photonic crystal structures. Though the e-beam lithography created a more precise and accurate pattern, it is not suitable for mass production, and while nanoimprinting technology is suitable for mass production, several issues remain. For example, the residual layer must be removed using etching, which limits the practical thickness of the photonic crystal layer, and the imprint material requires a high-etching

The photonic crystal structure can be applied quickly and accurately to the surface and the interior of the LED via the nanoimprint lithography technique [51, 52]. However, because the electrical characteristics of the LED will be degraded using dry etching of the p-GaN, T. A. Truong et al. [53] applied sol gel titania to the substrate (soft imprint plus a 300 C solidification process) to form titanium oxide photonic crystal structures on the LED

Motivated by the great potential of photonic crystal application on LED, several equipment manufacturers have devised intriguing strategies to develop this field. Molecular imprints Inc. [54] sprays the imprint material to a 6" quartz substrate. Using a step-and-repeat exposure procedure, they progressively transfer the structure of a hard mother mold (5×5 mm2) to the entire area of the quartz substrate, which now acts as a substitute mold. The nanoimprint of the 3" GaN LED occurs from the quartz mold. Due to the hard imprint nature of this technique, the cleanness of the mold and the substrate play crucial roles. Suss MicroTec Lithography GmbH [55] uses a PDMS soft mold adhered to the substrate by the vacuum force before imprinting. To prevent trapping air between the mold and the substrate during imprinting, the PDMS mold is gradually released from the vacuum force until the mold and the imprinting material are held together by the capillary force. After UV exposure, the gradual vacuum force removes the mold once more. EV Group Inc. [56] also adopted PDMS molds. The difference is that the mold comprises two soft PDMS layers with different degrees of hardness. The harder layer is the outer layer that directly touches the imprinting materials, which is believed to solve the rigidity issue of a typical soft mold, and prevent the deformation of the mold. Obducat AB [57] uses non-reusable polymer stamp as the mold. The polymer stamp is fabricated by pressing a hard master stamp against the polymer film. Once the polymer stamp is used, it is disposed. Due to its single usage nature, the stamp has no lifetime issues. In addition, the flexibility of the polymer stamp allows

attributed to the intermixing of light emitted from the side and front of the chip.

LED. The optical efficiency exceeded that of a single photonic crystal LED.

increased by 88 %, with a collimated far-field light pattern.

maximally by 25 % under a 20 mA current injection.

without damaging the LED surface.

selectivity.

NPSS has three distinct advantages:

### 1. **Enhancing the light extraction efficiency**

The PSS on the micrometer scale has been experimentally demonstrated to improve the light extraction efficiency of LEDs significantly. Furthershrinking the size of PSS can increase the structural density, and hence,enhancing the light extraction efficiency even more.

### 2. **Simplifying the process parameters**

Changing the condition on the current micron scale PSS process usually requires careful/tedious optimization of the subsequent epitaxial process parameters. By shrinking the patterning size into nanoscales, this optimization step can be neglected, facilitating a more rapid LED growth process.
