**4.2. Angularly uniform white LED**

**Figure 3.** SEM images of micro-trenches formed by laser micromachining at different focus offset planes (with the pulse energy, pulse repetition rate and scan speed fixed at 23 µJ, 1 kHz, and 25 µm/s, respectively.): (a) small offset of 300 µm; (b) optimal offset of 450 µm; (c) large offset of 600 µm, and (d) pulse energy of 45µJ, (e) 23 µJ, and (f) 7 µJ (with the focus off‐

The development of LEDs with high optical output power has been the driving force of next generation solid-state lighting [13]. On the other hand, the optical output power of the-stateof-art LEDs is still insufficient for making them practically viable. The large refractive index difference between nitride material (ηGaN = 2.585) and air (ηAir = 1), giving rise to a total internal reflection at the interfaces, is the major cause for the lower-than-expected light extraction efficiency. In addition, conventional LED chips with a cuboid geometry and a Lambertian emission pattern often have a light extraction efficiency of < 20%. Several methods have been proposed to alleviate these issues, such as flip-chip LEDs [14], photonic crystals [15] and surface texturing [16]. These proposed methods, however, are energy consuming, low

set level, pulse repetition rate, and scan speed are fixed at 450 µm, 1 kHz, and 25 µm/s, respectively.)

**4. Laser micromachining applications**

144 Advances in Micro/Nano Electromechanical Systems and Fabrication Technologies

**4.1. Geometrical shaped LED**

White LEDs are used widely in commercial applications, such as solid-state lighting, liquid crystal display (LCD) backlighting and signaling, owing to their energy efficiency and mercury-free composition. Currently, color down-conversion and color-mixing are the two mainstream methods of producing white LEDs. The use of phosphors as a conversion agent is used widely in commercial products. Nevertheless, the limited conversion efficiency from shorter wavelengths (typically blue at approximately 470 nm) to a longer wavelengths means the benefits of LEDs can never be fully achieved. Placing three LEDs (red, green and blue) into a single package (the RGB approach) resolves this deficiency but introdu‐ ces severe issues with color uniformity and homogeneity. For phosphor-coated LEDs, the placement and method of the phosphor coating will also affect the color uniformity considerably, whereby emission homogeneity is an important attribute for many applica‐ tions. For example, the phosphor coating process went through a reflow process to cover both the top and sidewalls, resulting in a non-uniform distribution and coating thickness, particularly at the edge of the chip. Such coating thickness non-uniformity, coupled with the unequal light emission from the top and sidewall, results in a non-uniformity of color emission from different viewing angles [18]. Therefore, tailoring the light emission pattern from the point of view of geometrically-shaped LED plays an important role in achieving high angular color uniformity. L. Zhu et al. [19] found that a truncated cone (TC) struc‐ ture integrating an Al mirror reflectors on the sidewall and bottom surfaces is an effec‐ tive approach for improving the angular color uniformity of white LEDs, showing 37% enhancement in color uniformity, compared to the conventional cuboid structure. Figure 6 presents three different coating profiles, phosphor-slurry coating, conformal coating and remote phosphor coating, along with the proposed white LED employing a TC structure.

**Figure 4.** a) Schematic diagram showing an exemplary light ray within a TP-LED. The arrows in blue indicate the addi‐ tional extracted rays due to the inclined sidewalls. (b) Light extraction efficiency of an inclined sidewall LED as a func‐ tion of the inclination angle of the sidewall. (c) SEM image of an InGaN LED die with a truncated pyramidal geometry.

tive approach for improving the angular color uniformity of white LEDs, showing 37% enhancement in color uniformity, compared to the conventional cuboid structure. Figure 6 presents three different coating profiles, phosphor-slurry coating, conformal coating and remote phosphor coating, along with the proposed white LED employing a TC structure.

146 Advances in Micro/Nano Electromechanical Systems and Fabrication Technologies

**Figure 4.** a) Schematic diagram showing an exemplary light ray within a TP-LED. The arrows in blue indicate the addi‐ tional extracted rays due to the inclined sidewalls. (b) Light extraction efficiency of an inclined sidewall LED as a func‐ tion of the inclination angle of the sidewall. (c) SEM image of an InGaN LED die with a truncated pyramidal geometry.

**Figure 5.** Optical micrographs showing cuboid LED (a) and TP-LED (b) biased at 10 mA, and (c) L-I curve comparing the performances of the TP-LED and cuboid LED.

**Figure 6.** Schematic diagram showing the cross-sectional views of LED coating using three different methods: (a) phos‐ phor-slurry coating; (b) conformal coating; (c) remote phosphor coating; and (d) 3D schematic diagram of a TC-LED.

Figure 7 shows operational images of a TC-LED, together with the reference LED (circu‐ lar LED without TC structure). Quantum dots (QDs) consisting of green (540 nm) and yellow (560 nm) light-emitting QDs (Evident Technologies) are used as color-conversion agents, which are mixed together to a 7 : 5 volume ratio for balanced white light emis‐ sion. The mixture is then blended with a transparent UV epoxy (Norland 61), following which a small volume of the slurry is dispensed onto the chips. The chips are packaged into standard TO-cans. To minimize these effects, the epoxy is spin-coated onto the chips to ensure evenness of the coating. The completed devices were tested at a bias current of 20 mA. Figure 7(c) and 7(e) shows the emission from the reference LED structure from two different angles. A ring of yellow light is clearly observed at the periphery of the circular chip, which was attributed to the relatively thicker coating on the sidewalls. This effect is suppressed with the TC-LED structure, as shown in Figure 7(d) and 7(f), where uniform white light emission can be observed from different angles.

**Figure 7.** Operational images of the packaged devices: (a) and (b) are the circular reference LED and TC-LED; (c) and (e) show the QD-coated circular reference LED, (d) and (f) show the QD-coated TC-LED.
