**4. Laser micromachining applications**

### **4.1. Geometrical shaped LED**

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 throughput, and often utilize expensive equipment, highlighting the need to search for alternative low cost methods that can be fully adopted in industrial mass production and enhance the light output intensity of LEDs significantly. Recently, the effect of geometrical chip-shaping realized with laser micromachining or other methods is being gradually recognized as a promising alternative technique for optimizing the efficiency and for modu‐ lating the emission pattern [17]. W.F. Fu et al. [8] reported that the geometrical shaping of LEDs by laser micromachining is an effective approach for enhancing the light extraction efficiency of a conventional cuboid LED (inclination angle of 90°) of 18.3% up to 33.9% in truncated pyramidal (inclination angle of 50°) LED geometry. This approach offers significant increases in light extraction efficiency of up to 85.2%, which is the highest value reported thus far.

Figure 4(a) presents the mechanism of enhanced light extraction with tiled sidewalls, showing the additional light extraction channel from the top surface as well as from the sidewalls due to reflections on the tilted sidewalls. According to the ray-tracing simulation, the light extraction efficiency depends on the inclination angle. Figure 4(b) shows the light extraction efficiency as a function of the inclination angle. Figure 4(c) shows a SEM image of an InGaN LED die with a truncated pyramidal geometry (TP-LED) fabricated by laser micromachining. Figure 5(a) and 5(b) show the operation images of a cuboid LED and a TP-LED, respectively, and Figure 5(c) presents their light output–current (*L-I*) characteristics. At lower driving currents (50 mA), the average light enhancement factor was 88.6%, which is consistent with the theoretical prediction of 85.2%. Such significant improvement in light extraction efficiency highlights the effectiveness of geometrical chip-shaping, particularly with the present ap‐ proach based on laser micromachining.
