**2. Process of laser micromachining**

One attractive feature of laser processes is its ability to remove material. Termed laser ablation, material removal can be achieved by physical or chemical microscopic mechanisms. Because lasers can be focused on a small spot with high energy density, precision machining of the features on the micrometer or tens of micrometer scale is possible. For example, E. Gu et al. examined the drilling of holes and micro-trenches in a free-standing GaN substrate by pulsed UV laser ablation4 . Another use of laser ablation in LED industries is wafer dicing. As GaN is normally grown on sapphire, and sapphire is the second hardest material in the world, a diamond blade is the only viable tool for mechanical dicing. On the other hand, a diamond blade often deviates from its intended dicing direction when the blades are thin, causing chipping or even device damage. With laser dicing, the dicing path can be controlled with high precision. In addition, the spacing between the individual LED dies can be reduced to a size comparable to the laser spot size, leading to an increase in die density.

Simple laser micromachining consists of a UV laser source, beam focusing optics and an x-y motorized translation stage, as shown in Figure 1. The laser source is a third harmonic ND:YLF diode-pumped solid state (DPSS) laser manufactured by Spectra Physics. The laser emits at 349 nm, and the pulse repetition rate ranges from single pulse to 5 kHz. At a reference diode current of 3.2 A, the pulse energy is 120 µJ at a repetition rate of 1 kHz, with a pulse width of approximately 4 nanoseconds. The TEM00 beam allows for tight focusing, offering a high spatial resolution. After beam expansion and collimation using a beam expander, the laser beam is reflected 90° using a dielectric laser line mirror and is focused onto the horizontal machining plane to a very tiny spot, several micrometers in diameter, with a focusing triplet. All optics used are made from UV-fused silica and are anti-reflection (AR) coated. The additional feature of this set-up, as illustrated in the schematic diagram of Figure 1, is the insertion of a UV mirror at an oblique angle within the optical path between the focusing optics and machining plane, which deflects the convergent beam to strike the sample at an oblique angle to the horizontal working plane. The size of the beam at the focal point is not only limited by the capability of the UV objective lens but is also sensitive to the coaxiality of the optics. With this modified set-up, it is relatively easy to optimize and monitor the beam through the tube lens imaged with a CCD camera. Once the optical setup is optimized before inserting the tilting mirror, the mirror can be inserted without affecting the coaxiality of the laser beam, so that the dimensions of the beam spot are unaffected.

**Figure 1.** Experimental setup of laser micro-machining.

cannot mount the translation module at a tilted angle because it will result in severe beam distortion. In the proposed approach, a laser beam turning mirror was introduced to the optical path to achieve a continuously-tunable range of tilting angles for beam projection, while retaining the beam quality. Laser micromachining is a potential simple, inexpensive and highthroughput alternative method for creating geometrically-shaped GaN LEDs compared to other available technologies [6,7,8]. K.N. Hui et al. reported the effectiveness of laser micro‐ machining incorporated with GaN semiconductors to achieve high light extraction GaN LEDs [7,9] and color tunable vertically-stacked LEDs in solid-state lighting applications [7,10].

This chapter examines the experimental process of laser micromachining, and the structural and optical properties of laser micromachining LED chips with a range of geometries. The optical characterization of LED, particularly the light extraction efficiency of geometricallyshaped LEDs, is discussed because the light extraction efficiency plays an important role in achieving high luminous efficacy LEDs. Finally, several applications derived from the utilization of laser micromachining, e.g. geometrically-shaped LED, angularly uniform white

One attractive feature of laser processes is its ability to remove material. Termed laser ablation, material removal can be achieved by physical or chemical microscopic mechanisms. Because lasers can be focused on a small spot with high energy density, precision machining of the features on the micrometer or tens of micrometer scale is possible. For example, E. Gu et al. examined the drilling of holes and micro-trenches in a free-standing GaN substrate by pulsed

normally grown on sapphire, and sapphire is the second hardest material in the world, a diamond blade is the only viable tool for mechanical dicing. On the other hand, a diamond blade often deviates from its intended dicing direction when the blades are thin, causing chipping or even device damage. With laser dicing, the dicing path can be controlled with high precision. In addition, the spacing between the individual LED dies can be reduced to a size

Simple laser micromachining consists of a UV laser source, beam focusing optics and an x-y motorized translation stage, as shown in Figure 1. The laser source is a third harmonic ND:YLF diode-pumped solid state (DPSS) laser manufactured by Spectra Physics. The laser emits at 349 nm, and the pulse repetition rate ranges from single pulse to 5 kHz. At a reference diode current of 3.2 A, the pulse energy is 120 µJ at a repetition rate of 1 kHz, with a pulse width of approximately 4 nanoseconds. The TEM00 beam allows for tight focusing, offering a high spatial resolution. After beam expansion and collimation using a beam expander, the laser beam is reflected 90° using a dielectric laser line mirror and is focused onto the horizontal machining plane to a very tiny spot, several micrometers in diameter, with a focusing triplet. All optics used are made from UV-fused silica and are anti-reflection (AR) coated. The additional feature of this set-up, as illustrated in the schematic diagram of Figure 1, is the

. Another use of laser ablation in LED industries is wafer dicing. As GaN is

LEDs, and vertically-stacked polychromatic LEDs are presented.

140 Advances in Micro/Nano Electromechanical Systems and Fabrication Technologies

comparable to the laser spot size, leading to an increase in die density.

**2. Process of laser micromachining**

UV laser ablation4

The angle of incidence of the deflected laser beam on the wafer is 2*θ*, where *θ*, as indicated in Figure 1, is the angle between the plane of the mirror and the normal. This angle can be precisely adjusted by mounting the mirror onto a rotation stage. Therefore, the incident angle can be varied over a wide range. In this experiment, a UV objective with a focal length of 75 mm was used based on two considerations. First, the focal length should be long enough to accommodate the mirror in the optical path. Secondly, an ideal tool for the fabrication of microstructures should have a very long penetration depth and negligible lateral dispersion. Nevertheless, an objective lens with a longer focal length also produces a larger focused beam spot. The two parameters are related by the following equation:

$$d = \frac{4\lambda M^2 f}{\pi D} \tag{1}$$

where *M2* quantifies the beam quality, *λ* is the wavelength of the laser beam, *f* is the focal length and *D* is the diameter of the incident beam.
