**3.2. Topography of laser micromachining**

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

*<sup>d</sup>* <sup>=</sup> <sup>4</sup>*λ<sup>M</sup>* <sup>2</sup> *<sup>f</sup>*

quantifies the beam quality, *λ* is the wavelength of the laser beam, *f* is the focal length

After laser micromachining, the depth of the micro-trenches pattern can be observed by scanning electron microscopy (SEM) to check the effectiveness of the approach. The quality of the cleave can be quantified by the width, depth, linearity and sidewall roughness of the trench formed by the laser beam. Because the focal length of the focusing lens (*f* = 75 mm) is much longer than the thickness of the GaN layer on the sapphire wafer (*t* = 420 µm), the depth of the trench depends mainly on the number of micromachining cycles. The number of cycles is controlled by configuring the translation stage to repeat its linear path several times. As the position reproducibility of the stage is better than 5 µm, increasing the number of cycles should not contribute significantly to the width of the feature. X.H. Wang et al. [11] reported the cross-sectional optical image of a GaN layer on a 420 µm thick sapphire wafer that had been micro-machined with an incident beam inclined at 45° with scan cycles ranging from 1 to 10. These incisions were carried out by setting the laser pulse energy to 54 µJ at a repetition rate of 2 kHz. Figure 2 shows the relationship between the inclined cutting depth and the number of passes of the beam. After the first pass of the beam, a narrow trench with a width of ~20 µm and depth of ~220 µm was formed. Successive scans of the beam along the trench resulted in further deepening and widening but the extent was increased at a decreasing rate. The depth of the trench depends on the effective penetration of the beam. From the second scan onwards, the beam needs to pass through the narrow gap before reaching the bottom of the trench for further machining. The energy available at this point was attenuated, which is partly due to lateral machining of the channel (causing undesirable widening), absorp‐ tion and diffraction effects. Therefore, the depth of the trench tends to saturate after

*<sup>π</sup><sup>D</sup>* (1)

spot. The two parameters are related by the following equation:

142 Advances in Micro/Nano Electromechanical Systems and Fabrication Technologies

and *D* is the diameter of the incident beam.

**3. Characterization of laser micromachining**

**3.1. Depth of micro-trenches as a function of the scan cycles**

where *M2*

multiple scans.

In addition to the scan speed and number of scan cycles, the focus offset and pulse energy are two important parameters controlling the quality and topography of the micro-trenches of GaN structures. The focus offset level is defined as the distance shifted away from the focal plane; the downward direction is positive. Y.H. Mak et al. [12] showed that micro-trenches with different topographies can be obtained precisely by controlling the focus offset and pulse energy of the laser beam. For example, in Figure 3(a), the sample is positioned near the focal plane (300 µm from focal plane); the laser beam ablates both the GaN and sapphire layers. A V-shaped valley is formed in the sapphire layer due to the Gaussian beam shape. At the optimal focal offset plane of 450 µm, as shown in Figure 3(b), ablation terminates automatically at the GaN/sapphire interface because the laser fluence decreases below the ablation threshold value for sapphire, resulting in the exposure of a flat and smooth sapphire bottom surface. At a larger focus offset plane of 600 µm, the GaN layer is not removed completely, leaving a shallow and rugged trench on the surface (Figure 3(c)).

Figure 3(d-f) illustrates the laser micromachined micro-trenches formed at three different pulse energies (45, 23, and 7 µJ) with the other parameters constant, and the focus offset is kept at the optimal value of 450 µm. When the pulse energy is set to 45 µJ, GaN and sapphire are ablated to form a V-shaped trench (Figure 3(d)), which is similar to that with a smaller focus offset. On the other hand, a low pulse energy results in shallow micro-trenches, which is similar to the large focus offset.

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