# Groove Formation in Glass Substrate by a UV Nanosecond Laser
## 3 Results and Discussion
### Groove Formation by Multiple Laser Scans  
In order to produce the groove or channel structures with various widths and depths in the glass substrate, multiple laser scans are necessary. In this section, the groove formation by the multiple laser scans is described. One of the process conditions to produce the crack-free groove is used in the experiment for multiple laser scans. The process condition selected for the experiment is a laser power of 2.28 W, scan speed of 25 mm/s, and pulse repetition rate of 30 kHz. Figure 9 shows the result of multiple laser scans with zero scan spacing. In this experiment, a laser beam scans the same scan track several times. From Figure 9, it is confirmed that the increase of the scan number does not induce cracking around the irradiation region.  
Figure 10 shows the measured depth and width of the groove as a function of the number of scans. The optical images (cross sectional view of the grooves) used to measure the depth and width of the groove are shown in Figure 11. The width of the groove does not nearly change as the scan number increases, whereas the depth of the groove tends to increase with the scan number, as seen in the figure. In high power density laser-material interaction, the absorption of the laser beam in the material dramatically increases once a laser-induced crater is produced as a result of the multiple reflection of the laser beam in the crater [24]. This could help the material removal in multiple laser scans, although the intensity of the laser beam decreases owing to continuous beam defocusing with the depth of the groove.  
Figure 10: Depth and width of the groove as a function of the number of scans.  
Figure 9: SEM images of the grooves produced by multiple laser scans with zero scan spacing (laser power: 2.28 W, scan speed: 25 mm/s, and pulse repetition rate: 30 kHz). Number of scans: (**a**) 1, (**b**) 5, (**c**) 10, and (**d**) 20.  
The multiple laser scans with a non-zero scan spacing is also tested to produce a wide groove. Two different scanning schemes are tested to evaluate the efficiency in material removal (see Figure 2). Figure 12 shows the cross section of the groove produced with two different scanning schemes using the same laser parameters. It is found that more material can be removed with method 3. In method 3, a rough surface morphology is uniformly produced for the whole scanned area after the first set of the laser scan is finished. This contributes to the acceleration of material removal owing to more laser energy absorption through the multiple reflection of the laser beam from the second set of the laser scan.  
Figure 13 shows the groove produced by multiple laser scans using method 3. A near rectangular-shaped groove is produced to imitate the microchannel of a bio-chip. It is seen that the groove has a quite smooth inner surface. The width and depth of the groove are \(\sim\)220 \(\upmu\)m and \(\sim\)500 \(\upmu\)m, respectively. No visible crack is observed around the groove wall along the direction of scanning. This result may suggest that a UV nanosecond laser could be one of the potential tools for the fabrication of bio-chip channels.  
Figure 11: Optical images of the grooves (cross-sectional view) produced by multiple laser scans with a zero scan spacing (laser power: 2.28 W, scan speed: 25 mm/s, and pulse repetition rate: 30 kHz). Number of scans: (**a**) 1, (**b**) 2, (**c**) 3, (**d**) 4, (**e**) 5, (**f**) 6, (**g**) 7, (**h**) 8, (**i**) 9, (**j**) 10, and (**k**) 20.  
Figure 12: Cross section of the grooves produced by multiple laser scans with a non-zero scan spacing (laser power: 2.28 W, scan speed: 25 mm/s, pulse repetition rate: 30 kHz, and scan spacing: 30 \(\upmu\)m). Scanning scheme: (**a**) method 2 in Figure 2 and (**b**) method 3 in Figure 2 (number of scans per scan track: 5 and number of scan tracks: 130).