# Groove Formation in Glass Substrate by a UV Nanosecond Laser
## 3 Results and Discussion
### Analysis of Glass Surface Morphology Produced by Single Laser Scan  
The experiment using single laser scan is carried out by varying the laser power, scan speed, and pulse repetition rate. Figure 4 shows the optical images of the glass surface damaged by the single laser scan at various laser powers and scan speeds under the fixed pulse repetition rate of 30 kHz. It is seen that cracks are generated along the direction of scanning owing to the thermal stress induced by rapid heating and cooling during the process. It is clearly observed that the size of the crack increases with decreasing scan speed and increasing laser power owing to greater laser energy deposition on the glass. The size of cracks produced is several tens of microns to several hundreds of microns depending \begin{table}
\begin{tabular}{c c} \hline
**Property** & **Value** \\ \hline Thermal conductivity & 0.94 W/(m K) [21] \\ Specific heat & 0.88 kJ/(kg K) [21] \\ Coefficient of linear expansion & \(8.3\times 10^{-6}\)\({}^{\circ}\)C [21] \\ Refractive index & 1.54 (at 355 nm) [22] \\ Absorption coefficient & 1.58 cm\({}^{-1}\) (at 355 nm) [22] \\ Transmittance & 0.85 (at 355 nm) [22] \\ \hline \end{tabular}
\end{table} 
Table 2: Thermophysical and optical properties of the soda lime glass. Figure 3: Result of damage threshold test for the soda-lime glass substrate. Laser power: (**a**) 0.33 W, (**b**) 0.72 W, (**c**) 1.22 W, and (**d**) 1.76 W. The circle in the figure denotes a crack. \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Material** & **SiO2** & **Al2O3** & **MgO** & **CaO** & **Na2O** & **K2O** & **Fe2O3** & **TiO2** \\ \hline Composition, mol. \% & 71.86 & 0.08 & 5.64 & 9.23 & 13.13 & 0.02 & 0.04 & 0.01 \\ \hline \end{tabular}
\end{table} 
Table 3: Chemical composition of the soda lime glass [23]. on the process condition. The crack has a seagull shape, and its one end propagates across the direction of scanning. Figure 5 shows the glass surface damaged by the single laser scan with higher pulse repetition rate. The aspect of the crack propagation is similar up to 50 kHz pulse repetition rate; however, the significant increase of the crack size is found from the pulse repetition rate of 60 kHz, as shown in the figure. Owing to the large growth of the cracks, neighboring cracks are linked to each other like in a chain, and their size reaches several millimeters. At a very high pulse repetition rate (80 kHz), the long crack of a line shape is eventually produced along the direction of scanning. The detailed morphology of the glass surface is investigated with SEM. Figure 6 shows SEM images of the glass surface scanned by a laser beam at a pulse repetition rate of 30 kHz. At the laser power of 3.32 W and scan speed of 5 mm/s, the glass surface irradiated by the laser beam shows a smooth wave morphology. This indicates that material removal does not nearly occur, and the  
Figure 4: Optical images of the glass surface damaged by a single laser scan. Laser power: (**a–d**) 2.28 W, (**e–h**) 2.8 W, and (**i–l**) 3.32 W. Scan speed: (**a,e,i**) 5 mm/s; (**b**), (**f,j**) 15 mm/s; (**c,g,k**) 25 mm/s; and (**d,h**), and (l) 35 mm/s. The circle in the figure denotes a crack. Figure 5: Optical images of the glass surface damaged by a single laser scan with a higher pulse repetition rate. Scan speed: (**a–d**) 5 mm/s and (**e–h**) 15 mm/s. Laser power and pulse repetition rate: (**a,e**) 2.48 W and 50 kHz, (**b,f**) 2.65 W and 60 kHz, (**c,g**) 2.39 W and 70 kHz and (**d,h**) 2.12 W and 80 kHz. The circle in the figure denotes a crack. interaction between the laser beam and glass only induces volume melting underneath the glass surface. The evidence of the volume melting is clearly observed at the end facet of the glass plate (the end point of the scanning path, see Figure 6i). As the laser power decreases and scan speed increases, the morphology of the glass surface is turned into a groove shape through the material removal process, as shown in the figure.