# Femtosecond Laser-Inscripted Direct Ultrafast Fabrication of a DNA Distributor Using Microfluidics
## 3 Results and Discussions  
### Analysis of Cross-Sectional Images of the Micro-channel  
Figure 2 shows the cross-sectional images of the femtosecond laser micro-scribed micro-channels. As the energy doses increased, the width and depth of the micro-channels increased. Specifically, for a larger energy dose, a larger bulging area was observed near the micro-scribed surface due to thermal damage. In addition, the shape of the bottom changed from a sharp to blunt edge as the thermal energy from the laser beam accumulated. The number of laser beam passes was one of the major parameters that affected the micro-channel geometry. When the laser beam passes were repeated, the width and depth were wider and deeper, respectively. As the laser beam pass increased, the equilateral triangle was stretched in the depth direction to form an isosceles triangle. Similar to the energy dose, the sharpness of the bottom area was gradually changed from a sharp to a blunt edge. The repeated laser beam passes caused a larger bulging area because of thermal damage.  
Increasing the energy dose and number of laser beam passes had a synergic effect on the generation of the micro-channel. A deeper ablation was observed when the energy dose was 15 laser beam passes than at 5 laser beam passes. This could occur because the accumulated energy from the repeated laser beam passes did not have time to dissipate to the surroundings, thus increasing the thermal damage.  
Figure 3a,b show the measured depth and width of the micro-channels using SEM images, respectively. Overall, the depth and width of the micro-channels were proportional to the energy  
Figure 2: Cross-sectional SEM images of the femtosecond laser micro-scribed channels according to the energy doses and number of laser beam passes. All scale bars are 10 \(\upmu\)m.  
dose and number of laser beam passes. In Figure 3a, the depth at five laser beam passes linearly increased from 1.20 \(\pm\) 0.40 to 12.99 \(\pm\) 2.12 \(\upmu\)m when the energy dose changed from 1 to 20%. The slope, the ratio of the channel depth to the energy dose, at five laser beam passes was 0.619 (\(R^{2}\) = 0.998). The slopes at 10 and 15 laser beam passes were 1.7 and 3 times greater the slope at five laser beam passes, respectively. Even though the energy dose increased equally, a nonlinear accumulation of total thermal energy could occur as the laser beam passes were repeated.  
In Figure 3b, the width at 5 laser beam passes linearly increased from 3.36 \(\pm\) 0.51 to 7.52 \(\pm\) 0.63 \(\upmu\)m when the energy dose increased from 1 to 20%. The slope, the ratio of the channel width to the energy dose, at 5 laser beam passes was 0.215 (\(R^{2}\) = 0.910). The change in the channel width was smaller than that of the depth of the micro-channels due to the Gaussian distribution and straight laser beam. The slopes at 10 and 15 laser beam passes were 1.6 and 2.5 times greater than the slope at 5 laser beam passes, respectively, due to the analysis previously discussed for Figure 3a.  
Figure 3: Characterization of the (**a**) depth and (**b**) width of the femtosecond laser micro-scribed channels.  
Figure 4 shows the cross-section of the micro-channel that was normalized to a triangle at 5, 10 and 15 laser beam passes. When comparing the shape of the triangles, the aspect ratios of the depth to width were proportionally increased with an increase in the laser beam passes.  
In Figure 4a at a laser beam passes of 5, the aspect ratio changed from 0.71 \(\pm\) 0.13 to 3.45 \(\pm\) 0.28 when the energy dose was increased from 1 to 20%. Therefore, the larger energy dose generated a higher aspect ratio. In Figure 4b,c, as the laser beam passes increased to 15 passes, the aspect ratios were increased from 1.33 \(\pm\) 0.29 to 4.07 \(\pm\) 0.27 and 1.71 \(\pm\) 0.79 to 5.50 \(\pm\) 0.24 at 10 and 15 laser beam passes, respectively. The depth and width of the laser micro-scribed channels were 3 times greater with an increase of the laser beam passes; however, the aspect ratio was approximately 1.6 times greater.