# Research on Microhole Processing Technology Based on the Femtosecond-Laser Spiral Trepanning Method
## 3 Results and Discussion  
The microholes of a femtosecond laser-processed stainless steel are displayed in Figure 4. The average diameters of the microhole entrance and exit were 0.3056 and 0.3524 mm, respectively. Meanwhile, there was almost no slag at the microhole entrance after processing (Figure 4a). However, due to the thermal effect, a circle of black oxide was formed on the surface near the microhole entrance, mainly because the outer ring of the laser scanning path was relatively dense and the power was relatively large. [PERSON] et al. [24] found that in microhole drilling via laser trepanning, the oxygen amount decreased with further movement away from the microhole entrance. Figure 4b is a microscopic picture of the microhole exit in this study. No obvious slag occurred at the exit after processing, but a small amount of debris existed in the lower right corner. Moreover, the black oxide circle around the microhole exit was larger than that around the microhole entrance, but the oxide color around the exit was relatively light. The mass ratios of different elements observed by energy-dispersive X-ray spectroscopy (EDX) are presented in Table 1. In the table, A and C denote areas near the microhole, while B and D are areas away from the microhole, about 100 \(\upmu\)m from A and C. The oxygen mass ratios of areas A and C (6.35% and 6.32%, respectively) were higher than those of areas B and D (5.17% and 5.07%, respectively), which proves the above explanation. In addition, the main reason for the difference in black oxide area between the microhole entrance and exit was that the air pump could blow away the debris and reduce the surface temperature, but with the increase in the laser processing depth, a small amount of residual debris occurred at the inner wall and back surface of the microhole exit. Moreover, an oxide layer was generated on a relatively large area of the microhole exit, due to the poor slag-discharge quality and the continuous accumulation of temperature. By simultaneously studying the morphology of the microhole and the mass ratio of different elements, it is possible to have a more comprehensive understanding of the effect of femtosecond laser processing of the microhole than simply observing the surface morphology of the microhole [22]. The error of the average entrance and exit diameters of the microholes processed by this spiral machining method was less than 6 \(\upmu\)m, and the precision of the micro-holes is higher than that previously reported [21; 22; 23]. Roundness and taper are important indexes to estimate the microhole quality. The maximum and minimum diameters of the microhole entrance were 0.3082 mm and \begin{table}
\begin{tabular}{c c c c c} \hline
**Element** & **A** & **B** & **C** & **D** \\ \hline C & 0.88 & 0.82 & 1.32 & 1.08 \\ \hline O & 6.35 & 5.17 & 6.32 & 5.07 \\ \hline Si & 0.92 & 0.94 & 1.37 & 1.14 \\ \hline Cr & 16.03 & 16.19 & 16.90 & 16.98 \\ \hline Fe & 68.13 & 68.87 & 66.30 & 68.32 \\ \hline Ni & 7.69 & 7.90 & 7.80 & 7.41 \\ \hline \end{tabular}
\end{table} 
Table 1: Mass ratios of different elements in different regions of the microhole entrance and exit. Figure 4: Inverted cone hole processed by femtosecond: (**a**) microhole entrance; (**b**) microhole exit. 0.3012 mm, respectively. The maximum and minimum diameters of the microhole exit were 0.3600 mm and 0.3364 mm, respectively.