# Research on Microhole Processing Technology Based on the Femtosecond-Laser Spiral Trepanning Method
## 3 Results and Discussion
Finally, the shape of the processed microhole is also a normal cone, and the morphology is similar to that of Figure 7c. The cross section of the microhole is consistent. In the process of microhole fabrication via femtosecond-laser processing, the factors affecting the microhole taper include the following: the initial phase, transverse displacement of the beam, the focal length of the focusing lens, beam quality, and defocusing amount. The initial phase, transverse displacement, and focal length of the focusing lens determine the laser incident angle. The beam quality is equivalent to the tool head. A good tool head will improve the processing efficiency. A poor beam quality will affect the material removal by the laser, resulting in a poor taper. The scanning path, air pressure, defocusing amount, and other factors will affect the microhole taper. This study mainly investigated the influence of the processing technology on the microhole taper. The experiments clearly showed that the different microhole taper was processed by adjusting the distribution of scanning path and power. Moreover, in addition to the influences of the scanning path and power, the influences of the pressure of air pump and defocusing amount on the microhole taper was also studied. The effects of air pressure and defocusing amount on the microhole taper is shown in Figure 9. With the increase in air pressure from 0.2 Pa to 0.4 Pa, the microhole tapers were slight differences. The air pressure mainly cools down the temperature of the microhole and blows away the melt caused during the laser processing microhole. The lack of obvious slag at the entrance demonstrates this point. Besides, the air pressure has an effect on the refractive index, which affects the formation of micropores when the laser propagates through the expanding plasma and gas [26]. However, according to the previous research [27], with the air pressure continuing to increase, the heat dissipation from the molten material will increase, and thereby causing re-solidification, as a result of which obvious slag occurs at the entrance of microhole. Besides, the microhole tapers were marked variation with the change of defocusing amount. The trend of results illustrates that the negative defocus could produce the inverted cone hole, while the positive defocus could produce the normal cone hole. Moreover, the taper increased with the increase in defocusing amount. The method enlarges the taper of microholes by varying the defocusing amount, which is very important for fabricating a larger angle taper microhole. In addition, with the defocusing amount continuing to increase, the laser energy will be low and the microhole is difficult to process. Furthermore, according to the method of varying defocusing amount, we provide some experimental results. Figure 10a shows a \(-1.24^{\circ}\) taper microhole obtained with a negative defocus of 0.5 mm based on the laser processing parameters of 0.03\({}^{\circ}\) in Table 2. Figure 10b shows a \(5.24^{\circ}\) taper microhole obtained with a positive defocus of 0.5 mm based on the laser processing parameters of 3.09\({}^{\circ}\) in Table 2. The decrease in the defocusing amount will weaken the center energy of the laser spot, but the laser processing range will radiate outward. Therefore, for the case of the \(-1.24^{\circ}\) taper hole, the change in the defocusing amount will cause the energy of the laser outer ring to radiate more broadly, and a microhole with an inverted cone hole can be obtained. For the \(5.24^{\circ}\) taper hole, the change in the defocusing amount will disperse the energy of the outer ring with weak energy, so it is more difficult to remove the material. However, the inner ring has higher energy, and the influence of defocusing on the inner ring is smaller than that on the outer ring. Finally, a normal cone hole with a larger taper is formed.