Objective Taper-free or negative conical micro-holes with a diameter of tens to few hundreds of microns are of great interest in a variety of industries. Conventional methods, such as electro-discharge drilling, mechanical drilling, electrochemical drilling, and continuous or pulsed laser drilling, have their limitations, including poor accuracy, low efficiency, as well as they are incapable of drilling in non-conductive materials, such as glass. Although ultrafast laser is believed to be a reliable tool for drilling processes due to its unique characteristics, the control of the diameter and taper of micro-holes is one of the greatest challenges encountered in the process. At present, the diameter and taper of micro-holes can be controlled using a 5-axis Galvano scanner, but the equipment is expensive, and a complex drilling strategy is required. Ultrafast laser helical drilling technology is an effective and simple way to adjust the taper and diameter of micro-holes since the laser spot is rotated at a high speed, and the roundness of the holes can be improved in the meantime. However, the taper and diameter are affected by several factors in helical drilling, and these factors vary with material, aspect ratio, laser characteristics, and drilling strategy. Hence, it is important and necessary to investigate the relationship between the diameter, taper, and beam rotation of micro-holes and to show the control mechanism of the diameter and taper in ultrafast laser helical drilling.

Methods With the ultrafast laser focused in the middle of a 0.5 mm thick copper plate, various micro-holes with different diameters and taper are drilled by adjusting the angle of the focused beam and rotation diameter of the laser spot. Then the rotation diameter of the laser spot at a focal plane, ±250 μm above and below the focal plane, is in situ monitored by a CCD. The diameters of the micro-holes in the entrance and exit are compared with the CCD measurement results at ±250 μm above and below the focal plane, respectively. The dependence of the spot rotation diameter on the angle of the wedge prism is analyzed based on geometric optics.

Results and Discussions Micro-holes with an aspect ratio of 6∶1 and taper of -2.6°-2.3° are obtained, and the relationship between the micro-hole diameter, taper, and beam rotation characteristics is shown (Table 1). The rotation diameter of the spot measured by the CCD and the diameter of the micro-hole drilled under the same parameters are investigated (Fig. 6(a)). When the position of the movable mirror changes from -4 mm to 3 mm, the diameter of the rotation beam changes from a small value to a large value at 250 μm above the focal plane and from a large value to a small value at 250 μm below the focal plane, while the diameter remains constant at the focal plane. This means that the focused beam rotates around the focus point (Fig. 6(b)). However, the angle of the focused beam does not equal the actual taper. Under a fixed wedge prism angle, the change in the hole diameter on the exit side is similar to that of the spot rotation diameter on the focal plane, the changes vary slightly, and the difference between them is almost constant. In contrast, the diameter of the hole entrance increases with the beam rotation diameter at 250 μm above the focal plane. That is, the hole diameter of the exit side remains unchanged, and the diameter change of the entrance side will lead to the change of the taper (Fig. 6(c)). It is based on this principle that micro-holes with a taper varying from -2.6° to 2.3° are machined in which a negative taper represents that the entrance of the micro-hole is smaller than the exit. Besides, when the rotation diameter ratio of the spot at ± 250 μm from the focal plane is 0.66, the hole taper is close to 0°, and a negative taper can be obtained when the ratio is smaller. When the ratio is 1, although the spot rotation diameter is constant, there is still a positive taper in the micro-hole. The diameter of the exit changes marginally no matter how much the rotation diameter changes because the laser energy is attenuated during the transmission in the micro-hole. The rotation diameter of the spot at the entrance must be smaller than that at the exit to get a straight hole with zero tapers. The pulse energy, frequency, and focus position will affect the processing efficiency and the shape of the hole. If the laser is focused on the surface of the workpiece, the micro-holes cannot penetrate completely under the same drilling time.

Conclusions In this study, we showed the relationship between the micro-hole diameter, taper, and beam rotation in a Dove-prism-based helical drilling system. The results show that the micro-hole size is dominated by the angle of the wedge prism. As for the taper of the micro-hole, adjusting the position of the movable mirror can change the exit angle of the focused beam, and then the taper can be adjusted. Since the laser energy will affect the ablation rate of material, the focus position, laser power, and spot overlap rate will slightly contribute to the micro-hole size and taper.