Significance Micro-holes with a diameter of tens to few hundreds of microns are widely used in different industrial fields, such as injection nozzles in the automotive industry, cooling holes in jet engine components, and interconnecting micro-via in electronic packages. Methods, such as electro discharge, mechanical, electrochemical, and continuous or pulsed laser drilling, are used for micro-hole machining. However, these methods cannot be used in the micro-hole drilling with a diameter smaller than 100 mu m. For micro-holes larger than this size, these methods have their limitations, such as poor accuracy, low efficiency, and incapable of drilling in non-conductive materials (e.g., glass). For the past decades, ultrafast laser has been a reliable tool for such processing due to its unique characteristics. Various hard-to-machine and newer materials, such as glass, diamond, biological materials, and superalloy, can be ablated by ultrafast laser through nonlinear absorption of energy. The pulse is too short that only a small amount of heat transfers to the surrounding of the irradiated zone, leading to minimized heat-affected zone and high processing precision. Although the ultrafast laser has advantages in micro-drilling, some points need to be carefully investigated for practical applications. The quality and processing efficiency of ultrafast laser drilling are affected by processing methods, materials, auxiliary methods and beam characteristics such as pulse energy, frequency, pulse width, polarization, etc. Different process parameters will result in different roundness, taper, defects, and surface quality. The study on process technology parameters is one of the core issues of ultrafast laser micro-hole machining. Recently, many advances have been achieved in the processing technology of ultrafast laser drilling in various materials. However, due to the differences in materials, processing, and auxiliary methods, different studies have different conclusions about the same process parameters. Thus, it is essential to summarize and compare the results of the same process parameter. Progress The study of ultrafast laser micro-hole machining began in the 1990s. Early studies focused on the interaction mechanism between the laser and material. A series of models of interaction between ultrafast laser and material were proposed. Many studies have focused on the investigation of aspect ratio and surface morphology of the micro-holes. With the progress of laser technology, the laser power has been increased, and the pulse width is further compressed. Besides, high-speed and high-precision laser processing technologies, such as helical drilling, have appeared. The study focuses on micro-hole processing gradually turn to the acquisition of higher precision, larger aspect ratio, and drilling on various hard-to-machine materials. Recently, due to its super-short time resolution, several research groups have employed the high-speed observation technique, such as pump-probe, to study the mechanism of laser-matter interaction. There has been an increasing interest in parallel processing, highspeed scanning, and other technologies with high efficiency suitable for micro-hole array drilling. Typical materials and applications of ultrafast laser micro-hole drilling are summarized (Table 1). The influence of main process parameters in ultrafast laser micro-hole drilling is summarized ( Table 2). The pulse energy and frequency are the dominant factors ( Figs. 1 and 3). Increasing the pulse energy and frequency can increase the drilling speed, but the defects become more prominent at same time. Thus, the determination of process parameters in processing is the process of finding the balance point of pulse energy and frequency. The effect of polarization on high aspect ratio micro-holes is more obvious (Fig. 4) since the high-speed and stable rotation of linear polarization is not easy to achieve. Thus, it is preferable to use circularly polarized light for processing. There are many auxiliary ways (Fig. 8); the most economical and convenient way is the coaxial air-assist, which can increase heat conduction and reduce debris adhesion. For stable energy transmission in the micro-hole, some researchers have proposed water-guided laser processing technology. The laser beam is coupled into the water jet, and the laser travels forward due to total reflection in the water jet. The water-guided laser has great advantages in high aspect ratio micro-holes processing. Recently, multi-beam parallel processing and high-speed scanning technology have appeared in the field of efficient micro-hole drilling. For example, the diffractive optical element (DOE), spatial light modulator (SLM), and acousto-optic modulator (AOM) are used for beam splitting and energy modulation. By using these devices, parallel processing of 16 laser beams is realized. Polygon laser scanning technology is another approach to enhance the processing speed by increasing the laser scanning speed from several meters per second to a hundred meters per second. Conclusions and Prospects With the development of laser technology, the pulse width is getting shorter, while the frequency and power are getting higher. The ultrafast laser has gradually become a reliable tool for high aspect ratio and precision micro-hole machining; however, several problems remained. Thus, in-depth and detailed explorations are essential to enhance the development of this micro-hole drilling technology.
