Fig. 1. Schematic diagrams of micropore quality characteristics. (a) Taper; (b) roundness; (c) recast layer; (d) heat-affected zone; (e) microcracks; (f) spatter deposition around periphery of holes

Fig. 2. Influence of pulse width on quality of micro-holes^[47-48]. (a) Relationship between pulse width and hole diameter; (b) relationship between pulse width and hole depth; (c) micro-hole morphologies under different pulse widths

Fig. 3. Effect of repetition frequency on quality of micro-holes^[51-53,55]. (a) Relationship between repetition frequency and diameter of micro-holes; (b) relationship between repetition frequency and depth of micro-holes; (c) relationship among repetition frequency, taper, and heat-affected zone; (d) effect of repetition frequency on roundness of micro-shallow holes

Fig. 4. Influence of laser energy on quality of micro-holes^[57-58,60]. (a) Relationship between laser energy and taper; (b) images of entrance and exit of micro-holes under different power; (c) relationship between laser power and roughness of micro-hole

Fig. 5. Influence of focal position on quality of micro-holes^[63-64]. (a) Relationship between focus position and hole taper; (b) relationship between focus position and splash area

Fig. 6. Effect of laser wavelength on quality of micro-holes^[67-68]. (a) Micro-hole section view obtained by 532 nm laser processing; (b) micro-hole section view obtained 1064 nm laser processing; (c) composite material electron microscope image obtained by 1066 nm laser processing; (d) composite material electron microscope image obtained by 266 nm laser processing

Fig. 7. Schematic diagrams of single pulse drilling and processing effect^[70-71]. (a) Schematic diagram of single-pulse processing and drilling; (b) micro-holes produced at rates of 300, 500, and 700 hole/s

Fig. 8. Schematic diagrams of multi-pulse drilling and processing effect^[76-79]. (a) Schematic diagram of multi-pulse drilling; (b) SEM images of micro-hole processing of polycarbonate materials with different pulse numbers (pulse numbers are 1, 5, and 100 from left to right); (c) SEM images of titanium foil perforated by ultraviolet femtosecond pulses using multi-beam interference technique; (d) scanning electron microscope image of micro-hole array

Fig. 9. Schematic diagram of rotary cutting and drilling and processing effect diagram^[83-84]. (a) Schematic diagram of rotary cutting and drilling; (b) micro holes processed by rotary cutting and drilling

Fig. 10. Schematic diagram of spiral drilling and processing effect diagram^[91]. (a) Schematic diagram of spiral drilling; (b) micro holes processed by spiral drilling

Fig. 11. Influence of vacuum environment on quality of micro-holes fabricated by laser processing^{[92-93,95-96]}. (a) Micro-holes processed in air; (b) micro-holes processed in vacuum environment; (c) effect of processing under vacuum and air environments on depth of micro-holes; (d) (e) micro-holes formed in air and vacuum corresponding to Fig. 11(c)

Fig. 12. Effect of auxiliary gas on quality of micro-holes^[103]. (a) Influence of gas jet type on depth of micro-holes; (b) influence of gas jet type on splashes of micro-holes

Fig. 13. Underwater laser processing^[104]. (a) Schematic diagram of laser underwater processing device; (b) micro-holes processed by laser underwater processing; (c) micro-holes processed in air

Fig. 14. Laser water jet processing^［109］. (a) Schematic diagram of low-pressure water jet assisted laser processing device; (b) thickness of recast layer of micro-holes processed in air and water

Fig. 15. Micro-holes obtained by water guided laser processing^[110]. (a) Principle of water guided laser processing; (b) inlet (top) and outlet (bottom) of microholes processed by water-guided laser

Fig. 16. Laser chemistry asynchronous processing of micro-holes^[113]. (a) Schematic diagram of laser chemical asynchronous processing micro-hole device; (b) orifices before and after chemical etching

Fig. 17. Laser chemical simultaneous processing of micro-holes^[116]. (a) Schematic diagram of laser chemical simultaneous processing of micro-holes; (b) morphologies of micro-hole sidewalls before and after etching

Fig. 18. Alcohol-assisted laser processing^[117]. (a) Schematic diagram of alcohol-assisted laser processing micro-hole device; (b) laser-processed micro-hole in air; (c) alcohol-assisted laser-processed micro-hole

Table 1. Diameter and degree of taper of micro-holes fabricated with different assisted gas pressure^[58]