Results and Discussions The prepared sapphire through hole diameter is 200 μm, the taper is 2?, the maximum hole edge damage width is 5.74 μm, and there is almost no heavy condensate at the hole edge (Fig. 9). In general, a suitable laser energy density enables the sample to melt and vaporize, which can reduce the splash of the molten material and improve the quality of the through-hole surface (Fig. 4). The higher the repetition frequency of the laser, the longer the corresponding pulse width, and the thermal effect of the laser is enhanced. Owing to the influence of the thermal action, spatter is produced around the processing zone, affecting the roughness of a through-hole surface (Fig. 6). The laser scanning speed directly affects the pulse overlap rate, and an appropriate pulse overlap rate can effectively remove materials and play an important role in reducing the taper of the through hole (Fig. 7).
Sapphire crystals are commonly used in high temperature pressure sensor chips because of their excellent physical and chemical properties. In particular, in the manufacturing process of an all-sapphire fiber Fabry-Perot cavity high-temperature pressure sensor, the sapphire base must be drilled, and the quality of the through-hole is an important factor to ensure that the fiber can be vertically inserted to realize the effective transmission of optical signals. It is difficult to ensure the quality of sapphire through-holes using traditional mechanical and chemical processing methods. Laser machining technology has non-contact, non-mask, and simple process characteristics, which make it prominent in the field of hard brittle material micro-machining. Recently, there have been reports on the use of ultrashort pulse lasers to punch holes in sapphire; however, its high cost and low processing efficiency make this machining method mainly used in high-precision surface microstructure etching. Short pulse lasers, such as 355 nm ultraviolet nanosecond lasers, are widely used in industrial production owing to their low cost and high efficiency; however, there are few reports on their application in sapphire through hole processing. In this study, we analyze the impact mechanisms of different laser energy densities, repetition frequencies, and scanning speeds on sapphire processing using the control variable method, as well as the parameter space of sapphire through hole processing. We aim to fabricate through holes with diameters of approximately 200 μm and tapering angles of less than 5? on 500 μm thick sapphires without notches, heavy coagulation, or other damages. We hope that our method can provide a new, low-cost, and efficient way to process and cut the internal structure of sapphire.
This study uses sapphire wafers with a diameter of 5.08 cm and thickness of 500 μm. Before and after the experiment, the sapphire is ultrasonically cleaned with anhydrous ethanol to remove impurities on the surface and avoid effects on the laser processing results. First, using the control variable method, different laser energy densities, repetition frequencies, and scanning speeds are adjusted to drill holes into the sapphire. Subsequently, scanning electron microscopy and laser scanning confocal microscopy are used to analyze the surface morphology and 3D structure of the through-hole under different conditions. The effects of different laser parameters on the surface morphology and taper of the through-hole are studied. Finally, the optimal parameter space to obtain high-quality sapphire through holes is selected based on the experimental results.
In this study, a 355 nm ultraviolet nanosecond laser is used to successfully prepare a through hole on a 500 μm-thick sapphire wafer with a quality identical to or better than that of the ultra-short pulse laser (Table 4). Through the analysis of the drilling results assisted by laser scanning confocal microscopy and scanning electron microscopy, we determine a combination of laser energy density of 31.12 J/cm2, laser frequency of 30 kHz, and laser scanning speed of 0.5 mm/s. The results show that a micro through hole with good surface morphology and small taper can be prepared on the sapphire wafer by adjusting the appropriate combination of laser parameters. The results indicate a certain reference value for processing and cutting sapphire internal structures with a 355 nm all-solid-state ultraviolet nanosecond laser.
