As a core material in the microelectronics industry, monocrystalline silicon has long been the focus and challenge of precision micro/nano machining technologies within the manufacturing sector. During conventional laser processing, the pronounced thermal accumulation effect often leads to extensive heat-affected zones (HAZ), dense microcracks, and surface slag accumulation, which severely limit its reliable application in high-performance microelectronic devices such as integrated circuits and MEMS sensors. Although water-jet-guided laser technology has improved processing quality to some extent through water-based cooling, it still faces inherent challenges, such as the limitation of laser power density due to the diameter of the water jet, the difficulty in achieving total internal reflection for water-optical coupling, and the susceptibility of nozzles to damage. To address these technical bottlenecks, this study aims to develop a novel coaxial and stable water jet-assisted 355 nm ultraviolet nanosecond laser machining system. Through innovative coupling device design and machining strategies, high-quality, low-thermal-damage micro/nano machining of monocrystalline silicon is expected to be achieved, thereby providing a new technological pathway for the precision machining of hard and brittle materials.

The core of this research was a self-designed axisymmetric buffer channel coupling device, which was refined using computational fluid dynamics (CFD) simulations that considered various structural parameters. The optimized design facilitated the creation of a water jet with a fine diameter (less than 1 mm) and a highly uniform radial flow velocity, along with superior stability. A 355 nm ultraviolet nanosecond laser source (with a pulse width of 14 ns and a maximum power of 26.5 W) was utilized, and the laser beam was coaxially coupled to the water jet via a precise optical focusing system, independent of total internal reflection within a water-fiber medium. High-purity deionized water was chosen as the auxiliary medium to minimize absorption losses of laser energy due to impurities. Employing Snell’s law, the focal shift of the laser across multiple layers of media—air, sapphire window, and water—was precisely calculated and adjusted, resulting in a focused spot size of approximately 42 μm within the water. To assess the system's performance, micro-groove machining experiments were carried out on monocrystalline silicon wafers (9.0 mm × 9.0 mm × 0.7 mm), with two groups formed: the experimental group (using coaxial water jet assistance at 1.5 MPa water pressure) and the control group (dry machining without water assistance). The differences between the two groups were systematically compared in terms of HAZ width, groove depth, surface morphology, microcrack formation, and oxidation levels, with quantitative analysis conducted using microscopy.

Both fluid simulations and experimental tests demonstrated that the designed coupling device was capable of generating an ultra-stable water jet featuring a dense segment length of up to 37 cm and a uniform radial flow velocity distribution (refer to Fig.5 and Fig.6). The machining results indicated substantial improvements over conventional dry laser processing: the HAZ width reduced from approximately 53 mm to 12 mm, marking a 77% decrease (refer to Fig.10a and 10c). The single-pass machining depth increased from 205 mm to 510 mm, an improvement of about 149% (refer to Fig.10b and 10d). Microscopic observations confirmed the effective suppression of microcracks and surface oxidation, along with a notable improvement in groove wall quality. Notably, the groove mouth width increased from 31 mm to 51 mm, which was primarily attributed to the mechanical erosion of thermally softened material by the high-speed water jet and the cooling gradient variation at the jet boundary. Such effects could be mitigated through the optimization of process parameters such as water pressure and scan speed. The introduction of the water jet had a triple synergistic effect—active cooling, efficient removal of molten debris, and suppression of plasma shielding—thereby significantly enhancing the utilization efficiency of laser energy and machining precision.

A coaxial and stable water jet-assisted 355 nm ultraviolet laser machining system has been successfully designed and constructed in this study. Through an axisymmetric buffer channel coupling device, precise coaxial alignment between the laser and an ultra-long stable water jet is achieved. The system demonstrates excellent performance in the micro-groove machining of monocrystalline silicon, effectively reducing the HAZ and microcrack defects while substantially increasing machining depth and surface quality. Moreover, the system overcomes the limitations of traditional water-guided laser techniques, such as restricted power density and nozzle vulnerability to damage, thus exhibiting strong potential for engineering applications and industrial scalability. Future research will focus on exploring the process adaptability of the system in complex three-dimensional microstructure machining. Furthermore, the coupled influences of multi-parameters—including water pressure, water quality, and laser parameters—on machining stability and quality will be systematically investigated. All of these will provide theoretical foundations and technical support for the green, efficient, and low-damage manufacturing of high-performance silicon-based micro-devices.