Abstract
Laser-induced plasma micromachining (LIPMM) can be used to fabricate high-quality microstructures of hard and brittle materials. The liquid medium of the LIPMM process plays a key role in inducing the plasma and cooling the materials, but the liquid medium is overheated which induces lots of bubbles to defocus the laser beam and reduce machining stability. In this paper, a comparative investigation on bubble behavior and its effect on the surface integrity of microchannels in three types of liquids and at different depths during LIPMM has been presented. Firstly, the formation mechanism of microbubbles was described. Secondly, a series of experiments were conducted to study the number and maximum diameter of the attached bubbles and the buoyancy movement of floating bubbles in the LIPMM of single-crystal silicon under deionized water, absolute ethyl alcohol, and 5.6 mol/L phosphoric acid solution with a liquid layer depth of 1–5 mm. It was revealed that the number and maximum diameter of attached bubbles in deionized water were the highest due to its high tension. Different from the continuous rising of bubbles at the tail of the microchannels in the other two liquids, microbubbles in 5.6 mol/L phosphoric acid solution with high viscosity rose intermittently, which formed a large area of bubble barrier to seriously affect the laser focus, resulting in a discontinuous microchannel with an unablated segment of 26.31 μm. When the depth of the liquid layer was 4 mm, absolute ethyl alcohol showed the advantages in narrow width (27.15 μm), large depth (16.5 μm), and uniform depth profile of the microchannel by LIPMM. This was because microbubbles in the anhydrous ethanol quickly and explosively spread towards the edge of the laser processing zone to reduce the bubble interference. This research contributes to a better understanding of the behavior and influence of bubbles in different liquid media and depths in LIPMM of single-crystal silicon.
Highlights
Single-crystal silicon is the key essential material of semiconductors with outstanding electrical properties and photovoltaic characteristics and has been widely used in the fields of optoelectronics and microelectronic devices [1]
Ehmann et al [3] proposed a novel type of underwater micromachining process, laser-induced plasma micromachining (LIPMM), to fabricate microchannels on the silicon surface through high-temperature spot plasma in the liquid
Compared with conventional mechanical machining technologies like turning, milling, or grinding which generate considerable cutting force leading to severe tool wear and surface damage, as well as low machining accuracy and efficiency, the proposed LIPMM is not restricted by the hardness and brittleness of materials to without contact machining for the reason that the principle of this process is to focus a laser beam on a liquid medium to induce a high-temperature plasma which is used to remove material
Summary
Single-crystal silicon is the key essential material of semiconductors with outstanding electrical properties and photovoltaic characteristics and has been widely used in the fields of optoelectronics and microelectronic devices [1]. Compared with conventional mechanical machining technologies like turning, milling, or grinding which generate considerable cutting force leading to severe tool wear and surface damage (microcracks), as well as low machining accuracy and efficiency, the proposed LIPMM is not restricted by the hardness and brittleness of materials to without contact machining for the reason that the principle of this process is to focus a laser beam on a liquid medium to induce a high-temperature plasma which is used to remove material. The formation of the microchannel constrains the bubbles and results in different sizes of attached bubbles [6] The existence of these microbubbles causes defocus and energy dissipation of the pulsed laser, which reduces machining stability and the quality of the surface microstructures.
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