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1. Introduction

Ultrafast laser ablation has attracted increasing attention in fabrication of structures at micrometer or even nanometer scales due to its unique advantages of non-contact, minimal damage and precise processing in almost any materials. However, the problems including recast layers, redeposition of ablation debris, phase transition, and induced cracks still limit its application prospects. To expel the ablation debris and heat accumulation instantaneously during laser ablation is essential for improving surface quality. Liquid-assisted laser ablation has the advantage of relieving thermal effects, whereas the light scattering and shielding effects caused by laser-induced cavitation bubbles, suspended debris, and turbulent liquid flow generally deteriorate laser beam transmission stability, decreasing processing efficiency and quality. Recently, Yang Guo, Pei Qiu, and Prof. Shaolin Xu, from Southern University of Science and Technology, China, and Prof. Gary J. Cheng, from Purdue University, USA, published a research article "Laser-induced microjet-assisted ablation for high-quality microfabrication" on IJEM. In this article, the authors proposed laser-induced microjet-assisted ablation (LIMJAA) technology to improve laser ablation performances by relieving the above-mentioned problems in traditional liquid-assisted laser ablation methods. For difficult-to-process materials, the LIMJAA method has been proven to be capable of fabricating high-quality microstructures with largely improved material removal efficiency.

Highlights

A laser-induced microjet-assisted ablation approach is developed for achieving high-quality micromachining in this article:

● A continuous and directional high-speed microjet generates from the asymmetrical collapse of sequentially laser-induced cavitation bubbles in a critical thin liquid film.

● Cavitation bubbles and ablation debris can be instantaneously expelled by the directional microjet for achieving high-quality laser ablation.

● The proposed approach is capable of machining high-quality arrays of micro-channels and micro-through-holes on difficult-to-process materials.

Figure 1.   (a) Schematic of laser-induced microjet-assisted ablation, (b) photograph by high-speed camera recorded laser-induced microjet near the air-liquid-solid interface.

2. Background

In conventional liquid-assisted laser ablation processes, the scattering and shielding effects of suspended bubbles and debris can reduce the processing efficiency and surface quality. Flowing liquid or waterjets are widely applied to eliminate the negative effects of bubbles and debris, including waterjet-assisted underwater laser cutting, coaxial waterjet-assisted laser processing, waterjet-guided laser processing, overflow-assisted laser machining, and hybrid laser-waterjet ablation technology. In these technologies, liquid flows or jets without turbulence and free of bubbles are preferred, but hard to obtain. It is very interesting to find that laser-induced microjet can be formed by asymmetrical collapse of cavitation bubbles if laser focuses near the interface of liquid and air. This phenomenon has been widely used in the fields of needle-free drug injection and microdroplet deposition for 3D printing. The properties of laser-induced microjets are similar to the above-mentioned external liquid jets, and they are expected to be useful to expel bubbles and debris during laser processing, which has not yet been clarified and practically used.

3.  Recent Advances

In this work, the effects of laser-induced cavitation bubbles on laser ablation performances under a critical thin liquid film was studied. It is found that a continuous and directional high-speed laser-induced microjet can be formed with continuous asymmetrical collapse of laser-induced cavitation bubbles. The microjet can carry off suspended secondary bubbles and ablation debris instantaneously to improve laser ablation performances. The temporal evolution of the continuous microjet and materials removal mechanisms are systematically studied. It is proven that the LIMJAA method is capable of fabricating high-quality microstructures with largely improved material removal efficiency on difficult-to-process materials. 

Principle of LIMJAA

A pulsed laser focusing near the liquid-air interface boundary will produce a pulsed downward microjet due to asymmetrical collapse of laser-induced cavitation bubble. As the pulsed laser constantly irradiates inside the liquid film, the pulsed microjet can accumulate and accelerate to form a rapid and steady continuous liquid jet as shown in Figures 2(a)-(d). In practical LIMJAA process, the continuous microjet will change its direction to be just opposite to the laser scanning direction after encountering the laser ablated curved surface as shown in Figure 1. This helps to instantaneously expel the suspended bubbles and debris in the ablation zone to achieve high-quality microfabrication. To make use of the laser-induced continuous microjet, researchers have also studied the effect of focal plane position on the microjet's initial speed, which achieves a peak value at a specific laser focal depth as shown in Figure 2(e). The higher microjet initial speed is beneficial to the formation of a steady continuous liquid jet. This revelation helps in determining the thickness of the liquid film used in the practical LIMJAA process.

Figure 2. Generation and evolution of continuous laser-induced microjet: (a) evolution of secondary bubbles within 100 ms, (b) trajectory of secondary bubbles induced by the first 5 laser pulses, (c) trajectory of 61-65 pulses induced secondary bubbles, (d) change of microjet speed with increasing pulses number, (e) initial speeds of laser-induced microjet versus dimensionless standoff distance.

Material removal mechanism of LIMJAA

Although liquid-assisted laser processing can relieve the ablation debris re-deposition and recast inside the machined grooves, there are still differences between laser processing in conventional immersed liquid and the thin liquid film. A cluster of bubbles in immersed liquid leads to unstable processing, which results in irregular ablation traces besides the grooves. In contrast, LIMJAA with a critical thin liquid film expels bubbles, debris, and molten materials instantaneously from the ablation zone by the mechanical impact of laser-induced high-speed microjet. Thus, smooth grooves free of recast layer and redeposition of debris are obtained by LIMJAA as shown in Figure 3. The ablation depth and material removal rate by LIMJAA are also improved as shown in Figure 4. Here, the researchers demonstrate that a groove with a width of 19 μm and a depth of 98 μm, namely, a depth-to-width ratio of 5.2 can be fabricated by a single-pass laser scanning process strategy as shown in Figure 5.

Figure 3. Surface morphologies of microgrooves processed by laser ablation in air, immersed liquid and LIMJAA, (a)-(c) on-line optical photograph of laser scanning processing on 4H-SiC wafer, (d)-(f) surface morphology of processed microgrooves on SiC wafer, (g)-(i) surface morphology of processed microgrooves on stainless steel.

Figure 4. Comparison of (a) ablation depth of microgrooves and (b) MRRs by LIMJAA, laser ablation in air, and immersed liquid with varying scanning speed.

Figure 5. The fabricated high depth-to-width ratio microgroove by LIMJAA with a single-pass laser scanning process on 4H-SiC wafer.

Practical applications of LIMJAA

It is proven that the novel LIMJAA technology can be used to process different types of materials, including hard-to-machine materials, thermal- and stress-sensitive materials, with high repeatability and stability. For example, micro-channel array structures are fabricated on the 4H-SiC wafer surface, which shows good application prospects in micro-channel heat dissipation systems of microelectronic devices and microfluidic molds for glass molding, as shown in Figure 6. For micro-drilling of thin wafers and metal strips, the LIMJAA approach can not only achieve efficient material removal rates but also obtain a very high surface quality as depicted in Figure 7.

Figure 6. Micro-channel array structures on 4H-SiC wafer. (a) Surface morphology and (b) 3D profile.

Figure 7. Square and circle micro-through-hole array structures on (a) 316 stainless steel strip with a thickness of 100 μm, (b) Fe-based metallic glass strip with a thickness of 25 μm, (c) silicon carbide wafer with a thickness of 200 μm, and (d) 4H-SiC wafer with a thickness of 100 μm.

4. Perspectives

In this work, the researchers systematically demonstrated the effectiveness of laser-induced microjet in liquid-assisted laser ablation for machining difficult-to-process materials with extremely sharp edges. To accurately control the local liquid thickness in the LIMJAA technology is still urgent for further improving its machining efficiency and stability. The technology and mechanism model established in this work can promote a key step for liquid-assisted ultrafast laser ablation in high-quality micromachining of various difficult-to-process materials. 

5. Supplementary Video

6. About the Authors

Dr. Xu is an assistant professor (PI) at Southern University of Science and Technology (Sustech). He got his PhD degree and worked as a JSPS research fellow and an assistant professor at Tohoku University from 2011 to 2017. In early 2017, Dr. Xu returned to China and began to establish an ultrafast laser micro-/nanofabrication Lab at Sustech, focusing on developing new ultrafast laser micro-/nanofabrication technologies, the principles of interactions between ultrafast laser and materials, and the applications of laser-structured surfaces.