The mechanism of micro-cracks formation in ultrasonic-assisted water confined laser micromachining silicon
Introduction
Silicon is one of the most common semiconductors, and it is widely used in microelectronics, 3C (computer, communication, and consumer electronics) products, and other high precision fields. However, silicon has obvious characteristics that are difficult to process. Laser micromachining is one of the most common methods to process brittle materials, such as silicon [1], [2], [3], [4], [5]. However, the heat-affected zone (HAZ), microcrack, recasting layer, and debris deposition are always accompanied by laser processing.
In order to further reduce the defects caused by thermal, the ultrasonic vibration-assisted long-pulse laser has become an alternative method. Some ultrasonic vibration-assisted millisecond pulse Nd: YAG laser processes have proposed, e.g., laser percussion [6], laser trepanning [7], single-pulse laser drilling [8], and the ultrasonic vibration is uniformly applied to the whole workpiece through the water medium in these methods. They found the ultrasonic can decrease the hole taper, clean the inner hole wall surface, stop abrupt accumulation micro ripples, grain refinement, reduce the recast layer thickness, and decrease the hole sidewall surface roughness.
A novel ultrasound-assisted water-confined laser micromachining was made a preliminary investigation of this technique by Wu et al. [9], and Liu et al. [10]. They found less debris deposition and a much higher material removal rate than laser ablation in water without ultrasound. According to the time-resolved observations and more detailed research, the underlying mechanism for the enhanced material removal rate is the in-situ ultrasonic wave enhances the water cleaning effect and reduced the material cloud, which reduces the interference with a subsequent laser pulse(s)’ energy to the workpiece [11]. Charee et al. proposed another ultrasonic-assisted laser ablation performing in the flowing water condition [12]. The ultrasonic vibration was applied directly to the bottom of the closed water container. They found that the ultrasound can energize the bubble (which is induced by laser processing), broken up, and then expelled from the work surface. The ultrasound can also enhance flush the cut debris, push the molten material away from the cut surface, and accelerate workpiece cooling. Sun et al. [13] reported ultrasound-assisted femtosecond laser drilling stainless steel. They found the no slag splash, burrs, and redeposition on the workpiece surface, and the bottom of a clean and flat hole is obtained. Sun et al. attributed this to the ultrasonic vibration that can break the larger cavitation bubbles induced by water and material evaporation. Zhou et al. have investigated the in-situ ultrasonic-assisted nanosecond pulse laser cutting silicon in water [14]. They found the material removal rate increased significantly. In addition, there was a systematic discussion on the effects of the acoustic streaming and laser-induced bubbles on the laser micromachining silicon. They found the effect of bubbles played a dominant role in the in-situ ultrasonic-assisted laser processing silicon, and the acoustic streaming had little effect on the groove temperature field.
In ultrasonic-assisted water-constrained laser micromachining brittle materials, micro-cracks is likely to generate due to ultrasonic vibration, bubble pulsation, and temperature gradient. However, there are few analyses of the micro-cracks formation in ultrasonic-assisted water confined laser micromachining. In order to control the microcrack and improve processing quality, the mechanism of micro-cracks formation in ultrasonic-assisted water confined laser micromachining silicon was investigated. Based on the authors’ previous publication [14], micro-cracks formation in ultrasonic-assisted water confined laser micromachining silicon was analyzed. In order to obtain an insight into the mechanism of microcrack propagation, the bubble dynamics change was observed by the high-speed camera, and the effect of the bubble on the workpiece was calculated numerically. In addition, a heat transfer and solid mechanics coupling model had been set up to analyze the effect of laser intensity on microcrack formation qualitatively. This study’s findings can contribute to the potential application of a micromachining method in processing brittle materials.
Section snippets
Experiment setup
The diagrammatic of the experimental setup is shown in Fig. 1, and Table 1 shown the experiment parameters. The single-crystalline silicon (the crystal orientation is 100) with a dimension of substrates. It is placed inside the fixture. Its machined surface and the upper fixture surface are maintained at the same level. A nanosecond laser (a wavelength of 1.06 , the laser pulse frequency is 20 kHz, the maximum laser output power is 20 W, the focal length is 75.202 mm, the
Model assessment
In order to analyze the influence of the laser beam scattering and blockage on the initial micro-cracks formation, A 2D finite element (FEM) model was developed, and solid mechanics and heat transfer coupling methods were applied in the numerical simulation task. Fig. 3 Shows the schematic of the computation model, and the physical boundary conditions are also presented. The governing equations are solved using the finite element method through COMSOL Multiphysics software.
Results and discussion
There is a direct relationship between micro-crack formation and stress. In this study, there are two main forces: shock force by bubble collapse and thermal stress. Different bubble sizes have different shock forces, and different the interference coefficient of bubbles leads to various thermal stresses. The influence of ultrasonic power and water layer thickness on bubble size and bubble interference coefficient is significant. The following is a detailed discussion of the formation of
Conclusions
The effect of ultrasonic power and the water layer thickness on the micro-cracks formation has been investigated in ultrasonic-assisted water confined laser processing single-crystalline silicon. The main findings and implications are drawn from this study are as follows:
1. The micro-cracks formed at the tip of the groove bottom, and no micro-cracks were in other positions of the groove cross-section.
2. The larger the bubble size, the shorter the micro-cracks formed. The smaller the bubble
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was financially supported funded by the National Natural Science Foundation of China (NSFC) (62004050), the key project of Guangxi Natural Science Foundation (2019JJD160010), the GUET Excellent Graduate Thesis Program (18YJPYBS01,18YJPYSS02), the Innovation Project of GUET Graduate Education (2020YCXS010), the innovation team of Guangxi high school and outstanding scholar program, and Innovation Project of Guangxi Graduate Education (YXYJYX25, JGY2018061, JGY2019074).
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