The mechanism of micro-cracks formation in ultrasonic-assisted water confined laser micromachining silicon

Highlights

• The effects of ultrasonic power and water layer thickness on the micro-crack formation were experimentally investigated.

• The influence of the laser-induced bubble size on micro-crack formation is analyzed by experiment and theoretical calculation.

• The influence of the interference coefficient of bubbles on micro-crack formation is analyzed by numerical simulation.

Abstract

In the ultrasonic-assisted water confined laser micromachining silicon, micro-cracks may generate. The effect of ultrasonic power and thickness of the water layer on the groove section’s micro-cracks is discussed systematically to control the micro-cracks formation and improve the processing quality. The experimental and simulational results indicate the maximum radius of the bubbles and the bubble interference coefficient have an important effect on the micro-cracks formation. The larger the maximum radius of the bubble, the shorter the micro-cracks formed, and the smaller the bubble interference coefficient, it is easier to produce micro-cracks. The bubble interference coefficient and the maximum radius of the bubble can be affected by the water layer thickness and ultrasonic power. Therefore the micro-cracks can be effectively controlled by selecting the appropriate water layer thickness of 1.5mm and ultrasonic power of 195W in this study. The study is obtained from this study provides a guide for micro-cracks under ultrasonic-assisted solution confined laser processing.

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.