Study On Laser Induced Cavitation Bubbles And Flow Field Distribution During Laser-Induced Backside Wet Etching Sapphire Substrates

Sapphire has many advantages including high hardness, high melting point, high wear resistance, good chemical stability under high temperature and wide optical transmission belt. It is the most important commercial substrate for the third generation of semiconductor materials GaN, new materials for making the smart phone camera, screen protection components, in industry, military, scientific research and high-tech fields has been widely used.Laser-induced backside wet etching (LIBWE) has a good application prospect with advantages of small heat affected zone and good processing quality, it is suitable for punching, cross cutting, cutting of transparent material. The Laser induced bubbles and micro jets in LIB WE has a great influence on the processing, when the liquid gap height is set at a certain value, the etching rate can be enhanced, so it is necessary to study how to make full use of the jets to improve the laser etching processing effect.In this paper, the laser induced bubbles and the micro flow field distribution in the process of laser-induced backside wet etching sapphire in copper sulfate solution have been studied in theory and experiment method. Firstly, the significance and the existing problems of LIB WE have been summarized, Secondly, experiment researches the laser induced bubbles and the micro flow characteristic during the processing, Thirdly, the numerical simulation has been conducted to study the laser induced bubbles and the micro flow characteristic at various gap heights, then forecast the influence of the micro jets on sapphire processing by combining the experimental results with the simulation results. Finally, Verification tests have been conducted according to the predicted results, the enhanced effect mechanism of micro jets has been revealed, some good quality micro-channels are machined. The main research methods and conclusions are listed as follows:(1) Use the high speed photography imaging to explore the bubble dynamic characteristics and the surrounding flow field distribution at various liquid gaps, whose height H is of10mm,3.75mm,2.5mm and1.25mm and0.45mm, respectively, in addition, the influence on the laser induced cavitation dynamic characteristic of the laser pulse energy density and the concentration of solution have been studied, then research the bubble behavior in the manufacturing process. The results showed that the bubble in liquid gap height0.45mm suffers the maximum limitation in the Y direction; the maximum volume of the bubble both increases with the laser energy density and the concentration of cupric sulfate solution, the maximum bubble volume and liquid gap height are two key factors of influencing the dynamic characteristics of the bubbles. When the laser fixed-point process the Sapphire in a mall liquid gap height, the vertical downward jet impacts on the confined plate and then reflect to form two tributaries, interacting with the jets on the bubble left and right, the liquid convection become quicker, this is good for the fluid bring heat to the processing surface and reduce the chances of developing steam bubbles, improving the processing quality, this is good for flow heat dissipation, During the laser linear processing, the jet impacting on the grooving frontier will decrease the jet intensity, so the jet strength in the laser moving direction is significantly less than the jet in the opposite direction.(2) Theoretically research the mechanism of laser induced bubble, explore the internal connection of the laser and the bubble, and use the FLUENT software to simulate the bubble and the surrounding flow field distribution at various liquid gaps during LIB WE, The results show that with the decrease of the liquid gap height, the maximum diameter of the bubble in the incident direction of laser is reduced, in the horizontal direction, it is more easier to form pit on the biggest diameter of the bubbles. The flow field distribution of H=0.45mm which is different from H=10mm includes the following several aspects:In the early stage of the bubble expansion, there is a high-speed flow points to sapphire workpieces. In the process of expansion, the liquid distributed between the bubble and sapphire is squeezed more heavily than other liquid gap, In the process of collapse, the left and right jets obliquely strike on the substrate surface. the fluid field distribution of H=0.45mm is conductive to the composition deposit on the surface. In addition, the maximum pressure in the small liquid gap is bigger than in other situations, it is advantageous to impact, remove and soften the sapphire material, according to this, we can know that decreasing the liquid gap height to a certain value can increase the material removal rate, improve the laser induced backside wet etching sapphire quality.(3) Carry out the laser etching sapphire surface groove process experiment, using the single factor method with the scanning speed, scanning frequency and liquid concentration respectively, and then manufacture the surface microstructure. With laser confocal microscope and scanning electron microscope inspect the machined surface morphology, using the X-ray Energy spectrum of the scanning electron microscope to the processing sapphire sediment content at variable gap height.The Result shows that the sapphire etching depth increase with the increase of scanning frequency, the increase of solution concentration and the decrease of the liquid gap height. The optical microscope and SEM pictures show that the sapphire processing quality of H=0.45mm is better than H=10mm. The solid-liquid interface temperature distribution shows that the highest liquid temperature is below the sapphire melting point. The material is directly removed with a single laser pulse in small probability, the first few laser pulses mainly activated material surface and produce sediments on the material surface, the sediment increase the absorption of laser and the material is removed after the temperature reaching the melting point. EDS composition test results show that with the decrease of the liquid gap height, the deposition amount on whole groove surface increase accordingly. The deposition amount is related to the absorption of laser energy, so decrease the liquid gap can use the bubble dynamics and the flow field to effectively enhance sapphire etching rate.