Research On Liquid-assisted Laser Cutting Of Silicon Wafer

With the rapid development of the integrated circuit industry,the demand for precise cutting technology of monocrystalline silicon wafer is increasingly urgent.Those defects such as chipping and microcracks are inevitably induced due to the contact stress generated in the traditional mechanical sawing process,so it is difficult to adapt to cutting the thin wafer.Laser technology is a non-contact processing method,the materials are removed instantaneously by using the focused high-energy beam.Therefore,the laser has been widely used in cutting of hard-brittle materials at present.In this paper,a preliminary survey in traditional laser cutting was carried out and summarized several common defects,for example,thermal damage,recast layer and the deposition of ablated material.Therefore,this paper is devoted to improve the processing quality by utilizing liquid-assisted laser cutting.The influences of laser parameter and processing condition on cutting performance were investigated,and the mechanism of liquid-assisted laser machining was analyzed macroscopically and microscopically by combining the cutting process and machining morphology.Eventually,the correlation between bubble behaviors and laser precision cutting was established.The detailed works studied in the paper are as follows:(1)Experimental study on liquid-assisted nanosecond(ns)laser cutting of silicon wafer.The influences of repetition frequency and scanning speed on cutting properties were studied,the influence mechanism of bubble behaviors on cutting quality was analyzed macroscopically and the effect of ethanol concentration on the behaviors of adhered bubble and dispersed bubble was revealed.The results show that both the groove width and the ablated zone were proportional to the repetition frequency and inversely proportional to the scanning speed.The improvement of surface quality was prevented by the adhered bubbles and the ethanol solution was found to be effective in changing the surface wettability.In this way,the dwell time and number of adhered bubbles were decreased significantly.Meanwhile,the density of dispersed bubbles was increased,conversely,causing the stronger laser extinction effect.(2)Experimental study on liquid-assisted femtosecond(fs)laser cutting of silicon wafer based on time-resolved shadow imaging.The time-resolved shadow imaging system was set up and applied to study the microscopic processes of fs laser ablation of liquid-immersed silicon wafer.The influence of pulse energy on the evolution property of cavitation bubbles(CB)was revealed,the behaviors of persistent bubbles(PB)were described,the influence mechanism of CB and PB on microgroove machining was analyzed microscopically and the corresponding solutions of the quality improvement were proposed accordingly.The results show that both the size and lifetime of CB were proportional to the pulse energy and the defocusing effect arisen from CB could lead to the formation of non-uniform microgrooves.The strategy of adjusting the repetition frequency on the basis of the lifetime of CB was proposed.Under the experimental conditions in this paper,the defocusing effect was greatly reduced when the repetition frequency is lower than 10 kHz.Moreover,the PB adhered on the microgroove would bring about more serious defects in the reciprocating machining process.The adhered PB would explode when reacting with the laser beam.The strong shock waves were released after the explosion of PB,and leading to severe damage to the surface morphology.The adhered PB could peel off timely from the surface when the ethanol solution was added,the explosion and impact could be avoided effectively and the processing quality could be improved significantly.(3)Experimental study on the effects of focusing condition on the processing properties in laser micro-grooving of silicon wafer in liquid.The processing efficiency and stability of fs laser ablation of liquid-immersed silicon wafer under three focusing conditions were studied.The surface morphology and sidewall quality of the microgrooves were compared,and the influences of the focusing condition on the behaviors of CB and PB were revealed.The results show that both the width and depth of the microgrooves machined under negative defocusing condition were increased,representing a higher processing efficiency.The microgroove profile was more uniform and its sidewall was much smoother as well,meaning a better processing stability.Both the size and lifetime of CB induced in negative defocusing condition were enhanced and the corresponding evolution process followed the Rayleigh distribution law.Moreover,the movement tendency of the dispersed PB was similar to backward flush,and the distance between the adhered PB and the microgroove was relatively enlarged.