Study On The Mechanism And Technological Rule Of Dicing Sapphire Substrate By Pulsed Green Laser

Sapphire is characterized by high wear resistance,high hardness,high brittleness,chemical stability and so on.It is the most important commercial substrate for blue LED,window material in industrial and national defense field and bearing material.The traditional mechanical dicing has the inherent disadvantages of chipping,crack and tool wear.Furthermore,it is strongly resistant to dry and wet etching.Hence,laser-based dicing techniques have aroused and there is a good prospect of application and development because it has incomparable virtues,such as high speed,narrow groove and non-contact.Experimental and theoretical research of dicing sapphire substrate by pulsed green laser(?=532 nm)have been conducted in this thesis.Firstly,the meaning and importance of laser dicing sapphire,the drawbacks of the traditional dicing methods,the character of the laser dicing technique and the current statues of development home and abroad have been summarized.Secondly,the temperature field and thermal stress field when laser interact with sapphire were researched from the theoretical aspect,and then the ablation threshold and crack threshold were deduced and compared with the experimental ones.Simultaneously,single pulse and multiple pulses experiments were conducted and the micro-morphology of ablated crater and diced groove was investigated to testify the underlying laser-sapphire interaction mechanism is dominated by the thermal one.Thirdly,the effects of process parameters(polarization,focus location,laser pulse energy,scanning velocity and scanning times)on the groove size and dicing quality have been explored based on a single factor experiment.Fourthly,the RSM(Response Surface Methodology)has been applied to model the relationship between groove size(depth and width)and the input parameter(laser pulse energy,scanning velocity and scanning times),optimize the responses,and hence obtain the optimized parameter combination.The main research methods and conclusions are listed as follows:1.The single pulse and multiple pulses ablation experiments were conducted,and the relation between ablated crater size(depth and diameter)and laser fluence and pulse number are obtained.Based on the Fourier heat conduction model,the temperature and stress field model when laser ablate sapphire were obtained,temperature and stress at different laser fluences versus time and location away from the surface were studied.The fitting methods via diameter square and via depth were adopted to determine the ablation threshold from experimental aspect.Results find the calculated ablation thresholds determined by the thermal model,micro-morphology method and fitting via diameter agree with each other,while differ with the one determined by fitting via depth.Hence,ablation threshold and crack threshold were determined at 15 J/cm2 and 84 J/cm2 respectively.Additionally,there are many witnesses of thermal phenomena,such as melting,evaporation hole,crack and sputtering material,when the micro-morphology of ablated crater and groove was checked.To sum up,the underlying green laser-sapphire interaction mechanism is a photothermal one.The XRD experiment found the sapphire single crystalline turned into amorphous,which promoted the absorption and hence the ablation rate.2.The main effect factors during laser dicing sapphire were analyzed,a series of single factor experiments have been designed and conducted to explore the effects of polarization,focus location,pulse energy,scanning velocity and scanning times on the groove size and quality.The following conclusions can be drawn:(1)polarization has a great effect on groove size,a narrow and deep groove can be obtained when the polarization direction is parallel to the incidence plane,while a wide and shallow when vertical to the incidence plane;(2)when focusing away from the front surface of the sample,groove width and depth increases and decrease with the increasing defocus distance,respectively.A deep groove can be obtained when the laser focuses inside the sample to some location,in our experiment,50 ?m is preferable;(3)groove width increases with the increasing pulse energy and is characterized by a linear law,and the groove width increase sharply and then tend to saturate;(4)both groove depth and groove width decrease with the increasing scanning velocity;(5)both groove depth and width increase with the increasing scanning times and follow a linear law.3.The RSM is used to explore the quantitative relationship between process parameters(laser pulse energy,scanning velocity and scanning times)and the groove size(depth and width).The main effects,interaction effects and the second order effects have been analyzed.Consideration the groove size and dicing quality,the optimized parameter combination is obtained and the following verification experiment was conducted.The result found when laser pulse energy is of 150 ?J,scanning velocity of 0.55 mm/s and scanning times of 3,a groove of depth of 148 ?m,width of 19 ?m with good quality can be obtained.