Research On High Efficiency And Low Damage Grinding Mechanism And Experiments For SiC Single Crystal

With the rapid development of technology,SiC single crystal become a important third generation of semiconductor materials,because of its superior attributes,such as wide band-gap,high-breakdown electric field strength,high-saturated drift velocity,high thermal conductivity,low coefficient of thermal expansion,strength at elevated temperatures.However,as a typical material difficult to machine,there is no mature machining technology for SiC single crystal,high efficiency and low damage machining technology has become the main obstacle for its industrial application.In this paper,based on the study on the material removal mechanism of SiC single crystal,aiming at the surface quality and material removal rate,an innovative ultra-precision machining process,ELID(Electrolytic in-process dressing)for SiC single crystal,which enables the improvement of surface quality and the elevation of material removal rate is put forward.ELID grinding technology is new technology of ultra-precision grinding,and the oxide film is formed on grinding wheel by electrolytic in-process technology,thus the wheel is in-process dressed.Performances of the oxide film of wheel surface on grinding process and grinding surface quality have important influence.Otherwise,ultrasonic lapping,mechanical polishing and chemi-mechanical polishing(CMP)also are discussed.they have great theoretical significance and applied value to promote the application of SiC single crystal in the semiconductor field.The main research contents are as follows:(1)The oxide layer formed on the wheel surface in electrolyzation plays an important part in ELID grinding.This paper try to modeling and simulating the phenomenon of the oxide layer formed on the diamond grinding wheel surface during pre-dressing in ELID.A general model for ELID pre-dressing process is developed based on fundamental electrochemical laws and ELID precision grinding principle,the effect of voltage and duty ratio of the power supply on the oxide layer formation characteristics of is investigated.With the simulation results effects of these factors on oxide layer growth characteristics are theoretically analyzed.To validate the model and simulation results ELID pre-dressing experiments which have the same pre-dressing condition with the simulation are done.Good correlation of the oxide layer growth behavior and thickness given by the model and the experiment is found.(2)SiC material removal mechanism and ELID grinding mechanism is analyzed,the character and condition of brittle to ductile transition of SiC single crystal,the critical depth of cut,and surface formation mechanism of ductile mode grinding of SiC single crystal are studied,the results show that ELID grinding can realize ductile grinding,this will lower the surface damage and improve the machining eff-iciency.Base on different damage forms,SiC surface layer damage measurement methods are analyzed.Damage measurement methods and the implementation schemes are determined by doing abundant experiments.(3)According feature of the ELID grinding,mechanism of the ELID grinding is analyzed and thermal source and thermal partition models on ELID grinding among work-piece,grinding wheel and electrolyte are established and numerical calculation is carried on.Calculating indicates,ELID grinding thermal mainly are produced by function between grit and work piece,partition of grinding thermal among work piece,grinding wheel,electrolyte is distribution function changed in position,electrolyte and grinding wheel take away most the thermal of grinding.(4)A new electrolysis cathode was successfully developed and used as a core component in the ELID system,including ELID lathe,Cast-iron-bonded Diamond Grinding Wheel,power source.A series of experiments of SiC single crystal substrates are carried out to find the effects of grain dimension,depth of cutting.The relationship between surface roughness,material removal rate and processing parameters are obtained.(5)This paper did a comprehensive analysis on ultrasonic lapping mechanism in theory.The mathematical models of material removal rate(MRR)are established and the nonlinear finite element program Ls-dynais used to simulate the process of ultrasonic lapping to describe the mechanism in further detail.The experimental results show that the smooth high quality surface and material removal rate can be obtained under ultrasonic lapping.