Study on Chemical Magnetorheological Compound Finishing Mechanism of Single Crystal SiC

Author:Liang Hua Zhuo

Supervisor:yan qiu sheng


Degree Year:2019





With the rapid development of high and new technologies such as 5G communication and artificial intelligence(AI),higher requirements are put forward for the development and manufacture of integrated circuits(IC).Single crystal SiC is the representative of the third generation semiconductor materials with wide bandgap,which has huge demand in LED lighting,military defense,power electronics,5G communication,automotive electronics and other fields.However,due to the high hardness,brittleness and chemical stability of SiC materials,the application of single crystal SiC as a substrate is limited by the difficulty and high cost of global ultra-smooth flattening.In this paper,a method of chemical magnetorheological compound finishing(CMRF)based on Fenton reaction is proposed to process SiC,which combines the flexibility,high efficiency and controllability of MR polishing with the characteristics of low damage and low surface roughness of CMP.An ultra-smooth polishing technology with high efficiency and high precision is explored.Firstly,the feasibility and effectiveness of Fenton reaction on oxidation of single crystal SiC were analyzed.The concentration of hydroxyl radicals produced by Fenton reaction and its effect on the chemical reaction rate of SiC were determined by the corrosion mode of static immersion in Fenton reaction solution on the surface of single crystal SiC and the means of visible spectrophotometry.Through energy spectrum analysis,it was found that a large proportion of oxygen elements appeared on the surface of SiC.The hydroxyl radicals produced by Fenton reaction could oxidize SiC effectively,and the hardness and elastic modulus of the oxide layer were much lower than those of the original surface.Experiments results show that carbonyl iron powder as a catalyst has better effect.The lower the pH,the larger amount of hydroxyl radicals,and the hydroxyl radicals can react with SiC surface to form a soft SiO2 oxide layer.In acidic condition,the lower the pH value,the better the polishing effect.Under alkaline conditions,the SiO2 oxide layer can be converted into silicate polishing and polishing effect is also betterSecondly,according to the material removal behavior of single crystal SiC on chemical magnetorheological compound finishing,different components of polishing fluids were designed to carry out polishing experiments,and the role of mechanical,chemical and their interaction in polishing process was quantitative studied.The results show that the material removal form is abrasive mechanical removal.Among the factors affecting material removal,mechanical removal plays a dominant role,which is far greater than chemical removal.Abrasive concentration in polishing fluid determines the ability of material mechanical removal.The concentration of carbonyl iron powder affects the stiffness of magnetorheological polishing pad,the binding force of abrasive particles,the contact state between catalyst and workpiece surface and the ability of chemical reaction.The greater concentration of abrasive and carbonyl iron powder,the greater the material removal ability of CMRF,the greater the mechanical removal proportion,and the smaller the material removal proportion caused by chemical reactionThirdly,aiming at the process of chemical magnetorheological compound finishing,we analysed the characteristics of polishing forces(normal force Fn and tangential force Ft)in a machining process by using the Kistler 9171A 3-D rotating dynamometer.The influences of rotation speeds of magnetic poles and the workpiece,abrasive concentration,concentration of carbonyl iron powders,working gaps,and deflection distance of the workpiece on magnetorheological polishing forces with dynamic magnetic fields were investigated.The results showed that polishing forces of polishing pads with static magnetic fields were significantly attenuated.For MR polishing pads under a dynamic magnetic field,polishing force signals showed remarkable changes in dynamic stability,showing a significant dynamic magnetorheological effect.The polishing force Fn and Ft increases with the increase of rotational speed,decreases with the increase of processing gap,increases with the increase of abrasive particle concentration and carbonyl iron powder concentration,and increases slightly with the increase of swing amplitude.The variation of workpiece rotational speed and processing gap has a great influence on the range of polishing force.Fourthly,in order to study the material removal characteristics of chemical magnetorheological compound finishing,a force model for polishing pads based on magnetorheological(MR)effects was built.Through the theory of solid-phase particles,this study conducted a force analysis of carbonyl iron powders and abrasives and calculated polishing forces of a single polishing pad based on MR effects on a workpiece surface.Based on this,according to the Preston equation,a material removal model was established.By conducting the CMRF test on single crystal SiC wafers,it is found that the test results were consistent with theoretical calculations.Finally,through the systematic experiment of chemical magnetorheological compound finishing on single crystal SiC substrate,the optimum technological parameters were obtained.After 120 minutes chemical magnetorheological compound finishing of single crystal SiC with original roughness of about Ra 40 nm,the surface roughness of 2" single crystal SiC is below Ra 0.1 nm,achieving global super smooth flattened machining surface.