Controllable Synthesis Of Ceria Abrasive And Study On Its Chemical Mechanical Polishing Performance

As the third generation semiconductor material,silicon carbide（SiC）has the advantages of high breakdown electric field strength,high saturated electron drift velocity,wide band gap,low dielectric constant,high thermal conductivity,and so on.It has important applications in space exploration,nuclear energy development,communication and other fields.However,SiC wafer has the characteristics of brittlenes,high hardness,and strong chemical inertia,so it is difficult to process.Chemical mechanical polishing（CMP）is the only method whcih can achieve global flattening of SiC wafer.Cerium oxide（CeO₂）is a common polishing abrasive.The chemical mechanical polishing performance of SiC wafer is affected by the morphology,size and size distribution of CeO₂.The regular morphology and uniform size distribution of CeO₂ abrasive can improve the polishing performance of SiC wafer..However,commercial CeO₂ abrasive particles are always in irregular shape and uneven in size distribution,which makes it difficult to obtain good polishing performance.Therefore,in this paper,octahedral and spheroid CeO₂ abrasives with uniform particle size distribution were prepared by solvothermal method.The effects of different types of surfactants on the dispersion stability and polishing performance of the two kinds of CeO₂ abrasives were studied.The removal mechanism of CeO₂ abrasive particles on Si surface of SiC wafer was analyzed by finite element simulation,friction and wear experiment and material removal model.Firstly,CeO₂ were synthesized by solvothermal method,and the morphology of CeO₂ was controlled by changing the reaction parameters.The synthesized CeO₂ was characterized by Infrared spectroscopy（FTIR）,X-ray diffraction（XRD）,Transmission electron microscopy（TEM）,Scanning electron microscopy（SEM）and UV-vis spectroscopy.It is found that the morphology of CeO₂ can be controlled by changing the concentration of Ce³⁺.With the increase of Ce³⁺concentration,the morphology of CeO₂ can change from spherical to octahedral.When Ce³⁺concentration is 0.05 mol/L,alcohol-water volume ratio is 3:1,and PVP dosage is 1 g,spheroid CeO₂ with particle size of about 150 nm can be obtained.When the concentration of Ce³⁺is 0.10 mol/L,alcohol-water volume ratio is 3:1,and PVP dosage is 2 g,octahedral CeO₂ with an edge length of about 150 nm can be obtained.The surface of octahedral CeO₂ particles has more Ce³⁺and oxygen vacancy than that of spheroid CeO₂ particles,and of which the surface chemical activity is stronger.Secondly,the effects of different physical dispersion modes,p H values,different surfactants and their concentrations on the dispersion stability of the two kinds of CeO₂abrasives were studied by UV-vis spectroscopy.The chemical mechanical polishing experiments on Si surface of SiC wafer were carried out by different polishing slurry with different surfactants.It is found that when p H value is 2,CTAB concentration is0.015wt%or SDBS concentration is 0.005wt%or PEG-2000 concentration is0.015wt%,the dispersion stability of the polishing abrasives is better.As using octahedral CeO₂ abrasive,the polishing effect is better when 0.015wt%CTAB is added.As using spheroid CeO₂ abrasive,the polishing effect is better when 0.010wt%PEG-2000 is added.Compared with the octahedral CeO₂ abrasive,the spherical CeO₂abrasive has lower polishing efficiency,but can achieve better surface quality.In addition,the mechanism of chemical mechanical polishing of CeO₂ abrasive on SiC wafer was studied by finite element simulation and contact mechanics theory.By finite element simulation,it is found that when the octahedral CeO₂ is in point contact with SiC wafer,the stress and strain are the largest,and the maximum shape variable of the wafer is 5.2330 nm.When spherical CeO₂ particles contact with SiC wafer,the maximum shape variable of the wafer is 3.7205 nm.Based on contact mechanics theory,it is found that the deformation between abrasive particles and polishing pad is elastic,and the deformation between abrasive particles and wafer is elastic as well under the conditions of polishing test in this paper.There are three states of non-contact,partial contact and complete contact between polishing pad and wafer.The three states of contact are analyzed respectively,and the influence factors of the indentation depth of abrasive particles embedded in wafer under different contact states are obtained.Finally,the tribological behavior on Si surface of SiC wafer was studied by friction and wear experiments,which were influenced by the morphology of CeO₂ and the type of surfactant.It is found that the friction coefficient can be improved by adding CTAB into the polishing slurry with octahedral CeO₂ as abrasive.The friction coefficient reaches the maximum with no surfactant into the polishing slurry with spheroid CeO₂as abrasive.The forming process of soft layer and its effect on polishing performance are analyzed.It is found that the soft layer is mainly produced by the chemical corrosion of KMn O₄ on the wafer surface.When the indentation depth of CeO₂ on wafer is equal to or close to the thickness of the soft layer,excellent polishing performance can be obtained.