Study On The Ultra-precison Polishing Technology And Mechanism Of Silicon Carbide Substrates

As one of the most representative of the third generation wide band gap semiconductor materials,silicon carbide(SiC)has been considered as an ideal material for high-temperature and high-frequency optoelectronic devices because of its outstanding mechanical and physical properties,such as wide bandgap,high breakdown electic field,high thermal conductivity and high saturation electron transfer rate,low dielectric constant and high temperature tolerance.It has stringent surface quality requirements for the application of SiC substrate,due to the fact that the surface quality of SiC substrate is crucial for its device quality.Currently,chemical mechanical polishing(CMP)is one of the most effective way to produce ultra-smooth undamaged SiC substrate surface.However,it is difficult to obtain this desired surface with good material removal rates(RRs)because of its high mechanical hardness and strong chemical inertness.Furthermore,the polishing mechanism of the SiC substrate during CMP process was not fully understood.In this paper,the chemical and mechanical mechanism of the 6H-SiC substrate during CMP process were analyzed and the material removal model for 6H-SiC substrate was established based on the optimization of 6H-SiC wafer polishing slurry,which was of great theoretical significance and application value to improve the ultra-precision machining technology and application level of SiC wafer.Firstly,the influence of the polishing slurry compositions on the polishing performance was studied based on the results of orthogonal experiments during 6H-SiC substrate CMP.The results indicated that the pH value of the polishing slurry has the greatest influence on the material removal rate,followed by particle concentration,and oxidant concentration has the least impact.The material removal rate of the KMnO4-based slurry was higher than the H2O2-based slurry under the same silica particle concentration and pH value due to the stronger oxidation ability of KMnO4.Both of the ceria-based slurry and alumina-based slurry exhibit a higher material removal rate in acidic KMnO4 environment,while the material removal rate of the silica-based slurry was relatively low under the acidic condition,Furthermore,the polished substrate surface quality could be deteriorated due to the high hardness of the alumina particle,which was easy to cause surface damage on the polished surface.The material removal rate of C-face was significant higher than that of the Si-face.Based on the XPS analysis of the static corrosion and the polished 6H-SiC surface,the changes of the elements on the substrate surface in composition,content and chemical state were studied.The results indicated that the atoms on the substrate surface could be further oxidized into the oxidation species(e.g.Si-C-O,Si-Ox-Cy,Si-O2,Si4-C4-x-O2,Si4-C4-O4 C-O and C=O)in the presence of oxidizer,and the higher relative proportion of oxidation species on the substrate surface correlated with the higher oxidizing strength of the oxidant.It was found that there were differences between the different crystal faces of the 6H-SiC wafers in the degree of the oxidative properties of the atoms and the difficulty of the removal of the oxidized products.The atoms on C-face are more susceptible to be oxidized than that on Si-face and the oxidation products were more easily removed,causing the C-face was easy to obtain a higher material removal rate than Si-face.The interaction forces between the substrate surfaces and the abrasive nanoparticles were investigated using contact angle measurement,zeta potential measurements,scanning electron microscope(SEM)observations.Meanwhile,these attractive/repulsive forces were also estimated using the Derjaguin-Landau-Verwey-Overbeek(DLVO)theory.The results indicated that the wettability of SiC wafer/water interface was better in strong acid or base environment than in mild conditions,especially in strong base conditions.Silica particles are expected to be attracted by SiC surface in the pH range of 2-5,while ceria particles surface is expected to be attracted by SiC for the pH values from 5 to 7.Silica particles are prone to adhere to the Si-face 6H-SiC surface at pH 2 and pH 4 and repel at pH 6,pH 8 and p H 10,while ceria particles tend to adhere to the substrate surfaces in acid or base environment.The influence of different oxidants and abrasive particles on the tribological behavior of 6H-SiC wafers during CMP were investigated through friction tests using pin-on-disc technique;the effects of different polishing pressure and frequency on the tribological behavior of 6H-SiC wafers in CMP were investigated through friction tests using ball-on-disc technique.In the absence of abrasive particles,the presence of oxidants would cause a new oxide layer to be formed on 6H-SiC substrate surface,reducing the friction coefficient between the pad and the substrate surface,and the stronger the oxidation capacity,the lower the friction coefficient.The friction coefficients between the pad and the 6H-SiC substrate surface could be reduced by the adhesion of silica or ceria particles which acted as ball bearings in the absence of oxidants.However,there was a significant difference in the friction coefficients and polishing performance between the adhesion of silica and ceria particles in the presence of KMn O4 below pH 5.The friction coefficients between the ball and the 6H-SiC substrate surface lubricated under aq.KMnO4 was higher than those under DI water,indicating that the chemical oxidation of the surface atoms was strengthened in the presence of the oxidant,improving the machinability of the wafer surface and facilitating the surface mechanical wear of the abrasive abrasion.The mechanical removal of the abrasive particles during sliding process increased with increase in polishing pressure and decreased with the increasing sliding frequency.Finally,based on the micro-contact mechanism between the polishing pad and the wafer,considering the deformation of the micro asperity on the polishing pad,the actual contact area between the polishing pad and wafer surface was compared under the elastic-plastic deformation and the elastic deformation of the asperity;The influence of the intermolecular force DLVO force on the total bearing external force of the abrasive particle and the oxidized surface indention depth pressed by the particle was studied based on the abrasive particle in the polishing pad/abrasive particle/wafer three-body force balance;Furthermore,the effect of the DLVO on the coordination of chemical and mechanical during CMP process was analyzed.The results indicated that the contact area under the elastic-plastic deformation was smaller than that under the elastic deformation.The total bearing external force of the abrasive particle and the oxidized surface indention depth pressed by the particle could be enlarged by the DLVO attractive interaction between the abrasive particle and oxidized surface,while this external force and indention depth could be reduced by the repulsive interaction.When the repulsion intermolecular DLVO force existed between the particle and the wafer,the oxidation proportion of the surface atoms could be improved.And the mechanical removal of the abrasive particle could be increased by the attraction intermolecular DLVO force existed between the particle and oxidized wafer surface.