Research On The Stability Of Acidic Alumina Nanoparticles And Its Application In Chemical Mechanical Polishing

SiC,the third generation semiconductor material,has been widely used in many aspects.With its widely application,the higher degree of surface smoothness is required.And so far,chemical mechanical polishing is the only technique which can realize SiC surface smooth without defect.However,CMP is a complex process which is vulnerable to many factors such as the polishing liquid pH,oxidant,polishing parameters and electrolyte.Certainly,the abrasive and its stability are the main influence factors.Scratches caused by.polishing or grinding during polishing which are strongly related to the abrasive and its stability are the main factors that causing surface defects.It will cause a series of irreversible defects such as scratches and spots on the surface of the wafer If the abrasive is too hard and inversely,the material removal rate is slow so the uneven material cannot be removed.In addition,if the abrasive particles gather to form large aggregates,even only a very small amount,can cause the scratches of the wafer.Therefore,it is the primary problem of chemical mechanical polishing to select the abrasive with suitable hardness and improve its dispersion stability.Nowadays,alkaline silica polishing slurries are widely used in SiC CMP.Bu the material removal rate is too low due to its hardness,causing a series drawbacks such as higher costs and long time.So,the alumina abrasive(mohs hardness 9)becomes the research hot spot.At present,many researchers focuses on the alumina stability in alkaline conditions.However,there are less research on acidic alumina stability especially in strong oxidizer solution.But studies have shown that potassium permanganate is one of the most effective antioxidant especially under the acidic condition.However,the surfactants and polymer dispersants which are commonly used as alumina dispersants can be easily oxidized by potassium permanganate.Therefore,it is of great practical significance and application value to study the stability of alumina nanoparticles in the water dispersion system of strong oxidants under acidic conditions.Based on this,this paper introduces a new method that can be used to improve the stability of alumina nanoparticles in silicon carbide CMP slurry and simulate the effect of silica and Mn oxide which are produced by grinding or polishing on alumina dispersion stability.This article mainly includes three parts:The first part,we studied the acidic alumina nanoparticles dispersion stability in four salts solution which are commonly used in CMP and discussed the effect of pH,ionic strength,electrolyte valence and aluminum nitrate on the stability of alumina.The critical concentration concentrations of the four salts were KNO3 0.15M,KMnO40.04M,K2S2O8 0.0015M,and K3Fe(CN)6 0.003mM.Adding aluminum nitrate can make alumina dispersion slurry pH stable in 24h and have acid-base buffering effect.In addition,adding suitable amount of aluminum nitrate can improve the alumina dispersion stability in CMP slurry.In the second part,we simulated the effect of silica and Mn oxide adsorption which was produced in grinding or polishing on the alumina stability as well as silica concentration,size,ascorbic acid concentration,aluminum nitrate and surfactant.We proved that the stability of alumina became worse due to silica and manganese oxide adsorption.We can improve the system stability by adding aluminum nitrate and surfactant because aluminum nitrate can adsorb the silica and Mn oxide to reduce the interaction between alumina and them.For the system of alumina-silica,the best concentration of aluminum nitrate is 0.3wt%.In the third part,we studied the the application of acidic alumina slurry in SiC chemical mechanical polishing.The surface roughness and smoothness of silicon carbide were characterized by atomic force microscope and optical microscope.In this method,the material removal rate increased significantly especially the C surface MRR can reach 4.85 mu m/h in 0.4wt%KMnO4 solution.There are still some scratches because of the large pH drift 3.5-5.2.The best KMnO4 concentration is 0.25wt%s.After adding aluminum nitrate,the MRR of Si face was 1u m/h and pH drift is small from 3.5 to 3.9 so the alumina nanoparticles are stable and the surface smoothness and roughness are obviously improved.