Study Of Effects And Mechanism Of Chemical Additives On Ultrasonic Chemical-Mechanical Polishing For Sapphire

In recent years,China’s LED chip market size,output value and number of enterprises have maintained a sustained growth trend.As one of the most important substrate materials for LED chips,sapphire has always been a hot research topic.With the development of technology,higher requirements are put forward for chip substrate materials.Sapphire wafer is a typical hard and brittle material.Thus the traditional chemical mechanical polishing(CMP)has been difficult to meet the market’s high requirements for its processing efficiency and polished surface quality.The research has shown that ultrasonic machining can achieve efficient and high-quality fabrication of hard and brittle materials such as sapphire,silicon,and silicon carbide.Therefore,our team has proposed a new process of ultrasonic assisted chemical mechanical polishing(UV-CMP)for sapphire,and proved the effectiveness of the new process.Polishing fluids are crucial during sapphire UV-CMP.They are composed of a variety of substances,including deionized water,abrasive particles,dispersants,pH regulators,and surfactants.The main function of dispersant is to prevent agglomeration of abrasive particles and enhance the stability of polishing fluid.The role of a pH regulator is to adjust the pH value of the polishing solution and maintain it stable.It also promotes the formation and removal of a reactive layer on the surface of the material.The action of surfactants is to reduce the surface tension of the polishing liquid and increase the spatial resistance between the abrasive particles.In this thesis,the effects of dispersants,alkaline pH regulators and surfactants in slurries on chemical corrosion and material removal mechanism in sapphire UV-CMP were studied.Firstly,the influences of three dispersants,polyethylene glycol,sodium polyacrylate,and sodium hexametaphosphate,on sapphire UV-CMP were investigated.Their effects on the chemical reaction and mechanical interaction between silica particles and sapphire surface were explored through X-ray photoelectron spectroscopy(XPS),scanning electron microscopy(SEM),zeta potential and particle size analysis.The results indicate that ultrasound and polyethylene glycol can synergistically promote the chemical reaction and sapphire removal rate.However,sodium polyacrylate and sodium hexametaphosphate will inhibit chemical reactions.Polyethylene glycol can affect the aggregation or dispersion of polishing particles in sapphire UV-CMP,which in turn influences chemical reactions and mechanical removal.Different concentrations of polyethylene glycol have different effects on sapphire UV-CMP.Appropriate concentrations are conducive to accelerating both chemical and mechanical action.However,too low or too high concentrations will cause a reduction in material removal rates.When the content is 0.2%,the synergistic effect of chemical and mechanical interaction between the abrasive and sapphire surface is the best.The sapphire removal rate reaches 48.5 nm/min,and the surface roughness after polishing is 0.16 nm.The feasibility of using green organic bases instead of traditional inorganic bases in sapphire UV-CMP was further probed.The effects of green organic bases sodium metasilicate(SMSN),aminopropanol(AMP)and traditional inorganic base KOH on chemical and mechanical properties of pH regulators were explored by XPS,SEM,zeta potential and particle size analysis.Compared with SMSN and KOH,green organic alkali AMP has the best polishing effect when the slurry pH value is 11.Therefore,AMP can replace KOH in sapphire UV-CMP as a green alkaline pH regulator.However,with the increasing slurry pH value adjusted by the three basic reagents,the sapphire removal rate significantly decreases.The removal rate of the organic base declines even more.The main reason for the decrease in removal efficiency is the increase in both particle aggregation and electrical potential between abrasive particles.It results in an increase in the distance between particles.This apparently reduces the amount of abrasive particles involved in mechanical wear during mechanical removal,weakens the mechanical effect and leads to an obvious reduction in the removal rate.Finally,the impacts of cationic surfactant cetyltrimethylammonium bromide(CTAB),anionic surfactant sodium dodecylbenzene sulfonate(SDBS),and nonionic surfactant fatty alcohol polyoxyethylene ether(AEO)on sapphire UV-CMP were researched.The functions of three different types of surfactants on the chemical products,abrasive adsorption and material removal rate of sapphire surface were investigated by XPS,SEM,surface tension,zeta potential and particle size analysis.The results imply that the hydrogen bonding of non ionic surfactant AEO causes wear particle aggregation and particle size increase to some extent.The electrostatic repulsion of anionic surfactant SDBS results in liitle change in the size of the abrasive particles.The electrostatic attraction of cationic surfactant CTAB brings apparent aggregation of wear particles and increase in particle size.The surface tension is proportional to the material removal rate within a certain range.The polishing effect is the best when 0.1% CTAB is added,with a sapphire material removal rate of 80.50nm/min and the polished surface roughness of 0.20 nm.