Controllable Synthesis Of Silica Sol And Application In Chemical Mechanical Polishing

In the integrated circuits（IC）industry,monocrystalline silicon has become the most commonly used wafer material due to its high purity,consistent electrical properties and chemical stability.With the rapid development of IC industry,the reduction of feature size of devices and the realization of high-density devices,the requirements of surface planarization of monolithic silicon wafers for IC are becoming higher and higher.Chemical mechanical polishing（CMP）is a combination of chemical polishing and mechanical polishing,which has the advantages of high removal rate,high surface smoothness and fewer surface defects.As the only effective method currently to achieve global planarization,CMP is widely used in the precision processing of monocrystalline silicon substrates.Silica and cerium dioxide are the most common abrasives for CMP,in which silica is the best candidate for its moderate hardness,good dispersion,good stability and easy storage.In the process of fine polishing,the large size silica abrasives are easy to come into being scratches and polishing haze on the silicon wafers,while the small size（<50 nm）silica abrasives can achieve the global flattening of the silicon wafers.Therefore,it is the most critical to control the size and dispersion uniformity of abrasives.Among the reported synthesis technologies of small size nano silica,there are some technical difficulties such as complex preparation process,difficult removal of additives and low silica content.And silica fine polishing abrasives for domestic chip manufacturing are basically dependent on imports.In order to solve the technical difficulties and domestic supply problems of CMP fine polishing abrasive silica,the silica sols（<50 nm）with uniform size,good dispersion and stability were prepared successfully with improved the traditional sol-gel method by adjusting the pH of initial water phase,reaction temperature,stirring rate and other factors,which were used as the abrasive for the polishing of monocrystalline silicon wafers to improve the surface quality of the wafers.（1）Regulated synthesis of silica sols.In the reaction system of ammonium-ammonium chloride buffer solution,silica sols with uniform particle size and good dispersion and stability below 50 nm were successfully synthesized using tetraethyl orthosilicate（TEOS）as silicon source without any other additives.By adjusting the influence factors such as pH of initial water phase.reaction temperature and stirring rate,the influence of various factors on the particle size of nano silica was investigated by means of TEM.DLS.XRD,XPS and other characterization methods.The experimental results show that the pH of initial water phase is the major factor affecting the particle size of nano silica.When the pH of initial water phase is in the range of 10.0-10.5,the particle size of nano silica decreases with the increase of pH,which reduces from 64.6 nm to 30.5 nm.The particle size of nano silica and solid content of synthetic silica sols can be adjusted by changing adding amount of TEOS.Under experimental conditions in this paper,the minimum particle size of the synthesized silica sol is 11.8 nm,and the maximum solid content is 13 wt%.After six months,the silica sol can still maintain the original size and morphology,which proves that its stability is excellent.（2）Silica sols were used as abrasive to prepare polishing slurry for fine polishing of silicon wafers.The polishing slurry was blended by mixing the synthesized silica sol with oxidant.By changing polishing pressure,polishing time,oxidant concentration,abrasive concentration and abrasive particle size,the influence of polishing conditions on polishing rate and surface quality of silicon wafers were explored by means of AFM,optical microscope and analytical scales equipment.When the polishing pressure is 36.8 kPa,the concentration of oxidizer H₂O₂ is 5 wt%,and the concentration of abrasive is 11 wt%,the silicon wafers are polished for 1.5 h by using the prepared silica sols of 22.4 nm,30.5 nm and 42.0 nm,respectively,resulting that the smooth surface of silicon wafers without scratches,pits and other defects has the surface roughness of less than 0.1 nm.Hence,the silica sols synthesized by the improved sol-gel method can be used as fine polishing abrasive for monocrystalline silicon wafers,showing good polishing performance and achieving precision machining of monocrystalline silicon wafers,which lays a foundation for the localization of key grinding materials.