Study On Chemical Mechanical Polishing Of Semiconductor Silicon Wafer With Nano-SiO₂ Slurries And Its Application

With the rapid development of the integrated circuit(IC),the requirement for high surface quality of single crystals silicon wafer keeps continuous increasing.At present,chemical mechanical polishing(CMP)is the only way that can realize global planarization.The study concerning the properties of the CMP slurry,the interactions between slurry and silicon wafer during CMP,the remove rate,and the CMP mechanism is of importance for both theoretical guide and practical application.In this paper,the several important aspects concerning the CMP process of silicon wafer have been investigated using the principles and methods of colloid chemistry,electrochemistry and quantum chemistry.The dispersion and stability properties of the nano-SiO₂ slurries in an aqueous phase has been systematically studied.The wetting and stability of the nano-SiO₂ particles in an aqueous system at different pH were investigated.The influence of dispersion method and addition of surfactants on the stability of the nano-SiO₂ slurries was also investigated. The dispersion behavior and mechanism of the nano-SiO₂ at different conditions were analyzed and discussed using surface zeta-potential, absorbency and adsorption and calculated interaction energy between particles.The research results indicate that the isoelectric point(pH_IEP)of the nano-SiO₂ particles is approximately 2.In acidic media,the particles show a better wetting,while at basic media,a better stability.Their dispersion behavior correlated well with surface Zeta potential.With increasing pH,the surface Zeta potential of the particles increases,leading to a stable dispersion due to electrostatic exclusion.Mechanical stirring and ultrasonic dispersion both promote the dispersion of the particles.However, to keep the stability of the slurries it is necessary to use surfactants. Different types of surfactant show different dispersion mechanism. Non-ionic surfactant,Triton X-100,forms an adsorption layer on the particle surface,which produces pace steric hindrance effect and changes the surface Zeta potential which leads to electrostatic exclusion.Both of them contribute to the prevention of the particles aggregation.Cationic surfactant CPB and ionic surfactant SDBS stabilize the particle dispersion mainly via electrostatic exclusion effect.Using a mixture of Triton X-100/SDBS(1:1)enhances electrostatic the exclusion effect and space steric hindrance effect,significantly improves the dispersion of nano-SiO₂ particles,produces a slurry with 30 day stability,In terms of electrochemical experimental methods,the characteristics and the mechanism of corrosion film-forming on different types of semiconductor wafers and different crystal surfaces were systematically studied with a rotation disc electrode.The relationship between the film-forming and pH,the content of SiO₂,the concentration of H₂O₂ as well as the duration has been analyzed.With a self-made CMP device,the electrochemical behavior of the wafer during dynamic CMP process was further studied.The influence and mechanism of the pressure,rotation rate, the content of SiO₂,pH and concentration of H₂O₂ on the corrosion potential and electrical current density was investigated.The results indicate that the film-forming rate of the Si(100)face is higher than that of the Si(111)face.Film-forming is consistent with Miiller model.The pH of the slurry shows a significant influence on the corrosion potential and electrical current density.At pH=10.5,a highest passivation film was obtained,corresponding to a highest corrosion electrical current density, indicating that the rate of corrosion film-forming and film-removing of polishing is highest.Addition of H₂O₂ in the slurries as an oxidant accelerates film-forming and raises corrosion potential of CMP process, leading to a higher corrosion electric current and accelerating the removal of film.Increasing the pressure,rotation rate,and SiO₂ content enhances the removal of the passivation film.The optimal polishing conditions obtained via experiments are as following:n(100):40kPa,100rpm,5～10wt%SiO₂,pH10.5,1vol%H₂O₂n(111):40kPa,200rpm,5～10wt%SiO₂,pH10.5,1vol%H₂O₂p(100):40kPa,200rpm,5～10wt%SiO₂,pH10.5,2vol%H₂O₂p(111):60kPa,200rpm,5～10wt%SiO₂,pH10.5,2vol%H₂O₂Based on the study of electrochemistry,the polishing rates of n(100) and n(111)type single crystal silicon wafers with nano-SiO₂ slurries at different pressures,rotation rates,SiO₂ contents,pH values,H₂O₂ concentrations and durations were investigated.It is observed that increasing content of SiO₂ in the slurries results in a removal saturation. The polishing rate increases quasi-linearly with increasing pressure and rotation rate,indicting that CMP is a synergic process involving both mechanical and chemical effects.The polishing rate decreases gradually with increasing duration and finally reaches a constant value.On the other hand,a maximum of the polishing rate occurs with increasing pH and H₂O₂ concentration,indicating that a dynamic equilibration established between chemical and physical effect.Under the same conditions,the fact that the polishing rate of Si(100)face is significantly higher than that of Si(111) implies that the CMP process of the silicon wafer comprises of a cycle including forming-removal-reforming of film.The consistence between the study on the dynamic electrochemistry and the study on the remove rate in the CMP process of silicon wafer indicates that electrochemistry can be used as a reliable method to investigate the CMP process and its mechanism,thus providing a new route for the study of the CMP process.Using quantum chemistry calculation method,the mechanism of the chemical reactions involved in CMP process of the silicon wafer was discussed.A silicon cluster model simulating Si(111)surface has been established,allowing us to speculate that the silicon surface obtained in CMP is H-ended.In the simulation,the geometrical conformations of the reactants,product,intermediates and transition states in the reaction potential surfaces were optimized in order to find the reaction routes in the CMP process.The effect of CMP using different bases has been compared. The reaction mechanism of H₂O in the CMP process was studies based on thermodynamics.To simulate the properties of≡Si-O-Si≡cluster,group structure models have been constructed.With these models the dissolving free energy and equilibrium coefficient of the main reactions in this hydration system have been calculated,providing a theory foundation for further investigation.A coarse polishing and a fine polishing slurry(GRACE2040)have been formulated,which are now applied on an industrial scale in the CMP process in GRINM Semiconductor Materials Co.,Ltd,located in Beijing. As coarse polishing and fine polishing slurry,the coarse polishing removal rate of GRACE2040 completely meets the quality demand of GRINM.The life time of the coarse and intermediate polishing pads is higher than the normal values(20h).The geometrical parameter,surface quality,surface roughness and eligibility of the polished silicon wafer are higher than the national and GRINM quality standard.According to GRINM,GRACE2040 coarse and intermediate polishing slurries completely meet the demand of the CMP process.GRINM will suggest to its purchasing department the addition of GRACE2040 to the list of qualified product.