Research On Step Flow Rate Model And Process Model For The Wet Etching Of Sapphire

Sapphire is widely used in the microelectromechanical systems(MEMS)for its excellent transparency,insulation and chemical stability.Wet etching is the preferred method for fabricating patterned sapphire substrates.The etchant is usually a mixed solution of concentrated sulfuric acid and concentrated phosphoric acid with different volume ratios,and the etching temperature is usually above 200°C.Due to the complicated atomic structure and anisotropy of sapphire,more structural planes are produced in wet etching compared with other crystal materials,and will evolve with the etching process.Therefore,it is particularly important to predict and control the wet etching of sapphire.The mechanism,simulation tool and experimental analysis of the wet etching of sapphire also need to be further studied.In this thesis,firstly,etch rate distributions of sapphire are obtained by the wet etching experiments of sapphire hemispheres,and the anisotropy of etch rates is analyzed according to the atomic structure of sapphire.The Monte Carlo simulation model of wet etching of sapphire is established to realize the effective simulation of etch rates and etched structures.Finally,surface morphologies of sapphire hemispheres as well as the formation and evolution of etched structures on different tangential wafers under different experimental conditions are analyzed.The main research contents of this thesis are as follows:(1)The wet etching experiments of sapphire hemisphere are carried out,etch rate distributions at different temperatures(H2SO4:H3PO4=3:1,202°C,223°C and 236°C)and concentrations(236°C,H2SO4:H3PO4=1:1,3:1 and 6:1)are obtained,and the influence laws of temperatures and concentrations of the etching solution on etch rate distributions are obtained.(2)Based on the crystal structure and atomic composition of sapphire,compound atomic groups(crystal units)that constitute the step-terrace structures of crystallographic planes of sapphire are identified and defined.Based on the positions and interactions of crystal units on the step-terrace structures,types of crystal units are further divided.(3)The general formula for calculating etch rates of crystallographic planes is derived based on the etching process of crystallographic planes,and the relationship between etch rates and atomic structures of crystallographic planes is established.By comparing the calculated and experimental results of etch rates,the effectiveness of the step flow rate model in calculating wet etch rates of sapphire is verified,and the factors affecting the anisotropy of wet etch rates of sapphire are summarized.(4)The six-index classification method which can effectively classify the types of surface atoms of sapphire and the removal probability function(Al-RPF)of surface atoms are proposed,and the relationship between the removal probabilities and the surrounding neighbor configurations of surface atoms is established.The Monte Carlo simulation tool for the wet etching of sapphire is developed to realize the effective simulation of etch rates and etched structures.(5)According to the errors between the simulation and experiment of the etch rate distribution,assisted crystallographic planes are added to further improve the simulated results of etching rate distributions and etched structures under different concentration conditions.According to the errors between the simulated and experimental rates under different temperature conditions,the removal probability function(M3-Al-RPF)of surface atoms of sapphire based on Transition State Theory is proposed to realize the accurate simulation of etch rates under different temperature conditions,so as to improve the efficiency of the Monte Carlo simulation.Finally,the modified method of the removal probability function of surface atoms of sapphire is successfully applied to the Monte Carlo simulation field of the wet etching of monocrystalline silicon and quartz.(6)The formation law of surface morphologies of sapphire hemispheres under different concentration conditions is obtained based on atomic structures and etch rates of crystallographic planes.The undercutting rate distribution of characteristic planes on C-plane wafers is obtained by the maximum positive curvature(MPC)recognition method,mask shapes and sizes of etched structures on C-plane wafers are reasonably designed,the formation and evolution laws of etched structures on C-plane wafers of sapphire are obtained.The influence law of concentrations of etchants on etched structures is futher obtained based on the undercutting rate distributions of characteristic planes at different concentrations.Finally,the undercutting rate distribution of characteristic planes on R-plane wafers is obtained by the maximum positive curvature(MPC)recognition method and the rotation matrix,and the formation and evolution laws of etched structures on R-plane wafers are obtained.In this thesis,the established step flow rate model explains the anisotropy of etch rates of sapphire,the established Monte Carlo simulation model realizes the effective simulation of wet etching of sapphire,and the analysis of surface morphologies and etched structures reveals the wet etching law of sapphire.These provide a foundation for the application and development of sapphire.