The Etching Mechanism And Process Model Research For Quartz Anisotropic Wet Etching

Although the anisotropic etching of quartz has been widely used in the fabrication of microelectromechanical systems(MEMS),the etching process and results are difficult to control and predict because of its complicated anisotropic etching characteristics.This study combines wet etching experiments and the Evolutionary Monte Carlo(EMC)method to describe the global etch rates,etched structures and surface morphologies of quartz in different etching conditions,as well as reveals the anisotropic etching mechanism of quartz from the perspective of microscopic atomic energy.In addition,the etching process of quartz is analyzed based on the step flow theory at the atomic level,and the roles of different types of atoms in the etching process of crystal plane are also explained.The main research contents are as follows:Firstly,based on the particular atomistic structure of quartz and the relationship between crystal plane atomic arrangements and corresponding etch rates,a suitable classification of the neighborhood of the surface atoms and a corresponding quartz-based removal probability function(QUARTZ-RPF)are put forward,which enables describing the removal rate of every surface atom as an explicit function of the occupation state of the neighborhood.Besides,the step flow etching theory is introduced to explain the etching process of quartz,and the roles of some typical surface atoms are identified in terms of their special position in etching substrate.The removal probabilities of surface atoms located in step are much higher than the probabilities of atoms in terrace.In fact,one of the primary causes that result in quartz anisotropic etching is surface atom types and distributions in crystal planes.Secondly,An EMC etching model of quartz is proposed to carry out simulation calculations on the etched microstructures and surface morphologies of quartz crystal on the basis of above researches,which clearly confirms the relationship among the macroscopic etch rates of crystal planes,microscopic activation energy and atomic removal probability.Furthermorc,an evolutionary algorithm is used to properly transform the(facet specific)macroscopic etch rates of several crystal planes parallel to the X(electric)and Y(mechanical)axes into suitable values for the(atom specific)microscopic removal probabilities.To improve the computational efficiency of the evolutionary search we make use of a transformation matrix(TM),effectively constraining the evolutionary search space.Thirdly,a large number of etching experiments were implemented to explore the etch rate anisotropy of quartz in saturated NH4HF with different concentration ratios and temperatures for the first time by using the single-crystal quartz hemisphere,and the influences of etching conditions on the anisotropic etching characteristics of quartz are analyzed as well in detail.Further more,the three-dimensional structures and surface topography etched on Z_cut,AT cut and BT_cut substrates using some mask patterns are also obtained in above experiments.Based on above experimental results,the effects of crystal planes with local extreme etch rates on the formation of etched structures as well as the apparent activation energy of a plane on its etch rate are all identified,which provides the experimental and theoretical basis for the establishment of the EMC etching model.Lastly,lots of simulation calculations of the EMC etching model on the etched micro structures and surface morphologies of quartz have been carried out in this research,and the final results indicate the EMC etching model has a good ability to predict the etch rates of a wide range of crystallographic facets as well as the three-dimensional microstructures and topography etched on Z_cut,AT_cut and BT_cut substrates using some mask patterns.In addition,the proposed Microscopic Activation Energy Evaluation function(MAEE)derived from Monte Carlo etching theory transforms the apparent activation energy of crystal planes into suitable values for the(atom specific)microscopic activation energy,which is the first to explain the cause of quartz anisotropic etching mechanism from the perspective of microscopic atomic energy,and specifies the roles of different types atoms in the anisotropic etching process of quartz.