| Anisotropic wet etching is an important process for the processing of MEMS devices,which has different effects on the preparation of different microstructures.The research of anisotropic wet etching technology is of great significance especially for the development of sapphire patterning and the processing of gallium nitride high aspect ratio structures.The formation of the wet etching structure is closely related to the etch rate.As an influencing factor of the anisotropic wet etching process,the etch rate is an important criterion for measuring the design of the etching structure.In the present situation,this paper proposes an anisotropic wet etch rate model based on the atomic arrangement of different materials and the step-flow mechanism.The model takes sapphire and gallium nitride as the main research subjects,distinguishes the surface atomic configurations of different crystal materials,and obtains the etch rate of ordinary crystal planes based on a limited number of key crystal planes.The rate model in this paper combines theoretical etching mechanism with experimental research and analysis to explain the microscopic reasons of crystal anisotropy on the atomic scale.According to the characteristics of the crystal lattice structure,the method of atomic structure modeling and step-terrace identification has been optimized,and the calculation principle of crystal plane etch rate has also been proposed.Finally,the accuracy of the etch rate model is demonstrated by comparing the experimental results in different crystals and etching systems to provide a reliable rate model for the development of etching process simulation tools.The main content of the paper is as follows:(1)In view of the current research status of anisotropic etching of different crystals and the etch rate research issues that restrict the development of this process technology,by comparing and analyzing the basic principles of anisotropic etching models,this paper proposes a MEMS wet etching rate model based on the step-flow mechanism.Later,from the study of the atomic arrangement characteristics and atomic type composition of different materials including silicon,sapphire and gallium nitride,the key main crystal planes and the ordinary crystal planes are distinguished,and the step-terrace structures between the key crystal planes and the ordinary crystal planes are explored.The formation law finally reveals the internal relationship between the macro-etching rate reflected by the crystal plane anisotropy of different materials and the surface atomic arrangement characteristics.At the same time,in order to show the composition and arrangement of surface atoms between crystal planes more vividly and concretely,this paper also simplifies the sapphire and gallium nitride atomic models with silicon-like simplifications to realize the reconstruction of atomic models and step-terrace structures of crystal planes.Finally,the complexity of the atomic model is reduced,which provides the possibility for more complex crystal atomic model analysis.(2)In this paper,an etch rate model is established based on the identification of the crystal plane step-terrace structure and the classification of basic crystal units.For sapphire and gallium nitride,a general expression for the etch rate of arbitrary crystal surface is proposed for the first time.The solution of the crystal plane etch rate is transformed into a matrix equation representation,which improves the calculation efficiency of the model.Among them,according to the crystal unit composition and ratio of the key crystal planes,the basic removal parameters corresponding to the key crystal planes are solved,and the matching relationship between the microscopic removal rate and the macroscopic etch rate is established.(3)The experimental study in this paper is carried out on the basis of etch rate model construction,and the rate model of sapphire is experimentally verified by different concentration ratios of acid etching conditions,and the accuracy of the rate model is verified against the etching results of Ga N presented in the literature.It is proved that the wet etching rate model based on the atomic arrangement anisotropy at the crystal surface is not only reasonable and reliable,but also has strong adaptability from various aspects such as temperature,etching solution and crystal materials. |
