COMSOL Multiphysics Simulations of Wet Etching of High-Aspect-Ratio Structures with Surface Charge

Abstract

In semiconductor manufacturing, the miniaturization of devices has always been a strong driving force in the industry. Making the individual transistors smaller makes it possible to increase the number of transistors on a given size chip, resulting in greater processing power and data storage capabilities. As the critical dimensions of today’s technology nodes approach the atomic scale, it is no longer feasible to obtain the same progression of increased transistor count through simple scaling. In the memory industry, new innovations are being incorporated into the manufacturing process, such as the verticalization of NAND memory to keep up with the demand for increased transistor density. With these innovations come new processing challenges, such as wet etching in extremely high-aspect-ratio (HAR) structures while maintaining top-to-bottom uniformity. The objective of this work was to create simulations in COMSOL Multiphysics that depict the time-dependent etching progress in HAR structures. A dynamic etching model was developed to predict the lateral recession profile of a HAR polysilicon trench bounded on the top and bottom by silicon dioxide (SiO2) etched by tetramethylammonium hydroxide. The model considered the diffusion of active etchant species, surface charging of polysilicon and SiO2, and reaction byproduct accumulation on the top-to-bottom etching profile. The results of the simulations suggest that the diffusion of reactant species into the trench could influence the top-to-bottom etch uniformity if the reaction order is greater than 1. The surface charging on polysilicon and SiO2 does not appear to affect the overall top-to-bottom uniformity but leads to a localized reduction in the etch rate in the corners of the trench where the polysilicon meets the SiO2. The secondary etching simulation was developed to model the wet etching of thin SiO2 films sandwiched between silicon layers, a structure that is often used to create cantilever microstructures. Etching models of SiO2 in hydrofluoric acid (HF) from the literature were utilized to provide boundary velocities in moving-boundary simulations of a dynamic etching process as a function of local concentrations of etchant species. The influence of the surface charge present on silicon in the Si/SiO2/Si stacked structures on the etch rate of SiO2 was modeled for varied thicknesses of SiO2. The etch rate of thin films of SiO2 was compared to the bulk etch rate without electrostatic interference. The simulations did not suggest that the SiO2 thickness in the stacked structure has a strong impact its etch rate because etch rate is dominated by neutral species.