Aqueous Wet Etching of Silicon With Alkali Metal Hydroxides

Abstract Silicon etching is a process that is regularly employed in the semiconductor industry for many applications ranging from micromachining on the sub-millimeter scale to intricate pattern formation in tens of nanometer regime. Wet etching is used for bulk silicon removal. Such etching processes, generally, involve multiple chemical reactions where a liquid solution of etching agents removes surface atoms of silicon through chemical or electrochemical redox reactions at the substrate-etchant interface. Many wet etching processes are anisotropic, and thus the wet etching of silicon with hydroxide solutions tends to be direction or crystalline plane specific, allowing for directional control and reduced undercutting of any etch mask. An additional advantage of metal hydroxide etchants rise from their selective nature of extremely poor etch rate towards the basic oxide of silicon. This makes growing the hard mask of SiO2 a very cost effect step in the wet etching process. NaOH and KOH solutions are well known wet etching agents that offer inexpensive alternatives to anisotropic dry etching but are often either difficult to control (etch rate-wise) or result in rough etched surfaces. Only through better understanding of the chemical reaction mechanisms, i.e. the role of alkali metal cations and water molecules during the wet etching process more controllability and desired etch surface profiles can be achieved. This study summarizes the results from a series of wet etch processes with all the five different group 1 alkali metal oxides and discusses in detail how different types of group 1 metal ions impact the dynamics of the chemical reactions in the aqueous medium. Wet etching assisted by smaller cations like Li+ tend to be more controllable while larger cations like Cs+ result in much faster etch rates. Surfactants added to fast etching metal hydroxides result in finer sidewalls and smoothened etched surface on silicon. Later in the paper, the effect of microwave irradiation on the surface tension, structural arrangement of water molecules and its application in anisotropic etching are highlighted. Step by step details of the experiments are provided in the materials and methods section making the study easy to understand. Line profile plots, optical images, SEM micrographs and XRD intensity plots are presented throughout the report to support the given explanations. Research findings from the study experimentally establish a trend for anisotropic etch rates of silicon substrate with aqueous solutions of all the group 1 alkali metal hydroxides. Silicon etch rate increases down the group.