Wet etching is the removal of material by immersing the wafer in a liquid bath of chemical etchant. There are two kinds of wet etching etchants, isotropic etchants and anisotropic etchants: Etching is an intaglio method of printmaking in which the image is incised into the surface of a metal plate using an acid. ... See Wafer (cooking) for the original meaning of the word. ... Isotropic means independent of direction. Isotropic radiation has the same intensity regardless of the direction of measurement, and an isotropic field exerts the same action regardless of how the test particle is oriented. ... This article is being considered for deletion in accordance with Wikipedias deletion policy. ...
Isotropic etchants attack the material being etched at the same rate in all directions.
Anisotropic etchants attack the silicon wafer at different rates in different directions. On wafers the most used etchant is KOH.
Anisotropic etching does not cause undercutting, and is preffered in applications where straight side walls are essential. See also: rates (tax) A rate is a special kind of ratio, of two measurements with different units. ... A right-handed Cartesian coordinate system, preseting the z (up) vector and y (forward) vector, the right is defined to be the positive x vector. ... The chemical compound potassium hydroxide, (KOH) sometimes known as caustic potash, potassa, potash lye, and potassium hydrate, is a metallic base. ...
Anisotropic etching requires a substrate with a well defined crystalline structure such as silicon. The etch is directional and proceeds along the exposed plane in the crystal lattice. As atoms are removed from the crystal lattice, different planes are exposed to the etchant. Since the density of atoms on the planes varies, the etch rate varies significantly.
In silicon, a popular anisotropic wet etch substrate, the initial (0 degree) surface is called 100 and etches a hundred times faster that the 110 (45 degree) and 111 (54 degree) surfaces. This results in a consistent wedge shaped etch pattern.
Contrary to popular belief, glass is not crytalline and cannot be anisotropically etched.
A simple wetetching process may just consist of dissolution of the material to be removed in a liquid solvent, without changing the chemical nature of the dissolved material.
A basic wetetching process may be broken down into three (3) basic steps: 1) diffusion of the etchant to the surface for removal; 2) reaction between the etchant and the material being removed; and 3) diffusion of the reaction byproducts from the reacted surface.
Reduction-oxidation (redox) reactions are commonly encountered in wafer fab wetetching processes, i.e., an oxide of the material to be etched is first formed, which is then dissolved, leading to the formation of new oxide, which is again dissolved, and so on until the material is consumed.
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