Wet Etching; Wet Processing

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Wet Etching

In wafer fabrication, etching refers to a process by which material is removed from the wafer, i.e., either from the silicon substrate itself or from any film or layer of material on the wafer. There are two major types of etching: dry etching and wet etching.

Wet Etching is an etching process that utilizes liquid chemicals or etchants to remove materials from the wafer, usually in specific patterns defined by photoresist masks on the wafer. Materials not covered by these masks are 'etched away' by the chemicals while those covered by the masks are left almost intact. These masks were deposited on the wafer in an earlier wafer fab step known as 'lithography.'

A simple wet etching 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. In general, however, a wet etching process involves one or more chemical reactions that consume the original reactants and produce new species.

A basic wet etching 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 wet etching 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.

Wet etching is generally isotropic, i.e., it proceeds in all directions at the same rate. An etching process that is not isotropic is referred to as 'anisotropic.' An etching process that proceeds in only one direction (e.g., vertical only) is said to be 'completely anisotropic'.

Figure 1. Example of a Wet Etching Station;

source: www.futurefab.com

In semiconductor fabrication, a high degree of anisotropy is desired in etching because it results in a more 'faithful' copy of the mask pattern, since only the material not directly under the mask are attacked by the etchant. Isotropic etchants, on the other hand, can etch away even the portion of material that's directly under the mask (usually in the shape of a quarter-circle), since its horizontal etching rate is the same as its vertical rate.

When an isotropic etchant eats away a portion of the material under the mask, the etched film is said to have 'undercut' the mask. The amount of 'undercutting' is a measure of an etching parameter known as the 'bias.' Bias is simply defined as the difference between the lateral dimensions of the etched image and the masked image. Thus, the mask used in etching must compensate for whatever bias an etchant is known to produce, in order to create the desired feature on the wafer.

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