Dry etching VS Wet etching

Etching is a process used in microfabrication, which consists of removing one or more layers of material from the surface of a wafer. Etching is a critical and extremely important step in the manufacture of microelectronic components, as each wafer can undergo numerous etching steps. For many etching steps, part of the wafer is protected from the etching reagent by a so-called masking material, which is resistant to etching. In some cases, the masking material is a photoresist that has been modelled by photolithography. Other situations require a more durable mask, such as silicon nitride.

If the etching is performed to make a cavity in a material, the depth of the cavity can be controlled by, among other things, modulating the etching time and etching speed. Nevertheless, etching usually requires the entire top layer of a multilayer structure to be removed without damaging the underlying or masking layers. The ability of the etching system to do this depends on the ratio of etching speeds in the two materials: this is called selectivity.

The two basic types of etching agents are the liquid phase and the plasma phase. The etching process of using liquid chemicals or etching agents to remove material from the substrate is called wet etching. In the plasma etching process, also known as dry etching, plasmas or etching gases are used to remove material from the substrate. Dry etching produces gaseous products, which must diffuse into the bulk gas and be expelled by the vacuum system. There are three types of dry etching: chemical reactions (using a plasma or reactive gases), physical removal (usually by momentum transfer), and a combination of chemical reactions and physical removal.  Wet etching, on the other hand, is only a chemical process.

Wet etching

The main wet etching technique is the so-called "chemical wet etching" technique. The substrate, which is partially protected, is immersed in a solution that will chemically etch the surface of the unprotected wafer. This is generally an acid such as hydrofluoric acid for a silicon substrate (the only one capable of reacting with the layer of silicon dioxide that naturally forms on the surface of silicon), or hydrochloric acid for a gallium arsenide substrate (the chloride ion reacts strongly with the gallium) or weaker, such as citric acid.

This process is less and less used in view of its disadvantages in favour of dry etching. 

Dry etching

The etchings made with plasma may differ depending on the plasma parameters used. Ordinary plasma etching works between 0.1 and 5 Torr. ( about 133.3 Pa). The plasma produces energetic, neutrally charged free radicals that react on the wafer surface. As the neutral particles attack the wafer from all angles, this process is isotropic.

The plasma gas contains molecules rich in chlorine or fluorine. For example, carbon tetrachloride (CCl4) is known to attack silicon and aluminium, while trifluoromethane attacks silicon dioxide and silicon nitride. An oxygen plasma will be used instead to oxidise the photoresist and facilitate its removal.

Another technique is also widely used: ion milling. Also known as sputter etching, this technique uses lower pressures of around 10^-4 Torr (10 mPa). It consists of bombarding the wafer with energetic ions of noble gases such as Ar+, which drop the atoms from the substrate by transferring kinetic energy. As the etching is performed by the unidirectional bombardment of the ions onto the wafer, this process is anisotropic. Nevertheless, this process has a low selectivity.

The well-known technique using reactive ions (RIE for Reactive Ion Etching) works under intermediate conditions between sputtering and plasma etching (between 10^-3 and 10^-1 Torr). Deep reactive ion etching (DRIE) is similar to RIE but can produce deeper and narrower features.