Etching is used in microfabrication to chemically remove layers from the surface of a wafer during manufacturing. Etching is a critically important process module, and every wafer undergoes many etching steps before it is complete. For many etch steps, part of the wafer is protected from the etchant by a "masking" material which resists etching. In some cases, the masking material is a photoresist which has been patterned using photolithography. Other situations require a more durable mask, such as silicon nitride. Orientation-Dependent Etching KOH pellets dissolved in water (self-heating) Etch Rate {110} > {100} >> {111} KOH has a slower etching orientation for the {111} planes You cannot use this KOH photoresist as a etching mask, because the oxide attacks too slowly, so this resist will not survive Photoresist can be used a etching mask, and the best photoresist for etching is nitride For example, the etch rate of Si in KOH Depends on Crystallographic Plane At low temperature you have high selectivity (etching rate is slower), at high temperature your selectivity will drop (higher etching rate) By increasing the temperature, the etch rate increases, but the selectivity decreases. There is a trade off between etch rate and etch selectivity. Figures of merit If the etch is intended to make a cavity in a material, the depth of the cavity may be controlled approximately using the etching time and the known etch rate. More often, though, etching must entirely remove the top layer of a multilayer structure, without damaging the underlying or masking layers. The etching system's ability to do this depends on the ratio of etch rates in the two materials (selectivity). Some etches undercut the masking layer and form cavities with sloping sidewalls. The distance of undercutting is called bias. Etchants with large bias are called isotropic, because they erode the substrate equally in all directions. Modern processes greatly prefer anisotropic etches, because they produce sharp, well-controlled features. Etching media and technology The two fundamental types of etchants are liquid-phase ("wet") and plasma-phase ("dry"). Each of these exists in several varieties. Wet etching The first etching processes used liquid-phase ("wet") etchants. This process is now largely outdated, but was used up until the late 1980s when it was superseded by dry plasma etching. The wafer can be immersed in a bath of etchant, which must be agitated to achieve good process control. For instance, buffered hydrofluoric acid (BHF)