Microtechnology - Wet etching

This is a short review about some wet etching processes. I will never say it enough: don't expect a full encyclopedia about wet etching processes here. I have no time for this, and there are a lot of resources elsewhere for that.
Use this to understand possibilities and problems linked to wet etching! For that, I will concentrate only on a few examples to explain different situations. Wet etching is widely used in microelectronics and MEMS devices microfabrication. It is largely the most widely used method to release MEMS microactuator, since fluids are able to get under microstructures and etch non visible material areas, so that MEMS mobile parts are free to move.

A chemistry science

Wet etching relies completely on chemical reactions. Where dry etching can include physical etching, wet etching cannot rely on the products kinetic energy to etch a material!
All the task consists here in finding chemical products or combinations so that the target is etched faster than the mask disappears! Of course, there are other parasitic chemical reactions that can happen.
So, chemistry is undoubtely a useful tool to predict the best etchant for each material, and for which material it should be avoided.

Parameters and considerations

Temperature

Chemical reactions cannot be changed in any way, except by changing the products concentrations. However, temperature can be changed as a parameter for the reaction.

Most of the time, increasing temperature will provide more energy to molecules to react, and accelerate the etching. Of course, this is true as long as you don't vaporize the products, in which case things are different.
In fact, some reactions need to be done with thermal heating otherwise they are too long to be useful. This is the case, for example, of KOH etching, that is generally done at 80°C. Another example of heating use is for removing photoresist with aceton. If the photoresist has been altered during, let's say a plasma etching, the removing can fail at room temperature and be successful with a light heating.
Since materials used in microtechnology are hardly detroyed at boiling temperature of acid/basic solutions, temperature should not change anything in the materials properties, but it can alters selectivity. For the KOH example, the highest selectivity is reached for particular concentration and temperature.

Concentration

Like temperature, concentration can, of course, change the etching rate. It can also affects selectivity. But there are other factor to take care when manipulating concentration.
Most of time, wet etching reactions are quite simple from a chemistry point of view. This means that you have the etching product, the target material(s), and after the reactions, ions and molecules generated don't react anymore.
At a local level, this means that with no agitation, concentration will get down on some place, depending on the geometric configuration of the structures. Here we talk about underetching again. There exists buffered solutions (a second product in the etching solution is added so that the etching ion is renew in real-time thanks to another reaction with this added product, see BHF). Another possibility to reduce that phenomenon is agitation of the solution.

Geometry

As explained for the concentration, geometry can affect the etching rate at a local level. Large areas are always etched slightly faster than very small patterns. Once the strucure has already several material layers, things get even more complicated, with the various angles of the underlying layers, etching can let a almost perfect pattern compared to the mask design, and in the same time has destroyed a small structure.

Mesh selectivity

A particular case in wet etching is the mesh selectivity that is observed with etchants like KOH or TMAH on crystal silicon.

Crystal silicon, like a lot of crystal materials, offer a different molecules density according to the face you look. This property has the advantage of allowing a special etching result.