What is Dry Etching?

Dry Etching vs. Wet Etching

Chemical wet etching

Wet etching is another strategy to create microstructural designs on surfaces. This process involves using chemical solutions in a bath type setting. This type of etching is faster than dry etching; however, it is not as precise. One large problem connected to liquid etching agents is the undercutting of the etching mask. This leads to drop shape cavities beneath the covered areas on the substrate. There is also the downside that some of these processes involve highly toxic or otherwise hazardous substances.

Dry etching

Dry etching has the advantage of being able to define small features of sizes below 0.1 micrometers, whereas wet etching cannot adequately define features below one micrometer. Also, wet etching requires the use of hazardous chemicals and can also cause water contamination.  Dry etching is completely safe and an environmentally friendly process. Dry etching is also ideal for large scale production, while still being reasonably sized.

Differences of both processes

Dry and wet etching are both technologies that remove atoms or molecules from a substrate via chemical bonding. The main difference is that wet etching needs liquid chemicals whereas dry etching processes rely on gaseous agents, mostly in the form of plasma. Dry etching usually has lower etch rates but much higher precision. This means that smaller structures can be etched and the whole process is much better to control.

There is also the aspect that wet etching is spatially isotropic, which means that the removal of material is the same in all directions. Plasma etching, on the other hand, can be made an-isotropic by applying magnetic or electric fields. This is important when structures with high aspect ratios shall be created. For example, precise deep trench structures, which are commonly needed in the semiconductor industry, can only be achieved with ion beam etching (a form of dry etching).

Another point is the amount of chemicals needed: wet etching requires a quite large amount of liquid reactants. Some of those are hazardous or even toxic. Furthermore, the products of the etching process can be poisonous or corrosive as well. Plasma etching, on the other hand, only requires small amounts of gases. The products of the reactions are gaseous as well and, thus, can be easily captured if needed.