Basic Concepts and Terminology in Plasma Etching

In my personal opinion, learning about such a rather complex system with many interactions as plasma processing is not 100% straight forward. It's more like solving a self consistent equation system by starting with a rough estimate and refining it step by step. Means you need to know something about all single concepts, like electrical discharge parameters, gas flow dynamics, surface chemistry and so on before you can get a better understanding of the interactions among them. But without knowing the links you cannot get a deeper knowledge of the single topics in relation to plasma processing. I am now several years in this business and the loop seems to continue forever on.

The most fundamental term in this context is ``plasma etching'' itself, it denotes the continues removal of material from a surface by physical and/or chemical processes. Etching in a discharge environment is achieved by providing active species, which react with the substrate forming volatile products. In plasma etching the reactive species are ions and activated neutrals, created in an electrical discharge. The process takes usually place inside an reactor under well controlled environmental parameters like e. g. pressure, temperature and feed gas flow. In more detail, the etching process can be divided into four steps (see also fig. ):

Figure: schematic view of the etching process

Formation of active gas species.

The source gas mixture is feed to the discharge chamber, where it is partially converted to ions and reactive neutrals.

Transport of the active species to the surface.

The active species are created in the bulk plasma or ion sheats, from where they are transported mainly by diffusion, to the substrate surface. Charged particles are also influenced by drift motions due to electric and/or magnetic fields.

Reaction at the surface.

This step it can be further split into three sub-steps, namely the adsorption of the precursors, the surface reaction and desorption of the products. Especially for the reaction sub-step a wide variety of mechanisms exists. For example, processes that depends mainly on the energy of the impinging ions are said to have a big physical component. The opposite is a chemical etch, where mainly activated neutrals react with the substrate independent of their kinetic energy. In practice most processes have both physical and chemical aspects.

Pump down of the reaction products.

After the desorption the volatile reaction products diffuse back to the bulk plasma. Here they exhausted by a vacuum pump. The different diffusion directions of etchants and reactions products are caused by the locations of the maximum concentration of both species, which are the bulk plasma and substrate surface, respectively.

Figure: profile evolution

isotropic/anisotropic/notching/trenching,residues loading/microloading/arde mechanisms to achieve defined profiles/defined particle beam IBE, RIBE / sidewall passivation/ low temperature processing combination of physical/chemical aspects different nature Al/Cl SiO/CF