Plasma Etching
Plasma etching is the removal of material from surfaces through plasma processes. It is also called dry etching because conventional etching processes are carried out with wet chemical methods using corrosive acids. The plasmas of the process gases change the aggregate state of the material to be etched from solid to gaseous phase, and the vacuum pump extracts the gaseous products. Masks can be used to etch partial areas or structures only. Plasma etching is only carried out in low-pressure plasma because:
A longer treatment duration is necessary to achieve the desired etching effects.
Also, most etching gases can only be used in low-pressure plasma.
Plasma etching is suitable for a wide range of applications. To tailor the etching process optimally to the application, a variety of process gases are available, and one of the following three basic etching processes can be used.
Ion Etching
Depending on the application, it is also called “physical etching”, “sputtering” or “micro sandblasting”.
Argon or other noble gases which form ions but no radicals are used as processes gases. The etching effect is realized by knocking out atoms or molecules from the substrate by the kinetic energy of the electrons accelerated in the electric field.
Applications:
As ion etching does not have a chemical effect, it works on almost every substrate (practically non-selective). The etching effect of the plasma acts almost exclusively in the acceleration direction of the ions. The effect is highly anisotropic.
Chemical Plasma Etching
The molecules of process gases used are mostly broken down into radicals in the plasma. The etching effect is mainly based on the reaction of these radicals with the atoms or molecules of the substrate and their conversion to gaseous degradation products.
Primary Applications:
Decomposition of oxide layers
Removal of photoresist (“stripping”)
Ashing of matrices for analysis
Etching of PTFE
Structuring and microstructuring of semiconductors
Plasma etching is highly selective, i.e., process gases and substrates must be perfectly matched. The etching effect is isotropic, i.e. has the same effect on all sides.
Reactive Ion Etching
In the plasma, gas molecules form radicals and positively charged ions. For etching, the reactive effect of the radicals as well as the kinetic energy of the ions can be used, if the plasma excitation occurs by accelerating the ions in the electric field and using them to bombard the substrate.
Reactive ion etching combines the effects of ion etching and plasma etching: There is a certain anisotropy, and materials which do not react chemically with the radicals are etched. Most importantly, the etching rate is considerably increased. By bombardment with ions, the substrate molecules are put into an excited state which makes them much more reactive.
Applications:
PTFE Etching
Plasma technology can provide adhesiveness to plastics which are normally “non-glueable” due to their low surface energy. With polypropylene (PP), polyethylene (PE) or polyoxymethylene (POM), this is achieved by activation in the oxygen plasma. For PTFE, the plastic material with the smallest surface energy, the activation process is not sufficient to achieve the desired result. The fluorocarbon bonds cannot be broken in oxygen plasma.
In hydrogen plasma, however, hydrogen radicals combine with the fluorine atoms of PTFE and thus break the carbon bonds. When hydrogen fluoride gas is extracted, unsaturated carbon compounds remain, to which polar liquid molecules can easily attach.
A brownish discoloration on the PTFE surface shows that etching has taken place.
Example POM: Before plasma treatment
Example POM: After plasma treatment
Example PTFE: Before plasma treatment
Example PTFE: After plasma treatment
