Etching with plasma
Decomposition of oxide layers, removal of photoresist, ashing of matrices, ...
Plasma etching is the removal of material from surfaces via plasma processes. It is also described as dry etching because conventional etching processes are carried out with wet chemical methods using aggressive acids. The plasmas of the process gases change the aggregate state of the material to be etched from solid to gaseous, 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 noticeable etching effects, and because 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, choose from a variety of process gases and select one of three basic etching processes.
Substrate atom
Plasma treatment process gas
Etching mask
Ion etching
Depending on the application, 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 based on the knocking out of atoms or molecules from the substrate by the kinetic energy of the electrons accelerated in the electric field.
Applications:
- Micro-structuring of surfaces e.g. for adhesion improvement (“micro sandblasting”)
- Bombarding of a vapour deposition source ("sputtering")
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
Process gases are used whose molecules in the plasma are mostly broken down into radicals. 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 application:
- 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 match perfectly. The etching effect is isotropic, i.e. has the same effect on all sides.
Reactive ion etching
In the plasma, molecular gases 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 are etched which do not react chemically with the radicals. Above all, however, 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.
Application:
- Primarily in semiconductor etching
PTFE etching
With Diener electronic plasma technology, plastics can be made gluable which are considered “non-gluable” 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. The hydrogen fluoride gas is extracted, and unsaturated carbon compounds remain to which polar liquid molecules can attach excellently.
A brownish discolouration on the PTFE surface shows that etching has taken place.
Plasma
Carbon polymer
Fluorine
Hydrogen
FAQ Frequently Asked Questions
Is it possible to etch metals?
Etching metals is possible in principle, but only with highly corrosive gases, which in turn can cause the metal to corrode. To enhance the etching effect, the parts can be preheated – or, if a chamber heater is installed in the plasma system, they can be heated continuously.
Which plastics can be etched?
These processes are used primarily to etch plastic surfaces. Etching is very important for plastics that are difficult to paint or bond, such as POM, PPS and PTFE. The increased surface area results in better adhesive bonding. Typical etching gases include oxygen, fluorine and hydrogen.
Can glass and ceramics be etched?
Plasma etching of glass in a vacuum is time-consuming and expensive. The glass is eroded only slowly by the ionised gas particles. Glass consists largely of SiO₂ and can therefore, in principle, be etched using fluorinated hydrocarbons (with the addition of oxygen).
The low etching rate and the associated long process time are responsible for the high costs of such processes.
Ceramics (such as Al₂O₃) can be etched using corrosive and non-corrosive gases. Corrosive gases include all fluorine-containing gases. Argon is one of the non-corrosive gases. Generally speaking, fluorine-containing gases have a higher ablation rate than other non-corrosive gases. Al₂O₃ is best etched using fluorine-containing gases.
How much can be removed during etching?
Depending on the substrate, a low-pressure plasma system can etch away material from the surface down to the lower micrometre range (< 1 micrometre).
How long does an etching last?
When the surface is etched, material is removed and is therefore permanently retained.
What can be etched?
All plastics and organic materials can be etched. Inorganic materials on request.
