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.
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.
- 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.
- 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.
- Primarily in semiconductor 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.
Generally, etching of metals is possible but only with highly corrosive gases, which in turn may corrode the metal. To intensify the etching effect, the parts can be preheated, or – if a chamber heating is installed in the plasma system – heated continually.
Primarily plastic surfaces are etched slightly with these processes.
For plastics with poor painting or gluing properties such as POM, PPS and PTFE, etching is very important. The surface enlargement improves the adhesiveness.
Gases typically used for etching are oxygen, various fluorine-chlorine gas compounds, and also hydrogen.
Plasma etching of glass in a vacuum is time-consuming and expensive. The ionised glass particles are very slow in decomposing the glass. Since glass primarily consists of SiO2, it can generally be etched with fluorinated hydrocarbons (with the addition of oxygen).
The low removal rate and resulting long process time make these processes very expensive.
Ceramic materials (e.g. Al2O3) can be etched with corrosive and non-corrosive gases.
All gases containing chlorine and fluorine are corrosive gases. Argon is a non-corrosive gas.
As a general rule, fluorinated gases have a higher removal rate than other, non-corrosive gases; chloric gases have a better etching effect than non-corrosive gases.
The best effects when etching Al2O3 are achieved with fluorinated gases.