A prerequisite for the adhesion of binding partners for painting, gluing, printing or bonding is a good wettability of the surface. Not only is wetting prevented by oil and grease marks, but also the clean surface of many materials cannot be sufficiently wetted by many liquids, adhesives and inks. The liquid rolls off. It will also not adhere to the surface after curing or drying.
The cause is a low surface energy of the substrate. Substances having a low surface energy wet those with a high surface energy, but not vice versa. The surface energy of the applied liquid, also referred to as surface tension, must be lower than that of the substrate in each case.
Most plastics have a very low surface energy, too low for wetting by adhesives and coatings. The reason is the non-polar surface. The molecules of the liquid cannot find connection points where they can accumulate.
The surface energy of a surface is increased by being activated. This ensures that attachment sites are created for the applied liquid.
Activation is traditionally achieved using chemical primers, liquid adhesion promoters. They are often highly corrosive and harmful to the environment. On the one hand you need to allow adequate drying time before further processing and on the other hand the surface often does not remain active for long. Non-polar materials such as polyolefins are not sufficiently activated by chemical primers.
You can also activate surfaces in an electric arc corona. This is a form of atmospheric pressure plasma treatment. However, only flat or convex surfaces that can be introduced into the electric arc can be treated in this way.
With Diener electronic atmospheric pressure plasma systems, the plasma for the electric arc is blown out through a nozzle. This also allows the surface of complex curved components to be activated.
Upon activation by air or oxygen plasma, non-polar hydrogen bonds of the plastic polymers are replaced by oxygen bonds. These can provide free valence electrons for the binding of liquid molecules.
Plasma activation under low pressure or atmospheric pressure also allows "non-adhesive" plastics such as POM, PE and PP to have very good bondability or be paintable. The desired surface energy can be adjusted very precisely, so that over activation that leads to etching can be avoided.
In low-pressure plasma, other gases can be used instead of oxygen or air, for example by use in place of oxygen, nitrogen (N 2), amines (NHx) or carboxyl groups (-COOH) are attached as reactive groups.
The activation of plastic surfaces remains effective over weeks and months. Further processing should still take place rapidly, since with increasing aging new contaminants are deposited.
PTFE can also be glued by plasma treatment. However that is not through activation, but rather due to etching.
Metals, ceramics and glasses generally have higher surface energies than plastics. However, there are also applications for these materials, in which plasma activation creates advantages. The surface tension of solder alloys is high and they roll off on many metal surfaces. Therefore, the plasma activation of metals also improves wetting during soldering. However, the activation of metals is usually only effective for a few minutes and they must be installed directly upstream of the soldering process (in-line).
Plasma etching is the material removal of surfaces by plasma processes. It is also referred to as dry etching, since conventional etching processes are carried out with wet-chemical corrosive acids. The plasmas of the process gases convert the material to be etched from the solid to the gaseous phase and the vacuum pump extracts the gaseous products. The use of masks can also ensure the etching of only parts of the surface or structures. Plasma etching is only performed as low-pressure plasma because
- significant etching effects require longer treatment time
- Almost all etching gases can be used in low-pressure plasma.
There are a variety of applications for plasma etching. For application-specific optimization of the etching process a variety of possible process gases and the selection of 3 basic etching methods are available.
Depending on the application, this is also known as "physical etching", "sputtering" or "micro-sandblasting".
Process gases are argon or noble gases, but the ions do not form free radicals. The etching effect is based on the ejection of atoms or molecules from the substrate through the kinetic energy of the accelerated electrons in the electric field.
- Microstructuring of surfaces such as for improving adhesion ("micro-sandblasting")
- Bombardment of an evaporation source ("sputtering")
Since ion etching does not act chemically, it works on almost any substrate (hardly selective). The etching effect of the plasma occurs almost exclusively in the acceleration direction of the ions. The effect is strongly anisotropic.
Chemical plasma etching
Process gases are used whose molecules in plasma are mainly split into radicals. The etching effect is mainly based on the reaction of these radicals with atoms or molecules of the substrate, converting them into gaseous breakdown products.
- Removal of oxide layers
- Removing photoresist ("stripping")
- Ashing of matrices for analysis
- Etching of PTFE
- Structuring and microstructuring of semiconductors
Plasma is very selective, i.e. the process gases and substrates must be very well matched. The etching is isotropic, i.e. it acts equally on all sides.
Reactive ion etching
Molecular gases form radicals and positively charged ions in the plasma. The reactive effect of the radicals can be used for the etching process, as well as the kinetic energy of the ions. When the plasma excitation is performed in this way, the ions are accelerated in the electric field and are fired onto the substrate.
Reactive ion etching combines the effects of ion etching and plasma etching: A certain amount of anisotropy is created and materials which do not chemically react with the radicals can also be etched by this plasma. Above all, the etching rate is significantly increased. The substrate molecules are excited by the ion bombardment and are thus much more reactive.
- Especially during the etching of semiconductors
At Deiner electronic we also use plasma technology to make plastics bondable, which would otherwise be considered as "non-bondable"due to their low surface energy. For polypropylene (PP), polyethylene (PE) or Polyoxymethylene (POM), this is achieved by activation in an oxygen plasma. For the plastic material with the lowest surface energy, PTFE, an activation process is not sufficient. The fluorine-carbon bonds can not be broken in an oxygen plasma.
In hydrogen plasma, however, hydrogen radicals combine with the fluorine atoms of PTFE and so break the carbon bonds. The hydrogen fluoride gas is exhausted off, and unsaturated carbon bonds remain, to which polar liquid molecules can strongly attached.
The successful etching is recognizable by a brown discolouration on the PTFE surface.
POM example: Before plasma treatment
POM example: After plasma treatment
POM example: Before plasma treatment
POM example: After plasma treatment
With low-pressure plasma, workpieces can be improved with various coatings. To achieve this, gaseous and liquid raw materials are fed into the vacuum chamber. In plasma cross-linking, the raw materials, mostly short-chain monomers, are converted to long-chain polymers. The selection of raw materials then determines the coating properties:
- Hydrophobic (water repellent)
- Hydrophilic (water-attracting/wetting)
- Scratch protection
- Corrosion protection
- Carbon coating
- Barriers/diffusion barriers
- Frictionless coatings/non-stick coatings
- Adhesion promoter/primer
- Water/steam barriers
Advantages of plasma-coating:
- Extremely thin coatings are possible on the nanometre scale
- Series ready, steady processes are possible through full automation
- Variety of options are feasible
- No temperature loading
- No solvents
- Very good gap penetration properties
- Suitable for general items and bulk
Very tiny amounts of contamination, invisible to the eye, are always present on all surfaces. The removal of these contaminants is almost always a prerequisite for proper further processing of the surface by methods such as:
- Coating process
Plasma technology offers solutions for any type of contamination, for any substrate and for any treatment. Molecular contamination residues are also removed. Various cleaning methods are available for different requirements in individual cases. The most important are:
1. The removal of hydrocarbons in oxygen plasma
Micro-cleaning - degreasing in oxygen plasma
Hydrocarbons such as residues of fats, oils or release agents are found on virtually all surfaces. These coatings drastically reduce the adhesion of other materials in subsequent processing of the surface. Therefore, the chemical removal of hydrocarbons in oxygen plasma is a standard treatment before any painting, printing or gluing.
The plasma reactions in this purification process are demonstrated, as an example, in "Small plasma physics".
Ions, radicals, and UV radiation act together. High-energy UV radiation splits macromolecules. Oxygen radicals, ions and split off hydrogen radicals occupy free chain ends of the polymer chains to H2O and CO2.
The degradation products of the hydrocarbons are gaseous in the low-pressure plasma and are removed by suction.
On polymeric surfaces, activation starts in parallel with the reduction of surface contamination by oxygen radicals. This activation is a prerequisite for proper adhesion on non-polar plastics. For details see Activating materials.
Oil, grease or release agents containing additives cannot always be completely removed in oxygen plasma. Solid oxides can form which adhere to the substrate. These can be purified in additional downstream purification processes, if necessary.
Cleaning in oxygen plasma works on virtually all materials. Purified dry air can often be used instead of oxygen. The removal of hydrocarbons is therefore carried out both in low-pressure plasma and atmospheric pressure plasma.
2. Mechanical cleaning by micro-sandblasting
A particularly simple plasma is an inert gas plasma. It consists only of ions, electrons and noble gas atoms. As the gas is always atomic, there are no radicals and, since noble gases do not react chemically, there are also no reaction products. Argon plasma is still active because of the kinetic energy of the heavy ions.
Due to the kinetic energy of impacting ions, atoms and molecules forming the coating are chipped away, so that they are gradually removed.The treatment acts on almost all surfaces, and thus on any kind of contamination. Almost all contamination that resists chemical attack can be removed by micro-sandblasting.As the positively charged ions are accelerated to a negatively charged electrode, plasma excitation occurs in a parallel plate reactor.
Structuring - physical etching
High-energy ions knock fragments out of the substrate material itself, and not only from the surface coating. This leads to an increasing molecular scale patterning and structuring of the surface. As with sand blasting or grinding, this leads to an increase in surface area and possibly also to back tapering which increases the adhesion of subsequently applied coatings.
In contrast to chemical etching effects, in low-pressure plasma, micro-sandblasting is not isotropic, i.e. it is not evenly applied to all surfaces of a component, but mainly in the direction of the electric field because the ions are accelerated in this direction.
3. Reduction of oxide layers
Oxide layers are found on many surfaces. Only a few metals have no tendency to form oxides after long storage. On many metals, oxide layers form during plasma cleaning in oxygen plasma. These oxide layers interfere with all further processing stages:
- Adhesion of electrical contacts during bonding, soldering
- Poor electrical contact
- Poor adhesion when gluing or painting
Non-metals can also often form oxidized solid deposits, which sometimes only form due to cleaning in an oxygen plasma. Oxide layers often oppose any attack by conventional solvents. Even their mechanical removal is often difficult, because of their high hardness. They are removed by reduction in hydrogen plasma.
In oxygen or air plasma, extremely thin metal layers of only a few atoms thick can also be targeted for oxidization. These invisible coatings harden and protect the metal from chemical and mechanical attack and against further oxidation. They ensure a permanent metallic lustre.
The surface oxidation is often carried out in atmospheric pressure plasma.
Because often various contaminants need to be removed from a surface, different cleaning processes are applied in sequence, such as:
- 1. The removal of separating agents (hydrocarbons) in oxygen plasma
- 2. Micro-mechanical precision cleaning by micro-sandblasting in argon plasma
- 1. Degreasing in oxygen plasma
- 2. Reduction of oxide films in hydrogen plasma
On the other hand, oxygen purification after activation of non-polar surfaces by the process of oxygen radicals continues for a long period after cleaning. For details, see Activating materials and for even more prolonged reaction downstream Etching of materials.
Plasma cleaning has unique advantages over other cleaning methods:
- Cleaning even in the finest cracks and gaps
- Cleaning of all component surfaces in a single step, even on the inside of hollow bodies
- Residue-free removal of degradation products by vacuum suction
- No damage to solvent-sensitive surfaces by chemical cleaning agents
- Also removes fine molecular residues
- Immediate further processing is possible (and beneficial). No venting and removal of solvents required
- No storage and disposal of hazardous, environmentally damaging and harmful cleaning agents is required
- Very low process costs