The qualified team consults on appropriate choice of available plasma reactor for smooth, reliable and repeatable treatment of various materials from polymers and metals to ceramics and composite materials. The suitable technologies are:
Discharge cleaning is a well-established technique that allows for excellent cleanliness of processed materials and thus suitable surface finish for any further treatment. It is an ecologically benign alternative to wet chemical cleaning suitable for removal of small amounts of surface impurities. Organic impurities are removed by oxidation of hydrocarbons to volatile products and inorganic impurities are removed by reduction of oxides, chlorides and alike using hydrogen plasma. While removal of organic impurities is often accomplished at room temperature, elevated temperatures are needed for reduction of oxides.
Plasma functionalization is a widely used technique for modification of surface properties of polymers and similar materials. The wettability of such products depends on the type and concentration of functional groups on the very surface. Improved wettability is obtained by functionalization with polar groups, while the wettability is suppressed by non-polar groups. In the first case users often take advantage of oxygen plasma, but water or even oil retarding properties are obtained by a brief treatment with plasma created in fluorocarbon or more complex gases. A simple method for a quick determination of surface wettability is a droplet of a liquid, usually a water droplet. If the droplet stands on the surface the surface finish of the material is hydrophobic. If it spills on a large surface it is hydrophilic.
Selective plasma etching is a suitable technology for tailoring surface properties of composite materials, in particular polymer composites. Most plastics are actually composites – various fillers are embedded into the polymer matrix in order to improve mechanical, physical, chemical, electrical etc. properties, or merely to make the product cheaper. The properties of such composite materials depend on the type and concentration of fillers as well as on distribution and sometimes even orientation of fillers in the polymer matrix. Namely, fillers tend to agglomerate inside the polymer matrix. Co-polymers or more complex materials are used sometimes. The products are usually made by extrusion or injection molding so the fillers do not stretch from the surface but are rather covered with the polymer. The surface properties of a composite product are thus dictated by the surface properties of the polymer which are usually inadequate. Plasma technology is usually used for selective removal of the polymer from the surface of the composite thus changing the surface properties. After accomplishing plasma treatment, the fillers stretch from the surface making surface properties of a composite product similar to properties of fillers. The surface becomes also rough upon plasma treatment what is beneficial for further treatment such as deposition of a suitable coating.
Plasma-induced nano-structuring is a popular method for increasing the surface of a product what is beneficial in numerous applications. The morphology of almost all solid materials can be structured using plasma technology. A conservative method is deposition of a mask followed by the technology of reactive ion etching. This technique is widely used in microelectronics but is not always practical because it is expensive. An alternative is application of plasma technology for stimulation of self-organization. Upon certain conditions the solid material becomes very rough upon plasma treatment, frequently covered with dense nanowires, nanowalls etc. One approach is deposition of the nanostructured film and another etching that leads to spontaneous formation of the surface with rich morphology.
Frequently asked questions
By exposure to oxygen plasma at moderate power. Many metals form oxide nanowires, nanobelts or similar structures spontaneously upon treatment with oxygen plasma at elevated temperatures. The growth rate increases with increasing temperature, however, if the temperature is too high, the growth of oxide is rather isotropic so non-specific nanostructures are formed.
Yes, within a limited range of plasma parameters. The treatment by oxygen plasma at moderate power causes formation of dense nano-cones of sub-micrometer dimensions and aspect ratio 2-3.
Mix nanoparticles into the polymer matrix, then apply the technology of selective plasma etching – removal of polymer from the surface so that nanoparticles stretch from the surface.
Difficult because the formation of nano-features on polymer surface upon treatment of polymer materials with a simple oxygen plasma is a spontaneous process. Typically, nanofeatures are of conical shape with aspect ratio about 2. Such morphology is sufficient to obtain super-hydrophilic effect (if oxygen plasma is the sole treatment) or super-hydrophobic elect (if oxygen plasma treatment is proceeded with a brief treatment in plasma of hydrocarbons).
Very stable as long as the polymer products are not exposed to heat.
Treatment of polymers in continuous mode is recommended. Best results are obtained using rather powerful oxygen plasma so that the surface of polymer products assumes elevated temperature. The surface layer becomes soft enough to facilitate mobility of polymer chains. Since upon plasma treatment the surface becomes also highly polar, the soft layer organizes spontaneously into a nano-cone morphology. The plasma treatment time of few seconds is sufficient for many polymers.
Treatment of a material in continuous mode is recommended. Some materials form self-organized nanostructures even in few seconds of plasma treatment providing plasma is powerful enough.
The fillers in the paint are not properly distributed. Apply moderately powerful oxygen plasma to remove polymer from the surface thus revealing the fillers. Keep the sample temperature below about 60 °C during plasma treatment. If the fillers are distributed inappropriately, apply pre-treatments. An appropriate surfactant will do the job.
Apply moderately powerful oxygen plasma for selective removal of polymer from the surface of the composite. Once the surface is polymer free you can metallize the product by electrochemical methods.
Plasma will not improve these properties. Use plasma for removal of the polymer from the surface so that fillers will become exposed. If they form agglomerates, you have to treat the fillers prior to mixing with molten polymer. Apply brief treatment with oxygen plasma.
Briefly treat the products with oxygen plasma. The reactive plasma radicals will activate the surface so the wettability and thus adhesion of the paint will improve. If this is not sufficient (still poor adhesion) use powerful oxygen plasma to remove practically all polymer from the surface so that fillers will appear on the surface. The wettability of plasma-treated inorganic fillers is usually excellent.
For each type of a product there are optimal treatment conditions so it is highly recommended to construct the plasma system adopted to each product’s desired modifications.
One or two dimensional products such as wires, foils, fabrics and alike.
Use a low-pressure plasma instead. The life time of radicals suitable for functionalization of polymers at low temperature at atmospheric conditions is about a microsecond, while at low pressure it is more than a millisecond. Atmospheric-pressure plasma is therefore restricted to the volume of large electric field, whereas there is virtually no restriction in the case of low-pressure plasma.
Plasma treatment just before further treatment(s) is always recommended. If this is not possible, store the plasma-treated products at low temperature to slow down the hydrophobic recovery.
Surface reactions are always exothermic so any polymer exposed to oxygen plasma is heated. The polar functional groups are unstable and decay spontaneously. The higher the temperature, the faster decay of polar functional groups.
The functionalization of many products of large surfaces is notoriously slow. First, measure the density of radicals in your system. If it is below, say, 1019 m−3, change the discharge configuration to create plasma of much higher density of radicals.
Some polymers are made super-hydrophilic upon treatment with oxygen plasma at elevated temperature. The surface layer becomes soft enough to facilitate mobility of polymer chains. Since the surface becomes also highly polar upon plasma treatment, the soft layer organizes spontaneously into nano-cone morphology.
Super-hydrophilic surface finish is typical for nanorough materials with a high concentration of polar functional groups. Make your material rough on the sub-micrometre scale and functionalize it with polar groups. Best method for nano-structuring of many materials is a brief treatment (around a second) with a rather powerful plasma.
The water contact angle of about 30° is typical for well-functionalized polymer surfaces with polar groups. If you want to have much better wettability, you have to nanostructure the surface. A combination of rough surface on the sub-micrometer level and saturation with polar groups leads to super-hydrophilic surface finish.
Poor adhesion on previously plasma treated polymers is often due to ageing effects. The hydrophobic recovery is a well-known obstacle. Coat your products immediately after plasma treatment.
Oxygen plasma at the pressure of around 1 mbar created by an electrodeless high-frequency discharge is the best for rapid hydrophilization of most polymers. Such plasma is characterized by extremely high density of neutral reactive species.
It depends on the type of polymer and plasma parameters. Many polymers become hydrophilic upon treatment with oxygen plasma for about one millisecond providing plasma is rich in neutral oxygen atoms.
Super-hydrophobic surface finish is typically obtained on materials of extremely rough surface on the sub-micrometer scale. Try nano-carbon in various forms, such as nanowalls, nanomesh and alike.
This is about the maximum what one can obtain when your materials have rather smooth surface. Much higher contact angles are obtained on rough surfaces on the sub-micrometre scale. Apply technology of selective plasma etching first to obtain rough surface finish and then functionalize your material with non-polar groups.
Oxygen plasma usually causes slow etching of such materials so the oxygen functional groups do not remain on the surface. If the water contact angle increased upon plasma treatment you probably removed the surface impurities which are more hydrophilic than the base material.
Such products release water vapor upon evacuation so you have treated your samples with plasma created in a mixture of CF4 and water. The water molecules dissociate to O and OH radicals, which then cause etching. Dry your products thoroughly before employing plasma technology.
Check the residual atmosphere in the plasma reactor. If the reactor is not pumped to very low pressure, there is a significant amount of air remaining in the reactor so you create plasma in a mixture of air and CF4 rather than pure CF4. The net result is etching of your polymer rather than functionalization with non-polar groups. Analyze the surface by XPS or SIMS to see if there is any fluorine on the surface.
Yes, but you need strong differential pumping to prevent mixing of processing gas with ambient atmosphere.
Yes. The volume of oxide is typically much larger than the volume of metal so cracks or pores or nano-features appear on the surface upon plasma treatment.
Employ flowing afterglow instead of glowing plasma. Neutral reactive plasma species will do the job without measurable modification of other properties.
Only those that do not form very stable oxides. Aluminium, magnesium etc. oxides are very difficult to reduce using hydrogen plasma. On the other hand, copper, nickel, tungsten, let alone noble metals are made perfectly clean using hydrogen plasma. One may consider using oxygen plasma first (to remove organic impurities) followed by treatment with hydrogen plasma. Cool the products down to room temperature before venting the plasma reactor.
All metals and alloys.
The reactive plasma species will reduce only the surface film of thickness of few nanometers. Reduction of thicker films requires elevated temperature to facilitate thermal diffusion. If your material withstands elevated temperature you are still encouraged to use plasma technology since the required temperature is still lower than using reactive gas in a furnace.
Plasma will penetrate inside gaps only if the Debye length is much shorter than the width of the gap. For treatment of gaps it is recommended to generate plasma with a discharge that enables hollow-electrode effect. Best results are obtained using asymmetrical capacitively coupled radiofrequency discharge.
It depends on the type of impurities, processing parameters and temperature of products during plasma treatment. The removal rate increases with temperature. At room temperature it is between about 1 and 10 nm/s using mild plasma treatment. Powerful plasmas remove impurities much faster but beware of thermal decomposition. Powerful plasma causes heating of your products leaving low-hydrogen-content impurities that are very difficult to remove completely.
Probably not. If the organic impurities are thick, firstly use detergent-water solution to remove majority of impurities. Then, use plasma cleaning as the final step to assure for excellent cleanliness.
No, but it is possible to minimize it. If an oxide film is an obstacle use oxygen-free plasma for removal of organic impurities. Best results are often obtained using mixtures of nitrogen and hydrogen, or ammonia. The surface of many metals is supposed to be free from oxygen after plasma treatment but a native oxide film is formed immediately after exposing the treated material to ambient conditions. The native oxide film is extremely thin (usually around one nanometer) and its thickness slowly increases with time. The thickness of the oxide film depends also on the temperature. Cool the products down to room temperature before venting the plasma reactor.
Clean surfaces obtained by plasma treatment absorb organic impurities which are always in air. Try the wettability immediately after plasma treatment since the organic impurities adsorb in minutes if not seconds.
There are always traces of impurities on surfaces. The surface properties such as wettability (ability to bind foreign material) are often governed by organic impurities. Characterize your materials with an appropriate surface-sensitive method (such as XPS or SIMS) and you will find carbon on the surface of your metals, glasses or ceramics. A brief treatment of such samples with oxygen plasma will allow for removal of such organic impurities.