Plasma technology can be used in circumstances where you would like to improve a material surface properties. Standard applications include
- Cleaning of residues such as hydrocarbons
- Activation before gluing, painting or printing
- Etching to partially remove surfaces as often used in semiconductor processes
- Coating of surfaces with PTFE like layers as protective barrier or to reduce friction
Plasma vs. Flame / Chemical Treatment
Compared to other methods, like flame treating or using chemicals to treat a surface, plasma technology shows many significant advantages:
- Many surface properties can be obtained exclusively with plasma treatment
- Can be used in online production or be operated independently and environmentally friendly process
- Regardless of geometry you are able to treat powder, small parts, discs, fleece, textiles, tubing, bottles, circuit boards, etc.
- Fabricated parts will not be mechanically changed
- Heating of the parts is minimal
- Operating costs are very low
- Extremely safe to operate
- Process is extremely energy efficient
Unlike low-pressure plasma or high-pressure plasma, in atmospheric-pressure plasma, no chamber is required to maintain a certain pressure level. Therefore, no costly chamber for producing a partial vacuum is needed.
Atmospheric-pressure plasma are a continuous process and need more measurement to control them than batch processes traditionally used in low pressure application, as atmospheric plasma are less contained and more difficult to control.
Dusty plasma, or complex plasma, contains particles sized between a millimetre and a nanometre, which can also combine to form larger particles known as “grain plasmas”. These particles are charged, and both the particles and the plasma behave as a plasma.
Measuring the dust directly is difficult because of the small size; in addition, measuring the charge on each dust particle is tricky because of the tiny charge. The most straightforward way to measure the dust density and charge is to measure the plasma density and temperature and calculate the dust parameters.
A method for physical vapour deposition of thin films, High-Power Impulse Magnetron Sputtering (HiPIMS), or High-Power Pulsed Magnetron Sputtering, uses extremely high power densities in short pulses of tens of microseconds at low duty cycle. The sputtered metal features a high degree of ionisation and a high rate of molecular gas dissociation, which result in high density deposited films. In HiPIMS, there is a need to measure time-resolved ion flux, neutral to ion ratio and degree of ionisation at substrate. It is also important to distinguish metal ions from background gas such as helium or argon.
Similar in concept to sandblasting, the ion beam application uses individual atoms in an ion beam to ablate a target. The physical sputtering effect is improved by using chemical reactivity in a process known as reactive ion etching. Focused ion beam instruments use a high-brightness beam in a scanned raster pattern to remove material in exact rectilinear patterns.
Broad ion beams are used to coat optical components where the ion beam assists the chemical reaction and helps remove non-volatile material during the process.
Plasma-enhanced chemical vapor deposition (PECVD) is used to deposit thin films from a gas state to a solid state on a substrate. The process uses a plasma generally created by RF frequency or DC discharge between two electrodes. There is a need to measure the plasma uniformity of plasma density and electron temperature to understand the reaction rate of processes of interest.
Used in integrated circuits, plasma etching shoots an appropriate gas mixture at high speed at a chosen sample. The plasma, or etch species, can be either charged ions or neutral atoms and radicals. The physical properties of the target will be modified as the atoms of the shot element will embed themselves at or just underneath the surface of the target.
There is a need to measure ion and neutral flux, energy, and angle arriving at the etch surface. In addition, for pulsed shots, these parameters need to be time-resolved.
Much of the baryonic matter of the universe is believed to consist of plasma, or an ionized gas consisting of negatively charged electrons and positively charged ions. These particles are charged, and are therefore strongly influenced by electromagnetic forces. All known astrophysical plasmas are influenced by magnetic fields. Scientists use plasma chambers to replicate the plasma state in space to understand how the plasma and the spacecraft interact. Therefore, diagnostics are needed to confirm that the plasma has the correct parameters to replicate what is found in space.
In sputtering, energetic particles bombard the target, therefore removing atoms. The kinetic energy of the incoming particles must be much higher than conventional thermal energies (≫ 1 eV). The process is commonly used for thin-film deposition, etching and analytical techniques.
Measuring the time resolved and space resolved ion distribution and energy is important in understanding the functioning of any specific sputter process.
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 times.
- 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.
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-chainpolymers. 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