- Vacuum evaporation
- Cathodic Sputtering
- Cathodic arc deposition
- Ion beam sputtering
- Pulsed laser deposition
Vacuum evaporation was one of the first methods developed in the category of physical vapor deposition. It was developed with the aim of obtaining thin films and is still one of the most commonly used methods in industry today (although it is being replaced by other processes such as magnetron sputtering, which provides better coverage of the surface of the parts and better adhesion).
Its principle is based on the evaporation, under vacuum, of a source material that we wish to coat a part with. The presence of a high vacuum within the chamber is important for this process because it allows a reduction in the sublimation temperature of the material. This material, subjected to an energy source, is then brought to its saturating vapour pressure and evaporated to then is deposited on the surface of the substrate (a more or less direct trajectory linked to the average free path of the species made possible by the vacuum).
It should be noted that the notion of high vacuum is important for a good number of PVD type processes for reasons that may vary but which induce a high level of purity of these deposits and allow a relatively straight trajectory of the elements.
Various means can be used to provide the energy necessary for the sublimation of the material such as :
- Joule effect (or resistive)
- Ion or laser bombardment
- Electronic bombardment (or EBPVD) assisted or not by ion beam.
In fact, this technique makes it possible to deposit a large majority of unalloyed materials (difficulties in the case of evaporation of alloyed materials whose constituents may have very different values of saturating vapor pressure) but is limited for the most refractory materials. Moreover, the relatively low energy of the evaporated particles results in no damage to the most sensitive substrates but also in lower adhesion compared to other processes.
Examples of applications :
- Use in the field of optics (e.g. filters)
- In the plastic packaging industry (metallization of plastics)
Cathodic sputtering is one of the many processes affiliated to the family of vacuum depositions known as PVD. It itself includes a series of sub-techniques, some examples of which are listed here:
- Magnetron cathodic sputtering: control of the plasma distribution on the target surface using a system called magnetron consisting of a series of magnets. The magnetic field generated gives a helical trajectory to the electrons close to the target, thus increasing the number of collisions and consequently the density of the plasma within this area.
- Reactive sputtering: this is a process whose operating principle is close to that of PECVD. In fact, in addition to the plasma gas, a reactive gas is injected which will react with the species present and thus create a third body which will form the desired deposit. This may be, for example, nitrogen or a carbon-based compound necessary for the formation of metal nitrides or carbides.
- High-power pulse magnetron sputtering (or “HIPIMS”): the principle is close to that of magnetron sputtering except that the power densities present are much higher (several MW) with pulses of a few microseconds (1 to 50 µs). This technique allows a strong ionisation of the species (> 99 %) contrary to other techniques (< 5 %). It thus allows a high level of control of the energy and direction of the species that participate in the growth of the layer.
The operating principle of cathodic sputtering is based on the creation of a localised plasma around the target of the material to be deposited thanks to a series of magnets. The ions of the plasma thus created and accelerated bombard the negatively polarised material and sputter it. Once vaporised, the material condenses on the surface of the part to be coated. Thus, as stated in the introductory section, the phenomena involved in obtaining the vapour (except for reactive spraying) are essentially physical.
The gases used in this type of process are inert plasmagen gases, which the most frequently used is argon (because it is easily ionised and for economic reasons). Once the gas is injected at very low pressure into the chamber, the target (cathode) is negatively polarized, allowing an electric field to circulate between the cathode and the grounded anode. Then, this causes the argon to ionise and consequently creates a cold plasma consisting of electrons, ions, photons and neutral species.
The Ar+ ions thus created are attracted and accelerated towards the target in the cathode sheath acquiring kinetic energy which they release by impacting the cathode surface. The cathode is then sputtered through a purely mechanical process of kinetic energy transfer. The material thus ejected is then deposited on the opposite substrate along a unidirectional trajectory linked to the free mean path of the species.
Advantages of this technique
- Possibility of spraying materials with very high melting points.
- Layer composition identical to that of the target serving as a material source at the coating scale.
- Good adhesion of the developed layers.
- Wide range of metallic and ceramic coatings possible.
- Increased density of the growing layers.
- Removal of impurities and insufficiently adherent species.
Drawbacks of this technique
- Difficult layer-by-layer growth control.
- Limited growth rate in relation to evaporation.
- Possible degradation for the most sensitive substrates.
Examples of applications :
- Optics (anti-reflective)
- Mechanics (cutting tools, machining)
- Production line (moulds, matrices)
- Automotive, aeronautics (wear resistance, friction)
- Metallization of plastics
Cathodic arc deposition is a type of process belonging to the family of Physical Vapor Deposition. An electric arc is used to sublimate a material from a target (cathode). In the same way as for all other PVD techniques, the vaporised material then condenses on the surface of the part to be coated, forming a thin layer.
The use of a high-power electric arc (high current and low voltage) directed at the target material during this process generates a localised plasma (or “spot cathode”) with a high rate of ionisation of the species present (30 to 90%). It is then possible to reach very high average temperatures within the “cathode spot” of up to 15,000 °C. Due to the phenomena involved, the material sublimates and is ejected from the target with a certain amount of energy allowing the material to reach speeds of around ten kilometres per hour. Once the material has sublimated, the “spot” disappears, leaving a crater on the surface of the target, then the arc forms again nearby and recreates another “spot”. The cycles (of very short duration) are thus repeated until the coating is complete.
This process can be adapted by applying an electromagnetic field to the target to direct the point of impact of the arc and thus over time cover the entire surface of the target to avoid eroding a particular area of the material only. In addition, reactive gas injection can be used to produce compound coatings.
For example: the injection of nitrogen during titanium sublimation results in the production of a titanium nitride coating. The gases thus injected react with the evaporated material to form the coating.
One of the disadvantages of arc deposition is that if one of the “spots” is concentrated for too long on a particular area, macro-particles (droplets) may be ejected from the target surface and deposited on the surface of the parts. This type of particle is relatively detrimental to the properties of the coating. Furthermore, in the worst case, this can result in significant damage to the target holder or the deposition chamber.
Cathodic arc deposition is often used to produce films with a very high hardness used mainly for the protection of cutting tools (thus significantly increasing their service life). It is thus possible to produce a wide range of hard coatings ranging from conventional nitride or carbide coatings (TiN, TiAlN, CrN, TiC, CrC, TiAlSiN) to nanostructured coatings and coatings known as DLC (Diamond Like Carbon).
Ion beam sputtering is a PVD technique in which the vapour of the material to be deposited is generated by interaction between the target and an ion source with high kinetic energy.
The particularity of this technique lies in the fact that the ions are not generated from or around the target but within an ion gun. Thus, it is possible to use ions with a higher energy than in sputtering, for example, and to control their energy and flow independently.
On the other hand, this requires the use of more complex technology making this process still little used industrially.
Pulsed laser deposition has the following general mechanism: a high-powered laser ablates the material to be deposited (usually in the form of a target) in a high-vacuum chamber and then passes it into the vapor phase so that it condenses on the surface of the workpiece to be coated in a face-to-face relationship. Compared to other PVD techniques, they allow more particularly the production of thin to very thin layers.
The coatings obtained can be made under secondary vacuum or in the presence of a low-pressure gas. For example, oxygen can be used to form thin layers of oxides.
It should be noted, however, that although the standard parameters for making deposits are simpler to set, the phenomena involved in this type of process are much more complex than those of other vacuum deposition techniques. Indeed, the pulsed laser beam is first absorbed by the material.
The energy thus acquired is then converted in several stages: into electronic excitation, into thermal energy, then chemical energy and finally mechanical energy, allowing the material to evaporate, be ablated and the formation of a plasma.
The species ejected from the target are present in many forms ranging from atoms to molten macro-particles, electrons, ions, molecules and other small particles. These are then deposited on the surface of the heated substrate.
In general, the process of a conventional coating using this type of process can be divided into four major steps, all of which play a major role in the properties of the coatings (e.g. crystallinity, uniformity and stoichiometry):
- Ablation of the target material by laser pulses and creation of the plasma
- Plasma dynamics
- The migration of the sprayed species to the surface to be coated
- The phenomena of nucleation and growth of the film on the substrate surface
This last step depends mainly on the following elements:
- Laser parameters
- Substrate temperature
- The surface condition of the workpiece to be coated
- The value of the pressure in the chamber
It should be noted that it is possible to obtain different modes of growth by this type of process :
- Growth by step
- Layer by layer growth
- 3D growth