Plasma processes


Plasma

In only two decades plasma processes have evolved to almost versatile tools for industrial applications. Plasma is used to manage a lot of tasks in surface treatment, i.e. coating, activation of polymers, etching of micro electronic components and cleaning.

But what is plasma technology?

By definition plasma is a partially ionized gas and often called the fourth state of matter. Phenomena like lightning and aurora borealis (northern lights) belong to natural plasmas. Technical generation can be done by application of electric fields, mostly in vacuum systems.

It is of interest for its exceptional inherent characteristics. For example the generation of highly excited particles, atoms, and radicals. These species can initiate chemical reactions not attainable under normal conditions. Thereby, the temperature of substrates can be kept at values not significantly higher than room temperature.

In addition, one can use the radiation of a plasma. By this attribute it is often called 'burning plasma'. The emission covers the range from vacuum ultraviolet (VUV) up to infrared (IR). A fluorescent lamp is an example for a technical plasma which has been optimized to gain maximum emission in the visible range.

Accelerated fast ions in the plasma enable features like sputtering, ion implantation and heating.

These physical and chemical properties make plasma processes outstanding in science and manufacturing.

Plasma activation

Activation of polymer surfaces is a certain application of the plasma technology. The meaning of activation is increasing the surface energy to enhance the adhesion and hence the wetting property. Adhesion is usually good if the surface energy is high. The contact angle of a droplet on the polymer is a convenient measure of adhesion.

In many cases polymers have low surface energies, respectively, are hydrophobic which may cause problems when using water-based inks for example. Thus, they must be chemically modified prior to paint work. For that purpose, plasma has proved to be very efficient. Plasma activation provides a substitute for current bonding agents.

Additionally, there is no limitation concerning the geometry of the substrate as well as the type of polymer, since temperature does not increase significantly during the process. And the topography keep almost unchanged. Just the uppermost monolayers are involved.

For activation processes the plasma gases consist typically of oxygen and sometimes nitrogen or ammonia. If oxygen is the gas, it is possible to modify organic surfaces in such way that they carry certain groups like hydroxyl, carboxyl, carbonic acid and peroxide. Using nitrogen or ammonia leads to amine and imine groups, which may also be of interest in certain applications. These polar hydrophilic groups change the surface from not wettable to wettable. Activation process duration is as short as some seconds. After the activation the work pieces are prepared for direct subsequent treatment, possibly in the same plasma system.

Typical utilizations for this well established technique are construction units in the automotive production, i.e. varnished polymer parts.

Here some advantages are listed :

Process

very good adhesion
better uniformity, reduced varia-bility of the paintwork results
high stability
easy control
applicable to sensitive, thermo-labile materials
Economy

no cost of disposal
no need for drying
low consumption of plasma gases
no need for adhesion primer
Ecology

solvent free
no critical waste
integrated environmental protection
Plasma cleaning

The cleaning of surfaces is also possible by means of plasma technology. Therefore, different plasma generation methods are currently in use: radio frequencies (40 kHz and 13,56 MHz) and micro waves. Typical gas mixtures contain oxygen, argon and sometimes hydrogen and carbontetrafluoride. The mixture depends on your certain cleaning problem: contamination, substrate material.

There is no limitation concerning the material to clean. Sensitive matter like thermolabile polymers is as possible as metal, glass and ceramics. But the plasma system has to be adapted to the appropriate task.

Depending on the gas mixture the plasma environment becomes oxidative or reductive. Oxidative plas-mas are advantageous in usage against organic impurities, i.e. oil and grease, while reductive plasmas remove inorganic deposits like metal oxides.

Almost every surface becomes dirty after processing. For further treatment, i.e. paint work, finishing, hard coating etc., the work pieces have to be cleaned. Plasma cleaning has proved to be the best way to get really clean, absolute grease-free surfaces.

Classical cleaning methods often utilize wet bath chemistry with hydrocarbons (HC), chlorinated hydrocarbons (CHC) or aqueous detergent solutions. All of them cause more or less environmental problems especially its disposal. Pollution control is done by end-of-pipe techniques.

In opposition to wet bath chemistry plasma cleaning fulfils the conditions of integrated environmental protection.

To understand how the plasma cleaning works one should look on a microscopic scale. What happens on and in the vicinity, respectively, of the surface?

In the case of organic contaminants the plasma gas will contain oxygen and argon. The cleaning effect is the sum of at least two processes, a physical and a chemical one.

Physical:
If the work piece is conductive it will be connected like a cathode (negative electrode). Then positive ions are accelerated towards the work piece by an electric field. They hit the surface and transfer their momentum onto it. Thus molecules and atoms are removed. These events are comparable to billiard with ions.

Chemical:
Activated oxygen, oxygen atoms and their ions react with hydrocarbons to form carbon dioxide and water. If the reaction is complete, it gets the following form :


Hydrocarbons are converted into comparably harmless gaseous compounds. Remaining products of incomplete combustion, if there are any, can be easily removed from the exhaust gases by activated carbon. Further filtering is not mecessary. Therefore, plasma cleaning is indeed an ecologically clean process.

Plasma cleaning offers some remarkable advantages in process, economy, ecology, and last but not least safety at work :

Process

high degree of degreasing
good fissure cleaning capability
ready for further plasma treat-ment, i.e. coating
high stability
easy control
applicable to sensitive, thermo-labile materials
Economy

low costs of disposal
no need for drying
low consumption of plasma gases
Ecology

solvent free
no critical waste
minimum disposal
integrated environmental protection
Working safety

only little efforts necessary for storing the compressed gases oxygen and argon
Sputtering

A COATING PROCESS WITH PLASMA

Sputtering generally means to eject atoms from a solid state target by "bombarding" it with accelerated gas ions. This technique is often used for the deposition of thin films.

Therefore a gas discharge is ignited in an inert gas (i.e. argon). The positive gas ions are accelerated towards a negative charged target where they eject atoms by direct momentum transfer. These atoms diffuse into the vacuum chamber surrounding the sputtering arrange-ment and condense as a thin film on substrate and chamber walls. Furthermore secondary electrons which keep the gas discharge running are set free.

In the following the most important sputtering techniques are explained:

1. DC-SPUTTERING

Fig. 1 shows the arrangement used for DC-sputtering. In this case target and substrate oppose each other in the vacuum chamber having a distance of a few centimetres. The target is connected to the negative output of a DC power supply, acting as the cathode whereas substrate and chamber walls act as the anode. After the creation of an argon atmosphere with a pressure of about 10-3 to 10-2 mbar the gas discharge is ignited by applying a DC voltage. The created Ar+-ions are now accelerated towards the target and eject atoms from it which then make the thin film on the substrate. 

Fig 1

DC-sputtering works with all conductive target materials. It is impossible to sputter insulating materials with this technique because the positive charged ions can't flow through the insulator. The electric circuit is interrupted. So the potential at the cathode drops and the positive ions are no longer accelerated towards the target. The process ends.

2. RF-SPUTTERING

To sputter insulators as well as conducting materials another tech-nique is used, the RF-sputtering (see Fig.2). In the RF-circuit a negative DC-potential builds up itself at the target (self-bias). This phenomenon is caused by the different mobility of heavy ions and light electrons in a high-frequency electric field.

So the Ar+-ions are accelerated towards the target again and eject atoms from it. Because the positive charges don't need to flow through the target the sputtering of non-conducting materials is also possible. Furthermore the plasma has a higher level of ionization caused by the oscillating movement of the electrons in a radio-frequency field. Therefore the current conduction is higher and lower voltages can be used consuming the same power. This has an important advantage: less damages in the deposited film than with the DC-process.

Fig 2

REACTIVE SPUTTERING

Another variation of sputtering is the reactive process which also can be realised with the DC- as well as with the RF-technique.

In the conventional process the target is made of the same material as the deposited film and the used gas is normally pure argon. The thin films which are made with the reactive process are a chemical compound of the target material and a reactive gas. This gas (i.e. oxygen or nitrogen) is mixed with the argon during the process. The atoms or molecules of both substances react in the plasma and the thin film is made of the reaction product. So we can deposit TiN- or ITO-films for example.

Except for sputtering there are some other coating processes using plasma such as PECVD (plasma enhanced chemical vapour deposition) or ion plating.

Article from Aurion website

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