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PLASMA PROCESSING
Plasma processing is a plasma-based material processing technology that aims at modifying the chemical and physical properties of a surface.
Plasma processing techniques include:
Plasma activation (or Plasma functionalization). It is done with the intent to alter or improve adhesion properties of surfaces prior to coating, painting, etc. In most cases, the surface in question is surface of a polymer material and weakly ionised oxygen plasma is used.
Surface activation is a result of following processes:
Removal of weak boundary layers. Plasma removes surface layers with the lowest molecular weight, at the same time it oxidises the uppermost atomic layer of the polymer.
Cross-linking of surface molecules. Oxygen radicals (and UV radiation, if present) help break up bonds and promote the three dimensional cross bonding of molecules
Generation of polar groups. Oxidation of the polymer is responsible for the increase in polar groups which is directly related to the adhesion properties of the polymer surface.
Plasma activation (or Plasma functionalization). It is done with the intent to alter or improve adhesion properties of surfaces prior to coating, painting, etc. In most cases, the surface in question is surface of a polymer material and weakly ionised oxygen plasma is used.
Surface activation is a result of following processes:
Removal of weak boundary layers. Plasma removes surface layers with the lowest molecular weight, at the same time it oxidises the uppermost atomic layer of the polymer.
Cross-linking of surface molecules. Oxygen radicals (and UV radiation, if present) help break up bonds and promote the three dimensional cross bonding of molecules
Generation of polar groups. Oxidation of the polymer is responsible for the increase in polar groups which is directly related to the adhesion properties of the polymer surface.
Plasma polymerization uses plasma sources to generate a gas discharge that provides energy to activate or fragment gaseous or liquid monomer, often containing a vinyl group, in order to initiate polymerization.
Plasma polymerization can be used to deposit polymer thin films. By selecting the monomer type and the energy density per monomer, known as Yasuda parameter, the chemical composition and structure of the resulting thin film can be varied in a wide range.
Plasma cleaning involves the removal of impurities and contaminants from surfaces through the use of an energetic plasma created from gaseous species. Gases such as argon and oxygen, as well as mixtures such as air and hydrogen/nitrogen are used. The plasma is created by using high frequency voltages (typically kHz to >MHz) to ionise the low pressure gas (typically around 1/1000 atmospheric pressure), although atmospheric pressure plasmas are now also common.
In a plasma gas atoms are excited to higher energy states and also ionised. As the atoms and molecules 'relax' to their normal, lower energy states they release a photon of light, this results in the characteristic “glow” or light associated with plasma. Different gases give different colours. For example, oxygen plasma emits a light blue color.
A plasma’s activated species include atoms, molecules, ions, electrons, free radicals, metastables, and photons in the short wave ultraviolet (vacuum UV, or VUV for short) range. This 'soup', which incidentally is around room temperature, then interacts with any surface placed in the plasma.
If the gas used is oxygen, the plasma is an effective, economical, environmentally safe method for critical cleaning. The VUV energy is very effective in the breaking of most organic bonds (i.e., C-H, C-C, C=C, C-O, and C-N) of surface contaminants. This helps to break apart high molecular weight contaminants. A second cleaning action is carried out by the oxygen species created in the plasma (O2+, O2-, O3, O, O+, O-, ionised ozone, metastably-excited oxygen, and free electrons). These species react with organic contaminants to form H2O, CO, CO2, and lower molecular weight hydrocarbons. These compounds have relatively high vapour pressures and are evacuated from the chamber during processing. The resulting surface is ultra-clean.
If the part to be treated consists of easily oxidised materials such as silver or copper, inert gases such as argon or helium are used instead. The plasma activated atoms and ions behave like a molecular sandblast and can break down organic contaminants. These contaminants are again vapourised and evacuated from the chamber during processing.
Most of these by-products are small quantities of harmless gasses such as carbon dioxide, and water vapor with trace amounts of carbon monoxide and other hydrocarbons. To put this in perspective, 10 minutes of automobile exhaust is approximately equivalent to one year of plasma cleaning exhaust.
Whether or not organic removal is complete can be assessed with contact angle measurements. When an organic contaminant is present, the contact angle of water with the device will be high. After the removal of the contaminant, the contact angle will be reduced to that characteristic of contact with the pure substrate.