What Is Vacuum Plasma Treatment?
Plasma as the fourth state of matter introduced in 1927 by Langmuir, is observable in various places, from Vacuum plasma in a glow discharge tube in a lab to lightning and flames in the atmospheric presuure. Vacuum plasma treatment is a common method for creating surfaces with desired properties through coating, cleaning, etching, surface activation and functionalization.
Plasma can be generated by applying an electric current across a dielectric gas (arcing) or increasing the gas temperature by heating (like a flame) to take out the electrons from atoms and ionizing the gas, making it conductive. These charged particles can affect the electrical characteristics of the gas or be manipulated by external electric/magnetic fields.
Plasmas are described by many characteristics, such as gas pressure, temperature, density, and degree of ionization. Different models describing the plasma give rise to different plasma classifications: cold, warm, and hot plasmas.
Cold Plasma vs Hot Plasma
If the high-energy particles within the plasma are mostly electrons and other gas molecules are at room-temperature energy, then we have a cold plasma, however, the electron’s temperature is still so high, around several thousand degrees centigrade. Cold plasma typically exists in a low-pressure glow discharge tube and Earth’s ionosphere. Increasing the plasma pressure results in elevated electron-ion collision, reaching thermodynamic equilibrium, hence higher plasma temperature, called hot plasma. The degree of ionization in a cold plasma is about 1%, whereas a hot plasma is fully ionized.
Glow Discharge Plasma
Glow Discharge Plasma is a soup of low-pressure ionized gases created by applying a high voltage between two electrodes in a low-pressure chamber or tube. When the voltage surpasses the striking value, the gas between the electrodes is ionized and a colored light starts to glow, which color depends on the gas type within the tube. The glow discharge can be controlled to form a sustainable plasma with high voltage and low current magnitudes for research and industrial purposes. Nowadays, the glow discharge phenomenon is created in the atmospheric pressure, useful for applications like plasma cleaning of large area surfaces (You can read more about glow discharge plasma here).
Application of Vacuum Plasma Treatment
Plasma contains energetic charged particles (and ions) that can act like a sandblast or can sputter the surface of objects. This capability makes plasma a useful tool for surface treatment technologies such as cleaning, etching, coating, and surface modifications. Here are some of the plasma applications in the science and industry world:
- Plasma Coating
- Plasma Cleaning (Cold Plasma Cleaning)
- Dry Etching
- Activating Surface
- Surface Functionalization
- Hydrophobic Treatment
- Hydrophilic Treatment
1- Plasma Coating
Plasma coating is a controlled process to deposit thin films of desired materials on a substrate. Plasma coating methods are outlined below:
- PECVD (Plasma-Enhanced Chemical Vapor Deposition)
- Plasma Spraying
- Sputtering
PECVD is a subdivision of the chemical vapor deposition method in which plasma helps the chemical bonding of reactive gases. In plasma spraying, a high-temperature (~10000 K) plasma jet is fed with the powders of the material to coat a surface. Sputtering is a Physical Vapor Deposition (PVD) method in which a plasma environment produces ionized particles to eject target materials to be deposited on a substrate to form a thin film. Vac Coat Ltd. Systems offer Sputtering technology to prepare glow-discharge coated surfaces (You can view the products through the link below).
2- Plasma Cleaning
In a plasma of ionized atoms and electrons, these energetic particles can interact in several manners to clean a surface from contaminations and organic bonds. Electron-ion recombination leads to the release of ultra-violet high-energy photons, which break the organic bonds; whereas plasma oxygen species react with organic bonds and form hydrocarbon vapors pumped out of the chamber. Also, plasma particles act as a molecular sandblast to decompose the surface contaminations, then the chamber is evacuated from the organic bonds.
The typical pressure to create a plasma for cleaning a sample surface is about 1 mbar (0.001 atmospheric pressure). The plasma at atmospheric pressure is also used in some cases.
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Removal of Oxidized Layer
A vacuum plasma could be used for oxidizing metal surfaces or removing the oxide layers from a surface. The difference in the process depends on the selected gas ionization. Oxide layers could be removed with Hydrogen (H2) plasma gas. Energy from the UV radiation in plasma weakens the surface oxide bonding. Then ionized hydrogen reacts with the oxygen of the oxide layer to form a vapor. The vapor is then removed from the surface and discharged by the vacuum pump. For oxidizing metal surfaces, it is required to have oxygen plasma gas.
3- Plasma Etching
One of the main applications of vacuum plasma is plasma etching in which the glow discharge plasma bombards the substrate with energetic ionized particles to form a nanoscale pattern. Plasma etching is also called Dry Etching because it doesn’t use any liquid bath or wet solution to remove the patterned surfaces. In plasma etching, the substrate plays the role of the cathode. The chemical nature of the process gases like Ar, CF4, SF6, O2, Cl2, etc., determines how its plasma reacts with the surface of a material and thus plasma etching effectiveness. This technique is commonly used in semiconductor device manufacturing.
4- Plasma Activation
Plasma vacuum systems are widely used in activating surfaces. The process gas which creates the plasma can also be used for surface modifications. The plasma state of gaseous species (oxygen, argon, nitrogen, hydrogen, helium, etc.) can change the properties of the substrate surface. This process occurs by developing reactive species and free radicals on the surfaces by removing atoms from surfaces. These atoms and charged species eject from the surface by the collision between inert gas and surface atoms. Different surface properties can be modified as:
- Surface tension/surface energy/contact angle
- Improved interfacial bonding and adhesion, used in different applications such as microfluidic device pattering (You can read Plasma Treatment in Microfluidics)
- The wettability properties and hydrophobicity or hydrophilicity of the surfaces
- Enabled surface bonds for the bonding process
5-1- Hydrophilic Treatment
Hydrophilic surface sciences have received the attention of many researchers nowadays. These surfaces have very high surface energy that could allow water to spread uniformly. The contact angle between water and these surfaces is less than 90 degrees (Contact angle shows wettability; lower than 90° means better wettability, and higher than 90° shows lower wettability). One of the main applications of plasma treatment is surface hydrophilicity.
In this method, the desired sample exposes to air plasma or oxygen plasma for a few minutes, and then it becomes hydrophilic. In addition, the plasma coating of polymers by films of carboxyl and amide groups can cause better wetting and increase surface hydrophilicity.
5-2- Hydrophobic Treatment
A thin film of polymer is required to prepare a hydrophobic surface. Plasma coating technology can produce a few layers of polymer to cover a surface. The vacuum-coated polymer can prepare surfaces that are resistant to dust, water, oil, and aqueous solutions.
A hydrophobic surface layer has many applications in various industries, such as self-cleaning and protecting surfaces from dust, oil, and water. It offers protective effects on textiles, plastic, ceramics, medical devices, and electronics.
Plasma Treatment Advantages
Surface Cleaning Using Plasma in a Low-Pressure Environment (vacuum) is an economical way to clean specimens uniformly and securely. Here some benefits of plasma cleaning over wet chemical cleaning are numbered:
- More controllable through power, pressure, gas type, and processing time compared to very sensitive wet cleaning processes to time and chemical concentrations
- No organic residues remain in plasma cleaning while further processing steps may be required in wet cleaning
- Environment-friendly with harmless gaseous waste in contrast to large volumes of mostly toxic liquid waste produced in wet cleaning
- Applicable to a wide range of materials (metals, plastics, glass, ceramics, etc.)
- More effective in the removal of microbial contamination on medical equipment than aggressive agents and organic solvents
Surface Treatment Applications
Plasma treatment has many applications due to its advantageous features over other surface treatment agents. Nowadays, plasma treatment is used in research and industry in various fields including but not limited to:
- Ultra-fine cleaning of metal surfaces
- Transmission Electron Microscopy (TEM) grids treatment
- Electronic industry: cleaning the PCB board and the lead frames
- Surface preparation and cleaning of glass products
- Surface preparation for ink printing
- Cleaning ceramic surfaces
- Removal of oxidized layers on surfaces
- Better bonding or strong bonding
- Increasing surface adhesion
- Enhance polymer surface adhesion
These applications could be applied to many different materials such as metals, glass, ceramics, plastics, woods, and textiles.
RF or DC Plasma Treatment?
The voltage to ignite a cold plasma can be applied in two ways: DC or RF. RF plasma can be excited under lower pressure, compared to DC plasma. The lower pressure of the RF process leads to fewer residual contaminations after the plasma treatment. In the table below, some specific features of the plasma torches created by RF and DC sources are compared.
Table 1. DC vs. RF plasma torch characteristics
| Property | DC Plasma Torch | RF Plasma Torch |
| Power range | Up to 106 Watts | Up to 105 Watts |
| Efficiency | High | Low |
| Residual impurity | Yes | No |
| Temperature | High | Low |
| Electrode erosion | Yes | No |
| Price | Low | High |
Also, it should be noted that the RF plasma has a higher ionization rate than the pulsed DC plasma.
Vac Coat Plasma Cleaners
Vac Coat Ltd. designs and manufactures sputtering deposition systems based on the plasma coating technique. Deposition of various metal and compound targets is made possible by DC and RF sputtering processes through semi/full-automatic Vac Coat user-friendly coating systems. Vac Coat also offers vacuum coating systems that can be equipped with plasma cleaners to treat the substrate surface before thin film deposition.
The Magnetron Desk Sputter Coater model DST1-300, Desk Sputter Carbon Coater equipped to Turbo Pump model DSCT, Desk Sputter Carbon Coater model DSCR, and Triple Target Desk Sputter Coater model DST3 are among the best-selling models that are capable of sample pretreatment prior to sputtering deposition of thin films and TEM grids treatment for TEM sample preparation, without having to break the vacuum or remove the sample from the vacuum condition. See the Vac Coat Company website for more information.
Some of Vac Coat Products
References
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- https://www. plasmaetch.com/plasma-cleaning.php
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