Surface Cleaning Using Plasma in a Low Pressure Environment (vacuum) is an economical way to clean specimens uniformly and securely. Removal of contaminants from the studied substrates without affecting the overall properties of the material is one of the benefits of plasma surface cleaning method. Plasma is widely used in circuit industry, including cleaning the PCB board before coating and cleaning the lead frames during the packaging process.
Plasma sample cleaning has significant advantages over other surface cleaning methods:
- Applicable to a wide range of materials (metals, plastics, glass, ceramics, etc.)
- Eco friendly. This method eliminates the need for hazardous chemical solvents, which saves considerable costs because it does not need to eliminate environmental hazards like other cleaning methods.
- The solvents leave behind the cleaning process while the plasma cleaner is able to perform the cleaning process without any residual effect. This process destroys antioxidants, carbon residues, oils and various types of organic compounds.
- Plasma surface cleaning is the best solution for microbial contamination. Many medical and manufacturing equipment depends on plasma because it is more effective than aggressive agents and organic solvents.
Plasma surface cleaning is a process in which impurities and contaminants of the sample surface are removed by the creation of high-energy plasma from gaseous particles. Gases such as oxygen, air and a combination of air, nitrogen or hydrogen are used for this purpose. High frequency voltage (in the range of kHz to several MHz) is usually used to ionize these gases and generate plasma. Created plasma to clean the sample surface is usually formed in a vacuum medium (pressure about 1 milli bars). The plasma at atmospheric pressure is also used in some cases. Plasma may be produced with DC or RF voltage. Low frequency sources are cheaper but less efficient. Choosing the appropriate source for surface cleaning application depends on a variety of factors, including cost. In order to properly select the source, the user must know which factors are more important to eliminate the contaminants in question: time, power, consumption gas, etc.
In plasma, gaseous atoms are excited and energized and often ionized. By returning atoms and molecules to their bases, they emit photons themselves. These photons give rise to the brightness and color of the plasma. Different gases lead to different colors of plasma, for example oxygen plasma emits light blue.
Plasma active components include atoms, molecules, ions, electrons, free radicals and high-energy photons with low wavelengths in the UV range. Once formed, this compound encounters all exposed surfaces of plasma.
If the used gas in the plasma is oxygen, the created plasma is an economical, efficient and environmentally friendly means of thoroughly cleaning the studied surfaces. Ultraviolet energy in plasma is effective in breaking most of the organic bonds (C – H, C – C, C = C, C – O, C – N) of the surface contaminants and causes the separation of high molecular weight pollutants. The second stage of purification involves ionized ozone, free electrons, and oxygen particles created in the plasma medium, such as O2+, O2−, O3, O, O+,and O−. These particles react with organic contaminants and produce H2O, CO, CO2 and low molecular weight hydrocarbons. These compounds have relatively high vapor pressure and are evacuated from the chamber during the process. As a result of these reactions, the exposed surface to the plasma reaches an extremely clean state. Figure 1 shows the relative amount of carbon before and after plasma cleaning process.
Figure 1: Carbon content at the z-depth of the material before and after the plasma cleaning process
If the sample is composed of oxidizing materials such as copper, inert gases such as argon or helium are used to plasma cleaning process. In these cases plasma-activated atoms and ions behave like molecular sandblasts instead of chemical reactions and can decompose organic contaminants. These pollutants evaporate during the process and are evacuated from the chamber.
If hydrogen plasma is used in the surface cleaning process, it will be very suitable and effective for the removal of glass oxide or metals.
Using different gaseous species (oxygen, argon, nitrogen, hydrogen, helium, etc.), plasma can change different properties at the substrate surface. These features include:
- Surface tension / surface energy / contact angle
- Improved interfacial bonding and adhesion
- Change the wettability properties and hydrophobicity or hydrophilicity of the surfaces
- Enable surface bonds for the bonding process
The quality of the surface cleaning process and removal of organic matter can be monitored by measuring the contact angle of the water droplet. In the case of organic contamination, the water droplet contact angle with the sample surface increases, and the contact angle of the droplet decreases as the contamination decreases to reach the contaminant-free contact surface. Figure 2 shows the contact angle of a drop of water with the glass sample surface before and after cleaning. In addition to the water contact angle, XPS and AFM tests are also used to check the quality of sample surface cleaning.
Figure 2: Contact angle of a drop of water with the glass sample surface before and after the plasma cleaning
The plasma sample cleaning process is often required to remove contaminants from surfaces prior to use in the fabrication process. This process can be applied to a set of materials along surfaces with complex geometries. Plasma cleaning can be a good alternative to wet chemical processes, such as piranha etching, which contain hazardous chemicals, increase the risk of contamination with chemical agents, and subject the process surfaces to the risk of etching.
In surface coating processes, if the surface is cleaned prior to the plasma coating process, it will have a significant effect on the quality of the created thin film. The plasma cleaning process results in a uniform film coating and better adhesion to the substrate.
Among the products of the Vaccoat Company, some vacuum coating system models are capable of performing the plasma surface cleaning process prior to the deposition. One of the models which are equipped to the plasma cleaner option is DST1-300. In this device, the user is able to the sputtering deposition process in order to deposit the desired material after the plasma cleaning process, without having to break the vacuum or remove the sample from the vacuum condition. Figure 3 shows the vacuum coating system model DST1-300. See the Vaccoat Company website for more information.
Figure 3: High vacuum Single magnetron cathode model DST1-300
Figure 4: Plasma cleaning process
- Evgeny V. Shun’ko & Veniamin V. Belkin (2007). “Cleaning Properties of atomic oxygen excited to metastable state 2s22p4(1S0)”. J. Appl. Phys. 102 (8): 083304–1– 4. Bibcode: 2007JAP…102h3304S. doi: 10.1063/1.2794857
- Banerjee, K. K.; Kumar, S.; Bremmell, K. E.; Griesser, H. J. (2010-11-01). “Molecular-level removal of proteinaceous contamination from model surfaces and biomedical device materials by air plasma treatment”. Journal of Hospital Infection. 76 (3): 234–242. doi:10.1016/j.jhin.2010.07.001
- Evgeny V. Shun’ko & Veniamin V. Belkin (2012). “Treatment Surfaces with Atomic Oxygen Excited in Dielectric Barrier Discharge Plasma of O2 Admixed to N2“. AIP Advances. 2 (2): 022157–24. Bibcode : 2012AIPA….2b2157S
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