What is Plasma Cleaning?
Surface cleaning using plasma is an economical way to clean specimens uniformly and securely through plasma treatment. Removal of contaminants from the studied substrates without affecting the overall properties of the material is one of the benefits of the plasma surface cleaning method. Plasma cleaning is widely used in the 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 as it applies to a wide range of materials, is eco-friendly, residual-free, and effective in highly sensitive applications like cleaning medical equipment.
Plasma Cleaning Process
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. Applying a high DC voltage or AC voltage (in the range of kHz to several MHz) ionizes these gases and generate plasma. In plasma, gaseous atoms are excited and energized and often ionized. Plasma active components include atoms, molecules, ions, electrons, free radicals, and high-energy photons with low wavelengths in the UV range, as the last one is the origin of plasma color.
These constituents encounter the exposed surfaces to perform various vacuum plasma treatment procedures like plasma coating, plasma functionalization, and plasma cleaning processes such as plasma etching and ashing.
Created plasma to clean the sample surface is usually formed in a vacuum medium (pressure about 1 millibars). The plasma at atmospheric pressure is also used in some cases (You can read more about pressure gauges).
How to Effectively Clean the Sample Surface by Glow Discharge Plasma?
There are many factors in creating a plasma that can control the productivity of the plasma cleaning procedure. The type of these items are among the key parameters.
- Power source
- Pressure level
- Sample location
- Gas
- Operation time
Plasma Power Source
Plasma may be produced with DC or RF voltage. Choosing the appropriate source for surface cleaning applications depends on a variety of factors, including cost and power, as low-frequency sources are cheaper but less efficient. 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.
Pressure in Plasma Cleaning Procedure
The plasma for the surface cleaning procedure can be created under atmospheric or vacuum conditions. Plasma cleaning parameters are different as classified by the range of applied voltage, chamber pressure, gas, sample position, etc. Some of them are summarized in the chart below.
The atmospheric pressure plasma cleaning uses weak and non-uniform corona discharge plasma at short electrode-surface distances to clean a surface, while low-pressure plasma cleaning benefits varied control factors such as sample position.
Sample Location in the Plasma Chamber
The position of the sample inside the vacuum chamber affects the cleaning procedure either. The energy of the plasma ions and electrons strongly depends on the sample distance from the cathode/anode. Also, the sample surface temperature rises as the electron bombardment increases on the anodic plate, while the sample placed on the cathode, is bombarded with energetic ions which etch or react with the surface. If the sample is placed somewhere between the two electrodes, the bombardment intensity can be tuned with a biasing voltage.
Plasma Gas
Using different gaseous species (oxygen, argon, nitrogen, hydrogen, helium, etc.), plasma can change different properties of 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 common gas types used to create a plasma for cleaning purposes are Oxygen, Hydrogen, and Argon gas.
Oxygen 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. This picture, shows the relative amount of carbon before and after plasma cleaning process.
Hydrogen Plasma
There exists an oxide layer on almost all metal surfaces exposed to the air atmosphere, which helps to prevent the corrosion of the metal bulk. However, these oxide layers act as a barrier that obstructs electrical contacting processes such as soldering and bonding. Hydrogen molecules, ions, and radicals in the hydrogen plasma create vapor with the oxygen of the oxide layer, which is easily pumped out by the system vacuum pump.
Argon Plasma in Plasma Cleaner
Some substances, like salts, inorganic materials, or ceramics, cannot be removed by the plasmas created from oxygen or hydrogen gases. Here, the ion bombardment with inert gas (like argon or helium) plasmas is used to eliminate almost all substances through physical etching, or the so-called micro-sandblasting. In these case, plasma-activated argon atoms and ions behave like molecular sandblasts instead of chemical reactions and can etch the substrate contaminants, and even its surface, leading to elevated surface roughness, which should be optimally controlled if low surface roughness is required. These pollutants evaporate during the process and are evacuated from the chamber.
Cleaned Surface Characterizations
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’s 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. This picture, 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 justify the quality of sample surface after cleaning.
Plasma Cleaning Applications
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.
Vac Coat Vacuum Coating Systems with Plasma Cleaner Option
Almost all vacuum coating system models produced by the Vac Coat Company, including Sputter Coaters and Carbon Coaters, are capable of performing the plasma surface cleaning process prior to the deposition. The plasma cleaner option is provided as an optional feature on low and high-vacuum SEM Coaters such as Magnetron Desk Sputter Coater models DSR1, DST1-300, Desk Sputter and Carbon Coater equipped with Turbo Pump model DSCT, Desk Sputter Carbon Coater model DSCR, and the research-grade sputter coater, Triple Target Desk Sputter Coater model DST3.
In DST1-300, the user is able to do 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. See the Vac Coat Company website for more information.
References
- Evgeny V. Shun’ko & Veniamin V. Belkin (2007). “Cleaning Properties of atomic oxygen excited to metastable state 2s22p4(1S0)”. 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: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 O2Admixed to N2“. AIP Advances. 2 (2): 022157–24. Bibcode : …2b2157S
- https://www.plasmaetch.Com/plasma–cleaning.php
- https://engineering.jhu.edu/labs/wp-content/uploads/sites/76/2016/04/All-About-Plasma-Cleaning.pdf
- Dinesh P.R. Thanu, Endu Sekhar Srinadhu, Mingrui Zhao, Nikhil V. Dole, Manish Keswani, Chapter 8 – Fundamentals and Applications of Plasma Cleaning, Editors: Rajiv Kohli, K.L. Mittal, Developments in Surface Contamination and Cleaning: Applications of Cleaning Techniques, Elsevier, 2019, Pages 289-353,
- https://www.plasma.com/en/plasma-technology-glossary/plasma-cleaning/
- Gupta, V., Madaan, N., Jensen, D. S., Kunzler, S. C., & Linford, M. R. (2013). Hydrogen plasma treatment of silicon dioxide for improved silane deposition. Langmuir, 29(11), 3604-3609.
Can we use plasma cleaner with lacey carbon substrates on TEM grids without damaging them?
According to the scientific literature on the plasma cleaning of carbon lacey TEM grids, plasma cleaning generally damages the carbon layer and makes it thinner, hence a short-term (< 80 s), low-power plasma (<10 W) of low-weight particles like hydrogen/oxygen may be helpful in removing the carbon lacey contaminants.