Plasma Physics
Plasma is the fourth state of matter that can be reached in high energies. Normally, when atoms in a solid gain enough energy it goes to liquid phase, and then the phase transition is continued to the gas phase at higher energy levels, and then plasma is formed at extremely high temperatures. In a typical plasma, some or all of the electrons in an atom gain enough energy to overcome nucleus potential barrier and an ionized gas is formed. So, plasma contains free electron-ions and is able to conduct electricity. Since atoms are normally neutral, the plasma is electrically neutral. Plasma may contain magnetic fields as moving electric charges create them.
Plasma Temperature
Plasma is not condensed enough that its constituent particles collide with each other frequently, so energy transfer and thermal equilibrium in plasma is not the same as the gas phase. Applying electromagnetic fields to the plasma, it may go unstable and a bunch of very fast particles appear in it, hence the plasma can possess different temperatures in different parts at a time; as the temperature of sun’s corona, made of plasma, can reach to millions of degree, while the temperature of the surface of the sun is about thousands of degrees (Figure 1).
Electromagnetic Interactions
Electromagnetic interactions are so important in the plasma as a result of electron-ion separation. Moreover, these interactions can occur at larger distances compared to the gas phase. So, waves in plasma, or organized motion of it, is very significant. Another noticeable feature of it is that it can be confined by using a magnetic field (Figure 2), a capability used in fusion energy research.
Plasma in the Universe
Plasma is the most abundant form of matter in the universe, as 99% of the seen universe is in the form of plasma. Stars, nebulas, and the aurora seen in the north and south pole are all made of plasma. We can also see it in everyday life in the form of a lightening, conductive gas in a fluorescent light bulb or a neon light bulb. In addition, it is a favorite field for the scientists to research.
Plasma Generation
Nowadays, plasma is generated by applying a high voltage between two electrodes, mostly in a controlled low-pressure gaseous environment, as a routine method for various applications in different fields. The plasma acquires different properties and applications depending on applying a DC (Direct Current) or AC (Alternative Current) voltage to the electrodes. The plasma generated using an AC voltage is usually called RF plasma, referring to the frequency range of the applied voltage, which is in the range of Radio Frequency (RF) ranging from 13.56 MHz to several GHz, created by the RF signal generators.
DC vs. RF Plasma Generation
The DC and RF plasma possess different operational characteristics, which suit them for processing different types of matters. The main dissimilarities are the working pressure, target materials type, and plasma stability which are discussed below.
Process Pressure
RF plasma can be generated in lower chamber pressure, under 15 mTorr, compared to the conventional DC plasma, which enhances the mean free path (MFP) of the sputtered target particles and faster deposition rate.
Type of Target Material
RF plasma has the capability for sputter coating conducting and insulating materials, due to continuous electric pole alteration and eliminating charge buildup on the insulating target surface. In contrast, DC plasma can only sputter conducting sputtering targets. Also, DC self-bias creation on the target surface during RF sputtering has a positive effect on the sputtering rate.
Maintenance and Stability
RF plasma can provide longer operational times without the need for maintenance breaks with more reliability, compared to DC plasma. The influencing factors in plasma formation and stability, like signal characteristics such as pulse duration, frequency, and power, along with chamber pressure are more controllable in RF signal generators utilizing precise RF matching network. However, DC plasma is more common for straightforward and easy operation.
Applications
Plasma can be utilized in many areas from the edge of science to medical applications and industry. Plasma generators are mainly used in:
Surface Plasma Treatment
Surface treatment of different materials with plasma can help to functionalize different surfaces, leading to enhanced surface hydrophilic/hydrophobic properties, corrosion resistance, and adhesion of deposited layers onto it.
Semiconductor Industry
Plasma coating and etching can be used in the semiconductor electronic industry, such as manufacturing computer chips, employing different deposition and surface modification processes to obtain various semiconductors with diverse properties, like plasma-enhanced chemical vapor deposition (PECVD).
Medical Applications
Plasma application is a useful method to sterilize medical instruments and eliminate biological hazards. It can also be employed in medical sciences to develop new medical treatments, like healing wounds, cancer therapy, and improving bone healing.
Environmental Remediation
Protection of the environment, cleaning natural resources (Air/water/soil) from hazardous wastes and pollutants, like microplastic, and removing bacteria and viruses are achievable through plasma.
Other Applications
- Plasma TV
- Plasma lamp or plasma globe
- Nuclear energy reactors
- Electron guns
- Rocket propulsion
- Plasma knife
- …
Vac Coat Coating Systems
Vacuum coating systems made by Vac Coat Ltd. are optionally capable of performing the plasma surface cleaning process prior to the deposition. Some of the models that are equipped with the plasma cleaner option are magnetron sputter coater model DST1-300, desk sputter carbon coater equipped to turbo pump model DSCT, desk sputter carbon coater model DSCR, and triple target sputter coater model DST3.
In DST1-300, 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. See the Vac Coat Company website for more information.
References
- https://www.psfc.mit.edu/vision/what_is_plasma
- https://en.wikipedia.org/wiki/Plasma_(physics)
- https://www.livescience.com/54652-plasma.html
- http://thescienceexplorer.com/technology/fourth-state-matter-plasma-technology-improve-bone-healing
- https://www.nasa.gov/mission_pages/sdo/multimedia/potw/potw30.html
- https://www.nasa.gov/mission_pages/hinode/solar_017.html
- https://vactechniche.com/rf-plasma-formation-process/#:~:text=Mechanism%20of%20Plasma%20Generation%20with%20RF&text=Gas%20Ionization%3A%20RF%20energy%20is,ionization%20of%20atoms%20or%20molecules.
- Sima, J., Wang, J., Song, J., Du, X., Lou, F., Pan, Y., … & Zhao, G. (2023). Dielectric barrier discharge plasma for the remediation of microplastic-contaminated soil from landfill. Chemosphere, 317, 137815.
- Du, ChangMing, and JianHua Yan. Plasma remediation technology for environmental protection. Springer Singapore, 2017.
- https://kindle-tech.com/faqs/what-is-the-difference-between-rf-plasma-and-dc-plasma#:~:text=RF%20plasma%20operates%20at%20lower,pressures%2C%20typically%20under%2015%20mTorr.
- Park, Sang Eun, et al. “Properties of gallium-doped zinc-oxide films deposited by RF or DC magnetron sputtering with various GZO targets.” Journal of the Korean Physical Society 54.3 (2009): 1283-1287.