Plasma Technologies

What is Plasma?

Plasma can be defined as "the fourth state of matter" because it is neither liquid, solid, nor gas. Generally, at low pressures (as in the upper atmosphere and in fluorescent lamps) or at very high temperatures (as in stars and nuclear fusion reactors), it is an ionized gas composed of positive ions and free electrons in proportions that result in almost no net electrical charge. A plasma contains ions, electrons, neutral gas atoms or molecules, UV light, and also excited gas atoms and molecules that can carry large amounts of internal energy. All of these components can interact with the surface during the plasma process. The effects of the plasma process can be precisely adjusted or determined by changing the gas mixture, power, pressure, etc. Although plasma is abundant in the universe, it is relatively rare on Earth. Lightning and auroras are the primary examples of naturally occurring plasma.

Use of Plasma Technology

Organic surface contaminants are not easily visible to the untrained eye, but these contaminants greatly affect an object's ability to interact with other materials. When we subject a surface to plasma treatment, we can remove these contaminants. Additionally, through plasma treatment, the surface can be shaped to increase or decrease wettability. This ensures that any type of printing, painting, or coating adheres to the object's surface. Many universities conduct plasma treatment on various substrates for educational and laboratory work. Plasma treatment is also applied by research laboratories at medical facilities and those working on new medical procedures. Manufacturing facilities use plasma processes to thoroughly clean their products before adhesion or printing operations. For example, a facility producing medical catheters will likely use vacuum plasma technology to clean the catheter and tubing before they are bonded together. This strengthens the bond and provides a durable, high-quality product. When manufacturing small integrated circuit (IC) chips, plasma etching is used to etch away a material layer only a few atoms thick. Nano IC chips are produced using plasma in clean rooms where spacing between transistors is as little as 10nm. This requires a very precise and smooth production process. A photoresist is used to block the etching process in the desired pattern, thus allowing only the areas without photoresist to be etched or abraded. This process allows any pattern to be processed with meticulous precision, with minimal or no errors.

How is Vacuum Plasma Treatment Performed?

To subject a product to plasma treatment, plasma is first created. A gas or gas mixture is introduced into a closed, low-pressure vacuum plasma chamber. This gas is then energized with RF (radio frequency) power generated between an array of electrodes. The active ions in these gases accelerate and begin to vibrate. This vibration removes contaminants from the surfaces in the chamber. During this process, UV light is emitted by the excited gas molecules and atoms in the plasma. This is what causes the plasma to glow. Temperature control systems are typically used to control the etching rate. Treatment at temperatures between 60 and 90 degrees Celsius can be four times faster than plasma treatment at ambient temperature. For parts that are temperature-sensitive or use temperature-sensitive components, plasma etching can be reduced to as low as fifteen degrees. Pre-programmed versions of all temperature control systems integrated into the plasma system's software are also commonly used in the industry. Settings are recorded with each plasma recipe for easy process replication. Changes in the process can be made by introducing different gases into the chamber. Commonly used gases include O2, N2, Ar, H2, and CF4. These five gases, used individually or in combination, are used in most laboratories worldwide for plasma treatment. Plasma treatment typically takes approximately five to eight minutes. When the plasma treatment is complete, a vacuum pump removes the contaminants resulting from the process and exhausts them from the environment. The products remaining in the chamber are now clean, sterilized, and ready for adhesion or further processing.

What is Plasma Cleaning?

Plasma cleaning is the process of removing all organic matter from an object's surface using an ionized gas called plasma. This is typically performed in a vacuum chamber using oxygen and/or argon gas. The cleaning process is environmentally safe because it does not contain solid chemicals. Plasma often leaves a free radical on the surface being cleaned to further enhance surface bondability.

What is Plasma Ashing?

Plasma ashing is the removal of carbon from products during cleaning or manufacturing processes. In a high-frequency plasma cleaner, oxygen is used to ensure that 100 percent of organic matter is removed by the plasma. All removed contaminants are pumped out of the chamber by the vacuum system. The purpose of plasma ashing is to completely remove organic matter, including water vapor and volatile carbon oxides. Oxygen plasma removes 100 percent of organic matter and leaves no residue on the sample. However, if there are inorganic contaminants on the sample before processing, they will still be present on the sample. Plasma ashing is always performed with oxygen (O₂) gas. A meteorite slice is placed inside an oxygen plasma cleaner that removes organics from outer surfaces. Image Source: NASA JPL-Caltech

Removal of Contamination

Low-pressure plasma cleaning is a smooth, safe, and economical method. It serves to remove contaminants from the surface of processed substrates without affecting the properties of the cast material. Plasma is widely used in the circuit board industry, including PCB cleaning before conformal coatings and cleaning of lead frames during packaging. Plasma treatment provides significant advantages over other surface cleaning techniques. It can be used for a wide variety of materials (metals, plastics, glass, ceramics, etc.). It is an environmentally friendly option. Plasma cleaning eliminates the need for hazardous chemical solvents. Significant cost savings can be achieved without having to address the environmental hazards of other processing methods. Oxygen is generally a suitable process gas. In cases where solvents may leave a residue after processing, a plasma cleaner can provide a completely residue-free end product. The process eliminates mold release agents, antioxidants, carbon residues, oils, and all varieties of organic compounds.   The surface of a MEMS device is cleaned with bright blue oxygen plasma to remove carbon contaminants. (100mTorr, 50W RF) Image Source: Maxfisch / CC 0 1.0

Plasma Sterilization

A plasma cleaner is the best solution for eliminating microbial contamination. Many medical and manufacturing facilities rely on plasma because it far exceeds aggressive surfactants and organic solvents. It provides a level of consistency and precision that other sterilization methods cannot easily compete with. Plasma systems can produce superior results by penetrating areas that solvents cannot reach due to surface tension limitations.

Oxygen Argon Plasma Cleaning vs. Hydrogen Plasma Cleaning

By using different types of gases (oxygen, argon, nitrogen, hydrogen, helium, etc.), plasma can modify various properties on the substrate surface.

These properties include but are not limited to:

• Modify surface tension / surface energy / contact angle properties. • Improved inter-surface adhesion and adhesiveness. • Hydrogen plasma is highly effective in removing oxide from glass or metal products. • Modified surface wettability to create hydrophilicity or hydrophobicity (increasing or decreasing liquid adhesiveness) - useful in pre-treatment, painting, and coating applications. • Coating processes: adhesion, wettability, corrosion and wear resistance, electrical conductivity and insulation, magnetic response, reflective/reflectance, anti-reflective, antimicrobial, scratch-resistant, water-resistant, coloration, etc.

Plasma Activation

Plasma activation is the process of treating a polymer to increase its ability to be painted or printed on. This is done by using oxygen plasma to oxidize the outer layer of the polymer. On metals that oxidize easily, argon is used. This not only produces a clean product but also increases polar groups, thus directly increasing the printability and interoperability of the polymer product. Oxygen argon plasma is also used for plasma activation. Prepared by: B. Serhat Cengiz References • www.tstar.com/materials-surface-enhancements • www.sciencedirect.com/topics/engineering/chemical-finishing • www.henniker-plasma.nl/en/plasma-technology • tantec.com/the-basics-of-plasma-treatment.html • pubs.acs.org/doi/10.1021/acsenergylett.8b00184 • pluto.space.swri.edu/image/glossary/plasma.html5