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Upload organization:Tokyo Institute of Technology Department of Energy Sciences  Upload date:2011/11/10

New Atmospheric-Pressure Plasma Sources and their Industrial Applications

New Atmospheric-Pressure Plasma Sources and their Industrial Applications


Plasmas generated in atmospheric pressure have attracted increasing attention in recent years since they do not require vacuum chambers or pumping systems, and are capable of continuous plasma processing. Atmospheric pressure plasma devices developed and made available commercially thus far can generate plasmas from room temperature helium, argon, or air. We are beginning to see the application of these devices to the cleaning of semiconductor surfaces and to enabling improvements to the adhesiveness of plastics and other materials. However, the current technology is subject to various limitations, in terms of which gases can be used and the size and shape of the plasmas generated. Damage to processed items through spark discharge is also a problem.

Taking ‘multi-gas plasma’ as one of its core research keywords, the Okino Laboratory is currently developing an atmospheric plasma source able to generate plasma from any gas. Developing this multi-gas capability allows the generation of any plasma using any desired gas, thereby significantly widening the scope of applications, beyond improvements to hydrophilization and adhesiveness, to coating processes using chemical vapor deposition (CVD) and similar methods. The Laboratory is also working on the development of damage-free plasmas, characterized by low temperatures and no risk of discharge damage. In addition to conventionally processed objects, such as semiconductors and plastics, damage-free plasmas can be used for the high-density plasma processing of metal, fiber, textile, paper, and biological organisms. In addition, the Laboratory is developing handheld plasma sources, where plasma irradiation can be carried out with the plasma irradiating device held in the hand. Robotization to allow precision movement and irradiation is also, of course, being utilized in our various research activities. These developments will moreover enable the plasma irradiation of complex shapes, as well as flat plates and sheets. The Laboratory is also working on the development of technology for high-speed/low-temperature coating, hydrophilization, cleaning, and sterilization.

Multi-Gas Damage-Free Plasma Jet
Atmospheric plasma jets generate no external electric discharge, even in proximity to metals or biological organisms, meaning there is no risk of discharge damage. Because they are damage-free it is even possible, as shown in the image to the right, to direct the high-density plasma directly onto human skin. Nor will they ignite when brought into proximity with acetone or similar compounds. These plasmas have a low temperature, around room temperature, and therefore they can be used to realize the high-density plasma irradiation of materials such as metal, semiconductors, fiber, textile, paper, biological organisms, and material with a low melting point. 

Figure 1. World’s first successful development of an atmospheric-pressure multi-gas damage-free plasma jets.

Furthermore, this type of plasma source is a multi-gas plasma source, capable of the stable generation of atmospheric plasma using nearly all the gases, as shown above. It is also able to blend organic gases, which are used as source materials for CVD, meaning that it is possible to generate a plasma using any composition of gases required. For these reasons, the atmospheric-pressure multi-gas damage-free plasma jet is an extremely useful tool for simple surface treatments such as surface washing, hydrophilization, hydrophobic finishing, and disinfection, as well as advanced treatments such as coatings and CVD. Moreover, it is a remote plasma, a type where the plasma is emitted directly from the plasma source, meaning it presents distinct advantages in terms of industrial applications since there are no limitations on the materials, thickness, or shape which can be processed. 

Industrial Application: Hydrophilizing Treatments for Polyimides
Polyimides are widely used as insulating material for electronic circuits and as protective film for the surface of semiconductor devices. The increasing integration and complexity of electronic circuits, however, have created a number of new issues, including problems of adhesion to metal used for wiring materials in circuits, and adhesiveness in the face of multi-layered circuits and devices. Findings from the Laboratory’s research on irradiation using plasmas generated from multiple different gases clearly show that hydrophilization is achieved with the greatest efficacy when plasma generated from gas to which oxygen has been added is used for target material irradiation. Under optimal circumstances, perfect wettability can be achieved with less than 0.1 seconds of irradiation. 

Figure 2. Low temperature and discharge-damage-free plasmas pose no risk from contact with skin, nor will they ignite in proximity to acetone or similar compounds.

Linear-Type Damage-Free Plasma
Recent years have seen linear-type atmospheric-pressure plasma generators becoming commercially available. These are used for a number of different purposes, including the cleaning of flat panels. Until now, however, these devices have been nonmobile, requiring installation to a fixed position. Okino Laboratory has modified the high-frequency matching circuit used in these devices and simultaneously managed to make the plasma generator lighter, thereby enabling the development of a linear-type handheld damage-free plasma generator that measures 335 mm (13 inch) in length. The plasma generator itself weighs just 0.8 kg, meaning that materials can be irradiated as the device is held in the hand.

Moreover, design innovations such as a unique electrode configuration and optimal power source characteristics mean that the electric field is entirely contained within the plasma source. As a result, there is no electrical discharge even when the plasma is brought into proximity with metal or biological organisms. This in turn makes possible the high-density plasma irradiation of materials such as metal, semiconductors, fiber, paper, biological organisms, and material with a low melting point. The Laboratory has also succeeded in developing a large experimental device measuring 1 m. 

Industrial Application: High-Speed Metal Cleaning
Atmospheric-pressure plasmas can be used to treat the surfaces of various materials, in order to remove organic materials (cleaning) or generate –COOH and –C=O. Such treatments can be used to render the target material hydrophilic, thereby improving adhesiveness and paintability. Today, atmospheric-pressure plasma treatments are used for the cleaning of flat panels and the adhesion of automobile components. 

Figure 3. Cleaning speed of copper plate surface

Figure 4. The linear-type handheld damage-free plasma generator developed by the Okino Laboratory. 

The 335-mm linear-type plasma generator developed at the Laboratory was used on copper plating to render the surface more hydrophilic. Not only did the plasma and radicals generated show a greater density, by regulating plasma flow and the incorporation of external air, the Laboratory recorded treatment speeds of up to 900 mm/sec (3.2 km/h). 

Multi-Gas High-Purity Thermal Plasma

Conventional atmospheric-pressure thermal plasma devices (with power consumption of around 1 kW) have, until now, only been compatible with argon. The Okino Laboratory, however, has shown the importance of gas stream in the stable generation of atmospheric-pressure plasma, and consequently has developed a method for the stable generation of plasmas from multiple gases. The atmospheric-pressure multi-gas high-purity thermal plasma is capable of rendering a number of gases other than argon into high-temperature, high-density plasmas of between 1,500–6,000°C, including helium, nitrogen, oxygen, carbon dioxide, nitrous oxide, and air. This method of plasma generation does not require the use of electrodes, which results in the generation of plasmas of extremely high purity. With this method, it is also possible to directly introduce fluids and powders into the plasma. 

Today, therefore, plasmas can be generated from diverse gases, and fluids, powders and gases can be introduced directly into these plasmas. Consequently, it is possible to produce atmospheric-pressure plasmas with the ideal atomic/molecular composition for various plasma treatments. In addition to improvements in treatment speed and in the precision of generated materials, these plasmas also require less material, which in turn means lower costs. At the Okino Laboratory, these plasma sources are being used to conduct various research projects, including studies on high-speed semiconductor processing under atmospheric pressure, CVD, direct decomposition processing of fluids and gases, nanoparticle production, ultra-high temperature quenching, surface oxidation treatments, and single cell ultra sensitive elemental analysis. 

Figure 5. Atmospheric-pressure multi-gas high-purity thermal plasma

Industrial Application: Highly Efficient Decomposition of Greenhouse Gas Laughing gas, which is used as an anesthetic gas for patients undergoing surgery, contains 300 times the global warming potential of carbon dioxide. Despite this, this greenhouse effect gas is used widely in Japan, with around 1,000 tons being released into the atmosphere every year (the equivalent of 300,000 tons of carbon dioxide). The Okino Laboratory is currently developing an energy-efficient atmospheric-pressure plasma treatment device that generates thermal plasma from surgical-use anesthetic gas and the compressed air blended in at the time of gas emission. In addition to the thermal radiation, radicals, ultraviolet light, and high-energy charged particles all contribute to the decomposition treatment, enabling higher efficiency. 

Research results so far have recorded a decomposition rate of 99.98%, and a decomposition efficiency of 1,180 g/kWh. The Laboratory is currently considering the development of a device for the commercial market. 

Figure 6. Atmospheric-pressure high-temperature plasma generated from anesthetic gas

Associate Professor Akitoshi OKINO
Department of Energy Sciences
Interdisciplinary Graduate School of Science and Engineering
Tokyo Institute of Technology

J2-32, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan

Okino Laboratory

Plasma Concept Tokyo, Inc. (Japanese only)

Akitoshi OKINO. Born in Kyoto in 1965. Graduated from Kyoto Municipal Horikawa Senior High School. Received Bachelor and Master degree in Applied Physics from Osaka University. Received Doctoral degree from the Department of Nuclear Engineering, Tokyo Institute of Technology. Doctor of Engineering. Before taking up his current position, worked as a Research Assistant in the Department of Electrical & Electronic Engineering, Tokyo Institute of Technology. Specializes in the development of world-leading atmospheric-pressure plasma sources based on plasma engineering and spectroscopic measurements, and research on analysis of trace elements in single cells, decomposition treatments of greenhouse gases, surface treatments using plasma, and atmospheric-pressure plasma applications for medical field. Established Plasma Concept Tokyo, Inc., in 2008, and is currently Director.