Microwave plasmas – all-rounders with huge market potential
Low-pressure plasmas have been used as a technical tool for quite some time, but only since plasmas can be generated also under atmospheric conditions new fields of application have opened up. Cold plasmas, for example, are highly attractive in industrial production when treating temperature-sensitive materials and activating surfaces. Recently, plasma processes have been established also in medicine and medical technology, to support wound healing and for sterilization of medical instruments, for instance.To generate a plasma, power is supplied to a gas by applying an electric or magnetic field. Plasma properties can be controlled through the energy supply, the composition of the gas and the pressure. What makes microwave plasmas specific is their high excitation frequency, typically in the 2 GHz range. Basically, frequency influences plasma applications in three ways. First, plasma characteristics change with growing frequency because the ionized atoms cannot follow the electromagnetic field and thus do not move any more while the lighter electrons can. This effect can be exploited to generate cold plasmas, for instance. Second, the electrical resonators needed to couple the energy into the plasma shrink in size, which allows realizing very compact plasma sources not feasible at lower frequencies. Third, the electron density is quasi-constant during a microwave period, thus making the plasma very efficient and avoiding generation of harmonics.
Compact microwave plasma generators for atmospheric and low-pressure conditions
FBH's atmospheric plasma sources require neither a vacuum chamber nor a high-voltage supply. They work under normal ambient air conditions. An oscillator integrated in the source generates a microwave signal in the 25-watt range, using an FBH high-performance gallium nitride transistor, which delivers high power at high efficiency. The oscillator feeds its 2.45 GHz signal through a resonant structure to the gas and thus ignites and maintains the plasma. Altogether, this results in a source with very small form factor. Electrically, only a DC supply is required. In a joint effort with the in-house Prototype Engineering Lab, this source was developed as a modular building block that can be tailored to the needs of the respective application.
The second topic of microwave plasma work at FBH is less mature, but very promising. It is about generating an inductively coupled plasma (ICP) within a small volume with an edge length of 10 cm, which was demonstrated in a DFG project together with Ruhr-Universität Bochum for the first time ever. Such ICP plasmas feature highly attractive properties such as high electron density and high plasma purity. Again, microwave excitation allows shrinking down bulky equipment to handy sizes.
Further information on FBH's microwave plasma sources
What is plasma?
Plasma is an ionized gas which consists partly or entirely of free charge carriers (ions and electrons). It is considered to be the fourth state of matter and occurs when substances are heated up. More than 90 % of the visible universe is in plasma state, such as the sun and the stars. On Earth, thermal plasmas occur in fire, and electric plasmas in lightning or polar lights. Characteristic is their typical glow, which is caused by radiation emitted from excited gas atoms, ions or molecules.
Technical gas discharge plasmas must be permanently supplied with energy, otherwise the plasma extinguishes. The positive and negative charge carriers then combine to form neutral atoms and molecules. The electrons released in plasmas give rise to new physical properties and affect their environment. Both electric and magnetic fields can control plasmas in such a way that they are directed towards a certain surface or kept away from another surface. This is used, among other things, for material coating or surface hardening. The application spectrum is broad and ranges from atomic layer deposition to material processing such as cutting or welding. In the medical sector, they are used for disinfection and wound healing.
Microwave plasmas are characterized by low plasma temperature, excellent intensity and good homogeneity. Furthermore, when generated under atmospheric pressure, they can be used efficiently and cost-effectively in a variety of applications, preferably for surface treatment such as activation, coating, and cleaning.
For this purpose, the FBH has developed an extremely compact plasma source that provides a microwave plasma with up to 20 W power, which is sufficient for many applications. With it, even temperature-sensitive materials like plastics can be treated without any problems. The source is also suitable for medical applications.
The small-sized plasma source contains a resonator for plasma excitation, a power oscillator operating in the 2.45 GHz ISM band as well as control and monitoring electronics – all integrated into a very handy housing of only 114 x 33 x 25 mm³. Center piece of the efficient power oscillator is an FBH GaN transistor.
Design and development of the plasma source rely on FBH’s comprehensive know-how in microwave technology. As a first step, a novel nonlinear plasma model was developed which predicts the electrical properties of the capacitively coupled microwave plasma (CCP) as a function of the absorbed power. Only with this knowledge it was possible to realize a microwave plasma source that both safely ignites the plasma and maintains it in a power-efficient way. Similarly, the resonator used for plasma excitation had to be carefully characterized, modelled and optimized. Among others, different electrode shapes were investigated to obtain the most suitable construction. Last but not least, the microwave power oscillator was optimized in terms of output power and efficiency as well as robustness against tolerances. One result of these comprehensive design efforts is that the plasma source can be easily adapted to different application scenarios, since it offers various tuning possibilities. For example, the power fed to the plasma can be adjusted over a wide range, also during operation.
Beyond offering outstanding properties and versatile operation, this new generation of FBH plasma sources is very easy to put into operation. Basically, only a 48 V DC voltage, gas supply and water cooling are required. Due to the special microwave power oscillator design, the plasma source works with a variety of gases including argon, air, oxygen, and nitrogen. The standard water cooling can be replaced by air cooling, if required. Because the built-in control and monitoring electronics ensure safe operation in any case, the plasma source can either work completely independently or be controlled and monitored externally by the user. Therefore, the source can be used both as a stand-alone hand-held solution and as part of industrial equipment.
Further information on our µPQ plasma source:
Due to their low surface tension, plastic surfaces can hardly be glued, coated or printed satisfactorily without pre-treatment. In order to increase adhesion, the surface is either treated with a primer, a chemically active adhesion promoter, or activated with a low-pressure plasma. In both cases, adhesion points are created by changing the surface bonds. With the ultra-compact atmospheric microwave plasma source µPQ developed by FBH, a cost-effective alternative to increase surface tension and improve adhesion has become available.
The new plasma source has already been successfully integrated and tested in several digital printer models in cooperation with TECHNOPLOT CAD Vertriebs GmbH, a digital printing specialist. Due to its extremely compact dimensions and the resulting low weight, the plasma source can be mounted directly in front of the print head. Therefore, activation of the material surface and subsequent printing are performed in a single step.
Improved printability of the plastics polyethylene (PE), polypropylene (PP), Teflon® (PTFE), and acrylic glass (PMMA) was successfully demonstrated. Also, printability of glass could be significantly increased. Since ambient air is used as process gas, operating costs are kept low. During the tests, the plasma source proved to operate stable at any time with either air or water cooling.
FBH’s µPQ has been optimized by integrating all components into one compact housing. Therefore, it can be used very flexibly with different gases and tailored to customer-specific requirements. It is ideal for challenging applications in the fields of surface treatment, cleaning and disinfection. µPQ is particularly suited for
- applications where a precise localization of the treatment area is necessary
- treatment of thermally sensitive materials
- complex geometries with hard-to-reach details
- attachment on moving machine parts
- flexible usage by hand during manual assembly or medical treatment
The Prototype Engineering Lab is specialized in developing the most suitable set-up according to customer’s needs. For example, the plasma technology can be arranged as an array to treat larger areas or special geometries. Also, combinations with UV LEDs are an option in order to improve printing or coating of highly sensitive materials.
FBH’s Prototype Engineering Lab complements the institute’s scientific competence. Based on the research results, it develops prototypes that can be tested in industrial applications. This way, the institute ensures rapid transfer into market-oriented products, processes, and services.
With its team of engineers and technicians, the Prototype Engineering Lab performs systematic device engineering of user-friendly prototypes for customized applications:
- Transforming research modules into stand-alone equipment that can be operated outside a lab environment
- Miniaturization of laboratory set-ups, utilizing dedicated electronics and optimized mechanical design
- Easy-to-use prototypes integrating power supply, sensors, and control unit
The standard way of generating a plasma, using the discharge between two electrodes, creates a large space-charge region that limits the density of free electrons. Inducing currents in the plasma by magnetic coupling eliminates this disadvantage. This approach is known as Inductively Coupled Plasma (ICP). Moreover, exciting plasmas with high frequency fields such as microwaves keeps the ions almost unmoved, while the electrons are strongly accelerated. Thus, they can create a high number of ions despite their short mean free path. This feature allows generating a cold and stable plasma even at atmospheric pressure.
The FBH has developed a new miniature source based on a microwave ICP which combines these advantages. In the course of a project with Ruhr-Universität Bochum, FBH has proven for the first time worldwide the existence of a high-confinement ICP mode in a compact microwave source. This new type of source opens up a broad field of applications in plasma chemistry. In addition, it exhibits a record efficiency: More than 60% of the incident microwave power is absorbed in the plasma.
The source also allows to create an inductively coupled plasma at atmospheric pressure, which has been successfully demonstrated with argon. Jointly with Ruhr-Universität Bochum, such sources have been characterized extensively in the pressure range 100 - 1000 Pa.
Two further distinct features need to be emphasized: First, the sources can be arranged in parallel as an array in order to obtain a plasma which is uniform over a large area. While classical sources have a limited processing surface due to the inhomogeneity of the generated plasma, such an array can extend this area significantly. Second, certain chemical processes require plasma generation and reaction of components at different places for optimum control. For this purpose, FBH has realized already a double ICP source, which offers the possibility to generate two plasma jets with two different gases and the chemical reaction occurring far away from the sources.
Such parallelization has been developed further in cooperation with the Berlin-based company SENTECH. Quadruple sources were built together with an FBH GaN-based oscillator and tested with oxygen for atomic layer deposition processes.
- Correlated Mode Analysis of a microwave driven ICP source (2019)
- Optical characterization of a novel miniature microwave ICP plasma source in nitrogen flow (2018)
- Electronic Frequency Tuning of a High-Power 2.45GHz GaN Oscillator (2015)
- An inductively coupled miniature plasma jet source at microwave frequencies (2013)