Plasma research contributes to new Research Center “Future Energy Materials and Systems”
The state NRW will fund four research centers and one research college during the next years in the framework of the funding instrument "Ruhr Konferenz". One research center “Future energy materials and systems” will support the plasma science at RUB in the area of synthetic plasma chemistry. (Image (c) hagenvontroja)
Boosting the efficiency of plasmas tenfold with the same energy input.
Plasmas are called the fourth state of matter: in the solid phase, their molecules occupy solid places, whereas there is some freedom of movement in the liquid phase and a lot of more freedom of movement in the gas phase. If more energy is supplied to a gas, the molecules break up and a plasma is created. The negatively charged electrons separate from the positively charged atomic nuclei and turn them into ions. These free electrons and ions can be accelerated by electromagnetic fields. If the fast electrons collide with other molecules, they can change them in turn by ionising or breaking them down. This process can result in different, sometimes short-lived reactive neutral particles and ions that may be useful for a variety of applications.
Smartphones, laptops, wafers
In the industry, plasmas are used, for example, to modify surfaces in a specific way, including coating spectacle lenses or displays or etching microscopic channels into silicon wafers. This results in fine structures in the nanometre range. “Every smartphone, every laptop contains components that have gone through such processes,” as Dr. Julian Schulze from the RUB Institute for Electrical Engineering and Plasma Technology outlines the use of plasmas in industrial applications. His aim is to better understand the details of processes in plasmas in order to render them more efficient. How is energy supplied to electrons and ions? How are they accelerated? How can this process be optimised?
In order to explore these questions, he concentrates on plasmas that are generated at room temperature, so-called low-temperature plasmas. They are often used for medical and industrial applications, because they do not damage or destroy their surrounding surfaces. In the medical field, this applies to e.g. human skin.
Such plasmas can, for example, be ignited between two electrodes, one of which is grounded, while a voltage is applied to the other. Applying the voltage leads to the generation of an electric field at the electrodes, which repels the negatively charged electrons from the surfaces while attracting positively charged ions. Because of the rapid movement of the electrons as a result of this acceleration, neutral particles can be excited by collisions with fast electrons. This ultimately makes plasmas glow. The acceleration of the ions to the surface can be used for surface modification.
Typically, a sinusoidal voltage is applied to the electrode – including industrial applications. This accelerates the electrons in a specific way. This, in turn, causes the plasma to emit light – which can be measured – at certain times and positions. Furthermore, positive ions reach the surfaces with a very specific energy distribution. However, Julian Schulze is not satisfied with this result: “Many electrons receive some of the energy that is fed in, but many of them do not receive enough to efficiently break up other molecules and, thus, produce high reactive particle densities. Also, the ion energy distribution at the interfaces cannot be controlled efficiently,” he points out. “We do not want to distribute the energy to electrons and ions with a watering can, but to deploy it in a more controlled manner so that fewer electrons receive more energy and the ion energy at interfaces can be precisely adjusted. This will enable plasmas to work more efficiently.”
The energy input to the electrons of the plasma can be controlled and improved
To this end, his team experimented with the form of the voltage waveform applied to the electrode and used in simulations. The process developed at RUB is called Voltage Waveform Tailoring, VWT for short. By superimposing several frequencies, different voltage waveforms can be generated, and the researchers have studied their effects on the plasma.
It emerged that, by tailoring this voltage waveform, the temporal and spatial distribution of the energy input to the electrons of the plasma can be controlled and improved. In the same way, the ion energy distribution at interfaces can be adjusted. “While in the case of the sinusoidal voltage many electrons are weakly accelerated at different points in time and over wide spatial areas, we have succeeded in using such optimised voltages to cause a much stronger acceleration of certain electrons at only one point in time within a period of the applied voltage and at specific spatial positions within the plasma,” explains Schulze. Thus, while the same amount of energy is dissipated, the amount of reactive species generated in the plasma increases. In principle, the situation is similar with ions: here, too, the acceleration of the charge carriers can be tailored with regard to time and space.
The mechanisms underlying these changes are the subject of in-depth studies. The effect of the voltage waveform on the plasma is rooted in the change in the relevant boundary zones near the electrodes, called plasma sheaths: the area in which a strong electric field exists, which is created by applying the voltage to the electrode, is a taboo zone for electrons; they are repelled. The positively charged, heavier, slower and colder ions, on the other hand, are attracted. The form of the applied voltage waveform modulates this boundary zone: it expands according to a pattern influenced by the voltage waveform and collapses again. “The electrons are kicked back into the plasma from the moving boundary region like a tennis ball from a racket,” as Julian Schulze illustrates one of several mechanisms that take place in the plasma. If the boundary layer is modulated so that it moves very fast, the electrons cannot keep up with it as quickly. “You have to imagine this in a similar way to when you pull a tablecloth away from under dishes so quickly that they simply remain where they were, even though you pull the floor away from beneath them. Then, you have to collect the dishes by hand, they do not follow the motion of the tablecloth,” says Schulze.
The positively charged ions perform the reverse motion, because they are accelerated towards the interfaces by the electric fields in the plasma boundary layer. Fast high-energy ions can be used to etch the interfaces, slower low-energy ions for coating. In any case, their energy and, thus, the processes on the surfaces can be tailored by VWT.
In order to find out exactly which mechanisms are at work when such tailored voltage waveforms are applied to the plasma, the engineers use e.g. high-resolution cameras to measure the electron dynamics. Moreover, laser measurements can be deployed to detect excited helium species and other reactive particles such as atomic oxygen, which are generated by collisions between neutral particles and energetic electrons. Since the excited helium particles absorb the laser radiation, it is possible to draw conclusions about the number of particles present by measuring the laser light that is transmitted by the plasma.
By superimposing two frequencies of the voltage applied to the electrode, the researchers measured a density of the particles that was more than five times greater. “Compared to applying the sinusoidal voltage, it is possible to increase the density of such particles tenfold and to control it by adjusting the voltage waveform,” Julian Schulze points out. Given the extent to which plasma is used in the semiconductor industry, the significance of this improvement is evident. “This is a billion-dollar industry,” says the researcher. “Every increase in efficiency has a huge economic impact.”
Once the inhomogeneous character of plasmas became apparent, not everyone was pleased. However, this characteristic has some advantages, for example for the industry.
They are often invisible to the naked eye: the wafer-thin layers that are deposited on surfaces with the help of plasmas. For example, on architectural glass to control its reflectivity, on tools to protect them from wear and tear, or on plastics to make them more impermeable to gases. Plasma coatings have become indispensable in industrial applications. While surfaces can also be coated using chemical processes, this would sometimes require such high temperatures that the coated objects would melt. Plasmas, on the other hand, generate the required energy not through heat, but through the reactive particles they contain.
In a plasma, matter is partially or completely ionised. By applying electric fields to electrodes in a plasma chamber, the introduced gas, such as argon, can be ionised and the charged particles are accelerated towards a metal electrode. The ions impinging on the metal knock individual atoms out of the material, which are then deposited on a workpiece that is located across from it and is to be coated. The team of the Collaborative Research Centre SFB-TR 87 explores what exactly happens in the plasmas during such coating operations. The researchers have been studying the underlying processes for years.
Hype and valley of tears
“At our Collaborative Research Centre, we experienced both the hype surrounding high power pulsed plasmas for superior coatings a good ten years ago and the subsequent deep valley of tears,” recalls Professor Achim von Keudell, who holds the professorship for Experimental Physics of Reactive Plasmas at RUB. In 1999, the so-called High-power Impulse Magnetron Sputtering was established. The process uses fully ionised plasmas whose surfaces have a power density that pretty much equals that of rocket engines. In contrast to conventional plasmas, these high-power plasmas can’t be operated continuously, because they would destroy the materials of the plasma chamber. Therefore, they are repeatedly switched on and off, i.e. run in pulsed mode.
These dense plasmas can also be used to produce correspondingly dense high-quality coatings. Consequently, this technology immediately sparked interest in the industry. “Then, disillusionment hit,” says von Keudell. This is because the higher quality of the layers came at the expense of the coating rate, which in some cases was only 30 percent of those achieved by traditional processes at the same electrical power input.
While conventional processes mainly use uncharged atoms, radicals or molecules for coating, high-performance plasmas primarily produce ions. Since the particles are charged, they are heavily affected by external and internal electric fields. Due to the direction of these electric fields, a large proportion of the ions that travel towards the workpiece simply turn back halfway and fly back. This phenomenon is called the return effect and can’t be circumvented because it is simply due to the property of fully ionised plasmas. “For ten years, the plasma community has been struggling and failing to achieve both a high-quality layer and a high growth rate. It doesn’t work, you have to decide which is more important depending on the application,” elaborates von Keudell.
However, a larger number of the ionised particles make it to the other side through the electric fields and settle there as a layer than one would expect according to simple plasma models. This is thanks to another pheno
menon that the Bochum team has now researched in depth: although the high-power plasmas look very uniform when viewed with the naked eye, they actually contain structures that help the ions to get to the other side.
Croissants and comets
Viewed from above, the plasmas are toroidal, they resemble a glowing donut. This glow of the excited particles forms structures that move in circles at ten kilometres per second and can only be detected by high-speed cameras. “One of the first structures we observed in an aluminium plasma at that time looked like a comet flying backwards,” describes Achim von Keudell. In subsequent analyses, the Bochum-based researchers found that different structures were formed, depending on the type of plasma. In a titanium plasma, for example, not a single comet is formed, but rather something that looks like a croissant. The number of structures in the torus also changes depending on the plasma conditions. Because of the regularity and rotation of the structures, the term “rotating plasma spokes” has been adopted.
The description of these inhomogeneities in technical plasmas caused a stir in the research community, especially among scientists working in applied fields. Today, Achim von Keudell is convinced: “It was probably unfortunate that we basic researchers spoke of inhomogeneities or instabilities. A process engineer doesn’t like the sound of that.”
It turned out that although these structures are created by instability at the beginning of each plasma pulse, the final structure is very stable. Structure formation is an essential property of high-power density plasmas and can’t be entirely prevented, but it doesn’t pose a problem for coating processes, either. On the contrary, it even helps to mitigate the return effect. Without the inhomogeneities, there would be no efficient layer growth at all in fully ionised plasmas, because many ions deflected by the electric fields would never reach the surface.
Because the luminous plasma spokes are electrically charged, they don’t interlock; once the structure is formed, it is stable. And because it moves at high speed, it does not affect the coating finish. Only under certain conditions can the structures freeze in their motion, which would indeed lead to an uneven coating. “In the meantime, however, we have understood the plasmas to such an extent that we know how to choose the parameters to prevent such an outcome,” says Julian Held, PhD researcher at the Chair of Experimental Physics II. “We can even specifically grow certain structures in the lab.”
Julian Held has perfected the method. He developed a technique that allows him to analyse structures in a plasma synchronised with high-speed cameras. Thus, he can also render the different plasma components visible separately from each other, such as the glow of certain atoms or ions, and correlate their movements in time and space. “For years, we had tons of individual images but never knew how to superimpose them,” Held recalls. “This project was an essential step in creating detailed plasma models.”
7th International Plasma Science & Entrepreneurship Workshop
On November 2 & 3, 2020, the 7th International Plasma Science & Entrepreneurship Workshop took place in an online format of the Ruhr-Universität Bochum. The event was organized by Hugo de Haan (programme, Vision Dynamics), Guus Peemen (chair, TuE), and Achim von Keudell (host and chair, RUB).
Plasma medicine, medical and healthcare
Plasma Surface modification & thin films
Atmospheric pressure plasma @ Micro/Nano scale
Atmospheric pressure plasma jet (APPJ)
Nanoparticles generation and particlesurface treatment
Surface diagnostics, energetics, analytics and –metrology
Plasma parameterization, diagnostics, simulation
Plasma for Emission Abatement & CO2 Plasma parameterization, diagnostics, simulation
The workshop is a PhD (student) expert-level workshop focused on the achievements, challenges and opportunities for the scientific- and entrepreneurial community working in the field.
Successful summer school online in 2020
Due to the current situation, this year's summer school did not take place in physics center in Bad Honnef, but online. The regular program consisting of basic plasma physics lectures and a master class on special topics could not take place as usual. Nevertheless, all teachers have agreed to present the basic lectures via video conferencing. The summer school was extended to two weeks with two lectures per day. This year more people could participate in the summer school due to its online format.
The lectures were technically flawless and the feedback from students and teachers was very positive. Many discussions and interactions could be made possible due to the high commitment of the teachers. Two practical workshops were also held by L. L. Alves on a Boltzmann solver and N. Braithwaite on the Paschen curve.
We hope for another summer school in 2021, then again on site at the physics center in bad Honnef. The latest information on planning will be published on the summer school homepage in March 2021.