Advancing Plasma Technology: A Conversation with Stefan Ries, M.Sc. from AEPT, Ruhr-Universität Bochum, Germany

Stefan Ries, M.Sc. discusses the challenges, research and accomplishments of their research group at the world renowned Institute for Electrical Engineering and Plasma Technology in Ruhr-Universität Bochum, Germany.


What research are you conducting and for which sector?
At the Institute of Electrical Engineering and Plasma Technology (AEPT) at the Ruhr-Universität Bochum, Germany, we investigate the physics of plasma sources in different areas: plasma in liquids, atmospheric plasmas, low pressure plasmas for coating applications. Our expertise is using plasma diagnostics such as Langmuir probe, multipole resonance probe [1], optical emission spectroscopy [2, 3], phase resolved optical emission spectroscopy [4], Retarding Field Energy Analyser [5] and the V-I probe to characterize different plasma sources. Particularly, our group, which has a good relationship with Impedans, investigates capacitively coupled plasmas (CCP) with one or more frequencies.

The aim is to understand the physics of these plasmas in order to correlate its characteristics with coating properties on substrates such as mechanical stress, hardness, topography, structure, etc. Using multiple frequency CCPs the electrical asymmetry effect [6,7] is investigated and shall be correlated to coating properties. Also the magnetic asymmetry effect [8, 9, 10] is investigated in a RF- magnetron in recent measurements. During the deposition process in a CCP the growing coatings are strongly influenced by the ion bombardment, namely the ion flux and the ion energy onto the substrate. With the electrical asymmetry effect the ion energy and ion flux can be manipulated to get a control over the resulting coating properties. For these intentions, it is indispensable to measure the ion flux and ion energy with an retarding field energy analyzer to understand the plasma processes. Therefore, the possibility of using retarding field energy analyzers from Impedans is crucial for our scientific work.

Before using Impedans products, what challenges did you face?
In the past, there was the challenge to measure ion energy distribution functions in CCPs to verify the electrical asymmetry effect in CCPs.

How did you hear about Impedans?
We heard about Impedans from scientific colleagues within our collaborative research centre SFB-TR 87 in Germany. Also, at our institute Dr. Schulze has maintained a good scientific contact with Impedans for several years.

What Impedans product are you using?
We are using the V-I-Probe system Octiv V-I (Poly + Suite) and , the older RFEA Semion pDC for pulsed DC plasmas and the newer RFEA system (single button sensor probe). Also the Langmuir probe by Impedans is being used at the Department of Physics at the West Virginia University, USA .

What capabilities did you gain when using this product?
With the Retarding Field Energy Analyzer we are able to measure the ion energy and ion flux within a CCP. Especially, Dr. Schulze and coworkers were able to measure experimentally the decoupling of the ion energy and ion flux using the electrical asymmetry effect, which was predicted by simulations. In recent experiments we were also able to study the electrical asymmetry effect in our large-area multiple frequency CCP, which is used for deposition of ceramic coatings.

What industry problem were you trying to address?

The physics of plasmas, especially low pressure plasmas, for deposition of thin films in CCPs are not fully understood. Although CCPs are used for etching processes and sputter deposition processes, there are only a few correlations between the plasma and the growing coatings. According to the “trial and error” principle, plasma processes are adjusted to optimize the desired coating properties in industry.

At this point, we want to replace this empirical procedure with a fundamental understanding of the plasma physics and the interaction with the growing films.

For example, a key aspect for the industry is the decoupling of the ion energy and ion flux onto the chamber surfaces, where the variation of the ion energy at a constant ion flux can be reached. This property could lead to very flexible plasma processes for etching/cleaning applications with high energetic ions and thin film depositions with desired properties using low ion energy bombardment.

Also, in large CCPs the standing-wave effect, skin effect and the edge effect cause large plasma inhomogeneities and finally inhomogeneities of the film properties (for example film thickness). Measuring the ion flux and ion energy at different positions in radial direction onto the electrodes with the Impedans RFEA systems, like the multi sensor RFEA system, we can investigate these inhomogeneties and their physical origins.

What were you able to accomplish?

We were able to measure the predicted decoupling of the ion flux and ion energy using the electrical asymmetry effect in CCPs with laboratory and industrial dimensions.

First measurements in our RF-magnetron CCP verify the magnetic asymmetry effect, which is predicted by simulations [8,9,10]. We were also able to show an effect of the varied ion energy on film properties in our large-area multiple frequency CCP.

Would you recommend this product to your industry?
The RFEA system is an important diagnostic plasma tool to measure the properties of the ion bombardment and can help the industry to achieve a better process control and interpretation of the resulting film properties. The RFEA can also be used to check the reproducibility of those plasma processes, which is very important for the industry with the aim to produce high quality films. Additionally, the Impedans products and software handling, are easy to use for customers. Even in reactive sputter processes (AlN, Al2O3) the button probe sensors of the RFEA are reliable and durable and withstand the reactive environment. In the case of problems with the systems or technical questions the Impedans support is always dedicated to help the customers to get rid of any technical problems.

Where do you see the industry in the next few years?
With the cooperative fundamental scientific work done by a large number of plasma and material scientists around the world, the industry will be able to get a better understanding of the nature of the plasma itself and its interaction with the growing films. This fundamental knowledge can be transferred to industry applications to advance existing coating processes, but also can be used to get rid of recent problems of many plasma processes in industry such as plasma inhomogeneities.

Further examples are process optimization based on predictive control of energy distribution functions of different particle species, diagnostics for plasma based process control and voltage waveform tailoring to customize distribution functions [11].

Last words
We are always interested in a collaboration with Impedans. At the one hand our feedback can help Impedans to improve their products and on the other hand we are able to use high performance products to characterize our plasma sources to achieve a better understanding of our sputtering systems. Also, we support future internships for upcoming students from our institute or student employees from Impedans at both sides, where students get the possibility to collect experiences with Impedans products and scientific experimental working. For example, our student Jonathan Jenderny spent several months at Impedans working on products such as Semion RFEA systems.

Bio: Stefan Ries is a master’s student at AEPT supervised by Dr. Ing. Peter Awakowicz, Head of Institute and Phd. J. Schulze, Lead Scientist.


[1] Styrnoll et al., Plasma Sources Sci. Technol._23_025013 (2014),
[2] Fiebrandt et. al., J. Phys. D Appl. Phys. 50 355202 (2017),
[3] Bobzin et al., J. Phys. D Appl. Phys. 50 075203 (2017),
[4] Berger et al., Appl. Phys. Lett. 111 201601 (2017),
[5] Bienholz et al., Plasma Sources Science and Technology, 21 015010 (2012),
[6] J Schulze et al., Journal of Physics D: Applied Physics, vol. 42 9 092005 (2009),
[7] Bienholz et al., Journal of Physics D: Applied Physics, vol. 47, 6, 065201 (2014),
[8] Yang et al., (2017),
[9] Yang et al., (2018),
[10] Trieschmann et. al, J. Phys. D: Appl. Phys. 46 (2013) 084016
[11] Schüngel et al., Journal of Physics D: Applied Physics, vol. 49, 26 265203 (2016),

This project is a subproject within the SFB-TR 87, which is fun­ded by the Deut­sche For­schungs­ge­mein­schaft (DFG)

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