Low Temperature Plasma Physics

**Prof. Julian Schulze**

West Virginia University, USA

and **Dr. Thomas Mussenbrock**

Theoretical Electrical Engineering

**Heating dynamics and process control in radio frequency plasmas driven by customized voltage waveforms (visit of M.Sc. Birk Berger to West Virginia University)**

Since 01/2015 and funded by a Research School PR.INT grant M.Sc. Birk Berger has been visiting Prof. Schulze’s laboratory at West Virginia University (WVU) in Morgantown, USA. Mr. Berger is a PhD student at the department of Electrical Engineering at the Ruhr-University Bochum. His research is focused on coupling mechanisms and the control of energy distribution functions of different particle species in inductively and capacitively coupled radio frequency (RF) plasmas driven by multiple consecutive harmonics of a fundamental frequency and adjustable harmonics’ voltage amplitudes and phases. His thesis work is based on a combination of experiments and simulations to understand the physics of such plasmas. He is co-supervised by Prof. Dr. Thomas Mussenbrock (RUB), who is an expert for simulations of such discharges, and Prof. Dr. Julian Schulze (WVU), who is an expert for experimental investigations of RF plasmas. At WVU Birk’s work is based on a unique experiment and team of international experts (see figure 1 right). He has been studying the electron power absorption dynamics by Phase Resolved Optical Emission Spectroscopy, the ion energy distribution by a Retarding Field Energy Analyzer, the plasma density and electron energy distribution function by Langmuir Probe measurements, and has performed current and voltage measurement (figure 1 left) as a function of the gas pressure, harmonics’ phases and voltages amplitudes. By doing this Birk has revealed a variety of fundamental coupling mechanisms in such plasmas that drastically affect their performance in applications ranging from deposition to etching processes on microscopic scales in semiconductor manufacturing.

Based on this project Mr Berger has published 4 papers in high impact international peer reviewed journals and has contributed 6 talks to international conferences

**Fig. 1, left:**

DC self bias as a function of the harmonics’ phases measured in a multi-frequency capacitive radio frequency plasma driven by 3 consecutive harmonics of 13.56 MHz. Right – M.Sc. Birk Berger, Dr. Edmund Schuengel, Prof. Dr. Julian Schulze (from left to right) in front of a radio frequency plasma source at West Virginia University.

**Publications resulting from this project:**

1. Experimental investigations of electron heating dynamics and ion energy distributions in capacitive discharges driven by customized voltage waveforms – B. Berger, S. Brandt, J. Franek, E. Schüngel, M. Koepke, T. Mussenbrock and J. Schulze; J. Appl. Phys. 118 (2015) 223302

2. Power supply and impedance matching to drive technological radio-frequency plasmas with customized voltage waveforms - J. Franek, S. Brandt, B. Berger, M. Liese, M. Barthel, E. Schüngel, J. Schulze; Rev. Sci. Instr. 86 (2015) 053504

3. Electron power absorption dynamics in capacitive radio frequency discharges driven by tailored voltage waveforms in CF4 – S. Brandt, B. Berger, E. Schüngel, I. Korolov, A. Derzsi, B. Bruneau, E. Johnson, T. Lafleur, D. O’Connell, M. Koepke, T. Gans, J.-P. Booth, Z. Donkó, J. Schulze; submitted to Plasma Sources Science and Technology

4. A Computationally Assisted Spectroscopic Technique to measure secondary electron emission coefficients in radio frequency plasmas – M. Daksha, B. Berger, E. Schüngel, I. Korolov, A. Derzsi, M. Koepke, Z. Donkó, J. Schulze; submitted to Journal of Physics D: Applied Physics

** Contributions to international conferences resulting from this project:**

1. Control of electron heating and ion energy distributions in capacitive plasmas by voltage waveform tailoring based on a novel power supply and impedance matching – B. Berger, J. Franek, S. Brandt, M. Liese, M. Barthel, E. Schüngel, M. Koepke, and J. Schulze, Conference talk at 68th GEC 2015

2. Control of electron heating and ion energy distributions in capacitive plasmas by voltage waveform tailoring based on a novel power supply and impedance matching – B. Berger, J. Franek, S. Brandt, M. Liese, M. Barthel, E. Schüngel, M. Koepke, and J. Schulze; Conference talk at 62nd AVS symposium 2015

3. Electron power absorption dynamics and ion energy distributions in capacitive discharges driven by customized voltage waveforms in argon and CF4 – B. Berger, S. Brandt, J. Franek, E. Schüngel, M. Koepke, J. Schulze, T. Mussenbrock, B. Bruneau, E. Johnson, T. Lafleur, J.-P. Booth, D. O’Connell, T. Gans, I. Korolov, A. Derzsi, Z. Donkó; Conference talk scheduled at 43rd ICOPS 2016

4. A Computationally Assisted Spectroscopic Technique to measure secondary electron emission coefficients in technological rf plasmas – M. Daksha, B. Berger, E. Schüngel, M. Koepke, J. Schulze, I. Korolov, A. Derzsi, Z. Donkó; Conference talk scheduled at 43rd ICOPS 2016

5. Control of charged particle dynamics in electronegative multi-frequency plasmas – J. Schulze, B. Berger, S. Brandt, B. Bruneau, Y. Liu, J. Franek, I. Korolov, A. Derzsi, E. Schüngel, M. Koepke, T. Mussenbrock, E. Johnson, T. Lafleur, J.-P. Booth, D. O’Connell, T. Gans, Y. Wang, Z. Donkó; Invited conference talk scheduled at 43rd EPS Conference of Plasma Physics 2016

6. Heating dynamics and control of energy distribution functions in capacitive discharges driven by tailored voltage waveforms – J. Schulze, B. Berger, S. Brandt, J. Franek, E. Schüngel, M. Koepke, T. Mussenbrock, B. Bruneau, E. Johnson, T. Lafleur, J.-P. Booth, D. O’Connell, T. Gans, I. Korolov, A. Derzsi, Z. Donkó; Invited conference talk scheduled at 18th ICPP 2016

**20th - 27th March 2016Prof. Schulze visits the Ruhr-University to present at a workshop and to support the research of a local graduate student**

From March 20th – March 27th Prof. Dr. Julian Schulze visited the department of electrical engineering at the Ruhr-University Bochum to give a talk at a local workshop on low temperature plasma science entitled: “Charged particle dynamics in technological RF plasmas: From fundamental understanding to predictive control”. Prof. Schulze also supported the research of Sebastian Wilczek on the kinetic interpretation of resonance phenomena in radio frequency low temperature plasmas by writing a manuscript together with him, which will be submitted to an international peer-reviewed journal.

**Publications resulting from this project:**

1. Kinetic Interpretation of Resonance Phenomena in Low Pressure Capacitively Coupled Radio Frequency Plasmas, S. Wilczek, J. Trieschmann, D. Eremin, R. P. Brinkmann, J. Schulze, E. Schuengel, A. Derzsi, I. Korolov, P. Hartmann, Z. Donko, and T. Mussenbrock (2016) Phys. of Plasmas to be submitted

**October - Decemver 2015Visit of M.Sc. Sarah Siepa to the group of Prof. Schulze at West Virginia University**

From October to December 2015 and funded by a Research School PR.INT grant, M.Sc. Sarah Siepa visited Prof. Schulze’s group at West Virginia University (WVU), USA, to perform spectroscopic measurements in chemically active electronegative radio frequency plasmas in his laboratory. Sarah is a graduate student in the department of Physics at the Ruhr-University Bochum. In her research, she develops a diagnostic to measure the plasma density and the electron temperature by optical emission spectroscopy based on a rate equation model to describe the population dynamics of the observed excited states of neutral gas atoms. At WVU she performed measurements at two unique experiments, namely a multi-frequency capacitive discharge and a hybrid inductive/capacitive plasma driven at phase-locked harmonics of a fundamental frequency, to test her diagnostic. She also attended two international conference together with Prof. Schulze and his group members, where she presented contributions summarizing her research performed at the Ruhr-University Bochum.

**12th - 21st July 2015Prof. Schulze gives a lecture on technological high frequency plasmas at the Ruhr-University Bochum**

From July 12th – July 21st Prof. Schulze visited the department of electrical engineering at the Ruhr-University Bochum to give a block lecture on “Technological High Frequency Plasmas” (http://www.tet.ruhr-uni-bochum.de/lehre/746/). This class was focused on theoretical models required to understand the physics of such plasmas, a detailed discussion of diagnostic and simulation methods as well as an introduction of different types of such discharges and their applications ranging from capacitively to inductively coupled micro- and macroplasmas to plasma etching and deposition. About 15 graduate students from different departments attended this class that was taught on 5 days from 8:30 am – 4 pm. In the following week the students took an oral graded exam. This class is analogous to a classical 3 SWS lecture and the students received regular credit for attending it and passing the final exam.

**23rd-27th March 2015Prof. Schulze visits the department of electrical engineering to give a colloquium talk and to support the research of Sebastian Wilczek on electron heating in capacitive RF plasmas**

From March 23rd – 27th 2015 Prof. Dr. Julian Schulze visited the department of Electrical Engineering to give a colloquium talk on “Heating dynamics and control of distribution functions in capacitive RF plasmas” and to support the research of Sebastian Wilczek on electron heating in capacitive radio frequency (RF) plasmas. Based on kinetic simulations Sebastian studies the power absorption of electrons in such low pressure plasmas driven by a single frequency sinusoidal voltage waveform. In contrast to previous assumptions he found that the plasma density does not increase quadratically as a function of the driving frequency, but discovered a drastic step-like increase at distinct frequencies. Moreover, he observed the formation of multiple energetic electron beams during a single RF period. Prof. Schulze helped to explain these observations by developing a semi-analytical model that demonstrates that the density increase is caused by a modulated confinement quality of these beam electrons. Both scientists could show that the plasma density will be low, when an electron beam generated at one electrode by the expanding sheath hits the opposing sheath during its collapse, and high, when a given beam hits the opposing sheath and is reflected back into the plasma. Prof. Schulze has been supporting the writing of a manuscript, that has been published in a special issue on electron heating in technological plasmas in an international peer-reviewed journal and where Sebastian is the first author.

**Publications resulting from this project:**

1. The effect of the driving frequency on the confinement of beam electrons and plasma density in low-pressure capacitive discharges - S Wilczek, J Trieschmann, J Schulze, E Schuengel, R P Brinkmann, A Derzsi, I Korolov, Z Donkó and T Mussenbrock 2015 Plasma Sources Sci. Technol. 24 024

**1st - 22nd August 2014Sebastian Wilczek visits the group of Prof. Schulze at West Virginia University**

From August 1st to August 22nd M.Sc. Sebastian Wilczek visited the group of Prof. Schulze at West Virginia University (WVU), USA, to work on common research projects in the field of low temperature plasma science. Sebastian is a graduate student in the department of electrical engineering at the Ruhr-University Bochum and his computational work is focused on the dynamics of energetic electron beams generated by the expanding boundary sheaths in capacitive radio frequency plasmas. At WVU Sebastian worked on simulations to understand the generation of multiple such beams during a single phase of sheath expansion and the kinetic interpretation of resonance phenomena related to such beams in these discharges.

**Publications resulting from this project:**

1. The effect of the driving frequency on the confinement of beam electrons and plasma density in low-pressure capacitive discharges - S Wilczek, J Trieschmann, J Schulze, E Schuengel, R P Brinkmann, A Derzsi, I Korolov, Z Donkó and T Mussenbrock 2015 Plasma Sources Sci. Technol. 24 024002

2. Kinetic Interpretation of Resonance Phenomena in Low Pressure Capacitively Coupled Radio Frequency Plasmas, S. Wilczek, J. Trieschmann, D. Eremin, R. P. Brinkmann, J. Schulze, E. Schuengel, A. Derzsi, I. Korolov, P. Hartmann, Z. Donko, and T. Mussenbrock (2016) Phys. of Plasmas to be submitted