The impedance probe: a possible industry compatible diagnostic tool in metallic depositing plasmas

• Applicant: Professor Dr.-Ing. Jens Oberrath
• Subject Area: Electronic Semiconductors, Components and Circuits, Integrated Systems, Sensor Technology, Theoretical Electrical Engineering
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 444843785

Nowadays, further improvement of products, which are produced by plasma processes, is challenging. A promising approach is active monitoring or control of these processes. However, this approach requires an industry compatible diagnostic concept, which measures inner plasma parameter like electron density and temperature without a significant perturbation of the process. A suitable diagnostic concept is active plasma resonance spectroscopy, which excites plasma resonances by means of a high frequency signal coupled to the plasma via an electrical probe. Several probe designs for this concept were invented, but the requirements in metallic depositing plasmas are difficult to fulfill. An excellent candidate for these plasmas is the impedance probe (IP). Its first design was invented in 1960, but it could not be improved to a standard industrial diagnostic tool, yet. Within this project the IP will be intensively investigated. In the first part of the project the spherical IP (sIP) will be analyzed, which allows the measurement of admittance and impedance spectra. In both spectra a resonance occurs, respectively, which allow for the simultaneous measurement of electron density and temperature by means of a simple model. However, a simple model for the impedance resonance is difficult to derive, because the underlying phenomenon is local and requires a kinetic description in low pressure plasmas. Such a kinetic model exists and will be analyzed in detail to understand the physical mechanism of the resonance behavior. The corresponding damping is known as Herlofson paradox, which is not yet fully understood, and will also be explained. Furthermore, all available models of the IPs, also the kinetic model, are based on simplifications: they are idealized (neglecting the holder) and described in electrostatic approximation. However, both simplifications are not validated, yet. By means of three dimensional full electromagnetic simulations the simplified kinetic model will be justified within this project.If the resonance behavior of the sIP is fully understood, the work of the project in the second part will be focused on a planar version of the IP, which can be integrated into the chamber wall. Such a planar IP (pIP) is not developed, yet. Thus, the probe behavior will theoretically be analyzed to investigate its usability for measurement purposes and a kinetic model for the pIP geometry has to be derived. A unique admittance resonance probably occurs and also a simple relation between the resonance frequency and the electron density can be determined. More challenging will be the analysis of the impedance resonance. It has to be proven, if a unique resonance will be excited and if the simplified model is able to describe it correctly. In addition, three dimensional full electromagnetic simulations are necessary to validate the simplified models, which justifies a development of the pIP in reality subsequent to this project.

• DFG Programme: Research Grants
• International Connection: USA
• Cooperation Partners: Professor Dr.-Ing. Peter Awakowicz; Dr. David Blackwell; David Walker