Elsevier

Vacuum

Volume 193, November 2021, 110502
Vacuum

Cylindrical inertial electrostatic confinement plasma source for surface treatment

https://doi.org/10.1016/j.vacuum.2021.110502Get rights and content

Abstract

The inertial electrostatic confinement (IEC) has been commonly researched in the scope of application for space propulsion and fusions reactors since many years. The utilization as thruster is possible due to a free emitting stream in a so-called jet mode of the IEC source. Depending on the setup, this jet can be operated as high energetic electron beam (tight jet) with electron energies of several keV or as low energetic plasma jet close to quasineutrality (spray jet). These modes are of high interest for thin film applications and plasma treatment. However, the IEC source is still not used in this field of application. Relevant application scopes can be electron substrate heating, surface plasma pretreatment, ionized physical vapor deposition and plasma enhanced chemical vapor deposition. In this paper, a cylindrical IEC source is presented that emits the plasma in both known jet modes, spray jet and tight jet. The jet emission occurs in radial outlet direction along the entire source height (“curtain-like” emission). Furthermore, an ignition test and operational behavior test is introduced. A stable spray jet operation region was examined and dependencies on pressure and voltage are investigated.

Introduction

The inertial electrostatic confinement (IEC) has been extensively researched for fusion (inertial electrostatic confinement fusion, IECF) applications. Universities and institutes around the world, such as the US (Illinois [1], Wisconsin-Madison [2], etc.) and Japan (Tokyo Institute of Technology [3], Kyoto University etc. [4]), have been researching the feasibility of IEC for fusion application for many years. Moreover, the implementation fields of IEC sources are constantly growing. For example the IEC source is considered to be an alternative to the Hall-Thrusters [5]. Universities and institutes such as the Institute of Space Systems in Stuttgart, Germany [6] and in the US (University of Kentucky [7], University of Illinois [8,9] etc.) are already researching different approaches of IEC-based thrusters.

The IEC sources are also interesting for other applications. Miley et al. mentioned in early studies the possible application for welding and cutting operations [10]. Due to its simple robust design and easy controllability, the IEC source can be used as a portable neutron/proton source [11]. Furthermore, the mean output energy of the charge carriers is low and therefore suitable for short-range applications such as the detection of landmines, oil, gold, explosive materials and illegal drugs. In addition, it is suitable for medical positron emission tomography for the detection of tumors [[11], [12], [13], [14], [15]].

The working principle of the IEC source is not bound to a specific geometry. In the recent years, primarily spherical and cylindrical IEC sources were investigated with regards to fusion [14,16] and thruster [6,7,[17], [18], [19]] scopes. Using the IEC source as thruster and in comparison to the IECF, the setup needs to be modified to generate a free jet for thrust. The Institute of Space Systems in Stuttgart, Germany [6] and the University of Illinois, US [9] investigate spherical IEC sources. The University of Kentucky, US [7] and University of Liverpool, UK [20] developed an approach using an cylindrical IEC source with axial directed output of the jet. In the aforementioned studies, the plasma output of the jet can be considered as “line-like”. This means the jet mode (spray jet and tight jet) originates from one spot with negligible radial flux distribution. Another approach for space propulsion using a cylindrical IEC-Source with radially emitting jet is introduced by Petkow and Rouwette [21].

The jet operation of IEC sources also has potential for applications in the field of thin-film technology. In particular, the jet can be utilized for surface pre-treatment, as secondary ionization source for sputtering like in ionized physical vapor deposition IPVD [22], or as plasma enhanced chemical vapor deposition (PECVD) source itself. The widely researched IEC-thrusters have a jet outlet. In order to maximize the coated components per batch, a higher plasma outlet area is favored. This paper introduces a new cylindrical IEC source with a directed radial spray jet or tight jet output over the entire source height (which will be considered in the following as “curtain-like” output).

Within the scope of the presented investigation, the concept of a new type of IEC source is examined with regard to ignition characteristics, operating range and jet behavior. The source is modular, hence many configurations can be tested. As a result, a stable operating range for the curtain-like output was found.

Section snippets

Theory of the IEC plasma source

This chapter analyses the theoretical background of the IEC plasma source with a link to resulting potential applications based on an extensive literature review.

Experimental method and setup

For the analysis and operation of the novel cylindrical IEC source an industrial batch coating facility from Bosch Manufacturing Solutions GmbH is used. The volume of the chamber is 0.8 m3 with a height of 1000 mm. The schematic cross section is sketched in Fig. 3. The chamber can be evacuated down to 1 mPa base pressure (i.e. without mass flow rate) through two turbomolecular pumps (Pfeiffer Vacuum PMP03714) in combination with a root pump and rotary vane pump. It can be separated from the

Results

The development of the cylindrical IEC source used in this activity is of an original nature and, of course, new if one refers to the type built here. Correspondingly, the design chapter forms a part of the result section, together with the results of the experimental investigation that follows in section 4.2.

Discussion

The newly designed cylindrical IEC source with radially emitting curtain-like jet shows similar behavior to conventional IEC sources referring to the jet modes. Three operating modes have been observed: In the first mode the glow discharge occurs. This is followed by the operation of the tight-jet mode. The third mode is described by the spray jet. The pressure region, where the tight jet operates, is rather small whereas the region for spray jet is wider compared to the work of Chan et al. [28

Conclusion

A novel cylindrical IEC source with radially emitting jet was designed. The plasma flow is emitted along the cylindrical height of the IEC source and will be utilized for thin film applications and surface treatment. Possible scope of applications are electron heating of the substrate, surface etching, PECVD-source, secondary ionization source in combination with magnetron sputtering (IPVD) and as surface post treatment. Furthermore, the design enables further studies in change of geometry,

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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