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Plasma, excited with microwave energy, has very interesting properties which are in direct correlation with the excitation frequency. This is due to the reachable high electron density, which itself has an influence on the density of further generated particles like ions, radicals, excited atoms and molecules or UV light. Altogether they are responsible for the high chemical reactivity of microwave plasmas attaining their maximum in the pressure regime between 1 and a few hundred Pa. Therefore, comparatively low end pumping systems can be used to achieve good results in etching, activation and deposition processes. Together with the availability of microwave generators, grown with the market of microwave heating technology, microwave plasma is an interesting tool for industrial applications. Scalability of power up to 100 kW per single magnetron tube and lateral dimensions of a few square meters per single plasma source have opened the field to industrial thin film technology for typical large area applications as solar or other flat panel industries. Roth & Rau Muegge is utilizing these advantages to supply microwave generators and plasma technology for various fields of applications which will be provided as an overview to give insight into running activities in industry. Although the presented applications are part of publicly funded programs and projects together with partners from several research institutes and universities, the technical depth of this contribution has to consider running contracts and non disclosure agreements (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A new type of plasma source, the Microwave Concentrator Stack, presented in this paper is a device of a geometry that can be described as an elliptical cylinder with additional metallic structures for shaping the electric field in its inside. The basic idea is to concentrate the electric field strength of microwaves on one of its focal lines by coupling microwave power into it on the other focal line. Even though the dimensions of the Concentrator Stack are too small compared to the wavelength of the microwaves to assume wave-optics behavior, a pronounced maximum of the electric field intensity is created on the focal line. By placing an evacuated glass tube around the focal line, free-standing plasmas can be generated by the strong electric field. In the low pressure range (10 to 200 Pa), the plasma displays very good one-dimensional homogeneity along the focal line of the Concentrator Stack. At higher pressures the plasma contracts to several ball-shaped discharges, whose positions correspond to the aforementioned metallic structures inside the Concentrator Stack. The electric field distribution was calculated and optimized for various versions of the Concentrator Stack using the finite element software Comsol MultiphysicsTM and also measured with PET absorber sheets. Measurements and calculations are in excellent agreement. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Experiments on laboratory scale carried out by use of different microwave-driven plasma sources proved that low-pressure plasmas represent an excellent means to efficiently sterilise materials for all kind of applications. Based on experimental results obtained for four different test spores, more detailed experiments on Bacillus subtilis with initial contaminations between 106 and 107 spores and varying spore densities were carried out showing the adaptability of the process parameters microwave power, continuous wave and pulsed power supply, and distance between plasma source and substrate to the individual requirements of the aspired sterilisation process. Spore reduction kinetics reproduced by a low-pressure microwave plasma source designed for industrial applications demonstrated the easy scalability of this plasma sterilisation technique.

Microwave plasmas have some outstanding features like high charge carrier concentrations at ion- and electron energies below 10 eV, which predestine these plasmas for thin film deposition technologies requiring a non-remarkable ion impact. The plasma deposition of the passivating Si 3 N 4 -layers on multi-crystalline solar cells without formation of ion impact induced surface recombination centres is an example for such kind of process. By using the process gases SiH 4 , NH 3 and H 2 a hydrogen rich plasma is generated resulting in excellent passivation properties of the silicon nitride layers on photovoltaic cells. In order to achieve stable plasma conditions on large-scale industrial deposition dimensions (150 nm/min at an area of 20×100 cm 2 ) over long times, magnetic field enhanced linear microwave plasma sources based on the plasmaline ® principle were applied. Following, some results for this remote microwave PECVD of Si 3 N 4 on multi-crystalline silicon solar cells are presented.

Gas mixtures of silane (SiH4) and ammonia (NH3) are used for plasma enhanced chemical vapour deposition (PECVD) of silicon nitride films (SiN). These films were deposited in a plasma reactor, called Plasmodul®. The plasma is excited by 2.45 GHz microwaves in the pressure range of 5–20 000 Pa. The silicon nitride films were deposited on silicon wafers, aluminium foils and molybdenum mirrors on a heatable substrate holder. The silicon wafers were used for electron lifetime determination. Aluminium foils and molybdenum mirrors were used for Fourier transform infrared (FTIR) spectroscopy and refractive index measurements. The plasma was characterised by measurements of the electron density distribution using Langmuir probes. The chemical composition of the films was investigated by FTIR spectroscopy and X-ray photoelectron spectroscopy (XPS) in dependence of the SiH4:NH3 monomer mixture ratio. The optical properties were determined by refractive index measurements. To investigate the passivation effect on silicon, the electron lifetime was measured by microwave detected photo conductance decay (MWPCD).

Fluoropolymers with their unique properties (chemical inertness, thermal stability, low surface tension, mechanical stability) are used in many industrial applications. One disadvantage of these fluoropolymers is their poor adhesion to other materials. In this work a short-time plasma pre-treatment of polytetrafluoroethylene (PTFE) with a low pressure microwave plasma was investigated. The Plasmodul ® source allows the pre-treatment of PTFE substrates with an ammonia plasma (NH 3 ). The newly developed plasma source Planartron ® , which is derived from the Duo-Plasmaline ® , was used for improving the adhesion properties of PTFE by generating oxygen and nitrogen plasmas. PTFE foils were modified on both sides by plasma treatment for typically 30 s. After bonding the foils to aluminum dollies using a 2-K-epoxy resin the bonding strength of the adhesive joint was measured directly by pull off tests. Some modified foils were additionally investigated by Fourier-transform infrared spectroscopy.

The methanol permeation through fuel cell relevant Nafion® membranes is investigated at different concentrations of methanol in aqueous solutions. Thin plasma polymerized barrier films are deposited on Nafion® membranes in a low pressure microwave generated plasma to reduce their methanol permeability. The methanol permeability was measured as a function of time using a gas chromatograph with a flame ionisation detector. It is shown that a plasma polymer layer with a thickness of approximately 0.27 μm on Nafion® membranes reduces the permeability to methanol by a factor of approximately 20.

A new kind of linearly extended plasma source was developed. The prototype of such a device was built up as a barrel with elliptical cross-section. This bifocal shape has two focus lines. A linearly extended microwave antenna is placed along one focus line and the sector of plasma treatment is placed along the second focus line. The antenna is a linear emitter of radial divergent microwave radiation. In this work, a frequency of 2.45 GHz in pulsed and continuous wave (CW) mode was used. The power up to 6 kW CW and 10 kW pulsed is adjustable, pulse on/off times are variable in a wide field. The microwave is reflected by the metallic walls of the barrel and concentrated in the second focus line apart from the antenna. There a quartz tube is placed, that can be evaporated and filled with a working gas at a pressure between 10 2 and 10 5 Pa. The presented experiments have been done with Argon up to atmospheric pressure at a gas flow up to 6 slm.

Gas mixtures of hexamethyldisiloxane (HMDSO) and oxygen (O 2 ) are used for plasma-enhanced chemical vapour deposition of silicon oxide films. These films were deposited in a newly developed plasma reactor, called Plasmodul ® . The plasma is excited by 2.45 GHz microwaves in the pressure range of 0.05–200 mbar. The Plasmodul is a modular device consisting of a plasma source, a gas inlet system, a reaction and substrate chamber and a diagnostic port. It is a flexible equipment, comfortable in handling and simple in construction. The special arrangement allows a film deposition remote from the plasma source. The chemical composition of the films was investigated in dependence on the HMDSO : O 2 monomer mixture ratio by Fourier-transform infrared spectroscopy.

Deposition of cubic boron nitride in hydrogen plasma is thought to proceed as a result of selective etching of sp2-hybridized boron nitride from the surface of the growing boron nitride film, the impact of hydrogen atoms or ions leading to formation of c-BN sp3 bonds on the surface and formation of metastable BNHx species in the gas phase. The mechanism of nucleation and growth of c-BN crystals in hydrogen plasmas is proposed.

The permeability of Nafion® 117 and some types of acid-base and covalently crosslinked blend membranes to methanol was investigated. The methanol crossover was measured as a function of time using a gas chromatograph with a flame ionization detector. In comparison to Nafion, the investigated acid-base and covalently crosslinked blend membranes show a significant lower permeation rate to methanol. Additionally, another method to reduce the methanol permeability is presented. In this concept a thin barrier layer is plasma polymerized on Nafion 117 membranes. It is shown that a plasma polymer layer with a thickness of 0.3 μm reduces the permeability to methanol by an order of magnitude.

Thin hydrocarbon films are deposited on Nafion™ membranes in a low-pressure plasma excited by microwaves. Gas mixtures of hexane (C 6 H 14 ) with hydrogen (H 2 ) were used as monomers. The permeability of methanol through the Nafion membranes modified by plasma polymer films is investigated as a function of the C 6 H 14 /H 2 ratio of the gas mixture. The methanol permeability was measured as a function of time using a gas chromatograph with a flame ionisation detector. It is shown that a plasma polymer film reduces the permeability of methanol by a factor of about 15.

A linearly extended plasma source — the Duo-Plasmaline — is characterised. The plasma is excited by microwaves of 2.45 GHz in a pressure range between 5 and 500 Pa. The device is similar to a coaxial wave guide. A quartz tube conveys through a vacuum chamber. A copper rod centered in the quartz tube is the inner conductor, and the plasma produced in the low-pressure regime outside the quartz tube represents the outer conductor. The microwaves are fed from both ends of the quartz tube into the wave guide and propagate mainly along the inner rod and the tube, filled with air at atmospheric pressure. The device generates a linearly extended plasma up to several meters, mainly controlled by the pressure and microwave power. The plasma source used here is expanded by a second parallel quartz tube both mounted and supplied parallel as a double line system. A sample stage movable perpendicular to the plasma source was mounted. The plasma was characterised for different plasma conditions by measurements of the electron density in relation to the axial and radial distances from the plasma source. The excellent axial homogeneity of the electron density is reflected in the homogeneous axial distribution of the etch rate of polymethylmetacrylate in an oxygen plasma. Also, the axial homogeneity of the deposition rate of quartz-like films polymerised in a plasma from hexamethyldisiloxane (HMDSO) and oxygen, is demonstrated. The newly designed linear plasma source is well suited for large-area plasma treatment and coating.

The plasma deposition of organic fluorescent films from volatile aromates is investigated. The plasma is produced in the pressure range of about 10 Pa by microwave excitation ( f =2.45 GHz) at electron cyclotron resonance. The fluorescence properties of the deposited films on glass slides at an excitation wavelength of 350 nm were studied. The spectral response is wide and shifted to the visible spectral regime, depending on the plasma conditions and the monomer mixtures used. The fluorescence of plasma polymerised benzene is orange, the polymer of toluene yellow and that of styrene yellow–green. The addition of hydrogen or ethylene changes the fluorescence colour and intensity. Highest intensities were obtained with mixtures of benzene and hydrogen, the strongest red shift was obtained with pure benzene. The fluorescence can be explained as a superimposed fluorescence emission of a sum of aromatic luminophores formed in the plasma and deposited on the substrate.

Thin fluoropolymer films are deposited on polyethylene foils in an electron cyclotron resonance plasma. Trifluoromethane (CF 3 H) and ethylene (C 2 H 4 ) are used as monomers. The permeability of toluene through the plasma polymer films is investigated in dependence on the CF 3 H:C 2 H 4 monomer mixture ratio. It is shown that thin fluorinated hydrocarbon layers strongly reduce the permeation, e.g. for toluene the permeability was decreased by a factor of about 1600. The amount of the permeated liquid was measured as a function of time using a flame ionisation detector. Permeability measurements were performed at temperatures of 22 °C, 40 °C and 50 °C.

Thin fluorocarbon polymer films are prepared on PE-foils in low-pressure electron cyclotron resonance plasmas using ethylene (C2H4) and trifluoromethane (CHF3) as monomers. The thin fluorinated hydrocarbon layers strongly reduces the permeability of polyethylene to alkanes. For example, the permeation of toluene was decreased by a factor of about 100 by a single, thin fluorocarbon layer. A further reduction of the permeation down to a factor of 1600 can be obtained by a multilayer coating. X-ray photoelectron spectroscopy and Fourier transform IR spectroscopy are used to characterize the plasma polymerized films. It is shown that the addition of CHF3 to a C2H4 plasma leads to an increase of CF3—, CF2—, and CF— groups and to a decrease of CH3— and CH2— groups in the film. The chemical composition of the polymer layers and their toluene permeabilities are discussed. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 717–722, 1997

The barrier properties of thin fluorocarbon films toward toluene are studied. Mixtures of trifluoromethane and ethylene are used as monomers in an electron cyclotron resonance plasma discharge. The measured permeation fluxes of toluene through polyethylene show that thin fluorocarbon layers strongly reduce the permeation, e. g. by a factor of about 3000 when an appropriate tapered layer is deposited on polyethylene foils. Different tapering of the barrier layers was investigated with respect to low permeation and good adhesion of the films. The amount of permeated toluene through the untreated and treated polyethylene foils are measured as a function of time using a gas chromatograph with a flame ionization detector.

Thin quartz-like polymer films are prepared in a low pressure electron cyclotron resonance plasma operated at 2.45 GHz. Hexamethyldisiloxane is used as monomer in a mixture with oxygen or hydrogen. The adhesion of the polymer films on metal surfaces is studied by measuring the adhesion power with an adhesion test instrument. It is shown that a plasma surface pre-treatment is necessary to obtain good film adhesion. The deposited films are characterized by their IR spectrum measured by the Fourier transform IR technique. The adhesion power depends on the chemical nature of the film; in some cases the surface energy determined by the test ink method correlates with the adhesion power.

The adsorption and in-situ polymerization of monothiophene on Ag(111) has been studied by X-ray photoemission (XPS), programmed thermal desorption (TDP) and mass spectrometry. Thiophene is physisorbed on Ag(111) and desorbs entirely below 250 K. Three different desorption peaks representing multilayers and two different monolayer states can be distinguished by TPD. X-ray irradiation leads to polymerization in multilayers which is accelerated by enhanced substrate temperatures, whereas the polymerization process is quenched at the Ag surface.

Summary form only given, as follows. Plasma technology is used in a wide field of applications for example for PECD-deposition, activation and etching. In particular microwave enhanced plasmas are very effective for activation of surfaces. Our objective targets are to construct plasma sources for large area application and plasma sources which can be used in a wide pressure regime as possible up to atmospheric pressure. In this presentation two sources for large area and wide pressure range plasma are discussed. They are based on a coaxial coupling of the magnetron and the antenna.