A von Keudell

Characterization of the effluent of a He/O2 microscale atmospheric pressure plasma jet by quantitative molecular beam mass spectrometry

The effluent of a microscale atmospheric pressure plasma jet (μ-APPJ) operated in helium with a small admixture of molecular oxygen (<1.6%) has been analyzed by means of two independent diagnostics, quantitative molecular beam mass spectrometry (MBMS) and two-photon absorption laser-induced fluorescence spectroscopy (TALIF). The atomic oxygen density, the ozone density and the depletion of molecular oxygen have been measured by MBMS and the atomic oxygen density has been validated by TALIF. Absolute atomic oxygen densities in the effluent up to 4.7×1015 cm-3 could be measured with a very good agreement between both diagnostics. In addition, ozone densities in the effluent up to 1.4×1015 cm-3 and an O2 depletion up to 10% could be measured by MBMS. The atomic oxygen density shows a maximum value at an O2 admixture of 0.6%, whereas the ozone density continues to increase toward higher O2 admixtures. With increasing distance from the jet, the atomic oxygen density decreases but is still detectable at a distance of 30 mm. The ozone density increases with distance, saturating at a distance of 40 mm. By applying higher powers to the μ-APPJ, the atomic oxygen density increases linearly whereas the ozone density exhibits a maximum.

Atmospheric pressure microplasma jet as a depositing tool

An atmospheric pressure microplasma jet is developed for depositing homogeneous thin films from C2H2. The adjustment of the gas flow through the microplasma jet assures optimal flow conditions as well as minimizes deposition inside the jet. In addition, the formation of an argon boundary layer surrounding the emerging plasma beam separates the ambient atmosphere from the flow of growth precursor. Thereby the incorporation of nitrogen and oxygen from the ambient atmosphere into the deposited film is suppressed. Soft polymerlike hydrogenated amorphous carbon (a-C:H) films are deposited at the rate of a few nm/s on the area of a few square millimeters.

Optical and electrical characterization of an atmospheric pressure microplasma jet for Ar∕CH4 and Ar∕C2H2 mixtures

A rf microplasma jet working at atmospheric pressure has been characterized for Ar, He, and Ar∕CH4 and Ar∕C2H2 mixtures. The microdischarge has a coaxial configuration, with a gap between the inner and outer electrodes of 250μm. The main flow runs through the gap of the coaxial structure, while the reactive gases are inserted through a capillary as inner electrode. The discharge is excited using a rf of 13.56MHz, and rms voltages around 200–250V and rms currents of 0.4–0.6A are obtained. Electron densities around 8×1020m−3 and gas temperatures lower than 400K have been measured using optical emission spectroscopy for main flows of 3slm and inner capillary flows of 160SCCM. By adjusting the flows, the flow pattern prevents the mixing of the reactive species with the ambient air in the discharge region, so that no traces of air are found even when the microplasma is operated in an open atmosphere. This is shown in Ar∕CH4 and Ar∕C2H2 plasmas, where no CO and CN species are present and the optical emission sp...

Quadrupole mass spectrometry of reactive plasmas

Reactive plasmas are highly valued for their ability to produce large amounts of reactive radicals and of energetic ions bombarding surrounding surfaces. The non-equilibrium electron driven plasma chemistry is utilized in many applications such as anisotropic etching or deposition of thin films of high-quality materials with unique properties. However, the non-equilibrium character and the high power densities make plasmas very complex and hard to understand. Mass spectrometry (MS) is a very versatile diagnostic method, which has, therefore, a prominent role in the characterization of reactive plasmas. It can access almost all plasma generated species: stable gas-phase products, reactive radicals, positive and negative ions or even internally excited species such as metastables. It can provide absolute densities of neutral particles or energy distribution functions of energetic ions. In particular, plasmas with a rich chemistry, such as hydrocarbon plasmas, could not be understood without MS. This review focuses on quadrupole MS with an electron impact ionization ion source as the most common MS technique applied in plasma analysis. Necessary information for the understanding of this diagnostic and its application and for the proper design and calibration procedure of an MS diagnostic system for quantitative plasma analysis is provided. Important differences between measurements of neutral particles and energetic ions and between the analysis of low pressure and atmospheric pressure plasmas are described and discussed in detail. Moreover, MS-measured ion energy distribution functions in different discharges are discussed and the ability of MS to analyse these distribution functions with time resolution of several microseconds is presented.

A robust method to measure metastable and resonant state densities from emission spectra in argon and argon-diluted low pressure plasmas

A simple robust method is presented to determine the densities of metastable and resonant species in low temperature, low pressure argon and argon-diluted plasmas. The ratios of spectral lines which correspond to transitions from common upper states to resonant or metastable lower states are measured with low resolution optical spectrographs. Photon reabsorption makes these ratios sensitive to the population densities of the lower states. The concept of escape factors is used to develop a set of nonlinear equations for the line ratios, which does not depend on the densities of the upper states. By means of a least squares method, the equations can be solved for metastable and resonant state population densities. The method does not depend on the nature of the excitation process, which makes it superior to other spectroscopic techniques in situations where the electron energy distribution is not known.

Inactivation of Bacillus atrophaeus and of Aspergillus niger using beams of argon ions, of oxygen molecules and of oxygen atoms

The inactivation of spores of Bacillus atrophaeus and of Aspergillus niger using beams of argon ions, of oxygen molecules and of oxygen atoms is studied. Thereby, the conditions occurring in oxygen containing low pressure plasmas are mimicked and fundamental inactivation mechanisms can be revealed. It is shown that the impact of O atoms has no effect on the viability of the spores and that no etching of the spore coat occurs up to an O atom fluence of 3.5 ? 1019?cm?2. The impact of argon ions with an energy of 200?eV does not cause significant erosion for fluences up to 1.15 ? 1018?cm?2. However, the combined impact of argon ions and oxygen molecules or atoms causes significant etching of the spores and significant inactivation. This is explained by the process of chemical sputtering, where an ion-induced defect at the surface of the spore reacts with either the incident bi-radical O2 or with an incident O atom. This leads to the formation of CO, CO2 and H2O and thus to erosion.

Deposition of silicon dioxide films using an atmospheric pressure microplasma jet

Organic and inorganic silicon dioxide films have been deposited by means of an atmospheric pressure microplasma jet. Tetramethylsilane (TMS), oxygen, and hexamethyldisiloxane (HMDSO) are injected into argon as plasma forming gases. In the case of TMS injection, inorganic films are deposited if an admixture of oxygen is used. In the case of HMDSO injection, inorganic films can be deposited at room temperature even without any oxygen admixture: at low HMDSO flow rates [ 0.1 SCCM,>32 ppm), SiOxCyHz with up to 21% of carbon are obtained. The transition from organic to inorganic film is confirmed by Fourier transform infrared spectroscopy. The deposition of inorganic SiO2 films from HMDSO without any oxygen admixture is explained by an ion-induced polymerization scheme of HMDSO.

Deposition of carbon-free silicon dioxide from pure hexamethyldisiloxane using an atmospheric microplasma jet

Carbon-free silicon dioxide has been deposited at room temperature by injection of pure hexamethyldisiloxane (HMDSO) into an atmospheric pressure microplasma jet from argon. At low HMDSO flow rates [ 0.1SCCM), SiOxCyHz films with a carbon content of up to 21% are obtained. The transition between organic to inorganic film is confirmed by Fourier transformed infrared spectroscopy. The deposition of inorganic films without oxygen admixture is explained by an ion-induced polymerization scheme of HMDSO.

Heating of a dual frequency capacitively coupled plasma via the plasma series resonance

The behavior of dual frequency capacitively coupled plasma discharges (2f-CCP) is experimentally studied by Langmuir probe and rf current measurements and is compared with simulations from the literature. The driving frequency ratio, system pressure, high frequency (HF) power and low frequency (LF) power are varied in the experiments. An increase in LF power causes a moderate increase in electron density but a significant decrease in electron temperature. An increase in HF power causes a strong increase in electron density and populates the high energy part of the electron energy distribution function. These dependences can be explained on the basis of a global model. It is shown that the ratios of HF/LF power and driving frequency are the most important parameters. At integer frequency ratios a significant increase in electron density was found, which is explained by the indirect heating at the plasma series resonance. Several design guidelines are derived which address industrial applications and process stability.

Unexpected O and O3 production in the effluent of He/O2 microplasma jets emanating into ambient air

Microplasma jets are commonly used to treat samples in ambient air. The effect of admixing air into the effluent may severely affect the composition of the emerging species. Here, the effluent of a He/O2 microplasma jet has been analyzed in a helium and in an air atmosphere by molecular beam mass spectrometry. First, the composition of the effluent in air was recorded as a function of the distance to determine how fast air admixes into the effluent. Then, the spatial distribution of atomic oxygen and ozone in the effluent was recorded in ambient air and compared with measurements in a helium atmosphere. Additionally, a fluid model of the gas flow with reaction kinetics of reactive oxygen species in the effluent was constructed. In ambient air, the O density declines only slightly faster with distance compared with a helium atmosphere. In contrast, the O3 density in ambient air increases significantly faster with distance compared with a helium atmosphere. This unexpected behavior cannot be explained by simple recombination reactions of O atoms with O2 molecules. A reaction scheme involving the reaction of plasma-produced excited species of unknown identity with ground state O2 molecules is proposed as a possible explanation for these observations.