Reaction chemistry in oxygen or hexamethyldisiloxane containing noble gas microplasma jets: a quantitative molecular beam mass spectrometry study Dissertation zur Erlangung des Grades „Doktor der Naturwissenschaften“ an der Fakultät für Physik und Astronomie der Ruhr-Universität Bochum von Dirk Ellerweg aus Troisdorf Bochum 2012 1. Gutachter: Prof. Dr. Achim von Keudell 2. Gutachter: Prof. Dr. Peter Awakowicz Datum der Einreichung: 11. November 2011 Datum der Disputation: 02. Februar 2012 Contents 1 Introduction and Motivation 5 2 Fundamentals 9 2.1 Microplasma . 9 2.1.1 Applications . 12 2.1.1.1 Biomedical applications . 12 2.1.1.2 Deposition of silicon dioxide film . 13 2.2 Mass spectrometry for microplasma diagnostics . 16 2.2.1 Threshold ionization mass spectrometry . 23 2.2.2 Calibration of a mass spectrometer . 24 2.2.3 Molecular beam mass spectrometry . 25 2.2.4 Molecular beam . 28 2.2.5 Composition distortion . 32 2.3 Laser induced fluorescence spectroscopy . 35 2.4 Comparison between MBMS and TALIF . 36 3 Experiment 37 3.1 Atmospheric pressure microplasma jet . 37 3.2 X-Jet . 39 3.3 Molecular beam mass spectrometer . 39 4 Fluid model of the plasma euent 47 4.1 Model of the gas flow . 47 4.2 Model of chemical kinetics . 49 5 Results 53 5.1 Performance of the molecular beam mass spectrometer . 53 5.1.1 Influence of the chopper design on the signal . 53 5.1.2 Analysis of air . 59 5.1.3 Mass dependence of the peak shape . 61 5.1.4 Gas flow rate into the MBMS system . 62 5.2 Calibration of the MBMS system . 63 3 Contents 5.3 Production of reactive oxygen species . 68 5.3.1 Measurement of neutral species . 68 5.3.2 Effluent chemistry in ambient air . 74 5.3.3 Measurement of ions . 82 5.4 Analysis of hexamethyldisiloxane containing microplasmas . 90 5.4.1 Measurement of positive ions . 90 5.4.2 Measurement of neutral species . 93 6 Conclusion and Outlook 107 Bibliography 111 4 1 Introduction and Motivation Cold atmospheric plasmas exhibit many unique properties, which make them highly attractive for a broad field of applications. This kind of plasma combines the advan- tages of low pressure and atmospheric pressure plasmas without their respective dis- advantages. Low pressure plasmas are low temperature, non-equilibrium plasmas. They are used commonly in industry for e.g. thin film deposition or etching especially in semicon- ductor industries. However, a complicated vacuum system is needed to sustain the plasma. Atmospheric pressure plasmas, on the contrary, are operated at atmospheric pressure with no need for a vacuum system. These plasmas are used for several ap- plications like ozone generation, activation and cleaning of surfaces in the case of cold non-equilibrium plasmas or plasma spraying and plasma welding in the case of hot thermal plasmas. The key advantage of atmospheric pressure plasmas is that they can be easily integrated in existing production lines. However, they tend to form thermal arcs and filaments, which restricts the field of applications. Overall, cold atmospheric plasmas offer non-equilibrium chemistry at atmospheric pressure with high densities of reactive neutral and charged species and high fluxes of photons. Especially, treatment of temperature and vacuum sensitive materials is made possible. Prominent applications are e.g. treatment of polymers or living tis- sues and localized deposition of thin films. However, considerable differences exist between cold atmospheric plasmas and low pressure plasmas. The short mean free path and the high collision rate due to the high pressure have a high impact on the plasma chemistry. Therefore, the plasma chemistry at atmospheric pressure is, in contrast to low pressure plasmas, only poorly understood. This work focuses on microplasma jets, a special kind of cold atmospheric plasmas with a high gas flow through a micro-scaled discharge. The reactive species generated in the micro discharge are delivered by the gas flow to a surface to be treated. In re- cent years, many different sources have been developed and studied for a variety of applications. A promising field of applications of microplasma jets that has to be highlighted is the relatively new field of plasma medicine. It has been observed that pathogens are 5 1 Introduction and Motivation deactivated under plasma exposure. Consequently, microplasma jets can be used for a localized treatment of living tissue and wounds to accelerate wound healing. How- ever, since plasmas are complex systems consisting of electrons, ions, reactive species, and photons, it is challenging to reveal the precise bacteria deactivation mechanism. Reactive species, like atomic oxygen and ozone, are supposed to play a crucial role during the inactivation of bacteria. A synergism between the different plasma con- stituents is also likely. The measurement of absolute densities of reactive species generated by a microplasma jet are of high interest in the study of these reaction mechanisms. However, it is a chal- lenging task due to the high pressure and the small dimensions of the jet. Many of the standard diagnostics of low pressure plasma cannot be applied. A specially designed molecular beam mass spectrometer has therefore been developed within the scope of this work. This mass spectrometer is perfectly suited for analysis of atmospheric pressure plasmas due to its very high sensitivity. In this thesis, the production of reactive oxygen species in a helium/oxygen micro- plasma jet is studied in detail by molecular beam mass spectrometry. The questions of particularly interest are: • What are the optimal conditions for generation of atomic oxygen and ozone? • How do these densities behave with increasing distance from the nozzle of the plasma jet? For reasons of simplicity, all prospective microplasma jet applications take place in air ambiance. The effect of admixing air into the effluent may severely affect the compo- sition of the emerging species. The questions, which are to be solved are: • How much air does admix into the effluent of the microplasma jet? • Does the air admixture has a big influence on the atomic oxygen and ozone densities? • Which are the important reactions in the effluent and does UV radiation play a crucial role? Especially the last question can be solved by comparing the measured densities to results of a fluid model of the plasma effluent. The developed molecular beam mass spectrometer is also capable to measure positive and negative ions. • However, are ions still left in the effluent of the plasma jet? • If so, how are they produced? The gained knowledge can be used to optimize the process of surface treatment and, especially, bacteria deactivation. 6 Plasmas are also widely used for deposition of surface coatings, for example SiO2-like films of good quality are desired in industry. Over the past years, the research fo- cused on deposition of silicon oxide films at atmospheric pressure to achieve as good films as in low pressure plasmas. However, the studies are often limited to an analysis of the resulting silicon oxide film, such as measurements of the deposition rate and the elemental composition of the film. For a precise comprehension of the deposition mechanism of silicon oxide films, the plasma itself must be analyzed. A He/O2 microplasma jet with admixtures of hexamethyldisiloxane is used in combi- nation with the molecular beam mass spectrometer to study the microplasma chem- istry leading to depositions of SiO2-like films. The questions of particularly interest are: • Under which conditions is the hexamethyldisiloxane consumption maximized and how large is the consumption? • Which reaction products are generated in the hexamethyldisiloxane plasma and what are their absolute densities? • Do ions contribute to the film growth at atmospheric pressure? The answers to these questions can contribute to a more precise understanding of the deposition mechanism at atmospheric pressure. Outline of the thesis The following chapter provides the necessary background knowledge regarding mi- croplasmas, their applications, and the diagnostics used. The principle of mass spec- trometry, and particularly molecular beam mass spectrometry and composition dis- tortion are explained here in detail. Chapter three introduces the experimental setup used in this work. The different ver- sions of the microplasma jet and the design of the molecular beam mass spectrometer are described here. A fluid model of the plasma effluent is defined in the next chapter. The fluid model has been developed for a better understanding of the reaction chemistry going on in the effluent. The results of the measurements are presented and discussed in chapter five. First, the performance of the molecular beam mass spectrometer is demonstrated by means of several test measurements and the calibration procedure is explained. Then the measurements of oxygen and hexamethyldisiloxane microplasma jets are shown. The measurements of reactive oxygen species in the effluent are also compared to the re- sults of the fluid model. The final chapter summarizes the thesis and gives a brief outlook on prospective in- vestigations. 7 2 Fundamentals In this thesis, the plasma chemistry of a microplasma jet has been analyzed by means of mass spectrometry. The necessary fundamentals are provided in this chapter. First, an introduction into microplasmas is given. Their possible applications are explained in detail with the help of two examples, which are of particular interest for this work. The following section focuses on the principle of mass spectrometry. For the analy- sis of atmospheric pressure plasmas, molecular beam mass spectrometry needs to be applied and, therefore, is explained here. When absolute densities should be deter- mined, one needs to understand the behavior of a molecular beam and its influence on the initial gas composition. These informations are provided here. The principle of two-photon absorption laser induced fluorescence spectroscopy is described as well. 2.1 Microplasma »Except near the electrodes, where there are sheaths containing very few electrons, the ionized gas contains ions and electrons in about equal num- bers so that the resultant space charge is very small.