From Thin Films to Nanoparticles : Investigation of Polymerization Processes in Capacitively Coupled Hydrocarbon Plasmas

다국어 초록 (Multilingual Abstract)

In recent years, low temperature plasmas have received great attention mainly in the different fields of plasma processing. Different types of plasmas such as glow-, arc-, RF-, and microwave plasmas have been used for deposition of thin films, for etching, and for various types of surface treatments. In order to develop new industrial applications and to improve existing products, investigations of plasma-wall interactions and plasma polymerization are necessary. Based on the knowhow and knowledge obtained by researches, more precise and controlled deposition/etching techniques can be achieved.
One important problem for the application of reactive low temperature plasmas is the formation of nanoparticles in the plasma volume. It is well known that such particles may act as "killer particles" which are responsible, for example, for the malfunction of integrated circuits. On the other hand a controlled growth of particles is desired for applications in catalysis and pharmacy and for the fabrication of nano-crystalline materials. In the latter case nanoparticles are incorporated into the growing film for a controlled modification of film properties. Especially in gas mixtures which are used for thin film deposition, polymerization processes in the plasma volume - leading to the formation of particles - are competing with "surface polymerization" processes which are leading to the formation of thin films.
In this work both processes were studied in a capacitively coupled GEC Cell in mixtures of argon and different hydrocarbon gases. Capacitively coupled discharges are commonly used for deposition and etching of thin films.
On the other hand they are also an appropriate tool for the study of polymerization processes in the plasma volume since they guarantee a very good con?nement of negatively charged clusters which is a precondition for the growth of nano- or micro-sized particles.
An important parameter for such kind of discharges is - beside the choice of the precursor gas - the rf power. Depending on power input and power coupling two types of hydrogenated carbon films with di�erent mechanical and optical properties can be deposited: polymer-like and diamond-like carbon ?lms.
The polymer-like films have low refractive index, low carbon density and a low hardness. The diamond-like films have, on the other hand, high refractive index, high carbon density and a high hardness. The decisive parameter which determines the properties of the deposited films in these experiments is the energy of the ions that are impinging on the substrate.
The importance of the ion energy for the properties of amorphous hydrogenated carbon ?lms (i.e. refractive index, hardness, carbon density, etc) was also nstrated in another discharge type. Experiments performed in an expanding microwave plasma confirm that refractive index and carbon density can be drastically increased by applying an external (pulsed DC) bias, i.e. by increasing the ion energy.
Another important factor is the choice of the precursor gas. The experiments show that the film growth is much faster in C2H2 plasmas than in CH4 plasmas. A similar result is obtained for the formation of nanoparticles in the plasma volume. The particle growth is much faster in C2H2 plasma than in CH4 plasmas. oreover the initiation of the particle growth is di�erent for both kind of gases. For identical process conditions the particles are formed spontaneously in the Ar/C2H2 discharge whereas in the Ar/CH4 discharge the formation of particles could be observed only either after a certain amount of C2H2 (from an external source) was transiently added to the discharge at a constant low input power or transient high input power at the ignition phase. From these experiments it can be concluded that the presence of a critical density of C2H2 is a decisive parameter for the initiation of the particle growth. This is a very important result, since the initial phase of the particle formation can be detected and controlled by monitoring the C2H2 amount in hydrocarbon containing plasmas.
The formation of particles was studied by two di�erent methods: one direct method and one indirect method. The direct method is based on laser light scattering and gives information about the temporal and spatial evolution of the dust formation. The indirect method is based on the fact that the plasma parameters (electron density , electron temperature, light emission from the plasma, etc) are influenced by the presence of dust particles. Although this response of the plasma to the formation of particles gives only global information about the presence of particles it can be used as a tool for process controlling.
It is known for example that the presence of dust particles in the discharge has a dramatic e�ect on the discharge impedance [92]. Consequently the Fourier pectrum of the voltage that is applied to the upper electrode is strongly influenced by the formation of particles. Especially the 7th harmonic of the voltage signal was found to correlate strongly with the presence of particles in the discharge chamber which was cross checked with Mie-ellipsometric measurements.
The results show that the measurement of the 7th harmonic can be used as a sensitive method for the detection of dust particles. With respect to applications in the microelectronic industry the detection of very small killer particles is an important precondition for the development of future devices. The use of the 7th harmonic is a possible alternative for methods based on laser light scattering which cannot be applied for very small particles, since the scattered laser light intensity is proportional to r6.
For given process parameters (pressure, gas mixture, etc) this method can be used even as an indicator for the temporal development of the particle size. An application which combines the formation of particles in the plasma volume and the deposition of thin films on a substrate is the controlled incorporation of carbon-nanoparticles in DLC ?lms. Despite the useful properties of DLC ?lms such as wear resistance, smoothness, low friction and bio-compatibility they have one important disadvantage. Their internal stress can reach up to several GPa when the film is thicker than a few hundred nm. As a result the stability of the film is dramatically reduced. One possibility to overcome this problem is to embed hydrogenated carbon particles into the ?lm. First experiments show that the internal
stress of DLC ?lms was indeed reduced due to the incorporation of nano particles. Further experiments especially for the quanti?cation of this e�ect have to be done in the future. Another interesting and still open problem for bio-medical applications is the control of nanoporosity of DLC ?lms. The nanoporosity can be measured by using a positron scanning microscope, i.e. characterization of the defects via positron annihilation life time [137]. Our preliminary results [138] show that the positron life time in polymer-like ?lms is relatively long, about 1700-1900 ps. It is found that these ?lms have similar structure as polyethylene [139,140]. The positron life time in diamond-like ?lms is short, about 300-400 ps, which is a typical positron life time in graphite with large defects [137,141]. These results are consistent with previous research of others [137,139,140,142] on carbon materials.
Another interesting application would be incorporation of foreign nanoparticles into DLC films to change their electrical properties. DLC films can have either p- or n-type characteristics depending on hydrogen contents in the film [143]. Combining with metal or emiconductor-like nanoparticles, one might be able to enhance the field emission properties, for example.

목차 (Table of Contents)

• Contents = iii
• List of Figures = vii
• List of Tables = xiii
• Chapter 1 General Introduction = 1
• Chapter 2 Standard GEC Reference Cell = 3
• 2.1 Introduction = 3
• 2.2 Experimental Arrangement = 5
• Chapter 3 Diagnostics = 9
• 3.1 Introduction = 9
• 3.2 Ellipsometry = 9
• 3.2.1 Introduction = 9
• 3.2.2 Rotating Element Ellipsometer = 11
• 3.2.3 Jones Matrix Analysis of the Rotating Analyzer Ellipsometer = 14
• 3.2.4 Modeling of the Sample Matrix = 18
• 3.2.5 Optical Models = 25
• 3.2.6 Mie-Ellipsometry = 31
• 3.3 Mass Spectrometry = 37
• 3.3.1 Introduction = 37
• 3.3.2 The Measurement Principles: Quadrupole Mass Spectrometer = 37
• 3.4 Rutherford Backscattering = 41
• 3.4.1 Introduction = 41
• 3.4.2 The Measurement Principles = 41
• 3.5 X-Ray Photoelectron Spectroscopy = 45
• 3.5.1 Introduction = 45
• 3.5.2 The Measurement Principles = 45
• 3.6 Atomic Force Microscopy = 48
• 3.6.1 Introduction = 48
• 3.6.2 The Measurement Methods = 48
• Chapter 4 Thin Film Processing = 53
• 4.1 Introduction = 53
• 4.2 Deposition of Thin Films in GEC Cell = 54
• 4.2.1 Infuence of the Input Power = 55
• 4.2.2 Infuence of the Precursor Gas = 59
• 4.2.3 Temperature E�ect and Surface Structures of Films = 62
• 4.2.4 Summary = 65
• 4.3 Etching of Thin Films in GEC Cell = 66
• 4.3.1 Infuence of the Chamber Condition = 66
• 4.3.2 Summary = 69
• Chapter 5 Dust Particle Formation = 71
• 5.1 Introduction = 71
• 5.2 Experimental Results = 72
• 5.3 Temporal and spatial evolution of the dust formation = 72
• 5.3.1 Forces acting on Dust Particles = 76
• 5.3.2 Motion of the Dust Particles = 78
• 5.4 The nucleation/Growth process in Ar/CH_(4) and Ar/C_(2)H_(2) plasmas = 80
• 5.5 The response of the plasma to the formation of dust particles = 84
• 5.5.1 Presence of Harmonics = 86
• 5.6 Summary = 91
• Chapter 6 Application; Dispersion Coatings = 93
• 6.1 Introduction = 93
• 6.2 Experimental Setup = 94
• 6.3 Detection of the Particles and Their Properties = 96
• 6.3.1 Correlation between Global and Local Measurements = 96
• 6.3.2 Determination of the Particle Properties = 97
• 6.4 Deposition of Size-Controlled Particles = 100
• 6.5 Dispersion Coatings: Incorporation of the Particles in a DLC Film = 104
• 6.6 Summary = 107
• Chapter 7 Summary and Outlook = 109
• Bibliography = 113
• Appendix A: Thin Film Processing using Surface Wave Sustained Discharge = 123
• A.1 Introduction = 123
• A.2 Deposition of Thin Films using SWSD = 124
• A.2.1 Experimental Setup = 124
• A.2.2 Infuence of the Substrate-Nozzle Distance = 126
• A.2.3 Infuence of the CH_(4) Flow = 128
• A.2.4 Infuence of the Ar Flow = 129
• A.2.5 Infuence of an External Bias = 130
• A.2.6 Deposition of a-C:H:N Films = 132
• A.2.7 Surface Structures and Carbon Density = 133
• A.2.8 Summary = 135
• A.3 Etching of Thin Films using SWSD = 137
• A.3.1 Etching by an Expanding Ar Plasma = 137
• A.3.2 Etching by an Expanding Ar/N2 Plasma and N2 Post-Discharge = 139
• A.3.3 Infuence of the Incorporated Nitrogen Atoms = 141
• A.3.4 Summary = 143
• References in Appendix A = 145
• Danksagung = 147
• Curriculum Vitae = 149