Dissertation. Michael Deilmann. Bochum, 2008

Ruhr-Universit¨at Bochum

der Fakult¨at f¨ur Elektrotechnik und Informationstechnik an der Doktor-Ingenieurs

Dissertation zur Erlangung des Grades eines

Bochum, 2008

Michael Deilmann

by pulsed low-pressure microwave plasmas

sterilization of PET bottles

Silicon oxide permeation barrier coating and

Dissertation

Tag der m¨undlichen Pr¨ufung: 14.08.2008

1. Berichter: Prof. Dr.-Ing. Peter Awakowicz 2. Berichter: Prof. Dr. rer. nat. J¨org Winter

Modern packaging materials such as polyethylene terephthalate (PET) offer various advan-tages over glass or metal containers and are gaining in importance for food and beverage packaging. PET bottles are nonbreakable and light weight compared to established ma-terials, but they only offer minor barrier properties against gas permeation. Therefore, the shelf-life of packaged food and beverages is limited. Additionally, common sterilization methods like heat, hydrogen peroxide or peracetic acid may not be applicable due to reduced heat or chemical resistance of the plastic material.

In the field of plasma sterilization, a process is developed on the basis of investigations into plasma sterilization mechanisms. Two relevant test spores for food packaging applications are considered and the capability of an optimized plasma for plasma sterilization of PET bottles is demonstrated. The achieved sterilization efficiency with treatment times below five seconds is in accordance with regulations of FDA (Food and Drug Administration, USA) and VDMA (Verband Deutscher Maschinen- und Anlagenbauer e.V., Germany). Further-more, the developed process is characterized concerning phase and spatially resolved electron temperature and density profiles. Additionally, absolutely calibrated emission spectroscopy is applied for process optimization.

For the permeation barrier coating of PET substrates, a silicon oxide (SiO_x) barrier coat-ing based on oxygen diluted hexamethyldisiloxane (HMDSO) plasmas is investigated. To achieve homogeneous treatment of three dimensional substrates, a pulsed plasma process is developed for SiO_x film deposition. A criterion correlating pulse conditions and residence times of the process gases in the packaging is deduced allowing for a process scale-up. The influence of all relevant plasma conditions, namely gas composition, flow rates, process pres-sure and pulsed microwave power properties is investigated in detail. Their effect on barrier properties, surface morphology and coating composition are analyzed. It can be stated, that good permeation barrier coatings have to exceed a critical thickness of 60 nm, are carbon free and as smooth as possible. Besides the coating analysis, the neutral gas composition and the ion bombardment onto the surface are considered by means of mass spectrometry. It is revealed that the deposition process of SiO_x films is based on the deposition of SiO_xC_yH_z like HMDSO fragments on the surface and oxygen etching to form SiO_x. As oxidation prod-ucts mainly CO₂, OH and H₂O are observed. Furthermore, the permeation mechanisms of deposited films are analyzed revealing defect permeation as a dominant mechanism for the observed residual permeation. These defects are visualized by means of capacitively coupled oxygen etching of deposited barrier layers.

Concluding, a plasma sterilization process is successfully developed and characterized for the treatment of PET bottles. It is easily combinable with a permeation barrier coating, which is developed and characterized.

Contents i

List of Figures iii

List of Tables v

Introduction 1

1 Surface wave excited plasmas 5

1.1 Plasma ignition by means of a Plasmaline antenna . . . 6

1.2 Pulsed plasmas . . . 9

2 Experimental setup 11 2.1 Reactor setup . . . 11

2.1.1 Generator characteristics . . . 12

2.1.2 Automatization of reactor setup . . . 13

2.1.3 Parameter ranges . . . 13

2.2 Plasma diagnostic methods . . . 14

2.2.1 Langmuir probe measurements . . . 14

2.2.2 Energy mass spectrometry . . . 15

2.2.3 Optical emission spectroscopy . . . 19

2.3 Coating analysis . . . 20

2.3.1 Permeation measurement . . . 20

2.3.2 Fourier transform infrared spectroscopy . . . 22

2.3.3 Stylus profilometry . . . 25

2.3.4 Atomic force microscopy . . . 25

2.3.5 Scanning electron microscopy, Energy dispersive x-ray spectroscopy . 26 2.4 Microbiological methodology . . . 26

2.4.1 Fraunhofer Institute for Process Engineering and Packaging . . . 26

2.4.2 Count reduction test . . . 26

2.5 Substrate materials . . . 27

2.5.1 Polyethylene terephthalate . . . 27

2.5.2 Silicon wafer . . . 29

2.5.3 Glass . . . 29

3 Characterization of reactor setup 31 3.1 Continuous wave characterization . . . 31

3.1.1 Electron density and electron temperature . . . 31

3.1.2 Ion energy distribution functions . . . 34

3.1.3 Power influence on electron density and electron temperature . . . 35

3.2 Pulsed characterization . . . 35 i

3.2.1 Electron density and electron temperature . . . 36

3.2.2 Ion energy distribution functions . . . 37

3.3 Influence of reactor coating on plasma parameters . . . 41

4 Plasma sterilization for aseptic filling of beverages 43 4.1 Classification and definition of aseptic filling . . . 43

4.1.1 Demands of aseptic filling . . . 44

4.1.2 Established sterilization processes for aseptic packaging . . . 45

4.1.3 Selection of test spores and artificial contamination . . . 46

4.2 Mechanisms of plasma sterilization . . . 47

4.3 Definition of plasma parameters . . . 49

4.4 Characterization of sterilization plasma . . . 49

4.5 Sterilization results . . . 50

4.5.1 Sterilization of specimen holders . . . 50

4.5.2 Sterilization of PET bottles . . . 50

4.6 Conclusion . . . 53

5 Permeation barrier coating 55 5.1 Mathematical description of permeation . . . 55

5.2 Plasma polymerization of hexamethyldisiloxane . . . 59

5.2.1 Analysis of HMDSO vapor . . . 60

5.2.2 Analysis of ions . . . 64

5.2.3 Fragment ambiguities of oxygen diluted HMDSO plasmas . . . 65

5.3 Pulsed coating deposition . . . 65

5.4 Development of barrier layer system on PET foils . . . 67

5.4.1 Adhesion promoting layer . . . 67

5.4.2 Influence of oxygen dilution during barrier coating deposition . . . 73

5.4.3 Influence of pulse power and duration . . . 86

5.4.4 Influence of process pressure . . . 99

5.4.5 Influence of layer thickness on barrier properties . . . 102

5.4.6 Optimized plasma parameters for deposition of barrier layer system . 103 5.5 Investigation of permeation mechanisms . . . 103

5.5.1 Analysis of coating defects . . . 105

5.5.2 Discussion of permeation mechanisms . . . 106

5.6 Coating of bottles . . . 108

5.7 Conclusion . . . 112

Summary 113

1.1 Illustration of cylinder symmetric inner and outer surface wave configuration 5

1.2 Electron density and electrical field for Plasmaline configuration . . . 7

2.1 Schematic of the reactor system . . . 11

2.2 Noise of microwave generator. . . 12

2.3 Power characteristic of microwave generator . . . 13

2.4 Scheme of Langmuir probe setup. . . 14

2.5 Schematic of energy mass spectrometer . . . 16

2.6 Ion energy distribution functions regarding aberrations . . . 17

2.7 Mass dependent transmission function of HIDEN EQP 300 . . . 19

2.8 Permeation measurement setup . . . 21

2.9 FTIR spectroscopy setup. . . 22

2.10 Penetration depth of infrared beam into coating versus wave number . . . . 23

2.11 Gaussian fits to FTIR absorption spectrum of plasma polymerized SiO_x film 24 2.12 Polyethylene terephthalate. . . 28

2.13 Foil carrier for treatment of PET foils . . . 28

2.14 FTIR absorption spectrum of PET and peak assignment . . . 29

3.1 Radial electron density and electron temperature profiles . . . 32

3.2 Axial Electron density and electron temperature profiles . . . 33

3.3 Ion energy distribution functions of cw argon plasmas . . . 34

3.4 Influence of power on electron density and electron temperature . . . 35

3.5 Time resolved electron density and electron temperature profiles . . . 36

3.6 Time resolved ion energy distribution functions for various process pressures 38 3.7 Time resolved count rate of Ar+ ions . . . 39

3.8 Time dependent electron density behavior and ion flux of Ar+ ions . . . 40

3.9 Time dependent ion flux of Ar+ ions of argon and argon:oxygen plasma . . . 41

3.10 Influence of SiO_x coating of the reactor chamber on potentials . . . 42

4.1 SEM pictures of B. atrophaeusand A. niger . . . 47

4.2 Radial and temporal behavior of electron density . . . 50

4.3 Optical emission spectrum of H₂ : N₂ : O₂ plasma . . . 51

4.4 Time dependent reduction ofB. atrophaeus and A. niger on specimen holders 52 4.5 Time dependent reduction ofB. atrophaeus inside PET bottles . . . 52

5.1 Illustration of four steps of permeation. . . 55

5.2 Boundary conditions and steady state solution of polymer film permeation . 57 5.3 Measurement and simulation of time dependent oxygen flux . . . 59

5.4 Chemical structure of HMDSO . . . 60

5.5 Schematic of plasma polymerization of HMDSO . . . 60

5.6 Fragmentation pattern of HMDSO . . . 61 iii

5.7 Fragmentation paths of HMDSO . . . 63

5.8 Influence of electron impact energy on fragmentation pattern of HMDSO . . 64

5.9 Deposition rate of SiO_x coatings for various inter-pulse durations . . . 66

5.10 SEM pictures of SiO_x coatings on pristine PET . . . 67

5.11 Permeation rates for various adhesion promoting layers . . . 69

5.12 FTIR absorption spectrum of adhesion promoting layer . . . 71

5.13 Ions spectrum during deposition of adhesion promoting layer . . . 72

5.14 Permeation rates of SiO_x coatings for various oxygen fluxes . . . 73

5.15 EDX investigation of layer composition for various oxygen fluxes . . . 74

5.16 FTIR absorption spectra of SiO_x coatings for various oxygen dilutions . . . . 75

5.17 Peak position of SiCH₃ vibrations for various oxygen fluxes . . . 75

5.18 Ratiosr_SiCH3 and r_SiOH for various oxygen fluxes . . . 76

5.19 Surface morphology of SiO_x coatings for various oxygen fluxes . . . 77

5.20 Normalized mass spectra of neutrals for various oxygen fluxes . . . 79

5.21 Relative behavior of neutrals for various oxygen fluxes . . . 80

5.22 Normalized mass spectra of positive ions for various oxygen fluxes . . . 81

5.23 Relative behavior of ions for various oxygen fluxes . . . 82

5.24 Optical emission spectrum of deposition plasma . . . 84

5.25 Behavior of optical emission related to significant ions and molecules . . . . 85

5.26 Permeation rates of SiO_x coatings for various pulse powers . . . 86

5.27 FTIR absorption spectra of SiO_x coatings for various pulse powers . . . 86

5.28 Surface morphology of SiO_x coatings for various pulse powers . . . 87

5.29 Permeation rates of SiO_x coatings for various pulse durations . . . 88

5.30 FTIR absorption spectra of SiO_x coatings for various pulse durations . . . . 88

5.31 Normalized mass spectra of neutrals for various pulse powers . . . 90

5.32 Relative behavior of neutrals for various powers . . . 91

5.33 Normalized mass spectra of neutrals for various pulse durations . . . 92

5.34 Relative behavior of neutrals for various pulse durations . . . 93

5.35 Fragmentation rates of HMDSO for variation of power and pulse duration . . 93

5.36 Normalized mass spectra of positive ions for various pulse powers . . . 94

5.37 Relative behavior of ions for various powers . . . 95

5.38 Normalized mass spectra of positive ions for various pulse durations . . . 97

5.39 Relative behavior of ions for pulse durations . . . 98

5.40 Time resolved relative optical emission during microwave power pulse . . . . 99

5.41 Permeation rates of SiO_x coatings for various process pressures . . . 100

5.42 FTIR absorption spectra of SiO_x coatings for various process pressures . . . 100

5.43 Ratiosr_SiOH and r_ss for various oxygen fluxes . . . 101

5.44 Surface morphology of SiO_x coatings for various process pressures . . . 102

5.45 Permeation rates of SiO_x coatings for various coating thicknesses . . . 103

5.46 Arrheniusplot of oxygen permeation . . . 104

5.47 Activation energy of oxygen permeation for various oxygen fluxes . . . 105

5.48 SEM pictures of SiO_x coatings on PET after etching in CCP oxygen plasma 105 5.49 Visualization of different permeation mechanisms . . . 107

5.50 SEM pictures of different coating defects . . . 108

5.51 Permeation rates of coated PET bottles as function of pump area . . . 109

5.52 Permeation rates of bottle walls as function of pump area . . . 110

2.1 Parameter ranges of microwave reactor setup. . . 14

2.2 Electron impact ionization cross sections of the rare gases . . . 18

2.3 Infrared absorption peaks in FTIR spectra of SiO_xC_yH_z like films . . . 25

3.1 Decay times for various pressures in the afterglow . . . 40

4.1 Minimum requirements of germ reduction for aseptic packaging machines . . 45

4.2 Recommended test spores for various sterilization mediums . . . 48

5.1 Relative intensities of fragments of HMDSO . . . 62

5.2 Bond energies of HMDSO and electronegativity of the elements . . . 62

5.3 Optimized plasma parameters for deposition of barrier layer system . . . 103

Modern packaging materials such as polyethylene terephthalate (PET) offer various advan-tages over glass or metal containers and are gaining in importance for food and beverage packaging. PET bottles are nonbreakable and light weight compared to established mate-rials, but they only offer minor barrier properties against gas permeation. Therefore, the shelf-life of packaged food is limited. Additionally, common sterilization methods like heat, hydrogen peroxide or peracetic acid may not be applicable due to reduced heat or chemical resistance of the plastic packaging material.

Industrial scale applications cope these disadvantages using e.g. gas scavengers or blends as part of the PET melt to reduce the gas permeation or even develop multilayer bottles. These bottles consist of several layers, where at least one layer is a high barrier material, such as ethylene vinyl alcohol (EVOH), responsible for a reduction of gas permeation. The production of these multilayer bottles is challenging, because a homogeneous distribution of the high barrier material has to be ensured during the production of a bottle preform and the stretch blow molding process. Additionally, the usage of various barrier materials can lead to problems of bottle recyclability due to an increased amount of impurities within the PET raw material.

In contrast, different plasma based coating processes are topic of ongoing investigations to decrease the permeation through PET for different applications like foils or bottles [1, 2, 3]. Plasma polymerized layers are deposited on the packaging material as permeation barriers. E.g. amorphous carbon hydrogen (a-C:H) coatings are known to act as good permeation barriers for food packaging applications, but they tend to show a light brown color on the substrate. In comparison, plasma polymerized silicon oxide (SiO_x) coatings have advan-tageous properties concerning transparency, recyclability and microwave use. Thus, they are gaining in importance in industrial processes. The influence of plasma parameters on permeation properties of SiO_x coatings is investigated by different groups for various se-tups like capacitive [4, 5, 6] or microwave plasmas [3, 7] and are employed for permeation barrier coatings. Additionally, microwave plasmas in combination with biased substrate holders [8, 9, 10, 11, 12, 13, 14] or magnetic field assisted plasma enhanced chemical vapor deposition of permeation barriers [15, 16, 17, 18] are investigated. An expanding thermal plasma can also be used for the deposition of high performance SiO_x coatings on flat sam-ples [19, 20, 21, 22, 23, 24].

Typically, these investigated plasmas are used for a continuous deposition of barrier coat-ings on flat samples like packaging foils by means of silicon containing monomers, e.g. hexamethyldisiloxane (HMDSO). Contrary, pulsed plasmas gain in importance for indus-trial processes and allow for a further degree of freedom by varying the pulse parameters. The pulsed deposition of SiO_x coatings is established for diverse applications [25, 26, 27, 28, 29], which show major influence of the pulse parameters on coating properties. Therefore, the

investigation of pulsed plasmas for the deposition of permeation barrier coatings is promis-ing. Typically, pulsed plasmas are characterized by a pulse frequency and a duty cycle. A more detailed investigation is required to understand the correlations of plasma pulsing and layer deposition for process optimization and scalability.

In the field of packaging material sterilization, various methods are established mainly based on the usage of toxic chemicals like hydrogen peroxide or peracetic acid. The influence of these substances on the packaging material are not extensively investigated. Furthermore, residues of the sterilants can interact with packaging materials leading to modifications of packaging composition, color or transparency and to the production of off-flavors.

Objective For the plasma treatment of bottle shaped packaging materials, a microwave driven low-pressure plasma reactor system is developed based on a Plasmaline antenna [30]. It allows for the ignition of a plasma inside bottles for various purposes based on the prop-agation of outer surface waves along an antenna. The objective of this thesis is to develop and characterize a combined sterilization and permeation barrier coating process for PET packaging materials.

In detail, in the field of plasma sterilization, a plasma process is developed in accordance with today’s regulations of validation of aseptic filling machines. Therefore, the results of investigations of sterilization mechanisms by means of low-pressure plasmas as performed by Halfmann et al. [31, 32] and the European BIODECON project [33] are adapted for the sterilization of PET bottles and the process is characterized in terms of plasma density, electron temperature and optical emission.

The deposition of transparent permeation barrier silicon oxide coatings represents the main focus of this thesis. They are designed to reduce the oxygen permeation of PET packaging materials and to allow for a homogeneous substrate treatment. The influence of the deposi-tion parameters namely oxygen diludeposi-tion, pulse power, process pressure and pulse condideposi-tions on the barrier properties, layer composition and morphology are investigated. Additionally, the deposition plasma is analyzed regarding composition and surface ion bombardment and insights into the deposition chemistry are revealed.

Outline of the thesis The first chapter Surface wave excited plasmasdescribes the real-ization of plasma ignition by means of surface waves and concentrates on the plasma creation by means of a Plasmaline antenna.

The experimental setup is described in chapter 2 regarding the description of the reac-tor setup, the plasma diagnostic methods, the coating diagnostics and the microbiological methodology. Additionally, the used substrate materials are introduced.

The plasma is characterized in chapter 3 considering electron density and electron temper-ature as well as ion energy distribution functions of continuous and pulsed argon plasmas. Chapter 4 Plasma sterilization for aseptic filling of beverages describes the development of a plasma based sterilization process based on the fundamental research of sterilization mechanisms. The capabilities of this new sterilization method are discussed and a treatment according to the requirements for validation of aseptic filling machines is designed.

The development of a permeation barrier coating system on PET constitutes the focus of chapter 5. A mathematical description of time dependent permeation is presented allowing for the determination of permeation constants of silicon oxide coatings. The plasma poly-merization of hexamethyldisiloxane as silicon containing monomer is afterwards discussed and the fundamentals of plasma enhanced chemical vapor deposition processes based on organic monomers are introduced. Thereafter, a criterion for the homogeneous deposition of barrier coatings depending on the pulse parameters of a hexamethyldisiloxane-oxygen plasma is deduced. The design of a permeation barrier coating on PET foils builds the main focus of this thesis. It is discussed in section 5.4. The influence of main plasma pa-rameters on coating properties and plasma composition is investigated and insight into the process chemistry are revealed. It is followed by the investigation of the permeation mecha-nism of plasma polymerized barrier coatings regarding coating defects and bulk permeation. Finally, the results of the adaption of the developed barrier coating system for the coating of PET bottles is discussed.

Electrical gas discharges are utilized for a wide range of application such as microelectron-ics, material processing or lightning. Various kinds of electrical coupling like capacitive or inductive are established using different frequencies ranging from ”direct current” to several Megahertz. Beside these kinds of plasma, surface wave excited plasmas are investigated since the 1970s and gain in importance for e.g. surface processing applications. Moisan et al. [34] review surface wave sustained plasmas driven in a frequency ranging from 1 MHz to 10 GHz. Typically a microwave frequency is used for the ignition of the plasma. Particularly, a frequency off = 2.45 GHz is widely utilized due to the usage of microwave magnetrons for other applications like microwave ovens resulting in reduced equipment costs. Surface wave sustained plasmas can be easily operated and are not affected by changes in the discharge conditions and plasma parameters [35]. With a proper designed wave launching system, the propagation of the surface wave is monomode allowing for excellent reproducibility of the plasma properties like electron density and temperature [34, 35]. Additionally, surface wave excited plasmas are characterized by an extraordinary flexibility in terms of the applied frequency, the gas pressure range and usage for manifold applications. Moisan et al. de-veloped the surfatron in the 1970s as a first simple and efficient surface wave launcher for the generation of long plasma columns at microwave frequencies [34, 36]. It consists of an evacuated quartz tube, in which a process gas is ignited by means of an electrical wave launching system adapted to the outside of the tube. The plasma is sustained due to an inner surface wave propagating along the inner surface of the quartz tube as visualized in figure 1.1(a).

metallic wave guide air (atmospheric pressure) quartz tube

plasma (low pressure)

(a) Inner surface wave excited plasma.

metallic wave guide air (atmospheric pressure) quartz tube

plasma (low pressure)

(b) Outer surface wave excited plasma.

Figure 1.1: Illustration of cylinder symmetric inner and outer surface wave configuration [37].

1.1

Plasma ignition by means of a Plasmaline antenna

R¨auchle [37] developed an outer surface wave sustained plasma called Plasmaline based on a kind of 1/r transformation of the inner surface wave system leading to a plasma ignition in the outer area of a quartz cylinder as shown in figure 1.1(b). A metal rod is surrounded by a quartz cylinder building a microwave antenna for the plasma ignition in the outer volume of the quartz cylinder within an evacuated vessel. Electromagnetic waves propagate mainly within the tube along the antenna and within the plasma as radially decaying sur-face waves [37]. This plasma generation is comparable to the propagation of electromagnetic waves in a coax cable. The inner conductor is represented by the Plasmaline antenna and the outer is built by the reactor vessel for a cylinder symmetric system. In between these conductors, the media can be described as dielectrics influencing the wave propagation and the characteristics of the electromagnetic field.

In dependence on the radiusr, the axial elongation z and the azimuthal directionϕ, a time dependent propagation of the electric field can be described by [37]

E(r, z, ϕ, t) = ⎛ ⎝EEr_z E_ϕ ⎞ ⎠=E₀(r)·ei(kz−ωt), (1.1)

where k and ω denote the complex wave number and the frequency ω = 2πf, respectively. Additionally, symmetry is assumed regarding the azimuthal angle ϕ for the cylinder sym-metrical system.

Based on Maxwell’s equations ∇ ×_{E =−µ} 0 ∂ H ∂t and ∇ × H =₀∂ E ∂t +J , (1.2)

a wave equation (1.3) is derived describing the wave propagation [37, 38] ∇ ×∇ ×_E=µ

00ω2r(r)E . (1.3) Solving this equation for the wave form described by equation (1.1), a differential equation for the radial and axial component of the electric field, E_r and E_z, respectively, is derived to be [37, 38] ∂2_E r ∂r2 + 1 r + 1 _r(r) ∂_r(r) ∂r ∂E_r ∂r (1.4) + µ₀₀ω2_r(r)−k2− 1 r2 + 1 _r(r) ∂2 r(r) ∂r2 − ∂_r(r) ∂r 2 E_r = 0 and E_z = 1 µ₀₀ω2 r(r) 1 r ∂ ∂r µ₀₀ω2r_r(r)E_r . (1.5)

Based on this description, the wave propagation in a coaxial arrangement consisting of an inner and outer conductor separated by a homogeneous dielectric _r(r) =const is revealed to be

_r(r) = const ⇒ E_r ∼ 1

The radial component of the propagating waveE_r shows an 1/r characteristic leading to a vanishingE_z as described by equation (1.5). Therefore, a transversal electromagnetic wave (TEM) is propagating along the antenna for a homogeneous dielectric.

In contrast, the characteristic of the electromagnetic wave becomes more complex consider-ing a coaxial arrangement with an ignited plasma in between the two conductors. For the wave propagation within a plasma, a complex dielectric permittivity _r depending on the plasma parameters has to be taken into account. The dielectric constant of a plasma can be derived to be [39] _r = 1− ω 2 pe ω2₍₁−_iν ω) , (1.7)

where ω_pe and ω denote the electron plasma frequency and the microwave frequency ω = 2πf, respectively. Additionally, the electron collision frequency ν describes the momentum transfer between electrons and neutrals. The electron plasma frequency is given by [39]

ω_pe=

n_ee2

₀m_e. (1.8)

Therefore, the plasma densityn_e strongly influences the electron plasma frequency ω_pe, the dielectric constant_rand the electrical field distribution in the plasma volume depending on the spatial coordinates. By means of this relation, a critical densityn_e,crit can be determined leading to _r = 0 for neglected friction conditions ν = 0 Hz [37]. For a microwave plasma with f = 2.45 GHz, the critical density is n_e,crit = 7.45·1016m−3 for ω = ω_pe and _r = 0. These conditions lead to a strong influence on the wave propagation properties of electric fields due to a vanishing dielectric constant.

E_r E_r, n_e n_e,crit n_e r 0 quartz cylinder inner conductor ∼ 1 r

Figure 1.2: Electron density ne profile and radial electrical field component Er for Plasmaline

configuration [38]. The level of the critical electron densityne,crit is indicated.

Figure 1.2 illustrates the characteristics of the radial component of the electric field E_r and the electron density n_e for a cylinder symmetric arrangement. Additionally, the value of the critical electron density n_e,crit is indicated. As it will be determined in chapter 3, the plasma density n_e exceeds this critical density n_e,crit as shown in figure 1.2 for the investigated experimental conditions. The radial component of the electric field E_r shows an 1/rcharacteristic within the space between the powered inner conductor and the quartz

cylinder due to a constant _r. Due to the continuity condition of the normal component of the electrical displacement field

_r(r)·E_r(r) = _r(r+∆r)·E_r(r+∆r), (1.9) discontinuities of E_r are observed at the boundaries of the quartz cylinder. For the quartz cylinder, the dielectric constant is approximately _r = 3.8 [37, 38]. Therefore, the electric field drops within the material. The electron density n_e increases for increasing radiuses r, reaches a maximum and then vanishes for further increasing radiuses. Thereby, the level of the critical density n_e,crit is passed twice. As afore mentioned, the dielectric constant _r of the plasma vanishes at the position of the critical density n_e,crit leading to a peak of the electric field due to continuity condition (1.9) at the radiuses, where n_e(r) =n_e,crit. Typi-cally, this peak is observed only for the smaller radiuses, where n_e(r) = n_e,crit, because the electron density gets much higher than the critical density (n_e n_e,crit) leading to strong shielding effects for the electrical field. For the second position, the electrical field is already vanished and typically, no second peak occurs for high density plasmas [37].

The axial component of the electric fieldE_z can be calculated for the revealed radial compo-nent by means of equation (1.5). Due to a complex characteristic ofE_r, the axial component E_z does not vanish as previously described for homogeneous dielectric materials. Therefore, an axial componentE_z is present leading to the propagation of a transverse magnetic (TM) wave along the Plasmaline antenna. The axial component E_z undergoes damping phe-nomena and decreases along the antenna depending on the applied power. Therefore, the elongation of the plasma can be adjusted by the applied power and plasma conditions. Due to the damping of the microwave power along the antenna, a nearly linear decrease of the electron density is observed and theoretically described along the axial direction of the antenna for a Plasmaline setup [37, 35, 34]. Therefore, R¨auchle et al. developed a Duo-Plasmaline for a homogeneous treatment of webs. Two microwave magnetrons are used at both sides of an antenna to realize a homogeneous electron density profile along the antenna by superimposing the two sources [37, 40, 41, 42, 43, 44, 45, 46].

In addition to the described wave propagation comparable to a coax cable, there exists a series of other TM modes able to propagate along the antenna. But due to the direct cou-pling of the wave launcher to the inner conductor of the antenna, the coax cable mode is exited in preference according to the theoretical description within this chapter [37]. A further description of the wave penetration of a surface wave exited plasma can be given by an analysis of the skin depthsδ [39, 47]:

δ = c ω

1

Im{1_r/2}, (1.10)

wherec denotes the speed of light c= (µ₀₀)−1/2_{. For low-pressure microwave plasmas, the} electron plasma frequencyω_pe is much higher than the applied microwave frequencyω and the collision frequency ν [47]. Regarding equation (1.7) and (1.10), the skin depth can be simplified for ων to be [39, 47] δ≈ c ω_pe = m_e µ₀n_ee2 . (1.11)

Typically, the skin depth δ is belowδ ≤19.5 mm for a microwave plasma at f = 2.45 GHz. Therefore, an estimate for the position of the maximum of the electron density as illustrated in figure 1.2 is revealed.

1.2

Pulsed plasmas

Typically, electrical discharges are driven in a continuous wave (cw) mode by applying the desired frequency. Recently, pulsed plasmas gain in importance for manifold processes like etching or deposition applications and constitute a fundamental topic considered from a scientific and technological point of view. A pulsing of the applied power leads to a further degree of freedom and represents a huge advantage over cw plasmas [48]. Typically, pulsed plasmas are characterized by a pulse frequency and a duty cycle describing the repetition rate and the relation between pulse duration and inter-pulse period, respectively. Variations of these two additional external controllable parameters allow for influencing the plasma and substrate parameters, e.g. deposition rate, film composition, etch selectivity, dust forma-tion or heat load. Furthermore, charging effects of substrates can be handled responsible for damaging of processes micro-structures. Bousquet et al. [25, 26, 49] investigate e.g. the influence of plasma pulsing on the layer properties of deposited SiO_x films. They show that the pulse parameters significantly determine the film properties like film composition and refractive index of the coating. Bouchoule et al. [50] show the influence of pulse parame-ters on the particle formation of an argon silane plasma. They reveal a strong correlation between the pulse parameters and the dust formation within the plasma responsible for undesirable dust particles deposited on the substrate. Additionally, Wenig et al. [51, 52] investigate in detail the behavior of electron density and electron temperature during the afterglow of pulsed plasmas. Especially, the behavior of electron reheating in the afterglow is investigated responsible for measurable residual energies of electrons in the late afterglow. They show, that in pulsed discharges, the mean electron energy can be tuned to influence the plasma chemistry, whereas in cw discharges, the mean electron energy is fixed by the gas composition, the neutral gas pressure and the discharge geometry. Therefore, the electron temperature, that is correlated to the mean electron energy for an assumedMaxwell distri-bution of the electrons, can be strongly influenced by means of power modulated discharges. As previously discussed, the axial elongation of plasma along a Plasmaline antenna is in-fluenced by the applied microwave power. A certain level of power has to be applied for a homogeneous plasma and surface treatment. Plasma pulsing allows for a homogeneous ignition during the pulse duration by applying higher pulse powers. Furthermore, plasma pulsing enables to reduce the mean power applied to the system by reducing the duty cy-cle. This capability can be important for the treatment of thermolabile materials, such as plastics or even biodegradable polymers, to reduce the heat load of the substrates [53]. Concluding the properties of surface wave exited plasmas, they are characterized by a high electron density leading to high plasma activity in terms of e.g. deposition rate, light emission and chemical activity. The power modulation of plasmas allows for a proper tuning of plasma and coating properties and gives two further parameters in terms of the duty cycle and the pulse frequency. Therefore, pulsed surface wave excited plasma are used within this thesis for the treatment of PET bottles and foils for sterilization and permeation barrier coating purposes.

The experimental setup for the treatment of PET bottles and foils is introduced in this chapter. The ignition of a plasma in a bottle is based on the propagation of an outer surface wave along a Plasmaline antenna as fundamentally described in chapter 1. Furthermore, the applied plasma diagnostics and the substrate analysis tools are described in this chapter, which are used within this thesis. The last section characterizes the substrate materials used for the sterilization and barrier coating process.

2.1

Reactor setup

process gases inner quartz tube copper tube outer quartz tube

Plasmaline antenna bottle cage

gas connectors z

z

Diagnostic flanges (here e.g. Langmuir probe)

Figure 2.1: Schematic of the reactor system including definition of z-axis and positions of radial

and axialLangmuirprobe measurements (height of vacuum vessel: 400 mm, diameter

of vacuum vessel: 140 mm, diameter of bottle cage: 85 mm, diameter of Plasmaline antenna: 12 mm).

The experimental setup for plasma treatment of PET bottles and foils is schematically shown in figure 2.1 [54]. It is composed of a vacuum chamber with a volume of 6 l and is capable of treating various bottle sizes up to 1.5 l. The reactor chamber has a height of 400 mm and a diameter of 140 mm. The bottles are inserted upside down in the vacuum chamber and fixed by a metal bottle cage, which has a diameter of 85 mm for the considered one liter bottles. In case of treating PET foils, a carrier is used, which fixes the substrate at the position of a bottle wall inside the metal bottle cage. The reactor chamber can be evacuated to a base pressure of 0.1 Pa. Microwave power is applied to the system by means of a modified Plasmaline antenna. This antenna consists of a copper tube with surrounding inner and outer quartz tube. The copper tube forms the inner conductor of a coaxial wave guide and acts as a microwave antenna. The plasma extends along the outer quartz tube due to the propagation of outer surface waves and forms the outer conductor by itself [37, 55]. Furthermore, the inner quartz tube is used to provide process gases, which flow through it into the reactor system. Liquid HMDSO is evaporated as process gas for deposition of barrier coatings and is fed into the chamber as a mixture with oxygen. The gas tubes are heated to prevent condensation of HMDSO. The vacuum system is pumped by a combination of a rotary and a roots pump whereas a gate valve is responsible for pressure control.

2.1.1

Generator characteristics

A microwave power source (Muegge Electronic GmbH, Reichelsheim, Germany) provides microwave energy at f = 2.45 GHz with a maximum power of P_cw = 2 kW. The source is capable of being pulsed within 1 ms ≤ t_on ≤ 24 ms and 1 ms ≤ t_off ≤ 250 ms, where t_on and t_off denote the pulse duration and the inter-pulse period, respectively. The generator is equipped with a trigger pulse output for time resolved measurements.

Generator noise 0 2 4 6 8 10 −30 −25 −20 −15 −10 −5 0 t / ms I / mA

Figure 2.2: Noise of microwave generator.

The microwave generator is not a research grade generator to be comparable to industrial scale processing applications. Therefore, a noise signal is present, which has to be analyzed prior to the measurements. Figure 2.2 shows the current signal of an electrical probe at a potential of Φ = 14 V during a cw plasma ignition to illustrate the generator noise.

A periodic noise signal is observed. It is characterized by a frequency of f = 300 Hz (T = 3.3 ms) due to the commutation of the three phase net signal (f = 50 Hz) by means of a rectifier circuit.

Power characteristic

For a time resolved investigation of the power characteristics of the microwave generator, the matching system is tuned to reflect nearly all power. The reflected microwave power is measured by means of a microwave detector (Muegge Electronic GmbH, Reichelsheim, Germany) connected to an oscilloscope. Exemplarily, the time resolved behavior of the mi-crowave generator for at_on = 4 ms pulse is illustrated in figure 2.3. A transient characteristic

0 1 2 3 4 5 0 0.5 1 1.5 2 2.5 t /ms normalized v o ltage signal

Figure 2.3: Power characteristic of microwave generator for ton= 4 ms and toff = 40 ms. is observed within 0 ms ≤ t ≤ 1 ms. The generator power drops twice at t = 0.25 ms and t = 0.5 ms. The initial maximum is reached after t = 0.1 ms and a second maximum is observed at t= 0.35 ms. These characteristic behavior has to be kept in mind for the time resolved investigations as performed in the following chapters.

2.1.2

Automatization of reactor setup

The reactor setup is completely computer controlled by means of a Labview [56] program. It allows for time dependent control of all external parameters, such as gas flows, power settings and pressure control. Additionally, recipes can be defined for complex processes, which are executed computer controlled. A process protocol records all parameters for later investigations.

2.1.3

Parameter ranges

Table 2.1 summarizes the possible parameter ranges for operation of the experimental setup for sake of clarity.

parameter symbol range

cw power P 0 W≤P ≤2000 W

pulse power P 0 W≤P ≤4000 W

pulse duration t_on 1 ms≤t_on ≤24 ms inter-pulse period t_off 1 ms≤t_off ≤250 ms working pressure p 10 Pa≤p≤100 Pa HMDSO gas flow Φ_HMDSO 0 sccm≤Φ_HMDSO≤50 sccm oxygen gas flow Φ_O2 0 sccm≤Φ_O2 ≤800 sccm nitrogen gas flow Φ_N2 0 sccm≤Φ_N2 ≤200 sccm hydrogen gas flow Φ_H2 0 sccm≤Φ_H2 ≤100 sccm argon gas flow Φ_Ar 0 sccm≤Φ_Ar ≤200 sccm Table 2.1: Parameter ranges of microwave reactor setup.

2.2

Plasma diagnostic methods

2.2.1

Langmuir probe measurements

For the analysis of electrical discharges, the determination of the electron densityn_e and the mean electron energy ¯E_eis essential, where ¯E_eis represented by the electron temperatureT_e for an assumed Maxwell distribution of the electron. Electrical probe measurements ac-cording to Langmuir [57, 58, 59] allow for a time and spacial resolved measurement of these important parameters. The measurements are based on the analysis of a voltage-current characteristic of the discharge using the orbital motion limited (OML) theory for the movement of electrons and ions towards a cylindrical probe tip as described in [60]. The Langmuir probe used in this thesis is a tungsten wire with a diameter of 50µm and a length of 5 mm. A voltage-current characteristic is measured by means of an electrical setup as schematically shown in figure 2.4 [61]. A voltage ramp is applied to the probe tip related to the continuously measured floating potential of the plasma and the probe cur-rent is measured. Afterwards, the voltage-curcur-rent characteristic is analyzed and statistically evaluated as described by Wenig and Schulze [51, 62] to determine the electron densityn_e, the electron temperatureT_e, the plasma potential U_pl and the floating potential U_fl.

control unit 16 Bit 12 Bit ramp voltage -80V - +80V 16 Bit floating potential probe current; < 1 mA - 100 mA V = 1 probe tip band-stop filter low-pass filter

f = 1 Hz - 1 kHz_g floating potential_amplifier

floating probe

Druyvesteyn[63] gives a relation of the measured characteristic of the electron currentI_e and probe voltage U relative to plasma potential U_pl and the electron energy distribution function f_e(E) f_e(E) = √ 8m_eE Ae3 d2I_e dU2 E=eU , (2.1)

whereAdenotes the surface area of the probe. The electron densityn_eand the mean electron energy ¯E_e can be determined using the moments µk of the electron energy distribution function: n_e =µ0 and E¯_e = µ 1 µ0 for µ k _{= ∞} 0 Ekf_e(E) dE . (2.2) The electron temperatureT_e is revealed under the assumption of aMaxwell distribution by

T_e = 2 3k_B

¯

E_e. (2.3)

Using this theory for the evaluation of the Langmuir probe characteristics, a phase and spatial resolved measurement of these important plasma parameters is possible. Within this thesis, the spatial measurements are performed radial and axial at position of z = 105 mm and radius of r = 28 mm, respectively (cp. figure 2.1). They characterize the profiles and homogeneity of the low-pressure surface wave excited plasma.

Due to the noise of the microwave generator as illustrated in figure 2.2, the measurements of the current-voltage characteristics are averaged regarding the measurement time of 20µs per voltage step and the generator noise off = 300 Hz. The time resolved measurements are also performed with a time resolution of 20µs and averaging over about 200 characteristics.

2.2.2

Energy mass spectrometry

Energy resolved mass spectrometry is a versatile diagnostic method to analyze the process chemistry of plasmas. In this work, energy mass spectrometry is applied for the analy-sis of neutrals and ions during barrier film deposition regarding their composition and to determine energy distribution functions of ions. A HIDEN EQP 300 [64] (HIDEN Ana-lytical Ltd., Warrington, UK) spectrometer is used, which is schematically illustrated in figure 2.5. It allows for a mass selective measurement to determine the intensities of ions or neutrals versus the mass related to the charge of the species as expressed by m/z.1 _The spectrometer is mounted at the same heights as radialLangmuir probe measurements are performed (cp. figure 2.1). A sampling orifice with a diameter ofd_orifice = 60µm separates the vacuum inside the spectrometer (typicallyp≈10−11_{Pa) and the process chamber. The} sampling orifice is mounted in-plain of the metal bottle cage and is electrically grounded. The system is pumped by a combination of a membrane pump and a turbo molecular pump. The HIDEN EQP is equipped with an ion extraction system for the analysis of ions pro-duced in the plasma and an electron impact ionization source emitting and accelerating

1_{The three-character symbol m/z is used to denote the dimensionless quantity formed by dividing the}

orifice extractor lens1 ionization source axis lens2 energy filter mass filter detector

Figure 2.5: Schematic of Energy mass spectrometer HIDEN EQP 300 [64].

electrons perpendicular to the line of sight of the sampling orifice. Therefore, neutrals can be ionized inside the mass spectrometer at various conditions and be detected. For the neu-tral gas analysis, the produced ions are accelerated to an energy of 1.8 eV and drift tolens2, where they are matched to the energy filter for efficient ion transfer [64]. During neutral gas analysis, a positive potential is applied to the ion extraction system to repel positive ions of the plasma. The filament current is held at a low level, typically below 100µA, to prevent thermal dissociation effects.

The energy filter of the EQP 300 consists of a 45◦ sector field energy analyzer and the mass separation is realized by a quadrupole mass filter. The detector is based on a continuous dynode electron multiplier (channeltron).

The mass spectrometer is heated prior the measurements to reduce the residual amount of water inside the spectrometer influencing the signal of hydrogen and oxygen containing molecules. Additionally, the filament temperature is chosen as small as possible and constant by means of low filament currents during the measurements to prevent thermal influences on e.g. molecule dissociation.

Tuning of the mass spectrometer for ion analysis

In case of extracting ions produced in the plasma by means of electrical filters, effects due to different trajectories of ions inside the mass spectrometer have to be taken into account. The extraction unit of the EQP 300 consists of an electrode extractor with tuneable neg-ative potential for positive ion analysis (cp. figure 2.5). Therefore, positive ions from the plasma are accelerated into the mass spectrometer and refocused by means oflens1onto the exit aperture of the electron-impact ionization source [64]. Similar to chromatic aberration phenomena of optical lenses, focusing effects of electrostatic lenses occur due to different ion energies. This aberration is observed, if the position of the focal length of an electrostatic lens is changed by various energies of charged particles leading to beam focusing at different positions. For the investigation of ion energy distribution functions of plasmas by means of energy resolved mass spectroscopy, this chromatic aberration has to be prevented, because

otherwise ions can be lost inside the mass spectrometer influencing the obtained count rate of ions versus the ion energy.

This problem is discussed in detail by Hamers [66] and successfully adapted for a determi-nation of ion energy distribution functions [67, 68]. For a proper ion extraction by the mass spectrometer, the ion beam has to be focused in the way, that the focal point of lens1 is localized in the exit aperture of the ionization source for all ion energies. This configuration leads to a mostly parallel ion beam passing the entrance of the energy filter and no ion losses due to collisions with the electrode aperture at the exit of the ionization system. The given mass spectrometer allows for a proper tuning by adjusting the lens parameters of the extracting system represented by theextractor and thelens1 voltages. Experimentally, the optimized configuration of extractor and thelens1 are revealed for the EQP 300 by tuning according to the procedure described by Hamers [66]. The determined voltages ofextractor and lens1 are V_ext = −40 V andV_L1 = −40 V, respectively. Figure 2.6 shows the influence of settings of the ion extraction system on the revealed ion energy distribution functions including and excluding chromatic aberrations. For example, a strong over estimation of ions depending on their energy is observed as illustrated in figure 2.6, when aberration is present. 0 5 10 15 0 0.2 0.4 0.6 0.8 1 energy / eV

normalized count rate / a.u.

Figure 2.6: Ion energy distribution functions for different lens settings of extraction optics:

(——) with and (· · · ·) without aberrations.

Mass dependent transmission of energy mass spectrometer

For an analysis of plasma polymerization of organosilicon compounds with high monomer masses, a knowledge about the mass dependent transmission function is essential for an adequate data analysis (cp. table 5.1, page 62). The mass dependency of the used mass spectrometer is revealed by means of a residual gas analysis of noble gases and a comparison of the measured count rates. Therefore, the reactor is consecutively filled with one of the noble gases helium, neon, argon, krypton and xenon. The pressure inside the reactor is kept constant at p = 10 Pa and the count rate revealed by the channeltron detector is determined. The count rateCof the detector at a specific mass is proportional to the particle density n_EMS of these particles inside the mass spectrometer, the ionization current I_e of the filaments responsible for electron impact ionization, the cross section of electron impact

ionization σ_ei of the element and the mass dependent transmission ˜T_EMS [69]. Thus, it can be described as

C = ˜T_EMSn_EMSI_eσ_ei. (2.4)

The density n_EMS and the partial pressure p_EMS = n_EMSk_BT of particles inside the mass spectrometer has to be determined for a determination of the transmission function T_EMS as follows. The number of particles ˙N_in entering the mass spectrometer from the reactor volume via a sampling orifice with area A are ˙N_in =ΦA, where Φ denotes the particle flux density corresponding to thermal velocity at reactor pressurep:

Φ= √ p

2πmk_BT ∼ p √

m . (2.5)

Additionally, the pumping system of the mass spectrometer influences the pressure p_EMS. The particle flux ˙N_out through the pumping system is described depending on effective throughputS_eff =SL/(S+L) as ˙ N_out = pEMS k_BT Seff = p_EMS k_BT SL S+L, (2.6)

where S and L denote the throughput and particle conductance, respectively [69, 70]. Bal-ancing particle numbers ˙N_in and ˙N_out reveals

n_EMS = 1 L 1 + L S ΦA≈ ΦA L for LS . (2.7)

The throughput of the used pumping system is S = 360 l s−1 and the conductance of the mass spectrometer is rather small due to many installations and can be estimated to be L ≈ 0.5 l s−1 [69]. For the given conditions of a molecular stream inside the spectrometer, the conductance is proportional to velocity of the particles L∼ √1

m [70]. Using this result

and solving equation (2.4), the mass dependent transmission function T_EMS is determined as function of known quantities:

T_EMS = C

p I_eσ_ei , (2.8)

whereT_EMS = ˜T_EMS·const. Therefore, the transmission function can be estimated by filling the reactor at a certain pressurepwith gases characterized by an electron impact ionization cross section σ_ei and the factor describing the isotope abundance for a certain mass as merged in table 2.2 for the rare gases.

element m/z isotope abundance E_ei/eV σ_ei/cm2

He 4 1.000 70 2.96·10−17

Ne 20 0.905 70 4.75·10−17

Ar 40 0.996 30 1.75·10−16

Kr 84 0.570 30 2.55·10−16

Xe 132 0.269 30 4.01·10−16

Table 2.2: Electron impact ionization cross sectionsσEI for single ionization of the rare gases [71] and isotope abundances.

The energies used for electron impact ionization are below measurable threshold values for multiple ionization to eliminate phenomena related to production of double ionized

atoms [71]. For the used mass spectrometer and lenses settings, an exponential transmission function is determined described by

T_EMS = 1.445·exp −m/z 22 . (2.9)

Figure 2.7 shows the results of the determination of T_EMS normalized to the transmission of helium. The exponential behavior of the mass transmission function is due to the mass separation by means of a quadrupole filter as expected and experimentally confirmed by Pecher [69]. 0 50 100 10-3 10-2 10-1 100 101 m/z relative transmission / a.u.

Figure 2.7: Mass dependent transmission function TEMS(m) of HIDEN EQP 300: (∗) relative transmission of noble gases, (——) exponential fit according to equation (2.9).

Time resolved measurement

For time resolved measurements of ion energy distribution functions, an experimental setup is used based on a multichannel scaler card FAST ComTec MCA-3 (FAST ComTec GmbH, Oberhaching, Germany) [72]. The multichannel scaler card is connected to the pulse output of the mass spectrometer and the trigger signal of the microwave generator to count the pulses with respect to pulse period. The time resolution used for the measurements is∆t= 10µs and typically, the signal is acquired over more than 500 pulses. A Labview program controls the lense settings of the mass spectrometer to set the required mass and energy ranges and is responsible for data storage. The energy resolution is chosen to be 0.1 eV.

2.2.3

Optical emission spectroscopy

For the spectroscopic investigation of light emission from the plasma, a broadband echelle spectrometer ESA 3000 (LLA Instruments GmbH, Berlin) is used. It allows for simultane-ous detection of emission in the wavelength range 200 nm ≤ λ ≤ 800 nm with a spectral resolution of 15 pm and 60 pm atλ = 200 nm andλ = 800 nm, respectively, due to measure-ment in different orders of the spectra. The spectrometer exhibits wavelength ranges, which can not be detected for wavelengthsλ ≥500 nm. These regions with zero efficiency are not significant for the spectral analysis within this thesis, but have to be kept in mind during data analysis. The spectrometer is absolutely calibrated using a tungsten-ribbon lamp and

branching ratios of N₂ and NO bands as described by Bibinov et al. [73].

For the measurements with the spectrometer, an optical fiber is used for transferring the light emission from the plasma to the spectrometer. An aperture blend mounted on the end of the fibre defines the measurement cone with an aperture angle of 2.09◦ to determine the volume of the light emission from the plasma. For measurements, a quartz window is used at the heights of flanges used for Langmuir probe and mass spectrometry investigations (z = 105 mm, cp. figure 2.1). The optical fibre is mounted in front of the quartz glass window at a distance of r= 125 mm related to the center of the reactor. Regarding the aperture of the measurement blend, a radius ofr_c = 4.6 mm of the measurement cone at the Plasmaline is determined. It is below the radius of the Plasmaline antenna (r = 6 mm). Therefore, the measurement cone is well defined and emission from areas behind the Plasmaline in the line of sight of the spectrometers do not influence the measurement. The optical fibre is adjusted using a Laser to ensure a proper alignment.

2.3

Coating analysis

2.3.1

Permeation measurement

For an evaluation of gas permeation through pristine and coated PET substrates, oxygen is used as test gas. Oxygen is a relevant gas for food packaging applications due to reactions with packaged food or beverages like oxidation of vitamin containing fruit juices. Compared to other gases like carbon dioxide or water vapor used for determination of permeation, oxy-gen constitutes a better model permeant because of its known transport properties [16]. E.g. water vapor is known to induce structural changes in SiO_x coatings through stress crack-ing [74]. Therefore, oxygen is used for the quantification of the permeation through pristine and coated PET.

The oxygen permeation rate is determined by a MOCON OX-TRAN 2/61 (MOCON Inc., Minneapolis, USA) [75] oxygen transmission rate system. The determination of oxygen transmission rates with this instrument is approved in industry and research institutes. The measurement principle is carrier gas method according to ASTM D 3985-81 and DIN 53380-3 [76]. The test substrates can be foils or three-dimensional packages like containers or bottles.

Figure 2.8 schematically shows the setup for testing of foils and bottles. The MOCON OX-TRAN 2/61 is equipped with six measurement cells. Therefore, six substrates, either foils or bottles, can be analyzed in parallel. For the testing of foils, a circular test area of 10 cm2 _{is considered and pure oxygen is used as test gas. The measurements are performed} at a temperature of T = 23◦C [76] and a relative humidity of 0%. The oxygen partial pressure difference used for the permeation measurement by carrier gas method is 1 bar. All permeation rates of coated and uncoated foils are tested at these conditions. Additionally, the temperature of the measurement chamber can be varied in the range of T = 20..50◦C to qualify temperature influence on permeation properties as considered in section 5.5. The calibration of the measurement setup is confirmed by means of certified oxygen transmis-sion reference films traceable to the National Institute of Standards and Technology (NIST). Furthermore, the determination of oxygen permeation through bottles is performed with a package adapter as shown in figure 2.8(b). Therefore, air is used as test gas and the inside

temperature control O2 O₂

carrier gas carrier gas to detector foil

O-ring

(a) Testing of foils.

air

carrier gas carrier gas to detector epoxy glue

(b) Testing of bottles.

Figure 2.8: Permeation measurement setup for determination of oxygen permeation through pack-aging materials [76, 75].

of the package is flushed by the carrier gas. The oxygen concentration in air is assumed to be 21%.

Nitrogen with an admixture of 2% hydrogen is used as carrier gas. Before this gas mixture is feed into the measurement cells, it is necessary to remove residues of oxygen from the carrier gas as a precaution by means of a catalyst. This catalyst removes the oxygen by producing water using the hydrogen content of the carrier gas [76].

The quantification of oxygen, which is flushed to the sensor by nitrogen:hydrogen mix-ture is based on an electrochemical detector consisting of a nickel-cadmium and a graphite electrode, which are drenched in caustic potash. Oxygen molecules, which penetrate the detection system react at the surface of the graphite cathode under consumption of four electrons and produce 4OH−:

O₂+ 2 H₂O + 4 e− →4 OH−. (2.10) The created 4OH− ions produce four electrons at the porous cadmium anode according to

2 Cd + 4 OH−→2 Cd(OH)₂+ 4 e−, (2.11) which can be measured as current via a calibration resistor. Therefore, each oxygen molecule, that penetrates the electrochemical detector system produces four electrons. This relation allows for a linear measurement of the oxygen permeation and a zeroing of the detector signal for different measurement cells. The permeation flux densityJ describes the volume of oxygen molecules∆Qreaching the electrochemical detector per measurement area Aand time ∆t:

J = ∆Q

A·∆t. (2.12)

Therefore, an adequate unit of permeation flux densityJ for testing of foils can be given as

Customary, the unit

[J] = cm3pck−1day−1 (2.14)

is used for the testing of packages and the permeating volume of oxygen flux is normalized to the package surface without exact determination of this value.

Regarding potential differences and small permeation leakages of the six measurement cells, both parts of the measurement cell can be flushed with nitrogen:hydrogen mixture and the detector signal is constituted as individual zerovalue. These values of the six measurement cells are determined prior to the measurement of foils or bottles. For the identification of the individual zero values for the analysis of packages, small glass bottles (V = 50 ml) are mounted on the package adapter, which are assumed to be impermeable.

2.3.2

Fourier transform infrared spectroscopy

Fourier transform infrared spectroscopy is a versatile diagnostic method to analyze bond compositions of solids, liquids or gaseous molecules. It is based on the absorption of infrared radiation due to vibrations within molecules, when they are irradiated. The frequency of molecular vibrations depends on the masses of atoms, bond forces and geometry of the mole-cule. Therefore, it allows for a classification of different bond structures within molecules and a distinction of various compositions. Typically, molecular vibrations are described as function of the wave number k = λ−1 = νc−1 of irradiating infrared beam and can be found in a range of k = 400. . .4000 cm−1_{, where c denotes the speed of light, λ and ν the} wavelength and frequency of infrared beam, respectively [77].

infrared source

δ movable mirror

L beam splitter

L mirror

substrate and detector

(a) Scheme of FTIR spectrometer based on an

Michelsoninterferometer [77]. n₁ n₂ Θ d_p substrate crystal

(b) Principle of ATR FTIR spectroscopy [77].

Figure 2.9: FTIR spectroscopy setup.

For the measurement of an infrared absorption spectrum, a setup as schematically shown in figure 2.9(a) is used. It consists of a polychromatic infrared source and aMichelson inter-ferometer. A polychromatic infrared beam is emitted by the source and applied to a beam splitter with a transparency of 50%. A fixed and a moveable mirror reflect the beam and guide it to the substrate and the detector. The distance between the beam splitter and the mirrors is equal, if the moveable mirror is not displaced leading to a positive interference of the splitted beams. A periodic displacement of the moveable mirror by a distanceδleads to

intensity variation in detected signal I(δ) depending on δ due to constructive and destruc-tive interferences depending of the wavelengths within the polychromatic source, which is called interferogram. A Fourier transform of detector signal allows for a determination of fourier spectra depending on wave numberk. For the analysis of substrates, the absorption spectrum can be determined with regard to a background spectrum containing the charac-teristic absorption of the measurement cell.

The substrates can be analyzed by transmitting the infrared beam directly through the sample for an analysis of the substrate composition. For an analysis of thin films, a method based on total reflection of the infrared beam is more established called attenuated total reflection (ATR) FTIR spectroscopy. Therefore, the sample is pressed on a crystal with a high refractive index and the infrared beam incidences at an angle above the critical angle

Θ_c = sin−1 n₂ n₁ , (2.15)

wheren₁ andn₂ denote the refractive index of the crystal and the sample, respectively [77]. Figure 2.9(b) schematically shows the setup for sample positioning of an ATR unit. The penetration depth d_p of the infrared beam penetrating the sample is determined by

d_p = λ

2πn₁(sin2Θ−(n₂/n₁)2₎ (2.16) and is exemplarily shown in figure 2.10 for the given experimental conditions [77]. A strong dependency of d_p on the wavelength is revealed, which is regarded in the evaluation of the spectra.

In this thesis, the composition of SiO_xC_yH_z coatings and PET is analyzed using a Bruker Vector 33 FTIR spectrometer (Bruker Optik GmbH, Ettlingen, Germany) equipped with a PIKE MIRACLE (PIKE Technologies, Madison, USA) Θ = 45◦ diamond ATR unit with a refractive index of n₁ = 2.4. The measurements are performed with a resolution of∆k= 2 cm−1and 32 scans are executed per spectrum. By reason of the transmission of the diamond ATR crystal, the spectra are presented in the range of 750 cm−1 _{≤k ≤4000 cm}−1_.

1000 2000 3000 4000 0.5 1 1.5 2 x 10-6 k /cm−1 dp / m

Figure 2.10: Penetration depthdpof infrared beam versus wave number forn2 = 1.41 andn1 = 2.4