Simon Steves

Silicon oxide barrier films deposited on PET foils in pulsed plasmas: influence of substrate bias on deposition process and film properties

A widely used plastic for packaging, polyethylene terephtalate (PET) offers limited barrier properties against gas permeation. For many applications of PET (from food packaging to micro electronics) improved barrier properties are essential. A silicon oxide barrier coating of PET foils is applied by means of a pulsed microwave driven low-pressure plasma. While the adjustment of the microwave power allows for a control of the ion production during the plasma pulse, a substrate bias controls the energy of ions impinging on the substrate. Detailed analysis of deposited films applying oxygen permeation measurements, x-ray photoelectron spectroscopy and atomic force microscopy are correlated with results from plasma diagnostics describing the deposition process. The influence of a change in process parameters such as gas mixture and substrate bias on the gas temperature, electron density, mean electron energy, ion energy and the atomic oxygen density is studied. An additional substrate bias results in an increase in atomic oxygen density up to a factor of 6, although plasma parameter such as electron density of ne = 3.8 ± 0.8 × 1017 m−3 and electron temperature of kBTe = 1.7 ± 0.1 eV are unmodified. It is shown that atomic oxygen densities measured during deposition process higher than nO = 1.8 × 1021 m−3 yield in barrier films with a barrier improvement factor up to 150. Good barrier films are highly cross-linked and show a smooth morphology.

Surface pre-treatment for barrier coatings on polyethylene terephthalate

Polymers have favourable properties such as light weight, flexibility and transparency. Consequently, this makes them suitable for food packaging, organic light-emitting diodes and flexible solar cells. Nonetheless, raw plastics do not possess sufficient barrier functionality against oxygen and water vapour, which is of paramount importance for most applications. A widespread solution is to deposit thin silicon oxide layers using plasma processes. However, silicon oxide layers do not always fulfil the requirements concerning adhesion and barrier performance when deposited on films. Thus, plasma pre-treatment is often necessary. To analyse the influence of a plasma-based pre-treatment on barrier performance, different plasma pre-treatments on three reactor setups were applied to a very smooth polyethylene terephthalate film before depositing a silicon oxide barrier layer. In this paper, the influence of oxygen and argon plasma pre-treatments towards the barrier performance is discussed examining the chemical and topological change of the film.It was observed that a short one-to-ten-second plasma treatment can reduce the oxygen transmission rate by a factor of five. The surface chemistry and the surface topography change significantly for these short treatment times, leading to an increased surface energy. The surface roughness rises slowly due to the development of small spots in the nanometre range. For very long treatment times, surface roughness of the order of the barrier layers thickness results in a complete loss of barrier properties. During plasma pre-treatment, the trade-off between surface activation and roughening of the surface has to be carefully considered.

Characterization of low-pressure microwave and radio frequency discharges in oxygen applying optical emission spectroscopy and multipole resonance probe

Optical emission spectroscopy (OES) and multipole resonance probe (MRP) are adopted to characterize low-pressure microwave (MW) and radio frequency (RF) discharges in oxygen. In this context, both discharges are usually applied for the deposition of permeation barrier SiOx films on plastic foils or the inner surface of plastic bottles. For technological reasons the MW excitation is modulated and a continuous wave (cw) RF bias is used. The RF voltage produces a stationary low-density plasma, whereas the high-density MW discharge is pulsed. For the optimization of deposition process and the quality of the deposited barrier films, plasma conditions are characterized using OES and MRP. To simplify the comparison of applied diagnostics, both MW and RF discharges are studied separately in cw mode. The OES and MRP diagnostic methods complement each other and provide reliable information about electron density and electron temperature. In the MW case, electron density amounts to ne = (1.25 ± 0.26) × 1017 m−3, and kTe to 1.93 ± 0.20 eV, in the RF case ne = (6.8 ± 1.8)×1015 m−3 and kTe = 2.6 ± 0.35 eV. The corresponding gas temperatures are 760±40 K and 440±20 K.

Combined in situ FTIR-spectroscopic and electrochemical analysis of nanopores in ultra-thin SiOx-like plasma polymer barrier films

Plasma polymerized SiOx barrier films were investigated by means of in situ spectroscopic and electrochemical methods to correlate the process parameters such as applied substrate bias with the resulting barrier properties. SiOx layers with various hexamethyldisiloxane/oxygen ratio were deposited with and without applied substrate bias. The resulting film morphologies were characterized by means of atomic force microscopy, and the presence of nanopores was analysed by cyclic voltammetry. In order to compare the film density and the presence of nanopore structure, evaluation of interfacial hydroxyl groups was performed by means of discrete polarization modulation Fourier transform infrared reflection-absorption spectroscopy in atmospheres with controlled partial pressures of H2O or D2O. It could be shown that the electrochemical and in situ spectroscopic approach allows for the analysis of nanopores and that a clear correlation of process parameters and film structure can be established.

Numerical simulations of a microwave driven low pressure plasma

Summary form only given. The market shows in recent years a growing demand for bottles made of polyethylene terephthalate (PET). Therefore, fast and efficient sterilization processes as well as barrier coatings to decrease gas permeation are required. A specialized microwave plasma source - referred to as the plasmaline - has been developed to allow for depositing thin films of e.g. silicon oxid on the inner surface of such PET bottles. The plasmaline is a coaxial waveguide combined with a gas-inlet which is inserted into the empty bottle and initiates a reactive plasma. To optimize and control the different surface processes, it is essential to fully understand the microwave power coupling to the plasma and the related heating of electrons inside the bottle and thus the electromagnetic wave propagation along the plasmaline. In this contribution, we present a fully electromagnetic numerical approach performed by means of the Hybrid Plasma Equipment Model (HPEM). Plasmas at different pressures and input powers are examined. The numerical results are compared with experimentally obtained data and show very good agreement.

Numerical simulation of a microwave driven low pressure plasma for pet bottle treatment

Summary form only given. Due to a growing demand for bottles made of polyethylene terephthalate (PET) fast and efficient sterilization processes as well as barrier coating to decrease gas permeation are required. Plasma sterilization is an alternative way of sterilizing PET without using toxic ingredients (e.g. hydrogen peroxide or peracetic acid). To allow investigations in the field of plasma sterilization of PET bottles, a microwave plasma reactor has been developed. A coaxial waveguide combined with a gas-inlet, a modified plasmaline, is used for both coupling the microwave power and injecting the gas mixture into the bottle. One key parameter in the context of plasma treatment of bottles is the ion energy distribution function (IEDF) at the inner surface of the bottle. Numerical results for IEDFs performed by means of the Hybrid Plasma Equipment Model (HPEM) are presented. Plasmas with relevant gas mixtures (Ar, ArH 2 and ArO 2 ) at different pressures and input powers are examined. The numerical results are compared with experimentally obtained data.