Exploring the Structure of the Modified Top Layer of Polypropylene During Plasma Treatment

Abstract

Plasma modifications of polypropylene (PP) surfaces are analyzed by means of vacuum beam experiments. A plasma source provides Ar ion beams and a background of UV photons. Additionally, neutral oxygen beams are sent to perform reactive sputtering of PP. The etch rate and chemical state are monitored in real time by in situ Fourier transform infrared (FTIR) spectroscopy. At the onset of Ar bombardment, PP shows high sputter yields, which decrease down to a constant etch rate indicating the formation of a modified top layer. The stationary top layer is modeled as combination of a pristine fraction plus a cross‐linked fraction of amorphous hydrocarbon. Photon‐ and ion‐dominated etch processes provide different cross‐linking fractions, whereas the sputter efficiency is maximized at intermediate ion energies (200 eV).

Validation of etching model of polypropylene layers exposed to argon plasmas

“…Again, as in the UV irradiation case (Figure b), the modified surface model fits very well with the experimental data. As shown in the etching curves, ion fluencies of about 10 17 cm −2 correspond to the formation of the DAC layer, which is consistent with the ion fluencies measured by Corbella et al Please note that the transition to stationary etching is not modeled here. Actually, the ellipsometric model is valid up to the complete formation of the DAC layer.…”

Fundamentals and Applications of Reflection FTIR Spectroscopy for the Analysis of Plasma Processes at Materials Interfaces

“…In order to investigate plasma modification on polymer surfaces, very high sensitivity is required, because ions and neutrals affect the surface only within a penetration depth of the order of a few nanometers at most. The enhancement of sensitivity can be achieved by using the previously described OCS, or by depositing ultra‐thin polymer films on smooth gold substrates to allow for the application of PM‐IRRAS …”

“…In Große‐Kreul et al and Corbella et al, the etching and chemical modification of different polymers was monitored in real‐time by in situ IRRAS using OCS substrates, with a thickness of the oxide layer to 1 μm in order to achieve the condition of resonance with the infrared wavelength of the probing light (Figure ). With this arrangement they could resolve reflectivity changes of the order of corresponding to the removal of sub‐monolayers of polymer (where R 0 corresponds to the spectrum of the pristine polymer).…”

Evolution of photoresist layer structure and surface morphology under fluorocarbon‐based plasma exposure

“…However, maintaining a controlled and consistent etching selectivity depends on the stability of the etch resistance of the photoresist material. For a 193 nm photoresist exposed to energetic plasmas, the etch resistance is proportional to the developed thickness of the surface DAC layer . However, the DAC layer is susceptible to chemical sputtering from reactive plasmas, which depletes the layer and decreases the etch resistance .…”