Abstract
Recent developments in mid-infrared (MIR) spectroscopic ellipsometry enabled by quantum cascade lasers (QCLs) have resulted in a drastic improvement in signal-to-noise ratio compared to conventional thermal emitter based instrumentation. Thus, it was possible to reduce the acquisition time for high-resolution broadband ellipsometric spectra from multiple hours to less than 1 s. This opens up new possibilities for real-time in-situ ellipsometry in polymer processing. To highlight these evolving capabilities, we demonstrate the benefits of a QCL based MIR ellipsometer by investigating single and multilayered polymer films. The molecular structure and reorientation of a thin biaxially oriented polyethylene terephthalate film is monitored during a stretching process lasting s to illustrate the perspective of ellipsometric measurements in dynamic processes. In addition, a polyethylene/ethylene vinyl alcohol/polyethylene multilayer film is investigated at a continuously varying angle of incidence (0– 50) in s, highlighting an unprecedented sample throughput for the technique of varying angle spectroscopic ellipsometry in the MIR spectral range. The obtained results underline the superior spectral and temporal resolution of QCL ellipsometry and qualify this technique as a suitable method for advanced in-situ monitoring in polymer processing.
Highlights
Polymer thin films have played an important role in many areas of modern life
We investigate the molecular structure of a 2.5 μm thin biaxially oriented polyethylene terephthalate (BOPET) film (Mylar® Thin-Film, Chemplex Industries, Inc., Palm City, FL, USA) during a stretching process by means of quantum cascade lasers (QCLs) ellipsometry
Since the data analysis, modelling and interpretation of ellipsometry spectra are entirely independent of the applied instrument—whether QCL based or conventional Fourier-transform infrared (FTIR) ellipsometer—we limit data analysis to a minimum and focus strictly on the significant benefits provided by QCL based instrumentation
Summary
Polymer thin films have played an important role in many areas of modern life. The importance and necessity of energy efficient technologies—in which polymer thin films might be key elements—is notably emphasized by the current developments related to climate change. Spectroscopic ellipsometry is a well-known non-destructive optical method and a powerful analytical tool to precisely quantify a large set of material parameters. The investigated change in polarization is typically represented by the amplitude ratio and the phase shift between two orthogonal polarization states—two parameters which ellipsometry measures simultaneously. By applying fundamental physics and data modeling, the measured phase shifts and amplitude ratios can be translated to much more comprehensible quantities such as layer thicknesses, optical properties (e.g., complex refractive index or dielectric function), anisotropy, roughness, geometry factors, defects, and quantum confinement effects of nanostructures [14]
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