Porous thin films are used in a variety of applications to provide enhanced electrical, sensing, or thermal characteristics dictated by the porous structure of the film. Mixing a skeletal material with air or other fillers, reduces the density compared to the bulk material effectively reducing the refractive index, dielectric constant, and other relevant parameters to levels not achievable with bulk materials. Traditional porosimetry techniques such as high-pressure mercury or liquid nitrogen porosimetry are not applicable to the small volumes of thin porous films.
Ellipsometric porosimetry utilizes the excellent sensitivity to small changes in a thin film’s refractive index when a solvent condenses in the pores during the adsorption process. In an ellipsometric porosimetry experiment, we combine standard ellipsometry measurements with our Environment Cell accessory which features a patented vapor-delivery system to create controlled relative pressure sample environments using a wide variety of solvents. Performing dynamic ellipsometry measurements while the relative pressure of a solvent is varied allows extraction of the relevant porous film parameters, porosity and pore size distribution.
In this webinar, the fundamentals of ellipsometric porosimetry will be covered. Attendees will gain an understanding of the hardware involved in the measurements and an introduction to the underlying theory relating standard ellipsometry measurements to adsorption isotherms and pore characteristics. Example analyses of mesoporous and microporous thin-film samples will be shown.
Jeremy Van Derslice
Mueller matrix ellipsometry is one of the hottest topics in the ellipsometry field. Just like “standard ellipsometry”, this non-destructive, optical characterization technique measures the change of polarization of light upon reflection from or transmission through a sample. However, Mueller matrix ellipsometry is not limited to measuring isotropic samples where film thickness and optical constants are the primary interest. On the contrary, the Mueller matrix contains all essential optical information, from intensity propagation, over cross-polarization due to linear and circular birefringence (LB and CB), linear and circular dichroism (LD and CD), retardance, to depolarization. Essentially, any optical effect possible will be described by a Mueller matrix. By measuring the Mueller matrix, we can characterize the most advanced applications. Some examples include arbitrarily anisotropic, crystalline substrates and films, birefringence in stretched polymer foils as found in roll-to-roll applications, polarization filters in AR/VR devices or cameras, entire liquid crystal cells, oriented nanostructures, bio and metamaterials, or periodic 3d structured surfaces in semiconductor metrology.
In this webinar, the basic concept of the Mueller matrix and measurement approaches using any J.A. Woollam ellipsometer will be introduced. We identified a few representative cases to demonstrate extraction of the desired material and device properties from the raw data.
Nina Hong. Ph.D.
For more than 23 years the J. A. Woollam IR-VASE®, FTIR-based ellipsometer (covering 1.7 to 30 µm wavelengths) has been used to study mid- and far-IR optical coatings and devices, semiconductors, photovoltaics, surface chemistry, and other applications. In contrast to standard FTIR techniques based on measuring absolute intensities, infrared spectroscopic ellipsometry (IR-SE) is a self-referencing technique that measures the change of polarization expressed as amplitude ratio tan(Ψ) and phase difference Δ between the p- and s-polarization components of a reflected or transmitted light beam. As a result, the measurement is highly accurate, precise, and repeatable. Furthermore, no reference sample is required, there is little sensitivity to fluctuations in ambient CO2 and water vapor levels or source intensity, and only a portion of the light beam needs to be collected to obtain accurate data. Model-based data analysis yields quantitative values for thicknesses as well as the refractive index and extinction coefficient, and dielectric function. For thin films on a substrate, measurement of the phase difference Δ greatly improves sensitivity for small layer thickness, often in the sub-nm range. The longer wavelengths of IR-SE (compared to NIR-Vis-UV ellipsometry) also allow measurement of thicker films up to several tens of microns. Physical sample properties such as phonon and other vibrational absorptions, free carrier density profiles, anisotropy, and gradients in the optical properties can be determined for bulk materials and thin films. Access to the depolarization of the light beam after interaction with the sample allows characterization of the film thickness uniformity over the measurement spot from a single measurement.
After a brief introduction of IR-SE and the IR-VASE® instrument, we will show examples for several applications such as IR-optical coatings and substrates, doped and undoped semiconductor substrates and coatings, polymer films, and anisotropic materials.
Tom Tiwald, Ph.D.