Imaging Ellipsometry (IE)

 
 

Advantages of Imaging Ellipsometry

Imaging Ellipsometry (IE) is an indispensable technique for characterizing thin films at interfaces in and out solution. First popular in semiconductor metrology, imaging ellipsometry is beginning to find wide-spread use in soft materials and biological applications. However, this technique remains underutilized. This is unfortunate because, IE is capable of filling a noticeable gap between fluorescence microscopy (FM) and atomic force microscopy (AFM). It is able to image fields-of-view comparable to FM while also measuring feature thicknesses comparable to AFM. An additional advantage of IE is that it does not require fluorescent labels like those needed for FM nor does it interact or perturb the system, which one must be cautious of with AFM.


Of particular interest to our lab are supported lipid membranes, lipid-lipid interactions, and lipid-protein interactions.


For additional and more thorough information on ellipsometry, visit


Background to IE: Equipment

Ellipsometric angles and spatially-resolved ellipsometric contrast images are acquired using a commercial Elli2000 imaging system (Nanofilm Technologie, Göttingen, Germany). The ellipsometer employs a frequency-doubled Nd:YAG laser (adjustable power up to 20 mW) at 532 nm and equipped with a motorized goniometer for an accurate selection of the incidence angle and corresponding detector positions. The ellipsometer employed the typical PCSA (polarizer-compensator-sample-analyzer) nulling-configuration in which a linear polarizer (P) and a quarter-wave plate (C) yields an elliptically polarized incident beam. Upon reflection from the sample (S), the beam is gathered via an analyzer (A) and imaged onto a CCD camera through a long working distance 10X objective (see figure below). The P, C, and A positions that yield the null condition are then converted to the ellipsometric angles, Δ and Ψ.




    Measurements are generally taken at an incidence angle of 60°. Silicon substrates with native oxide overlayer (SiO2/Si) whose surface chemistry is comparable to that of glass are used to enhance the optical contrast with the lipid phase. For characterization under aqueous conditions, a fluid cell is used (Nanofilm Technologie, Göttingen, Germany). The cell consists of a Teflon chamber (~3 ml volume) with glass windows fixed at 60° (incidence angle) to the substrate normal. A home-built cell is also used for some measurements. This cell consists of a glass and epoxy chamber with windows fixed at 60°. The field of view and lateral resolution of acquired images is limited by the objective and CCD used.

     Topographical maps of Δ can be generated using the micromapping feature of the Elli2000 software suite. This method assumes that Ψ is constant. In our specific experimental configurations, Ψ is relatively constant with respect to changes in Δ. Typically, 70-140 contrast images are scanned incrementally over a 4-8° change in polarization angle, while maintaining the analyzer angle at a constant value. These scans are then assembled to determine the null for each point comprised of a 2 x 2 region of pixels binned together. Δ values estimated for the individual null conditions can be mapped two-dimensionally. These maps are then transferred to a computer as ASCII files from which the final images can be constructed using commercial plotting software (Matlab).



Background to IE: Theory & Modeling

The technique of ellipsometry evolved in semiconductor metrology as a quantitative method for the determination of thin-film properties. In most general terms, it is based on polarization changes, which occur upon reflection of a polarized monochromatic light at an oblique incidence.


Ellipsometry is an indirect technique and extracting relevant physical information about the sample requires the use of optical models typically based on classical electromagnetic theory and the approximation of the sample in terms of parallel optical slabs of defined thicknesses (d) and refractive indices (n + ik). The quantitative accuracy of the physical properties determined directly depends upon how faithfully the slab model depicts the optical properties of the actual experimental sample. Over the past several decades, a range of models have been developed that are capable of capturing many complex physical properties, e.g. anisotropy, heterogeneity, and molecular orientation. From the vantage of supported membrane research, it offers a non-perturbative, quantitative method and allows in-situ, label-free, spatially-resolved, and large-area measurements of small spatial or temporal differences in the optical functions following bilayer depositions, phase separation, or protein-binding with reasonably short collection times determined by the video rate of the CCD detector.