Determining Optical Material Parameters With Motion in Structured Illumination

A set of power measurements as a function of controlled nanopositioner movement of a planar film arrangement in a standing wave field is presented as a means to obtain the thicknesses and the dielectric constants to a precision dictated by noise in an exciting laser beam and the positioning and detector process, all of which can be refined with averaging.

Purdue University, West Lafayette, IN

The broad need for determining the optical properties of thin films in a multitude of applications is usually served by ellipsometry. Practical application of ellipsometry generally requires prior constraints, typically in the form of a frequency-dependent model. To provide for a suitable solution of the inverse problem, where film parameters are determined from a set of optical measurements. We present motion in structured illumination as a means to obtain additional information and hence avoid the need for a material response model. Using this approach, inversion for multiple parameters at each wavelength becomes possible, and in a mutual information sense, this is achieved by taking intensity measurements at a known set of displacements in a cavity.

Ellipsometry measures the amplitude ratio and the phase difference between polarized light reflected from the surface of a film and determines the refractive index or thickness by fitting the experimental data to an optical model that represents an approximated sample structure. Generally, a model of the frequency-dependent dielectric constant is used for successful parameter extraction, in order to constrain the inversion. For example, such a model may represent a Lorentzian resonance or impose a Drude model. While simplifying the extraction, this imposes an approximate but not necessarily the correct description. Otherwise, a careful choice of the initial variables is needed in ellipsometry.