Spectroscopic ellipsometry is a non-destructive, noncontact, and non-invasive optical technique which is based on the change in the polarization state of light as it is reflected obliquely from a thin film sample. Ellipsometry uses a modelbased approach to determine thin film, interface, and surface roughness thicknesses, as well as optical properties (and much more!) for thin films ranging in thickness from a few A to several tens of microns.
Also, spectroscopic ellipsometry can be performed either ex-situ or in-situ, in static or kinetic mode, for various application needs.
Spectroscopic ellipsometry measures ψ and Δ, both of which describe the output elliptical polarization state after linearly polarized light is reflected obliquely off of a thin film sample.
The parameters ψ and Δ are related to the complex Fresnel reflection coefficients according to: ρ = tan ψ eiΔ = rp / rs. After collecting ψ and Δ, a model representing the thin film structure must be built in order to determine thickness and/or optical constants.
Typically, ellipsometers do not measure ψ and Δ directly. Instead, they measure functions of ψ and Δ. In the case of phase modulated ellipsometers, such as the UVISEL PLUS and UVISEL 2, the three measureables are: Is, Ic, and Ic', which are functions of ψ and Δ according to Is = sin2ψ.sin Δ, Ic = sin 2ψ cos Δ, and Ic' = cos 2ψ. When combined, Is and Ic provide an accurate measurement of Δ over the full range from 0° to 360° and Is and Ic' provide an accurate measurement of ψ over the full range from 0° to 90°.
It is important to note that spectroscopic ellipsometry is an indirect technique which does not measure thin film thickness and/or optical properties directly. In order to determine thin film thickness and/or optical properties, a model-based approach must be used. Ex-situ spectroscopic ellipsometry allows for the characterization of a range of thin film properties including layer thickness, surface roughness thickness, interface thickness, optical constants, composition, band gap, composition, crystallinity, grading, anisotropy, and uniformity by depth and area. It can also be used to calculate the depolarization factor and the Mueller Matrix coefficients.
It is important to note that spectroscopic ellipsometry is an indirect technique which does not measure thin film thickness and/or optical properties directly. In order to determine thin film thickness and/or optical properties, a model-based approach must be used. Besides thin film thickness and optical properties, in-situ spectroscopic ellipsometry can also be used to determine nucleation and growth parameters, precise optical properties without significant surface roughness or oxides, and film growth profiles.
Since ellipsometry is a model-based approach, it is helpful to know something about your sample (number of layers, materials, etc.). However, if little to nothing is known about the sample, it can still be studied with ellipsometry, as long as it is a simple single layer on a known substrate. Ellipsometry can also be used to determine the optical properties of the substrate if its material is unknown.
Polarization is defined by the orientation and phase of the electric field vector. We can describe polarization as a superposition of two orthogonal waves. The most general state of polarization, known as elliptical, allows for an arbitrary phase difference and arbitrary relative amplitudes of the two orthogonal waves.
Two special cases of elliptical polarization are known as circular and linear polarization. To obtain circular polarization, the two orthogonal waves must be 90° out of phase and have equal amplitudes.
To obtain linear polarization, the two orthogonal waves must be in phase but they may have arbitrary amplitudes.
Images taken from Fujiwara, H. “Spectroscopic Ellipsometry Principles and Applications,” John Wiley and Sons, 2007.
Optical properties are comprised of two components: the refractive index, and the extinction coefficient. The refractive index, denoted by n, is the ratio of the speed of light in a vacuum to the speed of light in the material. The extinction coefficient, denoted by k, is related to the absorption loss in the material. Together, these two components make up the complex refractive index, given by N = n-ik, which describes electromagnetic radiation’s interaction with materials (speed change and absorption loss).