Ellipsometry is a very versatile optical technique that has applications in many different fields, from the microelectronics and semiconductor industries (for characterizing oxides or photoresists on silicon wafers, for example) to biology. This very sensitive measurement technique provides unequalled capabilities for thin film metrology. As an optical technique, spectroscopic ellipsometry is non-destructive and uses polarised light to probe the dielectric properties of a sample.

Through the analysis of the state of polarisation of the light that is reflected from the sample, ellipsometry can yield information about layers that are thinner than the wavelength of the light itself, down to a single atomic layer or less. Depending on what is already known about the sample, the technique can probe a range of properties including the layer thickness, morphology, or chemical composition. It is commonly used to characterize with an excellent accuracy film thickness for single layer or complex multilayer stacks ranging from a few angstroms to several micrometres.

The name "ellipsometry" stems from the fact that the most general state of polarization is elliptic. The technique has been known for almost a century, and today has many standard applications. However, ellipsometry is also becoming more interesting to researchers in other disciplines such as biology and medicine. These areas pose new challenges to the technique, such as measurements on unstable liquid surfaces and microscopic imaging.

Ellipsometry Definitions

Basic principles

An ellipsometer functions by reflecting a beam of light of known polarization off of a sample, and measuring the polarization change upon reflection. The exact nature of the polarization change is determined by the sample's properties (thickness and refractive index). Ellipsometry is a specular optical technique (the angle of incidence equals the angle of reflection). In its modern incarnation, ellipsometry uses a laser as the illumination source, usually a HeNe laser which has a wavelength of 632.8 nm. Although optical techniques are inherently diffraction limited, ellipsometry exploits phase information and the polarization state of light, and can achieve angstrom resolution.

In its simplest form, the technique is applicable to thin films with thickness less than a nanometre to a micrometre. The sample must be composed of a small number of discrete, well-defined layers that are optically homogeneous, isotropic, and non-absorbing. Violation of these assumptions will invalidate the standard ellisometric fitting procedure, although more advanced variants of the technique have been designed (such as spectroscopic or multi-angle ellipsometry).

Details

Ellipsometry measures two of the four Stokes parameters, which are conventionally denoted by Ψ and Δ . The polarization state of the light incident upon the sample may be decomposed into an s and a p component (the s-component is oscillating parallel to the sample surface, and the p-component is oscillating parallel to the plane of incidence). The intensity of the s and p component, after reflection, are denoted by R_s and R_p. The fundamental equation of ellipsometry is then written:

Thus, tanΨ is the amplitude change upon reflection, and Δ is the phase shift. Since ellipsometry is measuring the ratio of two values (rather than the absolute value of either), it is very robust, accurate, and reproducible. For instance, it is insensitive to scatter and fluctuations, and requires no standard or calibration.

The measured Ψ and Δ can be converted to optical constants if a layer model is assumed. Directly inverting Ψ and Δ is not possible. Instead, an iterative procedure (least-squares minimization) is used: various values of the optical constants are considered, Ψ and Δ are then calculated using Fresnel reflection theory. The optical constants which come closest to the experimental Ψ and Δ are then considered to be the correct values for the sample.

References

External Links

General Information

Research

Surrey University
Photonics & Optoelectronics Research Laboratory
Newcastle University
Uta University
University of Wales Aberystwyth

Manufacturers

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Sopra
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