Spectroscopic Ellipsometry for Characterizing Surfaces and Thin Films
Spectroscopic Ellipsometry (SE) is a powerful surface characterization technique used to determine the optical properties and thicknesses of thin films. It is based on the principle of polarization, and it allows for the precise measurement of the ellipticity (the ratio of the amplitudes of the p- and s-polarized components of light).
Basic Concepts
SE relies on the interaction of polarized light with a sample. When light is polarized, its electric field oscillates in a single p or s-polarization. When polarized light interacts with a material surface, the components of the light's electric field parallel and perpendicular to the surface interact differently with the electrons in the material. This interaction changes the p- and s-polarization components of the light in amplitude and phase.
The ellipticity is calculated from the p- and s-polarization components. It is a convenient metric for surface and thin film characterization because it is sensitive to changes in the surface or thin film's optical properties. The ellipticity as a function of polarization angle (or wavelength, depending on the instrumentation used) is referred to as a psychrometric curve. It contains information about the optical properties of the layers in contact with the electric field vector of the light.
Equipment and Techniques
An SE instrument consists of the following components:
light source (e.g., laser or white light source),
polarizing optics, sample stage,
analysis optics (e.g., analyzer, modulator), and detection
system (e.g., photodiodes).
The polarizing optics are used to polarize the light before it interacts with the sample and to
analyze the p- and s-polarization components of the light after it has
interacted with the sample. The sample stage is used to position the sample with
respect to the light beam. The analysis optics are used to
separate the p- and s-polarization components of the light and to direct
each component to a photodiode. The detection system is used to measure the
intensities of the p- and s-polarization components of the light.
Types of Experiments
There are two main types of SE experiments: angle-of-incidence
(AOI)-dependent SE and wavelength-dependent SE.
AOI-dependent SE experiments vary the angle of-incidence
of the light on the sample at a single wavelength. Wavelength-dependent SE
experiments vary the wavelength of the light on the sample at a single
angle of-incidence.
Data Analysis
The psychrometric curves obtained from SE experiments contain information
about the optical properties of the layers in contact with the electric
field vector of the light. The data can be fitted to a suitable model to
determine the layers' optical properties, such as refractive index and
extinction coefficient.
Applications
SE is a versatile technique with a wide range of applications in various fields, including semiconductor manufacturing, materials science, and surface chemistry. It's used to characterize thin films, measure film thickness, determine optical constants, and study surface roughness and morphology.