Ellipsometry

AbstractEllipsometry, also known as reflection polarimetry or polarimetric spectroscopy, is a classical and precise method for determining the optical constants, thickness, and nature of reflecting surfaces or films formed on them. Ellipsometry derives its name from the measurement and tracking of elliptically polarized light that results from optical reflection. Typical experiments undertaken with this technique consist of measuring the changes in the state of polarization of light upon reflection from the investigated surfaces. Since this technique is very sensitive to the state of the surface, many experiments that involve such surface changes can be detected and followed. Hence ellipsometry finds applicability in a wide variety of fields, such as physical and chemical adsorption; corrosion; electrochemical or chemical formation of passive layers on metals, oxides, and polymer films; dissolution; semiconductor growth; and immunological reactions.The main strength of this technique lies in its capability not only to allowin situmeasurements and provide information about the growth kinetics of thin films but also simultaneously to allow the determination of many or all of the optical parameters necessary to quantify the system. Ellipsometry can be used to detect and follow film thickness in the monoatomic range, which is at least an order of magnitude smaller than what can be studied by other optical methods (e.g., interferometry and reflection spectroscopy).When linearly polarized light having an arbitrary electric field direction is reflected from a surface, the resultant light is elliptically polarized due to the difference in phase shifts for the components of the electric field that are parallel and perpendicular to the plane of incidence. According to the principle of reversibility, light of this resultant ellipticity, when incident on a reflecting surface, should produce linearly polarized light. This is the principle under which the ellipsometer operates. The angle of incidence of light, its wavelength, and the ellipticity of the incident light are related by means of certain theoretical relations to the optical parameters of the substrate and any film that might exist on it. By knowing the optical properties of the substrate and by carrying out ellipsometric measurements, one can find the thickness of a film and its refractive index with relative ease based on certain well‐known relations that are described here.Ellipsometry can be broadly divided into two categories. One class of ellipsometry, called null ellipsometry, concerns itself with performing a zero signal intensity measurement of the light beam that is reflected from the sample. In the other class of ellipsometry, measurements of the ellipticity and intensity of light reflected from the sample are performed and correlated to the sample properties. This technique is referred to as photometric ellipsometry. The techniques and methodology described in this article will concentrate mainly on the principles of null ellipsometry. However, references to the photometric techniques will be provided in those cases where they would provide an advantage over nulling methods.