The CMMP ellipsometer is what is called a nulling single wavelength ellipsometer. A HeNe laser with a linear polarizer and a 1/4 wave compensator is used to generate circularly polarized light. This circularly polarized light has two orthogonal components-- an s-component that is perpendicular to the sample surface, and a p-component that is parallel to the sample surface. Through the Fresnel equations, surfaces have different reflectivities in the s- and p-directions, and in the presence of thin-films and multilayers, there are also the effects of interference from reflection from multiple interfaces. This results in a change in the polarization of the reflected radiation-- instead of being circularly polarized, the reflected radiation is elliptically polarized.
What is measured in ellipsometry is the parameter, rho, which is the ratio of the reflectivities in the p- and s-directions: rho = rp/rs. In general rho is a complex number that is represented in ellipsometry in the following form: rho = tan (psi)*exp(i*delta). As such, psi is a measure of the amplitude ratio change upon reflection-- a measure of the ellipticity of the reflected radiation. delta is a measure of the phase shift between the p- and s-components. In the nulling ellipsometer psi and delta are determined using a rotating linear analyzer before a photodiode or CCD detector. Given information about the substrate and the sample model-- e.g. thin film, multilayer, etc.-- psi and delta are then used to determine such information as:
- n & k: the real and imaginary parts of the index of refraction of a substrate.
- nf & t: the real index of refraction and film thickness of a thin film on a substrate.
- t1 & t2: the thicknesses of a bilayer system