Any ellipsometric sample characterization comprises at least two steps:
In a first step, the ellipsometer measures the sample-induced change of polarization, which can be cast into two real numbers per probing photon energy. These are the so-called ellipsometric parameters Δ (“Delta”) and Ψ (“Psi”). These parameters may be understood as a “fingerprint” of how the investigated sample interacts with polarized light.
In a second step, the measured values of Δ and Ψ are used to calculate the sample properties of interest such as layer thickness, refractive index and/or absorption. A specialized data-analysis software uses a computational model of the sample to calculate the expected values for Δ and Ψ and compares it against the measured values. The properties of interest are then used as floating parameters to find the best match of experimental and model data. Hence, ellipsometry is a model-based (indirect) technique of optical thickness and refractive index measurement.
In order to perform an ellipsometric measurement, any ellipsometer setup consists at least of four components: A light source, a polarization state generator (PSG), which is made of some polarization optics and placed before the sample, a polarization state analyzer (PSA; more polarization optics) placed in the beam path behind the sample, and a photo detector. The PSG and PSA control and analyze the polarization states of the probing and the reflected light, respectively.
The measurement of the Δ- and Ψ-values is based on a sequence of intensity measurements for different settings of the polarization control and/or the polarization analysis (polarization modulation). From these intensity values and the known settings of the PSG and PSA, the Δ- and Ψ-values can be calculated directly.
There are various types of PSG and PSA designs and even more patterns of the applied polarization modulation. Accurion’s ellipsometers apply the very common design that uses a linear polarizer and a quarter-wave plate (λ/4-plate) as PSG, and a sole linear polarizer as PSA. For the polarization modulation, they apply the techniques of nulling ellipsometry and rotating compensator ellipsometry.