To understand how ellipsometry works, we must start with a description of polarized light.

Light is an electromagnetic wave.

This visualization shows a beam of light propagating towards you.

The electric field, shown as red vectors, oscillates perpendicular to the light beam direction.

The magnetic field, shown as blue vectors, oscillates perpendicular to both the light beam direction and the electric field.

The polarization state of a light beam is defined by its electric field, and therefore the magnetic field will not be displayed.

Imagine putting a screen at a fixed location in the beam path and watching the tip of the electric field vector versus time, as shown by the yellow ball.

In this example, the electric field oscillates up and down in a line, and the beam polarization state is called linear.

Waves can be added together to create new polarization states.

Here, the red and green waves are added in phase, resulting in a linear polarized beam with a different orientation.

When two waves with equal amplitudes are added 90 degrees out of phase, that is, the maximum amplitude of the first wave occurs when the amplitude of the second is zero, circularly polarized light is generated.

If the two waves have different amplitudes or arbitrary phases, elliptically polarized light is generated.

This is where the term ellipsometry comes from.

Now let's consider the reflection of polarized light from a sample.

When light is obliquely reflected from a sample, the incident and reflected beams define a plane of incidence.

Light with its electric field vector oscillating in the plane of incidence is called p-polarized light.

Light with its electric field vector oscillating perpendicular to the plane of incidence is called s-polarized light.

The enabling principle of ellipsometry is that p- and s-polarized light reflect differently.

Ellipsometry measures the complex reflectivity ratio of p- and s-polarized light, and typically reports the results in terms of the ellipsometric psi and delta parameters.

Tan psi is the magnitude of the ratio, and delta is the phase difference between the p- and s-reflected light.

To measure the ellipsometric parameters, an ellipsometer system must set the polarization state of the incident beam and detect the polarization state of the reflected beam.

In the FilmSense FS1 ellipsometer, a polarizer in the source unit sets the incident beam to a linear polarization state, rotated 45 degrees from the plane of incidence.

The FilmSense FS1 detector unit contains a proprietary combination of beam splitters and optics, which split the beam into multiple beams and detectors, each sensitive to a different polarization component of the incoming beam.

This provides fast, accurate, and reliable measurements of the ellipsometric parameters without any moving parts.

The FS1 software plots the measured ellipsometric data as colored points on a 2D plane.

The four colors correspond to the four FS1 measurement wavelengths.

To determine film thickness or optical constants, it is necessary to analyze the measured ellipsometric dataset.

An optical model is defined, corresponding to the structure of the sample.

Model parameters, such as film thickness, are specified.

The optical model can generate an ellipsometric dataset, which is shown on the plot as open circles.

At this specified thickness, the model-generated data does not fit the measured data very well, as quantified by the large fit difference value.

When the Fit Data button is clicked, the software automatically searches for the parameter values, which minimize the difference between the measured and model-generated ellipsometric datasets.

For a good fit, four bullseyes appear in the plot, the fit diff value is low, and sample parameters, such as film thickness, are determined with excellent accuracy.

Thanks for watching this introductory video on ellipsometry and polarized light.

To learn more about the features and capabilities of the FilmSense FS1 Banded Wavelength Ellipsometer, visit our website at www.film-sense.com