Ellipsometry is a powerful technique used for the non-contact and non-destructive optical analysis of thin films that is utilised in multiple parts of the Sêr SAM project's research activity.
Background - Polarization of Light
Many of us may be familiar with polarized sunglasses. These are often used to reduce glare from surfaces, for example during driving. A polarizer is an optical filter only allows light with a specific polarization to pass through while blocking others. By convention, the polarization of light (or any electromagnetic wave) is given by the direction of its electric field. Light can be linearly, circularly, or elliptically polarized. The analysis of elliptically polarized light gives ellipsometry its name.
Figure 1 An example of a polarization ellipse
Ellipsometry measures the change in polarization of light as it interacts with a sample. Linearly polarized light is directed at the sample. This interacts with the material, which then can be reflected, transmitted, or absorbed. Some of the light that has interacted with the material is then reflected into a detector, which measures the polarization and intensity of the light.
Figure 2 Schematic view of an ellipsometric measurement
This setup provides two reference frames for us to understand how the light is interacting with the sample. These are the planes of incidence of light and the sample surface, called the p and s components. These components reflect differently with the angle of incidence. An ellipsometer will measure the ratio of these components, and relate these to two parameters- psi (amplitude), and delta (phase). By measuring these components as a ratio, the measurements are accurate, reproducible, and referencing is not required. This makes ellipsometry a very robust optical characterisation technique.
When the incident angle between the light source and the detector is changed, it is called variable angle ellipsometry. When the incident light source is broad band (as opposed to a single wavelength), it is called spectroscopic ellipsometry.
Uses of Ellipsometry
Ellipsometry is typically to study films with thicknesses between sub-nanometres to a few microns (~10-9 – 10-6 m). The ellipsometer measures change in polarization by a film, this is fit to a suitable model to extract useful parameters. These can include optical constants, roughness, uniformity, and thickness. This can be useful in situations such as:
Film thickness calibration: Accurate thicknesses of materials are required for device fabrication. Ellipsometry enables quick and precise measurement of film thicknesses to calibrate the deposition technique (for example, spin coating).
Measurement of optical constants: The optical constants of a material depends on its composition. For example, while developing organic semiconductor materials, the ratio of a blend may be varied. Similarly, variations of the stoichiometry of inorganic films can be tuned via their process parameters. The corresponding changes in optical constants from these variations can be measured using ellipsometry. This can be combined with other techniques (e.g. UV-Vis spectrometry), to gain a deep insight into the optical behaviour of new materials.
Graded materials: Certain types of films (such as blends) may have optical constants that vary througout the layer. In these cases, ellipsometry can be used to understand the layer properties.
Film uniformity measurement: The uniformity of a film on a substrate is defined as the variation in its thickness across the surface. In developing a thin film deposition technique (for example, an MVD process), the uniformity of the film is a critical part of process development.
Statistical process control: Materials and processes can drift over time. Ellipsometry can be used as a routine measurement technique to track and correct any issues.
In the Sêr Sam research group, we use a J.A Woolham M-2000 spectroscopic ellipsometer. This includes an 8-inch sample stage, enabling the mapping of thickness and optical constants at this scale. The system also includes an accurate temperature-controlled stage.