The characterization of the optical constants and thickness of organic thin films is a major part of our research, and ellipsometry is the primary method of determining these quantities. The instrument used is a Variable Angle Spectroscopic Ellipsometer (VASE©) ellipsometer made by J.A. Woollam Co. Ellipsometry is sensitive to several material characteristics, such as
For the purposes of our research we are most interested in determining film thickness, optical constants, and optical anisotropy. The information that is obtained from the ellipsometry data is used to analyze the results from other experiments such as Maker fringe experiment, and waveguide coupling.
There are three types of data typically acquired with the ellipsometer, transmission and reflection intensity and of course ellipsometry.
Transmission and Reflection
An illustration of the transmitted, reflected, and incident beams is shown in Fig. 1. A beam of light is incident on a sample at at some arbitrary angle of incidence , the angle of incidence is defined as the angle between the input beam direction and the direction normal to the sample surface. At the boundary of the medium, part of the light will be reflected at angle while the other part will be transmitted through the sample at angle . Snell's law requires that all three beams be in the plane of incidence (shaded green in Fig. 1). The plane of incidence is defined as that plane which contains the input beam, the output beam, and the direction normal to the sample surface.
The transmission and reflection measurements acquire the intensity ratios, T and R respectively, over a given range of wavelengths. T and R are defined as the ratio of the light intensity being transmitted or reflected over the incident light intensity on the sample, as shown in Eqs. (1) and (2)
Ellipsometry measures the change in polarization state of light reflected from the surface of a sample. The measured values are expressed as and . These values are related to the ratio of Fresnel reflection coefficients, and for p and s-polarized light, respectively.
Because ellipsometry measures the ratio of two values, it can be highly accurate and very reproducible. From Eq. (3) the ratio is seen to be a complex number, thus it contains “phase” information contained in , which makes the measurement very sensitive. In the Fig. 2, a linearly polarized input beam is converted to an elliptically polarized reflected beam. For any angle of incidence greater than 0° and less than 90°, p-polarized light and s-polarized will be reflected differently.
The coordinate system used to describe the ellipse of polarization is the p-s coordinate system. The s-direction is taken to be perpendicular to the direction of propagation and parallel to the sample surface. The p-direction is taken to be perpendicular to the direction of propagation and contained in the plane of incidence.
The optical constants define how light interacts with a material. The complex refractive index is a representation of the optical constants of a material, it is represented by
The real part or index of refraction, n, defines the phase velocity of light in material:
where v is the speed of light in the material and c is the speed of light in vacuum. The imaginary part or extinction coefficient, k, determines how fast the amplitude of the wave decreases. The extinction coefficient is directly related to the absorption of a material and is related to the absorption coefficient by:
were is the absorption coefficient and is the wavelength of light.
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