How Light Is Used For Chemical And Structural Analysis Essays

How Light Is Used For Chemical And Structural Analysis Essays

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Light has always been one of the most efficient methods for chemical and structural analysis of materials. From techniques such as UV-Vis and IR spectroscopy to x-ray photoelectron spectroscopy to many others, varying energies of light are used to analyze a material. Spectroscopic ellipsometry also utilizes light to measure different optical constants of materials. In UV-Vis and IR spectroscopy, light in those respective ranges is used and the intensity of the light is measured to determine the absorption and transmission of the materials of that energy of light. Similarly, light in those wavelength ranges is used in ellipsometry but rather than just obtaining information from the intensity of the light, the polarization change of the light upon reflection or transmission at the sample is measured. From the polarization change, the complex refractive index can be measured.
The equations describing ellipsometry were first derived in 1887 by Paul Drude, who is well known for his ‘Drude Model’ that expresses the optical properties of metals and semiconductors. However, it was not until 1975 when Apnes merged spectroscopy and ellipsometry into a crude version of the system used today. Further advances were made in the 1980s and 1990s to improve the real-time monitoring and precision of the device. Today, spectroscopic ellipsometry is used to measure the optical constants of materials and once those are known, the thicknesses of sub-monolayer films can be determined.
Ellipsometry is an optical measurement technique that characterizes materials based on the changes in the light after reflection or transmission at a sample. Ellipsometry, named because light often takes on an elliptical polarization state upon reflection or t...

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...etup is mathematically equivalent to that of a rotating polarizer setup. In a rotating polarizer setup, tan(P) is replaced with tan (A). The optical configuration for both experimental setups is exactly the same. The only change is to which part optical part rotates during the analysis. In RAE, S3 is not measured and thus the error of the measurement increases as Δ approaches 0 and 180 degrees. To overcome this problem, a compensator can be introduced to the instrument design. A compensator is another optical tool that introduces as phase shift, δ and is typically placed after the sample but prior to the analyzer in the setup. The compensator allows for all of the Stokes parameters to be measured which drastically reduces the error in the measurement. This also provides an additional method of measurement for ellipsometers, rotating compensator ellipsometry.

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