Thickness and Index from Transparent Films - CompleteEASE Training Series - Video 3/11

Summary

This session guides users through determining the thickness and refractive index of transparent films using ellipsometry. It covers interpreting oscillation patterns on different substrates, employing the Cauchy model to describe refractive index versus wavelength, and a step-by-step fitting procedure. Users learn to estimate the refractive index from oscillation amplitudes, determine film thickness from the number of oscillations, and refine parameters for a precise fit. The tutorial demonstrates the process on an organic film on silicon, culminating in visualizing optical constants. The session emphasizes practical techniques for accurate measurement and analysis of transparent thin films, providing a foundation for further exploration.

Takeaways

• 😀 Transparent films are a common application for ellipsometry to measure film thickness and refractive index.
• 😀 The Cauchy (Koschi) dispersion model is used to describe the refractive index as a function of wavelength using parameters A, B, and C.
• 😀 The amplitude of oscillations in ellipsometry data helps estimate the refractive index of the film.
• 😀 Substrate type significantly affects oscillation behavior: silicon (high index) vs. glass (low index).
• 😀 Increasing the film's refractive index on silicon dampens peak amplitudes, while on glass it affects the valleys differently.
• 😀 Film thickness can be estimated from the number of oscillations in Ψ and Δ curves.
• 😀 Adjusting thickness shifts peak positions without changing their amplitudes.
• 😀 After initial estimation, a fit can refine thickness and Cauchy parameters A, B, and optionally C for better data matching.
• 😀 Additional model complications like roughness, grading, or anisotropy can further improve fit but are secondary steps.
• 😀 Graphing the optical constants allows verification of the physical plausibility of the refractive index across wavelengths.
• 😀 A stepwise fitting approach—index estimation first, then thickness, then parameter fitting—is critical for accurate results.

Q & A

What is the main goal of the session described in the transcript?

The main goal is to determine both the film thickness and the refractive index for transparent films using ellipsometry.

Why are transparent films commonly analyzed using ellipsometry?

Because many layers, including dielectrics, organics, and some semiconductors, are transparent at certain wavelengths, making ellipsometry effective for measuring their thickness and refractive index.

How does the substrate affect the observed oscillations in ellipsometry data?

A high-index substrate like silicon dampens the peaks as the film index increases, while a low-index substrate like glass can make the peaks appear as an upper envelope and cause valleys to lower with increasing film index.

Which model is used to describe the refractive index versus wavelength for transparent films?

The Cauchy (Koschi) dispersion model is used, which is an empirical equation with three parameters (A, B, and C) to describe the wavelength-dependent refractive index.

What do the A, B, and C parameters of the Cauchy model represent?

A represents the base amplitude of the refractive index, B provides curvature versus wavelength, and C adds further wavelength-dependent correction to improve the fit.

What are the first three critical steps to fit a transparent film according to the transcript?

1) Estimate the refractive index using the A parameter, 2) Estimate the film thickness using the number of oscillations, and 3) Fit the thickness and A/B parameters to match the data.

How can the number of oscillations in the data help in ellipsometry analysis?

The number of oscillations is directly related to film thickness, so counting them allows for an initial estimate of thickness before fitting.

Why is it useful to look at a single curve instead of all curves during fitting?

Focusing on a single curve simplifies the visualization and makes it easier to adjust parameters accurately without being distracted by multiple overlapping curves.

What is the process for refining the fit after initial estimates of index and thickness?

After estimating the index and thickness, the fit button is used to simultaneously refine thickness, A, B, and optionally C parameters, and additional features like roughness or anisotropy can be added for improved accuracy.

How can one visualize the optical constants after fitting?

By right-clicking on the Cauchy (Koschi) layer and selecting 'Graph layer optical constants,' which shows the refractive index versus wavelength, ensuring the result is physically plausible.

What is a physically plausible behavior for the refractive index of a transparent film?

The refractive index should generally decrease slightly as the wavelength increases, which is common for transparent organic or dielectric layers.

What happens to the peaks in ellipsometry data as the index of refraction of the film increases on a silicon substrate?

The peaks become smaller and dampen in amplitude.

What additional factors can be included in the model to improve the fit beyond the first three steps?

Additional factors include the C parameter in the Cauchy model, surface roughness, grading of the layer, and anisotropy of the film.