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Spectroscopy Based Modeling for Layer Thicknesses

IP.com Disclosure Number: IPCOM000239597D
Publication Date: 2014-Nov-18
Document File: 7 page(s) / 351K

Publishing Venue

The IP.com Prior Art Database

Abstract

Thickness of one or more thin film layers or coatings is an often desired quantity that requires consideration to be measured accurately. White Light Interferometry may be used to reliably measure a large range of film thicknesses. However, specialized processing techniques may be required to isolate individual layers of film thickness consuming extensive resources and time. For full stacks and film layers 1) above approximately one full wavelength (e.g. 500 nm) Fast Fourier Transform (FFT) processing is sufficient, 2) less than a half wavelength model fitting techniques may be used to measure the thickness and, 3) less than a quarter wavelength model fitting techniques may be the only method to determine the thickness. This paper describes a complex model to represent multilayer films and non-linear fitting techniques to find the thickness of each layer. This could potentially be expanded to find the refractive index of the layers as well, amongst other potential uses for the information captured by this type of model.

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Spectroscopy Based Modeling for Layer Thicknesses

Abstract

Thickness of one or more thin film layers or coatings is an often desired quantity that requires consideration to be measured accurately. White Light Interferometry may be used to reliably measure a large range of film thicknesses. However, specialized processing techniques may be required to isolate individual layers of film thickness consuming extensive resources and time. For full stacks and film layers
1) above approximately one full wavelength (e.g. 500 nm) Fast Fourier Transform (FFT) processing is sufficient, 2) less than a half wavelength model fitting techniques may be used to measure the thickness and, 3) less than a quarter wavelength model fitting techniques may be the only method to determine the thickness. This paper describes a complex model to represent multilayer films and non-linear fitting techniques to find the thickness of each layer. This could potentially be expanded to find the refractive index of the layers as well, amongst other potential uses for the information captured by this type of model.

Introduction - White Light Interferometry

In the case of a thin layer illuminated by white light, interference patterns may be observed due to the thickness and the refractive index of the layers as shown in the Figure 1. Part of the incident beam will be reflected at the interface between two layers constructed with a first (n1) and second (n2) medium. The rest of the beam will be transmitted. The transmitted beam in the second medium (n2) will be partially reflected at the next interface and then transmitted again to the first medium (n1). This scenario generates two beams that are delayed by the extra path travelled by the beam in the second medium.
This extra path is called the optical path difference (OPD). As represented in Figure 1:

ܱܲܦ ൌ 2݊݀ ൈ cos ߠ (1) ߠൌ sinିଵ

sin ߠ(2)

For a beam at normal incidence,ܱܲܦ ൌ 2݊݀, which is the case with most measurement systems.

Figure 1: Thin Film Interference Diagram

Therefore, a white light beam showing a broad spectral range, including ߣ and ߣ may constructively interfere for ߣ ൌ ߣ if ݉ߣൌ 2݊݀ cos ߠ and may destructively interfere for ߣ ൌ ߣ if

         ߣൌ 2݊݀ cos ߠ where m is a positive integer. As the white light is made of a continuum of wavelength in the visible range, the reflection measurement of the white light from such a layer will show a waveform as represented in Figure 2.

 



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Figure 2: Interference Pattern of Two Silica Films

The position of minima and maxima or the natural frequency of the waveform are directly related to the film's thickness and can be used to measure it.

As the film gets thinner, no minimum or maximum appears in the spectrum although there are still features due to interferences within the layers. Another configuration where minimum or maximum detection cannot help m...