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Apparatus and Method for Film Thickness and Optical Constant Determination

IP.com Disclosure Number: IPCOM000034264D
Original Publication Date: 1989-Jan-01
Included in the Prior Art Database: 2005-Jan-27
Document File: 7 page(s) / 212K

Publishing Venue

IBM

Related People

Rosen, HJ: AUTHOR [+3]

Abstract

A new method is described for measuring the thickness and optical properties of thin films. This method depends on the fact that the reflectivity and transmissivity of a thin film as a function of angle of incidence are strongly dependent upon the thickness and the real and imaginary indices of refraction of the media. In this method the angular reflectivity (or transmissivity) spectra is measured by illumination with a finite band of angles and detected by means of a suitable detector such as a diode array. Specific embodiments of the apparatus used to practice the method are shown in Figs. 1 and 2. In Fig. 1, narrow band, polarized and collimated radiation from a laser 1 or a filtered light source is reflected by a 50% beam splitter 2 and focused by a low f/number cylindrical lens 3 to a small focal spot on the thin film 4.

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Apparatus and Method for Film Thickness and Optical Constant Determination

A new method is described for measuring the thickness and optical properties of thin films. This method depends on the fact that the reflectivity and transmissivity of a thin film as a function of angle of incidence are strongly dependent upon the thickness and the real and imaginary indices of refraction of the media. In this method the angular reflectivity (or transmissivity) spectra is measured by illumination with a finite band of angles and detected by means of a suitable detector such as a diode array. Specific embodiments of the apparatus used to practice the method are shown in Figs. 1 and 2. In Fig. 1, narrow band, polarized and collimated radiation from a laser 1 or a filtered light source is reflected by a 50% beam splitter 2 and focused by a low f/number cylindrical lens 3 to a small focal spot on the thin film 4. The cylindrical lens 3 creates the band of angles required (in this case

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symmetric to the sample normal) and the small spot required for high spatial resolution. The radiation reflected from the thin film is collimated by this same lens and passed back through the beam splitter 2 to the diode array 5 for detection of the angular reflection spectrum. In a transmissive mode (not shown), the beamsplitter can be eliminated and a similar cylindrical lens on the opposite side of the film can be used to collect, collimate and direct the light to the array. In Fig. 2, the beam splitter is eliminated and one half of the cylindrical lens 6 aperture is used for illumination and the other half for collection. In either configuration, each diode in the array will measure radiation from a small angular range and the reflectivity as a function of angle is obtained directly from the output of the array. As an example, an array of 200 diodes would give an angular resolution of 1/4 degree for a 50-degree illumination angle. The wavelength of the illumination can be selected to match or fall within a transmission band of the thin film material.

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The reflection coefficients in both s- and p-polarizations depends strongly on film thickness and the optical properties of the film.

In Figs. 3a and 3b the s-polarized reflectivity as function of angle is shown for various film thicknesses for a non-absorbing media with a real index of 1.75 and an illumination wavelength of 500 nm. It is apparent, for thicknesses greater than 0.1 micron, that the shape of these curves can be used to uniquely determine the thickness of the film when the real part of the refractive index is known. When more than one interference fringe is present, for t > 1 micron in the case shown, the interference maxima occur when: 2nt cos rM = Mg where: t = film thickness

1

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n = real part of refractive index

rM = angular location of a peak (internal)

g = illumination wavelength

M = interference order

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By measuring the angular location of...