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Optical Thickness Measurement Analysis Applicable to Multilayer Films

IP.com Disclosure Number: IPCOM000088977D
Original Publication Date: 1977-Aug-01
Included in the Prior Art Database: 2005-Mar-04
Document File: 3 page(s) / 33K

Publishing Venue

IBM

Related People

Dill, FH: AUTHOR [+2]

Abstract

The IBM 7840 Film Thickness Analyzer (FTA) measures the visible spectral reflectance of filmed surfaces and determines film thickness from analysis of these spectra. Before the availability of the below-described system, only single layer films (with one exception) could be analyzed. In principle, the described system allows analysis of an arbitrary multilayer film to give the top layer thickness - (a) when it is transparent and (b) when the thicknesses and optical properties of all underlying layers are known. Important types of multilayer films made measurable by this system include combinations of SiO(2) and Si(3)N(4), and oxidized polysilicon films.

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Optical Thickness Measurement Analysis Applicable to Multilayer Films

The IBM 7840 Film Thickness Analyzer (FTA) measures the visible spectral reflectance of filmed surfaces and determines film thickness from analysis of these spectra. Before the availability of the below-described system, only single layer films (with one exception) could be analyzed. In principle, the described system allows analysis of an arbitrary multilayer film to give the top layer thickness - (a) when it is transparent and (b) when the thicknesses and optical properties of all underlying layers are known. Important types of multilayer films made measurable by this system include combinations of SiO(2) and Si(3)N(4), and oxidized polysilicon films.

The main feature of this system is a mathematical concept of "effective substrate", including the actual substrate plus all but the top layer of the film. This is illustrated in the figure. A complex refractive index n(3)(eff) = n(3)(eff) - ik(3)(eff) (1 is found at each wavelength a so that the reflectance R(Lambda) of the effective substrate plus the top layer is equal at each wavelength to the measured reflectance of the actual substrate plus multilayer film. Here, n denotes real (in the mathematical sense) refractive index, and k denotes extinction coefficient.

The quantities above are determined from the optical properties of the actual substrate and the thicknesses and optical properties of the underlying film layers. The reflection coefficient Rho of the substrate plus underlying layers is calculated at each wavelength Lambda. The complex refractive index of a homogeneous material, having Rho as its reflection coefficient, is given by n(3)(eff) (Lambda) = n(1) 1 - Rho(Lambda) over 1 + Rho(Lambda) (2)

where n(1) is the refractive index of the surrounding medium (e.g., for air, n(1) = 1). This defines mathematically the necessary optical properties of the effective substrate. The values assumed by n(3)(eff) and k(3)(eff) may be physically unrealizable, but only mathematical significance is attached to them.

Basically, once n(3)(eff) is found, analysis proceeds as in the single layer film case: the top layer becomes the entire film, and the underlying layers plus substrate become the effective substrate. The calculation of envelope curves to determine order changes and to determine absolute reflectance from relative reflectance i...