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Thickness and Refractive Index Determination for Thin Films

IP.com Disclosure Number: IPCOM000047212D
Original Publication Date: 1983-Oct-01
Included in the Prior Art Database: 2005-Feb-07
Document File: 3 page(s) / 23K

Publishing Venue

IBM

Related People

Coyard, M: AUTHOR

Abstract

The present method permits the refractive index of thin films used in the semiconductor industry to be easily and precisely measured simultaneously with the film thickness (in the approximate range of 100 nm to 4 mm). There are known methods which allow measuring both the refractive index and the film, such as ellipsometry, but they are cumbersome to use and require expensive equipment. The method proposed below is based on the extrema of the reflected beam whose wavelengths meet the fundamental equation: (Image Omitted) where N is an integer for maxima and half integer for minima. This equation may be found in [1], Chapter 11, section 1b1. In this equation, n and d are, respectively, the refractive index and the thickness of the film to be measured, and 0 the angle of the incident beam.

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Thickness and Refractive Index Determination for Thin Films

The present method permits the refractive index of thin films used in the semiconductor industry to be easily and precisely measured simultaneously with the film thickness (in the approximate range of 100 nm to 4 mm). There are known methods which allow measuring both the refractive index and the film, such as ellipsometry, but they are cumbersome to use and require expensive equipment. The method proposed below is based on the extrema of the reflected beam whose wavelengths meet the fundamental equation:

(Image Omitted)

where N is an integer for maxima and half integer for minima. This equation may be found in [1], Chapter 11, section 1b1. In this equation, n and d are, respectively, the refractive index and the thickness of the film to be measured, and 0 the angle of the incident beam. From this equation d may be determined if n is known, or vice versa, according to CARIS (Constant Angle Reflection Interference Spectroscopy) and VAMFO (Variable Angle Monochromatic Fringe Observation) methods. According to the proposed method, at a fixed angle, the spectrum yields maxima and minima. Let us consider an extremum corresponding to an interference order N. If the angle 0 is changed, the wavelength g of that extremum will be shifted. The variations of g vs 0 may be used to determine d and n as demonstrated below. The basic equation may be written:

(Image Omitted)

So the variations of g2 as a function of sin2 0 yields a straight line whose slope a and ordinate at origin b give the values of d and n, provided that the order N is known. The order N for any angle - is easily determined from 2 consecutive extrema using the equation: (See 2.) The measurement may be performed for several orders, depending on the thickness of the film, providing several pairs of values (d, n). It is an advantage of the proposed method to use standard spectrophotometer equipment with the UV/visible feature, such as the Beckman Model 5270 with the specular reflectance attachment, which has an incidence angle variable from 5OE to 60OE. The value of 0 does not need to be known with great...