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Transparent Film Thickness Measurement Disclosure Number: IPCOM000052339D
Original Publication Date: 1981-Jun-01
Included in the Prior Art Database: 2005-Feb-11
Document File: 5 page(s) / 126K

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Case, WR: AUTHOR [+2]


This measurement technique provides a nondestructive method of measurin transparent film thickness on a reflective substrate [1].

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Transparent Film Thickness Measurement

This measurement technique provides a nondestructive method of measurin transparent film thickness on a reflective substrate [1].

Method Summary: A. Signals from a normal incidence spectrophotometer (NIS) are converted into reflectivity values at N different wavelengths (Fig. 1).

B. An approximate film thickness is estimated (START). A set of 2N+1 thickness values are created around START. Intervals of H angstroms separate each of the 2N+1 values.

C. Theoretical reflectivities are calculated for each of the 2N+1 thickness values.

D. The measured reflectivity is then correlated to each theoretical reflectivity. This yields a correlation coefficient for each of the 2N+1 theoretical reflectivities. A weighting vector (WV) excludes bad data points.

E. The peak of the correlation coefficients is located using a quadratic fit. The thickness corresponding to the peak is the measured thickness of the sample.

This modification provides several unique processing features:
1. method of creating transfer function,
2. method of determining the estimated thickness,
3. linear correlation and interpolation technique to determine

4. use of weighting vector to exclude bad data,
5. elimination of determining order number to determine

6. correlation coefficient indicator of measurement quality, and
7. can be implemented on any spectrophotometer.

A. Reflectivity: Two (Z) signals are required to obtain a reflectivity (R). 1. Zb or D is the signal created with a black body sample (Fig. 2).

2. Zs is the signal created by the bare reflective substrate.

From D and Zs the system transfer function is determined by the following: ## where: n(o), k(o) and n(s)k(s) are the index of refraction real and imaginary for air and the substrate, respectively.

The reflectivity can now be calculated: R = Z(m) - D over CAL (Fig. 5) where: Z(m) is the measurement signal of the sample (Fig. 4).

B. Approximation:

An approximation of film thickness (START): START = KxJxn(i) where: K is a constant relating wavelength displacement tp thickness,


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n(i) is the real index of refraction of the film at the

center of the wavelength band used, and

J is an integer and is calculated one of three ways:

1. J = NJ when NJ is >/- 20.

2. J = one less than the number of zero crossings of the

first derivative of R when 3<NJ>20.

3. J = 3 or K(1)xR or K(2)xR, whichever is greater when

when NJ</-3. where: NJ is the index of the maxima of the power spectrum of the Fourier transform of R when 3</-NJ, and

K(1), K(2) are constants relating value derived to

proper J.

C. Theoretical Reflectivities: Theoretical reflectivities are generated at (N) intervals of (H) angstroms each side of START. This data is in matrix form 2Nr by N, where N is the number of elements in the reflectivity (R). There are variables required to generate theoretical reflectivities. These are listed: NE Index of refraction of the film being measured for each...