RU2395788C2 - Method to measure thickness of thin films on substrate - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: proposed contactless method consists in irradiating the surface by optical radiation on various sounding wavelengths, registering signal reflected from said surface and determining film thickness by analyzing dependence of reflected signal intensity upon sounding wavelengths. Note that here is used laser source, smoothly or discretely (over six sounding wavelengths) readjustable in narrow range of wave length λ. Reflected signal measurement results are used to additionally determine first R'ref(λ,d) and second R"ref(λ,d) derivatives of coefficient Rref(λ,d) of reflection of three-layer system "air-film-substrate" to calculate film thickness d by the formula:

, where n₂ is refractivity index of film material; r₁₂, r₂₃ are coefficients of reflection on the boundary of spheres "air-film" and "film-substrate"

EFFECT: higher accuracy of measurement.

2 dwg

Description

The invention relates to measuring technique and can be used, in particular, for rapid express control of the thickness of the films of oil products in treatment facilities, in inland waters, port waters, etc.

Known methods for measuring the thickness of the film on the surface of the material (see, for example, [1-5]), which consist in the fact that optical radiation is directed to the film surface, the wavelength of the radiation incident on the film surface is tuned, the signal reflected from the surface is recorded and determined film thickness d according to the analysis of the dependence of the intensity of the reflected signal on the wavelength.

где n₂ - показатель преломления пленки.Closest to the proposed one is a remote three-wave method for measuring the thickness of thin films [5], which consists in the fact that the surface is irradiated with optical radiation at three wavelengths λ _{1, 2, 3} , the signal reflected from the surface is recorded and the film thickness d is determined from the analysis of the dependence the intensity of the reflected signal at wavelengths λ _{1, 2, 3} , selected so that λ ₁ = λ ₂ -Δλ, λ ₃ = λ ₂ + Δλ, and Δλ is chosen so as to ensure the inequality

where n ₂ is the refractive index of the film.

The disadvantage of this method is its unstable operation (very large errors in determining the film thickness) in the presence of random errors in the values of the measured signals (which always occur due to measurement errors, noise of the receiving path, etc.). This leads to the need to use a very long averaging - to suppress random errors in the values of the measured signals, averaging over hundreds of thousands of single measurements is used. However, long-term averaging can be used only in the case of a very slow change in the thickness of the measured film (otherwise it causes distortion of the determined thickness).

This drawback can be avoided by the fact that according to the method of measuring the film thickness on the surface of the material, including irradiating the surface with optical radiation at different sensing wavelengths, registering the signal reflected from the surface and determining the film thickness from the results of analyzing the dependence of the reflected signal intensity on the sensing wavelengths, for measuring film thicknesses, the first and second derivatives of the reflection coefficient of the three-layer system “air-film-substrate” are calculated and the thickness of the plate is determined CQM value of the first and second derivative of reflectance.

The presence of a distinguishing feature indicates compliance with the criterion of "novelty."

The specified distinguishing feature is unknown in the scientific, technical and patent literature, and therefore, the proposed technical solution meets the criterion of "inventive step".

Figure 1 schematically shows a device that implements the proposed method.

The device contains a wavelength tunable radiation source 1, a photodetector 2, a unit 3 for calculating the first and second derivatives of the reflection coefficient of a three-layer system “air-film-substrate”, a unit 4 for calculating the thickness of the film 5 on the surface of the material 6.

The device operates as follows.

The optical radiation of the source 1 is reflected by the surface of the film 5 (thickness d) and the substrate 6, the intensity of the reflected radiation is detected by the photodetector 2, the signal from the photodetector enters the unit 3 for calculating the first and second derivatives of the reflection coefficient of the three-layer system “air-film-substrate”, the values of the first and the second derivative of the reflection coefficient is supplied to block 4 to determine the film thickness.

The radiation wavelength λ of source 1 is tuned (discrete or smoothly) to calculate, according to the measurements of the first and second derivatives of the reflection coefficient of the three-layer system “air-film-substrate”.

The radiation source 1 irradiates the surface with a narrow beam at an angle close to the vertical. A photodetector 2 at a radiation wavelength λ detects the radiation power P (λ) reflected by the surface under study. It is believed that the surface under study is quite smooth and the nature of reflection is close to mirror (with its reflection coefficient R _ref (λ)). The radiation source is located at a small distance from the surface, and the dimensions of the receiving optics are quite large, so that the receiver intercepts all the radiation reflected from the surface. Then the received power P (λ) can be represented in the form (see, for example, [6]):

Where:

P _o - power radiated by the source;

R _ref (λ) is the reflection coefficient of the surface.

When the radiation is incident vertically on the surface, for the reflection coefficient of the three-layer system “air-film-substrate” R _ref (λ, d) in the case of thin films (when the transmission of the film differs little from unity), we have (see, for example, [7]):

Where:

n _2,3 (λ), k _2,3 (λ) are the refractive indices and absorption of the film and substrate material, respectively; r ₁₂ , r ₂₃ are the reflection coefficients at the interface between the air-film and the film-substrate, respectively.

и

в знаменателе выражения (2) малы по сравнению с 1):Often, for the reflection coefficient R _ref (λ, d), instead of formula (2), a simpler approximate expression is used (given that in many cases the quantities

and

in the denominator, expressions (2) are small compared to 1):

Formulas (1) - (3) show that if the receiver intercepts all the radiation specularly reflected from the surface, then the reflection coefficient R _ref (λ, d) and the value of R _ref (λ, d) can be determined from the measurement results P (λ) find the film thickness d. However, the value of R _ref (λ, d) at the same sounding wavelength λ due to the interference of radiation reflected from the air-film and film-substrate interfaces does not uniquely determine the film thickness d.

This ambiguity for thin films can be eliminated by a method based on the determination of the first and second derivatives (with respect to the wavelength) of the reflection coefficient of the air-film-substrate system.

We find the first and second derivatives of R _ref (λ, d) with respect to λ, taking into account that the changes in λ of the values of r ₁₂ , r ₂₃ and n ₂ are many times slower than the changes in λ of the value of cos [2β (λ, d)]:

From (3) - (5) after simple transformations we have:

Thus, the measurement of the reflection coefficient of the air-film-substrate system, its first and second derivatives (values of R _ref (λ, d), R ' _ref (λ, d), R ” _ref (λ, d) on the right side (6)) allow us to find the film thickness d on the substrate.

A more accurate expression (based not on approximate formula (3), but on formula (2)) for determining the film thickness d has the form:

Where:

Thus, in order to determine the film thickness, it is necessary to measure the reflection coefficient of the air-film-substrate system and determine its first and second derivatives, which can be realized, for example, for a water surface with an oil film, using one tunable wavelength in a narrow Laser range near or mid-infrared.

Figure 2 shows the results of mathematical modeling of the described method for measuring the thickness of thin oil films. The dependence of the film thickness d _n determined (determined by algorithm (7)) on the film thickness d specified in the simulation in the case of measurement noise with a relative rms value of 1% is shown here. To calculate the first and second derivatives, we used discrete values of the reflection coefficient at six closely spaced (spaced apart by Δλ = 7.5 nm) wavelengths near λ = 0.8 μm with averaging the results over a series of only 300 single measurements.

In the drawing, a dashed line intersecting the drawing diagonally is a relationship for which the found value of the film thickness coincides with the actual one. The other two dashed lines are the 20% difference between the found value of the film thickness d _n and the actual value of the thickness d.

Thus, the described method allows under real conditions of measurement noise to provide a stable measurement of the thickness of thin films with an accuracy of at least about 20% in the range of film thicknesses from fractions of microns to ~ 5 microns.

The claimed invention is directed, in particular, to solving the problem of rapid express control of the thickness of the films of oil products, which is especially important in wastewater treatment plants when controlling the degree of water purification.

The measuring device can be assembled at the enterprises of the Russian Federation from components and components manufactured in the Russian Federation, and meets the criterion of "industrial applicability".

Information sources

1. Device for automatic measurement of film thickness. Patent 3-57407. Japan. 1993 Cl. G01B 11/06 (Russian Railways Inventions of the World, 1993, issue 82, N3, p. 45).

2. Method of measuring film thickness. United States Patent. Patent Number: 4,645,349. Date of Patent: Feb. 24, 1987. Int. Cl. G01B 11/06.

3. The remote method of measuring the thickness of the films. RF patent for the invention No. 2168151 dated 05/27/01. MKI G01B 11/06.

4. The method of measuring the thickness of the films on the substrate. RF patent for invention No. 2207501 dated 06/27/03. MKI G01B 11/06.

5. Remote three-wave method for measuring the thickness of thin films. RF patent for the invention No. 2304759. class G01B 11/06, G01N 21/17.

6. Grigoriev P.V., Lomonosov A.M., Solntsev M.V. Investigation of the statistical properties of the reflected signal during laser sensing of the sea surface // Izvestiya AN SSSR. Series Physical. 1987. V. 51, No. 2, S.210-214.

7. Bourne M., Wolf E. Fundamentals of Optics, Moscow: Nauka, 1970, 855 pp.

Claims (1)

A non-contact method for measuring the film thickness on the surface of the substrate material by irradiating the surface with optical radiation at different sensing wavelengths, recording the signal reflected from the surface and determining the film thickness according to the analysis of the dependence of the reflected signal intensity on the sensing wavelengths, characterized in that they are used smoothly or discretely ( from six sensing wavelengths) a laser radiation source tunable along the wavelength λ in a narrow range, according to the measurement data of the signal further comprises determining the first R _'ref (λ, d) and a second _{R''ref (λ, d} ) derivatives coefficient R _ref (λ, d) reflection sandwich "air-film-substrate" system and is calculated by the formula film thickness d :

where n ₂ is the refractive index of the film material; r ₁₂ , r ₂₃ are the reflection coefficients at the interface between the air-film and the film-substrate, respectively.

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