JPH0435683B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an apparatus for measuring film thickness using optical interference.

[Prior art] To measure the film thickness of an object to be measured using optical interference, light from a light source is projected onto the object to be measured, and the transmitted or reflected light is separated to form interference fringes. The film thickness of the object to be measured is detected by a detector, but
When the refractive index n of the object to be measured changes depending on the wavelength λ, there is a problem that accurate film thickness measurement becomes difficult.

In order to solve this problem, the applicant filed a patent application in 1983-
In No. 196148, we provided a film thickness measuring device that enables accurate film thickness measurement even if the refractive index of the object to be measured has wavelength dependence.

[Problems to be Solved by the Invention] However, even with this method, the refractive index of the object to be measured and the wavelength dependent coefficient (dispersion) of the refractive index must be set in advance. For objects with small dispersion and for rough measurements, it is sufficient to measure only the refractive index in advance, but for more accurate measurements, accurate values of dispersion are required. Although refractive index data is relatively well known, dispersion data is sparse. Also,
The refractive index and dispersion of plastic films vary depending on the type and proportion of additives, manufacturing method, etc., and it is often difficult to know the refractive index and dispersion of the actual object to be measured.

In view of the above points, it is an object of the present invention to provide a film thickness measuring device that measures the film thickness by determining the refractive index and dispersion of a sample whose thickness is known.

[Means for Solving the Problems] This invention projects light from a light source onto an object to be measured, separates the transmitted light or reflected light, and detects interference fringes with a detector. In a device for determining film thickness, at least two of the interference fringe patterns of the detector are
A film thickness measuring device that calculates the film thickness of the object to be measured by calculating the wavelength dependence coefficient of the refractive index on the assumption that each film thickness obtained from the wavelength or wave number or order difference that gives the extreme value in two different wavelength ranges is equal. It is.

[Embodiment] FIG. 1 is a configuration explanatory diagram showing an embodiment of the present invention.

In the figure, 1 is a light source, and the light from light source 1 is
The light is projected by a lens onto an object to be measured 4 such as a film through a half mirror 3, and the light transmitted or reflected by the object to be measured 4 is transmitted through a half mirror 3, a lens 5, and a shutter means 6 such as a chopper in this figure. , an aperture 7, a lens 8, and a spectroscopic means 9 such as a diffraction grating.
is incident on the detector 11 of an image sensor such as . Light corresponding to each wavelength separated by the spectroscopic means 9 is incident on each element of the detector 11 of this image sensor, and the intensity of the interference fringe pattern is detected. The output of the detector 11 is amplified by an amplifier 12, and a predetermined calculation is performed by a calculation means 13, whereby the film thickness d of the object to be measured 4 is calculated.

As shown in Fig. 2, parallel rays L ₁ from the light source,
L ₂ is a film thickness (thickness) d, which is reflected by the front and back surfaces of the object to be measured 4 whose refractive index has wavelength dependence,
Both rays L ₁ and L ₂ have an optical path difference 2nd/cos θ', and when this optical path difference is an integral multiple of the wavelength of the light, they interfere to form interference fringes as shown in FIG.

Assume that an interference fringe pattern as shown in Fig. 3 is obtained, and for the minimum wavelength λ ₁ and maximum wavelength λ ₂ that give the extreme values of the measurement area, each order of interference is set to m+N, m, wavelength λ ₁ , 〓 _2. The refractive index of the corresponding object to be measured 4 is n ₁ ,
Assuming n ₂ , the following formula holds.

(m+N)λ ₁ =2n ₁ d/cosθ' (1) mλ ₂ =2n2d/cosθ' (2) Obtaining m from equation (2) and substituting it into equation (1) to organize, d=Ncosθ'/21/ n ₁ /λ ₁ −n ₂ /λ ₂ (3). When θ'=0 and the light is projected perpendicularly to the object to be measured 4, cos θ'=1 and equation (3) becomes d=N/21/n ₁ /λ ₁ −n ₂ /λ (4). In this way, from the wavelengths λ ₁ , λ ₂ , the order difference N of the extreme values, and the refractive indices n ₁ , n ₂ corresponding to the wavelengths λ ₁ , λ ₂ , the film thickness of the object to be measured 4 is (3), (4 ) can be found from the formula.

Here, when equation (4) is expressed in terms of wave number γ, which is the reciprocal of wavelength λ, the following equation is obtained (maximum wave number γ ₁ =1/λ ₁ , minimum wave number γ ₂ =1/λ ₂ ).

d=N/21/n ₁ γ ₁ −n ₂ γ ₂ (5) Here, let us assume that the refractive index n is approximated by a linear expression of the wave number γ, and let the refractive index at the wave number γ ₀ be n ₀ . , n=n ₀ (1+α(γ−γ ₀ )) (6). α is a wavelength dependence coefficient (dispersion coefficient) indicating the wavelength dependence of the refractive index, where γ=γ ₀ and n=n ₀ , and γ ₀ is the reference wave number.

Substituting equation (6) into equation (5) and organizing it gives the following equation.

d=N/{2 (γ ₁ − γ ₂ ) n ₀ · (1 + α (γ ₁ + γ ₂ − γ ₀ ))} (7) By the way, assuming that n ₀ and α are known, the original wave number When the range (wavelength range) is divided into two and the film thickness is measured in each wave number range, the film thicknesses d ₁ and d ₂ are determined by the following equations. The wave number ranges were γ ₁₁ to γ ₁₂ and γ ₂₁ to _{γ 22} , and the order differences were N ₁ and N ₂ .

d ₁ = N ₁ / {2 (γ ₁₁ − γ ₁₂ ) n ₀・(1 + α (γ ₁₁ + γ ₁₂ − γ ₀ ))} (8) d ₂ = N ₂ / {2 (γ ₂₁ − γ ₂₂ ) n ₀・(1+α(γ ₂₁ + γ ₂₂ −γ ₀ ))} (9) Since the same location is measured, d ₁ and d ₂ are equal, and the following equation is obtained from d ₁ = d ₂ .

α={N ₂ (γ ₁₁ − γ ₁₂ ) −N ₁ (γ ₂₁ − γ ₂₂ )} / {N ₁ (γ ₂₁ − γ ₂₂ ) (γ ₂₁ + γ ₂₂ − γ ₀ ) −N ₂ (γ ₁₁ − γ ₁₂ )(γ ₁₁ +γ ₁₂ −γ ₀ )} (10) Since all the right-hand sides of α are measurable values, α can be found for known samples.

Also, n ₀ is given by the true thickness d _s , (8)
It can be obtained from either equation (9) or from the results of remeasurement over the entire wavenumber range.

Using equation (8), n ₀ is obtained as n ₀ =N ₁ /{2ds(γ ₁₁ −γ ₁₂ )·(1+α(γ ₁₁ +γ ₁₂ −γ ₀ ))} (11).

Using α, n _0, etc. obtained in this way in equation (6),
The refractive indexes n ₁ , n ₂ , . . . of arbitrary wave numbers (wavelengths) are determined, and the film thickness is determined from equation (5) and the like.

In other words, since the wavelengths incident on each element of the detector 11 are determined in advance by the spectroscopic means 9,
The relationship between each element number of the detector 11 and the wavelength (wave number) is stored and set in the memory of the calculation means 13.
Further, the relationship between the refractive index and the wavelength is also stored in the memory in the form of a table or equation. For example, it is something like equation (6) using the above α and n ₀ determined by measurement.

Then, during measurement, the output of each element of the detector 11 is read out sequentially, and as shown in FIG. 3, wavelengths λ ₁ and λ ₂ (wave number γ ₁ , γ ₂ ), and calculate these λ ₁ , λ ₂ (γ
₁ ,
The refractive indices n ₁ and n ₂ corresponding to γ ₂ ) are determined from the memory equation (6), etc., and the order difference N is determined from the difference in the number of extreme values, (3),
The film thickness d of the object to be measured 4 is determined by calculations such as equations (4) and (5) or (7), (8), and (9). Incidentally, due to variations in the film thickness of the object to be measured 4, the interference fringe patterns may shift and overlap, resulting in weak portions.
In order to avoid this, the film thickness is measured as described above from the output of the extreme value when the difference between the maximum value and the minimum value of the output for each wavelength of the detector 11 is greater than a predetermined value. . Thereby, the film thickness d of the object to be measured 4 can be measured from strong and reliable interference fringes without picking up uncertain and weak interference fringes.

In addition, the detector 11 is equipped with a charge storage type image sensor CCD.
When an image sensor such as the one shown in FIG. , the overall contrast deteriorates.

For this reason, the incident light is interrupted by a shutter means 6 such as a chopper whose sectors are rotated by a motor to limit the time for one measurement, thereby reducing the influence of fluctuations in the incident light on the detector 11 and reducing interference fringes. To prevent deterioration of contrast and ensure reliable measurement.

[Effects of the Invention] As described above, in this invention, the wavelength-dependent coefficient (dispersion) that determines the refractive index corresponding to the wavelength is determined from measurement, and is stored in memory in advance for use. Even if the refractive index of the film has wavelength dependence, it is possible to accurately measure the film thickness.

[Brief explanation of drawings]

FIGS. 1 and 2 are block diagrams showing one embodiment of the present invention, and FIG. 3 is an explanatory diagram of interference fringes. 1... Light source, 2, 5, 8, 10... Lens, 3
... Half mirror, 4 ... Object to be measured, 6 ... Shutter means, 7 ... Aperture, 9 ... Spectroscopic means, 1
1...Detector, 12...Amplifier, 13...Arithmetic means.

Claims (1)

[Claims of Claims] 1. A light source that projects light onto an object to be measured, a detector that uses a spectrometer to separate transmitted light or reflected light from the object to be measured and detects interference fringes, and interference fringes of this detector. A calculation that calculates the film thickness of the object to be measured by determining the wavelength dependence coefficient of the refractive index on the assumption that each film thickness obtained from the wavelength or wave number or order difference that gives the extreme value for at least two different wavelength ranges of the pattern is equal. A film thickness measuring device characterized by comprising: means. 2. The film thickness measuring device according to claim 1, wherein an image sensor is used as the detector.