JPH0543963B2 - - 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] In order 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 into interference fringes ( The interference waveform) is detected by a detector and the film thickness of the object to be measured is measured.

[Problems to be Solved by the Invention] However, in order to measure the film thickness of the object to be measured, it is necessary to know the refractive index in advance. If the value changes, it is necessary to change the refractive index settings of the measurement system accordingly, which is very inconvenient. Furthermore, it has been impossible to measure a film whose refractive index changes during operation (for example, a silicon diffusion film).

In view of the above points, an object of the present invention is to provide a film thickness measuring device that automatically measures and corrects the refractive index of an object to be measured such as a transparent film.

[Means for Solving the Problems] The present invention projects light from a light source onto an object to be measured at a constant angle, and separates the transmitted light or reflected light from the object to be measured into polarized light components of the P component and the S component. The resulting light is separated by a spectrometer, the interference waveform is detected by a detector, and the P component and S
The refractive index is determined from the output ratio of the components, and this refractive index and P
This is a film thickness measuring device in which the film thickness is measured by a calculating means based on the wavelength that gives the extreme value of the component or S component.

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

In the figure, reference numeral 1 denotes a light source, and the light from the light source 1 is projected by a lens 2 onto an object to be measured 3 such as a transparent film. The light is separated into P and S polarized components by a polarizing means 5 such as a polarizing plate rotated by a means, and separated into polarized light components through a lens 6 by a spectroscopic means 7 such as a diffraction grating or prism, and then transmitted through a lens 8 to a CCD. The light enters the detector 9 of an image sensor such as , and the intensity of the interference fringe pattern (interference waveform) as shown in FIG. 2 for the P component and the S component is detected. The output of this detector 9 is amplified by an amplifier (not shown), and predetermined calculations are performed by a calculation means 10 to determine the refractive index n and film thickness d of the object to be measured 3.
is calculated. Note that a synchronization signal for identifying whether the light separated by the polarization means 5 is a P component or an S component is also sent to the calculation means 10 and used for calculation.

As shown in FIG. 3, the light L from the light source 1 is reflected and transmitted by the front and back surfaces of the object to be measured 3 at an incident angle i ₁ and a refraction angle i ₂ . If the reflected light R and the transmitted light L are the refractive index of the object to be measured 3 and the thickness is d, for example, if the transmittance of the S component and the P component are Ts and Tp, as a result of interference, the following equation is obtained. It works.

Ts=C16n ² θ ₁ ² θ ₂ ² / {(θ ₁ + nθ ₂ ) ⁴ + (θ ₁ − nθ ₂ ) ⁴ −2 (θ ₁ + nθ ₂ ) ²・(θ ₁ − nθ ₂ ) ² cos(ηn) } …(1) Tp=C16n ² θ _{1 2} ^θ ₂ ² / {(nθ ₁ +θ ₂ ) ⁴ + (nθ ₁ −θ ₂ ) ⁴ −2(nθ ₁ +θ ₂ ) ²・(nθ ₁ −θ ₂ ) ² cos(ηn)} …(2) Here, the symbols are as follows.

θ ₁ = cosi ₁ , θ ₂ = cosi ₂ , η = 4πdθ ₂ /λ, λ: wavelength C: constant determined by optical system Equations (1) and (2) become minimum when cos (ηn) = -1. The following formula holds.

Ts＝C4n ² θ _{1 2} ^θ ₂ ² / (θ ₁ ² + n ² θ ₂ ² ) ² …(3) Tp＝C4n ² θ ₁ ² θ ₂ ² / (n ² θ ₁ ² + θ ₂ ² ) ² …( 4) Ts/Tp=(n ² θ ₁ ² + θ ₂ ² ) ² / (θ ₁ ² +n ² θ ₂ ² ) ² …(5) On the other hand, from Snell's law etc., nsini ₂ = sini ₁ θ ₂ = cosi ₂ = cos {sin ^-1 (sini ₁ /n)}...(6).

The left side of equation (5) can be found by measurement, i ₁ is settable and known (θ ₁ = cosi ₁ is also known), and θ ₂ is n based on equation (7).
It is a function of only . By substituting equation (7) into equation (5) and solving for n, the refractive index n can be found.

In addition, the appropriate minimum wavelength λ ₁ and maximum wavelength λ ₂ that give the extreme value of equation (1) or (2), and each order of interference are m+N,
When m is set, the following formula holds true.

(m+N) λ ₁ = 2nd/cosi ₂ (7) mλ ₂ = 2nd/cosi ₂ (8) m is obtained from equation (9), substituted into equation (8), and rearranged to give the following equation.

d=Ncos ₂ λ ₁ λ ₂ /2n (λ ₂ - λ ₁ ) = Ncosi ₂ /2n (1/λ ₁ -1/λ ₂ )...(9) In this way, the wavelengths λ ₁ , λ ₂ , and the extreme values The order difference N, (5)
The film thickness d of the object to be measured 3 is determined from the refractive index n according to the formula.

Although equations (1) and (2) have been described for transmitted light R, the same applies to reflected light T.

That is, since the wavelength of each element of the detector 9 is determined by the spectroscopy means 7, the relationship between each element number and wavelength is stored in the memory of the calculation means 10.

Then, at the time of measurement, the output signal of the interference waveform as shown in FIG. 2 when separated into the P component and the S component of the polarization means 5 is read out, and equations (3) and (4) for the wavelength that gives the minimum value are calculated. Equation (5) is calculated from the corresponding signal, and a predetermined incident angle i ₁ is substituted into the right side of equation (5) using equation (7) to calculate the refractive index n.

Next, from the interference waveform in Fig. 2, the wavelengths λ ₁ and λ ₂ are determined using memory from the element numbers that give the extreme values, the order difference N is determined from the difference between the extreme values, and the refraction calculated from equation (5) is calculated. The film thickness d of the object to be measured 3 is determined using the ratio n.

Incidentally, as shown in FIG. 4, the polarizing means 5 may be placed on the light source 1 side to project polarized light onto the object to be measured 3.

[Effects of the Invention] As described above, the present invention calculates the refractive index n using the ratio of the minimum values of the two polarization components of the P component and the S component. Even if the type or refractive index of the object to be measured changes, the refractive index can always be automatically corrected to accurately measure the film thickness. Furthermore, since the refractive index is determined from the ratio of polarized light components, the influence of fluctuations in optical constants due to optical paths, etc. is removed, and the refractive index can be determined with high precision.

[Brief explanation of the drawing]

FIG. 1, FIG. 2, FIG. 3, and FIG. 4 are configuration explanatory diagrams showing one embodiment of the present invention. DESCRIPTION OF SYMBOLS 1... Light source, 2, 4, 6, 8... Lens, 3... Measured object, 5... Polarizing means, 7... Spectroscopic means, 9... Detector, 10... Calculating means.

Claims (1)

[Claims of Claims] 1. A light source that projects light onto an object to be measured at a constant angle, and light that is time-divisionally separated into polarized light components of P and S components of transmitted light or reflected light from the object to be measured. A spectroscopic means for spectroscopy, a detector for detecting the interference waveform separated by the spectroscopic means, and a refraction of the object to be measured from the output ratio of the P component and the S component of the wavelength that gives the minimum value of the interference waveform of this detector. What is claimed is: 1. A film thickness measuring device comprising: calculation means for determining the refractive index and calculating the film thickness of the object to be measured based on the refractive index and the wavelength that gives the extreme value of the P component or the S component. 2. Claim 1, characterized in that a polarizing means for separating the light into polarized components is provided on the light source side that projects the light onto the object to be measured or on the side where the light is transmitted or reflected from the object to be measured. The film thickness measuring device described. 3. The film thickness measuring device according to claim 1 or 2, wherein an image sensor is used as the detector.