JPH01131405A - Film thickness measuring instrument - Google Patents

Abstract

PURPOSE:To automatically measure and correct the refractive index of an object of measurement such as an opaque film which has a coefficient of absorption by finding the refractive index from the output ratio of a P and an S component which provide the maximum value of an interference waveform and the output ratio of a P and an S component which provide the minimum value. CONSTITUTION:Light from a light source 1 is projected on the object 3 of measurement like the opaque film having the coefficient of absorption and the light which is transmitted through it is split into a P-polarized component and an S-polarized component By a polarizing plate 5 which is rotated by a driving means. Then they are diffracted spectrally by a prism 7 and incident on a CCD 9 to detect the intensity of interference fringe patterns as to the P and S components. The output of the CCD 9 is processed by an arithmetic means 10 as specified to find the refractive index and film thickness 10 the object 3. Consequently, the refractive index is found by utilizing the ratio of the P and S components which provide the maximum value and the ratio of the P and S components which provide the minimum value, so even if the kind and refractive index of the object 3 are changed, the refractive index is corrected automatically at all times and the accurate film thickness can be measured.

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 a target using optical interference,
Light from a light source is projected onto the object to be measured, the transmitted light or reflected light is separated, and interference fringes (interference waveforms) are detected by a detector to measure the film thickness of the object to be measured.

[Problem 1 to be solved by this invention However,
To measure the film thickness of the object to be measured, it is necessary to know the refractive index in advance, and if the type of object to be measured changes and the refractive index changes, the refractive index of the measurement system will change accordingly. It was very inconvenient as it required changing the setting values. Furthermore, it has been impossible to measure a film whose diffraction index fluctuates during operation (for example, a silicon diffusion film). For this reason,
Although the applicant describes a refractive index and film thickness measuring device for a transparent film in the specification filed at the same time, it is difficult to measure an opaque film that has an absorption coefficient.

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 an opaque film having an absorption coefficient.

[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 spectroscopic means, and the interference waveform is detected by a detector. From the output ratio of the P component and the S component that give the maximum value of the interference waveform and the output ratio of the P component and the S component that give the minimum value of the interference waveform, Find the refractive index, and combine this refractive index with 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, 1 is 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 an opaque film having an absorption coefficient, and the light transmitted or reflected from the object to be measured 3 is The polarization means 5 such as a polarizing plate rotated by a drive means (not shown) separates the light into P and S polarization components, which are separated into polarized light components through a lens 6 by a spectroscopic means 7 such as a diffraction grating or a prism, and are separated by a lens 8. The interference fringe pattern (interference waveform) as shown in FIG. 2 for the P and S components is incident on the detector 9 of an image sensor such as a COD.
intensity is detected. The output of the detector 9 is amplified by an amplifier (not shown), and a predetermined calculation is performed by the calculating means 10, whereby the refractive index n1 and the thickness d of the object to be measured 3 are 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 from the light source 1 has an incident angle of 1
1. It is reflected on the front and back surfaces of the object to be measured 3 at a refraction angle of 12. The reflected light R1 is refracted light, and the refractive index n of the object to be measured 3 is
, absorption coefficient χ, and thickness d. For example, if the transmittances of the S component and the P component are S and Tp, the following equation holds true as a result of interference.

TI)-016(n2+22)B12θ22/Z+
(1) Ts -ci 6
(n 2-142) B12θ22, / Z 2

(2>Here, the symbols are as follows.

Zl-(A12+B+2)2+ (C12+8+2)2
eXp (-277χ)+2 [(At2-8+2><
8+2-C+")-4A+B, 2C+)cos
(ηn )+2B+ (At (at2-C+2
>+C+ (At2-812) ) sin (ηn
] −exp(−ηχ) Z2− <A22+822)” +(C22+B22
) 2 eXl) (-277χ) +2
[(A2 2-822>(B22-022)+4A2
822C2) CO3(ηn) +282 (A2 (
B22-C22)-C2(A22-822) sin
(77n)] ・eXp(-ηχ) A+=n B1 +θ2, A2-θl+n θ2B
+=χθ1, B2−χθ2C+=nθ
1-θ2, C2-θl -n θ2θl-C09i
l, θ2=CO! 1ii2η−4πdθ2
/λ λ: Wavelength C: Constant absorption coefficient determined by optical system When χ is small, assuming that χ is sufficiently smaller than 1 and ignoring the second-order term of χ, equations (1) and (2) become as follows. .

Tp -C16n 2θ12θ22 /[A1' +CI4 exp (-277χ)+2
<-A12C12cos (ηn) +2A+B
+C+ (At-C+) sin (ηn))・
exp (−ηχ)] (3) TS
-C16n 2θ12θ22 / [A2'+C2' eXp (-27774)
+2 t, -A22C22cos (ηn) -2A
2B2C2(A2+C2) sin (ηn)) ・
exp (-ηχ)] <A4
Near the minimum values of Tp and Ts, sin (ηn)
-Q1CO8 (770) --1, so (3), (4
) formula becomes the following formula.

Tp -C16n 2θ12θ22 / (A+2+012eXD (-772:) )2
Ts=C16n2θ12θ22/(A22+C22ext)(-77χ))2 The ratio B of equations (5) and (6) for this minimum value is as follows.

8-TS /TO -(△+2+Cl2exp (-77χ) )2/(
A22+C22exa <-ηz>'t2 η in equation (7) corresponds to the minimum wavelength.

Also, near the maximum value, sin <ηn>-o.

Since cos(ηn)-1, equations (3) and (4) are expressed and similarly, the ratio of the S component to the P component is expressed by the following equation: % Equation % η in Equation (8) corresponds to the maximum wavelength.

Eliminate χ from equations (7) and (8).

B-<A+2+C+2 Rゞl/") / (A22 +C22Rゞ"') (9) Here, R-(A+ 2-P -A 22)/ (C+
l −P ・C22). Also, from Snell's law etc., n5ini 2-3inil θ2=cosi2 -cos(sin (s+ni1/n)) (
10).

Here, i is a known set value (θ1-cosi1 is also known)
, B, and P can be determined by measurement, so if the wavelength that gives <7> and (8) is determined, 8 in equation (9) becomes a function of the refractive index n, and the refractive index n can be determined from this. In addition, (7), (8)
The equation is the same even if it is solved for P.

In addition, the appropriate minimum wavelength λ1 and maximum wavelength λ2 that give the extreme value of equation (1) or (2), and each order of interference are m + N,
When ra is set, the following formula holds true.

1+N) A1 ” 2nd/QO8i2 (
11) 1λ2 = 2nd/cosi2 (
12) Calculating the value from equation (9) and substituting it into equation (8) yields the following equation.

d-Ncosi2λ1λ2/2n (λ2-λ1)=
Ncosi2/2n (1/λ1-1/λ2) In this way, the film thickness d of the object to be measured 3 is determined from the refractive index n using the wavelengths λ1, A2, the order difference N1 of the extreme values, and the equation (9).

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 of the interference wavelength as shown in FIG. 2 when separated into the P component and S component of the polarization means 5 is read out,
(7) of the S component and P component for the wavelength that gives the minimum value
Calculate the output ratio B of the equation, and calculate the output ratio of the S component and the P component of the equation (8) for the wavelength that gives the maximum value, and calculate the output ratio of the equation (8).
) The refractive index n of the object to be measured 3 can be determined using each of the 81 and 12 of the equation.

Next, from the interference wavelength in Fig. 2, the wavelengths λ1 and A2 are obtained from the element signal giving the extreme value using memory, the order difference N is obtained from the difference between the extreme values, and the refractive index n obtained from equation (9) is obtained. to find the film thickness d of the object to be measured 3.

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 provides a ratio of the P and S components that gives the maximum value of the polarization components of the two P and S components,
Since the refractive index n is determined using the ratio of the PSS component that gives the minimum value, the refractive index is always automatically calculated even if the type or refractive index of the object to be measured, such as an opaque film with an absorption coefficient, changes. It is possible to accurately measure the film thickness by correcting the

Furthermore, since the refractive index is determined from the ratio of polarized light components, the influence of fluctuations in the light source constant due to the optical path, etc. is removed, and the refractive index can be determined with high accuracy.

[Brief explanation of the drawing]

1, 2, and 4 are configuration explanatory diagrams showing one embodiment of the present invention, and FIG. 3 is an explanatory diagram of interference waveforms. 1...Light source, 2.4.6.8...Lens, 3...
Object to be measured, 5... Polarizing means, 7... Spectroscopy means, 9
...Detector, 10...Calculating means

Claims (1)

[Claims] 1. A light source that projects light at a fixed angle onto an object to be measured, and spectrally separates the light that is transmitted or reflected from the object to be measured into polarized components of the P component and the S component. A spectroscopic means, a detector for detecting the interference waveform separated by the spectroscopic means, an output ratio of the P component and the S component that give the maximum value of the interference wavelength of this detector, and the P component and the S component that give the minimum value. The refractive index of the object to be measured is determined from the output ratio of Film thickness measuring device. 2. 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.
Film thickness measuring device described in Section 2. 3. The film thickness measuring device according to claim 1 or 2, wherein an image sensor is used as the detector.