JPH0444202B2 - - 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 light interference, light is applied to the object to be measured from a light source, and the transmitted or reflected light is separated to detect interference fringes. The film thickness of the object to be measured is measured.

[Problems to be Solved by the Invention] However, if there are slight variations in the refractive index, film thickness, etc. within the measurement spot of the object to be measured,
The pattern of interference fringes differs depending on the difference in film thickness, etc., and when these are overlapped and synthesized at the same time, as shown in Figure 4, it can cause beats and weaken the contrast, making measurement difficult. be.

In addition, when measuring a continuously flowing object such as a polymer film using an image sensor such as a charge accumulation type imaging device CCD as a detector, it is necessary to cover a long distance within one scanning period of charge accumulation. When measuring the part, the interference fringe pattern that is spectrally separated and projected onto the image sensor differs depending on the difference in the film thickness of the object to be measured.
Beating as shown in the figure may occur, or the contrast may be weakened due to averaging, making measurement difficult.

In view of the above points, an object of the present invention is to provide a film thickness measuring device that enables measurement with sufficiently high accuracy even in continuous measurement.

[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 an image sensor. This film thickness measuring device is provided with a reset means for changing the reset time of an image sensor.

[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
A lens 2 projects light onto an object to be measured 4 such as a film through a half mirror 3.
In this figure, the light transmitted or reflected is separated by a spectroscopic means 9 such as a diffraction grating via a half mirror 3, a lens 5, an aperture 7, and a lens 8.
The light enters an image sensor 11 having a reset function, such as a one-dimensional CCD. Light corresponding to each wavelength separated by the spectrometer 9 is incident on each element of the image sensor 11, and the intensity of the interference fringe pattern is detected. The output of the image sensor 11 is amplified by an amplifier 12, and a predetermined calculation is performed by a calculation means 13 including a memory, etc., to calculate the film thickness d of the object to be measured 4. Further, a reset means 6 is provided for generating a reset pulse to the image sensor 11 to reset it.

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

Assuming that an interference fringe pattern as shown in FIG. 4 is obtained, and for the minimum wavelength λ ₁ and maximum wavelength λ ₂ that give the extreme values of the measurement region, the following equation holds, assuming that the orders of interference are m+N and m.

(m+N)λ ₁ = 2nd/cosθ' (1) mλ ₂ = 2nd/cosθ' (2) Obtaining m from equation (2) and substituting it into equation (1), we get d=Ncosθ'/2n λ ₁ λ ₂ /λ ₂ −λ ₁ (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/2n λ ₁ λ ₂ /λ ₂ −λ ₁ (4). In this way, the film thickness of the object to be measured 4 can be determined from the wavelengths λ ₁ and λ ₂ , the order difference N between the extreme values, and the known refractive index n.
It can be found from equations (3) and (4).

That is, since the wavelength incident on each element of the image sensor 11 is determined in advance by the spectroscopic means 9, the relationship between each element number of the image sensor 11 and the wavelength is stored in the memory of the calculation means 13.

Then, during measurement, the output of each element of the image sensor 11 is sequentially read out, and as shown in FIG. 4, the wavelengths λ ₁ and λ ₂ are determined using memory from the element number that can give an extreme value within the measurement range. , calculate the order difference N from the difference in the number of extreme values, use the refractive index n stored in a memory, etc., and calculate the film thickness d of the object to be measured 4 by performing calculations such as equations (3) and (4). demand.

By the way, as shown in FIGS. 4A and 4B, interference fringe patterns may shift and overlap due to variations in the film thickness of the object to be measured 4, resulting in weak portions. In order to avoid this, the film thickness is measured as described above from the output for the extreme value when the difference between the local maximum value and the local minimum value is greater than or equal to 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.

Furthermore, when a charge accumulation type image sensor CCD is used as the image sensor 11, if the object to be measured 4 moves and its thickness etc. deviate, a slightly different interference fringe pattern will appear on each element of the image sensor 11 in one scan. They are incident within the period and are combined, resulting in poor contrast as a whole.

Therefore, by applying a reset using the reset means 6, the accumulation time of the image sensor 11 is limited, that is, the measurement time within one scan is limited, the influence of fluctuations in the incident light on the image sensor 11 is reduced, and the interference is reduced. To prevent deterioration of stripe contrast and ensure reliable measurement. As shown in FIG. 3, each pixel output of the image sensor 11 is transferred every predetermined time T, during which charge is accumulated in each pixel.
If the reset is performed by 1, no charge will be accumulated in each pixel during that time, and the charge will be accumulated in each pixel for the accumulation time t2 after the reset is released. This reset time t1 can be changed and set to an optimal value.

[Effects of the Invention] As described above, the present invention limits the charge accumulation time of the image sensor by the reset means,
Since the measurement is performed at predetermined time intervals, sufficient contrast of interference fringes can be obtained, and stable and highly accurate film thickness measurement is possible.

[Brief explanation of drawings]

1 and 2 are configuration explanatory diagrams showing one embodiment of the present invention, FIG. 3 is an explanatory diagram of signals, and FIG. 4 is an explanatory diagram of interference fringes. DESCRIPTION OF SYMBOLS 1... Light source, 2, 5, 8, 10... Lens, 3... Half mirror, 4... Measured object, 6... Reset means, 7... Aperture, 9... Spectroscopic means, 11... Image sensor, 12... Amplifier, 13... calculation means.

Claims (1)

[Claims] 1. In an apparatus that projects light from a light source onto an object to be measured, separates the transmitted light or reflected light, and detects interference fringes with an image sensor, and measures the film thickness of the object to be measured, the image sensor is reset. A film thickness measuring device characterized by being provided with a reset means for changing time.