JPH0429364Y2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention relates to a device that measures 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. It is detected by a detector and the film thickness of the object to be measured is measured.

[Problems to be Solved by This Invention] However, if there is even a slight variation in the refractive index, film thickness, etc. within the measurement spot of the object to be measured,
The interference fringe patterns differ depending on the film thickness, etc., and when these overlap and are synthesized, as shown in Figure 3, beats may occur or the contrast may become weak, making measurement difficult.

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 separated and projected onto the image sensor differs depending on the film thickness of the object to be measured, and these are synthesized to form the third
Beating as shown in the figure may occur, or the contrast may become weak due to averaging, making measurement difficult.

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

[Means for solving the problem] This idea 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. This is a film thickness measuring device in which a shutter means is provided in an optical path between an object to be measured and a detector.

[Example] FIG. 1 is a configuration explanatory diagram showing an example of this invention.

In the figure, 1 is a light source, and the light from the light source 1 is projected by a lens 2 through a half mirror 3 onto an object to be measured 4 such as a film, and the light transmitted or reflected from the object to be measured 4 is as follows: In this figure, the light is separated by a spectroscopic means 9 such as a diffraction grating through a half mirror 3, a lens 5, a shutter means 6 such as a chopper, an aperture 7, and a lens 8.
The light enters a detector 11 of an image sensor such as a 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.

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 and form interference fringes as shown in FIG.

Assuming 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 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 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 detector 11 is determined in advance by the spectroscopy means 9, the relationship between each element number of the detector 11 and the wavelength is stored in the memory of the calculation means 13.

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

By the way, as shown in FIGS. 3A and 3B, 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.

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

For this reason, the incident light is interrupted by a shutter means 7, 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 contrast from worsening and to ensure measurement.

[Effects of the invention] As described above, in this invention, since the incident light is interrupted by the shutter means and measurements are taken at predetermined time intervals, sufficient contrast of interference fringes can be obtained.
Stable and highly accurate film thickness measurement becomes possible.

[Brief explanation of the drawing]

1 and 2 are configuration explanatory diagrams showing one embodiment of this 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)

[Scope of utility model registration request] In a device 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 a detector to measure the film thickness of the object to be measured.
An image sensor is used as a detector, and in order to limit the time for one measurement by intermittent incident light, a chopper means consisting of a chopper whose sectors are rotated by a motor is provided in the optical path between the object to be measured and the detector. Film thickness measuring device.