JPH0429002B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention relates to a film of, for example, a stretched polymer film, which has optical anisotropy and whose main refractive index value differs depending on the vibration direction of incident light. The present invention relates to an optical interference type film thickness measuring device suitable for measuring thickness.

PRIOR ART Many devices for measuring the film thickness of polymer films utilize the absorption of beta rays or infrared rays by the film, but none of these methods are suitable for highly accurate measurements. For more accurate measurements, for example, JP-A-56-115905 and JP-A-61-235707 are recommended.
The so-called optical interference method described in the above publication is adopted.

An optical interference type film thickness measuring device measures the amount of light that is reflected by or transmitted through a film.
Utilizing the fact that changes in spectral intensity due to interference phenomena depend on the incident angle of white light, the thickness of the film, and the refractive index, changes in the spectral intensity of the reflected light or transmitted light are detected, and the changes are detected. The film thickness is determined from However, when measuring the thickness of a film with optical anisotropy using such a device, there is a problem in that the signal-to-noise ratio decreases significantly.

That is, in the above-mentioned device, specifically, detecting a change in spectral intensity is as follows.
This is nothing more than knowing at what wavelength bright or dark areas occur due to interference phenomena. Therefore, in order to detect the position of a bright area or a dark area with high precision, it is necessary that the intensity ratio between the bright area and the dark area, that is, the contrast, be sufficiently large. However, since the film is generally stretched, it has optical anisotropy, and therefore the film is parallel to the plane containing the incident light and reflected light, or the incident light and transmitted light (hereinafter referred to as the incident plane). Except for certain special cases, the light component having a vibration direction perpendicular to the plane of incidence (hereinafter referred to as the S-wave component) and the light component having a vibration direction perpendicular to the plane of incidence (hereinafter referred to as the S-wave component), The dominant refractive index values are different, and the wavelengths at which bright and dark areas appear are different between the P-wave component and the S-wave component. Therefore, when using light with a mixture of P-wave and S-wave components, such as white light, two different spectral intensities overlap for bright and dark areas, making it impossible to obtain sufficient contrast. It will disappear. In particular, as the film thickness increases, the contrast between bright and dark areas in the spectral intensity of such films is originally small, and the wavelengths of adjacent bright or dark areas are close to each other. Bright areas and dark areas that contain information cancel each other out, resulting in a significant drop in contrast.

Problems to be Solved by the Invention The purpose of the present invention is to solve the above-mentioned drawbacks of conventional devices, and to greatly increase the contrast between bright and dark areas even when the object to be measured has optical anisotropy. An object of the present invention is to provide an optical interference type film thickness measuring device that can perform highly accurate measurements.

Means for Solving the Problems In order to achieve the above object, in the present invention, parallel white light is made incident on a measurement object at a constant angle of incidence, and the reflected light or transmitted light from the measurement object is sent to a spectrometer. The device is designed to measure the film thickness of the object to be measured from the spectral intensity of the white light, and a polarizer is placed on the incident optical path of the white light so that its polarization axis is in an arbitrary linear polarization direction. An optical interference type film thickness measuring device is provided. However, in the present invention, a polarizer may be placed on the reflection or transmission optical path of white light so that its polarization axis is in any linear polarization direction.

In this invention, on the incident optical path of white light,
By arranging a polarizer on the optical path reflected or transmitted by the measurement target so that its polarization axis is in any linear polarization direction, that is, in the vibration direction of the P-wave component or S-wave component, only arbitrary linearly polarized light can be generated. is made incident on or received by the object to be measured. Here, it is most preferable to arrange the polarizer so that its polarization axis correctly coincides with an arbitrary linear polarization direction, but a range of ±30° with respect to the linear polarization direction is acceptable. This range is the amplitude ratio of the P wave component (or S wave component)
is a range in which it is more than twice the S-wave component (or P-wave component). More preferably, the angle is within a range of ±15°.

A polarizing plate is usually used as the polarizer. Therefore, it is preferable that the polarizing plate has high absorbance in the wavelength range in which the spectral intensity is detected and has small color distortion. In addition, it is preferable that the material has high transmittance.

As a light source of white light, a tungsten lamp,
Xenon lamps, halogen lamps, etc. can be used.

As a spectroscope for reflected light or transmitted light, a prism type, a diffraction grating type, etc. can be used. Further, as the light receiving element, for example, a linear image sensor or the like can be used.

Function The function of the apparatus of the present invention will be explained below with respect to an apparatus in which a polarizer is placed on the incident optical path and the reflected light from the measurement target is received. There is essentially no difference even if the polarizer is placed in the polarizer and the reflected or transmitted light is received through the polarizer.

Now, when parallel white light is incident on a polarizer, only its P-wave component or S-wave component is transmitted. That is, only linearly polarized light is transmitted.

The linearly polarized light then enters the object to be measured at a constant angle of incidence and is reflected, but at this time the spectral intensity changes due to interference phenomena. As is well known, this change depends on the film thickness of the object to be measured if the incident angle and the refractive index of the object to be measured are constant. Therefore, if the incident angle θ is kept constant and the refractive index n of the object to be measured is measured in advance by some method, then from the change in the spectral intensity of the reflected light, the formula, d = [λ _n · λ _{n +1} / (λ _n −λ _n+1 )] / [2・(n ² −
sin ² θ) ^1/2 ] Where, d: Film thickness λ _n , λ _n+1 : The film thickness can be determined from the wavelength of adjacent bright or dark areas (peaks) (λ _n > λ _n+1 ). can.

Embodiment Hereinafter, this invention will be explained in more detail based on one embodiment thereof.

FIG. 1 shows an apparatus of the present invention in which a polarizer is disposed on the incident optical path and receives reflected light from an object to be measured.

In FIG. 1, light emitted from a white light source 1 passes through a condenser lens 2, a pinhole plate 3, a collimator lens 4, a polarizing plate 5, and an entrance window 8 opened in a condenser 7, which will be described later. The measuring object 6 is guided through the measuring object 6. Thus, the polarization axis of the polarizing plate 5 is aligned with the vibration direction of the P-wave component or the S-wave component determined by the plane of incidence of the white light onto the measurement object 6.

A light condenser 7 is arranged to face the measurement object 6 . This condenser 7 has a hemispherical support 9 whose center of curvature is at the point on the measurement target 6 where the film thickness is to be measured, and a large number of optical fibers are inserted into holes provided in the support 9. 10 are fixed at one end and bundled at the other end. One end of every optical fiber 10 is such that the fiber axis of the optical fiber 10 at one end is
It is fixed so as to face the point on the measurement target 6 where the film thickness is to be measured.

At the rear of the condenser 7, a separation lens 11 is arranged facing the other end of the optical fiber 10, and further includes an extraction pinhole plate 12, a collimator lens 13,
A plane diffraction grating 14, an imaging lens 15, an image intensifier 16, an imaging lens 17, and a linear image sensor 18 are arranged in this order.

Now, to explain the operation of the above-mentioned device, the white light emitted from the white light source 1 is focused onto the pinhole of the pinhole plate 3 by the condenser lens 2, and then collimated by the collimator lens 4. After that, the polarizing plate 5 produces only linearly polarized light with the vibration direction of the P-wave component or the S-wave component, which passes through the entrance window 8 of the condenser 7 and reaches the point to be measured on the measurement object 6. It is made to be incident.

The light incident on the measurement object 6 is reflected by the measurement object 6, and at this time, the spectral intensity can change due to an interference phenomenon. As described above, this change depends on the film thickness of the measurement object 6 if the incident angle of light and the refractive index of the measurement object are constant.

By the way, the light incident on the measurement object 6 is usually
Although it is regularly reflected, if the object to be measured has wrinkles, vertical movement, or inclination, the direction of reflection may change or the reflected light may be diffused. However, in this embodiment, a large number of optical fibers 10
Since a condenser 7 is used in which one end is arranged such that the fiber axis at one end faces the point to be measured on the measurement object 6, extremely efficient light reception is possible.

Now, the reflected light is guided to a classification lens 11 by an optical fiber 10, and further to an extraction pinhole plate 12. The separation lens 11 and the extraction pinhole plate 12 separate and extract only the components emitted from the other end of each optical fiber 10 in the vertical direction. Thus, the extracted outgoing light is made into parallel light by the collimator lens 13, guided to the plane diffraction grating 14, and split into spectra to form the same plane.
The light is separated into each wavelength.

The light emitted from the plane diffraction grating 14 is transferred to an image intensifier 15 by an imaging lens 15.
The light that is imaged on the image and has bright and dark areas caused by the interference phenomenon is amplified, and then the light that is focused on the image forming lens 17 is amplified.
An image is formed on the linear image sensor 18, and the intensity of the light for each wavelength is detected. From this detected intensity, the wavelength of the bright or dark area caused by the interference phenomenon is determined, and the film thickness of the measurement object 6 is calculated based on the above-mentioned equation as dependent on the wavelength.

2 to 4 show measurement examples using the apparatus of the above-described embodiment, that is, the relationship between the wavelength λ (nm) and the spectral intensity I. In each figure, a is a case where no polarizing plate is installed, b is a case where a polarizing plate is installed and only the P wave component is incident, and c is a case where a polarizing plate is installed and the S wave component is incident. This is the case where only the The objects to be measured were all polyester films, and the thickness measured using a stylus thickness meter was 4.84 μm in the one in Figure 2.
The one in Figure 3 is 14.7μm, and the one in Figure 4 is 25μm.
belongs to. In these examples, the actually detected spectral intensity is normalized by the spectral characteristics specific to the device, that is, the spectral characteristics specific to the light source, planar diffraction grating, linear image sensor, etc., and the spectral intensity received by the interference phenomenon is normalized. Only changes in intensity are extracted. Note that a 100W halogen lamp was used as the white light source. Furthermore, the size of the entrance window of the condenser, that is, the diameter of the measurement spot, was set to 8 mm. Plastic optical fibers with a diameter of 1 mm were used as the optical fibers, and 600 of them were arranged as shown in FIG.
In addition, a plane grating has a blaze wavelength.
A material with a wavelength of 750 nm and a groove count of 1200/mm was used. A 2048-bit CCD (Charge Coupled Device) element was used as the linear image sensor.

From FIGS. 2 to 4, when using the apparatus of the present invention using a polarizing plate, the intensity ratio between the bright part and the dark part of the spectral intensity, that is, the contrast, is greatly improved compared to the apparatus without using a polarizing plate. It can be seen that the detection accuracy of wavelengths in bright or dark areas is greatly improved. It should be noted here that the measurement result for a is a superposition of the measurement results for b and c with respect to the wavelengths of the bright and dark areas.

Effects of the Invention This invention provides a polarizer on the incident optical path of white light, or on the optical path reflected or transmitted by the measurement object, so that the polarization axis can be set in any linear polarization direction, that is, the vibration of P-wave component or S-wave component. As shown in the measurement example, when measuring the film thickness of a polymer film with optical anisotropy, the intensity of the bright and dark parts of the spectral intensity is It becomes possible to increase the ratio, that is, the contrast. Therefore, it is now possible to accurately detect the wavelengths of the bright and dark areas that contain film thickness information, making it possible not only to perform highly accurate film thickness measurements, but also to
It becomes possible to measure even relatively thick materials with optical anisotropy. This also means that the types of objects that can be measured expand.

[Brief explanation of drawings]

FIG. 1 is a schematic side view showing an apparatus according to an embodiment of the present invention, and FIGS. 2 to 4 are graphs showing different measurement examples. 1: white light source, 2: condenser lens, 3: pinhole plate, 4: collimator lens, 5: polarizing plate (polarizer), 6: measurement object, 7: condenser, 8: entrance window, 9: support , 10: optical fiber, 11: lens for classification, 12: pinhole plate for extraction, 13: collimator lens, 14: plane diffraction grating (spectroscope),
15: Imaging lens, 16: Image intensifier, 17: Imaging lens, 18: Linear image sensor.

Claims (1)

[Claims] 1. Parallel white light is made incident on the measurement target at a constant angle of incidence, the reflected light or transmitted light from the measurement target is guided to a spectrometer, and the film thickness of the measurement target is measured from the spectral intensity. An optical interference type film thickness measuring device characterized in that a polarizer is arranged on the incident optical path of the white light so that its polarization axis is in an arbitrary linear polarization direction. . 2 A device that makes parallel white light incident on the measurement target at a constant angle of incidence, guides the reflected light or transmitted light from the measurement target to a spectrometer, and measures the film thickness of the measurement target from the spectral intensity. and a polarizer on the reflection or transmission optical path of the white light,
An optical interference type film thickness measuring device characterized in that the polarization axis is arranged in an arbitrary linear polarization direction.