JP3714490B2 - Surface condition measurement method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a surface state measuring method for measuring a surface state of a transparent substrate such as a glass substrate used for a liquid crystal display and having a comparatively smooth surface shape.
[0002]
[Prior art]
In recent years, in a liquid crystal display device as a display device, it is required that the glass substrate used for the surface has good flatness. For this reason, when manufacturing a liquid crystal display device, it is necessary to inspect the surface state of the glass substrate to be used.
As methods for optically measuring the surface of this transparent body without being affected by reflection from the back surface, the following methods have been proposed.
First, a method using the difference in optical path.
Second, a method using polarized light.
A third method uses a low coherency light source.
[0003]
As a method using the difference in the first optical path, there is a distance measuring device using a triangulation method as shown in FIG.
After passing through the lens 52, the light from the light source 51 is reflected by the front and back surfaces of the glass substrate 53 to be measured, collected by the lens 54, and on a PSD (Position Sensitive Device) 55 from both the front and back surfaces. Is reflected.
However, in this state, the PSD 55 determines the position of the center of gravity of the intensity of the light incident on the light receiving surface, so that an accurate distance to the surface of the glass substrate 53 cannot be determined.
Here, the slit 56 prevents the reflection from the back surface from reaching the PSD 55, so that the distance to the surface of the glass substrate 53 can be measured with high accuracy.
[0004]
Next, as a method of using the above-mentioned second polarized light, there is a distance measuring device by a triangulation method using a polarizing plate as shown in FIG.
In the figure, 61 is a polarizing plate disposed on the light source side, 62 is a polarizing plate disposed on the light receiving side, and other reference numerals are the same as those in FIG.
Thus, this apparatus utilizes the fact that the polarization state of the reflected light from the surface of the glass substrate 53 is different from the polarization state of the reflected light from the back surface of the glass substrate 53. That is, the polarizing plates 61 and 62 prevent the reflected light from the back surface of the glass substrate 53 from entering the PSD 55 so that the distance to the surface of the glass substrate 53 is accurately measured.
[0005]
Further, as a method of using the third low-coherency light source described above, there is a Twiman-Green interference type flatness meter as shown in FIG.
The light from the light source 71 is collimated by the lens 72, passes through a bandpass filter 73 having an extremely narrow band, and is divided into two by a beam splitter 74.
One of the two divided by the beam splitter 74 is reflected by the glass substrate 75 to be measured, condensed by the lens 76, and taken into the image sensor 77.
The other half divided by the beam splitter 74 is reflected by the reference surface 78, the optical path is changed by the beam splitter 74, the light is condensed by the lens 76, and taken into the image sensor 77.
[0006]
Here, the difference in the optical path length is twice the difference between the distance L1 from the beam splitter 74 to the glass substrate 75 and the distance L2 from the beam splitter 74 to the reference surface 78.
Then, the flatness of the glass substrate 75 is measured by analyzing the interference fringes caused by the optical path length difference and the light wavelength.
If twice the difference between the distance L1 and the distance L2 is made shorter than the coherency of the light source light that has passed through the bandpass filter 73, the reflected light from the surface of the glass substrate 75 causes interference, but the back surface of the glass substrate 75 The reflected light from will not cause interference.
For this reason, according to this flatness meter, it is possible to perform measurement without the influence of reflection from the back surface of the glass substrate 75.
[0007]
[Problems to be solved by the invention]
Conventionally, the configuration as described above has the following problems.
First, in the method using the first optical path difference, it is necessary to adjust the position and size of the slit in advance according to the measurement target, which is troublesome.
This can be used as long as it is a line measurement type such as a point measurement type or a light cutting method. However, the surface measurement cannot be used because the optical paths of reflection from the front surface and reflection from the back surface overlap.
Further, the method using the second polarized light cannot be used unless there is a difference in the polarization state of the reflected light from the front surface and the back surface of the glass substrate.
If a metal thin film or the like is formed on the back surface, a difference occurs in the polarization state, but there is no difference in a normal glass substrate.
[0008]
On the other hand, the third method using a low-coherency light source is widely used for interference measurement targeting a transparent body and has been put into practical use.
However, it is necessary to match the distance L1 and the distance L2 with high accuracy, and the adjustment is complicated and time-consuming.
In addition, this is not applicable except for optical measurement using interference.
[0009]
The present invention has been made to solve the above-described problems, and an object thereof is to enable accurate measurement of the flatness of the surface of a transparent body such as a glass substrate.
[0010]
The surface state measuring method of the present invention is a method for optically measuring the surface state of a substrate made of a transparent material, a light source, a light projecting lens for converting light from the light source into parallel light having a predetermined area, A grating that is provided between the light projecting lens and the substrate made of a transparent material and that allows the light transmitted through the light projecting lens to enter the substrate made of the transparent material and to transmit the reflected light from the substrate made of the transparent material; A light receiving lens that collects the light reflected from the substrate, a slit that passes only the diffracted light of the specific order of the light that has passed through the light receiving lens, and moire fringes that incorporate the diffracted light of the specific order that has passed through the slit An image sensor that picks up an image and an optical sensor that is placed on the optical path from the light source to the slit and transmits only the wavelengths of the light that is difficult to transmit through the substrate made of a transparent material. The surface condition of the substrate made of a transparent material using a surface measuring device and a band-pass filter that has been adapted to measure optically.
For this reason, most of the light incident from the substrate surface made of the transparent material is attenuated and disappears.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
Embodiment 1
FIG. 1 is a configuration diagram showing the configuration of a surface shape measuring apparatus according to Embodiment 1 of the present invention.
In the figure, 1 is a light source composed of a high-pressure mercury lamp, 2 is a projection lens that converts light from the light source 1 into parallel light having a predetermined area, 3 is a grating, 4 is a glass substrate to be measured, and 5 is a glass substrate 4. A light-receiving lens that collects the light reflected from the light, 6 a band-pass filter that selectively transmits light having a wavelength of 313 nm, 7 a slit that allows only 0th-order diffracted light to pass through, and 8 a two-dimensional image sensor. Reference numeral 9 denotes an arithmetic processing unit.
Here, in the optical components such as the light projecting lens 2 and the light receiving lens 5 that transmit light, a material such as synthetic quartz that transmits light having a wavelength of 313 nm is used.
[0012]
The light from the light source 1 that has passed through the grating 3 projects a grating-like shadow onto the surface of the glass substrate 4. Then, the light reflected including the shadow from the surface of the glass substrate 4 passes through the grating 3.
As a result, the shadow of the grating 3 projected onto the glass substrate 4 is superimposed on the grating 3 and reflected. The light reflected and transmitted through the grating 3 again includes the surface shape information of the glass substrate 4. For this reason, an image reflected from the glass substrate 4 and transmitted through the grating 3 is formed with moire fringes of contour lines corresponding to the unevenness of the surface.
[0013]
This image is taken into the image sensor 8 through the light receiving lens 5 and the band pass filter 6. At this time, it is necessary to arrange the grating 3 at an angle different from that of the glass substrate 4 so that the reflected light from the grating 3 does not enter the image sensor 8.
By the way, the light passing through the light receiving lens 5 includes diffracted light of 0th order, ± 1st order, ± 2nd order,... It becomes heavy and blurry. For this reason, it is necessary to prevent diffracted light other than the 0th order from passing through the slit 7.
In addition, not only 0th order diffracted light but also other order diffracted light such as ± 1st order diffracted light may be used. In this case, diffracted light other than the specific order is not allowed to pass. deep.
[0014]
As described above, the moire fringe image captured by the image sensor 8 is captured and stored in the arithmetic processing unit 9.
The arithmetic processing unit 9 uses a plurality of moire fringe images obtained by changing (phase shifting) the distance between the lattice 3 and the glass substrate 4 by 1/4 of the contour line interval of the moire fringes. The amount of waviness on the surface of the substrate 4 is calculated.
In the above description, the interval between contour lines is changed by ¼. However, the present invention is not limited to this. For example, it may be changed by ３, or may be changed at a predetermined interval. That's fine.
[0015]
Here, the glass substrate 4 to be measured is used as, for example, a member of a liquid crystal display. Usually, as shown in FIG. 2A, the light transmittance (plate thickness: 1 mm) from around a wavelength of 360 nm. ) Starts to decrease, and at a wavelength of 300 nm, it becomes 10% or less.
Therefore, if light having a wavelength with low transmittance of the glass substrate 4 is used for the above-described measurement, the reflected light from the back surface of the glass substrate 4 is not taken into the image sensor 8.
[0016]
That is, by using a light source that uses light having a shorter wavelength that is difficult to transmit through the transparent body to be measured, optical measurement of the surface state is performed without being affected by the reflected light from the back surface of the transparent body. It becomes possible.
Glass that is normally used that only needs to transmit the visible light region contains B ₂ O ₃ or P ₂ O ₅ in addition to SiO ₂ , so that light with a short wavelength in the ultraviolet region is difficult to transmit. Yes.
[0017]
For example, in the surface shape measuring apparatus of FIG. 1, only the light having a wavelength of around 313 nm is guided to the image sensor 8 by the bandpass filter 6 so as not to be affected by the reflected light from the back surface of the glass substrate 4. I have to.
It should be noted that in the surface shape measuring apparatus of FIG. 1, light having a wavelength to be used is sufficiently transmitted in optical components such as the light projecting lens 2, the light receiving lens 5, and the image capturing portion of the image sensor 8. It is necessary to use what to do.
[0018]
For example, in a normal silicon-based CCD image sensor (image sensor), the sealing glass (window) of the module hardly transmits 313 nm light.
Therefore, if synthetic quartz having optical characteristics shown in FIG. 2B is used as the window material, the light reaches the light receiving surface on which the photodiodes of the CCD image sensor are arranged. As shown by the solid line in FIG. 3, with 313 nm light, a sensitivity of about 30% of the sensitivity at the peak wavelength can be obtained, and the image sensor 8 can be used.
By using an image sensor using a silicon-based light receiving element such as a CCD image sensor, the apparatus can be configured at a lower cost.
Note that an image pickup tube may be used as the image sensor 8 and the sensitivity can be made higher. However, since this is expensive, an increase in cost is inevitable in configuring the apparatus.
[0019]
By the way, if the wavelength to be used is, for example, to reduce the reflection from the back surface to a quarter of the light intensity of the reflection on the front surface, the glass substrate having the optical characteristics shown in FIG. In this case, the thickness may be about 320 nm or less if the plate thickness is 0.7 mm, and about 325 nm or less if 1.1 mm.
That is, when the transmittance of light of a certain wavelength when passing through 1 mm of glass is T, the transmittance of the glass having a plate thickness x (mm) is represented by T ^x .
Therefore, in the above-described case, a wavelength ^satisfying T ^x ≦ ¼ may be obtained.
For example, in the case of a plate thickness of 0.7 mm, the reflection from the back surface of the light irradiated on the glass substrate is transmitted twice through the glass substrate itself before and after the back surface reflection compared to the reflection on the surface. And that light is incident at an angle of 45 degrees, x = 0.7 × 2 ÷ sin45 °, which is about 2 (mm).
Therefore, the wavelength at which T ² = ¼ is about 320 nm, and the above is derived.
[0020]
Hereinafter, the results of measuring the surface state (flatness) of a glass plate having a thickness of 1.1 mm having the optical characteristics shown in FIG.
Here, the incident angle of light was set to 60 °. Therefore, in the light taken into the image sensor 8, the reflection from the back surface of the glass plate (glass substrate 4) is about 3.5% with respect to the reflection from the front surface.
FIG. 4 is an explanatory diagram showing the measurement result of the flatness, and the undulation of the surface of the glass substrate 4 is represented by contour lines. In the figure, the height of the contour line is shown in μm units on the right side of the contour map.
[0021]
In FIG. 4, (a) shows the result of using light having a wavelength of 313 nm for measurement by the band-pass filter 6 as described above.
On the other hand, FIG. 4B shows the result of measurement without using the bandpass filter 6.
In FIG. 4B, there is a thick vertical black line in the lower left or the like, and this is the influence of reflection from the back surface of the glass substrate 4.
[0022]
Moire fringes are also formed by reflection from the back surface of the glass substrate 4, but if the band pass filter 6 is not used, it overlaps with the moire fringes formed on the surface and is taken into the image sensor 8. For this reason, in the phase shift calculation process, the phase cannot be calculated and the above-described black line is generated, and the accurate measurement cannot be performed.
Here, in the measurement without using the band-pass filter 6, when antireflection measures were taken on the back surface of the glass substrate 4, the same result as shown in FIG. 4A was obtained. As an anti-reflection measure, there is a method of forming a light absorption film on the back surface of the glass substrate 4.
[0023]
In the above embodiment, the bandpass filter is arranged immediately before the image sensor. However, the present invention is not limited to this, and the bandpass filter may be arranged immediately after the light source.
However, in this case, if the band-pass filter is arranged, the light cannot be seen and it becomes difficult to adjust the optical path. Therefore, it is necessary to arrange the band-pass filter after adjusting the optical path.
Further, as the light source, for example, a hollow cathode lamp that emits light having a wavelength of 320 nm or less may be used. In this case, it is not necessary to use a bandpass filter.
However, in this case, as described above, since the light cannot be seen, it is difficult to adjust the optical path.
[0025]
【The invention's effect】
As described above, in the present invention, light having a wavelength in a region that is difficult to transmit through the transparent material is used for measurement.
In other words, light having a wavelength equal to or less than a wavelength at which the reflected light intensity from the back surface is, for example, ¼ of the reflected light intensity of the surface due to the light incident on the substrate surface is used for measurement.
For this reason, according to this invention, it becomes possible to eliminate the influence of the reflection from the back surface of the substrate, and there is an effect that the flatness of the surface of a transparent body such as a glass substrate can be accurately measured.
[Brief description of the drawings]
FIG. 1 is a configuration diagram showing a configuration of a surface shape measuring apparatus according to an embodiment of the present invention.
FIG. 2 is a characteristic diagram showing optical characteristics of normal glass and synthetic quartz.
FIG. 3 is a characteristic diagram showing sensitivity characteristics of a CCD image sensor.
FIG. 4 is a contour map showing the results of measuring flatness.
FIG. 5 is a schematic configuration diagram of a distance measuring apparatus using a conventional triangulation method.
FIG. 6 is a schematic configuration diagram of another distance measuring device using a conventional triangulation method.
FIG. 7 is a block diagram showing a schematic configuration of a Twiman-Green interference type flatness meter.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Light source, 2 ... Projection lens, 3 ... Grating, 4 ... Glass substrate, 5 ... Light receiving lens, 6 ... Band pass filter, 7 ... Slit, 8 ... Image sensor, 9 ... Arithmetic processing part.

Claims (1)

In a method for optically measuring the surface state of a substrate made of a transparent material,
A light source;
A light projection lens that converts the light from the light source into parallel light having a predetermined area;
A grating that is provided between the light projecting lens and a substrate made of a transparent material, and that allows the light transmitted through the light projecting lens to enter the substrate made of the transparent material and transmit the reflected light from the substrate made of the transparent material ;
A light receiving lens that collects light reflected from the substrate made of a transparent material through the grating;
A slit that passes only the diffracted light of a specific order of the light transmitted through the light receiving lens;
An image sensor that takes in a moiré fringe image by taking in a diffracted light of a specific order that has passed through the slit,
A transparent material using a surface measuring device that is disposed on an optical path from a light source to a slit and has a band-pass filter that transmits only a wavelength of a region that is difficult to transmit through a substrate made of a transparent material among light emitted from the light source A surface state measuring method, comprising: optically measuring a surface state of a substrate comprising :

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