JPS61277026A - Method and apparatus for detecting polarization angle - Google Patents

Abstract

PURPOSE:To make it possible to detect the polarization angle of an object to be measured in a non-contact state with high accuracy, by a method wherein P-polarized light and non-polarized light or P-polarized light and S-polarized light are alternately change over to be incident to an object to be measured at the same incidence angle and the intensities of the reflected lights of both lights to calculate the ratio of both intensities and the incident angle minimizing said ratio is detected as a polarization angle. CONSTITUTION:A photoelectric converter element 22 respectively outputs a P-polarized light signal I'rp and a non-polarized light signal I'r proportional to the light receiving intensities of the P-polarized light and the non-polarized light incident while alternately changed over by a rotary disc 8 and both signals are inputted to a ratio operation circuit 23 which, in turn, operates the ratio I'rp/I'r. A min. value detection circuit 24 detects the min. value of the ratio I'rp/I'r inputted from the ratio operation circuit 23. Then, the incident angle minimizing the ratio of the intensities of reflected lights is detected as a polarization angle.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention is applicable to substances for which the refractive index of light is not known, and even when the refractive index is known, it is difficult to detect the angle between the measurement light and the object to be measured. The present invention relates to a method for detecting the polarization angle of a flexible substance such as a resin film, and a detection device therefor.

(Prior art) In recent years, with the advancement of film manufacturing technology, it has become possible to manufacture ultra-thin films on the order of several microns.
In order to control the film thickness of Shishisha, it is necessary to detect the film thickness with high accuracy.

To detect the thickness of a relatively thick film, infrared rays (hereinafter referred to as measurement light) having a wavelength in the characteristic absorption band of the material forming the object whose thickness is to be measured (hereinafter referred to as measurement light) and characteristic absorption band A reference infrared ray (hereinafter simply referred to as reference light) having a wavelength appropriately separated from the reference infrared light is incident on the measured object from the perpendicular direction, and the transmitted intensity of the measurement light transmitted through the measured object is calculated from the reference infrared light. The absorbance of the measurement light by the object to be measured is determined from the transmitted intensity, and the thickness of the object to be measured is calculated based on this absorbance. It is difficult to ensure detection accuracy that can cope with ultra-thin films.

This is largely influenced by interference caused by multiple reflected light within the object to be measured such as a film.

That is, when the film is thick, the phase of the light flux is randomized within the film, reducing interference, and the absorbance of the measurement light is sufficiently large, so that the influence of interference due to multiple reflections can be virtually ignored. This is because when the thickness becomes thinner, the influence of interference becomes impossible to ignore, the S/N ratio decreases, and the detection accuracy decreases.

The applicant makes P-polarized light incident on the object to be measured while changing the incident angle, detects the angle at which the reflected light reaches the minimum value, and measures the thickness of an ultra-thin film using the transmitted light at that angle. A device for this purpose was disclosed in Japanese Patent Application No. 183636/1983.

However, since this device detects the polarization angle only by changes in the intensity of the reflected P-polarized light, the intensity of the reflected light changes due to the surface condition of the object to be measured and the optical axis. This was insufficient for accurate detection of corners.

The polarization angle is defined as the angle at which the reflected light is zero when a P-polarized ray is incident at that angle, in other words, all of the incident light is transmitted into the material, and generally the refractive index of the object is n1 When the polarization angle is θb, the relationship between the refractive index n and the polarization angle θb is θb = tan7' n - (1)
It is known that there is an open door, and conventionally, the polarization angle θb of an object has been calculated from the refractive index n of the object using the above equation (1).

However, in order to calculate the polarization angle θb from the refractive index n of the object to be measured using the above equation (1), the refractive index n of the object to be measured must be known. Moreover, the measurement site, ie, the light irradiation site, must have a polarization angle with respect to the measurement light. However, for flexible items such as resin films, items that run during the production process, or items that have fluctuations due to running, their polarization angle θb or refractive index n
It has been practically difficult to detect this in real time without contact.

On the other hand, according to the already mentioned definition of polarization angle (9), when the refractive index n is unknown, the angle of incidence of the P-polarized light beam is varied according to the definition of polarization angle, and the angle at which the reflected light intensity is minimized is detected. For example, it is conceivable that this angle can be detected in real time in a non-contact manner as a polarization angle.

The following considerations were made regarding the detection of the polarization angle.

Generally, as shown in FIG. 2, an incident angle θ
When incident light Ii is incident at point i, part of it is reflected by the surface of object M as reflection *; r r, and part of it becomes transmitted light IO that passes through object M. Further, a part of this transmitted light Io is repeatedly reflected within the object M and is emitted from the object M as reflected light I'r and transmitted light I'o.

Now, when the incident angle θi is changed for natural light, P-polarized light, and S-polarized light, the reflectance of these natural light, P-polarized light, and S-polarized light becomes curves R, Rp, and Rs, respectively in FIG.
Changes as shown in . As can be seen from the curve Rp in Figure 3, it is optically known that when P-polarized light is incident on the material at a polarization angle θb, the reflectance becomes zero and no reflection occurs (reverse In other words, this gives the definition of the polarization angle.) Therefore, to measure the polarization angle θb, P polarized light is incident on the object M to be measured, and the P polarization is adjusted so that the reflected light Ir becomes zero. Adjust the incident angle θi of polarized light-t-4-11
-1'l""/7'lλI) + Can A i #% Q
ff1-! -(h 4 h, ?1m speed is possible.

Theoretically, it is possible to measure the polarization angle θb as described above, but if the surface of the object M is not flat, or if there is dirt on the surface or turbidity inside, it is possible to measure the polarization angle θb. It is difficult to accurately detect the incident angle θi at which the reflected light Ir from the ray becomes zero. This is because the curvature, unevenness, dirt, etc. on the surface of the object M act as error elements when detecting the polarization angle θb, and the incident angle θi changes and the reflectance changes depending on the incident position of the P-polarized light incident on the object M. This is because the true polarization angle θb cannot be detected.

By the way, with respect to the above-mentioned noise factor of the object M, the reflectance of natural light and S-polarized light shown by curves R and Rs in FIG. 3 change similarly.

Therefore, it is considered that the above noise can be canceled out by taking the ratio of the reflectances of P-polarized light and natural light or P-polarized light and S-polarized light. The ratio Rp/R of the reflectance of P polarized light and natural light and the ratio R of the reflectance of P polarized light and S polarized light with respect to the incident angle θi
The results of determining p/Rs are shown in Fig. 4 as curves Rp/Rs, respectively.
Indicated by R and Rp/Rs.

From FIG. 4 above, when θi=θb, the ratios R1)/R and Rp/Rs also become zero, and this ratio Rp/R or R
If an incident angle θi at which p/Rs becomes zero is detected, this incident angle θi gives the polarization angle θb. In this case, the reflectance R1) of P-polarized light changes depending on the surface condition of the object M, and the reflectance R1) of natural light and the reflectance Rs of S-polarized light change at the same ratio, so the ratio R1)/R or Rp/
It is clear that the incident angle θi at which Rs becomes zero can be detected with a sufficiently high S/N ratio.

Furthermore, as is clear from Figure 3, in the region of 0 < θi < θb, as the incident angle θi increases, the reflectance Rp decreases toward zero, whereas the reflectances R and Rs increase. Therefore, the ratios Rp/R and Rp/Rs are the fourth
As can be seen from the figure, as the incident angle θi increases, the ratios Rp/R and Rp/Rs rapidly decrease. on the other hand,
In the region of θb<θi, the reflectance Rp increases rapidly as the incident angle θi increases, but the reflectances R and Rs also increase rapidly with almost the same tendency as the reflectance Rp, so the ratio Rp
/R and R1)/Rs also increase rapidly, similar to the reflectance R[). Therefore, it can be seen that when the incident angle θi approaches the polarization angle θb, the ratios Rp/R and Rp/Rs also rapidly approach zero. Thereby, the polarization angle θb can be detected sharply.

(Purpose of the Invention) The present invention has been made based on the above considerations, and does not require detection of the refractive index of the object to be measured. It is an object of the present invention to provide a polarization angle detection method and a polarization angle detection device that can detect the polarization angle of an object to be measured with high accuracy in a non-contact manner without being substantially influenced.

(Structure of the Invention) The first invention of the present application is to alternately switch between a P-polarized light beam and an unpolarized light beam or a P-polarized light beam and an S-polarized light beam and make them incident at the same incident angle on the object to be measured whose polarization angle is to be measured. Detect the reflected light intensity of the two light rays, calculate the ratio of both intensities, and detect the incident angle at which the ratio of the reflected light intensities is the minimum as the polarization angle. It is characterized by

The second invention of the present application also provides a light source, a polarizer that polarizes the light emitted from the light source into P-polarized light or S-polarized light, and a P-polarized light beam and an unpolarized light or P-polarized light beam on an object to be measured whose polarization angle is to be measured. A switching means for alternately switching between a polarized light beam and an S-polarized light beam, an incident angle adjustment means for continuously changing the incident angle of light, and a switching means for changing the incident angle of the light beam by alternating between the polarized light beam and the S-polarized light beam, and an incident angle adjustment means for continuously changing the incident angle of the light beam, and reflected light detection means for detecting the intensity of reflected light; timing generation means for detecting the timing of switching between P-polarized light and non-polarized light or S-polarized light; and P-polarized light according to a timing signal output from the timing generation means. A ratio calculation means for storing the reflection intensity of the light beam and the reflection intensity of the unpolarized or S-polarized light beam and calculating the ratio of the two, and detecting the minimum value of the ratio output from the ratio calculation means corresponding to the above-mentioned incident angle. The invention is characterized in that it is equipped with a means for detecting the minimum value.

(Example) Hereinafter, the present invention will be specifically explained with reference to Examples.

As shown in FIG. 1, this polarization angle detection device includes a measurement section A and a calculation section B, and a display and recording section C displays and records the detected polarization angle.

The measuring section A alternately switches between P-polarized light and unpolarized light or P-polarized light and S-polarized light to enter the object to be measured 1 held in a sample holder (not shown), and the angle of the sample holder is indicated by arrows A and A, In order to detect the intensity of reflected light from the object to be measured l while changing the incident angle of P-polarized light and non-polarized light or P-polarized light and S-polarized light incident on the object to be measured l as shown in belongs to.

The light incident on the object to be measured 1 may be a combination of P-polarized light and non-polarized light or a combination of P-polarized light and S-polarized light, but in the following, the light incident on the object to be measured 1 is P-polarized light. and non-polarized light.

The measuring section A is equipped with a light source 2, and the light emitted from the light source 2 enters a lens 3 and is converted into a parallel beam.
is projected on. The light projected onto beam splitter 4 is 2
The light is divided into two optical paths, one of which enters the lens 5, and the other enters the lens 7 from the reflecting mirror 6. The light incident on the lens 5 is focused into a spot by the lens 5 and projected onto a rotating disk (chopper) 8. Also, lens 7
Similarly, the light incident on the lens 7 is focused into a spot by the lens 7 and projected onto the rotating disk 8.

The rotating disk 8 is made of a light-shielding material, and has a slit 8 at a position where the light focused into a spot by the lens 5 or the light focused into a spot by the lens 7 is projected.
a is formed. The rotating disk 8 is rotationally driven by an electric motor 9. Therefore, at the timing when the light focused by the lens 5 is transmitted through the slit 8a, the light focused by the lens 7 is blocked by the rotating disk 8, and conversely, the light focused by the lens 7 is blocked by the rotating disk 8. At the timing when the light is passing through the slit 8a, the light collected by the lens 5 is blocked by the rotating disk 8. That is, the light condensed by lens 5 and the light condensed by lens 7 are alternately switched by rotating disk 8 and are incident on lenses 10 and 11 arranged behind this rotating disk 8. Note that the timing of the switching is determined by a notch (not shown) provided on the outer periphery of the rotating disk 8, a timing detector 12 such as a photointerrupter placed on the outer periphery of the rotating disk 8, The timing signal is detected by the timing generation circuit 13 which generates a timing signal based on the signal output from the timing detector 12.

The light that passes through the slit 8a of the rotating disk 8 and enters the lens 1° is converted into parallel rays and enters the polarizer 14.
P-polarized light is emitted from this polarizer 14 to a beam splitter 15. On the other hand, the light that passes through the slit 8a of the rotating disk 8 and enters the lens 11 is converted into parallel rays and is reflected by the reflecting mirror 1.
6 and enters the beam splitter 15. This reflector 1
6 to Beehup 11+, t1ζ1-λ+1↓1) Howbei-lazypuu2 The above P-polarized light and non-polarized light are alternately switched by the rotation of the rotating disk 8, and are transmitted from the beam splitter 15 to the lens 17. The light enters the beam and is focused into a spot. The P-polarized light and the non-polarized light that have exited the lens 17 are incident on the object to be measured l through the calibration 18 and the lens 19, and part of them is reflected by the object to be measured i. Hereinafter, the P-polarized light and the non-polarized light incident on the object to be measured l will be referred to as Iip and Ii, respectively.

The P-polarized light and non-polarized light reflected from the above-mentioned object to be measured are:
The light enters the photoelectric conversion element 22 through the lens 20 and the bandpass filter 21. The bandpass filter 21 may be of a suitable transmission wavelength band in which non-polarized optical interference among the reflected light can be practically ignored. The photoelectric conversion element 22 receives a P-polarized light signal r″r that is alternately switched by the rotating disk 8 and is proportional to the received light intensity of the incident P-polarized light and non-polarized light.
p and an unpolarized signal 1'r, respectively. As the photoelectric conversion element 22, a photodiode, a phototransistor, etc. can be used. Furthermore, when detecting the polarization angle θb using infrared rays, a pyromagnetic infrared sensor can be advantageously used.

It is not limited to this.

The P-polarized light signal I''rp and the non-polarized light signal I'r are manually input to a ratio calculation circuit 23 of the calculation section B, which will be described next.

Note that the P-polarized signal I'rp and the non-polarized signal I'r
may include a dark current component va depending on the type of element used as the photoelectric conversion element 22, the circuit used, etc.

In this case, the detection signal is input to the ratio calculation circuit 23 through a dark current cancellation circuit (not shown), and is input to the ratio calculation circuit 23 after canceling the dark current component va.

Next, the configuration of the calculation section B will be explained.

The calculation section B includes a ratio calculation circuit 23 and a minimum value detection circuit 24.
Consisted of.

The ratio calculation circuit 23 receives P input from the photoelectric conversion element 22.
After amplifying the polarized light signal I'rp and the non-polarized signal 1'r, the P-polarized signal I'rp and the non-polarized signal I'r are individually sampled and held at the timing of the timing signal input from the timing generation circuit 13. , calculates the ratio I'rp/1'r between the individually sampled and held P-polarized signal Birp and the non-polarized signal I'r.

On the other hand, the minimum value detection circuit 24 moves the object to be measured l to an arrow A.
, , When the incident angle θi of P-polarized light and non-polarized light is changed by rotating as shown by A2, the minimum value of the ratio 1'rp/I'r manually input from the ratio calculation circuit 23 is detected.

The display section C includes a first display device 25 and a second display device 26.

The first display device 25 displays the minimum value of the ratio I'rp/I'r detected by the minimum value detection circuit 24 during the first scan of rotating the object to be measured I in one direction and its incident angle θi. Display. In addition, the second display device 26 displays all of the ratio 1'rp/I'r and the corresponding incident angle θi calculated in the return scan of the object to be measured l from the first scan to the original position. indicate.

If the calculation result of the ratio I'rp/I'r between the P-polarized signal I'rp and the unpolarized signal 1'r detected as described above is the minimum value, R1)/R will also be the minimum value. .

That is, the proportion of P-polarized light and non-polarized light reflected by the object to be measured 1 that reach the photoelectric conversion element 22 is α.
and β, I'rp = αRp I ip −” (2
)I'r -βRI i -(3), and when the P-polarized light and the non-polarized light are incident on the object to be measured l under the same conditions, they can be regarded as α-β.

Therefore, from equations (2) and (3), Rp/Rocr
'rp/I'r - (4). From this equation (4), the ratio of the P-polarized signal I'rp and the unpolarized signal I'r output from the photoelectric conversion element 22 is 1'rp/
If the calculation result of I'r is the minimum, R[)/R will also be the minimum value. As a result, the ratio 1 output from the ratio calculation circuit 23
The polarization angle θb can be detected from the incident angle θi at which the calculation result of 'rp/I'r is the minimum.

When the polarization angle θb of the glass (BK7) was measured using the polarization angle detection device shown in FIG. 1, it was possible to detect the polarization angle θb with an accuracy of ±0.1 degree or less. In this measurement, the measurement wavelength range was 750 to 100 nm, and a silicon photodiode was used as the photoelectric conversion element.

In the above embodiment, P-polarized light and non-polarized light were used, but P-polarized light and S-polarized light may also be used.

When using P-polarized light and S-polarized light, use lens 1 in Figure 1.
A polarizer for S-polarized light (not shown) may be placed after the polarizer 1. Moreover, by rotating the polarizer 14 shown in FIG. 1, P-polarized light and S-polarized light can be generated. In this way, the rotating disk 8, the beam splitter 4.15, etc. can be omitted, and the optical system of the polarization angle detection device is simplified.

The polarization angle detection device shown in FIG. 1 can also be applied to an infrared thickness meter or spectroscopic analysis that detects the thickness of one layer of a resin film or the like using characteristic absorption in the infrared region.

In addition, the ratio calculation circuit 23 and the minimum value detection circuit 24 are replaced with a microcomputer, and in addition, the angle of the light incident on the object to be measured l, such as the angle of the sample holder of the object to be measured l, is controlled by a pulse motor or the like. By doing this, you can also automatically set the polarization angle. The amount of infrared rays transmitted through the object to be measured l is detected by a light receiving section 31 equipped with a spectroscope and a photoelectric detector, and based on this amount of transmission, the thickness of the object to be measured l is calculated by the calculation section 32. Detected.

Regarding the old light receiving section 31 and calculation section 32, the patent application No. 59-80976 filed by the present applicant (name of the invention "
This method is explained in detail in "Infrared Thickness Gauge").

(Effects of the Invention) According to the present invention, noise components that change at the same rate are canceled out and are not affected by polarization angle detection. S at the time of detection
/N ratio can be increased, and polarization angle detection accuracy can be improved.

Furthermore, according to the present invention, the polarization angle of a flexible measurement object can be detected without contact by simply projecting light onto the measurement object, and from this detected polarization angle, conversely,
It is also possible to detect the refractive index of the object to be measured.

Further, according to the present invention, automatic setting of the polarization angle can be easily performed by combining with an incident angle adjustment means for changing the incident angle of light incident on the object to be measured or a setting angle adjustment means for the object to be measured. realizable. In addition, the applicant filed a patent application in 1973.
By combining with the follow-up control means disclosed in Nos. 8 to 183,636, the polarization angle or refractive index can be detected in real time without contact.

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram showing the principle configuration of an embodiment of the polarization angle detection device according to the present invention, Fig. 2 is an explanatory diagram of reflected light and transmitted light of incident light incident on an object, and Fig. 3 is an explanatory diagram showing the reflected light and transmitted light of incident light incident on an object. FIG. 4 is an explanatory diagram showing the relationship between angle and reflectance, and FIG. 4 is an explanatory diagram of changes in the ratio of P-polarized light to non-polarized light and P-polarized light to S-polarized light with respect to the incident angle. 1... Object to be measured, 8... Rotating disk, 8a.
...Slit, 9...Electric motor,! 2...
・Timing detector, 13... Timing generation circuit, 14... Polarizer, 22... Photoelectric conversion element,
23... Ratio calculation circuit, 24... Minimum value detection circuit.

Claims (2)

[Claims] (1) Alternately switch between P-polarized light and unpolarized light or P-polarized light and S-polarized light so that they are incident on the object to be measured whose polarization angle is to be measured at the same angle of incidence, and then change the angle of incidence. A polarization angle detection method comprising: detecting the reflected light intensities of two light rays, calculating the ratio of both intensities, and detecting the incident angle at which the ratio of the reflected light intensities is the minimum as the polarization angle. (2) A light source and the light emitted from this light source as P polarized or S
A polarizer that polarizes the light, a switching means that alternately switches between P-polarized light and non-polarized light or P-polarized light and S-polarized light to be incident on the object to be measured whose polarization angle is to be measured, and a switching means that changes the incident angle of the light continuously. an incident angle adjusting means for changing the angle of incidence; a reflected light detecting means for detecting the intensity of reflected light of P-polarized light, non-polarized light or S-polarized light incident on the object to be measured; and a timing generation means for detecting the timing of switching, and storing the reflection intensity of the P-polarized light beam and the reflection intensity of the unpolarized light or the S-polarized light according to the timing signal output from the timing generation means, and calculates the ratio of the two. Ratio calculation means;
A polarization angle detection device comprising minimum value detection means for detecting the minimum value of the ratio outputted from the ratio calculation means in accordance with the incident angle.