JP5482195B2 - Retardation measuring device - Google Patents

Description

  The present invention relates to an apparatus for measuring retardation of a birefringent film or the like, and relates to a retardation measuring apparatus suitable for use in on-line measurement.

Retardation is used as an index for evaluating the characteristics of a film having birefringence. Retardation is a physical quantity defined by the following formula (1). Here, δn is the difference in refractive index and d is the film thickness. Since the direction of birefringence and the retardation value vary depending on the stretch ratio of the film, the quality in the film production process can be maintained by managing the retardation.
Retardation = δn × d (1)

  FIG. 4 shows the configuration of a conventional retardation measuring device. In FIG. 4, a polarizing plate (polarizing filter) 11, a sample 12 for measuring retardation, and a light detecting plate (light detecting filter) 13 are arranged in this order. White light emitted from the light source 10 passes through the polarizing plate 11, the sample 12, and the analyzer plate 13 and enters the spectroscope 14. The optical axis of the analyzer plate 13 is arranged so as to be parallel to the optical axis of the polarizing plate 11.

  The spectroscope 14 splits the incident light to calculate a spectral spectrum, and outputs the spectral spectrum to the calculation unit 15. The calculation unit 15 calculates retardation from the input spectrum and outputs it.

  The spectroscopic spectrum calculated by the spectroscope 14 has a periodic interference waveform with peaks and valleys, and the period of this interference waveform is proportional to the retardation. The computing unit 15 computes a power spectrum by performing Fourier transform on the input spectral spectrum, searches for a peak appearing in the power spectrum, and calculates a retardation value from the position of this peak.

  In order to measure retardation, it is necessary to include interference fringes of one period or more in the spectrum. The retardation value calculated from the spectral spectrum including one period of interference fringes is the minimum retardation value. When a visible light source is used as the light source 10, the measurement lower limit is about 1 μm.

  The amplitude of the interference fringes varies depending on the angle formed by the optical axis of the polarizing plate 11 and the optical axis of the sample 12. That is, the amplitude becomes 0 when the angle of the optical axis of the polarizing plate 11 and the sample 12 is 0 degree or 90 degrees, and becomes maximum when the angle is 45 degrees.

  When the amplitude of the interference fringes becomes 0 or 90 degrees, the retardation cannot be measured. For this reason, the sample 12 is fixed to the rotary stage, and the rotary stage is rotated to select and measure the point where the amplitude becomes maximum.

  Patent Document 1 describes a birefringence evaluation apparatus that can measure retardation. The birefringence evaluation apparatus according to Patent Document 1 has a configuration similar to that shown in FIG. 4, where a polarizing plate, a sample, and an analyzer plate are arranged, white light is irradiated, and a spectrum of transmitted light that has passed through the polarizer, sample, and analyzer plate is obtained. Create with a spectrometer. Since the intensity of the transmitted light changes in the form of a cosine wave of retardation, the retardation value is calculated by obtaining the period of the cosine wave.

  Patent Document 2 describes a film thickness measuring apparatus that irradiates a multilayer thin film with light and measures the film thickness from the reflected light of the multilayer thin film. In this film thickness measurement apparatus, a reflection spectrum is obtained from the reflected light from the multilayer thin film, a power spectrum is calculated by performing Fourier transform on the reflection spectrum, and each layer constituting the multilayer thin film is calculated from the peak position of the power spectrum. Measure the film thickness.

JP 2001-141602 A JP 2008-292473 A

However, such a retardation measuring apparatus has the following problems.
The retardation measuring apparatus in FIG. 4 must rotate the sample 12 in order to align the optical axes of the polarizing plate 11 and the sample 12. However, in the case of online measurement in the film manufacturing process, the film is moved, so the sample is moved. Can not rotate. For this reason, when the optical axis of the film to measure and the optical axis of the polarizing plate 11 correspond, there existed a subject that retardation could not be measured.

  The same effect can be obtained by rotating the polarizing plate 11 and the analyzing plate 13 in synchronism instead of rotating the sample 12. However, in the on-line measurement, since the film as the sample is moving, a high degree of control is required to rotate the polarizing plate 11 and the analyzer plate 13 on the opposite sides of the film in synchronization with each other. There has been a problem that the configuration of the apparatus becomes complicated. In addition, if the accuracy of the synchronization control between the polarizing plate 11 and the light detection plate 13 is low, there is a problem that these optical axes are shifted and the measurement error increases.

  Since the arrangement of the polarizing plate, the sample, and the analyzer plate of the birefringence evaluation apparatus of Patent Document 1 is the same as that of the retardation measuring apparatus of FIG. 4, the sample or the polarizer and the analyzer plate must also be rotated. For this reason, there existed a subject that it was difficult to use for online measurement.

  Patent Document 2 is an apparatus for measuring the thickness of a multilayer thin film, not an apparatus for measuring retardation.

  An object of the present invention is to realize a retardation measuring apparatus suitable for online measurement, which does not require a complicated synchronization mechanism and does not reduce the measurement accuracy.

In order to achieve such a problem, the invention according to claim 1 of the present invention is:
In the retardation measuring apparatus that irradiates the object to be measured with polarized light and measures the retardation of the object to be measured using the light returned from the object to be measured.
A light source that outputs white light to irradiate the object to be measured;
Polarizing the output light of the light source, and a polarizing plate on which the light returned from the object to be measured is incident,
A light returning from the object to be measured and transmitted through the polarizing plate is incident, and a spectroscopic unit that generates a spectral spectrum of the light;
A rotation mechanism for rotating the polarizing plate;
A spectral spectrum generated by the spectroscopic unit is input, and retardation is calculated and output from the spectral spectrum, and a power spectrum is calculated from the spectral spectrum. An arithmetic unit that controls the rotation mechanism so that the height of two peaks caused by the difference between the refractive index felt by the parallel component and the refractive index felt by the component perpendicular to the optical axis is the same;
It is equipped with. Since the polarizing plate can also serve as the analyzer, it is not necessary to install the analyzer on the opposite side of the object to be measured. Moreover, retardation can be measured even for a measurement object whose optical axis changes. Furthermore, since the angle of the polarizing plate can always be kept in an optimum state, the retardation can be measured with high accuracy.

The invention according to claim 2 is the invention according to claim 1,
The polarizing plate is disposed obliquely with respect to the light irradiated to the object to be measured. Since the light reflected on the surface of the polarizing plate is not mixed with the light transmitted through the object to be measured, the measurement accuracy can be improved.

The invention according to claim 3 is the invention according to claim 1 or 2,
A reflecting mirror is provided on the side opposite to the polarizing plate with respect to the object to be measured, and reflects the light transmitted through the object to be measured. Since all the light transmitted through the object to be measured is reflected, the retardation can be measured with high accuracy.

The present invention has the following effects.
According to the first, second, and third aspects of the invention, the object to be measured is irradiated with white light through the polarizing plate, and a spectral spectrum of the light transmitted through the polarizing plate by the light returned from the measuring object is created. Then, the retardation of the object to be measured is calculated from the spectrum. Also, a rotation mechanism for rotating the polarizing plate is provided, and the rotation is performed so that the heights of the two peaks are the same due to the difference in refractive index between the component parallel to the optical axis and the component perpendicular to the optical axis of the object to be measured. The mechanism was controlled.

  Since the polarizing plate can also serve as the analyzer, the retardation can be measured only by arranging the polarizing plate on one side of the object to be measured. For this reason, there is an effect that the angle between the polarizing plate and the analyzer plate is not shifted and the measurement accuracy is not lowered.

  Moreover, the relationship between the optical axis of a polarizing plate and the optical axis of a to-be-measured object can be united only by rotating the polarizing plate arrange | positioned at the one side of a to-be-measured object. For this reason, there is no need to rotate the polarizing plate and the light detecting plate in synchronization, so that there is an effect that complicated synchronous control is not required even when used for on-line measurement in which the object to be measured moves.

  In addition, since the light passes through the object to be measured twice, the sensitivity can be doubled compared to the conventional case. For this reason, there also exists an effect that the lower limit of the retardation which can be measured can be extended.

  Further, by disposing the polarizing plate obliquely with respect to the light irradiating the object to be measured, the light reflected from the surface of the polarizing plate is reflected in a direction different from the light transmitted through the object to be measured. For this reason, since the light reflected from the surface of the polarizing plate and the light transmitted through the object to be measured are not mixed, there is also an effect that the S / N ratio of the measurement can be increased.

  In addition, by providing a reflecting mirror on the opposite side of the polarizing plate with respect to the measured object, almost all of the light transmitted through the measured object is reflected and transmitted again through the measured object, further increasing the measurement accuracy. There is also an effect that can be done.

  In addition, by providing a rotation mechanism for rotating the polarizing plate, there is an effect that even if the optical axis of the object to be measured changes, the optical axis of the polarizing plate can be adjusted to be measured.

  In addition, by controlling the angle of the polarizing plate so that the two peaks caused by the film thickness in the power spectrum have the same height, the rotation angle of the polarizing plate can always be kept in an optimum state. There is also.

  In addition, since the direction of the optical axis of the film and the size of the retardation can be measured online, it is possible to know the orientation and alignment of the resin molecules in the film being manufactured, and characteristics such as film strength. This is useful for knowing and adjusting (quality control).

It is the block diagram which showed one Example of this invention. It is a block diagram of a calculating part. It is a characteristic view of reflected light. It is a block diagram of the conventional retardation measuring apparatus.

  Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of the retardation measuring apparatus according to the present invention. In FIG. 1, the retardation measuring apparatus includes a light source 20 that emits white light, optical fibers 21 and 27, an optical fiber probe 22, a polarizing plate 23, a polarizing plate rotating unit 24, a reflecting mirror 26, a spectroscopic unit 28, and a calculating unit 29. Is done. 25 is a film for measuring retardation and has birefringence. The film 25 corresponds to an object to be measured. The object to be measured does not necessarily have to be a film.

  The film 25 and the reflecting mirror 26 are disposed in parallel, and the polarizing plate 23 is disposed obliquely with respect to the film 25. The polarizing plate 23 is rotated by the polarizing plate rotating unit 24. Thereby, the angle of the optical axis of the polarizing plate 23 can be changed.

  The polarizing plate rotating unit 24 may rotate the polarizing plate 23 around an axis parallel to the optical axis of the light emitted from the optical fiber probe 22, and rotate about the normal direction of the polarizing plate 23. May be.

  White light emitted from the light source 20 is guided to the optical fiber probe 22 via the optical fiber 21, and is emitted from the optical fiber probe 22 toward the film 25. The light passes through the polarizing plate 23 and the film 25 and is reflected by the reflecting mirror 26, and then passes through the film 25 and the polarizing plate 23 again to enter the optical fiber probe 22. That is, light that has been reflected by the reflecting mirror 26, returned from the film 25, and transmitted through the polarizing plate 23 is incident on the optical fiber probe 22.

  The reflected light incident on the optical fiber probe 22 enters the spectroscopic unit 28 via the optical fiber 27. The spectroscopic unit 28 converts the incident light into spectroscopic and electrical signals to generate a reflection spectroscopic spectrum. This reflection spectrum is equivalent to the spectrum.

  The reflection spectrum generated by the spectroscopic unit 28 is input to the arithmetic unit 29. The calculation unit 29 calculates a power spectrum by Fourier-transforming the input reflection spectrum and measures the retardation of the film 25 and the angle of the optical axis from the peak of the power spectrum.

  FIG. 2 shows the configuration of the calculation unit 29. In FIG. 2, reference numeral 30 denotes a reflection spectrum acquisition unit that acquires the reflection spectrum generated by the spectrum unit 28. Reference numeral 31 denotes a power spectrum calculation unit, to which the reflection spectrum acquired by the reflection spectrum acquisition unit 30 is input. The power spectrum calculation unit 31 performs Fourier transform on the input reflection spectrum and calculates the power spectrum.

  Reference numeral 32 denotes a retardation peak detection unit, to which the power spectrum calculated by the power spectrum calculation unit 31 is input. The retardation peak detection unit 32 searches the input power spectrum and searches for a peak due to retardation. Reference numeral 33 denotes a retardation output unit, which calculates a retardation value from the position of the peak detected by the retardation peak detection unit 32, and outputs this retardation value.

  Reference numeral 34 denotes a film thickness peak detection unit to which a power spectrum calculated by the power spectrum calculation unit 31 is input. The film thickness peak detector 34 searches for the peak of the input power spectrum and detects the peak due to the film thickness of the film 25. The light reflected from the front surface and the back surface of the film 25 interferes to generate interference fringes. For this reason, a peak corresponding to this interference fringe appears in the power spectrum. The film thickness peak detector 34 detects this peak.

  Reference numeral 35 denotes a polarizing plate adjustment unit, to which peak data detected by the film thickness peak detection unit 34 is input. The polarizing plate adjusting unit 35 controls the polarizing plate rotating unit 24 and adjusts the angle of the polarizing plate 23 so that the input peak is appropriate. The polarizing plate adjusting unit 35 and the polarizing plate rotating unit 24 constitute a polarizing plate rotating mechanism.

  Reference numeral 36 denotes a birefringence angle calculation / output unit to which the output of the polarizing plate adjustment unit 35 is input. The birefringence angle calculation / output unit 36 calculates the angle of the birefringence axis of the film 25 from the input data and outputs it.

  Next, the operation of the calculation unit 29 will be described based on FIG. FIG. 3A is a graph of the reflection spectrum acquired by the reflection spectrum acquisition unit 30, where the horizontal axis represents the wavelength of the reflected light and the vertical axis represents the reflectance. This reflection spectrum includes fluctuations with a long period and fluctuations with a short period.

  FIG. 3B is a power spectrum calculated by Fourier transforming the reflection spectrum of FIG. 3A. The horizontal axis represents the optical film thickness, and the vertical axis represents the intensity. This power spectrum includes three peaks 40 to 42.

  40 is a peak due to retardation. As can be seen from the equation (1), the retardation is expressed by the product of the refractive index difference δn and the film thickness of the film 25. Since the refractive index difference δn is usually smaller than the refractive index, the peak 40 appears where the optical film thickness is small. The position of the peak 40 (optical film thickness) is the retardation value.

  41 and 42 are peaks that appear as a result of interference between the light reflected from the front surface of the film 25 and the light reflected from the back surface. Since the film 25 has birefringence, the refractive index felt by the polarized component parallel to the optical axis of the film 25 and the refractive index felt by the polarized component perpendicular to the optical axis of the film 25 are different. . For this reason, it is separated into two peaks 41 and 42. The optical film thickness is the product of the refractive index and the physical film thickness.

  Of the light incident on the film 25, the ratio of the polarization component parallel to the optical axis of the film 25 and the polarization component perpendicular to the optical axis of the film 25 can be changed by rotating the polarizing plate 23. For this reason, when the polarizing plate 23 is rotated, the ratio of the heights of the peaks 41 and 42 changes.

  When the polarizing plate 23 is rotated by the polarizing plate adjustment unit 35 so that the peaks 41 and 42 have the same height, the difference between the optical axes of the film 25 and the polarizing plate 23 becomes 45 degrees. As described above, in this state, the height of the peak 40 resulting from retardation is maximized, and the retardation value can be measured with high accuracy.

  For this reason, when the angle of the polarizing plate 23 is adjusted so that the heights of the peaks 41 and 42 are the same in the polarizing plate adjustment unit 35, the retardation can always be measured in an optimum state. For example, when the height of the peak with the smaller optical film thickness (peak 41) is higher than the height of the other peak (peak 42), the polarizing plate 23 is rotated to the right, and vice versa. When the rotation is rotated to, the difference between the optical axes of the film 25 and the polarizing plate 23 can always be maintained at 45 degrees.

At this time, the following equation (2) is established. Since the direction of the optical axis of the polarizing plate 23 is known from the rotation angle of the polarizing plate 23, the angle of the optical axis of the film 25 can be calculated from the following equation (2). The birefringence angle calculation / output unit 36 calculates and outputs the angle of the optical axis of the film 25 from the following equation (2).
Angle of optical axis of film 25 = angle of optical axis of polarizing plate 23 + 45 degrees (2)

  If the refractive index of the film 25 is known, the physical film thickness of the film 25 can be calculated using the optical film thickness calculated from the position of the peak 41 or 42. This can be performed, for example, by the method described in Patent Document 2.

  In the conventional example of FIG. 4, the sample 12 for measuring retardation is sandwiched between the polarizing plate 11 and the analyzer plate 13. In the online measurement, the sample 12 cannot be rotated, and it is difficult to rotate the polarizing plate 11 and the analyzer plate 13 in synchronization. Therefore, there is a problem that it is difficult to use this retardation measuring apparatus for online measurement. It was.

  In the embodiment of FIG. 1, the light transmitted through the polarizing plate 23 and the film 25 is reflected by the reflecting mirror 26 and transmitted through the film 25 and the polarizing plate 23 again. Since the polarizing plate 23 also operates as an analyzer plate, the analyzer plate is not necessary.

  For this reason, the retardation of the film 25 can be measured only by disposing the polarizing plate 23 on one side of the film 25. Since there is no need to rotate the film 25 and no analyzer plate is required, advanced control such as synchronous control is not necessary, and the configuration is simplified. Since it has such characteristics, it can be used for on-line measurement, and an angle deviation between the polarizing plate and the analyzer plate does not occur.

  Since the light emitted from the optical fiber probe 22 passes through the film 25 twice, the retardation value can be doubled and measured as compared with the conventional example of FIG. For this reason, retardation can be measured with high accuracy. 4 has twice the sensitivity as compared with the conventional example, so when using visible light, the lower limit of measurement can be reduced to about 0.5 μm, which is half of the conventional one.

  In addition, since the polarizing plate 23 is inclined with respect to the film 25, the light reflected by the surface of the polarizing plate 23 is reflected in an oblique direction and is not incident on the optical fiber probe 22. For this reason, the light transmitted through the film 25 and the light reflected by the surface of the polarizing plate 23 are not mixed and the S / N ratio of the measurement is not lowered.

  When the film 25 is thick, the peaks 41 and 42 due to the film thickness may not appear. In such a case, the angle of the polarizing plate 23 may be controlled so that the height of the peak 40 resulting from retardation is maximized. In the configuration of FIG. 2, the film thickness peak detection unit 34 and the birefringence angle calculation / output unit 36 are omitted, and the output of the retardation peak detection unit 32 is input to the polarizing plate adjustment unit 35. In this case, the optical axis and film thickness of the film 25 cannot be measured.

  Further, the reflecting mirror 26 can be omitted. In this case, interference occurs due to the reflected light reflected from the back surface of the film 25, and a peak 40 is generated. Since the reflectance of the back surface of the film 25 is low, the intensity of the interference signal becomes weak and the height of the peak 40 becomes low. However, since the same height as the peaks 41 and 42 due to the film thickness can be secured, it is rather convenient when measuring both retardation and film thickness. Also in this case, light that has returned from the film 25 and transmitted through the polarizing plate 23 enters the optical fiber probe 22.

  Further, when the birefringence axis angle can be predicted in advance, such as a uniaxially stretched film, the polarizing plate 23 can be fixed at an optimal angle in advance. In this case, a mechanism for rotating the polarizing plate 23 (the polarizing plate adjusting unit 35 and the polarizing plate rotating unit 24) is unnecessary.

  Further, when the reflected light reflected from the surface of the polarizing plate 23 does not affect the measurement, the polarizing plate 23 can be arranged in parallel with the film 25.

  Further, in these examples, the reflection spectrum was Fourier-transformed to calculate the power spectrum, and the retardation value was calculated from the peak position of the power spectrum. However, the present invention is not limited to this, and the reflection spectrum is used. The retardation value may be calculated by other methods.

DESCRIPTION OF SYMBOLS 20 Light source 21, 27 Optical fiber 22 Optical fiber probe 23 Polarizing plate 24 Polarizing plate rotation part 25 Film 26 Reflector 28 Spectral part 29 Calculation part 30 Reflectance spectral spectrum acquisition part 31 Power spectrum calculation part 32 Retardation peak detection part 33 Retardation output part 34 Film thickness peak detection unit 35 Polarizing plate adjustment unit 36 Birefringence angle calculation / output unit

Claims (3)

In the retardation measuring apparatus that irradiates the object to be measured with polarized light and measures the retardation of the object to be measured using the light returned from the object to be measured.
A light source that outputs white light to irradiate the object to be measured;
Polarizing the output light of the light source, and a polarizing plate on which the light returned from the object to be measured is incident,
A light returning from the object to be measured and transmitted through the polarizing plate is incident, and a spectroscopic unit that generates a spectral spectrum of the light;
A rotation mechanism for rotating the polarizing plate;
A spectral spectrum generated by the spectroscopic unit is input, and retardation is calculated and output from the spectral spectrum, and a power spectrum is calculated from the spectral spectrum. An arithmetic unit that controls the rotation mechanism so that the height of two peaks caused by the difference between the refractive index felt by the parallel component and the refractive index felt by the component perpendicular to the optical axis is the same;
A retardation measuring apparatus comprising:   The retardation measuring apparatus according to claim 1, wherein the polarizing plate is disposed obliquely with respect to the light irradiated to the object to be measured.   The retardation measuring apparatus according to claim 1, further comprising a reflecting mirror that is disposed on the opposite side of the polarizing plate with respect to the object to be measured and reflects light transmitted through the object to be measured.

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