JP2012112760A - Method and apparatus for measuring film thickness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method and an apparatus for measuring a film thickness capable of accurately measuring even a film having a thick film thickness and birefringence, to solve such a problem of a film thickness measuring apparatus for measuring a film thickness of a film from a peak of a power spectrum by an optical spectrum of light reflected from the surface and rear surface of the film that the thickness of the film cannot be measured when it is thick and since the peak in the film having the birefringence has bimodality, an error becomes large.SOLUTION: This method for measuring a film thickness includes: applying polarized light to a film, calculating a retardation from the light passing through the film, and calculating the film thickness from the retardation and a refractive index difference of the film. This method can accurately measure the thickness of a film having a thick film thickness and the birefringence.

Description

  The present invention relates to a method and apparatus for measuring a film thickness of a film or the like, and more particularly to a film thickness measuring method and apparatus suitable for use in measuring the thickness of a film having birefringence with a thickness of several tens of μm or more. is there.

  FIG. 4 shows the configuration of a film thickness measuring apparatus using light interference. In FIG. 4, the white output light of the light source 10 is irradiated to the film 12 for measuring the film thickness via the optical fiber probe 11.

  The reflected light reflected from the front and back surfaces of the film 12 is input to the spectroscopic unit 13 via the optical fiber probe 11. The spectroscopic unit 13 splits the input reflected light, converts it into an electrical signal, generates a reflected spectral spectrum, and outputs it to the computing unit 14. The calculation unit 14 performs Fourier transform on the input reflection spectrum and calculates a power spectrum, and measures the film thickness of the film 12 from the peak position of the power spectrum.

  The reflected light reflected from the front surface of the film 12 and the reflected light reflected from the back surface interfere with each other to generate interference fringes. For this reason, a peak due to this interference fringe appears in the power spectrum of the reflection spectrum. The calculation unit 14 calculates the optical film thickness from the position of this peak, and calculates the physical film thickness from the optical film thickness and the refractive index of the film 12.

  Patent Document 1 describes an invention of a film thickness measuring device that measures the film thickness of a film based on such a principle.

Retardation is used as an index of quality control in the production process of a film having birefringence. A film having birefringence has a characteristic that a refractive index felt by a polarized component parallel to the optical axis is different from a refractive index felt by a polarized component perpendicular to the optical axis. Retardation can be calculated by the following equation (1). Here, δn is the difference in refractive index, and d is the film thickness of the film to be measured.
Retardation = δn × d (1)

  FIG. 5 shows the configuration of the retardation measuring device. In FIG. 5, the white light which is the output light of the light source 20 is polarized by the polarizing plate 21 and irradiated to the film 23 for measuring retardation. A polarizing plate rotating unit 22 rotates the polarizing plate 21.

  The light transmitted through the film 23 is detected by the light detection plate 24 and input to the spectroscopic unit 28 via the probe 26 and the optical fiber 27. Reference numeral 25 denotes a light detection plate rotating unit that rotates the light detection plate 24.

  The spectroscopic unit 28 splits the input light, converts it into an electrical signal, generates a spectroscopic spectrum, and outputs it to the arithmetic unit 29. The computing unit 29 computes a power spectrum by performing Fourier transform on the input spectral spectrum, and calculates retardation from the peak position of the power spectrum.

  In order to measure retardation, the optical axes of the polarizing plate 21 and the analyzer plate 24 must be matched. When the angle formed by the optical axis of the polarizing plate 21 and the film 23 is 0 degree or 90 degrees, the peak of the power spectrum does not occur, so that the retardation cannot be measured. For this reason, the calculating part 29 controls the direction of the optical axis of the polarizing plate 21 and the analyzing plate 24 through the polarizing plate rotating part 22 and the analyzing plate rotating part 25 so that the optimum measurement conditions are obtained.

  In Patent Document 2, a sample to be measured is sandwiched between a polarizer placed in a parallel Nicol state and an analyzer, and the sample to be measured is obtained from the direction and intensity of the polarized light transmitted through the polarizer, the sample, and the analyzer. Describes a method of measuring the orientation and the direction of the main refractive index.

  Patent Document 3 discloses a thickness measuring device for measuring a film thickness from a ratio of transmitted light intensity of near infrared light in which light is absorbed by a film to be measured and near infrared light in which light is not absorbed, a polarizer, An apparatus is described in which a measurement film and an analyzer are arranged in this order, and the birefringence and the degree of orientation of the film are measured from the intensity spectrum of the transmitted light.

JP 2008-292473 A JP 2006-84268 A Japanese Patent Laid-Open No. 11-10728

  However, such a film thickness measuring apparatus and retardation measuring apparatus have the following problems.

  The film thickness measuring apparatus of FIG. 4 can accurately measure the film thickness when the film 12 is a thin film having a thickness of about 0.1 to 200 μm, but interference is less likely to occur with a film having a film thickness larger than that. Therefore, there is a problem that it is difficult to accurately measure the film thickness.

  In addition, since the birefringent film has two different refractive indexes, the peak of the power spectrum becomes bimodal, and there is a problem that it is difficult to measure an accurate film thickness. Conventionally, measurement accuracy has been improved by taking a weighted average of bimodal peaks, but it has been difficult to measure an accurate film thickness.

  The thickness measuring device described in Patent Document 3 requires light of a wavelength that is absorbed by the film and light of a wavelength that is not absorbed by the film. However, since it may be difficult to select these wavelengths, the measurement is limited. There was a problem that there was.

  The retardation measuring apparatus in FIG. 4 is an apparatus for measuring retardation, and has a problem that the film thickness cannot be directly measured.

  Further, since the polarizing plate 21 and the analyzing plate 24 are disposed on both sides of the film 23 and the polarizing plate 21 and the analyzing plate 24 must be rotated in synchronization with each other so that the optical axes coincide with each other, the film to be measured moves. When used for on-line measurement, there is a problem that sophisticated synchronization control is required and the apparatus becomes complicated.

  Furthermore, there has been a problem that measurement error increases if the polarization plate 21 and the light detection plate 24 are out of synchronization.

  An object of the present invention is to realize a film thickness measuring method and apparatus capable of measuring a film thickness with high accuracy even with a film having birefringence and a film thickness of 200 μm or more.

In order to achieve such a problem, the invention according to claim 1 of the present invention is:
In the film thickness measuring method for measuring the film thickness of the object having birefringence,
Irradiating the object to be measured with polarized light, spectroscopically analyzing the light transmitted through the object to be measured, generating a spectrum, and measuring retardation from the spectrum;
From the measured retardation and the refractive index difference of the measured object, a step of calculating the film thickness of the measured object;
Is provided. An accurate film thickness can be measured even for a thick object.

The invention according to claim 2 is the invention according to claim 1,
A power spectrum is calculated from the spectral spectrum, and a refractive index difference of the measured object is calculated from a peak due to the film thickness of the measured object of the power spectrum. It is not necessary to measure the refractive index of the object to be measured with another device.

The invention described in claim 3
In a film thickness measuring device that measures the film thickness of an object having birefringence,
A retardation measuring unit that irradiates the object to be measured with polarized light, generates a spectrum by spectroscopically analyzing the light transmitted through the object, and measures the retardation of the object to be measured from the spectrum;
The retardation measured by the retardation measuring unit is input, and from the inputted retardation and the refractive index difference of the measured object, a film thickness calculating unit that calculates the film thickness of the measured object,
It is equipped with. The film thickness can be measured even with a thick object to be measured.

The invention according to claim 4 is the invention according to claim 3,
The retardation measuring unit is
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 spectral spectrum generated by the spectroscopic unit is input, a retardation calculation unit that calculates and outputs a retardation from the spectroscopic spectrum, and
It is equipped with. Since the analyzer plate can be omitted, complicated synchronization control is not required, and the configuration of the apparatus can be simplified.

The invention according to claim 5 is the invention according to claim 4,
With respect to the object to be measured, a reflecting mirror disposed on the opposite side of the polarizing plate is provided. Since most of the light transmitted through the object to be measured can be returned to the object to be measured again, accurate measurement becomes possible.

The invention according to claim 6 is the invention according to claim 4 or claim 5,
The retardation calculator is
A power spectrum is calculated from the spectral spectrum, and a refractive index difference of the measured object is calculated from a peak due to the film thickness of the measured object of the power spectrum. It is not necessary to measure the refractive index of the object to be measured with another device.

The present invention has the following effects.
According to the first, second, third, fourth, fifth, and sixth aspects of the invention, the object to be measured is irradiated with polarized light, the light returned from the object to be measured is dispersed, and a spectrum is generated. The retardation was calculated based on the spectral spectrum, and the film thickness was calculated from the retardation and the refractive index difference between the object to be measured.

  Even with an object to be measured having birefringence, there is an effect that an accurate film thickness can be measured. In addition, there is also an effect that an accurate film thickness can be measured even with an object to be measured having a film thickness of 200 μm or more, which is difficult to measure with an optical interference type film thickness measuring apparatus.

  Further, as the retardation measuring unit, the analyzer can be omitted by using a configuration in which white light is polarized by the polarizing plate and irradiated on the object to be measured, and light returned from the object to be measured is incident on the same polarizing plate again. . For this reason, since it is not necessary to rotate the polarizing plate and the light detection plate in synchronization, there is also an effect that the configuration of the apparatus can be simplified even when used for on-line measurement in which the object to be measured moves.

  For this reason, the cost and the failure rate of the apparatus can be reduced, and the angle deviation between the polarizing plate and the light detection plate does not occur, so that the measurement accuracy can be improved. In addition, since the light passes through the object to be measured twice, there is an effect that the measurement sensitivity can be increased.

  Furthermore, by calculating the refractive index difference of the object to be measured from the peak due to the film thickness of the power spectrum of the spectral spectrum, there is also an effect that it is possible to save the trouble of measuring the refractive index separately.

It is the block diagram which showed one Example of this invention. It is a characteristic view for demonstrating operation | movement of the apparatus of FIG. It is a characteristic view for demonstrating operation | movement of the other Example of this invention. It is a block diagram of the conventional film thickness measuring apparatus. 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. The present invention targets a film having birefringence, measures the retardation of the film, and calculates the film thickness of the film from the refractive index difference of the film measured separately from the retardation value.

  FIG. 1 is a block diagram showing an embodiment of a film thickness measuring apparatus according to the present invention. In FIG. 1, the film thickness measuring apparatus includes a light source 30 that outputs white light, optical fibers 31 and 36, an optical probe 32, a polarizing plate 33, a polarizing plate rotating unit 34, a reflecting mirror 35, a spectroscopic unit 37, a retardation calculating unit 38, And a film thickness calculator 39. 40 is a film for measuring the film thickness.

  The film 40 corresponds to an object to be measured. In addition, the light source 30, the optical fibers 31 and 36, the optical probe 32, the polarizing plate 33, the polarizing plate rotating unit 34, the reflecting mirror 35, the spectroscopic unit 37, and the retardation calculating unit 38 constitute a retardation measuring unit.

  The polarizing plate 33, the film 40, and the reflecting mirror 35 are arranged in this order. Further, the polarizing plate 33 is disposed so as to be oblique to the incident light. The polarizing plate 33 is rotated by a polarizing plate rotating unit 34. When the polarizing plate 33 is disposed obliquely, the light reflected by the surface of the polarizing plate 33 is reflected leftward and does not enter the optical probe 32, so that the S / N ratio of the measurement can be improved.

  The white output light of the light source 30 is guided by the optical fiber 31 and input to the optical probe 32. The optical probe 32 emits the input light toward the polarizing plate 33. This light passes through the polarizing plate 33 and the film 40 and is reflected by the reflecting mirror 35, and then passes through the film 40 and the polarizing plate 33 again and enters the optical probe 32.

  That is, the light returned from the film 40 is incident on the optical probe 32. The polarizing plate 33 polarizes incident light and irradiates the film 40.

  The light transmitted through the film 40 incident on the optical probe 32 is guided by the optical fiber 36 and is incident on the spectroscopic unit 37. The spectroscopic unit 37 converts the incident light into spectroscopic and electrical signals, and generates a reflection spectroscopic spectrum. This reflection spectrum is equivalent to the spectrum.

  The reflection spectrum generated by the spectroscopic unit 37 is input to the retardation calculation unit 38. The retardation calculating unit 38 calculates the retardation of the film 40 from the input reflection spectrum.

  The retardation calculation unit 38 performs a Fourier transform on the input reflection spectral spectrum to calculate a power spectrum, and calculates retardation from the peak position of the power spectrum.

  The light incident on the film 40 and the light reflected by the reflecting mirror 35 and transmitted through the film 40 interfere with each other to generate interference fringes due to retardation. For this reason, a peak due to this interference fringe appears in the power spectrum of the reflection spectrum. The position (wavelength) of this peak is the retardation value.

  The amplitude of interference fringes resulting from retardation varies depending on the polarization direction of light incident on the film 40. The polarization direction of light incident on the film 40 can be changed by rotating the optical axis of the polarizing plate 33.

  The direction of polarization of light incident on the film 40 can be changed by rotating the polarizing plate 33 and changing the angle of its optical axis. When the angle formed by the optical axis of the polarizing plate 33 and the optical axis of the film 40 is 0 or 90 degrees, the amplitude of the interference fringes is 0, and is maximum when the angle is 45 degrees. The retardation calculating unit 38 controls the polarizing plate rotating unit 34 and is sufficient for measurement at the position where the amplitude of the interference fringes, that is, the peak height of the power spectrum is maximized, or in the intermediate state between the parallel Nicols state and the orthogonal Nicols state. The optical axis of the polarizing plate 33 is adjusted to a position where a height peak occurs.

  The polarizing plate rotating section 34 may rotate the polarizing plate 33 about an axis parallel to the optical axis of the light emitted from the optical probe 32, and may rotate about the normal direction of the polarizing plate 33 as an axis. Also good.

  The retardation value calculated by the retardation calculation unit 38 is input to the film thickness calculation unit 39. Further, the refractive index of the film 40 is input to the film thickness calculator 39. The refractive index is measured with a refractometer after cutting out a part of the film having the same characteristics as the film 40.

  Since the film 40 has birefringence, the refractive index for incident light parallel to the optical axis of the film 40 is different from the refractive index for incident light perpendicular to the optical axis. These two refractive indexes are input to the film thickness calculator 39.

  The film thickness calculator 39 calculates and outputs the film thickness of the film 40 from the refractive index of the film 40 input separately from the retardation of the film 40 input from the retardation calculator 38.

As explained in the equation (1), the retardation value is represented by the product of the refractive index difference and the film thickness. The film thickness calculator 39 calculates the refractive index difference δn from the two input refractive indexes, and calculates the film thickness of the film 40 by the following equation (2).
Film thickness of film 40 = (retardation value) / δn (2)
Note that the refractive index difference δn may be input to the film thickness calculator 39.

  Next, the present invention will be described in more detail with reference to FIG. FIG. 2A shows a reflection spectral spectrum created by the spectroscopic unit 37, where the horizontal axis represents wavelength and the vertical axis represents amplitude. In this wavelength spectrum, periodicity corresponding to the interference fringes is recognized.

  FIG. 2B is a power spectrum calculated by the retardation calculation unit 38, where the horizontal axis represents frequency and the vertical axis represents intensity. 41 is a peak due to retardation. The retardation computing unit computes a power spectrum by performing Fourier transform on the input reflection spectral spectrum, searches the power spectrum to obtain a peak 41 resulting from retardation, and calculates a retardation value from the position (wavelength) of this peak. .

  Next, another embodiment of the present invention will be described with reference to FIG. The film thickness measuring apparatus shown in FIGS. 1 and 4 is similar in that the sample to be measured is irradiated with light from the optical probe and the peak of the power spectrum caused by the interference fringes of the reflected light is detected. Therefore, when the film thickness of the film 40 is not so thick, like the film thickness measuring apparatus of FIG. Appears. In this embodiment, the refractive index difference δn is calculated using interference fringes resulting from this film thickness.

  FIG. 3 shows an example of a power spectrum in such a case. The horizontal axis is the optical wavelength, and the vertical axis is the intensity. The optical film thickness is the product of the physical film thickness and the refractive index.

  In FIG. 3, 42 is a peak due to retardation of the film 40, and 43 and 44 are peaks due to the film thickness of the film 40. Since the film 40 is birefringent and has two refractive indexes, the peak due to the film thickness is separated into two peaks 43 and 44.

When the optical film thickness obtained from the position of the peak 43 is T1, the optical film thickness obtained from the position of the peak 44 is T2, and the film thickness of the film is d, the refractive index difference δn is obtained from the following equation (3).
δn = (T2−T1) / d (3)

  In this way, since the refractive index difference δn can be measured with the configuration of FIG. 1, there is an advantage that it is not necessary to separately measure the refractive index of the film 40 using a refractometer. The refractive index difference δn and the film thickness may be measured separately, or may be measured simultaneously using one power spectrum.

  In particular, when continuously measuring on-line in a film production apparatus, the refractive index difference δn is calculated from the equation (3) using the film thickness d measured last time, and the equation (2) is calculated using the δn and the retardation value. The film thickness may be measured from Since the film thickness does not change abruptly, the refractive index difference δn can be accurately calculated by using the immediately preceding film thickness.

  In the film thickness measuring apparatus shown in FIG. 4, when the thickness of the sample to be measured increases, the optical film thickness increases and interference does not occur, which sometimes makes measurement impossible. In addition, in the sample having birefringence, the peak of the power spectrum becomes bimodal, so that an accurate film thickness cannot be measured.

  In the present invention, the retardation is measured, and the film thickness is calculated from the retardation. Since the refractive index difference is usually smaller than the refractive index, the retardation that is the product of the refractive index difference and the film thickness is smaller than the optical film thickness that is the product of the refractive index and the film thickness. For this reason, there is an advantage that a peak due to retardation can be detected even in a sample having a film thickness exceeding 200 μm where the peak cannot be detected by the film thickness measuring apparatus of FIG. 4, and the film thickness can be measured accurately.

  Moreover, since the peak resulting from retardation is unimodal, the position of the peak can be accurately calculated. For this reason, there also exists an advantage that the film thickness of the film which has birefringence can be measured correctly.

  Further, the retardation measuring apparatus of FIG. 5 has a polarizing plate 21 and an analyzing plate 24 arranged on both sides of the film 23, and these plates have to be rotated in synchronization, so the film 23 moves and cannot be rotated online. The measurement has a drawback that the apparatus becomes complicated.

  The film thickness measuring apparatus of FIG. 1 has a configuration in which light transmitted through the polarizing plate 33 and the film 40 is reflected by the reflecting mirror 35 and transmitted through the film 40 and the polarizing plate 33 again. Since the polarizing plate 33 can also serve as the analyzer, the retardation can be measured simply by disposing the polarizing plate 33 on one side of the film 40. For this reason, the configuration of the apparatus can be greatly simplified.

  In the embodiment of FIG. 1, the light transmitted through the film 40 is reflected by the reflecting mirror 35 and is transmitted through the film 40 again. Since the light passes through the film 40 twice, the retardation value can be doubled and measured as compared with the retardation measuring device of FIG. For this reason, there exists an advantage that an exact retardation value is obtained and a film thickness can be measured more correctly.

  Moreover, the film thickness measuring apparatus of FIG. 1 and FIG. Therefore, when the expected film thickness of the film 40 for measuring the film thickness is thin or does not have birefringence, the polarizing plate 33 is moved so as to be removed from the optical axis of the emitted light of the optical probe 32 and the conventional example shown in FIG. The film thickness is measured according to the same principle as above. When the expected film thickness is thick, when the film has birefringence, the retardation is measured by returning the polarizing plate 33 to the position shown in FIG. 1, and the film thickness is calculated from the retardation value. In this way, the film thickness measurement range can be greatly expanded.

  The reflecting mirror 35 can be omitted. In this case, interference occurs due to the reflected light reflected from the back surface of the film 40, and a peak 41 is generated. However, since the reflectance of the back surface of the normal film 40 is low, there is a disadvantage that the intensity of the interference signal is weak and the height of the peak 41 is low.

  When the birefringent axis angle can be predicted in advance, such as a uniaxially stretched film, the polarizing plate 33 may be fixed at an optimal position. This eliminates the need for a mechanism for rotating the polarizing plate 33 (polarizing plate rotating section 34).

  Further, when the reflected light reflected from the surface of the polarizing plate 33 does not affect the measurement, the polarizing plate 33 can be arranged in parallel with the film 40.

  Moreover, the structure which arrange | positions a polarizing plate and a light-analysis board to the both sides of a to-be-measured film like the example of FIG. Such a configuration is not suitable for on-line measurement, but there is no problem in off-line measurement in which a sample is cut and measured. Moreover, the structure which fixes a polarizing plate and an analyzer plate and rotates a to-be-measured film may be sufficient.

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

DESCRIPTION OF SYMBOLS 30 Light source 31, 36 Optical fiber 32 Optical probe 33 Polarizing plate 34 Polarizing plate rotating part 35 Reflecting mirror 37 Spectroscopic part 38 Retardation calculating part 39 Film thickness calculating part 40 Film 41-44 Power spectrum peak

Claims (6)

In the film thickness measuring method for measuring the film thickness of the object having birefringence,
Irradiating the object to be measured with polarized light, spectroscopically analyzing the light transmitted through the object to be measured, generating a spectrum, and measuring retardation from the spectrum;
From the measured retardation and the refractive index difference of the measured object, a step of calculating the film thickness of the measured object;
The film thickness measuring method characterized by comprising.   2. A power spectrum is calculated from the spectral spectrum, and a refractive index difference of the measured object is calculated from a peak due to the film thickness of the measured object of the power spectrum. Film thickness measurement method. In a film thickness measuring device that measures the film thickness of an object having birefringence,
A retardation measuring unit that irradiates the object to be measured with polarized light, generates a spectrum by spectroscopically analyzing the light transmitted through the object, and measures the retardation of the object to be measured from the spectrum;
The retardation measured by the retardation measuring unit is input, and from the inputted retardation and the refractive index difference of the measured object, a film thickness calculating unit that calculates the film thickness of the measured object,
A film thickness measuring apparatus comprising: The retardation measuring unit is
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 spectral spectrum generated by the spectroscopic unit is input, a retardation calculation unit that calculates and outputs a retardation from the spectroscopic spectrum, and
The film thickness measuring apparatus according to claim 3, comprising:   5. The film thickness measuring apparatus according to claim 4, further comprising a reflecting mirror disposed on the opposite side of the object to be measured from the polarizing plate. The retardation calculator is
The power spectrum is calculated from the spectral spectrum, and the refractive index difference of the measured object is calculated from the peak due to the film thickness of the measured object of the power spectrum. The film thickness measuring apparatus according to claim 5.

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