JP6239909B2 - Film thickness measuring method and apparatus - Google Patents

Description

  The present invention relates to a method and apparatus for measuring the thickness of a sheet-like polymer film, a thin film coated on a substrate, and the like by utilizing light interference.

  As a method for measuring the film thickness of various films such as a polymer film and a thin film coated on a substrate, a method using light interference is known. This method irradiates the film with white light containing light of continuous wavelengths, etc., and calculates the film thickness from the spectrum of the reflected or transmitted light based on the wavelength that shows the maximum or minimum value due to interference. It is. For example, Patent Document 1 describes a film thickness measuring method and apparatus using such light interference.

  There are various methods for calculating the film thickness from the spectral spectrum, and in particular, the power spectrum is obtained by fast Fourier transform (FFT) of the spectral spectrum with the wave number (reciprocal of wavelength) as the horizontal axis. A method of calculating the film thickness based on the wavelength giving the peak is often used. For example, the peak on the power spectrum based on the reflected light from the film corresponds to the optical path difference of the reflected light from the front and back surfaces of the film, that is, the optical film thickness of the film. From the average refractive index, the physical film thickness of the film can be determined.

  Here, one peak usually appears on the power spectrum for one film, but when the film has birefringence, the peak is split according to the polarization direction of the irradiated light. At this time, if the film thickness is calculated based on only one peak, the calculated film thickness includes an error. As one solution to this problem, Patent Document 2 describes obtaining the center of gravity position of a split peak and obtaining the thickness of the object to be measured based on the center of gravity position.

JP 2003-240515 A JP 2009-198361 A

  In an ideal situation, the peaks split by birefringence have almost the same height and spread, so that the film thickness measurement error can be reduced by obtaining the film thickness based on their centroids.

  However, in actual measurement, split peaks are often different in height and spread and are often asymmetric. The causes include the structural state of the crystalline and amorphous parts of the film, the orientation of the crystal, the deviation of the polarization direction of the light source, the vibration of the measurement object, the disturbance of the waveform caused by the FFT process, and their synergistic effects. Responsible. However, the actual film thickness does not change depending on the structural state or the like, and is considered to be approximately an intermediate value of the film thickness value calculated from the split peak position. Therefore, when the split peaks are greatly different in height, they are attracted to the peak with the higher center of gravity, so even if the film thickness is calculated based on the center of gravity, an error due to the influence of birefringence cannot be sufficiently eliminated.

  In the present invention, when split peaks appear on the power spectrum with respect to the above problem, the present invention determines characteristic points for the peaks at both ends, and calculates the film thickness based on the midpoints.

  The film thickness measuring method of the present invention includes a step of irradiating an object to be measured with continuous light to obtain a spectrum of reflected or transmitted light, a step of obtaining a power spectrum from the spectrum by Fourier transform, and the power spectrum. The thickness of the object to be measured is determined based on the midpoint between the first characteristic point for the shortest wavelength side peak and the second characteristic point for the longest wavelength side peak. And obtaining a step.

  Here, examples of the object to be measured include thin films such as so-called films and sheets. Examples of the film include a polymer film itself and a film attached to a substrate. Furthermore, a thin film coated on a substrate by dipping, coating, spraying, or the like is also included in the object to be measured. The thickness of the thin film comprising these films, sheets or coatings is usually about 0.1 μm to 1000 μm, and this film thickness measuring method is particularly suitable when the thickness of the thin film is about 2 μm to 200 μm. Continuous light refers to light that includes light of continuous wavelengths, that is, light that exhibits a continuous spectrum.

In the film thickness measurement method, the first characteristic point is the intersection of the shortest wavelength side of the intersection of the straight line indicating the power spectrum with a threshold, the second characteristic points are the power spectrum with a threshold value Of the intersections with the straight line shown, it can be the intersection on the longest wavelength side.

  In the film thickness measurement method, the first characteristic point may be a peak peak of the shortest wavelength side, and the second characteristic point may be a peak peak of the longest wavelength side.

  In the film thickness measurement method, the first characteristic point is a vertex of a figure obtained by waveform separation of the shortest wavelength peak, and the second characteristic point is waveform separation of the longest wavelength peak. It can be the vertex of a figure.

  The film thickness measuring apparatus of the present invention includes a light source that generates continuous light, an irradiation unit that irradiates the object to be measured with light from the light source, reflected light from the object to be measured with respect to the light irradiated from the irradiation unit, or A light receiving unit that receives the transmitted light, a light splitting unit that splits the light received by the light receiving unit to generate an electrical signal corresponding to the intensity, and an arithmetic unit. The calculation unit receives an electrical signal from the spectroscopic unit, creates a spectroscopic spectrum, creates a power spectrum from the spectroscopic spectrum by Fourier transform, and has the shortest wavelength side with respect to the splitting peak appearing in the power spectrum. The film thickness of the film is calculated based on the midpoint between the first characteristic point for the first peak and the second characteristic point for the peak on the longest wavelength side.

  According to the film thickness measuring method or apparatus of the present invention, the film thickness can be accurately measured even when the peak on the power spectrum is split by utilizing the interference of light. Furthermore, even when the split peaks on the power spectrum have greatly different heights, the film thickness can be measured with high accuracy. In particular, the film thickness measuring method of the present invention is useful for a thin film in which one peak and a splitting peak are mixed when measuring an object to be measured.

It is a block diagram of the film thickness measuring apparatus of one Embodiment of this invention. It is an example of the spectrum of reflected light. It is an example of the power spectrum obtained by FFT. It is a figure explaining the film thickness calculation method of 1st Embodiment. It is a figure explaining the film thickness calculation method of 2nd Embodiment. It is a figure explaining the film thickness calculation method of 3rd Embodiment. It is a figure explaining the film thickness calculation method based on the gravity center of the split peak. It is an experimental result which measured the film thickness of the polyimide film. It is an experimental result which measured the film thickness of the polyimide film.

A first embodiment of the present invention will be described with reference to FIGS. The film thickness measuring device of this embodiment is a reflective device that measures the reflected light from the film. Film thickness measurement method of this embodiment employs as the characteristic point of the peak split, the intersection of the shortest wavelength side and longest-wavelength side of the intersection of the straight line indicating the power spectrum with the threshold.

  In FIG. 1, the structure of the film thickness measuring apparatus of this embodiment is shown. The film thickness measuring apparatus 20 includes a light source 21, an irradiation optical fiber 22, a light receiving optical fiber 23, a spectroscopic unit 24, and a calculation unit 25.

  In FIG. 1, a light source 21 that emits continuous light such as white light is used. Various lamps can be used as the light source. The wavelength range of continuous light can be determined according to the thickness of the film to be measured. The irradiation unit and the light receiving unit use the irradiation optical fiber 22 and the light receiving optical fiber 23 in FIG. 1, respectively. However, the irradiation unit and the light receiving unit are not limited thereto. For example, the irradiation unit and the light receiving unit may be configured by a lens system. It may be configured. As the spectroscopic unit 24, various structures such as a monochromator and a polychromator using a diffraction grating can be used. The spectral range can be determined according to the thickness of the film.

  Next, a film thickness measuring method according to the first embodiment using the apparatus of FIG. 1 will be described.

  In FIG. 1, light is irradiated vertically from the irradiation optical fiber 22 onto the film 30 to be measured, and the reflected light is guided to the spectroscopic unit 24 via the light receiving optical fiber 23. The spectroscopic unit 24 generates an electrical signal corresponding to the light intensity and transmits it to the calculation unit 25.

In the calculation unit 25, a spectrum with the horizontal axis as the wavelength (λ) is generated. An example is shown in FIG. In the reflection spectrum, vibration due to interference of reflection from the front and back surfaces of the film is observed. In this embodiment, since light enters the film 30 perpendicularly, the optical path difference (Δ) of the reflected light from the film front surface and the back surface is Δ = 2dn. Here, d is the film thickness and n is the refractive index of the film. In the present embodiment, since the phase of the light reflected from the back surface is inverted, the spectrum becomes maximum at the wavelength λ _m where Δ = (m + 1/2) λ _m . Here, m is the order of interference and is a natural number.

Next, the computing unit 25 generates a spectrum having the horizontal axis as the wave number (1 / λ) from the spectrum having the horizontal axis as the wavelength (λ). In the spectrum with the wave number on the horizontal axis, the interval between the extreme values is constant as is apparent from the following equation. 2dn = 1 / (1 / λ _{m + 1} −1 / λ _m ).

Next, the calculation unit 25 generates a power spectrum by performing a fast Fourier transform (FFT) on the spectrum having the wave number on the horizontal axis. If the period of vibration of the original spectral spectrum is P (= 1 / λ _{m + 1} −1 / λ _m ), a peak appears on the power spectrum at a position where 2dn = 1 / P holds. In the power spectrum, the horizontal axis may be expressed as 2dn (= 1 / P), or may be converted using a preset average refractive index (n) of the film, and the horizontal axis may be expressed as d.

  If the film is completely isotropic, a single peak appears on the power spectrum as shown in FIG. When the film has birefringence, as shown in FIG. 3B, two split peaks appear on the power spectrum. These two peaks appear at positions corresponding to different optical film thicknesses caused by birefringence for one film.

  Furthermore, in the actual measurement, as shown in FIG. 3C, the split peaks may differ in height and spread and appear asymmetric. Further, when both a crystalline part and an amorphous part exist in the measurement range, as shown in FIG. 3 (d), a peak caused by the amorphous part is sandwiched by a split peak caused by the crystalline part. Three peaks may appear.

Next, a method of obtaining the split peak characteristic point will be described with reference to FIG. In the present embodiment, as the characteristic point of the split peaks, among the intersection points of the straight line showing the power spectrum and the threshold, employing the most of the short-wavelength side intersection point (S), and most long wavelength side of the intersection point (L). Then, the film thickness is calculated based on the midpoint (M).

  Here, the threshold value needs to be large enough to eliminate background noise and the like on the power spectrum. The threshold value may be set in advance, or may be calculated each time by the calculation unit according to the measurement data, for example, as 30% of the height of the lower peak.

  As shown in FIG. 4, by using the middle point of the characteristic point of the split peak, the film thickness can be obtained with high accuracy by eliminating the influence of the birefringence of the film. For comparison, FIG. 7 shows a method based on the center of gravity (G) of the splitting peak. In the method based on the center of gravity, since the center of gravity is closer to the peak than the middle of the two peaks, if the heights of the two peaks are greatly different, the influence of birefringence remains somewhat. On the other hand, in the method of the present embodiment (FIG. 4), the film thickness is calculated using almost the intermediate value of the split peak regardless of the peak height, so that the influence of birefringence is reduced. can do.

The method of the present embodiment, the power spectrum and is light processing computing section for obtaining the intersection of the straight line indicating the threshold value, the processing time is shortened, a polymer film production process, to transport the thin film, such as It is also suitable for applications that require high speed such as monitoring.

  Next, since the film thickness of the polyimide film was measured using the apparatus and method of the first embodiment, the experimental results will be described. In addition, as a film and coating layer which have birefringence, not only the said polyimide but a polyethylene terephthalate, a polypropylene, etc. are mentioned. However, in the polyimide thin film, a phenomenon in which one peak and a splitting peak coexist at the time of thickness measurement occurs, so that the present invention can be used more effectively.

  In the experiment, a ribbon-like single layer thermosetting polyimide film having a width of about 5 cm, a length of about 215 cm, a film thickness of about 12.5 μm, and an average refractive index of about 2 is conveyed in the length direction. The film thickness was measured every 1 cm using the apparatus shown in FIG. The measurement results were compared with values measured separately using a contact-type film thickness meter. In addition, white LED which emits continuous light was used for the light source, and the wavelength range which took the spectrum was 540 nm-680 nm.

  In the measurement position shown in FIG. 8, there was one peak on the power spectrum at positions 24 to 27 cm and 31 cm from the end of the ribbon, whereas a split peak was observed on the power spectrum in the range 28 to 30 cm. It was done. Thus, the peak shape changed from one to two, from two to one again. This is thought to be because the measurement range (light irradiation diameter on the film) was about 40 μm in this experiment, and the individual sizes of the crystalline and amorphous regions distributed in the film were larger than the measurement range. It is done.

In FIG. 9, the film thickness measurement result in a 24-31 cm part from the edge of a ribbon is shown. In the figure, “contact type” is a value measured by a contact type film thickness meter. The “interference type (comparative example)” is a film thickness calculated using the interference type film thickness meter of FIG. 1 based on the higher peak for the splitting peak. "Interference (Example)" is also using an interference film thickness meter of Figure 1, by the method of this embodiment for dividing the peak, i.e. the shortest among the intersection of the straight line showing the power spectrum and the threshold It is the film thickness calculated based on the midpoint of the intersection on the wavelength side and the longest wavelength side. The “difference (comparative example)” and “difference (example)” are values obtained by subtracting the “contact type” film thickness from the “interference type (comparative example)” and “interference type (example)”, respectively. is there.

  In FIG. 9, when there was one peak (positions 24 cm and 31 cm), there was no significant difference in the measured film thickness values for any of the contact type, interference type (comparative example), and interference type (example). . On the other hand, when a split peak appears (positions 28, 29, and 30 cm), the measured value of the interference type (comparative example) is significantly different from that of the contact type, and a difference of 0.51 μm maximum is generated, whereas the interference type The difference between the measured value of (Example) and that of the contact type was within 0.20 μm.

  Of course, the measurement result of the contact-type film thickness meter also includes an error, but the contact-type film thickness meter is not influenced by birefringence in principle. Therefore, regardless of the presence or absence of peak splitting, the measured value of the interference type (example) showed almost the same value as that of the contact type in the whole area of FIG. 9 because of the birefringence by the method of the interference type (example). This indicates that the impact has been sufficiently eliminated.

  In this experiment, the number of peaks changed from one to two. On the other hand, according to the method of this embodiment, even if the number of peaks changes from 1 to 3, it is not necessary to change the calculation method, and there is an advantage that the same calculation method can be used.

  Next, a second embodiment of the present invention will be described with reference to FIG. The film thickness measuring apparatus of this embodiment is the same as that shown in FIG. The film thickness measurement method of this embodiment is different from that of the first embodiment in that the peak apex is adopted as the characteristic point of the split peak.

  In this embodiment, a power spectrum is created as in the first embodiment, and in FIG. 5, the midpoint of the peak apex (P1) on the shortest wavelength side and the peak apex (P2) on the longest wavelength side in FIG. The film thickness is calculated based on M ′).

  Next, a third embodiment of the present invention will be described with reference to FIG. The film thickness measuring apparatus of this embodiment is the same as that shown in FIG. The film thickness measurement method of this embodiment is different from the first and second embodiments in that the vertex of a figure obtained by waveform-separating the peak is adopted as the characteristic point of the split peak.

  In the present embodiment, a power spectrum is created as in the first and second embodiments. Next, in FIG. 6, the peaks are curve-fitted into a Gaussian function to separate the waveforms. Next, the film thickness is calculated based on the apex (P3) of the figure from which the peak on the shortest wavelength side is waveform-separated and the midpoint (M ") of the figure (P4) from which the peak on the longest wavelength side is waveform-separated. For waveform separation, a method of waveform separation by curve fitting a peak into a Lorentz function can be used.

  The present invention is not limited to the above embodiments, and various modifications can be made within the scope of the technical idea.

  For example, in the said embodiment, light was irradiated to the film perpendicularly | vertically and the film thickness was calculated | required using the reflected light. However, the light irradiation angle may not be perpendicular to the film. Alternatively, the light receiving unit may be provided on the opposite side of the irradiation unit and the film, and the film thickness may be obtained using transmitted light.

  Moreover, in the said embodiment, although the thickness of the thin film which consists of polymer films itself was measured, this invention is applicable also to the film thickness measurement of the thin film coated on the base material. Furthermore, the present invention can be applied to a multilayer film coated on a substrate as long as the peak on the power spectrum for each layer can be resolved.

20 Film thickness measuring device 21 Light source 22 Optical fiber for irradiation (irradiation part)
23 Optical fiber for light receiving (light receiving part)
24 Spectrometer 25 Calculation Unit 30 Polymer Film (Measurement Object)

Claims (6)

Irradiating the object to be measured with continuous light to obtain a spectrum of the reflected or transmitted light;
Obtaining a power spectrum from the spectrum by Fourier transform;
For the asymmetric splitting peak appearing in the power spectrum, based on the midpoint between the first characteristic point for the shortest wavelength side peak and the second characteristic point for the longest wavelength side peak. Determining the thickness of the object,
A method for measuring a film thickness. The object to be measured is a polymer film or a thin film coated on a substrate.
The film thickness measuring method according to claim 1. Said first characteristic point is the intersection of the shortest wavelength side of the intersection of the straight line indicating the power spectrum with a threshold, the second characteristic point of intersection of the straight line indicating the power spectrum with a threshold value Of which is the intersection on the longest wavelength side,
The film thickness measuring method according to claim 1 or 2 . The first characteristic point is the peak peak of the shortest wavelength side, and the second characteristic point is the peak peak of the longest wavelength side.
The film thickness measuring method according to claim 1 or 2 . The first characteristic point is a vertex of a figure obtained by waveform separation of the peak on the shortest wavelength side, and the second characteristic point is a vertex of a figure obtained by waveform separation of the peak on the longest wavelength side,
The film thickness measuring method according to claim 1 or 2 . A light source that generates continuous light;
An irradiation unit for irradiating the object to be measured with light from the light source;
A light receiving unit that receives reflected light or transmitted light from the object to be measured with respect to light irradiated from the irradiation unit;
A spectroscopic unit that splits the light received by the light receiving unit and generates an electrical signal corresponding to the intensity;
A spectral spectrum is generated by receiving an electrical signal from the spectroscopic section, a power spectrum is generated from the spectral spectrum by Fourier transform, and a peak on the shortest wavelength side is compared with an asymmetric split peak appearing in the power spectrum. A calculation unit for calculating the film thickness of the object to be measured based on the midpoint between the first characteristic point of the first characteristic point and the second characteristic point for the peak on the longest wavelength side;
A film thickness measuring apparatus.

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