JP2002243415A - Film thickness measuring method and sheet manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a film thickness measuring method by optical interference that can execute measurement with high accuracy even when a measured object has double refraction and an interference spectral spectrum is distorted. SOLUTION: An optical spectrum at a measuring point on a sheet is measured and a wavelength that gives an extreme value of this optical spectrum is then determined. An estimated film thickness is determined and an interference order of the wavelength that gives the extreme value of the optical spectrum is then determined. A film thickness of a sheet is calculated based on the wavelength that gives the extreme value and the interference order.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a film thickness measuring method based on the principle of light interference and a sheet manufacturing method.

[0002]

2. Description of the Related Art The uniformity of thickness of a sheet such as a polymer film is an important quality. If the thickness of a product is not uniform, poor handling, poor conveyance, etc. may occur at the stage of using the product. Insufficient application of magnetic material, poor evaporation, etc.
It causes various troubles. Therefore, in order to control the means for forming the sheet to produce a product with excellent thickness uniformity, and to control the quality and process control so that products with poor thickness uniformity are not provided to users, It is important to measure the thickness profile.

Many methods for measuring the thickness of a sheet utilize the absorption of β-rays or infrared rays by the sheet, but none of these methods is suitable for high-precision measurement. For more accurate measurement, a so-called optical interference method disclosed in, for example, JP-A-56-115905 and JP-A-63-163105 is adopted.

[0004] The thickness measurement method of the optical interference method is a method of measuring white light
(Refer to electromagnetic waves including visible light, infrared light, ultraviolet light, etc. in a wavelength range having a certain width) The spectral spectrum due to the interference phenomenon when reflected or transmitted by the film, the incident angle of white light, the film thickness of the film And the refractive index are used to detect the spectral spectrum of the reflected light or transmitted light, and obtain the film thickness from the spectrum.

That is, as shown in FIG. 3, when a film having a thickness d and a refractive index n is irradiated with white light at an incident angle θ, a light beam I reflected on the front surface and a light beam II reflected on the rear surface are

[0006]

(Equation 1)

[0007] The optical path difference Δ represented by Therefore, when the reflected light is spectrally separated, a spectral spectrum having a dark part (valley) and a light part (peak) is obtained as shown in FIG. 4, and the wavelength giving the extreme value is determined by using an integer m called the order of interference. It is expressed by the following equation.

[0008]

(Equation 2)

[0009]

(Equation 3)

That is, considering that the phase of the light beam I is inverted, at a wavelength where Δ is m times the wavelength, the light beam I and the light beam II are in opposite phases and the spectrum is minimized. At a wavelength that is (m + 1/2) times, the light beam I and the light beam II have the same phase, and the spectrum is maximized. In the following, for the sake of simplicity, a description will be given of the case of the wavelength λ _m giving the minimum, but the same applies to the wavelength λ _{m + 1/2} giving the maximum.

From equation (2), if the wavelength λ _m and the order m are determined,

[0012]

(Equation 4)

Thus, d can be determined.

On the other hand, wavelengths λ _m , λ _{m + 1} (λ _m >) of adjacent valleys
λ _{m + 1} ), as described above

[0015]

(Equation 5)

Therefore, by eliminating m from the two equations,

[0017]

(Equation 6)

## EQU1 ##

In the method of measuring the film thickness d from the equation (6), it is not necessary to find the absolute value of the order m, and it can be calculated by detecting adjacent extreme wavelengths from the spectrum. On the other hand, in order to measure the film thickness with high accuracy, it is necessary to detect the extreme wavelength of the spectral spectrum with high accuracy.

[0020]

However, in a sheet having birefringence, the spectral spectrum may be distorted, and it may not be possible to detect an extreme value of the spectral spectrum with high accuracy.

That is, the interference light reflected or transmitted by the sheet-like object has a bright portion and a dark portion at a wavelength corresponding to the optical path difference expressed by the equation (1). Since the optical path difference differs depending on the light source, the spectral intensities where the wavelengths of the bright part and the dark part deviate overlap and the spectral spectrum is distorted.

FIG. 6 is an example of a distorted spectral spectrum. As a result of a beat phenomenon in which spectral patterns in which the pitch of repetition of a bright part and a dark part are shifted overlap each other, a wave whose amplitude in the bright part and the dark part is modulated is obtained. ing.

In such a spectrum, the wavelength giving the extreme value is deviated from the original wavelength, and as a result, the detection accuracy of the wavelength giving the extreme value of the spectrum is deteriorated.

[0024] In this case, for example, the lambda _m and was larger by detecting [Delta] [lambda], the film thickness d calculated by the equation (6) is [Delta] [lambda] / (lambda _m -
λ _{m + 1} ). Further, if the spectral spectrum is distorted, it becomes impossible to detect a wavelength at which some peaks or valleys have an extreme value, and it becomes impossible to detect one wavelength of an adjacent peak or valley, and it is difficult to obtain the film thickness d itself.

The degree of distortion of the spectrum due to birefringence is determined by the magnitude of birefringence and the relationship between the direction of the principal axis of birefringence and the direction of vibration of incident light. For example, in a biaxially stretched polymer film, the magnitude of birefringence and the principal axis direction differ depending on the position of the measurement point on the sheet in the sheet width direction, and thus the degree of distortion of the spectral spectrum also differs depending on the position in the sheet width direction. For this reason, depending on the position in the sheet width direction, a measurement error may increase or measurement may be difficult.

That is, in the conventional method, the thickness profile of a sheet having birefringence cannot be measured with high accuracy over the entire width in some cases.

An object of the present invention is to solve the above-mentioned drawbacks of the conventional film thickness measuring method, and to provide a high-precision optical interference measurement even when the object to be measured has birefringence and the spectral spectrum of interference is distorted. To provide a method for measuring a film thickness by using the method described above.

Further, a second object of the present invention is to control the process by measuring the film thickness of the sheet with high precision in the manufacturing process of a sheet such as a polymer film, or to control the process, thereby improving the quality and quality. An object of the present invention is to provide a sheet manufacturing method capable of improving the yield.

[0029]

According to the present invention, there is provided a film thickness measuring method for measuring the thickness of a sheet using an optical interference method, wherein a spectral spectrum of a measuring point on the sheet is measured. Measuring the spectral spectrum, measuring the wavelength that gives the extreme value of the spectral spectrum, determining the estimated film thickness, determining the estimated film thickness, the determined from the determined estimated film thickness Determining an interference order of a wavelength giving an extreme value of the obtained spectral spectrum, and calculating a film thickness of the sheet based on the determined wavelength giving the extreme value of the spectral spectrum and the determined interference order. And a film thickness calculating step.

In a preferred aspect of the film thickness measuring method according to the present invention, the estimated film thickness determining step is based on a film thickness calculated based on a wavelength giving a plurality of extreme values obtained in the wavelength determining step. It is characterized in that the estimated film thickness is determined.

In another preferred aspect of the film thickness measuring method of the present invention, the step of determining the estimated film thickness is based on a measured value at one or more other measuring points on the sheet adjacent to the measuring point. It is characterized in that the estimated film thickness is determined by the above method.

In another preferred aspect of the film thickness measuring method according to the present invention, the estimated film thickness determining step includes a step of determining a film thickness calculated based on a wavelength giving a plurality of extreme values obtained in the wavelength determining step. And determining an estimated film thickness based on a measurement value at one or more other measurement points on the sheet adjacent to the measurement point.

In the method of manufacturing a sheet according to the present invention, the film thickness at a number of measurement points on the sheet is measured using the above-described film thickness measurement method, and the obtained film thickness at each of the measurement points is determined. The method is characterized in that a film thickness profile of the sheet is calculated based on the sheet thickness, and a sheet forming process is controlled so that the obtained film thickness profile falls within a predetermined range.

In the method of manufacturing a sheet according to the present invention, the film thickness at a number of measurement points on the sheet is measured using the above-described film thickness measurement method, and the obtained film thickness at each of the measurement points is calculated. A sheet thickness profile of the sheet is calculated based on the sheet thickness, and quality control and / or process control of the sheet is performed based on the obtained film thickness profile.

In a preferred embodiment of the method for producing a sheet according to the present invention, the sheet is a polymer film.

The sheet of the present invention is characterized by being manufactured by the above-described sheet manufacturing method.

[0037]

DETAILED DESCRIPTION OF THE INVENTION The present invention detects the wavelength lambda _m giving its extreme from the spectral spectrum of the reflected light and the approximate value i.e. the estimated thickness was estimated thickness d to be measured d
Determine the order m from _E ,

[0038]

(Equation 7)

Thus, the film thickness d is determined.

Hereinafter, an embodiment of the film thickness measuring method of the present invention will be described with reference to the drawings.

FIG. 1 is a flowchart of a film thickness measuring method according to the present embodiment. The measuring procedure includes a spectral spectrum measuring step of measuring a spectral spectrum, a wavelength determining step of determining a wavelength giving an extreme value of the spectrum, An estimated film thickness determining step of determining an estimated film thickness; an order determining step of determining an interference order of a wavelength giving an extreme value determined in the wavelength determining step from the estimated film thickness determined in the estimated film thickness determining step; It is composed of a wavelength giving an extreme value determined in the determination step and a film thickness calculation step of calculating a film thickness based on the interference order determined in the order determination step.

FIG. 2 is a schematic configuration diagram showing an example of an optical interference type film thickness measuring device used in the present embodiment, and includes a light source 21, a light projecting unit 22, an optical path changing unit 25, a spectral unit 26, and a data processing unit 27. Be composed. In the figure and the following description, the incident light 23 is reflected light 2 reflected by the measurement object 20.
The reflection type optical interference type film thickness measuring apparatus using No. 4 will be described, but the same applies to a transmission type optical interference type film thickness measuring apparatus using transmitted light transmitted through the measurement target 20.

Hereinafter, each step of FIG. 1 will be described in detail.

The step of measuring the spectrum is performed by the optical interference type film thickness measuring apparatus shown in FIG. That is, the white light 23 emitted from the light source 21 and made parallel by the light projecting unit 22
Is incident on the measurement target 20 at a constant incident angle θ. The incident light is reflected (or transmitted) by the measurement object, and at this time, the intensity of each wavelength component changes due to an interference phenomenon. The reflected light 24 enters the spectroscopy unit 26 via the optical path conversion unit 25, is converted into an electric signal, and is input to the data processing device 27, so that the intensity for each wavelength, that is, the spectrum can be measured. (JP 63-163105 is described in detail in Japanese.) From the spectrum measured by the spectrum measuring step in wavelength determination step, determines the wavelength lambda _m giving its extremes. As is well known, the wavelength giving the extremum is determined by the interference phenomenon, and the film thickness d, the refractive index n, the incident angle θ
When,

[0045]

(Equation 8)

There is the following relationship. Here, m is an integer called an interference order.

[0047] In the wavelength determination step may determine the wavelength lambda _m giving its extreme by directly using spectrum measured by the spectrum measurement stage, optical interference type film thickness measuring device shown in FIG. 2 spectral absorption characteristics and having a respective optical system, from seeking spectrum after compensating the spectral absorption characteristics of the film itself, it is more preferable to determine the wavelength lambda _m giving its extremes.

In the step of determining the estimated film thickness, the estimated film thickness d _E is determined.

As a method of determining the estimated film thickness d _E , there is a method in which an approximate value d _P of the known film thickness itself is used as the estimated film thickness. That is,

[0050]

(Equation 9)

As another method for determining the estimated film thickness d _E , there is a method using a film thickness calculated using a conventional film thickness measuring method. That is,

[0052]

(Equation 10)

As another method for determining the estimated film thickness d _E , there is a method based on a measurement value at another measurement point on one or more sheets adjacent to the measurement point at which the film thickness is to be measured. is there. That is, assuming that K film thickness measurement values at a measurement point from the nearest measurement point are d _N−1 , d _N−2 , d _{N−3 ,.}

[0054]

[Equation 11]

The value estimated by the above is used. Where a
_{1 to} a _K are coefficients of linear prediction.

In this special case, if the thickness of the sheet changes gradually and the detection error of the wavelength λ _m giving the extreme value of the spectral spectrum at each measurement point is small, a ₁ = 1.0, a _{1 2} = ... a _K = 0

[0057]

(Equation 12)

That is, the measured value of the film thickness at the closest measurement point may be used as the estimated film thickness.

As another method of determining the estimated film thickness d _E , there is a method of determining the estimated film thickness d _E based on a measured value at an adjacent measurement point and an estimated film thickness at an adjacent measurement point.
That is, when the measured value at the adjacent measurement point is d _N−1 and the estimated film thickness at the adjacent measurement point is d _E ′,

[0060]

(Equation 13)

The value estimated by the above is used. Where b
Is a first-order lag coefficient, and 0 <b <1.

In the order determining step, the interference order m corresponding to the wavelength λ _m giving the extreme value determined in the wavelength determining step is determined. When the wavelength lambda _m the interference order m and the film thickness d which is a relationship of formula (2), was used an estimated thickness d _E in place of the film thickness d was determined by the estimated thickness determining step,

[0063]

[Equation 14]

M _E that satisfies is a good approximation of m if d _E is a good approximation of d. Therefore, the above equation was solved for m _E

[0065]

(Equation 15)

M _E is obtained by the calculation, and m is determined as a value obtained by rounding m _E to an integer value.

In the film thickness calculation step, the film thickness is calculated using the wavelength giving the extreme value determined in the wavelength determination step and the interference order m determined in the order determination step. That is,

[0068]

(Equation 16)

From the above, the film thickness is calculated.

[0070] At this time, the a lambda _m was larger by detecting [Delta] [lambda], the error of the thickness d calculated by the above equation is proportional to [Delta] [lambda] / lambda _m, with thicknesses determined in such conventional formula (6) Error Δλ
/ (Λ _m −λ _{m + 1} ) is sufficiently small. Therefore,
Even if the contrast is reduced due to birefringence and the accuracy of detecting the wavelength of the peak or valley of the spectral spectrum is deteriorated, the present invention can measure the film thickness with higher accuracy than before.

The wavelengths of two adjacent extreme values λ _m and λ
_{Even when} either _one of _{m + 1} cannot be detected, the film thickness can be determined according to this method. Therefore, even if the contrast is lowered due to birefringence and the wavelength of a peak or valley adjacent to the spectral spectrum cannot be detected, if the wavelength of a single peak or valley can be detected, the film can be formed with high accuracy. Thickness measurement is possible.

By the way, of the above methods for determining the estimated film thickness d _E , the method using the measured values at adjacent measurement points, that is, the method using the equation (9), the equation (10), or the equation (11), The measuring method at the adjacent measuring point is not particularly limited. That is, the measurement method at adjacent measurement points may be a value measured by a conventional method using Equation (6), or a value measured using the method of the present invention. Particularly in the latter case, the film thickness measurement is performed by determining the estimated film thickness of the adjacent Nth measurement point using the film thickness measurement value at the (N-1) th measurement point measured using the present film thickness measurement method. Performing the film thickness measurement by determining the next (N + 1) th estimated film thickness from the film thickness measurement value at the Nth measurement point.
The measurement method may be repeatedly used.

Further, the film thickness measuring device is reciprocated in the sheet width direction while moving the sheet in the longitudinal direction, and the film thickness is measured at many measurement points of the sheet while moving the measurement points.
The film thickness profile of the sheet may be obtained. In this case, the same position in the sheet width direction is used, not only when measuring the film thickness profile of the sheet whose thickness varies depending on the position of the measurement point in the sheet width direction. The measurement method according to the present invention is also applicable to the case where a measurement point at which the film thickness gradually changes over time (that is, according to a change in the position of the measurement point in the longitudinal direction of the sheet). Can be used. In this case, in the estimated film thickness determination method using the above formulas (9) and (10), instead of using the film thickness measurement value of the adjacent measurement point, the film thickness measurement value at the time before that may be used. .

In addition to measuring the film thickness profile of a sheet having a different thickness depending on the measurement location, even at the same measurement point, the thickness of the sheet may be increased due to, for example, lamination of components forming the sheet. The measurement method according to the present invention can also be used when measuring a substance which gradually changes with time. In this case, the above equations (9) and (1
In the estimated film thickness determination method using (0), instead of using the film thickness measurement value at the adjacent measurement point, the film thickness measurement value at the time before that may be used.

Hereinafter, a case where a sheet is manufactured by applying the method for measuring a film according to the present invention will be described.

FIG. 5 is a diagram schematically showing the configuration of a general sheet manufacturing facility.

The polymer extruded from the extruder 53 is spread in a sheet width direction perpendicular to the plane of FIG. 5 by a die 54 and extruded into a sheet 51, and is stretched by a stretching machine 52 in a longitudinal direction (longitudinal direction) and a transverse direction. Winding machine 5 which is stretched (in the width direction)
It is wound by 6.

In this sheet manufacturing method, the stretching machine 5
A thickness gauge 58 (for example, having a measuring unit as shown in FIG. 2) using the film thickness measuring method of the present invention is installed between the winding machine 56 and the winder 56, and the thickness profile of the sheet in the width direction is measured. Measure. The lip gap of the die 54 is controlled via the control means 59 so that the width direction film thickness profile has a desired shape.

The sheet manufactured in this way generally has birefringence, but depending on the conditions of the manufacturing process, the magnitude of the birefringence and the principal axis direction may differ depending on the position of the sheet in the sheet width direction. Accordingly, the degree of distortion of the spectral spectrum due to birefringence may vary depending on the location in the sheet width direction.

For this reason, when measuring the film thickness profile in the width direction of the sheet using the conventional film thickness measuring method, the measurement error of the film thickness is large in a place where the distortion of the spectral spectrum is large, and therefore the measured film thickness profile Accuracy deteriorates. As a result, since the measured film thickness profile does not represent the actual film thickness profile, it cannot be used for control or even if the measured film thickness profile is controlled to have a desired shape, In some cases, the thickness profile of the formed sheet may be different from a desired shape.

Therefore, in the sheet manufacturing method of the present invention, the width direction film thickness profile of the sheet is calculated using the above-described film thickness measuring method of the present invention, and the obtained film thickness profile is formed into a desired shape. Control so that As a result, the film thickness can be measured with high accuracy even if the spectral spectrum is distorted, and therefore, the film thickness profile can be measured with high accuracy over the entire width. As a result, by controlling the measured thickness profile to a desired shape, the thickness profile of the manufactured sheet can be set to a desired shape.

The manufactured sheet was evaluated from the width-direction film thickness profile of the sheet measured using the film thickness measuring method of the present invention.
By not using sheets as products, the quality of sheets shipped to customers can be maintained above a certain level, and by adjusting and correcting the processes that cause such problems, it is possible to prevent continuous production of defective products. .

[0083]

EXAMPLE An example of measuring the film thickness using the present invention will now be described. (Example 1) The thickness measured by a contact thickness gauge was 7.25 μm
m, a polyester film having a refractive index of 1.65, as shown in FIG.
The thickness was measured using a thickness gauge having a measuring section shown in FIG. The measurement spot diameter of the thickness gauge was 2 mm, the incident angle θ was 0 °, and the reflected light from the film was spectrally separated in the range of 730 to 860 nm, and as a result, the spectral intensity spectrum shown in FIG. 6 was obtained. Under the influence of birefringence, the spectrum has a small amplitude at a wavelength of 820 nm or more.

Table 1 shows the results of measuring the film thickness from the spectral intensity spectrum according to the present invention.

[0085]

[Table 1]

First, the extreme wavelength is detected from the spectral intensity spectrum, and here, 746.8, 770.3, 795.
Four minimum values of 6, 823.3 (nm) were obtained. Next, assuming that d _E = 7.25 (μm) using the value previously measured by the contact thickness gauge as the estimated film thickness, the order of each extreme value was calculated. As a result, m _E was as shown in the table. 31.81, 3
The values were 0.84, 29.86, and 28.85, and were rounded off to give m of 32, 31, 30, and 29, respectively. As a result of calculating the film thickness from the order m and the extremal wavelength using Expression 16, the values are 7.24, 7.24, 7.23, and 7.2, respectively.
4 (μm), which was 7.24 μm on average, a value close to the contact type.

On the other hand, as a comparative example, in the film thickness measurement results using the conventional method from the same spectral intensity spectrum, the film thickness obtained from each interval between the four extreme values was 7.41.
7.34, 7.16 (μm), 7.30 on average
(Μm) and the contact thickness gauge. (Example 2) The thickness measured by a contact thickness gauge was 14.15.
A polyester film having a μm and a refractive index of 1.65 was measured using the same thickness gauge as in Example 1, and a spectral intensity spectrum shown in FIG. 7 was obtained. Under the influence of birefringence, the spectrum has a small amplitude at a wavelength of 760 nm or less.

Table 2 shows the results of measuring the film thickness from the spectral intensity spectrum according to the present invention.

[0089]

[Table 2]

First, the extreme wavelength was detected from the spectral intensity spectrum, and nine minimum values shown in the table were obtained. Next, the film thickness obtained by the conventional method was used as the estimated film thickness. That is, from 14.15 to 14.15 calculated from the neighbors of the above nine minimum values.
D _E from the average value of eight film thicknesses of 14.99 (μm)
= 14.34 (μm), the order of each extreme value was calculated. As a result, m _E was 63.0 as shown in the table.
The value was 9 to 55.18, which was rounded off, and m was determined to be 63 to 55, respectively. As a result of calculating the film thickness from the order m and the extremal wavelength by the equation (16), 1
The value was 4.14 to 14.16 (μm), which was 14.15 (μm) on average, which was a value close to that of the contact type.

As described above, the average of the results of the film thickness measurement using the conventional method is 14.34 (μm), which is also different from the contact-type thickness gauge. (Example 3) A polyester film having a target thickness of 11.6 µm was manufactured using the sheet manufacturing apparatus shown in Fig. 5, and the same thickness gauge as used in Example 1 was used to shift the width direction position from 0m to 4.4.
FIG. 8 shows the result of measuring the film thickness at intervals of 2 mm up to 5 m and obtaining the film thickness profile in the width direction. When the film width position was 0 to 40 mm, the film thickness obtained by the conventional method was used as the estimated film thickness, and thereafter, the measured value one time before was used as the estimated film thickness. The film thickness profile was obtained over the entire width of the film, and it could be used effectively for controlling the film thickness profile.

As a comparative example, FIG. 9 shows a film thickness profile in the width direction when only the same calculation method as that shown in FIG. 8 is replaced with the conventional method. Film width position 0.
3m to 1.3m, 3.3m to 4.3m, measured value is about 1.
Although it fluctuates greatly in the range of 5 μm, the film produced was cut out and measured with a contact-type thickness gage, and it was found that there was no such a large fluctuation, and a correct film thickness profile was not obtained. Therefore, in the case of the polyester film shown in this embodiment, the film thickness profile measured by the conventional method cannot be used for controlling the film thickness profile.

[0093]

As described above, the film thickness measuring method of the present invention can measure the film thickness with high accuracy even for a sheet having birefringence. Therefore, by accurately measuring the film thickness profile of the sheet by the sheet manufacturing method of the present invention and controlling this to a desired shape, it is possible to accurately control the film thickness profile even for a sheet having birefringence. And the quality of the manufactured sheet is improved. In addition, by accurately measuring the film thickness profile of the sheet by the sheet manufacturing method of the present invention and performing quality control using the same, it is possible to prevent a defective product from flowing out to a customer and to perform a process that causes a defect. Is corrected, further production of defective products can be prevented, and as a result, sheet cost can be reduced.

[Brief description of the drawings]

FIG. 1 is a flowchart of a film thickness measuring method according to an embodiment of the present invention.

FIG. 2 is a diagram showing an optical interference type film thickness measuring apparatus according to an embodiment of the present invention.

FIG. 3 is a principle diagram of film thickness measurement using optical interference.

FIG. 4 is a diagram showing a spectral intensity spectrum in film thickness measurement using optical interference.

FIG. 5 is a diagram showing an overall schematic configuration of a sheet manufacturing facility in an embodiment of the present invention.

FIG. 6 is a spectral intensity spectrum according to one embodiment of the present invention.

FIG. 7 is a spectral intensity spectrum according to one embodiment of the present invention.

FIG. 8 is a measurement result of a thickness direction profile of a sheet according to an embodiment of the present invention.

FIG. 9 is a measurement result of a film thickness profile in a width direction of a sheet according to a conventional method.

[Explanation of symbols]

 Reference Signs List 20: measurement target 21: light source 22: light projecting part 23: incident light 24: reflected light 25: optical path changing part 26: spectral part 27: data processing device 51: sheet 52: stretching device 53: extruder 54: base 55: Cooling roll 56: Winding machine 57: Conveying roll 58: Thickness gauge 59: Control means

Claims (8)

[Claims] 1. A method for measuring a film thickness of a sheet using an optical interference method, comprising: a spectrum spectrum measuring step of measuring a spectrum spectrum at a measurement point on the sheet; and a wavelength giving an extreme value of the spectrum spectrum. Determining a wavelength,
An estimated film thickness determining step of determining an estimated film thickness, an order determining step of determining an interference order of a wavelength giving an extreme value of the determined spectral spectrum from the determined estimated film thickness, and the determined spectral spectrum A film thickness calculating step of calculating a film thickness of the sheet based on the wavelength giving the extreme value of the above and the determined interference order. 2. The method according to claim 1, wherein in the step of determining the estimated film thickness, the estimated film thickness is determined on the basis of the film thickness calculated based on the wavelength giving a plurality of extreme values obtained in the wavelength determining step. Item 2. The film thickness measuring method according to Item 1. 3. The method according to claim 1, wherein the step of determining an estimated film thickness determines an estimated film thickness based on a measured value at one or more other measurement points on the sheet adjacent to the measurement point. 2. The film thickness measuring method according to 1. 4. The method according to claim 1, wherein the step of determining the estimated film thickness includes the step of determining a film thickness calculated based on the wavelengths giving a plurality of extreme values obtained in the step of determining the wavelength; The method according to claim 1, wherein the estimated film thickness is determined based on a measured value at another measurement point. 5. A film thickness at a number of measurement points on a sheet is measured by using the film thickness measurement method according to claim 1, 2, 3 or 4, and based on the obtained film thickness at each measurement point. A film thickness profile of the sheet, and controlling a sheet forming process so that the obtained film thickness profile falls within a predetermined range. 6. A film thickness at a number of measurement points on a sheet is measured using the film thickness measurement method according to claim 1, and based on the obtained film thickness at each measurement point. The sheet thickness profile of the sheet is calculated, and based on the obtained film thickness profile, quality control of the sheet and / or
Alternatively, a method for producing a sheet, comprising performing step control. 7. The sheet according to claim 5, wherein the sheet is a polymer film.
Or the method for producing a sheet according to 6. 8. A sheet produced by the method for producing a sheet according to claim 5, 6 or 7.