JP4813292B2 - Organic thin film thickness measuring device and organic thin film forming device - Google Patents

Description

  The present invention relates to an organic thin film thickness measuring apparatus and an organic thin film forming apparatus, and more particularly to an optical film thickness measuring apparatus for organic EL that optically measures the film thickness.

Organic EL devices are attracting attention as next-generation displays because of their high-speed response, high visibility, and thinness. As shown in FIG. 9, in general organic EL, after forming the transparent conductive film 31 on the glass substrate 30, the organic thin films 32 to 34 are formed, and then the electrode 35 is laminated on the surface of the organic thin film 34, Finally, the whole is protected by sealing with the can 36.
In the organic EL device fabricated in this manner, when each organic thin film 32 to 34 functions as a hole transport layer, a light emitting layer, and an electron transport layer, a positive voltage is applied to the transparent conductive film 31 and a negative voltage is applied to the electrode 35. Then, the holes injected from the transparent electrode and the electrons injected from the cathode reach the organic thin film 33 to recombine the electrons and holes. At that time, the electric energy is converted into light energy and the organic light emission is performed. Light is emitted from the layer, and EL light 37 transmitted through the glass substrate 30 is emitted to the outside.

  The transparent conductive film 31 generally uses an ITO (Indium-Tin Oxide) thin film. When the organic thin films 32 to 34 are laminated on the surface, a glass substrate 30 on which the transparent conductive film 31 is formed is prepared, and after the surface treatment of the transparent conductive film 31 is performed, the inside of the vacuum chamber of the organic thin film forming apparatus Install in. At least one organic material evaporation source is disposed in the vacuum chamber, the transparent conductive film 31 of the installed glass substrate 30 is opposed to the organic material evaporation source, and the inside of the vacuum chamber reaches a predetermined pressure. Exhaust. When the organic material evaporation source is filled in advance with the organic material evaporation source and the organic material evaporation source is heated, the vapor of the organic evaporation material is released into the vacuum chamber.

  The film thickness of each functional layer constituting the organic EL device is one of the main factors governing the efficiency and color purity of the device, and it is extremely important to control the film thickness with high accuracy. Conventionally, a crystal thickness meter has been used to control the film thickness of each layer, and it is common to control the film thickness from the correlation between the film thickness deposited on the crystal sensor and the film thickness on the actual substrate. The film thickness correlation between the quartz sensor and the actual substrate may be disrupted due to changes in the evaporation rate due to fluctuations in the evaporation rate and changes in the surface of the evaporation material in the crucible used as the evaporation source. It was.

  Therefore, as a film thickness measurement method that does not use a quartz sensor, a method for directly and non-contactly measuring the film thickness on an actual substrate has been proposed (for example, Patent Document 1). In Patent Document 1, an organic material that forms an organic thin film is irradiated with excitation light during the film formation process, and fluorescence or reflected light excited thereby is detected. Based on the amount of fluorescence or the amount of reflection. A configuration for calculating and controlling the film thickness is disclosed.

More specifically, for example, as shown in FIG. 10, when Alq3 is used as an organic substance, it is known that fluorescence is remarkably generated in the vicinity of a wavelength of 530 nm when excitation light having a wavelength of 400 nm is irradiated. The fluorescence intensity and the film thickness are in a monotonically increasing relationship, and the correlation between the temporal change in fluorescence intensity and the temporal change in film thickness at a wavelength of 530 nm is calculated and stored in advance, and excitation at a wavelength of 400 nm. Simultaneously with the light irradiation, the fluorescence intensity at a wavelength of 530 nm is observed. Then, the film thickness at that time is specified from the stored correlation, and the end time of the film forming operation is determined.
JP 2006-16660 A

  However, the measuring method of Patent Document 1 has no problem in measuring the thickness of a single layer film made of a single organic material, but is suitable for a case where the organic material contains a doping material or a multilayer film made of a plurality of organic materials. Absent. Explaining the case where the organic material contains a doping material. For example, when Alq3 is doped with DCM2, the Alq3 single substance generates fluorescence at a wavelength of 530 nm with respect to irradiation light with a wavelength of 400 nm, whereas it is doped with DCM2. Thus, it is shown in Patent Document 1 that DCM2 absorbs the fluorescence and no fluorescence is generated at 530 nm (see FIG. 8). Similarly, when the object to be measured is composed of a stack of a plurality of organic substances, it is expected that accurate measurement cannot be performed because other organic substances absorb the fluorescence emitted by one organic substance.

  In this document, since it is difficult to measure fluorescence when a doping material is included, the reflectance with respect to excitation light is separately measured and compared with the film thickness (theoretical value) corresponding to the measured value. ing. In addition, FIG. 11 shows the relationship between the film thickness and the reflectance when the above-mentioned Alq3 is doped with DCM2. However, as shown in the figure, the reflected light from the host material is absorbed by the doping material, and the reflectance amplitude with respect to the increase in film thickness is attenuated (that is, the graph approaches a straight line parallel to the x-axis). To go). Therefore, this attenuation makes it more difficult to specify the film thickness accurately as the film thickness increases.

Naturally, not only when the organic material of a single layer contains a doping material but also in the case of a multilayer film composed of a plurality of organic materials, the other organic materials absorb the reflected light from the certain organic material. As a result, the amplitude of the reflectivity with respect to the increase in the film thickness is attenuated. Therefore, as the film thickness increases due to the increase in the number of layers as well as the film thickness in a certain layer, the accurate film thickness increases. Measurement becomes more difficult.
In particular, the thin film formed in the present invention is used for organic EL and the like, and it is necessary to cope with the case of forming a multilayer thin film related to three emission colors of red, blue and green. Dealing with issues is important.

Moreover, since many calculation elements (irradiation light intensity, fluorescence intensity of each organic substance with respect to the irradiation intensity, absorbance of each organic substance with respect to the fluorescence intensity, etc.) are required to obtain this function, an error factor increases. It becomes difficult to calculate accurately.
Furthermore, as another problem, there is a concern about deterioration of the structure caused by emitting fluorescence by irradiating the structure being formed with excitation light. Although it is not clear in principle, in a structure once excited, even if the film thickness can be accurately controlled, desired chemical performance or desired light emission characteristics as an organic EL with respect to characteristics other than the film thickness of the structure There was a case that I could not get.

In the present invention, in order to solve the above problems, the basic concept is to measure and control the film thickness using irradiation light that does not include an excitation wavelength.
A first aspect of the present invention is a film thickness measuring device for measuring the film thickness of an organic thin film formed on a structure, and projects irradiation light of a predetermined wavelength onto the structure at least during the film formation. Means for detecting the reflected light intensity or transmitted light intensity from the structure with respect to the irradiation light, and means for specifying the film thickness of the organic thin film based on the reflected light intensity or transmitted light intensity, and the predetermined wavelength is The absorption spectrum of the organic matter constituting the organic thin film was included in a wavelength range that gives an absorbance of 20% or less, preferably 10% or less, with respect to the absorbance peak value.

  Here, the means for specifying the film thickness is a memory in which a theoretical value indicating the relationship between the reflected light intensity or transmitted light intensity of the organic substance and the film thickness of the organic thin film is stored, and the reflected light intensity detected by the light receiver. Or it was set as the structure provided with the processor which calculates | requires the film thickness of organic substance based on a transmitted light intensity and a theoretical value. The function representing the theoretical value is a function without (or small) attenuation of the reflected light intensity or the transmitted light intensity with respect to the film thickness.

  Furthermore, when an organic thin film composed of a plurality of organic layers is formed on the structure, the predetermined wavelength is 20% or less, preferably 10%, with respect to the absorbance peak value for each absorption spectrum of the plurality of organic materials. In the following, it was included in the wavelength range giving the absorbance.

  A second aspect of the present invention is an organic thin film forming apparatus, the film thickness measuring apparatus of the first aspect, a vacuum chamber, one or more evaporation sources for evaporating organic matter inside the vacuum chamber, and a film thickness measuring device. It is an organic thin film formation apparatus which consists of a control means which determines the completion | finish timing of organic thin film formation based on the measurement result by.

  According to a third aspect of the present invention, there is provided a film thickness measuring method for measuring a film thickness of an organic thin film formed on a structure, wherein a light having a predetermined wavelength is projected onto the structure during film formation by a light projecting unit. A step of detecting light, a step of detecting a reflected light intensity or transmitted light intensity from the structure with respect to irradiation light by the light receiving means, and a reflected light intensity or transmitted light intensity detected by the light receiving means by a computer connected to the light receiving means; It consists of the step of specifying the film thickness of the organic thin film based on the theoretical value of the relationship between the reflected light intensity or transmitted light intensity and the film thickness stored in advance in the computer, and absorbs a predetermined wavelength of the organic matter constituting the organic thin film. The spectrum was selected so as to be included in a wavelength range giving an absorbance of 20% or less, preferably 10% or less, with respect to the absorbance peak value.

Since the present invention uses irradiation light that does not include an excitation wavelength when measuring the film thickness, absorption of irradiation light does not occur, and a multilayer film made of a plurality of organic substances or a thin film made of a doped organic substance is used. Can accurately measure and control film thickness.
Furthermore, since a wavelength at which irradiation light absorption does not occur is used, there is no attenuation (or less) in reflectance even when the film thickness increases. For example, even if the film thickness increases as a result of forming a multilayer film, it is accurate. The film thickness can be measured. Accordingly, even when the film thickness is large, high film formation accuracy can be obtained.
Therefore, the present invention is particularly useful when a multilayer film having a relatively large thickness is formed using a plurality of organic substances. In addition, since organic substances are not excited, deterioration of the structure due to fluorescence can be avoided.

  FIG. 1 shows an organic thin film forming apparatus for explaining an embodiment of the present invention, which has a vacuum chamber 1. A plurality of independent organic evaporation sources 4 are disposed on the bottom surface of the vacuum chamber 1, and a substrate holder 3 is disposed on the ceiling side. Although two organic evaporation sources 4 are provided in the figure, the number of evaporation sources may be appropriately selected. Each organic evaporation source 4 evaporates or sublimates the organic evaporation material 5 filled in the container, and generates a vapor 17 in the vacuum chamber 1. For the organic evaporation source 4, for example, an evaporation source that wraps a resistance heater around a crucible made of ceramics and generates steam by energization heating may be used. Alternatively, other heating methods, materials, and structures may be used. In the container, for example, Alq3 [Tris (8-hydroxy-quinoline) alminium] or α-NPD [N, N'-di-α-naphthyl-] which is a powdery sublimable organic compound shown in FIGS. 3A and 3B is used. An organic evaporation material such as N, N′-diphenyl- [1,1′-biphenyl] -4,4′-diamine may be filled.

  A substrate 2 is placed on a close position under the substrate holder 3, and a mask 6 for restricting a film formation region is placed under the substrate 2. Hereinafter, the portion constituting the organic thin film functional element deposited on the substrate 2 is hereinafter referred to as a structure including the element forming stage. If the combinatorial mask disclosed in Japanese Patent Application No. 2003-390319 proposed by the present applicant is used for the mask 6, a plurality of elements can be formed efficiently.

  A shutter 7 that shields the vapor is disposed between the substrate 2 and the organic evaporation source 4 so as to be openable and closable, and is controlled by the control device 16. Further, a deposition rate detecting means 15 for detecting the evaporation rate of the vapor 17 is disposed between the substrate evaporation sources. For the vapor deposition rate detecting means 15, for example, a quartz resonator is used as a suitable means capable of detecting a time-series change in the amount of deposited material, and the vapor deposition rate is determined from the change in the natural frequency of the quartz resonator. What is necessary is just to detect. The vapor deposition rate detection means 15 outputs the vapor deposition rate to the control device 16, and the control device 16 controls the evaporation source 4 to stabilize the evaporation rate.

  Near the glass substrate 2, a bundled tip 11 of the Y-branch optical fiber 8 is disposed. The branched optical fiber 9 at one end of the Y-branched optical fiber 8 is connected to a light source 12, and the optical fiber 10 at the other end is connected to a photodetector 13. The light source 12 has means for selecting and projecting light of an arbitrary wavelength range or light of an arbitrary single wavelength, and the light detector 13 is a spectroscope that senses light intensity by wavelength-separating the received light. Although it has a light intensity sensing means such as a photometer, the function of wavelength separation is not necessarily required. The optical fiber 8 irradiates the glass substrate 2 with light from the tip 11, and collects reflected light from the glass substrate 2 at the tip 11 of the optical fiber 8. Light irradiation and light collection may be performed, for example, according to a dead point of a structure formed on the substrate 2. In the embodiment, the film forming accuracy is improved by actually measuring the film thickness or material composition of the actual substrate. However, a monitor glass is prepared, and the optical fiber 8 is installed in the vicinity of the monitor glass to perform measurement and control. It is also possible to do this. The light detector 13 is connected to a computer 14, and the computer 14 is connected to a control device 16.

The computer 14 has a processor (not shown) to which the time-series change of the reflected light intensity from the light detector 13 is inputted, and a memory (not shown) in which the relationship between the reflected light intensity and the film thickness is stored as a theoretical value. Including. The measurement is performed by sequentially comparing a theoretical value calculated by a program stored in the memory in advance or a theoretical value based on a table stored in the memory with the input of the processor as an actual measurement value.
Here, the time series change means a change in incident light intensity with respect to a parameter obtained by multiplying a preset deposition rate by an elapsed time from the start of deposition. Similarly, the theoretical value is a value of incident light intensity calculated or referred to based on the above parameters.

  The control device 16 controls the evaporation source 4 or the shutter 7 based on the actually measured value of the film thickness output from the computer 14. Specifically, a target range is set in advance for the theoretical value calculated by the computer 14 using the program, and the measurement is performed when the measured value falls within the target range or when the measured value falls within the target range. What is necessary is just to control so that a film | membrane may be complete | finished.

  In addition, by using the Y branch optical fiber 8 in the embodiment, it is possible to make one introduction port into the vacuum chamber and contribute to simplification of the configuration. However, the optical fiber 9 derived from the light source and the light detection are used. The optical fibers 10 to be introduced into the device 13 may be provided independently to provide two introduction ports. In this case, the light emission position and the incident position may be provided separately. It is also conceivable to arrange a condensing lens or a wavelength cut filter between the optical fiber 10 connected to the light detector 13 and the substrate 2.

Here, selection of irradiation light will be described.
In the present invention, the wavelength of light applied to the structure is selected in a band that does not cause absorption. More specifically, the wavelength of the irradiation light is selected so as to be included in a wavelength range that gives an absorbance of a predetermined value or less with respect to the absorbance peak value in the absorption spectrum of the organic matter.

First, the case of a single layer of organic substance Alq3 will be described. FIG. 2 shows the absorbance with respect to the irradiation light wavelength when the organic substance is Alq3. Absorbance peaks when the wavelength of the irradiated light is around 400 nm, and the absorption decreases when the wavelength is 450 nm or more, and there is almost no absorption when the wavelength is 500 nm or more. FIG. 3 shows the relationship between the film thickness of the organic substance Alq3 and the reflectance when irradiation light with a wavelength of 500 nm is projected. As shown in the figure, the reflectivity with respect to the film thickness is a function with almost no attenuation (in both actual measurement and theoretical calculation). And as shown in FIG. 4, FIG. 3 can also be made into a time-axis graph based on the preset evaporation rate.
Incidentally, the numerical value (%) of the reflectance shown in FIG. 3 or FIG. 4 is appropriately scaled for convenience of measurement, and indicates a relative value in the graph. The same applies to each graph showing the subsequent reflectance.

  In this way, by using irradiation light having a wavelength that does not cause absorption, the film thickness can be controlled based on a function without attenuation (or little). Accordingly, accurate film thickness measurement can be performed even when the film thickness is large, that is, when the target point is several waves ahead from the origin of the horizontal axis of the graph.

Table 1 shows the wavelength of the irradiated light, the ratio of the absorbance to the peak, and the evaluation thereof. In the evaluation, ○ is a function with almost no attenuation in the graph corresponding to FIG. 3 or 4, and Δ is the same film formation condition as that of a general crystal type film thickness meter although there is some attenuation in the function. The film thickness accuracy below is high, and x indicates that the film thickness accuracy is lower than that of a quartz-type film thickness meter.

  As described above, since the structure of the present invention is mainly used for an organic EL or the like, usually, organic thin films for emitting red, green and blue light are laminated. Then, the case of a multilayer film will be described next. The Alq3 described above is for green light emission, the organic material for blue light emission is α-NPD, the organic material for red light emission is Alq3 as the host material, and the doping material is 10% DCM (hereinafter referred to as “Alq3 + DCM10%”). ) Is used. Here, in the case of a multilayer film in which the above three organic substances are laminated, it is necessary to select an irradiation light wavelength that does not cause absorption (that is, fluorescence) with respect to all the organic substances.

Here, as in the case of Alq3, the characteristics of the α-NPD single layer and the Alq3 + DCM 10% single layer will be described. FIG. 5 shows absorption spectra with respect to the irradiation light wavelength of α-NPD and Alq3 + DCM 10%. For comparison, the absorption spectrum of Alq3 is also plotted.
Tables 2 and 3 show the wavelength of irradiated light, the ratio to the absorbance peak, and the evaluation in the case of α-NPD and Alq3 + DCM 10%, respectively. The evaluation criteria are the same as in Table 1 shown above.

  FIGS. 6A and 6B show the relationship between the film thickness of the organic α-NPD and the reflectance when irradiation light having a wavelength of 400 nm and a wavelength of 500 nm is projected, respectively. As shown in FIGS. 6 (a) and 6 (b), it can be seen that when the absorbance is 24% of the peak (see Table 2), the attenuation is large, and when the absorbance is 0% (see Table 2), there is no attenuation. .

  As shown in Tables 1 to 3, in the case of a single layer, the wavelength of irradiation light may be selected so as to be included in a wavelength range that gives an absorbance of 20% or less with respect to the peak value of absorbance in the absorption spectrum of each organic matter. It is essential, and more preferably, it is selected to be included in a wavelength range that gives an absorbance of 10% or less. Even in the case of a multilayer film, in the case of a multilayer film in which the above three organic substances are laminated, it is only necessary to select an irradiation light wavelength that does not cause absorption (that is, fluorescence) with respect to all the organic substances.

Specifically, referring to FIG. 5 and Tables 1 to 3, when the irradiation light wavelength is 575 nm, the absorbance is about 0% with respect to the absorbance peaks of Alq3 and α-NPD, and with respect to Alq3 + DCM of 10%. Since it is 19%, it can be measured without problems. Furthermore, if the irradiation light wavelength is set to 590 nm or more, it is more preferable because it becomes 10% or less with respect to the absorbance peak of all organic substances.
In the above embodiment, the upper limit of the irradiation light wavelength to be selected may be in the visible light range, that is, about 800 nm or less.

Note that the relationship between the film thickness and the reflectance at a wavelength that does not cause absorption, a n _g refractive index of the _substrate, the refractive index n _m of the thin _film, the thickness of the thin film d, the wavelength lambda,
When δ = (2π / λ) n _m d,
The reflectance R (d) with respect to the film thickness d is
^{_{R (d) = 1-4n m 2}} n g / {n m 2 (1 + n g) 2 + (1-n m) 2 (n g 2 -n m 2) Sin 2 δ}
It is known that
As can be seen from the above equation, the amplitude of the reflectivity R (d) of the single layer film does not attenuate as the film thickness d increases.
Here, since the reflectance of the multilayer film is expressed by the product of the four-terminal matrix corresponding to the reflectance R (d) of each single layer, even in the multilayer film, the increase in the number of layers or the film thickness d The amplitude of the reflectance is not attenuated. In addition, even if there is a slight absorption and an attenuating factor is added to the reflectance, a function that hardly attenuates can be obtained as long as the above equation is dominant.

  In this way, even in the formation of a multilayer film using a plurality of organic materials, if irradiation light having a wavelength that does not absorb any of the organic materials is used, the reflection is hardly attenuated even when the film thickness is increased. Rate can be obtained. And even if a film thickness increases as a result of laminating | stacking an organic thin film, the precision of a film thickness measurement is maintained and the film-forming precision does not fall.

  FIGS. 7A and 7B show a case where irradiation light having a wavelength of 400 nm is projected onto a multilayer film composed of α-NPD having a thickness of 40 nm, Alq3 having a thickness of 60 nm, and a charge generation layer having a thickness of 20 nm. Reflectance is shown. FIG. 7 (a) shows a stack of 1 unit of the above three types, and FIG. 7 (b) shows a stack of 10 units. 8A and 8B show the reflectance when the wavelength of irradiation light is 500 nm (other conditions are the same as those in FIGS. 7A and 7B). Referring to Tables 1 and 2, since absorption occurs in both Alq3 and α-NPD at a wavelength of 400 nm, the reflectance increases as the number of layers increases and the film thickness increases as shown in FIG. Attenuates greatly. Similarly, referring to Tables 1 and 2, at a wavelength of 500 nm, Alq3 has an absorbance of about 7% of the peak, and α-NPD has an absorbance of 0% of the peak and no absorption occurs. Therefore, as shown in FIG. 8B, it can be seen that the reflectance is not attenuated even when the number of layers is increased and the film thickness is increased, which is suitable for film thickness measurement.

  In the above embodiment, the tip 11 measures the reflected light from the substrate 2. However, the tip 11 may measure the transmitted light of the substrate 2. In this case, the light projecting side and the light receiving side of the tip 11 may be separated, and the light projecting side and the light receiving side may be arranged so as to face each other with the substrate interposed therebetween. Since the reflectance and the transmittance are in an approximately inversely proportional relationship, the characteristics may be used.

The figure which shows the organic thin film formation apparatus by this invention Diagram showing absorption spectrum of organic matter The figure which shows the relationship between the film thickness of the organic thin film by this invention, and a reflectance The figure which shows the time-sequential reflectance of the organic thin film by this invention Diagram showing absorption spectrum of organic matter Diagram for explaining the present invention Diagram for explaining the present invention Diagram for explaining the present invention Diagram showing the structure of a general organic EL Diagram showing the relationship between organic excitation and fluorescence The figure which shows the relationship between the film thickness of organic thin film by conventional film thickness measurement, and reflectance

Explanation of symbols

1. Vacuum chamber 2. Substrate 3. Substrate holder 4. 4. Organic evaporation source 5. Organic evaporative material Mask 7. Shutter 8. Y-branch shaped optical fibers 9, 10. Optical fiber 11. 11. Y branch optical fiber tip Light source 13. Photodetector 14. Computer 15. Deposition rate detection means 16. Control device 17. Steam 30. Glass substrate 31. Transparent conductive film 32. Hole transport layer 33. Light emitting layer 34. Electron transport layer 35. Cathode metal 36. Can 37. EL light

Claims (7)

A film thickness measuring device for measuring a film thickness of an organic thin film formed on a structure, at least means for projecting irradiation light of a predetermined wavelength onto the organic thin film during film formation, the organic to the irradiation light Comprising: means for detecting reflected light intensity or transmitted light intensity from the thin film; and means for determining the film thickness of the organic thin film based on the reflected light intensity or transmitted light intensity;
The film thickness measuring apparatus characterized in that the predetermined wavelength is included in a wavelength range that gives an absorbance of 20% or less with respect to a peak value of absorbance of an absorption spectrum of an organic substance constituting the organic thin film.   2. The film thickness measuring apparatus according to claim 1, wherein the predetermined wavelength is included in a wavelength range that gives an absorbance of 10% or less with respect to the peak value of the absorbance. In the film thickness measuring device according to claim 1 or 2, an organic thin film composed of a plurality of organic layers is formed on the structure,
The film thickness measuring apparatus, wherein the predetermined wavelength is included in a wavelength range that gives an absorbance of 20% or less with respect to an absorbance peak value in the absorption spectrum of the plurality of organic substances.   4. The film thickness measuring apparatus according to claim 3, wherein the predetermined wavelength is included in a wavelength range that gives an absorbance of 10% or less with respect to a peak value of absorbance for each absorption spectrum of all the organic substances. A film thickness measuring device.   It is an organic thin film forming apparatus, Comprising: The film thickness measuring apparatus as described in any one of Claims 1-4, a vacuum chamber, 1 or more evaporation sources which evaporate organic substance inside this vacuum chamber, and this film thickness measurement An organic thin film forming apparatus comprising control means for determining an end timing of organic thin film formation based on a measurement result by the apparatus. A film thickness measuring method for measuring a film thickness of an organic thin film formed on a structure,
Projecting irradiation light of a predetermined wavelength onto the structure during film formation by light projecting means;
Detecting the reflected light intensity or transmitted light intensity from the structure with respect to the irradiation light by the light receiving means, and the reflected light intensity or transmitted light intensity detected by the light receiving means by a computer connected to the light receiving means; And the step of specifying the film thickness of the organic thin film based on the theoretical value of the relationship between the reflected light intensity or transmitted light intensity and the film thickness stored in advance in the computer, and the predetermined wavelength A method for measuring a film thickness, wherein an absorption spectrum of an organic substance to be constituted is selected so as to be included in a wavelength range that gives an absorbance of 20% or less with respect to a peak value of absorbance. 7. The film thickness measuring method according to claim 6, wherein the predetermined wavelength is selected so as to be included in a wavelength range that gives an absorbance of 10% or less with respect to the peak value of the absorbance. Thickness measurement method.

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