JP2017211293A - Image acquisition device and film thickness measurement method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image acquisition device which is of a simple construction, less affected by a variation in the refractive index of a substrate and small in measurement error, and a film thickness measurement method.SOLUTION: An image acquisition device of the present invention performs film thickness measurement utilizing optical interference, the image acquisition device comprising: a light source for radiating a measurement light; a sample stage on which a sample having a measurement object arranged on a substrate is placed; a low reflection body arranged between the bottom of the substrate and the sample stage, for reducing the measurement light reflected in the direction of the measurement object from the bottom of the substrate and the sample stage; a camera for receiving the measurement light reflected by the measurement object and converting the same to data; and control means for measuring the film thickness of the measurement object on the basis of the data acquired by the camera.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an image acquisition apparatus and a film thickness measurement method for measuring a film thickness of a paint film, a coating, or the like using optical interference.

  An electromagnetic wave having a frequency of 0.1 THz (terahertz) to 10 THz (wavelength 30 μm to 3 mm) is called a “terahertz wave”. In recent years, compact and easy-to-use terahertz light sources and light receivers have been developed, and spectroscopes and inspection systems obtained by upgrading these light sources and light receivers have been put into practical use (for example, see Patent Document 1).

  One of the characteristics of terahertz waves is that they have material permeability to materials such as paper and plastic. Non-Patent Document 1 discloses a method for measuring the coating film thickness of a vehicle using this property and using a terahertz wave pulse excited by an ultrashort pulse laser as a light source. In Non-Patent Document 1, as a method of measuring the film thickness, a so-called time-of-flight method (Time of Flight Method) that identifies the film thickness from the time difference between the return light from the front surface and the back surface when the terahertz wave pulse is irradiated. TOF method) is used.

  As another method for measuring the film thickness of a paint film or a coating, there is a method using optical interference. This method utilizes the fact that the reflected light intensity changes according to the film thickness because the reflected light reflected on the surface of the film interferes with the reflected light reflected on the back surface of the film. Patent Document 2 describes a method for measuring a film thickness of a photoresist using optical interference. Visible light and infrared light are generally suitable for measuring a relatively thin film thickness of several μm or less. For the measurement of a relatively thick film thickness of about several tens of μm to several mm, it is preferable to use optical interference of terahertz waves having a wavelength longer than these lights. Further, Patent Document 3 describes a method and apparatus for measuring film thickness using visible light based on an angle at which the reflectance of specific polarized light is minimized.

Japanese Patent Laying-Open No. 2015-122354 JP-T-2002-523963 JP 2007-078608 A

T. Yasui et al., Terahertz paintmeter for noncontact monitoring of thickness and drying progress in paint film, Applied Optics, Vol. 44, No. 32, pp. 6849-6856, November 10, 2005

  When terahertz waves are used as measurement light and the thickness of a coating film or coating deposited on the substrate is measured, the measured value of the film thickness is less when the measurement light is absorbed by the substrate at the frequency of the measurement light. May vary. This is because the terahertz wave irradiated to the object to be measured reaches the back surface of the base material, and new reflected light is generated on the back surface of the base material. As a result, the total reflected light intensity observed fluctuates due to slight fluctuations in the refractive index and thickness of the substrate, resulting in variations in the measured film thickness. Absorption of measurement light by the substrate is small, and there is a large difference between the refractive index of the sample stage on which the measurement sample is placed and the refractive index of the substrate (that is, the reflection of the measurement light at the interface where both are in contact) This phenomenon is noticeable in large cases. Patent Documents 1 to 3 and Non-Patent Document 1 do not disclose a technique for solving such a problem in film thickness measurement of a sample to be measured using a terahertz wave.

(Object of invention)
An object of the present invention is to provide an image acquisition apparatus and a film thickness measuring method with a simple configuration and a small measurement error in a film thickness measuring method using optical interference.

  An image acquisition apparatus according to the present invention is an image acquisition apparatus that performs film thickness measurement using optical interference, and includes a light source that emits measurement light and a sample stage on which a sample on which a measurement object is arranged is mounted on a substrate And a low reflector that is disposed between the bottom surface of the substrate and the sample stage and reduces the measurement light reflected from the bottom surface of the substrate and the sample stage toward the object to be measured, and is reflected by the object to be measured A camera that receives measurement light and converts it into data; and a control unit that obtains the film thickness of the object to be measured based on the data acquired by the camera.

  The film thickness measurement method of the present invention is a film thickness measurement method using optical interference, in which measurement light is emitted from a light source, and a sample in which an object to be measured is arranged on a substrate is mounted on a sample stage. Between the bottom surface of the material and the sample stage, a low reflector that reduces the measurement light reflected from the bottom surface of the substrate and the sample stage in the direction of the object to be measured is arranged, and the measurement light reflected by the object to be measured is arranged. The method is characterized in that light is received and a film thickness of the object to be measured is obtained based on the light reception result.

  INDUSTRIAL APPLICABILITY The present invention can provide an image acquisition apparatus and a film thickness measuring method capable of measuring a film thickness with high accuracy with a simple configuration in film thickness measurement using optical interference.

It is a figure which shows the structural example of the image acquisition apparatus 100 of 1st Embodiment. It is a figure which shows the simulation result of the relationship between the film thickness of the to-be-measured object 5 on the base material 6, and reflected light intensity at the time of using the image acquisition apparatus 100. FIG. It is a figure which shows the structure of the image acquisition apparatus 200 contrasted with the image acquisition apparatus 100. FIG. It is a figure which shows the simulation result of the relationship between the film thickness of the to-be-measured object 5 on the base material 6, and reflected light intensity at the time of using the image acquisition apparatus 200. FIG. It is a figure which shows the structure of the image acquisition apparatus 300 contrasted with the image acquisition apparatus 100. FIG. It is a figure which shows the simulation result of the relationship between the film thickness of the to-be-measured object 5 on the base material 6, and reflected light intensity at the time of using the image acquisition apparatus 300. FIG. It is a figure which shows the structural example of the image acquisition apparatus 400 of 2nd Embodiment. It is a figure which shows the structural example of the image acquisition apparatus 500 of 3rd Embodiment.

Next, embodiments of the present invention will be described in detail with reference to the drawings. In the following embodiments, an image acquisition apparatus using optical interference of terahertz waves having a frequency of 0.1 THz to 10 THz, which is particularly suitable for measuring a thickness of about several tens of μm to several mm will be described.
(First embodiment)
FIG. 1 is a diagram illustrating a configuration example of an image acquisition apparatus 100 according to the first embodiment of the present invention. The image acquisition device 100 is used to measure the film thickness of a measurement object using optical interference. The image acquisition device 100 includes a terahertz wave light source 1, a terahertz camera 2, a terahertz lens 3, a sample stage 7, a low reflector 8, and a control unit 9. The low reflector 8 is disposed between the sample 4 and the sample stage 7. The sample 4 includes a base material 6 and an object to be measured 5 applied on the base material 6. The object to be measured 5 is a measurement target of the film thickness by the image acquisition device 100, and is, for example, a coating or coating applied to the base material 6.

  The terahertz wave light source 1 is a light source that emits terahertz waves. As the terahertz wave light source 1, a light source that emits a terahertz wave having a narrow wavelength band and a substantially single wavelength (that is, substantially monochromatic light) may be used. By using a terahertz wave having a narrow wavelength band, it is possible to reduce a measurement error of the film thickness of the DUT 5 due to wavelength variation.

  The low reflector 8 is disposed in close contact between the bottom surface of the substrate 6 and the sample stage 7. The low reflector 8 is a material that suppresses reflection of terahertz waves in the direction of the substrate 6 at the boundary between the substrate 6 and the low reflector 8. The low reflector 8 may include an antireflection film for suppressing the reflection of the terahertz wave emitted from the terahertz wave light source 1 on the contact surface with the substrate 6. In addition, the low reflector 8 may have absorptivity with respect to terahertz waves emitted by the terahertz wave light source 1. That is, the low reflector 8 may be a radio wave absorber that absorbs the terahertz wave incident from the substrate 6 and the terahertz wave reflected by the sample stage 7.

  A part of the terahertz wave emitted from the terahertz wave light source 1 is reflected on the front and back surfaces of the object to be measured 5 and enters the terahertz lens 3. The terahertz lens 3 collects the terahertz wave and inputs it to the terahertz camera 2. By analyzing the intensity of the terahertz wave input to the terahertz camera 2 by the control unit 9, the film thickness of the object to be measured 5 can be obtained. For example, the terahertz wave light source 1 irradiates the measurement object 5 with the terahertz wave, and the terahertz camera 2 receives the terahertz waves reflected on the front surface and the back surface of the measurement object 5. Then, the terahertz camera 2 generates data of a two-dimensional distribution of the reflection intensity of the terahertz wave on the object to be measured 5 and outputs the data to the control unit 9.

  The controller 9 has a film thickness calculation function using optical interference. The control unit 9 processes the terahertz wave reflection intensity data obtained by the terahertz camera 2 to obtain the distribution of the film thickness of the object to be measured. Since a method for measuring a film thickness using optical interference is known, detailed description thereof is omitted. The control unit 9 may include a CPU (central processing unit) and a recording device such as a semiconductor memory, and the function of the control unit 9 may be realized by the CPU executing a program recorded in the recording device. The function of the control unit 9 may be included in the terahertz camera 2. The control unit 9 may have a function of controlling the entire image acquisition device 100 including the terahertz wave light source 1.

  In FIG. 2, the simulation result of the relationship between the film thickness of the to-be-measured object 5 on the base material 6 and reflected light intensity at the time of using the image acquisition apparatus 100 is shown. In the simulation of FIG. 2, the sample stage 7 is made of metal (aluminum). Aluminum has a refractive index of 133 and an extinction coefficient of 209. The measured object 5 has a refractive index of 1.49 and is slightly absorbed (extinction coefficient 0.12). Moreover, the thickness of the base material 6 is 3.2 mm. Although the refractive index of the base material 6 is about 1.6, a maximum value of about 1.62 is assumed. The frequency of the terahertz wave is 2 THz (wavelength 150 μm). The incident angle of the terahertz wave (the angle formed with the surface normal direction of the DUT 5) is 20 degrees. The polarization direction is p-polarized light. FIG. 2 shows a simulation result in the case where the low reflector 8 that sufficiently suppresses the generation of reflected light from the bottom surface of the substrate 6 toward the object to be measured 5 is used.

  In the image acquisition apparatus 100 having the configuration shown in FIG. 1, even when the refractive index of the base material varies, the low reflector 8 suppresses the generation of reflected light from the bottom surface of the base material 6 toward the object to be measured 5. Is done. For this reason, as shown in FIG. 2, it turns out that a difference hardly arises in the relationship between the film thickness of the to-be-measured object 5 on the base material 6, and the reflected light intensity from the to-be-measured object 5. FIG. That is, the image acquisition apparatus 100 can measure the film thickness with high accuracy with a simple configuration in the film thickness measurement using optical interference.

  As described above, the image acquisition apparatus 100 has a simple configuration and high accuracy by disposing the low reflector 8 between the bottom surface of the substrate 6 and the sample stage 7 in film thickness measurement using optical interference. An image acquisition apparatus capable of measuring a film thickness can be provided. The image acquisition apparatus 100 can measure the film thickness without depending on the polarization direction of the terahertz wave.

  In the first embodiment, the case where the light source is a terahertz wave light source has been described. However, the frequency band of the light source to which the configuration of the first embodiment is applicable is not limited to the terahertz band. The object to be measured 5 is not limited to a film, and the thickness of the object to be measured 5 can be measured by the image acquisition device 100 as long as it is a substance that transmits light emitted from the terahertz wave light source 1.

(Minimum configuration of the first embodiment)
The effects of the image acquisition device 100 of the first embodiment are also brought about by the following minimum configuration. Reference numerals of constituent elements of the first embodiment are shown in parentheses.

  That is, the image acquisition device (100) having the minimum configuration performs film thickness measurement using optical interference. The image acquisition device includes a light source (1), a terahertz camera (2), a sample stage (7), a low reflector (6), and a control unit (9). The light source (1) emits measurement light. The sample stage (7) carries the sample (4). The sample (4) is a measurement sample in which the object to be measured (5) is arranged on the substrate (6). The low reflector (8) is disposed between the bottom surface of the substrate (6) and the sample stage (7). The low reflector (8) has a function of reducing measurement light reflected from the bottom surface of the substrate (6) and the sample stage (7) toward the object to be measured (5). The terahertz camera (2) receives the measurement light reflected by the object to be measured (5) and generates light reception data. The control unit (9) processes the light reception data to determine the film thickness of the object to be measured (5).

  In a minimum configuration image acquisition apparatus having such a configuration, a low reflector is disposed between the bottom surface of the substrate and the sample stage. The low reflector reduces light emitted from the bottom surface of the substrate toward the object to be measured. With this configuration, it is possible to suppress the reflected light on the bottom surface of the base material or the light that has been transmitted through the base material, reflected outside, and returned to the base material from returning in the direction of the object to be measured. As a result, the influence of the reflected light generated at other locations on the intensity of the reflected light from the object to be measured can be suppressed. That is, the image acquisition apparatus having the minimum configuration can measure the thickness with high accuracy with a simple configuration in the thickness measurement using optical interference.

(First form of comparative example)
FIG. 3 is a diagram showing a configuration of the image acquisition device 200 compared with the image acquisition device 100. In the following description of the drawings, the same elements and reference numerals are assigned to the already described elements, and duplicate descriptions are omitted. Referring to FIG. 3, the image acquisition device 200 includes a terahertz wave light source 1, a terahertz camera 2, a terahertz lens 3, and a control unit 9. In the measurement using the image acquisition device 200, the sample 4 is held in the air. That is, in FIG. 3, the bottom surface of the substrate 6 is in contact with air. Note that the structure for holding the sample 4 in the air is omitted in FIG. FIG. 4 shows a simulation result of the relationship between the film thickness of the object to be measured 5 on the substrate 6 and the reflected light intensity of the terahertz wave when the image acquisition device 200 is used. Compared with the image acquisition apparatus 100 shown in FIG. 1, the image acquisition apparatus 200 of FIG. 3 does not include the sample stage 7 and the low reflector 8. The other simulation conditions are the same as the simulation of the image acquisition apparatus 100.

  FIG. 4 is a diagram showing a simulation result of the relationship between the film thickness of the measurement object 5 on the substrate 6 and the reflected light intensity when the image acquisition device 200 is used. Since the image acquisition device 200 does not include the low reflector 8 shown in FIG. 1, the reflected light from the back surface of the base material 6 due to the difference in refractive index between the bottom of the base material 6 and air is measured. 5 has a great influence on the intensity of reflected light on both sides. As a result, the measured value of the film thickness of the object to be measured 5 varies greatly due to the variation in the refractive index of the substrate 6. FIG. 4 shows that when the refractive index of the base material varies from 1.6 to 1.62, the reflectance when the object to be measured 5 is viewed from above is greatly changed. That is, FIG. 4 shows that when the refractive index of the substrate 6 varies, it is difficult for the image acquisition device 200 to accurately measure the film thickness of the object to be measured 5 unless the refractive index of the substrate is accurately known. Indicates.

(Second form of comparative example)
The configuration of another image acquisition device compared to the image acquisition device 100 will be described. FIG. 5 is a diagram illustrating a configuration example of the image acquisition device 300 compared with the image acquisition device 100. Referring to FIG. 5, the image acquisition apparatus 300 includes a terahertz wave light source 1, a terahertz camera 2, a terahertz lens 3, a sample stage 7, and a control unit 9. In the image acquisition device 300, the sample 4 is directly placed on the sample stage 7.

  FIG. 6 is an example of a simulation result of the relationship between the film thickness of the measurement object 5 on the substrate 6 and the reflected light intensity of the terahertz wave when the image acquisition device 300 is used. Compared to the image acquisition device 100 shown in FIG. 1, the image acquisition device 300 of FIG. 5 does not include the low reflector 8 between the sample 4 and the sample stage 7. The other simulation conditions are the same as the simulation conditions in the image acquisition apparatus 100.

  In the image acquisition device 300, the bottom of the substrate 6 and the sample stage 7 in FIG. For this reason, the reflected light from the back surface of the base material 6 to the terahertz camera 2 due to the difference in refractive index between the base material 6 and the sample stage 7 greatly affects the intensity of the reflected light on both surfaces of the object to be measured 5. FIG. 6 is a diagram showing a simulation result of the relationship between the film thickness of the measurement object 5 on the substrate 6 and the reflected light intensity when the image acquisition device 300 is used. FIG. 6 shows that also in the image acquisition device 300, when the refractive index of the base material varies from 1.6 to 1.62, the reflectance viewed from the upper surface of the DUT 5 greatly changes. As described above, also in the image acquisition device 300 according to the second embodiment of the comparative example, the measured value of the film thickness of the DUT 5 greatly changes due to the variation in the refractive index of the substrate 6. That is, FIG. 6 shows that when the refractive index of the base material varies, it is difficult for the image acquisition device 300 to accurately measure the film thickness unless the refractive index of the base material is accurately known. Yes.

(Second Embodiment)
Referring to FIG. 7, the image acquisition apparatus 400 according to the second embodiment includes a terahertz light source 1, a collimating lens 10, a terahertz camera 2, a terahertz lens 3, a sample stage 7, and a control unit 9. The image acquisition device 400 includes a low reflector 8 between the sample 4 and the sample stage 7. The collimating lens 10 converts the terahertz wave emitted from the terahertz wave light source 1 into a terahertz wave that propagates substantially in parallel. By disposing the collimating lens 10 between the terahertz wave light source 1 and the object 5 to be measured, the terahertz wave emitted from the terahertz wave light source 1 is irradiated to the sample as substantially parallel light. As a result, in addition to the effects of the image acquisition device 100, the image acquisition device 400 affects the measurement result of the film thickness of the measurement target 5 due to the dispersion of the incident angle of the terahertz wave on the measurement target 5. There is an effect that it can be reduced.

(Third embodiment)
Referring to FIG. 8, an image acquisition apparatus 500 according to the third embodiment includes a terahertz light source 1, a terahertz camera 2, a terahertz lens 3, a sample stage 7, a low reflector 8, and a control unit 9. A low reflector 8 is disposed between the sample 4 and the sample stage 7. The sample stage 7 of the image acquisition device 500 has a function of moving in a direction substantially parallel to the surface of the DUT 5 with the low reflector 8 and the sample 4 placed thereon. The movement of the sample stage 7 may be controlled by the control unit 9. Or you may control by the stage drive device which is not illustrated. The image acquisition apparatus 500 having such a configuration can acquire an image of the DUT 5 while moving the sample stage 7. As a result, in addition to the effect of the image acquisition device 100 of the first embodiment, the image acquisition device 500 has an effect that it is possible to measure the film thickness of a large-area sample. In the present embodiment, an example in which the sample stage 7 is moved has been described. However, the same effect can be obtained by moving the terahertz wave light source 1, the terahertz camera 2, and the terahertz lens 3 together with the object to be measured 5. It is clear.

  Embodiments of the present invention can be described as the following supplementary notes, but are not limited thereto.

(Appendix 1)
An image acquisition device that performs film thickness measurement using optical interference,
A light source that emits measurement light;
A sample stage on which a sample in which an object to be measured is arranged on a substrate is mounted;
A low reflector that is disposed between the bottom surface of the base material and the sample stage and reduces the measurement light reflected from the bottom surface of the base material and the sample stage toward the object to be measured;
A camera that receives the measurement light reflected by the object to be measured and converts it into data;
Control means for measuring the film thickness of the object to be measured based on data acquired by the camera;
An image acquisition apparatus comprising:

(Appendix 2)
The image acquisition apparatus according to appendix 1, wherein the low reflector includes an absorber having optical absorptivity with respect to the frequency range of the measurement light.

(Appendix 3)
The image acquisition apparatus according to appendix 1 or 2, wherein the measurement light is substantially monochromatic light having a frequency of 0.1 THz to 10 THz.

(Appendix 4)
4. The image acquisition apparatus according to claim 1, wherein the light source includes a collimator lens that irradiates the measurement object with the measurement light as substantially parallel light. 5.

(Appendix 5)
The sample stage is movable in a direction parallel to the surface of the object to be measured with respect to the position irradiated with the measurement light, and the camera is reflected by the object to be measured during the movement of the sample stage. The image acquisition device according to any one of appendices 1 to 4, wherein the measurement light is received and converted into data.

(Appendix 6)
A film thickness measuring method using optical interference,
Radiates measurement light from the light source,
Mount the sample with the object to be measured on the substrate on the sample stage,
Between the bottom surface of the base material and the sample stage, a low reflector that reduces the measurement light reflected from the bottom surface of the base material and the sample stage toward the object to be measured is disposed,
Receiving the measurement light reflected by the object to be measured and converting it into data;
Obtain the film thickness of the object to be measured based on the data,
Film thickness measurement method.

(Appendix 7)
The film thickness measuring method according to appendix 6, wherein the low reflector includes an absorber having optical absorptivity with respect to the frequency range of the measurement light.

(Appendix 8)
The film thickness measuring method according to appendix 6 or 7, wherein the measuring light is substantially monochromatic light having a frequency of 0.1 THz to 10 THz.

(Appendix 9)
The film thickness measuring method according to any one of appendices 6 to 8, wherein the measurement light is irradiated as substantially parallel light onto the object to be measured.

(Appendix 10)
Moving the sample stage in a direction parallel to the surface of the object to be measured with respect to the position irradiated with the measurement light;
Receiving the measurement light reflected by the object to be measured during the movement of the sample stage and converting it into the data;
10. The method for measuring a film thickness according to any one of appendices 6 to 9, characterized in that:

  Although the present invention has been described with reference to the embodiments, the present invention is not limited to the above embodiments. Various changes that can be understood by those skilled in the art can be made to the configuration and details of the present invention within the scope of the present invention.

  Further, the configurations described in the respective embodiments are not necessarily mutually exclusive. The operation and effect of the present invention may be realized by a configuration in which all or part of the above-described embodiments are combined.

DESCRIPTION OF SYMBOLS 1 Terahertz wave light source 2 Terahertz camera 3 Terahertz lens 4 Sample 5 Measured object 6 Base material 7 Sample stage 8 Low reflector 9 Control part 10 Collimating lens 100-500 Image acquisition apparatus

Claims (10)

An image acquisition device that performs film thickness measurement using optical interference,
A light source that emits measurement light;
A sample stage on which a sample in which an object to be measured is arranged on a substrate is mounted;
A low reflector that is disposed between the bottom surface of the base material and the sample stage and reduces the measurement light reflected from the bottom surface of the base material and the sample stage toward the object to be measured;
A camera that receives the measurement light reflected by the object to be measured and converts it into data;
Control means for measuring the film thickness of the object to be measured based on data acquired by the camera;
An image acquisition apparatus comprising:   The image acquisition apparatus according to claim 1, wherein the low reflector includes an absorber having optical absorptivity with respect to a frequency range of the measurement light.   The image acquisition apparatus according to claim 1, wherein the measurement light is substantially monochromatic light having a frequency of 0.1 THz to 10 THz.   4. The image acquisition apparatus according to claim 1, wherein the light source includes a collimator lens that irradiates the measurement object with the measurement light as substantially parallel light. 5.   The sample stage is movable in a direction parallel to the surface of the object to be measured with respect to the position irradiated with the measurement light, and the camera is reflected by the object to be measured during the movement of the sample stage. The image acquisition apparatus according to claim 1, wherein the measurement light is received and converted into data. A film thickness measuring method using optical interference,
Radiates measurement light from the light source,
Mount the sample with the object to be measured on the substrate on the sample stage,
Between the bottom surface of the base material and the sample stage, a low reflector that reduces the measurement light reflected from the bottom surface of the base material and the sample stage toward the object to be measured is disposed,
Receiving the measurement light reflected by the object to be measured and converting it into data;
Obtain the film thickness of the object to be measured based on the data,
Film thickness measurement method.   The film thickness measuring method according to claim 6, wherein the low reflector includes an absorber having optical absorptivity with respect to a frequency range of the measurement light.   The film thickness measurement method according to claim 6 or 7, wherein the measurement light is substantially monochromatic light having a frequency of 0.1 THz to 10 THz.   The film thickness measuring method according to claim 6, wherein the measurement light is irradiated as substantially parallel light onto the object to be measured. Moving the sample stage in a direction parallel to the surface of the object to be measured with respect to the position irradiated with the measurement light;
Receiving the measurement light reflected by the object to be measured during the movement of the sample stage and converting it into the data;
The film thickness measuring method according to claim 6, wherein the film thickness is measured.

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