JP2021076557A - Method for evaluating optical characteristics - Google Patents

Abstract

To provide a method for evaluating optical characteristics in a shorter time at less cost.SOLUTION: The method for evaluating optical characteristics includes: a first step of causing light to enter a periodic structure and obtaining a diffraction image; a second step of adding information of a phase to the diffraction image and obtaining a reconstruction image formed by reconstructing the surface shape of the periodic structure; a third step of extracting defects in the reconstruction image and determining the ratio of the defects in the reconstruction image; and a fourth step of calculating the ratio of an effective region in the reconstruction image from the ratio of the defects and expecting an optical characteristic of the periodic structure from the ratio of the effective region.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for evaluating optical characteristics.

Metamaterials are known as artificial substances that behave in response to light, which are not found in substances in the natural world. The optical properties of a substance are determined by the material's unique properties and surface conditions. For example, when minute unit structures are periodically arranged on the surface of a substance, the substance exhibits specific optical properties. By designing the geometric size of the unit structure, the optical properties of the material can be controlled. The metamaterial can be applied to, for example, a member having wavelength selectivity (for example, Patent Document 1), and is expected to be applied to a solar cell or the like.

Japanese Unexamined Patent Publication No. 2010-027831

The specific optical properties of metamaterials are realized by periodic arrays of minute unit structures. If the unit structure is defective, the desired optical characteristics may not be obtained. Therefore, there is a need for a method that can evaluate metamaterials appropriately and efficiently.

For example, Patent Document 1 observes the surface state of a metamaterial using a scanning electron microscope (SEM) and evaluates the optical characteristics of the metamaterial by an infrared absorption spectrum. The infrared absorption spectrum can be measured using a Fourier transform infrared spectrophotometer (FT-IR).

The SEM is a device for observing the structure of a minute region and is not suitable for evaluation of a large area. Further, FT-IR is not suitable as a measurement method for quickly evaluating a large area because it interferes with infrared light while moving the moving mirror. Therefore, for example, it is not realistic to incorporate SEM or FT-IR into a mass production line to evaluate a large area measurement target.

The present invention has been made in view of the above problems, and an object of the present invention is to provide an inexpensive method for evaluating optical characteristics that can shorten the time required for evaluation.

The present invention provides the following means for solving the above problems.

(1) The method for evaluating optical characteristics according to the first aspect is a first step of incident light on a periodic structure to obtain a diffraction image and adding phase information to the diffraction image to add phase information to the surface of the periodic structure. The second step of obtaining a reconstructed image in which the shape is reconstructed, the third step of extracting defects in the reconstructed image and obtaining the ratio of the defects in the reconstructed image, and the reconstructed image from the ratio of the defects. It has a fourth step of calculating the ratio of the effective region in the above and predicting the optical characteristics of the periodic structure from the ratio of the effective region.

(2) In the second step of the method for evaluating optical characteristics according to the above aspect, the inverse Fourier transform from the diffraction image to the real image and the Fourier transform from the real image to the diffraction image are performed while changing the phase of the diffracted wave. The phase information may be optimized by repeating it a plurality of times.

(3) In the method for evaluating optical characteristics according to the above aspect, the first step is a step of projecting a diffraction pattern of light reflected by the periodic structure onto a projection surface, and a predetermined step with respect to the perpendicular direction of the projection surface. A step of detecting a projected image projected on the projection surface with a detector arranged at the position of the elevation angle of the above, and a step of correcting the projected image based on the elevation angle of the detector and obtaining the diffraction image. , May have.

(4) In the method for evaluating optical characteristics according to the above aspect, the third step is a step of discriminating the defect from the length, peripheral length, and area of the unit structure in the reconstructed image, and is determined to be the defect. It may have a step of dividing the number of the unit structures by the number of the unit structures in an ideal state without the defects to obtain the ratio of the defects.

According to the method for evaluating optical characteristics according to the above aspect, it is inexpensive and the time required for evaluation can be shortened.

It is a schematic diagram for demonstrating the 1st step in the evaluation method of the optical property which concerns on 1st Embodiment. It is a top view of the periodic structure evaluated by the evaluation method of the optical property which concerns on 1st Embodiment. It is a schematic diagram for demonstrating the 2nd step in the evaluation method of the optical property which concerns on 1st Embodiment. It is an example of a plan view of a periodic structure, a diffraction image of the periodic structure, and a reconstruction image from the diffraction image. It is an example of a plan view of a periodic structure, a diffraction image of the periodic structure, and a reconstruction image from the diffraction image. It is the surface image of the periodic structure of Example 1 and the reconstruction image of the periodic structure of Example 1. It is an evaluation result of the optical property of the periodic structure of Example 1. It is the surface image of the periodic structure of Example 2 and the reconstruction image of the periodic structure of Example 1. It is an evaluation result of the optical property of the periodic structure of Example 2.

Hereinafter, the present embodiment will be described in detail with reference to the drawings as appropriate. The drawings used in the following description may be enlarged for convenience in order to make the features of the present invention easy to understand, and the dimensional ratios of the respective components may differ from the actual ones. is there. The materials, dimensions, etc. exemplified in the following description are examples, and the present invention is not limited thereto, and the present invention can be appropriately modified without changing the gist thereof.

The method for evaluating optical characteristics according to the first embodiment includes a first step, a second step, a third step, and a fourth step. The first step is a step of incident light on the periodic structure to obtain a diffraction image. The second step is a step of adding phase information to the diffraction image to obtain a reconstructed image in which the shape of the periodic structure is reconstructed. The third step is a step of extracting defects in the reconstructed image and obtaining the ratio of defects in the reconstructed image. The fourth step is a step of calculating the ratio of the effective region in the reconstructed image from the ratio of defects and predicting the optical characteristics of the periodic structure from the ratio of the effective region. Hereinafter, each step will be described in detail.

"First step"
FIG. 1 is a schematic diagram for explaining a first step in the method for evaluating optical characteristics according to the first embodiment. The evaluation device 100 shown in FIG. 1 is a device having a light source 20, a screen 30, and a detector 40 to obtain a diffraction image of the periodic structure 10. The direction from the light source 20 toward the periodic structure 10 is the y direction, one direction of the plane orthogonal to the y direction is the x direction, and the x direction and the direction orthogonal to the y direction are the z directions.

FIG. 2 is a plan view of the periodic structure 10 evaluated by the method for evaluating optical characteristics according to the first embodiment. FIG. 2 is a plan view of the periodic structure 10 from the y direction. The periodic structure 10 is an aspect of a metamaterial. The periodic structure 10 has a plurality of unit structures 11, and the unit structures 11 are arranged periodically. In FIG. 2, the unit structures 11 are arranged in a matrix in the x-direction and the z-direction. The unit structure 11 is, for example, a hole recessed from the surface of the periodic structure 10. The size of one side of the unit structure 11 is, for example, 100 nm or more and 100 μm or less. In FIG. 2, the case where the plan view shape of the unit structure 11 is a quadrangle is illustrated, but the shape of the unit structure 11 does not matter.

The light source 20 irradiates the periodic structure 10 with visible light or infrared light, for example. The light source 20 is, for example, a CW laser that oscillates a continuous wave. The wavelength of the light L1 emitted from the light source 20 to the periodic structure 10 can be set according to the size of the periodic structure of the periodic structure 10, and is, for example, 532 nm. The spot diameter of the light L1 oscillating from the light source 20 is, for example, 1 mm or more and 10 mm or less.

The screen 30 spreads in the x-direction and the z-direction. The screen 30 has an opening 32, and the light L1 is irradiated to the periodic structure 10 through the opening 32. The screen 30 is an example of a projection surface.

The detector 40 is, for example, an image sensor such as a camera. The detector 40 is located behind the periodic structure 10 with respect to the incident direction of the light L1 and at an elevation angle θ tilted from the y direction. The elevation angle θ is, for example, 15 °.

In the first step, a diffraction image of the periodic structure 10 is obtained. First, the periodic structure 10 is irradiated with light L1 from the light source 20. The light L1 enters the surface of the periodic structure 10 through the opening 32 of the screen 30. The light L1 is reflected on the surface of the periodic structure 10. The reflected diffracted light L2 is projected on the screen 30. The projected image projected on the screen 30 shows a diffraction pattern corresponding to the periodic structure on the surface of the periodic structure 10.

Next, the detector 40 detects the projected image projected on the screen 30. The detector 40 detects the light L3 reflected from the screen 30 and images the intensity distribution of the projected image. Since the detector 40 is tilted by an elevation angle θ with respect to the perpendicular direction (y direction) of the screen 30, the image detected by the detector 40 is distorted with respect to the actual projected image projected on the screen 30. I'm out. By image-correcting the image detected by the detector 40 based on the elevation angle θ, the diffraction image becomes a diffraction pattern symmetrical with respect to the center. The diffraction pattern after this correction is a diffraction image of the periodic structure 10.

"Second step"
Next, in the second step, phase information is added to the diffraction image obtained in the first step to obtain a reconstructed image in which the shape of the periodic structure 10 is reconstructed. In the diffraction image, information on the intensity of the diffracted light L2 remains, but information on the phase does not remain. In order to reconstruct the surface shape of the periodic structure 10 from the diffraction image, phase information is added to the diffraction image.

First, the disturbance caused by the cause other than the surface shape of the periodic structure 10 is removed from the raw data of the diffraction image. The disturbance is, for example, scattering of light L1 by the opening 32 of the screen 30 or Gaussian noise of the light source 20 and the detector 40. Disturbances can be removed by subtracting the darkfield background. Gaussian noise can be removed by using a Gaussian filter.

Next, phase information is added to the diffraction image. FIG. 3 is a schematic diagram for explaining a second step of the method for evaluating optical characteristics according to the first embodiment. Information of the phase to be given to the diffraction image, the Fourier transform from the inverse Fourier transform and real g _k to 'real g _{k from'} diffraction image G on the diffraction image G _k, the phase of the diffracted wave applied to the diffraction image G ' It is preferable to repeat it a plurality of times while changing and optimize it.

First, as described above, the diffraction image G'obtained in the first step does not have phase information. Therefore, a random phase is added to the diffraction image G'and an inverse Fourier transform is performed. _{The real image g k'is} obtained by performing the inverse Fourier transform. Then, _'by imposing a constraint in the real space, real image g _k' real image g _k is corrected in real g _k. The constraints in real space are, for example, that the electron density is zero outside the region where the sample is presumed to exist, and that the electron density is not physically negative. Next, the real image g _k is Fourier transformed to obtain _{a diffraction image G k.} The diffraction image G _k by imposing a constraint in the imaginary space, the diffraction image G _k is corrected to the diffraction image G '.

The diffraction image G'obtained in the first step and the diffraction image G'obtained by giving a random phase and being calculated coincide with each other so that the randomly given phases are correct. The above Fourier transform and inverse Fourier transform are performed a plurality of times while changing the phase to be given, and the deviation between the diffraction image G'obtained in the first step and the diffraction image G'obtained by giving a random phase is reduced. By going, the phase is optimized. The Fourier transform and the inverse Fourier transform are repeated, for example, 1500 times or more. Phase optimization may be streamlined by machine learning.

4 and 5 are an example of a plan view of the periodic structure 10, a diffraction image of the periodic structure 10, and a reconstruction image 50 from the diffraction image. FIG. 4 shows the result of the ideal periodic structure 10 without defects, and FIG. 5 shows the result of the periodic structure 10 having defects contaminated by the deposit 12. 4 (a) and 5 (a) are SEM images of the periodic structure 10. 4 (b) and 5 (b) are diffraction images of the periodic structure 10. 4 (c) and 5 (c) are reconstructed images 50 of the periodic structure 10.

Both FIGS. 4 (b) and 5 (b) show diffraction patterns in which diffraction spots Sp are regularly arranged. Comparing the diffraction image shown in FIG. 4 (b) with the diffraction image shown in FIG. 5 (b), the diffraction image shown in FIG. 5 (b) has a diffraction spot as compared with the diffraction image shown in FIG. 4 (b). Light is scattered around Sp. It is considered that the diffraction pattern was disturbed by the deposit 12.

4 (c) and 5 (c) are reconstructed images 50 in which the periodic structure 10 is reconstructed based on these diffraction patterns. In the reconstructed image 50 of FIG. 4 (c), the unit structures 51 are periodically arranged, and the surface shape of the periodic structure 10 shown in FIG. 4 (a) is appropriately reconstructed. On the other hand, in the reconstructed image 50 of FIG. 5C, the unit structures 51 are arranged periodically, but there are defects 52, 53, 54. The periodic structure 10 shown in FIG. 5A has a defect, and the reconstructed image shown in FIG. 5C appropriately reconstructs the surface shape of the periodic structure 10. It is difficult to obtain a pinpoint diffraction pattern for photographing the SEM, and the plan view of FIG. 5 (a) and the reconstructed image 50 of FIG. 5 (c) do not completely match.

As described above, by performing the first step and the second step, the surface shape of the periodic structure 10 can be reconstructed from the diffraction image regardless of whether it has a defect or a defect.

"Third step"
In the third step, the defects 52, 53, 54 in the reconstructed image 50 are extracted, and the ratio of the defects 52, 53, 54 in the reconstructed image 50 is obtained.

The defect 52 is formed by connecting adjacent unit structures 51. The defect 53 is such that the size of the unit structure 51 is smaller than a predetermined size. The defect 54 is a modification of the unit structure 51. The defects 52, 53, and 54 are distinguished from the ideal unit structure 51 without defects by image processing the reconstructed image 50 with a predetermined threshold value. In the image processing, ideally, threshold values are set for each parameter of the area, the peripheral length, and the length of the portion (black portion in FIGS. 4C and 5C) corresponding to the unit structure 51, and the threshold range is set. Those that do not fit inside are determined to be defects 52, 53, 54.

For example, the threshold value of the length of the portion corresponding to the unit structure 51 of the reconstructed image 50 is set to be 1/2 times or more and 2 times or less of one side of the unit structure 11 of the actual periodic structure 10. The defect 52 is likely to have a side length of twice or more, and the defect 52 can be distinguished. Further, the defect 53 is likely to have a side length of 1/2 times or less, and the defect 53 can be distinguished.

Further, for example, the threshold value of the peripheral length of the portion corresponding to the unit structure 51 of the reconstructed image 50 is set to 1/2 times or more and 2 times or less the peripheral length of the unit structure 11 of the actual periodic structure 10. Defects 52 and 54 are likely to have a circumference that is more than doubled, and defects 52 and 54 can be distinguished. Further, the defect 53 is likely to have a peripheral length of 1/2 times or less, and the defect 53 can be distinguished.

Further, for example, the threshold value of the area of the portion corresponding to the unit structure 51 of the reconstructed image 50 is set to 1/2 or more and 2 times or less the area of the unit structure 11 of the actual periodic structure 10. The defects 52 and 54 are likely to have more than double the area, and the defects 52 and 54 can be distinguished. Further, the defect 53 is likely to have an area of 1/2 times or less, and the defect 53 can be distinguished.

By going through the above procedure, the ideal unit structure 51 and the defects 52, 53, 54 can be distinguished. Then, the number of unit structures determined to be defects 52, 53, 54 is divided by the number of unit structures 51 in the ideal state without defects to obtain the ratio of defects 52, 53, 54.

Here, the defect 52 is a defect in which n unit structures (n is an integer) are connected, and the number of defects is one. However, it is a part that originally has n unit structures, and assuming that each of the n unit structures is determined as a defect, the number of unit structures determined to be defects 52 is n. The "number of unit structures 51 in an ideal state without defects" is the number of unit structures 51 extracted when there are no defects on the entire surface of the periodic structure 10, for example, there are no defects in FIG. 5 (c). It is not the number of unit structures 51 of the part.

"Fourth step"
The fourth step predicts the optical properties of the periodic structure 10 based on the percentage of defects. First, 1- "Percentage of defects" is performed to calculate the ratio of the effective region of the periodic structure 10. It is assumed that the effective region of the periodic structure 10 is flawless and exhibits ideal optical properties. Therefore, the optical characteristics of the periodic structure 10 can be predicted only by multiplying the optical characteristics of the periodic structure in the ideal state without defects and the ratio of the effective region.

As described above, in the method for evaluating the optical characteristics according to the first embodiment, the diffraction image is measured in the first step, and the subsequent second to fourth steps are calculated by using a computer or the like. The optical characteristics of the structure 10 are expected. Therefore, the method for evaluating the optical characteristics according to the first embodiment is inexpensive because it is only necessary to prepare a simple evaluation device 100 for obtaining a diffraction image as a measurement system. Further, the method for evaluating optical characteristics according to the first embodiment can shorten the time required for evaluating optical characteristics and can be applied to mass production and the like.

(Example 1)
First, the evaluation device 100 shown in FIG. 1 was prepared. As the light source 20, a TTL-modulated CW laser having a wavelength of 532 nm and an output of 9.62 mW was used. The spot diameter of the laser was approximately 1 mm. The detector 40 was arranged at an elevation angle of 15 ° with respect to the y direction. The exposure of the detector 40 was set to 0.2 seconds.

In the periodic structure of Example 1, a periodic microcavity was formed on a Si substrate by reactive etching (RIE), and Pt was coated on the periodic microcavity by sputtering. The pitch of the adjacent unit structures of the periodic structure was 2.6 μm, one side of the opening was 1.6 μm, and the depth of the opening was 5 μm. Then, the surface of the periodic structure was contaminated and defects were provided. FIG. 6A is an SEM image of the surface of the periodic structure of Example 1.

Next, the periodic structure was installed in the evaluation device 100, and a diffraction image was obtained. Then, a reconstructed image of the periodic structure was prepared from the diffraction image. FIG. 6B is a reconstructed image of the periodic structure of Example 1.

Next, the thresholds of the unit structure length, the perimeter, and the area of the reconstructed image were set, and image processing was performed to determine the proportion of defects in the reconstructed image. The length threshold was set to be 1/2 times or more and 2 times or less of one side of the unit structure of the actual periodic structure. The threshold value of the perimeter was set to be 1/2 times or more and 2 times or less the perimeter of the unit structure of the actual periodic structure. The area threshold was set to be 1/2 times or more and 2 times or less the area of the unit structure of the actual periodic structure. In FIG. 6B, the portion determined as a defect is shown in white. The percentage of defects in the reconstructed image of Example 1 was 25.7%. The percentage of defects in the reconstructed image showed a value close to the percentage of defects in the actual periodic structure.

Next, the ratio of the effective region was calculated from the ratio of defects. The proportion of effective domain was 74.9%. FIG. 7 is an evaluation result of the optical characteristics of the periodic structure of Example 1. In FIG. 7, “Designed” is an optical characteristic in an ideal state without defects, “Measured” is an actually measured value obtained by measuring the optical characteristic of an actual periodic structure by FT-IR, and “Calculation” is the above effective region. It is a value calculated from the ratio of, and "Damage" is a conversion of the optical characteristics of the defective part. The optical characteristics of the defective portion were treated as assuming that the optical characteristics were not modulated from the optical characteristics of Pt on the substrate.

As shown in FIG. 7, “Measured” and “Calculation” show very close values, and it can be seen that the method for evaluating the optical characteristics according to the present embodiment is functioning.

(Example 2)
Example 2 was the same as that of Example 1 except that the periodic structure to be measured was changed.

In the periodic structure of Example 2, a periodic microcavity was formed on a Si substrate by reactive etching (RIE), and Pt was coated on the periodic microcavity by sputtering. The pitch of the adjacent unit structures of the periodic structure was 2.7 μm, one side of the opening was 2.1 μm, and the depth of the opening was 5 μm. Then, there was a region in which an opening was not provided in a part of the periodic structure. FIG. 8A is an SEM image of the surface of the periodic structure of Example 2.

Next, the periodic structure was installed in the evaluation device 100, and a diffraction image was obtained. Then, a reconstructed image of the periodic structure was prepared from the diffraction image. FIG. 8B is a reconstructed image of the periodic structure of Example 2. In FIG. 8B, the portion determined as a defect is shown in white.

The percentage of defects in the reconstructed image of Example 2 was 6.6%. The percentage of defects in the reconstructed image showed a value close to the percentage of defects in the actual periodic structure. The proportion of effective domain was 93.4%.

FIG. 9 is an evaluation result of the optical characteristics of the periodic structure of Example 2. In FIG. 9, "Designed" is an optical characteristic in an ideal state without defects, "Measured" is an actually measured value obtained by measuring the optical characteristic of an actual periodic structure by FT-IR, and "Calculation" is the above effective region. It is a value calculated from the ratio of, and "Damage" is the optical characteristic of the defective part.

As shown in FIG. 9, “Measured” and “Calculation” show very close values, and it can be seen that the method for evaluating the optical characteristics according to the present embodiment is functioning.

10 Periodic structure 11, 51 Unit structure 12 Adhesion 20 Light source 30 Screen 32 Aperture 40 Detector 50 Reconstructed image 52, 53, 54 Defect 100 Evaluation device L1, L3 Light L2 Diffraction light Sp Diffraction spot

Claims (4)

The first step of injecting light into the periodic structure to obtain a diffraction image,
A second step of adding phase information to the diffraction image to obtain a reconstructed image in which the surface shape of the periodic structure is reconstructed.
A third step of extracting defects in the reconstructed image and determining the proportion of the defects in the reconstructed image, and
A method for evaluating optical characteristics, which comprises a fourth step of calculating the ratio of an effective region in the reconstructed image from the ratio of the defects and predicting the optical characteristics of the periodic structure from the ratio of the effective region. In the second step, the inverse Fourier transform from the diffraction image to the real image and the Fourier transform from the real image to the diffraction image are repeated a plurality of times while changing the phase of the diffracted wave, and the information on the phase is optimized. The method for evaluating optical characteristics according to claim 1. The first step is
The process of projecting the diffraction pattern of the light reflected by the periodic structure onto the projection surface,
A step of detecting a projected image projected on the projection surface with a detector arranged at a predetermined elevation angle with respect to the perpendicular direction of the projection surface.
A step of correcting the projected image based on the elevation angle at which the detector is arranged to obtain the diffraction image, and
The method for evaluating optical characteristics according to claim 1 or 2. The third step is
A step of discriminating the defect from the length, perimeter and area of the unit structure in the reconstructed image, and
A step of dividing the number of the unit structures determined to be the defects by the number of the unit structures in an ideal state without the defects to obtain the ratio of the defects.
The method for evaluating optical characteristics according to any one of claims 1 to 3.

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