Observation method

Abstract

An observation system comprising a light source (101), an acquisition device (102), and a display device (103). The light source (101) irradiates, with ultraviolet or visible light, a layered material (113) which grows on a substrate (112) being heated inside a furnace (111). The acquisition device (102) is disposed outside the furnace (111), and acquires reflected light reflected by the surface of the substrate (112) on which the layered material (113) irradiated with light grows. The display device (103) displays the state of the reflected light acquired by the acquisition device (102). The display device (103) displays a reflected light image captured by an image capture device.

Description

[Technical field]

[0001] The present invention relates to a method for observing layered materials grown by CVD or the like. By law Regarding. [Background technology]

[0002] Layered materials such as graphene are two-dimensional materials with a thickness of an atomic or molecular layer. In the case of a layered material, for example, a laminate of two or more layers, adjacent unit layers in the stacking direction are bonded by van der Waals forces. Layered materials are also called atomic layer thickness two-dimensional materials, and some have electrical conductivity, some have semiconducting properties, and some have insulating properties, and some also have high mechanical flexibility and excellent transparency. Due to these characteristics, layered materials have been attracting attention in recent years as next-generation materials for flexible electronics applications.

[0003] An example of a conductive layered material is graphene. Examples of semiconductor layered materials include transition metal dichalgogenides such as molybdenum disulfide (MoS2) and tungsten disulfide (WS2) and phosphorene. An example of an insulating layered material is hexagonal boron nitride (h-BN).

[0004] Taking advantage of the structural characteristic of atomic layer thickness, these layered materials can be scaled down to below 10 nm, which is the limit of current silicon devices, and have the advantage that they can be processed into a variety of structures using conventional silicon processes such as lithography and etching.

[0005] In order to put devices made of layered materials into practical use, it is essential to fabricate large areas of wafer-scale materials with uniformity, high quality, and low cost, and chemical vapor deposition (CVD) is known as a representative technology for achieving this. For example, it is possible to fabricate graphene or h-BN on a large area at low cost on the surface of a metal substrate such as copper (Cu), nickel (Ni), or cobalt (Co) (Non-Patent Document 1). In the case of graphene, wafer-scale fabrication on copper foil or thin copper film has been demonstrated (Non-Patent Document 2).

[0006] To produce layered materials with good reproducibility, it is effective to build a system that observes the production process in real time and provides feedback to the production parameters. Examples of observations of the production process of layered materials in the CVD method include methods using electron beams, synchrotron radiation, and thermal radiation (infrared rays) (Non-Patent Documents 3-5).

[0007] However, both observation techniques have limited scope of use, such as being available only in ultra-high vacuum or cold-wall furnaces. For example, it is difficult to apply the above-mentioned observations to hot-wall furnaces and the pressure range from atmospheric pressure to low vacuum (several thousand Pa) that is widely used in the CVD production of large-area graphene. In addition, observation techniques using synchrotron radiation also have the problem of requiring large-scale experimental equipment. Currently, there is a demand for a system that can observe the production process of layered materials in real time under practical production conditions for layered materials. [Prior art documents] [Non-patent literature]

[0008] [Non-Patent Document 1] X. Li, et al., "Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils", Science, vol. 324, pp. 1312-1314, 2009. [Non-Patent Document 2] S. Bae et al., "Roll-to-roll production of 30-inch graphene films for transparent electrodes", Nature Nanotechnology, vol. 5, pp. 574-578, 2020. [Non-Patent Document 3] ZJ Wang et al., "Direct Observation of Graphene Growth and Associated Copper Substrate Dynamics by in Situ Scanning Electron Microscopy", ACS Nano, vol. 9, no. 2, pp. 1506-1519, 2015. [Non-Patent Document 4] PR Kidambi et al., "Observing Graphene Grow: Catalyst-Graphene Interactions during Scalable Graphene Growth on Polycrystalline Copper", Nano Letters, vol. 13, pp. 4769-4778, 2013. [Non-Patent Document 5] T. Terasawa and K. Saiki, "Radiation-mode optical microscopy on the growth of graphene", Nature Communications, vol. 6, Article number 6834, 2015. Summary of the Invention [Problem to be solved by the invention]

[0009] When fabricating large-area, wafer-scale layered materials at low cost using the CVD method, hot-wall electric furnaces are widely used, with fabrication conditions ranging from atmospheric pressure to low vacuum (several Pa). In this pressure range, the mean free path of electrons becomes significantly shorter, making observation and analysis using the aforementioned electron beam impossible. When using synchrotron radiation, large-scale equipment such as an electron accelerator is required, and it is not practical to use it in conjunction with a hot-wall furnace or other equipment for fabricating layered materials.

[0010] In addition, when preparing a layered material in a hot-wall furnace, the temperature of the entire equipment rises, resulting in a large background of thermal radiation, making it difficult to make observations using the difference in thermal radiation between the layered material and the substrate as proposed in Non-Patent Document 5.

[0011] As described above, in the past, there was a problem in that it was difficult to observe the production process of layered materials in real time under practical production conditions for layered materials, such as by CVD, which allows large-area layered materials to be produced at low cost.

[0012] The present invention has been made to solve the above problems, and has an object to make it possible to observe the production process of a layered material in real time under practical production conditions for the layered material. [Means for solving the problem]

[0013] The observation method according to the present invention includes a first step of irradiating a layered material growing on a heated substrate inside a furnace with ultraviolet or visible light, a second step of acquiring, outside the furnace, the light reflected from the surface of the substrate on which the layered material is growing, and a third step of observing the state of the acquired reflected light. In the second step, a reflected light image of the surface of the substrate on which the layered material is growing is captured, and in the third step, the captured reflected light image is observed to determine whether the layered material is graphene and whether the number of graphene layers is single-layered or double-layered based on the difference between the reflectance ratio of graphene to the substrate (reflectance ratio = 1). .

[0014] In addition, the observation according to the present invention method In the method, ultraviolet or visible light is irradiated onto a layer of material growing on a heated substrate inside a furnace. The first step and lightThe light reflected from the surface of the substrate on which the layered material is growing is captured outside the furnace. The second step, The state of the reflected light obtained The third step is to observe Equipped with In the second step, a reflected light image of the surface of the substrate on which the layered material is growing is captured by irradiating light. In the third step, the captured reflected light image is observed, and the wavelength is scanned in the observation of the reflected light image to detect a boundary point where the light and dark of the image are inverted, where a reflectance ratio of the growing layered material to the substrate > 1 indicates that the area where the layered material is formed is bright and the substrate is dark, and a reflectance ratio < 1 indicates that the area where the layered material is formed is dark and the substrate is bright, thereby identifying the layered material. . Effect of the Invention

[0015] As described above, according to the present invention, a layered material growing on a heated substrate is irradiated with ultraviolet or visible light and the reflected light is acquired outside the furnace, so that the production process of the layered material can be observed in real time under practical production conditions for the layered material. [Brief description of the drawings]

[0016] [Figure 1] FIG. 1 is a configuration diagram showing the configuration of an observation system according to an embodiment of the present invention. [Diagram 2] FIG. 2 is a flowchart illustrating the observation method according to the embodiment of the present invention. [Diagram 3] FIG. 3 is a characteristic diagram showing the wavelength dependence of the reflectance ratio of graphene to a copper substrate when graphene is produced on the surface of a copper substrate using a thermal CVD apparatus. [Figure 4A] FIG. 4A is a photograph showing the result of capturing a reflected image of graphene formed on the surface of a copper substrate using an imaging device with an area image sensor. [Figure 4B] FIG. 4B is a photograph showing the result of capturing a reflected image of graphene formed on the surface of a copper substrate using an imaging device with an area image sensor. [Figure 4C] FIG. 4C is a photograph showing the result of capturing a reflected image of graphene formed on the surface of a copper substrate using an imaging device with an area image sensor. [Figure 4D] FIG. 4D is a characteristic diagram showing a comparison of contrasts in the dotted line portions in FIG. 4B and FIG. 4C. [Figure 5A] FIG. 5A is a photograph showing the result of capturing a reflected image of graphene formed on the surface of a copper substrate using an imaging device with an area image sensor. [Figure 5B] FIG. 5B is a photograph showing the result of capturing a reflected image of graphene formed on the surface of a copper substrate using an imaging device with an area image sensor. [Figure 5C] FIG. 5C is a photograph showing the result of capturing a reflected image of graphene formed on the surface of a copper substrate using an imaging device with an area image sensor. [Figure 5D] FIG. 5D is a photograph showing the result of capturing a reflected image of graphene formed on the surface of a copper substrate using an imaging device with an area image sensor. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] An observation system according to an embodiment of the present invention will be described below with reference to Fig. 1. This system includes a light source 101, an acquisition device 102, and a display device 103. This system is an observation system for observing in real time the production process of a layered material using a practical production device such as a thermal CVD method.

[0018] The light source 101 irradiates the layered material 113 growing on the heated substrate 112 in the furnace 111 with light in the ultraviolet or visible range. The light source 101 irradiates the layered material 113 with light, for example, from outside the furnace. The light emitted by the light source 101 is reflected by the reflecting mirror 121, passes through the beam splitter 122, passes through the quartz window 114 of the furnace 111, and enters the substrate 112. In this way, by using the reflecting mirror 121, the beam splitter 122, and the like, the light source 101 can irradiate the substrate 112 with light from a direction perpendicular to the surface of the substrate 112. The reflecting mirror 121 has a structure that reflects light in the visible range and the ultraviolet range. The beam splitter 122 also corresponds to light in the visible range and the ultraviolet range.

[0019] Here, the furnace 111 is, for example, a hot-wall type reaction furnace used in a well-known thermal chemical vapor deposition (CVD) apparatus. The substrate 112 can be made of, for example, a metal such as copper, an insulator such as sapphire or silicon oxide, or a semiconductor such as Si. Also, a substrate having a silicon oxide layer formed on a Si substrate can be used.

[0020] For example, graphene can be produced as a layered material by using a thermal CVD apparatus. When producing graphene, a hydrocarbon gas such as methane or an alcohol such as ethanol can be used as a raw material. For example, a quartz tube is placed inside a furnace 111, a substrate 112 is placed inside the quartz tube, and raw material gas is made to flow through the quartz tube. Also, for example, a so-called vacuum pump can be used to selectively set the pressure inside the furnace 111 to a wide range from atmospheric pressure to vacuum. Also, the temperature during growth is selected depending on the substrate material and raw material gas, and is, for example, about 600-1200°C.

[0021] Here, the heating of the substrate 112 is often performed by a heater arranged around the substrate 112. Due to the presence of the heater or the like, it may not be easy to irradiate the surface of the substrate 112 with light from an oblique direction. In contrast, by irradiating the substrate 112 with light from a direction perpendicular to the surface of the substrate 112, the presence of the heater or the like described above can be prevented from interfering with the light irradiation.

[0022] The acquisition device 102 is disposed outside the furnace 111 and acquires reflected light reflected by the surface of the substrate 112 on which the layered material 113 is growing when irradiated with light. The acquisition device 102 is an imaging device that captures an image of reflected light from the surface of the substrate 112 on which the layered material 113 is growing when irradiated with light. The imaging device includes, for example, a photographing lens, an area image sensor sensitive to visible and ultraviolet light, and an image processing circuit. The acquisition device 102 may also be a spectroscope that acquires the spectrum of reflected light reflected by the layered material 113 on which the layered material 113 is irradiated with light.

[0023] Here, in the preparation of a layered material by thermal CVD, infrared radiation is generated due to heating during preparation. If this infrared radiation is acquired by the acquisition device 102, the background for the reflected image signal becomes large, making it difficult to acquire an accurate signal. In this case, a bandpass filter 124 is disposed before the optical path of the acquisition device 102 to remove the infrared radiation. Also, since chromatic aberration becomes large in a reflected light image containing ultraviolet light, when the light source light has a broad wavelength range, a clear reflected image can be obtained by selecting the wavelength to be imaged (acquired) using the bandpass filter 124 and suppressing the aberration. Note that the above-mentioned problem of chromatic aberration is solved by using a single wavelength as the light source light.

[0024] The light reflected from the surface of the substrate 112 on which the irradiated layered material 113 is growing passes through the quartz window 114, is reflected by the beam splitter 122, and is taken into the acquisition device 102. In this manner, by using the beam splitter 122, the acquisition device 102 can acquire the reflected light reflected in a direction perpendicular to the surface of the substrate 112.

[0025] Here, the dimensions of the layered material in the process of growth that is the object of observation are minute regions on the order of μm in plan view. Therefore, for example, by disposing the microscopic optical system 123 immediately before the acquisition device 102 in the optical path of the reflected light, the acquisition device 102 can be configured to acquire reflected light of an enlarged region. For example, the microscopic optical system 123 causes an image capture device to capture a reflected light image of the enlarged region.

[0026] The display device 103 displays the state of the reflected light acquired by the acquisition device 102. The display device 103 displays the reflected light image captured by the imaging device. The display device 103 also displays the spectral state of the reflected light acquired by the spectroscope. The display device 103 can be configured, for example, from a liquid crystal display. This system observes the reflected image using visible and ultraviolet light as a light source, does not require a vacuum environment as with electron beams, and can be used in a wide range of pressure environments.

[0027] Next, an observation method according to an embodiment of the present invention will be described with reference to Fig. 2. This method is an observation method for observing in real time the production process of a layered material using a practical production device such as a thermal CVD method.

[0028] First, in a first step S101, a layered material 113 growing on a heated substrate 112 inside a furnace 111 is irradiated with ultraviolet or visible light. For example, the light is irradiated by the above-mentioned light source 101. Alternatively, by using an optical system including a reflecting mirror 121, a beam splitter 122, etc., the substrate 112 can be irradiated with light in a direction perpendicular to the surface of the substrate 112.

[0029] Next, in a second step S102, the light reflected from the surface of the substrate 112 on which the irradiated layered material 113 is growing is acquired outside the furnace 111. For example, by using an optical system such as a beam splitter 122, the light reflected in a direction perpendicular to the surface of the substrate 112 can be acquired.

[0030] Here, in the second step S102, an acquisition device 102 including an imaging device can be used to capture a reflected light image of the surface of the substrate 112 on which the irradiated layered material 113 is growing. Also, in the second step S102, an acquisition device 102 including a spectroscope can be used to obtain a spectrum of the reflected light reflected by the irradiated layered material 113.

[0031] Next, in a third step S103, the state of the acquired reflected light is observed. For example, a reflected light image captured by an acquisition device 102 equipped with an imaging device is observed on a display device 103. In addition, the state of the spectrum of the reflected light acquired by an acquisition device 102 equipped with a spectroscope can be observed on the display device 103.

[0032] As described above, according to the embodiment, for example, the state of layered material 113 growing on heated substrate 112 inside furnace 111 can be observed in real time as an image. According to the embodiment, the state of the layered material during the preparation process can be observed in real time over a wide range of pressures and temperatures during preparation of the layered material by thermal CVD. For example, a change in reflectance due to light absorption or interference that occurs when a layered material having a refractive index different from that of the substrate is prepared on a substrate can be easily determined by observing an image captured by an imaging device or a spectrum obtained by a spectroscope, and the preparation process of the layered material can be continuously and directly observed.

[0033] When a layer of material with a thickness of d is layered on a substrate, the total reflectance R f can be expressed using the overall Fresnel reflection coefficients as follows:

[0034]

number

[0035] In this case, the Fresnel reflection coefficient ρ f , and the Fresnel reflection coefficient ρ at the layered material-substrate interface i , and the phase difference Δ can be expressed as follows:

[0036]

number

[0037] However, the complex refractive index is f =n f -ik f , substrate N s =n s -ik s The refractive index of the layered material n f , substrate n s , the incident medium is n0. The extinction coefficient is f , substrate k s It is.

[0038] Reflectance of the substrate only R s can be expressed as follows:

[0039]

number

[0040] Here, the reflectance ratios R of single-layer graphene and double-layer graphene to the copper substrate were calculated using a thermal CVD apparatus. f / R s An example of this is shown by the solid line in Figure 3. The dashed line in Figure 3 shows the wavelength dependency of thermal radiation energy at 1050°C.

[0041] For example, when observing a two-dimensional image (reflection image) using an imaging device with an area image sensor, the contrast between the layered material and the substrate depends on the wavelength of the irradiating visible / ultraviolet light source. f / R s >1, the area where the layered material is made is bright and the substrate is dark. f / R s At R < 1, the area where the layered material is made is dark and the substrate is light. f / R s At 1, the layered material is invisible. From the difference in reflectance ratio, the number of graphene layers can also be determined.

[0042] Furthermore, looking at the wavelength dependence of the reflectance ratio shown by the solid line in Figure 3, we can see that the brightness of the graphene image is inverted at the boundary of 282 nm. The wavelength at this boundary is a value specific to each material, and by scanning the wavelength when observing the reflected image and detecting the boundary point where the brightness of the image is inverted, it is possible to identify the material.

[0043] As described above, the present invention allows the process of the layered material being formed on the surface of the substrate to be directly observed in real time by obtaining a two-dimensional image of the change in reflectance of the substrate on which the layered material is growing, and further allows the number of layers of the layered material to be determined and the material to be identified.

[0044] Here, when producing a layered material in a high-temperature environment using a hot-wall furnace, it is preferable to use a light source in the visible / ultraviolet region with a wavelength of 600 nm or less in order to reduce the influence of thermal radiation (infrared rays) from the heater. As shown by the dashed line in Figure 3, it is clear that the influence of thermal radiation becomes large from the vicinity of 600 nm wavelength to the infrared region on the longer wavelength side.

[0045] In this invention, by using a light source wavelength of 600 nm or less in the visible / ultraviolet region, it is possible to achieve real-time direct observation of processes over a wide temperature range, including high temperatures of over 1000° C. General image sensors have a visible light source with a wide wavelength range and have high sensitivity in the near-infrared region centered on the visible light region, so they cannot eliminate the effects of visible light and infrared light with wavelengths of 600 nm or more, and are therefore unsuitable for real-time direct observation of high-temperature processes.

[0046] Furthermore, regarding the lenses (objective lenses) that make up the microscopic optical system, general ones are not suitable in two respects: the working distance between the lens and the sample, and the wavelength of the light that passes through. The working (moving) distance of the objective lens of a general microscopic optical system is several to several tens of mm, but in high-temperature processes using hot-wall furnaces, taking into account the structure of the furnace body and changes in the refractive index of the lens due to heat, an operating range of several hundred mm or more is required.

[0047] In addition, the wavelength of light that passes through a typical objective lens is in the visible range of about 400 to 700 nm, and cannot be used in the visible and ultraviolet ranges below 400 nm. Therefore, it is important to select and combine optical components that make up the optical system, including the microscopic optical system, between the light source and the acquisition device that are compatible with light in the visible and ultraviolet ranges.

[0048] Below, we will explain in more detail using the results of the experiment. In the following, we will describe an example in which the production process of single-layer graphene on a copper thin film by thermal CVD was observed in real time using a visible and ultraviolet light source. A substrate with a copper thin film formed on it was placed inside a quartz tube placed in a hot-wall furnace, and the temperature was raised under an Ar atmosphere. The substrate was made of MgAl2O4 single crystal. The production temperature was 1050°C and the pressure was 9000 Pa. After these production conditions were reached, the supply of methane gas, the raw material, was started.

[0049] Figures 4A, 4B, and 4C show the results of capturing reflected images of graphene fabricated on the surface of a copper substrate using an imaging device with an area image sensor. In Figure 4A, light in the visible range was used as the light source. The area image sensor was also sensitive to the infrared range to some extent. In this image, the influence of thermal radiation (infrared rays) is large, and it is not possible to distinguish between the graphene and the copper thin film.

[0050] In contrast, when a light source having a wavelength of 365 nm is used as light source light, an imaging device with an area image sensor having sensitivity in the ultraviolet range is used, and a reflected image is obtained through a bandpass filter that cuts the infrared range, an image is obtained in which the area where graphene is produced is distinguished as a dark area and the copper substrate is distinguished as a bright area, as shown in Figure 4B. Similarly, when a light source having a wavelength of 470 nm is used as light source light, an imaging device with an area image sensor having sensitivity in the ultraviolet range is used, and a reflected image is obtained through a bandpass filter that cuts the infrared range, an image is obtained in which the area where graphene is produced is distinguished as a dark area and the copper substrate is distinguished as a bright area, as shown in Figure 4C.

[0051] FIG. 4D shows a comparison of the contrast of the dotted line parts in FIG. 4B and FIG. 4C. The two dotted lines in FIG. 4D indicate the boundary between graphene and Cu. It can be seen that the contrast intensity difference between the graphene region and the copper thin film region is larger with the light source of 365 nm wavelength than with the light source of 470 nm wavelength. This contrast coincides with the difference in reflectance shown by the solid line in FIG. 3.

[0052] Figures 5A, 5B, 5C, and 5D show reflection images observed every 8 minutes using a 365 nm light source during graphene production (growth). Figure 5A is a reflection image at the start of the supply of raw material gas (0 min). Figure 5B is a reflection image 8 minutes (8 min) after the start of the supply of raw material gas. Figure 5C is a reflection image 16 minutes (16 min) after the start of the supply of raw material gas. Figure 5D is a reflection image 24 minutes (24 min) after the start of the supply of raw material gas.

[0053] It was observed that, over time, the nucleated graphene domains spread laterally (towards the substrate surface), and the area of ​​the growing graphene increased. In the figure, the area enclosed by the dotted line indicates the growing graphene. It was possible to observe the size and shape of the graphene domains, as well as to measure the coverage and production speed.

[0054] As described above, according to the present invention, the process of producing graphene by the CVD method can be observed in real time.

[0055] As described above, according to the present invention, the layered material growing on the heated substrate is irradiated with ultraviolet or visible light and the reflected light is acquired outside the furnace, so that the process of producing the layered material can be observed in real time under practical production conditions for the layered material. Thus, according to the present invention, the growth process of the layered material can be observed in real time, so that the controllability of the domain size, coverage, and number of layers of the growing layered material can be improved.

[0056] It should be noted that the present invention is not limited to the above-described embodiments, and it is apparent that many modifications and combinations can be made by a person having ordinary knowledge in the art within the technical concept of the present invention. [Explanation of symbols]

[0057] 101...light source, 102...acquisition device, 103...display device, 111...furnace, 112...substrate, 113...layered material, 114...quartz window, 121...reflecting mirror, 122...beam splitter, 123...microscopic optical system, 124...bandpass filter.

Claims

1. a first step of irradiating a layered material growing on a heated substrate in a furnace with ultraviolet or visible light; a second step of acquiring, outside the furnace, light reflected by the surface of the substrate on which the layered material is growing, to which the light has been irradiated; The third step is to observe the state of the acquired reflected light. Equipped with the second step is to capture a reflected light image of the surface of the substrate on which the layered material is growing, the reflected light image being irradiated with the light; The third step includes observing the captured reflected light image; The layered material is graphene, and the number of graphene layers is determined to be single layer or double layer based on the difference between the reflectance ratio of the graphene to the substrate and the reflectance ratio of 1. An observation method comprising:

2. a first step of irradiating a layered material growing on a heated substrate in a furnace with ultraviolet or visible light; a second step of acquiring, outside the furnace, light reflected by the surface of the substrate on which the layered material is growing, to which the light has been irradiated; The third step is to observe the state of the acquired reflected light. Equipped with the second step is to capture a reflected light image of the surface of the substrate on which the layered material is growing, the reflected light image being irradiated with the light; The third step includes observing the captured reflected light image; An observation method characterized in that the layered material is identified by scanning the wavelength during observation of a reflected light image and detecting a boundary point where the brightness of the image is inverted, whereby when the reflectance ratio of the layered material being grown to the substrate is >1, the area where the layered material is formed is bright and the substrate is dark, and when the reflectance ratio is <1, the area where the layered material is formed is dark and the substrate is bright.

Images