JP4450979B2 - Optical aperture fabrication method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for manufacturing an optical aperture. In particular, the present invention relates to a method for manufacturing an aperture used in a near-field optical device that irradiates and detects near-field light.
[0002]
[Prior art]
A scanning probe microscope (SPM) typified by a scanning tunneling microscope (STM) or an atomic force microscope (AFM) is used to observe a minute region on the order of nanometers on the sample surface. SPM scans a sample surface with a probe with a sharpened tip, and observes the interaction between the probe and the sample surface, such as tunneling current and atomic force, with an image of resolution depending on the probe tip shape. However, restrictions on the sample to be observed are relatively severe.
[0003]
Therefore, the near-field optical microscope (SNOM), which enables observation of a minute region on the sample surface by focusing on the interaction between the near-field light generated on the sample surface and the probe, is drawing attention. Has been.
[0004]
In the near-field optical microscope, the surface of the sample is irradiated with near-field light from an opening provided at the tip of a sharpened optical fiber. The aperture has a size equal to or smaller than the diffraction limit of the wavelength of light introduced into the optical fiber, and has a diameter of about 100 nm, for example. The distance between the opening formed at the probe tip and the sample is controlled by the SPM technique, and the value is equal to or smaller than the size of the opening. At this time, the spot diameter of the near-field light on the sample is substantially the same as the size of the opening. Therefore, it is possible to observe the optical properties of the sample in a minute region by scanning near-field light irradiated on the sample surface.
[0005]
In addition to being used as a microscope, by introducing relatively high intensity light toward the sample through the optical fiber probe, near-field light having a high energy density is generated at the opening of the optical fiber probe, and the sample surface is generated by the near-field light. The present invention can also be applied as a high-density optical memory recording that locally changes the structure or physical properties. In order to obtain near-field light with high intensity, attempts have been made to increase the tip angle of the probe tip.
[0006]
In these devices using near-field light, the formation of openings is the most important. As one of the methods for forming the opening, a method disclosed in Japanese Patent Laid-Open No. 5-21201 is known. In the method of manufacturing an opening disclosed in Japanese Patent Laid-Open No. 5-21201, a sample in which a light shielding film is deposited on a sharpened light wave guide is used as a sample for forming an opening. In the manufacturing method of the opening, the light shielding film at the tip is plastically deformed by pressing a sharpened light wave guide with a light shielding film against a hard flat plate with a very small pressing amount well controlled by a piezoelectric actuator.
[0007]
As a method for forming the opening, there is a method disclosed in JP-A-11-265520. In the method of manufacturing an opening disclosed in Japanese Patent Laid-Open No. 11-265520, a target for forming an opening is a projection tip formed by a focused ion beam (FIB) on a flat plate. The opening is formed by irradiating the light shielding film at the tip of the protrusion with FIB from the side surface and removing the light shielding film at the tip of the protrusion.
[0008]
[Problems to be solved by the invention]
However, according to the method disclosed in Japanese Patent Laid-Open No. 5-21201, an opening can be formed only for each light guide. Further, according to the method of Japanese Patent Laid-Open No. 5-21201, it is necessary to control the pushing amount by a piezoelectric actuator having a moving resolution of several nanometers, so that the opening forming device is less influenced by vibrations of other devices and air. It must be smelled. In addition, it takes time to adjust the light propagating rod so that it is perpendicular to the flat plate. In addition to a piezoelectric actuator with a small amount of movement, a mechanical translation table with a large amount of movement is required. Furthermore, a control device is required to control the push-in amount using a piezoelectric actuator having a small moving resolution, and it takes several minutes to control and form the opening. Therefore, a large-scale device such as a high voltage power supply or a feedback circuit is required for manufacturing the opening. Further, there is a problem that the cost for forming the opening is increased.
[0009]
Further, according to the method of JP-A-11-265520, the object to be processed is a projection on a flat plate, but since the opening is formed using FIB, the time required for forming one opening is as long as about 10 minutes. . Also, in order to use FIB, the sample must be placed in a vacuum. Therefore, there is a problem that the manufacturing cost for the opening manufacturing becomes high.
[0010]
[Means for Solving the Problems]
The present invention has been made in view of the above problems,
In a method for producing an optical aperture in which an optical aperture is formed at the tip of a cone-shaped chip,
The chip;
The contact that is arranged in the vicinity of the chip and has substantially the same height as the chip,
At least with respect to an opening forming body made of a light shielding film formed on the chip,
A pusher that covers at least the tip and at least a portion of the degree;
The optical opening is formed by displacing with a force having a component toward the chip to form an optical opening at the tip of the chip.
[0011]
Therefore, according to the method for producing an optical aperture of the present invention, the displacement of the plane is controlled by the degree of contact having substantially the same height as the chip. Therefore, simply pressing the plane with a predetermined force makes it easy. An optical aperture can be created. In addition, the opening can be formed under various environments such as vacuum, liquid, and air. In addition, since no special control device is required when producing the optical aperture, the device for producing the optical aperture can be simplified. In addition, the time for applying the predetermined force can be easily shortened, and the time required for opening preparation can be shortened. Therefore, the cost required for opening preparation can be reduced.
[0012]
In addition, a method for producing an optical aperture is provided, wherein the shape of the optical aperture is measured, and the force is controlled based on the measurement result.
[0013]
Therefore, the openings having the same shape can be stably produced without being affected by factors such as the thickness distribution of the light shielding film, the warpage of the substrate, and the operation variation of the mechanism that applies force.
[0014]
In addition, a method for producing an optical aperture is characterized in that the size of the optical aperture is measured and the force is controlled based on the measurement result.
[0015]
Therefore, the openings of the same size can be stably produced without being affected by factors such as the thickness distribution of the light shielding film, the warp of the substrate, and the operation variation of the mechanism that applies force.
[0016]
In addition, an optical aperture manufacturing method is characterized in that the measurement is performed by a microscope.
[0017]
Accordingly, openings having the same shape or size can be stably produced with a simple apparatus configuration.
[0018]
Further, an optical aperture manufacturing method is characterized in that the measurement is measurement of electric resistance of the light shielding film.
[0019]
Therefore, the measurement can be automatically performed at a high speed, and an opening suitable for mass production can be manufactured at a low cost.
[0020]
Further, the optical aperture manufacturing method is characterized in that the measurement is measurement of the light transmission amount of the aperture.
[0021]
Therefore, high-accuracy and high-speed aperture measurement can be performed, and aperture fabrication that is inexpensive and suitable for mass production is possible.
[0022]
Further, the method for producing an optical aperture is characterized in that the control of the force is control of the magnitude of the force.
[0023]
Therefore, it is possible to stably produce a large number of openings with an apparatus having a simple configuration.
[0024]
Further, the method for producing an optical aperture is characterized in that the control of the force is control of the number of times of displacement.
[0025]
Accordingly, a large number of openings can be manufactured in a short time at a low cost.
[0026]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, a method for forming an opening according to the present invention will be described in detail with reference to the accompanying drawings.
(Embodiment 1)
1 to 3 are diagrams for explaining a method of forming an opening according to the first embodiment of the present invention. A workpiece 1000 shown in FIG. 1 includes a transparent layer 5 formed on a substrate 4, a weight-shaped chip 1 formed on the transparent layer 5 and a ridge-shaped degree 2, a chip 1, a degree 2 and a transparent The light shielding film 3 is formed on the layer 5. In the workpiece 1000, the transparent layer 5 is not necessarily required. In this case, the light shielding film 3 is formed on the chip 1, 2 per degree, and the substrate 4. Further, the light shielding film 3 may be deposited only on the chip 1.
[0027]
The height H1 of the chip 1 is several mm or less, and the height H2 of 2 per degree is several mm or less. The difference between the height H1 and the height H2 is 1000 nm or less. The distance between the tip 1 and 2 per degree is several mm or less. Further, the thickness of the light shielding film 3 is several tens nm to several hundreds nm although it varies depending on the material of the light shielding film 3.
[0028]
The chip 1, 2 per degree and the transparent layer 5 are made of a dielectric material having a high transmittance in the visible light region such as silicon dioxide and diamond, a dielectric material having a high transmittance in the infrared light region such as zinc selenium and silicon, and fluoride. A material having a high transmittance in the ultraviolet region such as magnesium or calcium fluoride is used. The material of the chip 1 can be any material as long as it transmits the chip 1 even in the wavelength band of light passing through the opening. The chip 1, 2 per degree, and the transparent layer 5 may be made of the same material, or may be made of different materials. For the light shielding film 3, for example, a metal such as aluminum, chromium, gold, platinum, silver, copper, titanium, tungsten, nickel, cobalt, or an alloy thereof is used.
[0029]
FIG. 2 is a view showing a state in which the light shielding film 3 on the chip 1 is plastically deformed in the method of forming the opening. On the workpiece 1000 shown in FIG. 1, a plate 6 that covers a part of chip 1 and at least 2 per degree and is flat on at least 2 sides per chip 1 and degree is placed. The pushing tool 7 is placed. At this time, transparent glass was used for the plate 6. Even after the plate 6 is placed, the position of the chip 1 can be confirmed by a microscope or visually. In the present embodiment, the positions of the chip 1 and the pushing tool 7 are confirmed from two directions with a microscope, and the pushing tool 7 is arranged directly above the chip 1. By applying a force F to the pushing tool 7 in the direction of the central axis of the chip 1, the plate 6 moves toward the chip 1. Compared to the contact area between the chip 1 and the plate 6, the contact area between the plate 2 and the plate 6 is several hundred to several tens of thousands times larger. Thus, the applied force F is distributed by 2 per degree, resulting in a smaller displacement of the plate 6. Since the displacement amount of the plate 6 is small, the plastic deformation amount that the light shielding film 3 receives is very small. Also, tip 1 and 2 per degree only undergo very little elastic deformation. The method of applying force F includes a method of lifting a weight having a predetermined weight by a predetermined distance and allowing it to fall freely, or a method of attaching a spring having a predetermined spring constant to the pushing tool 7 and pressing the spring by a predetermined distance. is there. When the plate 6 is harder than the light-shielding film and is softer than the chip 1 and 2 per degree, the force received by the chip 1 and 2 per degree is absorbed by the plate 6, so that the displacement amount of the plate 6 is further increased. It becomes small and it becomes easy to make the plastic deformation amount of the light shielding film 3 small.
[0030]
FIG. 3 is a view showing a state in which the plate 6 and the pushing tool 7 are removed after the force F is applied. Since the amount of plastic deformation of the light shielding film 3 is very small and the tip 1 and 2 per degree are displaced only in the elastic deformation region, an opening 8 is formed at the tip of the tip 1. The size of the opening 8 is about the diffraction limit of the wavelength of light passing through the chip 1 from several nm. In the above description, the plate 6 is inserted between the pushing tool 7 and the workpiece 1000. However, the opening 8 can be similarly formed by removing the plate 6 and directly pushing it with the pushing tool 7. In order to introduce light into the opening 8, the transparent body 5 or at least a part of the chip 1 is exposed by etching the substrate 4 from the side opposite to the formation surface of the chip 1, so that the light introduction port into the opening 8 is formed. Form. Further, by forming the substrate 4 from the transparent material 103, the step of forming the light inlet can be omitted.
[0031]
As described above, according to the opening manufacturing method of the present invention, the displacement amount of the plate 6 can be favorably controlled by 2 per degree, and the displacement amount of the plate 6 can be very small. Can be easily produced at the tip of the chip 1. Further, near-field light can be generated from the opening 8 by irradiating light from the substrate side.
[0032]
Next, the manufacturing method of the workpiece | work 1000 is demonstrated using FIGS. FIG. 4 shows a state in which the transparent material 103 is formed on the substrate material 104, and then the chip mask 101 and the contact mask 102 are formed. FIG. 4A shows a top view, and FIG. 4B shows a cross-sectional view at a position indicated by AA ′ in FIG. 4A. The transparent material 103 is formed on the substrate material 104 by chemical vapor deposition (CVD) or spin coating. The transparent material 103 can also be formed on the substrate material 104 by a method such as solid phase bonding or adhesion. Next, the chip mask 101 and the contact mask 102 are formed on the transparent material 103 by a photolithography process. The chip mask 101 and the contact mask 102 may be formed simultaneously or separately.
[0033]
The chip mask 101 and the contact mask 102 depend on the material of the transparent material 103 and the etchant used in the next process, but use a photoresist or a nitride film. The transparent material 103 is made of a dielectric material having high transmittance in the visible light region such as silicon dioxide or diamond, a dielectric material having high transmittance in the infrared light region such as zinc selenium or silicon, or magnesium fluoride or calcium fluoride. A material having high transmittance in the ultraviolet region is used.
[0034]
The diameter of the chip mask 101 is, for example, several mm or less. The width W1 of the contact mask 102 is, for example, the same as or smaller than the diameter of the chip mask 101 by several tens of nm to several μm. Further, the width W1 of the contact mask 102 may be several tens nm to several μm larger than the diameter of the chip mask 101. Further, the length of the contact mask 102 is several tens of μm or more.
[0035]
FIG. 5 shows a state in which the chip 1 and 2 per degree are formed. 5A is a top view, and FIG. 5B is a cross-sectional view at a position indicated by AA ′ in FIG. 5A. After the chip mask 101 and the degree mask 102 are formed, the chip 1 and 2 per degree are formed by isotropic etching by wet etching. By adjusting the relationship between the thickness of the transparent material 103 and the height of the chip 1 and 2 per degree, the transparent layer 5 shown in FIG. 1 may or may not be formed. The tip radius of the chip 1 is several nm to several hundred nm. Thereafter, a work 1000 shown in FIG. 1 can be formed by depositing a light shielding film by a method such as sputtering or vacuum evaporation. When the light shielding film 3 is deposited only on the chip 1, in the deposition process of the light shielding film 3, a metal mask having a shape such that the light shielding film is deposited on the chip 1 is placed on the chip 1 and sputtering or vacuum evaporation is performed. Further, even if a photolithography process in which the light shielding film 3 remains only on the chip 1 after the light shielding film 3 is deposited on the entire surface of the workpiece 1000 on which the chip is formed, the light shielding film 3 is formed only on the chip 1. Can be formed.
[0036]
6 and 7 are diagrams for explaining the relationship between the tip 1 and the height of 2 per degree in the method for manufacturing the workpiece 1000 described above. Hereinafter, a case where the diameter of the chip mask 101 is smaller than the width of the contact mask 102 will be described. 6 is a view showing only chip 1 and 2 per degree in the process described in FIG. 5A, and FIG. 7 is a view of chip 1 at a position indicated by BB ′ in FIG. It is sectional drawing of 2 per degree of the position shown by middle CC '.
[0037]
FIG. 7A shows a state in which the chip 1 has just been formed. Since the width of the contact mask 102 is larger than the diameter of the chip mask 101, in the state of FIG. 7A, a flat portion remains on the upper surface of the contact 2 in the state shown in FIG. The winning mask 102 remains. However, the chip mask 101 comes off because the contact area with the chip 1 is very small. In the state of FIG. 7A, the height H11 of the chip 1 and the height H22 of 2 per degree are the same.
[0038]
FIG. 7B shows a state in which etching is further advanced from the state of FIG. When further etching is performed from the state of FIG. 7A, the height H111 of the chip 1 without the chip mask 101 gradually decreases. On the other hand, the height H222 of 2 per degree where the perimeter mask remains remains the same as H22. The width of the flat portion of the upper surface at 2 per degree gradually decreases, and the cross-sectional shape becomes a triangle as shown in FIG. At this time, the difference ΔH between the height of chip 1 and 2 per degree differs depending on the difference between the diameter of chip mask 101 and the width of mask 102 and the tip angle of chip 1 and 2 per degree. It is about 1000 nm or less.
[0039]
FIG. 7C shows a state where the etching is further advanced from the state of FIG. 7B. The height H1111 of the chip 1 is lower than the height H111. Similarly, the height of H2222 per degree is also smaller than the height H222. However, since the amount of decrease of the height H1111 and the height H2222 is the same, the difference ΔH between the height of the chip 1 and 2 per degree does not change. When the width of the contact mask 102 is smaller than that of the chip mask 101, the relationship between the height of the chip 1 and the contact height 2 is only reversed. When the chip mask 101 and the degree mask 102 are equal, the height of the chip 1 and the degree 2 is equal.
[0040]
According to the method for manufacturing the workpiece 1000 of the present invention, the height difference ΔH between the chip 1 and 2 per degree can be satisfactorily controlled by the photolithography process. Therefore, in the opening manufacturing method described with reference to FIGS. 1 to 3, the displacement amount of the plate 6 can be well controlled.
[0041]
As described above, according to the first embodiment of the present invention, the height of the chip 1 and 2 per degree can be controlled well, and the amount of displacement of the plate 6 can be reduced by providing 2 per degree. Since it can be made small, it is easy to form a fine opening 8 having a uniform size at the tip of the chip 1 without using an actuator with high resolution. In our experiment, it was possible to form an opening 8 having a diameter of 100 nm or less simply by striking the pushing tool 7 with a hammer held in the hand. Further, since the height of the chip 1 and 2 per degree is well controlled, the production yield of the openings 8 is improved. In addition, since the workpiece 1000 described in the first embodiment of the present invention can be manufactured by a photolithography process, a plurality of workpieces 1000 can be manufactured on a sample having a large area such as a wafer, and the force F is constant. By doing so, it is possible to form the openings 8 having a uniform opening diameter for each of the plurality of workpieces 1000 produced. In addition, since it is very easy to change the magnitude of the force F, it is possible to form the openings 8 having different opening diameters individually for a plurality of workpieces 1000 produced. Further, since the opening 8 is formed simply by applying the force F, the time required for opening preparation is very short, from several seconds to several tens of seconds. Moreover, according to Embodiment 1 of this invention, a processing atmosphere is not ask | required. Therefore, it can be processed in the atmosphere, and the processing state can be immediately observed with an optical microscope or the like. Further, by processing in a scanning electron microscope, it is possible to observe the processing state with a higher resolution than that of an optical microscope. Moreover, since the liquid functions as a damper by processing in the liquid, processing conditions with improved controllability can be obtained.
[0042]
It is also possible to produce a plurality of apertures 8 having the same aperture diameter at a time by applying force F to a sample on which a plurality of workpieces 1000 are produced. In the case of batch processing, although depending on the number of workpieces 1000 per wafer, the processing time per opening is extremely short, being several hundred milliseconds or less.
(Embodiment 2)
FIG. 8 shows an apparatus 500 for producing an optical aperture by the method according to Embodiment 2 of the present invention. A substrate 508 on which a cone-shaped chip (not shown) is formed is fixed on an xy stage 509. The xy stage 509 is controlled by the computer 501 and controls the horizontal (xy) position of the substrate 508. The pushing tool 507 is connected to a gear 506 by an arm 505, and the arm 505 can move in an arc as indicated by a broken line in the drawing. The measurement light 511 emitted from the laser 512 is collected by the lens 510 and applied to a chip (not shown). The transmitted light 504 from the opening at the tip of the chip is collected by the lens 503 and detected by the light detection unit 502. The computer 501 receives the transmitted light amount data detected by the light detection unit 502 and determines the aperture size. If this size is smaller than the desired size, the arm 505 is moved further upward by the gear 506 and then the push-out tool 507 is dropped to apply force to the chip. On the contrary, when the opening size is larger than the desired size, the arm 505 is controlled to move only downward.
[0043]
In this way, the size of the formed opening is evaluated by measuring each time, and the result is fed back to control the force applied to the chip, so that an optical opening having a desired size can be produced stably at all times. Can do. In the present embodiment, the light transmission amount is measured as a method for measuring the opening size. However, it can also be measured by measuring the electrical resistance of the light shielding film as shown in FIG. In FIG. 9, the conductive plate 601 is brought into contact with the chip, and the electric resistance between A and B is measured. FIG. 9A shows a case where the opening size is small, and FIG. 9B shows a case where the opening size is large. In FIG. 9A, since the contact area between the conductive plate 601 and the light shielding film 3 is small, the electrical resistance between A and B is large. On the contrary, in FIG. 9B, the electrical resistance is reduced. Thereby, it is possible to determine whether the opening size is larger or smaller than the desired size, and by controlling the movement distance of the arm 505 by feeding back the information, the opening of the desired size can be stably produced. Can do. It is also possible to perform feedback control by evaluating the aperture size by observation with a microscope.
[0044]
As a control form of the force F, there is a method of applying the force F in a plurality of times. For example, when a method of creating one opening by pushing the pushing tool 7 twice is used, the first time is pushed with a fixed force, the opening size is measured, and a second force is required. Calculate the size. Thereby, it is possible to produce an opening having a constant size at all times. There is also a method in which a constant force F is applied in a plurality of times, the size is measured each time, and the process ends when the desired size is reached. There can be many kinds of embodiments for feedback control in detail.
[0045]
【The invention's effect】
By controlling the tip 1 and the height of 2 per degree and the force F, the opening 8 can be easily formed without using an actuator with high resolution. Further, since the height of the chip 1 and 2 per degree is well controlled, the production yield of the openings 8 is improved. In addition, since the workpiece 1000 described in the first embodiment of the present invention can be manufactured by a photolithography process, a plurality of workpieces 1000 can be manufactured on a sample having a large area such as a wafer, and the force F is constant. By doing so, it is possible to form the openings 8 having a uniform opening diameter for each of the plurality of workpieces 1000 produced. In addition, since it is very easy to change the magnitude of the force F, it is possible to form the openings 8 having different opening diameters individually for a plurality of workpieces 1000 produced. Further, since the opening is formed by simply applying the force F, the time required for opening preparation is as short as several tens of seconds or less. Moreover, according to Embodiment 1 of this invention, a processing atmosphere is not ask | required. Therefore, it can be processed in the atmosphere, and the processing state can be immediately observed with an optical microscope or the like. Further, by processing in a scanning electron microscope, it is possible to observe the processing state with a higher resolution than that of an optical microscope. Moreover, since the liquid functions as a damper by processing in the liquid, processing conditions with improved controllability can be obtained.
[0046]
It is also possible to produce a plurality of apertures 8 having the same aperture diameter at a time by applying force F to a sample on which a plurality of workpieces 1000 are produced. In the case of batch processing, although depending on the number of workpieces 1000 per wafer, the processing time per opening is extremely short, being several hundred milliseconds or less.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating a method for forming an opening according to a first embodiment of the present invention.
FIG. 2 is a diagram for explaining a method of forming an opening according to the first embodiment of the present invention.
FIG. 3 is a diagram illustrating a method for forming an opening according to the first embodiment of the present invention.
FIG. 4 is a diagram illustrating a method for manufacturing a workpiece 1000;
FIG. 5 is a diagram illustrating a method for manufacturing a workpiece 1000;
FIG. 6 is a diagram for explaining a relationship between a chip 1 and a height of 2 per degree in a method for manufacturing a workpiece 1000;
7 is a diagram for explaining a relationship between a chip 1 and a height of 2 per degree in a method for manufacturing a workpiece 1000;
FIG. 8 shows an apparatus for producing an optical aperture by the method according to Embodiment 2 of the present invention.
FIG. 9 is a diagram for explaining a method for measuring an opening size by a method according to Embodiment 2 of the present invention;
[Explanation of symbols]
1 chip 2 degrees 3 light-shielding film 4 substrate 5 transparent layer 6 plate 7 pushing tool 8 opening 101 chip mask 102 degrees mask 103 transparent material 104 substrate material 500 optical aperture forming apparatus 501 computer 502 light detection unit 503 Lens 504 Transmitted light 505 Arm 506 Gear 507 Push-off tool 508 Substrate 509 xy stage 510 Lens 511 Measuring light 512 Laser 601 Conductive plate 1000 Workpiece F Force H1 Chip height H2 Height per degree

Claims (4)

An optical opening that forms an opening by plastically deforming the light-shielding film by bringing a pusher into contact with the tip of the near-field optical device having a cone-shaped chip and the light-shielding film provided on the chip. In the production method of
Forming an opening forming body including the near-field optical device and a measure having a measure portion having a height substantially the same as the chip on a substantially flat surface;
The pusher has a substantially flat surface that covers the chip and covers at least a part of the degree,
An opening forming step of forming the opening at the tip of the chip by controlling the displacement of the pusher by bringing the pusher into contact with the contact portion;
A force adjustment step of measuring the shape of the opening, feeding back the measurement result, and adjusting the magnitude of the force so that the measurement result approaches the desired result,
The method for forming an optical opening, wherein the opening forming step is repeated after the force adjusting step .   The method for producing an opening according to claim 1, wherein the measurement is performed by a microscope.   The method for manufacturing an opening according to claim 1, wherein the measurement is measurement of electric resistance of the light shielding film.   The method for manufacturing an opening according to claim 1, wherein the measurement is measurement of a light transmission amount of the opening.

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