JP2016004098A - Optical element and imaging apparatus including the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical element formed with an antireflection structure having fine structures on the surface capable of achieving a higher antireflection function.SOLUTION: The optical element is formed of a molding material which is injected into a molding die to fill the same via a gate after being heated and melted. The optical element includes: an optical part 11 which has an optical functional surface including the optical axis; an antireflection concave-convex structure 13 formed on the optical part 11, which has a fine structure unit formed in a period of a predetermined wavelength or less; and an edge part 12 which is formed on the circumference of the optical part 11 and has a gate trace 16. The optical part 11 is arranged so that the period of the fine structure units in an area adjacent to the gate trace 16 is larger than the period of the fine structure units in an area separated away from the gate trace 16.

Description

  The present technology relates to an optical element having an antireflection concavo-convex structure for suppressing reflection of incident light on a surface, and an imaging apparatus including the optical element.

  In recent years, various optical elements in which antireflection treatment for suppressing light reflection is performed on the surface have been proposed. As one of the antireflection treatments, a technique for forming a fine structure on the surface of an optical member with a pitch equal to or less than the wavelength of incident light has been proposed. For example, there is a technique for forming a fine structure composed of regularly arranged linear concave portions or linear convex portions, a regularly arranged cone-shaped or columnar concave portion, or a fine structure composed of convex portions, etc. Proposed. A structure in which a plurality of such fine structures are arranged may be referred to as an “antireflection concavo-convex structure (SWS: Substructure Structured Surface)”.

  For example, Patent Document 1 discloses an optical element having an antireflection portion composed of a fine periodic concavo-convex structure formed at a pitch equal to or less than the wavelength of a light beam to be antireflection on at least a part of a virtual curved surface forming a member. doing.

JP 2006-053220 A

  In patent document 1, it has the structure which provided the reflection preventing part in the area | region of the curved surface of an optical effective surface and its extension part. Generally, an optical element made of resin is manufactured by injection molding. In this case, a general method is that the molten resin is poured into a cavity for molding the optical element through a gate which is a passage for the molten resin, and the molten resin is solidified.

  An object of the present technology is to realize an antireflection function superior to that of a conventional optical element in which an antireflection structure having a fine structure is formed on the surface.

  The present technology is an optical element obtained by heating and melting a molding material and injection-filling it into a mold through a gate, the optical element having an optical functional surface including an optical axis, and the optical part. An antireflection concavo-convex structure composed of a fine structural unit formed with a period of a predetermined wavelength or less, and an edge portion provided on the outer periphery of the optical portion and having a gate trace portion, and the optical portion is in the vicinity of the gate trace portion It is characterized in that the period of the fine structure unit in the region is larger than the period of the fine structure unit in the region away from the gate trace portion.

  According to the present technology, an optical element having an antireflection function superior to that of the related art can be realized.

It is sectional drawing which shows the structure of the optical element by one embodiment of this technique. It is the elements on larger scale and explanatory drawing which expand and show the gate trace part vicinity of an optical element. It is the schematic when an optical element is seen from an optical axis direction. It is a schematic sectional drawing which shows an example of the injection molding apparatus used for manufacture of the optical element by this Embodiment. It is sectional drawing which shows an example of the shaping | molding die for shape | molding the optical element by this Embodiment. It is the schematic which shows the process for producing the 1st shaping | molding die for shape | molding an optical element. In the optical element by this technique, it is the schematic which shows the other example of an antireflection structure. It is the schematic of the camera provided with the optical element by this technique.

  Hereinafter, an optical element according to an embodiment of the present technology will be described with reference to the drawings. However, more detailed description than necessary may be omitted. For example, detailed descriptions of already well-known matters and repeated descriptions for substantially the same configuration may be omitted. This is to avoid the following description from becoming unnecessarily redundant and to facilitate understanding by those skilled in the art.

  In addition, the inventors provide the accompanying drawings and the following description in order for those skilled in the art to fully understand the present technology, and these are intended to limit the subject matter described in the claims. is not.

[1. Configuration of optical element]
FIG. 1 is a cross-sectional view illustrating a configuration of an optical element according to an embodiment of the present technology. As shown in FIG. 1, the optical element 10 includes an optical unit 11 having an optical functional surface including the optical axis X, and an edge unit 12 provided on the outer periphery of the optical unit 11.

  The optical unit 11 includes a first optical surface 14 and a second optical surface 15 on which an antireflection uneven structure 13 made of SWS (Subwavelength Structured Surface) is formed. In the present embodiment, the first optical surface 14 and the second optical surface 15 are formed in a curved surface shape. The shapes of the first optical surface 14 and the second optical surface 15 may be aspherical shapes, flat surfaces, or free curved surfaces. The antireflection concavo-convex structure 13 is configured by regularly arranging a plurality of fine structural units. The antireflection concavo-convex structure 13 can suppress reflection of light having a wavelength longer than the period of the fine structure unit.

  The edge portion 12 has a gate trace portion 16. In the case of injection molding, after molding, the gate portion is cut and removed with a blade or the like, but a part of it remains. This remaining portion is the gate trace portion 16.

  FIG. 2A is a partially enlarged view showing the vicinity of the gate trace portion of the optical element in an enlarged manner. FIG. 2B is an explanatory diagram in which the spherical surface of the base surface L in FIG.

  As shown in FIG. 2A, the antireflection concavo-convex structure 13 includes an antireflection concavo-convex structure 13a and an antireflection concavo-convex structure 13b. The antireflection concavo-convex structure 13 b is formed on the optical surface near the gate trace 16. The antireflection concavo-convex structure 13 a is formed on the optical surface at a position away from the gate trace 16. Specifically, the antireflection concavo-convex structure 13 is formed in a region including the optical axis X.

  The antireflection concavo-convex structure 13 is formed to extend in the optical axis direction with the base surface L as a reference. The base surface L is a virtual surface formed by connecting the points where the convex portions and the convex portions forming the antireflection concavo-convex structure 13 intersect. The base surface L is designed in a shape necessary for realizing optical characteristics required for the optical element 10.

  The height of the antireflection concavo-convex structure 13 is defined as follows. The vertex of the convex portion constituting the antireflection concavo-convex structure 13 is defined as a vertex A. A line segment extending from the vertex A in the direction of the optical unit 11 and parallel to the optical axis X is defined as a line segment M. An intersection point between the line segment M and the base surface L is defined as an intersection point B. The length from the vertex A to the intersection B is defined as the height h of the antireflection uneven structure 13.

  As shown in FIG. 2B, in the state in which the spherical component of the base surface L is removed, the height of the antireflection uneven structure 13a and the height of the antireflection uneven structure 13b are substantially the same. In addition, the period (pitch) of the antireflection concavo-convex structure 13 is formed at a pitch equal to or less than the wavelength for which reflection is to be reduced. The pitch P of the antireflection concavo-convex structure 13 is defined by the distance between adjacent concave portions. Specifically, assuming that the most valley portion of the recess is the point a1, and the most valley portion of the recess adjacent to the a1 is the point a2, a line segment passing through the point a1 and parallel to the optical axis X, and the point a2 And the width of the line segment parallel to the optical axis X is defined as the pitch P1 of the antireflection concavo-convex structure 13a. The pitch P2 of the antireflection concavo-convex structure 13b is similarly defined.

  In the present embodiment, the pitch P2 of the antireflection concavo-convex structure 13b is formed to be larger than the pitch P1 of the antireflection concavo-convex structure 13a. For example, the pitches P1 and P2 are 200 nm to 320 nm. The value of the pitch P2 is set to be a value larger than the pitch P1. When the pitch is smaller than 200 nm, the aspect ratio, which is the ratio of the pitch and the height, increases, and processing becomes difficult. In addition, when the pitch is larger than 320 nm, a diffraction phenomenon occurs at a short wavelength, which makes it difficult to use as an optical product such as a camera.

  FIG. 3 is a schematic view of the optical element as viewed from the optical axis direction. A region near the gate trace 16 will be described with reference to FIG.

  As shown in FIG. 3, the outer diameter of the optical element 10 is defined as an outer diameter Y. The outer diameter Y is illustrated as a line segment Y. A line segment passing through the optical axis X and perpendicular to the line segment Y is defined as a line segment Z. The line segment Z passes through the midpoint of the line segment Y. That is, the line segment Z is a line segment that divides the optical element 10 into two. Further, as shown in FIG. 3, the line segment S divides the optical axis X to the end of the edge portion 12 in half. That is, the length from the end of the edge portion 12 to the line segment S is ¼ of the outer diameter L. In the present embodiment, a region included in the range of ¼ of the outer diameter L from the end of the edge portion 12 is defined as a region near the gate trace portion 16. An antireflection concavo-convex structure 13 b is formed in the region near the gate trace 16.

  Although details will be described later, it is preferable that the area near the gate trace 16 is smaller. Therefore, it is preferable that the region near the gate trace portion 16 is a region included in the range of 1/5 of the outer diameter L from the end portion of the edge portion 12.

[2. Manufacturing method of optical element]
FIG. 4 is a schematic cross-sectional view illustrating an example of an injection molding apparatus used for manufacturing an optical element according to the present embodiment. As shown in FIG. 4, the injection molding apparatus 20 includes a hopper 22 into which a resin material 21 such as a pellet material is charged, a screw 23, a sprue 24, a runner 25, a gate 26, and a first mold 27. The second mold 28 and the temperature adjusting unit 29 are included. Hereinafter, the manufacturing method of the optical element 10 will be described with reference to FIGS.

  First, the resin material 21 is put into the hopper 22. The charged resin material 21 is heated while being measured by the screw 23. The heated resin material 21 is plasticized and melted. The molten resin material 21 passes through the sprue 24, the runner 25, and the gate 26 and is injected and filled into a space (cavity) surrounded by the first mold 27 and the second mold 28. Thereafter, the resin material 21 filled in the cavity is solidified by being cooled by the temperature adjusting unit 29. Thereafter, the first mold 27 and the second mold 28 are opened, and the optical element 10 is taken out. When the optical element 10 is taken out, the resin material solidified in the shape of the gate 26, the runner 25, and the sprue 24 is also taken out into the optical element 10 as a whole, so the position of the gate 26 is cut with a blade or the like. At this time, the gate trace 16 is formed in the optical element 10.

  The resin material 21 can be selected from any type as long as it satisfies the refractive index and dispersion value required for the optical element 10.

  The optical element 10 obtained by injection molding shrinks in the process of cooling and solidifying, but the shrinkage differs between the vicinity of the gate trace portion 16 and the portion other than the vicinity of the gate trace portion 16. Similarly, the magnitude of the internal strain differs between the vicinity of the gate trace 16 and other portions. Hereinafter, the reason why the shrinkage amount is different and the reason why the internal strain is different will be described.

  First, the reason why the shrinkage amounts are different will be described. Heated and melted resin material 21 (hereinafter, simply referred to as molten resin) enters the gate 26 from the runner 25. When the molten resin passes through the gate 26, a shearing force is generated in the molten resin in a region near the gate 26 in the cavity. Hereinafter, the shearing force generated in the molten resin will be described.

  There are the following two types of flow of the molten resin when the molten resin is filled in the cavity.

  One is that the molten resin that has entered the cavity reaches a portion of the cavity opposite to the gate 26 side, and then flows into the molding surface of the first molding die 27 and the molding surface of the second molding die 28. Is the case. The first mold 27 and the second mold 28 are set to a temperature lower than that of the molten resin. Therefore, the molten resin that has contacted the molding surfaces of the first mold 27 and the second mold 28 is cooled by the respective molds. At this time, a thin and solidified layer called a skin layer is formed on the surface of the molten resin. However, since the molten resin flows inside the skin layer, a shearing force is generated between the molten resin and the skin layer.

  The second is a case where the molten resin is gradually filled from the gate 26 to the opposite side of the gate 26. Also at this time, a skin layer is formed on the molten resin in contact with the molding surface. Since the molten resin continues to flow after the skin layer is formed in the vicinity of the gate 26, a shearing force is generated between the skin layer and the molten resin.

  As shown in FIG. 4, the diameter of the runner 25 is larger than the diameter of the gate 26. Therefore, the molten resin enters the gate 26 having a small diameter from the runner 25 having a large diameter. That is, when the runner 25 enters the gate 26, the flow rate of the molten resin increases. Thereby, the shearing force described above is more likely to be generated.

  This shear force remains as internal strain in the process of melting and solidifying the molten resin.

  In the injection molding, the resin filling rate is higher in the back of the cavity. Therefore, the density of the resin filled in the vicinity of the gate 26 is smaller than the density of the resin in other portions. For this reason, the amount of shrinkage when the resin filled in the vicinity of the gate 26 is cooled and solidified is larger than that in the portion other than the vicinity of the gate 26. As a result, the shape of the antireflection concavo-convex structure 13b is easily deformed.

  In the present embodiment, the pitch P2 of the antireflection concavo-convex structure 13b is formed larger than the pitch P1 of the antireflection concavo-convex structure 13a. Thereby, even if the antireflection concavo-convex structure 13b contracts, it is possible to maintain a sufficient size for realizing optical performance. Further, since the pitch is large, it is easy to release from the mold.

  On the other hand, the antireflection concavo-convex structure 13a other than the vicinity of the gate 26 is axisymmetric and has a desired height dimension.

  FIG. 5 is a sectional view showing an example of a molding die for molding the optical element according to the present embodiment. On the molding surfaces of the first molding die 27 and the second molding die 28, fine processing for molding the antireflection concavo-convex structure 13 is performed.

  As shown in FIG. 5, an inverted shape 13A of the antireflection concavo-convex structure 13a is formed in a region corresponding to the antireflection concavo-convex structure 13a. A reverse shape 13B is formed in a region corresponding to the antireflection concavo-convex structure 13b. The period (pitch) of the inverted shape 13B is formed larger than the pitch of the inverted shape 13A.

  By the way, the anti-reflection concavo-convex structure 13 has a period equal to or less than a solution obtained by dividing the wavelength of light incident on the optical element 10 by the refractive index of the optical element 11 and is 0.3 times or more of the wavelength of incident light. If it has a height, it is said to exhibit an antireflection effect. In the optical element 10 of the present embodiment, when used in the optical system of the imaging apparatus, it is desirable that the reflectance is 0.5% or less with respect to the wavelength of visible light of 400 nm to 700 nm, and the antireflection uneven structure is used. It is desirable to ensure a height of 210 nm or more. More preferably, the height of the antireflection concavo-convex structure 13 is not less than 0.4 times the wavelength used. At this time, the height of the antireflection uneven structure 13 is desirably 280 nm or more. The relationship between the height and the reflectance is derived from a general theory in which the shape of the unit structure of the antireflection uneven structure 13 is a cone. However, depending on the shape of the unit structure, the reflectance may be different even at the same height. There is. In general, the molding shrinkage of the resin material for optical elements is 0.1 to 1.0%, and varies depending on the type of resin, molding conditions, and the shape of the molded product. Here, the molding shrinkage means the ratio of the dimension of the optical element after molding to the dimension of the mold.

  Next, a method for processing the first mold 27 and the second mold 28 will be described.

  FIG. 6 is a schematic view showing a process for producing a first mold for molding an optical element. In addition, although the 1st shaping | molding die 27 is mentioned as an example and demonstrated here, the 2nd shaping | molding die 28 can also be obtained by the same process.

  First, a mold base material 31 is prepared. Then, as shown in FIG. 6A, an optical element shape is formed on the mold base material 31 by machining. Next, as shown in FIG. 6B, a metal mask 32 is formed on the surface of the mold base material 31. As a method for forming the metal mask 32, a sputtering method, a vapor deposition method, or the like is preferable.

  Next, as shown in FIG. 6C, a resist mask 33 is formed on the metal mask 32. As a method for forming the resist mask 33, a spin coating method or a spray coating method is preferable.

  Next, as shown in FIG. 6D, a resist dot pattern 34 corresponding to the antireflection uneven structure 13 is formed on the resist mask 33. As a method for forming the resist dot pattern 34, an electron beam drawing method, an interference exposure (hologram exposure) method, or the like is preferable.

  Next, as shown in FIG. 6E, the resist dot pattern 34 is transferred to the metal mask 32 by dry etching. Thereby, a metal mask dot pattern 35 is formed. The metal mask dot pattern 35 may be formed by wet etching.

  Next, as shown in FIG. 6F, the metal mask dot pattern 35 is transferred to the mold base 31 by dry etching. At this time, by increasing the hole diameter of the resist dot pattern 34, the inverted shape of the antireflection concavo-convex structure 13 can be deepened. Thereby, the inverted shape 36 of the antireflection concavo-convex structure 13 is formed on the surface of the mold base material 31.

Next, specific examples and comparative examples according to the present technology will be described.
(Example)
The optical element of the embodiment is a biconvex optical element as shown in FIG. The optical element has an outer diameter of 10 mm, a thickness of 3 mm at the center, and an edge thickness of 1 mm. The effective optical diameter was 6 mm. A polyolefin resin APL5014 (refractive index nd = 1.60) manufactured by Mitsui Chemicals, Inc. was used as the resin material. The pitch of the anti-reflection concavo-convex structure is such that the periodic pitch near the optical axis on the incident surface side is 230 nm, the periodic pitch near the gate portion is 300 nm, the pitch near the optical axis on the exit side is 200 nm, and the periodic pitch near the gate portion is 230 nm. It was.

  Silicon carbide (SiC) was prepared as the mold base material 31. An optical element shape was formed on the mold base material 31 by machining, tungsten silicide (WSi) was formed by sputtering, and then an electron beam resist (positive type) was applied by spin coating. Thereafter, a dot shape was drawn by electron beam drawing.

  Using the resist dot pattern as a mask, a dot pattern was formed on the WSi mask by dry etching using argon gas. Subsequently, an inverted shape of the antireflection concavo-convex structure was formed on the SiC surface of the mold base 31 by dry etching using a fluorocarbon-based gas.

  A molding die obtained by finely processing the inverted shape of the antireflection concavo-convex structure was immersed in a fluorine-based mold release agent and subjected to a mold release treatment. An optical element was produced by injection molding a polyolefin-based resin using a mold subjected to a release treatment.

  The molding conditions were such that the resin temperature was 260 ° C., the mold temperature was 135 ° C., and the tact was 90 seconds. Further, an injection molding machine was provided with an eight-piece mold and injection molded. The holding pressure of the mold was 100 MPa.

  After molding, the shape of the antireflection uneven structure on the surface of the optical element was measured. An optical element having a pitch of the antireflection concavo-convex structure larger in the vicinity of the gate portion than in the vicinity of the optical axis on both the incident surface side and the output surface side was obtained. The period of the antireflection concavo-convex structure 13 was 250 nm over the entire surface of the optical element. The height of the antireflection concavo-convex structure 13 was 280 nm or more over the entire optical effective surface, and it was confirmed that it had a desired shape.

As a result of measuring the reflectance of the obtained optical element, the reflectance was 0.3% or less over the entire optical effective surface, and good reflectance characteristics were obtained (Comparative Example).
Next, using the same injection molding apparatus, molding was performed using a molding die in which the period (pitch) of the fine structure unit of the antireflection uneven structure was constant.

  As a result of measuring the reflectance characteristics of the obtained optical element, the reflectance of the portion away from the gate trace 16 was good at 0.3%, but the reflectance near the gate trace 16 was 1.2%. The desired properties were not met.

  As described above, according to the present technology, it is possible to realize an antireflection function superior to that of the related art in an optical element in which an antireflection structure having a fine structure is formed on the surface.

  In the present embodiment, the antireflection uneven structure 13 is provided only on the first optical surface 14 and the second optical surface 15, but the surface on which the antireflection uneven structure 13 is formed is not limited thereto. . The antireflection concavo-convex structure 13 may be provided only on the first optical surface. Further, the antireflection uneven structure 13 may be provided only on the second optical surface 15.

  Moreover, although the optical element 10 of this Embodiment was biconvex shape, this technique is not restricted to this. For example, the optical element may have a biconcave shape, a convex meniscus shape, or a concave meniscus shape.

  In the present embodiment, the antireflection concavo-convex structure 13 is described by taking an antireflection structure having a fine structure unit of a conical shape (see FIG. 7A) as an example. The configuration is not limited. For example, the fine structural unit may be a regular hexagonal pyramid shape or a pyramid shape such as a quadrangular pyramid shape (see FIG. 7B). Further, the shape of the fine structure unit of the antireflection structure is not necessarily limited to a cone or a pyramid shape, and may be a columnar shape (see FIG. 7C) or a prismatic shape (see FIG. 7D). It may be columnar. Furthermore, even in the shape of a bell with a rounded tip (see FIGS. 7 (e) and 7 (f)), a truncated cone (see FIG. 7 (g)) or a truncated pyramid (see FIG. 7 (h)). The shape may be either a truncated cone or a truncated pyramid. Moreover, each fine structural unit does not need to be a strict geometric shape, and may be substantially conical or pyramidal, columnar, bell-shaped, truncated cone or truncated pyramid. In short, the antireflection concavo-convex structure 13 only needs to be arranged at a pitch smaller than the wavelength of light at which each fine structure unit should reduce reflection.

  Next, a configuration of a camera will be described as an example of an imaging apparatus including an optical element according to the present technology. FIG. 8 is a schematic view of a camera including an optical element according to the present technology.

  As shown in FIG. 8, the camera 100 includes a camera body 110 and an interchangeable lens 120 attached to the camera body 110.

  The camera body 110 has an image sensor 130, and the interchangeable lens 120 is configured to be detachable from the camera body 110. The interchangeable lens 120 is, for example, a telephoto zoom lens. The interchangeable lens 120 has an imaging optical system 140 for focusing the light beam on the image sensor 130 of the camera body 110. The imaging optical system 140 includes the optical element 10 according to the present technology that functions as a lens element, and refractive lenses 150 and 160.

  With such a configuration, a camera having an excellent antireflection function for light can be realized.

  As described above, the embodiments have been described as examples of the present technology. For this purpose, the accompanying drawings and detailed description are provided. Accordingly, among the components described in the attached drawings and detailed description, not only the components essential for solving the problem, but also the components not essential for solving the problem in order to exemplify the above technique. May also be included. Therefore, it should not be immediately recognized that these non-essential components are essential as those non-essential components are described in the accompanying drawings and detailed description.

  Moreover, since the above-mentioned embodiment is for illustrating this technique, various changes, replacements, additions, omissions, and the like can be made within the scope of the claims or an equivalent scope thereof.

  As described above, according to the present technology, various optical systems such as a high-quality imaging optical system, objective optical system, scanning optical system, and pickup optical system, optical element barrel unit, optical pickup unit, imaging unit, and the like. An optical unit, an imaging device, an optical pickup device, an optical scanning device, and the like can be realized.

DESCRIPTION OF SYMBOLS 10 Optical element 11 Optical part 12 Edge part 13, 13a, 13b Anti-reflective uneven structure 13A, 13B Reverse shape 14 1st optical surface 15 2nd optical surface 16 Gate trace part 20 Injection molding apparatus 21 Resin material 22 Hopper 23 Screw 24 Sprue 25 Runner 26 Gate 27 First mold 28 Second mold 29 Temperature adjusting unit 31 Mold base 32 Metal mask 33 Resist mask 34 Resist dot pattern 35 Metal mask dot pattern 36 Inverted shape 100 Camera 110 Camera body 120 Interchangeable lens 130 Imaging Element 140 Imaging optical system 150, 160 Refractive lens

Claims (4)

An optical element obtained by heating and melting a molding material and injection-filling it into a mold through a gate,
An optical part having an optical functional surface including an optical axis, an antireflection uneven structure comprising fine structural units provided in the optical part and formed with a period equal to or less than a predetermined wavelength, and provided on the outer periphery of the optical part; And an edge portion having a trace portion,
The optical element is configured such that a period of the fine structure unit in a region near the gate trace portion is larger than a period of the fine structure unit in a region away from the gate trace portion. 2. The optical element according to claim 1, wherein the region in the vicinity of the gate trace portion is a region included in a range of ¼ of the outer diameter of the optical element from an end portion of the edge portion. The fine structure unit provided in the optical unit is provided such that the height in the optical axis direction of the region in the vicinity of the gate trace portion and the height in the optical axis direction of the region away from the gate trace portion are substantially the same height. The optical element according to claim 1. An imaging apparatus comprising the optical element according to claim 1 in an imaging optical system.