Metal mold manufacturing method, metal mold, optical element, and optical element manufacturing method

Abstract

A material of a cemented carbide having tungsten carbide as a major ingredient is cut and polished into a shape, a silicon coating as a protecting layer is applied thereto by the plasma CVD method, and a chromium nitride coating with a thickness of 1 &mgr;m is applied thereto to form a preliminary shape metal mold. The shape of the formed preliminary shape metal mold or the shape of a sample formed with the preliminary shape metal mold is measured, and from the manufacture error thereof, the shape correction amount of a preliminary shape metal mold 30 for use as a drag is obtained. Based on the obtained shape correction amount, correction by etching is performed on a drag molding surface 301 of the drag preliminary shape metal mold 30 so that a predetermined precision is obtained, thereby completing the metal mold. An optical element or a part is manufactured by use of the metal mold.

Description

[0001] This application is based on application No. 2001-144661 filed in Japan, the content of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a technology to manufacture a small-size optical element and the like with high precision.

[0004] 2. Description of the Related Art

[0005] In the fields of optical pickups, optical communications and the like, elements such as high-precision and small-size lenses and mirrors are required as information capacity increases. As a method of manufacturing such elements, molding to transfer the shape to the optical base material by use of a metal mold has been proposed.

[0006] For example, as a lens manufacturing method using molding, a method has been proposed such that after a prototype of the metal mold is obtained by cutting or grinding the metal base material, the shape of the metal mold itself or the shape of a lens molded with the metal mold is measured, and the manufacture error of the metal mold is corrected by machining in accordance with the result of the measurement.

[0007] As a microlens manufacturing method using lithography, for example, a method using isotropic etching has been proposed. Further, a method using reflow has been proposed.

[0008] Metal molds for molding are formed by machining such as polishing, grinding and cutting, and the machining precision is limited by the tip curvature of the cutting tool for machining and the mechanical positioning precision of the cutting tool for the metal mold. Because of this, it is difficult to machine or correct metal molds with high precision in manufacturing tiny optical elements with a diameter of not more than 1 mm.

[0009] For example, a high-numerical-aperture (NA) reflective-type element which is an optical element requires a profile irregularity not less than three times stricter than that of a refractive-type element. Uses requiring a high-precision wave front with a wave front distortion of approximately 0.1 of the wavelength requires a surface machining precision of as high as not more than 0.05 &mgr;m. However, with the prior art, because of the above-mentioned reasons, it is difficult to obtain metal molds with which such high-precision elements can be formed.

[0010] According to the method using reflow, the obtained shapes are limited because of the nature of the method that a heat deformation due to surface tension is used. In addition, it is difficult to obtain high-precision surfaces with stability.

[0011] When etching is used, machining can be performed with high precision. However, since the depth of one etching is comparatively small, if all the process of obtaining a final three-dimensional shape out of a base material is performed only by etching, it is necessary to repeat etching a multiplicity of times, and manufacture efficiency is low.

[0012] These problems arise not only with optical elements but also with various manufacture objects such as parts requiring high shape precision.

SUMMARY OF THE INVENTION

[0013] The present invention is made in view of the above-mentioned problems, and an object thereof is to efficiently form manufacture objects such as optical elements with high precision.

[0014] One aspect of the present invention is a method of manufacturing a metal mold for molding an object, comprising: preparing a preliminary shape metal mold having a shape approximate to an outside shape of the object to be molded; and processing for obtaining a molding surface by selectively removing a surficial part of the preliminary shape metal mold by etching.

[0015] Another aspect of the present invention is a metal mold for molding an object, comprising: a first portion of molding surface obtained by a preliminary manufacturing stage; and a second portion of molding surface obtained by selectively removing a surficial part obtained by the preliminary manufacturing stage by etching in accordance with a predetermined shape correction amount.

[0016] Still another aspect of the present invention is a method of manufacturing an optical element, comprising: preparing a preform having a shape approximate to an outside shape of the optical element; and selectively etching a surface shape of the preform for obtaining the optical element.

[0017] Further aspect of the present invention is an optical element comprises: a first surface portion obtained by a preliminary manufacturing stage having a shape approximately corresponding to a predetermined optical function; and a second surface portion obtained by selectively removing a surficial part obtained by the preliminary manufacturing stage by etching.

[0018] These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings, which illustrate specific embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In the following description, like parts are designated by like reference numbers throughout the several drawings.

[0020] FIG. 1 showing a first embodiment of the present invention shows a high-NA reflective-type lens;

[0021] FIG. 2 is a flowchart showing a process of manufacturing the high-NA reflective-type lens;

[0022] FIG. 3 shows a manner of manufacturing a sample by use of a preliminary shape metal mold for use as a cope and a preliminary shape metal mold for use as a drag;

[0023] FIG. 4 shows a manner of manufacturing the high-NA reflective-type lens by molding;

[0024] FIG. 5 shows a manner of measuring the manufacture error of the high-NA reflective-type lens with a Mach-Zehnder interferometer;

[0025] FIG. 6 shows an example of interference fringes obtained by the Mach-Zehnder interferometer;

[0026] FIG. 7 shows a ray tracing technique using a calculator;

[0027] FIG. 8 shows a relationship between a corrected molding surface and an ideal molding surface;

[0028] FIG. 9 shows an error evaluation of the corrected molding surface;

[0029] FIGS. 10A to 10F show a manner of correcting the preliminary shape metal mold for use as a drag by etching;

[0030] FIG. 11 shows a manner of measuring the manufacture error of a metal mold of a third embodiment with a laser interferometer;

[0031] FIG. 12 shows a ray tracing technique using a calculator;

[0032] FIG. 13 shows a microlens of a fourth embodiment;

[0033] FIG. 14 is a flowchart showing a process of manufacturing a preform of the microlens;

[0034] FIGS. 15A to 15D show a manner of manufacturing the preform of the microlens by reflow;

[0035] FIG. 16 is a flowchart showing a process of manufacturing the microlens from the preform of the microlens; and

[0036] FIGS. 17A to 17F show a manner of correcting the preform of the microlens by etching.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] Preferred embodiments of the present invention will be described in detail with reference to the attached drawings.

[0038] <1. First Embodiment>

[0039] In a first embodiment, a preliminary shape metal mold formed by a manufacturing process such as cutting and grinding is corrected by etching, and a high-NA reflective-type lens is formed with the corrected metal mold. The NA means “numerical aperture” which is the product of the sine of the angle between the light ray and the optical axis and the refractive index of the medium.

[0040] FIG. 1 shows a high-NA reflective-type lens 1 formed in the first embodiment. The high-NA reflective-type lens 1 has an NA of 1.3 and a diameter of several millimeters, and is used, for example, as a solid immersion lens generating near-field light for optical pickups for optical memories or in optical communications. A light ray incident through a first aperture 11 of the lens 1 is reflected at a second reflecting surface 14, and undergoes a condensing function at a first reflecting surface 13 to be condensed to a second aperture 12. As described above, optical elements having a plurality of reflecting surfaces require an accuracy of a wave front distortion of as high as approximately 0.1 of the wavelength of the light used. For example, when the refractive index n of the optical member in the lens 1 is 1.6 and the wavelength &lgr; of the light used is 0.4 &mgr;m, considering the fact that the error of the reflecting surface exerts a twofold influence on the optical axis, the maximum permissible error d is 0.0125 &mgr;m from the expression 1.

d×n×2=0.1×&lgr;  (1)

[0041] In the present embodiment, a metal mold and the lens 1 are formed so that the maximum error is not more than d by the following method:

[0042] FIG. 2 is a flowchart showing a process of manufacturing the lens 1 in the first embodiment. FIG. 3 shows a manner of manufacturing a sample prototype 100A by use of a preliminary shape metal mold 20 for use as a cope and a preliminary shape metal mold 30 for use as a drag (these will be abbreviated together as preliminary shape metal molds 20 and 30) which are a prototype of a metal mold for forming the lens 1 in the present embodiment. FIG. 4 shows a manner of manufacturing the lens 1 by molding by use of the corrected metal mold. With reference to FIGS. 2 to 4, the manufacturing method of the present embodiment will be described.

[0043] First, the preliminary shape metal molds 20 and 30 shown in FIG. 3 to be the prototypes of a cope 2 and a drag 3 shown in FIG. 4 are formed (step S11). The cope 2 is a metal mold for forming the shape of mainly a first reflecting surface 13 of the lens 1 with a cope molding surface 201 thereof. The drag 3 is a metal mold for forming the shape of mainly a second reflecting surface 14 of the lens 1 with a drag molding surface 301 thereof.

[0044] The preliminary shape metal mold 30 to become the drag 3 later is formed in the following manner: The upper surface (a drag preliminary molding surface 300) of a base material of a cemented carbide having tungsten carbide as a major ingredient is cut and polished into a plane, a silicon coating as a protecting layer is applied thereto by the plasma Chemical Vapor Deposition (CVD) method, and a chromium nitride coating with a thickness of 1 &mgr;m is further applied thereto. The reason why the thickness of the chromium nitride layer is 1 &mgr;m is that since the manufacture error amounts of metal molds are generally smaller than 1 &mgr;m, it is sufficient to presume the amount of shape correction by etching in the processing step described later to be not more than 1 &mgr;m. The preliminary shape metal mold 20 to become the cope 2 later is formed by cutting and polishing a base material of the cemented carbide into a desired aspherical concave surface (a cope preliminary molding surface 200) and applying a silicon coating as a protecting layer and a chromium nitride coating thereto.

[0045] By this method, preliminary shape metal molds having a shape approximate to the outside shape of the lens 1 can be prepared.

[0046] The method of manufacturing the preliminary shape metal molds 20 and 30 is not limited to the above-described example. That is, it is desirable that the following conditions be satisfied: the base material can be polished into a smooth specular surface with no pore; oxidation resistance at high temperatures is high; there is no change in structure and the like; the surface quality is maintained; the ingredients of the product neither fuse nor react with other materials; the product is easily parted from the metal molds; and the hardness and the strength are high at high temperatures, and as long as these conditions are satisfied, the preliminary shape metal molds 20 and 30 may be manufactured out of a different material or by a different method.

[0047] Then, using the preliminary shape metal molds 20 and 30, the sample prototype 100A of the high-NA reflective-type lens 1 is formed by molding shown in FIG. 3 (step S12). A reflective coating is selectively applied to a surface of the sample prototype 100A to obtain a sample 100 (see FIG. 5) having a first aperture 11, a second aperture 12, a first reflecting surface 13 and a second reflecting surface 14 like the high-NA reflective-type lens 1 shown in FIG. 1.

[0048] After the sample 100 is manufactured, the transmission wave front of the sample 100 is measured with a measuring device to obtain the shape correction amount of the preliminary shape metal molds (step S13). FIG. 5 shows an out line of a Mach-Zehnder interferometer 4 which is the measuring device for measuring the manufacture error of the formed sample 100. The Mach-Zehnder interferometer 4 shown in FIG. 5 comprises: a laser generator 41 generating a laser beam for the measurement; a beam splitter 42 splitting the incident laser beam into reflected light 400 and transmitted light 401; a mirror 43 further reflecting the reflected light 400; a microscope objective 44 directing the transmitted light 401 to the first aperture 11 of the sample 100; a hemispherical mirror 45 condensing the transmitted light 401 exiting through the second aperture 12 of the sample 100, again to the second aperture 12; a taking lens 46 for taking the interference fringes between the reflected light 400 and the transmitted light 401; and a CCD 47.

[0049] If a virtual sample having no manufacture error and formed to have the exact shape as designed is measured here, an image with no interference fringes or with interference fringes like straight lines arranged in parallel will be obtained on the CCD 47. However, since the sample 100 generally has a manufacture error, an image with curved interference fringes is obtained on the CCD 47.

[0050] FIG. 6 shows an example of the interference fringes observed due to the interference action of the Mach-Zehnder interferometer 4 shown in FIG. 5. A light ray 402 shown in FIG. 6 is, of the laser beam from the laser generator 41 shown in FIG. 5, a light ray incident on a point P on the second reflecting surface 14 of the sample 100, and part of the light ray 402 is reflected at the beam splitter 42 and is further reflected at the mirror 43 to reach the CCD 47.

[0051] Of the light ray 402, the light transmitted by the beam splitter 42 is condensed to the first aperture 11 of the sample 100 by the microscope objective 44, and is reflected at the point P on the second reflecting surface 14. Then, the light undergoes a condensing function at a point Q on the first reflecting surface 13 to be condensed to the second aperture 12. The light condensed to the second aperture 12 is reflected at the hemispherical mirror, follows the same optical path again, and is reflected at the beam splitter 42 to reach the CCD 47. The two light rays obtained from different optical paths interfere with each other to form a plurality of interference fringes on the CCD 47. An interference fringe 403 represents one of the interference fringes. Data of the observed interference fringe 403 and the like is input to a calculator or computer 48, and used for the processing described later. When a high-NA reflective-type lens with an NA of higher than 1 is measured, by bringing the hemispherical mirror 45 and the high-NA reflective-type lens into contact with each other or filling the gap therebetween with an oil of a high refractive index, even such a lens can be measured by the method shown here.

[0052] The shape of the actual transmission wave front of the sample 100 is easily measured from the image of the interference fringe 403 thus obtained on the CCD 47. For example, fringe scanning to measure the relative phase of each point on the interference fringe 403 by moving the mirror 43 in the direction of the optical axis is used.

[0053] Then, the manufacture error of the second reflecting surface 14 of the sample 100 is obtained by use of a ray tracing technique. FIG. 7 conceptually shows the ray tracing technique. A plane 501 is a plane supplied to the calculator 48 on the assumption that the second reflecting surface 14 of the sample 100 is a horizontal plane. Further, the light ray 402 is supplied to the plane 501 and the position of the point P at which the light ray 402 is reflected on the plane 501 is shifted downward in FIG. 7 to obtain a point S where the actually measured shape of the transmission wave front is obtained.

[0054] By doing this, the shape error of the transmission wave front is obtained as the distance between the points P and S. This operation is performed for all the points P on the plane 501 and all the corresponding points S are obtained to thereby obtain a plane 502. That is, the overall correction amount of the second reflecting surface 14 is obtained as the difference between the planes 501 and 502.

[0055] In the actual manufacture of optical elements having a plurality of optical surfaces, a manufacture error occurs in both the first reflecting surface 13 and the second reflecting surface 14. However, under a condition where the directions of the light rays incident on the reflecting surfaces are substantially the same (only one light ray is incident on each point on each reflecting surface) like in the present embodiment, by correcting only the drag 3 for forming the shape of the second reflecting surface 14 on the assumption that the second reflecting surface 14 includes all the manufacture errors, the errors of a plurality of optical reflecting surfaces (in the present embodiment, the first reflecting surface 13 and the second reflecting surface 14) can be equivalently corrected all together, whereby the transmission wave front of the formed high-NA reflective-type lens 1 can be corrected.

[0056] Since the light to be used is used for measuring the manufacture error of the optical element in this manner, more practical measurement can be performed. Assuming now that the maximum correction amount &dgr; in the present embodiment is 0.2 &mgr;m, a method of correcting this by etching will be described.

[0057] First, in performing etching, the minimum etching amount is obtained. FIG. 8 shows a corrected molding surface 503 having undergone etching together with an ideal molding surface 504. The ideal molding surface 504 is a surface providing the design optical performance of the manufactured high-NA reflective-type lens 1.

[0058] When correction is performed by etching, the corrected molding surface 503 is stepped as shown in FIG. 8. When the minimum etching amount in the etching (the size of the smallest step of the corrected molding surface 503 shown in FIG. 8) is a minimum etching amount h, the maximum error d between the corrected molding surface 503 and the ideal molding surface 504 is half the minimum etching amount h. Since the maximum permissible error d is 0.0125 &mgr;m, the minimum etching amount h is obtained as 0.025 &mgr;m.

[0059] FIG. 9 shows, together with the ideal molding surface 504, an estimated shape of the corrected molding surface 503 corrected so that etching is not performed more deeply than the ideal molding surface 504 at all times in etching. Since the minimum etching amount h is 0.025 &mgr;m, the corrected molding surface 503 is formed so as to be shifted from the ideal molding surface 504 by 0.025 &mgr;m at the maximum.

[0060] For the precision of an optical element like the high-NA reflective-type lens 1, since the influence of the shape error of the reflecting surface is sufficiently large compared to that of the position error (position error in the vertical direction in FIG. 9) of the reflecting surface, the error evaluation may be performed by regarding a plane 505 as the new ideal molding surface. The plane 505 is the ideal molding surface 504 parallelly shifted by 0.0125 &mgr;m in the vertical direction in FIG. 9. Since the maximum error d between the corrected molding surface 503 and the plane 505 is 0.0125 &mgr;m as shown in FIG. 9, the above-mentioned target value is achieved. That is, the maximum amount to be removed by etching is 0.175 &mgr;m.

[0061] Then, obtaining the number of times of etching and the etching amount of one etching so that the number of times of etching is minimized, since the minimum etching amount h when the maximum depth is 0.175 &mgr;m is 0.025 &mgr;m, the etching amount is 0.1 &mgr;m for the first etching, 0.05 &mgr;m for the second etching and 0.025 &mgr;m for the third etching.

[0062] As described above, the sample 100 of the high-NA reflective-type lens 1 is formed by use of the preliminary shape metal molds 20 and 30 not having undergone etching yet, the shape correction amounts of the preliminary shape metal molds 20 and 30 (only the metal mold 30 in the present embodiment) can be determined through measurement of the amount which corresponds to the surface shape of the sample 100, and correction corresponding thereto can be easily performed.

[0063] FIGS. 10A to 10F show a manner of correcting the drag preliminary shape metal mold 30 by dry etching. A process of correcting the drag preliminary shape metal mold 30 by etching will be concretely described with reference to FIG. 2 and these figures.

[0064] First, as shown in FIG. 10A, a resist 31 is applied to the drag preliminary molding surface 300 of the drag preliminary shape metal mold 30 (step S14). As the material of the resist 31, for example, in the case of laser beam writing, AZ1500 of Hoechst Aktiengesellschaft is used, and in the case of electron beam writing, ZEP-520 of Nippon Zeon Co., Ltd. is used. Then, as shown in FIG. 10B, a latent image is scanned by a laser beam condensed by a condenser lens 32 of a laser beam writing device, and a resist pattern is drawn on the resist 31 (step S15). In the case of electron beam writing, an electron beam exposure device is used instead of the laser beam writing device. The resist pattern may be drawn by a combination of a light source and a spatial light modulation element.

[0065] Then, as shown in FIG. 10C, the resist 31 is developed (step S16), and as shown in FIG. 10D, dry etching is performed on the drag preliminary shape metal mold 30 (step S17). The amount of the first etching is 0.1 &mgr;m as mentioned above. Then, as shown in FIG. 10E, the resist 31 is removed (step S18), and the first etching process is finished.

[0066] Then, steps S14 to S18 are repeated to perform dry etching until correction is completed (step S19). In the present embodiment, dry etching is performed to a depth of 0.05 &mgr;m in the second etching and to a depth of 0.025 &mgr;m in the third etching as mentioned above to complete correction, and the drag 3 having a corrected drag molding surface 301 is completed (see FIG. 10F). In the present embodiment, the cope preliminary shape metal mold 20 is used as the cope 2 as it is.

[0067] Thus, by correcting the shape of the surface of the drag preliminary shape metal mold 30 by selectively etching the surface of the drag preliminary shape metal mold 30 in accordance with the shape correction amount determined through the prior measurement, the drag molding surface 301 can be obtained, so that the cope 2 and the drag 3 with high precision can be easily manufactured.

[0068] When the drag 3 is completed, as shown in FIG. 3, the high-NA reflective-type lens 1 is formed by use of the cope 2 and the drag 3. Then, a reflective coating is selectively applied to the surface of the lens 1 so that the lens 1 has a structure as shown in FIG. 1.

[0069] By thus correcting by etching the formed preliminary shape metal molds 20 and 30 (in the present embodiment, of them, the drag preliminary shape metal mold 30 having a plane) in accordance with the shape correction amount obtained by the measurement value, the cope 2 and the drag 3 of high precision compared to that of the cope and the drag manufactured when the correction is performed by the conventional method using machining can be manufactured, and by forming the high-NA reflective-type lens 1 by use of the cope 2 and the drag 3, a high-precision and inexpensive high-NA reflective-type lens 1 can be provided.

[0070] <2. Second Embodiment>

[0071] In the first embodiment, the shape correction amount is obtained from the manufacture error of the sample 100 by measuring the shape of the sample 100. However, in the case of small-size lenses, measuring the optical performance is often easier than measuring the shape itself, and is higher in practical utility.

[0072] First, like in the first embodiment, the sample 100 is formed from the cope preliminary shape metal mold 20 and the drag preliminary shape metal mold 30, the manufacture error of the formed sample 100 is obtained by measuring the optical performance of the sample 100, and the shape correction amount is determined. As a method of obtaining the manufacture error by measuring the optical performance of the sample 100, for example, a method is used that is shown in “Evaluation of Laser Optics from the Spot Image,” Yasuhiro TANAKA, Kogaku (Optics, Journal of the Optical Society of Japan), Vol. 22, No. 8 (August, 1993), pp.456-461.

[0073] After the manufacture error is obtained by the above-described method, the shape correction amount is determined based on it, and the drag preliminary shape metal mold 30 is corrected by etching shown in FIGS. 10A to 10F in the first embodiment to form the drag 3. After the cope 2 and the drag 3 are formed, using them, the high-NA reflective-type lens 1 is formed by the method shown in FIG. 4.

[0074] By thus determining the shape correction amount through measurement of the optical performance of the sample 100, metal molds with high precision like that of the first embodiment can be easily manufactured, and by using them, a high-precision optical element can be manufactured.

[0075] <3. Third Embodiment>

[0076] In the above-described embodiment, the shape correction amount to be removed by etching when the metal mold is manufactured is obtained by measuring the sample 100 formed from the preliminary shape metal molds 20 and 30. However, the shape correction amount may be obtained by directly measuring the surface shape of the preliminary shape metal molds 20 and 30. In this case, since the object to be corrected is measured, a higher-precision metal mold can be manufactured.

[0077] FIG. 11 shows a laser interferometer 61 which is a device for measuring the preliminary shape metal molds. In FIG. 11, a diffraction optical element (zone plate 62) for a sphere measurement comprises concentric diffraction patterns designed so as to generate a wave front having the same shape as the asphere (the cope molding surface 201 of the cope 2) to be measured. When the drag preliminary shape metal mold 30 is measured, a zone plate designed so as to generate a wave front having the same shape as the drag molding surface 301 is used. The preliminary shape metal molds 20 and 30 are prepared by a similar method to that of the above-described embodiments.

[0078] First, the shapes of the cope preliminary molding surface 200 and the drag preliminary molding surface 300 are measured with the laser interferometer 61, and the shape errors of the surfaces are calculated. Then, the position of a point Y on the second reflecting surface 14 corresponding to an arbitrary point X on the first reflecting surface 13 is obtained by ray tracing with the calculator or computer 63 so that correction amount of the shape errors of the cope preliminary molding surface 200 and the drag preliminary molding surface 300 are reflected in the shape of the second reflecting surface 14. That is, the position of the point Y on the second reflecting surface 14 is obtained such that, in the high-NA reflective-type lens 1 in FIG. 12, even if the point X on the first reflecting surface 13 includes an error, light reflected at the point X and reaches the second aperture 12 (the influence of the error of the point X is canceled).

[0079] Then, the point corresponding to the point Y is obtained on the measured drag preliminary molding surface 300, and the distance between the points is the shape correction amount of the second reflecting surface 14 at the point Y. By obtaining the shape correction amount by the above-described method for all the points on the first reflecting surface 13, the overall shape correction amount of the second reflecting surface 14 can be obtained. When the overall shape correction amount of the second reflecting surface 14 is obtained, the shape correction amount of the drag preliminary molding surface 300 for molding the second reflecting surface 14 is obtained based on it.

[0080] When the shape correction amount of the drag preliminary molding surface 300 is obtained, etching is performed by a similar method to that of the first embodiment to correct the drag preliminary molding surface 300, so that the cope 2 and the drag 3 are obtained. By using the cope 2 and the drag 3, the high-NA reflective-type lens 1 is manufactured (see FIG. 4).

[0081] As described above, by using the laser interferometer 61 and the zone plate 62, the shape correction amount is determined through measurement of the surface shapes of the preliminary shape metal molds 20 and 30 not having undergone etching yet, and the prototypes of the cope 2 and the drag 3 to be corrected are directly measured, so that a higher-precision metal mold can be manufactured.

[0082] <4. Fourth Embodiment>

[0083] While examples manufacturing optical elements with metal molds are described in the above-described embodiments, when elements and parts not more than 1 mm are manufactured, lithography being inexpensive and with which micromachining is possible is generally used.

[0084] FIG. 13 shows a microlens 7 which is an optical element in a fourth embodiment. Although having a first aperture 71, a second aperture 72, a first reflecting surface 73 and a second reflecting surface 74 like the high-NA reflective-type lens 1 in the above-described embodiments, the microlens 7 is different from the lens 1 in that it is approximately 1 mm in diameter and smaller than the lens 1.

[0085] FIG. 14 is a flowchart showing a process of manufacturing a preform of the microlens 7 (hereinafter, abbreviated as preform) by reflow which is a method of lithography. FIGS. 15A to 15D show a manner of manufacturing the preform by reflow. With reference to these figures, a method of manufacturing the preform will be described.

[0086] First, as shown in FIG. 15A, a resist 76 is applied to a glass plate 75 which is the base material of the preform (step S21). Then, as shown in FIG. 15B, resist patterns are formed on the surface of the glass plate 75 by an exposure and development process (step S22).

[0087] After the resist patterns are formed, as shown in FIG. 15C, they are heated to approximately 140° C. to soften the resist 76 so that the resist 76 is curved (step S23). Then, as shown in FIG. 15D, the shape of the resist 76 is transferred to the glass plate 75 by etching (step S24) to obtain a preform main body 77a having the outside shape of the microlens 7.

[0088] A reflective coating is applied to the preform main body 77a (step S25) to form reflecting surfaces corresponding to the first reflecting surface 73 and the second reflecting surface 74 shown in FIG. 13. In this manner, a preform 77 having a shape corresponding to the outside shape of the microlens 7 can be prepared.

[0089] FIG. 16 is a flowchart showing a process of manufacturing the microlens 7 according to the fourth embodiment from the preform 77. FIGS. 17A to 17F show a manner of correcting the preform 77 by etching to manufacture the microlens 7. With reference to these figures, a method of manufacturing the microlens 7 will be described.

[0090] First, like in the first embodiment, the shape correction amount of the preform 77 is obtained by a measurement method using the Mach-Zehnder interferometer 4 (step S31). The shape correction amount may be obtained by the method shown in the second embodiment.

[0091] In this manner, the shape correction amount can be determined through measurement of the surface shape of the preform of the optical element or measurement of the optical performance of the preform of the optical element, so that a high-precision optical element can be manufactured by the etching described later.

[0092] Here, description will be given on the assumption that the maximum correction amount &dgr; is 0.2 &mgr;m like in the above-described embodiments.

[0093] After the shape correction amount of the preform 77 is obtained, the reflective coating corresponding to the second reflecting surface 74 of the microlens 7 is removed to return the preform 77 to the preform main body 77a (step S32), and as shown in FIG. 17A, a resist 78 is applied to the surface on the substantially plane side of the perform main body 77a (the surface corresponding to the second reflecting surface 74 of the microlens 7) (step S33).

[0094] Then, as shown in FIG. 17B, a pattern is drawn on the resist 78 by a condenser lens 79 of a laser beam writing device (step S34), and as shown in FIG. 17C, the resist 78 is developed (step S35).

[0095] Then, as shown in FIG. 17D, with the resist 78 as the mask, dry etching to a desired depth is performed on the preform main body 77a (step S36), and as shown in FIG. 17E, the resist 78 is removed (step S37). Steps S33 to S37 are repeated until correction is completed like in the first embodiment (step S38), and a reflective coating is selectively applied again onto the surface of the preform main body 77a (step S39) to form the microlens 7 as shown in FIG. 17F.

[0096] By the above-described method, a small-size optical element like the microlens 7 having a diameter of not more than 1 mm and on which correction cannot be performed by the conventional method can be obtained by selectively etching the surface of the preform of the optical element in accordance with the shape correction amount determined through prior measurement, so that a high-precision small optical element can be manufactured.

[0097] Moreover, by selectively etching only a specific optical surface, which is substantially plane, of a plurality of optical surfaces, a plurality of optical surfaces can be corrected all together, so that an optical surface having a complicated shape can be corrected and correction can be performed more easily than when correction is performed on all the optical surfaces.

[0098] <5. Modification>

[0099] While embodiments have been described, the present invention is not limited to the above-described embodiments and various modifications are possible.

[0100] For example, while dry etching is used as the method of etching for correction in the above-described embodiments, correction may be performed by wet etching as long as the preliminary shape metal mold or the optical element can be corrected in accordance with the obtained shape correction amount.

[0101] While the high-NA reflective-type lens 1 is described as an example of the object manufactured with the metal mold in the first to the third embodiments, the object manufactured with the metal mold is not limited to such an optical element. For example, the object may be a small-size metal part, and may be anything that is generally molded with a metal mold.

[0102] While the preform of the microlens is prepared by use of reflow in the above-described fourth embodiment, other method can be employed. For example, isotropic etching or the like may be used, and any method may be used as long as the preform of the optical element is manufactured with a predetermined precision.

[0103] As described above, by preparing a preliminary shape metal mold having a shape approximate to the outside shape of an object to be molded and selectively removing the surficial part of the preliminary shape metal mold by etching to obtain a molding surface, a metal mold with a higher precision can be manufactured than when the molding surface is obtained by machining. In addition, manufacture efficiency is higher than when a three-dimensional shape of an optical element or the like is formed by repeating only lithography with a small processing amount a multiplicity of times.

[0104] Further, since the shape correction amount is determined through measurement of the surface shape of the preliminary shape metal mold not having undergone etching yet, the object to be corrected is directly measured, so that a higher-precision metal mold can be manufactured.

[0105] Further, with the molding surface obtained by selectively removing, by the shape correction amount, the surficial part of the preliminary shape metal mold having a shape approximate to the outside shape of the object to be molded by etching, a high-precision optical element and part can be effectively manufactured.

[0106] Further, the optical element as the object obtained by the above-described method can be used for purposes requiring high precision.

[0107] Further, by preparing a preform having a shape approximate to the outside shape of an optical element and selectively etching the surficial part of the preform to obtain the optical element, a high-precision optical element can be inexpensively manufactured.

[0108] Further, by correcting a plurality of optical surfaces all together by selectively etching a specific one of a plurality of optical surfaces, correction can be performed more easily than when correction is performed on all the optical surfaces.

[0109] Further, by the substantially plane part being selected as the specific optical surface, correction can be performed more precisely and easily than when etching is performed on an optical surface having a complicated shape.

[0110] Further, by the above-described method, an optical element with a diameter of not more than 1 mm can be manufactured with high precision.

[0111] Further, by providing an optical surface obtained by selectively removing, by etching, the surficial part of a preform molded into a shape approximate to a predetermined optical function, the optical element can be used for purposes requiring high precision.

[0112] Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.

Claims

1. A method of manufacturing a metal mold for molding an object, comprising:

preparing a preliminary shape metal mold having a shape approximate to an outside shape of the object; and

a processing to obtain a molding surface by selectively removing a surficial part of the preliminary shape metal mold by etching.

2. A method according to claim 1, wherein said processing includes:

correcting a shape of a surface of the preliminary shape metal mold by selectively etching the surface of the preliminary shape metal mold in accordance with a shape correction amount determined through prior measurement.

3. A method according to claim 2, wherein the shape correction amount is determined through measurement of a surface shape of the preliminary shape metal mold not having undergone etching yet.

4. A method according to claim 2, wherein said processing includes:

forming a sample of the object by use of the preliminary shape metal mold not having undergone etching yet; and

determining the shape correction amount through measurement of an amount in which a surface shape of the sample is reflected.

5. A method according to claim 2, wherein said processing includes:

forming a sample of the object by use of the preliminary shape metal mold not having undergone etching yet; and

determining the shape correction amount through measurement of optical performance of the sample.

6. A metal mold for molding an object, comprising:

a first portion of molding surface obtained by a preliminary manufacturing stage; and

a second portion of molding surface obtained by selectively removing a surficial part obtained by the preliminary manufacturing stage by etching in accordance with a predetermined shape correction amount.

7. A metal mold according to claim 6, wherein the shape correction amount is determined through prior measurement.

8. A metal mold according to claim 7, wherein the shape correction amount is determined through measurement of shape of a surface obtained by the preliminary manufacturing not having undergone etching yet.

9. A metal mold according to claim 7, wherein a sample of the object is formed by use of the mold obtained by the preliminary manufacturing not having undergone etching yet and the shape correction amount is determined through measurement of an amount in which a surface shape of the sample is reflected.

10. A metal mold according to claim 7, wherein a sample of the object is formed by use of the mold obtained by the preliminary manufacturing not having undergone etching yet and the shape correction amount is determined through measurement of optical performance of the sample.

11. A method of manufacturing an optical element, comprising:

preparing a preform having a shape approximate to an outside shape of the optical element; and

processing for obtaining the optical element by selectively etching a surface shape of the preform.

12. A method according to claim 11, wherein said processing includes:

selectively etching a surface of the preform in accordance with a shape correction amount determined through prior measurement.

13. A method according to claim 12, wherein said processing further includes:

determining the shape correction amount through measurement of an amount in which the surface shape of the preform is reflected.

14. A method according to claim 12, wherein said processing further includes:

determining the shape correction amount through measurement of optical performance of the preform.

13. A method according to claims 11, wherein the optical element has a plurality of optical surfaces successively acting on incident light, and

in the etching, the optical surfaces are corrected all together by selectively etching a specific one of the optical surfaces.

14. A method according to claim 13, wherein the optical surfaces include a substantially plane part, and the substantially plane part is selected as the specific optical surface.

15. A method according to claim 11, wherein the optical element is not more than 1 mm in diameter.