Abstract:
A planar lens capable of compensating for chromatic aberration and which is easy to manufacture and enables easy assembling of optical pickups and a method for fabricating the planar lens are provided. The planar lens includes a transparent substrate with a lens cavity in a surface of the transparent substrate, and a lens element formed in the lens cavity with a first refracting surface in contact with the bottom of the lens cavity and a second diffracting surface having a diffraction grating opposite to the first refracting surface.

Description:
BACKGROUND OF THE INVENTION 
     This application claims priority from Korean Patent Application No. 2002-70662, filed on Nov. 14, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     1. Field of the Invention 
     The present invention relates to a planar lens and a method for fabricating the same, and more particularly, to an easy-to-fabricate planar objective lens capable of compensation of chromatic aberration, as an essential part of an optical pickup that is used in an optical disc drive (ODD) to write information in and/or reproduce information from an optical disc, such as a compact disc (CD) or a digital versatile disc (DVD), and an easy method for fabricating the planar microlens. 
     2. Description of the Related Art 
     Objective lenses are used to write information in an optical disc by focusing a laser beam emitted from a semiconductor laser used as a light source on a recording surface of the optical disc and to read information from an optical disc by condensing light reflected from its recording surface toward a photodetector. Conventionally, objective lenses for use in writing and reading information with respect to optical information storage media have been fabricated by mechanically cutting and polishing solid glass or plastics to a desired shape. Another conventional methods for fabricating such objective lenses use compression molding or injection molding in which objective lenses are fabricated from molten or semi-molten glass or plastics using molds formed by mechanical processing. 
     To fabricate an objective lens by compression molding, as shown in  FIG. 1 , upper and lower molds  2   a  and  2   b , which are located above and below a target object to be manufactured, i.e., a lens  1 , respectively, a ring  3  interposed between the upper and lower molds  2   a  and  2   b , and a sleeve  4  as a support for the upper and lower molds  2   a  and  2   b  are necessary. These upper and lower molds  2   a  and  2   b  are commonly manufactured by a high precision lathe with a diamond wheel. However, this mold processing method is limited as to processing a curved mold surface corresponding to a spherical or aspheric lens surface. In particular, to form a spherical or aspheric lens surface, a mold needs to be machined for a concave surface that matches the desired spherical or aspheric lens surface. However, it is difficult to machine a concave mold for lenses under about 1 millimeter in diameter with the mechanical mold machining method, because cutting tools and the curvature of diamond wheels therefore are limited. 
     Recent trends toward portable, miniature, and high-density optical discs necessitate smaller optical pickups compatible with these miniature optical discs. Smaller optical pickups should be assembled with smaller constituent optical elements, such as a laser diode, a collimator, a mirror, an objective lens, a photodetector, and the like. However, it is more difficult to manufacture and assemble smaller optical elements, especially with microlenses having a diameter of 1 mm or less because of difficulties in mold machining therefor, as described above. Due to the limitation in mold machining, the smallest known lenses manufactured thus far, for example, by injection molding with a numerical aperture (NA) of 0.85, have a diameter of about 4 mm. Even though such microlenses of 1 mm or less in diameter can be manufactured, it is difficult to handle such a small microlens and to assemble an optical pickup with the microlens. 
     General objective lenses have two opposing surfaces that are spherical or aspheric and which protrude closer to an optical disc, compared to other adjacent optical elements, so they are likely to be abraded or damaged when collided with the optical disc. 
     To eliminate these problems, there have been suggested techniques for fabricating objective lenses in an array in a planar substrate using, for example, conventional semiconductor manufacturing processes. These methods are advantageous for mass production and enable easy assembling of optical pickups at low costs. 
       FIGS. 2A through 2D  illustrate a process of fabricating a microlens array suggested by Akira Kouchiyama et al. (Japanese J. Appl. Phys., Part 1, Vol. 40, No. 3B, p. 1792, 2001). 
     Briefly, in the method suggested by Akira Kouchiyama, a glass substrate  5  is coated with a photoresist  6 , as shown in  FIG. 2A . After patterning, as illustrated in  FIG. 2B , the resulting photoresist pattern  6 ′ is reflowed by heating to a predetermined temperature, for example, about 150° C. Through this reflow process, a hemispheric photoresist pattern  6 ″ is formed, as shown in  FIG. 2C . Next, reactive ions are supplied into a plasma etching chamber to etch the substrate  5  through the hemispheric photoresist pattern  6 ″ so that a microlens array is formed in the glass substrate  5 , as shown in  FIG. 2D . As the photoresist pattern  6  is reflowed by heating, the photoresist pattern  6 ′ becomes hemispheric by surface tension generated during the reflow. As the plasma dry etching is performed on the glass substrate  5  under appropriate conditions with the hemispheric photoresist  6 ″ serving as a mask, the hemispheric shape of the mask is transferred into the glass substrate  5 . The resulting microlens array has a spherical refractive surface. 
     In this microlens array manufacturing method by plasma etching, it is difficult to uniformly deposit the photoresist to a sufficient thickness for an objective lens of appropriate thickness so that the sag height of the resulting microlens is limited. In addition, since the hemispheric photoresist pattern formed by reflowing is directly transferred into a glass substrate, the method cannot be applied for general aspheric lenses excluding spherical lenses. Dry etching applied to manufacture the microlens array takes a comparatively longer duration. 
     According to a microlens manufacturing method suggested by Masahiro Yamada et al., Technical Digest of ISOM/ODS 2002, p. 398, a substrate with a cavity for a lens used for optical control is manufactured by injection molding, and the cavity is filled with a high refractive index material and subjected to polishing to form a planar lens. 
     Masahiro&#39;s method requires a mold of a desired shape to form the cavity in the substrate. Unlike general molding processes where a cavity is formed in a substrate to correspond to a convex surface of a desired lens, a mold shaped like a plug or insert with a convex surface that matches a desired cavity of a substrate is formed by mechanical machining. Over the substrate with the cavity formed through injection molding using the plug-like mold, a high refractive index material is deposited to a predetermined thickness by sputtering and polished such that the high refractive index material remains only in the cavity of the substrate. The cavity filled with the high refractive index material substantially acts as a lens. 
     This method is suitable to manufacture single microlens but not for an array of multiple lenses. Also, the thin film deposition technique, i.e., sputtering, used in the method takes a long duration for a lens layer having an appropriate thickness. Furthermore, it is also time consuming to polish the deposited high refractive index material lens layer. 
     SUMMARY OF THE INVENTION 
     The present invention provides a planar objective lens that is easy to manufacture, enables easy assembling of optical pickups, and has good optical performance, and a method for fabricating the planar objective lens. 
     According to an aspect of the present invention, there is provided a planar lens comprising: a transparent substrate with a lens cavity in a surface of the transparent substrate; and a lens element formed in the lens cavity with a first refracting surface in contact with the bottom of the lens cavity and a second diffracting surface having a diffraction grating opposite to the first refracting surface. 
     In a planar lens according to the present invention, the lens element may be formed of a higher refractive index material than a material for the transparent substrate. 
     According to another aspect of the present invention, there is provided a method for fabricating a planar lens, the method comprising: (a) forming a lens cavity in a transparent substrate; (b) filling the lens cavity with a lens forming material; and (c) forming a diffraction grating in the surface of the lens cavity filled with the lens forming material. 
     In a method for fabricating a planar lens according to the present invention, step (b) may be performed by a sol-gel method including spin coating and baking of the lens forming material. In step (a), the lens cavity may be formed with a spherical or aspheric bottom surface. 
     Alternatively, the present invention provides a method for fabricating a planar lens comprising a lens element formed in a lens cavity of a transparent substrate, the lens element having a first refracting surface in contact with the bottom of the lens groove and a second diffracting surface having a diffraction grating opposite to the first refracting surface, the method comprising: (a) forming the lens cavity in the transparent substrate; (b) bringing a mold with a pattern region that matches the diffraction grating of the lens element to the transparent substrate such that the pattern region of the mold is aligned above with the lens cavity; (c) injecting a liquid state lens forming material containing a high refractive index material precursor into the lens cavity; (d) curing the liquid state lens forming material in the lens cavity; and (e) separating the mold from the transparent substrate. 
     In another embodiment of the above method, the liquid state lens forming material may be solid-nucleated by UV irradiation and is fully cured by heating. Preferably, the liquid state lens forming material is an alkoxide solution comprising a precursor of (Si,Ti)O 2 . 
     In step (a), the lens cavity may be formed with a spherical or aspheric bottom surface. 
     In still another embodiment of the above method according to the present invention, step (a) of forming the lens cavity in the transparent substrate may comprise: forming on a substrate a mask with a window through which a lens cavity region of the substrate is exposed; etching the lens cavity region of the substrate exposed through the window by isotropic dry or wet etching to form the lens cavity below the window; and removing the mask from the substrate. 
     In yet still another embodiment of the above method according to the present invention, step (a) of forming the lens cavity in the transparent substrate comprises: depositing photoresist over the top surface of the transparent substrate; forming in the deposited photoresist a groove which is the same in shape as the lens cavity to be formed; and etching the deposited photoresist to a depth in the substrate to transfer the shape of the groove into the transparent substrate, thereby resulting in the lens cavity in the transparent substrate. In this case, forming the groove in the deposited photoresist may be performed by exposure to light through a gray scale mask or by direct exposure to laser beams or electron beams. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: 
         FIG. 1  illustrates an example of a conventional method for fabricating an objective lens; 
         FIGS. 2A through 2D  are sectional views illustrating a conventional method for fabricating a microlens array; 
         FIG. 3  is a sectional view of a planar objective lens according to an embodiment of the present invention; 
         FIGS. 4A through 4D  are sectional views illustrating a method for fabricating a planar objective lens according to an embodiment of the present invention; 
         FIGS. 5A through 5D  are sectional views illustrating an embodiment of a method for forming a lens cavity in a substrate in the manufacture of a planar objective lens according to the present invention; 
         FIGS. 6A through 6D  are sectional views illustrating another embodiment of the method for forming a lens cavity in a substrate in the manufacture of a planar objective lens according to the present invention; and 
         FIGS. 7A through 7E  are sectional views illustrating a method for fabricating a planar objective lens according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Exemplary embodiments of an objective lens a method for fabricating the same according to the present invention will be described with reference to the appended drawings. 
     Referring to  FIG. 3 , an objective lens  10  according to an embodiment of the present invention has a configuration including a lens element  11  in a lens cavity  12   a  formed in the middle of the top surface of a substrate  12  with a concave aspheric or spherical surface. The lens element  11  has a first surface corresponding to the bottom of the lens cavity  12   a  and a second surface opposite to the first surface. The first surface of the lens element  11  is a refracting surface  11   b  having a spherical or aspheric curvature that matches the bottom contour of the lens cavity  12   a , and the second surface is a diffracting surface with a diffraction grating. 
     The objective lens  10  of  FIG. 3  is a planar lens formed by filling the lens cavity  12   a  of the substrate  12  with the lens element  11 , unlike general convex objective lenses whose refracting surfaces are convexed outward. In this objective lens  10  having the above configuration, for example, when a laser beam is incident on the bottom of the substrate  12  from a light source (not shown), the laser beam is primarily focused by being refracted at the reflecting surface  11   b  of the lens element  11  and secondarily focused at the diffracting surface  11   a  into a smallest light spot possible close to the diffraction limit. This diffracting surface  11   a  of the lens element  11  is designed to make it enable to compensate for lens aberration, and particularly, chromatic aberration, as well as to focus light, with the same shape as general Fresnel lenses. 
     Hereinafter, a method for fabricating a planar objective lens, having a configuration as described above, according to the present invention will be described in detail. Although the formation of a single lens is described in the following, the method described below can be applied to fabricate an array of multiple lenses in a single substrate. 
       FIGS. 4A through 4D  illustrate a method for fabricating a planar objective lens according to an embodiment of the present invention. Referring to  FIG. 4A , initially, a transparent glass substrate  12  is prepared. Next, a lens cavity  12   a  is formed in the glass substrate  12 , as shown in  FIG. 4B . The lens cavity  12   a  is formed by photolithography followed by wet etching or dry etching with a mask. 
     As shown in  FIG. 4C , the lens cavity  12   a  is filled with a high refractive index material to form a lens element  11 . In particular, over the surface of the substrate  12  with the lens cavity  12   a , the high refractive index material is deposited by a sol-gel method, in which spin coating and baking of the high refractive material are repeatedly performed to form a coated layer of a desired thickness. Next, the coated layer of high refractive material is polished such that the high refractive material remains only in the lens cavity  12   a , forming the lens element  11 . 
     As shown in  FIG. 4D , a diffraction grating is formed on the surface of the lens element  11  in the lens cavity to provide a desired objective lens according to the present invention. 
     Hereinafter, the method for fabricating an objective lens described with reference to  FIG. 4  will be described in more detail.  FIGS. 5 and 6  illustrate embodiments of a method for forming the lens cavity  12   a  in the substrate  12  in the fabrication of the objective lens illustrated in  FIG. 4 . 
     Embodiment 1 of Lens Cavity Formation 
     Referring to  FIG. 5A , the entire surface of a prepared substrate  12  is coated with photoresist  13  and subjected to light exposure through a mask  14  with a window  14   a  as a light transmission path. In this embodiment, positive photoresist is used so that a region to be etched is exposed to form the lens cavity  12   a  therebelow. 
     Next, the exposed region of the photoresist  13  is removed through a development process, as shown in  FIG. 5B , to expose a cavity region of the substrate  12 . 
     The exposed cavity region of the substrate  12 , which is uncovered with the photoresist  13 , is subjected to isotropic dry or wet etching so that the lens cavity  12   a  with a spherical bottom surface is formed, as shown in  FIG. 5C . 
     After the remaining photoresist  13  is stripped away, as shown in  FIG. 5D , and the process illustrated in  FIG. 4C  is performed. 
     Embodiment 2 of Lens Cavity Formation 
     Referring to  FIG. 6A , the entire surface of a prepared substrate  12  is coated with photoresist  13  and subject to light exposure through a gray scale mask  15  with a window  15  whose light transmittance locally varies. In this embodiment, positive photoresist is used so that a region to be etched is exposed to form the lens cavity  12   a  therebelow. 
     Since the window  15   a  of the gray scale mask  14  used in this embodiment has locally varying light transmittance, the amount of light radiated on the photoresist  13  during the exposure process through the window  15   a  is not uniform throughout the exposed region. The light transmittance of the window of the gray scale mask  14  needs to be appropriately controlled according to the curvature of a desired spherical or aspheric lens. 
     Next, a spherical or aspheric groove  13   a  is formed in the photoresist  13  through a development process, as shown in  FIG. 6B . 
     Next, as shown in  FIG. 6C , dry etching, for example, reactive ion etching, is performed on the photoresist  13  and into the substrate  12  to transfer the spherical or aspheric groove  13   a  in the photoresist  13  into the substrate  12 . As a result, a lens cavity  12   a  is formed in the substrate  12 , as shown in  FIG. 6D . After the formation of the lens cavity  12   a  is completed, the process illustrated in  FIG. 4C  above is performed. 
     In the process of  FIG. 4C , as the high refractive index material to fill the lens cavity  12   a , a material having a refractive index of 2.0 or greater and a high transmittance to blue light, for example, having a wavelength of 405 nm, such as SiO 2 , TiO 2 , ZrO 2 , HfO 2 , SrTiO 3 , BaTiO 3 , PbTiO 3 , PZT, PLZT, and the like, and combination of two or more substances from these materials and which is compatible with a sol-gel technique may be used. 
     In this sol-gel method, a relatively uniform and thick layer can be formed within a short time, compared to general physical deposition methods, such as sputtering. Since the top surface of the lens element formed as a result of filling the lens cavity  12   a  with the high refractive index material is uneven, the lumpy top surface of the lens element is polished ( FIG. 4C ). Next, a diffraction grating is formed on the surface of the lens element formed of the high refractive index material ( FIG. 4D ). In forming a refracting surface of the lens element, methods applied to form general diffraction optical elements (DOE), including photolithography using binary optics, gray scale photolithography, and direct lithography using electron beams or laser beams, may be used. 
       FIGS. 7A through 7E  illustrate a method for fabricating a planar objective lens according to another embodiment of the present invention. Referring to  FIG. 7A , initially, a substrate is prepared. Next, a lens cavity  12   a  is formed in the substrate  12 , as shown in  FIG. 7B , using any one of the methods described in Embodiments 1 and 2 above. 
     As shown in  FIG. 7C , a cover mold  16  having a pattern region  16   a  that matches the diffracting surface  11   a  (see  FIG. 3A ) of a desired lens element (diffractive lens) is prepared and aligned on and assembled with the substrate  12 . To form this cover mold  16 , a master substrate is manufactured with the same pattern as a diffraction grating of the diffraction surface  11   a  of a desired diffraction lens or a desired diffraction lens array, and the cover mold  16  having the pattern region  16   a  that has an inverse pattern with respect to the diffraction grating of the desired diffraction lens and matches it is manufactured from the master substrate, for example, by nickel plating. 
     Next, as shown in  FIG. 7D , a lens forming material, for example, an alkoxide solution containing a precursor of (Si,Ti)O 2  is injected into the lens cavity  12   a  covered with the cover mold  16 . Any solution containing any precursor of high refractive index material, excluding (Si,Ti)O 2 , may be used as the lens forming material. After injecting the lens forming material, the bottom surface of the substrate  12  is irradiated with ultraviolet (UV) rays for nucleation of the lens forming material into a semi-solid state. Next, the substrate  12  is heated to a predetermined temperature to fully cure the lens forming material. 
     After the lens forming material is fully cured into the lens element  11 , the cover mold  16  is removed to provide a desired objective lens or objective lens array, as shown in  FIG. 7E . 
     According to the method described in this embodiment, neither repeatedly spin coating the substrate with a lens material solution and baking the same, as in the sol-gel method described in the above embodiment with reference to  FIGS. 4A through 4D , nor accompanying surface polishing are necessary. When a previously manufactured cover mold is available, a desired lens element can be manufactured within a short time by merely applying the above UV curing technique. Therefore, the method described in this embodiment is highly advantageous for mass production. In addition, the use of a cover mold as described above makes it to form a desired diffracting surface easier, compared to photolithography using binary optics, gray scale photolithography, and direct lithography using electron beams or laser beams. 
     A planar objective lens according to the present invention manufactured by the method as described above provides structural advantages as follows. 
     First, unlike general objective lenses manufactured by compression or injection molding, since the objective lens according to the present invention is formed planar with a lens element buried in a planar substrate, it is easy to handle, and optical pickups can be easily assembled with this objective lens. In addition, the thickness of the objective lens can be minimized for miniature optical pickups. When a plurality of objective lenses are simultaneously manufactured on a wafer level, integrated optical pickups can be easily assembled with high efficiency, by easily aligning with another wafer on which other optical elements are mounted and binding thereto. 
     Second, the lens element of an objective lens according to the present invention has a refracting surface at one surface and a diffracting surface at the opposite surface, providing a great light focusing effect by the two lens surfaces. Therefore, an objective lens having a large numerical aperture (NA) can be manufactured. In addition, through proper combination of the refracting and diffracting surfaces, chromatic aberration-free achromatic lenses can be fabricated. 
     A method for fabricating a planar objective lens according to the present invention is advantageous in the following aspects. 
     First, according to the present invention, in forming a lens cavity in a substrate, rather than molding techniques using an embossed mold, wet or dry etching techniques, which are commonly used in the manufacture of semiconductor devices and enable microprocessing, are applied. Therefore, miniature microlenses in an array can be manufactured on a wafer level. 
     Second, since a high refractive index material for lenses are applied by sol-gel methods or UV curing methods, instead of physical thin film deposition techniques, such as sputtering, the overall processing time can be reduced. In addition, UV curing methods need no subsequent surface polishing following deposition of high refractive index materials, so that the overall manufacturing time is markedly reduced with high productivity. 
     A planar objective lens according to the present invention can be used in a portable miniature drive for optical discs having a small diameter of 30 mm or less and using semiconductor laser light of a blue wavelength of 400–410 nm as a light source for recording and reproduction. This planar objective lens according to the present invention has a numerical aperture (NA) of 0.85 and can be used to record and reproduce audio and video data in a portable minidisk (MD) player, a digital camcoder, a digital camera, a personal digital assistant (PDA), a MP3 player, etc. The planar objective lens according to the present invention can be applied to any kinds of optical pickups of different sizes for various disc drives as well as for optical discs having a 1-GB data capacity and a 30-mm or less diameter that need a blue wavelength light source. 
     While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that the present invention is not limited to a particular configuration and arrangement described above and various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.