Abstract:
A solid immersion lens has a positioning surface and a surface including a convex surface forming an effective optical path. The solid immersion lens has a hemispherical or hyperhemispherical shape provided with a convex surface on the light entering side and an approximate plane surface on the light exiting side.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on Patent Application No. 10-261887 filed in Japan, the content of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a solid immersion lens and production method thereof, and specifically relates to a solid immersion lens used as a recording/reading head for a high resolution microscope probe or high density optical memory (recording/reading) and production method thereof. 
     2. Description of the Related Art 
     In the field of optical memory for optically recording/reading information, the following two methods have been proposed in recent years for increasing the recording density of disks used as a recording media. 
     (1) Shorten the wavelength of the light used for recording/reading. 
     (2) Increase the aperture number NA of the object lens converging the light on the disk. 
     Among these methods, method (1) requires the development of a semiconductor laser or the like for emitting light of a shorter wavelength to reduce the wavelength of the light itself. The development of such a semiconductor laser is not yet a simple matter, however. 
     Focusing on method (2), there have been various proposals for emitting the light used for recording/reading on a disk using a solid immersion lens. This method is based on the principle of the liquid immersion microscope (i.e., improved resolution microscope via liquid immersion method). That is, this method utilizes the principle of the microscope obtaining a resolution at less than 100 nm in visible light. Prior applications of related art include U.S. Pat. Nos. 5,004,307 and 5,764,613. 
     Solid immersion lenses heretofore experimentally produced include the hemispherical solid immersion lens  41  shown in FIG. 14, and the hyperhemispherical solid immersion lens  42  having a cap piece remaining under the hemisphere as shown in FIG.  15 . In FIGS. 14 and 15, reference number  43  refers to a normal objective (condenser) lens, and reference number  44  refers to a recording medium (disk). 
     In the hemispherical solid immersion lens  41 , the light beam LB enters perpendicular to the concave surface  41   a,  and is converged by the plane surface  41   b  of the center area. When the refractive index of the solid immersion lens  41  is designated n, the wavelength within the lens  41  becomes 1/n, and as a result the aperture number NA of the condenser lens  43  becomes a multiple of n. The spot size of the light beam LB is reduced to 1/n, and the resolution becomes a multiple of n. That is, when the aperture number NA of the condenser lens  43  is 0.5, the refractive index n of the solid immersion lens  41  is 1.8, and the wavelength λ of the light beam LB is 780 nm, the spot size S is determined by equation (1) below; i.e., the spot size S is 430 nm. 
     
       
           S=λ/ (2  sin θ )  (1) 
       
     
     In the hyperhemispherical solid immersion lens  42 , the effective optical path is greater than the radius. In this instance, the spot size S of the light beam LB is determined by equation (2) below; i.e., the spot size S is 240 nm. 
     
       
           S=λ/ (2 n sin θ )  (2) 
       
     
     In this way, a condensed spot having a wavelength less than the used light can be obtained by using the solid immersion lenses  41  and  42 . Using these methods, the spacing (air gap) b between the recording medium  44  and the solid immersion lens  41  and  42  must be sufficiently reduced so as to be maintained at approximately 100 nm or less. In order to control and maintain this air gap, it has been proposed to install the solid immersion lens on a floating slider applying the art of the magnetic hard disk. Prior art citations include U.S. Pat. Nos. 5,125,750, 5,774,281 and 5,786,947, and prior art citations using other than a floating slider include Japanese Laid-Open Application No. 8-212579. 
     The three issues below must be resolved to practicalize a high density memory using a solid immersion lens. 
     (1) Lens Holder Mechanism 
     In the conventional hemispherical and hyperhemispherical solid immersion lenses  41  and  42 , it is difficult to prevent inclination (0 degree) of the plane surface  41   b  relative to the reference surface  49  of the support fixture  47  because there is no fixed reference to the support fixture  47  on the convex surface  41   a,  as shown in FIG.  16 . When the plane surface  41   b  has a standard inclination, the fixed part of the lens disadvantageously protrudes to the object (medium) side of the lens, such that the required air gap cannot be ensured. 
     (2) Installation on the Slider 
     When installing a solid immersion lens on a floating slider for use as the recording head of a high density memory, the lens must be compact so as to have a diameter of 2 mm or less to reduce the head weight, and the lens support fixture must be provided with an air-bearing surface for floating. In this case, inclination between the support fixture and the lens plane surface must be prevented to maintain a sufficiently precise position of the solid immersion lens relative to the recording medium. 
     (3) Manufacturing Cost 
     Conventionally, hemispherical and hyperhemispherical solid immersion lenses are manufactured by grinding a glass member into a spherical shape to obtain a ball lens, then sectioning or grinding the ball lens. Such a production method, however, entails a complex process which raises the cost and is presently only in the experimental stage which does not result in a usable product. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an improved solid immersion lens and production method thereof. 
     Another object of the present invention is to provide a solid immersion lens capable of being mounted so as to hold a predetermined position without inclination relative to a support fixture, and further capable of being mounted on a floating slider so as to sufficiently reduce the air gap with an object (medium). 
     Still another object of the present invention is to provide a production method capable of mass production of a solid immersion lens via a simple process at low cost. 
     These objects are attained by a first solid immersion lens of the present invention provided with a hemispherical shape or hyperhemispherical shape having a convex surface on the light entering side and an approximately plane surface on the light exiting side, wherein the convex surface comprises a positioning surface and a surface forming an effective optical path part. In this first solid immersion lens, it is desirable that the positioning surface is a curved surface having the opposite sign of the curvature of the surface forming the effective optical path part. Furthermore, includes a radius of curvature of infinity, i.e., a circular conical shape extending the tangent line of the radius at the effective optical path part. 
     A second solid immersion lens of the present invention provided with a hemispherical shape or hyperhemispherical shape having a convex surface on the light entering side and an approximately plane surface on the light exiting side, wherein a collar part having a positioning surface is provided outside the effective optical path part of the convex surface. 
     In the first and second solid immersion lenses, the positioning surface formed outside the effective optical path part of the convex surface comprises a fixed reference for the lens support fixture, and allows the solid immersion lens to be supported with excellent positioning (i.e., without inclination). Furthermore, the a sufficiently small air gap is ensured without the fixed part of the lens protruding on the object (medium) side. 
     Even when the lens is compact, the surface area of the approximate plane surface on the exit side is enlarged by providing the positioning surface, such that when the solid immersion lens is installed on a floating slider, the air-bearing surface is enlarged when floating. In addition to effectively preventing inclination, there is improved positioning accuracy of the solid immersion lens relative to the object (medium). 
     In the first solid immersion lens, the convex surface may comprise a first convex surface forming an effective optical path for forming a converged spot in the center of the approximate plane surface, and a second convex surface forming an effective optical path forming a converged spot at a location slightly separated from the approximate plane surface. In this way, a bifocal solid immersion lens can be obtained. 
     The first and second solid immersion lenses are manufactured by pressure molding a softened glass member using both a mold having a concave surface approximately inverting the convex shape, and a mold having a surface forming an approximate plane surface on the light exiting side. The pressure molding method using such molds allows inexpensive mass production of a solid immersion lens by a simple process which does not require cutting or grinding. If molten glass is dripped onto a mold having a surface for forming an approximate plane surface or having a concave surface approximately inverting the convex surface shape, the weight of the dripped glass can be evenly balanced so as to obtain a solaced immersion lens without dispersion. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and features of the present invention will become clear from the following description, taken in conjunction with the preferred embodiments with reference to the accompanying drawings, in which: 
     FIG. 1 shows an elevation view of an optical system including a solid immersion lens of a first embodiment; 
     FIG. 2 illustrates the positioning surface and the effective surface of the solid immersion lens shown in FIG. 1; 
     FIG. 3 is a section view of the support fixture and the solid immersion lens of FIG. 1; 
     FIG. 4 illustrates the solid immersion lens of FIG. 1 used as a bifocal lens; 
     FIG. 5 is an elevation view showing an optical system including a solid immersion lens of a second embodiment; 
     FIG. 6 shows a solid immersion lens of a third embodiment; 
     FIG. 7 is an elevation view showing an optical system including a solid immersion lens of a fourth embodiment; 
     FIG. 8 shows a section view of a support fixture and the solid immersion lens of FIG. 7; 
     FIG. 9 is a section view showing the float-regulating mechanism of the support fixture of FIG. 8; 
     FIG. 10 is an elevation view showing an optical system including a solid immersion lens of a fifth embodiment; 
     FIG. 11 is a section view of a support fixture and the solid immersion lens of FIG. 10; 
     FIG. 12 is a section view of a support fixture and the solid immersion lens of a sixth embodiment; 
     FIG. 13 illustrates a first embodiment of the process of the production method; 
     FIG. 14 is an elevation view of an optical system including a conventional solid immersion lens (hemispherical); 
     FIG. 15 is an elevation view of an optical system including a conventional solid immersion lens (hyperhemispherical); and 
     FIG. 16 is a section view of a support fixture and a conventional solid immersion lens. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments of the solid immersion lens and production method thereof of the present invention are described hereinafter with reference to the accompanying drawings. 
     Solid Immersion Lens of the First Embodiment (refer to FIGS.  1 ,  2 ,  3 ) 
     In FIG. 1, reference number  1  refers to the solid immersion lens of a first embodiment, reference number  43  refers to a condensing lens having a aperture number NA of approximately 0.6, reference number  44  refers to a recording medium, and reference number LB 1  refers to converged light condensed by the condensing lens  43 , and having a opening angle from the condensed spot C 1  of approximately 74°. An air gap b between the solid immersion lens  1  and the recording medium  44  is set at approximately 100 nm. 
     The solid immersion lens  1  comprises a convex surface having a hemisphere  3   a  of radius r 1  of 0.5 mm as a standard, and a plane surface  7  on the exit side. The effective surface  4  which transmits the converged light LB 1  is the spherical surface having hemisphere  3   a  of 0.4 mm, and the effective diameter a is 0.6 mm. A positioning surface  5  having a curvature of opposite sign is provided on the exterior side of the effective surface  4 . The diameter of the plane surface  7  is 2 mm. 
     The positioning surface  5  is defined as the “surface having a curvature of opposite sign.” In other words, the direction of the curvature center of the positioning surface  5  is inverted relative to the direction of the curvature center of the effective surface  4 . As shown in FIG. 2, the curvature center C 1  of the spherical surface  3   a  forming the effective surface  4  exists in the center of the plane surface  7 , and is positioned on the object side as the boundary of the convex surface  2 . On the other hand, the curvature center C′ of the spherical surface  6  having a radius r 2  of 1 mm and forming the positioning surface  5  exists on the light source side as the boundary of the convex surface  2 . 
     When the converged light LB 1  irradiates the recording medium  44  using the optical system shown in FIG. 1 including the solid immersion lens  1 , a recording spot 80 nm in diameter is obtained on the recording medium  44 . 
     The installation of the solid immersion lens  1  on the floating slider is accomplished using the support fixture  51  shown in FIG.  3 . The support fixture  51  has a concentric circle concavity  52 , and an opening  53 , and the solid immersion lens  1  is set in the concavity  52  such that the effective surface  4  protrudes from the opening  53 . At this time, the solid immersion lens  1  is positioned by abutting the positioning surface  5  against the ring edge  54  of the support fixture  51 . In the solid immersion lens  1  of the aforesaid shape, the lens  1  abutment line (i.e., the ring line on the positioning surface  5 ) is clearly delineated relative to the ring edge  54  which acts as a lens  1  position-regulating element, and the lens  1  is mounted on the support fixture  54  with excellent precision. In particular, the lens plane surface  7  can be mounted on the reference surface  55  of the bottom surface of the support fixture  51  with a precision on the order of micron to submicron. The surface area of the plane surface  7  is increased by forming the positioning surface  5  (approximately 4× the surface area of a conventional lens of the same diameter), such that the lens  1  itself receives a large floating force. 
     Relatedly, in the conventional support fixture shown in FIG. 16, innumerable abutment ring lines of the lens  41  are present relative to the ring edge  48  to abut the convex surface (spherical surface)  41   a  against the ring edge  48  of the support fixture  47 . Accordingly, the angle θ increases due to disruption of the parallel relationship of the lens plane surface  41   b  and the reference surface  49  of the support fixture  47 , such that the obtainable mounting precision is only at most on the order of submillimeter. 
     The solid immersion lens functioning as a bifocal lens is described below. 
     The solid immersion lens  1  has a convex surface  2  comprising an effective surface  4  and a positioning surface  5 , as previously mentioned. As shown in FIG. 4, a single converged light beam LB 2  on the outer surface of the converged light LB 1  is directed. The converged light LB 1  is transmitted through the effective surface  4  and forms a condensed light spot C 1  at the center of the plane surface  7 , and the converged light LB 2  is transmitted through the interface  8  of surfaces  4  and  5  and forms a condensed light spot C 2  at a position slightly separated from the spot C 1 . 
     The recording medium  44  is provided with a tracking layer  46  below a recording layer  45 . Using the solid immersion lens  1 , information is recorded/read on the recording layer  45  via the condensed light spot C 1 , and the tracking layer  46  can be accessed via the condensed light spot C 2 . In this case, the installation of the solid immersion lens  1  on the floating slider may be accomplished using the support fixture  51  shown in FIG.  3 . 
     Solid Immersion Lens of the Second Embodiment (Refer to FIG.  5 ) 
     In FIG. 5, the solid immersion lens  1  of the second embodiment comprises a convex surface  2  having hyperhemisphere  3   b  as a reference surface with a radius of 0.5 mm, and a plane surface  7  on the exit side, wherein the diameter of the effective surface  4  is approximately 0.7 mm. In other aspects the construction is identical to that of the first embodiment; parts common to FIG. 1 are designated by like reference numbers and are not described further. The condenser lens  43  has an aperture number NA of approximately 0.4, and when the recording medium  44  is irradiated by convergent light LB 1 , the obtained recording spot has a diameter of 80 nm on the recording medium  44 . 
     Installation of the solid immersion lens  1  on the floating slider may be accomplished using the support fixture  51  shown in FIG. 3, such that the abutment line of the positioning surface  5  is clearly delineated relative to the ring edge  54 , and the plane surface  7  of the lens  1  is supported with high precision so as to maintain a small air gap b of approximately 100 nm. 
     Solid Immersion lens of the Third Embodiment (Refer to FIG.  6 ) 
     In FIG. 6, the solid immersion lens  1  of the third embodiment comprises a convex surface  2  having hyperhemisphere  3   b  as a reference surface with a radius of 0.5 mm similar to the first embodiment, and a circular cone positioning surface  5   a  (the curvature center is on the top surface) having a curvature of zero on the exterior side of the effective surface  4 . 
     In this hemispherical solid immersion lens  1 , a recording spot 80 nm in diameter is obtained on the recording medium  44  when recording using the condenser lens  43  shown in FIG.  1 . Installation of the solid immersion lens  1  on the floating slider may be accomplished using the support fixture  51  shown in FIG. 3, such that the abutment line of the positioning surface  5  is clearly delineated relative to the ring edge  54 , and the plane surface  7  of the lens  1  is supported with high precision so as to maintain a small air gap b. 
     Solid Immersion Lens of the Fourth Embodiment (Refer to FIGS.  7 ,  8 ,  9 ) 
     In FIG. 7, the solid immersion lens  11  of the fourth embodiment is constructed of a convex surface  12  comprising an effective surface  14  and a collar  15  provided on the exterior side of the effective surface  14 , and a plane surface  17  on the exit side. The effective surface  14  is a hemisphere 0.5 mm in radius as a reference, and an effective diameter a of approximately 0.6 mm. A collar  15  is 0.2 mm in thickness, and has a positioning surface  15   a  which is perpendicular to the optical path of the converged light LB 1 . The diameter of the plane surface  17  is 2 mm. The condenser lens  43  has an aperture number NA of approximately 0.6 similar to that of the first embodiment, such that the convergent light LB 1  converges at an opening angle of approximately 74° from the condensed light spot C 1 . An air gap b of approximately 100 nm is set between the lens plane surface  17  and the recording medium  44 . 
     When the convergent light LB 1  irradiates the recording medium  44  using the optical system of FIG. 7 including the solid immersion lens  11 , a recording spot 80 nm in diameter is obtained on the recording medium  44 . 
     Installation of the solid immersion lens  11  on the floating slider may be accomplished using the support fixture  61  shown in FIG.  8 . That is, the support fixture  61  has a concentric circle concavity  62  and an opening  63 , and the solid immersion lens  11  is set in concavity  62  such that the effective surface  14  protrudes from the opening  63 . At this time, the solid immersion lens  11  is positioned by abutting the positioning surface  15   a  of the collar  15  against the plane surface  62   a  of the concavity  62 . In this way, the lens plane surface  17  can be fixed relative to the reference surface  65  of the bottom surface of the support fixture  61  with a precision on the micron to submicron order by positioning the lens  11  by abutting the positioning surface  15   a  against the plane surface  62   a  of the support fixture  62 . The surface area of the lens plane surface  17  is increased by providing the collar  15  (i.e., 4× the surface area of a conventional lens of identical diameter), and the lens  11  itself receives a large floating force. 
     To maintain an air gap b of approximately 100 nm between the recording medium  44  and the plane surface  17  of the lens  11 , the surface  64  of the support fixture  61  may be pressed by a suspension  66 , as shown in FIG.  9 . 
     Solid Immersion Lens of the Fifth Embodiment (Refer to FIGS.  10 ,  11 ) 
     In FIG. 10, the solid immers 9 oon lens  11  of the fifth embodiment has a construction basically similar to that of the fourth embodiment. A differing aspect is the formation of a single positioning surface  15   b  connected to the collar  15  parallel to the optical path on the exterior side of the effective surface  14 . 
     In this solid immersion lens  11 , in addition to the positioning surface  15   a  abutting the plane surface  62   a  of the support fixture  61 , the single positioning surface  15   b  is positioned by abutting the interior surface  63   a  of the opening, so as to regulate inclination and horizontal dislocation of the lens  11 . 
     Solid Immersion Lens of the Sixth Embodiment (Refer to FIG.  12 ) 
     In FIG. 12, the solid immersion lens  11  of the sixth embodiment is provided with an effective surface  14  identical to that of the fifth embodiment, and has a positioning surface  18   a  parallel to the optical axis, and a positioning surface  18   b  perpendicular to the optical axis. A support fixture  67  is provided with an opening  68  into which the positioning surface  18   a  is inserted, such that the solid immersion lens  11  can be accurately positioned by using the positioning surfaces  18   a  and  18   b  and the interior wall surface and opening top edge of the support fixture  67  as references. That is, the precision of the lens plane surface  17  is improved relative to the reference surface  69  of the support fixture  67 . 
     This solid immersion lens  11  is particularly effective when weight reduction of the lens unit is an important design issue. 
     Production Method of the First Embodiment (Refer to FIG.  13 ) 
     FIG. 13 illustrates the method for producing the solid immersion lens  11  of the fourth embodiment shown in FIG.  7 . Although the direct press method utilizing glass droplets is used, various well-known conventional methods may be used as a glass molding method using a mold. 
     In FIG.  13 (A), a glass material  25  is directed from a melting furnace (not illustrated) to a nozzle  20 . A dense flint glass SF57 (refractive index approximately if 1.85; Minolta Co., Ltd. optical glass product) may be used as the glass material  25 . The glass material  25  is pre-melted in a melting furnace at approximately 900° C., and dripped onto the mold  31  as glass droplets  26  from the tip  21  of the nozzle  20 . 
     Since the mold  31  forms the plane surface  17  of the solid immersion lens  11 , it is made of hardened alloy, and the surface  32  is surface polished (maximum surface roughness: 0.03 μm). Since the mold  35  shown in FIG.  13 (C) forms the convex surface  12  of the solid immersion lens  11 , it is formed of hardened alloy, and is provided with a surface  36  corresponding to the effective surface  14  and a surface  37  corresponding to the positioning surface  15   a  of the collar  15 , and the surfaces  36  and  37  are polished (maximum surface roughness: 0.03 μm). The molds  31  and  35  are heated when molding to approximately 380° C. by a heating means not shown in the drawings. 
     The glass droplet  26  dripped onto the mold  31  (Refer to FIG.  13 (B)) is pressed by the mold  35  while in a softened state, so as to form a predetermined shape (Refer to FIG.  13 (C)). At this time, the press pressure is set at approximately 50 kgf/cm 2 , and the molds  31  and  35  are opened after pressing for 10 sec. 
     Production Method of the Second Embodiment 
     This production method uses a lanthanum glass LaSF9 (refractive index: approximately 1.85; Minolta Co., Ltd. optical glass product) as a glass material; the molding device and process are identical to those of FIG.  13 . Since the glass material used in the second embodiment differs from the glass material used in the first embodiment, the melting temperature of the glass material is approximately 250° C., and the mold temperature is approximately 600° C., and the press pressure is set at approximately 60 kgf/cm 2 . 
     Other Embodiments 
     The solid immersion lens of the present invention and the production method thereof are not limited to the previously described embodiments, and may be variously modified insofar as such modification does not depart from the scope of the invention. 
     In particular, the shape of the solid immersion lens may have an effective surface, a plane surface, and a positioning surface for positioning on the support fixture, and the shape of the detail parts is optional. 
     The production method has been described by way of example of the solid immersion lens of FIG. 7, but the solid immersion lenses of FIGS. 1,  5 ,  6 ,  10 , and  12 , as well as solid immersion lenses of other configurations may be produced using the same press process and glass material as raw material. When producing the solid immersion lenses shown in FIGS. 10 and 12, it is desirable that the surfaces  15   b  and  18   a  are provided with a slight inclination to improve separation characteristics from the mold. In the aforesaid embodiments, glass material was dripped onto the plane surface of a mold, but the glass material may also be dripped onto the concave surface of a mold. 
     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 modification 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.