Abstract:
A micro-actuator, its manufacturing method, an optical pickup head of an optical recording/reproducing apparatus having the micro-actuator, and its manufacturing method are disclosed. A low voltage and low power bi-directional driving is accomplished so that a size and a weight of a system can be considerably reduced and a response speed can be improved. A uniformity and a production efficiency are heightened by reducing a alignment tolerance. In addition, a configuration tolerance caused due to an uneven thickness of a protection layer of the record layer on the disk and an uneven smoothness of the disk is corrected, so that a focal point of an objective lens can be optimally made on a record layer of the disk.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a micro-actuator, its manufacturing method, an optical pickup head of an optical recording/reproducing apparatus having the micro-actuator, and its manufacturing method, and more particularly, to a micro-actuator which can be bi-directionally driven, an optical pickup head of an optical recording/reproducing apparatus which can focus a focal point of an objective lens optimally on a record layer by using the bi-directional micro-actuator, and its manufacturing method. 
   2. Description of the Background Art 
   With the MEMS (Micro Electro Mechanical System) technologies advancing, a technique of design, manufacturing and application of a microstructure, a ultra-micro actuator, various ultra-micro sensors, micro optical parts, micro fluid device or the like, which was hard to be realized in the past, has been developed and commercialized. 
   In case of a comb-type micro-actuator using various metals such as a polycrystalline silicon, a single crystalline silicon, aluminum or nickel, a static electricity force generated from close side wall surfaces of comb fingers (comb type electrodes) is used as a driving force of the microstructure. The static electricity force has such characteristics that only attraction works between two electrodes charged by a difference in applied voltages, so the driving force of the micro-actuator is uni-directional. 
   Therefore, in the case of the micro-actuator, only the uni-directional driving is used, and in case where a bi-directional driving is required, the micro-actuator is disposed at both sides, which, however, results in increase of the size and weight of a system adopting the micro-actuator and degradation of response speed. 
   Consequently, a technique related to the bi-directional driving of the micro-actuator is in urgent need of developing. 
   Recently, popularization of a personal computer and generalization of a data transmission network and a mobile communication device such as personal information terminal according to advancement of a multimedia environment accompany requirement of a considerable increase in a capacity of information to be processed and stored in those devices. 
   In order to cope with the situation, researches are ongoing to increase a record density of an optical recording medium such as a CD or a DVD, increase a resolution of an optical pickup device, and implement small optical components. 
   One of the optical pickup device satisfying the requirement of the ultra-compact high density optical recording/reproducing apparatus is a slide type optical pickup head which is able to record a data on or reproduce and search a data from an optical disk. 
   The optical recording/reproducing apparatus and its optical pickup head will now be described with reference to  FIGS. 1 and 2 . 
     FIG. 1  is a plan view of an optical recording/reproducing apparatus in accordance with a conventional art, and  FIG. 2  is a sectional view of an optical pickup head of the optical recording/reproducing apparatus in accordance with the conventional art. 
   As illustrated, the conventional optical recording/reproducing apparatus includes: a swing arm  10  installed rotatable at a certain angle; an actuator  11  for rotatably driving the swing arm  10  and a head  20  installed at an edge portion of the swing arm  10  and scanning a track of a disk  12  by being floated on a disk  12  by virtue of pneumatics. 
   The head  20  includes a converging lens  21  mounted isolated as long as a focal distance from an objective lens  30  and a slider  22  for mounting the converging lens  21 . 
   An air-bearing surface  22   a  is installed at a bottom surface of the slider  22  to levitate the slider  22  on the disk  12 . 
   At an upper side of the head  20 , there are installed a reflection mirror  41  and an optical transmitting and receiving unit  40  for transmitting and receiving optical beam in order to record/reproduce information to/from the disk  12 , a recording medium. 
   An alignment tolerance between the position of the converging lens and the air-bearing surface is a crucial factor in determining a uniformity, a reliability and a resolution of an optical information signal recorded on and reproduced from the surface of the optical disk. 
   The alignment tolerance includes a evenness tolerance of the slider surface, a focal distance tolerance and a tilt tolerance in forward/backward and left/right directions of SIL (Solid Immersion Lens). 
   In the conventional optical recording/reproducing apparatus, because the converging lens, the slider including the air-bearing surface and the objective lens are separately manufactured and assembled. Thus, an alignment tolerance can not be avoided to degrade performance of the optical recording/reproducing apparatus. 
   Also, manufacturing of each component depends on grinding and cutting, so that mass-production can be hardly expected and uniformity between components is degraded, and the production efficiency is so low that a cost of production increases. 
   Moreover, difference of thickness of a protection layer formed on the record layer of the disk, a focal distortion due to an incomplete disk smoothness, and a change in a depth of focus cause a signal degradation. 
   SUMMARY OF THE INVENTION 
   Therefore, an object of the present invention is to provide a micro-actuator and its manufacturing method capable of considerably reducing a size and a weight of a system and improving a response speed by accomplishing a low voltage and low power bi-directional driving. 
   Another object of the present invention is to provide an optical pickup head of an optical recording/reproducing apparatus having a micro-actuator capable of heightening a uniformity and a production efficiency by reducing a alignment tolerance, and capable of optimally focusing a focal point of an objective lens on a record layer by correcting a tolerance caused due to an uneven thickness of a protection layer of the record layer on the disk and an uneven smoothness of the disk, and its manufacturing method. 
   To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, there is provided a micro-actuator including: a lower substrate having a plurality of lower fixed electrodes formed at regular intervals at one side; an upper substrate installed at an upper side of the lower substrate and having a plurality of upper fixed electrodes formed at regular intervals at one side to correspond to the configuration of the lower substrate; an insulation layer interposed between the lower substrate and the upper substrate; a moving substrate having moving electrodes formed at an outer circumferential surface so as to be alternately arranged between the lower fixed electrodes and upper fixed electrodes, and being installed to be driven in a direction of an optical axis; an elastic member installed at the moving substrate to elastically return the moving substrate to an initial position; and a power supply unit for supplying power to the lower substrate, the upper substrate and the moving substrate to drive the moving substrate. 
   The elastic member is one or more spring elements arranged in a beam type or plate type to which a bi-metal is added and is a beam type or plate type to which a conductive thin film layer is added. 
   To achieve the above objects, there is also provided a method for manufacturing a micro-actuator including: a first step of preparing a basic material formed with a lower material, an upper material and an insulation layer interposed between the lower material and the upper material; a second step of patterning an etching mask at a surface of the lower material, removing portions of the lower material exposed through the etching mask in a vertical direction to expose the insulation layer to form lower fixed electrodes; a third step of removing the etching mask patterned at the lower material, patterning an etching mask at a surface of the upper material and removing portions of the upper material exposed between the etching mask of the upper material in a vertical direction to expose the insulation layer to form an upper fixed electrodes; a fourth step of etching a portion of the insulation layer to separate the moving electrodes from the upper fixed electrodes; a fifth step of etching a residual portion of the upper material and align the upper material and the lower material by using the etching mask patterned on the upper material; and a sixth step of disposing the moving electrode between the upper fixed electrodes and the lower fixed electrodes. 
   The upper material and the lower material are one of silicon, a conductor or a semiconductor. 
   In the fourth step, in order to align the shape of the upper material and the shape of the lower material, a double side alignment method is used. 
   In the sixth step, in order to dispose the moving electrode between the upper fixed electrodes and the lower fixed electrodes, a compressive residual stress can be applied to the moving electrodes or a self-weight of the moving electrodes can be used. 
   To achieve the above objects, there is also provided an optical pickup head of an optical recording/reproducing apparatus having a micro-actuator, including: a slider having a converging lens integrally formed at a central portion, a magnetic field-generating coil formed around the converging lens and an air-bearing surface formed at a lower surface; and an objective lens micro-actuator for micro-actuating an objective lens for transmitting optical beam of a transmitting/receiving unit to the converging lens in an optical axial direction. 
   The objective lens micro-actuator includes: a lower substrate positioned at an upper portion of the slide, having a mounting hole at a central portion and a plurality of lower fixed electrodes formed at regular intervals at an inner circumferential surface of the mounting hole; an upper substrate having a mounting hole at a central portion corresponding to a shape of the lower substrate, having a plurality of upper fixed electrodes formed at regular intervals at an inner circumferential surface of the mounting hole, and being installed at an upper side of the lower substrate; an insulation layer interposed between the lower substrate and the upper substrate; a moving substrate inserted in the mounting holes of the lower substrate and the upper substrate to be actuated in an optical axial direction, and having moving electrodes formed at an outer circumferential surface so as to be alternately arranged between the lower and upper fixed electrodes; a plurality of electrode pads for supplying power to the lower substrate, the upper substrate and the moving substrate in order to drive the moving substrate. 
   In order to elastically return ore the moving substrate to its initial position, an elastic member is installed at the moving substrate. 
   An anti-refraction coating film is formed on the converging lens, and a protection layer or a lubrication layer as like DLC (Diamond-Like Carbon) is formed at the surface of the air-bearing surface. 
   To achieve the above objects, there is also provided a method for manufacturing an optical pickup head of an optical recording/reproducing apparatus having a micro-actuator, including: a first step of manufacturing a slider having a converging lens integrally formed at a central portion, a magnetic field-generating coil formed around the converging lens and an air-bearing surface formed at a lower surface, and an objective lens actuator for micro-actuating an objective lens for transmitting optical beam of a transmitting/receiving unit to the converging lens in an optical axial direction as components by using a micro-machining and a semiconductor device manufacturing process; a second step of aligning and bonding the slider and the objective lens actuator by using an alignment mark; and a third step of aligning the objective lens in the same optical axial direction as the converging lens. 
   To achieve the above objects, there is also provided a method for manufacturing an optical pickup head of an optical recording/reproducing apparatus having a micro-actuator, including: a first step of manufacturing a slider having a converging lens integrally formed at a central portion, a magnetic field-generating coil formed around the converging lens and an air-bearing surface formed at a lower surface, and an objective lens actuator for micro-actuating an objective lens for transmitting optical beam of a transmitting/receiving unit to the converging lens in an optical axial direction in a wafer form by using a micro-machining and a semiconductor device manufacturing process; a second step of aligning and bonding the slider and the objective lens actuator by using an alignment mark; a third step of cutting the bonded wafer to individual optical pickup head chips; and a fourth step of installing the objective lens and the converging lens to be aligned in an optical axial direction. 
   The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. 
     In the drawings: 
       FIG. 1  is a plan view of an optical recording/reproducing apparatus in accordance with a conventional art; 
       FIG. 2  is a sectional view showing an optical pickup head of the optical recording/reproducing apparatus in accordance with the conventional art; 
       FIGS. 3 to 8  show a micro-actuator in accordance with the present invention, of which 
       FIG. 3  is a perspective view of a portion of the micro-actuator; 
       FIG. 4  is a plan view of  FIG. 3   
       FIG. 5  is a sectional view taken along line A-A of  FIG. 4 ; 
       FIG. 6  is a sectional view taken along line B-B of  FIG. 4 ; 
       FIG. 7  is a view for explaining a principle of the micro-actuator; and 
       FIG. 8  is an enlarged view of a major part of  FIG. 7 ; 
       FIGS. 9A to 9G  are a sequential process of manufacturing the micro-actuator; 
       FIGS. 10 to 15  show an optical pickup head of an optical recording/reproducing apparatus in accordance with the present invention, of which 
       FIG. 10  is a separated perspective view showing the optical pickup head; 
       FIG. 11  is a plan view of  FIG. 10 ; 
       FIG. 12  is a sectional view taken along line C-C of  FIG. 11 ; 
       FIG. 13  is a perspective view of the micro-actuator of  FIG. 10 ; 
       FIG. 14  is a plan view of  FIG. 13 ; 
       FIG. 15  is a sectional view taken along line D-D of  FIG. 14 ; and 
       FIGS. 16 to 18  are vertical sectional views for explaining a correction principle of a depth of focus, of which 
       FIG. 16  shows a depth of focus when a protection layer of a disk has a suitable thickness; 
       FIG. 17  shows a depth of focus when the protection layer of the disk is thin; and 
       FIG. 18  shows a depth of focus when the protection layer of the disk is thick. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
   A micro-actuator in accordance with the present invention will now be described with reference to the accompanying drawings. 
     FIGS. 3 to 8  show a micro-actuator in accordance with the present invention, of which  FIG. 3  is a perspective view of a portion of the micro-actuator,  FIG. 4  is a plan view of  FIG. 3 ,  FIG. 5  is a sectional view taken along line A-A of  FIG. 4 ,  FIG. 6  is a sectional view taken along line B-B of  FIG. 4 ,  FIG. 7  is a view for explaining a principle of the micro-actuator, and  FIG. 8  is an enlarged view of a major part of  FIG. 7 . 
   As shown in the drawings, a micro-actuator of the present invention includes a lower substrate  110  having a plurality of lower fixed electrodes  112  formed at regular intervals at one side; an upper substrate  120  installed at an upper side of the lower substrate  110  and having a plurality of upper fixed electrodes  122  formed at regular intervals at one side to correspond to the configuration of the lower substrate  110 ; an insulation layer  130  interposed between the lower substrate  110  and the upper substrate  120 ; a moving substrate  140  having moving electrodes  142  formed at an outer circumferential surface so as to be alternately arranged between the lower fixed electrodes  112  and upper fixed electrodes  122 , and being installed to be driven in a direction of an optical axis; an elastic member  150  installed at the moving substrate  140  to elastically return the moving substrate  140  to an initial position; and a power supply unit Vd, Vu and Vm for supplying power to the lower substrate  110 , the upper substrate  120  and the moving substrate  140  to drive the moving substrate  140 . 
   That is, a plurality of lower fixed electrodes  112  are formed at regular intervals at one side of a lower substrate  110 . An upper substrate  120  is installed at an upper surface of the lower substrate  110  corresponding to the shape of the lower substrate  110  includes a plurality of upper fixed electrodes  122  formed at regular intervals at one side. 
   An insulation layer  130  is interposed between the lower substrate  110  and the upper substrate  120 . The insulation layer  130  can be a thermal oxide film, a low temperature oxide (LTO) film, a low temperature nitride film or a polyimide, or any other material can be adopted as the insulation layer so long as it has excellent electric insulation characteristics and can be easily deposited, bonded or coated at a surface of the upper and lower substrates. 
   A moving substrate  140  includes a moving electrodes  142  formed at an outer circumferential surface so as to be alternately arranged between the lower fixed electrodes  112  and upper fixed electrodes  122 , and is installed to be driven in a direction of an optical axis (in the focal direction). 
   The lower fixed electrodes  112 , the upper fixed electrodes  122  and the moving electrodes  142  are made of an electrolytic-plated metal. 
   In order to elastically restore the moving substrate  140  to an initial position and drive it in an optical axial direction, a beam type or a plate type spring  150  as an elastic member is suspended at one side of the moving substrate  140 . As the elastic member, besides the spring, a bi-metal or a conductive thin film layer can be also used. 
   The lower substrate  110  and the upper substrate  120  are fixed at a first fixing unit  181 , and a beam type or plate type spring  150  is fixed at a second fixing unit structure  182 . The moving electrodes  142  are isolated as much as a certain gap g 1  by a free space. 
   The free space of the edge can be vacuum, air or an insulation fluid. 
   The thickness “t” of the insulation layer  130  should be set suitable so that a breakdown may not occur even if a difference of voltage between the upper fixed electrodes  122  and the lower fixed electrodes  112  are maximized. A damper  170  may be installed at the other side of the moving substrate  140 . 
   In order to driving the moving substrate  140 , Vd, Vu and Vm are prepared as power supply units for supplying power to the lower substrate  110 , the upper substrate  120  and the moving substrate  140 . 
   A driving force working between the moving electrodes and the upper fixed electrodes, lower fixed electrodes in the micro-actuator is expressed by below equation (1): 
   
     
       
         
           
             
               
                 F 
                 = 
                 
                   
                     
                       ɛ 
                       · 
                       l 
                     
                     
                       2 
                       ⁢ 
                       g 
                     
                   
                   · 
                   
                     ( 
                     
                       
                         V 
                         u 
                       
                       - 
                       
                         V 
                         d 
                       
                     
                     ) 
                   
                   · 
                   
                     ( 
                     
                       
                         V 
                         u 
                       
                       + 
                       
                         V 
                         d 
                       
                       - 
                       
                         2 
                         ⁢ 
                         
                           V 
                           m 
                         
                       
                     
                     ) 
                   
                 
               
             
             
               
                 ( 
                 1 
                 ) 
               
             
           
         
       
     
   
   wherein “F” is a force working for the moving electrodes, “G” is a gap between the moving electrodes and the upper fixed electrodes, and a gap between the moving electrodes and the lower fixed electrode, ‘I’ is an overlap length of sections of the moving electrodes and the upper fixed electrodes, and an overlap length of the moving electrodes and the lower fixed electrodes in a vertical direction, Vd, Vu and Vm are voltages of the lower fixed electrodes, the upper fixed electrodes and the moving electrodes, and ε is a permittivity constant. 
   As noted in equation (1), if a difference between the voltages applied to the moving electrodes and the upper fixed electrodes are greater than a difference between voltages applied to the moving electrodes and the lower fixed electrodes, the moving electrodes are moved upwardly. If, however, the difference between the voltages applied to the moving electrodes and the upper fixed electrodes are greater than the difference between voltages applied to the moving electrodes and the lower fixed electrodes, the moving electrodes are moved downwardly. 
   In other words, at a point where a driving force of the moving electrodes and elastic force by an elastic coefficient “k” of the spring supporting the moving substrate are balanced, a displacement of the moving electrodes are determined. 
   When the voltages applied to the moving electrodes, the upper fixed electrodes and the lower fixed electrodes are removed, the moving electrodes are returned to its initial position by virtue of the restoration force of the spring. 
   A capacitance when the micro-actuator is applied to an electrode of a capacitance type sensor can be expressed by below equation (2): 
   
     
       
         
           
             
               
                 
                   C 
                   12 
                 
                 = 
                 
                   
                     
                       
                         ɛ 
                         · 
                         l 
                         · 
                         
                           d 
                           u 
                         
                       
                       g 
                     
                     · 
                     
                       C 
                       13 
                     
                   
                   = 
                   
                     
                       ɛ 
                       · 
                       l 
                       · 
                       
                         d 
                         d 
                       
                     
                     g 
                   
                 
               
             
             
               
                 ( 
                 2 
                 ) 
               
             
           
         
       
     
   
   wherein C 12  is a capacitance by the moving electrodes and the lower fixed electrodes, C 13  is a capacitance by the moving electrodes and the upper fixed electrodes, d u  and d d  are respectively a distance in an optical axial direction from an upper edge of the moving substrate to a lower edge of the upper fixed substrate, and a distance in an optical axial direction from the lower edge of the moving substrate to an upper edge of the lower fixed substrate. 
   The differential component capacitance C of the two capacitors is expressed by below equation (3): 
   
     
       
         
           
             
               
                 C 
                 = 
                 
                   
                     
                       ɛ 
                       · 
                       l 
                     
                     g 
                   
                   · 
                   
                     ( 
                     
                       
                         d 
                         u 
                       
                       - 
                       
                         d 
                         d 
                       
                     
                     ) 
                   
                 
               
             
             
               
                 ( 
                 3 
                 ) 
               
             
           
         
       
     
   
   When the moving electrodes is moved along a driving direction as much as a displacement Δd by a physical amount or a chemical amount to be detected, an amount of change in the differential component capacitance ΔC is expressed by below equation (4): 
   
     
       
         
           
             
               
                 
                   Δ 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   C 
                 
                 = 
                 
                   
                     2 
                     · 
                     
                       
                         ɛ 
                         · 
                         l 
                       
                       g 
                     
                     · 
                     Δ 
                   
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   d 
                 
               
             
             
               
                 ( 
                 4 
                 ) 
               
             
           
         
       
     
   
   That is, the differential component capacitance corresponds to double the amount of change in the capacitance of the conventional uni-directional capacity type sensor, and the increase in the amount of change in the capacitance means enhancement of a sensitivity and resolution of the capacitance type sensor. 
   A method for manufacturing the micro-actuator in accordance with the present invention will now be described. 
     FIGS. 9A to 9G  are a sequential process of manufacturing the micro-actuator. 
   Left sides (a) of  FIGS. 9A through 9G  correspond to  FIG. 5 , while the right sides (b) correspond to  FIG. 6 . 
   As illustrated, a method for manufacturing the micro-actuator including: a first step of preparing a basic material formed with a lower silicon (LS), a lower material, an upper silicon (US), an upper material, and an insulation layer  130  interposed between the lower silicon (LS) and the upper silicon (US); a second step patterning an etching mask (LM) at a surface of the lower silicon (LS), removing portions of the lower silicon (LS) exposed through the etching mask (LM) in a vertical direction to expose the insulation layer  130  to form lower fixed electrodes  112 , a third step of removing the etching mask patterned at the lower material, patterning an etching mask (UM) at a surface of the upper silicon (US) and removing portions of the upper silicon (US) exposed between the etching masks (UM) of the upper silicon (US) in a vertical direction to expose the insulation layer  130  to form a plurality of upper fixed electrode  122 ; a fourth step of etching a portion of the insulation layer  130  to separate the each moving electrode  142  from the each upper fixed electrode  122 ; a fifth step of etching a residual portion of the upper silicon (LS) and align the upper silicon (US) and the lower silicon (LS) by using the etching mask (UM) patterned on the upper silicon; and a sixth step of disposing the each moving electrode  142  between the each upper fixed electrode  122  and the each lower fixed electrode. 
   The method for manufacturing the micro-actuator will now be described in detail. 
   To begin with, the basic material is formed as a so-called wafer type SOI (Silicon on Insulator) substrate consisting of the upper silicon (US), the insulation layer  130  and the lower silicon(LS). 
   Through a follow-up process, the upper silicon (US) is formed as the fixed electrode  122  and the moving electrode  142 , while the lower silicon (LS) is formed as the fixed electrode  112 . 
   The lower material and the upper material of the basic material can be a semiconductor material or a conductor material as well as the silicon material. 
   The insulation layer  130  can be made of various materials such as a silicon oxide film, a silicon nitride film and a polymer thin film, and a suitable material is preferably selected according to a manufacturing method of the micro-actuator. 
   When the basic material of the wafer type is prepared, as shown in  FIG. 9A , a washing process is performed to remove a contaminant of the surface, and then a etching mask is patterned on the surface of the lower silicon (LS) by using a series of semiconductor manufacturing process such as a photolithography, a thin film deposition process and an etching process. 
   Next, as shown in  FIG. 9B , a portion of the lower silicon (LS) exposed through the etching mask is selectively removed in a vertical direction to expose the insulation layer  130  by using a silicon deep reactive ion etching technique, a sort of an anisotropic etching technique, to form the lower fixed electrode  112  of the lower substrate  110 . 
   After the lower fixed electrode  112  is completed by etching the lower silicon (LS), the etching mask (LM) is selectively removed. 
   And then, as shown in  FIG. 9C , an etching mask (UM) is patterned on the upper surface of the upper silicon (US) by using the above-mentioned semiconductor manufacturing process. And, a portion of the upper silicon (US) exposed through the etching mask (UM) is selectively removed in a vertical direction by using the silicon deep reactive ion etching technique until the insulation layer  130  is exposed. 
   As shown in  FIG. 9D  the insulation layer  130  of the silicon oxide film exposed by selectively removing the upper silicon is selectively removed by using a wet chemical etching or a dry etching to separate the moving electrode  142  of the moving substrate  140  and the upper fixed electrode  122  of the upper substrate  120 . 
   Thereafter, a residual silicon from the upper silicon to the lower silicon is removed downwardly by using the silicon deep reactive ion etching technique by using the etching mask (UM). Then, as shown in  FIG. 9E , the upper fixed electrode  122  of the upper substrate  120  and the lower fixed electrode  122  of the lower substrate  110  are self-aligned. 
   Subsequently, as shown in  FIG. 9F , the insulation layer  130  remaining at the lower side of the moving electrode  142  of the moving substrate  140  and the etching mask (UM) formed at the upper side of the upper fixed electrode  122  are removed. 
   In this respect, the insulation layer  130  remaining at the lower side of the moving electrode  142  may not be removed to be used as a micro-electrode structure. 
   Referring to  FIG. 9C , in order to align the upper silicon etching mask pattern to the shape of the lower silicon (LM), a double side alignment technique among micro-machining techniques is used. 
   Thereafter, as shown in  FIG. 9G , the moving electrode  142  of the moving substrate  140  is disposed or overlaps to be positioned at the center between the fixed electrode  122  of the upper substrate  120  and the lower fixed electrode  112  of the lower substrate  110 , thereby completing manufacturing of the micro-actuator. 
   Positioning or overlapping the moving electrodes at the center of the fixed electrode and the lower fixed electrode can be made by various ways: One example can be transforming of the elastic member. That is, the elastic member is connected to the moving electrode and transformed to dispose or overlap the moving electrode in the optical axial direction. 
   Another example can be using a bi-metal. That is, bi-metals having different thermal expansion rates are stacked, to which power is applied for a heat distortion of the bi-metals, and then the moving electrode is disposed. 
   Still another example is disposing the moving electrode by using a dead load of the moving electrode. 
   Yet another example is to use an electrostriction effect. In this method, a capacitor of piezoelectric material such as PZT or ZnO inserted between the conductive thin film layers of metal is stacked, and a certain voltage is applied between the two conductive thin film layers for their transformation, and then, the moving electrode is disposed. 
   In this manner the micro-actuator and its manufacturing method accomplishes a low voltage and low power bi-directional driving. By adopting this technique, a capacitance sensor can make a differential detection and thus heighten a sensitivity and resolution. And in case of a system adopting this technique, its size and weight can be considerably reduced and a response speed can be enhanced. 
   The optical pickup head of the optical recording/reproducing apparatus of the present invention will now be described. 
     FIG. 10  is a separated perspective view showing the optical pickup head,  FIG. 11  is a plan view of  FIG. 10 ,  FIG. 12  is a sectional view taken along line C-C of  FIG. 11 ,  FIG. 13  is a perspective view of the micro-actuator of  FIG. 10 ,  FIG. 14  is a plan view of  FIG. 13 , and  FIG. 15  is a sectional view taken along line D-D of  FIG. 14 . 
   As illustrated in the drawings, the optical pickup head of the optical recording/reproducing apparatus of the present invention includes: a slider  200  made having a substrate structure made of a transparent material with a converging lens  201  integrally formed at the central portion thereof; and a micro-actuator  300  for micro-actuating an objective lens  400  for transmitting optical beam of a optical beam transmitting and receiving unit  40  (refer to  FIG. 2 ) to the converging lens  201  in an optical axial direction. 
   An air-bearing surface  203  is formed at a lower surface of the slider  200 , on which a protection layer or a lubrication layer is formed. At both surface of the slider  200 , an alignment mark  204  is formed to align the micro-actuator  300  and the slider  200  in the optical axial direction. 
   The converging lens  201 , focusing optical beam on the record layer of the disk, is integrally formed at an upper surface of the slider  200 . Preferably, a non-reflection coating film is formed on the converging lens  201 . 
   A magnetic field generation coil  202  is formed around the converging lens  201 , for performing a direct-rewritable function of an optical magnetic recording. If the optical pickup head is applied to an optical storing unit which uses a phase change, not the optical magnetic recording mode, the magnetic field generation coil can be omitted. 
   The magnetic field generation coil  202  is formed as a spiral planar coil type to be concentric with an optical axis at a bottom surface of an etched recess  200   a  formed at the upper surface of the slider  200 . 
   The magnetic field generation coil  202  can be formed at the surface where the converging lens  201  is formed, and preferably, it is formed at a rear surface of the surface where the converging lens  201  is formed. 
   When a current is applied from an external power source to the magnetic field generation coil  202 , a magnetic field proportional to the value of the current is generated in the optical axial direction, changing a magnetic polarization of the optical magnetic material, the record layer of the disk. Accordingly, information can be recorded or rewritten. 
   The objective lens micro-actuator  300  includes: a lower substrate  310  positioned at an upper side of the slide  200  and having a mounting hole  311  at its central portion and a plurality of lower fixed electrode  312  formed at regular is intervals at an inner circumferential surface of the mounting hole  311 ; an upper substrate  320  having a mounting hole  321  at the central portion and a plurality of fixed electrodes  322  formed at regular intervals at an inner circumferential surface of the mounting hole  321  corresponding to the shape of the lower substrate  310 , and installed at an upper side of the lower substrate  110 ; an insulation layer  330  interposed between the lower substrate  110  and the upper substrate  120 ; a moving substrate  340  inserted between the mounting hole  311  of the lower substrate  310  and the mounting hole  321  of the upper substrate  320  so as to be installed to be driven in the optical axial direction, and having a moving electrode  342  formed at an outer circumferential surface and alternately arranged between the lower fixed electrode  312  and the upper fixed electrode  322 ; and a plurality of electrode pads Vd, Vu and Vm for supplying power to the lower substrate  310 , the upper substrate  320  and the moving substrate  340  in order to drive the moving substrate  340 . 
   An elastic member  341  is installed at an edge of the moving substrate  340  to elastically support the moving substrate  340 . An engaging jaw  341   a  is formed at an upper surface of the moving substrate  340  in order to mounted an objective lens  400  thereon. A void g 2  is formed between the moving electrode  342  and the upper fixed electrodes  312  and lower fixed electrodes  322 . 
   An operation for controlling a depth of focus of the optical pickup head in accordance with the present invention will now be described with reference to  FIGS. 16 to 18 . 
     FIGS. 16 to 18  are vertical sectional views for explaining a correction principle of a depth of focus, of which  FIG. 16  shows a depth of focus when a protection layer of a disk has a suitable thickness,  FIG. 17  shows a depth of focus when the protection layer of the disk is thin, and  FIG. 18  shows a depth of focus when the protection layer of the disk is thick. 
     FIG. 16  shows a case that the thickness of the protection layer  12   a  on the record layer  12   b  of the disk  12  is identical to an ideal value in an optical design without a thickness tolerance and a optical beam made incident from outside is an ideal parallel optical beam. 
   With reference to  FIG. 16 , “G” is a distance between an upper surface of the protection layer  12   a  of the disk  12  and the bottom of the slider  200 , t 0  is an ideal thickness of the protection layer  12   a  within a tolerance range in optical design, and l 0  is a distance between the objective lens  400  and the converging lens  201 . 
   In this state, even if an optical signal is recorded or reproduced at a null position without driving the object lens  400 , the optical beam is accurately focused on the record layer  12   b  of the disk  12  being rotated, so that the optical signal is not degraded and a stable operation is performed. 
     FIG. 17  shows a case that the thickness t 1  of the protection layer  12   a  of the disk  12  is thinner than the ideal value in design an optical system. 
   In  FIG. 17 , “G” is a distance between the upper surface of the protection layer  12   a  of the disk  12  and the bottom of the slider  200 , t 1  is the thickness of the protection layer formed thin beyond the ideal value in the optical design, and l 1  is a distance between the objective lens  400  and the converging lens  201 . 
   In this abnormal state, the objective lens  400  is moved upwardly by the micro-actuator  300  to increase the gap l 1 , thereby correcting the thickness of the protection layer  12   a  of the disk  12  which is thin as the depth of focus be far from the bottom surface of the slider  200 . 
   In this respect, if the protection layer  12   a  of the disk  12  is thin, the objective lens micro-actuator  300  corrects the focal distance displacement as much as the reduced thickness of the protection layer, provided that the rotation number per hour or rotation angular velocity of the disk per hour is constant. 
   Finally,  FIG. 18  shows a case that the thickness t 2  of the protection layer  12   a  on the record layer  12   b  of the disk  12  is greater than the ideal design value. 
   In  FIG. 18 , “G” is a distance between the upper surface of the protection layer  12   a  of the disk  12  and the bottom of the slider  200 , t 2  is the thickness of the protection layer formed greater than the ideal value in the optical design, and l 2  is a distance between the objective lens  400  and the converging lens  201   
   In this abnormal state, the objective lens  400  is moved downwardly by the micro-actuator  300  to reduce the gap l 2 , thereby correcting the thickness of the protection layer  12   a  of the disk which is thin as the depth of focus be near to the bottom of the slider  200  is distanced. 
   In order to determine an optimum position of the objective lens  400  driven by the micro-actuator, to detect an optical signal of a level determined by a detector for an optical focusing servo of the optical beam transmitting and receiving unit  40  (refer to  FIG. 2 ) after optical beam is reflected form the record layer of the disk, an optical signal detector and an input terminal of an actuator are controlled by a sub-feedback circuit. 
   Likewise, for an assembly error of an independent optical system and an tolerance of an incident culmination parallel optical beam resulted from a shape tolerance and a performance error of each optical factor, a correction of the depth of focus is performed by the micro-actuator so that the optical signal can be maintained on the record layer in an optimum state by the sub-feedback servo circuit. 
   One example of a method for manufacturing an optical pickup head of an optical recording/reproducing apparatus includes: a first step of manufacturing a slider having a converging lens integrally formed at a central portion, a magnetic field-generating coil formed around the converging lens and an air-bearing surface formed at a lower surface, and an objective lens actuator for micro-actuating an objective lens for transmitting optical beam of a transmitting/receiving unit to the converging lens in an optical axial direction as components by using a micro-machining and a semiconductor device manufacturing process; a second step of aligning and bonding the slider and the objective lens actuator by using an alignment mark; and a third step of aligning the objective lens in the same optical axial direction as the converging lens. 
   Another example of the method for manufacturing an optical pickup head of an optical recording/reproducing apparatus includes: a first step of manufacturing a slider having a converging lens integrally formed at a central portion, a magnetic field-generating coil formed around the converging lens and an air-bearing surface formed at a lower surface, and an objective lens actuator for micro-actuating an objective lens for transmitting optical beam of a transmitting/receiving unit to the converging lens in an optical axial direction in a wafer form by using a micro-machining and a semiconductor device manufacturing process; a second step of aligning and bonding the slider and the objective lens actuator by using an alignment mark; a third step of cutting the bonded wafer to individual optical pickup head chips; and a fourth step of installing the objective lens and the converging lens to be aligned in an optical axial direction. 
   As for the alignment and bonding technique in the second step, if the substrate constituting the converging lens is a glass substrate containing an impurity such as sodium or the like, that is, for example, Pyrex #7740, a silicon substrate constituting the micro-actuator and an anodic bonding technique can be used. 
   As so far described, the optical pickup head of the optical recording/reproducing apparatus having the micro-actuator and its MANUFACTURING method of the present invention have the following advantage. 
   That is, a deflection generated between the record layer and the optical pickup head due to the error generated between a processing tolerance in manufacturing of an optical disk and in assembling and mounting, an error between the uneven thickness of the protection layer of the record layer of the disk and the smoothness of the disk surface, and an error according to an eccentricity and vibration of a rotational shaft of a spindle motor can be corrected, so that an optimum focus is made on the record layer of the disk. 
   As the present invention may be embodied in several forms without departing from the spirit or essential characteristics thereof, it should also be understood that the above-described embodiments are not limited by any of the details of the foregoing description, unless otherwise specified, but rather should be construed broadly within its spirit and scope as defined in the appended claims, and therefore all changes and modifications that fall within the metes and bounds of the claims, or equivalence of such metes and bounds are therefore intended to be embraced by the appended claims.