Patent Publication Number: US-2005135012-A1

Title: Head slider, magnetic storage apparatus and head slider producing method

Description:
BACKGROUND OF THE INVENTION  
      This application claims the benefit of a Japanese Patent Application No. 2003-420080 filed Dec. 17, 2003, in the Japanese Patent Office, the disclosure of which is hereby incorporated by reference.  
      1. Field of the Invention  
      The present invention generally relates to head sliders, magnetic storage apparatuses and head slider producing methods, and more particularly to a flying type head slider for use in a hard disk drive or optical storage apparatus, a magnetic storage apparatus which uses such a head slider, and a head slider producing method for producing such a head slider.  
      2. Description of the Related Art  
      Magnetic storage apparatuses, such as hard disk drives, are popularly used as external magnetic storages for computers, household video storage units, navigation equipments and the like. Recently, there are increased opportunities to record extremely large amounts of information, such as dynamic images, and there are increased demands to increase the storage capacity, to increase the operation speed and to reduce the cost. In order to satisfy such demands, active research is being made to develop recording and reproducing techniques which enable high-density recording on magnetic recording media.  
      In the recording and reproducing techniques, it is important to improve the write and read performance of a magnetic head and to reduce the flying height of the magnetic head, in correspondence with the increased recording density of the magnetic recording medium. The magnetic head is provided on a head slider which is supported on a head suspension. The magnetic head carries out the recording and reproducing operations by floating from the surface of the magnetic recording medium by maintaining the flying height on the order of ten-odd μm. This flying height is maintained by the balance of an air force and a pushing force.  
      The air force is generated by an air flow that is generated by the rotation of the magnetic recording medium, such as a magnetic disk, and is received by a medium opposing surface of the head slider confronting the surface of the magnetic recording medium. On the other hand, the pushing force is generated by the head suspension and acts on the head slider in a direction so as to push the medium opposing surface of the head slider towards the surface of the magnetic recording medium. The air force is generated mainly by an air bearing surface which is located at a highest portion of the medium opposing surface of the head slider. In other words, the air bearing surface receives a floating force due to a pressure which increases due to the air flow sandwiched between the air bearing surface and the surface of the magnetic recording medium.  
      The flying height of the magnetic head refers to a distance from the surface of the magnetic recording medium to a magnetic head element of the magnetic head. In order to realize a high-density recording, it is necessary to reduce the flying height, because a spacing loss at the time of the recording and reproduction is directly related to the flying height. That is, when the flying height is small, the magnetic head element can sense more magnetic flux leaking from a recording layer of the magnetic recording medium at the time of the reproduction, and satisfactory record information on the recording layer at the time of the recording.  
      Recently, hard disk drives which enable a surface recording density exceeding 80 Gbit/in 2  have been developed. In such hard disk drives, the flying height of the magnetic head is set to an extremely small amount on the order of ten-odd nm in order to obtain a sufficiently high signal-to-noise ratio (SNR). When the flying height is extremely small, it is necessary to control various flying height varying factors which affect and vary the flying height with an extremely high precision compared to the case where the flying height is on the order of several tens of nm.  
      One of the flying height varying factors is the precision with which the air bearing surface and a step surface which is one step lower than the air bearing surface are formed when forming the medium opposing surface of the head slider. The medium opposing surface of the head slider may be formed by a process similar to a semiconductor forming process. That is, a resist layer is coated on a medium opposing surface of a wafer bar which is cut from a wafer, and a patterning is performed by an exposure apparatus using a photomask having the shape of the air bearing surface. When performing the patterning, the photomask and the wafer bar are aligned, but an alignment error on the order of several μm occurs. As a result, the area of the air bearing surface may deviate from a designed value due to the alignment error, and make it impossible to obtain a desired flying height and deteriorate the yield of the head slider. In addition, when such a head slider is used in the hard disk drive, the magnetic head may crash against the surface of the magnetic recording medium.  
      It is conceivable to modify the alignment technique used in the exposure apparatus to a technique which reduces the alignment error. However, this would require drastic modifications with regard to controlling the dimensions of a photomask alignment mechanism of the exposure apparatus, controlling the dimensions of the wafer bar, designing a fixing jig for the wafer bar, and the like. Hence, it is impractical to make such drastic modifications which are both troublesome and time-consuming.  
     SUMMARY OF THE INVENTION  
      Accordingly, it is a general object of the present invention to provide a novel and useful head slider, magnetic storage apparatus head slider producing method, in which the problems described above are suppressed.  
      Another and more specific object of the present invention is to provide a head slider which has a stable floating characteristic even when a flying height is small, is suited for high-density recording and can be produced with a high yield, and to provide a magnetic storage apparatus which uses such a head slider and a head slider producing method for producing such a head slider.  
      Still another object of the present invention is to provide a head slider comprising a medium opposing surface configured to confront and float from a surface of a recording medium, the medium opposing surface having an air inlet end and an air outlet end with respect to an air flow between the medium opposing surface and the recording medium; a front rail, disposed on the medium opposing surface in a vicinity of the air inlet end, and having a first air bearing surface and a first step surface which forms a stepped portion with the first air bearing surface, the first air bearing surface being higher than the first step surface relative to the medium opposing surface; and a rear rail, disposed on the medium opposing surface in a vicinity of the air outlet end, and having a second air bearing surface and a second step surface which forms a stepped portion with the second air bearing surface, the second air bearing surface being higher than the second step surface relative to the medium opposing surface, the first air bearing surface being substantially surrounded by the first step surface. According to the head slider of the present invention, it is possible to obtain a stable floating characteristic even when the flying height is small, and the head slider is suited for high-density recording. Further, the head slider can be produced with a high yield.  
      A further object of the present invention is to provide a magnetic storage apparatus comprising a recording medium having a surface; and a head slider configured to be movable with respect to the recording medium at a floating distance from the surface of the recording medium, the head slider comprising a medium opposing surface configured to confront and float from the surface of the recording medium, the medium opposing surface having an air inlet end and an air outlet end with respect to an air flow between the medium opposing surface and the recording medium; a front rail, disposed on the medium opposing surface in a vicinity of the air inlet end, and having a first air bearing surface and a first step surface which forms a stepped portion with the first air bearing surface, the first air bearing surface being higher than the first step surface relative to the medium opposing surface; and a rear rail, disposed on the medium opposing surface in a vicinity of the air outlet end, and having a second air bearing surface and a second step surface which forms a stepped portion with the second air bearing surface, the second air bearing surface being higher than the second step surface relative to the medium opposing surface, the first air bearing surface being substantially surrounded by the first step surface. According to the magnetic storage apparatus of the present invention, it is possible to obtain a stable floating characteristic of the head slider even when the flying height is small, thereby making the magnetic storage apparatus suitable for high-density recording. Further, the magnetic storage apparatus can be produced with a high yield because the head slider can be produced with a high yield.  
      Another object of the present invention is to provide a head slider producing method comprising the steps of (a) forming a first resist layer pattern on a medium opposing surface of a block which includes a head element and is to form a head slider; (b) etching the medium opposing surface using the first resist layer pattern as a mask to form an air bearing surface; (c) removing the first resist layer pattern and forming a second resist layer pattern which is larger than the air bearing surface and completely covers the air bearing surface; and (d) etching the medium opposing surface using the second resist layer pattern as a mask to form a step surface which forms a stepped portion with the air bearing surface, so that the air bearing surface is substantially surrounded by the step surface. According to the head slider producing method of the present invention, it is possible to produce a head slider which can obtain a stable floating characteristic even when the flying height is small, and is suited for high-density recording. Further, the head slider can be produced with a high yield.  
      Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a diagram showing an embodiment of a head slider according to the present invention in a state floating from a magnetic recording medium;  
       FIG. 2  is a perspective view showing the embodiment of the head slider;  
       FIG. 3  is a diagram showing a medium opposing surface of the head slider shown in  FIG. 2 ;  
       FIG. 4  is a cross sectional view of the head slider along a line X-X′ in  FIG. 3 ;  
       FIGS. 5A and 5B  are diagrams for explaining an embodiment of a head slider producing method for producing the embodiment of the head slider;  
       FIGS. 6A through 6D  are diagrams for explaining the embodiment of the head slider producing method for producing the embodiment of the head slider;  
       FIG. 7  is a diagram showing a medium opposing surface of a modification of the embodiment of the head slider according to the present invention;  
       FIG. 8  is a diagram showing a medium opposing surface of a comparison example of a head slider;  
       FIG. 9  is a diagram showing floating characteristics of the embodiment of the head slider and the comparison example of the head slider; and  
       FIG. 10  is a plan view showing a part of an embodiment of a magnetic storage apparatus according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      A description will be given of embodiments of a head slider according to the present invention, a magnetic storage apparatus according to the present invention and a head slider producing method according to the present invention, by referring to  FIGS. 1 through 10 .  
       FIG. 1  is a diagram showing an embodiment of the head slider according to the present invention in a state floating from a magnetic recording medium. A head slider  10  shown in  FIG. 1  is supported on a head suspension  14 , and confronts a surface  11   a  of a magnetic recording medium  11 . In this embodiment, the magnetic recording medium  11  is a magnetic disk. When the magnetic recording medium  11  rotates in a direction ROT indicated by an arrow, an air flow AF is generated as indicated by arrows. Hence, the head slider  10  floats in a position where an air inlet end  10   a  of the head slider  10  is floats from the surface  11   a  of the magnetic recording medium  11  by an amount larger than an air outlet end  10   b  of the head slider  10 . A medium opposing surface  13  of the head slider  10  has a head element  12 , provided in a vicinity of the air outlet end  10   b , for recording information on and reproducing information from the magnetic recording medium  11 . A flying height (or height) FH is a distance from the surface  11   a  of the magnetic recording medium  11  to the head element  12 . Stable recording and reproducing characteristics can be obtained for high-density recording, by stably maintaining the flying height FH to a desired value. The head slider  10  suppresses variation in the flying height FH even when a processing error is introduced during the production process of the head slider  10 .  
       FIG. 2  is a perspective view showing the embodiment of the head slider.  FIG. 3  is a diagram showing a medium opposing surface of the head slider shown in  FIG. 2 , and  FIG. 4  is a cross sectional view of the head slider along a line X-X′ in  FIG. 3 . As may be seen from  FIGS. 2 through 4 , the head slider  10  has an approximate parallelepiped shape having a length of 1.25 mm, a width of 1.00 mm and a height of 0.30 mm, for example. The length is taken in the horizontal direction in  FIG. 3 , the width is taken in a vertical direction in  FIG. 3 , and the height is taken in a vertical direction in  FIG. 4 . The head slider  10  is made of a material such as Al 2 O 3 —TiC (allitic). A protection layer made of Diamond-Like-Carbon (DLC) may be formed on the medium opposing surface  13  of the head slider  10 . In addition, an alumina (Al 2 O 3 ) layer  15  having a thickness of several tens of μm covers the head element  12  at the air outlet end  10   b  of the head slider  10 .  
      A front rail  16  is provided on the medium opposing surface  13  of the head slider  10  in a vicinity of the air inlet end  10   a . A rear center rail  18  is provided on the medium opposing surface  13  in a vicinity of the air outlet end  10   b . A pair of rear side rails  19  are provided on the medium opposing surface  13  on respective sides of the head slider  10  relative to the rear center rail  18 . The rear center rail  18  is closer to the air outlet end  10   b  than the rear side rails  19 .  
      The front rail  16  includes air bearing surfaces  16   a  each extending in a direction taken along the width of the head slider  10 , and a step surface  16   b  which is lower than the air bearing surfaces  16   a  and forms a stepped portion with the air bearing surfaces  16   a . In this embodiment, each air bearing surface  16   a  spreads towards the corresponding closer side of the head slider  10 , as shown in  FIGS. 2 and 3 . For example, the step surface  16   b  is approximately 0.2 μm lower than the air bearing surface  16   a , as shown in  FIGS. 2 and 4 . The air bearing surfaces  16   a  have the highest height of the medium opposing surface  13 . The shape and dimensions of each air bearing surface  16   a  may be appropriately set to suit the flying height and the floating position such as a pitch angle and a roll angle. The step surface  16   b  is formed so as to substantially surround each of the air bearing surfaces  16   a . In other words, the step surface  16   b  exists on both sides of each air bearing surface  16   a  in the direction taken along the width of the head slider  10 , and in front and rear of each air bearing surface  16   a  respectively closer to the air inlet end  10   a  and the air outlet end  10   b  of the head slider  10 .  
      The pitch angle is the angle formed by the medium opposing surface  13  of the head slider  10  and the surface  11   a  of the magnetic recording medium  11 . On the other hand, the roll angle is the angle for which the head slider  10  turns about a center axis of the head slider  10  extending along a longitudinal direction thereof.  
      The rear center rail  18  includes an air bearing surface  18   a  approximately at a center in the direction taken along the width of the head slider  10 , and a step surface  18   b  which is lower than the air bearing surface  18   a  and forms a stepped portion with the air bearing surface  18   a . The head element  12  is provided on the air bearing surface  18   a  closer to the air outlet end  10   b  of the head slider  10 . The step surface  18   b  is formed so as to substantially surround the front and sides of the air bearing surface  18   a , but not the rear closer to the air outlet end  10   b  of the head slider  10 . In other words, the step surface  18   b  exists on both sides of the air bearing surface  18   a  in the direction taken along the width of the head slider  10 , and in front of the air bearing surface  18   a  closer to the air inlet end  10   a  of the head slider  10 .  
      For example, the head element  12  is made up of a Giant Magneto Resistive (GMR) reproducing element and a thin film inductive recording element (both not shown) which are stacked in this order on the rear center rail  18 . Of course, a Ferromagnetic Tunnel Junction Magneto Resistive (TMR) element, a ballistic MR element and the like may be used in place of the GMR reproducing element of the head element  12 . Further, a ring head, a single-pole head for perpendicular magnetic recording and the like may be used in place of the thin film inductive recording element.  
      Each rear side rail  19  includes an air bearing surface  19   a , and a step surface  19   b  which is lower than the air bearing surface  19   a  and forms a stepped portion with the air bearing surface  19   a . In each rear side rail  19 , the step surface  19   b  is formed so as to substantially surround the air bearing surface  19   a.    
      A pair of side rails  20  on both sides in the direction taken along the width of the head slider  10 . Each side rail  20  extends from the front rail  16  towards the air outlet end  10   b  of the head slider  10 . Each side rail  20  has the same height as the step surface  16   b  of the front rail  16 . A width of each side rail  20  in the direction taken along the width of the head slider  10  is approximately 30 μm, for example.  
      A groove  21  is formed in the rear of the front rail  16  closer to the air outlet end  10   b . For example, the groove  21  has a depth of approximately 2 μm to approximately 3 μm from the air bearing surfaces  16   a.    
      Next, a description will be given of a floating mechanism of the head slider  10 . First, a basic floating mechanism generates an air flow along the surface  11   a  of the magnetic recording medium  11  when the magnetic recording medium  11  rotates. The air flow hits the stepped portions formed by the step surfaces  16   b ,  18   b  and  19   b  and the corresponding air bearing surfaces  16   a ,  18   a  and  19   a , and thereafter acts on the air bearing surfaces  16   a ,  18   a  and  19   a . Hence, a floating force corresponding to a sum of products of a pressure received from the air flow and areas of the air bearing surfaces  16   a ,  18   a  and  19   a  is generated. This floating force acts on the medium opposing surface  13  of the head slider  10 , so as to push the head slider  10  away from the surface  11   a  of the magnetic recording medium  11 . On the other hand, a negative pressure is generated by the groove  21 , so as to generate a force in a direction opposite to the floating force. This force in the direction opposite to the floating force acts on the medium opposing surface  13  of the head slider  10 , so as to draw the head slider  10  closer to the surface  11   a  of the magnetic recording medium  11 . The head slider  10  floats from the surface  11   a  of the magnetic recording medium  11 , with the desired flying height and floating position, due to the balancing of the floating force and the force in the direction opposite to the floating force. Accordingly, a change in the areas of the air bearing surfaces  16   a ,  18   a  and  19   a  causes the flying height of the head slider  10  to vary.  
      More particularly, the air flow first hits the stepped portion formed by the step surface  16   b  and the air bearing surfaces  16   a  of the front rail  16 , and the air flow is compressed by the collision with the stepped portion to increase the pressure. The increased pressure of the air flow acts on the air bearing surfaces  16   a  to thereby generate the floating force. Then, the air flow reaches the groove  21  and is expanded in the direction taken along the width of the head slider  10 , to thereby generate the negative pressure and the force acting in the direction opposite to the floating force. Thereafter, the air flow hits the stepped portions formed by the step surfaces  19   b  and the air bearing surfaces  19   a  of the rear side rails  19  and the stepped portion formed by the step surface  18   b  and the air bearing surface  18   a  of the rear center rail  18 . Hence, similarly to the air flow hitting the stepped portion of the front rail  16 , the floating forces are generated by the stepped portions of the rear side rails  19  and the rear center rail  18 . The floating force generated at the front rail  16  is greater than a combined floating force generated at the rear side rails  19  and the rear center rail  18 . Hence, when the surface  11   a  of the magnetic recording medium  11  shown in  FIG. 1  is taken as a reference height, the floating position of the head slider  10  is such that the air inlet end  10   a  is higher than the air outlet end  10   b  and the pitch angle is approximately 200 μrad, for example.  
      At the front rail  16 , the areas of the air bearing surfaces  16   a  are set large, so as to generate a floating force greater than the combined floating force generated at the rear side rails  19  and the rear center rail  18 . A portion of the step surface  16   b  partitions the central part of the air bearing surface of the front rail  16  into the two air bearing surfaces  16   a , as shown in  FIGS. 2 and 3 . However, it is not essential to provide such a portion of the step surface  16   b , and the front rail  16  may have a single air bearing surface  16   a . The distances from the surface  11   a  of the magnetic recording medium  11  to the rear side rails  19  and the rear center rail  18  are shorter than the distance from the surface  11   a  to the front rail  16 , as may be seen from  FIG. 1 . For this reason, the pressure of the air flow is greater at the rear side rails  19  and the rear center rail  18  when compared to that at the front rail  16 , and even a small change in the areas of the air bearing surfaces  19   a  and  18   a  greatly affects the floating forces that are generated.  
      In addition, the rear side rails  19  provided on both sides of the head slider  10  have the function of maintaining the floating position of the head slider  10  to a roll angle within a predetermined range, even when the magnetic recording medium  11  is a magnetic disk and the head slider  10  is located at the inner or outer peripheral portion of the magnetic disk and an angle of the air flow reaching the head slider  10  changes. This angle of the air flow reaching the head slider  10  is the angle formed by the direction of the air flow and the longitudinal direction of the head slider  10 . The longitudinal direction of the head slider  10  is the horizontal direction in  FIG. 3 . Accordingly, the floating stability of the head slider  10  is improved by controlling the floating force at each of the rear side rails  19  to a predetermined range.  
      As described above, in the head slider  10  of this embodiment, the air bearing surfaces  16   a  of the front rail  16  are surrounded by the step surface  16   b . For this reason, as will be described later in conjunction with the production process of the head slider  10 , the area of the air bearing surfaces  16   a  does not change even if an alignment error of a photomask for forming the air bearing surfaces  16   a  occurs. Accordingly, it is possible to prevent the floating force generated at the front rail  16  from varying.  
      Widths of the step surfaces  16   b ,  18   b  and  19   b  surrounding the corresponding air bearing surfaces  16   a ,  18   a  and  19   a  in  FIG. 3  are set as follows. The width of each of the step surfaces  16   b ,  18   b  and  19   b  surrounding the corresponding air bearing surfaces  16   a ,  18   a  and  19   a  refers to a distance between an end portion of each of the step surfaces  16   b ,  18   b  and  19   b  and a boundary portion of between each of the step surfaces  16   b ,  18   b  and  19   b  and the corresponding air bearing surfaces  16   a ,  18   a  and  19   a . For example, minimum widths X 1  through X 5  of the step surface  16   b  surrounding the air bearing surfaces  16   a  in  FIG. 3 , with respect to each side of the air bearing surfaces  16   a , are set to 1 μm or greater by taking into consideration the alignment error during the exposure process which will be described later. The minimum widths X 1  through X 5  are preferably 5 μm or greater, and more preferably 10 μm or greater. The upper limit of each of the minimum widths X 1  through X 5  may be set appropriately depending on the position and the designed flying height. For example, the upper limit of the minimum widths X 1  and X 2  is 30 μm, the upper limit of the minimum width X 3  is 100 μm, and the upper limit of the minimum widths X 4  and X 5  is 50 μm. The effects of surrounding the air bearing surfaces  16   a  by the step surface  16   b  are particularly notable, because the air bearing surfaces  16   a  of the front rail  16  are longer and wider than the air bearing surfaces  19   a  and  18   a  of the rear side rails  19  and the rear center rail  18 .  
      In the rear center rail  18 , the step surface  18   b  surrounds the air bearing surface  18   a  except for the end portion of the surface  18   a  on the side of the air outlet end  10   b  provided with the head element  12 . Similarly to the front rail  16 , a minimum width of the step surface  18   b  surrounding the air bearing surface  18   a  is set to 1 μm or greater, and preferably to 5 μm or greater, and more preferably to 10 μm or greater.  
      In each of the rear side rail  19 , the step surface  19   b  surrounds the air bearing surface  19   a . Similarly to the front rail  16 , a minimum width of the step surface  19   b  surrounding the air bearing surface  19   a  is set to 1 μm or greater, and preferably to 5 μm or greater, and more preferably to 10 μm or greater.  
      By setting the minimum widths of the step surfaces  16   b ,  18   b  and  19   b  surrounding the corresponding air bearing surfaces  16   a ,  18   a  and  19   a , it is possible to suppress variation of the flying height caused by an alignment error of the patterning at the time of forming the air bearing surfaces  16   a ,  18   a  and  19   a  and the step surfaces  16   b ,  18   b  and  19   b , as will be described later.  
      Of course, the shape of the air bearing surfaces  16   a  is not limited to that shown in  FIG. 2 . For example, the air bearing surfaces  16   a  may have an appropriate shape, such as a rectangular shape, in place of the trapezoidal shape shown in  FIG. 2 . In addition, the front or leading edge of the air bearing surface  16   a  may be perpendicular to or, inclined or curved with respect to the air flow. Similarly, the rear or trailing edge of the air bearing surface  16   a  may be perpendicular to or, inclined or curved with respect to the air flow. Furthermore, it is not essential for the side edges of the air bearing surface  16   a  to be parallel with respect to the air flow.  
      Next, a description will be given of an embodiment of the head slider producing method according to the present invention, by referring to  FIGS. 5A, 5B  and  6 A through  6 D.  FIGS. 5A, 5B  and  6 A through  6 D are diagrams for explaining this embodiment of the head slider producing method.  FIGS. 6A through 6D  show cross sections of a block which forms one head slider.  
      In a process shown in  FIG. 5A , an allitic wafer  30  is used as a substrate. An alumina layer  15   a  is formed on the wafer  30  to cover the surface of the wafer  30 . Then, a plurality of head elements  12  are formed in a matrix arrangement on the surface of the alumina layer  15   a . Each head element  12  is made up of a GMR reproducing element and a thin film inductive recording element which are stacked. An alumina layer  15   b  is formed on the alumina layer  15   a  to a thickness of 5 μm to 20 μm, for example, to cover the head elements  12 . An aluminum nitride layer may be used in place of the alumina layer  15   b.    
      Next, in a process shown in  FIG. 5B , the wafer  30  is cut into rows of head elements  12 , that is, wafer bars  31 . Each wafer bar  31  has the row of head elements  12 . In other words, each wafer bar  31  is made up of a plurality of blocks  32  which are to be later separated along dotted lines shown in  FIG. 5B . Each block  32  has one head element  12  provided on an end thereof, and in this particular case, a top cut surface  32   a  forms the medium opposing surface and a bottom cut surface  32   b  forms a surface which is to be fixed to the head suspension. The cut surface of the block  32 , which is to form the medium opposing surface, is determined by the orientation of the head element  12 .  
      Thereafter, in a process shown in  FIG. 6A , the cut surface  32   a  of the block  32  is polished and planarized, before forming a negative type resist layer  33  on the cut surface  32   a . The resist layer  33  is the patterned. This patterning is performed by aligning a photomask  34  to a predetermined reference position using a reducing optical system exposure apparatus, for example. The photomask  34  is printed with patterns of the air bearing surfaces  16   a ,  18   a  and  19   a . The resist layer  33  is exposed via the photomask  34  by irradiating an ultraviolet ray, for example, so as to form exposed portions  33   a  and a non-exposed portion  33   b.    
      In a process shown in  FIG. 6B , the resist layer  33  is developed by use of a dip developing apparatus or the like, so as to remove the non-exposed portion  33   b . As a result, the exposed portions  33   a  having the shapes of the air bearing surfaces  16   a ,  18   a  and  19   a  remain on the cut surface  32   a . Next, the exposed portions  33   a  are used as masks to remove a portion of the block  32  by Reactive Ion Etching (RIE), for example, including the alumina layers  15   a  and  15   b , as indicated by a dotted line. Hence, portions covered by the exposed portions  33   a , namely, air bearing surfaces  35 , remain after the RIE. The etching amount (depth) is set slightly larger than the heights of the stepped portions formed by the step surfaces  16   b ,  18   b  and  19   b  and the corresponding air bearing surfaces  16   a ,  18   a  and  19   a , so as to secure an amount (thickness) of the air bearing surface  35  that is finally polished in a later process. When the air bearing surface  35  will not be polished in a later process, the etching amount (depth) may be set equal to the height of the corresponding stepped portion.  
      In a process shown in  FIG. 6C , the exposed portions  33   a  of the resist layer  33  are removed, and a resist layer  36  is newly formed. The resist layer  36  is the patterned. This patterning is performed by aligning a photomask  38  to a predetermined reference position using the reducing optical system exposure apparatus, for example, similarly as in  FIG. 6A  when forming the air bearing surfaces  35 . The photomask  38  is printed with patterns of the front rail  16 , the rear center rail  18  and the rear side rails  19 . The resist layer  36  is exposed via the photomask  38  by irradiating an ultraviolet ray, for example, so as to form exposed portions  36   a  and a non-exposed portion  36   b.    
      In a process shown in  FIG. 6D , the resist layer  36  is developed by use of a dip developing apparatus or the like, so as to remove the non-exposed portion  36   b . As a result, the exposed portions  36   a  remain on the cut surface  32   a . Next, the exposed portions  36   a  are used as masks to remove a portion of the block  32  by RIE, for example, to a depth of the groove  21  shown in  FIG. 2 , so as to form step surfaces  39  which form stepped portions with the air bearing surfaces  35 . Consequently, the step surfaces  39  formed substantially surround the corresponding air bearing surfaces  35 . Although not shown in  FIG. 6D , the exposed portions  36   a  of the resist layer  36  are removed, and the air bearing surfaces  35  are polished, so as to adjust the tip end of the head element  12  and the stepped portions of the air bearing surfaces  16   a ,  18   a  and  19   a . The medium opposing surface  13  of the head slider  10  is completed in this manner.  
      The head slider  10  is then fixed to the head suspension  14 , and subjected to quality inspection. The quality inspection is performed to check whether or not the flying height of the head slider  10  is within a designed range, and only the head sliders  10  belonging to the acceptable wafer lot are used as parts for the magnetic storage apparatus.  
      When exposing the resist layers using the masks as described above, the photomasks  34  and  38  must be aligned when forming the air bearing surfaces  35  and when forming the step surfaces  39 . When aligning the photomask  34  for forming the air bearing surface and the photomask  38  for forming the step surface which forms the step portion with the air bearing surface, an alignment error on the order of several μm or greater may occur. But in the head slider  10  of this embodiment, even if a relative alignment error of the photomasks  34  and  38  occurs, the area of the air bearing surface  35  is virtually suppressed completely from varying, because the air bearing surface  35  is substantially surrounded by the step surface  39 . As a result, it is possible to substantially suppress the flying height of the head slider  10  from varying due to the alignment error of the photomasks  34  and  38 .  
      In addition, when performing the etching to form the step surface  39  in  FIG. 6D , even if the an error in the patterning of the resist layer  36  occurs due to the alignment error of the photomask  38 , it is possible to virtually avoid the air bearing surface  35  from being exposed. Thus, it is possible to prevent the air bearing surface  35  from being etched when forming the step surface  39 .  
      Furthermore, in a case where a sidewall surface  36   a - 1  at the opening of the resist layer  36  is tapered in  FIG. 6D , the step surface  39  at the tapered portion is etched due to the deteriorated resist performance at the sidewall surface  36   a - 1 . As a result, the end portion of the step surface  39  recedes, thereby making the area of the step surface  39  smaller than the designed value. But even in such a case, the air bearing surfaces  16   a  of the front rail  16  shown in  FIG. 3  having the large area (large length and width) are completely surrounded by the step surface  16   b  in the head slider  10  of this embodiment. Consequently, the air bearing surfaces  16   a  will not be etched simultaneously as the step surface  16   b , thereby making it possible to prevent the flying height from varying.  
      Therefore, it is possible to realize a head slider which is suited for high-density recording and has a flying height which is suppressed from varying greatly. Moreover, it is possible to realize a head slider which can be produced at a high yield.  
      In the processes described above in conjunction with  FIGS. 5A, 5B  and  6 A through  6 D, only the air bearing surfaces  35  and the step surfaces  39  are described, and the description of each of the air bearing surfaces  16   a ,  18   a  and  19   a  and the corresponding step surfaces  16   b ,  18   b  and  19   b  is omitted. However, as may be seen from these figures, all of the air bearing surfaces  16   a ,  18   a  and  19   a  may be formed simultaneously in the process shown in  FIG. 6B , and all of the corresponding step surfaces  16   b ,  18   b  and  19   b  which form the stepped portions with the air bearing surfaces  16   a ,  18   a  and  19   b  may be formed simultaneously in the process shown in  FIG. 6D . Hence, the air bearing surfaces  16   a ,  18   a  and  19   a  can be formed to the same height with a satisfactory planar characteristic. Similarly, the step surfaces  16   b ,  18   b  and  19   b  can be formed to the same height with a satisfactory planar characteristic.  
      Of course, at least one of the air bearing surfaces  16   a ,  18   a  and  19   a  may be formed separately to a height which his different from the height of the other air bearing surfaces. Similarly, at least one of the step surfaces  16   b ,  18   b  and  19   b  may be formed separately to a height which is different from the height of the other step surfaces. In other words, the air bearing surfaces  16   a ,  18   a  and  19   a  may have different heights, and the step surfaces  16   b ,  18   b  and  19   b  may have different heights.  
      Next, a description will be given of a modification of the embodiment of the head slider according to the present invention, by referring to  FIG. 7 .  FIG. 7  is a diagram showing a medium opposing surface of this modification of the embodiment of the head slider. In  FIG. 7 , those parts which are the same as those corresponding parts in  FIGS. 1 through 4  are designated by the same reference numerals, and a description thereof will be omitted.  
      This modification shown in  FIG. 7  is basically the same as the embodiment described above, except that a head slider  50  is provided with rear center rail  51  in place of the rear center rail  18 . The rear center rail  51  is provided on the medium opposing surface  13  in a vicinity of the air outlet end  10   b . The rear center rail  51  includes an air bearing surface  51   a  approximately at a center in the direction taken along the width of the head slider  50 , and a step surface  51   b  which is lower than the air bearing surface  51   a  and forms a stepped portion with the air bearing surface  51   a . The head element  12  is provided on the air bearing surface  51   a  closer to the air outlet end  10   b  of the head slider  50 . The step surface  51   b  is formed so as to substantially surround the front, rear and sides of the air bearing surface  51   a , and includes a step surface portion  51   b - 1  at the rear closer to the air outlet end  10   b  of the head slider  50 . In other words, the step surface  51   b  exists on both sides of the air bearing surface  51   a  in the direction taken along the width of the head slider  50 , and in the front and rear of the air bearing surface  51   a  respectively closer to the air inlet end  10   a  and the air outlet end  10   b  of the head slider  50 .  
      Accordingly, even if an alignment error is generated in the direction towards the air outlet end  10   b  when forming the air bearing surface  51   a , the effects on the flying height is virtually prevented by the provision of the step surface portion  51   b - 1 . The width of the step surface portion  51   b - 1 , in a direction taken along the width of the head slider  50 , is set to 1 μm or greater at the narrowest portion, and preferably to 5 μm or greater, and more preferably to 10 μm or greater.  
      When forming the head slider  50  this modification, an alumina layer  15 - 1  which covers the head element  12  is formed to a thickness which is thicker than the alumina layer  15  by an amount corresponding to the width of the step surface portion  51   b - 1 . For example, if the width of the step surface portion  51   b - 1  on the side of the air outlet end  10   b  is 10 μm, the alumina layer  15 - 1  is formed to a thickness of 30 μm. A portion of the alumina layer  15 - 1  or all of the alumina layer  15 - 1  may be replaced by an aluminum nitride layer. Since the thermal conductivity of the aluminum nitride layer is approximately 100 W/(m·K) and higher than the thermal conductivity of the alumina layer  15 - 1  which is approximately 15 W/(m·K), the aluminum nitride layer can easily release the heat generated from the thin film inductive recording element and avoid the tip end of the head element  12  from projecting due to thermal expansion and avoid thermal damage to the GMR reproducing element.  
      In the embodiment and modification described above, the head element  12  is provided on the read center rail  18  or  51  of the head slider  10  or  50 . However, the head element  12  may be provided on one of the two rear side rails  19  or, on both of the two rear side rails  19 . Furthermore, the present invention is not limited to the head sliders  10  and  50  having the rear center rails  18  and  51 , and the present invention is similarly applicable to head sliders which do not have a rear center rail.  
      Next, a description will be given of a comparison of the variations in the head flying heights of the embodiment of the head slider described above and a comparison example of a head slider, by referring to  FIGS. 8 and 9 .  FIG. 8  is a diagram showing a medium opposing surface of the comparison example of the head slider.  FIG. 9  is a diagram showing floating characteristics of the embodiment of the head slider and the comparison example of the head slider. As will be described later,  FIG. 9  shows the effects of an alignment error of a photomask on the flying height obtained by simulations.  
      The head slider  10  of the embodiment has the medium opposing surface  13  shown in  FIG. 3 . The medium opposing surface  13  was made to a length of 1.25 mm and a width of 1.00 mm, by taking the length and the width in the same directions as the length and the width of the head slider  10 . The air bearing surfaces  16   a ,  18   a  and  19   a  of the front rail  16 , the rear center rail  18  and the two rear side rails  19  were respectively made to have areas of 0.16 mm 2 , 0.016 mm 2  and 0.016 mm 2 . For example, the minimum width X 1  of the step surface  16   b  shown in  FIG. 3  was set to 30 μm. A step or height difference between the air bearing surfaces  16   a ,  18   a  and  19   a  and the corresponding step surfaces  16   b ,  18   b  and  19   b  was set to 0.12 μm.  
      On the other hand, a head slider  100  of the comparison example shown in  FIG. 8  had a front rail  116 , a rear center rail  118 , a pair of rear side rails  119 , a pair of side rails  120 , and a groove  121 . The front rail  116  includes a pair of air bearing surfaces  116   a  and a step surface  116   b  which is provided in front of the air bearing surfaces  116   a  on the side of an air inlet end  100   a . No step surface is provided on the sides of the air bearing surfaces  116   a  nor the rear of the air bearing surfaces  116   a  on the side of an air outlet end  100   b . Similarly, the rear center rail  118  includes an air bearing surface  118   a , and a step surface  118   b  provided in front of the air bearing surface  118   a  on the side of the air inlet end  100   a . No step surface is provided on the sides of the air bearing surface  118   a  nor the rear of the air bearing surface  118   a  on the side of the air outlet end  100   b . In addition, each rear side rail  119  includes an air bearing surface  119   a , and a step surface  119   b  provided in front of the air bearing surface  119   a  on the side of the air inlet end  100   a . No step surface is provided on the sides of the air bearing surface  119   a  nor the rear of the air bearing surface  119   a  on the side of the air outlet end  100   b . In other words, none of the air bearing surfaces  116   a ,  118   a  and  119   a  are surrounded by the corresponding step surfaces  116   b ,  118   b  and  119   b.    
      The flying height of the head slider  10  of the embodiment and the flying height of the head slider  100  of the comparison example were obtained by simulations using a known calculation program for calculating a flying height of a head slider. The simulations were performed for a Case A where the position of the pattern of each step surface deviates in the longitudinal direction of the head slider, a Case B where the position of the pattern of each step surface deviates in the direction taken along the width of the head slider, and a Case C where the width of the pattern of each step surface deviates. It was assumed for the sake of convenience that the pattern of each air bearing surface is formed at the predetermined designed position with the predetermined designed width, without deviation. For example, the deviation of the width of the pattern of each step surface occurs when the reduction ratio of the reducing optical system exposure apparatus deviates from a predetermined reduction ratio.  
      In  FIG. 9 , the ordinate indicates a flying height deviation of the embodiment and the comparison example in arbitrary units (A.U.), and the abscissa indicates the embodiment and the comparison example for the Cases A, B and C. The flying height deviation corresponds to the deviation or variance of the flying height. In  FIG. 9 , the flying height deviation of the embodiment is indicated by bars with hatching, and the flying height deviation of the comparison example is indicated by shaded bars.  
      As may be seen from  FIG. 9 , the flying height deviation of the embodiment is greatly reduced for all of the three Cases A, B and C when compared to the flying height deviation of the comparison example. More particularly, the flying height deviation of the embodiment for the Case A is reduced to approximately 30% of that of the comparison example. It may easily be understood that the reduction in the flying height deviation of the embodiment is due to the step surface  16   b  which is also formed in the rear of the air bearing surface  16   a  on the side of the air outlet end  10   b  as shown in  FIG. 3 , while the comparison example has the groove  121  in the rear of the air bearing surface  116   a  and the step surface  116   b  is only formed in front of the air bearing surface  116   a  on the side of the air inlet end  100   a . It may also be understood that the step surface  19   b  formed in the rear of the air bearing surface  19   a  on the side of the air outlet end  10   b  of each rear side rail  19  similarly contributes to the reduction in the flying height deviation of the embodiment.  
      On the other hand, the flying height deviation of the embodiment for the Case B is reduced to approximately 10% of that of the comparison example. It may easily be understood that the reduction in the flying height deviation of the embodiment is due to the step surface  16   b  which is also formed on both sides of the pair of air bearing surfaces  16   a  as shown in  FIG. 3 , while the comparison example does not have a step surface formed on both sides of the air bearing surface  116   a  and the step surface  116   b  is only formed in front of the air bearing surface  116   a  on the side of the air inlet end  100   a . It may also be understood that the step surface  18   a  formed on both sides of the air bearing surface  18   a  of the rear center rail  18  and the step surface  19   b  formed on both sides of the air bearing surface  19   a  of each rear side rail  19  similarly contribute to the reduction in the flying height deviation of the embodiment.  
      Furthermore, the flying height deviation of the embodiment for the Case C is reduced to approximately 30% of that of the comparison example. It may easily be understood that the reduction in the flying height deviation of the embodiment is due to the step surfaces  16   b ,  18   b  and  19   c  which substantially surround the corresponding air bearing surfaces  16   a ,  18   a  and  19   c  of the front rail  16 , the rear center rail  18  and the rear side rails  19  as shown in  FIG. 3 , while the comparison example has the air bearing surfaces  116   a ,  118   a  and  119   a  which have large portions that are not surrounded by the corresponding step surfaces  116   b ,  118   b  and  119   b.    
      Next, a description will be given of the embodiment of the magnetic storage apparatus according to the present invention, by referring to  FIG. 10 .  FIG. 10  is a plan view showing a part of this embodiment of the magnetic storage apparatus. In this embodiment of the magnetic storage apparatus, the present invention is applied to a magnetic disk drive.  
      A magnetic storage apparatus  60  shown in  FIG. 10  generally includes a housing  61 . Inside this housing  61 , there are provided a hub  62  which is driven by a known driving means such as a spindle motor (not shown), at least one magnetic disk  63  which is provided as the magnetic recording medium and is fixed on the hub  62  to be rotated thereby, an actuator unit  64 , an arm  65  which is mounted on the actuator unit  64  and is movable in a radial direction of the magnetic disk  63 , a head suspension  66  which is mounted on the arm  65 , and a head slider  68  which is supported by the head suspension  66 . The basic construction of this magnetic storage apparatus  60  is known, and a detailed description thereof will be omitted in this specification.  
      The magnetic disk  63  may be used for longitudinal magnetic recording or for perpendicular magnetic recording. The magnetic disk  63  used for the longitudinal magnetic recording has a recording layer having magnetizations parallel to a substrate surface, and this recording layer may be formed by a single magnetic or ferromagnetic layer, a stacked structure made up of a plurality of magnetic or ferromagnetic layers which are stacked or, a stacked ferrimagnetic structure made up of upper and lower magnetic or ferromagnetic layers which are exchange-coupled via a nonmagnetic layer such as a Ru layer which is 0.6 nm to 0.9 nm thick, for example. In the case of the stacked ferrimagnetic structure, magnetization directions of the upper and lower magnetic or ferromagnetic layers may be mutually antiparallel in a state where no recording magnetic field is applied on the magnetic disk  63 . An underlayer may be provided under the recording layer. This underlayer may be made of Cr or Cr alloy having an added element such as W and Mo, for example, so that the magnetization of the recording layer is oriented in an in-plane direction parallel to the substrate surface.  
      On the other hand, the magnetic disk  63  used for the perpendicular magnetic recording has a recording layer having magnetizations perpendicular to the substrate surface. An underlayer is provided below the recording layer. The underlayer is made of a nonmagnetic material such as Co, Cr, Ru, Re, Ri, Hf and alloys thereof. For example, the underlayer is may have a thickness of 2 nm to 30 nm when made of Ru, RuCo or CoCr.  
      The recording layer of the magnetic disk  63  may be made of Ni, Fe, Co, Ni alloy, Fe alloy or Co alloy, regardless of whether the magnetic disk  63  is for the longitudinal magnetic recording or for the perpendicular magnetic recording. The Co alloy used for the recording layer may be CoCrTa, CoCrPt or CoCrPt-M, where M denotes an element selected from a group consisting of B, Mo, Nb, Ta, W, Cu and alloys thereof. The recording layer may have a thickness of 3 nm to 30 nm.  
      The magnetic storage apparatus  60  of this embodiment is characterized by the head slider  68 . The head slider  68  has the structure of the head slider  10  of the embodiment described above or the structure of the head slider  50  of the modification of the embodiment described above.  
      The basic construction of the magnetic storage apparatus  60  is not limited to that shown in  FIG. 10 . In addition, the magnetic recording medium used in the present invention is not limited to the magnetic disk  63 . For example, the magnetic recording medium may be a magneto-optical disk.  
      According to this embodiment, the inconsistency in the flying height of each of the individual head sliders is suppressed, and the production yield of the head slider and the magnetic storage apparatus is improved. Hence, even if the head slider is designed to operate with an extremely small flying height, it is possible to avoid the head slider and the head element from crashing to the magnetic recording medium, and to realize a high-density recording.  
      The magnetic storage apparatus may employ the so-called ramp load and unload system which recedes the head slider in a region outside the region of the magnetic recording medium when the magnetic storage apparatus does not operate or is in a standby or sleep state. In this case, a ramp member or the like may be provided to lock the head suspension or the like in the receded position of the head slider.  
      In each of the embodiment and the modification of the head slider, each air bearing surface is substantially surrounded by the corresponding step surface in each of the front rail, the rear center rail and the rear side rails. However, the air bearing surface may be substantially surrounded by the corresponding step surface in at least one of the front rail, the rear center rail and the rear side rails.  
      Further, the present invention is not limited to these embodiments, but various variations and modifications may be made without departing from the scope of the present invention.