Abstract:
Disclosed is a method of manufacturing a magnetic field detector having various structures that can be used as a high-density magnetic biosensor. An embodiment of the invention provides a method of manufacturing a magnetic field detector including a magnetoresistive element using a magnetic bead detecting thin film. The method includes: preparing a substrate; depositing the thin film on an upper surface of the substrate; and etching the thin film to form a ring-shaped magnetoresistive element.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of manufacturing a magnetic field detector, and more particularly, to a method of manufacturing a magnetic field detector that detects a weak magnetic field generated from magnetic beads having a size of several tens of nanometers to several micrometers. 
     This work was supported by the IT R&amp;D program of MIC/IITA [2006-S-074-02, High Performance Bio-sensor System Using Nano-particles]. 
     2. Description of the Related Art 
     Micro devices and array devices using the same have had a great effect on the analysis of DNA, RNA, protein, viruses, and bacteria. In order to effectively analyze these bio-molecules, researches on magnetic biosensors using spherical magnetic particles (hereinafter, referred to as “magnetic beads”) having a size of several tens of nanometers to several micrometers have been conducted. 
     The magnetic biosensor includes a magnetic field detector provided with a biochemical layer capable of being coupled to specific molecules. The magnetic biosensor performs analysis using magnetic beads, which are superparamagnetic particles to which biochemical molecules are coupled and have a size of several nanometers to several micrometers. When a solution containing the magnetic beads is dropped on the magnetic field detector, specific binding occurs between capture molecules on the surface of the magnetic field detector and target bio-molecules on the surfaces of the magnetic beads. When an external magnetic field is applied to the magnetic beads to magnetize the magnetic beads, the magnetic field detector detects the magnetic field generated from the magnetic beads, thereby indirectly detecting the bio-molecules. 
     As the magnetic bead detecting device using the magnetoresistive element according to the related art, the following have been proposed: linear magnetic field detectors each having triangular structures at both ends and an array thereof (M. C. Tondra, U.S. Pat. No. 6,875,621B2); a linear magnetic field detector having a hemispherical structure at one end (G. Li, et al., Journal of Applied Physics 93, 7557 (2003)); a magnetic field detector including linear magnetoresistive elements connected to each other (J. C. Rife, et al., Sensors and Actuators, A107, 209 (2003); a magnetic field detector including linear magnetoresistive element having a spiral shape (Biosensors and Bioelectronics 19, 1149 (2004); and a magnetic field detector including a linear magnetoresistive element having a U shape (H. A. Ferreira et al., Journal of Applied Physics 99, 08P105 (2006)). 
     In general, the magnetic field detector for detecting the magnetic beads according to the related art uses the above-mentioned linear magnetoresistive element. When the magnetic field detector with the linear structure is used, mutual interference occurs due to a stray field that is generated from the magnetic field detector magnetized by an external magnetic field. That is, the linear magnetoresistive element having a flat upper surface is used in the magnetic field detector according to the related art. Therefore, when an external magnetic field is applied, the magnetization direction of the element is fixed from one end to the other end thereof, which causes the leakage of the magnetic field to the outside of the element, that is, a stray field. The stray field lowers the SN ratio and has an adverse effect on the stability of the magnetic field detector. As a result, the magnetic field detector using the linear magnetoresistive element is not suitable as a high-density magnetic field detector. 
     SUMMARY OF THE INVENTION 
     The invention is designed to solve the problems, and an object of the invention is to provide a method of manufacturing a magnetic field detector with various structures capable of being used as a high-density magnetic biosensor. 
     In order to achieve the object, according to an aspect of the invention, there is provided a method of manufacturing a magnetic field detector including a magnetoresistive element using a thin film for detecting magnetic beads. The method includes: depositing the thin film on a substrate; and etching the thin film to form a ring-shaped magnetoresistive element. 
     According to another aspect of the invention, there is provided a method of manufacturing a magnetic field detector using a thin film for detecting magnetic beads. The method includes: preparing a substrate; depositing the thin film on an upper surface of the substrate; etching the thin film to form a ring-shaped magnetoresistive element; forming, on the upper surface of a substrate, electrode pads connected to the magnetoresistive element; forming a protective layer that covers the magnetoresistive element and the electrode pads; and forming, on an upper surface of the protective layer, a magnetic bead limiting layer that covers the entire surface of the magnetoresistive element and portions of the electrode pads. 
     The thin film may be formed of any one of a giant magnetoresistive thin film, an anisotropic magnetoresistive thin film, a spin valve thin film, and a tunnel-type magnetoresistive thin film. 
     The thin film may include a pinned layer and a free layer. 
     The thin film may be a laminate of a seed layer, an antiferromagnetic layer, a pinned layer, a gap layer, a free layer, and a protective layer formed in this order. 
     The seed layer may be formed of Ta. 
     The antiferromagnetic layer may be formed of IrMn. 
     The pinned layer may be formed of Ni 80 Fe 20  or Co 80 Fe 20 . 
     The gap layer may be formed of Cu. 
     The free layer may be formed of Ni 80 Fe 20  or Co 80 Fe 20 . 
     The protective layer may be formed of Ta. 
     The magnetoresistive element may be formed in any one of a circular ring shape, an elliptical ring shape, a square ring shape, and a rectangular ring shape. 
     In the forming of the electrode pads, the electrode pads may be formed of Ta or Au. 
     In the forming of the electrode pads, the electrode pads may be formed so as to be horizontally connected to the magnetoresistive element. 
     In the forming of the electrode pads, the electrode pads may be formed so as to be vertically and horizontally connected to the magnetoresistive element. 
     In the forming of the protective layer, the protective layer may be formed of SiO 2  or Si 3 N 4 . 
     In the forming of the protective layer, the protective layer may be formed with a thickness of 50 to 300 nm at room temperature. 
     In the forming of the magnetic bead limiting layer, the magnetic bead limiting layer may be formed using a photosensitive thin film. 
     In the forming of the magnetic bead limiting layer, the magnetic bead limiting layer may be formed with a thickness of 1 to 2 μm at room temperature. 
     The outside diameter of the magnetoresistive element may be in a range of 100 nm to 30 μm. 
     The width of the magnetoresistive element may be in a range of 100 nm to 5 μm. 
     A plurality of magnetoresistive elements may be formed in a one-dimensional array, or a two-dimensional array, that is, in a matrix. 
     According to the manufacturing method of the above-described aspect of the invention, since a stray field is formed inside the magnetoresistive element having a circular ring shape, an elliptical ring shape, a square ring shape, or a rectangular ring shape, the stray field does not leak to the outside of the magnetic field detector. Therefore, there is no mutual interference due to the stray field. 
     In particular, since the magnetic bead limiting layer limits the generation of the stray caused by an external magnetic field inside the magnetoresistive element, the operation of the magnetic field detector can be sufficiently stable to detect bio-molecules on a magnetic biosensor chip. 
     According to the manufacturing method of the above-described aspect of the invention, it is possible to improve the performance of a magnetic field detector, and manufacture a high-density magnetic field detector. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A to 1C  are diagrams illustrating the laminated structure of a giant magnetoresistive thin film according to the invention; 
         FIGS. 2A to 8B  are diagrams sequentially illustrating a method of manufacturing a magnetic field detector according to a first embodiment of the invention; 
         FIGS. 9A to 11B  are diagrams illustrating modifications of the first embodiment; 
         FIGS. 12A to 12C  are graphs illustrating the relationships between voltages and magnetic fields applied to magnetic field detectors provided with magnetoresistive elements having a circular ring shape, an elliptical ring shape, and a square ring shape, respectively; 
         FIGS. 13A to 19B  are diagrams sequentially illustrating a method of manufacturing a magnetic field detector according to a second embodiment of the invention; 
         FIGS. 20A to 22B  are diagrams illustrating modifications of the second embodiment; 
         FIGS. 23A to 29B  are diagrams sequentially illustrating a method of manufacturing a magnetic field detector according to a third embodiment of the invention; 
         FIGS. 30A to 32B  are diagrams illustrating modifications of the third embodiment; 
         FIGS. 33A to 39B  are diagrams sequentially illustrating a method of manufacturing a magnetic field detector according to a fourth embodiment of the invention; 
         FIGS. 40A to 42B  are diagrams illustrating modifications of the fourth embodiment; 
         FIGS. 43A to 49B  are diagrams sequentially illustrating a method of manufacturing a magnetic field detector according to a fifth embodiment of the invention; 
         FIGS. 50A to 52B  are diagrams illustrating modifications of the fifth embodiment; 
         FIGS. 53A to 59B  are diagrams sequentially illustrating a method of manufacturing a magnetic field detector according to a sixth embodiment of the invention; and 
         FIGS. 60A to 62B  are diagrams illustrating modifications of the sixth embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, methods of manufacturing magnetic field detectors according to embodiments of the invention will be described with reference to the accompanying drawings. 
       FIGS. 1A to 1C  are diagrams illustrating the laminated structure of a giant magnetoresistive thin film according to an embodiment of the invention. In  FIG. 1C , a substrate  1  is a Si or SiO 2  single crystal substrate. A SiO 2  oxidation layer is formed on the surface of the substrate  1 . A giant magnetoresistive thin film  2  having a laminated structure of a seed layer, a free layer, a gap layer, a pinned layer, an antiferromagnetic layer, and a protective layer is formed on an upper surface of the substrate  1  by vapor deposition. For example, a seed layer  14  is formed on the upper surface of the substrate  1 , and an antiferromagnetic layer  15  is formed on an upper surface of the seed layer  14 . A pinned layer  16  is formed on an upper surface of the antiferromagnetic layer  15 , and a gap layer  17  is formed on an upper surface of the pinned layer  16 . A free layer  18  is formed on an upper surface of the gap layer  17 , and a protective layer  19  is formed on an upper surface of the free layer  18 . 
     The seed layer  14  and the protective layer  19  are formed of, for example, Ta film with a thickness of about 5 nm. The antiferromagnetic layer  15  is formed of, for example, IrMn film, and the thickness of the antiferromagnetic layer  15  is about 15 nm. The pinned layer  16  is formed of, for example, Ni 80 Fe 20  film, and the thickness of the pinned layer  16  is about 3 nm. The gap layer  17  is formed of, for example, Cu film, and the thickness of the gap layer  17  is about 3 nm. The free layer  18  is formed of, for example, Ni 80 Fe 20  film, and the thickness of the free layer  18  is about 6 nm. The magnetization direction of the pinned layer  16  is fixed, and the antiferromagnetic layer  15  is for fixing the magnetization direction of the pinned layer  16 . The magnetization direction of the free layer  18  is not fixed. 
     The giant magnetoresistive thin film  2  having the above-mentioned laminated structure and thicknesses is grown by a sequential sputtering deposition method. The pinned layer  16  and the free layer  18  may be formed of Co 80 Fe 20  film, instead of Ni 80 Fe 20  film.  FIG. 1A  is a plan view illustrating the free layer  18 , and  FIG. 1B  is a cross-sectional view illustrating the pinned layer  16  and the free layer  18  of the giant magnetoresistive thin film  2 . In the giant magnetoresistive thin film  2 , the layers may be laminated in a different order from the above, for example, in the order of the seed layer, the free layer, the gap layer, the pinned layer, the antiferromagnetic layer, and the protective layer, if necessary. 
     A magnetoresistive element shown in the following drawings is formed in a desired shape by etching the giant magnetoresistive thin film  2 . In the following drawings, as shown in  FIG. 1B , the magnetoresistive element is schematically illustrated to include the pinned layer  16  and the free layer  18 . Arrows shown in the pinned layer  16  and the free layer  18  in  FIGS. 1B and 1C  indicate the degree of magnetization. Meanwhile, the magnetoresistive element may be formed using, for example, an anisotropic magnetoresistive thin film, a spin valve thin film, or a tunnel-type magnetoresistive thin film other than the giant magnetoresistive thin film  2 . 
     First Embodiment 
       FIGS. 2A to 8B  are diagrams sequentially illustrating a method of manufacturing a magnetic field detector according to a first embodiment of the invention. The magnetic field detector according to the first embodiment is characterized in that it includes a magnetoresistive element with a single circular ring shape. 
     First, the giant magnetoresistive thin film  2  is formed on the substrate  1  by vapor deposition and then etched to form a magnetoresistive element  20  with a circular ring shape (see  FIGS. 2A and 2B ). In the etching process, dry etching, such as an Ar gas ion milling method, is performed on the giant magnetoresistive thin film  2  shown in  FIG. 1C  to etch all the portions of the film except a circular ring portion.  FIG. 2A  is a plan view, and  FIG. 2B  is a diagram illustrating the arrangement of the substrate  1  and the magnetoresistive element  20 . For example, in the first embodiment, the magnetoresistive element  20  has an outside diameter a of about 100 nm to 30 μm, and the magnetoresistive element  20  has a width b of about 100 nm to 5 μm. The dimensions of the magnetoresistive element  20  may be applied to magnetoresistive elements  20  according to other embodiments. 
     As shown in  FIGS. 3A and 3B , a metal thin film layer  22  formed of Au is deposited on the substrate  1  and the magnetoresistive element  20 . For example, Au is grown with a thickness of about 150 nm by a sputtering deposition method under the conditions of an argon gas pressure of about 3×10 −4  Torr, a sputtering power of about 60 W, and room temperature, thereby forming the metal thin film layer  22 .  FIG. 3A  is a plan view, and  FIG. 3B  is a diagram illustrating the deposited metal thin film layer  22 . The metal thin film layer  22  may be formed of Ta. 
     As shown in  FIGS. 4A and 4B , electrode pads  24  are formed. The electrode pads  24  are used as electrodes for applying a current and measuring a horizontal voltage. The electrode pads  24  are formed by a lift-up method, using dry etching or a negative photosensitive mask. In this case, all the portions of the metal thin film layer  22  except for portions serving as the electrode pads  24  are removed. For example, the electrode pads  24  are formed on the left and right of the magnetoresistive element  20  so as to face each other in the horizontal direction.  FIG. 4A  is a plan view, and  FIG. 4B  is a diagram illustrating the formed electrode pads  24 . 
     As shown in  FIGS. 5A and 5B , an insulating thin film layer  26  is deposited on the substrate  1 , the magnetoresistive element  20 , and the electrode pads  24 . The insulating thin film layer  26  is formed of SiO.sub.2 or Si.sub.3N.sub.4. In order to prevent the magnetoresistive element  20  and the electrode pads  24  from being corroded by a solution for analyzing, for example, SiO.sub.2 or Si.sub.3N.sub.4 is grown with a thickness of about 50 to 300 nm (preferably, 150 nm) by a sputtering deposition method under the conditions of an argon gas pressure of about 3.times.10.sup.-4 Torr, a sputtering power of about 100 W, and room temperature, thereby forming the SiO.sub.2 or Si.sub.3N.sub.4 insulating thin film layer  26 .  FIG. 5A  is a plan view, and  FIG. 5B  is a diagram illustrating the formed insulating thin film layer  26 . 
     As shown in  FIGS. 6A and 6B , the insulating thin film layer  26  is partially removed to form an insulating protective layer  28 . All the portions of the insulating thin film layer  26  except a portion serving as the insulating protective layer are removed by a lift-up method, using dry etching, such as an Ar gas ion milling method, or a negative photosensitive mask, thereby forming the insulating protective layer  28 .  FIG. 6A  is a plan view, and  FIG. 6B  is a diagram illustrating the formed insulating protective layer  28 . 
     As shown in  FIGS. 7A and 7B , a photosensitive magnetic bead thin film  30  is deposited on the substrate  1 , the electrode pads  24 , and the insulating protective layer  28 . The photosensitive magnetic bead thin film  30  is formed with a thickness of about 1 to 2 μm (preferably, about 1.5 μm) at room temperature by spin coating at a speed of about 3000 to 5000 rpm.  FIG. 7A  is a plan view, and  FIG. 7B  is a diagram illustrating the deposited photosensitive magnetic bead thin film  30 . 
     As shown in  FIGS. 8A and 8B , the photosensitive magnetic bead thin film  30  is selectively removed to form a magnetic bead limiting layer  32 . The magnetic bead limiting layer  32  is formed by removing all the portions of the photosensitive magnetic bead thin film  30  except a portion serving as the magnetic bead limiting layer  32 , using a lift-up method and a negative photosensitive mask. The magnetic bead limiting layer  32  can impound a magnetic bead analysis solution therein such that the magnetic bead analysis solution is positioned close to the magnetoresistive element  20 .  FIG. 8A  is a plan view and  FIG. 8B  is a diagram illustrating the formed magnetic bead limiting layer  32 . 
     The magnetic field detector manufactured through the above-mentioned processes includes the magnetoresistive element  20  with a circular ring shape that is grown on the Si single crystal substrate  1 , and the electrodes  24  for applying a current to the magnetoresistive element  20  and measuring a horizontal voltage, as shown in  FIGS. 8A and 8B . The insulating protective layer  28  is deposited on the entire surface of the magnetoresistive element  20  and portions of the electrodes  24 , and the magnetic bead limiting layer  32  is formed on the magnetoresistive element  20 , the electrodes  24 , and the insulating protective layer  28 . The electrodes  24  mean the electrode pads described with reference to  FIGS. 4A and 4B , and may be called horizontal electrodes. 
       FIGS. 9A to 11B  are diagrams illustrating modifications of the first embodiment of the invention. 
     A magnetoresistive element shown in  FIGS. 9A and 9B  is similar to that shown in  FIGS. 8A and 8B  except that it has an elliptical ring shape, and the magnetoresistive element is manufactured by the same method as described above. In  FIGS. 9A and 9B , reference numeral  34  denotes the magnetoresistive element with the elliptical ring shape.  FIG. 9A  is a plan view, and  FIG. 9B  is a diagram illustrating a magnetic field detector including the magnetoresistive element  34  with the elliptical ring shape. For example, the external diameter of the major axis of the magnetoresistive element  34  is in a range of about 100 nm to 30 μm, and the width of a ring of the magnetoresistive element  34  is in a range of about 100 nm to 5 μm. The ratio of the external diameter of the minor axis to the external diameter of the major axis of the magnetoresistive element  34  is in a range of about 1:2 to 1:3. 
     A magnetoresistive element shown in  FIGS. 10A and 10B  is similar to that shown in  FIGS. 8A and 8B  except that it has a square ring shape, and the magnetoresistive element is manufactured by the same method as described above. In  FIGS. 10A and 10B , reference numeral  36  denotes the magnetoresistive element with the square ring shape.  FIG. 10A  is a plan view, and  FIG. 10B  is a diagram illustrating a magnetic field detector including the magnetoresistive element  36  with the square ring shape. For example, the length of one side of the magnetoresistive element  36  is in a range of about 100 nm to 30 μm, and the width of a ring of the magnetoresistive element  36  is in a range of about 100 nm to 5 μm. 
     A magnetoresistive element shown in  FIGS. 11A and 11B  is similar to that shown in  FIGS. 8A and 8B  except that it has a rectangular ring shape, and the magnetoresistive element is manufactured by the same method as described above. In  FIGS. 11A and 11B , reference numeral  38  denotes the magnetoresistive element with the rectangular ring shape.  FIG. 11A  is a plan view, and  FIG. 11B  is a diagram illustrating a magnetic field detector including the magnetoresistive element  38  with the rectangular ring shape. For example, the external diameter of the major axis (length) of the magnetoresistive element  38  is in a range of about 100 nm to 30 μm, and the external diameter of the minor axis (width) of a ring of the magnetoresistive element  38  is in a range of about 100 nm to 5 μm. The ratio of the width to the length of the magnetoresistive element  38  is in a range of about 1:2 to 1:3. 
     The dimensions of the magnetoresistive elements according to the first embodiment and the modifications may be applied to the following other embodiments and modifications thereof. Of course, the dimensions of the magnetoresistive elements according to the first embodiment and the modifications thereof are just illustrative, but the invention is not limited thereto. 
     In the magnetic field detectors manufactured according to the first embodiment and the modifications thereof, since a stray field is formed inside the magnetoresistive element having a circular ring shape, the stray field is circulated in the magnetoresistive element, and does not leak to the outside of the element. As a result, there is no mutual interference due to the stray field. 
     When the magnetic bead is magnetized by a magnetic field applied from the outside, a weak magnetic field is generated, and the generated magnetic field has an effect on the magnetization direction of the free layer, which causes a variation in the output voltage of the magnetoresistive element. The variation in the output voltage makes it possible to detect the magnetic bead. 
       FIGS. 12A to 12C  are graphs illustrating the relationships between output voltages and magnetic fields applied to the magnetic field detectors provided with the magnetoresistive elements having a circular ring shape, an elliptical ring shape, and a rectangular ring shape, respectively. As can be seen from  FIGS. 12A to 12C , a rapid voltage variation occurs when the strength of the external magnetic field is around 0 (zero) oersted (Oe). These results prove that the magnetic field detector manufactured by the above-mentioned manufacturing method can detect a very weak magnetic field. 
     Second Embodiment 
       FIGS. 13A to 19B  are diagrams sequentially illustrating a method of manufacturing a magnetic field detector according to a second embodiment of the invention. The magnetic field detector according to the second embodiment differs from the magnetic field detector according to the first embodiment in that it further includes vertical electrodes (vertical electrode pads). 
     First, a giant magnetoresistive thin film  2  is formed on a substrate  1  by vapor deposition and then etched to form a magnetoresistive element  20  with a circular ring shape (see  FIGS. 13A and 13B ). In the etching process, dry etching, such as an Ar gas ion milling method, is performed on the giant magnetoresistive thin film  2  shown in  FIG. 1C  to etch all the portions of the film except a circular ring portion.  FIG. 13A  is a plan view, and  FIG. 13B  is a diagram illustrating the arrangement of the substrate  1  and the magnetoresistive element  20 . 
     As shown in  FIGS. 14A and 14B , a metal thin film layer  22  formed of Au is deposited on the substrate  1  and the magnetoresistive element  20 . For example, Au is grown with a thickness of about 150 nm by a sputtering deposition method under the conditions of an argon gas pressure of about 3×10 −4  Torr, a sputtering power of about 60 W, and room temperature, thereby forming the metal thin film layer  22 .  FIG. 14A  is a plan view, and  FIG. 14B  is a diagram illustrating the deposited metal thin film layer  22 . The metal thin film layer  22  may be formed of Ta. 
     As shown in  FIGS. 15A and 15B , horizontal electrode pads  24   a  and vertical electrode pads  24   b  are formed. The horizontal electrode pads  24   a  are used as electrodes for applying a current and measuring a horizontal voltage. The vertical electrode pads  24   b  are used as electrodes for measuring an output voltage (that is, a vertical voltage) in a direction orthogonal to the direction in which a current flows to the magnetoresistive element  20 . The horizontal and vertical electrode pads  24   a  and  24   b  are formed by a lift-up method, using dry etching or a negative photosensitive mask. In this case, all the portions of the metal thin film layer  22  except for portions serving as the electrode pads  24   a  and  24   b  are selectively removed. For example, the horizontal electrode pads  24   a  are formed in the horizontal direction so as to face each other with the magnetoresistive element  20  interposed therebetween (that is, so as to be positioned on the left and right of the magnetoresistive element  20 ). The vertical electrode pads  24   b  are formed in the vertical direction so as to face each other with the magnetoresistive element  20  interposed therebetween (that is, so as to be positioned on the upper and lower sides of the magnetoresistive element  20 ). That is, a line lining the horizontal electrode pads  24   a  in the horizontal direction is orthogonal to a line linking the vertical electrode pads  24   b  in the vertical direction.  FIG. 15A  is a plan view, and  FIG. 15B  is a diagram illustrating the formed electrode pads  24   a  and  24   b.    
     As shown in  FIGS. 16A and 16B , an insulating thin film layer  26  is deposited on the substrate  1 , the magnetoresistive element  20 , and the electrode pads  24   a  and  24   b . The insulating thin film layer  26  is formed of SiO 2  or Si 3 N 4 . In order to prevent the magnetoresistive element  20  and the electrode pads  24   a  and  24   b  from being corroded by an analytical solution, for example, SiO 2  or Si 3 N 4  is grown with a thickness of about 50 to 300 nm (preferably, 150 nm) by a sputtering deposition method under the conditions of an argon gas pressure of about 3×10 −4  Torr, a sputtering power of about 100 W, and room temperature, thereby forming the SiO 2  or Si 3 N 4  insulating thin film layer  26 .  FIG. 16A  is a plan view, and  FIG. 16B  is a diagram illustrating the formed insulating thin film layer  26 . 
     As shown in  FIGS. 17A and 17B , the insulating thin film layer  26  is partially removed to form an insulating protective layer  28 . All the portions of the insulating thin film layer  26  except a portion serving as the insulating protective layer are removed by a lift-up method, using dry etching, such as an Ar gas ion milling method, or a negative photosensitive mask, thereby forming the insulating protective layer  28 . The insulating protective layer  28  covers the entire surface of the magnetoresistive element  20  and portions of the horizontal and vertical electrode pads  24   a  and  24   b .  FIG. 17A  is a plan view, and  FIG. 17B  is a diagram illustrating the formed insulating protective layer  28 . 
     As shown in  FIGS. 18A and 18B , a photosensitive magnetic bead thin film  30  is deposited on the substrate  1 , the electrode pads  24   a  and  24   b , and the insulating protective layer  28 . The photosensitive magnetic bead thin film  30  is formed with a thickness of about 1 to 2 μm (preferably, about 1.5 μm) at room temperature by spin coating at a speed of about 3000 to 5000 rpm.  FIG. 18A  is a plan view, and  FIG. 18B  is a diagram illustrating the deposited photosensitive magnetic bead thin film  30 . 
     As shown in  FIGS. 19A and 19B , the photosensitive magnetic bead thin film  30  is selectively removed to form a magnetic bead limiting layer  32 . The magnetic bead limiting layer  32  is formed by removing all the portions of the photosensitive magnetic bead thin film  30  except a portion serving as the magnetic bead limiting layer  32 , using a lift-up method and a negative photosensitive mask. The magnetic bead limiting layer  32  can impound a magnetic bead analysis solution therein such that the magnetic bead analysis solution is positioned close to the magnetoresistive element  20 .  FIG. 19A  is a plan view and  FIG. 19B  is a diagram illustrating the formed magnetic bead limiting layer  32 . 
     The magnetic field detector manufactured through the above-mentioned processes includes the magnetoresistive element  20  with a circular ring shape that is grown on the Si single crystal substrate  1 , the horizontal electrodes  24   a  for applying a current to the magnetoresistive element  20  and measuring a horizontal voltage, and the vertical electrodes  24   b  for measuring a vertical voltage, as shown in  FIGS. 19A and 19B . The insulating protective layer  28  is deposited on the entire surface of the magnetoresistive element  20  and portions of the electrodes  24   a  and  24   b , and the magnetic bead limiting layer  32  is formed on the magnetoresistive element  20 , the electrodes  24   a  and  24   b , and the insulating protective layer  28 . The horizontal electrodes  24   a  mean the horizontal electrode pads described with reference to  FIGS. 15A and 15B , and the vertical electrodes  24   b  mean the vertical electrode pads described with reference to  FIGS. 15A and 15B . 
       FIGS. 20A to 22B  are diagrams illustrating modifications of the second embodiment of the invention. 
     A magnetoresistive element shown in  FIGS. 20A and 20B  is similar to that shown in  FIGS. 19A and 19B  except that it has an elliptical ring shape, and the magnetoresistive element is manufactured by the same method as described above. In  FIGS. 20A and 20B , reference numeral  34  denotes the magnetoresistive element with the elliptical ring shape.  FIG. 20A  is a plan view, and  FIG. 20B  is a diagram illustrating a magnetic field detector including the magnetoresistive element  34  with the elliptical ring shape. 
     A magnetoresistive element shown in  FIGS. 21A and 21B  is similar to that shown in  FIGS. 19A and 19B  except that it has a square ring shape, and the magnetoresistive element is manufactured by the same method as described above. In  FIGS. 21A and 21B , reference numeral  36  denotes the magnetoresistive element with the square ring shape.  FIG. 21A  is a plan view, and  FIG. 21B  is a diagram illustrating a magnetic field detector including the magnetoresistive element  36  with the square ring shape. 
     A magnetoresistive element shown in  FIGS. 22A and 22B  is similar to that shown in  FIGS. 19A and 19B  except that it has a rectangular ring shape, and the magnetoresistive element is manufactured by the same method as described above. In  FIGS. 22A and 22B , reference numeral  38  denotes the magnetoresistive element with the rectangular ring shape.  FIG. 22A  is a plan view, and  FIG. 22B  is a diagram illustrating a magnetic field detector including the magnetoresistive element  38  with the rectangular ring shape. 
     In the magnetic field detectors manufactured according to the second embodiment and the modifications thereof, since a stray field is formed inside the magnetoresistive element having a circular ring shape, the stray field is circulated in the magnetoresistive element, but does not leak to the outside of the element. As a result, there is no mutual interference due to the stray field. 
     When the magnetic bead is magnetized by a magnetic field applied from the outside, a weak magnetic field is generated, and the generated magnetic field has an effect on the magnetization direction of the free layer, which causes a variation in the output voltage of the magnetoresistive element. The variation in the output voltage makes it possible to detect the magnetic bead. 
     Third Embodiment 
       FIGS. 23A to 29B  are diagrams sequentially illustrating a method of manufacturing a magnetic field detector according to a third embodiment of the invention. The third embodiment differs from the first embodiment in that a plurality of magnetoresistive elements having circular ring shapes (that is, a one-dimensional array structure) are arranged in a line. Since a manufacturing method according to the third embodiment is most similar to that according to the first embodiment, those skilled in the art can easily understand the manufacturing method according to the third embodiment. 
     First, a giant magnetoresistive thin film  2  is formed on a substrate  1  by vapor deposition and then etched to form an array of a plurality of magnetoresistive elements  20  with circular ring shapes (see  FIGS. 23A and 23B ). The plurality of magnetoresistive elements  20  are arrayed in a line at equal distances. In the etching process, dry etching, such as an Ar gas ion milling method, is performed on the giant magnetoresistive thin film  2  shown in  FIG. 1C  to etch all the portions of the film except circular ring portions.  FIG. 23A  is a plan view, and  FIG. 23B  is a diagram illustrating the arrangement of the substrate  1  and the plurality of magnetoresistive elements  20 . 
     As shown in  FIGS. 24A and 24B , a metal thin film layer  22  formed of Au is deposited on the substrate  1  and the plurality of magnetoresistive elements  20 . For example, Au is grown with a thickness of about 150 nm by a sputtering deposition method under the conditions of an argon gas pressure of about 3×10 −4  Torr, a sputtering power of about 60 W, and room temperature, thereby forming the metal thin film layer  22 .  FIG. 24A  is a plan view, and FIG.  24 B is a diagram illustrating the deposited metal thin film layer  22 . The metal thin film layer  22  may be formed of Ta. 
     As shown in  FIGS. 25A and 25B , electrode pads  24  are formed. The electrode pads  24  are used as electrodes for applying a current and measuring a horizontal voltage. The electrode pads  24  are formed by a lift-up method, using dry etching or a negative photosensitive mask. In this case, all the portions of the metal thin film layer  22  except for portions serving as the electrode pads  24  are removed. For example, the electrode pads  24  include electrode pads connecting the plurality of magnetoresistive elements  20  in the horizontal direction, and electrode pads extending from the rightmost and leftmost magnetoresistive elements  20  to the outside in the horizontal direction.  FIG. 25A  is a plan view, and  FIG. 25B  is a diagram illustrating the formed electrode pads  24 . 
     As shown in  FIGS. 26A and 26B , an insulating thin film layer  26  is deposited on the substrate  1 , the magnetoresistive elements  20 , and the electrode pads  24 . The insulating thin film layer  26  is formed of SiO 2  or Si 3 N 4 . In order to prevent the magnetoresistive elements  20  and the electrode pads  24  from corroded by an analytical solution, for example, SiO 2  or Si 3 N 4  is grown with a thickness of about 50 to 300 nm (preferably, 150 nm) by a sputtering deposition method under the conditions of an argon gas pressure of about 3×10 −4  Torr, a sputtering power of about 100 W, and room temperature, thereby forming the SiO 2  or Si 3 N 4  insulating thin film layer  26 .  FIG. 26A  is a plan view, and  FIG. 26B  is a diagram illustrating the formed insulating thin film layer  26 . 
     As shown in  FIGS. 27A and 27B , the insulating thin film layer  26  is partially removed to form an insulating protective layer  28 . All the portions of the insulating thin film layer  26  except a portion serving as the insulating protective layer are removed by a lift-up method, using dry etching, such as an Ar gas ion milling method, or a negative photosensitive mask, thereby forming the insulating protective layer  28 .  FIG. 27A  is a plan view, and  FIG. 27B  is a diagram illustrating the formed insulating protective layer  28 . 
     As shown in  FIGS. 28A and 28B , a photosensitive magnetic bead thin film  30  is deposited on the substrate  1 , the electrode pads  24 , and the insulating protective layer  28 . The photosensitive magnetic bead thin film  30  is formed with a thickness of about 1 to 2 μm (preferably, about 1.5 μm) at room temperature by spin coating at a speed of about 3000 to 5000 rpm.  FIG. 28A  is a plan view, and  FIG. 28B  is a diagram illustrating the deposited photosensitive magnetic bead thin film  30 . 
     As shown in  FIGS. 29A and 29B , the photosensitive magnetic bead thin film  30  is selectively removed to form a magnetic bead limiting layer  32 . The magnetic bead limiting layer  32  is formed by removing all the portions of the photosensitive magnetic bead thin film  30  except a portion serving as the magnetic bead limiting layer  32 , using a lift-up method and a negative photosensitive mask. The magnetic bead limiting layer  32  can impound a magnetic bead analysis solution therein such that the magnetic bead analysis solution is positioned close to the magnetoresistive element  20 .  FIG. 29A  is a plan view, and  FIG. 29B  is a diagram illustrating the formed magnetic bead limiting layer  32 . 
     The magnetic field detector manufactured through the above-mentioned processes includes the plurality of magnetoresistive elements  20  with circular ring shapes that are grown on the Si single crystal substrate  1 , and the electrodes  24  for applying a current to the plurality of magnetoresistive elements  20  and measuring a horizontal voltage, as shown in  FIGS. 29A and 29B . The insulating protective layer  28  is deposited on the entire surface of each of the plurality of magnetoresistive elements  20  and portions of the electrodes  24 , and the magnetic bead limiting layer  32  is formed on the plurality of magnetoresistive elements  20 , the electrodes  24 , and the insulating protective layer  28 . The electrodes  24  mean the electrode pads described with reference to  FIGS. 25A and 25B , and may be called horizontal electrodes. 
       FIGS. 30A to 32B  are diagrams illustrating modifications of the third embodiment of the invention. 
     A magnetoresistive element shown in  FIGS. 30A and 30B  is similar to that shown in  FIGS. 29A and 29B  except that it has an elliptical ring shape, and the magnetoresistive element is manufactured by the same method as described above. In  FIGS. 30A and 30B , reference numeral  34  denotes the magnetoresistive element with the elliptical ring shape.  FIG. 30A  is a plan view, and  FIG. 30B  is a diagram illustrating a magnetic field detector including a plurality of magnetoresistive elements  34  with the elliptical ring shapes. 
     A magnetoresistive element shown in  FIGS. 31A and 31B  is similar to that shown in  FIGS. 29A and 29B  except that it has a square ring shape, and the magnetoresistive element is manufactured by the same method as described above. In  FIGS. 31A and 31B , reference numeral  36  denotes the magnetoresistive element with the square ring shape.  FIG. 31A  is a plan view, and  FIG. 31B  is a diagram illustrating a magnetic field detector including a plurality of magnetoresistive elements  36  with the square ring shapes. 
     A magnetoresistive element shown in  FIGS. 32A and 32B  is similar to that shown in  FIGS. 29A and 29B  except that it has a rectangular ring shape, and the magnetoresistive element is manufactured by the same method as described above. In  FIGS. 32A and 32B , reference numeral  38  denotes the magnetoresistive element with the rectangular ring shape.  FIG. 32A  is a plan view, and  FIG. 32B  is a diagram illustrating a magnetic field detector including a plurality of magnetoresistive elements  36  with the rectangular ring shapes. 
     In the magnetic field detectors manufactured according to the third embodiment and the modifications thereof, since a stray field is formed inside the magnetoresistive element having a circular ring shape, the stray field is circulated in the magnetoresistive element, and does not leak to the outside of the element. As a result, there is no mutual interference due to the stray field. 
     When the magnetic bead is magnetized by a magnetic field applied from the outside, a weak magnetic field is generated, and the generated magnetic field has an effect on the magnetization direction of the free layer, which causes a variation in the output voltage of the magnetoresistive element. The variation in the output voltage makes it possible to detect the magnetic bead. 
     Fourth Embodiment 
       FIGS. 33A to 39B  are diagrams sequentially illustrating a method of manufacturing a magnetic field detector according to a fourth embodiment of the invention. The magnetic field detector according to the fourth embodiment differs from that according to the second embodiment in that a plurality of magnetoresistive elements having circular ring shapes are arranged in a line (that is, a one-dimensional array structure). Since a manufacturing method according to the fourth embodiment is most similar to that according to the second embodiment, those skilled in the art can easily understand the manufacturing method according to the fourth embodiment. 
     First, a giant magnetoresistive thin film  2  is formed on a substrate  1  by vapor deposition and then etched to form an array of a plurality of magnetoresistive elements  20  with circular ring shapes (see  FIGS. 33A and 33B ). The plurality of magnetoresistive elements  20  are arrayed in a line at equal distances (that is, a one-dimensional array structure). In the etching process, dry etching, such as an Ar gas ion milling method, is performed on the giant magnetoresistive thin film  2  shown in  FIG. 1C  to selectively etch all the portions of the film except circular ring portions.  FIG. 33A  is a plan view, and  FIG. 33B  is a diagram illustrating the arrangement of the substrate  1  and the plurality of magnetoresistive elements  20 . 
     As shown in  FIGS. 34A and 34B , a metal thin film layer  22  formed of Au is deposited on the substrate  1  and the plurality of magnetoresistive elements  20 . For example, Au is grown with a thickness of about 150 nm by a sputtering deposition method under the conditions of an argon gas pressure of about 3×10 −4  Torr, a sputtering power of about 60 W, and room temperature, thereby forming the metal thin film layer  22 .  FIG. 34A  is a plan view, and  FIG. 34B  is a diagram illustrating the deposited metal thin film layer  22 . The metal thin film layer  22  may be formed of Ta. 
     As shown in  FIGS. 35A and 35B , horizontal electrode pads  24   a  and vertical electrode pads  24   b  are formed. The horizontal electrode pads  24   a  are used as electrodes for applying a current and measuring a horizontal voltage. The vertical electrode pads  24   b  are used as electrodes for measuring a vertical voltage. The horizontal and vertical electrode pads  24   a  and  24   b  are formed by a lift-up method, using dry etching or a negative photosensitive mask. In this case, all the portions of the metal thin film layer  22  except for portions serving as the electrode pads  24   a  and  24   b  are removed. For example, the electrode pads  24   a  include electrode pads connecting the plurality of magnetoresistive elements  20  in the horizontal direction, and electrode pads extending from the rightmost and leftmost magnetoresistive elements  20  to the outside in the horizontal direction. The vertical electrode pads  24   b  are formed for each of the magnetoresistive elements  20  so as to be orthogonal to the horizontal electrode pads  24   a . That is, a line lining the horizontal electrode pads  24   a  in the horizontal direction is orthogonal to a line linking the vertical electrode pads  24   b  in the vertical direction.  FIG. 35A  is a plan view, and  FIG. 35B  is a diagram illustrating the formed electrode pads  24   a  and  24   b.    
     As shown in  FIGS. 36A and 36B , an insulating thin film layer  26  is deposited on the substrate  1 , the magnetoresistive elements  20 , and the electrode pads  24   a  and  24   b . The insulating thin film layer  26  is formed of SiO 2  or Si 3 N 4 . In order to prevent the magnetoresistive elements  20  and the electrode pads  24   a  and  24   b  from being corroded by an analytical solution, for example, SiO 2  or Si 3 N 4  is grown with a thickness of about 50 to 300 nm (preferably, 150 nm) by a sputtering deposition method under the conditions of an argon gas pressure of about 3×10 −4  Torr, a sputtering power of about 100 W, and room temperature, thereby forming the SiO 2  or Si 3 N 4  insulating thin film layer  26 .  FIG. 36A  is a plan view, and  FIG. 36B  is a diagram illustrating the formed insulating thin film layer  26 . 
     As shown in  FIGS. 37A and 37B , the insulating thin film layer  26  is partially removed to form an insulating protective layer  28 . All the portions of the insulating thin film layer  26  except a portion serving as the insulating protective layer are removed by a lift-up method, using dry etching, such as an Ar gas ion milling method, or a negative photosensitive mask, thereby forming the insulating protective layer  28 .  FIG. 37A  is a plan view, and  FIG. 37B  is a diagram illustrating the formed insulating protective layer  28 . 
     As shown in  FIGS. 38A and 38B , a photosensitive magnetic bead thin film  30  is deposited on the substrate  1 , the electrode pads  24   a  and  24   b , and the insulating protective layer  28 . The photosensitive magnetic bead thin film  30  is formed with a thickness of about 1 to 2 μm (preferably, about 1.5 μm) at room temperature by spin coating at a speed of about 3000 to 5000 rpm.  FIG. 38A  is a plan view, and  FIG. 38B  is a diagram illustrating the deposited photosensitive magnetic bead thin film  30 . 
     As shown in  FIGS. 39A and 39B , the photosensitive magnetic bead thin film  30  is selectively removed to form a magnetic bead limiting layer  32 . The magnetic bead limiting layer  32  is formed by removing all the portions of the photosensitive magnetic bead thin film  30  except a portion serving as the magnetic bead limiting layer  32 , using a lift-up method and a negative photosensitive mask. The magnetic bead limiting layer  32  can impound a magnetic bead analysis solution therein such that the magnetic bead analysis solution is positioned close to the magnetoresistive element  20 .  FIG. 39A  is a plan view and  FIG. 39B  is a diagram illustrating the formed magnetic bead limiting layer  32 . 
     The magnetic field detector manufactured through the above-mentioned processes includes the plurality of magnetoresistive elements  20  with circular ring shapes that are grown on the Si single crystal substrate  1 , the horizontal electrodes  24   a  for applying a current to the plurality of magnetoresistive elements  20  and measuring a horizontal voltage, and the vertical electrodes  24   b  for measuring a vertical voltage, as shown in  FIGS. 39A and 39B . The insulating protective layer  28  is deposited on the entire surface of each of the plurality of magnetoresistive elements  20  and portions of the electrodes  24   a  and  24   b , and the magnetic bead limiting layer  32  is formed on the plurality of magnetoresistive elements  20 , the electrodes  24   a  and  24   b , and the insulating protective layer  28 . The horizontal electrodes  24   a  mean the horizontal electrode pads described with reference to  FIGS. 35A and 35B , and the vertical electrodes  24   b  mean the vertical electrode pads described with reference to  FIGS. 35A and 35B . 
       FIGS. 40A to 42B  are diagrams illustrating modifications of the fourth embodiment of the invention. 
     A magnetoresistive element shown in  FIGS. 40A and 40B  is similar to that shown in  FIGS. 39A and 39B  except that it has an elliptical ring shape, and the magnetoresistive element is manufactured by the same method as described above. In  FIGS. 40A and 40B , reference numeral  34  denotes the magnetoresistive element with the elliptical ring shape.  FIG. 40A  is a plan view, and  FIG. 40B  is a diagram illustrating a magnetic field detector including a plurality of magnetoresistive elements  34  with the elliptical ring shapes. 
     A magnetoresistive element shown in  FIGS. 41A and 41B  is similar to that shown in  FIGS. 39A and 39B  except that it has a square ring shape, and the magnetoresistive element is manufactured by the same method as described above. In  FIGS. 41A and 41B , reference numeral  36  denotes the magnetoresistive element with the square ring shape.  FIG. 41A  is a plan view, and  FIG. 41B  is a diagram illustrating a magnetic field detector including a plurality of magnetoresistive elements  36  with the square ring shapes. 
     A magnetoresistive element shown in  FIGS. 42A and 42B  is similar to that shown in  FIGS. 39A and 39B  except that it has a rectangular ring shape, and the magnetoresistive element is manufactured by the same method as described above. In  FIGS. 42A and 42B , reference numeral  38  denotes the magnetoresistive element with the rectangular ring shape.  FIG. 42A  is a plan view, and  FIG. 42B  is a diagram illustrating a magnetic field detector including a plurality of magnetoresistive elements  38  with the rectangular ring shapes. 
     In the magnetic field detectors manufactured according to the fourth embodiment and the modifications thereof, since a stray field is formed inside the magnetoresistive element having a circular ring shape, the stray field is circulated in the magnetoresistive element, but does not leak to the outside of the element. As a result, there is no mutual interference due to the stray field. 
     When the magnetic bead is magnetized by a magnetic field applied from the outside, a weak magnetic field is generated, and the generated magnetic field has an effect on the magnetization direction of the free layer, which causes a variation in the output voltage of the magnetoresistive element. The variation in the output voltage makes it possible to detect the magnetic bead. 
     Fifth Embodiment 
       FIGS. 43A to 49B  are diagrams sequentially illustrating a method of manufacturing a magnetic field detector according to a fifth embodiment of the invention. The fifth embodiment differs from the third embodiment in that a plurality of magnetoresistive elements having circular ring shapes are arranged in a two-dimensional array (that is, in a matrix). Since a manufacturing method according to the fifth embodiment is most similar to that according to the third embodiment, those skilled in the art can easily understand the manufacturing method according to the fifth embodiment. 
     First, a giant magnetoresistive thin film  2  is formed on a substrate  1  by vapor deposition and then etched to form a plurality of magnetoresistive elements  20  with circular ring shapes in a matrix (see  FIGS. 43A and 43B ). The plurality of magnetoresistive elements  20  are arrayed in a matrix at equal distances. In the etching process, dry etching, such as an Ar gas ion milling method, is performed on the giant magnetoresistive thin film  2  shown in  FIG. 1C  to etch all the portions of the film except circular ring portions.  FIG. 43A  is a plan view, and  FIG. 43B  is a diagram illustrating the arrangement of the substrate  1  and the plurality of magnetoresistive elements  20 . 
     As shown in  FIGS. 44A and 44B , a metal thin film layer  22  formed of Au is deposited on the substrate  1  and the plurality of magnetoresistive elements  20 . For example, Au is grown with a thickness of about 50 to 300 nm (preferably 150 nm) by a sputtering deposition method under the conditions of an argon gas pressure of about 3×10 −4  Torr, a sputtering power of about 60 W, and room temperature, thereby forming the metal thin film layer  22 .  FIG. 44A  is a plan view, and  FIG. 44B  is a diagram illustrating the deposited metal thin film layer  22 . The metal thin film layer  22  may be formed of Ta. 
     As shown in  FIGS. 45A and 45B , electrode pads  24  are formed. The electrode pads  24  are used as electrodes for applying a current and measuring a horizontal voltage. The electrode pads  24  are formed by a lift-up method, using dry etching or a negative photosensitive mask. In this case, all the portions of the metal thin film layer  22  except for portions serving as the electrode pads  24  are removed. For example, the electrode pads  24  are linearly formed with the magnetoresistive elements  20  interposed therebetween.  FIG. 45A  is a plan view, and  FIG. 45B  is a diagram illustrating the formed electrode pads  24 . Of course, the arrangement of the electrode pads  24  is not limited to that shown in  FIGS. 45A and 45B , but the electrode pads  24  extending to the outside in  FIGS. 45A and 45B  may be disposed at different positions as long as the electrode pads  24  can be linearly arranged. 
     As shown in  FIGS. 46A and 46B , an insulating thin film layer  26  is deposited on the substrate  1 , the plurality of magnetoresistive elements  20 , and the electrode pads  24 . The insulating thin film layer  26  is formed of SiO 2  or Si 3 N 4 . In order to prevent the magnetoresistive elements  20  and the electrode pads  24  from corroded by an analytical solution, for example, SiO 2  or Si 3 N 4  is grown with a thickness of about 50 to 300 nm (preferably, 150 nm) by a sputtering deposition method under the conditions of an argon gas pressure of about 3×10 −4  Torr, a sputtering power of about 100 W, and room temperature, thereby forming the SiO 2  or Si 3 N 4  insulating thin film layer  26 .  FIG. 46A  is a plan view, and  FIG. 46B  is a diagram illustrating the formed insulating thin film layer  26 . 
     As shown in  FIGS. 47A and 47B , the insulating thin film layer  26  is partially removed to form an insulating protective layer  28 . All the portions of the insulating thin film layer  26  except a portion serving as the insulating protective layer are removed by a lift-up method, using dry etching, such as an Ar gas ion milling method, or a negative photosensitive mask, thereby forming the insulating protective layer  28 .  FIG. 47A  is a plan view, and  FIG. 47B  is a diagram illustrating the formed insulating protective layer  28 . 
     As shown in  FIGS. 48A and 48B , a photosensitive magnetic bead thin film  30  is deposited on the substrate  1 , the electrode pads  24 , and the insulating protective layer  28 . The photosensitive magnetic bead thin film  30  is formed with a thickness of about 1 to 2 μm (preferably, about 1.5 μm) at room temperature by spin coating at a speed of about 3000 to 5000 rpm.  FIG. 48A  is a plan view, and  FIG. 48B  is a diagram illustrating the deposited photosensitive magnetic bead thin film  30 . 
     As shown in  FIGS. 49A and 49B , the photosensitive magnetic bead thin film  30  is selectively removed to form a magnetic bead limiting layer  32 . The magnetic bead limiting layer  32  is formed by removing all the portions of the photosensitive magnetic bead thin film  30  except a portion serving as the magnetic bead limiting layer  32 , using a lift-up method and a negative photosensitive mask. The magnetic bead limiting layer  32  can impound a magnetic bead analysis solution therein such that the magnetic bead analysis solution is positioned close to the magnetoresistive element  20 .  FIG. 49A  is a plan view, and  FIG. 49B  is a diagram illustrating the formed magnetic bead limiting layer  32 . 
     The magnetic field detector manufactured through the above-mentioned processes includes the plurality of magnetoresistive elements  20  with circular ring shapes that are grown on the Si single crystal substrate  1 , and the electrodes  24  for applying a current to the plurality of magnetoresistive elements  20  and measuring a horizontal voltage, as shown in  FIGS. 49A and 49B . The insulating protective layer  28  is deposited on the entire surface of each of the plurality of magnetoresistive elements  20  and portions of the electrodes  24 , and the magnetic bead limiting layer  32  is formed on the plurality of magnetoresistive elements  20 , the electrodes  24 , and the insulating protective layer  28 . The electrodes  24  mean the electrode pads described with reference to  FIGS. 45A and 45B , and may be called horizontal electrodes. 
       FIGS. 50A to 52B  are diagrams illustrating modifications of the fifth embodiment of the invention. 
     A magnetoresistive element shown in  FIGS. 50A and 50B  is similar to that shown in  FIGS. 49A and 49B  except that it has an elliptical ring shape, and the magnetoresistive element is manufactured by the same method as described above. In  FIGS. 50A and 50B , reference numeral  34  denotes the magnetoresistive element with the elliptical ring shape.  FIG. 50A  is a plan view, and  FIG. 50B  is a diagram illustrating a magnetic field detector including a plurality of magnetoresistive elements  34  with the elliptical ring shapes. 
     A magnetoresistive element shown in  FIGS. 51A and 51B  is similar to that shown in  FIGS. 49A and 49B  except that it has a square ring shape, and the magnetoresistive element is manufactured by the same method as described above. In  FIGS. 51A and 51B , reference numeral  36  denotes the magnetoresistive element with the square ring shape.  FIG. 51A  is a plan view, and  FIG. 51B  is a diagram illustrating a magnetic field detector including a plurality of magnetoresistive elements  36  with the square ring shapes. 
     A magnetoresistive element shown in  FIGS. 52A and 52B  is similar to that shown in  FIGS. 49A and 49B  except that it has a rectangular ring shape, and the magnetoresistive element is manufactured by the same method as described above. In  FIGS. 52A and 52B , reference numeral  38  denotes the magnetoresistive element with the rectangular ring shape.  FIG. 52A  is a plan view, and  FIG. 52B  is a diagram illustrating a magnetic field detector including a plurality of magnetoresistive elements  36  with the rectangular ring shapes. 
     In the magnetic field detectors manufactured according to the fifth embodiment and the modifications thereof, since a stray field is formed inside the magnetoresistive element having a circular ring shape, the stray field is circulated in the magnetoresistive element, and does not leak to the outside of the element. As a result, there is no mutual interference due to the stray field. 
     When the magnetic bead is magnetized by a magnetic field applied from the outside, a weak magnetic field is generated, and the generated magnetic field has an effect on the magnetization direction of the free layer, which causes a variation in the output voltage of the magnetoresistive element. The variation in the output voltage makes it possible to detect the magnetic bead. 
     Sixth Embodiment 
       FIGS. 53A to 59B  are diagrams sequentially illustrating a method of manufacturing a magnetic field detector according to a sixth embodiment of the invention. The sixth embodiment differs from the fourth embodiment in that a plurality of magnetoresistive elements having circular ring shapes are arranged in a two-dimensional array (that is, in a matrix). Since a manufacturing method according to the sixth embodiment is most similar to that according to the fourth embodiment, those skilled in the art can easily understand the manufacturing method according to the sixth embodiment. 
     First, a giant magnetoresistive thin film  2  is formed on a substrate  1  by vapor deposition and then etched to form a plurality of magnetoresistive elements  20  with circular ring shapes in a matrix (see  FIGS. 53A and 53B ). The plurality of magnetoresistive elements  20  are arrayed in a matrix at equal distances. In the etching process, dry etching, such as an Ar gas ion milling method, is performed on the giant magnetoresistive thin film  2  shown in  FIG. 1C  to etch all the portions of the film except circular ring portions.  FIG. 53A  is a plan view, and  FIG. 53B  is a diagram illustrating the arrangement of the substrate  1  and the plurality of magnetoresistive elements  20 . 
     As shown in  FIGS. 54A and 54B , a metal thin film layer  22  formed of Au is deposited on the substrate  1  and the plurality of magnetoresistive elements  20 . For example, Au is grown with a thickness of about 150 nm by a sputtering deposition method under the conditions of an argon gas pressure of about 3×10 −4  Torr, a sputtering power of about 60 W, and room temperature, thereby forming the metal thin film layer  22 .  FIG. 54A  is a plan view, and  FIG. 54B  is a diagram illustrating the deposited metal thin film layer  22 . The metal thin film layer  22  may be formed of Ta. 
     As shown in  FIGS. 55A and 55B , horizontal electrode pads  24   a  and vertical electrode pads  24   b  are formed. The horizontal electrode pads  24   a  are used as electrodes for applying a current and measuring a horizontal voltage. The vertical electrode pads  24   b  are used as electrodes for measuring a vertical voltage. The horizontal and vertical electrode pads  24   a  and  24   b  are formed by a lift-up method, using dry etching or a negative photosensitive mask. In this case, all the portions of the metal thin film layer  22  except for portions serving as the electrode pads  24   a  and  24   b  are removed. For example, the horizontal electrode pads  24   a  are formed in the horizontal (row) direction with respect to the magnetoresistive elements  20 , and the vertical electrode pads  24   b  are formed in the vertical (column) direction with respect to the magnetoresistive elements  20 .  FIG. 55A  is a plan view, and  FIG. 55B  is a diagram illustrating the formed electrode pads  24   a  and  24   b.    
     As shown in  FIGS. 56A and 56B , an insulating thin film layer  26  is deposited on the substrate  1 , the magnetoresistive elements  20 , and the electrode pads  24   a  and  24   b . The insulating thin film layer  26  is formed of SiO 2  or Si 3 N 4 . In order to prevent the magnetoresistive elements  20  and the electrode pads  24   a  and  24   b  from being corroded by an analytical solution, for example, SiO 2  or Si 3 N 4  is grown with a thickness of about 50 to 300 nm (preferably, 150 nm) by a sputtering deposition method under the conditions of an argon gas pressure of about 3×10 −4  Torr, a sputtering power of about 100 W, and room temperature, thereby forming the SiO 2  or Si 3 N 4  insulating thin film layer  26 .  FIG. 56A  is a plan view, and  FIG. 56B  is a diagram illustrating the formed insulating thin film layer  26 . 
     As shown in  FIGS. 57A and 57B , the insulating thin film layer  26  is partially removed to form an insulating protective layer  28 . All the portions of the insulating thin film layer  26  except a portion serving as the insulating protective layer are removed by a lift-up method, using dry etching, such as an Ar gas ion milling method, or a negative photosensitive mask, thereby forming the insulating protective layer  28 .  FIG. 57A  is a plan view, and  FIG. 57B  is a diagram illustrating the formed insulating protective layer  28 . 
     As shown in  FIGS. 58A and 58B , a photosensitive magnetic bead thin film  30  is deposited on the substrate  1 , the electrode pads  24   a  and  24   b , and the insulating protective layer  28 . The photosensitive magnetic bead thin film  30  is formed with a thickness of about 1 to 2 μm (preferably, about 1.5 μm) at room temperature by spin coating at a speed of about 3000 to 5000 rpm.  FIG. 58A  is a plan view, and  FIG. 58B  is a diagram illustrating the deposited photosensitive magnetic bead thin film  30 . 
     As shown in  FIGS. 59A and 59B , the photosensitive magnetic bead thin film  30  is selectively removed to form a magnetic bead limiting layer  32 . The magnetic bead limiting layer  32  is formed by removing all the portions of the photosensitive magnetic bead thin film  30  except a portion serving as the magnetic bead limiting layer  32 , using a lift-up method and a negative photosensitive mask. The magnetic bead limiting layer  32  can impound a magnetic bead analysis solution therein such that the magnetic bead analysis solution is positioned close to the magnetoresistive element  20 .  FIG. 59A  is a plan view, and  FIG. 59B  is a diagram illustrating the formed magnetic bead limiting layer  32 . 
     The magnetic field detector manufactured through the above-mentioned processes includes the plurality of magnetoresistive elements  20  with circular ring shapes that are grown on the Si single crystal substrate  1 , the horizontal electrodes  24   a  for applying a current to the plurality of magnetoresistive elements  20  and measuring a horizontal voltage, and the vertical electrodes  24   b  for measuring a vertical voltage, as shown in  FIGS. 59A and 59B . The insulating protective layer  28  is deposited on the entire surface of each of the plurality of magnetoresistive elements  20  and portions of the electrodes  24   a  and  24   b , and the magnetic bead limiting layer  32  is formed on the plurality of magnetoresistive elements  20 , the electrodes  24   a  and  24   b , and the insulating protective layer  28 . The horizontal electrodes  24   a  mean the horizontal electrode pads described with reference to  FIGS. 55A and 55B , and the vertical electrodes  24   b  mean the vertical electrode pads described with reference to  FIGS. 55A and 55B . 
       FIGS. 60A to 62B  are diagrams illustrating modifications of the sixth embodiment of the invention. 
     A magnetoresistive element shown in  FIGS. 60A and 60B  is similar to that shown in  FIGS. 59A and 59B  except that it has an elliptical ring shape, and the magnetoresistive element is manufactured by the same method as described above. In  FIGS. 60A and 60B , reference numeral  34  denotes the magnetoresistive element with the elliptical ring shape.  FIG. 60A  is a plan view, and  FIG. 60B  is a diagram illustrating a magnetic field detector including a plurality of magnetoresistive elements  34  with the elliptical ring shapes. 
     A magnetoresistive element shown in  FIGS. 61A and 61B  is similar to that shown in  FIGS. 59A and 59B  except that it has a square ring shape, and the magnetoresistive element is manufactured by the same method as described above. In  FIGS. 61A and 61B , reference numeral  36  denotes the magnetoresistive element with the square ring shape.  FIG. 61A  is a plan view, and  FIG. 61B  is a diagram illustrating a magnetic field detector including a plurality of magnetoresistive elements  36  with the square ring shapes. 
     A magnetoresistive element shown in  FIGS. 62A and 62B  is similar to that shown in  FIGS. 59A and 59B  except that it has a rectangular ring shape, and the magnetoresistive element is manufactured by the same method as described above. In  FIGS. 62A and 62B , reference numeral  38  denotes the magnetoresistive element with the rectangular ring shape.  FIG. 62A  is a plan view, and  FIG. 62B  is a diagram illustrating a magnetic field detector including a plurality of magnetoresistive elements  38  with the rectangular ring shapes. 
     In the magnetic field detectors manufactured according to the sixth embodiment and the modifications thereof, since a stray field is formed inside the magnetoresistive element having a circular ring shape, the stray field is circulated in the magnetoresistive element, but does not leak to the outside of the element. As a result, there is no mutual interference due to the stray field. 
     When the magnetic bead is magnetized by a magnetic field applied from the outside, a weak magnetic field is generated, and the generated magnetic field has an effect on the magnetization direction of the free layer, which causes a variation in the output voltage of the magnetoresistive element. The variation in the output voltage makes it possible to detect the magnetic bead. 
     In the above-described embodiments and modifications, as the size of the magnetoresistive element decreases, magnetic bead sensing capability (that is, sensitivity) is improved. However, when the size of the magnetoresistive element is excessively small, it is difficult to actually manufacture a magnetic field detector. Therefore, the above-mentioned dimensions of the magnetoresistive element are determined in consideration of the current capability of manufacturing the element and sensitivity. Of course, when the element manufacturing capability is improved, it may be possible to further reduce the size of the magnetoresistive element. In addition, sensitivity for sensing the magnetic bead is lowered when the size of the magnetoresistive element is out of the above-described numerical range. 
     While the invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. In the above-described embodiments and modifications, the magnetoresistive elements are formed in a circular ring shape, an elliptical ring shape, a square ring shape, and a rectangular ring shape. However, the magnetoresistive elements may be formed in various shapes, such as a pentagonal ring shape, a hexagonal ring shape, and an octagonal ring shape, as long as they can have ring shapes and electrodes can be provided.