Abstract:
A magnetic memory device having a packaged magnetic memory chip is disclosed, which comprises a package structure including a magnetic memory chip, and a magnetic guide of a high-permeability magnetic material, forming a structural member of the package structure.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2002-260656, filed Sep. 5, 2002, the entire contents of which are incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a magnetic memory device.  
           [0004]    2. Description of the Related Art  
           [0005]    A magnetic random access memory (hereinafter, referred to as MRAM) is a general term indicating a solid-state memory in which the recorded information can be rewritten, stored and read out utilizing the direction of magnetization of a ferromagnetic material as an information recording carrier. The memory cell of the MRAM normally has a stacked structure of a plurality of ferromagnetic layers.  
           [0006]    The information is recorded in accordance with whether the relative positions of magnetization of a plurality of ferromagnetic layers constituting the memory cell are parallel or not parallel which correspond to “1” or “0”, respectively, in binary information.  
           [0007]    The information is written by inverting the direction of magnetization of the ferromagnetic members of the cell by the magnetic field generated by the current supplied to the write lines arranged in cross stripes.  
           [0008]    The MRAM basically consumes no power in the storing mode of information, and is a nonvolatile memory in which information can be stored even when the power is switched off.  
           [0009]    The information is read out by the use of the phenomenon of what is called the magneto-resistance effect in which the electric resistance of the memory cell changes in accordance with the relative angle between the direction of magnetization of the ferromagnetic layers constituting the cell and the sense current or the relative angles of magnetization between a plurality of ferromagnetic layers.  
           [0010]    The functions of the MRAM, as compared with the functions of the conventional semiconductor memory using a dielectric material, have the following advantages:  
           [0011]    (1) MRAM is completely nonvolatile and rewritable at least 10 15  times.  
           [0012]    (2) Nondestructive read-out operation is possible and no refresh operation is required, thereby allowing the shortening of the read cycles.  
           [0013]    (3) As compared with the charge storage-type memory cell, the resistance to radiation is high.  
           [0014]    The packing density, the write time and the read-out time per unit area of MRAM are estimated to be generally the same as those for DRAM. Taking advantage of the great feature of nonvolatility, therefore, applications of MRAM are expected as an external storing device for portable equipment, a memory-mixed-LSI and a main memory of the personal computer.  
           [0015]    The MRAM now under study for commercialization includes a device exhibiting the tunnel magneto-resistance (hereinafter referred to as TMR) effect as a memory cell (See “ISSCC 2000 Digest Paper TA7.2”, for example).  
           [0016]    The device exhibiting the TMR effect (hereinafter, referred to as the TRM device) is mainly configured of three layers including a ferromagnetic layer, an insulating layer and a ferromagnetic layer, and the current flows through the insulating layer. The tunnel resistance value changes in proportion to the cosine of the relative angle of magnetization between the two ferromagnetic metal layers and assumes a local maximum value in the case where the two magnetization are not parallel to each other.  
           [0017]    In the NiFe/Co/A1 2 0 3 /Co/NiFe tunnel coupling, for example, the magneto-resistance change rate exceeding 25% is found in the low magnetic field of not more than 50 [Oe] (See “IEEE Trans. Mag., 33,3553 (1997)”, for example).  
           [0018]    Known structures of the TMR device include what is called a spin valve structure in which a counter ferromagnetic member is arranged adjacent to one ferromagnetic member to fix the direction of magnetization to improve the magnetic field sensitivity (See “Jpn. J. Appl. Phys., 36, L200 (1997),” for example), and a structure having double tunnel barriers to improve the bias dependency of the magneto-resistance change rate (See “Jpn. J. Appl. Phys. 36, L1380 (1997),” for example).  
           [0019]    Several problems have yet to be solved, however, to develop a MRAM having a packing density of not less than the order of Gigabits (Gb).  
           [0020]    One of the problems is the reduction in the write current. In the conventional MRAM proposed, a current is supplied to the wiring and the magnetic field generated thereby is used to invert the magnetization of the recording layer of MTJ (magnetic tunneling junction). The strength of the magnetic field generated from the wiring changes with the current value of the wiring and the distance between the wiring and MTJ. In the past reported cases, however, the strength of the magnetic field generated is about several [Oe/mA].  
           [0021]    Further, the threshold value of magnetization inversion of the recording layer of the TMR device (hereinafter, defined as the switching magnetic field Hsw) increases in inverse proportion to the size of the direction of the hard axis of magnetization of the TMR device (hereinafter, referred to as the cell width W) as shown by the equation below.  
           
         Hsw=HswO+A/W  
       
           [0022]    The value A known in the prior art is 10 to 20 [Oe·μm].  
           [0023]    The electromigration is a limiting factor against the reliability of the wiring. The electromigration is accelerated by the wiring current density. The upper limit of the current density of the Al—Cu wiring and the Cu wiring now in use for LSI fabrication is about 10 [mA/μm 2 ] and 100 [mA/μm 2 ], respectively.  
           [0024]    Consider the fabrication under 0.1 μm rule required for realizing the packing density of Gb order, for example. Even in the case where the Cu wiring is used, the upper limit of the current value that can be supplied in the wiring is about 1 mA, which generates a magnetic field of about several [Oe]. In other words, in order to obtain the MRAM of Gb order, the switching magnetic field of the TMR device is required to be reduced to several tens to several [Oe].  
           [0025]    When using the MRAM with such a reduced switching magnetic field, however, careful attention must be paid to avoid a writing error due to external magnetic fields. In mounting the MRAM on an electronic device, for example, it is necessary to consider the magnetic field leaking from the motor, the iron core of the speaker or the permanent magnet, the magnetic field leaking from the hollow core coil of the CRT or the like and the magnetic field leaking from the magnet clip used for the case open/close portion, etc. Also in other life spaces, the magnetic field leaking from the magnet clip may cause the writing error or destroy the data.  
           [0026]    [0026]FIGS. 1 and 2 schematically show the lines of magnetic flux leaking from the permanent magnet and the hollow coil. The survey conducted by the present inventors shows that the magnetic field along the moving radius of the magnet at a position 5 mm horizontally away from the center of a cylindrical ferrite magnet (surface magnetic pole 1300 kG) 5 mm in radius and 2 mm in thickness is about 30 [Oe].  
           [0027]    Generally, household appliances have many magnetic field sources as described above. In using the magnetic memory for these household appliances, therefore, a shield structure is required which protects the recorded magnetic information against the disturbing magnetic fields originating from the environment.  
           [0028]    A magnetic shield structure conventionally proposed is shown in FIG. 3, for example. In this example, a magnetic memory is arranged in a hermetically sealed package magnetically shielded with a high-permeability soft magnetic material such as permalloy (See “U.S. patent Ser. No. 5,939,772, for example).  
           [0029]    A package structure configuring a hermetically sealed space using a magnetic shield material, however, makes a bulky package and is undesirable from the viewpoints of both cost and the packaging technique. Especially, the use of such a package structure for household appliances poses a problem.  
         BRIEF SUMMARY OF THE INVENTION  
         [0030]    According to an aspect of the present invention, there is provided a magnetic memory device having a packaged magnetic memory chip, comprising a package structure including a magnetic memory chip; and a magnetic guide of a high-permeability magnetic material, forming a structural member of the package structure. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0031]    [0031]FIG. 1 is a diagram showing the distribution of leakage magnetic flux of a permanent magnet;  
         [0032]    [0032]FIG. 2 is a diagram showing the distribution of leakage magnetic flux of a hollow coil;  
         [0033]    [0033]FIG. 3 is a diagram showing a conventional magnetic memory of magnetic shield type;  
         [0034]    [0034]FIG. 4 is a schematic diagram showing a magnetic memory device according to each of embodiments of the present invention, arranged in a magnetic field;  
         [0035]    [0035]FIG. 5A is a diagram showing a magnetic memory device according to an embodiment of the present invention;  
         [0036]    [0036]FIG. 5B is a plan view showing a pattern of a lead frame of the magnetic memory device shown in FIG. 5A;  
         [0037]    [0037]FIG. 6 is a diagram showing another example of the pattern of the lead frame;  
         [0038]    [0038]FIG. 7 is a diagram showing a magnetic memory device according to another embodiment of the present invention;  
         [0039]    [0039]FIG. 8 is a diagram showing a die-bonding sheet;  
         [0040]    [0040]FIG. 9 is a diagram showing a magnetic memory device according to a further embodiment of the present invention;  
         [0041]    [0041]FIG. 10 is a diagram showing a magnetic memory device according to a further embodiment of the present invention; and  
         [0042]    [0042]FIG. 11 is a diagram showing a magnetic memory device according to a further embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0043]    [0043]FIG. 4 shows a schematic diagram of a magnetic memory device according to each of embodiments of the present invention, arranged in a magnetic field.  
         [0044]    In the magnetic memory packaging shown in FIG. 4, a magnetic guide  2  of a high permeability magnetic material is arranged in contact with or in close proximity to a magnetic memory  1 . In this way, the effect of the disturbing magnetic field on the magnetic memory  1  can be reduced by the passing of the magnetic flux leakage in the vicinity of the magnetic memory  1  through the magnetic guide  2  without the introduction of the magnetic flux leakage into the magnetic memory  1 .  
         [0045]    The requirements to attain the advantage are summarized into the following:  
         [0046]    (1) The magnetic guide of a high permeability magnetic material is arranged for the magnetic memory.  
         [0047]    (2) The permeability of the magnetic guide is at least ten times larger than that of the storing layer of the magnetic memory.  
         [0048]    (3) The magnetic memory is not hermetically sealed by the magnetic guide, but at least one side of the parallelepipedal magnetic guide is open.  
         [0049]    By meeting the requirements (1) and (2), the magnetic flux of the disturbing magnetic field are prevented substantially from passing through the storing layer of the magnetic memory. In the case where the requirement (3) is met, the package can be prevented from becoming bulky. Further, it is unnecessary to make measurable changes on the conventional packaging technique and thus a magnetic memory device for household use can be obtained without the increase of the cost.  
         [0050]    The optimum distance between the magnetic memory and the magnetic guide, the size, material, permeability, etc. of the magnetic guide are determined in accordance with the specific device structure of the magnetic memory.  
         [0051]    Embodiments of the present invention will be specifically explained below.  
         [0052]    [First Embodiment] 
         [0053]    [0053]FIG. 5A shows a magnetic memory device according to an embodiment of the present invention. The package structure is that of normal SIP package.  
         [0054]    A magnetic memory chip  11  is mounted on a die pad  12   a  of a lead frame  12  and bonded thereon by a die bonding agent (adhesive). The terminal pad of the magnetic memory chip  11  and the inner leads  12   b  of the lead frame  12  are connected to each other by bonding wires  14 , and then the die pad  12   a  of the lead frame  12 , the magnetic memory chip  11 , the inner leads  12   b  of the lead frame  12 , and the bonding wires  14  are molded with a resin  13 .  
         [0055]    [0055]FIG. 5B is a plan view showing a pattern of a lead frame  12  of the magnetic memory device shown in FIG. 5A.  
         [0056]    The lead frame  12  of the magnetic memory device includes the die pad  12   a,  the inner frames  12   b  and the outer frames  12   c.  In the magnetic memory device of this embodiment, the die pad  12   a  of the lead frame  12  is located at a center of the package. However, as shown in FIG. 6, a lead frame may be employed in which the die pad  12   a  of the lead frame  12  is located at a corner of the package. Generally, the material used for this type of lead frame is a Cu material or a Fe material (See Japanese Patent Application KOKAI No. 9-74159, for example).  
         [0057]    According to this embodiment, in contrast, the lead frame  12  is configured of a conductive magnetic material of high permeability. As a result, the lead frame  12  constitutes a magnetic guide and the effect of the disturbing magnetic field on the magnetic memory can be suppressed.  
         [0058]    In order to reduce the contact resistance of the bonding portions between the magnetic memory chip and the inner lead portions  12   a  of the lead frame  12 , the inner lead portions are plated with a precious metal. On the other hand, in order to improve the solderability for connection of the outer lead portions  12   c  of the lead frame  12  with connection pads of an external substrate, the outer lead portions are plated with a precious metal or solder.  
         [0059]    Preferable magnetic materials of the lead frame  12  include the grain-oriented electrical steel, permalloy, a permalloy alloy with various elements added, a metal crystal material such as sendust and Finemet, a metal amorphous foil, a ferrite material, etc. The shield performance is determined by the permeability of these magnetic materials. In a strong magnetic field, however, the saturation magnetization of the film should also be taken into consideration. Thus, a material may be selected in accordance with the required shield performance.  
         [0060]    Let B be the saturation magnetization of the film, μ the specific permeability of the shield material, and Hmax an expected maximum external magnetic field.  
         [0061]    Then, the relation B&lt;4 πμmax is the condition required of the shield material. In the case where Hmax is 20 Oe and μ is 10 3 , for example, B is about 2 T, in which case the grain-oriented electrical steel with Fe as a main component is useful. In the case where Hmax is 50 Oe and μ is 10 3 , on the other hand, B is about 0.7 T, in which case an alloy of the permalloy group is effective. Hmax is determined taking into consideration only the vector component of the direction of easy axis of magnetization of the storing layer of the memory.  
         [0062]    A resin mixed with a high-permeability magnetic particulate may be used as the resin  13 . A suitable high-permeability magnetic material includes an oxide such as ferrite of spinel type or ferrite of garnet type. More specifically, a resin with Mn—Zn ferrite and an additive, or a resin with yttrium iron garnet and an additive is used. The addition of these magnetic materials may reduce the insulation characteristic of the resin. Therefore, a normal resin may be used for the portions contacted by the outer lead portion while a high-permeability magnetic material is added only for the other portions.  
         [0063]    [Second Embodiment] 
         [0064]    The lead frame  12  described with reference to FIGS. 5A, 5B and  6  is a high-permeability magnetic material in its entirety. As an alternative, the surface of the conventional lead frame body of Cu or Fe is covered with a high-permeability magnetic material as a magnetic guide. The high-permeability magnetic film can be formed by plating, vacuum deposition or sputtering. As another alternative, a resin paste containing high-permeability magnetic powder such as ferrite can be coated.  
         [0065]    [Third Embodiment] 
         [0066]    [0066]FIG. 7 shows a magnetic memory device according to another embodiment of the present invention, in which the magnetic memory device is of a multi-chip package type.  
         [0067]    The magnetic memory chips  11   a,    11   b  are superposed on the die pad  12   a  of the lead frame  12  and bonded by die bonding agents  15   a,    15   b.  The chips  11   a,    11   b  may not always be both a magnetic memory chip, but the chip  11   a  may be a logic IC chip, while the chip  11   b  may be a magnetic memory chip.  
         [0068]    Also according to this embodiment, like in the aforementioned embodiments, the lead frame  12  is configured of a high-permeability magnetic material.  
         [0069]    As an alternative, a high-permeability magnetic material covered frame may be used, in which the surface of the conventional lead frame body of Cu or Fe is covered with a high-permeability magnetic material as a magnetic guide.  
         [0070]    The high-permeability magnetic film can be formed by plating, vacuum deposition or sputtering. As another alternative, a resin paste containing high-permeability magnetic powder such as ferrite can be coated.  
         [0071]    [Fourth Embodiment] 
         [0072]    In the configuration shown in FIG. 7, the lead frame  12  is made of a nonmagnetic metal of high heat radiation characteristic, and at least one of the die bonding agents  15   a,    15   b  for bonding the chip contains a high-permeability magnetic material. As a result, the die bonding agents  15   a,    15   b  act as a magnetic guide. Such die bonding agents may be a resin agent of coating type with particulates of a high-permeability magnetic material mixed in the resin agent. As another alternative, as shown in FIG. 8, a sheet member may be used, which comprises a foil member  22  of high-permeability magnetic material is held between the adhesive resin sheets  21   a,    21   b.    
         [0073]    [Fifth Embodiment] 
         [0074]    In the configuration of FIG. 7, the lead frame  12  and the die bonding agents  15   a,    15   b  may be the same as the conventional ones, while the sealing resin  13  may be modified to function as a magnetic guide. In this case, the resin with high-permeability magnetic particulates mixed therein is used for only one of the portion  13   b  of the resin  13  which covers the upper surface of the chip and the portion  13   a  of the resin  13  covering the lower surface of the chip. A suitable high-permeability magnetic material includes an oxide such as ferrite of spinel type or ferrite of garnet type. More specifically, a resin with Mn—Zn ferrite and an additive, or a resin with yttrium iron garnet and an additive is used. The addition of these magnetic materials may reduce the insulation characteristic of the resin. Therefore, a normal resin may be used for the portions contacted by the outer lead portion while a high-permeability magnetic material is added only for the other portions.  
         [0075]    [Sixth Embodiment] 
         [0076]    The third to fifth embodiments described above may be combined. Specifically, in the configuration of FIG. 7, the lead frame  12  is used as a magnetic guide, while the die bonding agents  15   a,    15   b  are also used as a magnetic guide. As an alternative, the lead frame  12  is used as a magnetic guide, while the upper portion  13   b  or the lower portion  13   a  of the sealing resin  13  is used as a magnetic guide. As another alternative, the die bonding agents  15   a,    15   b  are used as a magnetic guide, while the upper portion  13   b  or the lower portion  13   a  of the sealing resin  13  is used as a magnetic guide. As a further alternative, these members can all be used as a magnetic guide.  
         [0077]    [Seventh Embodiment] 
         [0078]    [0078]FIG. 9 shows a magnetic memory device according to a further embodiment of the present invention.  
         [0079]    In the package structure in this embodiment of FIG. 9, a ceramic laminate board  31  is fixed on the peripheral portion of a heat sink  33 , and a magnetic memory chip  11  is bonded to the central portion of the heat sink  33  by a die bonding agent  34 . The outer terminal of the magnetic memory chip  11  is connected by bonding wire  36  to each layer wiring  37  of the ceramic laminate board  31 , and the wiring of each layer is connected by a through-wiring  38  to solder balls  32  arranged on one surface of the laminate  31 . The magnetic memory chip  11  and its peripheral portion are sealed by resin molding  35 .  
         [0080]    In this package structure, according to this embodiment, the heat sink  33  is configured of a high-permeability magnetic material and used as a magnetic guide. In the case where a high heat radiation characteristic is required, the body of the heat sink  33  is formed of Cu, Al or the like, and a high-permeability magnetic film is formed on the surface of the heat sink body, as in the second embodiment, as a magnetic guide.  
         [0081]    [Eighth Embodiment] 
         [0082]    In the package structure shown in FIG. 9, the heat sink  33  is formed of a non-magnetic metal, and the die bonding agent  34  is mixed with a high-permeability magnetic material. As an alternative, a sheet member with a high-permeability magnetic foil member held by resin sheets as shown in FIG. 8 is used as a die bonding agent  34 . In this way, by using the die bonding agent  34  as a magnetic guide, the effect of the disturbing magnetic field to the magnetic memory can be suppressed.  
         [0083]    [Ninth Embodiment] 
         [0084]    In the package structure of FIG. 9, the heat sink  33  and the die bonding agent  34  are the same as the conventional ones. A resin with high-permeability magnetic particulates mixed therein is used as a sealing resin  35  which may function as a magnetic guide. A suitable high-permeability magnetic material is an oxide such as ferrite of spinel type or ferrite of garnet type. More specifically, a resin with Mn—Zn ferrite and an additive or a resin with yttrium iron garnet and an additive is used.  
         [0085]    [Tenth Embodiment] 
         [0086]    The seventh to ninth embodiments can be combined. Specifically, in the configuration of FIG. 9, the heat sink  33  is used as a magnetic guide, while the die bonding agent  34  is also used as a magnetic guide. As an alternative, the heat sink  33  is used as a magnetic guide, while the sealing resin  35  is also used as a magnetic guide. As another alternative, the die bonding agent  34  is used as a magnetic guide, while the sealing agent  35  is also used as a magnetic guide. Further, all of these members can be used as a magnetic guide.  
         [0087]    [11th Embodiment] 
         [0088]    [0088]FIG. 10 shows a magnetic memory device according to a further embodiment of the present invention.  
         [0089]    In the package structure in this embodiment of FIG. 10, a wiring  42  for leading a terminal of the magnetic memory chip  11  formed on a surface of a base board  41 , and a solder ball  43  is formed on the wiring at the peripheral portion of the wiring  42 . A magnetic memory chip  11  is face-down bonded on the surface of the base board  41 , and the chip portion is covered with the sealing resin  44 .  
         [0090]    In this package structure, the base board  41  is made of a high-permeability magnetic material and used as a magnetic guide.  
         [0091]    [12th Embodiment] 
         [0092]    In the package structure of FIG. 10, the base board  41  may be the same as the conventional ones containing no magnetic materials. A resin with high-permeability magnetic particulates mixed therein is used as a sealing resin  44 , which can thus be rendered to function as a magnetic guide. An oxide such as ferrite of spinel type or ferrite of garnet type is suitable as a high-permeability magnetic material. More specifically, a resin with Mn—Zn ferrite and an additive or a resin with yttrium iron garnet and an additive is used.  
         [0093]    As an alternative, the base board  41  is made of a high-permeability magnetic material while a resin with high-permeability magnetic particulates dispersed therein is used as a sealing resin  44 , and both of them are rendered to function as a magnetic guide.  
         [0094]    [13th Embodiment] 
         [0095]    [0095]FIG. 11 shows a magnetic memory device according to a further embodiment of the present invention.  
         [0096]    In the package structure in this embodiment of FIG. 11, a base board  51  having a chip mounting portion formed as a depression is a two-side wiring board, and the wirings  52  and  53  on the two sides thereof are connected by way of via-contact layer  54 . A magnetic memory chip  11  is bonded on the base board  51  by a die bonding agent  55 , and covered with a sealing resin  56 .  
         [0097]    In this package structure, the die bonding agent  55  is mixed with a high-permeability magnetic material. As an alternative, a sheet member having a high-permeability magnetic foil sandwiched by resin sheets is used as a die bonding agent  55 , as shown in FIG. 8. By using the die bonding agent  55  as a magnetic guide in this way, the effects of the disturbing magnetic field on the magnetic memory can be suppressed.  
         [0098]    [14th Embodiment] 
         [0099]    In the package structure of FIG. 11, a resin with high-permeability magnetic particulates are mixed in the sealing resin  56  is used, which functions as a magnetic guide. As an alternative, both the sealing resin  56  and the die bonding agent  55  can be used as a magnetic guide.  
         [0100]    It will thus be understood from the foregoing description that according to the embodiments of the present invention, a magnetic memory device free of the effect of the disturbing magnetic field can be easily provided.  
         [0101]    Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.