Abstract:
Disclosed are a method and apparatus which provide a magnetic shield for integrated circuits containing electromagnetic circuit elements. The shield is formed of a magnetically permeable material, which may be a non-conductive magnetic oxide, and either partially contacts or completely surrounds the integrated circuit.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for shielding electromagnetic integrated circuits from external magnetic fields. 
     BACKGROUND OF THE INVENTION 
     A conventional integrated circuit (IC) package typically comprises (1) an IC chip or die including a plurality of input/output terminals; (2) a support for the chip, such as a pad, substrate or leadframe, including electrically conductive leads; (3) electrical connections such as wire bonds or conductive bumps for electrically connecting the input/output terminals of the chip with the electrically conductive leads; and (4) a material for encasing or encapsulating the chip, the support and the electrical connections while leaving portions of the leads accessible outside the casing or encapsulation. Fabrication of such a conventional IC package requires attaching the IC chip to the support, connecting the input/output terminals of the chip to the electrically conductive leads, and encapsulating the IC chip, the support and the electrical connections in, for example, a plastic package. 
     Recently, very high-density magnetic memories, such as magnetic random access memories (MRAMs), have been proposed to be integrated together with CMOS circuits. Magnetic random access memories employ one or more ferromagnetic films as storage elements. A typical multilayer-film MRAM includes a plurality of bit or digit lines intersected by a plurality of word lines. At each intersection, a ferromagnetic film is interposed between the corresponding bit line and word line to form a memory cell. 
     When in use, an MRAM cell stores information as digital bits, the logic value of which depends on the states of magnetization of the thin magnetic multilayer films forming each memory cell. As such, the MRAM cell has two stable magnetic configurations, high resistance representing, for example, a logic state 0 and low resistance representing, for example, a logic state 1. The magnetization configurations of the MRAMs depend in turn on the magnetization vectors which are oriented as a result of electromagnetic fields applied to the memory cells. The electromagnetic fields used to read and write data are generated by associated CMOS circuitry. However, stray magnetic fields, which are generated external to the MRAM, may cause errors in memory cell operation when they have sufficient magnitude. 
     Very high-density MRAMs are particularly sensitive to stray magnetic fields mainly because the minuscule MRAM cells require relatively low magnetic fields for read/write operations which, in turn, depend upon the switching or sensing of the magnetic vectors. These magnetic vectors are, in turn, easily affected and have the magnetic orientation changed by such external stray magnetic fields. 
     To diminish the negative effects of the stray magnetic fields and to avoid sensitivity of MRAM devices to stray magnetic fields, the semiconductor industry could introduce memory cells requiring higher switching electromagnetic fields than a stray field which the memory cells would typically encounter. However, the current requirements for operating such memory cells is greatly increased because higher internal fields necessitate more current. Thus, both the reliability and scalability of such high current devices decrease accordingly, and the use of MRAMs which may be affected by stray magnetic fields becomes undesirable. 
     Accordingly, there is a need for an improved magnetic memory structure and a method of forming it, which shields against external magnetic fields. There is also a need of a packaging device for encasing a magnetic random access memory IC chip which reduces the effects of external magnetic fields on internal memory cell structures and operations. There is further a need for minimizing the cost of a packaging which shields a magnetic random access memory IC chip from external magnetic fields. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and apparatus which provide a packaging device for magnetic memory structures, such as MRAMs, which shields such memory structures from external magnetic fields. The invention employs a magnetic shield, preferably formed of non-conductive magnetic oxides, which either partially contacts or completely surrounds an integrated circuit chip which includes such magnetic memory structures. 
     These and other features and advantages of the invention will be more clearly apparent from the following detailed description which is provided in connection with accompanying drawings and which illustrates exemplary embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of an integrated circuit package assembly at an intermediate stage of processing and in accordance with a first exemplary embodiment of the present invention. 
     FIG. 2 is a cross-sectional view of the integrated circuit package assembly of FIG. 1 at a subsequent stage of processing to that shown in FIG. 1 
     FIG. 3 is a cross-sectional view of the integrated circuit package assembly of FIG. 1 at a subsequent stage of processing to that shown in FIG.  2 . 
     FIG. 4 is a cross-sectional view of the integrated circuit package assembly of FIG. 1 at a subsequent stage of processing and in accordance with a second embodiment of the present invention. 
     FIG. 5 is a cross-sectional view of the integrated circuit package assembly of FIG. 1 at a subsequent stage of processing and in accordance with a third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     In the following detailed description, reference is made to various specific embodiments in which the invention may be practiced. These embodiments are described with sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be employed, and that structural and electrical changes may be made without departing from the spirit or scope of the present invention. 
     The present invention provides a method for fabricating packaging devices for electromagnetic integrated circuit structures, such as MRAM structures, to provide electromagnetic shields and to a shielded packaged electromagnetic integrated circuit structure. The present invention employs a magnetic shield, preferably formed of electrically non-conductive magnetic oxides, which either partially contacts or completely surrounds an integrated circuit chip which contains electromagnetic structures. In one exemplary embodiment of the invention, the magnetic shield is formed as a glob or layer of magnetic field shielding material which is affixed to one or more surfaces of an integrated circuit chip. In another exemplary embodiment, an encapsulating material of the chip packaging includes magnetic field shielding material therein. 
     Referring now to the drawings, where like elements are designated by like reference numerals, FIGS. 1-4 illustrate exemplary embodiments of the present invention. FIG. 1 depicts an integrated circuit (IC) package assembly  10  at an intermediate stage of processing. A semiconductor chip or die  12  includes an array of input/output terminals  14  and internal electromagnetic structures, such as MRAM cells and access circuitry. 
     The chip  12  is supported by a die pad  16  (FIG. 1) which can be formed, for example, of a leadframe or a dielectric substrate. Each of the input/output terminals  14  is further electrically connected with respective conductive leads  22  by wire bonds  20 , or other suitable electrical connectors. 
     Referring now to FIG. 2, a magnetic shield is provided for shielding the chip  12  from external magnetic field disturbances. According to a first exemplary embodiment of the present invention, a glob top  33  is formed over the semiconductor die  12 , including the input/output terminals  14 , and portions of the wire bonds  20 . The glob top  33  comprises an electrically non-conductive magnetic shielding material  30 , which can be injected, for example, from a nozzle. If desired, a mold can be used to shape the magnetic shielding material  30 . If a mold is used, the magnetic shielding material  30  is injected into a cavity of the mold, and flows along the top of the chip  12 , the input/output terminals  14  and adjacent portions of the wire bonds  20  which are within the mold cavity. Subsequent to the injection of the magnetic shielding material  30  into the mold cavity, the magnetic shielding material  30  hardens to form the glob top  33 , as illustrated in FIG.  2 . If a mold is not used, a nozzle can simply deposit a glob top  33  of material on the upper surface of chip  12 . 
     The magnetic shielding material  30  may be formed, for example, of an electrically non-conductive material with permeability higher than that of air or silicon. As such, the preferred choice for the magnetic shielding material  30  is a non-conductive magnetic oxide, for example, a ferrite such as MFe 2 O 4 , wherein M=Mn, Fe, Co, Ni, Cu, or Mg, among others. Manganites, chromites and cobaltites may be used also, depending on the device characteristics and specific processing requirements. 
     Further, the magnetic shielding material  30  may be also composed of magnetic particles, for example nickel or iron particles, which are incorporated into a non-conducting molding material, for example a glass sealing alloy or a polyimide. Since nickel is conductive, the concentration of nickel particles in the glass alloy should be low enough so that shielding material  30  does not form a continuous conductor if the shield extends to the input/output terminals  14  or the wire bonds  20 . 
     Next, as illustrated in FIG. 3, the structure of FIG. 2 is fulrther encapsulated into a packaging material  35 , for example a plastic compound, which, as known in the art, may be injected into a mold cavity through a passage (not shown). As the packaging material  35  is injected, it flows around the glob top  33 , portions of the wire bonds  20  and conductive leads  22 , as well as around the die pad  16 . This way, the input/output terminals  14  of integrated circuitry including magnetic memory structures, such as MRAMs, are shielded by the glob top  33  and encapsulated in the packaging material  35  for enhanced protection from external stray magnetic fields. 
     Further, for even maximum protection, the packaging material  35  may also comprise a mold compound, such as a plastic compound, with conductive magnetic particles therein. For example, conductive magnetic particles of, for example, nickel, iron, and/or cobalt, may be suspended in a matrix material, such as a plastic compound, at a concentration that does not allow the particles to touch and form a continuous shorting conductor between the leads. Alternatively, the packaging material  35  may comprise a mold compound, such as a plastic compound, including non-conductive particles of, for example, non-conductive magnetic oxides and/or Mumetal alloys, which may comprise approximately 77% nickel (Ni), 4.8% copper (Cu), 1.5% chromium (Cr) and 14.9% iron (Fe). 
     Although FIGS. 2 and 3 show the magnetic shielding material  30  in the form of a rounded glob top  33  on only the top of chip  12 , it is also possible to apply a glob of shielding material  30  to the bottom surface instead, or to the top and bottom of chip  12 . Moreover, if the material of choice for the die pad  16  is a dielectric substrate, it is also possible to apply a flat layer  60  of shielding material  30  to the bottom of the chip  12 , as illustrated in FIG.  5 . In this case, the bottom flat layer  60  of shielding material  30  may be conductive or non-conductive as needed, depending on the characteristics of the IC device. A non-conductive magnetic shielding material may employ a non-conductive oxide, for example a ferrite such as MFe 2 O 4 , wherein M=Mn, Fe, Co, Ni, Cu, or Mg, among others, manganites, chromites and/or cobaltites. Similarly, a conductive magnetic shielding material may be composed of Mumetal alloys comprising approximately 77% nickel (Ni), 4.8% copper (Cu), 1.5% chromium (Cr) and 14.9% iron (Fe), or magnetic particles, such as nickel or iron particles, which are incorporated into a non-conducting molding material, for example a glass sealing alloy or polyimide. If, however, the material of choice for the die pad  16  is a lead frame comprising a magnetic material, such as the commonly used alloy  42  which already provides magnetic shielding, then the bottom flat layer  60  is optional. 
     FIG. 4 illustrates yet another exemplary embodiment of the present invention, in which a magnetic shielding material  50  is formed as the chip  12  encapsulating material which is used to form an IC packaging assembly  11 . The preferred material for the magnetic shielding material  50  is a non-conductive magnetic oxide, for example a ferrite such as MFe 2 O 4 , wherein M=Mn, Fe, Co, Ni, Cu, or Mg, among others. However, manganites, chromites and cobaltites may be used also, depending on the device characteristics and processing requirements. Further, conductive Mumetal alloys comprising approximately 77% nickel (Ni), 4.8% copper (Cu), 1.5% chromium (Cr) and 14.9% iron (Fe) may be used also, as well as conductive magnetic particles, such as nickel, iron or cobalt particles, incorporated into a molding material, for example a glass sealing alloy or a commercially available IC mold compound. 
     The magnetic shielding material  50  completely surrounds the semiconductor chip  12 . A protective plastic packaging  56  (FIG.  4 ), such as a commercially available IC mold compound, is next optionally provided to completely surround the magnetic shielding material  50  and to complete the fabrication of the IC package assembly  11 . 
     Although the exemplary embodiments described above refer to specific magnetic shielding materials it must be understood that the invention is not limited to the materials described above, and other magnetic shielding materials, such as ferromagnetics like nickel-iron (Permalloy), nickel or iron may be used also, as long as they are capable of shielding electromagnetic structures within chip  12  from external magnetic fields. 
     Further, although the exemplary embodiments described above refer to specific locations where the shielding material is applied to a die, it is also possible to apply the shielding material in other locations. For example, as described above, two globs  33  or layers of material could be employed for shielding the magnetic memories structures, one on each side of chip  12 , or multiple globs or layers of the same or different shielding material which overlap each other may be used on one or both sides of chip  12 . In addition, the specific shape of the shielding material is not limited to that shown in FIGS. 2-4 and other shapes, configurations, or geometries may be employed. 
     The present invention is thus not limited to the details of the illustrated embodiments and the above description and drawings are only to be considered illustrative of exemplary embodiments which achieve the features and advantages of the present invention. Modifications and substitutions to specific process conditions and structures can be made without departing from the spirit and scope of the present invention. Accordingly, the invention is not to be considered as being limited by the foregoing description and drawings, but is only limited by the scope of the appended claims.