Abstract:
A method and apparatus which provide one or more electromagnetic shield layers for integrated circuit chips containing electromagnetic circuit elements are disclosed. The shield layers may be in contact with the integrated circuit chip, including magnetic memory structures such as MRAMs, or in a flip-chip carrier, or both. A printed circuit board which supports the chip may also have one or more shield layers.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a method and apparatus for shielding electromagnetic integrated circuits from external magnetic fields.  
         BACKGROUND OF THE INVENTION  
         [0002]    In conventional packaging techniques, an integrated circuit chip or die is first attached to a carrier and then contacts of both the die and the carrier are electrically connected. One such packaged device, called a flip-chip device, requires a semiconductor chip to be flipped and bonded with a carrier, so that contacts of the chip directly bond to contacts of the carrier. Thus, both die bonding and interconnection are simultaneously accomplished.  
           [0003]    A conventional bond flip-chip device  10  including an integrated circuit chip or die  30  and a flip-chip carrier  20  is illustrated in FIG. 1. The flip-chip carrier  20  is fabricated from a substrate  12 , an insulating layer  14 , a plurality of conductive traces  15  (FIG. 2) and an elastomeric layer  16 . The conductive traces  15  may be located within or on the insulating layer  14  in a variety of ways, for example, by building up the conductive traces  15  on the insulating layer  14  through electrolytic deposition.  
           [0004]    The conductive traces  15  (FIG. 2) are each electrically connected to a solder ball  28  through an inset (not shown) in the substrate  12 . Although a single solder ball  28  is shown in FIG. 1, it must be understood that any number of solder balls  28  may be employed, as the solder balls  28  are used to mount the flip-chip device  10  to a circuit board or other electrical structure.  
           [0005]    The die  30  is shown in dotted line above the flip-chip carrier  20 . In use, the die  30  is positioned on the elastomeric material  16  of the flip-chip carrier  20 . The ffip-chip carrier  20  is electrically connected with the die  30  by way of suitable conductive connecting structures, such as, for example, solder balls  24  positioned within a gap  21  of the flip-chip carrier  20 . The solder balls  24  are in electrical connection with respective conductive traces  15  and with suitable contacts on the die  30 .  
           [0006]    Recently, very-high density magnetic memories, such as magnetic random access memories (MRAMs), have been proposed to be integrated with CMOS circuits. This integration has also complicated the packaging of such devices, as the packaging must have a longer lifetime, better electrical performance, as well as more efficient heat dissipation.  
           [0007]    A typical multilayer-film MRAM includes a plurality of bit or digit lines intersected by a plurality of word lines. At each intersection, layers of ferromagnetic film separated by a non-magnetic film are 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.  
           [0008]    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.  
           [0009]    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 produce 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, 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.  
           [0010]    Accordingly, there is a need for an improved magnetic memory packaging structure and a method of forming it, which shield against external magnetic fields and which permit use of lower power levels for circuit operations. There is also a need for a flip-chip packaging device for mounting 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  
         [0011]    The present invention provides a method for fabricating magnetically shielded electromagnetic integrated circuit structures, such as MRAM structures. The present invention employs one or more magnetic shields which are incorporated either on an integrated circuit chip which contains electromagnetic structures, or in a flip-chip packaging device, or in both. In one exemplary embodiment of the invention, the electromagnetic shield is formed as one or more layers of magnetic field shielding material incorporated on the integrated circuit chip or in a flip-chip carrier, or both. In another exemplary embodiment, a printed circuit board supporting the flip-chip packaging may also include shielding material.  
           [0012]    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  
       [0013]    [0013]FIG. 1 is a perspective view of a conventional flip-chip device.  
         [0014]    [0014]FIG. 2 is a top view of the conventional flip-chip device of FIG. 1.  
         [0015]    [0015]FIG. 3 is a perspective view of an integrated circuit package assembly in accordance with a first exemplary embodiment of the present invention.  
         [0016]    [0016]FIG. 4 is a side view of the package assembly of FIG. 3.  
         [0017]    [0017]FIG. 5 is a side view of the package assembly of FIG. 3 and in accordance with a second exemplary embodiment of the present invention.  
         [0018]    [0018]FIG. 6 is a side view of the package assembly of FIG. 3 and in accordance with a third exemplary embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]    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.  
         [0020]    Referring now to the drawings, where like elements are designated by like reference numerals) FIGS.  3 - 6  illustrate exemplary embodiments of the present invention. FIG. 3 depicts an integrated circuit (IC) package assembly  100  at an intermediate stage of processing. A semiconductor chip or die  300  includes an array of internal electromagnetic structures, such as MRAM cells and access circuitry, and is shown in dotted line above a flip-chip carrier  200 .  
         [0021]    The flip-chip carrier  200  is fabricated from a substrate  120 , an insulating layer  140 , a plurality of conductive traces (not shown) and an elastomeric layer  160 . The substrate  120  is typically formed of a material with high mechanical stability at high temperature. The substrate  120  may be a flexible tape such as, for example, a polyimide tape. Two commercially available polyimide tapes, KAPTON® from E. I. DuPont Nemours and Company and UPILEX® from Ube Industries, Ltd., can be employed to form the substrate  120 . The elastomeric layer  160  may be formed of a silicone or a silicone-modified epoxy.  
         [0022]    The conductive traces may be located within or on the insulating layer  140  in a variety of ways. One way, which is an addition method, is to build up the conductive traces on the insulating layer  140  through electrolytic deposition. The electrolytic deposition may be accomplished with a mask or, if performed without a mask, a subsequent etching step may be employed to create the conductive traces. Other suitable methods include sputter coating and laminating a sheet of conductive material, such as copper, and etching away excess copper to form the traces.  
         [0023]    The conductive traces are each electrically connected to solder balls  280  through an inset (not shown) in the substrate  120 . The solder balls  280  are used to mount the flip-chip device carrier  200  with the chip  300  to a circuit board or other electrical structure, as it will be described in more detail below.  
         [0024]    As shown in FIG. 3, the chip  300 , including magnetic memory structures such as MRAM cells, is shown in dotted line above the flip-chip carrier  200 . In use, the chip  300  is positioned on the elastomeric material  160  of the flip-chip carrier  200 , as illustrated in FIG. 4. As known in the art, the flip-chip carrier  200  is electrically connected with the chip  300  by way of suitable conductive connecting structures, such as, for example, solder balls  240  positioned within a gap  210  of the carrier  200 . The solder balls  240  are in electrical connection with respective conductive traces  150  (not shown) and with suitable contacts on the chip  300 .  
         [0025]    Referring now to FIGS.  3 - 4 , a first magnetic shielding layer  110  is provided for shielding the chip  300  from external magnetic field disturbances. According to a first exemplary embodiment of the present invention, the first magnetic shielding layer  110  is formed on the back surface of the semiconductor chip  300 , that is the surface that is opposite to a front surface of the semiconductor chip  300  or the surface that directly contacts the flip-chip carrier  200 .  
         [0026]    The first magnetic shielding layer  110  comprises a magnetic shielding material which can 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 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 may be also composed of conductive magnetic particles, for example nickel, iron or cobalt particles, which are incorporated into a non-conductive base material, such as a glass sealing alloy or a polyimide. Alternatively, the magnetic shielding material may be formed of a film or layer of conductive magnetic material, such as nickel, cobalt, iron, Permalloy, or Mumetal, among others.  
         [0027]    Next, as illustrated in FIG. 4, the flip-chip carrier  200  with the attached chip  300  is further attached to a portion of a printed circuit board  400  by surface mounting, for example. This way, the flip-chip carrier  200  is mounted flat on the printed circuit board  400  and contacts the printed circuit board  400  through the solder balls  280 , eliminating the need for holes through the printed circuit board.  
         [0028]    As shown in FIG. 4, a flat layer  111  of magnetic shielding material may be embedded within the printed circuit board, which can be otherwise formed of a resin compound, for example, or other known printed circuit board material. In this case, the magnetic memory structures of the chip  300  are first shielded by the first magnetic shielding layer  110  and further shielded by the flat layer  111  of the printed circuit board  400  for maximum protection from external stray magnetic fields.  
         [0029]    Although FIG. 4 shows the magnetic shielding material in the form of layer  110  on the backside of chip  300  and as layer  111  embedded within the printed circuit board  400 , it is also possible to apply a layer of shielding material on the bottom surface of the printed circuit board  400  instead. For example, FIG. 5 illustrates another exemplary embodiment of the present invention, in which a bottom magnetic shielding layer  112  is formed on the bottom surface of the printed circuit board  400 , together with the flat layer  111  embedded within the printed circuit board  400 .  
         [0030]    The preferred material for the bottom magnetic shielding layer  112  is a conductive Mumetal alloy comprising, for example, approximately 77% nickel (Ni), 4.8% copper (Cu), 1.5% chromium (Cr) and 14.9% iron (Fe), as well as conductive magnetic particles, such as nickel or iron particles, incorporated into a molding material, for example a glass sealing alloy or a commercially available mold compound. However, 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, may be used also, as well as manganites, chromites and cobaltites, depending on the device characteristics and processing requirements.  
         [0031]    In yet a third embodiment of the present invention, another magnetic shielding layer  113  is formed of a magnetic shielding material as part of the flip-chip carrier  200 , as shown in FIG. 6. The magnetic shielding layer  113  is formed between the substrate  120  and the insulating layer  140  to further protect the MRAM devices from external magnetic fields and to complete the fabrication of an IC package assembly  102  (FIG. 6). The magnetic shielding material may be different than, or similar to, the magnetic shielding materials for layers  110 ,  111  and  112 . Appropriate conductive vias, insulated from magnetic shielding layer  113 , are formed through layer  113 . Again, the magnetic shielding material may be preferably 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, or a manganite, chromite or cobaltite. Further, the magnetic shielding material may be also composed of magnetic particles, for example nickel, iron or cobalt particles, which are incorporated into a base material such as a glass sealing alloy or a commercially available mold compound. Since nickel is conductive, however, the concentration of nickel particles in the glass alloy should be low enough so that the shielding material does not form a continuous conductor, unless appropriate conductive vias electrically insulated from the magnetic layer are used.  
         [0032]    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), cobalt-nickel-iron, nickel or iron may be used also.  
         [0033]    In addition, the magnetic shielding material forming the various shielding layers described above 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 each others. Alternatively, the magnetic shielding material may comprise a molding material including non-conductive particles of, for example, non-conductive magnetic oxides and/or Mumetal alloys. Mumetal alloys may comprise, for example, approximately 77% nickel (Ni), 4.8% copper (Cu), 1.5% chromium (Cr) and 14.9% iron (Fe).  
         [0034]    Further, although the exemplary embodiments described above refer to specific locations where the shielding material is applied to a chip, it is also possible to apply the shielding material in other locations. For example, it is also possible to apply a layer of shielding material on both the top and bottom surfaces of the printed circuit board  400 , or on the top and bottom surfaces together with a shielding material embedded within the printed circuit board. Further, a plurality of layers of material could be employed for shielding the magnetic memories structures, one on each side of chip  300 , or layers of the same or different shielding materials which overlap each other may be used on one or both sides of chip  300 , flip-chip carrier  200  and/or circuit printed board  400 . In addition, the specific shape of the shielding material is not limited to that shown in FIGS.  3 - 6  and other shapes, configurations, or geometries may be employed. Also, the chip  300  and associated flip-chip carrier  100  may be encapsulated in a flip-chip package which is mountable through solder balls  180  to a printed circuit board.  
         [0035]    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.