Patent Publication Number: US-2006019422-A1

Title: Magnetic shield for integrated circuit packaging

Description:
RELATED APPLICATION  
      This application is a continuation application of U.S. application Ser. No. 10/719,419, filed Nov. 21, 2003, which is a divisional application of U.S. application Ser. No. 10/050,339, entitled “MAGNETIC SHIELD FOR INTEGRATED CIRCUIT PACKAGING,” filed Jan. 15, 2002, now U.S. Pat. No. 6,906,396, the disclosures of which are hereby incorporated by reference in their entirety. 
    
    
     FIELD OF THE INVENTION  
      The present invention relates to magnetic shielding for integrated circuits and, more particularly, to magnetic shielding for integrated circuits having magnetic materials used therein for which protection from stray external magnetic fields is desired.  
     BACKGROUND OF THE INVENTION  
      Magnetic materials are used, for example, in magnetic cell memories and magnetic field sensors. In random access magnetoresistive memories, data is stored by applying magnetic fields and thereby causing a magnetic material in a cell to be magnetized into either of two possible memory states. The information stored in the memory is contained in the orientations of the magnetization vectors of the magnetic material layers used in each memory cell. Such memory cells exhibit a pronounced decrease in electrical resistance when an applied magnetic field brings the magnetization vectors in different layers into alignment. Recalling data is accomplished by sensing resistance changes in the cell. The cells can be written or erased by applying magnetic fields created by passing currents through conducting lines external to the magnetic structures, or through the magnetic structures themselves.  
      There are often undesirable magnetic fields in and about the device, which are generated either as part of the device operation or from external sources. Such fields can have significant effects on the magnetization of the magnetic thin film. The field can contribute to a loss of information or to storage of erroneous information in the magnetic memory cells. Thus, magnetic memory cells function best when they are protected from external magnetic field disturbances.  
      A metal with a relatively high magnetic permeability can be used to form a shield for protection from magnetic fields. Metals that are used widely in magnetic shielding include soft magnetic or high permeability materials, such as NiFe, NiFeMo and NiFeCu. Such magnetic shielding materials, are generally available from metal supply companies, such as Carpenter Technology Corporation of Wyomissing, Pa.  
      U.S. Pat. No. 5,939,772 entitled “Shielded Package For Magnetic Devices,” issued Aug. 17, 1999, describes the use of magnetically permeable metal shields attached to the outside of a hermetically sealed ceramic package. The shields are electrically connected to the package ground plane. Laminated magnetic shielding for ceramic packages is also described in U.S. Pat. No. 5,561,265, issued Oct. 1, 1996.  
      Ceramic package technology can be expensive. Furthermore, as performance increases, the physical characteristics of ceramic packages may become limiting. Specifically, a ceramic material based on Al 2 O 3  has a relatively high dielectric constant (ε r ˜7-8). Additionally, because of the high-temperature processing, metallization is limited to refractory metals that are quite resistive, such as Mo and W.  
      Other references include application of magnetic shielding within a plastic package. U.S. Pat. No. 4,953,002, issued Aug. 28, 1990, for example, teaches magnetic shielding internal to a plastic encapsulated package.  
      Magnetic integrated circuit structures must also be housed in a way that minimizes cost if they are to be viable for the commercial memory market. Therefore, a shielding arrangement to protect magnetic films in magnetic integrated circuit structures from significant external adverse influences, including external magnetic fields, and which can be provided economically, would be desirable. Desirably, such a shielding arrangement should be flexible enough to meet the varied needs of integrated circuit users.  
     SUMMARY OF THE INVENTION  
      In accordance with one aspect of the invention, a housing is provided for protecting an integrated circuit device. The housing comprises a molded body that encapsulates the integrated circuit device. At least one magnetically permeable foil is applied to an outer surface of the molded body.  
      In accordance with another aspect of the invention, a method is provided for magnetically shielding a semiconductor die. The method includes forming a molded unitary housing around the semiconductor die. A film of magnetic shield material is applied to at least one outer surface of the molded unitary housing. The film is applied in a manner that such that it is approximately parallel to a major surface of the semiconductor die. Advantageously, the shield material can be degaussed just prior to application, after the package is subjected to high temperature processing.  
      In accordance with another aspect of the invention, an integrated circuit package is provided. The package includes an integrated circuit die, a molded body encapsulating the die, and a magnetic shield layer extending parallel to a major surface of the die over an outer surface of the molded body.  
      In accordance with still another aspect of the present invention, a method is provided for packaging an integrated circuit chip. The method includes mounting the chip on a die carrier. Epoxy is molded over the chip to form an encapsulant. A magnetic shield layer is then selected for a particular integrated circuit environment. This selected magnetic shield is applied over the encapsulant.  
      In accordance with still another aspect of the invention, an integrated circuit package is provided with an encapsulant surrounding an integrated circuit die. The encapsulant includes a recess on an outer surface thereof. The recess is configured for receiving and mechanically retaining a magnetic shield foil. In the illustrated embodiment, the recess includes overhanging tabs for removably trapping the foil within the recess. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic cross section of a packaged integrated circuit with magnetic shielding attached to outer surfaces of the package, according to an illustrated embodiment of the invention.  
       FIG. 2  is a schematic cross section of an integrated circuit encapsulated in a ball-grid array package that has magnetic shielding attached to an outer surface of the package, according to an illustrated embodiment of the invention.  
       FIG. 3  is a schematic cross section of a packaged integrated circuit with magnetic shielding set into recesses on outer surfaces of the package, according to an illustrated embodiment of the invention.  
       FIG. 4  is a perspective view of a ball-grid array package showing a recess in the top surface in which a magnetic shield is held mechanically, according to an illustrated embodiment of the invention.  
       FIGS. 5A and 5B  are schematic cross sections cut along lines  5 A- 5 A and  5 B- 5 B, respectively, of  FIG. 4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      Magnetic integrated circuits, such as MRAM (magnetic random access memory) devices, can be sensitive to external magnetic fields. Information is stored in MRAMs specifically as a direction of magnetization in a magnetic material layer. If the layer is exposed to an undesirable external magnetic field, the direction of magnetization can inadvertently change. Such exposure to stray fields can lead to memory erasure, accidental writing and/or reading errors.  
      Of course, the external environments for magnetic integrated circuit devices are not all the same. Some devices may be located in environments with strong external magnetic fields, and some may be located in environments where external magnetic fields are negligible. When magnetic shielding is incorporated inside the packaging of a magnetic device, a best guess is made as to the size and thickness of magnetic shielding to use. There are drawbacks to this “one size fits all” approach. The designer may choose to provide magnetic shielding for a worst-case scenario, thereby using more magnetic material than may be required for many applications. In this case, customers pay for more shielding than they might need. Additionally, customers may wish to have shielding for only some of their applications.  
      Perhaps more importantly, magnetic shielding should be degaussed, i.e., provided with random magnetic orientation. In order to keep the shield degaussed, the shield should be applied as late as possible in the packaging process. This is because during any high temperature steps, the chip must be exposed to a controlled magnetic field to ensure that the “pinned” or fixed magnetic layers within the chip maintain their desired magnetic alignment. Even soldering a package to a circuit board can raise temperatures high enough to risk alteration of the pinned layers&#39; magnetization. Thus, even packaging steps should be performed under a controlled magnetic field, if possible. Unfortunately, such a field would also tend to align the magnetic shield, if present, such that it would not remain degaussed.  
      It would be useful to have a system of magnetic shielding for magnetic integrated circuits that can be adapted easily for individual customer applications, is removable for certain applications and/or can be readily applied after all high temperature processing, particularly those steps in which magnetic fields are applied to maintain pinned layers within the chip.  
      The aforementioned needs are satisfied by the embodiments of the present invention, which provide package structures and methods for providing magnetic shielding to an integrated circuit after packaging is complete. Thus, the magnetic shielding can be tailored to meet the specific needs of the customer without incurring the expense of over-shielding or the risk of under-shielding. More importantly, the shield can be degaussed and applied after all high temperature packaging steps. Furthermore, in certain embodiments described herein, magnetic shielding is removably applied to an outside surface of an integrated circuit package, such that it can be removed and degaussed after packaging and even after mounting the package without degaussing pinned layers in the chip.  
      These and other objects and advantages of the present invention will become more fully apparent from the following description taken in conjunction with the accompanying drawings.  
       FIG. 1  is a cross-sectional, schematic drawing of a package or housing  10  for an integrated circuit device, according to an illustrated embodiment of the invention. The package comprises a magnetic integrated circuit  12  encapsulated within a plastic or epoxy encapsulant, preferably in the form of a molded body  14 . For purposes of the present description a magnetic integrated circuit is defined as an integrated circuit containing at least one magnetic thin film layer forming a part of an active device. Preferably, the molded body  14  comprises an organic material, more preferably, an elastomer or an epoxy mold compound. The skilled artisan will appreciate that the molded body  14  encapsulates the die  12 , in contrast to ceramic packages that are hermetically sealed around a die.  
      The integrated circuit  12  is encapsulated onto a die carrier or substrate  16 . Preferably, the die carrier  16  comprises electrically conducting leads  18 . Conducting wires  20  are bonded to bond pads  22  on the integrated circuit  12  and attached to the electrically conducting leads  18  of the die carrier  16 . In an alternative “flip chip” arrangement (not shown), solder bumps on the integrated circuit are bonded to the leads  18 , and conducting wires  20  are not used. In the illustrated embodiment, the leads  18  extend into electrodes  24  that protrude from the molded body  14  and can make connections to external circuitry. The electrodes  24  typically extend below the molded body  14 .  
      As will be appreciated by the skilled artisan, the features and advantages described herein will have application to numerous molded or encapsulated integrated circuit packages, such as lead frame packages. More recently, however, die carriers comprise plastic substrates. For such packages, the electrical leads  18  and electrodes  24  represent conductive traces on or in a plastic substrate extending out of the molded body  14  to form contacts that eventually form connections with larger circuits (e.g., a motherboard).  
      In  FIG. 1 , magnetically permeable foils  26 ,  28  are attached to both the top and bottom outer surfaces of the molded body  14 . The foils  26 ,  28  are thus electrically insulated from the packaged circuitry and leads. Preferably, the foils comprise soft magnetic or high permeability materials, such as nickel-iron based alloys, cobalt-iron based alloys, nickel-cobalt based alloys or amorphous ferromagnetics. More preferably, the foils comprise a NiFe-based alloy, such as mu metal or permalloy. Preferably, the foil thickness is between about 1 μm and 1000 μm. The foils  26 ,  28  are held onto the approximately flat surfaces by thin layers of adhesive  29 , preferably, an epoxy-based adhesive. The foils  26 ,  28  are arranged to be and larger than a major surface of the magnetic integrated circuit  12 . In an alternative arrangement, there is a magnetically permeable foil  26  on only one outer surface.  
       FIG. 2  is a schematic cross section of a ball-grid array housing or package  30  for an integrated circuit device  12 , according to another embodiment of the invention. The integrated circuit or die  12  is attached to a rigid substrate  32  with a die attach material  34 , preferably epoxy or elastomer. The rigid substrate  32  contains conductive traces  36  that connect to solder balls  38  arranged in an array on the bottom surface of the rigid substrate  32 . The solder balls  38  are configured to make electrical connections to external circuitry. Conductive wires  40  provide conductive paths between bond pads  42  on the integrated circuit  12  and the conductive traces  36  on the rigid substrate  32 . A molded body  44  encapsulates the integrated circuit  12  onto the rigid substrate  32 , with the solder balls  38  serving as the electrodes that are not covered by the molded body  44  and therefore are exposed on the outside of the package  30 . Preferably, the molded body  44  comprises an organic material, more preferably, an elastomer or an epoxy mold compound.  
      In  FIG. 2 , a magnetically permeable foil  46  is attached to an outer surface of the molded body  44 , held in place by a thin layer of adhesive  48 , preferably, an epoxy-based adhesive. The molded body  44  electrically insulates the foil  46  from the package circuitry. Preferably, the foils comprise “soft” magnetic or high permeability materials, such as nickel-iron based alloys cobalt-iron based alloys, nickel-cobalt based alloys or amorphous ferromagnetics. More preferably, the foils comprise NiFe-based alloys such as mu metal or permalloy. Preferably, the foil thickness is between about 1 μm and 1000 μm. The foil  46  is arranged to be approximately parallel to and larger than a major surface of the magnetic integrated circuit  12 .  
       FIG. 3  is a cross-sectional, schematic drawing of a housing or package  50  for an integrated circuit device  12 , according to another embodiment of the invention. The package  50  comprises the magnetic integrated circuit  12  encapsulated within a molded body  52 . Preferably, the molded body  52  comprises an organic material, more preferably, an elastomer or an epoxy mold compound.  
      As described above for  FIG. 1 , the integrated circuit  12  is encapsulated by the molded body  52  onto a die carrier  16 . Preferably, the carrier  16  includes electrically conducting leads  18 . Conducting wires  20  are bonded to bond pads  22  on the integrated circuit and attached to the electrically conducting leads of the die carrier  18 ,  16 . In an alternative arrangement (not shown), solder bumps on the integrated circuit are bonded to electrically conducting traces on a plastic substrate in a “flip chip” arrangement, and conducting wires  20  are not used. The electrically conducting leads  18  extend to form electrodes  24  that protrude from the molded body  52  and can make connections to external circuitry. The electrodes  24  themselves can comprise the contacts of a lead frame, but more preferably comprise conductive traces on or in a plastic substrate.  
      In  FIG. 3 , magnetically permeable foils  54 ,  56  are fitted into recesses  58 ,  60  in the top and bottom outer surfaces of the molded body  52 . Preferably, the foils comprise “soft” magnetic or highly permeable materials as described hereinabove. The foils  54 ,  56  are held in place by thin layers of adhesive  62 , preferably, an epoxy-based adhesive. The foils  54 ,  56  are arranged to be approximately parallel to and larger than a major surface of the magnetic integrated circuit  12 . In another arrangement, there is a magnetically permeable foil  54  and recess  58  on only one outer surface of the molded body  52 .  
      In accordance with one arrangement, the recesses  58 ,  60  are etched into the encapsulant  52  after molding. Preferably, however, the recesses  58 ,  60  are formed in the body  52  as molded.  
      Another preferred embodiment for attaching a magnetically permeable foil in a recess in the outer surface of a molded body can be understood with reference to  FIG. 4 . A finished ball-grid array type of package  70  ready for the addition of magnetic shielding is shown in a perspective view in  FIG. 4 . Only the molded body or encapsulant  71  is shown in  FIG. 4 .  
      The top surface  72  contains a recessed region  74  over most of its area. The recess  74  has two parallel edges  76  whose sidewalls  78  are approximately perpendicular to the top surface  72 , as is apparent in the cross-sectional view of  FIG. 5A . The remaining two parallel edges  80  of the recess  74  include an overhanging tab  82  at the top surface  72 , which protrudes into the region of the recess  74 , as is apparent from the cross-sectional view of  FIG. 5B . The recess  74  is preferably formed, including overhanging tabs  82 , during the molding process. One or more tabs  82  are preferred over a single overhanging ledge extending the length of the edge  80 , simply to facilitate removal of the mold.  
       FIGS. 5A and 5B  show only the top outer surface portion of a housing for an integrated circuit. It will be understood that the outer surface arrangement shown in  FIGS. 5A and 5B  can be used with any number of integrated circuit and wiring arrangements consistent with molded body packages, including those discussed above for  FIGS. 1 and 2 . Additionally, the outer surface arrangement shown in  FIGS. 5A and 5B  can be used either on only one package surface or on both major package surfaces, according to the requirements of the operating environment. Preferably, the molded body  71  comprises an organic material, more preferably, an elastomer or an epoxy mold compound.  
       FIG. 5A  is a cross section of the recess  74  cut through the recess edges  76  whose sidewalls  78  are approximately perpendicular to the top surface  72  of the molded body  71 . A sheet of magnetic shield material  84  lies within the recess  74  with its edges  86  adjacent to the sidewalls  78  of the recess  74 .  
       FIG. 5B  is a cross section of the recess  74  cut along a surface perpendicular to the surface shown in  FIG. 5A . The top edges  80  of the recess  74  have at least one overhanging tab  82  at the top surface  72  of the housing  70  and, deeper inside the recess  74 , sidewalls  88  that are approximately perpendicular to the plane of the top surface  72 . The overhanging tabs  82  protrude into the region of the recess  74 . A sheet of magnetic shield material  84  is trapped within the recess  74 , below the tabs  82 , with its edges  90  adjacent to the sidewalls  88  of the recess  74 . It will be understood that, in other arrangements, the tabs  82  can taper to the recess floor rather than having the illustrated perpendicular sections  88 . The illustrated tab configuration, tapering above and below the innermost protrusion, facilitates deflection to insert and/or remove the magnetic shield  84 .  
      In the illustrated embodiment, no adhesive is used to hold the sheet of magnetic shield material  84  in place within the recess  74  of the molded body  71  for the magnetic integrated circuit. The sheet of magnetic shield material  84  is cut to fit the size of the recess  74 . The sheet  84  is placed into the recess  74  by bending the sheet  84  slightly to fit under the overhangs  82  and then releasing the sheet  84  to fit into place against the sidewalls  88  of the recess  74 . The width of the recess opening within the overhang edges  82  is less than the width of the magnetic material sheet  84 , thus providing a mechanical means of keeping the magnetic material sheet  84  in place. It will be understood that, if desired, adhesive can additionally be employed.  
      Advantageously, the magnetic shield  84  can additionally be removed and replaced. Thus, a package can be shipped with the shield  84  in place. The customer can remove the shield  84 , conduct additional high temperature processing in a strong magnetic field (without affecting the shield), and replace the shield after completion of high temperature packaging steps. Alternatively, after installation and use, the shield  84  can be removed for degaussing again, should the need arise.  
      The embodiments of the invention have been described using examples of packages that contain one integrated circuit or die. The embodiments of the invention are equally useful for a multi-die package, wherein integrated circuits are arranged next to one another and/or stacked one over another within one molded package. Connections among the dies and between the dies and conducting traces connected to electrodes that protrude from the package can be made by wire bonding or by solder bump bonding as described above with respect to the illustrated embodiments.  
      The structures and methods described above in the illustrated embodiments offer many advantages for magnetic shielding of magnetic integrated circuits. Fully processed and packaged integrated circuit devices can be removed from the fab environment and inventoried. At this point, all high temperature processing has been completed. Magnetic shielding, tailored to meet a particular customer&#39;s requirements, can be added to the outside of the packages just prior to shipping. The magnetic shielding is preferably degaussed and/or given a particular magnetic alignment according to customer needs. This would not be possible if the magnetic shielding were introduced into the integrated circuit or the package before all high temperature processing was complete. Moreover, the embodiments described herein obtain magnetic shielding, post-processing tailoring and the benefits of low-dielectric epoxies and high conductivity copper metallization for IC packaging.  
      Although the foregoing description of the preferred embodiments of the present invention has shown, described and pointed out the fundamental novel features of the invention, it will be understood that various omissions, substitutions and changes in the form of the detail of the apparatus as illustrated as well as the uses thereof may be made by those skilled in the art, without departing from the spirit of the present invention. Consequently, the scope of the present invention should not be limited to the foregoing discussion, but should be defined by the appended claims.