Abstract:
A cap for a microelectromechanical system device includes a first layer of, e.g., Bismaleimide Triazine (BT) resin material in which a through-aperture is formed, laminated to a second layer of BT resin material that closes the aperture in the first layer, forming a cavity. The first and second layers are laminated with a thermosetting adhesive that is sufficiently thick to encapsulate particles that may remain from a routing operation for forming the apertures. The interior of the cavity, including exposed portions of the adhesive, and the exposed face of the first layer are coated with an electrically conductive paint. The cap is adhered to a substrate over the MEMS device using an electrically conductive adhesive, which couples the conductive paint layer to a ground plane of the substrate. The layer of conductive paint serves as a shield to prevent or reduce electromagnetic interference acting on the MEMS device.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    Embodiments of the present invention are directed to an electromagnetic shielding structure, and in particular, to a structure configured to be mounted to a substrate over a microelectromechanical system device to shield the device from electromagnetic interference. 
         [0003]    2. Description of the Related Art 
         [0004]    Microelectromechanical systems (MEMS) are mechanical devices formed using processes originally developed for manufacturing integrated circuits and the like. In the last decade, MEMS devices have become increasingly more diverse and complex, finding use in more and more consumer and industrial applications. A partial list of MEMS devices in common use includes accelerometers, gyroscopes, inclinometers, optical switches, fluid pumps, biological testing devices, pressure sensors, motors for aligning read/write heads for hard drives, vehicle airbag triggers, microphones, thermometers, etc. As MEMS devices become more sophisticated and sensitive, in many cases they have become susceptible to electromagnetic interference (EMI) generated by nearby circuits or other sources. This is especially true for sensors of various types, such as, e.g., microphones. To protect such devices from EMI, shield caps are sometimes provided, which are placed over MEMS devices to intercept and shunt to ground EM radiation that might otherwise affect the effectiveness of the particular device. 
         [0005]      FIG. 1  is a cross-sectional side view of a MEMS EMI shield cap  50  according to known art. Typically, the such devices are made from laminated sheets of Bismaleimide Triazine (BT). The shield cap  50  of  FIG. 1  has a first BT layer  52  in which an aperture  60  is formed, and a second BT layer  54  that closes the aperture  60 , forming a blind aperture, or cavity. The facing surfaces of the first and second layers  52 ,  54  have respective layers of copper foil  56 ,  58 . A thin adhesive layer  62  bonds the facing surfaces together. The interior of the cavity  60  and the bottom surface of the first BT layer  52  (as oriented in  FIG. 1 ) are plated in layers of copper  64 , nickel  66 , and gold  68 . 
         [0006]    A conductive adhesive is used on the bottom surface of the first layer  52  to mechanically couple the shield cap  50  over a MEMS device on a substrate, and electrically couple the conductive lining of the cap to a circuit ground. EM radiation cannot penetrate conductive plating in the shield cap, but is instead carried to ground by the device. 
         [0007]    The shield cap  50  is manufactured as one of hundreds that are made in sheets or wafers then cut into individual caps. The manufacturing process includes depositing a seed layer of copper on the interior of the cavity  60  and the bottom surface of the first BT layer  52 , then electroplating, in succession, copper, nickel, and gold layers  64 ,  66 ,  68 . 
         [0008]    The initial seed layer does not adhere to the adhesive layer  62  at A, but, provided the adhesive layer is sufficiently thin, the build-up of the copper layer  64  on the side wall surface  70  and back wall surface  72  of the cavity  60  will bridge the gap to form a continuous plated surface. This is important because if the plating on the back wall surface  72  is not electrically coupled to the plating on the side walls  70 , EMI that strikes the back wall will not be shunted to ground, but will instead be reradiated inside the shield cap  50 , which would, of course, defeat the purpose. 
       BRIEF SUMMARY 
       [0009]    According to an embodiment, an electromagnetic interference (EMI) shield cap is provided, that includes a first layer of polymeric material, preferably Bismaleimide Triazine (BT), with an aperture extending through the first layer. A second layer of polymeric material is positioned on a first face of the first layer and covers an opening of the aperture so that the aperture forms a cavity in the combined layers, with side walls and a back wall. The first and second layers are joined by an adhesive positioned between the layers, and an electrically continuous layer of conductive paint covers the back and side walls of the cavity and exposed portions of the adhesive, and also covers the exposed face of the first layer. 
         [0010]    According to an embodiment, a portion of the adhesive extends from between the first and second layers of polymeric material and forms a bead around the back wall of the cavity, which is also covered by the conductive paint. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0011]      FIG. 1  is a cross-sectional side view of a MEMS electromagnetic interference (EMI) shield cap according to known art. 
           [0012]      FIG. 2  is a perspective view of a MEMS device package that is partially cut away to show elements of an EMI shield cap  102 , according to an embodiment. 
           [0013]      FIGS. 3-8  are side cross-sectional views of showing respective stages in the manufacture of EMI shield caps and MEMS device packages according to the embodiment of  FIG. 2 , taken along lines  8 - 8  of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    Referring again to the prior art shield cap  50  of  FIG. 1 , the inventors investigated a large number of product failures of electromagnetic interference (EMI) shield caps in which the adhesive joint  62  between the first and second Bismaleimide Triazine (BT) layers  52 ,  54  failed, permitting the second layer  54  to separate from the device. It was found that there were a number of contributory causes to the adhesive failure, including contamination between the adhesive  62  and the copper layer  58 , and inadequate finishing of the surface of the copper layer  58 . In both of these cases, the result was that the adhesive had not properly bonded with the copper layer  58  of the second BT layer  54 . 
         [0015]    It was first determined that it would cost prohibitive to clean the BT layers to a degree sufficient to eliminate all potential contamination before laminating the layers. During the manufacturing process, the adhesive layer  62  is applied to the layer of copper foil  56  of the first BT layer  52  before formation of the apertures  60 . In this way, the adhesive layer  62  is automatically trimmed to size when the apertures  60  are made. However, the apertures  60  are formed by a router mechanism, which produces large quantities of tiny chips and fibers, many of which adhere to the edges of the adhesive layer  62 , and are very difficult to remove or prevent. Additionally, the routing process can cause occasional delamination along the edges of the aperture  60  between internal layers, permitting the chips and fibers to penetrate. When a protective liner is removed from the adhesive  62  it tends to dislodge some of the chips from the edges of the adhesive, so that during the subsequent laminating process, some of the chips will inevitably be captured between the adhesive  62  and the copper foil layer  58  of the second BT layer  54 . Because the adhesive layer  62  must be quite thin to enable the copper layer  64  to bridge the gap at A during plating, even very small chips can interfere with proper adhesion of the BT layers  52 ,  54 . 
         [0016]      FIG. 2  is a perspective view of a MEMS device package  100  that is partially cut away to show elements of an EMI shield cap  102 , according to an embodiment. The package  100  includes a substrate  104  on which a MEMS microphone  106  is mounted and over which the EMI shield cap  102  is coupled. The substrate  104  can be any appropriate material such as, for example, BT, molding compound, polyamide resin, etc. A conductive land  18  on the substrate  104  surrounds the microphone  106 , and is configured to be electrically coupled to a circuit ground. A conductive adhesive  116  couples the shield cap  102  to the conductive land  118 . 
         [0017]    The shield cap  102  includes a body comprising first and second BT layers  108 ,  110  coupled together by an adhesive layer  112 . The shield cap  102  has a layer of conductive paint  114  lining the interior of a cavity  135  and extending over the bottom surfaces of the first BT layer  108 . An aperture  134  is formed in the second BT layer  110  to permit proper operation of the MEMS microphone  106 , according to well known principles. 
         [0018]    A method of manufacturing the EMI shields cap  102  is described below with reference to the cross-sectional side views of  FIGS. 3-8 . 
         [0019]      FIG. 3  shows a carrier substrate  122  with an alignment pin  124 , which is one of a plurality of alignment pins coupled to the carrier substrate. Relief cavities  126  are provided in the carrier substrate to facilitate a later step in the process, as described below. A first BT sheet  117  is positioned on the carrier substrate  122  with a first face  121  in contact with the carrier substrate  122 . The alignment pins  124  extend through corresponding alignment apertures  127  formed in the first BT sheet. The first BT sheet  117  has an adhesive film  119  positioned on a second face  123  of the first BT sheet  117 . The adhesive film  119  includes an adhesive film layer  112  and a protective liner layer  120 . 
         [0020]    As shown in  FIG. 4 , apertures  130  are cut in the first BT sheet  117 . In the process shown, the apertures  130  are cut in a router process, as indicated at R. The adhesive film  119  is cut during the same process, so that the apertures  130  extend through the adhesive film. 
         [0021]    The relief cavities  126  formed in the carrier substrate  122  are in positions that correspond to the positions of the apertures  130 , so that, as the cutting bit penetrates the BT sheet  117  and travels laterally to form an aperture  130 , it does not contact the carrier substrate  122 . Thus, the carrier substrate  122  is not modified by the routing process and can therefore be reused without requiring deburring or resurfacing, and no chips are produced of the material of the carrier substrate. Additionally, workload on the tip of the cutting bit is reduced, bearing in mind that, assuming equal work, the sharp points that define the corners of the cutting bit will become dull more quickly than the cutting edges that define the flutes of the cutting bit. 
         [0022]    In the embodiment shown, a router process is employed to form the apertures  130  in the first BT sheet  117 . However, this is by way of example, only. Many other alternative processes can be used. According to various embodiments, the apertures  130  can be formed by laser drilling, water jet cutting, die cutting, press punching, fine blanking, electron discharge machining, etc. 
         [0023]    Following formation of the apertures  130 , the protective liner layer  120  is removed from the adhesive film  119 , exposing the adhesive film layer  112 . The first BT sheet and a second BT sheet are aligned on alignment pins like the pin  124  of  FIG. 3  in order to ensure that features of the second BT sheet  110 , such as, e.g., apertures  134 , are correctly positioned relative to the apertures  130  of the first BT sheet  117 . Then, as shown in  FIG. 5 , the first BT sheet  117  is pressed together with the second BT sheet  128 , with the adhesive film layer  112  pressed between the second face  123  of the first BT sheet and a first face  129  of the second BT sheet  128  to form an assembly  133  comprising the first and second BT sheets  117 ,  128  and the adhesive film layer  112 . The second BT sheet  128  covers one end of the apertures  130  to produce corresponding cavities  135 . 
         [0024]    The term aperture is used herein to refer to a hole that extends through a piece of material so as to be open at two ends. The term cavity is used to refer to a hole that is closed at one end, such as is sometimes referred to as a blind aperture. 
         [0025]    A chip  131  of BT material is shown trapped between the adhesive film layer  112  and the first face of the second BT sheet  128 . As previously noted, completely removing all such chips following the routing process can be difficult, and in the case of EMI shield caps like that of  FIG. 1 , can interfere with full attachment of the BT layers, causing wastage. According to an embodiment, a thickness of the adhesive film layer  112  is selected to exceed the average dimensions of chips produced in the routing process. Additionally, increased pressure is employed in pressing the first and second BT sheets  117 ,  128  together so that the adhesive film layer  112  is forced into contact with the first face  129  of the second BT sheet  128 , substantially encapsulating any chips that might be trapped. The increased compressive force also causes a portion of the adhesive film layer  112  to squeeze out from between the first BT sheet  117  and the second BT sheet  128 , producing beads  132  of adhesive material around the inside corners of the cavities  135 . 
         [0026]    According to an embodiment, the pressure applied to laminate the first and second BT sheets together is between 5 kg/cm 2  and 30 kg/cm 2 . According to another embodiment, the pressure applied to laminate the first and second BT sheets together is between 10 kg/cm 2  and 20 kg/cm 2 . According to another embodiment, the pressure applied to laminate the first and second BT sheets together is between 16 kg/cm 2  and 18 kg/cm 2 . 
         [0027]    According to an embodiment, the adhesive film layer  112  has a thickness of between 15 μm and 100 μm. According to another embodiment, the adhesive film layer  112  has a thickness of between 20 μm and 50 μm. According to an embodiment, the adhesive film layer  112  has a thickness of 25 μm. 
         [0028]    The adhesive film layer  112  is preferably a thermosetting adhesive such as is known in the art, and is cured under heat or a combination of heat and pressure. According to an embodiment, the adhesive film layer  112  is cured while the laminating pressure is applied, by the application of heat. According to another embodiment, following the process steps described with reference to  FIG. 5 , the assembly  133  is heated in an oven to a prescribed temperature to cure the adhesive film layer  112 . 
         [0029]    According to an embodiment, the material of the first and second BT sheets  117 ,  128  is initially only partially cured, in which case, the process employed to cure the adhesive film layer  112  also cures the first and second BT sheets. 
         [0030]    Turning now to  FIG. 6 , a coating of electrically conductive paint  114  is applied to one side of the assembly  133  so that the conductive paint coats the first face  121  of the first BT sheet  117 , side walls  136  of the cavities  135 , and exposed portions of the first face  129  of the second BT sheet  128 . The paint  114  is formulated to adhere to the BT material of the first and second BT sheets, and also to the material of the adhesive film layer  112 , and so forms an electrically conductive layer of paint over the interiors of the cavities  135 , extending over the beads  132  of adhesive and also over the remaining portions of the first face  121  of the first BT sheet  117 . 
         [0031]    According to an embodiment, the paint is sprayed onto the assembly  133 . The inventors were concerned that a coat of paint that was sufficiently thick to act as an EMI shield would cover or block very small features such as, e.g., the apertures  134 . However, the inventors determined that the paint can be applied in thin coats until an acceptable conductivity is achieved while preserving features like the apertures  134 . 
         [0032]    The conductive paint  114  is cured, and then, as shown in  FIG. 7 , the assembly  133  is cut into individual EMI shield caps  102 , each comprising a portion of the first BT sheet  117  forming a first BT layer  108 , and a portion of the second BT sheet  128  that forms a second BT layer  110 . Each shield cap  102  includes a cavity  135 , a first face  138 , and an electrically continuous layer of conductive paint  114  extending over the side and back walls of the cavity and onto the first face. Because the conductive paint is applied to the assembly before being cut into individual shield caps, there is no conductive layer on the outsides of the shield caps  102 . The term continuous is used to mean that there are no portions of the layer of paint  114  within the cavity  135  that are electrically isolated from the remainder of the layer. 
         [0033]    According to an embodiment, the shield caps  102  are installed over MEMS devices as described hereafter with reference to  FIG. 8 . One or more MEMS devices  106  are first manufactured according to known principles, and mounted to a substrate  104 . In the example shown in  FIG. 8 , the MEMS device  106  is a microphone in a flip-chip configuration and the substrate  104  includes the appropriate contact pads and an electrical wiring circuit configured to electrically couple the MEMS device as a component of another circuit. The substrate  104  also includes a layer  118  of conductive material that is electrically coupled to a circuit ground and patterned to receive the EMI shield cap  102 . 
         [0034]    Depending upon the requirements of the particular application, the layer  118  can be patterned to be coupled around the entire first face  138  of the shield cap  102 , or, alternatively, can be in the form of contact pads that make contact with the shield cap over a smaller portion of the first face  138 . The substrate  104  can also include a conductive ground plane coupled to the circuit ground and positioned to shield the MEMS device from below. 
         [0035]    Conductive adhesive  116  is positioned on the substrate  104  and the EMI shield caps  102  are pressed onto the conductive adhesive, typically in a pick-and-place operation. The conductive adhesive  116  is cured, after which the substrate is cut into individual shielded MEMS packages  100 , as shown in  FIG. 2 . 
         [0036]    According to an alternate embodiment, a single sheet of BT material is machined to form a plurality of cavities in one face, after which, the sheet is processed as described above, beginning with the steps described with reference to  FIG. 6 . The machining process can be a routing process similar that described above, or any process that can produce the cavities of the size and depth desired for a given application. By forming cavities in a single sheet, it is not necessary to laminate two or more sheets to form the cavities, which obviates the associated steps. 
         [0037]    The second BT sheet  128  and shield caps  102  are shown in the drawings as including apertures  134 . These are specific to an EMI shield cap intended for use with a MEMS microphone, and are merely exemplary. The shape and particular configuration of the shield cap, including features such as the apertures  134 , etc., are selected according to the intended end use of the shield cap, and are well understood in the art. 
         [0038]    The first and second BT sheets  117 ,  128  are shown as each having a single layer of BT material. According to other embodiments, either sheet or both sheets can comprises a plurality of layers. For example, according to an embodiment in which the device to be shielded requires a shield cap having a cavity with a depth that exceeds the thickness of a single layer of BT material, the first sheet  117  comprises a number of layers sufficient to meet or exceed the minimum cavity depth. Additionally, because the disclosed process does not require plating, copper layers, such as described with reference to the shield cap  50  of the prior art, are not necessary. However, BT sheets with copper layers can be used if desired. For example, a layer of copper under the paint, at least on the horizontal surfaces of a shield cap, may improve the conductivity of the paint layer. 
         [0039]    In tests conducted by the inventors, a number of shield caps were made, substantially as described above, to evaluate their effectiveness in overcoming the problems outlined above. An adhesive film having a thickness of 25 μm was used, and subjected to a laminating pressure of 16.6 kg/cm 2 . Two thin coats of conductive pint were applied. The test shield caps were found to provide adequate electromagnetic shielding, tolerated multiple thermal cycles at reflow temperatures, and to meet or exceed standards in mechanical strength and rigidity. No delamination was found to occur. 
         [0040]    BT is a class of resin that is very widely used in printed circuit boards and various semiconductor packaging applications because BT can be formulated to have a very high glass transition temperature and a very low dielectric constant. Both of these qualities are desirable in the semiconductor industry. Accordingly, embodiments are described as being made from BT material. However, the claims are not limited to BT material except where such limitation is explicit. The selection of material is a design consideration, and may depend on a number of factors, including the particular intended end use, availability and cost, etc. Alternative materials can include polyimide resin, epoxy resin, polyphenylether resin, etc. 
         [0041]    In describing the embodiments illustrated in the drawings, directional references, such as right, left, top, bottom, etc., are used to refer to elements or movements as they are shown in the figures. Such terms are used to simplify the description and are not to be construed as limiting the claims in any way. 
         [0042]    The term coupled, as used in the claims, includes within its scope indirect coupling, such as when two elements are coupled with one or more intervening elements even where no intervening elements are recited. 
         [0043]    The unit symbol “μm” is used herein to refer to a value in microns. One micron is equal to 1×10 −6  meters. 
         [0044]    The abstract of the present disclosure is provided as a brief outline of some of the principles of the invention according to one embodiment, and is not intended as a complete or definitive description of any embodiment thereof, nor should it be relied upon to define terms used in the specification or claims. The abstract does not limit the scope of the claims. 
         [0045]    The various embodiments described above can be combined to provide further embodiments. All of the U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet are incorporated herein by reference, in their entirety. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications and publications to provide yet further embodiments. 
         [0046]    These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.