Abstract:
Apparatus and methods for mounting micro-electromechanical (MEMS) sensors in three dimensions, using horizontal and vertical substrates.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to and the benefit of U.S. Provisional Application No. 61/074,051, filed on Jun. 19, 2008, the entire disclosure of which is hereby incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates, generally, to the packaging of micro-electromechanical (MEMS) devices and, more specifically, to the mounting of precision MEMS sensors in three dimensions. 
       BACKGROUND 
       [0003]    Modern consumer and commercial electronic device design has undergone a trend towards miniaturization and the incorporation of multiple disparate functionalities. In particular, single-chip-scale packages increasingly tend to include multiple small devices for a plurality of functions. Many applications call for the incorporation of two- and three-dimensional navigation technology into such compact devices. Two- and three-dimensional sensors suitable for that purpose include magnetic sensors (also referred to as magnetometers) and/or tilt sensors (also referred to as accelerometers). Preferably, these sensors are of minimum height along an axis perpendicular to the chip surface (the Z-axis) and compact in directions parallel to the chip surface (i.e., parallel to the XY-plane). Vertical mounting is needed to align sensors along the X-, Y-, and Z-axes. However, mounting Z-axis accelerometers or magnetometers along the z-axis can be very challenging for the packaging industry, especially for mass market applications that have space limitations. Current cost-sensitive, high-volume, standard mounting processes fail to facilitate mounting vertical sensors for applications with limited space. Accordingly, there is an emerging need for the simple, low-cost vertical and horizontal mounting of MEMS devices at precise angles in a chip-scale package. 
       SUMMARY 
       [0004]    The present invention provides, in various embodiments, processes and methods for integrating MEMS sensors with precise location and/or orientation in X-, Y-, and Z-direction into low-cost organic chip-scale packages. In certain embodiments, the method attains structures having an overall height of less than about 0.8 mm, a width of less than about 4 mm, and a length of less than about 6 mm. Various embodiments exploit the surface tension of solder to align a Z-axis-mounted MEMS device onto, or into a hole formed in, the X-Y surface plane of a substrate. 
         [0005]    In a first aspect, the invention provides an apparatus for precision MEMS mounting in organic packaging. In various embodiments, the apparatus includes a vertical sensor circuit assembly and a horizontal circuit assembly. The vertical sensor circuit assembly includes a MEMS device surface-mounted to a substrate. The substrate has an array of connection pads on a first side, a dummy set of connection pads on the opposite side, and conductive leads between the array and bottom edge lead pads of the substrate. The horizontal circuit assembly includes a horizontal die and the vertical sensor circuit assembly, both surface-mounted to a substrate having conductive leads with each of the horizontal die and the vertical sensor circuit assembly. Solder surface tension is used to align the horizontal die and the vertical sensor circuit assembly to the horizontal circuit assembly. In some embodiments, the distance between the bottom edge and the top edge of the vertical sensor circuit assembly is less than about 0.8 mm, and the height of the horizontal circuit assembly is less than about 0.8 mm. The MEMS device may be a hermetic sealed cavity device. 
         [0006]    In a second aspect, the invention provides a method for precision MEMS mounting in organic packaging. The method includes patterning a first side of a Z-axis substrate with bond pads corresponding to the MEMS device; patterning a first side of the MEMS device with metal contacts for mounting to the first side of the Z-axis substrate; placing the first side of the MEMS device in contact with the first side of the Z-axis substrate; reflowing the MEMS device and the Z-axis substrate at a first temperature, and patterning the first side of an XY-axis substrate with bond pads corresponding to the Z-axis substrate. The patterning on the first side of the XY-axis substrate is spread slightly outward from the center of mass of the Z-axis substrate. The MEMS device may be a hermetic sealed cavity device. While placing the MEMS device, optical pattern recognition may be used to orient the device. 
         [0007]    In various embodiments, the method further includes patterning the opposite side of the Z-axis substrate with dummy lead pad; reflowing the Z-axis substrate and the XY-axis substrate at a temperature lower than the first temperature; and/or adding solder mask to one or both sides of the Z-axis substrate. In certain embodiments, the method moreover includes patterning a second side of the MEMS device with metal contacts for mounting to the first side of a second Z-axis substrate; and patterning a first side of a second Z-axis substrate with bond pads corresponding to the second side of the MEMS device for mounting, wherein the reflowing of the MEMS device at a first temperature causes the MEMS device to assume a perpendicular orientation between the first and second Z-axis substrates. In these embodiments, a hole may be cut in the XY-substrate, and the MEMS device may be positioned in the hole in the XY-substrate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The foregoing discussion will be understood more readily from the following detailed description of the invention when taken in conjunction with the accompanying drawings, in which: 
           [0009]      FIGS. 1A and 1B  are schematic side views of apparatus in accordance with various embodiments; 
           [0010]      FIG. 2A  is a set of cross-sectional views illustrating the artwork on various surfaces of substrates utilized in the apparatus of  FIGS. 1A and 1B ; 
           [0011]      FIG. 2B  is a schematic drawing showing the different layers of a Z-axis substrate in accordance with various embodiments; 
           [0012]      FIG. 2C  is a schematic drawing showing the different layers of an XY-substrate in accordance with various embodiment; 
           [0013]      FIG. 3  is a flow chart illustrating a method in accordance with various embodiments; 
           [0014]      FIG. 4  is a schematic side view of an apparatus including a Z-axis MEMS sensor directly soldered to an XY-substrate in accordance with one embodiment; 
           [0015]      FIG. 5  is a schematic side view of an apparatus including a Z-axis MEMS sensor embedded in a hole formed in an XY-substrate in accordance with one embodiment; and 
           [0016]      FIG. 6  is a schematic side view of an apparatus including a Z-axis MEMS sensor directly soldered to an XY-substrate and including multiple rows of connections in accordance with one embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]      FIG. 1A  illustrates, in cross section, an exemplary apparatus  100  for integrating MEMS sensors in organic packaging. The apparatus includes a substrate  102  and, mounted thereon, various MEMS sensors  104 ,  106 ,  108 . A first MEMS sensor  104  is a magnetometer that is soldered to a bottom surface (hereinafter also referred to as side B) of the substrate  102  and oriented horizontally, i.e., parallel to the XY-substrate. A second MEMS sensor  106 , also a magnetometer, is vertically oriented and mounted flush to a Z-axis substrate  110 . The Z-axis substrate  110  itself, with a face perpendicular to the surface whereon the sensor is mounted, is soldered to side B of the XY-substrate  102 . The third MEMS sensor  108  is an accelerometer die that is horizontally mounted to side B of the XY-substrate  102 , and enclosed by a hermetically sealed cavity. Detail on the structure and manufacture of hermetic seal cavity devices is provided in a U.S. patent application Ser. No. 12/488,137, filed on even date herewith, which is hereby incorporated herein by reference in its entirety. 
         [0018]    Another exemplary apparatus ( 150 ) in accordance with various embodiments is illustrated in  FIG. 1B . In addition to a cavity-encapsulated, horizontal XY-accelerometer  108  and a horizontal XY-magnetometer  152 , apparatus  150  includes two vertical MEMS devices—a Z-magnetometer  154  and a Z-accelerometer  156 , each of which is attached to the XY-substrate  102  via a Z-axis substrate  110 . 
         [0019]      FIG. 2A  illustrates the artwork on the MEMS-die side of the Z-axis substrate  110  (side A z ) and on the opposite side of the Z-axis substrate (side B z ), as well as the artwork on side B of the XY-substrate  102  in a region where the Z-axis substrate  110  is attached. The die side of the Z-axis substrate is patterned with bond pads  200  corresponding to the Z-axis MEMS device mounted thereon. Conductive traces  202  redistribute the signals from these bond pads to the bottom edge lead pads  204  of the Z-axis substrate  110 . Dummy lead pads  206  are patterned on side A z  of the Z-axis substrate for symmetry and balance of force. The XY-substrate  102  is patterned with bond pads  208  corresponding to the Z-axis substrate  110 , and additional bond pads corresponding to the X- and Y-axis MEMS devices mounted thereon. Conductive traces redistribute the signals from these bond pads to the lead edge pads of the substrate  102 . The bond pads of the XY-substrate  102  spread slightly outward from the center of mass of the region wherein the horizontal MEMS devices and Z-axis substrates are placed. During assembly of the apparatus, this spreading creates surface tension when the solder is wetted, which in turn pulls the MEMS devices and Z-axis substrates flush against the XY-substrate, thereby forcing an automatic perpendicular alignment of the MEMS devices and the Z-axis substrates to within the tolerance of the diced surfaces.  FIG. 2B  illustrates in more detail the various layers and the artwork of the Z-axis substrate  110 ; and  FIG. 2C  shows the various layers of the XY-substrate  102  and the combined artwork for the apparatus. 
         [0020]    Apparatus  100 ,  150  and similar structures may be built according to the method illustrated in  FIG. 3 . The method includes patterning sides A z  and B z  of the Z-axis substrate  110  as illustrated in  FIG. 2  (step  300 ). In step  302 , solder mask is applied to both sides of the Z-axis substrate to control and isolate the bond pads  200  and the lead pads  204 ,  206  and constrain the wicking action of solder during reflow in a later step. Further, SnSbCu solder paste with a liquidus temperature from about 240° C. to about 260° C. is applied to side A z  over the bond pads  206  only. 
         [0021]    The vertical MEMS devices (e.g., magnetometers  154  and/or  156 ) are patterned and stud-bumped or plated with gold or copper (step  304 ), and then aligned and pick-and-placed onto the Z-axis substrate  110  (step  306 ). Optical pattern recognition may be used during placement to recognize and orient the rotation of each device around an axis normal to side A z  within less than one degree. A vacuum head may be used to pick up the MEMS devices and place them onto the Z-axis substrate  110 , maintaining a Z-tilt of less than one degree with respect to the substrate surface. Next, the Z-axis substrate  110  with the aligned devices is reflowed at a temperature between 260° C. and 340° C. (step  308 ), allowing for later end-customer green process assembly with SnAgCu solder having a liquidus temperature of from about 220° C. to about 230° C. The Z-axis substrate  110  with aligned MEMS devices is then diced, and transfer-rotated 90 degrees into holding trays in preparation of the pick-and-place onto the XY-substrate. 
         [0022]    In step  310 , the XY-substrate  102  is patterned with bond pads corresponding to the horizontal MEMS devices (e.g., magnetometers  104  and/or accelerometer dies  108 ) and the Z-substrates  110  to be mounted. Then, solder mask is added to sides A and B to isolate the wicking action of the solder. Solder paste is applied to side A of the XY-substrate (step  312 ), and the XY- and Z-mounted MEMS devices are pick-and-placed into position (step  314 ). Again, optical pattern recognition may be employed with the pick-and-place to recognize and orient the rotation of the devices and Z-substrates in X- and Y-direction (i.e., around an axis normal to side A) within less than one degree. The vacuum head picks up the devices and places them onto the XY-substrate, maintaining a tilt of less than 1 degree with respect to the substrate surface. In step  316 , the XY-substrate and devices mounted thereon is reflowed at a temperature between about 220° C. to about 230° C. (step  316 ). 
         [0023]    The apparatus and method described above may be modified in various ways. For example, the Z-mounted MEMS device may be a hermetic sealed cavity device, such as an accelerometer. The vertical mounting of a hermetic sealed cavity device may be accomplished as described above, using additional weights to balance the pick-and-place Z-substrate so that the solder wicking process results in perpendicular alignment to the XY-substrate. 
         [0024]    In an alternative embodiment, a Z-axis MEMS device may be directly soldered to the XY-substrate, without using a Z-axis substrate. The resulting structure  400  is illustrated schematically in  FIG. 4 . In this embodiment, the bond pads of the Z-mounted MEMS die  402  are arranged to be on one side of the die, and the back side of the die is plated with a similar dummy pattern. The die is stud-bumped or plated on the front and back sides. Then, the die is rotated by ninety degrees, and pick-and-placed onto the substrate  102 . The substrate is reflowed against the spread artwork  404  to force an automatic perpendicular alignment of the MEMS device  402  and the substrate  102 . 
         [0025]    In another embodiment, the Z-mounted MEMS device is integrated into a hole in the substrate, resulting in a structure  500  shown in  FIG. 5 . The hole  502  may be laser-cut or mechanically routed in the XY-substrate  102 . The Z-mounted MEMS device  504  is pick-and-placed into the hole  502 , and suspended by stud bumps  506  on the device  504 . Solder is dispensed over the bond pads with the stud bump wires and then reflowed. To guarantee perpendicular alignment, a boat is used under the substrate during reflow. 
         [0026]    In yet another embodiment, illustrated in  FIG. 6  by structure  600 , the Z-mounted MEMS device  602  is attached to the XY-substrate  102  as in  FIG. 4 , but includes addition electric connections. Again, bond pads are arranged to be on one side of the MEMS die  602 , and the back side of the die  602  is plated with a similar dummy pattern. The die  602  is stud-bumped or plated on both sides. The upper connections  604  to the die  602  are wire-bonded in an open ark  606  in a buttress fashion to make a contact on pads one row beyond the lower set of connections  608 . The die is rotated by ninety degrees, and pick-and-placed onto the substrate  102 . The substrate is reflowed against the spread artwork to force an automatic perpendicular alignment of the MEMS device  602  and the substrate  102 . 
         [0027]    Having described certain embodiments of the invention, it will be apparent to those of ordinary skill in the art that other embodiments incorporating the concepts disclosed herein may be used without departing from the spirit and scope of the invention. Accordingly, the described embodiments are to be considered in all respects as only illustrative and not restrictive.