Abstract:
A semiconductor device and method by which a device chip with a micromachine is directly surface mounted to a circuit board. A capping chip is bonded to the device chip and encloses the micromachine. The capping chip has a first surface facing the device chip, an oppositely-disposed second surface, and electrical interconnects through the capping chip between the first and second surfaces. The electrical interconnects electrically communicate with runners on the device chip that are electrically connected to the micromachine, thereby providing a signal path from the micromachine to the exterior of the device. The capping chip further includes bond pads for electrical communication with the electrical interconnects. With the bond pads, the capping chip can be surface mounted to a circuit board by reflowing solder bumps formed on the bond pads. Depending on the placement of the bond pads on the capping chip, the semiconductor device can be mounted to the circuit board with the capping chip between the device chip and circuit board, or the semiconductor device can be mounted with one side of the device attached to the circuit board.

Description:
TECHNICAL FIELD 
     The present invention generally relates to surface mount electronic devices. More particularly, this invention relates to a semiconductor device having a micromachine and capable of being surface mounted as a package to a circuit board. 
     BACKGROUND OF THE INVENTION 
     A variety of semiconductor micromachines are known, including yaw (angular rate) sensors, angular and linear accelerometers, pressure sensors, thermal sensors, and actuators such as nozzles and valves. Each of these devices typically involves one or more micromachined structures (micromachines) formed in or on a silicon chip (referred to herein as a device chip). The device chip is often placed within a protective subpackage and then wire bonded to electrically connect the device to bond pads on the subpackage. The bond pads of the subpackage can then be reflow soldered to conductors on a circuit board, electrically and physically interconnecting the device to the board circuitry. Alternatively, device chips can be glued to a ceramic substrate, and then wire bonded to a circuit board after other surface mount components have been reflow soldered to the board. 
     Another packaging alternative involves wafer bonding methods, in which the micromachine of a device chip is enclosed by a second chip (referred to herein as a capping chip), which is bonded to the device chip. A cavity is often formed in the capping chip to receive and/or provide clearance for the micromachine of the device chip. Absolute pressure sensors require that the cavity be evacuated and hermetically sealed, while the performance of yaw sensors and accelerometers with resonating and tunneling micromachines generally benefit if the cavity is evacuated so that the micromachine operates in a vacuum. Bonding is typically achieved by forming the capping chip of silicon or glass (e.g., Pyrex), which can be bonded to the silicon device chip by such known techniques as anodic bonding and silicon fusion bonding, or with the use of glass frit, adhesives and solder. An example of this method is represented in FIG. 1, in which a micromachine sensor  110  is shown to include a device chip  112  with a surface micromachine  114 , and a capping chip  116  with a cavity  118  in which the micromachine  114  is received. A portion of the capping chip  116  is removed by cutting or etching to allow for wire bonding of bond pads  120  on the device chip  112  to a ceramic substrate (not shown) to which the sensor  110  is attached by glueing or another suitable method. The substrate is then placed in a cavity package and mounted to a circuit board. 
     From the above, it can be appreciated that semiconductor micromachines have required various packaging and bonding steps that add significant cost. Accordingly, it would be desirable if semiconductor micromachines could be produced and packaged with reduced material and processing requirements, yet were produced in a form that protects the delicate micromachine from potential hazards within its operating environment. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a semiconductor device and method by which a device chip with a micromachine is directly surface mounted to a circuit board. Semiconductor devices in accordance with this invention generally entail a device chip with a micromachine and electrically-conductive runners that electrically connect the micromachine to appropriate signal conditioning circuitry. A capping chip is bonded to the device chip and encloses the micromachine. The capping chip has a first surface facing the device chip, an oppositely-disposed second surface, and electrical interconnects through the capping chip between the first and second surfaces. The electrical interconnects electrically communicate with the runners on the device chip, thereby providing a signal path from the micromachine to the exterior of the device. The capping chip further includes bond pads in electrical communication with the electrical interconnects. With the bond pads, the capping chip can be surface mounted to a circuit board by reflowing solder bumps formed on the bond pads. Depending on the placement of the bond pads on the capping chip, the semiconductor device can be mounted to the circuit board with the capping chip between the device chip and circuit board, or the semiconductor device can be mounted with one side of the device attached to the circuit board. 
     The method of this invention generally entails forming the device and capping chips in accordance with the above, and then bonding the capping chip to the device chip so as to enclose the micromachine within the semiconductor device and electrically connect the micromachine to the bond pads on the exterior of the capping chip. Bonding is preferably performed with solder bumps formed on the capping chip. The solder bumps are located on the capping chip so as to register with the runners on the device chip when the capping and device chips are mated. Reflowing causes the solder bumps to form solder connections that physically interconnect the runners to the electrical interconnects, and thereby electrically interconnect the micromachine to the bond pads of the semiconductor device. 
     In view of the above, a semiconductor device with a micromachine element can be manufactured and surface mounted to a circuit board without the additional steps of wire and adhesive bonding, without a chip for the sole purpose of enclosing the micromachine, and without a subpackage or cavity package to protect the micromachine. 
     Other objects and advantages of this invention will be better appreciated from the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 is a cross-sectional view of a wafer-bonded semiconductor micromachine sensor in accordance with the prior art. 
     FIG. 2 is a cross-sectional view of a wafer-bonded semiconductor micromachine sensor in accordance with a first embodiment of the present invention. 
     FIG. 3 is a cross-sectional view of the sensor of FIG. 2 surface mounted to a circuit board in accordance with a method of this invention. 
     FIG. 4 is a cross-sectional view of the sensor of FIG. 2 that has been surface mounted to a circuit board in accordance with an alternative method of this invention. 
     FIGS. 5 and 6 are cross-sectional views of wafer-bonded semiconductor micromachine devices in accordance with second and third embodiments of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 2 through 6 illustrate examples of semiconductor devices that can be fabricated and surface mounted in accordance with this invention. Each device is shown as being formed by solder bonding a device chip to a capping chip, such that a micromachine is protected in some manner by the capping chip, e.g., a micromachine is formed on the device chip and enclosed within a cavity formed by a recess in the capping chip. As evidenced from the Figures, the micromachine can have a variety of transduction configurations, including that of an actuator or a sensing element for motion, pressure, heat, light or chemical sensing. The device chips are preferably silicon, more preferably monocrystallographic silicon, though it is foreseeable that other materials could be used. The capping chips can be formed of ceramic, glass, silicon or another semiconducting material through which electrically conductive interconnects can be formed. Suitable ceramic materials include low temperature cofired ceramic (LTCC), high temperature cofired ceramic (HTCC), thick film ceramic with punched vias, thick or thin film on glass (e.g., Pyrex, etc.) or ceramic with machined vias. To better match the expansion coefficients of a ceramic capping chip with a silicon device chip, the composition of the ceramic can be modified with the addition of Pyrex or a glass frit mixed into the ceramic prior to green sheet fabrication. By matching the expansion coefficient of the device and capping chips, a more stable and durable device is produced. 
     Referring to FIG. 2, a semiconductor sensing device  10  is shown with a device chip  12  solder bonded to a capping chip  16 . A micromachine  14  formed on the device chip  12  is enclosed within a cavity  18  formed by a recess in a lower surface  22  of the capping chip  16 . The recess can be fabricated during the green tape portion of LTCC or HTCC fabrication, or formed by machining or etching after the material for the capping chip  16  is fired. As depicted, the micromachine  14  may be a resonating micromachine of a type used to sense motion, such as angular rate sensors for monitoring yaw, pitch or roll, angular and linear acceleration, and vibration sensors, as disclosed in U.S. Pat. No. 5,831,162 to Sparks et al., commonly assigned with this invention. Other types of sensing micromachines are also possible with the general configuration shown in FIG. 2, including but not limited to micromachined cantilevers for sensing motion. As known in the art, capacitive or piezoresistive sensing elements (not shown) can be used to sense motion of the micromachine  14 . 
     The micromachine  14  is shown as being electrically interconnected to bond pads  20  on the capping chip  16  by conductive runners  26  on the enclosed surface of the device chip  12  and by metal vias  28  through the thickness of the capping chip  16 , i.e., between the lower surface  22  and the upper surface  24  of the capping chip  16 . The runners  26  and metal vias  28  can be formed by any suitable method. As an example, the vias  28  may be formed during the green tape portion of LTCC or HTCC fabrication of the capping chip  16 . Alternatively, if the chip  16  is formed of thick-film ceramic, the vias  28  can be produced by punching or machining the chip  16 , and then filling with a suitable conductor material. With the bond pads  20 , the micromachine  14  and its corresponding sensing elements can be electrically interconnected with circuitry on a substrate to which the device  10  is mounted, as will be discussed in reference to FIGS. 3 and 4 below. Signal conditioning circuitry for the sensing elements can be formed on the device or capping chips  12  or  16 . 
     As shown in FIG.  2 . in a preferred embodiment of the invention, the metal vias  28  are physically and electrically connected to the runners  26  with solder connections  30  within the cavity  18 , and the capping chip  16  is attached to the device chip  12  with a solder seal ring  32  that surrounds the cavity  18  and the solder connections  30 , so that the solder connections  30  as well as micromachine  14  are protectively enclosed between the chips  12  and  16 . The chips  12  and  16  can be solder bonded in a vacuum with the seal ring  32 , with the result that the micromachine  14  is hermetically vacuum sealed within the cavity  18  to enhance the performance of the micromachine  14  if operated as a resonating or tunneling element of a yaw sensor or accelerometer. If a hermetic seal is not required, the seal ring  32  need not be continuous or even a ring. The solder bonding process by which the ring  32  bonds the chips  12  and  16  entails depositing a suitable solder composition on solderable regions of the device and capping chips  12  and  16 . These solderable regions are necessary as solder will not wet or metallurgically bond to the substrates of the chips  12  and  16 . A suitable process and materials for the solderable regions are disclosed in U.S. Pat. No. 6,062,461 to Sparks et al., commonly assigned with this invention. 
     Finally, solder bumps  34  are shown as being located on the bond pads  20 , allowing for the device  10  to be “flip-chip” mounted to an appropriate substrate, as depicted in FIGS. 3 and 4. In order to avoid remelting the solder connections  30  and seal ring  32  during solder bonding of the device  10 , the solder compositions for the solder connections  30  and seal ring  32  preferably have a higher melting or liquidus temperature than that of the solder bumps  34 . The device  10  can then be placed on a circuit board and reflowed along with other surface-mount components. In FIG. 3, the device  10  is shown placed next to a conventional surface-mount component  36  on a circuit board  38  of any suitable construction. The solder bumps  34  on the capping chip  16  are shown as having been reflowed to form solder connections  40  that physically and electrical connect the device  10  to the board  38 , so that the capping chip  16  is between the device chip  12  and the board  38 . 
     In FIG. 4, an alternative mounting orientation for the device  10  is shown, by which a side or the device  10  is attached to the circuit board  38 . By mounting the device  10  as depicted in FIG. 4, the device  10  can be oriented to respond in any axis (x, y or z) of motion. This embodiment of the invention is preferably achieved by forming wide electrical vias in the saw street of the wafer material from which the capping chip  14  is cut. The resulting metal regions  42  (one of which is shown in FIG. 4) can be plated with solder or a solderable material, and then joined with solder  44  to the circuit board  38 , so that the metal regions  42  are between the capping chip  16  and the board  38 . The metal regions  42  preferably do not contact the device chip  12  because the body of the chip  12  is typically at electrical ground. Conductive runners  46  arc shown on the surface of the capping chip  16  as electrically connecting the metal vias  28  to the metal regions  42 , in order to electrically interconnect the micromachine  14  to the circuit board  38 . Though not shown in FIG. 4, the bond pads or FIGS. 2 and 3 may also be present on the exposed (lefthand) surface of the capping chip  16 . so that the device  10  can be mounted in either manner shown in FIGS. 3 and 4. FIG. 4 also shows an optional plate  48  attached to the device chip  12  and joined with solder  50  to the circuit board  38  to provide greater stability for the device  10 . The plate  48  can be formed of any suitable, preferably nonconducting material, and may attached to the device chip  12  by gluing, solder or other suitable methods. 
     FIGS. 5 and 6 illustrate other sensing applications for a semiconductor micromachine device in accordance with this invention. In FIG. 5, a fluid-handling actuator  60  is shown mounted to a circuit board  88  in which an opening  86  has been formed through which a fluid passes before entering the actuator  60 . As shown, the actuator  60  is structured similarly to the sensing device  10  of FIGS. 2 through 4, including device and capping chips  62  and  66 , a solder seal ring  82  attaching the device chip  62  to the capping chip  66 , metal vias  78  through the capping chip  66 , and solder connections  70  and  80  by which the actuator  60  and its sensing elements are electrically interconnected with circuitry on the circuit board  88 . As with the previous embodiments, the solder connections  70  and  80  are originally in the form of solder bumps, enabling reflow soldering of the device chip  12  to the capping chip  16 , and enabling the device  60  to be “flip-chip” mounted to the circuit board  88 . In addition, the capping chip  66  is shown as being attached to the circuit board  88  with a second solder seal ring  84  that isolates the solder connections  70  and circuitry on the circuit board  88  from the fluid flowing through the actuator  60 . 
     The actuator  60  differs primarily from the sensing device  10  of FIGS. 2 through 4 by the presence of passages  64  and  68  formed in the device and capping chips  62  and  66 , respectively, which permit fluid flow to actuator elements  74  and  76  formed or attached to the device chip  62 . Suitable applications for the actuator  60  include but are not limited to ink jet printing, medical and chemical fluid analysis, and gas sensing. 
     Finally, FIG. 6 depicts an absolute pressure sensor  90  in accordance with this invention, by which a capping chip  96  is used to surface-mount a device chip  92  to a substrate, shown as the circuit board  88  of FIG.  5 . The device chip  92  is shown to have a thinned section that defines a diaphragm  94  for sensing pressure to which the thinned section is subjected. A solder seal ring  102  attaches the device chip  92  to the capping chip  96 , and defines a chamber  98  between the chips  92  and  96  that is evacuated during solder bonding and thereafter hermetically sealed under vacuum by the ring  102 , as required for sensing absolute pressure. As with the previous sensing devices of FIGS. 2-5, the sensor  90  is equipped with metal vias  108  through the capping chip  96  and solder connections  100  and  106  by which the sensor  90  and its associated sensing elements (not shown) are electrically interconnected with circuitry on the circuit board  88 . Signal conditioning circuitry for the sensing elements can be formed on the device chip  12  or a separate chip on the board  88 . The sensing elements can be of any suitable type, including capacitive and piezoresistive sensing elements of types known in the art. As with the actuator  60  of FIG. 5, the capping chip  96  is shown as being attached to the circuit board  88  with the solder connections  106  and a second solder seal ring  104 , the latter of which can be used to form an evacuated or otherwise protected region on the capping chip  96  in or on which circuits (not shown) can be formed. While described as sensing pressure, the diaphragm  94  can be equipped with heat sensing elements to provide a thermal sensing capability for such applications as bolometers and other temperature sensors, thermopiles and IR sensors. 
     Each of the semiconductor devices described above share the features of having a micromachine element and the ability to be manufactured and surface mounted to a circuit board without the additional steps of wire and adhesive bonding, without the use of a chip whose sole purpose is to enclose the micromachine, and without conventional subpackages or cavity packages for protecting the micromachine. Devices in accordance with the present invention achieve these advantages by employing a capping chip that not only provides support and protection for its device chip and micromachine, but also provides electrical interconnects that enable the device chip to be directly surface mounted (i.e., solder bonded, preferably flip-chip mounted) to a substrate without the requirement for additional packaging or bonding steps. The features of this invention are applicable to a variety of semiconductor micromachine applications in addition to those described above, and can be achieved with devices that differ in appearance from those shown in the Figures. 
     Additional advantages of this invention include the ability to stack sensing devices so that multiple devices are mounted to a substrate with a single solder-bonding operation. Another option is to enlarge the capping chip so that discrete components, such as capacitors, inductors and resistors, can be simultaneously solder-bonded to the capping chip with the device chip, or subsequently wire-bonded to the capping chip. An organic coating or soldered metal cap may be used to encapsulate or enclose the components on the capping wafer, to permit handling as a single surface-mount package. 
     While the invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art. Accordingly, the scope of the invention is to be limited only by the following claims.