Abstract:
Embodiments relate to sensor and sensing devices, systems and methods. In an embodiment, a micro-electromechanical system (MEMS) device comprises at least one sensor element; a framing element disposed around the at least one sensor element; at least one port defined by the framing element, the at least one port configured to expose at least a portion of the at least one sensor element to an ambient environment; and a thin layer disposed in the at least one port.

Description:
RELATED APPLICATION 
     This application is a continuation of application Ser. No. 12/835,111 filed Jul. 13, 2010, which is hereby fully incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The invention relates generally to integrated circuit (IC) packaging and more particularly to micro-electromechanical systems (MEMS) packages having one or more ports. 
     BACKGROUND 
     Integrated circuit micro-electromechanical system (MEMS) devices can require specialized packages or packaging processes. Some MEMS devices, such as pressure sensors, often require specialized packaging solutions to accommodate one or more external pressure ports because at least a portion of the sensor chip surface needs to be exposed to the ambient environment without the common protection of mold compound and/or a package wall. On the other hand, bond pads and bond wires of the sensor die present critical challenges with respect to external media compatibility and need adequate protection. These opposing needs present challenges for the design of package solutions, often leading to specialized solutions that are not standardized, necessitating changes to the wafer and assembly processes that are complicated and expensive. 
     For example, conventional MEMS pressure sensors often have a premolded cavity package, with the cavity opened or exposed on one side for the sensor die and bond wires. The chip surface with the sensing element faces the open portion of the cavity package. The sensor die and bond wires can be covered with a gel or other suitable material filled into the cavity package to protect the bond pads, bond wires and the sensing elements from external media. One disadvantage of such a conventional package design is that the gel often cannot provide adequate protection against aggressive media. As previously mentioned, this is most critical for the sensitive bond pads and bond wires. Another disadvantage is the relatively high material cost for the premolded package, leading to higher product cost. Further, this package type is not compatible with relative pressure sensor applications, discussed in more detail below. 
     Another conventional solution is to use a standard leadframe and to form a cavity during the mold process with a specially shaped mold tool. In this case the sensor die bonding into the formed cavity and the wire bonding are done after the mold process. This approach also usually requires a protective gel on the sensor die. The previously mentioned disadvantages relating to media compatibility and the incompatibility for relative pressure sensor applications are also applicable here. 
     Yet another package solution is to use a leadframe with a predefined pressure port in combination with a specially shaped mold tool to create a pressure port, for example at the backside of the package. In this case the sensing element faces the backside of the sensor chip in order to create a pressure port by attaching the sensor die with the sensing element on top of the pressure port in the package backside. The bond pads and the bond wires on the frontside of the chip can be covered with the mold compound. A disadvantage of this approach is a higher package cost compared with that of a standard package. Special and cost-intensive MEMS processes are required at the wafer process to create the MEMS structure with the backside opening on the sensor die. Absolute and relative pressure sensing applications require different wafer and assembly processes in this case, which is not desired. 
     SUMMARY 
     Embodiments relate to sensor and sensing devices, systems and methods. In an embodiment, a micro-electromechanical system (MEMS) device comprises at least one sensor element; a framing element disposed around the at least one sensor element; at least one port defined by the framing element, the at least one port configured to expose at least a portion of the at least one sensor element to an ambient environment; and a thin layer disposed in the at least one port. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which: 
         FIG. 1A  is a cross-sectional view of an absolute pressure sensor during fabrication according to an embodiment. 
         FIG. 1B  is a top view of the sensor of  FIG. 1A . 
         FIG. 2A  is a cross-sectional view of an absolute pressure sensor during fabrication according to an embodiment. 
         FIG. 2B  is a top view of the sensor of  FIG. 2A . 
         FIG. 3  is a cross-sectional view of an absolute pressure sensor during fabrication according to an embodiment. 
         FIGS. 4A and 4B  are cross-sectional views of an absolute pressure sensor in a mold tool according to an embodiment. 
         FIG. 5A  is a cross-sectional view of an absolute pressure sensor according to an embodiment. 
         FIG. 5B  is a top view of the sensor of  FIG. 5A . 
         FIG. 6  is a cross-sectional view of an absolute pressure sensor according to an embodiment. 
         FIG. 7  is a flowchart according to an embodiment. 
         FIG. 8A  is a cross-sectional view of an absolute pressure sensor according to an embodiment. 
         FIG. 8B  is a top view of the sensor of  FIG. 8A . 
         FIG. 9A  is a cross-sectional view of an absolute pressure sensor according to an embodiment. 
         FIG. 9B  is a top view of the sensor of  FIG. 9A . 
         FIG. 10A  is a cross-sectional view of a relative pressure sensor according to an embodiment. 
         FIG. 10B  is a top view of the sensor of  FIG. 10A . 
         FIG. 11A  is a cross-sectional view of a relative pressure sensor according to an embodiment. 
         FIG. 11B  is a top view of the sensor of  FIG. 11A . 
         FIG. 12A  is a cross-sectional view of an inertial pressure sensor according to an embodiment. 
         FIG. 12B  is a top view of the sensor of  FIG. 12A . 
     
    
    
     While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
     Embodiments of the invention relate to packages and packaging methods for IC sensors. In particular, embodiments can provide standardized and low-cost package solutions for MEMS pressure sensors, improving media compatibility and covering a wider range of sensor applications. Standardized package and packaging methods and processes in the assembly line are made possible at least in part by predefining a pressure port for the pressure sensing element at the wafer-level process before assembly, thereby enabling use of standard package solutions, mold tools and assembly lines and processes. 
     In one embodiment, the bond pads and bond wires of the IC sensor are covered and thus protected by a mold compound, similar to ICs in a standard package, while the predefined pressure port exposes necessary portions of the sensor to provide operability. The sensor device at the pressure port of the package is typically the only portion of the chip surface exposed to the environment. Because the sensor device itself is usually less sensitive to environmental conditions, the media compatibility of the pressure sensor is improved. To further improve the media compatibility, the package concept can be combined with additional features like a protective gel over the sensor device in the pressure port to avoid, for example, moisture, frozen media, debris and mechanical impact on the sensor membrane. 
     Embodiments also relate to relative pressure sensing, in which two pressure ports can be achieved by implementing at least two sensing elements with appropriate positioning on the chip surface and the corresponding predefined pressure ports. The wafer and assembly processes can be the same as for absolute pressure sensor embodiments having a single pressure port. An advantage for relative pressure sensor applications according to this approach is that the major impact of the package stress on the output signal is eliminated due to the fact that the two sensing elements corresponding to the different pressure ports are implemented in a differential signal processing scheme in order to measure only the differential pressure. Because both sensing elements are exposed to a similar mechanical stress level, the corresponding signal-shift is eliminated in the relative pressure signal output. Another advantage is that a wider range of sensor applications can be implemented with a common wafer technology and assembly line platform. 
     Embodiments also have applicability to other sensor applications. For example, optical sensors can also have a port or aperture in the sensor package and thereby benefit from embodiments disclosed herein. Therefore, the pressure sensing embodiments discussed herein are not limiting. 
       FIGS. 1-7  depict fabrication stages of an embodiment of an absolute pressure sensor  100 . Referring to  FIG. 1 , sensor  100  comprises a sensor die  102  having at least one pressure sensing device  104 . Each pressure sensing device  104  includes a cavity  106  and a membrane  108 . Die  102  includes bond pads  110  and, optionally, other electrical devices such as transistors  112 , resistors and/or capacitors  114 . At this stage, sensor  100  can be similar to or the same as conventional sensor devices. 
     In  FIG. 2 , however, sensor  100  is modified to comprise a frame  116  fabricated during the wafer process. Frame  116  establishes a predefined pressure port  118  by creating a tight structure, protecting the active membrane structure of sensing device  104 , as discussed in more detail below. Frame  116  can comprise a polymer or other suitable material, and pressure port  118  can comprise a polymer and have vertical dimensions (with respect to the orientation of the drawing on the page) in the range of about 100 micrometers (μm) to about 500 μm in embodiments. A fabrication process for pressure port  118  can be a wafer-coating with a photosensitive epoxy lacquer and subsequent photolithography steps for patterning the pressure port structure in an embodiment, though other processes can be used in other embodiments. For example, in an embodiment the pressure port material comprises a polymer and can be patterned by photolithography with an etch process omitted given the polymer material properties. Preparing sensor  100  with frame  116  at the wafer level enables use of a standard IC assembly line and package solution for sensor  100 . 
     In  FIG. 3 , sensor  100  is shown die-attached to leadframe  120 . Leadframe  120  is coupled by wire bonds  122  to bond pads  110 . Other coupling means and mechanisms can be used in other embodiments, as appreciated by those skilled in the art. 
       FIG. 4  depicts stages of a mold process. In  FIG. 4A , sensor  100  is depicted within a mold tool  124 . Mold tool  124  comprises an inlet  126  for a mold compound and an outlet  128  for air. In other embodiments, the positions and configurations of inlet  126  and outlet  128  can vary. Embodiments having frame  116  and predefined pressure ports enable use of standard rather than customized mold tools. As can be seen in  FIG. 4 , the top of frame  116  seals against an inner surface  125  of mold tool  124  during the mold process such that mold compound cannot enter predefined pressure ports  118 . In  FIG. 4B , a mold compound  130  has been filled into mold tool  124 , enclosing and protecting sensor  100  and bond wires  122  while leaving the chip surface at pressure port  118  exposed. 
       FIG. 5  depicts sensor  100  with mold tool  124  removed. Sensor  110  is now enclosed, or “packaged,” by mold compound  130 , with pressure port  118  uncovered and unfilled such that membrane  118  can be exposed to the ambient environment. 
       FIG. 6  depicts another embodiment of sensor  100 . In the embodiment depicted in  FIG. 6 , sensor  100  includes an optional package cover  132  and an optional gel  134 . Cover  132  can be formed of a typical cover material, for example metal, as understood by one skilled in the art and can be desired according to an application or end use of sensor  100 , for example, to create a port for coupling sensor  100  to a pipe or other structure. In an embodiment, optional cover  132  is glued or otherwise suitably affixed to sensor  100 . Cover  132  can be omitted, for example, when the application of sensor  100  is to measure barometric pressure, among others. 
     Optional gel  134  can provide protection of pressure sensing device  104  from external media, mechanical impact and the like. Various embodiments of sensor  100  can comprise one, both or neither of optional cover  132  and optional gel  134 . Further, embodiments of sensor  100  having a plurality of pressure sensing devices  104  can include optional gel  134  over some but not all pressure sensing devices  104 . In embodiments, frame  116  and predefined pressure port  118  enables the use of frame  116  as a predefined bound for gel  134 . In some embodiments, frame  116  can provide separate compartments or areas for gel  134 , as depicted in  FIG. 6 . 
       FIG. 7  is a flowchart of an embodiment of the above-discussed process. At  150 , at least one pressure port is defined at wafer level by adding a frame to the sensor. Refer also to  FIG. 2 . The port need not be a pressure port in every embodiment, however, as the process has applicability to, for example, optical sensors and inertia sensors, among others. At  160 , the sensor die is attached to the leadframe and coupled thereto via bond wires or other suitable coupling means. Refer also to  FIG. 3 . At  170 , the sensor is placed in standard mold tool, and at  180  the mold tool is filled with a mold compound. Refer also to  FIG. 4 . At  190 , the sensor is removed from the mold tool, now packaged in the mold compound. Optional post-processing, including placing a protective gel over the sensing membrane and/or adding a package cover, can also be done. Refer also to  FIG. 6 . 
     The concept of predefining a sensor port, such as for pressure sensors, optical sensors and other sensors in embodiments, can be flexible and adapted for different wafer technologies. While a surface micro-machining technology can be applicable to the embodiments of  FIGS. 1-6 , a bulk micro-machining technology can be used in other embodiments. Referring to  FIG. 8 , another embodiment of a pressure sensor  200  is depicted. Sensor  200  comprises a sensor die  202  having a top wafer  203  and a bottom wafer  205 . Top wafer  203  defines a sensor membrane portion  208  and cavity  206 , with bottom wafer  205  providing a hermetic seal of cavity  206 . Top wafer  203  also includes piezoresistors  207  and bond pads  210 . Similar to sensor  100  discussed above, sensor  200  comprises a frame  216  that isolates predefined pressure port  218  during fabrication, enabling use of standard processing and packaging methodologies and mold tools. Packaged in mold compound  230 , sensor  200  also comprises leadframe  220  and bond wires  222  and, optionally, package cover  232  and protective gel  234 . 
     Sensor  200  uses a piezoresistive effect to sense pressure. In an embodiment, piezoresistors  207  can measure the stress or strain in sensor membrane portion  208  of top wafer  203 . This type of sensor can have applicability in, for example, a tire pressure monitoring system (TPMS), though sensor  200  can also be suitable in other applications and uses.  FIG. 8B  is a top view of sensor  200  of  FIG. 8A , with optional cover  232  and gel  234  omitted. 
     The embodiment of sensor  200  depicted in  FIGS. 9A and 9B  is similar to that of  FIGS. 8A and 8B  but comprises two chips. In  FIGS. 9A and 9B , sensor  200  further comprises an application-specific integrated circuit (ASIC) die  236 . ASIC die  230  is attached to leadframe  220  and coupled to die  202  by a bond wire  222 . As in other embodiments, cover  232  and gel  234  are optional and omitted from the view of  FIG. 9B . 
     As can be seen in embodiments of sensors  100  and  200  discussed herein, the definition and use of a port predefined at the wafer level of a MEMS sensor device enables use of standardized packaging processes and solutions, avoiding the need for a special mold tool and thereby providing lower-cost solutions while at the same time not interfering with the function or use of the sensor device. Embodiments also provide increased flexibility with respect to wafer technologies and product and application concepts. While sensors  100  and  200  generally relate to absolute pressure sensors, embodiments also have applicability to relative or differential pressure sensing applications. 
     Referring to  FIG. 10 , an embodiment of a relative pressure sensor  300  is depicted. Sensor  300  is similar to sensor  100  of  FIG. 6  but configured for relative or differential pressure sensing applications. In particular, sensor  300  comprises at least two predefined pressure ports  318   a  and  318   b  and sensing devices  304   a  and  304   b  that are sufficiently separated such that relative pressure can be determined, as understood by one having skill in the art. In the embodiment of  FIG. 10 , sensor  300  can comprise an optional cover  332 , which can connect the interfaces of both ports  318   a  and  318   b  with the package, providing separately coupleable ports for each of sensing devices  304   a  and  304   b . The top view of sensor  300  in  FIG. 10B  omits optional gel  334  and package cover  332 . 
     As in other embodiments, sensor  300  comprises a frame  316  that provides a cost-effective way to use standard packaging processes and assembly lines. In the case of relative pressure sensor  300 , frame  316  also provides a simple way to form two pressure ports, avoiding the conventional approach of forming a pressure port on each of the front- and back-side of the chip surface. Relative or differential pressure sensor  300  can have applicability, for example, in measuring the pressure drop across a filter, among other applications and uses. 
     Referring to  FIG. 11 , another embodiment of a relative pressure sensor  300  is depicted. Sensor  300  of  FIG. 11  is similar to sensor  300  of  FIG. 10 , including at least two separated pressure ports  318   a  and  318   b , but differs by comprising piezoresistors  307 , similar to sensor  200  of  FIG. 8 . Similar reference numerals are used herein throughout to refer to similar features in various embodiments, such as here in  FIGS. 8 and 11  with the first number differing according to sensor  200  or sensor  300 . Cover  332  and gel  334  are, as in other embodiments, optional and are shown in  FIG. 11A  but omitted from  FIG. 11B . 
     Referring to  FIG. 12 , an embodiment of a packaged inertial sensor  400  having a predefined pressure port is depicted. Inertial sensor  400  comprises a sensor die  402  having at least one pressure sensing device  404 . As in other sensor embodiments discussed above, inertial sensor  400  can comprise a plurality of pressure sensing devices  404 . Each pressure sensing device  404  includes a cavity  406  and a sensor membrane  408 . Sensor die  402  also comprises a plurality of bond pads  410  and can include optional electrical devices, such as transistors  412 , resistors and/or capacitors  414  in embodiments. A frame  416  is used to predefine pressure port  418  during the mold compound package forming stages. 
     In an embodiment, sensor  400  can be configured as an inertial sensor by filling only one side of pressure port  418  with a material, such as a protective gel  434  or other suitable medium. Gel  434  thus covers only one sensing element  404   a  and acts as an inertial mass, causing an equivalent “inertial” pressure load on sensing element  404   a  under inertial load. By signal processing the differential signal between the covered ( 404   a ) and uncovered ( 404   b ) sensor devices, the inertial load can be sensed independently from any applied pressure load. This is due to the fact that in differential mode the load of physical pressure on covered and uncovered devices is equal and thus eliminated by differential signal processing. Only the inertial load leads to a non-zero differential signal because of the different inertial mass on covered and uncovered sensing elements  404   a  and  404   b . The inertial mass and thus the inertial sensor sensitivity of sensor  400  can be adjusted in various embodiments by modifying the type or amount of filling material  434 , for example by using a gel with embedded grains of copper or gold to provide a flexible filling material with high mass. 
     In an embodiment, pressure and inertia sensor features can be combined on a single die by signal processing in differential mode and standard mode with an added signal of covered and uncovered sensor devices, as understood by one skilled in the art. This provides a high degree of flexibility for a multitude of sensor applications and uses with reduced design effort, fabrication complexity and overall device cost. 
     Embodiments therefore relate to packages and packaging methods for IC sensors. In particular, embodiments can provide standardized and low-cost package solutions for MEMS pressure sensors, improving media compatibility and covering a wider range of sensor applications. Standardized package and packaging methods and processes in the assembly line are made possible at least in part by predefining a pressure port for the pressure sensing element at the wafer-level process before assembly, thereby enabling use of standard package solutions, mold tools and assembly lines and processes. In at least one embodiment, this is accomplished by implementing a frame structure at wafer level that seals a predefined pressure port to a mold tool, preventing the mold compound 
     Other embodiments relate to other types of sensors, including relative or differential pressure sensors, inertia sensors and combinations thereof, such as a single sensor die having both an absolute pressure sensor and an inertia sensor. Further, a plurality of single type of sensor can also be combined in a single sensor device. The flexibility provides applicability to a variety of 
     Various embodiments of systems, devices and methods have been described herein. These embodiments are given only by way of example and are not intended to limit the scope of the invention. It should be appreciated, moreover, that the various features of the embodiments that have been described may be combined in various ways to produce numerous additional embodiments. Moreover, while various materials, dimensions, shapes, implantation locations, etc. have been described for use with disclosed embodiments, others besides those disclosed may be utilized without exceeding the scope of the invention. 
     Persons of ordinary skill in the relevant arts will recognize that the invention may comprise fewer features than illustrated in any individual embodiment described above. The embodiments described herein are not meant to be an exhaustive presentation of the ways in which the various features of the invention may be combined. Accordingly, the embodiments are not mutually exclusive combinations of features; rather, the invention may comprise a combination of different individual features selected from different individual embodiments, as understood by persons of ordinary skill in the art. 
     Any incorporation by reference of documents above is limited such that no subject matter is incorporated that is contrary to the explicit disclosure herein. Any incorporation by reference of documents above is further limited such that no claims included in the documents are incorporated by reference herein. Any incorporation by reference of documents above is yet further limited such that any definitions provided in the documents are not incorporated by reference herein unless expressly included herein. 
     For purposes of interpreting the claims for the present invention, it is expressly intended that the provisions of Section 112, sixth paragraph of 35 U.S.C. are not to be invoked unless the specific terms “means for” or “step for” are recited in a claim.