Abstract:
A large-scale MEMS device includes a MEMS die supported by at least one compliant die mount. The compliant die mount couples the MEMS die to a support structure. The support structure is positioned within a package. In accordance with an aspect of the invention, the package is substantially symmetrical about the MEMS die. In accordance with another aspect of the invention, the support structure and/or the package is designed to have a neutral bend axis along the MEMS die.

Description:
BACKGROUND OF THE INVENTION 
     The present invention is related to packaging for MEMS devices. More particularly, the present invention is related to packaging for relatively large-scale MEMS devices where thermal expansion is problematic. 
     Microelectromechanical systems (MEMS) were first developed in the 1970&#39;s, and commercialized in the 1990&#39;s. Generally, MEMS devices are microscopic and are characterized by their ability to interact with the physical world on a small scale. MEMS devices can typically be categorized as either receiving some sort of mechanical input, such as sensors and the like, or devices that generate some sort of mechanical output, such as actuators. Given the extremely small scale of typical MEMS devices, such actuation may be extremely small-scale but still able to be used for a variety of purposes. Examples of MEMS sensors can be used to gather data related to thermal, optical, chemical, and even biological inputs. MEMS-based actuators can also be used for a variety of purposes, including, but not limited to, devices that respond and control the environment by moving, filtering and/or pumping materials. 
     While MEMS devices have traditionally been very small-scale devices (e.g. the size of a grain of sand) larger MEMS devices have also proven to be extremely useful. For example, in the field of optical communications, the movement provided by MEMS actuators can be extremely useful in providing optical switching, multiplexing and/or selective attenuation. Generally, optical MEMS devices are larger scale than the traditional microscopic structures. However, since these larger MEMS devices are electromechanical in nature, and use existing MEMS technology for fabrication, they are still considered microelectromechanical systems even though they may no longer be “micro.” 
     Thermal expansion in MEMS devices is well known and appreciated by those skilled in the art. In fact, some MEMS devices employ thermal expansion of dissimilar materials in order to generate actuation. Since MEMS devices are generally microscopic, and since thermal expansion is proportional to the size of the body expanding, expansion of non-actuating portions of MEMS structures has traditionally not been a problem. However, in larger-scale MEMS devices, such as those used in optical communication, the MEMS structure may have a dimension that exceeds 0.5 cm. This is an extremely large-scale MEMS device. Accordingly, the thermal expansion associated with such a large device can adversely impact the mechanical aspect of the MEMS device. For example, in a variable optical attenuator, actuation on the order of micro inches may make a significant difference in optical attenuation. If additional displacement is caused by undesirable displacement due to thermal expansion, non-linearities and/or unpredictable results may occur. Accordingly, there is a desire to minimize thermal effects on large-scale MEMS devices. 
     Another factor that can cause undesirable displacements in large-scale MEMS devices is the packaging itself. MEMS devices are generally relatively brittle and must be protected from the environment. Accordingly, they are generally disposed within a package of some sort. Due to constraints of size and budget, the packaging material itself is generally formed of a material that is not the same as that of the MEMS structure. Thus, the packaging will generally have a coefficient of thermal expansion that differs from the MEMS material. As the temperature of the entire package/MEMS assembly changes, thermally induced strains occur. Traditional electronics packages (including MEMS packages) generally use a die mount on a single side of the device. Differing coefficients of thermal expansion (CTE) between the die and the package can cause the die to bend. Bending the die for an electronic device is generally not a significant problem since electrical connections will usually accommodate some degree of bending. However, for MEMS devices a small bend or thermally induced strain can cause the MEMS device to malfunction or not perform its intended function as well. 
     Providing a large-scale MEMS device with improved behavior in response to thermal changes would be extremely useful. Such devices could provide more accurate optical communication devices, such as multiplexers, switches, and attenuators, for example, without significantly increasing the cost of those devices. Further, if the temperature behavior is improved significantly, thermal control of MEMS devices and even temperature sensing of such devices may be obviated. 
     SUMMARY 
     A large-scale MEMS device includes a MEMS die supported by at least one compliant die mount. The compliant die mount couples the MEMS die to a support structure. The support structure is positioned within a package. In accordance with an aspect of the invention, the package is substantially symmetrical about the MEMS die. In accordance with another aspect of the invention, the support structure and/or the package is designed to have a neutral bend axis along the MEMS die. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a MEMS structure and packaging in accordance with an embodiment of the present invention. 
         FIG. 2  is a top plan view of a MEMS die mounted within a support structure in accordance with an embodiment of the present invention. 
         FIG. 3  is a side elevation cross section view of a MEMS die and packaging in accordance with an embodiment of the present invention. 
         FIG. 4  is a perspective view of a MEMS die and packaging in accordance with another embodiment of the present invention. 
         FIG. 5  is a perspective view of yet another embodiment of a MEMS die and packaging in accordance with another embodiment of the present invention. 
         FIG. 6  is a perspective view of yet another embodiment of a MEMS die and packaging in accordance with another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention are highly useful for any large-scale MEMS device for which thermal expansion can cause undesirable dimensional changes. While embodiments of the present invention will be described with respect to an electronically variable optical attenuator MEMS device, those skilled in the art will recognize that embodiments of the present invention can be practiced with many other types of MEMS devices including, but not limited to, optical communication devices such as optical switches, multiplexers as well as any other suitable MEMS devices. 
       FIG. 1  is a perspective view of an electronically variable optical attenuator based upon MEMS technology in accordance with an embodiment of the present invention. Attenuator  10  includes a pair of fiberoptic waveguides  12  and  14  that are received through slots  16  and  18 , respectively, in support structure  20 . Waveguides  12  and  14  are coupled to MEMS die  22  which, in accordance with known MEMS technology, provides microactuation to adjust the optical coupling between ends of waveguides  12  and  14 . MEMS die  22  is a large-scale MEMS device. As used herein, large-scale MEMS device includes any MEMS structure that has a dimension that is greater than 1.0 millimeters. Additionally, a large-scale MEMS device includes traditional small-scale MEMS structures that are physically coupled to a structure having a dimension greater than 1.0 millimeters formed of a material which undergoes thermal expansion, and which expansion affects the small-scale structure. For example, the dimension between ends  24  and  26  of MEMS device  22  is preferably approximately 3 millimeters. MEMS structure  22  is preferably formed of a suitable semiconductor material, such as silicon, alumina, ceramics, etc. Generally, such materials are relatively expensive, and forming the entire device  10  out of the same material would be extremely cost prohibitive. Thus, support structure  20  is formed of a material that is different than MEMS device  22 . In a preferred embodiment, support structure  20  is actually a printed circuit board using suitable printed circuit board materials. One example of such materials is the well-known FR4 epoxy laminate material used for circuit boards. FR4 has a coefficient of thermal expansion of approximately 11 microns/m/° C. lengthwise, and 15 microns/m/° C. crosswise. FR5 epoxy laminate can also be used. Accordingly, as temperature changes, MEMS die  22  changes dimensions at a rate different than that of support structure  20 . In order to accommodate this differential thermal expansion, a relatively small gap is created between MEMS die  22  and support structure  20 . Support structure  20  includes cavity  21  within which MEMS die  22  is disposed.  FIG. 2  illustrates this gap in better detail. At least one, and preferably four, compliant die mounts are used to bridge the gap between MEMS die  22  and support structure  20 . Mounts  28  are illustrated in  FIG. 2 . Support structure  20  preferably has a number of bonding pads  30  that are coupled to associated bonding pads  32  on MEMS die  22  using known wire bonding techniques. Electrical connection from the complete device  10  to pads  30  is facilitated by lead frame  34 . Support structure  20  and each of covers  38  and  40  preferably include cooperative registration features  36  to facilitate a precise alignment of covers  38  and  40  with respect to support structure  20 . In one embodiment, support structure  20  includes alignment pins extending from both top and bottom surfaces  42 ,  44 , respectively that interact with alignment holes or recesses in covers  38  and  40 . Those skilled in the art will recognize that any suitable cooperative arrangement may be employed to effect precise alignment between support structure  20  and covers  38  and  40 . In some embodiments, covers  38  and  40  can, themselves, be circuit board populated with any suitable circuitry. 
     In order to enhance the robustness of the fiberoptic structure, a pair of strain relief boots  46  and  48  are also preferably provided for cables  14  and  12 , respectively. 
     In accordance with one aspect of the present invention, covers  38  and  40  are substantially identical to one another. Thus, they have a substantially identical shape, are formed of substantially the same materials, and have substantially the same thicknesses as one another. Accordingly, as covers  38  and  40  are mounted to support structure  20 , changes in temperature will cause equal expansions in both the covers  38  and  40  and thus will generate no, or substantially no, additional bending. While it is preferred that covers  38  and  40  be substantially identical, as long as due care is paid to each cover&#39;s contribution to bending, certain deviations can be permitted. For example, slight modifications in shape or size in one cover may be compensated by modifications in cover thickness or material in the cover. The important concept is that the relative ability of one cover to urge support structure  20  to bend in one direction as thermal expansion occurs is substantially cancelled by the opposite cover. Thus, the neutral bend axis of the entire device  10  should run substantially through the center of support structure  20  and MEMs device  22  in the plane of MEMS device  22 . 
       FIG. 2  is a top plan view of support structure  20  supporting MEMS device  22  by virtue of compliant die mounts  28 . Compliant die mounts  28  can be constructed from any suitable material that has a modulus of elasticity such that it will not impart substantially any deflection upon MEMS die  22 , but will instead accommodate dimensional changes between MEMS die  22  and support structure  20 . Preferably, compliant mounts  28  are constructed from an elastomeric material that has a thermal operating range suitable for the intended use of the finished device. While  FIG. 2  illustrates four compliant mounts  28 , it is expressly contemplated that the entire gap between die  22  and support structure  20  could be filled with a single compliant mount. Accordingly, any suitable number of compliant mounts  28 , including one, may be used in accordance with embodiments of the present invention. 
       FIG. 3  is a cross sectional elevation view of device  10  in accordance with an embodiment of the present invention.  FIG. 3  illustrates top cover  38  and bottom cover  40  each having a recess  50  to increase internal space within device  10 . The use of added internal space can help accommodate electronic circuitry which may be desirable to locate within device  10 .  FIG. 3  also illustrates compliant mounts  28  having a thickness that is substantially equal to that of support structure  20 . Compliant mounts  28  have a thickness that is preferably greater than that of MEMS die  22 . In some embodiments, compliant mounts  28  may be provided with a small channel or recess that envelops the edge of MEMS die  22  in order to increase the mechanical robustness of the structure. Finally,  FIG. 3  illustrates alignment members  41  extending both above and below the surface of support structure  20 . 
       FIG. 4  is a perspective view of an optical communication device  100  in accordance with an embodiment of the present invention. Some aspects of device  100  are similar to those of device  10 , and like components are numbered similarly. The primary difference between device  100  and device  10  is that device  100  includes circuitry  104  disposed on support structure  20 . Circuitry  104  is illustrated within region  102 . In order to accommodate circuitry  104 , support structure  20 , covers  38  and  40  have been widened to provide additional surface area for circuitry  104 . Providing electronics within device  100  can improve device response time as well as provide additional features and characteristics of the device. The addition of circuitry  104 , which is generally surface mount circuitry such as surface mount integrated circuits, surface mount resistors, surface mount capacitors, et cetera, will affect the thermal expansion characteristics of support structure  20 , it is preferred that the circuitry be applied to support structure  20  in a relatively symmetrical fashion. Accordingly, as illustrated in  FIG. 4 , similar circuitry  104  (similar at least in the sense of their physical dimensions) is disposed on opposite sides of MEMS die  22 . Further still, it is preferred that similar circuitry also be disposed on the underside of support structure  20 . As stated above, covers  38  and  40  preferably include recesses  50  that accommodate the added height of circuitry on support structure  20 . Circuitry  104  can be any suitable circuitry, including but not limited to, an analog-to-digital converter, an op amp, a capacitance measuring circuit, a piezoelectric measurement circuit, a Wheatstone resistor bridge, et cetera. 
       FIG. 5  is a perspective view of a MEMS-based optical communication device  200  in accordance with another embodiment of the present invention. Device  200  bears many similarities to devices  10  and  100 , and like components are numbered similarly. Device  200  illustrates another aspect of the present invention wherein one or both of the covers may also have electronic circuitry disposed thereon and/or therein. Notably, cover  202  is adapted to mount, either within or thereon, circuit board  204  having electronic circuitry  206 . While it is preferred that cover  40  then be similarly configured, it is possible that embodiments of the present invention can be practiced by adapting cover  40  such that its thermal expansion characteristics counteract the combined effects of thermal expansion of cover  202  and circuit board  204 . 
     While embodiments of the present invention have generally described a single-channel optical communication device, embodiments can also be practiced with multiple channel devices. For example, by maintaining relative symmetries, it may be possible to stack multiple MEMS devices on top of each other and enclose them within a pair of substantially identical covers.  FIG. 6  is a perspective view of yet another embodiment of a MEMS die and packaging in accordance with another embodiment of the present invention.  FIG. 6  illustrates large-scale MEMS device  300  having a pair of covers  38 ,  40  that enclose a plurality of support structures  20 , each having a MEMS die  22  disposed therein. Device  300  is a multi-device in that each MEMS die  22  can be used independently. Also, Additional channels can be created in side-by-side fashion as well. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. For example, While embodiments of the present invention have generally been described with respect to an electronically variable optical attenuator, those skilled in the art will recognize that embodiments of the present invention are applicable to all large-scale MEMS devices for which thermal expansion is problematic.