Abstract:
A packaging scheme for MEMS device is provided. A method of packaging MEMS device in a semiconductor structure includes forming an insulation fence that surrounds the MEMS device on the semiconductor structure. The method further includes attaching a wafer of dielectric material to the insulation fence. The lid wafer, the insulation fence, and the semiconductor structure enclose the MEMS device.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present disclosure generally relates to the field of micro-electro-mechanical systems (MEMS) and, more particularly, to the packaging of MEMS devices. 
         [0003]    2. Description of the Related Art 
         [0004]    In general, MEMS devices are miniature electro-mechanical devices of high-level integration for carrying out many different categories of functions. The various functions that can be performed in a MEMS device include sensing for motion, light, sound, radio waves, and so forth. MEMS devices can be made as standalone devices and coupled to a separate chip having circuit thereon, or can include integrated electronics and micromechanical components on a common silicon substrate. The electronic components of a MEMS device are typically fabricated using some of the same processes used for fabrication of semiconductor-based integrated circuits, but on a much larger scale. On the other hand, the micromechanical components of a MEMS are typically fabricated using micromachining processes that, for example, selectively add structural layers or etch away parts of the structure to form the mechanical and electro-mechanical portions of the device. 
         [0005]    MEMS devices typically contain delicate moving parts. Some modes of motion to be sensed include, for example, motion that causes touching of electrodes, moving parts with constant contact, moving parts without contact, and deformation. Depending on the mode and the purpose of motion, the packaging requirements of MEMS devices differ. The packaging of a MEMS device typically serves one or more functions, such as: protection of the MEMS device from the environment, provision of mechanical support, interfacing with the environment to be tested (e.g., for sensors and actuators), handling of the MEMS device after fabrication, and routing of electrical interconnections. 
         [0006]    Currently, there are a number of issues related to the packaging of MEMS devices. For instance, current passivation techniques for MEMS made by the front-side release micromachining processes require large topographies, making it difficult for passivation. Furthermore, the existing lid approach makes vacuum packaging complicated when there is huge topography in the device. Existing hermetic sealing approaches require additional contact levels defined well outside the MEMS device area in order to maintain vacuum seal for the MEMS device, consuming a lot of area on the die. 
       BRIEF SUMMARY 
       [0007]    In one embodiment, a sealed enclosure is constructed over a micro-electro-mechanical systems (“MEMS”) device. The enclosure includes a fence constructed around a perimeter of the MEMS device and a lid disposed on top of the fence to seal the enclosure. The fence is constructed in a trench so as to contain an outer dielectric portion and an inner bond-facilitating portion. The outer dielectric portion of the fence prevents the fence from conducting current between electrodes, contacts, or the like. The inner portion of the fence facilitates an adhesive bond between the fence and the lid placed on the fence. 
         [0008]    The lid hermetically seals the MEMS device while providing contact accesses to the top side of the MEMS components. In one embodiment of the invention, access openings are partially etched before bonding the wafer lid to the fence and the wafer lid is lapped back to expose the access openings to the MEMS device after bonding. In another embodiment, the access openings are completely etched prior to bonding. In another embodiment, the access openings are etched post bonding. The access openings are outside the perimeter of the sealed enclosure but are close enough to the enclosure to provide direct access to various electrodes of the MEMS device directly below it. Accordingly, the MEMS device is released prior to bonding the wafer lid to the fence of the enclosure. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0009]      FIG. 1  is a simplified schematic of a semiconductor-based device having a semiconductor structure that includes a MEMS device formed on a substrate. 
           [0010]      FIGS. 2-8  are simplified schematics progressively showing a fabrication process performed on semiconductor-based device of  FIG. 1  to provide packaging for the MEMS device according to an embodiment of the invention. 
           [0011]      FIG. 9  is a simplified schematic of a top level view of the semiconductor-based device on a wafer level after the fabrication process of  FIGS. 2-8  according to one embodiment of the invention. 
           [0012]      FIGS. 10-14  are simplified schematics progressively showing a fabrication process performed to package an example of a MEMS device. 
           [0013]      FIG. 15  is a simplified schematic of a top level view of the semiconductor-based device of  FIG. 14  after the fabrication process of  FIGS. 10-14  according to one embodiment of the invention. 
       
    
    
       [0014]    In the drawings, identical reference numbers identify similar elements or acts. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale. For example, the shapes of various elements and angles, and some of the elements are enlarged and positioned to improve understanding of the inventive features. 
       DETAILED DESCRIPTION 
       [0015]    In the description provided herewith, certain specific details are set forth in order to provide a thorough understanding of various disclosed embodiments. However, one skilled in the relevant art will recognize that embodiments may be practiced without one or more of these specific details, or with other methods, components, etc. In some instances, well-known structures or processes associated with fabrication of MEMS have not been shown or described in detail to avoid unnecessarily obscuring descriptions of the inventive embodiments. 
         [0016]    Unless the context requires otherwise, throughout the specification and claims that follow, the words “comprise” and “include” and variations thereof, such as “comprises,” “comprising,” and “including,” are to be construed in an open, inclusive sense, that is, as meaning “including, but not limited to.” 
         [0017]    Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
         [0018]    As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise. It should also be noted that the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise. 
         [0019]    As used in the specification and appended claims, the use of “correspond,” “corresponds,” and “corresponding” is intended to describe a ratio of or a similarity between referenced objects. The use of “correspond” or one of its forms should not be construed to mean the exact shape or size. 
         [0020]      FIG. 1  illustrates a semiconductor-based device  10  having a semiconductor integrated circuit  14  that is formed on a substrate  12  and includes a MEMS device  16 . The substrate  12  is a silicon wafer. The structure  18  is a topology structure that may be part of an adjacent device or that may be part of a MEMS device  16 . In one embodiment, structure  18  is an oxide structure. In another embodiment, it is a conductive polysilicon interconnect between the MEMS and the integrated circuit  14 . The semiconductor integrated circuit  14  may include any combination of CMOS transistors, bipolar transistors, and the respective layers of semiconductor material, one or more layers of dielectric material, and/or one or more layers of electrically conductive material used to form such structures and interconnect them. Likewise, the MEMS device  16  may include a structure having any combination of semiconductor material, dielectric material, and/or electrically conductive material. The general structure and fabrication process for MEMS devices and semiconductor structures that include MEMS devices are well known in the art. Therefore, detailed structures of the semiconductor structure  14  and the MEMS device  16  are not shown in  FIGS. 1-9  to avoid obscuring illustration of some of the inventive features. 
         [0021]      FIG. 2  illustrates a fabrication process performed on the semiconductor-based device  10  of  FIG. 1  according to one embodiment. An oxide layer  20  is formed on the MEMS device  16  and the semiconductor structure  14 . In one embodiment, the oxide layer  20  is a layer of tetraethyl orthosilicate (TEOS) deposited on the MEMS device  16  and the semiconductor structure  14 . In another embodiment, the oxide layer  20  is a layer of undoped silicate glass (USG) deposited on the MEMS device  16  and the semiconductor structure  14 . The oxide layer  20  is planarized after deposition on the MEMS device  16  and the semiconductor structure  14 . In one embodiment, the oxide layer  20  is planarized by chemical-mechanical polishing (CMP). 
         [0022]      FIG. 3  illustrates a fabrication process performed on the semiconductor-based device  10  of  FIG. 2  according to one embodiment. One or more openings, such as  25   a  and  25   b , are formed in the oxide layer  20 . In one embodiment, the openings  25   a  and  25   b  are formed by lithography and etching. In one embodiment, the oxide layer  20  is etched by dry etching to form the openings  25   a  and  25   b . Although two openings  25   a  and  25   b  illustrated in  FIG. 3  are examples of where the openings may be formed, the number of openings may be formed in the oxide layer  20  at different locations in various other embodiments. The locations of the openings  25   a  and  25   b  are chosen so as to allow the MEMS device  16  to be sealed by a fence structure at different possible boundary locations. A fence will be formed by materials deposited in the openings  25   a  and  25   b , as described in detail below. In one embodiment, the openings  25   a  and  25   b  surround an area of the oxide layer  20  that is directly above the MEMS device  16 . In one embodiment, the openings  25   a  and  25   b  constitute a continuous trench that surrounds an area of the oxide layer  20  that is directly above the MEMS device  16 . 
         [0023]      FIG. 4  illustrates a fabrication process performed on the semiconductor-based device  10  of  FIG. 3  according to one embodiment. A layer of dielectric material  30  is formed on the oxide layer  20  and in the openings  25   a  and  25   b  of the oxide layer  20 . Accordingly, the layer of dielectric material  30  is also deposited on the semiconductor structure  14  and the structure  18  in the openings  25   a  and  25   b  of the oxide layer  20  where portions of the oxide layer  20  have been etched away. 
         [0024]    The layer of dielectric material  30  that is deposited in the openings  25   a  and  25   b  will eventually form the outer surface of the insulation fence  50 , as will be shown in  FIG. 7 . In one embodiment, the layer of dielectric material  30  is a layer of nitride deposited on the oxide layer  20  and in the openings  25   a  and  25   b  of the oxide layer  20 . Examples of nitrides that are insulators include silicon nitride and boron nitride. In one embodiment, the layer of dielectric material  30  is a layer of silicon nitride deposited on the oxide layer  20  and in the openings  25   a  and  25   b  of the oxide layer  20 . Silicon nitride is a hard, solid substance having good shock resistance and other mechanical and thermal properties, and thus is believed to be a suitable material to form the dielectric layer  30 . The dielectric layer  30  prevents the fence from conducting current between structures of the semiconductor-based device  10 . In one embodiment, the thickness of the layer of dielectric material  30  is approximately 0.5 μm. 
         [0025]      FIG. 5  illustrates a fabrication process performed on the semiconductor-based device  10  of  FIG. 4  according to one embodiment. A layer of conductive material  40  is formed on the layer of dielectric material  30 . The layer of conductive material  40  also fills the openings  25   a  and  25   b  of the oxide layer  20 . The layer of conductive material  40  and the layer of dielectric material  30  together form the insulation fence  50 , see  FIG. 7 , to which a lid  60  will be bonded to form packaging for the MEMS device  16 . 
         [0026]    In one embodiment, the layer of conductive material  40  is a layer of polycrystalline silicon deposited on the layer of dielectric material  30  and in the openings  25   a  and  25   b  of the oxide layer  20 . In another embodiment, the layer of conductive material  40  is a layer of epitaxially-grown monocrystalline silicon. In yet another embodiment, the layer of conductive material  40  is a layer of metallic material. Because bonding with dielectric material is relatively more difficult, the use of polycrystalline silicon, metal, or epitaxially-grown monocrystalline silicon as a component of the insulation fence  50  will promote bonding between the insulation fence  50  and the lid  60 . In one embodiment, the thickness of the layer of conductive material  40  deposited on top of the layer of dielectric material  30  is approximately between 10 and 20 μm. 
         [0027]    In one alternative embodiment, the material  40  is a dielectric, such as an oxide or a nitride. The entire openings  25   a  and  25   b  are filled completely with an insulator, such as a nitride or an oxide or laminated layers of these two materials. 
         [0028]      FIG. 6  illustrates a fabrication process performed on the semiconductor-based device  10  of  FIG. 5  according to one embodiment. The portions of the layer of conductive material  40  and the layer of dielectric material  30  that are deposited on top of the oxide layer  20  are removed. In one embodiment, the layer of conductive material  40  and the layer of dielectric material  30  are etched back by CMP or an ion beam etch and etching stops at the surface of the oxide layer  20 . That is, the portions of the layer of conductive material  40  and the layer of dielectric material  30  that are deposited on top of the oxide layer  20  are etched away. The remaining portions of the layer of conductive material  40  and the layer of dielectric material  30  are those deposited in the openings  25   a  and  25   b  of the oxide layer  20 . 
         [0029]      FIG. 7  illustrates a fabrication process performed on the semiconductor-based device  10  of  FIG. 6  according to one embodiment. The oxide layer  20  is removed. In one embodiment, the oxide layer  20  is oxide etched away by hydrogen fluoride (HF). This process leaves behind a fence-like structure or the insulation fence  50 , which is formed by the remaining dielectric material  30  and conductive material  40  that were deposited in the openings  25   a  and  25   b  of the oxide layer  20 . In one embodiment, the hydrogen fluoride etch is used to release the MEMS device  16 . 
         [0030]      FIG. 8  illustrates a fabrication process performed on the semiconductor-based device  10  of  FIG. 7  according to one embodiment. It is taken generally along line  8 - 8  of  FIG. 9 . A lid  60  is attached to the insulation fence  50  to provide packaging for the MEMS device  16 . In one embodiment, the lid  60  includes a layer of dielectric material  62  and a layer of bonding material  64 . In one embodiment, the lid of dielectric material  62  is a portion of an undoped silicon wafer and the layer of bonding material  64  is a layer of glass frit. In other embodiments, the lid  60  is a glass, quartz, silicon carbide or some other airtight protective layer. In another embodiment, the layer of bonding material  64  includes a layer of eutectic material, such as gold for example. In yet another embodiment, the layer of bonding material  64  includes a layer of polycrystalline silicon. In other embodiments, the layer of bonding material  64  includes a material that is conducive to the bonding between the lid  60  and the insulation fence  50 . 
         [0031]    The lid  60  may also include a getter material  66 , which includes a reactive material to remove traces of gas and impurities from the MEMS  16  to help maintain a vacuum. 
         [0032]    In one embodiment, the space enclosed by the lid  60 , the insulation fence  50  and the semiconductor-based device  10 , which includes the MEMS device  16 , the semiconductor structure  14  and the structure  18 , is vacuum sealed. A vacuum seal such as the one disclosed herein may ensure a high vacuum over the life of the device. In other embodiments, a hermetic seal is formed by bond  64  to prevent an exchange of gasses, but there is no vacuum in the MEMS  16 . It may contain argon, ambient air or some other gas at standard atmospheric pressure. 
         [0033]    In one embodiment, the lid  60  includes one or more openings. Only two openings  51  and  55  are shown in  FIG. 8  for simplicity. The location of the openings  51  and  55  are chosen so that the openings  51  and  55  are aligned for a deep contact to be disposed after the lid wafer  60  is bonded to the insulation fence  50 . In embodiments where there are more than one locations where contact with semiconductor structure  14  is desired, there may be multiple openings. Each opening  51  and  55  in the lid wafer  60  allows access to semiconductor structure  14 , such as by a bonding wire, or deep contact, for example. In one embodiment, the lid wafer  60  is etched using tetramethylammonium hydroxide (TMAH) to create the openings  51  and  55 . In one embodiment, the openings  51  and  55  are created after the lid wafer  60  is bonded to the insulation fence  50 . In another embodiment, the openings  51  and  55  are partially created before the lid wafer  60  is bonded to the insulation fence  50 , and the lid wafer  60  is lapped back to fully expose the openings  51  and  55  after bonding. In one embodiment, the lid wafer  60  enables a generic process to be used to interface to a device created by micromachining. 
         [0034]      FIG. 9  is a simplified diagram of a top level view of the semiconductor-based device  10  on a wafer level after the fabrication process of  FIGS. 2-8  according to one embodiment. As shown in  FIG. 9 , in one embodiment, the lid  60  includes a number of openings  65  to allow access to areas of interest on semiconductor structure  14  of the semiconductor-based device  10 . 
         [0035]      FIG. 10  illustrates an example of a MEMS device  16  that may be constructed in a semiconductor-based device  70 , according to one embodiment of the invention. MEMS device  16  includes an oxide  11 , an electrode  13 , a buried oxide  15 , an oxide layer  18 , and an electrode  19 . In this embodiment, MEMS device  16  is a silicon resonator. Silicon resonators offer advantages over quartz resonators such as small size, greater robustness, and better aging performance. 
         [0036]      FIG. 10  also illustrates a fabrication process performed on the semiconductor-based device  70  according to one embodiment. An oxide layer  20  is formed on the MEMS device  16 . In other embodiment, the oxide layer  20  is a layer of tetraethyl orthosilicate (TEOS) deposited on the MEMS device  16 . In other embodiments, the oxide layer  20  is a layer of undoped silicate glass (USG) deposited on the MEMS device  16  or a buried oxide made by ion implantation of oxygen atoms through the substrate  12 . The oxide layer  20  is planarized after deposition. In one embodiment, the oxide layer  20  is planarized by chemical-mechanical polishing (CMP). 
         [0037]      FIG. 11  illustrates a fabrication process performed on the semiconductor-based device  70  of  FIG. 10  according to one embodiment. A layer of dielectric material  30  is formed on the oxide layer  20  and MEMS device  16 , and a layer of conductive material  40  is formed on the layer of dielectric material  30 . The dielectric material  30  and conductive material  40  fill openings formed within the oxide layer  20 . 
         [0038]    In one embodiment, the layer of dielectric material  30  is a layer of nitride deposited on the oxide layer  20  and the MEMS device  16 . Examples of nitrides that are insulators include silicon nitride and boron nitride. In one embodiment, the layer of dielectric material  30  is a layer of silicon nitride deposited on the oxide layer  20  and in the openings of the oxide layer  20 . The dielectric layer  30  prevents the fence from conducting current between structures of the semiconductor-based device  70 . In one embodiment, the thickness of the layer of dielectric material  30  is approximately 0.5 μm. 
         [0039]    In one embodiment, the layer of conductive material  40  is a layer of polycrystalline silicon deposited on the layer of dielectric material  30 . In another embodiment, the layer of conductive material  40  is a layer of epitaxially-grown monocrystalline silicon. In yet another embodiment, the layer of conductive material  40  is a layer of metallic material. Because bonding with dielectric material is relatively more difficult, the use of polycrystalline silicon, metal, or epitaxially-grown monocrystalline silicon as a component of the insulation fence  50  will promote bonding between the insulation fence  50  and the lid  60 . In one embodiment, the thickness of the layer of conductive material  40  deposited on top of the layer of dielectric material  30  is approximately between 10 and 20 μm. In one alternative embodiment, the material  40  is a dielectric, such as an oxide or a nitride. 
         [0040]      FIG. 12  illustrates a fabrication process performed on the semiconductor-based device  70  of  FIG. 11  according to one embodiment. The portions of the layer of conductive material  40  and the layer of dielectric material  30  that are deposited on top of the oxide layer  20  are removed. In one embodiment, the layer of conductive material  40  and the layer of dielectric material  30  are etched back by CMP or an ion beam etch and etching stops at the surface of the oxide layer  20 . 
         [0041]      FIG. 13  illustrates a fabrication process performed on the semiconductor-based device  70  of  FIG. 12  according to one embodiment. The oxide layer  20  is removed as is a portion of oxide layer  18 . In one embodiment, the oxide layer  20  is oxide etched away by hydrogen fluoride (HF). This process leaves behind a fence-like structure or the insulation fence  50 , which is formed by the remaining dielectric material  30  and conductive material. In one embodiment, the hydrogen fluoride etch is used to release the MEMS device  16  by etching away oxide  11 , part of buried oxide  15 , and part of oxide layer  18  to create MEMS space  17 . 
         [0042]      FIG. 14  illustrates further steps in the fabrication process performed on the semiconductor-based device  70  of  FIG. 13  according to one embodiment. It is taken generally along line  14 - 14  of  FIG. 15 . A lid  60  is attached to the insulation fence  50  to provide packaging for the MEMS device  16 . The lid  60  is formed and attached using the same techniques as previously described with respect to  FIGS. 7 and 8 . The lid  60  may also include a getter material  66 , which includes a reactive material to remove traces of gas and impurities from the MEMS  16  to help maintain a vacuum. In one embodiment, the space enclosed by the lid  60 , the insulation fence  50  and the MEMS device  16  is vacuum sealed. In one embodiment, a hermetic seal is formed by bond  64  to prevent an exchange of gasses, but there is no vacuum in the MEMS  16 . 
         [0043]    In one embodiment, the lid  60  includes one or more openings, such as openings  74 ,  76 , and  78 . The locations of the openings  74 ,  76 , and  78  are chosen so that the openings  74 ,  76 , and  78  are aligned for deep contacts  68  and  69  to be disposed after the lid wafer  60  is bonded to the insulation fence  50 . As shown, deep contact  68  makes contact with electrode  19 , and deep contact  69  makes contact with electrode  13 . The openings  74 ,  76 , and  78  enable electrical contact with the MEMS device  16  from above the lid wafer  60 . 
         [0044]    Conventional prior art lid approaches use contacts disposed well outside the device area in order to maintain an air tight seal on the MEMS device, therefore the embodiments of the present disclosure manifest the ability to build MEMS devices in less area, thereby increasing the number of die per wafer. 
         [0045]    In one embodiment, additional integrated circuits are constructed above lid wafer  60 , and the additional circuits utilize the deep contacts  68  and  69  to provide signals to and receive signals from the MEMS device  16 . In this embodiment, the lid  60  is a fully functional semiconductor silicon wafer having integrated circuits formed thereon. A group of CMOS logic circuits having full transistors with sources, drains and channel regions are formed in the upper side of the lid wafer  60 . One surface of the lid wafer may therefore be an active surface with integrated circuits formed therein. The lid wafer  60  in this embodiment performs two functions: an air tight seal as a lid to the MEMS and a semiconductor substrate for active transistors. 
         [0046]    In one embodiment, the openings  74 ,  76 , and  78  are partially created before the lid wafer  60  is bonded to the insulation fence  50 , and the lid wafer  60  is lapped back to fully expose the openings  74 ,  76 , and  78  after bonding. In this embodiment, the active surface of the lid wafer may be the downward facing side, in the regions near the through hole  74  and the other side of the MEMS. 
         [0047]      FIG. 15  is a simplified diagram of a top level view of the semiconductor-based device  70  after the fabrication process of  FIGS. 10-14  according to one embodiment. As shown in  FIG. 14 , in one embodiment, the lid  60  includes a number of openings  74 ,  76 , and  78  to allow access to areas of interest on MEMS device  16  of the semiconductor-based device  70 . 
         [0048]    Thus, a fabrication process for packaging MEMS devices, such as the MEMS device  16 , is disclosed. It is believed that the process will ensure high integrity of vacuum seal over the life time of the MEMS device  16 . The use of dielectrics in building the insulation fence  50  should ensure electrical contacts and the lid wafer  60  are isolated. Unlike prior art fabrication processes, packaging for MEMS devices built by embodiments of the disclosed fabrication process should require less area, thereby preserving die area for other use. Furthermore, the disclosed fabrication process, and the various embodiments thereof, is believed to be a generic process that can be adopted flexibly and easily for a wide range of MEMS devices fabricated by micromachining technology. 
         [0049]    The above description of illustrated embodiments, including what is described in the Abstract, is not intended to be exhaustive or to limit the embodiments to the precise forms disclosed. Although specific embodiments of and examples are described herein for illustrative purposes, various equivalent modifications can be made without departing from the spirit and scope of the disclosure, as will be recognized by those skilled in the relevant art. The teachings provided herein of the various embodiments can be applied to other context, not necessarily the exemplary context of completely sealing a MEMS device with an insulation fence and a lid wafer generally described above. For example, if a part of the MEMS device  16  is to be exposed and not sealed by the insulation fence  50  and the lid wafer  60 , the locations of the openings  25   a  and  25   b  in the oxide layer  20  can be chosen so that the resultant insulation fence  50  leaves the particular part of the MEMS device  16  exposed. 
         [0050]    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.