Abstract:
A packaged microelectromechanical system may be formed in a hermetic cavity by forming the system on a semiconductor structure and covering the system with a thermally decomposing film. That film may then be covered by a sealing cover. Subsequently, the thermally decomposing material may be decomposed, forming a cavity, which can then be sealed to hermetically enclose the system.

Description:
BACKGROUND  
         [0001]    This invention relates generally to microelectromechanical systems (MEMS) and particularly to packaging for such systems.  
           [0002]    A MEMS device is generally a delicate mechanical structure formed by an etching technique that allows the device to move freely. As a result, there is a need to encapsulate MEMS devices for controlling the pressure and composition of the environment in which those devices operate. The devices also need to be protected from destructive processes involved in standard packaging including dicing and cleaning. In addition, there is a need to reduce the cost of packaging MEMS devices by reducing the amount of die space used by the packaging. Generally the more die space that is utilized the more expensive the resulting MEMS.  
           [0003]    Thus, there is a need for better ways to package MEMS devices. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]    [0004]FIG. 1 is an enlarged cross-sectional view of a packaged MEMS device in accordance with one embodiment of the present invention;  
         [0005]    [0005]FIG. 2 is an enlarged cross-sectional view at an early stage of manufacturing of the device shown in FIG. 1 in accordance with one embodiment of the present invention;  
         [0006]    [0006]FIG. 3 is an enlarged cross-sectional view of a subsequent stage of manufacturing in accordance with one embodiment of the present invention.  
         [0007]    [0007]FIG. 4 is an enlarged cross-sectional view at a subsequent stage of manufacturing in accordance with one of the present invention;  
         [0008]    [0008]FIG. 5 is an enlarged cross-sectional view at a subsequent stage of manufacturing in accordance with one embodiment of the present invention;  
         [0009]    [0009]FIG. 6 is an enlarged cross-sectional view at a subsequent stage of manufacturing in accordance with one embodiment of the present invention;  
         [0010]    [0010]FIG. 7 is an enlarged cross-sectional view at a subsequent stage of manufacturing in accordance with one embodiment of the present invention;  
         [0011]    [0011]FIG. 8 is an enlarged cross-sectional view at a subsequent stage of manufacturing in accordance with one embodiment of the present invention; and  
         [0012]    [0012]FIG. 9 is an enlarged cross-sectional view of another embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0013]    Referring to FIG. 1, a package  10  may include a microelectromechanical system (MEMS) device  18  within a cavity  22  defined between a cover  20  and a semiconductor structure  12 . Openings  32  in the cover  20  may be plugged with the patch  24  in one embodiment of the present invention.  
         [0014]    Electrical connections from the outside world may be made to the MEMS device  18  through an interconnection layer  16  which is buried within the semiconductor structure  12 . In particular, the interconnection layer  16  may be above a layer  14  and below a layer  13  that may be formed of any dielectric material. In one embodiment, the layer  13  is an oxide. As a result, electrical connections can be made to the MEMS device  18 , bypassing the cover  20  and avoiding the need to penetrate the cover  20 . Penetrating the cover  20  may compromise the environment within the cavity  22 , and if the cover  20  is electrically conductive, the electrical connections  16  would be electrically shorted. In some embodiments, the cavity  22  may be a vacuum cavity but in general, it may be desirable in many embodiments to maintain a hermetic seal in the cavity  22 .  
         [0015]    Referring to FIG. 2, the fabrication of the package  10  shown in FIG. 1 begins by depositing a sacrificial layer  15  on the semiconductor structure  12 . The sacrificial layer  15  may include a thermally decomposing film that may be formed for example by a spin-on process. The film may be one that decomposes to form a gas at temperatures above 350° C. in one embodiment. In one embodiment the film may be polynorbornene that decomposes at a temperature of 425° C. The preparation of polynorbornene is described in Bhusari et al., “Fabrication of Air-Channel Structures for Microfluidic, Microelectromechanical, and Microelectronic Applications,” Journal of Microelectromechanical Systems, Vol. 10, No. 3, September 2001 at page 400. Polynorbornene functionalized with triethoxysilyl (TES) adheres to oxides so the layer  13  may be an oxide in one embodiment.  
         [0016]    Referring to FIG. 3, the film  15  may be patterned using conventional techniques to form an aperture through the film  26 . As shown in FIG. 4, the MEMS device  18  may be formed, for example, by depositing and patterning techniques.  
         [0017]    Referring to FIG. 5, a second layer  25  of the thermally decomposing film may then be formed as shown in FIG. 5. As a result of the imposition of the patterned layer  15  and the MEMS device  18 , a humped configuration may result in some embodiments. As shown in FIG. 6, the layer  25  may be patterned to form edges  28 .  
         [0018]    As shown in FIG. 7, a cover  20  may be formed, for example, by a deposition, encapsulating the MEMS device  18  and the layers  15  and  25 . Openings  32  may be formed in the cover using patterning techniques in one embodiment of the present invention. The cover  20  may be formed of a variety of materials including a metal or a dielectric or a combination of metals and dielectrics that can form a hermetic barrier. The openings  32  may be patterned so that the sacrificial layers  25  and  15  may be removed by thermal decomposition.  
         [0019]    Referring to FIG. 8, the structure shown in FIG. 7 may be exposed to elevated temperatures that cause the layers  15  and  25  to thermally decompose releasing the MEMS device  18  and creating a cavity  22  beneath the cover  20 . In one embodiment the thermally decomposed material sublimates in response to heating and passes as a gas through the openings  32 . Any technique for heating the layers  15  and  25  can be used including baking or exposure to infrared or other energy sources.  
         [0020]    Referring to FIG. 1, a patch  24  may simply be deposited or printed directly onto the holes  32  to seal the cavity  22 . In one embodiment the sealing process may be done in a controlled environment so that the cavity  22  contains the desired ambient gas at the desired pressure. The holes may be positioned far enough away from the device  18  that the device  18  is not affected by that deposition process. The patch  24  may be formed of epoxy, solder, or frit glass as three examples.  
         [0021]    Referring next to FIG. 9, in accordance with another embodiment of the present invention a sealing material  34  may be formed over the entire cover  20 , sealing the holes  32  at the same time. Sealing the entire cover  20  may improve the cover&#39;s ability to maintain the hermetic cavity  22 . In one embodiment, the cover  20  may be formed without openings  32  by making the cover  20  sufficiently porous to pass the decomposed layers  15  and  25 . In such an embodiment, the sealing material  34  thereafter provides the barrier needed to seal the cavity  22 .  
         [0022]    Some embodiments of the present invention may have various advantages. For example, some embodiments may be advantageous because the release process is done at the wafer level, eliminating the need for expensive die-level processing. Particularly, the embodiments shown in FIGS.  1 - 9  may be wafers that have not yet been severed into dice. As a result, all the processing shown in those figures, in some embodiments, may be done at the wafer level. This eliminates the need for expensive die-level processing in some embodiments.  
         [0023]    In accordance with some embodiments of the present invention, a relatively smaller amount of area on a die is dedicated to encapsulating the MEMS devices  18 . Again, reducing the amount of die area devoted to the encapsulation technique reduces the cost of the resulting packaged product.  
         [0024]    In some embodiments, the release process uses a thermal decomposition film, eliminating any stiction problem. Stiction occurs in processes where a liquid etchant is used to release a MEMS structure. The liquid-vapor meniscus forces delicate mechanical elements into contact, where solid bridging, van der Waals forces and/or hydrogen bonding may result in permanent bonding of the structures.  
         [0025]    In some embodiments, the packaging process may be performed using standard deposition and etch processes. Such processes may be readily integrated into existing process flows.  
         [0026]    In addition, in some embodiments, once the device  18  is sealed, conventional integrated circuit packaging techniques may be utilized. Therefore, expensive specialty processes for MEMS packaging such as wafer bonding may not be necessary.  
         [0027]    While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.