Abstract:
Systems and methods for reducing stiction between elements of a microelectromechanical systems (MEMS) device during anodic bonding. The MEMS device includes a substrate cover with an optional conductor on its interior surface and the cover is anchored to a first portion of a sensing element. The MEMS device further includes a second portion of the sensing element separated from the substrate cover with a space and an antistiction element disposed between the second portion and cover. The antistiction element can be formed of a material type with high electrostatic resistance, to prevent stiction between MEMS device elements during anodic bonding.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Certain microelectromechanical (MEMS) sensor devices include both an upper and a lower covering with a space-gap interposed therebetween. This space gap can contain a substrate wafer that acts as a sensing or actuating mechanism for the MEMS device. The gap is formed between recessed areas at the periphery of the upper and lower coverings, and the substrate wafer can be hermetically sealed between the two coverings in a very sensitive anodic bonding process. 
         [0002]    During an anodic bonding process, a secured substrate wafer is first bonded to the lower covering at raised contact regions at the covering&#39;s periphery edge. This process can involve the application of high temperatures and an electric potential of several hundred to a few thousand volts. Next, wafer elements that constrain device movement in a plane orthogonal to the covering are removed and the upper covering is similarly bonded to both the unsecured substrate wafer and the lower covering at raised contact regions at the covering&#39;s periphery edges. 
         [0003]    The physical bonding occurs as a result of a current that flows between the substrate wafer and the coverings at their points of contact. The strength of this bond is proportional to the magnitude of electric potential applied during the bonding process. 
         [0004]    Unfortunately, when too high an electric potential is applied across a covering, an undesirable electrostatic effect occurs, which is commonly known as stiction. For example, during the bonding of the upper covering, upwardly compliant component on the substrate wafer can adhere to a conductor component on the bottom surface of the upper covering. This stiction can render a MEMS device unusable. 
         [0005]    Therefore, there remains a need for an effective deterrent to stiction between sensitive MEMS device components in the anodic bonding process. It would be advantageous if this deterrent could increase the voltage threshold point at which stiction occurs, thereby increasing MEMS device production yield, while at the same time creating a more robust MEMS device. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention provides systems and methods for preventing stiction between MEMS device components in an anodic bonding process. In accordance with one aspect of the present invention a MEMS device includes a substrate cover with an interior surface, anchored to a first portion of a sensing or actuating element, an optional conductor residing on the interior surface of the substrate cover, a second portion of the sensing element separated from the substrate cover with a space, and an antistiction element disposed between the second portion of the sensing element and the cover to prevent stiction during anodic bonding. 
         [0007]    In accordance with further aspects of the invention, the conductor, the cover or the second portion of the sensing element can include the antistiction element. 
         [0008]    In accordance with another aspect of the invention, the conductor or cover can include bumples (small volumes that protrude from the surface) or strips that reduce a contact surface area between the second portion of the sensing element and the conductor. 
         [0009]    In accordance with other aspects of the invention, the antistiction element can be formed from Titanium Nitride, Titanium Tungsten, Tungsten, Ruthenium, Rhodium, or Iridium or other similar materials. 
         [0010]    In accordance with still further aspects of the invention, the first portion of the sensing element can be bonded to the periphery edge of the substrate cover with application of an electric potential. 
         [0011]    In accordance with still further aspects of the invention, the applied electric potential is a voltage greater than 200 volts. 
         [0012]    In yet further aspects of the invention, a method for preventing stiction between MEMS device components in an anodic bonding process includes bonding a first substrate cover to a first portion of a sensing or actuating element, disposing an antistiction element between a second portion of the sensing or actuating element and an interior surface of the second substrate cover, and bonding the first portion of the sensing or actuating element to a second substrate cover, such that the antistiction element prevents stiction of the second portion of the sensing element, when an electric potential is applied. 
         [0013]    As will be readily appreciated from the foregoing summary, the invention provides means for improving the production yield of sensitive MEMS devices by deterring stiction between device components during anodic bonding. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings: 
           [0015]      FIG. 1  is a cross-sectional view of a MEMS device in accordance with an embodiment of the present invention; 
           [0016]      FIG. 2  is a top perspective view of a MEMS device with its upper covering removed in accordance with an embodiment of the present invention; 
           [0017]      FIG. 3  is a top perspective view of a MEMS device with its upper covering removed in accordance with another embodiment of the present invention; and 
           [0018]      FIG. 4  is a cross-sectional view of a MEMS device in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    The present invention provides systems and methods for reducing stiction between elements of a microelectromechanical systems (MEMS) sensor or actuator device during anodic bonding.  FIG. 1  illustrates a MEMS device  10  in accordance with an embodiment of the present invention. The components of the MEMS device  10  include: an upper substrate cover  12 , a lower substrate cover  14 , an upper conductor  16 , a lower conductor  18 , a sensing or actuating element  20  in a device layer  22 , an upper anchor  24 , a lower anchor  26 , and one or more antistiction elements  28 . 
         [0020]    During fabrication of the MEMS device  10 , the lower substrate cover  14  can be configured to include a lower conductor  18  that resides on an interior surface of the lower substrate cover  14 . In an embodiment, the lower conductor  18  includes the antistiction elements  28 . In accordance with a first bonding process, a single wafer substrate that includes the sensing or actuating element  20  in the device layer  22  are bonded to the lower substrate cover  14  utilizing the lower anchor  26  and an anchor portion of the lower conductor  18  (this would be thermal compression bonding) as bonding agents. The first bonding process can include application of an electric potential of sufficient magnitude to induce a bonding current amongst the lower substrate cover  14 , including the lower anchor  26 , and the exterior portions of the single wafer substrate that include the sensing or actuating element  20  and the device layer  22 . 
         [0021]    The device layer  22  bonded to the lower substrate cover  14  and to the anchor  26  in the first bonding process can then be segmented through an etching process (or other type of removal process) that is irreversible. The segmented portions include the sensing or actuating element  20  and other components. In an embodiment, the sensing or actuating element  20  includes a first portion and a second portion. In this embodiment, the etching that separates the single wafer substrate, occurs after the first bonding process. Therefore, there is no chance of electrically induced stiction between device elements during bonding, as the MEMS device elements are each attached with no freestanding portions. 
         [0022]    The next step of the fabrication process can include a second bonding process where the first portion of the sensing or actuating element  20  bonded to the upper substrate cover  12  utilizing the upper anchor  24  as a bonding agent (see comments above). In one embodiment, the upper conductor  16  resides on the interior surface of the upper substrate cover  12 , and the upper conductor  16  includes the antistiction elements  28 . The antistiction elements  28  are designed to prevent stiction between the second portion of the sensing or actuating element  20  and the upper conductor  16 , when an electric potential is applied between these two features during the second bonding process. 
         [0023]    The MEMS device elements being bonded during the second bonding process can be bonded together through application of an electric potential of several hundred to a few thousand volts. Both the magnitude of a bonding current induced between MEMS device elements being bonded and the strength of the ensuing physical bond are proportionate to the magnitude of the electric potential applied across the upper substrate cover  12  during the second bonding process. The antistiction elements  28  are designed to prevent stiction as the second portion of the sensing or actuating element  20  deflects (arcing implies an electrical arc, in this case it is a physical deflection) towards the upper substrate cover  12  during application of a predetermined electric potential in the second bonding process. In one embodiment, the antistiction elements  28  are designed to resist stiction with the application of electric potential greater than  200  volts during the second bonding process. 
         [0024]    As shown in  FIGS. 2 and 3 , a MEMS device  29  includes a sensing or actuating element  20 - 1  that includes one or more bumple elements  28 - 1 . The bumple elements  28 - 1  reduce a contact surface area between a deflected portion of the sensing or actuating element  20 - 1  and upper conductor cover  12  during the second anodic bonding process. The bumple elements  28 - 1  are located on one or both sides of the sensing or actuating element  20 - 1 . The bumples or strips can be formed using one of several methods: 1) using photolithography and a subsequent “lift-off” process; 2) using an aperture or shadow mask. 
         [0025]    As shown in  FIG. 4 , a MEMS device  30  includes a sensing or actuating element  34  that includes strip elements  32 . The strip elements  32  reside on the surface of the sense or actuating element  34 . 
         [0026]    In an embodiment, a MEMS device component&#39;s material type can be fabricated from a plurality of materials having specialized conductive or insulating properties. In one embodiment, the antistiction elements ( 28 - 1  and  32  of  FIGS. 1-4 ) are formed from Titanium Nitride, Titanium Tungsten, Tungsten, Ruthenium, Rhodium, or Iridium. The antistiction elements  28  are formed of a conductive material such as Gold, but may be formed on a non-conductive material. In another embodiment, the upper and lower covers ( 12  and  14 ) are formed of a glass substrate and the sensing or actuating elements ( 20 ,  20 - 1  and  34 ) are formed of a Silicon substrate. 
         [0027]    In an embodiment, the bumples or strips are coated to include multiple layers of materials, such that the outer layer is more resistant to stiction induced bonding with the substrate (e.g., Silicon) of the sensing or actuating elements ( 20 ,  20 - 1  and  34 ). One Example of an outer layer that is particularly resistant to electrostatic bonding with Silicon is Graphite. In another embodiment, a sensing or actuating element is hermetically sealed between the covers of the MEMS device during fabrication. 
         [0028]    Example dimensions for the bumbles are ˜5 μm×5 μm×0.1 μm (height) and larger. Strips would be ˜5 μm wide×several hundred microns long×˜0.1 μm (height). 
         [0029]    While various embodiments of the invention have been illustrated and described, many changes can be made without departing from the spirit and scope of the present invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined by reference to the claims that follow.