Abstract:
A conductive bridge in a second conductive layer may be utilized to join a pair of spaced apart conductive strips in a first conductive layer. A gap between the first and second strips may be bridged by the bridge while isolating both the first and second strips and the bridge itself from another conductor which extends through the gap between the first and second strips.

Description:
BACKGROUND  
         [0001]    This invention relates generally to microelectromechanical structures (MEMS).  
           [0002]    Microelectromechanical structures are physical structures which may be fabricated using microelectronic fabrication techniques. In the fabrication of MEMS devices, it is often desirable to isolate different structures electrically from one another. To this end, an air gap may be positioned underneath an electrical connector. Such a structure may be called a bridge since it allows an electrical connection over an air gap and provides for isolation from underlying devices.  
           [0003]    For example, for multi-mode multi-band cell phone applications, an antenna switch multiplexer switches the antenna to a different mode or band, as well as between transmission and receiving. The multiplexer consists of many individual switches. To route the signal lines, ground lines, and actuation control lines across each other, more that two metal layers are needed.  
           [0004]    For example, an in-line cantilever beam metal contact series switch generally requires two metal lines in order to allow the connection. A first signal line may be in a first layer, a second signal line may also be in the first layer, an actuation element may be in the first layer, but the cantilever beam metal contact switch itself must be in at least a second layer.  
           [0005]    Thus, there is a need for better ways to allow connections in MEMS devices. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    [0006]FIG. 1 is a top plan view of one embodiment of the present invention;  
         [0007]    [0007]FIG. 2 is a top plan view of a second embodiment of the present invention;  
         [0008]    [0008]FIG. 3 is a top plan view of a third embodiment of the present invention;  
         [0009]    [0009]FIG. 4 is an enlarged cross-sectional view of a technique in accordance with one embodiment of the present invention;  
         [0010]    [0010]FIG. 5 is an enlarged cross-sectional view of the embodiment shown in FIG. 4 at a subsequent stage in accordance with one embodiment of the present invention;  
         [0011]    [0011]FIG. 6 is an enlarged cross-sectional view at a subsequent stage in accordance with one embodiment of the present invention;  
         [0012]    [0012]FIG. 7 is an enlarged cross-sectional view at a subsequent stage in accordance with one embodiment of the present invention;  
         [0013]    [0013]FIG. 8 is an enlarged cross-sectional view at a subsequent stage in accordance with one embodiment of the present invention;  
         [0014]    [0014]FIG. 9 is an enlarged cross-sectional view of a subsequent stage in accordance with one embodiment of the present invention;  
         [0015]    [0015]FIG. 10 is an enlarged cross-sectional view of a subsequent stage in accordance with one embodiment of the present invention;  
         [0016]    [0016]FIG. 11 is an enlarged cross-sectional view of a subsequent stage in accordance with one embodiment of the present invention;  
         [0017]    [0017]FIG. 12 is an enlarged cross-sectional view of a stage subsequent to the stage depicted in FIG. 8 in accordance with another embodiment of the present invention; and  
         [0018]    [0018]FIG. 13 is an enlarged cross-sectional view of a subsequent stage to that shown in FIG. 12 in accordance with one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0019]    Referring to FIG. 1, a basic switch/transmission line co-planar waveguide (CPW)  10  includes a control voltage line  18  that is routed under a bridge  16   e  across a ground line  12   a , including two strips separated by a gap  22 , in accordance with one embodiment of the present invention. The ground lines  12   a ,  12   b , and  12   c  may be formed in a first conductive layer. The signal line  16  may be made in a second, separate conductive layer. The control voltage line  18  may also be in the first conductive layer.  
         [0020]    The width of the CPW  10  generally scales with the width of the signal line  16 . The width of the signal line  16  may be reduced by using both of the first and second conductive layers in order to have the necessary conductivity. Thus, the ground lines  12  may be made using a thin bottom metal layer in one embodiment.  
         [0021]    The width “W” of the bridge  16 e may be small enough so that a sacrificial layer (not shown in FIG. 1) underneath the bridge  16   e  may be removed during a release step. In one embodiment, the span of the bridge  16   e  may be smaller than approximately five times the thickness of the second, upper conductive layer so that the bridge  16   e  is stiff enough not to collapse under voltage between the two conductive layers.  
         [0022]    [0022]FIG. 2 shows a multiplexer  10   a  which includes ground lines  12 , signal lines  16   f  and  16   g , as well as the signal lines  16   a  through  16   g  in accordance with another embodiment of the present invention. The bridge  20   a  bridges the ground lines  12   c , the bridge  26  bridges the elements  16   f  and  16   g , and the bridge  20  bridges the ground lines  12   a . Thus, the control voltage line  18   a  may span all the way through three separate ground lines,  12   a ,  12   b , and  12   c , to reach the ground line  12   d.    
         [0023]    Referring to FIG. 3, in accordance with still another embodiment of the present invention, the ground lines  12   a ,  12   b , and  12   c  may be crossed by control lines  18   b  and  18   c . The control voltage line  18   b  goes under a bridge  34  and the control voltage line  18   c  goes under a bridge  35 . The signal lines  32  and  36  are joined by the bridge  34 . The signal lines  36  and  38  are joined by the bridge  35 . By keeping the span of each bridge  34  and  35  relatively small, multiple bridges may be needed in some embodiments. Thus, the intermediate signal line portion  36  may provide an island which allows the length of the bridges  34  and  35  to be limited to the desired length.  
         [0024]    In accordance with one embodiment of the present invention, a bridge, such as a bridge  16   e ,  16   c ,  20 ,  26 ,  20   a ,  16   c ,  34 , or  35 , may be formed by forming a dielectric layer  42  over a semiconductor substrate  40 . The dielectric layer may be silicon dioxide or silicon nitride, as two examples.  
         [0025]    Then, as shown in FIG. 5, a first or bottom conductive layer  12  may be deposited on the dielectric layer  42  and patterned. The patterning of the layer  12  forms the central island  46  and the gaps  44 . The bottom conductive layer  12  may be a composite of titanium, nickel, and gold, in one embodiment.  
         [0026]    Referring to FIG. 6, the structure may then be covered with a sacrificial layer  48 . The sacrificial layer  48  may be deposited or spun-on in some embodiments. In one embodiment, the sacrificial layer  48  may be made of polymeric materials, such as polyimide, resist, or flowable glasses, that reflow, shrink, melt, or vaporize at elevated temperatures.  
         [0027]    Next, referring to FIG. 7, after lithography and etching, anchor holes  50  may be formed in the sacrificial layer  48 .  
         [0028]    A seed layer  52 , for facilitating plating, may then be coated over the structure shown in FIG. 7 to achieve the structure shown in FIG. 8. A thick resist  54  may be patterned as a mold for plating as shown in FIG. 9. Next, a bridge  16  may be plated, using the seed layer  52  to facilitate adherence of the bridge  16 , and using the resist  54  as a mold for defining the bridge  16 . The second or top conductive layer forming the bridge  16  may be gold in one embodiment of the present invention.  
         [0029]    After plating the bridge  16 , the resist  54  may be removed. The seed layer  52  may be etched away and the material  48  may be released, forming a void  58  under the bridge  16 . In one embodiment, the sacrificial material  48  is released through the application of heat.  
         [0030]    In another embodiment of the present invention, after the structure shown in FIG. 8 is formed, etching may be used to form the U-shaped metal structure  52 , as shown in FIG. 12. Instead of plating a seed layer, a heavier metal layer  52  may be formed in this embodiment. Thereafter, the air bridge may be formed by releasing the material  48 , forming the void  60 .  
         [0031]    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.