Abstract:
A microelectromechanical (MEMS) switch is described. The switch comprises a cantilever beam having a proximal end and a distal end. The cantilever beam is supported by its proximal end above a substrate by a raised anchor. An intermediate actuation electrode is placed beneath the cantilever beam and is separated from the bottom of the cantilever beam by a narrow gap. Finally, a contact pad or transmission line is placed beneath the cantilever beam and separated from the bottom of the cantilever beam by a larger gap.

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates to microelectromechanical system (MEMS) switches, and more particularly, to a MEMS switch using stepped actuation. 
     BACKGROUND OF THE INVENTION 
     The use of microelectromechanical (MEMS) switches has been found to be advantageous over traditional solid-state switches. For example, MEMS switches have been found to have superior power efficiency, low insertion loss, and excellent electrical isolation. However, for certain high-speed applications such as RF transmission/receiving, MEMS switches are in general too slow for many applications. This is primarily due to the speed of a MEMS switch being limited by its resonance frequency. To improve the speed of the MEMS switch, the stiffness of the MEMS structure must be increased. However, stiff structures require higher actuation voltages for the switching action to occur. 
     One possible solution is to simply reduce the gap between the structure and the actuation electrode. However, this is problematical because this will degrade electrical isolation. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     The invention is best understood by reference to the figures wherein references with like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the reference number in which: 
     FIG. 1 illustrates a prior art cantilever capacitive shunt MEMS switch. 
     FIG. 2 illustrates a prior art cantilever metal/metal contact MEMS switch. 
     FIG. 3 illustrates a prior art bridge beam capacitive shunt MEMS switch. 
     FIG. 4 illustrates a prior art bridge beam metal/metal contact MEMS switch. 
     FIGS. 5A-C illustrates a cantilever capacitive shunt MEMS switch formed in accordance with the present invention. 
     FIGS. 6A-C illustrates a bridge beam capacitive shunt MEMS switch formed in accordance with the present invention. 
     FIGS. 7A and 7B illustrates a cantilever contact MEMS switch formed in accordance with the present invention. 
     FIGS. 8A and 8B illustrates a bridge beam metal/metal contact MEMS switch formed in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of a MEMS switch are described in detail herein. In the following description, numerous specific details are provided in order to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, materials, components, etc. In other instances, well known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of various embodiments of the invention. Furthermore, it is understood that the various embodiments shown in the figures are illustrative representations, and are not necessarily drawn to scale. 
     Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, material, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrase “in one embodiment” or “in an embodiment” in various places throughout the specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments. 
     In general, the electrostatic actuation force between two parallel plate electrodes is given by the equation: 
     
       
           F=AV   2 /2 d   2   
       
     
     where is the electrical permeability of air, A is the overlapping area of the electrodes, d is the gap distance, and V is the actuation voltage. As seen from the above, to maintain the same actuation force, one can reduce the actuation voltage by reducing the gap distance. 
     Prior art cantilever and bridge beam based capacitive shunt and metal/metal contact MEMS switches are shown in FIGS. 1-4. In the simplest type of MEMS switch, in FIG. 1, a cantilever capacitive shunt switch  101  is shown in the “off” position and the “on” position. The switch  101  includes an actuation electrode  103 , a dielectric layer  105  formed atop the actuation electrode  103 , and a cantilever beam  107 . The cantilever beam has one end secured to an anchor  109  that is in turn anchored to a substrate  111 . A distance d separates the second end of a cantilever beam  107  from the actuation electrode  103  and dielectric layer  105 . In the “off” position, the cantilever beam  107  is not in contact with the dielectric  105 . Typically, in this type of switch, the actuation electrode  103  is also part of a transmission line that carries electrical signals. In the “on” position, the cantilever beam  107  is attracted to the actuation electrode  103  by electrostatic forces when a voltage is carried on the actuation electrode  103 . The “top electrode” formed by the cantilever beam  107  and the “bottom” actuation electrode  103  are separated by the dielectric layer  105 . 
     Turning to FIG. 2, a prior art cantilever metal/metal contact MEMS switch  201  is shown. The contact switch  201  includes a contact pad  203 , an actuation electrode  205 , and a cantilever beam  207 . One end of the cantilever beam  207  is connected to an anchor  209  that is fixed to a substrate  211 . In operation, the actuation electrode  205  is activated with a voltage, which creates an electrostatic attraction between the actuation electrode  205  and the cantilever beam  207 . This causes the cantilever beam  207  to deform downward into contact with the contact pad  203 . 
     Turning to FIG. 3, a prior art bridge beam capacitive shunt switch  301  is shown. The switch includes a bridge beam  303  suspended at its ends by anchors  305  and  307 . The anchors  305  and  307  are attached to a substrate  309 . Located underneath the bridge beam  303  and between the anchors  305  and  307  is an actuation electrode  311 . Formed atop of the actuation electrode  311  is a dielectric layer  313 . In the “off” position, the bridge beam  303  is suspended over the dielectric layer  313  and actuation electrode  311 . In the “on” position, a voltage is applied to the actuation electrode  311  which causes electrostatic forces to attract the bridge beam  303  into contact with the dielectric layer  313 . 
     Turning to FIG. 4, a bridge beam metal/metal contact MEMS switch  401  is shown. The switch  401  includes a bridge beam  403  that is suspended above a substrate  405  by anchors  407  and  409 . An actuation electrode  411  is disposed underneath the bridge beam  403  in between the anchor supports  407  and  409 . Further, a contact pad  413  is also disposed underneath the bridge beam  403  and between the anchor supports  407  and  409 . In the “off” position, the bridge beam  403  is suspended above the actuation electrode  411  and the contact pad  413 . In the “on” position, a voltage is applied to the actuation electrode  411  that causes electrostatic forces to draw the bridge beam  403  downward so that it contacts the contact pad  413 . In some embodiments, the bridge beam  403  has a contact button  415  that is used for contacting the contact pad  413 . 
     The present invention modifies the prior art MEMS switches shown in FIGS. 1-4 through the use of intermediate actuation electrodes. Specifically, turning to FIG. 5, a switch  501  formed in accordance with the present invention is shown. The switch  501  includes a cantilever beam  503 , a transmission line  505 , an intermediate actuation electrode  507 , and their corresponding dielectric layers  509  and  511 . The dielectric layers  509  and  511  serves to prevent short circuiting when the switch  501  is activated. The cantilever beam  503  has one end (the proximal end) secured to an anchor  513 . The anchor in turn is secured to a substrate  515 . Typically, the cantilever beam  503  and the anchor  513  are formed from polysilicon. Alternative materials may be used, but should preferably be easily formed using semiconductor processes and be conductive, such as copper, aluminum, or gold. Further, although not shown, the cantilever beam  503  is electrically connected to other circuitry that is selectively connectable to the transmission line  505  by means of the switch  501 . In other words, the switch connects the transmission line to other circuit devices when activated. Typically, the circuit devices are also formed on or in the substrate. Moreover, the term transmission line as used herein refers to any conductive device used for carrying electrical signals. Examples include, without limitation, metal interconnects and the like. 
     The substrate  515  is typically a semiconductor substrate (e.g. a silicon wafer). Alternatively, the substrate  515  may be an epitaxial silicon layer. Still alternatively, the substrate  515  may be a dielectric material. Thus, the term substrate as used herein means an underlying material that can serve as a support for the anchor  513 . 
     The distal end of the cantilever beam  503  is left unsupported and is free to move downwardly. However, in its undisturbed state, the cantilever beam  503  is substantially straight and suspended over the substrate  515 . Disposed underneath the distal end of the cantilever beam  503  are the transmission line  505  and its dielectric layer  509 . The transmission line  505  is also formed on the substrate  515  and is typically a conductive material, such as aluminum, copper, polysilicon, or gold. As will be seen below, the dielectric layer  509  serves to separate the cantilever beam  503  and the transmission line when the switch is “on” to effectuate capacitive coupling. 
     Disposed on the substrate  515  and between the transmission line  505  and the anchor  513  is intermediate actuation electrode  507  and it&#39;s corresponding dielectric layer  511 . Note that the height of the anchor is higher than that of the intermediate actuation electrode  507 . Further, the height of the intermediate actuation electrode  507  is higher than that of the transmission line  505 . As seen in FIG. 5A, the gap distance between the dielectric layer  509  and the cantilever beam  503  is denoted by distance D 2 . The distance between the dielectric layer  511  and the cantilever beam  503  is denoted by distance D 1 . In this embodiment, D 2  is greater than D 1 . 
     In operation, to turn the switch  501  to the “on” position, a DC actuation voltage is applied to the intermediate actuation electrode  507 . Moreover, the transmission line  505  should be carrying a voltage signal. A DC actuation voltage is also applied between cantilever beam  503  and transmission line  505 . The DC actuation voltage will not interfere with the AC signals carried on the transmission line  505 . This is because a DC voltage cannot penetrate through the dielectric layer  509 . However, AC signals will still be transmitted by capacitive coupling. These voltages on the actuation electrode  507  and the transmission line  505  tend to cause an electrostatic attraction between the cantilever beam  503  to the actuation electrode  507  and the transmission line  505 . Because the distance D 1  between the cantilever beam  503  and the actuation electrode  507  is relatively small, the electrostatic attraction force is sufficient to close the gap D 1  between the intermediate actuation electrode  507  and the cantilever beam  503 . This is shown in FIG.  5 B. Once this happens, the gap between the cantilever beam  503  and the transmission line  505  is reduced (to D 2 −D 1 ). By narrowing this gap, the electrostatic attraction force between the transmission line  505  and the cantilever beam  503  is then sufficient to close the gap between the cantilever beam  503  and the transmission line  505 , thereby forming a capacitive connection between the transmission line and the cantilever beam  503 . 
     The use of a two-step activation technique allows for the use of stiffer cantilever beams for the same activation voltage. The use of stiffer beams results in a higher resonance frequency, which in turn allows for higher switching speeds. 
     The concepts of the present invention can also be applied to other embodiments and types of MEMS switches. For example, a bridge beam shunt switch incorporating the present invention is shown in FIGS. 6A through 6C. The bridge beam shunt switch  601  is similar to that shown in FIG. 3 except that an intermediate actuation electrode is added. A bridge beam  602  is suspended above an intermediate actuation electrode  603  and a transmission line  605 . The bridge beam  602  is suspended by means of anchors  607  and  609 . The anchor in turn is secured to a substrate. 
     Typically, the bridge beam  602  and the anchors  607  and  609  are formed from polysilicon. Alternative materials may be used, but should preferably be easily formed using semiconductor processes and be conductive, such as copper or aluminum. Further, although not shown, the bridge beam  602  is connected to other circuitry that is selectively connectable to the transmission line  605  by means of the switch  601 . In other words, the switch connects the transmission line  605  to other circuit elements when activated. 
     Further, formed on the top surface of the intermediate actuation electrode  603  and the transmission line  605  are thin dielectric layers (similar to those of FIGS.  5 A- 5 C). As will be seen below, the dielectric layers serves to separate the bridge beam  602  and the transmission line  605  when the switch is “on” to effectuate capacitive coupling. 
     In the “off” position, the bridge beam  602  is suspended above the intermediate actuation electrode  603  and the transmission line  605 . The distance between the intermediate actuation electrode  603  and the bridge beam  602  is denoted by distance D 1 . The distance between the transmission line  605  and the bridge beam  602  is denoted by distance D 2 . In this embodiment, D 2  is greater than D 1 . To switch the shunt switch  601  on, a DC voltage is applied across both gaps D 1  and D 2 . Gap D 1  is closed first because the DC voltage on the intermediate actuation electrode  603  creates enough attraction force to close the gap D 1 . This is shown in FIG.  6 B. 
     Once this happens, the gap between the bridge beam  602  and the transmission line  605  is reduced (to D 2 −D 1 ). By narrowing this gap, the electrostatic attraction force between the transmission line  605  and the bridge beam  602  is then sufficient to close the gap between the bridge beam  602  and the transmission line  605 , thereby forming a capacitive connection between the transmission line  605  and the bridge beam  602 . Thus, in the “on” position, the switch appears as in FIG.  6 C. 
     Turning to FIGS. 7A and 7B, a cantilever contact switch  701  is shown. The contact switch  701  includes a cantilever beam  703  suspended above a substrate  705  by means of an anchor  707 . The cantilever beam  703  has one end (the proximal end) secured to anchor  707 . The anchor in turn is secured to a substrate  705 . Typically, the cantilever beam  703  and the anchor  707  are formed from polysilicon. Alternative materials may be used, but should preferably be easily formed using semiconductor processes and be conductive, such as copper or aluminum. Further, although not shown, the cantilever beam  703  is connected to other circuitry that is selectively connectable to a contact pad  709  by means of the switch  701 . In other words, the switch connects the contact pad  709  to other circuit elements when activated. 
     The distal end of the cantilever beam  703  is left unsupported and is free to move downwardly. However, in its undisturbed state, the cantilever beam  703  is substantially straight and suspended over the substrate  705 . Disposed underneath the distal end of the cantilever beam  503  is contact pad  709 . The contact pad  709  is also formed on the substrate  705  and is typically a metal conductive material, such as aluminum or copper. Alternatively, the contact pad may be polysilicon. 
     Disposed on the substrate  705  and between the contact pad  709  and the anchor  707  is intermediate actuation electrode  711 . Note that the height of the anchor  707  is higher than that of the intermediate actuation electrode  711 . Further, the height of the intermediate actuation electrode  711  is higher than that of the contact pad  709 . As seen in FIG. 7A, the gap distance between the contact pad  709  and the cantilever beam  703  is denoted by distance D 2 . The distance between the actuation electrode  711  and the cantilever beam  703  is denoted by distance D 1 . In this embodiment, D 2  is greater than D 1 . 
     In operation, to turn the switch  701  to the “on” position, a voltage is applied to the intermediate actuation electrode  711 . The voltage on the actuation electrode  711  tends to cause an electrostatic attraction between the cantilever beam  703  to the actuation electrode  711 . Because the distance D 1  between the cantilever beam  703  and the actuation electrode  711  is relatively small, the electrostatic attraction force is sufficient to reduce the gap between the intermediate actuation electrode  711  and the cantilever beam  703  until the cantilever beam  703  is in contact with the contact pad  709 . 
     In yet another alternative embodiment, a bridge beam metal/metal contact switch  801  is seen in FIGS. 8A and 8B. A bridge beam  803  is suspended above an intermediate actuation electrodes  809  and  811  and a contact pad  813 . The bridge beam  803  is suspended by means of anchors  805  and  807 . The anchor in turn is secured to a substrate. 
     Typically, the bridge beam  803  and the anchors  805  and  807  are formed from polysilicon. Alternative materials may be used, but should preferably be easily formed using semiconductor processes and be conductive, such as copper or aluminum. Further, although not shown, the bridge beam  803  is connected to other circuitry that is selectively connectable to the contact pad  813  by means of the switch  801 . In other words, the switch connects the contact pad  813  to other circuit elements when activated. 
     In the “off” position, the bridge beam  803  is suspended above the intermediate actuation electrodes  809  and  811  and the contact pad  813 . In this embodiment, two intermediate actuation electrodes  809  and  811  are shown. However, any number of actuation electrodes may be used as design requirements may require. The distance between the intermediate actuation electrode  809  and  811  and the bridge beam  803  is denoted by distance D 1 . 
     To turn the switch  801  on, a DC voltage is applied to the actuation electrodes  809  and  811 . Because the initial gap between the bridge beam  803  and the intermediate actuation electrodes  809  and  811  is much smaller, the DC voltage needed to bend the bridge beam is much less. Alternatively, for the same applied DC voltage, the bridge beam  803  may be made stiffer, resulting in a faster switch. The electrostatic attraction force generated is sufficient to draw the bridge beam  803  downwardly into contact with the contact pad  813 . Preferably, the contact pad is formed from a metal material, such as aluminum or copper. 
     The above description of illustrated embodiments of the invention, including what is described in the abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. For example, while the bending beam and breathing bar types of mechanical resonators have been described, other types of mechanical resonators may also be substituted into the concepts and ideas of the present invention. 
     These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctorines of claim interpretation.