Abstract:
The present invention generally relates to RF MEMS devices that are capable of hot switching. The RF MEMS devices, by utilizing one or more spring mechanisms, are capable of hot switching. In certain embodiments, two or more sets of springs may be used that become engaged at specific points in the displacement of the cantilever of the MEMS device. The springs allow for a significant increase in the release voltage for a given pull in landing voltage.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims benefit of U.S. Provisional Patent Application Ser. No. 61/247,852 (CK064L), filed Oct. 1, 2009, which is herein incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    Embodiments of the present invention generally relate to the field of micromechanical variable capacitors, specifically to the act of releasing a digital capacitor in the presence of a residual RF voltage. The inventions described herein can be applied to any micromechanical structure where the difference between latching voltage and actuation voltage needs to be minimized. It allows devices to be made where the spring constant of the cantilever can be engineered to be greater than the electrostatic attraction across the landing contact due to the voltage dropped across the capacitance contact, while the voltage required to turn the device on is unchanged. 
         [0004]    2. Description of the Related Art 
         [0005]    Micromechanical actuators are based on the simple principal that they will deflect or move in the presence of an external force. The deflection of these actuators typically follows a linear relationship between force and deflection. The slope of this relationship is defined by the materials used, the geometries of the switch and/or legs, and how the switch and/or legs are anchored (Hooke&#39;s Law in the general sense). 
         [0000]      Spring Force= F spring= K*X   (1)
 
         [0006]    Where K is the spring constant and X is the displacement. 
         [0007]    The external force normally does not follow a linear relationship between its magnitude and the position of the switch. For the case of electrostatics, the force will increase with the square of the position to the control electrode. This situation causes the phenomena of “snap-in” when a critical displacement is reached. The Electrostatic Force F E  is given by: 
         [0000]    
       
         
           
             
               
                 
                   
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                     E 
                   
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                           0 
                         
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         [0008]    Where A is the area of overlap between the pull down electrode and the micro electromechanical system (MEMS) device, Z is the starting position, ∈ o  is the permittivity of free space and V is the applied control voltage. Equilibrium is defined when the sum of all forces is zero which yields the classic snap-in behavior for electrostatic MEMS. Once the cantilever jumps to contact the separation between the cantilever and the pull in electrode is greatly reduced and from equation 2 the electrostatic force increases greatly. To allow the cantilever to pull off the control voltage has to be reduced greatly. 
         [0009]      FIG. 1A  shows the forces acting on the MEMS device vs. the displacement for an electrostatically actuated MEMS device with a linear spring. Note that the vertical scale of the figure has a logarithmic scale, to better show the various points on the graph. The curve labeled Fspring shows the mechanical force vs. the displacement of the spring which varies linearly with displacement. The curves labeled F@V 1 , F@V 2 , F@V 3  are the electrostatic forces acting on the MEMS device for different applied voltages V 1 , V 2 , V 3  where V 3 &gt;V 2 &gt;V 1 . 
         [0010]    The MEMS displacement at various applied voltages is found by finding the intersection of the mechanical force curve and the electrostatic force curves. For instance when voltage V 1  is applied, the MEMS device displaces to point p 1 . When the applied voltage is increased to V 2  the MEMS device displaces to point p 2  and when the voltage is increased to V 3  the MEMS device displaces to point p 3 . At this point when the voltage is increased any further there is no longer an intersection of the electrostatic force curve with the mechanical force curve, because the electrostatic force is always larger than the mechanical force. As a result, the device snaps in and displaces to point p 4 . 
         [0011]    When the voltage is subsequently reduced from V 3  to V 2  the electrostatic force F@V 2  at the displaced location p 4  is still larger than the mechanical force Fspring so that the device stays displaced at point p 4 . Once the voltage is reduced to V 1  the electrostatic force F@V 1  at the displaced location p 4  is as large as mechanical force Fspring. Any further reduction of the voltage would result in only one intersection with the mechanical force curve in point p 1  and the device will snap back from point p 4  to point p 1 . 
         [0012]      FIG. 1B  shows the MEMS displacement vs. applied voltage for the MEMS devices with a linear spring. Shown in this figure are the same points as shown in  FIG. 1A . During the up-sweep of the applied voltage, the displacement follows the curve labeled pull-in. At an applied voltage of 5V, the MEMS devices displaces to point p 1 . Then as the voltage is increased to 15V, the device displaces to point p 2 . When the voltage is increased to 25V, the MEMS device displaces to point p 3 . Any further increase in voltage would result in the device to snap in to the full displacement and end up in point p 4 . Then the voltage is reduced and the displacement follows the curve labeled release. When the voltage is reduced to 5V, the displacement stays at 100% of the gap (point p 4 ′). Any further reduction makes the device snap back down to point p 1 . Thus, the pull-in voltage is 25V and the release voltage is 5V.  FIG. 1B  shows the large difference between the release voltage (voltage at which the device snaps from position p 4 ′ to position p 1 ) and landing voltage (voltage at which the device snaps from position p 3  to position p 4 ). 
         [0013]    The large difference between the release voltage and the landing voltage creates a problem for capacitive RF MEMS when it comes to hot switching. Hot switching is defined as the largest RF voltage that can exist between, for example a MEMS cantilever switch and a landing electrode for which the spring constant of the cantilever is able to pull the contact apart when the control voltage is set to zero. 
         [0014]    Therefore, there is a need for RF MEMS that are capable of hot switching. 
       SUMMARY OF THE INVENTION 
       [0015]    The present invention generally relates to RF MEMS devices that are capable of hot switching. The RF MEMS devices, by utilizing one or more spring mechanisms, are capable of hot switching. In certain embodiments, two or more sets of springs may be used that become engaged at specific points in the displacement of the cantilever of the MEMS device. The springs allow for a significant increase in the release voltage for a given landing voltage. 
         [0016]    In one embodiment, a MEMS device is disclosed. The device includes a substrate, a first support post coupled to the substrate and extending vertically therefrom and a first cantilever coupled to the first support post. The device also includes a second support post coupled to the substrate at a location spaced from the first support post. The second support post extends vertically from the substrate. The device also includes a second cantilever coupled to the second support post, a pull-in electrode coupled to the substrate in a location between the first support post and the second support post and a contact electrode coupled to the substrate in a location between the pull-in electrode and the second support post. 
         [0017]    In another embodiment, a method of operating a MEMS device is disclosed. The method includes applying a voltage to a pull-in electrode. The method also includes moving a first cantilever a first distance and into contact with a second cantilever while the voltage is applied. The method also includes moving the first cantilever and the second cantilever a second distance such that the first cantilever contacts a contact electrode and the second cantilever is spaced from the contact electrode while the voltage is applied. The method also includes changing the voltage applied to the pull-in electrode and spacing the first cantilever from the contact electrode. 
         [0018]    In another embodiment, a MEMS device is disclosed. The MEMS device includes a substrate, a support post coupled to the substrate and extending vertically therefrom and a cantilever coupled to the support post, the cantilever having a first portion and a second portion extending from the first portion. The MEMS device also includes a contact post coupled to the substrate at a location spaced from the first support post. The contact post extends vertically from the substrate. The MEMS device also includes a pull-in electrode coupled to the substrate in a location between the support post and the contact post. The MEMS device also includes a contact electrode coupled to the substrate in a location between the pull-in electrode and the contact post. The first portion is movable from a position spaced from the contact electrode to a position in contact with the contact electrode and the second portion is movable from a position spaced from the contact post to a location in contact with the contact post. 
         [0019]    In another embodiment, a method of operating a MEMS device is disclosed. The method includes applying a voltage to a pull-in electrode. The method also includes moving a cantilever a first distance such that a first portion of the cantilever is spaced from a contact electrode and a second portion of the cantilever is in contact with a contact post while the voltage is applied. The method also includes moving the cantilever a second distance such that the first portion contacts the contact electrode while the second portion remains in contact with the contact post while the voltage is applied. The method also includes changing the voltage applied to the pull-in electrode and spacing the cantilever from the contact electrode and the contact post. 
         [0020]    In another embodiment, a device includes a substrate having one or more electrodes formed therein, an electrically insulating layer disposed over the substrate and the one or more electrodes, and one or more landing structures coupled to the electrically insulating layer. The micro electromechanical device also includes a MEMS element coupled to the electrically insulating layer. The MEMS element is movable from a first position to a second position spaced from the electrically insulating layer. The MEMS element includes a first portion that contacts the electrically insulating layer when the MEMS element is in the first position and a second portion that contacts the one or more landing structures when the MEMS element is in the second position. 
         [0021]    In another embodiment, a device includes a substrate having one or more electrodes formed therein, an electrically insulating layer disposed over the substrate and the one or more electrodes, and one or more spring elements coupled to the electrically insulating layer, the one or more spring elements movable from a first position to a second position. The device also includes a MEMS element coupled to the electrically insulating layer. The MEMS element is movable from a third position to a fourth position spaced from the electrically insulating layer. The MEMS element includes a first portion that contacts the electrically insulating layer when in the third position and a second portion that contacts the one or more spring elements and moves the one or more spring elements from the first position to the second position. 
         [0022]    In another embodiment, a method includes forming a MEMS element over a substrate having an insulating layer, one or more landing structures, and a first sacrificial layer disposed thereover. The forming a MEMS element includes forming a first structural layer over the first sacrificial layer, forming a second sacrificial layer over the first structural layer, removing at least one portion of the second sacrificial layer to expose a portion of the first structural layer, forming a structural element over the exposed first structural layer, and forming a second structural layer over the second sacrificial layer and the structural element. The method also includes removing the first sacrificial layer and the second sacrificial layer to free the MEMS element. The freed MEMS element is movable from a first position to a second position. The freed MEMS element has a first portion that contacts the insulating layer when the MEMS element is in the first position. The freed MEMS element also includes a second portion that contacts the one or more landing structures when the MEMS element is in the second position. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
           [0024]      FIG. 1A  shows the force vs. displacement for an electrostatically actuated MEMS device with a linear spring. 
           [0025]      FIG. 1B  shows the pull-in and release voltages of an electrostatically actuated MEMS device with a linear spring. 
           [0026]      FIGS. 2A-2C  are schematic representations of a MEMS device according to one embodiment. 
           [0027]      FIGS. 2D and 2E  are schematic representations of a MEMS device according to another embodiment. 
           [0028]      FIG. 3  shows the total restoring force versus displacement for a compound system containing two cantilevers. 
           [0029]      FIG. 4A  shows the force vs. displacement for an electrostatically actuated MEMS device with a compound spring. 
           [0030]      FIG. 4B  shows the pull-in and release voltages of an electrostatically actuated MEMS device with a compound spring. 
           [0031]      FIG. 5  shows the hot-switch comparison between a MEMS RF device with a linear spring and with a compound spring. 
           [0032]      FIG. 6A  shows the force vs. displacement for an electrostatically actuated MEMS device with a compound spring with a modified parameter set. 
           [0033]      FIG. 6B  shows the pull-in and release voltages of an electrostatically actuated MEMS device with a compound spring with a modified parameter set. 
           [0034]      FIG. 7  shows the hot-switch performance between of a MEMS RF device with a compound spring using a modified parameter set. 
           [0035]      FIGS. 8A and 8B  are schematic side views of a MEMS RF device according to one embodiment. 
           [0036]      FIG. 9  is a schematic top view of the MEMS RF device of  FIGS. 8A and 8B . 
           [0037]      FIG. 10  is a schematic side view of the MEMS RF device of  FIGS. 8A and 8B . 
           [0038]      FIG. 11  is a schematic top view of a MEMS RF device according to another embodiment. 
           [0039]      FIG. 12  is a schematic side view of the MEMS RF device of  FIG. 11 . 
           [0040]      FIG. 13  is a schematic top view of a MEMS RF device according to another embodiment. 
           [0041]      FIG. 14  is a schematic side view of the MEMS RF device of  FIG. 13 . 
           [0042]      FIG. 15  is a schematic top view of a MEMS RF device according to another embodiment. 
           [0043]      FIG. 16  is a schematic side view of the MEMS RF device of  FIG. 15 . 
           [0044]      FIG. 17  is a schematic top view of a MEMS RF device according to another embodiment. 
           [0045]      FIG. 18  is a schematic side view of the MEMS RF device of  FIG. 17 . 
           [0046]      FIG. 19  is a schematic top view of a MEMS RF device according to another embodiment. 
           [0047]      FIG. 20  is a schematic side view of the MEMS RF device of  FIG. 19 . 
       
    
    
     DETAILED DESCRIPTION 
       [0048]    The present invention uses two or more sets of springs that become engaged at specific points in the displacement. This allows for a significant increase in the voltage that can be applied to the RF electrode for a given pull in landing voltage. 
         [0049]    If the cantilever initially has a spring constant of k 1  and then after it has been pulled in a distance d it lands on an additional spring with a spring constant k 2 , then the spring constant of the combined system has a step wise increase. The second spring  2  can either be on the substrate or it can be attached to the first cantilever (as shown in  FIG. 2A  and  FIG. 2D  respectively) and make contact to a bump before the first cantilever makes contact to the substrate or contact. Thus there are two regions of displacement X for the end of the first primary cantilever, firstly from 0 to d where the restoring force is k 1 *X and then from d to D the restoring force is (k 1 *X+k 2 *(X−d)). Here, D is the total movement of the first cantilever until it makes contact with the substrate contact (we assume D is greater than d). 
         [0050]    For  FIG. 2A , X&lt;d: F=k 1 *X. For  FIG. 2C , X&gt;d: F=(k 1 *X+k 2 *(X−d)). For  FIG. 2D , X&lt;d: F=k 1 *X. For  FIG. 2E , X&gt;d: F=(k 1 *X+k 2 *(X−d)). The mechanical restoring force then follows the curve shown in  FIG. 3 . 
         [0051]    This process could be continued with more than one extra cantilever which would give a number of increasing slopes to the curve shown in  FIG. 3  with the third cantilever landing at d 3  where d 3  lies between d and D. 
         [0052]    The electrostatic pull in force is given by: 
         [0000]    
       
         
           
             
               
                 
                   
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         [0053]    For a plate of area A 3  moving to a pull in plate (3 in  FIGS. 2A-2E ) of the same area when they are separated by a gap (Z−X). For a cantilever, there is a slope change and so there is a modification to these formulae, but the essential feature that the electrostatic force increases as one over the gap squared remains. This means that there is a large attractive force when the cantilever is close to the pull in electrode (3 in  FIGS. 2A-2E ). 
         [0054]    If we are trying to hot switch a cantilever, then there will also be a voltage difference between cantilever  1  and the landing bump  4 . This puts an additional attraction which also follows the formula (4), though with Z replaced with D and the new area A 4  being the overlap of the contact  4  with the cantilever  1 . 
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         [0055]    For a capacitance switch there would be a thin insulator over contact  4  of thickness z. This would limit the electrostatic pull in force from the contact to a maximum: 
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         [0056]    The second cantilever can be designed so that the spacing and spring force are such that the voltage required to pull the cantilever  1  down to touch the contact  4  is unchanged, but the pull off force at contact  4  is large enough to overcome the electrostatic attraction due to the voltage drop across the dielectric at that contact due to the voltage V 4  at contact  4 . 
         [0057]    To illustrate this further, inspect  FIG. 4A  which shows a force vs. displacement curve for a MEMS system including a compound spring. Note that the vertical scale of the figure has a logarithmic scale, to better show the various points on the graph. The mechanical spring force is indicated by the curve labeled Fspring. The initial part of this curve follows the linear spring curve shown in  FIG. 1A . Then at the point marked A the compound spring is engaged and there is an increase in the spring constant indicated by a steeper slope of the force-vs.-displacement curve. The curves labeled F@V 1 , F@V 2 , F@V 3  are the electrostatic forces acting on the MEMS device for different applied voltages V 1 , V 2 , V 3  where V 3 &gt;V 2 &gt;V 1 . The MEMS displacement at various applied voltages is found by finding the intersection of the mechanical force curve and the electrostatic force curves. 
         [0058]    For instance when voltage V 1  is applied, the MEMS device displaces to point p 1 . When the applied voltage is increased to V 2  the MEMS device displaces to point p 2  and when the voltage is increased to V 3  the MEMS device displaces to point p 3 . At this point when the voltage is increased any further there is no longer an intersection of the electrostatic force curve with the mechanical force curve, because the electrostatic force is always larger than the mechanical force. As a result, the device snaps in and displaces to point p 4 . 
         [0059]    When the voltage is subsequently reduced from V 3  to V 2  the electrostatic force F@V 2  at the displaced location p 4  is as large as mechanical force Fspring. Any further reduction of the voltage would result in only one intersection with the mechanical force curve in point p 1  and the device will snap back from point p 4  to point p 2 . 
         [0060]    From the above analysis one can summarize that the pull-in voltage of the device with a compound spring is V 3  and the release voltage is V 2 . 
         [0061]      FIG. 4B  shows the MEMS displacement vs. applied voltage for the MEMS device with the compound spring. Shown in this figure are the same points as shown in  FIG. 4A . During the up-sweep of the applied voltage the displacement follows the curve labeled pull-in. At an applied voltage of 5V, the MEMS device displaces to point p 1 . Then as the voltage is increased to 15V, the device displaces to point p 2 . When the voltage is increased to 25V, the MEMS device displaces to point p 3 . Any further increase in voltage would result in the device to snap in to the full displacement and end up in point p 4 . Then the voltage is reduced and the displacement follows the curve labeled release. When the voltage is reduced to 15V the displacement stays at 100% of the gap (point p 4 ′). Any further reduction makes the device snap back down to point p 2 . Thus, the pull-in voltage is 25V and the release voltage is 15V. From this analysis the advantage of the compound spring is clear. There is an increase in the release voltage from 5V to 15V, while the pull-in voltage has not changed. 
         [0062]      FIG. 5  shows the hot-switch comparison between a MEMS RF device with a linear spring (curve labeled linear) and with a compound spring (curve labeled CST). The hot-switch voltage is defined as the maximum voltage that is allowed on the RF electrode while the device still can be released when the voltage on the pull-in electrodes is removed. 
         [0063]    The MEMS device was first pulled in using a large enough voltage on the pull-in electrodes, i.e. the displacement would follow the curves labeled pull-in in  FIGS. 1B  (linear spring) and  4 B (compound spring) and result in the MEMS device to be displaced to point p 4 . Subsequently the voltage on the RF-electrode was increased to 50V and the voltage on the pull-in electrodes was removed. This voltage applied on the RF electrode is large enough to keep the MEMS device displaced in point p 4  even though the pull-in voltage was removed. 
         [0064]    Subsequently, the voltage on the RF electrode was slowly reduced until the device snaps-back to find the maximum voltage on the RF that can hold the device in a displaced position (the hot-switch voltage). For the device with the compound springs, the voltage has to be reduced to 43V until the device releases and moves from point p 5  to p 6 . For the device with the linear springs, the voltage has to be reduced to 15V until the device releases and moves from point p 7  to p 8 . 
         [0065]    From the above analysis one can summarize that the device with the compound springs can thus handle a much larger voltage on the RF electrode and still be able to release when the voltage on the pull-in electrodes is removed. 
         [0066]    By adjusting Z, D, d, k 1  and k 2 , it is possible to obtain the situation where there is an initial pull-in of the cantilever to d, but the electrostatic force is not high enough yet to completely pull-in the cantilever to D. This is illustrated in  FIGS. 6A and 6B . The initial pull-in of the device happens when the MEMS displacement snaps from point p 3  to p 3 ′. At that point the non-linear spring is too stiff to allow a full pull-in of the device. A little more voltage on the control electrode is required to overcome the non-linear spring and pull the device in to point p 4 . Upon reduction of the control voltage, the MEMS displacement can also show 2 release points, first going from point p 4 ′ to point p 5  dominated by the high spring-constant section of the compound spring and a subsequent snap-back from point p 6  to p 2  dominated by the low spring-constant section of the compound spring. 
         [0067]    The resulting hot-switch curve of this device is shown in  FIG. 7 . It also shows a 2-stage release of the device. First going from point p 5  to point p 6 , dominated by the high spring-constant section of the compound spring and a subsequent snap-back from point p 7  to p 8  dominated by the low spring-constant section of the compound spring. The hot-switch voltage in this case is a bit higher compared to the hot-switch voltage in  FIG. 5 . 
         [0068]    The 2-stage landing concept with the compound spring also has another advantage. When the MEMS device is being pulled-in, it initially accelerates until it hits the compound spring element. At this point it is being decelerated by the compound spring element and a second pull-in behavior occurs. Since this secondary pull-in occurs over a much reduced distance, the velocity that the MEMS element develops is much reduced compared to an equivalent device with a linear spring. This reduced impact velocity leads to less wear and longer lifetimes. 
         [0069]    There are several advantages to the embodiments disclosed herein. In particular, the embodiments minimize the switch size to achieve a fixed hot switch voltage and minimizes the landing voltage and landing velocity which improves the device reliability. 
         [0070]    Now with reference to  FIGS. 8A ,  8 B,  9  and  10 , one embodiment is described that corresponds to the schematic view of  FIGS. 2D-2E . It shows a MEMS RF switch fabricated on substrate  101 . It contains pull-in (control) electrodes  102  and RF electrode  102 ′. Both layer  102  and  102 ′ are formed at the same time by standard CMOS fabrication techniques (material deposition, lithographic masking steps and etches). Suitable materials that may be utilized for the electrodes  102 ,  102 ′ include titanium, tantalum, titanium nitride, tantalum nitride, copper, aluminum, and combinations thereof. On top of substrate  101  and electrodes  102 ,  102 ′ an insulating layer  103  is deposited, which serves to prevent leakage currents between the electrodes  102 ,  102 ′ and the moveable MEMS element  106 - 109  once it is actuated in a landed position as shown in  FIG. 8B . Suitable material that may be utilized for the insulating layer  103  include silicon oxide, silicon nitride, silicon oxynitride, spin-on glass, and combinations thereof. 
         [0071]    On top of dielectric layer  103  a contact layer is deposited and patterned to form structures  104 ,  104 ′. Structures  104  serve as the landing points of the device anchor  105  and are optional. If structures  104  are not used device anchor  105  will anchor the MEMS element onto the insulating layer  103 . Structures  104 ′ (See  FIG. 9 ) serve as the landing points of the composite springs  110 . 
         [0072]    If the contact layer  104 ,  104 ′ is to be used to provide electrical connection to the substrate  101 , suitable material that may be used for the contact layer  104 ,  104 ′ include electrically conductive material such as a metal that may comprise a material selected from the group consisting of titanium, tantalum, titanium nitride, tantalum nitride, copper, aluminum, titanium-aluminum, aluminum-nitride, titanium-aluminum-nitride and combinations thereof. Alternatively, the contact layer  104 ,  104 ′ may comprise an insulating layer including silicon oxide, silicon nitride, silicon oxynitride, spin-on glass, and combinations thereof. 
         [0073]    A moveable MEMS element may be formed over the substrate  101  by depositing a sacrificial layer, patterning the sacrificial layer to expose the landing structure  104 . It is to be understood that should the structure  104  not be utilized, the insulating layer  103  would be exposed. After the MEMS element is formed, another sacrificial layer may be deposited and patterned such that collectively, the sacrificial layers form the shape of the cavity within which the MEMS element will move. The sacrificial layers will eventually be removed to free the MEMS element. The sacrificial layers may be deposited by conventional processed such as plasma enhanced chemical vapour deposition (PECVD), chemical vapour deposition (CVD), spin-on technologies, and physical vapour deposition (PVD) to name a few. Suitable materials for the sacrificial layers include silicon containing compounds such as silicon dioxide, spin-on glass, or spin-on dielectric containing a long chain molecule with a carbon backbone. Such a material would need to have a low silicon content, because the sacrificial etch to remove carbon based compounds often leaves residues if they contain silicon. The anchors  105  are formed by etching holes in the first sacrificial layer. The bottom layer  106 ,  107  will deposit inside the holes to form anchors  105 . 
         [0074]    The moveable MEMS element consists of a 2-layer structure. The bottom-layer is formed by  106 ,  107  and the top-layer is formed by  109 . Structure  106  and  107  are both part of the bottom structure but denoted with a separate index only to indicate the different function that these structures take. The bottom layer  106 ,  107  and top layer  109  are connected by support structures  108  in selected regions. In one embodiment these support structures  108  are formed by selectively etching holes in a sacrificial layer present on top of bottom layer  106 ,  107 . The sacrificial layer will eventually be removed when the other sacrificial layers are removed. During deposition of the top layer  109 , this layer will also deposit inside holes and land on the bottom layer  106 , thereby forming the 2-layer MEMS structures with support structures  108 . This method of forming the moveable MEMS element allows for the formation of a stiff membrane  106 ,  109  while still allowing for flexible support beams  107 . The support beams  107  are anchored to the substrate  101  or contact layer  104  with support structures  105 . 
         [0075]    The material for bottom layer  106 ,  107 , support structures  108 , and top layer  109  include electrically conductive materials such as titanium, tantalum, titanium nitride, tantalum nitride, copper, aluminum, titanium aluminum, titanium aluminum nitride, and combinations thereof. While each of bottom layer  106 ,  107 , and top layer  109  are shown as single layers, it is contemplated that each layer  106 ,  107 ,  109  may comprise a multi-layer structure. For example, a 5-layer stack (TiN—Al—TiN—Al—TiN) may be utilized for each layer  106 ,  107 ,  109 . A 3-layer stack is also contemplated for each layer  106 ,  107 ,  109  such as TiN then Al then TiN. The tri-layer structure combines the strength advantages of TiN which has a high resistivity with the low resistance properties of Al which has poor mechanical strength. By sandwiching a thin Al layer between two TiN layers any residual stress difference in the deposition of the two materials will not cause differential stress in the MEMS structure. An additional advantage of a 3-layer structure is that the mechanical strength of TiN films is reduced at greater thickness of film. This is because of increased voids at increasing deposition thicknesses. By stopping the deposition of the TiN at around 200 nm putting down a thin Al layer and commencing growth, this problem can be greatly reduced. Layers  106 ,  107 ,  109  may be deposited by well known techniques such as sputtering, electroless plating and electrochemical plating. Layers  106 ,  107  and  109  may be patterned to form the desired final structure. 
         [0076]    In the same bottom layer that forms structure  106  of the stiff membrane and the support beams  107 , also the compound spring elements  110  are formed. Thus, when layers  106 ,  107  formed by patterning the electrically conductive material, compound spring elements  110  are also formed.  FIG. 10  shows a cross-section view through the compound spring element  110  of  FIG. 9 . Landing structure  104 ′ is positioned underneath the tip of the compound spring element  110  in such a way that if the MEMS element  106 - 109  is pulled down, the compound spring element  110  contacts the landing structure  104 ′ before the membrane section  106  touches the insulating layer  103  over the RF electrodes  102 ′ and pull-in electrodes  102 . A further pull-in of the device will occur until the membrane  106  touches the insulating layer  103 , which will result in the compound spring elements  110  to deflect and create an extra restoring force acting on the membrane. 
         [0077]    The total restoring force acting on the membrane contains two components, the first component being the restoring force of the support beams  107  and the second component being the restoring force of the compound spring elements  110 . The restoring force of the support beams  107  can be tuned by the dimensional parameters (i.e., length, width, thickness) and are chosen such that the pull-in voltage has the desired level. The restoring force component of the compound spring element  110  can be tuned by the dimensional parameters as well (i.e., length, width, thickness) as well as by the thickness of the landing structure  104 ′. The values are chosen such that the hot-switch voltage meets the desired level. 
         [0078]    In order to obtain a high hot-switch voltage, it is important that the restoring forces created by these 2 components are effective over the area of the RF electrode  102 ′. For this reason, a stiff membrane (i.e., items  106 ,  108 ,  109 ) is required for obtaining a high hot-switch voltage. If, for instance, the MEMS structure would only contain the bottom layer  106 , then the situation may arise where the restoring forces generated by the compound spring elements  110  would not help in releasing the membrane section  106  from the insulating layer  103  when the control voltage applied to the pull-in electrodes  102  is removed because the membrane could just deflect locally and stay in contact with the insulating layer  103  above the RF electrode in the presence of a high RF signal. In this case, to obtain a high hot-switch voltage a much stiffer bottom layer  106  would be required which would increase the pull-in voltage considerably as well. 
         [0079]    The combination of the stiff 2-layer membrane structure  106 ,  109  with support structures  108  in combination with the support beams  107  (which deflect to move the membrane) and the compound spring elements  110  provide for a non-linear force-vs.-displacement curve as shown in  FIG. 3  that is effective over the RF-area and provides for a high hot-switch voltage while keeping the pull-in voltage to an acceptable lower value. The restoring forces are most effective if the compound spring elements  110  are placed at a position close to the RF electrode  102 ′. The farther away from the RF electrode these are placed (i.e., closer towards the end of the plate), the stiffer the 2-layer membrane structure has to be for the same hot-switch voltage to be maintained. 
         [0080]    It is to be understood that the MEMS elements described herein are enclosed within a cavity when complete. One or more encapsulating layers are formed over the topmost sacrificial layer before the MEMS element is released. One or more holes are formed through the encapsulating layer to permit the sacrificial layers to be exposed to an etchant and removed to free the MEMS element. 
         [0081]    Now with reference to  FIGS. 11 and 12 , another embodiment is described that corresponds to the schematic view of  FIGS. 2A-2C . The materials and fabrication processes utilized to fabricate the device include the materials and processes as described above with regard to  FIGS. 8A ,  8 B,  9  and  10 . However, in order to form the different structure, the patterning shape of the sacrificial layers and the structural layers that form the MEMS device are adjusted accordingly.  FIG. 12  shows a cross-section view through the compound spring elements  1210 . The top-layer  1209  is connected to the bottom layer  1206  by support structures  1208 . In this case the compound spring elements  1210  are anchored to the substrate  1201  via support structures  1205  and optionally contact layer  1204 . The top-layer  1209  of the membrane is shaped such that it overlaps the tip of the compound spring elements  1210  in such a way that if the MEMS element is pulled in, the top-layer  1209  contacts the compound spring element  1210  before the membrane section  1206  contacts the insulating layer  1203  over the RF electrodes  1202 ′ and pull-in electrodes  1202 . This is achieved by ensuring that the sacrificial layer that is deposited between bottom layer  1206 ,  1207 ,  1210  and top layer  1209  is thinner than the sacrificial layer that is deposited between bottom layer  1206 ,  1207 ,  1210  and insulating layer  1203 . 
         [0082]    A further pull-in of the device will occur until the membrane  1206  touches the insulating layer  1203 , which will result in the compound spring elements  1210  to deflect and create an extra restoring force acting on the membrane. 
         [0083]    The total restoring force acting on the membrane contains two components, the first component being the restoring force of the support beams  1207  and the second component being the restoring force of the compound spring elements  1210 . The restoring force of the support beams can be tuned by the dimensional parameters (i.e., length, width, thickness) and are chosen such that the pull-in voltage has the desired level. The restoring force component of the compound spring element  1210  can be tuned by the dimensional parameters (i.e., length, width, thickness) as well as by the difference in thickness of the sacrificial layer below and above the bottom layer  1206 . The values are chosen such that the hot-switch voltage meets the desired level. 
         [0084]    Now with reference to  FIGS. 13 and 14 , another embodiment is described. The materials and fabrication processes utilized to fabricate the device include the materials and processes as described above with regard to  FIGS. 8A ,  8 B,  9  and  10 . However, in order to form the different structure, the patterning shape of the sacrificial layers and the structural layers that form the MEMS device are adjusted accordingly. The device includes a substrate  1301 , electrodes  1302 ,  1302 ′, insulating layer  1303 , contacts  1304 ,  1304 ′, top layer  1309  and support structures  1308 . In this case the compound spring element  1310  is combined with the support beams  1307 . This is achieved by including the contact layer  1304 ′ underneath the support beam  1307  somewhere along the length between the anchor point  1305  and the attachment of the support beam  1307  to the membrane  1306 . 
         [0085]    The initial stiffness of the support beam  1307  is given by its total length, width and thickness which are targeted to result in the desired pull-in voltage. Once the device is pulled in and the support beam  1307  touches the contact structure  1304 ′, the stiffness of the leg increases, resulting in the non-linear force-vs.-distance curve of  FIG. 3 . The extra stiffness created by the contact structure  1304 ′ is given by the position of the contact structure underneath the support beam  1307  and by the thickness of the contact layer  1304 ′. A higher stiffness is achieved by locating the contact structure  1304 ′ closer to the attachment point of the support beam  1307  to the plate  1306 . The position of contact structure  1304 ′ is chosen such to result in the desired hot-switch voltage. 
         [0086]    Now with reference to  FIGS. 15 and 16 , another embodiment is described that includes a substrate  1501 , anchor points  1505 , and contacts  1504 . The materials and fabrication processes utilized to fabricate the device include the materials and processes as described above with regard to  FIGS. 8A ,  8 B,  9  and  10 . However, in order to form the different structure, the patterning shape of the sacrificial layers and the structural layers that form the MEMS device are adjusted accordingly. In this case the compound spring element  1510  is created at the back-end of the plate  1506 . Landing structure  1504 ′ is positioned underneath the tip of the compound spring element  1510  in such a way that if the MEMS element  1506 - 1509  is pulled down, the compound spring element  1510  contacts the landing structure  1504 ′ before the membrane section  1506  touches the insulating layer  1503  over the RF electrodes  1502 ′ and pull-in electrodes  1502 . 
         [0087]    The actual compound spring element that creates the extra restoring force in this case entails both the short stub  1510  at the plate-end as well as the complete stiff plate  1506 ,  1508 ,  1509 . Upon a further pull-in of the device both the section  1510  and the complete plate  1506 ,  1508 ,  1509  will deflect, until the membrane  1506  touches the insulating layer. The plate-bending deflection of the stiff plate and the stub  1510  will create an extra restoring force acting on the membrane to create the non-linear force-vs.-displacement curve as shown in  FIG. 3 , which will help it to pull-off once the pull-in voltage is removed from pull-in electrode  1502  in the presence of an RF voltage on the RF electrode  1502 ′. 
         [0088]    Plate-bending contributes a larger percentage to the total restoring force in this embodiment compared to the embodiments of  FIGS. 9-14 . The restoring force can be tuned by the thickness of the bottom layer  1506 ,  1507 ,  1510 , the thickness of the top layer  1509  and the thickness of the support structures  1508  (i.e., the separation between layers  1506  and  1509 ). The percentage of the total restoring force due to the plate-bending vs. the percentage of the total restoring force coming from the deflection of the short stub  1510  at the plate end depends on the dimensions of the plate and the stub, 
         [0089]    Now with reference to  FIGS. 17 and 18 , another embodiment is described that includes a substrate  1701 , anchor points  1705 , and contacts  1704 . The materials and fabrication processes utilized to fabricate the device include the materials and processes as described above with regard to  FIGS. 8A ,  8 B,  9  and  10 . However, in order to form the different structure, the patterning shape of the sacrificial layers and the structural layers that form the MEMS device are adjusted accordingly. In this case the compound spring element  1710  is created at the back-end of the plate  1709 . Landing structure  1711  which is created in the same layer as the bottom layer  1706  is positioned underneath the tip of the compound spring element  1710  in such a way that if the MEMS element  1706 - 1709  is pulled down, the compound spring element  1710  contacts the landing structure  1711  before the membrane section  1706  touches the insulating layer  1703  over the RF electrodes  1702 ′ and pull-in electrodes  1702 . This is achieved by ensuring that the sacrificial layer that is deposited between bottom layer  1706 ,  1707  and top layer  1709 ,  1710  is thinner than the sacrificial layer that is deposited between bottom layer  1706 ,  1707  and insulating layer  1703 . 
         [0090]    The actual compound spring element that creates the extra restoring force in this case entails both the short stub  1710  at the plate-end as well as the complete stiff plate  1706 ,  1708 ,  1709  and the stiffness of the landing structure  1711 . Upon a further pull-in of the device both the section  1710  and the complete plate  1706 ,  1708 ,  1709  as well as the landing structure  1711  will deflect, until the membrane  1706  touches the insulating layer. The plate-bending deflection of the stiff plate and the stub  1710  will create an extra restoring force acting on the membrane to create the non-linear force-vs.-displacement curve as shown in  FIG. 3 , which will help it to pull-off once the pull-in voltage is removed from pull-in electrode  1702  in the presence of an RF voltage on the RF electrode  1702 ′. 
         [0091]    Plate-bending contributes a larger percentage to the total restoring force in this embodiment compared to the embodiments of  FIGS. 9-14 . The restoring force can be tuned by the thickness of the bottom layer  1706 ,  1707 ,  1711 , the thickness of the top layer  1709 ,  1710  and the thickness of the support structures  1708  (i.e., the separation between layers  1706  and  1709 ). The percentage of the total restoring force due to the plate-bending vs. the percentage of the total restoring force coming from the deflection of the short stub  1710  at the plate end depends on the dimensions of the plate and the stub. 
         [0092]    The landing structure  1711  in this embodiment can be made stiff by placing a device anchor  1705  very close by or more compliant by placing a device anchor  1705  further away. In case of a more compliant landing structure, it reduces the effectiveness of the compound spring, but it also reduces the impact force of the stub  1710  with the landing structure which can improve the lifetime of the device. 
         [0093]    Now with reference to  FIGS. 19 and 20 , another embodiment is described. The materials and fabrication processes utilized to fabricate the device include the materials and processes as described above with regard to  FIGS. 8A ,  8 B,  9  and  10 . However, in order to form the different structure, the patterning shape of the sacrificial layers and the structural layers that form the MEMS device are adjusted accordingly. The device includes a substrate  1901 , electrodes  1902 ,  1902 ′, insulating layer  1903 , contacts  1904 ,  1904 ′, anchors  1905 ,  1905 ′, bottom layer  1906 ,  1906 ′, top layer  1907 ,  1909 ,  1910  and support structures  1908 . In this case the compound spring element  1910  is combined with the support beams  1907  which are now located in the top-layer. This is achieved by including the contact layer  1904 ′ underneath the support beam  1907  somewhere along the length between the anchor point  1905  and the attachment of the support beam  1907  to the membrane  1909 . Landing structure  1911  which is created in the same layer as the bottom layer  1906  is positioned above the contact layer  1904 ′ and anchored to it via support structure  1905 ′. 
         [0094]    The initial stiffness of the support beam  1907  is given by its total length, width and thickness which are targeted to result in the desired pull-in voltage. Once the device is pulled in and the support beam  1907  touches the landing structure  1911 , the stiffness of the leg increases, resulting in the non-linear force-vs.-distance curve of  FIG. 3 . The extra stiffness created by the landing structure  1911  is given by the position of the landing structure  1911  underneath the support beam  1907  and by the difference in thickness of the sacrificial layer below and above the bottom layer  1906 . A higher stiffness is achieved by locating the landing structure  1911  closer to the attachment point of the support beam  1907  to the plate  1909 . The position of landing structure  1911  is chosen such to result in the desired hot-switch voltage. 
         [0095]    The described embodiments above can also be combined to use various compound spring elements  110 ,  1210 ,  1310 ,  1910  placed close by the RF electrode  102 ′,  1202 ′ ( FIGS. 9-12 ) as well as placed underneath the support beams  1307 ,  1907  ( FIGS. 13-14 ,  19 - 20 ) or utilize the plate-bending embodiments ( FIGS. 15-18 ). In described embodiments of  FIGS. 9-10  and  FIGS. 13-16  the compound spring engagement depends on the thickness of the contact structure  104 ′,  1304 ′,  1504 ′. In described embodiments of  FIGS. 11-12  and  FIGS. 17-20  the compound spring engagement depends on the difference in thickness of the sacrificial layer below and above the bottom layer  1206 ,  1706 ,  1906 . By combining compound spring elements that depend on different dependencies it is possible to create a multi-stage compound system where first one set of compound springs is activated and then the next set. Such a multi-stage compound system may further reduce the impact velocity of the moveable MEMS structure  106 - 109 ,  1206 - 1209 ,  1306 - 1309 ,  1506 - 1509 ,  1706 - 1709 ,  1906 - 1909  and increase the hot-switch voltage more. 
         [0096]    While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.