Patent Abstract:
Systems and methods for providing high-capacitive RF MEMS switches are provided. In one embodiment, the invention relates to a micro-electro-mechanical switch assembly including a substrate, an electrode disposed on a portion of the substrate, a dielectric layer disposed on at least a portion of the electrode, a metal layer disposed on at least a portion of the dielectric layer, and a flexible membrane having first and second ends supported at spaced locations on the substrate base, where the flexible membrane is configured to move from a default position to an actuated position in response to a preselected switching voltage applied between the flexible membrane and the electrode, and where, in the actuated position, the flexible membrane is in electrical contact with the metal layer.

Full Description:
FIELD OF THE INVENTION 
     This invention relates in general to switches and, more particularly, to systems and methods for providing high-capacitive RF MEMS switches. 
     BACKGROUND 
     One existing type of switch is a radio frequency (RF) micro-electro-mechanical system (MEMS) switch. This existing type of switch typically has a substrate with two conductive posts spaced apart on the substrate. A conductive part (e.g., electrode) is provided on the substrate between the posts, and is covered by a layer of a dielectric material. A flexible and electrically conductive membrane extends between the posts, so that a central portion of the membrane is located above the conductive part on the substrate. An RF signal is applied to one of the conductive part and the membrane. 
     In the deactuated or non-actuated state of the switch, the membrane is spaced above both the conductive part and the dielectric layer covering it. In order to actuate the switch, a direct current (DC) bias voltage is applied between the membrane and the conductive part. This bias voltage produces charges on the membrane and the conductive part, and the charges cause the membrane and conductive part to be electrostatically attracted to each other. This attraction causes the membrane to flex, so that a central portion thereof moves downwardly until it contacts the top of the dielectric layer on the conductive part. This is the actuated position of the membrane. 
     In this actuated state of the switch, the spacing between the membrane and the conductive part is less than in the deactuated state. Therefore, in the actuated state, the capacitive coupling between the membrane and the conductive part is significantly larger than in the deactuated state. Consequently, in the actuated state, the RF signal traveling through one of the membrane and conductive part is capacitively coupled substantially in its entirety to signals traveling along the other part. 
     In order to deactuate the switch, the DC bias voltage is turned off. The inherent resilience of the membrane then returns the membrane to its original position, which represents the deactuated state of the switch. Because the capacitive coupling between the membrane and conductive part is much lower in the deactuated state, the RF signal traveling through one of the membrane and capacitive part experiences little or no capacitive coupling to signals traveling along the other part. 
     In certain applications, the ratio of capacitance in the actuated state to capacitance in the non-actuated or default state can be very important. In general, the greater the capacitance ratio is, the greater the bandwidth is that can be provided by the switch. The non-actuated capacitance, or off-capacitance, is a function of the switch membrane and parasitics when the membrane is in the non-actuated position. The actuated capacitance, or on-capacitance, is a function of the metal-insulator-metal (MIM) capacitor formed when the membrane snaps down to the actuated position on top of the dielectric covering the electrode. To provide a RF MEMS switch with better performance characteristics, it is therefore desirable to increase the on-capacitance of the switch. 
     SUMMARY OF THE INVENTION 
     Aspects of the invention relate to systems and methods for providing high-capacitive RF MEMS switches. In one embodiment, the invention relates to a micro-electro-mechanical switch assembly including a substrate, an electrode disposed on a portion of the substrate, a dielectric layer disposed on at least a portion of the electrode, a metal layer disposed on at least a portion of the dielectric layer, and a flexible membrane having first and second ends supported at spaced locations on the substrate base, where the flexible membrane is configured to move from a default position to an actuated position in response to a preselected switching voltage applied between the flexible membrane and the electrode, and where, in the actuated position, the flexible membrane is in electrical contact with the metal layer. 
     In another embodiment, the invention relates to a method for manufacturing a micro-electro-mechanical switch assembly including depositing an electrode material on a surface of a substrate, depositing a dielectric material on at least a portion of a surface of the electrode material, depositing a metal layer on at least a portion of a surface of the dielectric layer, depositing a plurality of posts on the substrate at positions spaced apart from the electrode material, depositing a spacer material on the metal layer and between the posts, depositing a flexible membrane on the spacer material and the posts, and etching the spacer material from the assembly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top view of a RF MEMS switch having a floating metal layer for reducing an air-gap in the actuated position of the switch in accordance with one embodiment of the invention. 
         FIG. 2  is a cross sectional view of the RF MEMS switch of  FIG. 1 , including the floating metal layer, in a non-actuated state in accordance with one embodiment of the invention. 
         FIG. 3  is a cross sectional view of the RF MEMS switch and floating metal layer of  FIG. 1  in an actuated state in accordance with one embodiment of the invention. 
         FIG. 4  is a cross sectional enlarged view of a section of the RF MEMS switch of  FIG. 3  illustrating the floating metal layer in contact with a portion of the membrane in accordance with one embodiment of the invention. 
         FIG. 5  is a top view of a RF MEMS switch having a patterned floating metal layer for switch biasing in accordance with one embodiment of the invention. 
         FIG. 6  is an diagrammatic illustration of a process for manufacturing a RF MEMS switch, including cross sectional views of the switch at various stages and corresponding process steps, in accordance with one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Capacitance of an RF MEMS switch is a characteristic that is important to performance of the switch. While not bound by any particular theory, capacitance of an RF MEMS switch in the actuated state, or on-capacitance, is a function of the dielectric constant and thickness of the dielectric. More specifically, the on-capacitance is proportional to the dielectric constant for a constant thickness. In RF MEMS capacitive switches, often there is a finite amount of air (e.g., air gaps), caused by surface roughness, between the membrane and dielectric that dramatically reduces the maximum obtainable on-capacitance because the air has a low dielectric constant and because these air gaps are not easily removed. In such case, the reduced on-capacitance undesirably limits the low-frequency broadband operation of the RF MEMS switch. 
     Referring now to the drawings, embodiments of RF MEMS switches include a substrate, an electrode positioned on the substrate, a dielectric positioned on the electrode, a flexible membrane and a floating (e.g., electrically isolated) metal layer positioned on the dielectric that substantially removes or eliminates the capacitive effects of any air gaps. The floating metal layer can be deposited onto the dielectric layer such that a minimal air gap exists between the floating metal layer and the dielectric. In several embodiments, the floating metal layer is deposited onto the dielectric layer such that no air gap exists between the floating metal layer and the dielectric. When the RF MEMS switches are actuated, the flexible membrane can make an ohmic contact with the floating metal layer. As such, despite any air gaps that might exist between the floating metal layer and the flexible membrane, the floating metal layer effectively becomes continuous with the flexible membrane. In such case, the on-capacitance becomes a function of only the dielectric constant and thickness of the dielectric material. Thus, the on-capacitance can be increased without limits caused by air gaps. 
     In a number of embodiments, the dielectric can have rough surfaces. However, in accordance with processes for manufacturing embodiments of RF MEMS switches described herein, the floating metal layer can be deposited directly on the dielectric, thereby substantially reducing or eliminating troublesome air gaps. While some air gaps may exist between the floating metal layer and the flexible membrane in the actuated position, ohmic contacts can be made between the floating metal layer and the flexible membrane. As such, any air gaps that might exist between the floating metal layer and the flexible membrane can have negligible effect on the capacitance seen by signals traveling through the RF MEMS switch and only affect contact resistance which has a much reduced impact on performance. 
     In several embodiments, the floating metal layers of the RF MEMS switches are patterned to provide sufficient electrical characteristics to enable biasing circuitry to apply an electric field that switches the flexible membrane to the actuated position and to enable the flexible membrane to return to the default position when the electric field is removed. 
       FIG. 1  is a top view of a RF MEMS switch  100  having a floating metal layer (not visible) for reducing an air-gap in the actuated position of the switch in accordance with one embodiment of the invention. The switch  100  includes a substrate  102 , an electrode layer  104  positioned on the substrate  102 , a dielectric layer  106  positioned on the electrode, a floating metal layer  110  (see  FIG. 2 ) positioned on the dielectric  106 , and a flexible membrane  108  positioned on posts  112  (see  FIG. 2 ) above the dielectric  106 . 
       FIG. 2  is a cross sectional view of the RF MEMS switch of  FIG. 1 , including the floating metal layer  110 , in a non-actuated state in accordance with one embodiment of the invention. As can be seen from  FIG. 2 , the electrode  104  is positioned on a portion of a top surface of the substrate  102 . The dielectric  106  is positioned on top and side surfaces of the electrode  104  and portions of the substrate  102 . The floating metal layer  110  is positioned on a top surface of the dielectric  106 . While the top surface of the dielectric  106  may be uneven and rough, the floating metal layer  110  can be deposited on to the dielectric  106  in a deposition process as described below. In such case, the floating metal layer  110  can be positioned on top of the dielectric  106  with very little or no separation that would provide an air gap between the floating metal layer and dielectric. 
     While not shown, a bias control circuit is typically coupled to the membrane  108  and electrode  104 . In operation, the bias control circuit can apply a DC bias voltage between the membrane  108  and electrode  104 , thereby creating an electric field that actuates the membrane from the default position (e.g.,  FIG. 2 ) to an actuated position (e.g.,  FIGS. 3 ,  4 ). In the actuated position, the switch can provide maximum capacitive coupling (e.g., closed position for RF MEMS switch). When the DC bias voltage is removed, the flexible membrane can return to the default or non-actuated position that provides minimal capacitive coupling (e.g., open position for RF MEMS switch). 
     In the embodiment illustrated in  FIG. 2 , the substrate can be made of alumina. In other embodiments, other suitable materials can be used, including, without limitation, a high resistivity silicon such as gallium arsenide, alumina, quartz, glass or combinations thereof. 
     In the embodiment illustrated in  FIG. 2 , components appear to have certain relative sizes. However,  FIG. 2  is not drawn to scale and other suitable component sizes can be used. 
     In the embodiment shown in  FIG. 1 , typical dimensions are 300 microns in length and 264 microns in width for the RF MEMS switch  102 . In the embodiment shown in  FIG. 2 , the typical thickness of the metal post  112  is 3 microns and is typically made out of gold. The electrode  104  is typically 0.5 microns thick and made of gold and/or other metals while the dielectric  106  is typically 0.25 microns and made of silicon nitride. The floating metal layer  110  is typically 0.25 microns and can be made out of titanium while the flexible membrane  108  is typically 0.5 microns thick and made of aluminum. In this paragraph, reference has been made to specific dimensions and materials. In other embodiments, other suitable dimensions and materials can be used. 
       FIG. 3  is a cross sectional view of the RF MEMS switch  100  and floating metal layer  110  of  FIG. 1  in an actuated state in accordance with one embodiment of the invention. In the actuated state or position, the membrane  108  of the switch  100  extends downward such that a center portion of the membrane  108  makes contact with the floating metal layer  110 . In the actuated position, the floating metal layer  110  can contact the membrane  108  at multiple contact points effectively forming an ohmic, or metal to metal, contact between two metallic components. In some embodiments, both the membrane and floating metal layer have approximately flat surfaces such that the contact therebetween is made surface to surface (e.g., a total number of contact points is substantial). 
     The embodiments illustrated in  FIGS. 2 and 3  can be modified to accommodate varying dimensions of the materials. For example, by changing the x-y-z dimensions of the electrode  104 , the dielectric  106  and the floating metal layer  110 , a larger or smaller capacitor can be formed without affecting the overall operation in accordance with the invention. In addition, the dimensions and thicknesses of the posts  112  and membrane  108  can be changed to increase or decrease the DC actuation voltage of the switch without affecting the overall operation in accordance with the invention. In several embodiments, the posts  112 , membrane  108 , electrode  104  and floating metal  110  are made out of conducting materials such as metals, but are not limited to a specific type of conductor. In a number of embodiments, the dielectric  106  is made of a non-conductive, low-loss RF dielectric material, but is not limited to any specific material. 
       FIG. 4  is a cross sectional enlarged view of a section of the RF MEMS switch  100  of  FIG. 3  illustrating the floating metal layer  110  in contact with a portion of the membrane  108  in accordance with one embodiment of the invention. In the embodiment illustrated in  FIG. 4 , the Top surface of the floating metal layer  110  is rough and includes a number of recesses  114 . As such, a number of contact points  116  and air gaps  118  exist between the top surface of the floating metal layer  110  and the membrane  108 . 
     While not bound by any particular theory of operation, these air gaps between the floating metal layer and membrane do not affect the on-capacitance of the switch in the actuated position as the ohmic contact between those components makes them appear electrically as one continuous component. Instead, the on-capacitance is a function only of the dielectric constant of the dielectric layer and not air. As such, the on-capacitance can be controlled as desired by selection of a dielectric material and a particular thickness for a particular application. 
       FIG. 5  is a top view of a RF MEMS switch  200  having a patterned floating metal layer  210  for switch biasing in accordance with one embodiment of the invention. The switch  200  includes a substrate (not shown), an electrode layer  204  positioned on the substrate, a dielectric layer  206  positioned on the electrode, the patterned floating metal layer  210  positioned on the dielectric  206 , and a flexible membrane  208  positioned on posts  212  extending above the dielectric  206 . The patterning of the floating metal layer  210  can allow the applied electric field to extend through the openings in the floating metal layer to reach the membrane and actuate it even after the floating metal layer has taken a charge during a previous actuation. 
     While not bound by any particular theory, because the floating metal layer is not electrically connected to anything it has no way to quickly dissipate charge after the flexible membrane  208  has been released to its unactuated state. Therefore, any charge left behind from the previous actuation will continue to reside on the floating metal layer. In the case of a continuous (non-patterned) metal sheet, this left-over charge can effectively shield out any bias voltage applied to the switch and keep the membrane from acquiring enough charge to actuate or, at a minimum, can require a higher bias voltage for switch actuation than before the left-over charge was stored. In order to keep a constant actuation voltage over many actuations, the openings in the patterned floating metal layer can allow the electric field to pull the membrane down to actuate the switch regardless of the left-over charge on the floating metal layer. 
     In the embodiment shown in  FIG. 5 , the floating metal layer has been patterned in a particular manner. However, in other embodiments, the style, spacing and quantity of patterning can be varied in accordance with a number of desired design parameters, including, for example, the actuation voltage, the ohmic contact quality, the on-capacitance, and/or other design parameters. 
       FIG. 6  is an diagrammatic illustration of a process for manufacturing an RF MEMS switch  300 , including cross sectional views of the switch at various stages and corresponding process steps, in accordance with one embodiment of the invention. In several embodiments, this process can be used to manufacture the RF MEMS switches of  FIGS. 1 to 5 . The process first deposits and patterns ( 350 ) an electrode  304  on a portion of a top surface of a substrate  302 . The process further deposits and patterns ( 350 ) a dielectric  306  on top and side surfaces of the electrode  304  and portions of the substrate  302 . The process then deposits and patterns ( 352 ) a floating metal sheet  310  on a top surface of the dielectric  306 . 
     The process then deposits and patterns ( 354 ) two metal posts  312  on the top surface of the substrate  302  at locations spaced apart from the electrode  304 , dielectric  306 , and floating metal layer  310 . In several embodiments, the locations of the metal posts  312  are spaced apart at distances from the electrode  304  that are about equal. The process then deposits ( 356 ) spacer material  314  between the metal posts  312  and on top of the floating metal layer  310  and portions of the dielectric  306 . The process then deposits and patterns ( 358 ) a metal membrane  308  on top surfaces of the spacer  314  and metal posts  312 . The process then etches or removes ( 360 ) the spacer material from the switch assembly  300 . 
     In some embodiments, the process does not perform all of the actions described. In other embodiments, the process performs additional actions. In one embodiment, the process performs the actions in a different order than illustrated in  FIG. 6 . In some embodiments, the process performs some of the actions simultaneously. 
     In one embodiment, the process adds an additional thin dielectric to the top of the floating metal layer or floating electrode. In this case, the membrane in the actuated position would form a capacitor rather than an ohmic contact with the floating electrode. The RF signal would see this capacitance in series with the capacitance between the floating electrode and the bottom or substrate electrode. In such case, the capacitance would still be increased over that for a standard RF MEMS switch with no floating electrode if the top dielectric is significantly thinner than the first (bottom) dielectric. If T bottom  represents the thickness of the bottom dielectric (e.g., between the bottom electrode and the floating electrode), T top  represents the thickness of the dielectric on top of the floating electrode, and R represents the ratio of the membrane capacitance with and without the air gaps, then the ratio of capacitance with a floating electrode to that without a floating electrode will be (T top +T bottom )/(R×T bottom +T top ), where T top +T bottom  equals the thickness of the MEMS dielectric without a floating electrode and R has a value such that 0&lt;R≦1. 
     While the above description contains many specific embodiments of the invention, these should not be construed as limitations on the scope of the invention, but rather as examples of specific embodiments thereof. Accordingly, the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims and their equivalents.

Technology Classification (CPC): 8