Methods of fabricating tunable capacitors

Abstract

A tunable capacitor having low loss and a corresponding high Q is provided. The tunable capacitor comprises first and second substrates having first and second capacitor plates disposed, respectively, thereon. The capacitor plates may comprise a high temperature superconductor material. A MEMS actuator, that is either driven by electrostatic force, heat or both, operably contacts the second substrate so that once the actuator is engaged it responds by displacing the second substrate, thereby varying the capacitance between said first capacitor plate and said second capacitor plate. As such, the capacitance can be controlled based upon the relative spacing between the first and second capacitor plates. The MEMS actuator may either be operably attached to the second substrate or detached, yet supporting, the second substrate. A method is also provided for micromachining or otherwise fabricating a tunable capacitor having high temperature superconductor capacitor plates and electrostatic and/or thermal MEMS actuation such that the resulting tunable capacitor has low signal loss and a corresponding high Q. The tunable capacitor can therefore be used in high frequency applications, such as those using radio frequencies.

That which is claimed:  
     
         1 . A tunable capacitor comprising: 
 a first substrate having a first surface;    a first capacitor plate disposed on the first surface of said first substrate;    a second substrate having a first surface;    a second capacitor plate disposed on the first surface of said second substrate, wherein said first and second substrates are positioned such that said first and second capacitor plates face one another in a spaced apart relationship; and    a microelectromechanical actuator operably contacting said second substrate for displacing said second substrate in response to electrostatic forces, thereby varying the capacitance between said first capacitance plate and said second capacitance plate.    
     
     
         2 . The tunable capacitor of    claim 1   , wherein said first and second capacitor plate further comprise a high temperature superconductor material.  
     
     
         3 . The tunable capacitor of    claim 2   , wherein said first and second capacitor plates further comprise a high temperature superconductor yttrium compound.  
     
     
         4 . The tunable capacitor of    claim 2   , wherein said first and second capacitor plates further comprise a high temperature superconductor thallium compound.  
     
     
         5 . The tunable capacitor of    claim 1   , wherein said microelectromechanical actuator further comprises: 
 at least one first electrode disposed on the first surface of said first substrate; and    at least one cantilever structure at least partially overlying said first electrode and operably contacting said second substrate, said at least one cantilever structure comprising a second electrode.    
     
     
         6 . The tunable capacitor of    claim 5   , wherein said at least one first electrode further comprises a high temperature superconductor material.  
     
     
         7 . The tunable capacitor of    claim 5   , wherein said second electrode further comprises silicon.  
     
     
         8 . The tunable capacitor of    claim 5   , further comprising a first intermediary layer disposed on the first surface of said first substrate intermediate said cantilever structure and said first substrate.  
     
     
         9 . The tunable capacitor of    claim 8   , wherein said first intermediary layer further comprises gold.  
     
     
         10 . The tunable capacitor of    claim 5   , further comprising a second intermediary layer disposed on the first surface of said second substrate intermediate said cantilever structure and said second substrate, wherein said second intermediary layer operably connects said second substrate to said cantilever structure.  
     
     
         11 . The tunable capacitor of    claim 10   , wherein said first intermediary layer further comprises gold.  
     
     
         12 . The tunable capacitor of    claim 5   , wherein said cantilever structure further comprises spring-like elements structurally patterned in said cantilever structure that provide elasticity for said cantilever structure.  
     
     
         13 . The tunable capacitor of    claim 5   , wherein said cantilever structure further comprises a first layer comprising the second electrode and a second layer disposed on the first layer comprising a biasing element.  
     
     
         14 . The tunable capacitor of    claim 13   , wherein said first layer further comprises silicon and said second layer comprises gold.  
     
     
         15 . The tunable capacitor of    claim 1   , wherein said microelectronic actuator is operably attached to said second substrate.  
     
     
         16 . The tunable capacitor of    claim 1   , further comprising at least one spring-like structure operably connected to said second substrate, whereby said at least one spring-like structure provides elasticity for said second substrate.  
     
     
         17 . The tunable capacitor of    claim 1   , wherein said first and second substrates further comprise low signal loss substrates.  
     
     
         18 . The tunable capacitor of    claim 17   , wherein said first and second substrates further comprise magnesium oxide (MgO).  
     
     
         19 . A tunable capacitor comprising: 
 a first substrate having a first surface;    a first capacitor plate disposed on the first surface of said first substrate;    a second substrate having a first surface;    a second capacitor plate disposed on the first surface of said second substrate, wherein said first and second substrates are positioned such that said first and second capacitor plates face one another in a spaced apart relationship; and    a microelectromechanical actuator operably contacting said second substrate for displacing said second substrate in response to thermal actuation, thereby varying the capacitance between said first capacitance plate and said second capacitance plate.    
     
     
         20 . The tunable capacitor of    claim 19   , wherein said first and second capacitor plates further comprise a high temperature super conductor material.  
     
     
         21 . The tunable capacitor of    claim 20   , wherein said first and second capacitor plates further comprise a high temperature superconductor yttrium compound.  
     
     
         22 . The tunable capacitor of    claim 20   , wherein said first and second capacitor plates further comprise a high temperature superconductor thallium compound.  
     
     
         23 . The tunable capacitor of    claim 19   , wherein said microelectromechanical actuator further comprises a moveable composite structure having at least two layers, including: 
 a first layer; and    a second layer disposed on said first layer and operably contacting said second substrate, wherein said first layer responds differently to thermal actuation than said second layer.    
     
     
         24 . The tunable capacitor of    claim 23   , wherein said first layer comprises one or more materials having a lower coefficient of thermal expansion than one or more materials comprising said second layer.  
     
     
         25 . The tunable capacitor of    claim 23   , wherein said first layer comprises silicon.  
     
     
         26 . The tunable capacitor of    claim 23   , wherein said second layer comprises gold.  
     
     
         27 . The tunable capacitor of    claim 23   , further comprising a first intermediary layer disposed on the first surface of said first substrate intermediate said moveable composite structure and said first substrate.  
     
     
         28 . The tunable capacitor of    claim 27   , wherein said first intermediary layer further comprises gold.  
     
     
         29 . The tunable capacitor of    claim 23   , further comprising a second intermediary layer disposed on the first surface of said second substrate intermediate said moveable composite structure and said second substrate wherein said second intermediary layer operably connects said second substrate to said moveable composite structure.  
     
     
         30 . The tunable capacitor of    claim 29   , wherein said first intermediary layer further comprises gold.  
     
     
         31 . The tunable capacitor of    claim 23   , wherein said second layer is operably attached to said second substrate.  
     
     
         32 . The tunable capacitor of    claim 23   , wherein said moveable composite structure further comprises spring-like elements structurally patterned in said second layer so as to provide elasticity for said moveable composite structure.  
     
     
         33 . The tunable capacitor of    claim 23   , further comprising at least one spring-like structure operably connected to said second substrate, whereby said at least one spring-like structure provides elasticity for said second substrate.  
     
     
         34 . The tunable capacitor of    claim 19   , wherein said first and second substrates further comprise a low signal loss substrate.  
     
     
         35 . The tunable capacitor of    claim 19   , wherein said first and second substrates further comprise magnesium oxide (MgO).  
     
     
         36 . A tunable capacitor comprising: 
 a first substrate having a first surface;    a first capacitor plate disposed on the first surface of said first substrate;    a second substrate having a first surface;    a second capacitor plate disposed on the first surface of said second substrate, wherein said first and second substrates are positioned such that said first and second capacitor plates face one another in a spaced apart relationship; and    a microelectromechanical actuator operably contacting said second substrate for displacing said second substrate in response to a motive force selected from the group consisting of electrostatic force and thermal actuation, thereby varying the capacitance between said first capacitance plate and said second capacitance plate.    
     
     
         37 . The tunable capacitor of    claim 36   , wherein said first and second capacitor plates further comprise a high temperature super conductor material.  
     
     
         38 . The tunable capacitor of    claim 36   , wherein said microelectromechanical actuator further comprises: 
 at least one first electrode disposed on the first surface of said first substrate; and    at least one cantilever structure at least partially overlying said first electrode and operably contacting said second substrate, said at least one cantilever structure comprising at least two layers and a second electrode.    
     
     
         39 . The tunable capacitor of    claim 38    wherein said cantilever structure further comprises a first layer and a second layer disposed upon said first layer, wherein said first layer responds differently to thermal actuation than said second layer.  
     
     
         40 . The tunable capacitor of    claim 38    wherein said first layer comprises one or more materials having a lower coefficient of thermal expansion than one or more materials comprising said second layer.  
     
     
         41 . A method for making a tunable capacitor comprising: 
 fabricating a first capacitor plate structure comprised of a first substrate and a first high-temperature superconductor capacitor plate disposed on the first substrate;    fabricating a micromechanical (MEMS) actuator that is responsive to electrostatic forces;    fabricating a second capacitor plate structure comprised of a second substrate, a second high-temperature superconductor capacitor plate disposed on the second substrate and at least one first actuator electrode disposed on the second substrate; and    connecting said first capacitor plate structure to said second capacitor plate structure such that the MEMS actuator is disposed between the first and second capacitor plate structures and in operable contact with the first capacitor plate structure.    
     
     
         42 . The method of    claim 41   , wherein the step of fabricating an electrostatic driven MEMS actuator further comprises the substeps of: 
 disposing a biasing layer on the surface of a third substrate;    attaching the biasing layer to the first substrate; and    etching back the underside of the third substrate to create a second actuator electrode layer.    
     
     
         43 . The method of    claim 41   , wherein the step of fabricating an electrostatic MEMS actuator further comprises the substeps of: 
 disposing a release layer on the surface of a third substrate;    disposing a biasing layer on the release layer;    bonding the biasing layer to the first substrate;    etching back the underside of the third substrate to create a second actuator electrode layer; and    releasing the biasing layer from the second actuator electrode layer in an area proximate the first substrate.    
     
     
         44 . The method of    claim 41   , wherein said connecting step further comprises: 
 disposing a first support layer on the surface of said second substrate;    disposing a second support, layer on the surface of said electrostatic driven MEMS actuator; and    attaching the first support layer to the second support layer.    
     
     
         45 . A method for making a tunable capacitor comprising: 
 fabricating a first capacitor plate structure comprised of a first substrate and a first high-temperature superconductor capacitor plate disposed on the first substrate;    fabricating a microelectromechanical (MEMS) actuator that is thermally actuated;    fabricating a second capacitor plate structure comprised of a second substrate and a second high-temperature superconductor capacitor plate disposed on the second substrate; and    connecting said first capacitor plate structure to said second capacitor plate structure such that the MEM actuator is disposed between the first and second capacitor plate structures and in operable contact with the first capacitor plate structure.    
     
     
         46 . The method of    claim 45   , wherein the step of fabricating a thermally activated MEMS actuator further comprises the substeps of: 
 disposing a first layer on the surface of a third substrate, the first layer comprising a material having a higher thermal coefficient of expansion than said third substrate;    attaching the first layer to the first substrate; and    etching back the underside of the third substrate to create a second layer of a thermal actuator.    
     
     
         47 . The method of    claim 45   , wherein the step of fabricating a thermally activated MEMS actuator further comprises the substeps of: 
 disposing a release layer on the surface of a third substrate;    disposing a first layer of a thermal acuator on the release layer;    bonding the first layer to the first substrate;    etching back the underside of the third substrate to create a second layer of the thermal actuator; and    releasing the first layer from the second layer in an area proximate the first substrate.    
     
     
         48 . The method of    claim 45   , wherein said connecting step further comprises: 
 disposing a first support layer on the surface of said second substrate;    disposing a second support layer on the surface of said electrostatic MEMS actuator; and    attaching the first support layer to the second support layer.