Abstract:
A micromechanical structure and device and methods of forming and using the structure and device are disclosed. The structure includes an ion conductor and a plurality of electrodes. Mechanical properties of the structure are altered by applying a bias across the electrodes. Such structures can be used to form device such as actuators and air-gap devices.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Patent Application Ser. No. 60/500136, entitled Programmable Metallization Cell Technology in Microactuators and Air Gap Switches, filed Sep. 3, 2003, claims priority to application Ser. No. 10/796,808, entitled Programmable Structure, An Array Including The Structure, And Methods Of Forming The Same, filed Mar. 17, 2003, and claims priority to application Ser. No. 10/282,902, entitled Tunable Cantilever Apparatus and Method of Making Same, filed Oct. 28, 2002. 
     
    
     FIELD OF INVENTION  
       [0002]     The present invention generally relates to micromechanical structures and to devices including the structures. More particularly, the invention relates to micromechanical structures including an ion conductor material, devices including the structures, and to methods of forming and using the structures and devices.  
       BACKGROUND OF THE INVENTION  
       [0003]     Micromechanical devices often include piezoelectric material, which undergoes a dimensional change upon application of a sufficient bias across the material. Although such piezoelectric material devices may work relatively well for some uses, such devices are problematic with regard to several applications. In particular, use of micromechanical structures that include piezoelectric material to effect a dimensional change typically requires application of a relatively large bias to cause a relatively small dimensional change in the material, and any dimensional change relaxes once the bias is removed. Thus, the devices require continuous application of a relatively large bias for so long as the material change is desired.  
         [0004]     Additional problems arise because devices including typical piezoelectric materials are difficult to integrate with integrated circuits and microelectomechanical systems (MEMS). Consequently devices using piezoelectric materials are often reserved for discrete as opposed to integrated devices.  
         [0005]     Other forms of piezoelectric material structures can be formed in such a way that a continuous bias is not required to sustain a dimensional change. However, these structures are relatively difficult to manufacture and are difficult to integrate with other devices.  
         [0006]     Accordingly, improved micromechanical structures and devices including the structures that are relatively easy to fabricate, that are relatively easy to integrate with other devices, and that are relatively non-volatile are desired.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention provides improved micromechanical structures and devices for use as, for example, microactuators and air-gap switches. More particularly, the invention provides structures that undergo a change resulting from migration of ions within the structure, to devices including the structures, and to methods of forming and using the structures and devices.  
         [0008]     The ways in which the present invention addresses various drawbacks of now-known micromechanical devices are discussed in greater detail below. However, in general, the present invention provides a structure and a device that are relatively easy and inexpensive to manufacture, which are relatively easy to integrate with other devices, and which maintain their state when the source of electrical energy is removed, i.e., are relatively non-volatile.  
         [0009]     In accordance with one exemplary embodiment of the present invention, a micromechanical structure includes an ion conductor and at least two electrodes. The structure is configured such that when a bias is applied across two electrodes, conductive material within the ion conductor migrates and thus alters a physical property of the ion conductor. In accordance with one aspect of this embodiment, the structure includes a conductive layer proximate the ion conductor and optionally includes an insulating layer between the ion conductor and the conductive layer. The conductive layer facilitates supplying electrons to the ion conductor when a bias is applied across the electrodes.  
         [0010]     In accordance with another exemplary embodiment of the invention, a structure includes an ion conductor, at least two electrodes, and a barrier interposed between at least a portion of one of the electrodes and the ion conductor. In accordance with one aspect of this embodiment the barrier material includes a material configured to reduce diffusion of ions between the ion conductor and at least one electrode. The diffusion barrier may also serve to prevent undesired electrodeposit growth within a portion of the structure. In accordance with another aspect, the barrier material includes an insulating material. In accordance with yet another aspect of this embodiment, the barrier includes material that conducts ions, but which is relatively resistant to the conduction of electrons. Use of such material may reduce undesired plating at an electrode and increase the thermal stability of the device.  
         [0011]     In accordance with another embodiment of the invention, a structure is formed on a surface of a substrate. In accordance with one aspect of this embodiment, the device includes an ion conductor layer and a first electrode and a second electrode overlying the ion conductor layer. In accordance with yet a further aspect of this embodiment, the structure is formed using deposition and etch processing.  
         [0012]     In accordance with another embodiment of the invention, an actuator includes a structure including a first electrode, an ion conductor, and a second electrode. Physical properties of the actuator are altered by applying a sufficient bias across the first and second electrodes to cause conductive material to migrate through a portion of the ion conductor.  
         [0013]     In accordance with another embodiment of the invention, an air-gap device includes a structure including a first electrode, an ion conductor, and a second electrode. Physical properties of the air-gap device are altered by applying a sufficient bias across the first and second electrodes to cause conducting material to migrate through a portion of the ion conductor. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     A more complete understanding of the present invention may be derived by referring to the detailed description and claims, considered in connection with the figures, wherein like reference numbers refer to similar elements throughout the figures, and:  
         [0015]      FIG. 1  illustrates a micromechanical structure formed on a surface of a substrate in accordance with the present invention;  
         [0016]      FIG. 2  illustrates a micromechanical structure in accordance with an alternative embodiment of the present invention; and  
         [0017]      FIGS. 3, 4 ,  5 ( a ) and  5 ( b ) illustrate various devices formed using the structures of the present invention.  
     
    
       [0018]     Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.  
       DETAILED DESCRIPTION  
       [0019]      FIG. 1  illustrates a micromechanical structure  100  formed on a surface of a substrate  110  in accordance with an exemplary embodiment of the present invention. Structure  100  includes electrodes  120  and  130 , an ion conductor  140 , and optionally includes buffer or barrier layer  150  and/or conducting layer  160 . As will be discussed in greater detail below, structure  100  can be used to form devices such as actuators, air-gap switches, and the like. Moreover, because structure  100  can be formed overlying a substrate, the structure allows for monolithic integration of single components and arrays of the structures described herein with electronic, MEMS, or similar devices.  
         [0020]     Generally, structure  100  is configured such that when a bias greater than a threshold voltage (V T ), discussed in more detail below, is applied across electrodes  120  and  130 , conductive material within ion conductor  140  migrates and thus alters a physical property of structure  100 . For example, as a voltage V≧V T  is applied across electrodes  120  and  130 , conductive material within ion conductor  140  begins to migrate and form an electrodeposit (e.g., electrodeposit  170 ) at or near the more negative of electrodes  120  and  130 . The term “electrodeposit” as used herein means any area within the ion conductor that has an increased concentration of reduced metal or other conductive material compared to the concentration of such material in the bulk ion conductor material. Electrodeposits  170  may have significant growth parallel to as well as normal to the electrolyte surface. As set forth in more detail below, electrodeposit  170  may be shaped to displace a cantilever or other movable components of devices such as air-gap devices and actuators.  
         [0021]     In the absence of any insulating barriers, which are discussed in more detail below, the threshold voltage required to grow the electrodeposit is approximately the redox potential of the system, typically a few hundred millivolts. If the same voltage is applied in reverse, the electrodeposit will dissolve back into the ion conductor.  
         [0022]     Referring again to  FIG. 1 , substrate  110  may include any suitable material. For example, substrate  110  may include semiconductive, conductive, semiinsulative, insulative material, or any combination of such materials. In accordance with one embodiment of the invention, substrate  110  includes an insulating material  180  and a portion  190  including microelectronic devices formed on a semiconductor substrate. Layers  180  and  190  may be separated by additional layers (not shown) such as, for example, layers typically used to form integrated circuits. Because the structures of the present invention can be formed over insulating or other materials, the structures are easily integrated with microelectronic or other devices and are particularly well suited for applications where substrate (e.g., semiconductor material) space is a premium.  
         [0023]     In accordance with one exemplary embodiment of the invention, one of electrodes  120  and  130  is formed of a material including a metal that dissolves in ion conductor  140  when a sufficient bias (V≧V T ) is applied across the electrodes (oxidizable electrode) and the other electrode is relatively inert and does not dissolve during operation of the device (an indifferent electrode). For example, electrode  120  may be an anode during a deposit  170  growth process and be comprised of a material including silver that dissolves in ion conductor  140  and electrode  130  may be a cathode during the deposit growth process and be comprised of an inert material such as tungsten, nickel, molybdenum, platinum, metal silicides, and the like. Having at least one electrode formed of a material including a metal which dissolves in ion conductor  140  facilitates maintaining a desired dissolved metal concentration within ion conductor  140 , which in turn facilitates rapid and stable electrodeposit  170  formation within ion conductor  140 . Furthermore, use of an inert material for the other electrode (cathode during an electrodeposit growth step) facilitates electrodissolution of any electrodeposit that may have formed. Various other configurations of ion conductor  140  suitable for use with the present invention are discussed in application Ser. No. 09/951,822, entitled Microelectronic Programmable Device And Methods Of Forming And Programming The Same, the contents of which are hereby incorporated herein by reference.  
         [0024]     Ion conductor  140  is formed of material that conducts ions upon application of a sufficient voltage. Suitable materials for ion conductor  140  include glasses, plastics, and semiconductor materials. In one exemplary embodiment of the invention, ion conductor  140  is formed of chalcogenide material.  
         [0025]     Ion conductor  140  may also suitably include dissolved conductive material. For example, ion conductor  140  may comprise a solid solution that includes dissolved metals and/or metal ions. In accordance with one exemplary embodiment of the invention, conductor  140  includes metal and/or metal ions dissolved in chalcogenide glass. An exemplary chalcogenide glass with dissolved metal in accordance with the present invention includes a solid solution of As x S 1-x —Ag, Ge x Se 1-x —Ag, Ge x S 1-x —Ag, As x S 1-x —Cu, Ge x Se 1-x —Cu, Ge x S 1-x —Cu, where x ranges from about 0.1 to about 0.5, other chalcogenide materials including silver, copper, zinc, combinations of these materials, and the like. In addition, conductor  140  may include network modifiers that affect mobility of ions through conductor  140 . For example, materials such as metals (e.g., silver), halogens, halides, or hydrogen may be added to conductor  140  to enhance ion mobility and thus increase formation and dissolution of electrodeposits  170 . In accordance with one exemplary embodiment of the invention, ion conductor  140  includes a germanium-selenide glass with silver diffused in the glass. Germanium selenide materials are typically formed from selenium and Ge(Se) 4/2  tetrahedra that may combine in a variety of ways. In a Se-rich region, Ge is 4-fold coordinated and Se is 2-fold coordinated, which means that a glass composition near Ge 0.20 Se 0.80  will have a mean coordination number of about 2.4. Glass with this coordination number is considered by constraint counting theory to be optimally constrained and hence very stable with respect to devitrification. The network in such a glass is known to self-organize and become stress-free, making it easy for any additive, e.g., silver, to finely disperse and form a mixed-glass solid solution. Accordingly, in accordance with one embodiment of the invention, ion conductor  140  includes a glass having a composition of about Ge 0.17 Se 0.83  to about Ge 0.25 Se 0.75 . Additional ion conductor materials and methods of forming the ion conductor are discussed in the &#39;822 Application.  
         [0026]     Contacts (not illustrated) may suitably be electrically coupled to one or more electrodes  120 ,  130  to facilitate forming electrical contact to the respective electrode. The contacts may be formed of any conductive material and are preferably formed of a metal such as aluminum, aluminum alloys, tungsten, or copper.  
         [0027]     Insulating material layers  150  and  180  suitably include material that prevents undesired diffusion of electrons and/or ions from or to structure  100 . In accordance with one embodiment of the invention, layers  150  and/or  180  include silicon nitride, silicon oxynitride, polymeric materials such as polyimide or parylene, or any combination thereof. Thus, when used, layer  150  helps prevent a short circuit between the anode and cathode via conducting layer  160 .  
         [0028]     Layer  160  includes material that can supply electrons to the electrolyte along its length between electrodes  120  and  130  to facilitate growth of electrodeposit  170 . Exemplary materials suitable for layer  150  include semi-insulators such as thin (less than 2 nm) silicon dioxide or silicon nitride, or semi-insulating polycrystalline silicon. Exemplary materials suitable for layer  160  include conductors such as doped silicon or polycrystalline silicon, titanium nitride, or metals such as tungsten or aluminum.  
         [0029]     By way of one specific example, silver in a GeSe ion conductor, will generate an ion current flow at an applied bias above approximately 300 mV. The electron current flow from the cathode reduces the excess metal due to the ion flux and hence a silver-rich electrodeposit  170  is formed on or in electrolyte  140 . This amount of electrodeposited material (metal in excess of the starting composition of the electrolyte) is determined by the ion current magnitude and the time the current is allowed to flow.  
         [0030]     A morphology of an electrodeposit generally depends on, among other things, the composition of the electrolyte. For example, for a given voltage, a starting glass Ge 0.3 Se 0.7  into which the silver was photodissolved produces a relatively continuous electrodeposit, whereas a starting glass of Ge 0.4 Se 0.6  showed a discontinuous growth. The Ge 0.4 Se 0.6  material has also allowed significantly more perpendicular growth due to significant electrodeposition at the interface between the electrolyte and the electrodeposit, resulting in a 100 nm high deposit as compared to around 20 nm for the Ge 0.3 Se 0.7  case.  
         [0031]     When the ion current flows, the silver is believed to move as a coordinated motion of ions—essentially a “ripple effect” analogous to a shift register. In order to be reduced, the ionic silver must combine with the electron current from the cathode which means that the ions nearest the cathode will be reduced first and the conducting electrodeposit so formed will become the source of cathodic electrons during subsequent reduction. In this way, the growing electrodeposit “harvests” ions from the electrolyte as it grows out from the cathode, supplying electrons for the reduction of the local silver near its boundaries and thereby further extending the electrodeposit across the electrolyte. The electrodeposition process is reversible upon application of a reverse bias which makes the electrodeposit the oxidizable anode and re-plates the excess silver back onto the silver electrode.  
         [0032]      FIG. 2  illustrates a structure  200  in accordance with another embodiment of the invention. Structure  200  is similar to structure  100 , except structure  200  includes optional buffer layers  210  and  220 .  
         [0033]     Layers  210  and/or  220  may include a material that restricts migration of ions between conductor  140  and the electrodes. In accordance with exemplary embodiments of the invention, a barrier layer includes conducting material such as titanium nitride, titanium tungsten, a combination thereof, or the like. The barrier may be electrically indifferent, i.e., it allows conduction of electrons through structure  200 , but it does not itself contribute ions to conduction through structure  200 . An electrically indifferent barrier may reduce undesired dendrite growth, and thus may facilitate dissolution of electrodeposit  170  when a bias is applied which is opposite to that used to grow the electrodeposit. In addition, use of a conducting barrier allows for the “indifferent” electrode to be formed of oxidizable material because the barrier prevents diffusion of the electrode material to the ion conductor.  
         [0034]     In accordance with one embodiment of the invention, structure  100  is formed by forming conductive layer  160  and insulating layer  150  overlying substrate  110 , using, for example, deposition and etch techniques. Ion conductor material is then deposited overlying insulating layer and patterned. Next, any barrier layers are formed by depositing the barrier material and etching the material to form the barrier structure (e.g., structures  210  and  220 ). Electrodes  120  and  130  may be formed using, for example, deposition and etch techniques.  
         [0035]     As noted above, an electrodeposit can be used in various micromechanical device applications, particularly those involving the elevation, tilting, translation, etc. of an object, structure, or other integrated component. Since the volume of the electrodeposit is dependent on the total Faradaic charge applied, a continuous range of non-volatile displacement is available from below one nm to in excess of 100 nm, controlled by an electrical signal. In addition, it is possible to use the perpendicular electrodeposit as a means for connecting two conductors separated by an air gap. This type of air-gap switch has great utility in, for example, rf switching applications. Note also that the switching surface is renewed by the electrodeposition effect which is also desirable for rf switching applications.  
         [0036]      FIGS. 3 and 4  illustrate respectively a lift actuator  300  and a tilt actuator  400  in accordance with the present invention. Each actuator  300 ,  400  includes a substrate  310 , which may be the same or similar to substrate  110 ; an ion conductor  320 ; an electrodeposit, respectively illustrated in  FIGS. 3 and 4  as element  330 ,  430 ; a movable element  340 ,  440 ; and electrodes, such as electrodes  120 ,  130  described above, on the surface of conductor  330  and perpendicular to the plane of the illustration. For illustration purposes, the electrodes are not shown in  FIGS. 3 and 4 . Movable elements may be formed of any suitable material such as semiconductive, conductive, or insulating material, and may be formed by, for example, deposition and etch processing.  
         [0037]     In operation, when a sufficient bias is applied across the electrodes, electrodeposits  330 ,  430  begin to grow, causing respective elements  340 ,  440  to move. In the case of lift actuator  300 , moveable element is supported by a cantilever or a sliding mount. Similarly, in the case of tilt actuator  400 , movable element  440  is anchored to substrate  310  via a hinge  450 . Note that the movement of elements  340 ,  440  can be translated into horizontal movement using suitable linkages.  
         [0038]     FIGS.  5 ( a ) and  5 ( b ) illustrate an air-gap switch  500  in accordance with the present invention. Switch  500  includes substrate  510 , electrodes  520 ,  530 , ion conductor  540 , electrodeposit  550 , and terminal  560 . As illustrated in  FIG. 5 ( a ), during operation of switch  500 , no current flows between electrode  520  and terminal  560  when no electrodeposit is present. As illustrated in  FIG. 5 ( b ), upon application of a sufficient bias across electrodes  520 ,  530  for a sufficient period of time, electrodeposit  550  grows and creates a short between electrode  520  and terminal  560 .  
         [0039]     Air-gap switches can also be formed using structures  300  and  400 , such that an electrical circuit is opened or closed by, for example, lifting, tilting, or translation of a conducting component which is moved via growth or dissolution of an electrodeposit.  
         [0040]     Although the present invention is set forth herein in the context of the appended drawing figures, it should be appreciated that the invention is not limited to the specific form shown. For example, while the micromechanical structures are conveniently described above in a horizontal configuration, the invention is not so limited; the structure of the present invention may suitably be formed as a vertical structure. Furthermore, although only some of the devices are illustrated as including buffer, barrier, or insulating layers, any of these layers may be added to the devices of the present invention. Various other modifications, variations, and enhancements in the design and arrangement of the method and apparatus set forth herein, may be made without departing from the spirit and scope of the present invention as set forth in the appended claims.