Abstract:
A MEMS device comprising a substrate; an anchored end connected to the substrate; and an actuator comprising a first electrode; a piezoelectric layer over the first electrode; and multiple sets of second electrodes over the piezoelectric layer, wherein each of the sets of second electrodes being defined by a transverse gap there between, and wherein one of the sets of second electrodes are actuated asymmetrically with respect to a first plane resulting in a piezoelectrically induced bending moment arm in a lateral direction that lies in a second plane. The device further comprises an end effector opposite to the anchored end and connected to the actuator; a ferromagnetic core support structure connected to the end effector; a movable ferromagnetic inductor core on top of the ferromagnetic core support structure; and a MEMS inductor coiled around the ferromagnetic core support structure and the movable ferromagnetic inductor core.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation-In-Part (CIP) of U.S. patent application Ser. No. 11/387,078 filed on Mar. 20, 2006, the complete disclosure of which, in its entirety, is herein incorporated by reference. 
    
    
     GOVERNMENT INTEREST 
     The embodiments described herein may be manufactured, used, and/or licensed by or for the United States Government. 
    
    
     BACKGROUND 
     1. Technical Field 
     The embodiments herein generally relate to microelectronic systems, and more particularly to microelectromechanical systems (MEMS) and MEMS inductor technology. 
     2. Description of the Related Art 
     MEMS devices are micro-dimensioned machines manufactured by typical integrated circuit (IC) fabrication techniques. The relatively small size of MEMS devices allows for the production of high speed, low power, and high reliability mechanisms. The fabrication techniques also allow for low cost mass production. MEMS devices typically include both electrical and mechanical components, but may also contain optical, chemical, and biomedical elements. Typically, an inductor is configured as a coil comprising conducting material. For example, copper wire may be wrapped around a ferromagnetic core. Such a core typically has a sufficiently high permeability to confine the magnetic field closely to the inductor, which increases the inductance of the device. 
     Miniaturization of radio frequency (RF) circuits has generally been limited to a degree by the lack of high performance on-chip inductors. The miniaturization thus far of RF circuits has been exploited by the cellular phone and wireless products markets. Military radios and radar systems also benefit from the further miniaturization of RF circuits. Inductors are found in RF matching networks and voltage controlled oscillators; critical components of RF front ends for transceivers and receivers. In some applications, the inductor may need to be tunable; i.e., the inductance of the inductor capable of being selectively modified. 
     Tunable RF MEMS inductors are an enabling technology for reconfigurable RF circuits. Reconfigurable RF circuits have received a great deal of attention in recent years and would, for example, enable filter bandwidths to be significantly manipulated as system requirements dictate. In addition, inductors in series or in parallel with filter elements also increase filter bandwidth. At present, integrated inductors, in silicon technologies, have produced inductor Q values of less than five. Moreover, MEMS inductors have shown inductor Q values an order of magnitude greater than this. While the industry has its choice of several designs of inductors to select when utilizing them for incorporation into an electromagnetic device, there remains a need for a novel piezoelectric MEMS inductor device which is capable of being tunable, and which can be incorporated in different types of electrical circuits. 
     SUMMARY 
     In view of the foregoing, an embodiment herein provides a MEMS device comprising a substrate; an anchored end connected to the substrate; and an actuator comprising a first electrode; a piezoelectric layer over the first electrode; and multiple sets of second electrodes over the piezoelectric layer, wherein each of the sets of second electrodes being defined by a transverse gap there between, and wherein one of the sets of second electrodes are actuated asymmetrically with respect to a first plane resulting in a piezoelectrically induced bending moment arm in a lateral direction that lies in a second plane. The device further comprises an end effector opposite to the anchored end and connected to the actuator; a ferromagnetic core support structure connected to the end effector; a movable ferromagnetic inductor core on top of the ferromagnetic core support structure; and a MEMS inductor coiled around the ferromagnetic core support structure and the movable ferromagnetic inductor core. 
     Preferably, the ferromagnetic core support structure comprises a base portion connected to the end effector; and a plurality of finger-like projections extending from the base portion. Moreover, the movable ferromagnetic inductor core is preferably on top of the plurality of finger-like projections of the ferromagnetic core support structure. The device may further comprise multiple actuation beams and multiple connection beams adapted to connect the multiple actuation beams to one another. Furthermore, each of the multiple actuation beams preferably comprise two sets of the second electrodes. Additionally, the set of second electrodes may comprise an extensional electrode and a contraction electrode. Also, the device may further comprise a spring attached to the end effector, wherein the spring comprises a residual stress deformation mitigation spring adapted to prevent out-of-plane stress deformation of the actuator. Furthermore, the device may comprise a spring attached to the end effector, wherein the spring comprises a residual stress deformation mitigation spring adapted to restrict translational motion of the end effector to be within the second plane, and wherein the first plane is transverse to the second plane. 
     Another embodiment provides a MEMS device comprising at least one actuation beam comprising a continuous lower electrode; a piezoelectric layer over the lower electrode; and at least one pair of upper electrodes over the piezoelectric layer. The device further comprises an anchored end connected to the at least one actuation beam; an end effector opposite to the anchored end and connected to the at least one actuation beam; a spring connected to the end effector; a ferromagnetic core support structure connected to the end effector; a movable ferromagnetic inductor core on top of the ferromagnetic core support structure; and a MEMS inductor coiled around the ferromagnetic core support structure and the movable ferromagnetic inductor core. 
     Preferably, the ferromagnetic core support structure comprises a base portion connected to the end effector; and a plurality of finger-like projections extending from the base portion. Also, the movable ferromagnetic inductor core is preferably on top of the plurality of finger-like projections of the ferromagnetic core support structure. Moreover, the device may further comprise connection beams adapted to connect multiple actuation beams to one another. Additionally, the at least one actuation beam may comprise multiple pairs of the upper electrodes. Moreover, the pair of upper electrodes may comprise a first electrode and a second electrode, wherein the pair of upper electrodes comprising a gap between the first electrode and the second electrode. Preferably, the pair of upper electrodes comprises an extensional electrode and a contraction electrode. Furthermore, the spring member may comprise a residual stress deformation mitigation spring adapted to prevent out-of-plane stress deformation of the actuation beam. Also, one of the multiple pairs of upper electrodes may be actuated asymmetrically with respect to a first plane resulting in a piezoelectrically induced bending moment arm in a lateral direction that lies in a second plane. Moreover, the spring member may comprise a residual stress deformation mitigation spring adapted to restrict translational motion of the end effector to be within the second plane, wherein the first plane is transverse to the second plane. Additionally, the device may further comprise a silicon substrate attached to the anchored end. 
     Another embodiment provides a MEMS device having a first end and a second end, wherein the device comprises a sensor comprising a piezoelectric layer; and multiple electrodes sandwiching the piezoelectric layer, the multiple electrodes comprising a continuous first electrode attached to a first side of the piezoelectric layer and at least one pair of second electrodes attached to a second side of the piezoelectric layer, wherein the pair of second electrodes comprises a primary electrode and a secondary electrode defined by a transverse gap there between. The device further comprises a substrate anchored to the first end; an end effector attached to the second end; a spring member attached to the end effector, an anchored end connected to the sensor; an end effector opposite to the anchored end and connected to the sensor; a ferromagnetic core support structure connected to the end effector; a movable ferromagnetic inductor core on top of the ferromagnetic core support structure; and a MEMS inductor coiled around the ferromagnetic core support structure and the movable ferromagnetic inductor core, wherein the multiple electrodes are adapted to receive voltage, the voltage causing the end effector to laterally deflect in a geometric plane of the substrate. 
     Preferably, the ferromagnetic core support structure comprises a base portion connected to the end effector; and a plurality of finger-like projections extending from the base portion. Additionally, the movable ferromagnetic inductor core is preferably on top of the plurality of finger-like projections of the ferromagnetic core support structure. Moreover, the device may further comprise multiple actuation beams; and multiple connection beams adapted to connect the multiple actuation beams to one another, wherein each of the multiple actuation beams comprise two pairs of the second electrodes. Preferably, the primary electrode is an extensional electrode and the secondary electrode is a contraction electrode. Moreover, the spring member may comprise a residual stress deformation mitigation spring adapted to prevent out-of-plane stress deformation of the actuator. Furthermore, one of the pairs of second electrodes may be actuated asymmetrically with respect to a first plane resulting in a piezoelectrically induced bending moment arm in a lateral direction that lies in a second plane. Additionally, the spring member may comprise a residual stress deformation mitigation spring adapted to restrict translational motion of the end effector to be within the second plane, wherein the first plane is transverse to the second plane. 
     These and other aspects of the embodiments herein will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however that the following descriptions, while indicating preferred embodiments and numerous specific details thereof, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the embodiments herein without departing from the spirit thereof, and the embodiments herein include all such modifications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments herein will be better understood from the following detailed description with reference to the drawings, in which: 
         FIG. 1(A)  is a perspective view of a cantilevered structure of a piezoelectric MEMS actuator device according to an embodiment herein; 
         FIG. 1(B)  is a top view of the cantilevered structure of  FIG. 1(A)  according to an embodiment herein; 
         FIG. 1(C)  is a top view of the cantilevered structure of  FIG. 1(A)  undergoing in-plane extensional deflection according to an embodiment herein; 
         FIGS. 2(A) and 2(B)  are top perspective views of a piezoelectric MEMS actuator device according to an embodiment herein; and 
         FIG. 3  is a top perspective view of a piezoelectric MEMS inductor device according to an embodiment herein. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The embodiments herein and the various features and advantageous details thereof are explained more fully with reference to the non-limiting embodiments that are illustrated in the accompanying drawings and detailed in the following description. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments herein. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein. 
     As mentioned, there remains a need for a novel piezoelectric MEMS inductor device which is capable of being tunable, and which can be incorporated in different types of electrical circuits. The embodiments herein achieve this by providing a lateral piezoelectric driven highly tunable MEMS inductor that enables as much as an order of magnitude increase in the tunability of high Q MEMS inductors and thus provides massive tunability and high Q in advanced RF circuits with applications in numerous military communications and radar systems. Referring now to the drawings, and more particularly to  FIGS. 1(A) through 3 , where similar reference characters denote corresponding features consistently throughout the figures, there are shown preferred embodiments. 
     There are multiple geometric configurations possible for the piezoelectric actuator/sensor used in accordance with the embodiments herein. One such configuration is illustrated as a cantilever beam  115  shown in  FIGS. 1(A) through 1(C) .  FIG. 1(A)  illustrates a piezoelectric actuator/sensor device  140  that comprises a pair of upper electrodes  116   a ,  116   b , which may comprise platinum or other suitable material, disposed over an active piezoelectric layer  114 , which is positioned above a lower electrode  112 . The piezoelectric layer  114  preferably comprises sol-gel PZ 0.52 T 0.48  (PZT). The configuration of the actuator/sensor device  140  enables proper device operation by having the upper electrodes  116   a ,  116   b  and the lower electrode  112  sandwich the piezoelectric layer  114 . The absence of the traditional MEMS piezoelectric out-of-plane piezoelectric actuator&#39;s structural layer (found in conventional devices) ensures the optimal condition of the piezoelectric moment arm (δ) (shown in  FIG. 1(B) ) residing in the x-y plane according to the embodiments herein. The actuator/sensor device  140  comprises a free end  120  and an anchored end  121  attached to a substrate  118 . 
       FIG. 1(B)  illustrates a top view of the actuator/sensor device  140  of  FIG. 1(A) . When a voltage is applied between the lower electrode  112  and one of the upper electrodes (shown here, for example, upper electrode  116   a ), a piezoelectrically generated strain induced axial force (F) offset from the neutral axis (N.A.) of the actuator/sensor device  140 , creates a bending moment (M) on the actuator/sensor device  140 , which is configured as a cantilever beam  115 . This bending moment (M) causes in-plane deflection (x-y plane) of the actuator/sensor device  140  with the direction of the generated displacement shown as offset distance δ.  FIG. 1(C)  illustrates the actuator/sensor device  140  undergoing bending thereby producing lateral in-plane (x-y plane) deflection of the actuator/sensor device  140 . 
     The configurations of the upper electrodes  116   a ,  116   b  are dependant upon actuator geometry. Unlike bulk piezoelectric actuators which may achieve bipolar actuation (piezoelectric strain may be compressive or tensile) through the application of the opposite polarity electric field, thin film piezoelectric actuators cannot achieve this for typical operating voltages. Typical thin film piezoelectrics (microns to sub-micron) operate above their coercive field experience only in-plane (x-y plane) contraction due to the high electric field nonlinearities associated with ferroelectric materials such as PZT. Therefore, large actuation in piezoelectric MEMS actuators only accommodates in-plane (x-y plane) compression and is largely independent of the polarity of the excitation electric field. Controlling the voltage-displacement response of the structure is therefore almost entirely  20  dependant upon the geometry and absolute value of the voltage. In the cantilever beam  115  illustrated in FIGS  1 (A) through  1 (C) only one upper electrode  116   a  (for example) is actuated; otherwise if both upper electrodes  116   a ,  116   b  were actuated, the generated bending moments (M) would cancel and no lateral bending would occur. 
       FIGS. 2(A) through 2(B)  illustrate another piezoelectric MEMS actuator/sensor device  150  used in accordance with the embodiments herein (the overall concept and principal of actuation is similar to the actuator  140  of  FIGS. 1(A) through 1(C) ). An anchored end  121  of the actuator/sensor device  150  is attached to the substrate  118  (of  FIG. 1(A) ) which fixes the actuator/sensor device  150  in place and an end effector  130  is positioned opposite the anchored end  121  . The end effector  130  is positioned on the free end  120  of the piezoelectric MEMS actuator/sensor device  150 . The displacement of the free end  120  largely remains in the x-y plane (plane of the substrate  118 ) upon actuation (i.e., application of voltage).  FIG. 2(B)  illustrates the general bidirectional actuation movement of the upper electrodes  116   a ,  116   b  where the “a” movement corresponds with the direction of movement of upper electrode  116   a  and the “b” movement corresponds with the direction of movement of upper electrode  116   b . Generally, the actuation of upper electrode  116   a  results in contraction of the actuator/sensor device  150  and actuation of upper electrode  116   b  results in extension of the actuator/sensor device  150 . 
     The actuation occurs similarly to the process described for the actuator/sensor device  140  of  FIGS. 1(A) through 1(C) , thus a voltage applied between the lower electrode  112  and one of the upper electrodes (shown here, for example, upper electrode  116   a  causes in-plane (x-y plane) deflection of the actuator/sensor device  150  with the direction of the generated displacement shown as “a” and “b” for the respective upper electrodes  116   a ,  116   b . Likewise, the converse effect is true for the structure to function as a sensor. An applied stress, causing bending, will cause the piezoelectric material to generate a voltage which may be detected with additional electronics (not shown). 
     Generally, the actuator/sensor device  150  further comprises multiple sets of preferably four parallel actuation beams  115   w ,  115   x ,  115   y ,  115   z  connected at their extreme ends by perpendicular connection beams  119 . Electrode traces (not shown) also run along the connection beams  119  to electrically connect all actuation beams  115   w ,  115   x ,  115   y ,  115   z  (shown in  FIGS. 2(A) and 2(B) ). Each set of four parallel actuation beams  115   w ,  115   x ,  115   y ,  115   z  may then be attached to the next set by additional connection beams  119  at the inner ends of the parallel actuation beams  115   w ,  115   x ,  115   y ,  115   z . For the optimal configuration, the upper electrodes  116   a ,  116   b  on each parallel actuation beam  115   w ,  115   x ,  115   y ,  115   z  are separated in order to achieve maximum lateral deflection. The end effector  130  is located at the connection point of the last set of parallel actuation beams  115   x ,  115   z . The end effector  130  remains in the x-y plane during actuation. 
       FIG. 3  illustrates another embodiment of a piezoelectric MEMS actuator/sensor device  160  used in accordance with the embodiments herein (the overall concept and principal of actuation is similar to the actuator  140  of  FIGS. 1(A) through 1(C)  and the actuator  150  of  FIGS. 2(A) and 2(B) ). As shown, n additional sets of actuation beams  115  provide n times the deflection. The actuator/sensor device  160  comprises a spring member preferably embodied as residual stress deformation mitigation springs  180 , which are configured to have a large out-of-plane stiffness (k), to resist residual stress deformation, and a large in-plane compliance that minimizes the influence of the springs  180  on the in-plane displacement of the actuator/sensor device  160 . The springs  180 , which may comprise single crystal silicon or other suitable material of minimal residual stress, are connected to the end effector  130  and are anchored (anchoring substrate not shown) at the ends  185  of the spring  180 . Furthermore, there exist multiple possible geometric configurations for the springs  180 . The various geometries are valid if they achieve large out-of-plane stiffness and large in-plane compliance such that they prevent out-of-plane stress deformation with minimal reduction of the in-plane displacement of the end effector  130 . 
     A ferromagnetic core  188  is connected to the end effector  130  and changes the magnetic flux density and thus the inductance of the device  160 . The preferable ferromagnetic material of the core  188  is characterized by low electrical conductivity and a high magnetic relative permeability. The material may be either laminated with a dielectric and/or is patterned to form numerous discrete sections so as to limit losses attributed to eddy currents generated within the ferromagnetic material. 
     The MEMS inductor  200  is the component of the device  160  that is to be electrically manipulated. The purpose of the device  160  is to alter the inductance of the MEMS inductor  200 . Preferably, the inductor  200  is a solenoid inductor having a primary axis parallel to the end effector  130 . The ferromagnetic core  188  comprises a base portion  187  and an interdigitated set of beams  189   a - 189   d  comprised of the active ferromagnetic material  195  atop a structural silicon layer  190 . Generally, the base portion  187  and the set of beams  189   a - 189   d  form a ferromagnetic core support structure  191 . Those skilled in the art would understand that multitudes of relative geometries are possible, and the embodiments herein are not necessarily limited to the example illustrated in  FIG. 3  where beam  189   d  is the length of the previous beam  189   c  minus the width of the inductor  200 , and beam  189   c  is the length of the previous beam  189   b  minus the width of the inductor  200 , and beam  189   b  is the length of the previous beam  189   a  minus the width of the inductor  200 . This particular case allows for a linear relationship between actuator displacement and the increase of the ferromagnetic mass with the core  188 . 
     The inductor  200  is to be connected to either a DC circuit (not shown) or transmission line (not shown) for operation at high frequencies. It may be DC or in a coplanar waveguide “CPW” configuration. In the CPW configuration, the inductor  200  would additionally have flanking ground planes attached thereon. The overall design is also amenable to implementation in a “micro strip” transmission line. 
     The device  160  may be fabricated as follows (the thicknesses described below are approximate and are examples of preferred embodiments; however the embodiments herein are not limited to these thicknesses). The starting material of the substrate  118  is a single crystal silicon wafer. Next, SiO 2  (˜1,000′) is deposited via Plasma Enhanced Chemical Vapor Deposition (PECVD). Then, via DC magnetron sputtering, the lower electrode  112  (˜200-800′) comprising Ta/Pt is deposited. Thereafter, sol-gel is spin coated or Lead-Zirconate-Titanate (PZT)  114  (˜5,000′)is sputter deposited. After this, a liftoff process occurs with sputtered Pt to define the top electrode  116   a ,  116   b  (˜800′). Upon completion of this step, the PZT layer  114  and TaPt (lower electrode  112 ) is ion milled down to the SiO 2  to define the actuator structure  140 . Reactive Ion Etching (RIE) of the SiO 2  occurs next down to silicon substrate  118  to define the actuator  140 . Next, a wet etching of the PZT  114  on bottom electrode bond pads (not shown) occurs. Thereafter, a PECVD process of the SiO 2  (˜10,000′) occurs. Then, the SiO 2  undergoes a RIE process to define a support cantilever structure for the inductor  200 . 
     The next step of the process is an anisotropic Si etch to define residual stress mitigation springs  180 , ferromagnetic core support structure  191 , and the rest of the actuator  160 . A liftoff process then occurs with evaporated Au to define lower segments of the inductor  200  on the predefined SiO 2  support cantilevers  189   a - 189   d . After this, a liftoff process is used to define the active ferromagnetic material  195 . Thereafter, a sacrificial layer (not shown) of sputtered silicon is deposited and patterned to open vertical posts  201  for the inductor  200 . A liftoff process then occurs with evaporated Au to define the vertical posts  201  and connection beams  202 , which connect adjacent turns of inductor  200 . Next, a XeF 2  isotropic release process of the deposited sacrificial layer occurs of the remaining silicon beneath the actuator  160 , residual stress mitigation springs  180 , the ferromagnetic core support structure  191 , and the silicon between the inductor turns so as to allow the core  188  to traverse the intended path. 
     Generally, the actuation of the device  160  occurs when the lateral piezoelectric MEMS actuator  160  actuates and interdigitates a multiple beam structure  191  with ferromagnetic material  195  atop into the core of a high Q RF MEMS inductor  200 . The internal magnetic flux density of the inductor  200  is enhanced by the large magnetic permeability of the ferromagnetic material  195 , thus altering the value of the inductance. 
     The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the appended claims.