Patent Publication Number: US-8536037-B2

Title: Electrically responsive device

Description:
RELATED APPLICATIONS 
     This application claims priority to U.S. provisional application Ser. No. 60/668,933, filed on Apr. 6, 2005, and entitled “Micromachined Electroactive Resonant Device,” the entirety of which is hereby incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to electrically responsive devices and methods for fabricating electrically responsive devices. 
     BACKGROUND OF THE INVENTION 
     Electrically responsive devices are those devices that generate an output signal in response to an electrical signal and/or generate an electrical signal in response to an input signal. Typical types of input and output signals include electrical, optical, electromagnetic, vibrational, and thermal signals. One type of electrically responsive devices is a resonant device. 
     Resonant devices, such as surface acoustic wave devices, bulk acoustic wave devices, flexural plate/lamb wave devices, and quartz crystal microbalance devices are fashioned substantially from monolithic materials such as quartz, or, from layered materials that include uniform thin films of electroactive materials (e.g., piezoelectric materials such as Zinc Oxide, Lead Zirconate Titanate—PZT, Aluminum Nitride, Indium Nitride and Sol-Gel piezoceramic materials) in combination with other micromachinable materials (e.g., Silicon, Silicon Oxides, Silicon Nitride and Nickel Irons). These resonant devices, when preferentially coated with appropriate materials and/or packaged in an appropriate environment are used as, for example, electrical filters (e.g., passive), gas phase detectors, and liquid phase sensors. 
     As an electrical filter the resonant device is used to transmit resonant energy and filter signals outside of the pass band of the device. Electrical filters (e.g., surface acoustic wave (SAW) devices and film bulk acoustic resonator (FBAR) devices) are typically characterized by their insertion, transmission and reflectance properties. High transmission, isolated narrow band and stable properties of resonance operation are desirable features for filters used in modulating and demodulating wired and wireless signals. Defined and stable narrow pass band filter properties allow for packing more carrier frequencies in a given bandwidth. 
     Resonant devices also are sometimes used to determine the presence and amount of various measurands (e.g., chemical or biological) in gas phase and liquid phase samples. Resonant sensors may be used, for example, to determine the presence and magnitude of chemical vapors and biological matter in aerosol, the bulk properties of liquids (e.g., density, viscosity, and speed of sound) and the concentration of analytes in solutions. 
     In gas phase operation, a surface of a resonant device is typically coated with an absorbent material that interacts selectively and binds with specific gas phase products passed over the surface of the resonant device. The gas phase products that bind to the absorbent material increase the mass loading of the resonant device. The increase in mass loading changes properties of the device (e.g., the stiffness or the resonance of the device). Electrical signals produced by the resonant device reflect the change in resonance of the device. 
     In liquid operation, a surface of a resonant device is exposed to a liquid. The surface of the resonant device interacts with and is subsequently loaded by the physical interaction of the liquid with the resonant device. In some devices the loading of the resonant device occurs through the coherent oscillatory compression and motion of the liquid near the surface of the device. The resonant device produces an electrical signal that varies as the loading varies. A detectable resonance response results if the motion of the liquid is stable and oscillatory, and the oscillatory motion dissipates substantially less than the peak stored potential energy of the moving surface of the resonant device. Changes in the properties of the liquid are determined based on the electrical signal produced by the resonant device. 
     Generally, it is desirable for resonant devices to have isolated resonance modes, narrow pass bands, low loss, and stable and repeatable operating characteristics when used in a variety of operating environments (e.g., vacuum, gas phase, and liquid phase). Variations in the devices (e.g., due to manufacturing tolerances) tend to result in non-isolated resonance modes, wide resonance bands, high loss, and unstable and non-repeatable operating characteristics. 
     A need therefore exists for improved electrically responsive devices and methods for fabricating electrically responsive devices. 
     SUMMARY OF THE INVENTION 
     The present invention features electrically responsive devices and methods for fabricating electrically responsive devices. In some embodiments, the present invention features resonant devices and methods for fabricating resonant devices. 
     The invention, in one aspect, features a device with spatially modulated structural properties. Excitation and/or sensing means of the device are substantially correlated with the modulated structural properties. Selective modulation of structural properties (e.g., stiffness and mass properties) results in preferred resonance mode shapes. For example, correlating the excitation means with the structural modulation selectively excites preferred resonances resulting in a preferred pass band. Correlating the sensing means with the structural modulation enables selective sensing of preferred resonances, also resulting in a preferred pass band. 
     In one embodiment, correlating excitation and sensing means with the structural property modulation of a device results in an electrical filter (e.g., a surface acoustic wave (SAW) device or a film bulk acoustic resonator (FBAR) device) having a preferred pass band, narrower bandwidth and lower transmission loss than would otherwise be achieved without structural modifications. The pass band includes at least one resonant mode. 
     In some embodiments, preferred stiffness and mass modulations are achieved by selectively removing and/or adding material to a substantially planar device. The modulation can be periodic with spatial periodicity set to match (or substantially match) one of the device&#39;s mode shapes. 
     In some embodiments, the device is a composite of materials that is fabricated having modulated structural properties. In some embodiments, the device has a substantially uniform thickness. By way of example, a device can be fabricated having uniform thickness by removing a first material of the device and then filling the region exposed by removing the first material with a second material that has a different stiffness than the first material. 
     In one embodiment, the device includes electrode material, electroactive material, and a substrate material, according to an illustrative embodiment of the invention. The electroactive material is combined with the substrate material to achieve composite structural properties in which the properties (e.g., stiffness and/or mass) vary along the device to achieve preferred device properties. A preferred excitation and sensing mode is achieved by applying (e.g., depositing) electrode material to the electroactive material over a substantial area of the device. In some embodiments, alternative materials are instead used to fabricate devices incorporating principles of the present invention. For example, materials that allow for electrostatic or capacitive excitation and/or sensing can be used to fabricate a device. 
     The invention, in another aspect, relates to a method for fabricating an electrically responsive (e.g., resonant) device. The method involves applying an electrically responsive (e.g., electroactive or electrooptical) material over at least a portion of a surface of a substrate material and applying an electrode material over at least a portion of a surface of the electrically responsive material. The method also involves selectively removing at least one region of the electrode material exposing the electrically responsive material. The method also involves selectively removing at least some of the electrically responsive material in a region corresponding to the at least one region of the electrode material (i.e., the region exposed by removing the electrode material). 
     In some embodiments, the selective removal of the electrode material and the removal of the electrically responsive material substantially improves the band pass resonant response (e.g., modes greater than the first mode) of the device. 
     In some embodiments, the invention relates to a method for modifying a resonance mode of the response of the device by enforcing that resonance mode of response of the device. In some embodiments, a particular pattern of the electrode material is used to enforce a resonance mode of response of the device. In some embodiments, removal of the electrically responsive material substantially modifies at least one resonance mode of response of the device by reducing modal overlap and spillover associated with other resonance modes of the device. 
     In some embodiments, the method also can involve selectively removing at least some of the substrate material in a region corresponding to the at least one region of the electrode material. The removal of the electrode material and the removal of the electrically responsive material can involve etch processing (one or more of wet etching, dry etching, plasma etching, laser assisted etching, laser ablation, ion beam milling, and electron beam etching). 
     Removal of the electrode material and removal of the electrically responsive material can be conducted with a single removal step (e.g., a single etching step is performed that removes the material). Removal of the electrode material can involve producing an interdigitated pattern in the electrode. Applying the electrically responsive material over the surface of the substrate material can involve applying the electrically responsive material on to the surface of the substrate material by, for example, reactive sputtering of the electrically responsive material. 
     Applying the electrode material over the surface of the electrically responsive material can involve applying the electrode material on to the surface of the electrically responsive material. In some embodiments, the electrode material is applied by physical vapor deposition (e.g., e-beam evaporation plating or sputtering). In some embodiments, an electrode material is also applied between the substrate material and the electrically responsive material. 
     In some embodiments, selectively removing the electrically responsive material in the region corresponding to the at least one region of the electrode material includes removing all the electrically responsive material in the region corresponding to the at least one region of the electrode material. In some embodiments, an alternative material is then applied to the regions from which the electrode material and the electrically responsive material were removed to produce a device having a substantially uniform thickness while still having modulated stiffness and/or mass properties, in accordance with principles of the present invention. 
     In some embodiments, the method also can involve selectively removing a portion of the substrate material. Selectively removing a portion of the substrate material can result in the electrically responsive material having at least one region that is unsupported by the substrate. In some embodiments, the unsupported electrically responsive material is a cantilever electrically responsive element. The electrically responsive material can include, for example, an electroactive material (e.g., a piezoelectric material, piezoceramic material, electroceramic material or a single crystal electroactive material). 
     The combination of the substrate, electrically responsive material and electrode material can be incorporated into an integrated circuit package. The device fabricated according to the method can be a filter or a sensor (e.g., physical, biological or chemical sensor). 
     In some embodiments, a filler material is applied in the region of the electrically responsive material that was previously selectively removed. The filler material can have a stiffness dissimilar to the stiffness of the electrically responsive material. 
     The invention, in another aspect, relates to a method for fabricating an electrically responsive device (e.g., a resonant device) that involves applying an electrode material over at least a portion of a surface of an electrically responsive material. The method also involves selectively removing at least one region of the electrode material exposing the electrically responsive material. The method also involves selectively removing at least some of the electrically responsive material in a region corresponding to the at least one region of the electrode material to substantially modify at least one resonance mode of the device. 
     The invention, in another aspect, relates to a method for fabricating an electrically responsive device. The method involves selectively applying electrically responsive material to a surface of a substrate material and selectively applying an electrode material to a surface of the electrically responsive material to substantially modify at least one resonance mode of response of the device. 
     The invention, in another aspect, features an electrically responsive device. The device includes a substrate material and at least one electrically responsive element on the substrate. The device also includes an electrode material on a surface of the at least one electrically responsive element, wherein the electrically responsive element and the electrode material are configured to modify at least one resonance mode of response of the device. 
     The invention, in another aspect, features an electrically responsive device that includes a substrate material and at least one electrically responsive element on the substrate. The device also includes an electrode material on a surface of the at least one electrically responsive element, wherein the electrically responsive element and the electrode material are patterned on the substrate layer to modify at least one resonance mode of response of the device by reducing modal overlap and spillover associated with other resonance modes of the device. 
     In some embodiments, the electrically responsive material and electrode material are patterned to produce at least one actuating element and at least one sensing element. In some embodiments, the electrically responsive material and electrode material are patterned to produce at least one launch element and at least one receiver element. The resonant device can include a conductive (e.g., metallic) material between the electrically responsive material and the substrate material. 
     The invention, in another aspect, features an electrically responsive device that includes a substrate material having a surface and defining a plane. The device also includes an electrically responsive material over at least a portion of the surface of the substrate material and varying in thickness in the plane of the substrate material. The device also includes an electrode material over portions of the surface of the electrically responsive material in an interdigitated pattern. 
     In some embodiments, the electrode material regions are located over thicker regions of the electrically responsive material. In some embodiments, the electrode material regions are located over thinner regions of the electrically responsive material. 
     The invention, in another aspect, features an electrically responsive device. The device includes a substrate material having a surface and varying in thickness in a plane of the substrate material. The device also includes an electrically responsive material over at least a portion of the surface of the substrate material. The device also includes an electrode material over portions of the surface of the electrically responsive material in an interdigitated pattern. 
     The invention, in another aspect, relates to a method for fabricating an electrically responsive device. The method involves applying an electrically responsive material over at least a portion of a surface of a substrate material. The method also involves selectively removing at least some of the electrically responsive material. The method also involves applying an electrode material over at least a portion of a surface of the electrically responsive material. 
     In some embodiments, applying the electrode material produces an interdigitated electrode pattern over the electrically responsive material. In some embodiments, applying the electrode material produces an annular-shaped electrode pattern over the electrically responsive material. 
     The invention, in another aspect, relates to a method for fabricating an electrically responsive device. The method involves applying a first electrode material over at least a portion of a surface of a substrate material and applying an electrically responsive material over at least a portion of a surface of the first electrode material. The method also involves applying a second electrode material over at least a portion of a surface of the electrically responsive material. The method also involves selectively removing some of at least one of the electrically responsive material or the substrate material to modify at least one resonance mode of response of the device. 
     The invention, in another aspect, features an apparatus for detection of an analyte. The apparatus includes a fluid channel. The apparatus also includes an electrically responsive device defining at least a portion of at least one surface of the fluid channel. The apparatus also includes a monitoring device to monitor at least one signal output by the device. The device includes a substrate material having a surface and defining a plane. The device also includes an electrically responsive material over at least a portion of the surface of the substrate material and varying in thickness in the plane of the substrate material. The device also includes an electrode material over portions of the surface of the electrically responsive material in an interdigitated pattern. 
     The invention, in another aspect, features a cartridge for detection of an analyte. The cartridge includes a fluid channel and an electrically responsive device (e.g., a resonant device) located within the fluid channel or defining at least a portion of at least one surface of the fluid channel. The device includes a substrate material having a surface and defining a plane. The device also includes an electrically responsive material over a portion of the surface of the substrate material and varying in thickness in the plane of the substrate material. The device also includes an electrode material over portions of the surface of the electrically responsive material in an interdigitated pattern. 
     The invention, in another aspect, features a kit used in detection of an analyte. The kit includes a cartridge that has a fluid channel and a first component which specifically binds the analyte to a surface of the device. The cartridge also includes an electrically responsive device (e.g., a resonant device) located within the fluid channel or defining at least a portion of at least one surface of the fluid channel. The device includes a substrate material having a surface and defining a plane. The device also includes an electrically responsive material over a portion of the surface of the substrate material and varying in thickness in the plane of the substrate material. The device also includes an electrode material over portions of the surface of the electrically responsive material in an interdigitated pattern. The kit also includes particles (e.g., magnetic beads) that have a second component that specifically bind the analyte. 
     The invention, in another aspect, relates to a method for fabricating an electrically responsive device. The method involves providing an electrically responsive material over at least a portion of a substrate material. The method also involves altering a property (e.g., a structural property) of at least one of the electrically responsive material or the substrate material to isolate a resonant mode of the device. 
     Altering a property can involve altering the stiffness over a portion of the electrically responsive material or the substrate material. Altering a property can involve altering distribution of mass over a portion of the electrically responsive material or the substrate material. Altering a property can involve removing a portion of the electrically responsive material to correspond substantially with an electrode pattern on a surface of the electrically responsive material. 
     The invention, in another aspect, features an electrically responsive device. The device includes a substrate material. The device also includes an electrically responsive material over at least a portion of the substrate, wherein a property of at least one of the substrate material or the electrically responsive material is altered to isolate a resonant mode of the device by altering the stiffness of the device. 
     The invention, in another aspect, relates to a method for fabricating an electrically responsive device. The method involves applying an electrically responsive material over at least a portion of a surface of a substrate material. The method also involves applying an electrode material over at least a portion of a surface of the electrically responsive material. The method also involves applying a material over at least a portion of the surface of the device to alter mass distribution over the device to substantially modify at least one resonance mode of response of the device. 
     The invention, in another aspect, features an electrically responsive device including a composite structure having spatially modulated properties. The device also includes means of exciting and sensing motion of the structure that is substantially correlated with the spatially modulated properties to produce a narrow (e.g., with respect to frequency) and low loss (e.g., having a more pronounced transfer function peak) pass band. Advantages according to the invention can be achieved in devices exposed to fluid loading or in devices not exposed to fluid loading. 
     In some embodiments, the composite structure includes an electroactive material. In some embodiments, the spatially modulated properties are periodic along a surface of the structure. In some embodiments, the composite structure includes a component capable of binding to biological or chemical matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, feature and advantages of the invention, as well as the invention itself, will be more fully understood from the following illustrative description, when read together with the accompanying drawings which are not necessarily to scale. 
         FIG. 1A  is a schematic illustration of an electrically responsive device. 
         FIG. 1B  is a schematic illustration of a portion of the device of  FIG. 1A . 
         FIG. 2  is a schematic illustration of a portion of an electrically responsive device, according to an illustrative embodiment of the invention. 
         FIG. 3  is a schematic illustration of different phases of a method for fabricating an electrically responsive device, according to an illustrative embodiment of the invention. 
         FIG. 4  is a schematic illustration of a portion of an electrically responsive device, according to an illustrative embodiment of the invention. 
         FIG. 5  is a schematic illustration of a portion of an electrically responsive device, according to an illustrative embodiment of the invention. 
         FIG. 6  is a schematic illustration of a pattern of electrode material, according to an illustrative embodiment of the invention. 
         FIG. 7  is a schematic illustration of a pattern of electrode material, according to an illustrative embodiment of the invention. 
         FIG. 8  is a graphical representation of transfer function magnitudes versus frequency for a resonant device incorporating principles of the present invention and a resonant device not incorporating principles of the present invention. 
         FIG. 9  is a graphical representation of transfer function-magnitudes versus frequency for a resonant device incorporating principles of the present invention and a resonant device not incorporating principles of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
       FIG. 1A  is a schematic illustration of an electrically responsive device  100 . In this embodiment, the electrically responsive device  100  is a resonant device constructed from a substrate  108  (e.g., a silicon wafer) using micro-fabrication techniques known in the art. Alternative methods of fabrication are possible without departing from the scope of the present invention. In this embodiment, a cavity  124  is etched into the substrate  108  to produce a thin, suspended membrane  104  that is approximately 1.6 mm long (along the X-Axis), 0.3 mm wide (along the Z-Axis) and 2 μm thick (along the Y-Axis). The overall substrate  108  thickness is approximately 500 μm, so the depth of the cavity  124  is just slightly less than the substrate  108  thickness. A 0.5 μm layer of an electrically responsive material  132  (e.g., an electroactive or electrooptical material) is deposited over an outer surface  160  (i.e., the surface opposite the cavity  124 ) of the membrane  104 , as shown in a region  120  of the device  100  shown in the expanded view insert of  FIG. 1B . In some embodiments, a fluid channel is used instead of a cavity  124 , in which the fluid channel, for example, delivers fluid to the membrane  104  and/or directs fluid away from the membrane  104 . 
     In some embodiments, the electrically responsive device is fabricated from a variety of materials forming a composite structure. The device also includes actuation and sensing structure that allows for actuation of the composite structure and sensing motion of the composite structure, respectively. In one embodiment, the composite structure is the suspended structure  142  shown in  FIG. 1  including region  120  of  FIG. 2 ). 
     The electrically responsive material  132  can be, for example, an electroactive material (e.g., a piezoelectric material, a piezoceramic material, an electroceramic material or a single crystal electroactive material). In one embodiment, the electroactive material is aluminum nitride (AlN). Electrode material in the form of two sets of interdigitated metal electrode material  140  are deposited over an outer surface  164  of the electrically responsive material  132 . In some embodiments, titanium and/or gold are suitable electrode materials. In one embodiment, a 100 Angstrom layer of titanium is used as the electrode material  140 . In some embodiments, a thin layer of metal (e.g., gold) is deposited over an outer surface  164  of the electroactive material  132  prior to deposition of the electrode material  140 . 
     In this embodiment, the substrate  108  forming the membrane  104  is silicon. The silicon membrane  104  is conductive preferably with a resistivity of less than about 0.01 ohm-cm. The membrane  104  functions as a lower electrode. A field is applied between one set of the interdigitated electrode material  140  and the membrane  104 . In this embodiment, the field between the electrode material  140  and the membrane  104  is substantially in the X-Y plane. In some embodiments, however, the membrane  104  is not conductive or no conductive material is located below the membrane (when viewed in the X-Y plane). In these embodiments, an additional separate electrode material lead or layer (not shown) is provided that is electrically isolated from the interdigitated electrode material  140 . The additional electrode material is located between each set of electrode material and a field is applied between each set of electrode material and the additional material to, for example, actuate the electrically responsive material. In this manner, the field is applied substantially in the X-Z plane. 
     A layer  136  of metal (e.g., approximately 500 angstroms of gold) is deposited on an inner surface  138  (i.e., the surface facing the cavity  124 ) of the membrane  104  to, for example, facilitate immobilization of capture agents. Biological or chemical matter binds to capture agents on the layer  136  under circumstances where the device  100  is used to quantify the matter in, for example, a fluid sample. In some embodiments, no layer  136  of metal is used. 
     In operation, instrument/control electronics  128  (referring to  FIG. 1A ) apply a time-varying electrical signal to one set of the electrode material  140  (relative to the membrane  104 ) to generate vibrations  112  in the suspended membrane  104 . The instrument/control electronics  128  also monitor the vibrational characteristics of the membrane  104  by receiving a sensor signal from the second set of electrode material  140  relative to the membrane  104 . 
     In one embodiment, several fluids are used in an application of the resonant device to detect the presence of biological molecules suspended in a fluid. A reference buffer fluid is exposed to the layer  136  to establish a baseline resonance response. A sample solution containing the biological molecules is flowed over the layer  136  of the device  100 . At least some of the biological molecules bind to the layer  136  causing the resonance characteristics of the device  100  to change. The resonance of the device  100  with bound biological molecules is compared with the baseline resonance to determine how much biological material is bound to the layer  136  of the device  100 . 
     In one embodiment, when liquid is in contact with the cavity side  124  of the membrane  104 , the maximal response of the plate structure is around 15-25 MHz as dictated by various properties of the device  100  (e.g., length, thickness and stiffness of the membrane  104 ). In one embodiment, the instrument/control electronics  128  compare a reference signal to the signal from the second set of electrode material  140  to determine the changes in the relative magnitude and phase angle of the signal as a function of frequency. The instrument/control electronics  128  interpret these changes to detect the presence of, for example, a targeted analyte that has attached to the layer  136  of the membrane  104 . In some embodiments, the instrument/control electronics  128  also determines, for example, the concentration of the targeted analyte on the layer  136  of the membrane  104 . 
     In some embodiments, the substrate  108 , electroactive material  132  and electrode material  140  are parts of a device  100  that are incorporated into an integrated circuit package using techniques known in the art. In some embodiments, the device  100  is a filter device (e.g., a surface acoustic wave (SAW) device or a film bulk acoustic resonator (FBAR) device). In some embodiments, the device  100  is a sensor used, for example, to detect or measure biological, chemical or physical properties of fluids or gases. 
     Devices, such as the device  100  of  FIGS. 1A and 1B , however, have suboptimal performance properties. The devices tend to have non-isolated resonance modes, high loss, wide resonance bands, and unstable and non-repeatable operating characteristics. Additionally, the response of these devices is sensitive to the boundary conditions of the structure defined by, for example, the suspended structure of  142  that is defined by the combination of cavity  124 , the membrane  104 , electrically responsive material  132 , and other materials located on, below, or between membrane  104  and electrically responsive material  132 . In some embodiments, it is desirable during fabrication to control the dimensions of the suspended structure  142  along the X-axis and the Y-axis, the alignment of the electrode material  140  relative to the boundaries of the suspended structure  142 , and the compliance of the regions in proximity to the boundaries of the suspended structure  142  to achieve repeatable performance of the device  100 . 
       FIG. 2  is a schematic illustration of a region  120  of a electrically responsive device (for example, the device  100  of  FIG. 1A ) incorporating principles of the present invention that overcome the limitations of prior art devices. The region  120  of the device has a membrane  104  and an electrically responsive material  132  (e.g., AlN) distributed over portions of the membrane  104 . In this embodiment, the region  120  of the electrically responsive device also has two sets of interdigitated electrode material  140  located over the electrically responsive material  132 . 
     This embodiment of the region  120  of the electrically responsive device is different from the region  120  of the device of  FIG. 1B  in that there are locations  244  that do not have electrically responsive material. The lack of electrically responsive material in locations  244  substantially modifies at least one resonance mode of response of the device by altering a structural property of the device. In this embodiment, the lack of material in locations  244  alters the stiffness of the device along the X-Axis of the device. Referring to  FIG. 2 , the locations  244  lacking electrically responsive material enforce a desired modal response of the electrically responsive device such that, when combined with electrical stimulus applied by electronics (e.g., the electronics  128  of  FIG. 1A ) to the electrode material  140 , results in improved modal isolation and dynamic amplification. 
     Modal isolation is the narrow band preferred response of one resonance mode over other resonance modes exhibited by the electrically responsive device in a particular frequency range. Improved dynamic amplification results from the improved alignment of the device mode shape with the transductions means (i.e., position of electrode material  140  relative to electrically responsive materials  132 ). Electrically responsive devices having locations  244  that lack electrically responsive material also improve dynamic amplification of signals associated with the devices due to the reduction in material damping that would otherwise occur if electrically responsive material was located in locations  244 . 
     Device locations  244  lacking electrically responsive material  132  are achieved in a variety of ways. By way of example, locations  244  can be created by selective removal of the electrically responsive material  132 . In some embodiments, the locations  244  are created by selective application or deposition of electrically responsive material  132  in only those locations where it is desirable to have the electrically responsive material  132 . 
     In this embodiment (referring to  FIG. 2 ) the locations  244  completely lack electrically responsive material  132 . Alternative embodiments are contemplated, however, where only a portion of the electrically responsive material is lacking in locations  244 . In some embodiments, the electrically responsive material remaining in locations  244  can have a, for example, square, rectangular, semicircular, or wedge shape. In some embodiments, the electrically responsive material  132  in locations  244  has an irregular shape (e.g., rough texture). 
     By way of example, an isotropic etching process can be used to produce a semicircular shape when viewed in the X-Y plane. In some embodiments, an etching process that removes some or all of the electrically responsive material  132  in locations  244  also removes some of the electrically responsive material  132  in adjacent locations (e.g., below locations of electrode material  140 ). Without departing from the scope of the invention, in some embodiments one or more materials used to fabricate an electrically responsive device that incorporates principles of the present invention can have a non-uniform dimension (e.g., thickness when viewed in the X-Y plane). By way of example, fabrication techniques exists that allow for producing a layer of electrically responsive material that has a generally smoothly varying thickness along the X-axis and/or Z-axis. In this manner, the stiffness of the device  100  can be varied along the X-axis and/or Z-axis in accordance with principles of the present invention. 
     In some embodiments, a controllable etch process is used to modify or modulate structural properties of one or more layers of the electrically responsive device. In some embodiments, uniformity and repeatability of the etched depth is controlled to within about 500 Angstroms. In some embodiments, about 1500 Angstroms of electrically responsive material is removed (e.g., etched) in locations  244  of the electrically responsive device. In some embodiments, the steps used to fabricate the electrically responsive device are performed in an order that protects the electrically responsive material during processing until it is desired that the electrically responsive material be, for example, etched. 
     In some embodiments, a filler material is subsequently applied in the locations  244 . The filler material can have a stiffness that is dissimilar to the stiffness of the electrically responsive material  132 . The filler material (or another material) can be applied to create a substantially flat or planar surface in the X-Z plane of the electrically responsive device. 
     The principles of the present invention can be achieved in other embodiments of the invention. One or more properties of the device can be altered along one or more of the X-Axis, Y-Axis or Z-Axis of the electrically responsive device. The stiffness of the electrically responsive device can be altered by removing material from, for example, the substrate material or the electrically responsive material (e.g., the substrate material  108  or the electrically responsive material  132  of  FIG. 2 ). The stiffness of the device can be altered by modifying or doping certain portions of the substrate material or the electrically responsive material. The distribution of mass over a portion of the device can also be altered to modify at least one resonant mode of the device. 
     In one embodiment, the distribution of stiffness and/or mass is altered by adding material (e.g., a material compatible with MEMS processing techniques) in a specific pattern over the device (e.g., over electrode material, electrically responsive material and/or substrate material). In some embodiments, material is added because it is easier to control the deposition process rather than an etching process where, for example, timing associated with etching has more variability. 
     In some embodiments, the electrically responsive device includes just a membrane (e.g., the membrane  104  of  FIG. 1A ). The membrane is adapted such that the portion  120  of the membrane has the features illustrated in  FIG. 2 . In some embodiments, a cartridge for detecting an analyte incorporates the electrically responsive device and a fluid channel. The electrically responsive device is located within the fluid channel or defines at least a portion of a surface of the fluid channel. The cartridge can be a consumable component of an apparatus and can be removed and replaced. Some embodiments also can include fluid control devices (e.g., plugs, obstructions and baffles) that alter the flow through the cartridge or an apparatus that incorporates the cartridge. 
     One embodiment of the invention is a kit used in detection of an analyte or other target material (e.g., chemical or biological matter). The kit includes the cartridge which includes the electrically responsive device and fluid channel. The electrically responsive device also includes a first component (e.g., material, film, substance or chemical) that is capable of binding to an analyte. The component or material can be a substance bound to a surface of the electrically responsive device (e.g., a surface of layer  136  of the membrane  104  of the device  100  of  FIGS. 1A and 2 ). In some embodiments, the kit also includes particles (e.g., beads) that include a second component capable of binding to the analyte. The particles can be, for example, separate from the cartridge or located within the cartridge in, for example, a cavity or channel located in the cartridge. In other embodiments, a sample containing the analyte is mixed with the particles in the cartridge or external to the cartridge. At least some of the analyte binds to the particles. The particles are then flowed past the surface of the electrically responsive device having the first component. The analyte bound to the particles then bind to the second component. Then, in the manner described herein previously, electronics (e.g., the electronics  128  of  FIG. 1A ) are used by, for example, an operator or automatically by a processor, to detect the presence and/or quantify the amount of analyte present. 
     In some embodiments, a gas phase selective absorptive layer is provided on one or more surfaces of the materials use to fabricate the electrically responsive device. The absorptive layer selectively binds or adheres to a gas that is exposed to the electrically responsive device. Structural properties (e.g., mass, stiffness, loss) of the absorptive layer change based on the degree of diffusion of the gas into the absorptive layer. These structural changes are determined based on changes in pass band characteristics of device. In this manner, the presence an d/or quantity of gas can be determined for a sample containing the gas that is provided to the device. 
     In some embodiments, liquid is provided to one or more surfaces of the electrically responsive device. Coupling (e.g., acoustic coupling) of the liquid to the electrically responsive device loads the device and produces a loaded device pass band (curves  912  and  916  of  FIG. 9  are transfer function plots for fluid loaded electrically responsive devices). Changes in the exposed liquid properties are determined from changes in device pass band characteristics of the curves as compared with similar curves obtained in the absence of fluid loading. 
       FIG. 3  is a schematic side illustration showing different phases of a method  300  for fabricating an electrically responsive device (e.g., a resonant device comprising the device region  120  of  FIG. 2 ), according to an illustrative embodiment of the invention. The method  300  involves applying (step  304 ) an electrically responsive material  132  (e.g., an electroactive material) over at least a portion of a surface  160  of a membrane  104  of a substrate material  108 . In one embodiment, the electrically responsive material  132  is applied to the surface  160  of the membrane  104  by reactive sputtering of the electrically responsive material  132 . Other suitable methods may, alternatively, be used to apply the electrically responsive material  132  over the substrate material  108 . 
     The method  300  also involves applying (step  308 ) an electrode material  140  over at least a portion of a surface  164  of the electrically responsive material  132 . Various methods may be used to apply the electrode material  140 . In one embodiment, the electrode material  140  is applied by physical vapor deposition (e.g., e-beam evaporation plating or sputtering). 
     In some embodiments, an optional step is performed prior to applying (step  308 ) the electrode material  140 . In some embodiments, an intermediate layer is first applied to the surface  164  of the electrically responsive material  132 , prior to application of the electrode material  140 . In one embodiment, the intermediate layer improves the subsequent bonding of the electrode material  140  to the electrically responsive material  132 . In some embodiments, additional layers of material are deposited over the electrode material  140  to coat or protect the underlying material or to alter structural properties according to principles of the present invention. 
     The method  300  also involves selectively removing (step  312 ) at least one region  320  of the electrode material  140  exposing the electrically responsive material  132 . The removal step (step  312 ) can involve any suitable removal process (e.g., a suitable semiconductor material removal process). In this embodiment, the electrode material  140  is removed using etch processing (e.g., one or more of wet etching, dry etching, plasma etching, laser assisted etching, laser ablation, ion beam milling, and electron beam etching). Removing the regions  320  of the electrode material  140  produces a plurality of locations of the electrode material  140  on the surface  164  of the electrically responsive material  132 . 
     The method  300  also involves selectively removing (step  316 ) at least some of the electrically responsive material  132  in the region  320 , producing regions  244 . Step  316  can be conducted using the various types of removal processes discussed herein (e.g., etch processing). In this embodiment, all the electrically responsive material is removed in the region  320  corresponding to the electrode material  140 . In this manner, regions  244  of the electrically responsive material  132  are substantially the same in geometry along the X-Axis and Z-Axis as the regions  320  of the electrode material  140 . 
     In some embodiments, however, only a portion of the electrically responsive material  132  is removed in the region  320 , resulting in regions  244  of the electrically responsive material  132  being smaller in geometry than the regions  320  of the electrode material  140  along the X-Axis and Z-Axis. In some embodiments, removal of the electrode material (step  308 ) and removal of the electrically responsive material (step  312 ) are conducted with a single removal step. 
       FIG. 4  is a schematic illustration of a region  120  of an electrically responsive device, according to an illustrative embodiment of the invention. In this embodiment, the substrate  108  is removed in regions  404  of the region  120  of the device. The regions  404  of the substrate  108  are substantially the same in geometry along the X-Axis and Z-Axis as both the regions  320  of the electrode material  140  and the regions  244  of the electrically responsive material  132 . Creating regions  404  of the substrate  108  enforces a desired modal response of the device. In other embodiments, regions  320 ,  244  and  404  have different geometries from each other along the X-Axis and Z-Axis. 
       FIG. 5  is a schematic illustration of a region  120  of a membrane  104  of an electrically responsive device, according to an illustrative embodiment of the invention. The region  120  of the membrane  104  has a layer of electrically responsive material  132  disposed over a substrate  108 . Electrode material  140  is located over the electrically responsive material  132 . In this embodiment, a modal response of the device comprising the region  120  of  FIG. 5  is modified by the removal of substrate material  108  in regions  504 . The regions  504  correspond to substantially the same geometry (along the X-Axis and Z-Axis) of the electrode material  140  in the regions  320  associated with the electrode material  140 . 
     In some embodiments, the modal response of the electrically responsive device is instead modified by creating regions (lacking substrate material) in the substrate  108  in regions located beneath the electrode material  140  along the Y-Axis. In some embodiments, at least one region  504  comprises no substrate material  108  below the electrically responsive material  132 . In this manner, the electrically responsive material is not supported in this region by the substrate material  108 . In this manner, the unsupported electrically responsive material can produce a cantilever electrically responsive element. In some embodiment, the cantilever electrically element is an cantilever electroactive element in which element may be actuated to bend or actuate the element. 
     In some embodiments, a filler material is subsequently applied in the regions  504 . The filler material can have a stiffness that is dissimilar to the stiffness of the substrate material  108 . In this manner, a property of the electrically responsive device also can be altered to substantially modify a resonance of the device. 
       FIG. 6  is a schematic illustration of a pattern  600  of electrode material, such as the electrode material  140  of  FIG. 2 , according to an illustrative embodiment of the invention. The pattern  600  has a sensing or receiving side  604  and an actuating or launching side  608 . The sensing or receiving side  604  provides output signals from the device (e.g., the device  100  of  FIGS. 1A and 1B ). The actuating or launching side  608  is used to provide input signals to the device. The side  604  has two sets of electrode material locations  140   a  and  140   b  (generally  140 ) similarly as described previously herein regarding, for example, the electrode material  140  of  FIG. 2 . Likewise, the side  608  has two sets of electrode material locations  140   c  and  140   d  (generally  140 ) similarly as described previously herein. 
     In operation in one embodiment, time varying, opposing polarity electrical signals are applied to the electrode material locations  140   c  and  140   d  of the side  608  to generate vibration in the membrane  104  of a resonant device. An outer edge  612  of the membrane  104  is also shown in  FIG. 6 . In operation, the electrode material locations  140   c  and  140   d  in combination with the electrode material locations  140   a  and  140   b  are used to measure the effects of, for example, a fluid in contact with the resonant device (e.g., a surface of the membrane  104  or a surface of a material located above or below the membrane  104 ). In some embodiments, the electrode material locations  140   c  and  140   d  in combination with the electrode material locations  140   a  and  140   b  are used to measure the effects of, for example, biological matter in contact with the surface of the membrane  104 . 
       FIG. 7  is a schematic illustration of a pattern  700  of electrode material, such as the electrode material  140  of  FIG. 2 , according to an illustrative embodiment of the invention. The pattern  700  has a sensing or receiving side  704  and an actuating or launching side  708 . The side  704  has two sets of electrode material locations  140   a  and  140   b . Likewise, the side  708  has two sets of electrode material locations  140   c  and  140   d . Devices according to the invention can be fabricated such that the combination of electrode material and electrically responsive material produce one or more sensing or receiving elements. Similarly, devices according to the invention, can be fabricated such that the combination of electrode material and electrically responsive material produce one or more actuating or launching elements. 
     In operation in one embodiment, time varying, opposing polarity electrical signals are applied to the electrode material locations  140   c  and  140   d  to generate vibrations in the membrane  104  of a resonant device. An outer edge  712  of the membrane  104  is also shown in  FIG. 7 . By way of example, in operation, the electrode material locations  140   c  and  140   d  in combination with the electrode material locations  140   a  and  140   b  are used to, for example, measure the effects of a fluid in contact with the resonant device. 
     In this embodiment, the membrane  104  has a rectangular shape as viewed in the X-Z plane. Alternative geometries for the membrane (and also, for example, the suspended structure  142  of  FIG. 1 ) are contemplated that incorporate principles of the present invention. Further, in this embodiment, the side  704  and side  708 , together, have a generally rectangular shape as viewed in the X-Z plane. Alternative geometries for one or both of sides  704  and  708  are contemplated that incorporate principles of the present invention. By way of example, the membrane  104 , suspended structure  142 , and the electrodes  140  (e.g., side  704  and side  708 ) can have a variety of shapes when viewed in the X-Z plane. 
     In one embodiment, the membrane  104 , suspended structure  142 , and sides  704  and  708  together, have a generally square shape when viewed in the X-Z plane. In another embodiment, the membrane  104  and suspended structure  142  have a generally circular shape when viewed in the X-Z plane. In this embodiment, the electrode material locations  140   a ,  140   b ,  140   c  and  140   d , together have a generally circular shape when viewed in the X-Z plane. In this embodiment, the electrode material locations  140   a ,  140   b ,  140   c  and  140   d  are individually annular in shape (when viewed in the X-Z plane) forming generally concentric rings relative to each other. 
     By way of illustration, an experiment was conducted in which data was acquired for a resonant device incorporating principles of the present invention. The experiment was conducted with a cavity and sensing surfaces of the device (e.g., the cavity  124  and surfaces within the cavity  124  of the device  100  of  FIG. 1A ) exposed to 1× phosphate buffered saline fluid sold by, for example, Sigma-Aldrich with offices in St. Louis, Mo.  FIG. 8  illustrates a plot  800  of the transfer function magnitude obtained using the device  100  of  FIG. 1A . The Y-Axis  804  of the plot  800  is the magnitude of an input signal applied to the device  100  relative to an output signal of the device  100 . The X-Axis  808  of the plot  800  is frequency in Hertz (Hz). 
     Curve  812  is the transfer function magnitude versus frequency for an electroactive material device  100  having a region  120  that does not incorporate principles of the present invention (i.e., the region  120  of  FIG. 1B ). Curve  816  is the transfer function magnitude versus frequency for an electroactive material device  100  having a region  120  that incorporates principles of the present invention (i.e., the region  120  of  FIG. 2 ). By way of reference, the resonant device  100  used in obtaining the data associated with curves  812  and  816  each had a pattern of electrode material corresponding to the pattern  600  of  FIG. 6 . 
     The curve  812  has two regions  820  and  824  corresponding to various dominant resonance modes of the electrically responsive device. Curve  816 , however, has one region  828  in which resonance modes of the resonant device are relatively dominant compared to other locations on the curve  816 . In this experiment, region  828  of curve  816  is a preferred pass band because it is generally isolated from adjacent resonant modes of the electrically responsive device as demonstrated by the observation that the curve  816  drops off to the left and right of region  828 . In this manner, the selective removal of at least some of the material  132  that creates regions  244  of the region  120  of the device  100  of  FIG. 2  substantially modifies at least one resonance mode of the device  100 . Modal overlap and spillover associated with some resonance modes between about 16 MHz and about 26 MHz is reduced thereby enforcing at least one resonance mode of response of the device  100  as illustrated in the region  828  of the curve  816 . 
     By way of illustration, another experiment was conducted in which data was acquired for a resonant device incorporating principles of the present invention. The experiment was conducted with a cavity and sensing surfaces of the device (e.g., the cavity  124  and surfaces within the cavity  124  of the device  100  of  FIG. 1A ) exposed to 1× phosphate buffered saline fluid sold by, for example, Sigma-Aldrich with offices in St. Louis, Mo.  FIG. 9  illustrates a plot  900  of the transfer function magnitude obtained using the device  100  of  FIG. 1A . The Y-Axis  904  of the plot  900  is the magnitude of an input signal applied to the device  100  relative to an output signal of the device  100 . The X-Axis  908  of the plot  900  is frequency in Hertz (Hz). 
     Curve  912  is the transfer function magnitude versus frequency for an electroactive material device  100  having a region  120  that does not incorporate principles of the present invention (i.e., the region  120  of  FIG. 1B ). Curve  916  is the transfer function magnitude versus frequency for a device  100  having a region  120  that incorporates principles of the present invention (i.e., the region  120  of  FIG. 2 ). By way of reference, the device  100  used in obtaining the data associated with curves  912  and  916  each had a pattern of electrode material corresponding to the pattern  700  of  FIG. 7 . 
     The curve  912  has a region  920  corresponding to various dominant resonance modes of the resonant device. Curve  916  has a region  928  in which at least one resonance mode of the resonant device has been modified to enforce at least one resonance mode illustrated in region  928  as compared to other locations on the curve  916 . In this experiment, region  928  of curve  916  is a preferred pass band because it is generally isolated from adjacent resonant modes of the electrically responsive device as demonstrated by the observation that the curve  916  drops off to the left and right of region  928  and there is no substantial resonant mode shown to the left and right of region  928 . In contrast, for example, region  920  of curve  912  does have at least one substantial resonant mode  940  that is adjacent to the region  920 . In this manner, the device corresponding to curve  912  has a less preferred pass band region  920  because of the presence of the at least one resonance mode  940 . The device has a preferred pass band, narrower bandwidth with respect to frequency and lower transmission loss, evidenced by a more pronounced peak in region  928  relative to the peaks in regions  920  and  940 ), than would otherwise be achieved without incorporating principles of the present invention. Preferred pass bands, narrower bandwidth and lower transmission loss are also achieved in the device in the absence of fluid loading. 
     Further, the region  928  of the curve  916  illustrates that at least one resonant mode in the region  928  has been substantially modified when compared with the comparable region  920  of curve  912 . The substantial modification is due to the removal of at least some of the electroactive material  132  that creates regions  244  of the region  120  of the device  100  of  FIG. 2 . Modal overlap and spillover associated with some resonance modes between about 16 MHz and about 26 MHz is reduced in the region  928  of the curve  916 , thereby enforcing at least one resonance mode of response of the device  100  in the region  928  of the curve  916 . 
     Variations, modifications, and other implementations of what is described herein will occur to those of ordinary skill in the art without departing from the spirit and the scope of the invention and are considered to be encompassed thereby. Accordingly, the invention is not to be defined only by the preceding illustrative description.