Microelectromechanical systems comprising differential inductors and methods for making the same

An integrated Microelectromechanical Systems (“MEMS”) device (100). The MEMS device comprises a substrate (200, 300), a transition portion (118), and a differential inductor (1000, 1100, 1300). The transition portion is connected to and at least partially extends transversely away from a major surface of the substrate. The differential inductor is mechanically suspended above a major surface of the substrate at least partially by the transition portion. The differential inductor is also electrically connected to an electronic circuit external thereto by the transition portion. A first dielectric gap exists between the major surface of the substrate and the differential inductor.

BACKGROUND OF THE INVENTION

1. Statement of the Technical Field

The inventive arrangements relate to Microelectromechanical System (“MEMS”) and methods for forming the same, and more specifically to differential inductors for MEMS devices.

2. Description of the Related Art

MEMS is a technology of very small devices typically between 2 micrometers to 2 millimeters in size. The MEMS devices can include one or more components between 1 to 100 micrometers in size. Conventional MEMS devices are fabricated using molding techniques, plating techniques, wet etching techniques, and/or dry etching techniques. Various materials can be used to create the MEMS devices. Such materials include silicon, polymers, metals and ceramics.

Radio Frequency (“RF”) filters typically occupy a relatively large amount of space in an RF system (i.e., >25%). As such, it has been desirable to miniaturize RF filters via MEMS technology. RF filters may comprise varactor devices, such as Gap Closing Actuator (“GCA”) varactors. GCA varactors generally operate on the principle of electrostatic attraction between adjacent interdigitating fingers of a drive comb structure and a movable truss comb structure. That is, motion of the truss comb structure can be generated by developing a voltage difference between the drive comb structure and the truss comb structure. The voltages applied at comb structures are also seen at the interdigitating fingers, respectively. The resulting voltage difference generates an attractive force between the interdigitating fingers. If the generated electrostatic force between the fingers is sufficiently large to overcome the other forces operating on truss comb structure (such as a spring constant of a resilient component), the electrostatic force will cause the motion of the truss comb structure between a first interdigitated position (resting position at a zero voltage difference) and a second interdigitated position (position at a non-zero voltage difference) among a motion axis. Once the voltage difference is reduced to zero, a resilient component (e.g., a spring) restores the position of the truss comb structure to the first interdigitating position.

RF filters typically comprise RF inductors built using thin films disposed on the substrate (e.g., Silicon or Silicon Germanium). Such RF inductors are typically limited in their performance which is proportional to their Quality (“Q”) factor. The limited performance of the RF inductors is a consequence of parasitic effects and dielectric losses from the substrate and is a function of frequency. As the frequency increases, more loss results due to coupling of the electric field through the substrate, instead of between the windings of the RF inductor.

SUMMARY OF THE INVENTION

The present invention concerns systems and methods for making an integrated MEMS device. The MEMS device comprises a substrate, one or more capacitive elements (such as a varactor), and one or more differential inductor elements. A transition portion is connected to and at least partially extends transversely away from a major surface of the substrate. The differential inductor is mechanically suspended above a major surface of the substrate at least partially by the transition portion. The differential inductor is also electrically connected to an electronic circuit external thereto by the transition portion. A first dielectric gap exists between the major surface of the substrate and the differential inductor. The first dielectric gap can include, but is not limited to, air or other dielectric fluid. A magnetic material may be disposed between coil windings of the differential inductor.

In some scenarios, the electronic circuit is formed on the major surface of the substrate so as to reside between the substrate and the differential inductor, wherein a dielectric gap exists between the electronic circuit and the differential inductor. In this case, a first conductive material used to fabricate the differential inductor has a first melting point that is different from second melting points of all second conductive materials used to form the electronic circuit by no more than 100° C.

The transition portion may comprise a three dimensional hollow ground structure in which an elongated center conductor is suspended. The elongated center conductor is separated from the three dimensional hollow ground structure via a dielectric gap on all sides. A first inductor port of the differential inductor is electrically connected to the three dimensional hollow ground structure of the transition portion. A second inductor port of the differential inductor is electrically connected to the elongated center conductor of the transition portion. Alternatively, both of the first and second inductor ports are electrically connected to the elongated center conductor of the transition portion. A periphery of the differential inductor may be separated from and at least partially surrounded by a ground structure of the transition portion. In this case, a magnetic material may be disposed between the differential inductor and the ground structure of the transition portion.

The differential inductor may comprise a first coil portion defined by windings traveling along a first serpentine path in a direction towards a center of the differential inductor and a second coil portion defined by windings traveling along a serpentine path in a direction away from the center of the differential inductor. The first coil portion and the second coil portion can be electrically connected to each other at the center of the differential inductor. At least a portion of each winding of the second coil portion is disposed between two adjacent windings of the second coil portion. In some scenarios, a first winding portion of the first coil portion may overlap a second winding portion of the second coil portion such that a dielectric gap exists between the first and second winding portions.

DETAILED DESCRIPTION

The present invention generally concerns MEMS devices which are integrated with ICs. The MEMS devices can be used in a variety of applications. Such applications include, but are not limited to, multi-band communication system applications, radar applications, wide-band tracking receiver applications, broadcast radio applications, television applications, wireless communication device applications (e.g., cellphone applications), Ultra-WideBand (“UWB”) communication applications, SATellite COMmunication (“SATCOM”) applications, Software Defined Radio (“SDR”) applications, microwave/mm-wave communication applications, military/space system applications, and/or telecommunication applications. The MEMS devices include, but are not limited to, varactors, inductors, and/or RF filters. As should be understood, RF filters are generally configured to combine and/or separate multiple frequency bands. Exemplary circuit architectures for such RF filters are shown inFIGS. 14 and 15.

The present invention will be described below in relation to exemplary RF filters. The present invention is not limited in this regard. Each of the electronic components of the RF filters (e.g., varactors and inductors) can be used independently or with other MEMS devices in accordance with particular applications.

A schematic illustration of an exemplary RF filter100is provided inFIG. 1. In some scenarios, the RF filter100comprises a 3-pole tunable bandpass filter designed to select a desired band of frequencies for a particular frequency range (e.g., the 1060-1370 MHz range). Embodiments of the present invention are not limited to 3-pole tunable bandpass filter architectures. The RF filter100can include any type of filter architecture suitable for a particular application, or have as few or as many poles as necessary for bandwidth. Notably, the tunable feature of the RF filter100offers significant size reduction over switch-type RF bandpass filter banks.

As shown inFIG. 1, the RF filter100is implemented using three shunt varactors102,104,106, three shunt inductors108,110,112, two series inductors114,116, and a transition portion118(e.g., a transmission line). Each of the listed components102-116can be fabricated using at least one conductive material, such as metal (e.g., gold, nickel, aluminum, copper, chromium, titanium, tungsten, platinum, and/or silver). The operation of inductors are well known in the art, and therefore will not be described in detail herein. Notably, the inductors are not limited to the architecture shown inFIG. 1. Accordingly, the inductors can have alternative designs, such as a differential inductor design as discussed below in relation toFIGS. 10-13. As will also be described below, in some scenarios, the RF filter100exhibits a 1.9 dB insertion loss across a 300 MHz bandwidth. This is a significant insertion loss improvement over conventional RF filter designs, such as those described above in the background section of the document.

The transition portion118is configured to electrically connect the RF filter100to external circuitry. Accordingly, the transition portion118comprises a ground structure120and a center conductor122. The center conductor122is electrically connected to the shunt varactors102,104,106at points190,192,194, inductors108,110,112at ends196,198,199, and inductors114,116at their outer ends and center ends. Pass-over conductive structures154are provided to facilitate the electrical coupling of the center conductor122to the center ends of the inductors114,116. Similarly, the ground structure120is electrically connected to the shunt varactors102,104,106via grounding portions150and inductors108,110,112via pass-over grounding portions152.

Operations of the shunt varactors102,104,106will be described below in relation toFIGS. 4-5. Still, it should be understood that each shunt varactor102,104,106comprises interdigitated drive comb structures and a truss comb structure. A voltage (e.g., 90 Volts) is applied to the drive comb structures via the center conductor122such that a gap between each drive comb structure and the truss comb structure is varied. For example, in some scenarios, the gap between respective comb structures is varied between 20 microns down to 5 microns. Notably, the drive comb structures170,172of each shunt varactor102,104,106are electrically connected to each other via a respective coupling structure180.

As shown inFIG. 1, the ground structure120comprises a plurality of straight portions defined by a three dimensional hollow structure with a generally rectangular cross-sectional profile. Notably, the three dimensional hollow structure can have a cross-sectional profile other than a rectangular cross-sectional profile. The center conductor122is disposed within the three dimensional hollow structure. In some scenarios, the center conductor122is suspended therein so as to extend along a center axis of each straight portion of the ground structure120. Accordingly, the center conductor122is encompassed by the ground structure120along at least a portion of its length and separated from the ground structure120via a dielectric gap124on all sides thereof. The dielectric gap124can comprise, but is not limited to, air or other dielectric fluid. The center conductor122and ground structure120are fabricated using at least one conductive material, such as metal (e.g., gold, nickel, aluminum, copper, chromium, titanium, tungsten, platinum, and/or silver).

In some scenarios, the center conductor122is suspended within the ground structure120via one or more dielectric straps (not shown inFIG. 1) connected between opposing sidewalls of the ground structure120. For example, a dielectric strap can be connected between sidewalls160,162of the ground structure120so as to mechanically support and/or suspend at least a portion of the center conductor122disposed within section164of the ground structure120. In some scenarios, a plurality of dielectric straps is disposed along the entire length of each straight portion of the ground structure120.

The ground structure120also comprises isolation portions101,103,105,107,109,111,113,115each defined by a plurality of sidewalls (e.g., two, three or four sidewalls). Each isolation portion101,103,105,107,109,111,113,115at least partially surrounds a respective component102,104,106,108,110,112,114,116so as to electrically isolate the same from other adjacent components. For example, as shown inFIG. 1, the ground structure120surrounds four sidewalls of each inductor108-116. The ground structure120also surrounds three sidewalls of each shunt varactor102-106. Embodiments of the present invention are not limited in this regard. Alternatively, the ground structure120can surround one or more sidewalls of one or more inductors108-116and/or shunt varactors102-106.

In some scenarios, a space151is provided between adjacent sidewalls130/132,134/136,138/140,142/144of the ground structure120. Notably, the space151has dimensions selected for ensuring that adjacent electronic components are placed in close proximity to each other. For example, in some scenarios, the adjacent sidewalls130/132,134/136,138/140,142/144are spaced 0.1-1.0 mm from each other. In other scenarios, no space151is provided between adjacent sidewalls130/132,134/136,138/140,142/144. Alternatively a single sidewall of the ground structure120(or a “common sidewall”) is used to separate two adjacent components102/110,102/114,104/112,104/114,104/116,106/116,108/104,108/114,110/106,110/114,110/116,112/116. In this case, the single sidewall has a thickness that is the same as or greater than that of one adjoining sidewalls of the ground structure120. The other adjoining sidewalls include, but are not limited to, a sidewall of the ground structure to which the common sidewall is adjacent and directly connected.

In some scenarios, the RF filter100has an overall size of 3.6 mm by 4.8 mm. Accordingly, each shunt varactor102,104,106has a size of 1.1 mm by 1.4 mm. Each shunt inductor108,110,112has a size of 1.1 mm by 1.1 mm. Embodiments of the present invention are not limited to the particularities of such scenarios. However, it should be reiterated that such an RF filer architecture exhibits a 1.9 dB insertion loss across a 300 MHz bandwidth. This is a significant insertion loss improvement over conventional RF filter designs, such as those described above in the background section of the document.

Notably, the RF filter100can be fabricated using a process which allows the RF filter100to be fabricated without the use of the high heat required to fabricate conventional polysilicon based MEMS devices. In this regard, the material used to fabricate the RF filter100and the material used to fabricate an IC have melting points that are the same (e.g., a values ≦100° C.) or that have no more than a 100° C. difference. An exemplary fabrication process will be described below in relation toFIGS. 9A-9Z.

The fabrication process also allows the RF filter100to be formed so as to be suspended above a substrate200,300. As will be described below, the RF filter100may be: (a) suspended over the substrate200exclusively by a rigid transition portion204as shown inFIG. 2; or (b) at least partially suspended above the substrate200via an isolation platform600as shown inFIGS. 6-8and resilient interconnections304,306with external circuitry as shown inFIGS. 3A-3B. Substrate200,300can include, but is not limited to, a flat semiconductor wafer. In this regard, a dielectric gap202,302is provided between the RF filter100and the substrate200,300.

By suspending the RF filter100above the substrate200,300, valuable space on the surface of the substrate200is made available for other circuitry206,306thereby providing a more compact MEMS device as compared to conventional MEMS devices including RF filters. Notably, there is relatively minimal coupling (cross talk) of a signal traveling through the filter onto circuitry206,306formed on the substrate200,300. For example, in some scenarios, the isolation is greater than 40 dB across 6 GHz.

As noted above, in some scenarios, the RF filter100is isolated from the substrate300via resilient interconnections304,306, as shown inFIG. 3A. The interconnections304,306between the RF filter100and an external device (e.g., a switch) may be designed to ensure that vibrations from an external environment are not connected to the RF filter100. In this regard, at least a portion of each interconnection304,306may be designed to move in the directions of the vibrations. Such an interconnection design can include, but is not limited to: a wire bond (not shown) or a spring (e.g., spring350shown inFIG. 3B) interconnection electrically connecting the center conductor122of the transition portion118to a center conductor of a transition portion308,310connected to the substrate300; and/or a plurality of spring interconnections312electrically connecting each of the four sidewalls of the ground structure120to respective sidewalls of the respective transition portion308,310connected to the substrate300. Alternatively or additionally, the material used to form interconnections304,306between the transition portion118is formed from a material with a relatively low mechanical stiffness.

Referring now toFIG. 4, there is provided a schematic illustration of an exemplary architecture for a drive portion400of a shunt varactor. Each of the shunt varactors102,104,106ofFIG. 1can have a drive portion that is the same as or similar to that ofFIG. 4. Drive portion400includes a drive comb structure402having a fixed position and extending along a longitudinal axis404. Drive portion400also includes a truss comb structure406that extends substantially parallel to axis404and that can elastically move in the x direction along a motion axis420substantially parallel to axis404of the drive comb structure402. For example, as shown inFIG. 4, truss comb structure406can include or be attached to at least one restorative or resilient component212connected to a fixed end. The resilient component212restores a position of truss comb structure406when no external forces are being applied. The drive comb structure402can have one or more drive fingers408extending therefrom towards truss comb structure406. The truss comb structure406can similarly include one or more truss fingers410extending therefrom towards the drive comb structure402.

As shown inFIG. 4, the drive comb structure402and the truss comb structure406can be positioned to be interdigitating. The term “interdigitating”, as used herein with respect to comb structures, refers to arranging comb structures such that the fingers extending from such comb structures at least partially overlap and are substantially parallel.

In the exemplary architecture ofFIG. 4, fingers408and410can each have a width and a height of a and b, respectively, and an overlap length of l. Although comb structures with multiple sets of fingers can be configured to have the same dimensional relationships (width, height, and overlap), the present invention is not limited in this regard and dimensional relationships can vary, even within a single shunt varactor. Furthermore, the portion shown inFIG. 4and the dimensional relationship shown inFIG. 4are only the electrically conductive portions of drive portion400. As one of ordinary skill in the art will recognize, comb structures can further include structural portions comprising non-conductive or semi-conductive materials extending in the z direction to provide structural support for the conductive portions shown inFIG. 4.

The drive portion400shown inFIG. 4operates on the principle of electrostatic attraction between adjacent interdigitating fingers. That is, motion of the truss comb structure406can be generated by developing a voltage difference between the drive comb structure402and the truss comb structure406. In the case of drive portion400, the voltages applied at comb structures402,406are also seen at fingers408,410, respectively. The resulting voltage difference generates an attractive force between fingers408and410. If the generated electrostatic force between fingers408and410is sufficiently large to overcome the other forces operating on truss comb structure406(such as a spring constant of resilient component412), the electrostatic force will cause the motion of the truss comb structure406between a first interdigitated position (resting position at a zero voltage difference) and a second interdigitated position (position at a non-zero voltage difference) among motion axis420. Once the voltage difference is reduced to zero, resilient component412restores the position of truss comb structure406to the first interdigitating position.

As shown inFIG. 4, each finger410in truss comb structure406can be disposed between two fingers408of drive comb structure402. Accordingly, an electrostatic force is generated on both sides of finger410when a voltage difference is developed between comb structures402and406. Therefore, to ensure movement of truss comb structure406in only one direction in response to a voltage difference, fingers410are positioned with respect to fingers408such that the electrostatic force in a first direction along the x-axis is greater than the electrostatic force in an opposite direction in the x-axis. This is accomplished by configuring the finger spacing (i.e., spacing between fingers of interdigitated comb structures) in the first direction along the x-axis (x0) and the finger spacing in the opposite direction along the x-axis (y0) to be different when the voltage difference is zero. Since the amount of electrostatic force is inversely proportional to the distance between fingers, the motion of truss comb structure will be in the direction associated with the smaller of x0and y0.

The drive portion400provides a control mechanism for horizontal actuation in a shunt varactor that can be precisely controlled by adjusting the voltage difference between the drive and truss comb structures. This allows continuous adjustment over a range of interdigitating positions (by adjusting the voltage continuously over a voltage range).

Although the drive portion described above could be connected to any variety of devices, using such a drive portion for various types of devices will only provide a partial improvement in manufacturing robustness and device reliability. In general, the robustness of the IC fabrication techniques used for fabricating MEMS devices and other types of devices is increased by reducing the variety of feature types and dimensional variation in each layer. The present invention exploits this characteristic. In particular, another aspect of the invention is to use the comb structure drive portion in conjunction with a comb structure based varactor portion, as shown below inFIG. 5.

FIG. 5shows a top-down view of an exemplary MEMS shunt varactor500that is useful for understanding the present invention. Each of the shunt varactors102,104,106ofFIG. 1can be the same as or similar to the MEMS shunt varactor500ofFIG. 5. As shown inFIG. 5, varactor500includes a drive portion501, similar to the drive portion400described above in relation toFIG. 4. That is, drive portion501includes drive comb structures502aand502b(collectively502), a truss comb structure504, drive fingers506, and truss fingers508.

Truss comb structure504also includes resilient portions510with fixed ends512aand512b(collectively512). Resilient portions510comprise resilient or flexible reed structures511mechanically coupling truss comb structure504to fixed ends512. Therefore, a leaf spring structure is effectively formed on the two ends of truss comb structure. In operation, as a force is exerted on truss comb structure504(by generating a voltage difference between fingers506and508) the reed structures511deform to allow truss comb structure to move along motion axis505from a first interdigitated position to at least a second interdigitated position. Once the force is no longer being exerted, the reed structures511apply a restorative force to restore the position of the truss comb structure504to a first interdigitated position. The operation and configuration of components502-510is substantially similar to that of components402,406,408,410,412ofFIG. 4. Therefore, the discussion ofFIG. 4is sufficient for describing the operation and configuration for components502-510ofFIG. 5. As described above, in addition to the drive portion501, varactor500also includes a variable capacitor or varactor portion514, as shown inFIG. 5. The varactor portion514includes input/output comb structures516aand516b(collectively516) having a fixed position. The input/output comb structures516can also have one or more sense fingers518extending therefrom. Within the varactor portion514of varactor500, the truss comb structure504can additionally include one or more additional truss fingers520extending therefrom and interdigitating sense fingers518. Therefore, the truss comb structure504interdigitates (via fingers508and fingers520) both the drive fingers506and the sense fingers518. As a result, the truss comb structure504mechanically connects and is part of both the drive portion501and the varactor portion514.

Fingers506,508,518and520are shown to be similarly dimensioned and having a same amount of overlap. However, the invention is not limited in this regard and dimensional relationships can be different in the drive portion501and varactor portion514. Furthermore, the dimensional relationship can also vary within the varactor portion514. Additionally, as described above with respect toFIG. 4, the comb structures502,504and516can further include conductive portions and structural portions, comprising non-conductive or semi-conductive materials, to provide structure support for the conductive portions.

As described above, varactor500is configured to provide functionality as a variable capacitor or varactor. In particular, the truss comb structure504is configured to provide an adjustable capacitance based on adjustment of the gap between the first capacitor plate, provided by fingers518, and a second capacitor plate, provided by fingers520. Therefore, varactor500forms a first adjustable capacitor or varactor between truss comb structure516aand truss comb structure504, with a capacitance of COUT1, and a second adjustable capacitor or varactor between comb structure516band truss comb structure504, with a capacitance of COUT2.

These first and second varactors can be used separately or in combination. In combination, these varactors can be connected to provide capacitance in series or parallel. For example, to provide a series capacitance, the capacitance can be measured between comb structures516aand516b. In contrast to provide a parallel capacitance, the capacitance can be measured between comb structures516a,516band fixed end512a(if electrically connected to fingers520).

In some scenarios, a discontinuity524is provided to isolate fingers520from fingers508. As described above, the discontinuity524can be provided to reduce any interference between the varactor portion514and the drive portion501. For example, to prevent the charge stored between fingers518and520from affecting a voltage difference between fingers506and508and vice versa. However, if fixed ends512aand512bare both connected to ground, isolation between drive portion401and varactor portion514is maintained without requiring such discontinuity524.

Varactor500operates as follows. A circuit (not shown) is connected to comb structures516a,516b, and fixed end512a(if necessary, as described above). To increase amount of capacitance at COUT1and COUT2, a voltage difference (VBIAS) is developed between fingers506and508to generate electrostatic attraction between these fingers. For example, VBIASis applied across drive comb structures502and fixed ends512b(which is electrically connected to fingers508) to cause sufficient electrostatic attraction between fingers506and508to induce motion of truss comb structure504, and consequently motion of fingers520towards fingers518, reducing a spacing X0—CAPbetween fingers518and520. Consequently, the changing of the spacing between the capacitor plates results in a different capacitance value for both COUT1and COUT2. Therefore, to increase capacitance, VBIASis selected to create an electrostatic force that is at least greater than the restorative force of reed structures511to cause motion of truss comb structure504along motion axis505. Afterwards, to decrease the capacitance, VBIASis reduced such that the electrostatic force is less than the restoring force applied by reed structures511. The restoring force then acts on truss comb structure504to increase the gap between fingers520and fingers518, and thus lower the capacitance.

Referring now toFIG. 6, there is provided a top-down view of the RF filter100with an isolation platform600that is useful for understanding the present invention. The isolation platform600is configured to protect the RF filter100by absorbing vibrations from an external environment prior to arrival thereat. In this regard, the isolation platform600is (a) coupled to the substrate300so as to be suspended above the substrate (as shown inFIG. 8) using one or more mechanical support structures802, (b) coupled to the RF filter100so as to suspend the RF filter100over the substrate300, (c) designed such that the RF filter100is isolated from external sources of vibration, and (d) designed to have a relatively large amount of natural damping such that damping devices are not needed.

As shown inFIG. 6, the isolation platform600comprises at least one sidewall framing or surrounding a periphery of the RF filter100. For example, in some scenarios, the isolation platform600comprises four adjoining sidewalls602,604,606,608defining a rectangular frame610, as shown inFIG. 6. In other scenarios, the isolation platform600comprises a single sidewall (not shown) defining a circular frame (not shown) which circumscribes or encircles the periphery of the RF filter100.

The isolation platform600also comprises one or more resilient components (e.g., springs)612,614,616,618for absorbing vibrations from an external environment. The resilient components are formed from a metal material. As such, the metal resilient components (e.g., springs)612,614,616,618store energy when compressed. However, the stored energy is not or only minimally coupled to the RF filter when the resilient components return to their pre-stressed shapes. This is because metal resilient components (e.g., springs) have a relatively large amount of natural damping in them. Accordingly, the amplitude of vibration is decreased by the resilient components (e.g., springs)612,614,616,618. Consequently, the force exerted when the resilient components (e.g., springs)612,614,616,618return to their pre-stressed shapes is not proportional to the amount it is compressed.

Each resilient component612,614,616,618is coupled between the frame610and a respective isolation portion101,103,105,107,109,111,113,115of the ground structure120. As such, the isolation platform600is grounded. The resilient components isolate the varactors102,104,106from vibrations in at least the x direction. In this regard, it should be understood that the leaf springs of the varactors are designed to have a low actuation voltage. As such, the leaf springs are relatively stiff in the y direction and z direction, and therefore are not or only minimally susceptible to vibrations in those directions. However, the leaf springs are relatively flexible in the x direction, and therefore are susceptible to vibrations in that direction. Therefore, the resilient components612,614,616,618of the isolation platform600provide a means for isolating the x direction vibrations from the varactors such that their performances are not affected thereby. In scenarios where the leaf springs of the varactors are susceptible to vibrations in the y and/or z directions, the isolation platform600can include additional resilient components (not shown) for absorbing such y and/or z direction vibrations prior to reaching the RF filter100. For example, the isolation platform600can alternatively or additionally include resilient components (not shown) coupled to sidewall604and/or sidewall608.

The resilient components612,614,616,618are designed to have the same or different geometries and/or orientations. For example, as shown inFIG. 6, the resilient components612,616and618have the same orientations. Resilient components612,616and618have the same serpentine shape. However, resilient component614has a different serpentine shape than that of resilient components612,616,618. Embodiments of the present invention are not limited in this regard.

The RF filter100and the isolation platform600can be simultaneously fabricated using a process which allows the structures thereof to be fabricated without the use of the high heat required to fabricate conventional polysilicon based MEMS devices. In this regard, the material used to fabricate the RF filter100, the material used to fabricate an IC, and the material used to fabricate the isolation platform600have melting points that are the same (e.g., a values ≦100° C.) or that have no more than a 100° C. difference. For example, in some scenarios, the RF filter, IC and isolation platform are fabricated using the same metal material, such as copper.

Referring now toFIG. 7, there is provided a perspective view of a portion of the RF filter100with the isolation platform600that is useful for understanding the present invention. As shown inFIG. 7, the frame610has an aperture700formed therethrough at a location where a first terminal702(e.g., an input terminal) of the transition portion118of the RF filter100resides. Although not shown inFIG. 7, another aperture is formed through the frame610at a location where a second terminal (e.g., an output terminal not shown inFIG. 7) of the transition portion118of the RF filter100resides. Each aperture700is sized and shaped to allow the transition portion118to pass therethrough, and also to provide a dielectric gap704between all four sides of the transition portion118and the frame610. The dielectric gap704ensures that vibrations from the external environment (e.g., the external transition portion and/or external electronic component to which the transition portion is electrically connected) are not directly coupled from the isolation platform600to the RF filter100.

The height706of each sidewall of the frame610can be the same as or different than the overall height of one or more components of the RF filter100. For example, as shown inFIG. 7, height706of each sidewall of the frame610is greater than that of the inductors108,110,112,114,116and the transition portion118, but less than that of the varactors102,104,106. Embodiments of the present invention are not limited to the particularities of this example architecture.

The RF filter structure described above can be fabricated using a MEMS fabrication technique. This is illustrated inFIGS. 9A-9Z.FIGS. 9A-9Zshow partial cross-sections of certain components of a MEMS device (e.g., the MEMS device shown inFIGS. 5-8) during various steps of a fabrication process. Notably, the fabrication process described below inFIGS. 9A-9Zis sufficient for understanding how an entire MEMS device can be fabricated in accordance with embodiments of the present invention.

Manufacture of the MEMS device begins with the formation of an interface layer902on a substrate900. An isolation layer904also exists on the substrate900. After the formation of the interface layer902, various steps are performed to fabricate an RF filter with an isolation platform that is suspended above the electronic circuit900. These steps are described below in relation toFIGS. 9B-9Z.

As shown inFIG. 9B, a first and second resist layer908aand908b(collectively908) is disposed on the top surface of the substrate900so as to cover the circuitry906. Next, the second resist layer908bis patterned to form at least partially an isolation platform (e.g., isolation platform600ofFIG. 6), shunt varactors (e.g., a shunt varactor102,104and/or106ofFIG. 1), inductors (e.g., inductor108,110,112,114and/or116ofFIG. 1), and a transition portion (e.g., transition portion118ofFIG. 1) of the RF filter (e.g., RF filter100ofFIG. 1). A schematic illustration of second resist layer908bwhich has been patterned is provided inFIG. 9C. As shown inFIG. 9C, at least four patterns have been formed in the second resist layer908b. A first pattern909is provided for forming a frame and a resilient component of the isolation platform. A second pattern910is provided for forming a lower portion of a comb structure of a shunt varactor. A third pattern912is provided for forming a lower portion of an inductor coil. A fourth pattern914is provided for forming a lower portion of a transition portion. Therefore, each pattern909,910,912,914is then filled with a conductive material915-920, as shown inFIG. 9D.

Notably, the relative orientations and spacing between the patterns909,910,912,914have simply been selected for ease of explanation. Embodiments of the present invention are not limited to the relative orientations and component spacing shown inFIGS. 9B-9Z.

InFIG. 9E, a third resist layer922is disposed over the resist layers908and conductive material915-920. The third resist layer922is then patterned inFIG. 9Ffor forming at least a portion of a middle section of the isolation platform, comb structure, inductor coil, and transition portion. As such, four patterns923-928are formed in the third resist layer922. Pattern923is provided for forming a portion of a middle section of the frame and resilient component of the isolation platform. Pattern924is provided for forming a portion of middle section of the comb structure of the shunt varactor. Pattern926is provided for forming a portion of a middle section of an inductor coil. Pattern928is provided for forming a portion of a middle section of a transition portion. Therefore, each pattern923-928is then filled with the conductive material935-940, as shown inFIG. 9G.

InFIG. 9H, a fourth resist layer937is disposed over the third resist layer922and conductive material935-940. The fourth resists layer937is then patterned inFIG. 9Ifor forming a dielectric strap which will support a center conductor (e.g., center conductor122ofFIG. 1) within the ground structure (e.g., ground structure120ofFIG. 1) of the transition portion. Therefore, pattern946is filled with a non-conductive material952as shown inFIG. 9J.

This process of disposing, patterning and filling of resists layers is repeated as shown inFIGS. 9K-9Yuntil the RF filter structure ofFIG. 9Yis formed. Subsequently, the resist layers are removed as shown inFIG. 9Z. As a result of removing the resists layers, the RF filter structure with the isolation platform is suspended over the substrate900. The RF filter is electrically isolated from the circuit902via air. A schematic illustration of an exemplary RF filter suspended over a substrate is shown inFIGS. 2 and 3Awhich were discussed above. A schematic illustration of an exemplary RF filter with an isolation platform is provided inFIGS. 6-8.

As noted above, the inductors108,110,112,114,116of the RF filter100are not limited to the architectures shown inFIGS. 1-7. Exemplary alternative architectures for the inductors108,110,112,114,116of the RF filter100will now be described in relation toFIGS. 10-13.

FIG. 10provides a top perspective view of an exemplary differential inductor1000. Inductors114,116of the RF filter100can be the same as or similar to differential inductor1000. In the case of inductors108,110,112of the RF filter100, the inductor architecture schematically illustrated inFIG. 10can be modified such that one of the two inductor ports1002,1004is connected to the ground structure1012as opposed to the center conductor1006of the transition portion1008. Notably, the differential inductor1000advantageously has a larger Q factor in a small area as compared to single-end inductors. Also, in some scenarios, the differential inductor1000has an aspect ratio of 20:1 for inductor windings and has well contained electric fields in the windings.

As shown inFIG. 10, differential inductor1000comprises a coil1010having a first inductor port1002and a second inductor port1004. Each inductor port1004is coupled to a center conductor1006of a transition portion1008. The coil1010is designed such that a pass-over structure is not required for connecting a respective inductor port to a transition portion. In this regard, the coil1010comprises a first coil portion1014defined by windings traveling along a first serpentine path in a direction towards a center1016of the inductor1000and a second coil portion1018defined by windings traveling along a second serpentine path in a direction away from the center1016of the inductor1000. At least a portion of each winding of the second coil portion1018is disposed between two adjacent windings of the first coil portion1014. As a result, there is an increased amount of inductance coupling effects between the windings of the inductor1000, and therefore a larger inductance value is obtained in the same or smaller total area as compared to the inductor architecture shown inFIG. 1. The first and second coil portions1014,1018are electrically coupled to each other at the center1016of the inductor1000.

In some scenarios, a magnetic material1100is disposed between the windings of the inductor1000as shown inFIG. 11, as well as between the coil1010and the ground structure1012of the transition portion1008. The magnetic material includes, but is not limited to, cobalt, platinum, iron, and/or any other ferrite-based material. The magnetic material1100increases the inductance value of the inductor1000. For example, if the magnetic material comprises an iron based material, then the inductance value of the inductor1000is increased from 1 nH to 62 nH.

During fabrication, the magnetic material1100is added: (a) after the inductor1000structure is fabricated and at least one resist layer is removed; or (b) via an interleaving process in which layers of the inductor1000and layers of magnetic material1100are alternatively built-up.

Referring now toFIG. 12, there is provided a graph illustrating Q factors for the two types of inductors of the present invention, namely an inductor114ofFIG. 1and differential inductor1000without the magnetic material1100. As evident fromFIG. 12, the Q factor of the differential inductor1000is significantly higher than the Q factor for the inductor114ofFIG. 1of the same size using the same fabrication technology. Still, the Q factors of both inductors114,1000are much higher than that of conventional inductors which are formed directly on a substrate. As should be understood, the Q factor is critical for providing an RF filter with a good rejection ratio.

Referring now toFIG. 13, there is provided a schematic illustration of another exemplary architecture for an inductor1300which can be used with the RF filter of the present invention. As shown inFIG. 13, the inductor1300includes a coil1302with a plurality of windings. Specifically, the coil1302comprises a first coil portion1304defined by windings traveling along a first serpentine path in a direction towards a center1308of the inductor1300and a second coil portion1306defined by windings traveling along a second serpentine path in a direction away from the center1308of the inductor1300. At least a portion of each winding of the second coil portion1306is disposed between two adjacent windings of the first coil portion1304. Also, the first and second portions1304,1306have overlap or cross-over portions1310. A dielectric gap (not shown inFIG. 13) is provided between respective overlap or cross-over portions1310of the first and second portions1304,1306. The dielectric gap provides an inductor with an increased Q factor as compared to conventional inductors in which overlap or cross-over portions are separated from each other by other dielectric materials (e.g., a dielectric substrate material).

Although the invention has been illustrated and described with respect to one or more implementations, equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application. Thus, the breadth and scope of the present invention should not be limited by any of the above described embodiments. Rather, the scope of the invention should be defined in accordance with the following claims and their equivalents.