WO2009092605A1

WO2009092605A1 - Microelectromechanical system tunable capacitor

Info

Publication number: WO2009092605A1
Application number: PCT/EP2009/000442
Authority: WO
Inventors: Conor O'mahony; Martin Hill
Original assignee: University College Cork, National University Of Ireland, Cork
Priority date: 2008-01-25
Filing date: 2009-01-23
Publication date: 2009-07-30

Abstract

The present invention provides a new type of microelectromechanical capacitor, that allows ' a first selective deflection of a movable capacitance electrode to form an arched shape over a fixed capacitance electrode upon the application of an electrostatic force between the movable capacitance electrode and fixed bias electrodes, and a second deflection of the arched movable capacitance electrode upon the application of a second electrostatic force between the movable capacitance electrode and fixed capacitance electrode; such that said first and second deflections control and/or tune the capacitance of said capacitor. By providing the first and second deflections of the movable capacitance electrode allows that (a) the capacitor is continuously tunable and (b) provides a high accurate tuning range. The first and second deflections are used to modify the capacitance between the movable capacitance electrode and fixed capacitance electrode(s). The tunable capacitor of the present invention can be used to optimise the operation of a radio-frequency circuit at multiple frequencies and power levels.

Description

Title

MICROELECTROMECHANICAL SYSTEM TUNABLE CAPACITOR

Introduction The invention relates' to a microelectromechanical systems (MEMS) tunable capacitor or "varactor" and method of controlling same.

Variable capacitors (varactors) are used in RF systems to tune the frequency of Local Oscillators (LO), to tune filters and to optimise the impedance match between circuit stages over a range of frequencies.

At present many systems use solid state silicon or GaAs varactor diodes. These devices operate under reverse bias condition and the junction capacitance is varied by the reverse bias voltage. These devices have a quality factor (Q) of in the range 30-60 with capacitance tuning ranges from 4-6. The quality factor is limited by the diode series resistance and the parasitic capacitance to the substrate. Varactor diodes are also limited in their RF power range to ensure there is no forward conduction and they suffer from harmonic distortion effects. To achieve high Q-factors these devices may require processing steps which are not compatible with CMOS processing and may therefore not be integrated on a single substrate with the CMOS RF circuits.

The most common type of MEMS variable capacitor operates by using electrostatic forces to vary the spacing between the capacitor electrodes. MEMS tunable capacitors offer the possibility to achieve a Q factor in excess of 100 and should achieve tuning ranges comparable to or in excess of semiconductor varactors. In addition, MEMS tunable capacitors should handle more power than varactor diodes as they cannot be forward biased. They also exhibit low harmonic distortion as the device inertia prevents it from responding to RF frequencies which are 10⁴ times the device resonant frequency. Integrated with CMOS circuitry and with MEMS switches, HF frequency filters, phase shifters, inductors and micromechanical resonators, MEMS variable capacitors could contribute towards the goal of developing a single chip radio system. Basic MEMS capacitors consist of a movable structure suspended over a fixed electrode, thereby comprising a capacitor with one movable plate. When a DC polarization voltage, Vp, is applied across the plates of this system, an attractive electrostatic force between them is induced, causing the upper movable electrode to move downwards towards the bottom electrode, until equilibrium between the electrostatic force and mechanical restoring force of the suspensions, is reached. At a certain bias voltage, the restoring force provided by the movable plate anchors no longer balances the electrostatic force and the structure becomes mechanically unstable, causing the top plate to spontaneously collapse onto the bottom electrode. This critical voltage is commonly referred to as the "pull-in voltage", Vpi, and has been well documented in the literature.

This pull-in voltage limits the stable tuning voltage of parallel-plate varactors to 50%, and represents a serious drawback to the use of such devices in telecommunications applications. A number of methods of extending the tuning range have previously been explored.

US6,744,335 describes a MEMS tunable capacitor having a movable structure supported by anchors at both ends. There are three electrodes under the movable structure. Upon application of a tuning voltage across the structure and the outer electrodes the structure deflects so that the central portion of the structure and the central electrode form a tunable capacitor. The arrangement of the electrodes means that a capacitance tuning range of greater than 50% can be achieved.

A thesis "Design and Investigation of Microelectromechanical (MEMS) Varactors" of Maxim Shakhray, 2005, (Saint Petersburg State Polytechnical University, Saint Petersburg, Russia), describes arrangements in which the deflecting structure is cantilevered. It is pulled down in a first phase at its extremity by a starter electrode, and the remainder is deflected down to varying extents according to tuning voltage applied to a working electrode. European Patent Publication Number EP 1 193 215 'Nokia Corporation' describes a tunable capacitor structure which is intended to eliminate the effects of thermal stress, i.e. the device will stay flat when the temperature varies. The capacitance plate is designed to stay rigid and not to bend. The Nokia patent publication describes an arrangement for minimising the effects of thermal mismatch on MEMS and thereby improving reliability. However a problem with the capacitor device disclosed in EP 1 193 215 is that the capacitance is difficult to tune and/or control.

European Patent Publication Number EP 1 562 207 'ST MICROELECTRONICS' discloses a switch that has only two states - on or off. There is no tuning or control associated with the switch and is thus not suitable as a tunable capacitor shaping.

The invention is directed towards providing an improved MEMS varactor to overcome the above mentioned problems.

Summary of the Invention

According to the present invention there is provided, as set out in the appended claims, a micromechanical capacitor, comprising: a substrate; at least one fixed capacitance electrode; at least two fixed bias electrodes, which may be electrically connected; a movable capacitance electrode being disposed opposite said fixed bias electrodes and the fixed capacitance electrode and suspended over said electrodes by a support assembly, such that the movable capacitance electrode and the fixed capacitance electrode form a capacitor; wherein the support assembly allows: a first selective deflection of the movable capacitance electrode to form an arched shape over said fixed capacitance electrode upon the application of an electrostatic force between the movable capacitance electrode and fixed bias electrodes, and a second deflection of the arched movable capacitance electrode upon the application of a second electrostatic force between the movable capacitance electrode and fixed capacitance electrode; such that said first and second deflections control and/or tune the capacitance of said capacitor

The two step-process of the invention allows for a curved arched shape using planar, flat fabrication technologies and electrostatic actuation using a first deflection, and then controllably bend this shape by a second deflection so that the movable capacitance electrode has no instability points, thus forming a tunable capacitor. By providing the first and second deflections of the movable capacitance electrode allows that (a) the capacitor is continuously tunable and (b) provides a high accurate tuning range. The first and second deflections are used to modify the capacitance between the movable capacitance electrode and fixed capacitance electrode(s). The tunable capacitor of the present invention can be used to optimise the operation of a radio-frequency circuit at multiple frequencies and power levels.

In one embodiment, parts of the movable capacitance electrode deflect into contact with the substrate fixed electrode, or topography defined on these regions, upon application of electrostatic forces between the movable electrode and the bias electrodes, and the support assembly limits the deflection of parts of the movable electrode, along one axis of the support assembly, to a displacement less than that of those parts in contact in order to create said arched shape. In one embodiment parts of the fixed electrodes and substrate comprise patterned surfaces with pimples and/or airgaps to reduce dielectric charging, or unwanted effects, that may take place between the movable capacitance electrode and the at least one or more of the bias electrodes.

In a further embodiment the capacitance is varied during a first phase from an initial value to another intermediate value by application of the electrostatic force between the fixed bias electrodes and the movable capacitance electrode to create said arched shape in the movable electrode, wherein said capacitor is further continuously tunable during a second phase to a final value by application of said electrostatic force between the fixed capacitance electrode and arched movable capacitance electrode such that increase in capacitance is achieved by narrowing and lowering the arched shape of the movable capacitance electrode with respect to the fixed capacitance electrode. According to the invention, there is provided a micromechanical capacitor, comprising: a substrate; at least one fixed capacitance electrode; and a movable capacitance electrode disposed opposite said fixed capacitance electrode, such that these two electrodes form a capacitor; wherein the movable capacitance electrode may be deflected upon the application of electrostatic forces between the fixed capacitance electrode and the movable capacitance electrode to change the capacitance between the movable capacitance electrode and the fixed capacitance electrode.

In one embodiment, there are a plurality of resilient supports connected to the movable capacitance electrode such that the deflected shape of the movable capacitance electrode upon the application of electrostatic forces between the movable capacitance electrode and fixed capacitance electrode is defined by the characteristics of the resilient supports.

In one embodiment, the capacitor further comprises: at least one fixed bias electrode, the movable capacitance electrode being disposed opposite said fixed bias electrode and the fixed capacitance electrode and suspended by a plurality of rigid and resilient supports, such that the movable capacitance electrode and the fixed capacitance electrode form a capacitor; wherein the supports allow: the selective deflection of the movable capacitance electrode into an arched shape defined by the characteristics of the resilient supports, the movable capacitance electrode, and the fixed bias electrode upon the application of electrostatic forces between the movable capacitance electrode and fixed bias electrode, and subsequent further deflection of the movable capacitance electrode upon the application of electrostatic forces between the movable capacitance electrode and fixed capacitance electrode; wherein these deflections change the capacitance between the movable capacitance electrode and the fixed capacitance electrode. In one embodiment, the movable capacitance electrode is deformable so that it can repeatably deflect between a position lying flat over the substrate and an arched shape.

In one embodiment, the capacitor characteristics provide a tunable capacitance to optimise operation of a radio-frequency circuit at multiple frequencies and power levels.

In one embodiment, parts of the movable capacitance electrode deflect into contact with the substrate, fixed electrode or topography defined on these regions upon application of electrostatic forces between the movable electrode and the bias electrode, and the resilient supports limit the deflection of some parts of the movable electrode to a displacement less than that of other parts in contact in order to create an arched shape.

In one embodiment, the movable capacitance electrode is coupled to movable bias electrodes so that the movable capacitance electrode can be deflected by application of voltage between the fixed bias electrodes and the movable bias electrodes without voltage on the movable capacitance electrode.

In one embodiment, a part of the movable capacitance electrode opposite the fixed capacitance electrode has limited deflection after an electrostatic force is applied between the fixed electrodes and the movable electrode, and this part of the movable electrode can be further deflected towards the fixed capacitance electrode after application of electrostatic force between the fixed capacitance electrode and movable capacitance electrode.

In one embodiment, capacitance between the movable capacitance electrode and the fixed capacitance electrode is increased by applying electrostatic forces between the movable electrode and the fixed electrode; and this increase in capacitance is achieved by narrowing and lowering arched shape of the movable capacitance electrode.

In one embodiment, the electrostatic force required to achieve an increase in capacitance is defined by the characteristics of the resilient supports, the movable capacitance electrode, the fixed capacitance electrode, and the substrate topography. In one embodiment, the capacitance is switchable during a first phase from an initial value to another intermediate value by application of electrostatic force between the fixed bias electrode and the movable capacitance electrode; and is further continuously tunable during a second phase to a final value by application of electrostatic force between the fixed capacitance electrode and movable capacitance electrode.

In one embodiment, the capacitor does not exhibit pull-in instability during the tuning phase when said first and/or said second electrostatic forces are applied.

In one embodiment, the electrodes are configured so that steps of causing selective deflection of the movable capacitance electrode and tuning of the capacitor take place in multiple stages according to characteristics of the structure and of method of application of electrostatic forces between the electrodes.

In one embodiment, the movable capacitance electrode comprises a beam and the resilient support is located approximately mid-way along the beam.

In one embodiment, the distance between the opposing surfaces of said movable capacitance electrode and said fixed capacitance electrode are on the order of tenths to tens of micrometres.

In one embodiment, said fixed electrodes, said capacitance electrodes, and said movable electrode are fabricated from a combination of thin film metal and thin film insulator layers.

In one embodiment, the resilient support comprises an anchor and an arm extending from the anchor to the movable capacitance electrode.

In one embodiment, there is at least one pair of resilient supports on opposite lateral sides of the movable capacitance electrode. In one embodiment, the forces needed to deflect the movable capacitance electrode and to tune the capacitor are defined by the geometrical shape of the movable capacitance electrode and of the bias electrode.

In another aspect, the invention provides a switchable, tunable micromechanical capacitor, comprising: a substrate; fixed bias electrode(s); fixed capacitance electrode(s); and a movable capacitance electrode disposed opposite both said fixed electrode(s) such that the movable capacitance electrode and the fixed capacitance electrode forms a capacitor; wherein the movable capacitance electrode has the shape of a beam, is supported at each end by rigid anchors attached to the substrate and is supported by resilient support(s) at approximately its mid-point; wherein there is a fixed capacitance electrode under the centre portion of the movable capacitance electrode, and additional fixed bias electrodes on both sides of the fixed capacitance electrode; wherein the movable capacitance electrode may be selectively switched into an arched shape defined by the characteristics of the resilient supports, the movable capacitance electrode, and the fixed bias electrode upon the application of electrostatic forces between the movable capacitance electrode and fixed bias electrode(s), and; wherein parts of the movable capacitance electrode deflect into contact with the substrate, fixed bias electrodes or topography defined on these regions upon application of electrostatic forces between the movable capacitance electrode and fixed bias electrodes, and the resilient supports limit the deflection of other parts of the movable capacitance electrode to a displacement less than that of the parts in contact, and the movable capacitance electrode may be subsequently further deflected upon the application of electrostatic forces between the movable capacitance electrode and fixed capacitance electrode, thereby tuning the value of the capacitance between the capacitance electrodes. In another aspect, the invention provides a system comprising a plurality of micromechanical capacitors of any preceding claim, arranged so that multiple values of capacitance may be achieved by suitable switching or tuning of the capacitors as appropriate.

In another embodiment there is provided a microelectromechanical capacitor comprising: a substrate; at least one fixed capacitance electrode; a movable arched capacitance electrode being disposed opposite said fixed bias electrodes and the fixed capacitance electrode and suspended over said electrodes by a support assembly in an arched shape, such that the movable capacitance electrode and the fixed capacitance electrode form a capacitor; wherein the support assembly allows a deflection of the arched movable capacitance electrode upon the application of an electrostatic force between the movable capacitance electrode and fixed capacitance electrode; such that said deflection controls and/or tunes the capacitance of said capacitor.

In a further embodiment of the invention there is provided a method of controlling and/or tuning a microelectromechanical capacitor comprising the steps of: suspending a movable capacitance electrode over at least two fixed bias electrodes and a fixed capacitance electrode by using a support assembly, such that the movable capacitance electrode and the fixed capacitance electrode form a capacitor; a first deflecting of the movable capacitance electrode to form an arched shape over said fixed capacitance electrode upon the application of a controllable electrostatic force between the movable capacitance electrode and fixed bias electrodes; and a second deflecting of the arched movable capacitance electrode upon the application of a second controllable electrostatic force between the movable capacitance electrode and fixed capacitance electrode, such that said first and second deflecting steps control and/or tune the capacitance of said capacitor.

In one embodiment, the fixed or movable electrodes form part of an electrical circuit for transmission of electrical signals.

Brief Description of the Drawings

The invention will be more clearly understood from the following description of some embodiments thereof, given by way of example only with reference to the accompanying drawings in which: -

Fig. 1 is a perspective view of a MEMS varactor of the invention.

Fig. 2 is a perspective view of the MEMS varactor of the invention where resilient supports lateral to the main device are used to define the deflection profile of the structure under an applied electrostatic force.

Fig. 3 is a perspective view of the MEMS varactor of the invention where the structure is fabricated as a planar structure supported by a combination of rigid and resilient supports and additional electrodes are added near the rigid supports to allow selective deflection of the planar movable electrode to form an arched movable electrode.

Figs. 4(a), 4(b), 5(a), 5(b), 6(a), 6(b), 7(a) and 7(b) show the varactor at various states for progressively increasing capacitance,

Fig. 8 is a plot of capacitance vs. tuning voltage (voltage applied between fixed and movable capacitance electrodes);

Fig. 9 is a set of plots showing electrode shape as a function of tuning voltage;

Fig. 10 is a plot of capacitance vs. actuation voltage (voltage between bias electrodes and movable electrode); Figs. 11 and 12 are views of an alternative varactor of the invention;

Fig. 13 is a diagram of a system comprising an array of varactors of the invention;

Figs. 14 and 15 are plots illustrating operation of the system; and

Fig. 16 is a perspective view of another embodiment of the MEMS varactor of the invention similar to Fig. 3.

Detailed Description of the Drawings

Overview A switchable and tunable micromechanical capacitor comprises a substrate, fixed bias electrode(s); fixed capacitance electrode(s); and a movable capacitance electrode opposite the fixed bias and fixed capacitance electrodes. The movable capacitance electrode is suspended by rigid and resilient supports, such that it forms a capacitor together with the fixed capacitance electrode. The supports allow selective deflection of the movable capacitance electrode into an arched shape defined by the characteristics of the resilient supports, the movable electrode, and the fixed bias electrodes upon the application of electrostatic forces between the movable capacitance electrode and the fixed bias electrodes. There is further deflection of the arched, movable capacitance electrode upon the application of electrostatic forces between the movable capacitance electrode and fixed capacitance electrode(s). These deflections are used to modify the capacitance between the movable capacitance electrode and fixed capacitance electrode(s). The tunable capacitance may be used to optimise the operation of a radio- frequency circuit at multiple frequencies and power levels.

Parts of the movable electrode may be deflected into contact with the substrate, fixed electrode or topography defined on these regions, and the resilient supports limit the deflection of other parts of the movable electrode to a displacement less than that of the parts in contact in order to create an arched shape. Alternatively, the movable capacitance electrode may be fabricated with an inherently arched shape, the ends of the arched shape being in contact with the substrate or topography defined on the substrate.

The movable capacitive electrode may be coupled to movable bias electrodes so that it can be deflected by application of voltage between fixed and movable bias electrodes without voltage on the movable capacitive electrode.

Description of the Embodiments

Referring to Fig. 1 a varactor 1 of the invention comprises a substrate 2 on which an arched movable capacitance electrode 3 is supported at its ends by the contact with the substrate 2 or topographical features on the substrate, and there is a fixed capacitance electrode 4 on the substrate 2 under the movable electrode 3. Application of electrostatic forces causes the electrode 3 to deflect so that the arch shape is lowered and narrowed until it is fully in contact with the fixed electrode 4.

Referring to Fig. 2 a varactor as illustrated in Fig. 1 with an additional pair of resilient supports 5 supporting the movable capacitance electrode 3 mid-way along its length. In this case, the supports 5 apply an additional level of control over the deflection of the movable capacitance electrode 3 towards the fixed capacitance electrode 4.

Referring to Fig. 3 a varactor 1 of the invention comprises a substrate 2 and a support assembly to support a movable capacitor electrode 3 over the substrate 2. A fixed capacitance electrode 4 and two fixed bias electrodes 7 are positioned on the substrate. The support assembly comprises a pair of fixed base supports 6, one positioned at either end of the movable capacitor electrode 3 to form a main axis such that the movable capacitor electrode can be coupled to the base supports 6 to suspend the movable capacitor electrode over the substrate along the main axis. It will be appreciated that the base supports are rigid to form fixed anchors 6 and the movable capacitor electrode 3 can be in the form of a beam that is flexible. The support assembly further comprises a pair of fixed posts 5, each post offset from each other and substantially orthogonal to the main axis, positioned approximately midway along a main axis defined by the pair of fixed base supports 6. A resilient flexible member 8 is coupled to the fixed posts 5. The resilient member 8 forms a support such that the movable capacitor electrode 3 is mechanically attached to the resilient member and supported by the resilient member 8. The resilient member 8 can be a single piece to form a bridge to support the movable capacitor electrode or in two separate parts, each part supporting a side of the movable capacitor electrode 3. The spring constant of the resilient member is selected to be in proportion with the spring constant of the movable capacitor electrode (beam) 3. The spring constant of the resilient member is designed such that the application of a first electrostatic force ensures that the part of the movable capacitor electrode in the region supported by the resilient member 8 does not materially bend (in comparsion to unsupported sections of the beam) and come into contact with the fixed electrode, so that upon application of electrostatic forces between the movable electrode and the bias electrodes, the deflection of part of the movable electrode is limited to a displacement less than that of other parts in order to create an arched shape.

The distance between the fixed capacitance electrode and fixed bias electrodes 7 can also be designed to take account of the spring constant and bending forces when said first and second electrostatic forces are applied. For example, when the first electrostatic force is applied to the two fixed bias electrodes 7 the spring constant of the resilient member 8 prevents the centre of the beam from being pulled towards the substrate, however the first electrostatic force pulls other parts of the beam 3 towards the substrate to define the arch according to a first aspect of the present invention. By appropriately designing the spring constant of the resilient member 8 it is then possible to apply the necessary second electrostatic force such that the second electrostatic force applied controls the shape of the arch over the fixed capacitance electrode to fine tune the microelectromechanical electrode. This construction and methodology allows for control and tuning the microelectromechanical capacitor of the present invention, as described in more detail below. The preferred substrate material is silicon but may also comprise glass, GaAs or other. The substrate may have been previously worked on, e.g. it may contain CMOS circuitry or bulk micromachined components. The anchors 6 and the movable electrode 3 and the resilient supports 5 are surface micromachined from a conductive material such as aluminium, titanium, gold, or a composite layer such as aluminium/oxide. The electrodes 4, 7, are formed from a conductive metal and may be passivated with an insulating layer such as oxide or nitride.

The varactor 1 may be manufactured in an integrated process by carrying out the deposition, etch and selective removal of a series of thin films. This is done using semiconductor-related photolithographic processes obvious to those skilled in the art.

Operation of the varactor shown in Fig.1 , Fig. 2 is the same as for the varactor shown in Fig.3 after sufficient voltage has been applied between the fixed bias electrodes 7, located near the rigid anchors, and the movable electrode 3 to cause the movable electrode to deflect to the arched shape shown in Fig. 4(a). An electrostatic force causes two plates to move together when a voltage is applied between the two plates. The device operation is illustrated in Figs. 4 to 7. The device capacitance is measured between the fixed capacitance electrode 4 and the movable capacitance electrode 3. The tuning voltage is applied between the fixed capacitance electrode 4 and the movable capacitance electrode 3. As the tuning voltage increases the capacitance between the fixed capacitance electrode 4 and the movable capacitance electrode 3 increases, and the increase is achieved by a combination of the reduction of the maximum gap between the arched, movable capacitance electrode 3 and fixed capacitance electrode 4 and an increase in the fraction of the movable electrode 3 which is in contact with the substrate 2 or the fixed capacitive electrode 4 or some layers interposed between the movable electrode 3 and the substrate 2 and the fixed capacitive electrode 4. At maximum tuning voltage the arched portion of the movable electrode 3 is completely deflected into contact with the substrate 2, the fixed capacitive electrode 4 or some layers interposed between the electrode 3 and the substrate 2 and the fixed capacitance electrode 4. This is illustrated in Fig. 7(a) and 7(b).

The key dimensions of the capacitor (for example only) as shown in Figs. 3-7. Dimensions are in micrometres

Table 1.

Referring to Figs. 8 and 9, it is clear that the structure of the varactor provides a much larger capacitance range than a parallel plate model, from 0.5pF to 2.6pF corresponding to a tuning voltage range of 10V.

In more detail, the structure shown in Fig.3 operates as a two-step tunable capacitor where in the first step of operation, a DC actuation voltage in excess of the pull-in voltage is applied between the fixed bias electrodes 7 and the movable electrode 3. The resultant electrostatic force causes the movable electrode 3 to pull-in over the electrodes 7. When the movable electrode 3 is pulled down over the actuation electrode 7 the resilient supports δperpendicular to the movable electrode restrain the centre of the movable electrode from deflecting to contact the surface resulting in an arched-shaped electrode as shown in Fig. 4. The change ih the capacitance between the movable capacitance electrode and the fixed capacitance electrode^' as the applied actuation voltage is increased from OV to a voltage in excess of the pull-in voltage is shown in Fig. 10. In the second step of operation, as a two-phase tunable capacitor, a tuning voltage is applied, as described above, between the fixed capacitance electrode 4 and the movable electrode 3 to deflect the arch- shaped centre region of the movable electrode 3, changing the capacitance between the movable electrode 3 and the fixed capacitance electrode 4. The change in capacitance as a result of increasing tuning voltage is shown Fig. 8.

An alternative tunable capacitor can reduce the voltage required for actuation by separating the actuation area and tunable capacitance area. Figs. 1 1 and 12 show the layout and 3D schematic of the structure. In this device the movable electrode is actuated into the arch-shaped position by applying an actuation voltage to switch structures connected to each end of the movable electrode. The switch structure dimensions are required to exert sufficient force on the movable electrode to actuate it to the arch-shaped position. This allows more flexibility in the design of actuation and tunable capacitance parts of the structure. The device actuation is determined by the switch plate area and support tether characteristics and by the length and width of the movable electrode. The required tunable capacitance characteristic can be achieved by setting the movable electrode length and width, the length of the lower centre electrode and the dimensions of the centre resilient supports. The movable capacitance electrode in this example is in the form of a beam that has a length of 600μm and width of 40μm with lOOxlOOμm² switch plates supported by four tethers of length and width 35μm and 5μm respectively. The tuned capacitor has a length of 390μm and the centre support tethers are 60μm in length and lOμm in width. The structure dimensions can be designed to modify the actuation voltage, base capacitance and tuning characteristic of the device.

After actuation to the arch-shaped profile, with an actuation voltage of 20V, with no voltage applied to the centre electrode, the beam profile is shown in Fig. 11.

The two-step tunable capacitor can be integrated in an array of tuned capacitor elements by connecting capacitors in parallel as shown in Fig. 13. If the capacitors VCl and VC2 of fig. 13 have the tuning and switched capacitance characteristics of the Fig. 8 and Fig. 10 respectively and have independent tuning voltage control then the combined capacitors could have the tuning characteristic shown in Fig. 14. In this figure the phase of operation indicated as A represents the capacitance of the capacitor pair with both VCl and VC2 in the up position (i.e. no actuation voltage applied to the devices). With an actuation voltage applied to VCl the capacitance of the combined capacitive pair can be tuned by varying the tuning voltage applied to VCl as shown in phase B. If the actuated VCl has a maximum tuning voltage applied and VC2 is actuated and has its tuning voltage gradually increased the capacitance of the combination can be tuned as shown in Phase C. If the capacitors VCl and VC2 have a common tuning voltage then the capacitance tuning operates in the same phases A, B and C but has the characteristic shown in Fig. 15.

Referring now to Figure 16 shows another embodiment of the micromechanical capacitor according to the present invention wherein the movable capacitance electrode is mechanically coupled to and electrically isolated from movable bias electrodes (10). A coupling beam (9) is used to mechanically couple but electrically isolate the movable capacitance electrode (3) from the movable bias electrodes (10) so that, the movable assembly (3) can be deflected into an arched shape over the fixed capacitance electrode by application of voltage between the fixed bias electrodes and the movable bias electrodes. In effect the movable capacitance electrode is mechanically coupled to and electrically isolated from at least one movable bias electrode positioned on a surface of, or within, the support assembly. As described above further deflection of the arched assembly and movable capacitance electrode can be achieved by the application of electrostatic forces between the movable capacitance electrode and fixed capacitance electrode. The advantage of the embodiment of Figure 16 is that due to the electrical isolation between the electrodes the biasing voltage applied does not affect the operation of the capacitor.

It will be appreciated that the at least two fixed bias electrodes described with respect to the operation of the invention can be electrically connected with each other to form a single electrical connection, however it is to be understood that the function of the at least two fixed bias electrodes is to provide enough electrostatic force to ensure that the movable capacitance electrode forms the arch shape as hereinbefore described with respect to the description and/or drawings.

It will be further appreciated that the steps of causing selective deflection, described above, of the movable capacitance electrode and tuning of the capacitor take place in multiple stages according to characteristics of the structure and the method of application of electrostatic forces between the electrodes.

The invention offers a micromechanical varactor realised using a novel structure that can be integrated in an RF system to tune the frequency of Local Oscillators (LO), to tune filters and antennae, to optimise the impedance match between circuit stages over a range of frequencies and to allow antenna optimisation. The device also exhibits advantages over existing devices as it has characteristics such as:

- low tuning voltage

- high tuning range

- high quality factor - high base capacitance

- low harmonic distortion, as the device inertia prevents it from responding to RF frequencies

- low sensitivity to residual stress and temperature - inherently switched operation which allows for operation in arrays as described in this document

- no pull- in instability over the complete range of operation

- the device may be fabricated on the same substrate as other passive or active components or may be integrated in a multi-chip system

- the device is suitable for wafer scale packaging

The performance of the capacitor in RF applications may be for efficient RF power transfer through impedance matching with input and output stages.

The invention is not limited to the embodiments described but may be varied in construction and detail.

Claims

1. A microelectromechanical capacitor, comprising: a substrate; at least one fixed capacitance electrode; at least two fixed bias electrodes, which may be electrically connected; a movable capacitance electrode being disposed opposite said fixed bias electrodes and the fixed capacitance electrode and suspended over said electrodes by a support assembly, such that the movable capacitance electrode and the fixed capacitance electrode form a capacitor; wherein the support assembly allows: a first selective deflection of the movable capacitance electrode to form an arched shape over said fixed capacitance electrode upon the application of an electrostatic force between the movable capacitance electrode and fixed bias electrodes, and a second deflection of the arched movable capacitance electrode upon the application of a second electrostatic force between the movable capacitance electrode and fixed capacitance electrode; such that said first and second deflections control and/or tune the capacitance of said capacitor.

2. A microelectromechanical capacitor as claimed in claim 1 wherein parts of the movable capacitance electrode deflect into contact with the substrate fixed electrode(s), or topography defined on these regions, upon application of electrostatic forces between the movable electrode and the bias electrodes, and the support assembly limits the deflection of parts of the movable electrode, along one axis of the support assembly, to a displacement less than that of those parts in contact in order to create said arched shape.

3. A microelectromechanical capacitor as claimed in any preceding claim, wherein the capacitance is varied during a first phase from an initial value to another intermediate value by application of the electrostatic force between the fixed bias electrodes and the movable capacitance electrode to create said arched shape in the movable electrode, wherein said capacitor is further continuously tunable during a second phase to a final value by application of said electrostatic force between the fixed capacitance electrode and arched movable capacitance electrode such that increase in capacitance is achieved by narrowing and lowering the arched shape of the movable capacitance electrode with respect to the fixed capacitance electrode.

4. A microelectromechanical capacitor as claimed in any preceding claim, wherein the movable capacitance electrode is deformable so that it can repeatably deflect between a position lying flat over the substrate and an arched shape.

5. A microelectromechanical capacitor as claimed in any preceding claim, wherein the capacitor characteristics provide a tunable capacitance to optimise operation of a radio-frequency circuit at multiple frequencies and power levels.

6. A microelectromechanical capacitor as claimed in any preceding claim, wherein the movable capacitance electrode is mechanically coupled to and electrically isolated from at least one movable bias electrode positioned on a surface of, or within, said support assembly.

7. A microelectromechanical capacitor as claimed in any preceding claim, wherein capacitance between the arched movable capacitance electrode and the fixed capacitance electrode is increased by applying electrostatic forces between the arched movable capacitance electrode and the fixed capacitance electrode; and this increase in capacitance is achieved by narrowing and lowering the arched shape of the movable capacitance electrode.

8. A microelectromechanical capacitor as claimed in any of claims 1 to 7, wherein the electrostatic force required to achieve an increase in capacitance is defined by the characteristics of the supports, the movable capacitance electrode, the fixed capacitance electrode, and the substrate topography.

9. A microelectromechanical capacitor as claimed in any preceding claim, wherein the capacitor does not exhibit pull-in instability during the tuning phase when said first and/or said second electrostatic forces are applied.

10.A microelectromechanical capacitor as claimed in any preceding claim, wherein the electrodes are configured so that the steps of causing selective deflection of the movable capacitance electrode and tuning of the capacitor take place in multiple stages according to characteristics of the structure and the method of application of electrostatic forces between the electrodes.

1 LA microelectromechanical capacitor as claimed in any of claims 1 to 11, wherein the support assembly comprises a pair of fixed base supports, one positioned at either end of the movable capacitor electrode to form a main axis such that the movable capacitor electrode can be coupled to the base supports to suspend the movable capacitor electrode over the substrate along the main axis.

12.A microelectromechanical capacitor as claimed in claim 11, wherein the base supports are rigid to form fixed anchors and the movable capacitor electrode can be in the form of a beam that is flexible.

13.A microelectromechanical capacitor as claimed in any of claims 1 1 or 12, wherein the support assembly further comprises a pair of fixed posts each post offset from each other and substantially orthogonal to the main axis, positioned approximately midway along the main axis defined by the pair of fixed base supports.

14. A microelectromechanical capacitor as claimed in claim 13 further comprising a resilient flexible member that is coupled to the fixed posts, such that the resilient member forms a support for said movable capacitor electrode.

15.A microelectromechanical capacitor as claimed in claim 14 wherein the spring constant of the resilient member is designed to be in proportion with the spring constant of the movable capacitor electrode, such that upon application of electrostatic forces between the movable electrode and the bias electrodes, the deflection of part of the movable electrode is limited to a displacement less than that of other parts in order to create said arched shape.

16.A microelectromechanical capacitor as claimed in any preceding claim, wherein the distance between the opposing surfaces of said movable capacitance electrode and said fixed capacitance electrode are on the order of tenths to tens of micrometers.

17.A microelectromechanical capacitor as claimed in any preceding claim, wherein said fixed electrodes, said capacitance electrodes, and said movable electrode are fabricated from a combination of thin film metal and thin film insulator layers.

18.A microelectromechanical capacitor as claimed in any preceding claim, wherein the electrostatic forces needed to deflect the movable capacitance electrode and to tune the capacitor are defined by the geometrical shape of the movable capacitance electrode and of the bias electrode.

19. A switchable, tunable microelectromechanical capacitor, comprising: a substrate; fixed bias electrodes; fixed capacitance electrode(s); and a movable capacitance electrode disposed opposite both said fixed electrode(s) such that the movable capacitance electrode and the fixed capacitance electrode forms a capacitor; wherein the movable capacitance electrode has the shape of a beam, is supported at each end by anchors attached to the substrate and is supported by central lateral support(s) at approximately its mid-point; wherein there is a fixed capacitance electrode under the centre portion of the movable capacitance electrode, and additional fixed bias electrodes on both sides of the fixed capacitance electrode; wherein the movable capacitance electrode may be selectively switched into an arched shape defined by the characteristics of the beam, the central lateral supports, the movable capacitance electrode, and the fixed bias electrodes upon the application of electrostatic forces between the movable capacitance electrode and fixed bias electrode(s), and; wherein parts of the movable capacitance electrode deflect into contact with the substrate, fixed bias electrodes or topography defined on these regions upon application of electrostatic forces between the movable capacitance electrode and fixed bias electrodes, and the central lateral supports limit the deflection of the centre parts of the movable capacitance electrode to a displacement less than that of the parts in contact, and the movable capacitance electrode may be subsequently further deflected upon the application of electrostatic forces between the movable capacitance electrode and fixed capacitance electrode, thereby tuning the value of the capacitance between the capacitance electrodes.

20. A system comprising a plurality of microelectromechanical capacitors of any preceding claim, arranged so that multiple values of capacitance may be achieved by suitable switching or tuning of the capacitors as appropriate.

2 LA system as claimed in claim 20, wherein the fixed or movable electrodes form part of an electrical circuit for transmission of electrical signals.

22. A microelectromechanical capacitor comprising: a substrate; at least one fixed capacitance electrode; a movable arched capacitance electrode being disposed opposite said fixed bias electrodes and the fixed capacitance electrode and suspended over said electrodes by a support assembly in an arched shape, such that the movable capacitance electrode and the fixed capacitance electrode form a capacitor; wherein the support assembly allows a deflection of the arched movable capacitance electrode upon the application of an electrostatic force between the movable capacitance electrode and fixed capacitance electrode; such that said deflection controls and/or tunes the capacitance of said capacitor.

23. A method of controlling and/or tuning a microelectromechanical capacitor comprising the steps of: suspending a movable capacitance electrode over at least two fixed bias electrodes and a fixed capacitance electrode by using a support assembly, such that the movable capacitance electrode and the fixed capacitance electrode form a capacitor; a first deflecting of the movable capacitance electrode to form an arched shape over said fixed capacitance electrode upon the application of a controllable electrostatic force between the movable capacitance electrode and fixed bias electrodes; and a second deflecting of the arched movable capacitance electrode upon the application of a second controllable electrostatic force between the movable capacitance electrode and fixed capacitance electrode, such that said first and second deflecting steps control and/or tune the capacitance of said capacitor.