Abstract:
Tunable MEMS resonators having adjustable resonance frequency and capable of handling large signals are described. In one exemplary design, a tunable MEMS resonator includes (i) a first part having a cavity and a post and (ii) a second part mated to the first part and including a movable layer located under the post. Each part may be covered with a metal layer on the surface facing the other part. The movable plate may be mechanically moved by a DC voltage to vary the resonance frequency of the MEMS resonator. The cavity may have a rectangular or circular shape and may be empty or filled with a dielectric material. The post may be positioned in the middle of the cavity. The movable plate may be attached to the second part (i) via an anchor and operated as a cantilever or (ii) via two anchors and operated as a bridge.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. application Ser. No. 12/473,882, filed May 28, 2009 and scheduled to issue on Jan. 29, 2013 as U.S. Pat. No. 8,363,380. The disclosure of U.S. application Ser. No. 12/473,882 is hereby incorporated by reference in its entirety. 
     
    
     FIELD 
       [0002]    The present disclosure relates generally to electronics, and more specifically to micro-electro-mechanical system (MEMS) varactors. 
       DESCRIPTION OF THE RELATED TECHNOLOGY 
       [0003]    MEMS is a technology used to form miniature electro-mechanical devices with mechanical moving parts. These devices may be used to implement various radio frequency (RF) circuit components such as variable capacitors (varactors), switches, resonators, inductors, etc. MEMS devices may have certain advantages over RF circuit components fabricated in other manners, such as higher quality factor (Q), lower insertion loss, better linearity, etc. 
         [0004]    A MEMS varactor typically includes two terminals or electrodes. One terminal is typically used for a common terminal, which may be for circuit ground or some other common connection. The other terminal may be used for both an RF signal and a direct current (DC) voltage. The DC voltage may be varied to mechanically move a plate within the MEMS varactor, which may then adjust the capacitance of the MEMS varactor. The RF signal may be passed through the MEMS varactor and may have its characteristics (e.g., frequency, amplitude, etc.) altered by the capacitance of the MEMS varactor. 
         [0005]    The 2-terminal MEMS varactor described above may be used for a low-power application with a small RF signal. In this case, the capacitance of the MEMS varactor may not be varied too much by the RF signal. However, the RF signal may be relatively large for a high-power application, such as a transmitter of a wireless communication device. If a large RF signal is applied to the MEMS varactor, then the capacitance of the MEMS varactor may be varied by a large amount due to a large root mean square (RMS) voltage of the RF signal, which may be undesirable. A MEMS varactor that can handle a large RF signal, with little or acceptable changes in capacitance due to the large RF signal, would be desirable. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIGS. 1 and 2  show a MEMS varactor with three terminals. 
           [0007]      FIGS. 3A to 3D  show different operational modes of the MEMS varactor in  FIGS. 1 and 2 . 
           [0008]      FIGS. 4 and 5  show another MEMS varactor with three terminals. 
           [0009]      FIGS. 6A to 6D  show different schemes for controlling changes to the capacitance of a MEMS varactor for a large RF signal. 
           [0010]      FIGS. 7 and 8  show two MEMS varactors with vertically stacked plates. 
           [0011]      FIG. 9  shows a block diagram of a wireless communication device. 
           [0012]      FIG. 10  shows a notch filter implemented with MEMS varactors. 
           [0013]      FIG. 11  shows a process for operating a MEMS varactor. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any design described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other designs. 
         [0015]    Various exemplary designs of MEMS varactors are described herein. These MEMS varactors may be used for various circuits such as tunable filters, tunable antennas, etc. Some of these MEMS varactors may be able to handle large RF signals and may be used for high-power applications. For example, the MEMS varactors may be used for a transmitter of a wireless communication device, which may be required to provide a large output power, e.g., 36 dBm for a power amplifier in GSM. The MEMS varactors may be able to handle a large RF signal and may have a small change in capacitance due to the large RF signal. Some of the MEMS varactors may also be able to achieve a high capacitance tuning range. 
         [0016]    In an aspect, MEMS varactors with three or more terminals may be implemented with a horizontal structure. For a MEMS varactor with the horizontal structure, bottom plates for an RF signal and a DC voltage may be formed on the same level. A movable top plate may be formed over the bottom plates and may be mechanically moved to vary the capacitance of the MEMS varactor. 
         [0017]      FIG. 1  shows a top view of an exemplary design of a MEMS varactor  100  implemented with the horizontal structure. MEMS varactor  100  includes three terminals. Different operating modes may be supported by applying an RF signal and a DC voltage to the three terminals in different manners, as described below. 
         [0018]      FIG. 2  shows a cross-sectional view of MEMS varactor  100  in  FIG. 1 . The cross-sectional view in  FIG. 2  is taken along line A-A′ in  FIG. 1 . 
         [0019]    As shown in  FIGS. 1 and 2 , MEMS varactor  100  includes a center bottom plate  120  and side bottom plates  130  and  132  formed on top of a substrate  110 . Substrate  110  may be glass, silicon, or some other material. Glass may have better performance as well as lower cost. Center bottom plate  120  may be formed in a metal layer or some other conductive layer. Side bottom plates  130  and  132  may also be formed in the metal layer or some other conductive layer. Center bottom plate  120  and side bottom plates  130  and  132  may be formed in the same layer (as shown in  FIG. 2 ) or in different layers. An insulation layer  140  may be formed over bottom plates  120 ,  130  and  132  with dielectric or some other non-conductive material that can provide electrical insulation. 
         [0020]    In the exemplary design shown in  FIGS. 1 and 2 , posts  142  and  144  may be formed over insulation layer  140  outside of bottom plates  130  and  132 , respectively. In another exemplary design, bottom plates  130  and/or  132  may extend underneath posts  142  and/or  144 , respectively. In any case, posts  142  and  144  may be formed with oxide or some other material. A moveable top plate  150  may be formed over posts  142  and  144  and may be separated from bottom plates  120 ,  130  and  132  by a gap  152 . Top plate  150  may be formed with a conductive material and may also be referred to as a mechanical membrane, a mechanical electrode, etc. 
         [0021]    As shown in  FIG. 1 , a first terminal  112  may be formed on one side of center bottom plate  120 . Side bottom plates  130  and  132  may be connected by a conductor  136 , and a second terminal  114  may be formed on conductor  136 . A third terminal  116  may be formed on one side of top plate  150 . 
         [0022]    MEMS varactor  100  operates as follows. A fixed DC voltage may be applied to terminal  116 . A variable DC voltage may be applied to terminal  112  or  114 . The voltage difference between the variable DC voltage applied to terminal  112  or  114  and the fixed DC voltage applied to terminal  116  causes top plate  150  to move down. A large voltage difference would cause top plate  150  to move down more, which would then result in a larger capacitance for MEMS varactor  100 . The converse would be true for a smaller voltage difference. 
         [0023]    For example, terminal  116  may be coupled to circuit ground, and a variable DC voltage may be applied to terminal  112  or  114 . A smallest capacitance C min  may be obtained with zero Volts applied to terminal  112  or  114 , which would cause top plate  150  to rest at its normal position that is farthest away from bottom plates  120 ,  130  and  132 . A largest capacitance C max  may be obtained with a sufficient voltage applied to terminal  112  or  114 , which would cause top plate  150  to move toward bottom plates  120 ,  130  and  132  and rest on insulation layer  140 . The voltage used to obtain C max  is referred to as a pull-in voltage V pull-in . 
         [0024]      FIG. 3A  shows a schematic diagram of MEMS varactor  100 . Center bottom plate  120  is coupled to terminal  112 , side bottom plates  130  and  132  are both coupled to terminal  114 , and top plate  150  is coupled to terminal  116 . An RF signal and a DC voltage may be applied to terminals  112  and  114  in several manners, as described below. 
         [0025]      FIG. 3B  shows a first operational mode (mode  1 ) for MEMS varactor  100 . In this mode, an RF signal is applied to terminal  112  coupled to center bottom plate  120 . A DC voltage is applied to terminal  114  coupled to side bottom plates  130  and  132 . The first mode may be used to obtain a large capacitance tuning range, i.e., a large C max /C min . 
         [0026]      FIG. 3C  shows a second operational mode (mode  2 ) for MEMS varactor  100 . In this mode, a DC voltage is applied to terminal  112  coupled to center bottom plate  120 . An RF signal is applied to terminal  114  coupled to side bottom plates  130  and  132 . Movable top plate  150  is more stiff at the two ends near posts  142  and  144  and is less stiff toward the central area. Thus, top plate  150  may move less when a large RF signal is applied to side bottom plates  130  and  132 , due to greater stiffness of top plate  150  at the two ends. This may result in less change in capacitance due to the larger RF signal. Operating MEMS varactor  100  in the second mode may result in less sensitivity to higher power (or higher RMS voltage) of the RF signal. 
         [0027]      FIG. 3D  shows a third operational mode (mode  3 ) for MEMS varactor  100 . In this mode, an RF signal and a DC voltage are both applied to terminal  112  coupled to center bottom plate  120 . Because movable top plate  150  is less stiff toward the central area, a smaller DC voltage may be used to obtain C max  in the third mode. 
         [0028]    Table 1 summarizes the three operational modes for MEMS varactor  100 . Table 1 also gives C min , C max , capacitance tuning range (C max /C min ), and pull-in voltage V pull-in  for each of the three modes for an exemplary design of MEMS varactor  100 . 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Mode 1 
                 Mode 2 
                 Mode 3 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 RF signal applied to . . . 
                 Terminal 
                 Terminal 114 
                 Terminal 112 
               
               
                   
                 112 
               
               
                 DC voltage applied to . . . 
                 Terminal 
                 Terminal 112 
                 Terminal 112 
               
               
                   
                 114 
               
               
                 C min  (with 0 V applied) 
                 0.05 pF 
                  0.1 pF 
                 0.08 pF 
               
               
                 C max  (with V pull-in  applied) 
                 0.91 pF 
                 0.16 pF 
                 0.94 pF 
               
               
                 Capacitance tuning range 
                 18.2 
                 1.6 
                 11.8 
               
               
                 (C max /C min ) 
               
               
                 Pull-in voltage V pull-in   
                 16.0 V 
                  8.5 V 
                  6.2 V 
               
               
                   
               
             
          
         
       
     
         [0029]    In general, C min  and C max  may be dependent on the size of the bottom plate(s) to which the RF signal is applied. A larger capacitance may be obtained with a larger plate size, and vice versa. C min  may be obtained with the top plate being farthest from the bottom plate(s) and may be further dependent on the gap distance between the top plate and the bottom plate(s). Smaller C min  may be obtained with a larger gap, and vice versa. 
         [0030]    In the first mode, C min  and C max  may be determined by the size of bottom plate  120 , and C min  may be determined further by the gap distance between bottom plate  120  and top plate  150 . In the second mode, C min  and C max  may be determined mostly by the size of bottom plates  130  and  132 . In the third mode, C min  and C max  may be determined by the size of bottom plate  120 , and C min  may be determined further by the gap distance between bottom plate  120  and top plate  150 . The desired C min  and C max  may be obtained (i) with an appropriate size for the bottom plate(s) to which the RF signal is applied, (ii) with an appropriate gap distance between the bottom plate(s) and the top plate, and/or (iii) by varying other characteristics or parameters of MEMS varactor  100 . 
         [0031]    As shown in Table 1, the first mode may be well suited for binary/digital applications, e.g., with the RF signal being switched on and off. The second mode may be well suited for high-power applications. The third mode may be well suited for low-bias applications. 
         [0032]      FIG. 4  shows a top view of an exemplary design of a MEMS varactor  400  with three terminals. 
         [0033]      FIG. 5  shows a cross-sectional view of MEMS varactor  400  in  FIG. 4 . The cross-sectional view in  FIG. 5  is taken along line A-A′ in  FIG. 4 . 
         [0034]    As shown in  FIGS. 4 and 5 , MEMS varactor  400  includes a first set of bottom plates  420 ,  422  and  424  and a second set of bottom plates  430  and  432  formed on top of a substrate  410 . Substrate  410  may be glass or some other material. Bottom plates  420  to  432  may be formed in a metal layer or some other conductive layer. An insulation layer  440  may be formed over bottom plates  420  to  432  with a non-conductive material. 
         [0035]    Posts  442  and  444  may be formed with oxide or some other material over insulation layer  440  outside of bottom plates  420  and  424 , respectively. A moveable top plate  450  may be formed with a conductive material over posts  442  and  444  and may be separated from bottom plates  420  to  432  by a gap  452 . Top plate  450  may move down when a DC voltage is applied, may be stiffer at the two ends near posts  442  and  444 , and may be less stiff near the central area. 
         [0036]    As shown in  FIG. 4 , a conductor  428  may connect bottom plates  420 ,  422  and  424  and may be coupled to a first terminal  412 . A conductor  438  may connect bottom plates  430  and  432  and may be coupled to a second terminal  414 . A third terminal  416  may be formed on one side of top plate  450 . 
         [0037]    In a first mode, an RF signal may be applied to terminal  412 , and a DC voltage may be applied to terminal  414 . In a second mode, the RF signal may be applied to terminal  414 , and the DC voltage may be applied to terminal  412 . In a third mode, the RF signal and the DC voltage may both be applied to terminal  412 . In a fourth mode, the RF signal and the DC voltage may both be applied to terminal  414 . Different varactor characteristics (e.g., C min , C max , capacitance tuning range, and V pull-in ) may be obtained for the four modes. 
         [0038]      FIGS. 1 and 2  show an exemplary design of MEMS varactor  100  with three bottom plates  120 ,  130  and  132  formed under top plate  150 .  FIGS. 4 and 5  show an exemplary design of MEMS varactor  400  with five bottom plates  420 ,  422 ,  424 ,  430  and  432  formed under top plate  450 . In general, any number of bottom plates may be formed under a top plate. More bottom plates may provide more freedom to obtain the desired varactor characteristics and may also allow for greater control of capacitance change due to signal swing. The bottom plate(s) for the RF signal and the bottom plate(s) for the DC voltage may be arranged in a comb-like structure, as shown in  FIGS. 1 ,  2 ,  4  and  5 , or may be arranged in other manners, e.g., with circular shape structures. 
         [0039]      FIG. 6A  shows an exemplary design for controlling changes in capacitance due to electro-static force from a large RF signal applied to MEMS varactor  400  in  FIG. 4 . In this exemplary design, the capacitance change may be controlled by selecting suitable sizes for the bottom plates and exploiting the greater stiffness of the top plate near the two posts. In particular, less capacitance change due to the large RF signal may be obtained by (i) applying the RF signal to bottom plates  430  and  432 , which are formed under a stiffer portion of top plate  450  than bottom plate  422 , and (ii) forming bottom plates  430  and  432  with smaller areas than bottom plates  420 ,  422  and  424  for the DC voltage. 
         [0040]      FIG. 6B  shows another exemplary design for controlling changes in capacitance due to electro-static force from a large RF signal applied to MEMS varactor  400  in  FIG. 4 . In this exemplary design, the capacitance change may be controlled by selecting suitable gap distance between the top plate and the bottom plates. In particular, less capacitance change due to the large RF signal may be obtained by having (i) a larger gap for bottom plates  430  and  432  to which the RF signal is applied and (ii) a smaller gap for bottom plates  420 ,  422  and  424  to which the DC voltage is applied. In general, a larger gap for the bottom plates for the RF signal may result in smaller capacitance change due to the RF signal. 
         [0041]      FIG. 6C  shows an exemplary design for obtaining larger capacitance and/or larger capacitance tuning range for MEMS varactor  400  in  FIG. 4 . In this exemplary design, the capacitance and/or capacitance tuning range may be increased by having (i) a larger gap for bottom plates  420 ,  422  and  424  to which the DC voltage is applied and (ii) a smaller gap for bottom plates  430  and  432  to which the RF signal is applied. 
         [0042]      FIG. 6D  shows another exemplary design for controlling changes in capacitance due to electro-static force from a large RF signal. In this exemplary design, a thin film resistor (TFR)  460  may be formed between bottom plate  420  and substrate  410 . A thin film resistor  464  may be formed between bottom plate  424  and substrate  410 . A thin film resistor  462  may be formed on substrate  410  instead of bottom plate  422 . A planarization layer  470  may be formed over bottom plates  420  to  432  with spin-coating techniques and may provide electrical isolation. 
         [0043]    As shown in  FIG. 6D , the gap may vary across top plate  450 , with bottom plates progressively closer to the center of the top plate having progressively larger gap. This may decrease the DC pull-in voltage and improve the tuning ratio. 
         [0044]    In another aspect, MEMS varactors with three or more terminals may be implemented with a vertical structure. For a MEMS varactor with the vertical structure, three (or possibly more) plates may be stacked vertically (i.e., placed in parallel) and coupled to three (or possibly more) terminals. A middle plate may be mechanically moved to vary the capacitance of the MEMS varactor. 
         [0045]      FIG. 7  shows a cross-sectional view of an exemplary design of a MEMS varactor  700  implemented with the vertical structure. MEMS varactor  700  includes a bottom plate  720  formed on top of a substrate  710 . Substrate  710  may be glass or some other material. Bottom plates  720  may be formed in a metal layer or some other conductive layer. An insulation layer  722  may be formed over bottom plate  720  with a non-conductive material. A moveable middle plate  730  may be formed with a conductive material over bottom plate  720  and may be separated from the bottom plate by a gap  732 . A top plate  740  may be formed with a conductive material over middle plate  730  and may be separated from the middle plate by a gap  734 . An insulation layer  742  may be formed below top plate  740  with a non-conductive material. 
         [0046]    In the exemplary design shown in  FIG. 7 , a terminal  712  may be formed on one end of bottom plate  720 , a terminal  714  may be formed on one end of middle plate  730 , and a terminal  716  may be formed on one end of top plate  740 . A first DC voltage V B1  may be applied via an RF choke  762  (or a resistor) to terminal  712 . An input RF signal (RFin) may be applied via a DC blocking capacitor  764  to terminal  712 . A second DC voltage V B2  may be applied via an RF choke  766  (or a resistor) to terminal  716 . An output RF signal (RFout) may be provided via terminal  714 . 
         [0047]    Middle plate  730  may move up or down due to the DC voltages applied to bottom plate  720  and top plate  740 . Insulation layers  722  and  742  prevent middle plate  730  from shorting to bottom plate  720  or top plate  740 , respectively. A variable capacitor C 1  may be formed between middle plate  730  and bottom plate  720 . The capacitance of C 1  may be determined by the sizes of middle plate  730  and bottom plate  720  as well as the gap distance between these two plates. 
         [0048]    Top plate  740  may be used to compensate or reduce capacitance change due to the input RF signal applied to bottom plate  720 . A power detection unit may measure the signal swing of the input RF signal. The second DC voltage may be adjusted based on the measured RF signal swing. For example, a larger input RF signal may pull middle plate  730  toward bottom plate  720  and may increase the capacitance C 1 . A larger DC voltage may then be applied to top plate  740  to pull middle plate  730  toward top plate  740  and counter the pull by the larger input RF signal. 
         [0049]      FIG. 8  shows a cross-sectional view of an exemplary design of a MEMS varactor  800  implemented with the vertical structure. MEMS varactor  800  includes a bottom plate  820  formed over a substrate  810 , a movable middle plate  830 , a top plate  840 , and insulation layers  822  and  842 , as described above for MEMS varactor  700  in  FIG. 7 . 
         [0050]    A terminal  812  may be formed on one end of bottom plate  820 , a terminal  814  may be formed on one end of middle plate  830 , and a terminal  816  may be formed on one end of top plate  840 . A first DC voltage V B1  may be applied via an RF choke  862  to terminal  812 . An input RF signal may be applied via a DC blocking capacitor  864  to terminal  814 . A second DC voltage V B2  may be applied via an RF choke  866  to terminal  716 . An output RF signal may be provided via DC blocking capacitors  872  and  874 , which may be coupled to bottom plate  820  and top plate  840 , respectively. 
         [0051]    Middle plate  830  may move up or down due to the DC voltages applied to bottom plate  820  and top plate  840 . A first variable capacitor C 1  may be formed between middle plate  830  and bottom plate  820 . A second variable capacitor C 2  may be formed between top plate  840  and middle plate  830 . The capacitance of C 1  may be determined by the sizes of plates  820  and  830  as well as the gap distance between these two plates. The capacitance of C 2  may be determined by the sizes of plates  830  and  840  as well as the gap distance between these two plates. The total capacitance between the output RF signal and the input RF signal may be given as C total =C 1 +C 2 . 
         [0052]      FIGS. 1 through 6B  show some exemplary designs of MEMS varactors with the horizontal structure. An RF signal may be applied to bottom plates located near the posts where the top plate has greater stiffness. This may result in smaller impact/deflection due to a large signal swing of the RF signal. Alternatively, the RF signal may be applied to bottom plates located away from the posts. This may result in a larger capacitance tuning range. 
         [0053]      FIGS. 7 and 8  show some exemplary designs of MEMS varactors with the vertical structure. An RF signal may be applied to a bottom plate or a middle plate of an MEMS varactor. A second DC voltage may be applied to a top plate formed over the movable plate to compensate for and reduce capacitance change due to a large signal swing of the RF signal. 
         [0054]    The MEMS varactors described herein may provide certain advantages over conventional MEMS varactors. First, the MEMS varactors described herein may be able to handle a larger signal swing. This capability may be especially beneficial for a high-power application such as a transmitter of a wireless communication device. Second, the capacitance tuning range may be controlled independently by a DC voltage with the horizontal structure. A larger capacitance tuning range may be obtained with the vertical structure. For both the horizontal and vertical structures, the varactor characteristics may also be controlled by selecting appropriate plate sizes, plate thickness, and gap distance, e.g., as described above for  FIGS. 6A to 6D . The vertical structure may have other advantages, such as better thermal stability, which may be desirable for practical product applications. Furthermore, varactor capacitance change due to temperature may be compensated with the top and the bottom plates. In comparison to a varactor with a single electrode (which can only increase the varactor capacitance), the vertical structure can compensate for both capacitance increase/decrease due to increase/decrease of temperature. 
         [0055]    The MEMS varactors described herein may be used for various electronics devices such as wireless communication devices, cellular phones, personal digital assistants (PDAs), handheld devices, wireless modems, laptop computers, cordless phones, broadcast receivers, Bluetooth devices, consumer electronics devices, etc. The use of the MEMS varactors in a wireless communication device, which may be a cellular phone or some other device, is described below. 
         [0056]      FIG. 9  shows a block diagram of an exemplary design of a wireless communication device  900 . In this exemplary design, wireless device  900  includes a data processor  910  and a transceiver  920 . Transceiver  920  includes a transmitter  930  and a receiver  950  that support bi-directional wireless communication. In general, wireless device  900  may include any number of transmitters and any number of receivers for any number of communication systems and any number of frequency bands. 
         [0057]    In the transmit path, data processor  910  processes data to be transmitted and provides an analog output signal to transmitter  930 . Within transmitter  930 , the analog output signal is amplified by an amplifier (Amp)  932 , filtered by a lowpass filter  934  to remove images caused by digital-to-analog conversion, amplified by a variable gain amplifier (VGA)  936 , and upconverted from baseband to RF by an upconverter  938 . The upconverted signal is amplified by a power amplifier (PA)  940 , further filtered by a filter  942  to remove images caused by the frequency upconversion, routed through a duplexer/switch  944 , and transmitted via an antenna  946 . Filter  942  may be implemented with a MEMS notch filter that can handle high power from PA  940 . Filter  942  may be located after PA  940  (as shown in  FIG. 9 ) or prior to PA  940 . 
         [0058]    In the receive path, antenna  946  receives signals from base stations and provides a received signal, which is routed through duplexer/switch  944  and provided to receiver  950 . Within receiver  950 , the received signal is amplified by a low noise amplifier (LNA)  952 , filtered by a bandpass filter  954 , and downconverted from RF to baseband by a downconverter  956 . The downconverted signal is amplified by a VGA  958 , filtered by a lowpass filter  960 , and amplified by an amplifier  962  to obtain an analog input signal, which is provided to data processor  910 . 
         [0059]      FIG. 9  shows transmitter  930  and receiver  950  implementing a direct-conversion architecture, which frequency converts a signal between RF and baseband in one stage. Transmitter  930  and/or receiver  950  may also implement a super-heterodyne architecture, which frequency converts a signal between RF and baseband in multiple stages. A local oscillator (LO) generator  970  generates and provides transmit LO signals for upconverter  938  and receive LO signals for downconverter  956 . A phase locked loop (PLL)  972  receives control information from data processor  910  and provides control signals to LO generator  970  to generate the transmit and receive LO signals at the proper frequencies. 
         [0060]      FIG. 9  shows an exemplary transceiver design. In general, the conditioning of the signals in transmitter  930  and receiver  950  may be performed by one or more stages of amplifier, filter, mixer, etc. These circuits may be arranged differently from the configuration shown in  FIG. 9 . Furthermore, other circuits not shown in  FIG. 9  may also be used to condition the signals in the transmitter and receiver. Some circuits in  FIG. 9  may also be omitted. All or a portion of transceiver  920  may be implemented on an analog integrated circuit (IC), an RF IC (RFIC), a mixed-signal IC, etc. 
         [0061]    Data processor  910  may perform various functions for wireless device  900 , e.g., processing for transmitted and received data. A memory  912  may store program codes and data for data processor  910 . Data processor  910  may be implemented on one or more application specific integrated circuits (ASICs) and/or other ICs. 
         [0062]    As shown in  FIG. 9 , a transmitter and a receiver may include various analog circuits. Each analog circuit may be implemented in various manners and may include one or more MEMS varactors described herein. For example, MEMS varactors may be used in power amplifier  940 , filter  942 , duplexer/switch  944 , LO generator  970 , etc. 
         [0063]      FIG. 10  shows a schematic diagram of an exemplary design of a notch filter  1000  implemented with MEMS varactors. Notch filter  1000  may be used for filter  942  in  FIG. 9 . Within notch filter  1000 , an input RF signal (RFin) is provided to node A, and an output RF signal (RFout) is provided via node C. A capacitor  1012  is coupled between node A and circuit ground, and a capacitor  1014  is coupled between nodes A and B. An inductor  1016  and a MEMS varactor  1018  are coupled in parallel between node B and circuit ground and form a first resonator. An inductor  1020  is coupled between nodes A and C. A capacitor  1022  is coupled between node C and circuit ground, and a capacitor  1024  is coupled between nodes C and D. An inductor  1026  and a MEMS varactor  1028  are coupled in parallel between node D and circuit ground and form a second resonator. 
         [0064]    MEMS varactors  1018  and  1028  may each be implemented with MEMS varactor  100 ,  400 ,  700  or  800  in  FIG. 1 ,  4 ,  7  or  8 , respectively. This may then allow notch filter  1000  to handle a large input RF signal and provide good performance. For example, notch filter  1000  may be used for GSM, CDMA, WCDMA or some other cellular applications, and MEMS varactors  1018  and  1028  may need to handle high RF power in a range of 28 to 35 dBm (or 0.6 to 3.2 Watt), which may translate to a peak voltage in the range of about 30 to 75V. In this case, the MEMS varactors should have a pull-in voltage of greater than 30V for CDMA/WCDMA and greater than 75V for GSM. This may allow the capacitance of the MEMS varactor to be tuned mostly by the DC voltage and to not be significantly influenced by a large RF signal. 
         [0065]    In an aspect, a MEMS varactor may comprise first and second bottom plates and a top plate. The first bottom plate (e.g., bottom plate  120  or  130  in  FIG. 1 ) may be electrically coupled to a first terminal, which may receive an input signal. The second bottom plate (e.g., bottom plate  130  or  120 ) may be electrically coupled to a second terminal, which may receive a DC voltage. The top plate (e.g., top plate  150 ) may be formed over the first and second bottom plates and may be electrically coupled to a third terminal. The DC voltage may cause the top plate to mechanically move and vary the capacitance observed by the input signal. The input signal may be an RF signal or a signal of some other type. The input signal may have a large signal swing, e.g., of more than 10 Volts. 
         [0066]    In an exemplary design, the MEMS varactor may further comprise a third bottom plate (e.g., bottom plate  132  in  FIG. 1 ) formed under the top plate and electrically coupled to the first terminal. In this exemplary design, which may correspond to mode  2  in  FIG. 3C , the input signal may be applied to the first and third bottom plates, and the DC voltage may be applied to the second bottom plate. The first and third bottom plates (e.g., bottom plates  130  and  132 ) may be formed on two sides of the second bottom plate (e.g., bottom plate  120 ), which may be formed under a central area of the top plate. 
         [0067]    In another exemplary design, the MEMS varactor may further comprise a third bottom plate (e.g., bottom plate  132  in  FIG. 1 ) formed under the top plate and electrically coupled to the second terminal. In this exemplary design, which may correspond to mode  1  in  FIG. 3B , the DC voltage may be applied to the second and third bottom plates, and the input signal may be applied to the first bottom plate. The second and third bottom plates (e.g., bottom plates  130  and  132 ) may be formed on two sides of the first bottom plate (e.g., bottom plate  120 ), which may be formed under the central area of the top plate. 
         [0068]    In yet another exemplary design, at least one additional first bottom plate may be formed under the top plate and electrically coupled to the first terminal, e.g., as shown in  FIGS. 4 and 5 . At least one additional second bottom plate may also be formed under the top plate and electrically coupled to the second terminal, e.g., as also shown in  FIGS. 4 and 5 . 
         [0069]    In an exemplary design, the first bottom plate (which receives the input signal) may have a smaller area than the second plate (which receives the DC voltage) in order to reduce changes in capacitance due to signal swing of the input signal, e.g., as shown in  FIG. 6A . In another exemplary design, the second bottom plate may be thicker than the first bottom plate in order to reduce changes in capacitance due to signal swing of the input signal, e.g., as shown in  FIG. 6B . In yet another exemplary design, bottom plates located progressively closer to the central area of the top plate may have progressively larger gap to the top plate, e.g., as shown in  FIG. 6D . 
         [0070]    In another aspect, a MEMS varactor may comprise first, second and third plates. The first plate (e.g., plate  720  in  FIG. 7  or plate  820  in  FIG. 8 ) may be electrically coupled to a first terminal, which may receive a first DC voltage. The second plate (e.g., plate  730  in  FIG. 7  or plate  830  in  FIG. 8 ) may be formed over the first plate and may be electrically coupled to a second terminal. The third plate (e.g., plate  740  in  FIG. 7  or plate  840  in  FIG. 8 ) may be formed over the second plate and may be electrically coupled to a third terminal, which may receive a second DC voltage. An input signal may be passed between the first and second terminals. The first and second DC voltages may cause the second plate to mechanically move and vary the capacitance observed by the input signal. 
         [0071]    In an exemplary design, the input signal may be applied to the first terminal and capacitively passed to the second terminal, e.g., as shown in  FIG. 7 . In this exemplary design, the input signal may observe the capacitance between the first and second plates. The second DC voltage may be determined based on signal swing of the input signal. In another exemplary design, the input signal may be applied to the second terminal and capacitively passed to both the first and third terminals, e.g., as shown in  FIG. 8 . In this exemplary design, the input signal may observe the capacitance between the first and second plates plus the capacitance between the second and third plates. 
         [0072]    In yet another aspect, an apparatus (e.g., a wireless communication device) may comprise a filter that receives an input signal and provides an output signal. The filter may include a MEMS varactor, which may comprise first, second and third plates. The first plate may be electrically coupled to a first terminal, the second plate may be electrically coupled to a second terminal, and the third plate may be electrically coupled to a third terminal. The MEMS varactor may have a variable capacitance determined by at least one DC voltage applied to at least one of the first, second and third terminals. The filter may further comprise an inductor coupled in parallel with the MEMS varactor and forming a resonator to attenuate the input signal at a designated frequency. 
         [0073]    In an exemplary design, the first and second plates may be formed on a common layer and under the third plate, e.g., as shown in  FIGS. 1 and 2 . The at least one DC voltage may comprise a DC voltage applied to the first terminal to cause the third plate to mechanically move and vary the capacitance of the MEMS varactor. In another exemplary design, the second plate may be being formed over the first plate, and the third plate may be formed over the second plate, e.g., as shown in  FIG. 7  or  8 . The at least one DC voltage may cause the second plate to mechanically move and vary the capacitance of the MEMS varactor. 
         [0074]      FIG. 11  shows an exemplary design of a process  1100  for operating a MEMS varactor. A DC voltage may be applied to a first plate of a MEMS varactor comprising the first plate, a second plate, and a third plate (block  1112 ). An input signal may be applied to at least one of the first, second, and third plates of the MEMS varactor (block  1114 ). The DC voltage may be varied to mechanically move the third plate of the MEMS varactor and vary the capacitance observed by the input signal (block  1116 ). 
         [0075]    In an exemplary design, the input signal may be applied to the second plate of the MEMS varactor. The first and second plates (e.g., plates  120  and  130  in  FIG. 1 ) may be formed under the third plate (e.g., plate  150  in  FIG. 1 ). 
         [0076]    In another exemplary design, the input signal may be applied to the first plate (e.g., plate  720  in  FIG. 7 ). A second DC voltage may be applied to the second plate (e.g., plate  740 ). The third plate (e.g., plate  730 ) may be formed over the first plate, and the second plate may be formed over the third plate. The second DC voltage may be generated based on the signal swing of the input signal. 
         [0077]    In yet another exemplary design, the input signal may be applied to the third plate (e.g., plate  830  in  FIG. 8 ). A second DC voltage may be applied to the second plate (e.g., plate  840 ). The third plate may be formed over the first plate (e.g., plate  820 ), and the second plate may be formed over the third plate. 
         [0078]    The MEMS varactors described herein may be fabricated with various MEMS process technologies known in the art. The MEMS varactors may be fabricated on a substrate (e.g., a glass or silicon substrate) and may be encapsulated in a suitable package. A substrate with MEMS varactors may also be packaged together with a semiconductor IC die. The MEMS varactors may also be fabricated on a semiconductor IC (e.g., a silicon based CMOS IC, a GaAs or InP based compound semiconductor IC, etc.) using semiconductor process technology. 
         [0079]    An apparatus implementing any of the MEMS varactors described herein may be a stand-alone device or may be part of a larger device. A device may be (i) a stand-alone IC package, (ii) a set of one or more IC packages that may include memory ICs for storing data and/or instructions, (iii) an RFIC such as an RF receiver (RFR) or an RF transmitter/receiver (RTR), (iv) an ASIC such as a mobile station modem (MSM), (v) a module that may be embedded within other devices, (vi) a receiver, cellular phone, wireless device, handset, or mobile unit, (vii) etc. 
         [0080]    In one or more exemplary designs, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on or transmitted over as one or more instructions or code on a computer-readable medium. Computer-readable media includes both computer storage media and communication media including any medium that facilitates transfer of a computer program from one place to another. A storage media may be any available media that can be accessed by a computer. By way of example, and not limitation, such computer-readable media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium that can be used to carry or store desired program code in the form of instructions or data structures and that can be accessed by a computer. Also, any connection is properly termed a computer-readable medium. For example, if the software is transmitted from a website, server, or other remote source using a coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technologies such as infrared, radio, and microwave, then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of medium. Disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk and blu-ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media. 
         [0081]    The previous description of the disclosure is provided to enable any person skilled in the art to make or use the disclosure. Various modifications to the disclosure will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other variations without departing from the scope of the disclosure. Thus, the disclosure is not intended to be limited to the examples and designs described herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.