Abstract:
According to one embodiment a microelectromechanical (MEMS) switch is disclosed. The MEMS switch includes a substrate, a plurality of actuation electrodes mounted on the substrate, a plurality of bottom electrodes mounted on the substrate, a capacitor having subcomponents mounted on the two or more bottom electrodes and a top bendable electrode mounted on the substrate. The top electrode collapses a first magnitude towards the actuation electrodes whenever a first voltage is applied to one or more of the actuation electrodes and collapses a second magnitude towards the actuation electrodes whenever a second voltage is applied to the actuation electrodes.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to micro-electromechanical systems (MEMS) and, more specifically, the present invention relates to a MEMS varactors.  
       BACKGROUND  
       [0002]     Micro-electromechanical systems (MEMS) devices have a wide variety of applications and are prevalent in commercial products. One type of MEMS device is a MEMS varactor (variable capacitor). A MEMS RF varactor may be used for RF filter frequency tuning to enhance the wireless system&#39;s capability. A tunable RF filter includes one or more MEMS varactors arranged in the filter circuit. The MEMS varactor is ideal for wireless devices because of their low power characteristics and ability to operate in radio frequency ranges. MEMS RF varactors show their promising applications in cellular telephones, wireless computer networks, communication systems, and radar systems. In wireless devices, MEMS RF varactors may be used for tunable antenna, tunable filter banks, etc.  
         [0003]     MEMS varactors may be implemented to provide solutions for achieving capacitance tuning for RF applications, such as tunable filters. Most varactors include a single gap, which limits tuning ratio. Thus, the gap is the same at both capacitor and actuation regions. Such structure has the advantage of simple fabrication. However, the top electrode can only be moved down to approximately one-third of the air gap before the “pull-in” occurs. This causes an abrupt increase of capacitance that cannot be used beyond this point for a continuous tuning application.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]     The present invention will be understood more fully from the detailed description given below and from the accompanying drawings of various embodiments of the invention. The drawings, however, should not be taken to limit the invention to the specific embodiments, but are for explanation and understanding only.  
         [0005]      FIG. 1  illustrates one embodiment of a wireless communications system;  
         [0006]      FIG. 2A  illustrates a cross section view of one embodiment of a RF MEMS varactor;  
         [0007]      FIG. 2B  illustrates a schematic of one embodiment of a RF MEMS varactor;  
         [0008]      FIG. 3  illustrates a top view of one embodiment of a RF MEMS varactor;  
         [0009]      FIG. 4  illustrates a cross section view of another embodiment of a RF MEMS varactor;  
         [0010]      FIG. 5  illustrates a cross section view of yet another embodiment of a RF MEMS varactor;  
         [0011]      FIG. 6  illustrates a cross section view of still another embodiment of a RF MEMS varactor;  
         [0012]      FIG. 7  illustrates a cross section view of another embodiment of a RF MEMS varactor;  
         [0013]      FIG. 8  is a graph illustrating one embodiment of simulation results;  
         [0014]      FIG. 9A  illustrates a cross section view of another embodiment of a RF MEMS varactor;  
         [0015]      FIG. 9B  illustrates a schematic of another embodiment of a RF MEMS varactor;  
         [0016]      FIG. 10  illustrates a top view of one embodiment of a RF MEMS varactor;  
         [0017]      FIG. 11  illustrates a cross section view of another embodiment of a RF MEMS varactor; and  
         [0018]      FIG. 12  illustrates a cross section view of yet another embodiment of a RF MEMS varactor.  
     
    
     DETAILED DESCRIPTION  
       [0019]     A zipper varactor for a MEMS switch is described. Reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment.  
         [0020]     In the following description, numerous details are set forth. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form, rather than in detail, in order to avoid obscuring the present invention.  
         [0021]      FIG. 1  illustrates one embodiment of a wireless system  100 . System  100  includes a RF filter  150 . In one embodiment, RF filter  150  is constructed of several specific inductors and capacitors, which exhibit a specific RF filtering characteristic at desired frequency range. For wireless applications, several filters with various frequency ranges are implemented to increase system  100  performance and functionality. Although shown as a tunable filter, RF filter may be implemented as other types of filters (e.g., a fixed-band filter).  
         [0022]     According to one embodiment, MEMS varactors are included in filter  150  to implement the capacitors. In such an embodiment, the varactor capacitances are adjusted to desired values to tune filter  150  for another frequency range. Voltage source controller  120  is electrically connected to the MEMS varactors.  
         [0023]     In one embodiment, voltage source controller  120  includes logic for selectively supplying voltages to actuation electrodes (not shown) within filter  150  to selectively activate switch  150 . Receiver  130  processes signals that are received at system  100  via antenna  110 . Transmitter  140  generates signals that are to be transmitted from system  100 .  
         [0024]      FIG. 2A  illustrates one embodiment of a RF MEMS varactor  200 , while  FIG. 2B  illustrates one embodiment of a schematic for varactor  200 . Varactor  200  includes a substrate/dielectric  205 , a bottom electrode  210  layered over the substrate  205 , and a top electrode  215  mounted on substrate  205 . Electrode  215  carries RF signal (“Vs”) that is received or transmitted from  150 . According to one embodiment, electrode  215  is a bendable/movable conductive beam that includes a thick metal (e.g., gold).  
         [0025]     Actuation electrodes  230  are also included. Actuation electrodes  230  are mounted on substrate  205 , and allow a signal to pass from electrode  215  upon becoming electrically charged (or actuated). In one embodiment, actuation electrodes  230  are inter-digit actuation electrodes that may be connected and actuated simultaneously for analog applied voltage, or may be actuated separately with individual digital applied voltages. In addition, stoppers  220  are included to maintain a predetermined gap between electrode  215  and electrodes  230  when varactor  200  is in a collapsed state.  
         [0026]     In a further embodiment, actuation electrodes  230  are distributed with several digits under electrode  215 . Each electrode  230  may have different sizes. The actuation region is constructed with physical stoppers  220  to enable collapsing zipping action during actuation. Such a method prevents DC actuation charging since no insulator is used.  
         [0027]     The collapsing action of bendable electrode  215  depends on the voltages applied to actuation electrodes  230 . As discussed above, inter-digit actuation electrodes  230  may be either connected and actuated simultaneously by analog voltage or completely separated by individual digital voltages. In various embodiments, analog actuation voltage is not available due to a system setup issue. Thus, multi-digit electrodes enables multi-stage actuation with separate fixed digital voltages.  
         [0028]     A capacitor is included within varactor  200 , which includes several parallel sub-capacitors which are distributed between the actuation electrodes as shown in  FIG. 3 , which is a top view of varactor  200 . The sub-capacitors (C 1 , C 2 , and C 3 , in this embodiment) have different sizes. The C1 capacitor has the smallest size, which is located corresponding to the lowest spring constant (k 1 ) of electrode  215 . This is because that the smallest capacitor has a smallest self-actuation force and the low k1 constant will be adequate to resist this force created by RF signal.  
         [0029]     In a further embodiment, the C3 capacitor is the largest, and is located corresponding to the highest spring constant (k 3 ) of electrode  215  so that the spring force can resist the larger self-actuation force from this large capacitor. Such an arrangement reduces the unwanted self-actuation at the capacitor region induced by RF signal.  
         [0030]      FIG. 4  illustrates a cross section view of another embodiment of a RF MEMS varactor  200 . In this embodiment, a dielectric layer  330  is deposited on each electrode  210 , and is coupled to the capacitor to increase the total capacitance.  
         [0031]      FIG. 5  illustrates one embodiment of varactor  200  during actuation of the collapsing zipper varactor. As shown in  FIG. 5 , electrode  215  is actuated with the tip collapsing on the first actuation electrode  230 (A). As a result, both C1 and C2 (and slightly on C3) have an increased capacitance due to the reduced air gap with bending of top plate electrode  215 .  
         [0032]     According to one embodiment, the change of capacitance is continuous if all electrodes  230  are connected (i.e., V1=V2=V3=Va) with a single analog actuation voltage (Va). The embodiment of  FIG. 5  may also be achieved by an alternative digital actuation scheme such as V1=Vb, V2=0, V3=0. The V1=Vb causes the beam electrode  215  to collapse at tip of the beam as shown in  FIG. 5 .  
         [0033]      FIG. 6  illustrates another embodiment of varactor  200  during actuation of the collapsing zipper varactor. For the case of all actuation electrodes  230  being connected (e.g., V1=V2=V3=Va), the increase of actuation voltage results in the further collapsing of top beam electrode  215  with an zipping action towards its beam anchor as shown in  FIG. 6 . The C1 capacitor reaches it maximum and does not contribute to the total increase of capacitance further.  
         [0034]     The increase of capacitance continues from the capacitor C 2  and C 3 . Although the beam  215  spring constant increases as the zipping action continues, the total capacitance may still increase linearly since the C2 capacitor is larger in size. The phenomenon shown in  FIG. 6  may also be achieved in the digital actuation scheme by addition of voltage to actuation electrode  230 (B) from  FIG. 5 , e.g., V1=V2=Vb, and V3=0. Note that the capacitance of the varactor illustrated is determined by the air gap defined from the physical stopper  220 . If the stopper  220  height is reduced, the total capacitance may be increased.  
         [0035]      FIG. 7  illustrates yet another embodiment of varactor  200  during actuation of the collapsing zipper varactor. For the case of all actuation electrodes  230  being connected (e.g., V1=V2=V3=Va), as the actuation voltage continues to increase, the top beam electrode  215  further collapses and the largest sub-capacitor C 3  has the major contribution to the further increase of capacitance as shown in  FIG. 7 . For the case of the digit actuation scheme, the occurrence shown in  FIG. 7  is achieved when all the electrodes are applied with the voltage, e.g., V1=V2=V3=Vb.  
         [0036]      FIG. 8  is a graph illustrating one embodiment of simulation results for the collapsing zipper varactor. As shown in  FIG. 8 , the capacitance ranges from approximately 0.28 pF to approximately 0.84 pF. The tuning ratio is approximately 3, which is much larger than the traditional single gap varactor with similar fabrication simplicity. Note that a stopper  220  height of 0.1 um was used in the simulation. With the reduction of stopper  220  height, the total capacitance can be more than 1 pF. Although not optimized, the simulation result also shows the high linearity of capacitance vs. applied voltage.  
         [0037]      FIG. 9A  illustrates a cross section view of another embodiment of a RF MEMS varactor  200  where top beam electrode  215  is made up of a low stress gradient polysilicon in order to achieve the ultra-low-voltage actuation (&lt;3V). In such an embodiment, the main actuation component of top beam  215  is composed of low stress gradient polysilicon for low voltage actuation.  
         [0038]     Further, the portion of electrode  215  not above actuation electrodes  230  is composed of metal  950  (e.g. for low resistivity) and is still used in order to have a high quality factor of capacitance. Note that electrode  215  (polysilicon) in such case is no longer used as part of RF signal path. Electrode  215  is the carrier structure and actuation electrode for varactor  200 . The actuation mechanism is same as the metal beam switch described previously.  FIG. 9B  illustrates one embodiment of a schematic for the varactor  200  shown in  FIG. 9A , and  FIG. 10  illustrates a top view of a varactor  150  with a top beam electrode  215  made up of polysilicon.  
         [0039]      FIG. 11  illustrates a cross section view of yet another embodiment of a RF MEMS varactor  200 . In this embodiment, a clamp-clamp beam type collapsing zipper varactor is implemented, where the top electrode  215  is anchored on both sides. In such an embodiment, the collapsing zipping action occurs from center of the top beam in contrast to the cantilever type varactor shown above with respect to the embodiments described above, where zipping action occurs from the edge of top beam  215 .  
         [0040]      FIG. 12  illustrates a cross section view of yet another embodiment of a RF MEMS varactor  200  where a clamp-clamp beam type collapsing zipper varactor is implemented with the polysilicon top beam electrode  215  described above in  FIGS. 9A, 9B  and  10 .  
         [0041]     The above described the varactor implements a parallel capacitor with a top bendable plate and inter-digit actuation electrodes to achieve high tuning ratio. The top movable/bendable plate is actuated by the actuation electrodes and collapses towards the bottom electrodes with zipping action. The amount of capacitance change can be achieved by either changing the voltage on all the actuation electrodes simultaneously or apply the fixed voltage on separate (inter-digit) actuation electrodes digitally.  
         [0042]     With the collapsing zipping action, the capacitance tuning may be increased continuously along with an increase of collapsing area. With the inter-digit actuation electrode configuration, each electrode can be individually size-optimized to reduce the required actuation voltage. Moreover, the capacitor is also divided into several plates with various sizes depending on the location on the top plate. The size of the separate capacitors can be optimized to increase the capacitance linearity and reduce the self-actuation due to RF signal across the capacitor.  
         [0043]     Whereas many alterations and modifications of the present invention will no doubt become apparent to a person of ordinary skill in the art after having read the foregoing description, it is to be understood that any particular embodiment shown and described by way of illustration is in no way intended to be considered limiting. Therefore, references to details of various embodiments are not intended to limit the scope of the claims, which in themselves recite only those features regarded as the invention.