Patent Publication Number: US-2011063068-A1

Title: Thermally actuated rf microelectromechanical systems switch

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 61/243,187 filed on Sep. 17, 2009 which is incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     This invention relates to micro electromechanical systems (MEMS) switches, and more particularly to a thermally actuated radio frequency (RF) MEMS switch. 
     RF MEMS switches have been used extensively in configurable circuits, antennas, and other RF applications. Examples of conventional MEMS switches include a Direct Current (DC) contact type switch and a capacitive type switch. The contact type switch is typically used for switching signal from DC to 60 GHz while capacitive switches are used for switching RF signals ranging between 6 GHz to approximately 120 GHz.  FIG. 1A  is a diagram illustrating a typical cantilever beam RF MEMS switch. As shown in  FIG. 1A , the RF MEMS switch  1  includes a cantilever beam  2  anchored to a substrate  4  via an anchor  6 . The cantilever beam  2  is pulled down by the use of a DC electrode  8  positioned beneath the cantilever beam  2 . When in an on-position, the cantilever beam  2  makes a metal-to-metal contact with an RF transmission line  10  as shown in  FIG. 1A .  FIG. 1B  is a diagram illustrating a conventional capacitive shunt switch in an on-position. As shown in  FIG. 1B , the switch  25  is formed on a substrate  12  and includes a lower electrode and a dielectric layer  15  formed on the lower electrode. A flexible bridge member  19  is anchored via two posts  20  positioned at an input  13  and an output  14 . The flexible bridge member  19  bends due to electrostatic actuation and the capacitance of the switch  25  changes between on and off states. 
     There are several problems associated with these types of switches. For example, the DC contact type switch has contact failures that include increased contact resistance from contamination build-up and shorting failures from micro-welding of the contacts. The capacitive type switch has problems such as the on-off ratio of the switch capacitance is limited by a small distance between two electrodes. The capacitive type switch also has problems with substrate loss and down-state capacitance degradation. 
     SUMMARY 
     To solve the above-identified problems, the present invention provides a low voltage thermally actuated RF MEMS switch and a thermal actuation method used to achieve a reliable, low-voltage switch operation. 
     According to an embodiment of the present invention, a radio frequency (RF) micro electromechanical system (MEMS) switch formed on a substrate. The RF MEMS switch includes a micromechanical member including a flexible switch membrane configured to move between an on state and an off state of the RF MEMS switch. The flexible switch membrane includes a first set of fingers on a sidewall thereof to be vertically coupled with a second set of fingers formed at an output of the RF MEMS switch on the substrate. The switch further includes an actuation member in operable communication with the micromechanical member and configured to thermally actuate the micromechanical member such that the first set of fingers electrically couple with the second set of fingers upon thermal actuation of the micromechanical member to enable transmission of an RF signal. 
     According to another embodiment of the present invention, a method for actuating a radio frequency (RF) micro electromechanical system (MEMS) switch formed on a substrate is provided. The method includes thermally actuating a micromechanical member having a first set of fingers on a sidewall thereof, and vertically coupling the first set of fingers with a second set of fingers formed at an output of the RF MEMS switch on the substrate based on the thermal actuation, to move the RF MEMS switch into an on state, thereby enabling transmission of an RF signal. 
     According to another embodiment of the present invention, a method for fabricating a radio frequency (RF) micro electromechanical system (MEMS) switch on a semiconductor substrate is provided. The method includes forming a plurality of dielectric layers and metal layers between the dielectric layers adjacent to semiconductor circuitry formed on an upper surface of the semiconductor substrate and etching of exposed dielectric material of the dielectric layers formed to form structural side walls of the switch. The structural side walls including a micromechanical member including a plurality of fingers on a sidewall of the micromechanical member to be vertically coupled with fingers formed on the semiconductor substrate. The method further includes depositing of an oxide layer on the structure sidewalls of the switch, and removing a portion of the semiconductor substrate beneath the micromechanical member to release the micromechanical member formed. 
     Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the invention with advantages and features, refer to the description and to the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIGS. 1A and 1B  are diagrams illustrating conventional RF MEMS switch. 
         FIG. 2  is a diagram illustrating a RF MEMS switch that can be implemented within embodiments of the present invention. 
         FIG. 3  is a top view illustrating the RF MEMS switch shown in  FIG. 2 . 
         FIGS. 4A and 4B  are top views, respectively, illustrating an off-position and an on-position of the RF MEMS switch shown in  FIGS. 2 and 3  that can be implemented within embodiments of the present invention. 
         FIG. 5  is a diagram illustrating the multiple layers of the RF MEMS switch shown in  FIGS. 2 and 3  that can be implemented within embodiments of the present invention. 
         FIGS. 6A through 6E  are diagrams illustrating a fabrication process of the RF MEMS switch shown in  FIGS. 2 and 3  before a final releasing operation. 
     
    
    
     The detailed description explains the preferred embodiments of the invention together with advantages and features, by way of example with reference to the drawings. 
     DETAILED DESCRIPTION 
     Turning now to the drawings in greater detail, it will be seen that  FIG. 2  illustrates a radio frequency (RF) microelectromechanical system (MEMS) switch  100  that can be implemented within embodiments of the present invention. In  FIG. 2 , the RF MEMS switch  100  is formed on a substrate  200 , i.e., a complementary metal-oxide semiconductor (CMOS) substrate, for example. However, the present invention is not limited to a CMOS substrate and may be vary as necessary. According to an embodiment of the present invention, the RF MEMS switch  100  is an alternative to a CMOS switch. 
     According to an embodiment of the present invention, the RF MEMS switch  100  is a vertical switch having sidewall coupling for capacitive switching. That is, the RF MEMS switch  100  is a vertically displaceable device which remains parallel to the substrate  200  and is rotatable with respect to the substrate  200 . The RF MEMS switch  100  may be utilized in band-configurable RF circuits such as configurable voltage-controlled oscillators (VCOs) and matching networks where the switch  100  can be used to change the value of the matching network to make it match different frequency points. In addition, the RF MEMS switch  100  may be utilized in configurable filtering arrays, and configurable antenna arrays. 
     According to an embodiment of the present invention, the RF MEMS switch  100  includes a ground plane  30  formed on the substrate  200 . The ground plane  30  has generally a rectangular shape. The ground plane  30  includes a plurality of metal layers (to be discussed below with reference to  FIG. 6 ). A RF inputting end (i.e., Port  1 ) and a RF outputting end (i.e., Port  2 ) are also provided. The plurality of metal layers of the ground plane  30  is etched to form RF inputting end and RF outputting end. The RF inputting end is electrically isolated from the ground plane  30  and the RF MEMS switch  100  is anchored at one side (e.g., at Port  1 ) to the substrate  200 , for example. As shown in  FIG. 2 , the RF MEMS switch  100  is etched such that beneath the RF MEMS switch  100 , a hollow area can be seen where bulk silicon of the substrate  200  has been removed (to be discussed later below). 
     The RF MEMS switch  100  further includes a micromechanical member (i.e., a flexible switch membrane  40 ) configured to be thermally actuated between an on position and an off position of the RF MEMS switch  100 . The RF MEMS switch  100  may be thermally actuated by sending an electrical current to a heating element as discussed below with reference to  FIG. 3 . The on position corresponds to an on-state of the switch  100  wherein the transmission line is closed and can be used for transmitting a RF signal. The off position corresponds to an off-state of the switch  100  where the transmission line is open and may not be used for transmitting a RF signal. 
     Further, the flexible switch membrane  40  is a floating beam that can freely move between both the on-position and off-position in a direction that is perpendicular to the substrate  200 . According to an embodiment of the present invention, the flexible switch membrane  40  comprises a plurality of bimorph beams  42  integrally combined and each comprising a plurality of vias  44  to facilitate a release of the flexible switch membrane  40  from the substrate  200  formed beneath the RF MEMS switch  100 . According to the current embodiment of the present invention, the plurality of bimorph beams  42  together forms an H-shape. Therefore, the RF MEMS switch  100  is in the form of an H-shape. However, the present invention is not limited to an H-shape and the shape may be varied accordingly. 
     The flexible switch membrane  40  comprises a first set of elongated fingers  46   a  on a sidewall  43  thereof for vertical coupling with a second set of fingers  46   b  formed at the RF outputting end (i.e., Port  2 ) of the RF MEMS switch  100 . According to an embodiment of the present invention, the first set of fingers  46   a  are arranged generally parallel to one another at spaced-apart positions and respectively attached to the sidewall  43  of the flexible switch membrane  40 . Likewise, the second set of fingers  46   b  are arranged parallel to one another at spaced-apart positions and respectively attached to the RF outputting end on the substrate  200 . That is, the second set of fingers  46   b  are fixed onto the substrate  200  at the RF outputting end. 
     According to an embodiment of the present invention, a number of fingers in the first set of fingers  46   a  are equal to that of the second set of fingers  46   b . As shown in  FIGS. 2 and 3  the first and second sets of fingers  46   a  and  46   b  each comprises a total of 25 fingers. Further, as shown in  FIG. 2 , the first set of fingers  46   a  and the second set of fingers  46   b  each comprise groups of fingers. Thus, in the current embodiment of the present invention, the first and second sets of fingers  46   a  and  46   b  each include five (5) groups of five (5) fingers (totaling  25  fingers in each set of fingers  46   a  and  46   b ). Further, a predetermined gap is formed between each finger of the first and second set of fingers  46   a  and  46   b  and each finger  46   a ,  46   b  is of a predetermined thickness. 
     According to an embodiment of the present invention, the number of fingers  46   a  and  46   b  may vary. According to an embodiment of the present invention, the fingers  46   a  and  46   b  may include between one to thousands, for example therefore the capacitance of the switch  100  is linearly configurable. For example, if the switch  100  includes six fingers  46   a ,  46   b  then the predetermined gap may be approximately 2 μm and the thickness may be approximately 4.2 μm. Thus, according to an embodiment of the present invention, the capacitance in the on state is approximately 1.02 pf while the capacitance in the off state is approximately 0.17 pf. On the other hand, if the switch  100  includes 12 fingers  46   a ,  46   b  and a predetermined gap of approximately 2 μm and the thickness of approximately 4.2 μm, the capacitance in the on state may be approximately 2 pf while the capacitance in the off state may be approximately 0.29 pf. Thus, the capacitance is configurable by changing the number of fingers  46   a ,  46   b  provided. 
     According to one embodiment of the present invention, further, as shown in  FIG. 3 , the RF MEMS switch  100  further includes an actuation member  50  configured to thermally actuate the flexible switch membrane  40 . The actuation member  50  is provided at RF inputting end. According to an embodiment of the present invention, the actuation member  50  may be a polysilicon heater however the present invention is not limited hereto and any suitable thermal heating device may be used. 
     According to an embodiment of the present invention, the first set of fingers  46   a  are electrically coupled with the second set of fingers  46   b  upon thermal actuation of the flexible switch membrane  40 . An on position and an off position of the RF MEMS switch  100  will now be discussed below with reference to  FIGS. 4A and 4B . 
       FIGS. 4A and 4B  are top views, respectively, illustrating an off-position and an on-position of the RF MEMS switch  100  shown in  FIGS. 2 and 3  that can be implemented within embodiments of the present invention. As shown in  FIG. 4A , when in an off position, the flexible switch membrane  40  is tilted upward in a position parallel to the substrate  200  such that the transmission line is open and an RF signal may not be transmitted. As shown in  FIG. 4B , when thermally actuated via the actuation member  50 , electrical current is applied and causes the flexible switch membrane  40  to bend downward. The bending action causes the flexible switch membrane  40  to tilt relative to the substrate  200  and the fingers  46   a  and  46   b  become interdigitated such that the transmission line is in a closed position to allow the transmission of an RF signal. According to an embodiment of the present invention, the displacement of the flexible switch membrane  40  is determined by temperature variation caused by electrical heating. 
     As an alternative embodiment of the present invention, thermal actuation may be used to turn on the switch  100  and an electrical field (i.e., DC voltage) is used to hold the switch  100  at in the on position. In this embodiment, no DC current is required after the switch  100  is turned on and the overall power consumption may be reduced. 
     As mentioned above, the ground plane  30  is formed of a plurality of metal layers.  FIG. 5  is a diagram illustrating an example of the multiple layers of ground plane  30  of the RF MEMS switch  100 . As shown in  FIG. 5 , the ground plane  30  may include, for example, three dielectric layers (e.g., silicon oxide (SiO 2 ) layers  32   a ,  32   b  and  32   c ) and three metal layers (e.g., aluminum layers  34   a ,  34   b , and  34   c ) however the present invention is not limited hereto, the number of layers may vary, accordingly. Since the dielectric layers  32   a ,  32   b  and  32   c  are of a material having a different thermal coefficient of expansion than that of the metal layers  34   a ,  34   b  and  34   c , the flexible switch membrane  40  bends from the heat upon thermal actuation. A fabrication process will now be described below with reference to  FIGS. 6A through 6E . 
       FIGS. 6A through 6E  are diagrams illustrating a fabrication process of the RF MEMS switch before a final releasing operation. According to an embodiment of the present invention, the RF MEMS switch  100  is CMOS process-compatible and uses a two step maskless reactive ion etching (RIE) technique for post-processing. The top-level metal is used as an etch-resistant mask to define the RF MEMS switch  100 . The RF MEMS switch  100  may be manufactured by using conventional surface micromachining technologies (i.e., by depositing and patterning several layers on a wafer). 
     As shown in  FIG. 6A , CMOS circuitry  300  is formed on an upper surface of the silicon (Si) substrate  200  and the ground plane  30  (i.e., microstructural region  310 ) is disposed on the silicon (Si) substrate  200 . As mentioned above with reference to FIG.  5 , the ground plane  30  may include three dielectric layers (e.g., silicon dioxide (SiO 2 ) layers  32   a ,  32   b  and  32   c ) and three metal layers  34   a ,  34   b  and  34   c  formed of Aluminum (Al), for example. Alternatively, the metal layers  34   a ,  34   b  and  34   c  may be formed of silicon germanium (SiGe), for example. A layer of polysilicon may be formed at an upper surface of the substrate  200 . A backside silicon deep reactive ion etching (RIE) operation and controls the thickness of the switch  100  and forms a cavity that allows the switch  100  to move freely. The multiple metal layer structure is etched to form the RF MEMS switch  100  in  FIGS. 6B through 6E . That is, the RF inputting and outputting ends (e.g., Ports  1  and  2 ) and the flexible switch membrane  40 , the fingers  46   a  and  46   b  as shown in  FIGS. 2 and 3  are etched. 
     In  FIG. 6B , next, exposed dielectric material (e.g., SiO 2 ) of the dielectric layers  32   a ,  32   b  and  32   c  not covered by a top level metal layer is removed via an anisotropic reactive ion etching (RIE) process to form structural side walls of the switch  100 . That is, this etching process forms the flexible switch membrane  40  and the fingers  46   a  and  46   b.    
     In  FIG. 6C , according to an embodiment of the present invention, an oxide material  35  such as silicon dioxide (SiO 2 ) is deposited along the side walls via a conformal plasma enhanced chemical deposition (PECVD) process, for example. This operation increases the impedance between the two electrodes. 
     In  FIG. 6D , the silicon (Si) is then etched to expose Si beneath the flexible switch membrane  40  and in between the groups of fingers  46   a  and  46   b  using CHF 3 /O 2  (for etching SiO 2 ). 
     In  FIG. 6E , an isotropic RIE process using SF6 plasma or XeF2 is then used to remove the bulk silicon of the substrate  200  to release the flexible switch membrane  40 . 
     Embodiments of the present invention provide a thermally actuated RF MEMS switch that is CMOS process compatible and provides linearly configurable capacitance by changing the number of fingers of the switch, thereby increasing the on/off capacitance ratio of the switch. 
     While the preferred embodiment to the invention has been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first described.