Abstract:
A radio frequency (RF) microelectromechanical systems (MEMS) switch is manufactured by independent processing and subsequent bonding together of a MEMS substrate in alignment with an RF substrate. The RF MEMS switch is designed so as to encapsulate a flexing diaphragm supporting a switch electrode used with electrostatic flexing potentials to move electrodes of the MEMS substrate up and down over an RF transmission line structure of the RF substrate. The bonded combined MEMS switch structure is used to create an encapsulated RF MEMS switch suitable for direct coupling, AC coupling, and direct modulation of RF signals. The resulting MEMS RF switch device provides a reliable, minimally distorting RF transmission line geometry, free of contamination for use in high speed RF signal switching applications well suited for advance communication RF switching requirements.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention was made with Government support under contract No. F04701-93-C-0094 by the Department of the Air Force. The Government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to the field of radio frequency (RF) devices and semiconductor manufacturing processes. More particularly, the present invention relates to RF switches using semiconductors microelectromechanical systems (MEMS) and semiconductor manufacturing processes. 
     BACKGROUND OF THE INVENTION 
     Radio frequency (RF) devices are commonly used in communication systems where high frequency operation is required. One device used in communication systems is an RF switch that is a mechanical switch switching at high-speed for use in RF communication systems. Microelectromechanical systems (MEMS) are miniature devices that are being manufactured in a wide variety of mechanical forms. MEMS devices are inherently both mechanical and electrical devices that are subject to wear and contamination and suffer from limited life times. Electrical functionality is often limited by the mechanical durability of the MEMS devices. RF MEMS switches offer high-speed operation for RF communication systems but suffer from speed limitations inherent in mechanical systems. 
     U.S. Pat. No. 5,578,976 issued Nov. 26, 1996 discloses an RF MEMS switch device. This switch device has a suspended arm that is attached on one side to a substrate and provides a conductive pad on another freely suspended side using a cantilever arm that extends over a ground line. The device is subject to contamination. The freely suspended cantilever arm suffers from an inherent mechanical weakness by virtue of flexing back and forth the cantilever arm at a single connection point. The switch device can be used as an AC capacitive coupler for communicating an RF signal across DC biased contacts on the cantilever arm and the supporting substrate. The RF MEMS switch with a suitable DC bias can also function as a DC coupled RF switch. 
     U.S. Pat. No. 5,638,946 issued Jun. 12, 1997 also discloses an RF MEMS switch and also discloses a suspended arm that is attached on one side to a substrate as a cantilever arm suffering from a single point of flexing wear and stresses on the attached side of the arm. This RF MEMS switch also suffers from contamination and limited lifetime. The RF MEMS device is suitable as a direct DC switch coupler or as an AC coupler with a limited operation frequency range. 
     These RF MEMS switches, though intended to operate at high switching speed, are limited in speed of actuation due to the inherent nature of the extended cantilever arm that must substantially flex up and down during operation over the electrical contacts and waveguides. The RF MEMS switches suffer from contamination due to exposure of debris formed during both manufacture and operational use. The asymmetric suspension mechanical configuration functions as an uncontrolled one ended suspension spring, providing uncontrolled mechanical oscillations during use, disadvantageously effecting the electrical performance of the RF switch. The RF switches are made larger than that minimally required due to the suspension cantilever arm, due to the use as a mechanical spring return, and due to the substrate pad placement being extended to the end of the cantilever arm. The physical arrangement of the RF switch electrodes significantly deviates from an ideal RF transmission line and consequently perturbs the propagation of RF signals due to impedance mismatch. Additionally, because of the inherent rotational operation of the contact end of the cantilever arm, the RF switch exhibits an asymmetric electrical performance as the arm rotationally flexes during operation when the contact pads are not consistently aligned with the substrate contact pads. The contact pads, when in contact with each other, suffer from stiction that slows the speed of operation and limits the effective operating range of the MEMS RF switches. These and other disadvantages are solved or reduced using the present invention. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a microelectromechanical systems (MEMS) radio frequency (RF) switch for high-speed electrical operation. 
     Another object of the invention is to provide a MEMS RF switch that is resistant to external contamination during use. 
     Yet another object of the invention is to provide a MEMS RF switch that can be controlled by opposing DC bias voltages for controlled electrical operation. 
     Still another object of the invention is to provide a MEMS RF switch having equilaterally suspended contacts for evenly distributed flexing and wear during operational use. 
     A further object of the invention is to provide a MEMS RF switch that is resistant to contamination through MEMS encapsulation of the operational contact pads of the MEMS RF switch during manufacture. 
     Another object of the invention is to provide a MEMS RF switch that has contact pads subject to both pull up and pull down biasing for controlled electrical operation. 
     Still another object of the invention is to provide a MEMS RF switch that can be operated as an RF AC coupler during operational use. 
     Still another object of the invention is to provide a MEMS RF switch that can be operated with direct con tact symmetric coupling during operational use. 
     Yet another object of the invention is to provide a MEMS RF switch having distortion free RF operation by virtue of equilateral coupling suspension with uniform signal propagation along an uninterrupted RF transmission. 
     The invention is directed to a MEMS RF switch that is optimized for operation over 1 GHz. The device has a vertical contact pad alignment configuration of electrodes and transmission lines such that minimal RF distortion, loss, and reflections will be created in the switch. Distortion free operation is accomplished by the use of a continuous grounded coplanar transmission line structure for the RF transmission line through the MEMS switch structure. The switch has on and off transition times that are symmetric and perfected by electrostatic actuation in both up and down directions. The vertical alignment configuration of electrodes contacts and waveguides have a minimal area switch contact with minimum moving mass during electrostatic actuation. The actuation electrodes are suspended above the transmission line and move up and down during minimal supporting spring forces during electrostatic actuation. The switch mechanical design provides rapid ON and OFF switching times. The switching speed is primarily a function of the inertia of the rest mass and switching potential of the electrostatic potentials that can further function to restore the switch to the ON or OFF conditions. 
     The RF switch is a MEMS device for switching signals through an RF transmission line. The RF MEMS switch is enclosed using two opposing substrates bonded together. A MEMS electrode substrate and an RF transmission line substrate are firstly separately manufactured and then bonded together to encapsulate, that is, entomb the composite RF MEMS switch. The opposing substrate switch design enables independent fabrication and process optimization of both the MEMS switch portion and RF transmission line portion of the composite RF MEMS switch. The composite substrate configuration of the RF MEMS switch increases manufacturing yields with improved performance. The RF MEMS switch is fabricated by wafer bonding the MEMS switch substrate in vertical alignment with the RF transmission line substrate so that electrostatic electrodes and switch coupler of the MEMS switch substrate are in respective vertical alignment with electrostatic electrodes and RF transmission lines of the RF transmission line substrate. The opposing substrate bonding process enables the RF transmission wafer and MEMS switch wafer, when bonded together, to be hermetically sealed from ambient dirt and contamination by encapsulation further increasing switch yield and long term switch reliability. After wafer bonding, the composite wafers maybe further processed using conventional packaging and wafer-sawing methods without risk of contaminating the delicate released MEMS structures. These and other advantages will become more apparent from the following detailed description of the preferred embodiment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A depicts a grounded coplanar waveguide pattern. 
     FIG. 1B depicts an actuator electrode pattern. 
     FIG. 1C depicts a bridge electrode pattern. 
     FIG. 2 is a top view of a diaphragm. 
     FIG. 3 is a side view of a RF MEMS switch. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of the invention is described with reference to the figures using reference designations as shown in the figures. Referring to all of the figures, various features of the RF MEMS switch are formed using semiconductor photolithographic patterns, designated with an “a” extension of the reference designations as used in the figures. Input and output conduction lines are electrically connected to external signals using electrical traces that are designated with a “b” extension of the reference designations as also used in the figures. 
     An RF grounded coplanar waveguide transmission line structure is formed by electrode photolithography patterns. A back left grounded coplanar waveguide  10  is formed using a back left grounded coplanar waveguide pattern  10   a  and extends externally for electrical connection. A left lower actuator electrode  12  is formed using grounded coplanar waveguide pattern  12   a  shown preferably extending to pattern  10   a , and having a left lower actuator electrode bias conductor  12   b . A front left grounded coplanar waveguide  13  is formed using a grounded coplanar waveguide pattern  13   a , shown extending through pattern  12   a  to pattern  10   a  and extends externally for electrical connection. A back right grounded coplanar waveguide  14  is formed using a back right grounded coplanar waveguide pattern  14   a  and extends externally for electrical connection. A front right grounded coplanar waveguide  15  is formed using a front right grounded coplanar waveguide pattern  15   a  and extends externally for electrical connections. A right lower actuator electrode  16  is formed preferably using a right lower actuator electrode pattern  16   a  and extends externally using a right lower actuator electrode bias conductor  16   b . A back center coplanar waveguide  18  is formed using a back center coplanar waveguide pattern  18   a  and extends externally for electrical connection. A front center coplanar waveguide  20  is formed using a front center coplanar waveguide pattern  20   a  and extends externally for electrical connection. Between the front and back center coplanar waveguides  18  and  20  is disposed a center coplanar waveguide gap  21 . Signal grounds are carried on electrodes  10   a ,  13   a ,  14   a , and  15   a . Grounded actuator electrode patterns are shown as  12   a  and  16   a , and are placed either adjacent to electrodes  10   a ,  13   a ,  14   a , and  15   a , or in direct electrical contact with the electrodes  10   a ,  13   a ,  14   a , and  15   a . RF signals are carried on a transmission line consisting of electrodes  20   a  and  18   a . The photolithographic patterns are fabricated using conventional integrated circuit and MEMS fabrication processes known to those skilled in the art. 
     A diaphragm  22 , preferably having a plurality of diaphragm apertures  23   a ,  23   b ,  23   c  and  23   d , is flexible and suspended above the coplanar waveguide consisting of traces  10 ,  13 ,  14 ,  15 ,  18  and  20 . The diaphragm  22  provides support for electrical elements operating in combination with the coplanar waveguide. A left upper diaphragm actuator electrode  24  is formed using a left actuator electrode pattern  24   a  and extends externally with a left upper diaphragm actuator electrode bias conductor  24   b . A right upper diaphragm actuator electrode  26  is formed using a right actuator electrode pattern  26   a  and extends externally using a right upper diaphragm actuator electrode bias conductor  26   b . A left lower diaphragm actuator electrode  28  is formed using the left actuator electrode pattern  24   a  and extends externally using a left lower diaphragm actuator electrode bias conductor  28   b . A right lower diaphragm actuator electrode  30  is formed using the right actuator electrode pattern  26   a  and extends externally using a right lower diaphragm actuator electrode bias conductor  30   b . A bridge electrode  32  is formed using a bridge electrode pattern  32   a  and does not extend externally and is suspended by the diaphragm to be centered over the gap  21 . 
     The diaphragm  22  is equilaterally suspended using a front diaphragm arm  42 , a left diaphragm arm  44 , a back diaphragm arm  46 , and a right diaphragm arm  48 . The diaphragm arms  42 ,  44 ,  46 , and  48 , are respectively used to suspend the diaphragm  22  using a front diaphragm arm anchor  56 , a left diaphragm arm anchor  50 , a back diaphragm arm anchor  52 , and a right diaphragm arm anchor  54 . The anchors  56 ,  50 ,  52 , and  54  extend into and become part of a diaphragm mounting frame  58  that supports the diaphragm  22  through the arms  42 ,  44 ,  46 , and  48  and respective anchors  50 ,  52 ,  54  and  56 . The diaphragm  22  and mounting frame  58  is suspended using a left upper diaphragm pedestal  60 , a left lower diaphragm pedestal  62 , a right upper diaphragm pedestal  64 , and a right lower diaphragm pedestal  66 . The pedestals  60  and  64  are used to suspend and offset in position the diaphragm  22  from a MEMS substrate  70  having a MEMS substrate grounded plane  72 . 
     The MEMS substrate also supports electrical elements for flexing the diaphragm  22 . A left upper actuator electrode  74  is formed using the left actuator electrode pattern  24   a  and extends externally through an upper actuator electrode bias conductor  74   b . A right upper actuator electrode  76  is formed using the right actuator electrode pattern  26   a  and extends externally through a right upper actuator electrode bias conductor  76   b . A left upper actuator electrode insulator  78  is preferably formed on the left upper actuator electrode  74  using the left actuator electrode pattern  24   a . A right upper actuator electrode insulator  80  is preferably formed on the right upper actuator electrode  76  using the right actuator electrode pattern  26   a . The left actuator electrodes  24 ,  28 ,  12 , and  74 , are in preferred vertical alignment respecting each other as are all of the right actuator electrodes  26 ,  30 ,  16 , and  76 . 
     The pedestals  62  and  66  are used to standoff the diaphragm  22  in respective directions of a left MEMS substrate bonding direction  81   a  and a right MEMS substrate bonding direction  81   b  extending equally and in parallel to standoff the diaphragm  22  from an RF substrate  82  having an RF substrate ground plane  84 . The coplanar waveguide traces  10 ,  13 ,  14 ,  15 ,  18 , and  20 , the lower actuator electrodes  12  and  16 , and the gap  21  are disposed on the RF substrate  82 . A left lower actuator electrode insulator  86  is formed using the left actuator electrode pattern  24   a  and disposed over the left lower electrode  12 . A center coplanar waveguide insulator  88  is formed using the bridge electrode pattern  32   a  and disposed over the gap  21  and over ends of the front and back center waveguides  18  and  20 , so as to function as a capacitive dielectric between the ends of the waveguides  18  and  20  to the bridge electrode  22  so as to provide two series capacitive coupling dielectrics at the two ends of front and back center coplanar waveguides  18  and  20 . A right lower actuator electrode insulator  90  is formed using the right actuator electrode pattern  26   a  and disposed over the right lower actuator electrode  16 . These insulators  78 ,  80 ,  86 , and  90  are respectively used for electrical conduction isolation between electrodes  24  and  74 ,  26  and  76 ,  28  and  12 , and  30  and  16 . The RF substrate  82  is bonded to the left lower diaphragm pedestal  62  and the right lower diaphragm pedestal  66  when moving the RF substrate  82  towards the pedestals  62  and  66  respectively along a left RF substrate bonding direction  92   a  and a right RF substrate bonding direction  92   b . The left and right RF substrate bonding directions  92   a  and  92   b  are in opposing alignment with the left and right MEMS substrate bonding directions  81   a  and  81   b . When the pedestals  60  and  64  are bonded to the MEMS substrate  70 , when the diaphragm  22  and frame  58  is supported between the pedestals  60 ,  62 ,  64 , and  66 , and when the RF substrate is bonded to the pedestals  62  and  66 , all of the electrodes  12 ,  16 ,  24 ,  26 ,  28 ,  30 ,  74 , and  76 , the coplanar waveguide  10 ,  13 ,  14 ,  15 ,  18 ,  20 ,  21 , bridge  32 , diaphragm  22  as well as the insulators  78 ,  80 ,  86 ,  88  and  90  are all entombed through encapsulation using the opposing MEMS substrate  22  as a ceiling, the RF substrate as a floor, and the pedestals  60 ,  62 ,  64 , and  66  as side walls. 
     In the preferred form, actuator electrodes  12  and  16  are shown as being grounded and attached to grounded waveguide  10  and  13 , and  14  and  15 , for ease of manufacture and simplistic control. It should be apparent that actuator electrodes  12  and  16  could be made separate and apart from the grounded waveguide  10 ,  13 ,  14 , and  15  to provide more flexible control of the RF switch operation by providing independent electrostatic control voltages on the actuator electrode  12  and  16  using control lines  12   b  and  16   b  and having the same contact area of and in alignment with actuator electrodes  28  and  30 , respectively. In the preferred form, ground control voltages are applied to lines  12   b  and  16   b  so that the left and right bottom actuator electrodes are grounded. In operation, electrostatic control voltages are applied to lines  24   b ,  26   b ,  28   b ,  30   b ,  74   b , and  76   b  to control the electrostatic forces between actuator electrode pairs  24  and  74 ,  28  and  12 ,  76  and  26 , and  30  and  16 . In controlling the electrostatic control voltages on lines  12   b ,  16   b ,  24   b ,  26   b ,  28   b ,  30   b ,  74   b ,  76   b , electrostatic push and pull forces are created between actuator electrode pairs  24  and  74 ,  28  and  12 ,  76  and  26 , and  30  and  16  causing the diaphragm  22  to move up and down in controlled motion so as to move the bridge  32  up and down in controlled motion in proximity over the gap  21  between the front and back waveguides  18  and  20  so as to couple and decouple the capacitive coupling between the front and back center waveguides  18  and  20 . 
     An RF signal can propagate between the back and front center waveguides  18  and  20  during coupling, and can not propagate from the back and front center waveguides  18  and  20  during decoupling. The grounded coplanar waveguide traces  10 ,  13 ,  14 ,  15 ,  18  and  20  are preferably made of gold and operate as transmission lines. The grounded coplanar waveguide formed by traces  10 ,  13 ,  14 ,  15 ,  18 , and  20  that are preferably covered by the optional thin dielectric over the area defined by the bridge electrode  32   a . With the dielectric, the RF MEMS switch is a capacitive AC coupling RF MEMS switch. Without the thin dielectric, the RF MEMS switch can operate as a DC coupling RF MEMS switch. 
     The RF switch OFF state impedance when the bridge electrode  32  raised is determined by the gap  21  in the center waveguides  18  and  20  and the parasitic capacitance to the bridge electrode  32  in the full up position. Center waveguides  18  and  20  function as RF inputs and RF outputs over an RF transmission line. The gap  21  is sized for large electrical isolation between the center waveguides  18  and  20 . The ON state impedance with the bridge electrode  32  lowered onto the dielectric  88  is determined by the two parasitic capacitors formed between respective center waveguides  18  and  20  and the bridge electrode  32  when the bridge electrode  32  is pulled down on top of the gap  21  to capacitively bridge the gap  21  with two series capacitors. 
     To turn ON the capacitive bridge in the RF MEMS switch, the actuator electrodes  24 ,  26 ,  28 ,  30 ,  74  and  76 , may be energized with an appropriate control electrostatic voltage. The bottom diaphragm electrodes  28  and  30  are controlled relative to the preferred ground control voltage on the lower electrodes  12  and  16 . The upper diaphragm actuator electrodes  24  and  26  are controlled with an electrostatic voltage relative to the top actuator electrodes  74  and  76 . With difference control voltages between the lower diaphragm actuator electrodes  28  and  30  and the lower electrodes  12  and  16 , and additionally between electrodes  24  and  26  relative to electrodes  74  and  76 , an electrostatic pull down force will exist to flex the diaphragm  22  to bring the bridge electrode  32  into capacitive coupling proximity with the gap  21  of center waveguides  18  and  20 . 
     The RF MEMS switch can be oppositely turned OFF in a similar manner by applying opposite control electrostatic voltages to the electrodes  28 ,  30 ,  24 ,  26 ,  74 , and  76  so as to discharge the turn on electrode capacitance while simultaneously energizing the electrodes to pull-up the MEMS diaphragm  22  and RF bridge electrode  32 . The polarities of the controlling voltages are reversed to achieve a similar but opposite pull-down or pull-up operation. The electrode areas for pull-down and pull-up can be made to any area desired to create the appropriate forces independent of the RF bridge electrode geometry. Similarly, the RF bridge electrode  32  can be made to an area to optimize the RF coupling independent of the MEMS actuator electrodes  28 ,  30 ,  24 ,  26 ,  74 , and  76 . The bridge electrode  32  may be in direct contact with center coplanar waveguides  18  and  20  as an alternative arrangement for direct contacting operation of the RF MEMS switch. 
     The RF MEMS switch may be operated in a linear mode where the MEMS diaphragm  22  may be driven to flex with an AC signal applied to the actuator electrodes  24 ,  26 ,  28 ,  30 ,  74 ,  76 ,  12 , and  16 , such that, an RF signal through the center waveguide  18  and  20  may be directly modulated. Modulation of an RF carrier may be achieved by varying the capacitive coupling from RF input to RF output by variably controlling the flexing distance between the bridge  32  and the gap  21 . In this direct modulation operation, the RF MEMS switch becomes an RF modulation element using the control actuator lines  12   b ,  16   b ,  24   b ,  28   b ,  26   b ,  30   b ,  74   b , and  76   b . Modulation from input to output of the center waveguides  18  and  20  is enabled by applying a DC bias on the diaphragm electrodes  24 ,  26 ,  28 ,  30 ,  74 ,  76 ,  12 , and  16 , to partially pull down the RF bridging electrode  32 , bringing the bridge electrode  32  in constant proximity to the RF center waveguides lines  18  and  20 , to set a DC biased capacitive coupling of the modulator electrode  32  to the center waveguide transmission lines  18  and  20  as DC biased coupling. AC modulation signal is then applied in addition to this DC biased coupling to the actuator electrode lines  12   b ,  28   b ,  26   b ,  30   b ,  74   b , and  76   b , resulting in AC linear motions of the bridging electrode  32 , and a time varying impedance of the RF transmission line of the center waveguide  18  and  20  with the varying impedance modulation in synchronism to the AC modulation signal. The controlling potentials can have a DC bias component for setting the amount of coupling by placing the bridge  32  at a DC bias distance from the gap  21  and having an AC modulation component superimposed upon the DC bias signal for AC modulation of a communication signal communicated between the front and back center waveguides  18  and  20 . 
     As may now be apparent, the RF MEMS switch is characterized as having an encapsulated diaphragm  22  with actuator electrodes  24 ,  26 ,  28 ,  30 ,  74 ,  76 ,  12 , and  16  equilaterally displaced about the center diaphragm positioned bridge  32  centered over a gap  21  of a center waveguide having two portions  18  and  20 . The two substrates  82  and  70  offer contamination free encapsulation for improved reliability. The RF MEMS switch offers an improved electrode configuration that is a hermetically sealed and self-enclosed MEMS structure using a minimal area and minimum mass suspension diaphragm  22 . The RF MEMS switch offers minimal RF distorting through transmission switch lines with minimal RF losses using symmetrical ON and OFF switching for improved speed. The two independent MEMS processes are used to fabricate the opposing wafers. The electrode area for RF coupling with MEMS actuation potentials and the DC and AC coupling of the transmission lines, can be independently controlled during design and manufacturing. The RF MEMS switch may be used for grounded microstrip operation, RF modulator operation, as well as ON and OFF switch operation. The RF MEMS switch has applications in communication systems where the device can be used as an RF transmission line switch, a variable RF attenuator, an RF modulator, or as part of beam forming and antenna diversity networks by forming RF MEMS switched time delay elements. Those skilled in the art can make enhancements, improvements, and modifications to the invention, and these enhancements, improvements, and modifications may nonetheless fall within the spirit and scope of the following claims.