Abstract:
Pilot switch circuitry grounds a hot node (an injection node) of a microelectromechanical system (MEMS) switch to reduce or eliminate arcing between a cantilever contact and a terminal contact when the MEMS switch is opened or closed. The pilot switch circuitry grounds the hot node prior to, during, and after the cantilever contact and terminal contact of the MEMS come into contact with one another (when the MEMS switch is closed). Additionally, the pilot switch circuitry grounds the hot node prior to, during, and after the cantilever contact and terminal contact of the MEMS disengage from one another (when the MEMS switch is opened).

Description:
RELATED APPLICATIONS 
     This application claims the benefit of provisional patent application Ser. No. 61/596,930, filed Feb. 9, 2012, the disclosure of which is hereby incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     The present invention relates to microelectromechanical system (MEMS) switches, and in particular to pilot switch circuitry that reduces or eliminates arcing between MEMS switch contacts when the MEMS switch is opened or closed. 
     BACKGROUND 
     As electronics evolve, there is an increased need for miniature switches that are provided on semiconductor substrates along with other semiconductor components to form various types of circuits. These miniature switches often act as relays, generally range in size from a micrometer to a millimeter, and are generally referred to as microelectromechanical system (MEMS) switches. 
     In some applications, MEMS switches are configured as switches and replace field effect transistors (FETs). Such MEMS switches reduce insertion losses due to added resistance, and reduce parasitic capacitance and inductance inherent in providing FET switches in a signal path. MEMS switches are currently being deployed in many radio frequency (RF) applications, such as antenna switches, load switches, transmit/receive switches, tuning switches, and the like. For instance, transmit/receive systems requiring complex RF switching capabilities may utilize a MEMS switch. 
     With the incorporation of WLAN (wireless local area network) circuitry into smartphones, hot switching is no longer the exception but the rule. Hot switching occurs when a switch is switched (changes state from ON to OFF, or vice versa) while some voltage exists across the switch. Specifically, many modern “smartphones” have at least two antennas: a cellular antenna and a WLAN antenna. Thus, most cycles required of a MEMS contact switch (or other switch) attached to the cellular antenna will have hot switching power due to power received from the WLAN antenna. This hot switching may be fatal to contact switches. Conversely, hot switching occurs in switches connected to the WLAN antenna due to power received from the cellular antenna. 
     Thus, these two antennas are presently separate but co-located in the handset such that power from one network may be coupled to the adjacent network. The attenuation between these two adjacent radios can conservatively be 5 dB. It must be assumed by component manufacturers that these two radios are in continuous operation for the lifetime of the phone, from 5 to 10 years. The peak power from a WLAN radio is known to be +28.5 dBm. The power incident to all devices directly connected to the cellular radio antenna is therefore +23.5 dBm (after subtracting the 5 dB isolation). This level of power has been shown in characterization to limit the useful lifetime of MEMS contact switches to less than 1e6 cycles. However, the required lifetime number of cycles of such a switch used in an antenna switch exceeds 1e9 cycles and is calculated to be as many as 130 B cycles. 
     Furthermore, MEMS contact switches are known to have a low tolerance for ESD (electro static discharge) stresses with HBM (human body model) durability not exceeding 150 V in most cases compared to a specification requirement of 250 V. No MEMS contact switch has ever been exposed to the 8 k contact discharge test common to handset ESD reliability requirements. But it is possible that a MEMS contact switch would fail this test even with the typical decoupling capacitor and shunt inductor in place. 
     Turning to  FIGS. 1A and 1B , a MEMS device  10  having a main MEMS switch  12  is illustrated according to the prior art. The main MEMS switch  12  is formed on an appropriate substrate  14 . The main MEMS switch  12  includes a cantilever  16 , which is formed from a conductive material, such as gold. The cantilever  16  has a first end and a second end. The first end is coupled to the substrate  14  by an anchor  18 . The first end of the cantilever  16  is also electrically coupled to a first conductive pad  20  at or near the point where the cantilever  16  is anchored to the substrate  14 . Notably, the first conductive pad  20  may play a role in anchoring the first end of the cantilever  16  to the substrate  14  as depicted. The first conductive pad  20  may form a portion of or be connected to a first terminal (not shown) of the main MEMS switch  12 . 
     The second end of the cantilever  16  forms or is provided with a cantilever contact  22 , which is suspended over a terminal contact  24  formed or provided by a second conductive pad  26 . The second conductive pad  26  may form a portion of or be connected to a second terminal (not shown) of the main MEMS switch  12 . Thus, when the main MEMS switch  12  is actuated, the cantilever  16  moves the cantilever contact  22  into electrical contact with the terminal contact  24  of the second conductive pad  26  to electrically connect the first conductive pad  20  to the second conductive pad  26 . The main MEMS switch  12  may be encapsulated by one or more encapsulating layers  30 , which form a substantially hermetically sealed cavity around the cantilever  16 . The cavity is generally filled with an inert gas and sealed in a near vacuum state. Once the encapsulation layers  30  are in place, an overmold  32  may be provided over the encapsulation layers  30 . 
     To actuate the main MEMS switch  12 , and in particular to cause the cantilever  16  to move the cantilever contact  22  into contact with the terminal contact  24  of the second conductive pad  26 , an actuator plate  28  is formed over a portion of the substrate  14 , preferably under the middle portion of the cantilever  16 . To actuate the main MEMS switch  12 , an electrostatic voltage is applied to the actuator plate  28 . The presence of the electrostatic voltage creates an electromagnetic field that effectively moves the cantilever  16  against a restoring force toward the actuator plate  28  from an “open” position illustrated in  FIG. 1A  to a “closed” position illustrated in  FIG. 1B . Likewise, removing the electrostatic voltage from the actuator plate  28  releases the cantilever  16  for return to the open position illustrated in  FIG. 1A . As illustrated, the open position occurs when the cantilever contact  22  is out of contact with the terminal contact  24 , and the closed position occurs when the cantilever contact  22  comes into contact with the terminal contact  24 . Other embodiments may differ. 
     In light of the electromechanical structure of the main MEMS switch  12 , the main MEMS switch  12  cannot provide switching action as fast as typical solid state switches, such as n-type metal-oxide-semiconductor (NMOS) FET switches. The switching time of the main MEMS switch  12  typically depends upon the electromagnetic field applied to the cantilever  16 , the mass of the cantilever  16 , and the restoring force of the cantilever  16 . However, an FET switch may generate higher insertion loss than is generated by the main MEMS switch  12 . Moreover, at high power levels in an RF circuit (not shown), parasitic capacitance at the semiconductor junctions of the FET switch may alter RF signals. 
     During switching events, a difference in potential between the cantilever contact  22  and the terminal contact  24  may cause an electrical arc resulting from an electrical current flowing through normally non-conductive media, such as air. Undesired or unintended electrical arcing may have detrimental effects on the cantilever contact  22  and the terminal contact  24  of the main MEMS switch  12 . For instance, as the main MEMS switch  12  is being either actuated to the closed position of  FIG. 1B  or released to the open position of  FIG. 1A , arcing from a difference in potential between the cantilever contact  22  and the terminal contact  24  may cause significant aging, unintended wear and tear, degradation, sticking, or destruction of the cantilever contact  22 , the terminal contact  24 , or both. Unintended power dissipation through arcing should be limited for optimum contact lifetime of the cantilever contact  22  and the terminal contact  24 . 
     Therefore, it is evident there is a need to reduce the amount of power incident to switches connected to a cellular antenna and particularly to MEMS contact switches which are connected to a cellular antenna in order that their lifetimes can be extended to cover the lifetime cycle requirements for this application and make available their extremely low loss and high linearity. 
     Decreasing the amount of power incident to these switches minimizes arcing, and thus decreases switch contact aging, degradation, sticking, and destruction. In order to utilize the low loss and high linearity possible with MEMS, some means to attenuate the power must be introduced. 
     Furthermore, there is a need to provide some improved tolerance in these same MEMS contact switches to ESD stresses which can occur as a result of their installation at the antenna of a mobile handset. 
     SUMMARY 
     The present disclosure relates to the addition of a pilot switch shunt branch to a common port (such as a “hot node” or an “RF injection node” of a MEMS contact or other switch) to attenuate power entering the switch through the common port during switching events. If the pilot switch is made in solid state, it can switch quickly and additionally offer ESD protection for the switch network it precedes. 
     In one embodiment, the pilot switch includes a solid state switch in series with a MEMS switch. 
     In one embodiment, two MEMS switches are transitioned substantially simultaneously during a period when the pilot switch shunts the hot node to ground. 
     In one embodiment, two MEMS switches are transitioned sequentially during a period when the pilot switch shunts the hot node to ground. 
     Those skilled in the art will appreciate the scope of the present disclosure and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying figures incorporated in and forming a part of this specification illustrate several aspects of the disclosure, and together with the description serve to explain the principles of the disclosure. 
         FIGS. 1A and 1B  illustrate a microelectromechanical system (MEMS) switch in an open and closed position, respectively, according to the prior art. 
         FIGS. 2A ,  2 B, and  2 C illustrate a solid state pilot switch linking a hot node to a ground. 
         FIG. 3  illustrates a solid state pilot switch in series with a MEMS pilot switch, linking a hot node to a ground. 
         FIG. 4  illustrates a bypass resistor R 1  in parallel with one of the MEMS. 
         FIG. 5  illustrates a two die configuration. 
         FIG. 6  illustrates simultaneous switching for two MEMS. 
         FIG. 7  illustrates sequential switching for two MEMS. 
         FIG. 8  illustrates exemplary calculations for a maximum resistance of the pilot switch. 
         FIGS. 9A and 9B  illustrate power flows effects due to a pilot switch. 
     
    
    
     DETAILED DESCRIPTION 
     The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the embodiments and illustrate the best mode of practicing the embodiments. Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the disclosure and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims. 
       FIGS. 1A and 1B  illustrate a microelectromechanical system (MEMS) switch in an open and closed position, respectively, according to the prior art. See Background section for additional discussion. 
       FIG. 2A  illustrates a solid state pilot switch linking a hot node to a ground. Control circuitry  38  controls: transmitters  34 , MEMS switches  42 , receivers  33 , and pilot switch circuit  48 A. Control circuitry  38  may be logically organized into the following portions (not shown): transmitter control circuitry, MEMS switches control circuitry, receiver control circuitry, and pilot switch control circuitry. In the embodiment of  FIG. 2A , a pilot switch circuit  48 A includes a solid state pilot switch SW 1  that links (connects) the hot node N 1  to a ground GND. The solid state pilot switch SW 1  is controlled by at least one control input node NSW 1 . Other embodiments of the pilot switch circuitry  48 A are discussed later. 
     Hot node N 1  is often called an “injection node,” and may be linked to antenna ANT 1 , or to a cable input (not shown). Hot node N 1  may or may not have voltage at any given time. Antenna ANT 1  is subject to receiving transmissions or interference INT (shown as a lightning bolt icon) from second antenna ANT 2 . Second antenna ANT 2  may belong to another device or may belong to the same device. In one embodiment, ANT 1  is a cellular antenna and ANT 2  is a WLAN (wide local area network) antenna, and both antennas are located in a single handheld communication device (this is a common configuration that causes many interference problems). 
       FIG. 2B  illustrates exemplary MEMS switches  42  in more detail. Specifically, in  FIG. 2B  MEMS switches  42  include a SPNT (Single Pole, N Throw) switch set on a single die. In this case, there are 14 MEMS switches, so N=14, and this is a SP14T (single pole, 14 throw) switch set. The N 1  node can be “thrown” to any of 14 nodes labeled M 1  through M 14 . In one embodiment, a first set of MEMS switches  44  is linked to various cellular transmission circuits (see transmitters  34  in  FIG. 2A ), and a second set of MEMS switches  46  is linked to various cellular receiving circuits (see receivers  36  in  FIG. 2A ). 
     The proper use of the pilot switch SW 1  in pilot switch circuitry  48 A would be to close (change from a high impedance to a low impedance state) the pilot switch SW 1  before a state change (open-&gt;close or close-&gt;open) of any of the connected MEMS switches (M 1 -M 14 ). The pilot switch SW 1  is controlled by node NSW 1 . 
       FIG. 2C  illustrates control circuitry  38  controlling MEMS switch M 12  through node NM 12 , and controlling pilot switch SW 1  through control node NSW 1 . 
       FIG. 3  illustrates a solid state pilot switch in series with a MEMS pilot switch, linking a hot node to a ground.  FIG. 3  is identical to  FIG. 2B , except that pilot switch circuit  48 B includes a MEMS pilot switch M 15  in series with solid state pilot switch SW 1 . The timing of these pilot switches will be discussed below in relation to other figures. The order of MEMS pilot switch M 15  in series with solid state pilot switch SW 1  may be reversed (not shown), so that the MEMS pilot switch M 15  may be on the ground side of the solid state pilot switch. Further, a resistor (not shown) may be inserted into this series. 
     The addition of a MEMS pilot switch in series with the solid state pilot switch eliminates the negative impact of the OFF capacitance and limits the impact of the non-linearity of the solid state device. On the minus side, the total transition time required now includes 1.5 MEMS cycles plus 1 solid state switch cycle. Also, the MEMS pilot switch may be less protected from power incident on the common port. 
       FIG. 4  illustrates a bypass resistor R 1  in parallel with one of the MEMS.  FIG. 4  is identical to  FIG. 3 , except that a bypass resistor R 1  has been placed in parallel with MEMS switch M 8 . A bypass resistor R 1  may be used in combination with pilot switch circuitry such as  48 B, or may be used by itself (not shown) without pilot switch circuitry. 
       FIG. 5  illustrates a two die configuration. In  FIG. 5 , a first die D 1  includes a SP14T (Single Pole, 14 Throw) switch set  42  with 14 MEMS switches and a contact pad N 1 A. This is a common commercial die. 
     Die D 2  includes pilot switch circuitry  48 C (or  48 B, not shown), and control circuitry CC for controlling the solid state pilot switch S 21  (and optionally the MEMS pilot switch, not shown). Die D 2  includes contact pads N 1 B and N 1 C. 
     Antenna ANT 1  is linked to contact pad N 1 D. 
     In the two die configuration of  FIG. 5 , contact pads N 1 A and N 1 B are bonded together using bond wire B 1 , contact pads N 1 B and N 1 C are contacted together internally inside of die D 2 , and contact pads N 1 C and N 1 D are bonded together using bond with bond wire B 2 . 
     After the bond wires are attached, contact pads N 1 A, N 1 B, N 1 C, and N 1 D all form a single hot node N 1  (not shown). 
     In this embodiment, a single die D 2  is conveniently inserted between the commercial SP14T die D 1  and the antenna ANT 1 . This single die D 2  contains both the pilot switch  48 C and the associated control circuitry CC. Thus, this embodiment is very efficient to implement in production. 
       FIG. 6  illustrates simultaneous switching for two MEMS. The MEMS switch state transitions can be coincident or occur in any beneficial order. In  FIG. 6 , the MEMS switch transitions occur simultaneously. The order of MEMS switch transitions may be adjusted as required by an application, and all such variations are considered to be within the scope of the present disclosure. 
     Of course any number of switches attached to the hot node may transition in any particular order (or simultaneously) during the single pilot switch cycle (Off/On/Off). And of course it is also possible to use more than one pilot switch Off/On/Off cycle if needed or if it is advantageous to switch multiple MEMS switches. 
     In  FIG. 6 , a switching sequence illustrates simultaneous switching for two MEMS (M 1  and M 2 ) while pilot switch SW 1  grounds hot node N 1 . 
     Graph G 1  illustrates pilot switch SW 1  turning on at time T 1 , thus grounding hot node N 1 . Switch M 1  transitions from On to Off at time T 2 , and switch M 2  transitions from Off to On simultaneously at time T 2  (while the hot node N 1  is grounded). Later, at time T 3 , the pilot switch SW 1  turns off, and isolates hot node N 1  from ground GND. 
     For example, before time T 1 , switch M 1  may be connected to a transmission circuit (not shown), and may be conducting a transmission signal to an antenna (not shown) through hot node N 1 . After time T 3 , switch M 2  may be connected to a receiving circuit (not shown), and may be conducting a received signal from an antenna (not shown) through hot node N 1  to the receiving circuit (not shown). In other words, this switching sequence may represent a change from transmitting to receiving by a cellular telephone. 
     Alternatively, this switching sequence may represent: a change from receiving to transmitting by a cellular telephone; or a change from transmitting in a first band (through M 1 ) to transmitting in a second band (through M 2 ); or a change from receiving in a first band (through M 2 ) to receiving in a second band (through M 2 ). In all of these cases, grounding hot node N 1  during the transitions of MEMS switches increases the lifespan of the MEMS switches. 
     Circuit CKT 1  shows the MEMS switches before transitioning, and circuit CKT 2  shows the MEMS switches after transitioning. 
     If the pilot switch  4 A is replaced by a pilot switch that includes a solid state pilot switch and a MEMS pilot switch (as shown in pilot switch  48 B in  FIG. 4 ), then switching the pilot switch requires that both the solid state pilot switch and the MEMS switch are switched. 
     In one embodiment, switching pilot switch  48 B (not shown) ON includes first switching the MEMS pilot switch ON, then switching the solid state pilot switch ON (this grounds hot node N 1 ). While the hot node is grounded, other MEMS switches are transitioned, either simultaneously or sequentially. Switching the pilot switch  48 B OFF includes first switching the solid state pilot switch OFF, then switching the MEMS pilot switch OFF (this isolates hot node N 1  from ground). 
     Alternatively (not shown), the pilot switch may be turned ON, MEMS switch M 1  transitioned, the pilot switch turned OFF, then the pilot switch may be turned ON, MEMS switch M 2  transitioned, and the pilot switch turned off. This alternative is not very efficient from a timing point of view. It is more efficient to transition M 1  and M 2  simultaneously as shown in  FIG. 6 , or sequentially (but during a single cycle of the pilot switch) as shown in  FIG. 7  below. 
       FIG. 7  illustrates sequential switching for two MEMS.  FIG. 7  is identical to  FIG. 6 , except that MEMS switch M 2  transitions from OFF to ON at time T 5 , and time T 5  is different from time T 2  (but during a single cycle of the pilot switch). Thus, switch M 2  transitions sequentially with respect to switch M 1  (and not simultaneously as shown in  FIG. 6 ). 
     Sequential switching of the MEMS switches has some advantages in comparison to simultaneous switching. Sequential switching reduces the maximum switching power requirements, because only one switch must be transitioned at a time. 
     If the pilot switch  4 A is replaced by a pilot switch that includes a solid state pilot switch and a MEMS pilot switch (as shown in pilot switch  48 B in  FIG. 4 ), then switching the pilot switch requires that both the solid state pilot switch and the MEMS switch are switched. One possible switching sequence for pilot switch  48 B was discussed above in relation to  FIG. 6 . 
       FIG. 8  illustrates exemplary calculations for a maximum resistance of a pilot switch in a 50 ohm system, for a power reduction of 13.5 dBm. 
     Assuming that a cellular antenna receives 23.5 dBm peak power from a nearby WLAN antenna, and allowing a design maximum of 10 dBm to reach the SP14T MEMS switches through the hot node, means that the pilot switch must attenuate the WLAN signal by 13.5 dBm (23.5−13.5=10). 
     Graph G 3  plots Gain versus Resistance Rshunt for a simulated circuit similar to  FIG. 2 . The gain reaches −13.5 (an attenuation of 13.5) at dB1, corresponding to a resistance R 1  of 6.7 ohms. Thus, as a design parameter, the pilot switch should have a resistance of 6.7 ohms or less when ON (when grounding the hot node). 
       FIGS. 9A and 9B  illustrate power flow effects due to a pilot switch. 
     In  FIG. 9A , pilot switch  48 D is open, antenna ANT 1  receives 23.5 dBm from a nearby WAN antenna (not shown), transmission circuit CKT 3  is transmitting at 35 dBm through MEMS switch M 5 . 
     In  FIG. 9B , pilot switch  48 F is closed (providing an attenuation of 13.5 dBm for a 6.7 ohms resistance to ground), antenna ANT 1  receives 23.5 dBm from a nearby WAN antenna (not shown), and 10 dBm of power reaches MEMS switch M 5 . 
     Those skilled in the art will recognize improvements and modifications to the preferred embodiments of the present disclosure. All such improvements and modifications are considered within the scope of the concepts disclosed herein and the claims that follow. In the claims, the term “solid state switch” refers to any semiconductor device capable of acting as a switch, including, but not limited to: field effect transistors (FET), complementary metal oxide semiconductors (CMOS), and so forth.