Abstract:
According to one embodiment a microelectromechanical (MEMS) switch is disclosed. The MEMS switch includes a pulse generator to provide a low voltage source, a transformer coupled to the pulse generator to boost a voltage received from the pulse generator and a switch component coupled to the pulse generator. The switch component includes an actuation capacitor to store charge associated with the voltage received from the transformer.

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 switch.  
       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 RF switch. A typical MEMS RF switch includes one or more MEMS switches arranged in an RF switch array. MEMS RF switches are ideal for wireless devices because of their low power characteristics and ability to operate in radio frequency ranges. MEMS RF switches are often found in cellular telephones, wireless computer networks, communication systems, and radar systems. In wireless devices, MEMS RF switches can be used as antenna switches, mode switches, and transmit/receive switches.  
         [0003]     MEMS RF switches typically implement cantilever beam switching mechanisms, for example conductive beam and the insulated beam). The actuator capacitor of the switch is formed between a conductive plate of the cantilever beam and a control electrode that runs under the beam. When a voltage is applied to the control electrode, an electric field develops between the two plates.  
         [0004]     The force of this electric field bends the cantilever beam down. When the beam deforms far enough, the switch makes contact, and is “closed”. The voltage that closes the switch is called V pull-in (V PI ). Often a larger voltage than V PI  is desirable to increase contact pressure and reduce contact resistance.  
         [0005]     To de-actuate the switch, the voltage across the actuation capacitor drops below significantly below V PI . There is inherent hysteresis between the actuation voltage and the de-actuation voltage. For instance, for a switch that has a actuator gap change from open to closed of g final &lt;(⅔) g 0 , the de-actuation voltage will be approximately ⅓ the actuation voltage. Once the switch is actuated, the actuation capacitor voltage can leak down significantly, and the switch will remain closed. The hysteresis however slows down de-actuation to open the switch.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]     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.  
         [0007]      FIG. 1  illustrates one embodiment of a wireless communications system;  
         [0008]      FIG. 2  is a block diagram illustrating one embodiment of a RF MEMS switch;  
         [0009]      FIG. 3  is an electrical representation of one embodiment of a RF MEMS switch;  
         [0010]      FIG. 4  illustrates one embodiment of a conductive beam MEMS RF switch; and  
         [0011]      FIG. 5  illustrates one embodiment of an insulating beam MEMS RF switch.  
     
    
     DETAILED DESCRIPTION  
       [0012]     A high speed, low voltage MEMS switch architecture 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.  
         [0013]     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.  
         [0014]      FIG. 1  is a block diagram of one embodiment of a wireless communication system  100 . System  100  includes an antenna  110  for transmitting and receiving signals. System  100  also includes a voltage source controller  120 , a receiver  130  a transmitter  140 , and a MEMS switch  150  electrically coupled to antenna  110 .  
         [0015]     Voltage source controller  120  is electrically connected to MEMS switch  150 . In one embodiment, voltage source controller  120  includes logic for selectively supplying voltages to actuation electrodes (not shown) within MEMS switch  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 .  
         [0016]     During operation, system  100  receives and transmits wireless signals. This is accomplished by voltage source controller  120  selectively activating MEMS switches  150  so that switch  150  is coupled to receiver  130  so that received signals can be transmitted from antenna  110  to receiver  130  for processing, and coupled to transmitter  140  so that transmitted signals generated by transmitter  140  can be passed to antenna  110  for transmission.  
         [0017]      FIG. 2  is a block diagram illustrating one embodiment of a RF MEMS switch  150 . Switch  150  includes a switch component  210 , a rectifier  215 , a transformer  220  and a pulse generator  225 .  FIG. 3  is an electrical diagram of one embodiment of switch  150 .  
         [0018]     Referring to  FIG. 3 , switch component  210  is a cantilever beam switch such as a conductive beam or insulated beam switch, which includes an actuary capacitor used to actuate component  210 .  FIG. 4  illustrated one embodiment of a conductive beam switch, while  FIG. 5  illustrated one embodiment of an insulating beam switch. In an alternative embodiment, other types of switching mechanisms may be included without departing from the true scope of the invention.  
         [0019]     Rectifier  215  is coupled to switch component  210 . Rectifier  215  permits current to travel in only one direction within switch  150 . In one embodiment, rectifier  215  is a diode that is the p-substrate on which switch  150  is fabricated. In such an embodiment, an n implant or diffusion may be used to implement the diode. Further, the substrate may be a lightly doped material, which enables the diode to be easily device engineered to have a high breakdown voltage.  
         [0020]     Transformer  220  is a step-up transformer that boosts an input voltage received at switch  150  from a low voltage to a voltage sufficient to actuate switch component  210 . In one embodiment, transformer  220  is implemented with an air core from the multilevel metallization available in the MEMS process along with available metal air bridges.  
         [0021]     Pulse generator  225  provides actuation and de-actuation voltage transitions for switch  150 . Pulse generator  225  receives an output phasing control signal that provides actuation and de-actuation voltage transitions. In one embodiment, pulse generator  225  is a digital pulsed wave modulator (PWM). However in other embodiments, pulse generator  225  may be implemented as a frequency variable generator.  
         [0022]     Further, pulse generator  225  includes phase multiplexers coupled to the primary side of the transformer  220  coils. One multiplexer, when enabled, allows pulses to be delivered to the input terminal of transformer  220 , while the other multiplexer allows pulses to be delivered to the output terminal of transformer  220 . According to one embodiment, pulse generator  225  is included on a separate digital integrated circuit from the other switch  150  components.  
         [0023]     In one embodiment, switch component  210  is actuated by a 0 to V DD  voltage transition from pulse generator  225  on the in-phase transformer primary input (e.g., the side with the dot). The positive transition of the digital input provided by pulse generator  225  generates a current in the transformer secondary that is proportional to the input current by I input /N (the turns ratio of the transformer).  
         [0024]     The current induced in the secondary is stored on the actuation capacitor, and generates a voltage. Subsequently, pulse generator  225  undergoes a negative transition back to ground. In response, the current is rectified out by the reverse biased diode. Afterwards another positive transition again occurs at the input, which results in the charge again being deposited on the actuation capacitor. This charge is added to the charge that was previously stored during the previous transition and results in an increased voltage. This is commonly referred to as “charge pumping”.  
         [0025]     Positive transitions on the input are continued until the voltage across the activation capacitor reaches its terminal value of V input *N. According to one embodiment, this final voltage value may be greater than the switch component  210  activation voltage plus some guard banding. The number of input transitions occurring once the switch is closed is a function of the transformer primary current, the on resistance of the diode, and the final value of the actuator capacitance.  
         [0026]     After the terminal voltage is achieved, the transitions on the input can be stopped, to reduce dynamic power dissipation. However in one embodiment, occasional single transitions on the input are implemented to maintain switch component  210  in the closed state. The frequency of these positive transitions is a function of the actuation capacitor value, and the reverse leakage current of the diode.  
         [0027]     To de-actuate switch component  210 , the multiplexer switches the pulse generator  225  output to the output of the phase input terminal on the transformer (the one without the dot). The positive transitions are rectified out, and the negative transitions “charge pump” down the voltage on the actuator capacitor. Pulses are applied until the approximate terminal value of 0V is reached. This process is the reverse of the actuation, with the exception that no maintenance pulses are required for the off state.  
         [0028]     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.