Abstract:
According to one embodiment a microelectromechanical (MEMS) switch is disclosed. The MEMS switch includes a top movable electrode, and an actutaion electrode with an undoped polysilicon stopper region to contact the top movable electrode when an actuation current is applied. The undoped polysilicon stopper region prevents actuation charging that accumulates over time in a unipolar actuation condition.

Description:
FIELD OF THE INVENTION 
   The present embodiments of the invention relate generally to micro-electromechanical systems (MEMS) and, more specifically, relate to a MEMS switch. 
   BACKGROUND 
   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 radio frequency (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 show their promising applications in cellular telephones, wireless computer networks, communication systems, and radar systems. In wireless devices, MEMS RF switches may be used as antenna switches, mode switches, and transmit/receive switches. 
   Traditionally, in MEMS switch architecture, dielectric such as oxide or nitride is used on the actuation electrode to prevent electric short when the movable top electrode makes contact with the actuation electrode. However, in a unipolar actuation condition, where voltage is applied in the same polarity, charges are constantly trapped in the non-conductive dielectric and accumulate there over time. This phenomenon is known as “actuation charging”. The result of actuation charging is device failure because the trapped charges produce adequate electrostatic force to hold the movable electrode closed. 
   In order to prevent the actuation charging problem in MEMS switches, bipolar actuation has been used to retrieve charges injected into the dielectric with the opposite polarized voltage. However, such an approach requires a special and expensive bipolar actuation chip design, sometimes costing more than the MEMS device itself. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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. 
       FIG. 1  illustrates one embodiment of a wireless communications system; 
       FIG. 2  illustrates one embodiment of a MEMS switch in an open state; 
       FIG. 3  illustrates one embodiment of a MEMS switch in a closed state; 
       FIG. 4  is a band diagram illustrating charges and potential through one embodiment of a MEMS switch with no actuation voltage being applied; 
       FIG. 5  is a band diagram illustrating charges and current through one embodiment of a MEMS switch with actuation voltage being applied; 
       FIG. 6  illustrates another embodiment of a MEMS switch with polysilicon on both the top and actuation electrodes, in an open state; 
       FIG. 7  illustrates another embodiment of a MEMS switch with polysilicon on both the top and actuation electrodes in a closed state; 
       FIG. 8  is a band diagram illustrating charges and potential flow through one embodiment of a MEMS switch with no actuation voltage applied; 
       FIG. 9  is a band diagram illustrating charges and current flow through one embodiment of a MEMS switch with actuation voltage being applied; 
       FIG. 10  is a band diagram illustrating charges and potential flow through one embodiment of a MEMS switch with no actuation voltage applied; 
       FIG. 11  is a band diagram illustrating charges and current flow through one embodiment of a MEMS switch with actuation voltage being applied; 
       FIG. 12  illustrates one embodiment of the configuration of the top and actuation electrodes in a MEMS switch; and 
       FIG. 13  illustrates one embodiment of the configuration of the top and actuation electrodes in a MEMS switch. 
   

   DETAILED DESCRIPTION 
   A mechanism to prevent actuation charging in 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. 
   In the following description, numerous details are set forth. It will be apparent, however, to one skilled in the art, that the embodiments of the 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. 
     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 . 
   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 . 
   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. 
     FIGS. 2 and 3  illustrate one embodiment of a MEMS switch  150 . In  FIG. 2 , switch  150  is in one depiction in an “open” state  200 . In  FIG. 3  switch  150  is depicted in a “closed” state  250 . In one embodiment, switch  150  includes an actuation electrode  205  having an undoped polysilicon (intrinsic, i) stopper region  210 , and doped (p-type or n-type) polysilicon region  215 . A top movable electrode  220  may be metal (for example, gold (Au)). According to one embodiment, the top movable electrode  220  also includes at least one stopper  225 . Stopper  225  will contact the undoped polysilicon region  210  of the actuation electrode  205  when actuation voltage is applied. In one embodiment, stopper  225  size and undoped polysilicon region  210  are much smaller than the size of the actuation electrode  205 . 
   When voltage is applied to actuation electrode  205 , an electrostatic force pulls down the top electrode  220 , which will seize its movement when stopper  225  makes contact to the undoped polysilicon region  210  (see “closed” state  250 ). However, the low-resistive doped polysilicon region  215  produces the main electrostatic actuation continuously. 
   Using the undoped polysilicon  210  to diffuse the actuation charges reduces the actuation charging problem. These charges will drift away towards the p-type, n-type, or metal electrodes due to their semiconductor property. Therefore, no charges will build up in the updoped polysilicon  210 . Only space charges in the depletion region remain as fixed charges between electrodes. However, the total amount of charges from this region does not increase over time and is too low to cause a problem. 
     FIG. 4  is a band diagram depicting charges and potential through one embodiment of the MEMS switch with an gold metal top electrode  220  and an actuation electrode  205  with n-type doped polysilicon (note that p-type may also be used)  215  and undoped polysilicon stopper  210  regions. This is illustrated by the magnified view of the “open” state  200  from  FIG. 2 . In  FIG. 4  the device is not actuated or “open” (V a =0). The current path through the top electrode at the actuation region is represented by C–C′ (Au/air/n interface). The current path through the undoped polysilicon stopper region is represented by B–B′ (Au/air/i-n interface). 
   The region B–B′ band diagram  260  illustrates the potential at the stopper region at equilibrium. Due to the work function difference between gold and undoped polysilicon, a small potential drop between the two electrodes is anticipated (&lt;=0.2V). The region C–C′ band diagram  270  illustrates the potential at the actuation region at equilibrium. The work function difference between gold and the n-type doped polysilicon creates a small potential drop between the two electrodes (&lt;=0.65V). These potential drops originate from the material work function difference and will not increase over actuation lifetime. The small potential should not cause a problem when the actuation is not in the same voltage range. In such cases, the restoring force of the top electrode overcomes this small potential and keeps the device open. 
     FIG. 5  is a band diagram illustrating the same embodiment of MEMS switch  150  depicted in  FIG. 4 , except that in this figure actuation voltage is being applied (V a &gt;pull-in voltage V p ) and therefore switch  150  is now “closed”. Top electrode  220  is pulled down and making contact to undoped polysilicon stopper region  210  of the actuation electrode  205  (illustrated by the magnified view of the “closed” state  250  from  FIG. 3 ). The region B–B′ diagram  280  depicts the results at the stopper region, which forms an Au/i-n junction with a very small contact area (&lt;1 um). Because undoped polysilicon is used, only a very small amount of leakage current is expected to flow through the stopper region (for example, ˜uA of V a =5V). In a further embodiment, the Au/i-n interface is under reversed bias similar to the metal-semiconductor schottky contact, which helps reduce the risk of current flow. Furthermore, the undoped polysilicon serves as a resistor (for example, &gt;500 k ohm) so that the actuation voltage V a  remains between the two electrodes even when they make contact at the stopper region B–B′. 
   The region C–C′ diagram  290  depicts the result at the actuation region, which forms an Au/air/n interface. The actuation voltage remains across the C–C′ actuation region to keep the movable top electrode closed. Moreover, any charges that are injected into the updoped polysilicon will drift toward either electrode, which means that no trapped charges are accumulated. When the applied actuation voltage is removed, top electrode  220  will be opened by its restoring force. A small intrinsic voltage may exist as described in the  FIG. 4  discussion, which should be considered in the design to ensure the restoring force overcomes this small voltage. 
     FIGS. 6 and 7  illustrate another embodiment of MEMS switch  150 , with polysilicon as both top movable  305  and actuation  310  electrodes. Undoped polysilicon stopper regions  315 ,  320  are found on both the top electrode  305  and the actuation electrode  310 . According to one embodiment, these undoped polysilicon stopper regions  315 ,  320  are strategically placed so as to provide further resistance when making contact with each other. The surface  325  of the top movable electrode  305  is doped (either p-type of n-type) for actuation current conduction. Similarly regions of actuation electrode  310  are doped (either p-type or n-type)  330  for actuation current conduction. Metal (for example, gold) is used at the RF signal contact region  335  as illustrated in the drawing. The embodiment of MEMS switch  150  depicted in  FIG. 6  is illustrated in an “open” state  300 , while  FIG. 7  shows switch  150  in a “closed” state  350 . 
     FIG. 8  is a band diagram depicting charges and potential through one embodiment of the MEMS switch  150  illustrated in  FIG. 6 . The device is not actuated (i.e., V a =0) and therefore in an “open” state. A magnified view of the “open” state  300  from  FIG. 6  depicts the current path at the stopper region, B–B′, and also the current path at the actuation region C–C′. Here, for exemplary purposes, the doped polysilicon surface  325  on the top electrode  305  is p-type doped, and the doped polysilicon region  330  in the actuation electrode  310  is n-type doped. 
   The region B–B′ (p-i/air/i-n interface) band diagram  360  illustrates the potential at the undoped polysilicon stopper region under equilibrium. Since undoped polysilicon is used on both electrodes  305 ,  310  in this region, there is no potential drop between the two electrodes. The region C–C′ (p-i/air/n interface) band diagram  365  in  FIG. 8  illustrates that there is potential drop from the material work function difference between the updoped polysilicon  315  and the n-type polysilicon  330  (&lt;=0.45V). 
   However, the potential in this case is smaller than that for the gold top electrode illustrated in  FIG. 4 . Therefore, the intrinsic potential issue is reduced with the polysilicon top electrode configuration. When the actuation voltage is applied, electrostatic charges will distribute to the bottom surface of the polysilicon top electrode  305  and to the top surface of the actuation electrode  310  to produce an actuation force similar to the structure with the gold top electrode. Some change of the structure configuration may be done to eliminate the intrinsic potential from the work function difference, which will be addressed below. 
     FIG. 9  is a band diagram illustrating the same embodiment of MEMS switch  150  depicted in  FIG. 8 , except that in this figure actuation voltage is being applied (V a &gt;pull-in voltage V p ) and therefore the switch is now “closed”. A magnified view of the device in the “closed” state  350 , as in  FIG. 7 , is included. Here, as in  FIG. 8 , the doped polysilicon surface  325  on the top electrode  305  is p-type doped, and the doped polysilicon region  330  of the actuation electrode  310  is n-type doped. The top poly electrode  305  is pulled down and making contact at the undoped polysilicon stopper regions  315 ,  320  of both electrodes  305 ,  310 . 
   The region B–B′ band diagram  370  shows the result at the undoped stopper contact regions  315 ,  320 , which forms a p-i/i-n interface junction with a very small contact area (actual &lt;1 um). Because undoped polysilicon is used on both sides of the stopper contact regions  315 ,  320 , the equivalent resistance is high and leakage current is further reduced. Furthermore, the p-i/i-n interface is under reversed bias similar to a p-n junction, which also helps to increase the resistance. 
   The actuation voltage across the C–C′ region (p-i/air/n interface) is illustrated in the C–C′ band diagram  375 . The voltage is retained between the top  305  and actuation  310  electrodes. The electrostatic charges remain on the electrode surfaces to keep the movable top electrode  305  closed. Again, any charges that are injected into the updoped polysilicon  315 ,  320  will drift away toward either electrode, which means that no trapped charges should be accumulated. When the applied voltage is removed, the top electrode will open through its restoring force. The intrinsic voltage (&lt;=0.45V) here is smaller than in the case with the gold top electrode (see  FIG. 5 ). Therefore, this structure is less sensitive to its intrinsic potential. 
     FIG. 10  is a band diagram illustrating another embodiment of a MEMS switch  150  similar to that depicted in  FIG. 8 . Here, the doped polysilicon region  330  on the actuation electrode  310  is p-type doped, instead of n-type doped. The doped silicon region  325  on the top electrode  305  remains p-type doped. The device is not actuated in this depiction (i.e., V a =0). A magnified view of the “open” state  300  from  FIG. 6  depicts the charges and potential at the stopper region, B–B′, and also the charges and potential at the actuation region C–C′. Corresponding band diagrams illustrate the results for the B–B′ path  380  and the C–C′ path  385 . 
   No potential drop is expected at the B–B′ stopper region (p-i/air/i-p interface). Similar to the analysis in  FIG. 8 , the same undoped polysilicon  315 ,  320  is used on the electrodes  305 ,  310  on either side of the air gap, resulting in no potential drop. For the case of C–C′ actuation region (p-i/air/p interface), there is still potential drop from the material work function difference between updoped  315  and p-type polysilicon  330  (&lt;=0.45V). However, the potential is in opposite polarity from the case of p-i/air/n shown and discussed with  FIG. 8 . 
     FIG. 11  is a band diagram illustrating a similar embodiment to the MEMS switch  150  depicted in  FIG. 10 , except that in this embodiment actuation voltage is being applied (V a &gt;pull-in voltage V p ) and therefore the switch is now “closed”. A magnified view of the device in the “closed” state  350  is included. Here, as in  FIG. 10 , the doped polysilicon surface  325  on the top electrode  305  is p-type doped, and the doped polysilicon region  330  of the actuation electrode  310  is p-type doped. The top poly electrode  305  is pulled down and making contact to the undoped polysilicon stopper regions  315 ,  320  of both electrodes  305 ,  310 . 
   The band diagram  390  for the B–B′ undoped polysilicon stopper region (p-i/i-p interface) illustrates that the undoped polysilicon acts as a resistor to reduce the risk of large current flow. With adequate small contact area found at the stopper region (and the long length of undoped polysilicon), the resistance at the stopper contact may remain very high. 
   The actuation voltage across the C–C′ region (p-i/air/n interface), as depicted in band diagram  395 , is retained between electrodes to keep the top movable electrode  305  closed. Again, any charges that are injected into the updoped polysilicon  315 ,  320  will drift away toward either electrode  305 ,  310 , which means that no trapped charges should be accumulated. When the applied voltage is removed, the top electrode  305  will open by its restoring force similar to the case described in  FIG. 9 . 
     FIGS. 12 and 13  illustrate other embodiments of the top electrode and actuation electrode configuration, which may be implemented to reduce the intrinsic potential found in other configurations. The material work function difference from different electrode materials creates an intrinsic potential between electrodes. In most cases, the intrinsic potential may be neglected. But, for the case that the actuation voltage is in the range of the intrinsic potential (&lt;1V), a change of the actuation configuration could be implemented to overcome such an issue. The main approach is to use the same material on both sides of the air gap as shown in both  FIGS. 12 and 13 . P-type polysilicon is illustrated in the drawing. N-type polysilicon may be used as well. 
     FIG. 12  shows that an undoped thin polysilicon is first deposited and locally p-type doped to form the desired undoped polysilicon stopper region  405  and doped polysilicon region  410  of the top polysilicon electrode. A second thick polysilicon  415  is then deposited and p-type doped for desired actuation conduction. As seen from the figure, p-type polysilicon is present on both sides of the actuation region (C–C′) and undoped polysilicon is present on both sides of the stopper region (B–B′). Furthermore, the regions of undoped and doped polysilicon are symmetrically configured so that the same material always corresponds on the top electrode and actuation electrode. As a result, the intrinsic potential that arises from the work function difference between different materials is eliminated in such a configuration. 
     FIG. 13  shows another embodiment of the top and actuation electrode configuration that may be used to reduce intrinsic potential. The undoped polysilicon layer is first deposited and patterned to form at least one stopper region  420 . Then, a thick polysilicon p-type (or n-type) doped layer  425  is deposited to form the remaining conductive top electrode. This doped polysilicon surrounds the stopper region of the undoped polysilicon on all sides of the stopper except the bottom surface. The result is similar to  FIG. 12 , with the same type of polysilicon material symmetrically corresponding on the top electrode and actuation electrode to reduce the intrinsic potential. 
   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.