Patent Publication Number: US-7911300-B2

Title: MEMS RF-switch using semiconductor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a divisional of application Ser. No. 11/179,460 filed Jul. 13, 2005. The entire disclosure of the prior application, application Ser. No. 11/179,460, is considered part of the disclosure of the accompanying divisional application and is hereby incorporated by reference. This application claims priority from Korean Patent Application No. 10-2004-0054449, filed on Jul. 13, 2004, the entire disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Apparatuses consistent with the present invention relate in general to a RF (Radio Frequency)-switch which allows an AC (alternating current) signal to pass therethrough by a bias voltage. More specifically, the present invention relates to a MEMS RF-switch using a semiconductor layer between a first electrode and a second electrode, thereby preventing charge buildup and sticking. 
     2. Description of the Related Art 
     Technical advances in MEMS (Micro Electro Mechanical System) have brought the development of a RF-switch based on the MEMS. In general, MEMS RF-switches have performance advantages over traditional semiconductor switches. For instance, the MEMS RF-switch provides extremely low insertion loss when the switch is on, and exhibits a high attenuation level when the switch is off. In contrast to semiconductor switches, the MEMS RF-switch features very low power consumption and a high frequency level (approximately 70 GHz). 
     The MEMS RF-switch has a MIM (Metal/Insulator/Metal) structure, that is, an insulator is sandwiched between two electrodes. Therefore, when a bias voltage is applied to the MEMS RF-switch, the switch acts as a capacitor, allowing an AC signal to pass therethrough. 
       FIG. 1  is a cross-sectional view of a related art MEMS RF-switch. As shown in  FIG. 1 , the MEMS RF-switch includes a substrate  11 , a first electrode  12 , an insulator  13 , and a second electrode  15 . Particularly, the MEMS RF-switch in  FIG. 1  has a cap structure where the second electrode  15  packages the first electrode  12  and the insulator  13 . Also, an air gap  14  exists between the second electrode  15  and the insulator  13 . 
     When a bias voltage V bias  is applied in the direction shown in  FIG. 1 , the second electrode  15  is thermally expanded and shifts in the direction of the arrow, thereby making contact with the insulator  13 . As such, the first electrode  12 , the insulator  13  and the second electrode  15  act as a capacitor together, and the RF-switch is turned on, which in turn allows an RF signal to pass therethrough at a predetermined frequency band. However, if the bias voltage V bias  is not applied, the second electrode  15  shrinks and is separated from the insulator  13 . As a result, the RF-switch is turned off and cannot allow the RF signal to pass therethrough. 
     When the bias voltage is applied, the second electrode  15  is charged positively resulting in a buildup of positive (+) charges, and the first electrode  12  is charged negatively resulting in a buildup of (−) charges. On the right hand side of  FIG. 1  is a graph illustrating charges, or the quantities of electric charges, on the first electrode  12 , the insulator  13  and the second electrode  15 , respectively, of an ideal RF-switch. Referring to the graph in  FIG. 1 , the first electrode  12  which corresponds to the interval (0˜x 1 ) is charged with −Q p , the second electrode  15  which corresponds to the interval (x 3 ˜x 4 ) is charged with +Q p . If the bias voltage is cut off in this state the charge turns back to 0. Meanwhile, the charge on the insulator  13  is maintained at 0, independent of the application of a bias voltage. 
     In practice, however, charge buildup often occurs to the insulator  13 . Thus, the detected charge on the insulator  13  is not always 0. 
       FIGS. 2A and 2B  are graphs for explaining charge buildup and sticking that occur to a non-ideal RF-switch.  FIG. 2A  illustrates a case when a bias voltage V bias  is applied. As shown, the first electrode  12  is charged with −Q p , the second electrode  15  is charged with +Q 1 . At this time, +Q 2  is built up on the insulator  13 . Q 1  and Q 2  satisfy a relation of Q 1 +Q 2 =Q p . As such, although the bias voltage V bias  may be applied, a repulsive force is generated by the insulator  13  which is charged positively with +Q 2  until the second electrode  12  is charged positively with greater than +Q 2 . Therefore, the RF-switch is not turned on until a bias voltage with a certain magnitude is applied. As a consequence, switching time is increased. 
     Meanwhile, once the RF-switch is on, the insulator is charged with +Q 2  and the first electrode  12  is charged with −Q 2  even though the bias voltage may be cut off. As a result, sticking occurs because the second electrode  15  and the insulator  13  are not separated. Moreover, the RF-switch may not be turned off at all even when the bias voltage is completely cut off. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an aspect of the present invention to provide a MEMS RF-switch using a semiconductor layer between a first electrode and a second electrode, thereby preventing charge buildup and sticking. 
     To achieve the above aspects of the present invention, there is provided a MEMS RF-switch, connected to an external power source, for controlling switching on or off of transmission of AC signals, the MEMS RF-switch including: a first electrode coupled to one terminal of the power source; a semiconductor layer combined with an upper surface of the first electrode, and forming a potential barrier to become insulated when a bias signal is applied from the power source; and a second electrode disposed at a predetermined distance away from the semiconductor layer, and being coupled to the other terminal of the power source, wherein the second electrode contacts the semiconductor layer when the bias signal is applied from the power source. 
     Also, the semiconductor layer may include a P-type semiconductor layer and an N-type semiconductor layer. 
     In addition, the MEMS RF-switch may further include: a substrate connected to a lower surface of the first electrode for supporting the first electrode, the semiconductor layer and the second electrode. 
     In this exemplary embodiment, the second electrode has a cap structure covering the first electrode and the semiconductor at the predetermined distance away from the semiconductor layer; or a cantilever structure, comprising a support part connected to a predetermined region of the substrate, and a protruded part supported by the support part for being a predetermined distance away from the semiconductor layer. 
     Additionally, the semiconductor layer may be made of one of intrinsic semiconductor, P-type semiconductor and N-type semiconductor. 
     Another aspect of the present invention provides a MEMS RF-switch comprising: a P-type substrate having a region on the upper surface doped by an N-type semiconductor; a first electrode connected to a lower surface of the P-type substrate and coupled to one terminal of an external power source; and a second electrode disposed at a predetermined distance away from the N-type semiconductor, and being coupled to the other terminal of the power source, wherein the second electrode contacts the N-semiconductor when a bias signal is applied from the power source. 
     Yet another aspect of the present invention provides a MEMS RF-switch comprising: an N-type substrate having a region on the upper surface doped by a P-type semiconductor; a first electrode connected to a lower surface of the N-type substrate and coupled to one terminal of an external power source; and a second electrode disposed at a predetermined distance away from the P-type semiconductor, and being coupled to the other terminal of the power source, wherein the second electrode contacts the P-type semiconductor when a bias signal is applied from the power source. 
     In addition, at least one of the first electrode and the second electrode may be made of one of metals, amorphous silicon and poly-silicon. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects of the present invention will become more apparent by describing certain exemplary embodiments of the present invention with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic cross-sectional view of a related art MEMS RF-switch; 
         FIGS. 2A and 2B  illustrate the operation of the MEMS RF-switch of  FIG. 1 ; 
         FIG. 3  is a schematic cross-sectional view of a MEMS RF-switch according to an exemplary embodiment of the present invention; 
         FIGS. 4A and 4B  a illustrate the operation of the RF-switch of  FIG. 3 ; 
         FIG. 5  illustrates the operational principle of the RF-switch of  FIG. 3 ; 
         FIGS. 6-8  illustrate, respectively, the structure of an RF-switch according to another exemplary embodiment of the present invention; and 
         FIG. 9  is a schematic cross-sectional diagram of a cantilever type RF-switch of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
     Exemplary embodiments of the present invention will now be described more fully with reference to the accompanying drawings. 
       FIG. 3  is a schematic cross-sectional view of a MEMS RF-switch according to an exemplary embodiment of the present invention. As shown in  FIG. 3 , the MEMS RF-switch includes a first electrode  110 , a semiconductor layer  120 , and a second electrode  130 . Also, the MEMS RF-switch further includes a substrate  100  for support. 
     The first electrode  110  and the second electrode  130  are coupled to both ends of an external power source  140 , respectively. Therefore, when a bias signal V bias  is applied from the external power source  140  the first electrode  110  and the second electrode  130  are charged with −Q and +Q, respectively. 
     The second electrode  130  is fabricated to be thinner than its surrounding support structure (not shown) so that it is thermally expanded by the application of the bias signal and makes contact with the semiconductor layer  120 . In this case, the bias signal is applied to the semiconductor layer  120  as a reverse bias signal. Thus, the semiconductor layer  120  generates a potential barrier by the layout of free electrons and holes therein and exhibits an insulating property. In result, the first electrode  110 , the semiconductor layer  120  and the second electrode  130  form a capacitor together, allowing an RF signal to pass therethrough at a predetermined frequency band. 
     Examples of the semiconductor layer  120  include intrinsic semiconductors, P-type semiconductors and N-type semiconductors. The P-type semiconductor or the N-type semiconductor can be obtained by carrying out a process of doping, i.e., adding donor impurity and acceptor impurity to the semiconductor, separately. Since the recombination of free electrons and holes takes place in the semiconductor layer  120  when the bias signal is cut off, charge buildup does not occur. 
       FIGS. 4A and 4B  are diagrams which depict the operation of the RF-switch of  FIG. 3 .  FIG. 4A  illustrates charge states of the first and second electrodes  110 ,  130  and the semiconductor layer  120  when the bias signal V bias  is applied. As shown in  FIG. 4A , the first electrode  110  is charged negatively, and the second electrode  130  is charged positively. The second electrode  130  is thermally expanded and makes contact with the semiconductor layer  120 . Free electrons are laid out on the upper portion of the semiconductor layer  120  due to the (+) charges on the second electrode  130 , and holes are laid out on the lower portion of the semiconductor layer  120  due to the (−) charges on the first electrode  110 . As such, the potential barrier is formed inside the semiconductor layer  120  and as a result, a depletion region is expanded between the first electrode  110  and the semiconductor layer  120 . In this manner, the semiconductor layer  120  becomes insulated and can allow the RF signal only to pass therethrough. Consequently, the MEMS RF-switch is turned on. 
       FIG. 5  graphically explains how the semiconductor layer  120  becomes insulated. Referring to  FIG. 5 , the energy levels on the semiconductor layer  120  are indicated by E c  (conduction band), E f  (Fermi level), and E v  (valance band). The first electrode  110  and the semiconductor layer  120  form a schottky diode structure. Accordingly, the semiconductor layer  120  becomes a cathode and the first electrode  110  becomes an anode. If the bias signal is applied to the second electrode  130  in this structure, a reverse bias is applied to the schottky diode. That is to say, as shown in  FIG. 5 , the potential barrier is created between the first electrode  110  and the semiconductor layer  120 . The energy level of the potential barrier is greater in the amount of e ΦBn  than that of the first electrode, and greater in the amount of e Vbi  than the conduction band E c  of the semiconductor layer. Thus, the movement of free electrons and holes between the first electrode  110  and the semiconductor layer  120  are interfered, and the semiconductor layer  120  becomes insulated. Additionally, the energy level of the first electrode  110  may be the same with the Fermi level. 
       FIG. 4B  illustrates charge states of the first and second electrodes  110 ,  130  and the semiconductor layer when the bias voltage V bias =zero, that is the external power source  140  is cut off. In this case, the charge on each of the first and second electrodes  110 ,  130  becomes zero, and the free electrons and holes having been spread out on both surfaces of the semiconductor layer  120  are now recombined inside the semiconductor layer  120 . Therefore, the second electrode  130  is normally separated from the semiconductor layer  120 , and no sticking occurs therebetween. In consequence, the MEMS RF-switch is normally turned off. 
       FIG. 6  illustrates the structure of an RF-switch according to another exemplary embodiment of the present invention. Referring to  FIG. 6 , the MEMS RF-switch in this exemplary embodiment includes a first electrode  210 , a P-type semiconductor layer  220 , an N-type semiconductor layer  230 , and a second electrode  240 . The first electrode  210  and the second electrode  240  are coupled to both ends of an external power source  250 , respectively. 
     The P-type and N-type semiconductor layers  220 ,  230  are combined with each other, forming the PN-junction diode. As depicted in  FIG. 6 , when the first electrode  210  and the second electrode  240  are coupled to the (−) terminal and the (+) terminal of the external power source  250 , respectively, a reverse bias is applied to the PN-junction diode. Therefore, a potential barrier is created between the PN junction diodes and the semiconductor layers become insulated. Consequently the MEMS RF-switch is turned on, allowing the RF signal to pass therethrough. 
       FIG. 7  illustrates the structure of an RF-switch according to yet another exemplary embodiment of the present invention. Referring to  FIG. 7 , the MEMS RF-switch in this exemplary embodiment includes a first electrode  310 , a P-type substrate  320 , an N-well  330 , and a second electrode  340 . The N-well  330  is fabricated by doping a certain portion of the upper surface of the P-type substrate  320 , thereby forming the structure of a PN junction diode. In short, when a bias signal is applied from the external power source  350 , the MEMS RF-switch starts operating based on the exactly same principle used for the MEMS RF-switch of  FIG. 6 . 
       FIG. 8  illustrates the structure of an RF-switch according to still another exemplary embodiment of the present invention. In  FIG. 8 , the bias direction of an external power source  450  is reversed. That is, a first electrode  410  and a second electrode  440  are coupled to the (+) terminal and the (−) terminal of the external power source  450 , respectively. A certain portion of the upper surface of an N-type substrate  420  is doped by a P-well  430 , thereby forming the structure of a PN-junction diode. As a result, when a bias signal is applied from the external power source  450 , the MEMS RF-switch starts operating based on the exactly same principle used for the MEMS RF-switch of  FIG. 6 . 
     In the exemplary embodiment of present invention, the first electrodes  110 ,  210 ,  310 ,  410  and the second electrodes  130 ,  240 ,  340 ,  440  are made of conductive materials including metal, amorphous silicon and poly-silicon. It is beneficial to fabricate electrodes by using the materials used in the CMOS (Complementary Metal-Oxide Semiconductor) fabrication because all the existing CMOS fabrication facilities and procedures can be compatibly used. 
     In addition, the second electrodes  130 ,  240 ,  340 ,  440  can have the cap structure or the cantilever structure. As the name implies, the second electrode  130 ,  240 ,  340  or  440  of the cap structure covers the first electrode and the semiconductor layer from a predetermined distance. The cap structure is well depicted in  FIG. 1 , so further details will not be necessary. 
       FIG. 9  is a schematic cross-sectional diagram of a cantilever type RF-switch according to the exemplary embodiment of  FIG. 3 . As shown in  FIG. 9 , a part of the second electrode  120  makes contact with the substrate  100  and forms a support part  500   a . Also, another part of the second electrode  130  forms a protruded part  500   b  being protruded from the support part  500   a  so that it is a predetermined distance away from the first electrode  110  and the semiconductor layer  120 . When a bias signal is applied from outside, the protruded part  500   b  moves downward and makes contact with the semiconductor layer  120 . 
     In conclusion, the MIM-structured RF-switch based on the MEMS utilizes the semiconductor layer instead of the insulator to allow AC signals to pass therethrough. Therefore, when the bias signal is applied, the potential barrier is formed on the semiconductor layer, thereby making the semiconductor layer insulated. In this manner, the semiconductor layer can transmit AC signals. When the bias signal is cut off, on the other hand, free electrons and holes in the semiconductor layer are recombined, whereby charge buildup and sticking can be prevented. In addition, by manufacturing the first and second electrodes out of poly-silicon or amorphous silicon, all the existing CMOS fabrication processes can be compatibly used with the exemplary embodiments of the present invention. 
     The foregoing embodiments are merely exemplary and are not to be construed as limiting the present invention. The present teaching can be readily applied to other types of apparatuses. Also, the description of the exemplary embodiments of the present invention is intended to be illustrative, and not to limit the scope of the claims, and many alternatives, modifications, and variations will be apparent to those skilled in the art.