Patent Publication Number: US-6218911-B1

Title: Planar airbridge RF terminal MEMS switch

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an RF switch and a process for making an RF switch and more particularly, to an RF switch fabricated by way of microelectromechanical system (MEMS) technology which includes a planar airbridge which allows for switch deflection in a single plane generally parallel to the substrate and thus only requires a single level of metallization, greatly simplifying the fabrication of the switch relative to known switches. 
     2. Description of the Prior Art 
     RF switches are used in a wide variety of applications. For example, such RF switches are known to be used in variable RF phase shifters; RF signal switching arrays; switchable tuning elements as well as in gang switching of voltage control oscillators (VCO). In order to reduce the size and weight of such RF switches, microelectromechanical system (MEMS) technology has been known to be used to fabricate such switches. MEMS technology is a process for fabricating various components using micromaching in a very similar manner as integrated circuits are fabricated. 
     Switches fabricated using MEMS technology normally include a substrate with one or more metal traces and control pads. An airbridged beam is known to be formed over the substrate in order to form one or more contacts with one or more of the metal traces; however, with only a single throw. Such switches normally require multiple levels of metallization. 
     Electrostatic forces are known to be used to control the opening and closing of the contacts. In particular, the control pad is connected to an external source of DC voltage. When the DC voltage is applied to the control contact, electrostatic forces cause the beam to deflect and make contact with one of the contacts, thus closing the circuit between the metal trace and the beam which define an RF contact. When the DC voltage is removed from the control pad, in some known switches, the resiliency of the beam causes it to deflect back to its normal position. In other known switches, electrostatic force is required to return the beam to the normal position. With such switches, the deflection of the beam is normally in a plane generally perpendicular to the plane of the substrate. 
     U.S. Pat. No. 5,619,061 and in particular FIGS. 18A-18D of the &#39;061 patent discloses an RF switch with a single pole configuration, formed from multiple levels of metallization. In particular, the &#39;061 patent discloses an RF switch which includes a beam suspended on opposing edges by thin metal hinges. More particularly, the beam is spaced apart from the substrate and suspended about midway along each edge by way of thin metal hinges. Metal traces are applied to the substrate and aligned with the edges of the beam. Control pads are disposed on the substrate adjacent the metal traces. Application of a DC voltage to the control pads causes an electrostatic attraction force to rotate the beam clockwise or counter clockwise and make contact with one of the metal traces on the substrate. 
     There are several known disadvantages of such RF switches. For example, such switches require a minimum of two levels of metal deposition, which adds to the complexity of the fabrication process. In addition, such switches are known to require relatively high voltages, typically 20-30 volts to operate. The relatively high voltage requirement is due to either the limited length of the airbridge, limited because of the possibility of collapsing, or due to the large distance between the beam and the DC control pad. Because of the possibility of foreign particles getting underneath the metal flap or membrane, such switches are normally limited to single throw designs because more throws normally require additional complicated metal deposition steps which could collapse onto lower levels. In addition, one of the failure mode for these kinds of switch is so called “sticking on”, the switches stay at “on” position permanently. Thus, there is a need to provide an RF switch which has multiple throws that is amenable to being fabricated using MEMS technology which is less complicated to fabricate, remedy “sticking on” problem, and only requires a single level of metallization. 
     SUMMARY OF THE INVENTION 
     Briefly, the present invention relates to an RF switch and a process for fabricating an RF switch which includes multiple throws that can be fabricated utilizing only a single layer of metallization. The switch in accordance with the present invention includes one or more airbridge suspended beams disposed adjacent one or more metal traces. One or more control pads are disposed adjacent the airbridged suspended beam to operate the switch electrostatically. The suspended beam as well as the metal traces and contact pads are all fabricated with a single metallization layer. The switch is configured such that deflection of the beam is in a plane generally parallel to the plane of the substrate. By eliminating multiple metallization layers, the complexity for fabricating the switch is greatly reduced. Moreover, the switch configuration also allows multiple throws and multiple poles using a single level of metallization. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     These and other advantages of the present invention will be readily understood with reference to the following specification and attached drawing wherein: 
     FIG. 1 is a perspective view of a single pole double throw capacitive type switch in accordance with the present invention. 
     FIG. 2 is a top view of the switch illustrated in FIG. 1, shown in an on position. 
     FIG. 3 is a top view of the switch illustrated in FIG. 1, shown in an off position. 
     FIGS. 4A-4H illustrate the processing steps for fabricating the switch in accordance with the present invention. 
     FIG. 5A is a top view of an alternate embodiment of the switch illustrated in FIG.  1 . 
     FIG. 5B is a top view of the switch illustrated in FIG. 5A shown with the switch in an on position. 
     FIG. 5C is similar to FIG. 5B but shown with the switch in an off position. 
     FIG. 5D is similar to FIG. 5A illustrating the use of insulated stoppers in accordance with one aspect of the invention. 
     FIG. 6 is a top view of another alternate embodiment of the switch in accordance with the present invention illustrating the switch with multiple throws and multiple poles. 
     FIGS. 7A and 7B are end views of an alternate airbridge for use with the present invention. 
    
    
     DETAILED DESCRIPTION 
     The present invention relates to an RF switch amenable to being fabricated using microelectromechanical switch (MEMS) technology. In accordance with an important aspect of the invention, the switch deflection is generally in a plane generally parallel to the plane of the substrate. The switch in accordance with the present invention can be fabricated using only a single level of metallization in various configurations including single pole single throw as well as multiple pole multiple throw, thus simplifying the fabrication process as well as reducing the cost of the switch. 
     Referring to FIG. 1, a perspective view of the switch in accordance with the present invention is illustrated and generally identified with the reference numeral  20 . The switch  20  is formed on a generally planar insulating substrate  22 , such as quartz or a semiconducting substrate, such as Gallium Arsenide (GaAs), which may be covered with a layer of insulating film (not shown) on the top to prevent current leakage. As shown, the switch  20  includes a beam  24  formed as an airbridge disposed adjacent to one or more spaced apart parallel metal traces  26  and  28 . Electrostatic forces may be used to deflect the airbridge  24  to make contact with one of the metal traces  26  or  28 . Portions of the traces  26  and  28  may be raised to the same height as the airbridge  24  to maximize the electrostatic force and contact area. More particularly, an RF input RF in  is applied to the beam  24 , for example, by way of an external blocking capacitor  30  which may be terminated by a choke  31  or terminating resistor  32  to ground. An RF output terminal RF out  is connected to the metal trace  26 . 
     In this embodiment, the metal traces  26  and  28  have a dual purpose. In particular, the metal traces  26  and  28  together with the beam  24  act as AC electrical contacts as well as DC control pads. In particular, as illustrated in FIGS. 2 and 3, the metal traces  26  and  28  may be connected to a pair of DC voltage sources  34  and  36  by way of a pair of relatively high value resistors  37 ,  39  which serve to insulate the RF signal from DC, and terminated by way of a pair of blocking capacitors  38  and  40  and termination resistor  42 . As shown in FIG. 2, when a DC voltage is applied to the metal trace  26 , the beam  24  is attracted and makes capacitive contact with the metal trace  26  through a thin layer of an insulator (not shown). The insulator layer is used to prevent the DC bias from being shorted to ground. Thus, applying a voltage to the metal trace  26  results in closing the RF switch to allow RF signals connected between the RF input terminal RF in  to be connected to the RF output terminal RF out . Similarly, as shown in FIG. 3, applying a DC voltage to the metal trace  28  causes the beam  24  to be deflected in order to make contact with the metal trace  28 , thereby opening the connection between the RF input terminal RF in  and the RF output terminal RF out . The termination resistor  42  can be removed allowing the blocking capacitor to be used to connect to another RF output. In this way the switch becomes a single pole double throw (spdt) switch. The switch illustrated in FIGS. 1-3 relies on a relatively thin layer of a high dielectric layer, such as 50 to 100 nanometers of silicon nitride with relative dielectric constant ∈ r  of 7, or aluminum nitride (∈ r  of 9) material coating on the beam  24  and metal traces  26  and  28  resulting in low reactance in an “on” position. The low dielectric constant of air (∈ r  of 1) results in the switch having a high reactance in the “off” position. For such switch, if it is sticking to one side (“sticking on”), a voltage can be applied to the other side to pull it off, thus reduce the “sticking on” problem. 
     The process diagram for fabricating the switch illustrated in FIGS. 1-3 is illustrated in FIGS. 4A-4H. Although the switch indicated in FIGS. 1-3 is a single pole single throw, it should be clear to one of ordinary skill in the art that the principles of the present invention are applicable to various switch configurations, for example, as illustrated in FIGS. 5 and  6 , which have multiple poles and multiple throws all using a single level of metallization, Turning to FIG. 4A, a substrate  50  is provided, such as a (GaAs) or other semiconducting or insulating type substrate. A first photoresist  52  is spun on top of the substrate  50 . As will be apparent below, the thickness of the first photoresist  52  determines the size of the air gap beneath the airbridge  24 . For example, the thickness of the first photoresist  52  may be 0.3-2 microns. After the first level of photoresist  52  is spun on top of the substrate  50 , the first photoresist  52  is exposed and developed by way of conventional photolithography techniques, to create a support  54  for the airbridge metal beam  24  and portions of the electrode  26  and  28  as shown in FIG.  1 . In particular, the device is exposed to a high temperature, for example 200° C., so that the edges of the first support  54  become rounded as shown in FIG.  4 B. The rounded shape of the first support  54  results in a gradual rise of the bridge  24  and portions of the electrodes  26  and  28  which provides additional mechanical strength of the raised metal as shown in FIG.  4 E. The high temperature treatment also prevents the first support  54  from being developed during development of the second photoresist  56 . Subsequently, as illustrated in FIG. 4C, a second photoresist  56  is spun on top of the support  54 . For example, 2.5 microns of the second photoresist  56  may be spun on top of the support  54  as shown. The second photoresist  56  is exposed and developed by conventional photolithography techniques using a suitable mask to form molds  58 ,  60  and  62  for the DC pads and the airbridge metal beam  24 . As shown in FIG. 4C, the molds  58  and  60  are used for the metal traces  28  and  26 , respectively, while the mold  62  is used for the airbridge metal beam  24 . After the molds  58 ,  60  and  62  are formed, a conductive metal layer  64 , for example, 2 microns of metal, such as aluminum, is deposited on top of the photoresist  56  as well as in the molds  58 ,  60  and  62  for the metal traces  28 ,  26  and the airbridge metal beam  24 , respectively, as illustrated in FIG.  4 E. Subsequently, in step  4 F, the excess metal and photoresist  56  is lifted off by a conventional process such as to soak the substrate in acetone to form the metal traces  28  and  26  and the airbridge metal beam  24 . Next, as illustrated in FIG. 4G, the support  54  is removed to define an air gap  66  beneath the airbridge metal beam  24 . The support  54  may be removed by oxygen plasma. Lastly, a layer of dielectric material, such as silicon dioxide or silicon nitride  68  is deposited onto the surface of the switch. A typical thickness of the layer is about 50 to 100 nanometers (FIG.  4 H). Thus, the switch  20 , as illustrated in FIGS. 1-3, is formed utilizing a single level of metallization to provide a single pole single throw switch or single pole double throw in which the deflection of the airbridge metal beam  24  is in a plane generally parallel to the plane of the substrate. 
     Alternate embodiments of the switch are illustrated in FIGS. 5A-5D and  6 . As discussed above, these embodiments as well as other configurations are amenable to being fabricated using the principles of the present invention in particular to being fabricated using a single metallization layer. Referring to FIG. 5A, an alternate configuration in the switch illustrated in FIG. 1 is illustrated and generally identified with the reference numeral  70 . In this embodiment, the switch  70  is formed on substrate  72  and includes an airbridge metal beam  74  disposed between a pair of spaced apart metal traces  76  and  78 . In this embodiment, the metal traces  76  and  78  do not have a dual function as the embodiment illustrated in FIGS. 1-3 and are used strictly for the switch contacts. As such, in this embodiment there is no need to have a layer of dielectric material between the airbridge and the contacts to prevent shorting out the DC voltage as in FIG.  1 . As shown in FIG. 5A, the metal traces  76  and  78  may be disposed generally perpendicular to the airbridge metal beam  74 . An RF input terminal RF in  is connected to one end of the airbridge metal beam  74  and terminated by way of an RF choke or termination resistor  75 . An RF output terminal RF out  is connected to one end of the metal trace  76 . 
     In this embodiment, separate control pads  80 ,  82 ,  84  and  86  are provided. As shown in FIG. 5A, the control pads  80  and  82  are disposed on one side of the airbridged beam  74  while the control pads  84  and  86  are disposed on the opposite side. A voltage applied to the DC control pads  84  and  86  causes the airbridge metal beam  74  to be deflected towards them as shown in FIG.  5 B and contact the metal trace  76  to provide a short circuit between the input terminals RF in  and the output terminal RF out . Similarly, when a DC voltage is applied to the control contact pads  80  and  82 , the airbridge beam  74  is deflected towards  80  and  82  as shown in FIG. 5C to open circuit the connection between the RF input terminal RF in  and the RF output terminal RF out . Unlike, the switch in FIG. 1 which works as a capacitive switch that cannot pass DC signal, this switch can work for both AC and DC. Again, the “sticking on” problem will be minimized due to the availability of two pairs of control pads,  80 ,  82 ,  84 , and  86 . 
     In this embodiment, the metal traces  76  and  78  may be formed with posts  88  and  90  on the ends to a height generally equal to the height of the airbridge beam  74 . In addition to enabling contact between the airbridge beam  74 , the posts  88  and  90  act as stops to prevent the airbridge beam  74  from contacting the DC control pads  80 ,  82 ,  84  and  86 . To further prevent the airbridge beam  74  from contacting the DC control pads, one or more isolated stoppers  87  can be placed along the DC control pads as showed on FIG. 5D. A portion  89  of the stoppers  87  is raised to the same height as the airbridge beam  74 . 
     An alternate embodiment of the switch is illustrated in FIG.  6 . In this embodiment, the switch generally identified with the reference numeral  100 , is configured as a single pole six throw switch and includes a plurality of airbridge beams  92 ,  94  and  96 . The airbridge metal beams  92 ,  94  and  96  are mechanically isolated from one another but are in electrical contact with each other. The airbridge beams  92 ,  94  and  96  are each disposed between a pair of metal traces  102  and  104 ,  106  and  108 ,  110  and  112 . Control pads  114 ,  116 ,  118 ,  120 ,  122 ,  124 ,  126 ,  128 ,  130 ,  132 ,  134  and  136  are disposed on opposing sides of the airbridge beams  92 ,  94  and  96 , respectively. An RF input terminal RF in  is connected to one end of the airbridge metal beams  92 ,  94  and  96 . A plurality of RF output terminals, RF out1 , RF out2 , RF out3 , RF out4 , RF out5  and RF out 6 , are connected to each of the metal traces  102 ,  104 ,  106 ,  108 ,  110  and  112 . 
     Each of the airbridge metal beams  92 ,  94  and  96  acts in the same manner by electrostatic forces as discussed above. For example, a DC voltage applied to the contact pads  118  and  120  will cause the airbridged level  92  to deflect to the right providing a short circuit between the RF input terminal and the RF output terminal RF out2  . Similarly, a DC voltage applied to the control pads  114  and  116  will cause the airbridge beam to deflect to the left causing a short circuit between the RF input terminal and the RF output terminal RF out1 . The balance of the switch outputs operate in the same manner. The switch shown in FIG. 6 may thus be used as a selector switch to connect an RF input source RF in  to any one of the six RF output ports RF out1 —RF out6 . 
     FIGS. 7A and 7B are top views of an airbridge beam  140  for use with the present invention. As shown, the bending stiffness of the bridge  140  can be varied along its lengths if desired for an arbitrary bending shape. As shown in FIGS. 7A and 7B, some portions  142 ,  144  of the airbridged beam bridge  140  can be formed as a relatively narrow region to form a thin compliant region, while other portions of the bridge portion can be formed as a relatively wider but stiff region. The advantage of it will be lower activation voltage while maintaining the conductivity of the bridge for a given bridge length. 
     Thus, it should be clear that the process in accordance with the present invention is amenable to forming various RF switches with multiple poles and multiple throws using only a single level of metallization. The fact that separate control sources are required to turn the switch on and off does not require additional levels of metallization. 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. Thus, it is to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described above.