Abstract:
A MEMS switch device including: a substrate layer; an insulating layer formed over the substrate layer; and a MEMS switch module having a plurality of contacts formed on the surface of the insulating layer, wherein the insulating layer includes a number of conductive pathways formed within the insulating layer, the conductive pathways being configured to interconnect selected contacts of the MEMS switch module.

Description:
BACKGROUND TO THE INVENTION 
       [0001]    Micro-electro-mechanical systems (MEMS) is a technology of very small devices. MEMS are typically made up of components in the range of 1-100 μm in size and MEMS devices generally range in size from 20 μm to 1 mm. An example of a MEMS device is a MEMS switch. Typically such MEMS switches are manufactured using a technique known as surface micromachining. In MEMS devices formed using surface micromachining the MEMS elements are formed on the surface of a substrate using conventional lithography and etching technology familiar from surface semiconductor processing. The interconnects and other circuit elements, such as resistors, for operating the MEMS switch are also formed on the surface of the substrate. However, forming the interconnects and desired resistors using conventional surface silicon processing techniques and materials may result in the interconnects being unreliable due to surface corrosion of the interconnects and leakage currents between adjacent surface interconnects. 
       SUMMARY 
       [0002]    According to various embodiments there is provided a MEMS switch device including a substrate layer, an insulating layer formed over the substrate layer, and a MEMS switch module having a plurality of contacts formed upon the surface of the insulating layer, wherein the insulating layer includes a number of conductive pathways formed within the insulating layer, the conductive pathways being configured to interconnect selected contacts of the MEMS switch module. 
         [0003]    At least one of the conductive pathways preferably includes a track of conductive material, such as aluminium, below the surface of the insulating layer and at least one conductive via, such as a tungsten via, extending from the track to the surface of the insulating layer. Furthermore, each conductive via may be electrically connected to one of the contacts of the MEMS switch module. 
         [0004]    At least one of the conductive pathways may include a track of resistive material, such as polysilicon, the conductive pathway preferably being configured as a resistive circuit element. The MEMS switch module may include a switch beam, wherein the resistive circuit element is preferably formed within the insulating layer and is aligned with the switch beam. 
         [0005]    The MEMS switch device may further comprise a control module in electric communication with the MEMS switch module. 
         [0006]    The substrate layer can comprise a high resistivity material, for example silicon, quartz, sapphire, gallium arsenide or glass. 
         [0007]    The MEMS switch device may further include a protective housing enclosing the MEMS switch module. The protective housing can comprise silicon bonded to the insulating layer. 
         [0008]    In some embodiments, a method of fabricating a MEMS switch device is disclosed. The method includes forming an insulating layer over a substrate layer and forming a number of conductive pathways within the insulating layer and subsequently forming on the surface of the insulating layer a MEMS switch module including a plurality of contacts, whereby selected contacts are configured to be interconnected by the conductor pathways within the insulating layer. 
         [0009]    The method may further include, after forming the MEMS switch module, forming a protective cap enclosing the MEMS switch module. 
         [0010]    The method may further include providing a control module arranged to be in electrical communication with the MEMS switch module. The MEMS switch device may be encapsulated with a protective plastic material. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    Embodiments of the present invention are described below, by way of non-limiting examples only, with reference to the accompanying figures of which: 
           [0012]      FIG. 1  is a schematic side cross section illustrating a MEMS switch device according to embodiments of the present invention; 
           [0013]      FIG. 2  is a schematic side cross section illustrating a MEMS switch module of the switch device shown in  FIG. 1 ; 
           [0014]      FIG. 3  is a schematic side cross section illustrating a substrate and insulating layer of the MEMS switch device shown in  FIG. 1 ; 
           [0015]      FIG. 4  is a schematic side cross section illustrating a further embodiment of the MEMS switch device with a protective cap formed over a portion of the MEMS switch module; 
           [0016]      FIG. 5  is a schematic side cross section illustrating the MEMS switch device and protective cap shown in  FIG. 4  packaged in combination with a control module; and 
           [0017]      FIG. 6  is a flowchart illustrating a method for manufacturing a MEMS switch device, according to one embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0018]      FIG. 1  schematically illustrates a MEMS switch device  1  according to one embodiment. The MEMS switch device  1  includes a substrate layer  2 , an insulating layer  4  formed over the substrate layer  2 , and a MEMS switch module  6  formed on the surface of the insulating layer  4 . The substrate layer  2  and the insulating layer  4  can be formed using equipment and materials typically employed for complementary metal-oxide-semiconductor (CMOS) backend processing techniques (e.g., oxide deposition, tungsten deposition, aluminum deposition, photolithography and etching, chemical mechanical planarization, etc.), although other suitable processing techniques may be used. The substrate layer  2  and insulating layer  4  are referred to together herein as a CMOS backend  8 . In the particular embodiment illustrated in  FIG. 1 , the CMOS backend  8  has a number of conductive pathways  10  formed within the insulating layer  4 . The conductive pathways  10  can interconnect selected contacts  12  of the MEMS switch module  6  formed on a surface of the insulating layer  4 . It will be understood that, in practice, the insulating layer  4  is typically formed from multiple oxide depositions. 
         [0019]      FIG. 2  schematically illustrates the MEMS switch module  6  formed on the surface of the insulating layer  4 . In  FIG. 2 , the conductive pathways  10  illustrated in  FIG. 1  are not illustrated for the purposes of clarity. As referred to above with reference to  FIG. 1 , the illustrated MEMS switch module  6  includes a number of electrical contacts  12  formed on the surface of the insulating layer  4 . In the illustrated embodiment each contact  12  includes a first layer  14  of electrically conductive material, such as any one of the noble metals, e.g. ruthenium or platinum, formed on the surface of the insulating layer  4 , and a second electrically conductive layer  16  can be formed over the top of the first layer  14 . The second layer  16  can be more electrically conductive than the first layer  14 . The second conductive layer  16  may be, for example, gold, which is highly conductive and highly resistant to corrosion. The first layer  14  can act to ensure a better electrical and physical contact with the insulating layer  4  than would be provided with a contact  12  formed solely of the second conductive material  16 , such as gold. In the embodiment illustrated in  FIG. 2 , the MEMS switch module  6  also includes a MEMS switch beam  18  that is electrically and physically connected to a further contact  12   a,  which is referred to herein as an anchor contact. The MEMS switch beam  18  can be formed of gold in some embodiments. The anchor contact  12   a  serves a dual purpose of physically supporting one end of the switch beam  18  on the surface of the insulating layer  4  and also providing an electrical connection to the switch beam  18  via the contact  12   a.  In the particular embodiment illustrated in  FIG. 2 , the switch beam  18  is cantilevered from anchor contact  12   a  so as to be separated from the surface of the insulating layer  4 . At the end of the switch beam  18  opposite to the anchor contact  12   a,  a contact tip  20  is formed on the underside of the beam  18 , the contact tip  20  being coated in a hard conductive material, such as one of the noble metals. The contact tip  20  is located above a further contact pad  22 , which may also be formed from a relatively hardwearing conductive material such as a noble metal. The provision of the hardwearing coatings  20 ,  22  on the contact tip  20  of the switch beam  18  and the corresponding contact pad  22  on the surface of the insulating layer  4  can prevent these surfaces from potentially softening and sticking together at high switching frequencies, which may occur if a softer conductive material, such as gold, were to be used. While not separately called out in  FIGS. 1 and 4 , the material choices for the MEMS switch module  6  of  FIG. 2  are also applicable to the MEMS switch modules of  FIGS. 1 and 4 . 
         [0020]    In operation, a voltage is applied to a gate electrode  24 , which can also be formed from a noble metal, such as ruthenium. Applying the voltage to the gate electrode  24  creates an electrostatic force attracting the switch beam  18  towards the gate electrode  24 . The electrostatic force causes the switch beam  18  to deform and the beam tip  20  to come into contact with contact pad  22 , thus closing an electric circuit between the contact pad  22  and anchor contact  12   a.  In the particular embodiment illustrated in  FIG. 2 , a further projection or protrusion  26  is formed on the underside of the switch beam  18  between the anchor pad  12   a  and the gate electrode  24 . The protrusion  26 , also referred to as a “bumper,” prevents the switch beam  18  from collapsing into contact with the gate electrode  24 , which may occur through metal fatigue or if an excessive electrostatic force is generated. 
         [0021]    The MEMS switch module  6  is fabricated on the insulating layer  4  by using surface processing techniques including deposition, masking and etching steps. In the illustrated embodiment, the switch module  6  does not include any conductive pathways formed on the surface (e.g., exterior surface) of the insulating layer  4  interconnecting any of the contacts  12 ,  12   a,    22 ,  24 . 
         [0022]      FIG. 3  schematically illustrates the CMOS backend  8 , including the substrate layer  2  and the insulating layer  4 . In some embodiments the insulating layer  4  is formed from silicon dioxide, but other insulating materials known to be used instead of silicon dioxide may be used as appropriate. Formed within the insulating layer  4  are a number of conductive pathways  10 . Typically the conductive pathways include a linear trace  30  formed of a resistant material, such as aluminum, formed horizontally within the insulating layer  4 . The conductive traces  30  can be buried within the insulating layer  4 . A number of conductive vias  32  can extend from the traces  30  to the surface of the insulating layer  4 . The conductive vias  32  may be formed from tungsten plugs, or other appropriate conductive material. The vias  32  may vary in size and number as appropriate. The vias  32  and traces  30  are formed within the insulating layer  4  using known bulk processing techniques, such as those typically used in the production of CMOS devices. The insulating substrate layer  2  is, typically, in preferred embodiments, formed from high resistivity silicon, but may be formed from other high resistivity substances, such as quartz, sapphire, gallium arsenide and glass etc. Other conductive pathways may be formed within the insulating layer  4 , as required by the desired electrical circuitry for the MEMS switch module  6 . An example of such a further conductive pathway is a higher resistance resistor  34 , which in some embodiments is formed from high resistance poly-silicon embedded within the insulating layer  4 . Such a resistor when formed underneath the switch beam  18  of the switch module  6  can serve the purpose of decoupling any undesirable capacitance between the switch beam  18  and the insulating layer  4 . 
         [0023]    With reference back to  FIG. 1 , it can be seen that the conductive vias  32  form electrical interconnections between the contact pads of the MEMS switch module  6  and the conductor traces  30 , thus forming the desired conductive pathways between the different elements of the MEMS switch module  6 . The conductive pathways of the illustrated embodiment are thus completely buried within the insulating layer  4  and thus protected from corrosion and other degradation. 
         [0024]    In further embodiments, further protection of the MEMS switch module is provided by the provision of a protective cap. Referring to  FIG. 4 , a protective silicon cap  40  is placed over the switch beam  18  of the switch module  6 , leaving the relevant contact pads  12  outside the protective cap  40 . The protective cap  40  can be formed from a single block of appropriate material, such as silicon, with an internal cavity  42  etched out. The cap  40  is bonded to the surface of the insulating layer  4  on which the MEMS switch module  6  is formed using appropriate bonding techniques, such as anodic bonding or using an appropriate adhesive, such as the illustrated glass  44 . The protective cap  40  helps to prevent moisture and other contaminants from interfering with the switch module operation and helps prevent subsequent plastic molding steps from damaging the switch module during the plastic package assembly process. The protective cap  40  can also prevent mechanical damage of the MEMS switch module and can prevent particles getting underneath the switch beam  18 . A further benefit of the protective cap  40  is to provide a controlled environment in which the actual MEMS switch operates. For example, the interior cavity  42  of the cap  40  may be filled with an inert gas. 
         [0025]      FIG. 5  schematically illustrates the MEMS switch device and protective cap shown in  FIG. 4  packaged in combination with a control module  46 . The control module  46  is preferably an application specific integrated circuit (ASIC) and is provided to generate appropriate high voltage (for example, about 80V) switching signals to be applied to the MEMS switch device in response to received logic signals, for example, about 3V. The lower voltage control logic signals are applied to the control module  46  by means of an input connection  48  and a wire bond connection  50  to the control module. A further wire bond  52  can connect an output of the control module  46  and one of the contact pads of the MEMS switch device. The output signals from the MEMS switch device can be output via a further wire bond  54  to an output  56  of the package. In some embodiments the input connection  48 , control module  46 , MEMS switch device  1  and output  56  are encapsulated in a typical packaging plastic material  58  in accordance with known packaging techniques. Whilst illustrated with the control module  46  and MEMS switch device package  1  side-by-side, alternatively the MEMS switch device  1  may be stacked on top of the control module  46 , and some or all of the wirebonds can be replaced with other forms of interconnection (e.g., flipchip, through silicon vias, etc.). 
         [0026]      FIG. 6  is a flowchart illustrating a method for manufacturing a MEMS switch device, according to one embodiment. In an initial step  60  of the illustrated embodiment, the substrate layer  2  of the CMOS backend is formed using conventional bulk processing/wafer processing techniques. The next step  62  is to form the insulating layer  4  over the substrate  2 , with the subsequent step  64  being to form the conductive pathways in the insulating layer. The skilled artisan will appreciate that, in practice, portions of the insulating layer  4  are formed before and after forming the conductive pathways. The initial steps used to form the backend, including the conductive pathways, can all be completed using conventional bulk processing technology. Subsequent to this, the backend wafer is then processed in accordance with surface processing techniques to form, at step  66 , the MEMS switch module on the surface of the insulating layer  4 . If desired, the protective cap may subsequently be formed over the switch module (at step  68 ), with the MEMS switch device and control module, if desired, subsequently packaged in accordance with conventional packaging techniques (step  70 ). Although the method illustrated in  FIG. 6  has been described in accordance with particular orders or sequences, it should be appreciated that other orders or sequences may be suitable. 
         [0027]    The MEMS switch device and method of fabricating the MEMS switch device according to the embodiments disclosed herein buries the conductive pathways (interconnects) in a manufacturable manner, using backend CMOS-type processing. This avoids the problems experienced with MEMS switch devices fabricated solely with surface micro-machine technology in which the interconnects were unburied and therefore were prone to corrosion and leakage currents between interconnects developing due to exposure. 
         [0028]    Furthermore, as explained above, in some embodiments, the MEMS switch beam  18  can be formed of a highly conductive and corrosion-resistant metal, particularly gold. Using gold for the switch beam  18  and the second conductive layer  16  can greatly reduce the amount of losses as compared to switches manufactured using a semiconductor material (e.g., silicon). Without being limited by theory, it is believed that a gold switch is advantageous compared to using a semiconductor switch because, when the gold switch is closed, the switch acts as a substantially loss-less analog device such that the input signal is substantially the same as the output signal (e.g., the gold switch in the closed configuration may act as a gold wire). In some embodiments, the MEMS switches described herein are integrated with high frequency circuits, such as RF arrays. For example, gold switch beams may be used in RF applications at operating frequencies between about 11 GHz and about 100 GHz. For example in some embodiments, the disclosed switch beams may be used in RF applications at operating frequencies of at least about 77 GHz. 
         [0029]    In other embodiments, MEMS switches as described herein are used to replace much larger mechanical relays in applications where a smaller profile is desired. For example, in medical applications in which the switch may be used inside the human body, highly reliable and loss-less MEMS switches disclosed herein may be made to be smaller than conventional mechanical relays. 
         [0030]    Accordingly, in various embodiments, as explained herein, it can be advantageous to use a MEMS switch with conductive parts including gold, platinum, and/or ruthenium. However, the use of such metals in conjunction with conventional CMOS fabrication facilities may be undesirable. For example, if a conventional CMOS fabrication facility were used to fabricate both the CMOS backend  8  and a MEMS switch module  6  that uses a gold switch beam  18 , gold used to form the switch beam  18  may contaminate the carefully calibrated CMOS facilities and any future CMOS processes used in the facility. 
         [0031]    Accordingly, it can be advantageous to manufacture the CMOS backend  8  in a first fabrication facility. The CMOS backend  8  can then be transported to a separate fabrication facility for the manufacture of the MEMS switch module  6 , for example, modules that include gold switch beams. The transfer can be conducted prior to dicing or after dicing wafers. The techniques taught herein enable both the use of buried interconnects and the use of exotic materials for the MEMS switch that may be incompatible with the facility (e.g., CMOS fabrication facility) used to create the buried interconnects.