Abstract:
Microelectromechanical system (MEMS) apparatus and methods for surface acoustic wave (SAW) switching are disclosed. The apparatus includes a piezoelectric substrate having spaced apart input and output SAW transducers. A MEMS switch is arranged between the input and output SAW transducers The MEMS switch has a deformable member in electromagnetic communication with one or more actuation electrodes formed on or above the substrate. The deformable member is deformable to mechanically contact the substrate to deflect or absorb a SAW generated by the input SAW transducer.

Description:
RELATED APPLICATION(S) 
   This application is a Continuation of U.S. application Ser. No. 10/198,503 filed Jul. 17, 2002 Now U.S. Pat. No. 6,933,808 which is incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The field of the invention relates to microelectromechanical systems (MEMS), and in particular relates to MEMS apparatus and methods for surface acoustic wave (SAW) switching. 
   BACKGROUND OF THE INVENTION 
   Filters and switches are often used in combination in electronic devices. In cell phones for example, radio frequency (RF) signals are detected by an antenna, converted to electrical signals, and then processed. To process the signals, a switch is needed to switch the RF antenna to a filter on the receiving side of the device, or to a filter on the transmission side of the device. In addition, switches are needed to change between frequency channels. In most electronic devices, the switches are in the form of transistors. It is known in the art of electronics that electrical signals suffer from “insertion loss” from passing through switching and filter circuitry. 
   SAW devices are used in certain electronic applications as resonators and filters. In a SAW filter, an electrical signal is inputted to an input SAW transducer formed on a piezoelectric substrate. The input electrical signal typically has a relatively wide range of frequencies. However, the input SAW transducer creates a SAW having only a narrow range of frequencies. The SAW then travels over the substrate and is detected by an output SAW transducer. The output SAW transducer only responds to a narrow range of SAW frequencies, further enhancing signal filtering. The detected SAW is then converted to an output electrical signal, which has a narrower frequency range than the input electrical signal. 
   MEMS switches are also used in select electronic applications. One example of a MEMS switch is a capacitor shunt switch, which includes a top electrode in the form of a membrane, and a bottom electrode in the form of a transmission line. In operation, when a direct current (DC) actuation voltage is applied across the top electrode (membrane) and the bottom electrode (transmission line), the membrane is deflected to make physical contact with the dielectric layer of the transmission line. This shorts the circuit to ground, thereby cutting off transmission of signals traveling through the transmission line. 
   Presently, both MEMS and SAW devices are employed in a variety of electronic devices as resonators, filters and switches. Yet, the general approaches to switching and filtering using SAW and/or MEMS devices involve switching in the electrical domain and filtering in the acoustic domain. This approach tends to be inefficient because of the associated insertion losses. Unfortunately, alternative approaches are currently lacking because of the dearth of efficient acoustic-based switches. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic plan view of a generalized example embodiment of a MEMS switching apparatus having an actuation electrodes with two electrode members; 
       FIG. 2  is a schematic plan view of another generalized example embodiment of a MEMS switching apparatus similar to that of  FIG. 1 , except that the actuation electrode includes a single electrode member located beneath the deformable member; 
       FIG. 3A  is a schematic plan view of an example embodiment of the MEMS switching apparatus of  FIG. 1 , wherein the MEMS switch includes a deformable member with a grating layer; 
       FIG. 3B  is a cross-sectional view of the deformable member of the MEMS switch of  FIG. 3A , illustrating in more detail the structural layer and the grating layer; 
       FIG. 3C  is a close-up plan view of the MEMS switch of  FIG. 3A , illustrating an example embodiment employing four actuation electrodes; 
       FIG. 4A  is a schematic plan view of an example embodiment of the MEMS switching apparatus of  FIG. 1 , wherein the MEMS switch includes a deformable member with an absorber layer; and 
       FIG. 4B  is a cross-sectional view of the deformable member of the MEMS switch of  FIG. 4A , illustrating in more detail the structural layer and the absorber layer. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In the following detailed description of the embodiments of the invention, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention embodiments may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice them, and it is to be understood that other embodiments may be utilized and that changes may be made without departing from their scope. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the embodiments of the present invention is defined only by the appended claims. 
     FIG. 1  is a schematic plan view of a generalized example embodiment of a MEMS switching apparatus  100 . Apparatus  100  includes an input SAW transducer  112  and an output SAW transducer  114 , each formed on or above an upper surface  117  of piezoelectric substrate  118 . Input SAW transducer  112  includes first and second sets  120  and  122  of interdigitally arranged electrode fingers  124  and  126 . Likewise, output SAW transducer  114  includes first and second sets  128  and  130  of interdigitally arranged electrode fingers  132  and  134 . 
   In an example embodiment, electrode finger sets  120  and  122  are made of a metal film formed using photolithographic and thin film processes, which may include either etching or lift-off techniques. In an example embodiment of input SAW transducer  112 , the width W 1  of each electrode finger  124  and  126  and the spacing S 1  between adjacent electrode fingers is on the micron or submicron level. Likewise in an example embodiment of output SAW transducer  114 , the width W 2  of each electrode finger  132  and  134  and the spacing S 2  between adjacent electrode fingers is on the micron or submicron level. 
   Input and output SAW transducers  112  and  114  define a SAW path  137  over which a SAW travels. SAW path  137  is defined as the region of substrate surface  117  between the input and output SAW transducers. The width of SAW path  137  is substantially the same as the width of the SAW transducers, so that the SAW path is defined essentially by the size and spacing of the SAW transducers and covers the area between the SAW transducers. 
   An electrical signal (e.g., voltage) source  140  is coupled to electrode finger sets  120  and  122  of input SAW transducer  112  via wires  141  and  142 , and serves to drive the input SAW transducer. In an example embodiment, electrical signal source  140  is an electronic element or device, such as an RF antenna or an amplifier. Further, an electronic element or device  144  is electrically coupled to electrode finger sets  128  and  130  of output SAW transducer  114  via wires  145  and  146 . In an example embodiment, electronic element or device  144  is an amplifier (e.g., a low-noise amplifier), an electronic filter, or an analog signal processing chip. Alternatively, electronic device  144  includes some or all of these (or like) elements. 
   Apparatus  100  further includes a MEMS switch  150  formed on piezoelectric substrate  118  between input SAW transducer  112  and output SAW transducer  114 . MEMS switch  150  includes anchors  160  connected to substrate  118  at upper surface  117 . Anchors  160  support a deformable member  166  adapted to mechanically contact upper surface  117  within SAW path  137 . In an example embodiment, deformable member  166  is a beam. In another example embodiment, deformable member  166  is a membrane. 
   MEMS switch  150  includes an actuation electrode  170  formed on substrate surface  117 . Actuation electrode  170  is arranged so as to be in electromagnetic communication with deformable member  166 . In particular, actuation electrode  170  is designed and arranged to electromagnetically engage deformable member  166  with sufficient strength to cause the deformable member to deform and contact substrate upper surface  117  when an electrical signal (e.g., a voltage signal) is applied to the actuation electrode. 
   Actuation electrode  170  can be made up of one or more electrode elements. For instance, in the example embodiment illustrated in  FIG. 1 , actuation electrode  170  is made up of two side actuation electrode elements  170 A and  170 B arranged on upper surface  117  beneath deformable member  166  and adjacent anchors  160 . In an example embodiment, electrode elements  170 A and  170 B lie entirely outside of SAW path  137 . In another example embodiment, the electrode elements making up electrode  170  lie at least partially outside of SAW path  137 . 
   In another example embodiment illustrated in  FIG. 2 , actuation electrode  170  of MEMS switch  150  includes a single electrode member  170 A located on substrate upper surface  117  directly beneath deformable member  166  within SAW path  137 . Actuation electrode member  170 A is conductive, and in example embodiments includes a wear-resistant metal such as Cr, or includes an insulator such as doped diamond. To minimize the loss of SAW energy when passing over the actuation electrode, actuation electrode  170 A should be relatively thin and uniform compared to the wavelength of the input SAW  210 . 
   Coupled to MEMS switch  150  and to actuation electrode  170  via a wire  188  is an actuation electrical signal (e.g., voltage) source  190  that periodically actuates (i.e., activates or “turns on”) the MEMS switch to deform deformable member  166  so that the deformable member is selectively mechanically contacted with and removed from a portion of substrate upper surface  117  within SAW path  137 . 
   With continuing reference to  FIG. 1 , apparatus  100  operates as follows. Electrical signal source  140  applies an input electrical signal  200  between sets  120  and  122  of electrode fingers  124  and  126 . This creates a periodic strain in piezoelectric substrate  118 , thereby creating an input SAW  210  that travels over substrate surface  117  and within SAW path  137 . The electrode finger width W 1 , electrode finger spacing S 1 , the interdigital pattern of the electrode fingers  124  and  126 , and the frequency content of the applied input electrical signal  200  determines the magnitude and phase of input SAW  210 . The input SAW propagates across upper surface  117  of substrate  118  to MEMS switch  150 . 
   When MEMS switch  150  is in a first state, deformable member  166  is not in contact with substrate surface  117 . This allows SAW  210  to propagate beneath the deformable member and through the MEMS switch without being disturbed. Input SAW  210  continues propagating along substrate surface  117  until it reaches output SAW transducer  114 , where it is converted to an output electrical signal  220 . Output electrical signal  220  is then further processed by electronic device  144 . 
   When MEMS switch  150  is switched to a second state via an electrical signal  226  from electrical signal source  190 , actuation electrode  170  electromagnetically engages and attracts deformable member  166 . This causes the deformable member to deform and make contact with substrate upper surface  117 . In one embodiment of apparatus  100 , deformable member  166  deflects most of or substantially all of input SAW  210 , thereby forming a deflected SAW  230 . This deflection prevents most of or substantially all of input SAW  210  from reaching output SAW transducer  114 . 
   Further in an example embodiment, deflected SAW  230  is optionally absorbed by an absorbing member  240  residing on or above substrate upper surface  117  and positioned to intercept the deflected SAW. Example materials for absorbing member  240  include silicone and silicone-based materials, such as RTV-3145 available from Dow-Corning, Inc. 
   In another example embodiment discussed in greater detail below, deformable member  166  includes an absorber layer that absorbs most of or substantially all of input SAW  210 , thereby prevents input SAW  210  from reaching output SAW transducer  114 . 
   The selective actuation of MEMS switch  150  causes deformable member  166  to interact with and modify the input SAW  210  in a manner that allows apparatus  100  to operate as an acoustic switch. Several specific example embodiments of the generalized example embodiment of apparatus  100  are now set forth in greater detail below. 
   MEMS Switch with Grating 
     FIG. 3A  is a schematic plan view of one example embodiment of the general example embodiment of the MEMS switching apparatus  100  of  FIG. 1 .  FIG. 3B  is a cross-sectional view of deformable member  166  of apparatus  100  of  FIG. 3A . Deformable member  166  includes in the present example embodiment a structural layer  254  with a lower surface  256 . Formed on lower surface  256  is a grating layer  260  having grating lines  262  with a grating spacing S G . Both structural layer  254  and grating layer  260  can be made of a number of materials. In example embodiments, structural layer  254  includes a metal such as Ni, Au, Ti or Al, and grating layer  260  includes a metal, a metal-coated dielectric, nitride, carbide, or an oxide such as SiO 2 . 
   In an example embodiment, grating layer  260  is oriented at an angle θ relative to axis A 1 . This results in input SAW  210  being deflected along an (imaginary) axis A 2  that intersects axis A 1 . In an example embodiment, absorber  240  lies along axis A 2  to intercept and absorb deflected SAW  230 . In an example embodiment, orientation angle θ is such that the deflection of input SAW  210  occurs at a right angle, i.e., such that axes A 1  and A 2  are at 90 degrees. 
   The particular grating angle θ needed to achieve a particular deflection direction depends upon the velocities of the input and deflected SAWs  210  and  230 . Consider V I  the velocity of incident SAW  210  and V D  the velocity of deflected SAW  230 . The velocity V D  may be different from V I  due to anisotropy of piezoelectric crystal substrate  118 . The pitch P of grating layer  260  is determined by P=V I Sin θ/f, where f is the frequency of incident SAW  210 . The condition for deflection at a right angle is given by tan θ=V I /V D . Further in the example embodiment, the number of grating lines and the grating spacing S G  are selected to maximally reflect incident SAW  210 . 
     FIG. 3C  is close-up plan view of the MEMS switch of  FIG. 3A , which includes four anchors  160  with suspension members  272  attached thereto and connected to deformable member  166 . In addition, actuation electrode  170  of MEMS switch includes four actuation electrode members  170 A,  170 B,  170 C and  170 D on substrate surface  117  arranged beneath deformable member  166  adjacent the deformable member&#39;s four corners. This arrangement allows for added flexibility of deformable member  166 , while also providing space to accommodate multiple actuation electrodes. 
   In the operation of MEMS switching apparatus  100  of  FIG. 3A , in a first state deformable member  166  is not in contact with substrate upper surface  117 . This allows input SAW  210  to propagate directly to output SAW transducer  114 . However, when MEMS switch  150  is switched to the second state via electrical signal  226  from actuation electrical signal source  190 , actuation electrode members  170 A,  170 B,  170 C and  170 D electromagnetically engage deformable member  116 , causing the deformable member to deform and make contact with substrate upper surface  117 . This allows the grating layer of the deformable member to intercept and deflect most of or substantially all of input SAW  210 . 
   In an example embodiment, deflected SAW  230  is optionally absorbed by absorbing member  240 . This deflection and absorption provides the selective isolation of output SAW transducer  114  from input SAW transducer  112  necessary for carrying out a switching operation. 
   MEMS Switch with Absorber Layer 
     FIG. 4A  is a schematic plan view of another example embodiment of the generalized example MEMS switching apparatus  100  of  FIG. 1 .  FIG. 4B  is a close-up cross-sectional view of deformable member  166 . 
   In apparatus  100  of  FIG. 4A , deformable member  166  is membranous and includes a structural layer  304  with a lower surface  306 , and an absorber layer  310  with a lower surface  312  formed on the structural layer lower surface. Absorber layer  310  is made of a material capable of absorbing a SAW. Example embodiments of absorber layer  310  include a polymer, or a soft metal. 
   In certain example embodiments, the material making up absorber layer  310  may be capable of damaging or contaminating substrate  118 . In such a case, an optional example embodiment includes a thin liner layer  316  formed over lower surface  312  to protect upper surface  117  from damage or contamination from absorber layer  310 . Thin liner layer  316  is made of a material compatible with the material making up substrate  118 , and in an example embodiment includes the same material as that making up substrate  118 . 
   Further in an example embodiment, substrate upper surface  117  includes an optional thin protective layer (not shown) to protect an underlying electrode or the piezoelectric substrate itself. 
   In the operation of MEMS switching apparatus  100  of  FIG. 4A , when MEMS switch  150  is in the first state, deformable member  166  does not contact substrate surface  117 . This allows input SAW  210  to propagate directly through MEMS switch  150  and to output SAW transducer  114 . However, when MEMS switch  150  is actuated via electrical signal  226  from actuation electrical signal source  190 , actuation electrodes  170 A and  170 B electromagnetically engage deformable member  166 , causing it to deform and make mechanical contact with substrate upper surface  117 . This allows deformable member  166  to intercept and absorb most of or substantially all of the input SAW in absorber layer  310 . This absorption provides the selective isolation of output SAW transducer  114  from input SAW transducer  112  necessary for carrying out switching operation. 
   While the present invention has been described in connection with preferred embodiments, it will be understood that it is not so limited. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention embodiments as defined in the appended claims.