Abstract:
In one aspect, a microelectronic device comprises: a suspended lithium-based thin film; and one or more electrodes disposed on the suspended lithium-based thin film, wherein the one or more electrodes comprises one or more fingers, and a width of at least one outer finger of the one or more fingers is smaller than a width of at least one inner finger of the one or more fingers.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of priority under 35 U.S.C. §119(e) to provisional U.S. Patent Application No. 61/797,166 filed Nov. 30, 2012, and provisional U.S. Patent Application No. 61/660,284 filed Jun. 15, 2012, the entire contents of each of which are hereby incorporated by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0002]    This invention was made with government support under contract number N66001-10-1-4005-00007274 awarded by the Defense Advanced Research Projects Agency (DARPA). The government has certain rights in the invention. 
     
    
     FIELD OF USE 
       [0003]    The present disclosure relates generally to microelectronic structures, and specifically to microelectronic structures with suspended lithium-based thin films. 
       BACKGROUND 
       [0004]    Resonators are used in radio frequency (RF) communication systems. Generally, resonators need to be high quality (high-Q), low loss, and stable, and have a low motional impedance. 
       SUMMARY 
       [0005]    The present disclosure describes methods and apparatus relating to suspended lithium-based membrane microelectronic structures. The suspended lithium-based membrane microelectronic structures may be implemented in resonant micro-electro-mechanical systems (MEMS), other moving structures, or both. The suspended lithium-based membrane microelectronic structures may be used for multi-frequency wideband multiplexers and reconfigurable RF front ends. Multi-frequency MEMS resonators produced from the suspended lithium-based membranes may simultaneously achieve high electromechanical coupling (k t   2 ) and a high quality factor (Q). 
         [0006]    In one aspect of the present disclosure, a microelectronic device comprises: a suspended lithium-based thin film; and one or more electrodes disposed on the suspended lithium-based thin film, wherein the one or more electrodes comprises one or more fingers, and a width of at least one outer finger of the one or more fingers is smaller than a width of at least one inner finger of the one or more fingers. 
         [0007]    Implementations of the disclosure can include one or more of the following features. The suspended lithium-based thin film comprises one or more of lithium niobate or lithium tantalate. An average thickness of the lithium-based thin film is between 100 nm and 30 μm. The one or more electrodes comprise one or more of aluminum, gold, platinum, molybdenum, or copper. Each of the one or more electrodes are configured to receive a signal. At least one of the one or more electrodes has an average thickness between 5 nm and 150 nm. The one or more fingers comprise one or more inter-digital fingers. A width of at least one outer inter-digital finger of the one or more inter-digital fingers is smaller than a width of at least one inner inter-digital finger of the one or more inter-digital fingers. A center frequency of the microelectronic device is based on a pitch of at least one finger of the one or more fingers. A frequency of operation of the microelectronic device is based on one or more of a number of the one or more fingers and a spacing among at least a portion of the one or more fingers. The frequency of operation of the microelectronic device is between about 1 MHz and 10 GHz. A chip comprising a plurality of microelectronic devices. A first one of the plurality of microelectronic devices has a first bandwidth and a second one of the plurality of microelectronic devices has a second bandwidth, and wherein the first bandwidth differs from the second bandwidth. 
         [0008]    In another aspect of the present disclosure, a method for forming a suspended lithium-based membrane semiconductor structure comprises: depositing a bonding agent on a surface of a lithium-based carrier substrate; implanting ions into a surface of a lithium-based donor substrate; forming, based on implanting, an ion-implanted surface of the lithium-based donor substrate; bonding the ion-implanted surface of the lithium-based donor substrate to the bonding agent; removing the lithium-based donor substrate from the ion-implanted surface that is bonded to the bonding agent, with at least a portion of the ion-implanted surface remaining bonded to the bonding agent following removal; forming, based on removing, a lithium-based membrane on the bonding agent, with the lithium-based membrane comprising the at least a portion of the ion-implanted surface that remains bonded to the bonding agent; forming one or more electrodes on the lithium-based membrane; etching at least one release window extending through the lithium-based membrane to the bonding agent; and removing, using the at least one release window, the bonding agent to suspend a portion of the lithium-based membrane with respect to the lithium-based carrier substrate. 
         [0009]    Implementations of the disclosure can include one or more of the following features. The bonding agent comprises one or more of an adhesive agent or an oxide. The adhesive agent comprises one or more of benzocyclobutene or polyimide. The lithium-based carrier substrate and the lithium-based donor substrate each comprise one or more of lithium niobate or lithium tantalate. A thickness of the ion-implanted surface of the lithium-based donor substrate is based on an amount of ions that are implanted and is further based on an energy of the ions that are implanted. The average thickness of the ion-implanted surface of the lithium-based donor substrate is between 100 nm and 30 μm. The method further comprises: prior to bonding the ion-implanted surface of the lithium-based donor substrate to the bonding agent, forming one or more additional electrodes on the ion-implanted surface of the lithium-based donor substrate. Removing the lithium-based donor substrate from the ion-implanted surface that is bonded to the bonding agent comprises: heating the lithium-based donor substrate at a temperature based on an amount and energy of the ions implanted on the lithium-based donor substrate; and removing, based on heating, the lithium-based donor substrate from the ion-implanted surface that is bonded to the bonding agent. The temperature is an average temperature between 200 degrees Celsius and 350 degrees Celsius. The lithium-based donor substrate is heated between two hours and twelve hours. The lithium-based donor substrate comprises an X-cut substrate. The method further comprises: after removal of the lithium-based donor substrate, polishing an exposed surface of the lithium-based membrane to reduce roughness of the exposed surface of the lithium-based membrane. The roughness of the exposed surface of the lithium-based membrane is less than 5 nm. Forming the one or more electrodes on the lithium-based membrane comprises: forming the one or more electrodes using one or more of physical sputtering, electron-beam evaporation, and thermal evaporation. The one or more electrodes comprise one or more of aluminum, gold, platinum, molybdenum, or copper. Each of the one or more electrodes comprise one or more fingers. At least one of the one or more fingers has an average thickness between 5 nm and 150 nm. The one or more fingers of the one or more electrodes comprise a plurality of inter-digital fingers. A width of at least one outer inter-digital finger of the plurality of inter-digital fingers is smaller than a width of at least one inner inter-digital finger of the plurality of inter-digital fingers. Etching the at least one release window extending through the lithium-based membrane to the bonding agent comprises: depositing an etch mask layer over the lithium-based membrane and the one or more electrodes; depositing a photo-resist layer on the etch mask layer; etching a pattern in the photo-resist layer; transferring the pattern of the photo-resist layer to the etch mask layer; and stripping away the photo-resist layer. The etch mask layer comprises one or more of an oxide or a nitride. Depositing an etch mask layer over the lithium-based membrane and the one or more electrodes comprises: depositing an etch mask layer over the lithium-based membrane and the one or more electrodes using plasma-enhanced chemical vapor deposition or physical sputtering. Transferring the pattern of the photo-resist layer to the etch mask layer comprises: transferring the pattern of the photo-resist layer to the etch mask layer using reactive ion etching with a fluorine-based etching recipe. The method further comprises: after removing the bonding agent, removing the etch mask layer. Removing the etch mask layer comprises: removing the etch mask layer using reactive ion etching with a fluorine-based etching recipe. Etching the at least one release window extending through the lithium-based membrane to the bonding agent comprises: etching the at least one release window using reactive ion etching or inductively-coupled plasma etching with a chlorine-based solvent. Removing the bonding agent comprises: releasing an etchant in the at least one release window using a liquid chemical etch or a vapor etch. The etchant comprises one or more of a sulfuric acid based etchant or a hydrofluoric acid based etchant. The method further comprises: performing drying after removing the bonding agent. Performing drying comprises: performing point drying after removing the bonding agent. A micro-electro-mechanical system (MEMS) comprising a suspended lithium-based membrane semiconductor structure formed according to the method. A moving structure comprising a suspended lithium-based membrane semiconductor structure formed according to the method. 
         [0010]    The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description, the drawings, and the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0011]      FIG. 1  shows an example of a suspended lithium-based membrane microelectronic device. 
           [0012]      FIG. 2  shows an example of a suspended lithium-based membrane microelectronic device with a weighted electrode configuration. 
           [0013]      FIG. 3  is a cross-sectional view of the device of  FIG. 2 . 
           [0014]      FIG. 4  is a flowchart of an example of a process for forming suspended lithium-based membrane microelectronic structures. 
           [0015]      FIGS. 5-7  show side views of a suspended lithium-based membrane microelectronic structure during different stages of fabrication. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]      FIG. 1  shows an example of a suspended lithium-based membrane microelectronic device  100 . The device  100  may include inter-digital transducer (IDT) metal electrodes  102  on a suspended lithium-based membrane  104 . The term “lithium-based” as used in this disclosure, refers to materials that include lithium as part of their composition. For example, the lithium-based membrane  104  may include lithium niobate (LN), lithium tantalite (LT), or both. The electrodes  102  may be evenly spaced and may each have identical width. 
         [0017]    The suspended lithium-based membrane microelectronic device  100  may be operated as a resonator by applying an alternating voltage across adjacent electrodes  102 . For example, the electrodes  102  may be alternatively connected different signals, such as a ground signal and an input signal, which may be applied to the device  100  to induce an electric field within the device  100 . The associated electric field generates a mechanical deformation or strain in the lithium-based membrane  104  through a piezoelectric effect and excites lateral expansion and compression of the membrane  104  (a mode of vibration known as S 0  lamb wave). A charge is generated when the membrane  104  vibrates. When the frequency of the applied electric field coincides with the mechanical resonance frequency of the suspended lithium-based membrane  104 , the mechanical vibrations are enhanced, and the device  100  effectively behaves as a resonator. The mode of vibration of the suspended lithium-based membrane  104  may depend on the geometry of the membrane  104  and the size and pitch of the electrodes  102 . Several modes of vibration may be excited, such as shear, length-extensional, width-extensional, lamb, and flexural acoustic waves. 
         [0018]    The device  100  with a membrane  102  of LN material may have a higher intrinsic electromagnetic coupling (k t   2 ) than an Aluminum Nitride (AlN) Contour Mode Resonator (CMR). A center frequency of the device  100  may be based on the lithographically defined pitch of the electrode fingers, which may enable multi-frequency resonators on a single chip. Full suspension of the device may efficiently trap energy and enhance the quality factor (Q) with respect to LN-saw acoustic wave (SAW) devices. The high velocity (e.g., approximately 6500 m/s) of the selected S 0  lamb wave mode may enable higher frequencies of operation (e.g., frequencies of approximately 1 MHz to 10 GHz) using the same lithographic resolution of SAW devices. 
         [0019]      FIG. 2  shows an example of a suspended lithium-based membrane microelectronic device  200  with a weighted electrode configuration on a suspended lithium-based thin film  201 . For the weighted electrode configuration, the widths of electrodes may be gradually reduced toward the edges of the device  200 . Center and inner electrodes having identical width are referred to as unweighted electrodes. Electrodes having smaller widths than the unweighted electrodes are referred to as weighted electrodes. A device  200  may have any number of weighted and unweighted electrodes. 
         [0020]    For the device  200  of  FIG. 2 , the two electrodes close to an edge of the device  200  are weighted electrodes. For example, the electrodes  202  and  204  close to the left edge of the device  200  are weighted electrodes. The width of electrode  202  is smaller than the width of electrode  204 . The width of the electrode  204  is smaller than the width of electrode  206 . Similarly, the electrodes  208  and  210  close to the right edge of the device are weighted electrodes. The width of electrode  208  is smaller than the width of electrode  210 . The width of electrode  210  is smaller than the width of electrode  212 . The electrodes  206  and  212  and electrodes between the electrodes  206  and  212  may be unweighted electrodes having identical width. 
         [0021]      FIG. 3  is a cross-sectional view of the device  200  of  FIG. 2  along line A. Examples of design parameters for the device  200  and values are listed in Table 1 below. In Table 1, λ refers to the acoustic wavelength of the desired design frequency. The unweighted electrodes  206  and  212  may be centered on their respective fingers. The weighted electrodes  202 ,  204 ,  208 , and  210  may be centered or off-centered on their respective fingers. Multiple design variations are possible to control the performance of the device  200 . For example, the number of fingers and relative spacing can be altered (increased or decreased) depending on the frequency of operation and the number of fingers forming the device  200 . 
         [0000]    
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Parameter 
                 Description 
                 Value 
               
               
                   
               
             
             
               
                 N 
                 Number of unweighted electrodes 
                  0-100 
               
               
                 Wp 
                 Unweighted electrode finger pitch 
                 λ/2 
               
               
                 Wf 
                 Unweighted electrode width 
                 λ/8-λ/2 
               
               
                 W1 
                 Weighted electrode 202 finger width 
                 3λ/16-5λ/16 
               
               
                 Wf1 
                 Weighted electrode 202 width 
                  λ/16-5λ/16 
               
               
                 W2 
                 Weighted electrode 204 finger width 
                 λ/2 
               
               
                 Wf2 
                 Weighted electrode 204 width 
                 λ/8-λ/2 
               
               
                 Ws12 
                 Spacing between weighted electrodes 202 
                 λ/8-λ/2 
               
               
                   
                 and 204 
               
               
                 Ws2u 
                 Spacing between weighted electrode 204 
                  λ/8-2λ/3 
               
               
                   
                 and last unweighted electrode 206 
               
               
                 Ws1e 
                 Spacing between weighted electrode 202 
                 0~3λ/16 
               
               
                   
                 and membrane 201 edge/acoustic boundary 
               
               
                   
               
             
          
         
       
     
         [0022]    Operation of the device  200  of  FIGS. 2 and 3  is similar to the operation of the device  100  of  FIG. 1 . The device  100 , however, may introduce unwanted spurious modes and overtone resonances due to the high intrinsic electromagnetic coupling (k t   2 ) of the LN material, which may be reduced or prevented by the weighted electrode configuration of the device  200 . The device  200  may be more efficient than the device  100  and may maximize the intrinsic electromagnetic coupling of the LN material by dispersing the energy in various modes. The mechanical energy within the device  200  is concentrated into the fundamental S 0  lamb wave mode, and unwanted parasitic modes may be significantly subdued. As a result, the electromagnetic coupling of the fundamental response may be significantly enhanced. 
         [0023]    The device  200  of  FIG. 2  meets temperature stability requirements for existing commercial RF bands. The temperature coefficient of frequency (TCF) for devices  200  with three orientations of electrode patterns, namely 30°, 50°, and 70° to +Y axis, are listed below in Table 2. For the device  200  at 30° orientation, the TCF is −74 ppm/K, which may be higher than for an AlN CMR (−30 ppm/K) but lower than for a LN-SAW device (−90 ppm/K). 
         [0000]    
       
         
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Orientation (to +Y) 
                 TCF (ppm/K) 
               
               
                   
                   
               
             
             
               
                   
                 30° 
                 −74 
               
               
                   
                 50° 
                 −76 
               
               
                   
                 70° 
                 −63 
               
               
                   
                   
               
             
          
         
       
     
         [0024]      FIG. 4  is a flowchart of an example of a process  400  for forming suspended lithium-based membrane microelectronic structures, e.g., the device  100  of  FIG. 1  and the device  200  of  FIG. 2 . Briefly, the process  400  includes depositing a bonding agent on a surface of a lithium-based carrier substrate ( 402 ), implanting ions into a surface of a lithium-based donor substrate to form an ion-implanted surface of the lithium-based donor substrate ( 403 ), bonding the ion-implanted surface of the lithium-based donor substrate to the bonding agent ( 404 ), removing the lithium-based donor substrate from the ion-implanted surface leaving a lithium-based membrane on the bonding agent ( 406 ), polishing an exposed surface of the lithium-based membrane ( 408 ), forming one or more electrodes on the lithium-based membrane ( 410 ), etching at least one release window extending through the lithium-based membrane to the bonding agent ( 412 ), and removing the bonding agent ( 414 ). The process  400  will not be described in more detail with reference to  FIG. 4 . 
         [0025]      FIGS. 5-7  show side views of a suspended lithium-based membrane microelectronic structure during different stages (a)-(j) of fabrication. A bonding agent  502  is deposited on a surface of a lithium-based carrier substrate  504  in stage (a). The lithium-based carrier substrate  504  may be LN, LT, or both. The bonding agent  502  may be either an adhesive agent, such as benzocyclobutene (BCB) or polyimide, or an oxide, such as Si0 2 . The bonding agent  502  may be deposited on the substrate  504  using any suitable deposition technique. The bonding agent  502  may serve as a sacrificial layer for suspending the microelectronic structure. 
         [0026]    In stage (b), a lithium-based donor substrate  506  is implanted with ions  508  to form an ion-implanted surface of the lithium-based donor substrate. The lithium-based donor substrate  506  may be an X-cut substrate, and implantation may be performed on the X-cut substrate. An X-cut substrate is a substrate which was grown with a particular orientation. The implanted ions  508  may penetrate the donor substrate  506  from a surface of the substrate  506  to a depth represented by an ion implantation line  510 . A dose of ions may be selected to enable high yield splitting of the donor substrate  506  at low temperatures, e.g., a temperature less than 350° C. A thickness of the ion implanted surface  512  may be controlled by the amount, energy, or both of the ions  508  implanted on the donor substrate  506 . The relationship between ion energy and thickness of the ion implanted surface  512  may be non-linear and regulated by ion scattering relationship. In some implementations, the average thickness of the ion-implanted surface  512  is in the range between approximately 100 nm and 30 μm. In some implementations, an electrode (not shown) may be formed on the ion-implanted surface  512  of the donor substrate  506  using, e.g., any suitable patterning technique. 
         [0027]    In stage (c), the ion-implanted surface  512  of the lithium-based donor substrate  506  is bonded to the bonding agent  502  on the lithium-based carrier substrate  504 . The ion-implanted surface  512  may be bonded to the bonding agent  502  using any suitable bonding technique. 
         [0028]    In stage (d), the lithium-based donor substrate  506  is removed from the ion-implanted surface that is bonded to the bonding agent. After removal of the lithium-based donor substrate  506 , at least a portion of the ion-implanted surface  512  remains bonded to the bonding agent as a lithium-based membrane  514 . In some implementations, the portion of the ion-implanted surface  512  left as the membrane  514  is the entire ion-implanted surface  512 . In other implementations, the portion of the ion-implanted surface  512  left as the membrane  514  may be less than the entire ion-implanted surface  512 . 
         [0029]    The lithium-based donor substrate  506  may be removed from the ion-implanted surface using a heat treating technique that causes the donor substrate  506  to split along the implantation line  510 . The amount and energy of the ion implantation on the lithium-based donor substrate  506  may be selected to enable splitting to occur at relatively low temperatures, e.g., temperatures less than 350° C. For example, the donor substrate  506  may be heated at an average temperature of 200° C. to 350° C. between two to twelve hours. 
         [0030]    After the donor substrate  506  splits leaving the lithium-based membrane  514 , the lithium-based membrane  514  may be chemically polished, mechanically polished, or both to reduce roughness of the exposed surface  516  of the membrane  514 . In some implementations, the roughness of the exposed surface  516  may be reduced to less than approximately 5 nm. 
         [0031]    At stage (e), electrodes  518  are formed on the lithium-based membrane  514 . The electrodes  518  may be formed using any suitable lithography or metal deposition techniques such as physical sputtering, electron-beam evaporation, thermal evaporation, or a combination of techniques. The electrodes  518  may include any suitable metal, metallic thin film, or a combination of metals or metallic thin films. Examples of suitable metallic materials include aluminum, gold, platinum, molybdenum, or copper. The electrodes  518  may be in the form of inter-digital electrode fingers. For example, the electrodes  518  may be inter-digital titanium electrodes having an average thickness of 5 nm, or inter-digital aluminum electrodes having an average thickness of 150 nm. 
         [0032]    Stages (f)-(h) show an example of a technique for etching release windows that extend through the membrane  514  to the bonding agent  502 . After the electrodes  518  are formed, an etch mask layer  520  may be deposited over the electrodes  518  and the lithium-based membrane  516  at stage (f). The etch mask layer  520  may prevent the erosion of the electrodes  518  by the etching chemicals during etching of the release windows. The etch mask layer  520  may serve as a protective layer for the electrodes  518  during removal of the bonding agent. The etch mask layer  520  may be deposited with a plasma-enhanced chemical vapor deposition system or a physical sputtering system. The etch mask layer  520  may include either an oxide or a nitride, such as Si0 2 . 
         [0033]    A photo-resist layer  522  may be deposited over the etch mask layer  520 . The photo-resist layer  522  may include polymers. Patterns  524  may be etched in the photo-resist layer  522  using any suitable lithography technique. 
         [0034]    At stage (g), the patterns  524  of the photo-resist layer  522  may be transferred to the etch mask layer  520 . A reactive ion etching (RIE) technique with a recipe using fluorine-based chemistry may be used to transfer the patterns  524  to the etch mask layer  520 . The recipe may be Cl 2  based. The photo-resist layer  522  may be stripped away after the transfer of the patterns  524  to the etch mask layer is complete. 
         [0035]    At stage (h), release windows  526  may be etched on two or four sides of the set of electrodes  518 . The release windows  526  define the boundaries for the microelectronic structure. The release windows  526  extend through the lithium-based membrane  514  to the bonding agent  502 . The release windows  526  may be etched using a chlorine-based solvent, and the etch mask layer  520  protects the electrodes  518  from the chlorine. The release windows  526  may be etched using reactive ion etching or inductively coupled plasma etching. 
         [0036]    At stage (i), the bonding agent  502  under the set of electrodes  518  is removed, resulting in the suspension of the lithium-based membrane  514  with respect to the lithium-based carrier substrate  504 . The bonding agent  502  may be removed through an etch release. The etch release may be performed by releasing an etchant in the release windows  526 . The etchant released may be a sulfuric acid-based etchant, such as Nanostrip or piranha, a hydrofluoric acid-based etchant, such as a buffered oxide etchant, or any other suitable etchant that will remove the bonding agent  502  without etching the etch mask layer  520 . When using a sulfuric acid-based etchant or a hydrofluoric acid-based etchant, the etch release process may be either a liquid chemical etch or a vapor etch. A critical point drying (CPD) may be performed to enable full suspension of the membrane  514  after removing the bonding agent  502 . The resulting membrane  514  may be fully suspended on the carrier substrate with four tethers placed near the corners of the structure. 
         [0037]    The etching technique shown in stages (f)-(h) may yield straighter sidewall definition for orientations with high electromechanical coupling (e.g., 20-40° to +Y axis), which may enhance the Q of the structure. The parameters for optimized RIE compared to original RIE using a Trion system are listed below in Table 3. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Parameter 
                   
                 Optimized 
                 Original 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Cl 2   
                 5 
                 sccm 
                 10 
                 sccm 
               
               
                   
                 BCl 3   
                 15 
                 sccm 
                 25 
                 sccm 
               
               
                   
                 Ar 
                 20 
                 sccm 
                 10 
                 sccm 
               
               
                   
                 Temp 
                 65° 
                 C. 
                 65° 
                 C. 
               
               
                   
                 RIE power 
                 280 
                 W 
                 250 
                 W 
               
               
                   
                 ICP power 
                 600 
                 W 
                 600 
                 W 
               
               
                   
                 Pressure 
                 10 
                 mT 
                 15 
                 mT 
               
               
                   
                 Etch rate 
                 45 
                 nm/min 
                 40 
                 nm/min 
               
               
                   
                   
               
             
          
         
       
     
         [0038]    By introducing a higher ratio of Ar to the gas mixture, a previously observed RIE dependence on orientation is mitigated. As a result of the better defined acoustic boundaries of the structure using optimized RIE, the Q may be improved from 300 to 1300. Combined with a high electromagnetic coupling, a FOM of 280 at 30° to +Y axis may be achieved. Such FOM may result in a low motional resistance of 18Ω. As the orientation of the structure increases to a larger angle, the motional impedance may increase due to the diminishing electromagnetic coupling. Despite the impedance increase, this feature may be used to design filters with different bandwidths on the same chip. 
         [0039]    In stage (j), the etch mask layer  520  is removed. The lithium-based membrane  514  with the electrodes  518  is suspended after the etch mask later  520  is removed. The etch mask layer  520  may be removed using reactive ion etching with a fluorine-based recipe. After the bonding agent  502  and the etch mask layer  520  are removed, any post etch residue and liquid remaining under the suspended lithium-based membrane  514  may be removed using a critical point dryer system. In implementations where electrodes (not shown) are formed on the ion-implanted surface  512  prior to the bonding at stage (c), electrodes  518  are formed on the top and electrodes (not shown) are formed on the bottom of the suspended lithium based membrane  514 . 
         [0040]    A number of implementations have been described. Nevertheless, various modifications can be made without departing from the spirit and scope of the processes and techniques described herein. In addition, the processes depicted in the figures do not require the particular order shown, or sequential order, to achieve desirable results. In addition, other steps can be provided, or steps can be eliminated, from the described processes, and other components can be added to, or removed from, the describe apparatus and systems. Accordingly, other embodiments are within the scope of the following claims.