Abstract:
A micro electrical-mechanical systems (MEMS) device INCLUDES a MEMS substrate and at least one MEMS structure on the MEMS substrate. In addition, there is at least one battery cell on the MEMS substrate coupled to the at least one MEMS structure. The at least one battery cell includes a support fin extending vertically upward from the MEMS substrate and a first electrode layer on the support fin. In addition, there is an electrolyte layer on the cathode layer, and a second electrode layer on the electrolyte layer. The support fin may have a height greater than a width. The first electrode layer may have a processing temperature associated therewith that exceeds a stability temperature associated with the second electrode layer.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to the field of micro electrical-mechanical systems (MEMS) and, more particularly, to battery cells for MEMS and related methods. 
       BACKGROUND OF THE INVENTION 
       [0002]    Micro electrical-mechanical systems (MEMS) are typically made from components of 1 to 100 micrometers in size, and MEMS devices generally range in size from 20 micrometers to a millimeter, involving the integration of both electrical and mechanical elements, sensors, actuators, and the like on a substrate utilizing micro-fabrication technology. MEMS devices are particularly useful because they may combine the computational ability of microelectronics with the perception capabilities of microsensors and the precise control capabilities of microactuators. Indeed, the fabrication and integration of these elements on a single substrate may allow the realization of complete systems on a single chip. 
         [0003]    MEMS technology has already become commonplace in today&#39;s world and is employed in a variety of applications, such as accelerometers that detect collisions in cars, pressure sensors that detect air pressure in car tires, and optical switching for communications. Components of MEMS devices, such as controllers and actuators, often require a power source. In some cases, it may not be desirable, convenient, or feasible to draw power from an external source. As such, battery cells sized for integration in MEMS devices have been developed. 
         [0004]    For example, U.S. Pat. Pub. 2004/0191626 to Lewis, Jr. et al. discloses a volumetric lithium-ion battery for use in MEMS. The battery is constructed from materials capable of providing one Joule per cubic millimeter and has a higher capacity than typical planar MEMS batteries because it is thicker in the dimension perpendicular to the plane of the electrodes, designated the Z dimension. The MEMS battery has a volume of approximately one cubic millimeter, and the layers thereof are arranged in a vertically stacked arrangement. 
         [0005]    U.S. Pat. No. 5,338,625 to Bates et al. discloses a thin film battery for use in MEMS devices. The battery includes a lithium anode, a vanadium oxide cathode, and an electrochemically stable electrolyte sandwiched therebetween so that the layers are arranged in a vertically stacked arrangement. In order for an implementation of either of these batteries to generate larger voltages, however, it may require many cells arrayed on a large percentage of the available space of the substrate upon which they are carried, which may not be desirable. 
         [0006]    U.S. Pat. No. 6,610,440 to LaFollette et al. discloses a battery for use in MEMS. Individual battery cells of this battery each include an anode, a cathode, and an electrolyte sandwiched therebetween so that these layers are arranged in a vertically stacked arrangement. The individual battery cells are arranged adjacent each other on a silicon substrate and may be coupled in series or parallel. The generation of higher voltages with such a battery may require numerous such battery cells, which, in the aggregate, may undesirably consume a large percentage of the available space on the silicon substrate. 
       SUMMARY OF THE INVENTION 
       [0007]    In view of the foregoing background, it is therefore an object of the present invention to provide a battery cell for a MEMS device that conserves valuable space on a MEMS substrate yet produces desired voltages. 
         [0008]    This and other objects, features, and advantages in accordance with the present invention are provided by a micro electrical-mechanical systems (MEMS) device that includes a substrate and at least one MEMS structure on the substrate. The MEMS device has at least one battery cell on the substrate that is coupled to the at least one MEMS structure. The at least one battery cell comprises a support fin extending vertically upward from the MEMS substrate and a first electrode layer on the support fin that comprises the cathode. There is an electrolyte layer on the cathode layer, and a second electrode layer on the electrolyte layer that comprises the anode. 
         [0009]    The support fin may have a height greater than a width. Indeed, the support fin may have a height greater than 30 μm and a width less than 20 μm. This battery cell advantageously conserves valuable space on the MEMS substrate while providing a greater voltage than prior batteries for MEMS devices. 
         [0010]    The first electrode layer may have a processing temperature associated therewith that exceeds a stability temperature associated with the second electrode layer. The at least one battery cell may comprise a plurality thereof, and a pattern of electrically conductive traces may couple the plurality of battery cells in series. This series connection of the plurality of battery cells advantageously allows for a greater output voltage than possible with either a single battery cell, or a plurality of battery cells connected in parallel. 
         [0011]    The first electrode layer may comprise a cathode layer, and the second layer may comprise an anode layer. The support fin may comprise an electrically conductive material, for example, copper. In addition, the first electrode layer may comprise Lithium Cobalt Oxide. The electrolyte layer may comprise a glass electrolyte, such as Lithium Phosphorous Oxynitride (LiPON). The second electrode layer may comprise Lithium. 
         [0012]    The MEMS device may include semiconductor circuitry on the MEMS substrate coupled to the at least one MEMS structure and the at least one battery cell. In addition, there may be at least one photovoltaic cell on the MEMS substrate for charging the at least one battery cell. The inclusion of a photovoltaic cell may be particularly advantageous in that it, together with the battery cell, can be used to construct a self contained MEMS device with a long lifetime, as this battery cell design may have an operating life of more than 50,000 charge and discharge cycles. 
         [0013]    A method aspect is directed to a method of making a micro electrical-mechanical systems (MEMS) device comprising forming at least one MEMS structure on a MEMS substrate and forming at least one battery cell on the MEMS substrate to be coupled to the at least one MEMS structure. The at least one battery cell is formed by at least forming a support fin extending vertically upward from the MEMS substrate and forming a first electrode layer on the support fin. An electrolyte layer may be formed on the cathode layer, and a second electrode layer may be formed on the electrolyte layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a perspective view of a MEMS device in accordance with the present invention. 
           [0015]      FIG. 2  is a perspective view of another embodiment of a MEMS device in accordance with the present invention. 
           [0016]      FIG. 3  is a flowchart of a method of making a MEMS device in accordance with the present invention. 
           [0017]      FIG. 4  is a more detailed flowchart of a method of making a battery cell for a MEMS device in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout, and prime notation is used to show similar elements in other embodiments. 
         [0019]    Referring initially to  FIG. 1 , a MEMS device  10  is now described. The MEMS device  10  includes a MEMS substrate  11  that carries two battery cells  13   a ,  13   b , a MEMS structure  12 , a photovoltaic cell  14 , and semiconductor circuitry  15 . The MEMS structure  12 , the photovoltaic cell  14 , and the semiconductor circuitry  15  are coupled to the battery cells  13   a ,  13   b  via conductive traces on the MEMS substrate  11 . The photovoltaic cell  14  is of a type known to those skilled in the art and is coupled to the battery cells  13   a ,  13   b  for recharging the battery cells through the semiconductor circuitry via conductive traces. The semiconductor circuitry  15  may include a processor that is coupled to the MEMS structure  12  for control thereof via conductive traces, and may also include power control circuitry to regulate battery charging and supply voltage for the MEMS circuitry. 
         [0020]    The MEMS substrate may typically be silicon, although in other embodiments it may be polymer, ceramic, metal, or other suitable materials. The MEMS structure  12  may be an actuator, accelerometer, pressure sensor, or gyroscope, for example. 
         [0021]    Further details of the battery cells  13   a ,  13   b  are now given with additional reference to  FIG. 1 . The battery cells  13   a ,  13   b  are coupled together in series via conductive traces  14 . This advantageously allows a high voltage MEMS battery to be constructed from a plurality of such battery cells  13   a ,  13   b . The conductive traces may be gold, copper, nickel or another suitable conductor. 
         [0022]    Each battery cell  13   a ,  13   b  includes a support fin  23   a ,  23   b  extending vertically upward from the MEMS substrate  11 . The support fins  23   a ,  23   b  are constructed from copper, although other suitable materials may also be used. The copper or other suitable metal that comprises the fin may also be coated by a second metal or alloy to improve the oxidation resistance or to impart a diffusion barrier. The height of the support fins  23   a ,  23   b  is illustratively greater than their width. The height of the support fins  23   a ,  23   b  is preferably greater than 30 micrometers and the width is preferably less than 20 micrometers, although support fins with other dimensions may be used. Consequently, the height of each battery cell  13   a ,  13   b  is greater than its width, advantageously conserving valuable space on the MEMS substrate  11 . In some applications, the side walls of the fins may not be vertical, but rather may be sloped in a manner that the base of the fin may be wider than the top, with a height greater than a width at the base. By shaping the battery cells  13   a ,  13   b  in such a fashion many such cells can fit on the MEMS substrate  11  and can be coupled in series to create a high voltage battery, such as 50 to 150 volts, or coupled in parallel to create a battery capable of delivering higher currents, or in a series-parallel configuration to balance both voltage and power capability. 
         [0023]    First electrode layers, illustratively cathodes  22   a ,  22   b , are on the support fins  23   a ,  23   b . The cathodes  22   a ,  22   b  may be formed from lithium cobalt oxide, although other suitable materials may be used. For example, a cathode constructed from any lithium intercalation compound having open channels or layers that can accommodate the diffusion and storage of lithium ions without inducing an irreversible change in the surrounding framework would be suitable. Such suitable materials include for example, lithium manganese oxide, lithium nickel oxide and lithium iron phosphate. 
         [0024]    Electrolyte layers  21   a ,  21   b  are on the cathode layers  22   a ,  22   b . The electrolyte layers  21   a ,  21   b  are preferably constructed from lithium phosphorous oxynitride glass, although other solid state electrolytes, such as a variety of lithium containing salts, ceramics, glasses and polymers, may be used. The use of a solid electrolyte instead of a liquid electrolyte reduces the chance of issues such as damage to the battery cell and its surroundings due to liquid leakage, and leaching of electrode material. 
         [0025]    Second electrode layers, illustratively anodes  20   a ,  20   b , are on the electrolyte layer  21   a ,  21   b . The anodes  20   a ,  20   b  are preferably constructed from metallic lithium, although other suitable materials capable of storing lithium ions or alloying with lithium metal, such as carbon, silicon, tin, or the like, may also be used. 
         [0026]    The cathode layers  22   a ,  22   b , and the anode layers  20   a ,  20   b  are preferably of uniform thicknesses, although need not be so. Likewise, the electrolyte layers  21   a ,  21   b  are also preferably of uniform thicknesses, although they also need not be so. 
         [0027]    Those of skill in the art will appreciate that, in some embodiments, the MEMS device  10 ′ need not include photovoltaic cells and semiconductor circuitry, as shown in  FIG. 2 . The elements of the MEMS device  10 ′ not specifically mentioned are similar to those of the MEMS device  10  as described above with respect to  FIG. 1  and require no further description herein. 
         [0028]    With reference to the flowchart  30  of  FIG. 3 , a method of making a MEMS device is now described. After the start (Block  31 ), at Block  32 , a MEMS structure is formed on a MEMS substrate. Next, a conductive trace is formed for contacting a first pole of the battery (Block  33 ). 
         [0029]    Thereafter, at least one battery cell is formed on the MEMS substrate to be coupled to the MEMS structure. The battery cell is formed by forming a support fin extending vertically upward from the MEMS substrate (Block  34 ), and forming a first electrode layer on the support fin (Block  35 ). An electrolyte layer is formed on the cathode layer (Block  36 ), and a second electrode layer is formed on the electrolyte layer (Block  37 ). A conductive trace is then formed for interconnecting battery cells and contacting a second pole of the battery (Block  38 ). Block  39  indicates the end of the method. 
         [0030]    Those of skill in the art will appreciate that the MEMS structure may be formed after all or part of the battery cell in some applications. For example, the MEMS structure may be formed after the cathode so the MEMS structure would not be exposed to the same process temperatures as the cathode. It may also be advantageous to form part of the battery cell, for example the support fin, during formation of the MEMS structure. 
         [0031]    Further details of the formation of the battery cells  13   a ,  13   b  are now given with reference to flowchart  40  of  FIG. 4 . After the start (Block  41 ), at Block  42 , a copper interconnection layer is formed on the MEMS substrate using techniques known to those of skill in the art. At Block  43 , a photoresist mask corresponding to desired locations and shapes of the support fins is formed on the MEMS substrate. 
         [0032]    At Block  44 , the support fins are formed from copper via electrodeposition. At Block  45 , the photoresist mask is then removed. At Block  46 , a lift-off resist mask corresponding to desired locations and shapes of the cathodes is formed, and at block  47 , lithium cobalt oxide is deposited via sputtering. At Block  48 , the lift-off resist mask, together with the excess lithium cobalt oxide is removed, thereby leaving lithium cobalt oxide formed into the desired shape of the cathodes. At Block  49 , the MEMS substrate is annealed to thereby form crystalline lithium cobalt oxide cathodes. 
         [0033]    At Block  50 , a photoresist mask is formed on the MEMS substrate that corresponds to desired locations and shapes of the electrolyte layer and the anodes. At Block  51 , lithium phosphorous oxynitride (LiPON) for the electrolyte layer is deposited via sputtering, and at Block  52 , lithium for the anode is deposited via sputtering. At Block  53 , the photoresist is removed to thereby form the LiPON into the electrolyte layer and the lithium into the anode. 
         [0034]    As will be appreciated by those skilled in the art, the lithium cobalt for the cathodes is annealed at a temperature greater than a stability temperature (such as a melting point) of the lithium for the anodes. For this reason, the cathodes are advantageously formed before the anodes. 
         [0035]    At Block  54 , a photoresist corresponding to desired locations and shapes of the connections between the battery cells is formed. At Block  55 , cooper is deposited via sputtering to complete the connections between the battery cells and optionally between the battery poles and the power buss for the device. At Block  56 , the photoresist is removed. At Block  57 , a barrier film is formed over the battery cells, and the method ends at Block  58 . 
         [0036]    Many modifications and other embodiments of the invention will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is understood that the invention is not to be limited to the specific embodiments disclosed, and that modifications and embodiments are intended to be included within the scope of the appended claims.