Abstract:
A microelectronic supercapacitor is amenable to being fabricated using micro electromechanical systems (MEMS) techniques. By utilizing MEMS techniques, the supercapacitor in accordance with the present invention can be formed with volumes &lt;1 mm 3 . As such, such microelectronic supercapacitor is suitable for use in applications in which only a few millimeters are available for both a supercapacitor and an energy storage device, such as a battery.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is related to commonly-owned co-pending patent application Ser. No. ______, entitled “Volumetric Micro Batteries”, attorney docket number 11-1186, filed on even date. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to supercapacitors and more particularly to microelectronic supercapacitors formed from micro electromechanical systems (MEMS) techniques formed with volumes ≦1 mm 3 .  
           [0004]    2. Description of the Prior Art  
           [0005]    Supercapacitors are generally known in the art. Examples of such supercapacitors are disclosed in U.S. Pat. Nos. 5,151,848 and 5,426,561, hereby incorporated by reference. In general, such supercapacitors relate to high surface area capacitors, such as double layer capacitors, which can provide up to 2000 times the volumetric capacitance of conventional electrolytic capacitors.  
           [0006]    Various applications exist which require power supplies including capacitors that are limited in size to a few cubic millimeters of volume. Examples of such applications include high-speed electronic applications that require on-chip/on-board power supplies to prevent local current starvation. Other applications include microsensors and military applications, such as precision munition systems.  
           [0007]    Supercapacitors are known to be used with power supplies to speed up the delivery of the electric energy stored in a battery to a load. More particularly, power supplies are formed from energy storage devices, such as a battery. Such energy storage devices are normally characterized by energy density (i.e. energy stored per unit of volume or mass) and power (how fast the energy can be delivered). In order to increase the power or rate at which the energy within the battery can be delivered to a load, supercapacitors are known to be coupled to the battery. Unfortunately, known supercapacitors are component size (i.e. much larger than a few millimeters of volume) and are thus not suitable for use in the various applications discussed above. Thus, there is a need for supercapacitor having a total volume of less than a few cubic millimeters.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention relates to a microelectronic supercapacitor that is amenable to being fabricated using micro electromechanical systems (MEMS) techniques. By utilizing MEMS techniques, the supercapacitor in accordance with the present invention can be formed with volumes ≦mm 3 . As such, such microelectronic supercapacitors are suitable for use in applications as discussed in which only a few cubic millimeters are available for both a supercapacitor and an energy storage device, such as a battery. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0009]    These and other advantages of the present invention will be readily understood from the following specification and attached drawings wherein:  
         [0010]    [0010]FIG. 1 is a sectional view of a microelectronic supercapacitor in accordance with the present invention.  
         [0011]    [0011]FIG. 2 further illustrates the cell separator, which forms a portion of the supercapacitor in accordance with the present invention.  
         [0012]    [0012]FIG. 3 illustrates an alternate embodiment of the supercapacitor with a jelly roll geometry which maximizes the surface area for increased capacitance energy storage.  
         [0013]    [0013]FIG. 4 illustrates another embodiment of the supercapacitor formed from planar layers.  
         [0014]    FIGS.  5 A- 5 G are process diagrams illustrating the process for fabricating a microcapacitor with an insulator in accordance with the present invention in which both electrodes are in the same plane.  
         [0015]    FIGS.  6 A- 6 E represent an alternate embodiment illustrating the process for fabricating a microcapacitor without an insulator in accordance with the present invention in which the electrodes are located in different planes.  
         [0016]    FIGS.  7 A- 7 H are process diagrams illustrating the process for fabricating a micro-supercapacitor having a jelly roll configuration in accordance with an alternate embodiment of the present invention.  
         [0017]    FIGS.  8 A- 8 G are process diagrams for fabricating an array of micro-supercapacitors in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0018]    The present invention relates to a supercapacitor and in particular to a microelectronic supercapacitor formed with a volume ≦1 mm 3 , making it suitable for applications which require power supplies including supercapacitors within a volume of less than a few cubic millimeters. The supercapacitors are formed using micro electromechanical systems (MEMS) manufacturing processes, such as soft lithography processes used for non-silicon materials, such as ceramics, polymers and plastics, and are thus about 1000 times smaller by volume than conventional supercapacitors. Another important aspect of the invention is that the supercapacitor has a relatively high specific power storage characteristic, for example approximately 10,000 watts/kg, higher than any other stored energy device including batteries and fuel cells. An exemplary supercapacitor in accordance with the present invention, operating at about 2.8 volts formed with an organic electrolyte provides 5 mF of capacitance with 10 mW power for about 0.7 seconds.  
         [0019]    [0019]FIGS. 1 and 2 represent one embodiment of the supercapacitor in accordance with the present invention. FIG. 3 illustrates a spiral or jelly roll configuration in which the surface area is maximized for increased capacitance and energy storage. FIG. 4 relates to yet another embodiment in which multiple supercapacitors in parallel are created using volume stacking.  
         [0020]    Referring to FIG. 1, the supercapacitor, generally identified with the reference numeral  20 , includes two electrodes  22  and  24  and a porous polymer separator  26 . The volume between the porous polymer separator  26  and the electrodes  22  and  24  form an electrolyte tank with two cavities for receiving of an electrolyte  28 . An insulator  30  is used to form a generally circular sidewall.  
         [0021]    The electrodes  22 ,  24 , polymer separator  26  and the insulator sidewall  30  form an electrolyte tank carried by a substrate  32 , for example silicon wafers, glass slides or polyimide flexible substrates. The substrate  32  closes the bottom of the electrolyte tank. The top of the electrolyte tank is closed by an insulated top  34  to provide a sealed device.  
         [0022]    The electrode  22  and  24  can be formed from various materials. Exemplary materials include carbon, carbon aerogel, metal oxides or a combination of these materials. Carbon and carbon aerogel electrodes provide relatively large internal surface areas, for example, approximately 2500 m 2  per gram of material, which provides a relatively large surface interface between the electrolyte  28  and the electrodes  22  and  24 .  
         [0023]    Various materials are suitable for the electrolyte  28 . For example, both solid and liquid electrolytes are suitable. Liquid electrolytes may include caustic or acid electrolytes. Examples of liquid electrolytes include aqueous or organic salt/solvent systems; the solvent can be selected from: water, propylene carbonate, propylene carbonate/diethylcarbonate, 1,3 dioxolane and tetrahydrofuran; the salts include: lithium perchlorate, tetrabutylammonium tetraphenylborate, tetraethylphosphonium tetrafluoroborate, and tetraethylammonium tetrafluoroborate. Examples of solid electrolytes include hydrated Nafion® (a registered trademark of DuPont) electrolyte. The specific electrolyte selected determines the operating voltage of the supercapacitor, which strongly contributes to the stored energy (i.e. E=½ CV 2 ).  
         [0024]    The separator  26  together with the electrodes  22  and  24  form two electrolyte cavities  28  to create a double layer of electrolyte  28  as shown. The separator  26  may be formed from an organic polymer, such as Celgard®, PTFE or any organic polymer that is charge permeable.  
         [0025]    As shown, in FIG. 2 the porous separator is used to enable electronic separation of the electrodes while allowing passage of ionic species. It is the accumulation of ions at the electrodes that provides the high charge storage capacity for supercapacitors.  
         [0026]    In accordance with an important aspect of the invention, the supercapacitor may be formed from MEMS processes and, in particular, soft lithography processes, as discussed above. Two embodiments of a micro-supercapacitor are illustrated in FIGS.  5 A- 5 G and  6 A- 6 E. The embodiment illustrated in FIGS.  5 A- 5 G illustrates an embodiment of a micro-supercapacitor with an insulator in which the electrodes are formed in a common plane. The embodiment illustrated in FIGS.  6 A- 6 E illustrates an embodiment without an insulator in which the electrodes are formed in different planes.  
         [0027]    Referring first to FIGS.  5 A- 5 G, initially a substrate  50 , for example, polyimide tape, is punched as illustrated in FIG. 5A. As shown, the holes, generally identified with the reference numeral  52 , are bridged by metal contacts, generally identified with the reference numeral  54 . The metal contacts  54  are deposited by conventional metal deposition techniques. After the metal contacts  54  are deposited on the substrate  50 , a plurality of electrodes, generally identified with the reference numeral  56 , for example, carbon metal oxide composite materials (C/MO x ) electrodes, as discussed above, are printed on top of the contacts  54  by conventional techniques, as generally illustrated in FIG. 5B. As shown in FIG. 5B, the electrodes  56  are separated defining cavities, generally identified with the reference numeral  58  therebetween. As shown in FIG. 5C, a separator  60 , for example, a PTFE slurry, is applied in each of the cavities  58 , as generally shown in FIG. 5C. Next, as illustrated in FIGS.  5 D, the separator  60  and electrodes  56  are impregnated with an electrolyte, for example, as discussed above, under vacuum.  
         [0028]    As shown in FIGS.  5 B- 5 D, the electrodes  56  are formed in a generally c-shape defining cavities, generally identified with the reference numeral  62 . These cavities  62  in the electrodes  56  are filled with an insulator, generally identified with the reference numeral  64 , as shown in FIG. 5E. The insulator  64  is applied by conventional techniques and may include any of the insulators discussed above. After the insulator is applied as shown in FIG. 5E, a cap layer  66  is formed over the entire array of capacitors. The cap layer  66  may be formed from materials, similar to the substrate, such as silicon, glass or polyimide tape. Lastly, the array of micro-supercapacitors are cut as generally shown in FIGS. 5F and 5G. As shown in FIG. 5G, the contacts identified with the reference numeral  54 A and  54 B are formed in the same plane.  
         [0029]    Referring to FIG. 6A a substrate  70 , for example, polyimide tape, is provided. A number of electrodes, generally identified with the reference numeral  72 , for example, C/MO x  electrodes, as discussed above, are printed on the substrate  70  by conventional techniques to form an array of electrodes with gaps therebetween, generally identified with the reference numeral  74 . As shown in FIG. 6B, a separator, for example, PTFE slurry, identified with the reference numeral  76 , is applied within the cavity  74 . The separator  76  is applied by conventional techniques. Subsequently, as shown in FIG. 6C, the electrodes  72  and separator  76  are impregnated with an electrolyte, for example, as discussed above, under a vacuum, as generally shown in FIGS. 6C. Next, as shown in FIG. 6D, a cap  78 , as discussed above, is applied to the top of the structure. Subsequently, as illustrated in FIG. 6E, the array of micro-supercapacitors are cut and contacts  80 A and  80 B are formed on opposing sides by conventional metal deposition techniques.  
         [0030]    In order to provide an increased surface area for increasing the capacitance of the device, a jelly roll geometry may be provided for increased capacitance for energy storage. As mentioned above, energy storage E=½ CV 2 . By increasing the capacitance, the energy storage capability is greatly increased.  
         [0031]    FIGS.  7 A- 7 H are process diagrams which illustrate the process for forming micro-supercapacitors with a spiral or jelly roll configuration. FIGS.  8 A- 8 G illustrate the process for forming an array of micro-supercapacitors with a jelly roll configuration. Referring to first to FIGS.  7 A- 7 H, a substrate  82 , for example, a Celgard® substrate, is provided. A metal contact layer  84  is deposited on the substrate  82  by conventional metal deposition techniques, as generally illustrated in FIG. 7A. As shown in FIG. 7B, an electrode layer  86  is deposited on top of the metallization layer  84 , such that the metallization layer  84  extends beyond the electrode layer  86  on one end and the electrode layer  86  overhangs the metallization layer  84  on the opposing end, as generally shown in FIG. 7B. Next, an insulator layer  48 , for example, PTFE slurry, is formed on top of the electrode layer  86 . As shown in FIG. 7D another electrode layer  90 , for example, a C/MO x  electrode layer, as discussed above, is formed on top of the insulator layer  88  by conventional techniques. A top metallization  92  is formed on top of the electrode layer  90  by conventional metal deposition techniques as generally shown in FIG. 7E. FIG. 7F is a top view of the structure illustrated in FIG. 7E.  
         [0032]    In order to create a spiral or jelly roll configuration as illustrated in FIG. 7G, the substrate illustrated in FIG. 7F is rolled up. The ends may be secured with a suitable epoxy.  
         [0033]    FIGS.  8 A- 8 G illustrate a process for forming an array of micro-supercapacitors with a jelly roll configuration. Initially, a substrate  96 , for example, a polyimide substrate, is provided. A metallization layer  98  is provided on top of the substrate  96  and covered with a conductive epoxy  100  as generally illustrated in FIG. 8B. An array of insulators  102 , for example, as illustrated in FIG. 8F is formed on top of the conductive epoxy  100  by conventional techniques. The micro-supercapacitors  94  with a jelly roll configuration are disposed within the cavities, generally identified with the reference numeral  104 , as generally shown in FIGS. 8B and 8G. As generally shown in FIG. 3, an electrolyte is disposed in the cavities of the before a top layer  106  is formed on top of the array. The top layer may be formed, for example, with metallized polyimide with conductive epoxy to form an array of micro-supercapacitors, as generally shown in FIG. 8E, with contacts  108   a  and  108   b.    
         [0034]    [0034]FIG. 4 illustrates another alternate embodiment of the invention in which the supercapacitor  20  is formed from relatively planar stacked layers separated by insulators. In particular, a first electrolyte  38 , formed on a substrate  40 , for example silicon wafers, glass slides or polyimide flexible substrates. The electrolyte  30  is connected to a bond pad  42  formed on the edge of the substrate  40  by conventional metal deposition techniques. An electrolyte layer  44  is disposed on top of the first electrode  38 . A second electrolyte  46  is formed on top of the electrolyte  44 . An insulator  50  is formed on top of the electrode  46 . The micro-supercapacitor illustrated in FIG. 4 may be formed by the process as generally illustrated in FIGS.  7 A- 7 F.  
         [0035]    Many modifications and variations of the present invention are possible in light of the above teachings. Thus, it is to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described above.  
         [0036]    What is claimed and desired to be covered by a Letters Patent is as follows: