Abstract:
An electrical energy storage device ( 20 ) includes a silicon substrate ( 22 ), which is formed so as to define a multiplicity of micro-containers ( 24, 26 ) having porous silicon walls ( 28 ), which are configured to serve as an anode layer. A cathode layer ( 34 ) overlies the micro-containers, with a separator layer ( 32 ) intermediate the anode layer and the cathode layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application 61/351,891, filed Jun. 6, 2010, which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates generally to electrical energy sources, and particularly to three-dimensional (3D) microbatteries. 
       BACKGROUND OF THE INVENTION 
       [0003]    Miniature, two-dimensional thin-film micro-batteries are known in the art. For example, U.S. Pat. Nos. 5,338,625 and 5,567,210, whose disclosures are incorporated herein by reference, describe a thin-film microbattery used as a backup or primary integrated power source for electronic devices. The battery includes a lithium anode, an electrochemically-stable electrolyte and a vanadium-oxide cathode. The battery is fabricated directly onto a semiconductor chip, onto a semiconductor die, or onto a portion of a chip carrier. 
         [0004]    U.S. Pat. No. 6,610,440, whose disclosure is incorporated herein by reference, describes microscopic batteries that are integratable or integrated with micro-electromechanical (MEMS) systems or other microscopic circuits. The inventors describe closed system microscopic batteries used for internal storage of electricity using interval reactants. The batteries comprise microscopic electrodes, electrolyte and a reservoir for the electrolyte. 
         [0005]    A three-dimensional thin-film microbattery is described in U.S. Pat. No. 6,197,450, whose disclosure is incorporated herein by reference. Thin-film micro-electrochemical energy storage cells (MEESC) such as microbatteries and double-layer capacitors (DLC) are described. The energy storage cells comprise two thin layer electrodes, an intermediate thin layer of a solid electrolyte and an optional fourth thin current collector layer. The layers are deposited in sequence on a surface of a substrate. The substrate comprises multiple through-cavities of arbitrary shape, with high aspect ratio, which increase the total electrode area per volume ratio. 
         [0006]    Three-dimensional microbatteries are also described by Long et al., in “Three-Dimensional Battery Architectures,” Chemical Review, volume 10, number 104 (October, 2004), pages 4463-4492, which is incorporated herein by reference. 
         [0007]    Geometric configurations of 3D microbatteries are described by Hart et al., in “3-D Microbatteries,”  Electrochemistry Communications,  volume 5 (2003), pages 120-123, which is incorporated herein by reference. This paper presents finite-element simulations showing current and potential distribution for several cathode-anode array configurations. 
         [0008]    A method for producing arrays of cavities in silicon is described by Kleimann et al., in “Formation of Wide and Deep Pores in Silicon by Electrochemical Etching,”  Materials Science and Engineering B,  volume 69-70 (2000), pages 29-33, which is incorporated herein by reference. Another process for producing micro-cavity arrays is described by Li et al., in “Microfabrication of Thermoelectric Materials by Silicon Molding Process,”  Sensors and Actuators A,  volume 108 (2003), pages 97-102, which is incorporated herein by reference. The authors describe a process for fabricating thermoelectric micro-modules with densely-aligned, fine-scale and high-aspect-ratio elements. 
         [0009]    U.S. Patent Application Publication 2009/0142656, whose disclosure is incorporated herein by reference, describes an electrical energy storage device, which includes a substrate formed so as to define a multiplicity of micro-containers separated by electrically-insulating and ion-conducting walls. A first plurality of anodes is disposed in a first subset of the micro-containers, and a second plurality of cathodes is disposed in a second subset. The anodes and cathodes are arranged in an interlaced pattern. 
       SUMMARY 
       [0010]    Embodiments of the invention described hereinbelow provide improved designs for 3D microbatteries. 
         [0011]    There is therefore provided, in accordance with an embodiment of the invention, an electrical energy storage device, including a silicon substrate, which is formed so as to define a multiplicity of micro-containers having porous silicon walls, which are configured to serve as an anode layer. A cathode layer overlies the micro-containers, with a separator layer intermediate the anode layer and the cathode layer. 
         [0012]    In some embodiments, the porous silicon is impregnated with lithium. The porous silicon may extend through an entire thickness of the walls or partially through the walls. 
         [0013]    In a disclosed embodiment, the device includes an anode current collector in electrical communication with the anode layer and a cathode current collector in electrical communication with the cathode layer. 
         [0014]    In some embodiments, the cathode and separator layers extend into the micro-containers. The micro-containers may include first and second sets of the micro-containers, which are respectively formed in opposing, first and second surfaces of the substrate, wherein the cathode and separator layers extend into only the first subset of the micro-containers. 
         [0015]    In an alternative embodiment, the separator layer includes an electrolyte, which fills at least some of the micro-containers, and the cathode layer is a planar layer covering the at least some of the micro-containers. 
         [0016]    There is also provided, in accordance with an embodiment of the invention, a method for fabricating an electrical energy storage device, which includes forming a multiplicity of micro-containers in a substrate, the micro-containers having walls including porous silicon configured to serve as an anode layer. A separator layer is formed over the walls in the micro-containers, and a cathode layer is disposed over the separator layer. 
         [0017]    There is additionally provided, in accordance with an embodiment of the present invention, a microelectronic device, including a silicon substrate and a microcircuit disposed on the substrate. An electrical energy storage device is disposed in the substrate and coupled to provide electrical power to the microcircuit. The storage device includes a multiplicity of micro-containers formed in the substrate and having walls including porous silicon configured to serve as an anode layer. A cathode layer overlies the micro-containers, with a separator layer intermediate the anode layer and the cathode layer. 
         [0018]    The invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a schematic, pictorial illustration of a substrate of a 3D microbattery, in accordance with an embodiment of the invention; 
           [0020]      FIG. 2  is a schematic, sectional illustration of a portion of a 3D microbattery, in accordance with an embodiment of the invention; 
           [0021]      FIG. 3  is a flow chart that schematically illustrates a method for producing a 3D microbattery, in accordance with an embodiment of the invention; 
           [0022]      FIG. 4  is a schematic, pictorial illustration of a 3D microbattery, in accordance with an embodiment of the invention; and 
           [0023]      FIG. 5  is a schematic top view of a microelectronic device, in accordance with an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0024]    Silicon is known as a material capable of providing one of the highest capacities for lithium-ion insertion (or intercalation). Use of silicon as an anode material in lithium-ion batteries, however, is problematic, because insertion of lithium ions into the silicon causes a huge volume expansion (approximately 400%) of the silicon. This expansion results in disintegration of the silicon, loss of its electrical conductivity, and loss of electrical contact with the current collector of the battery. The disintegration problem may be solved by use of porous silicon (PS), as described by Hyunjung Kim, et al., in “Three-Dimensional Porous Silicon Particles for Use in High-Performance Lithium Secondary Batteries,”  Angewandte Chemie,  volume 47, issue 52 (2008), pages 10151-10154, which is incorporated herein by reference. 
         [0025]    In embodiments of the invention, 3D microbatteries comprise high-capacity anodes made from porous silicon. In some embodiments, the microbatteries comprise 3D thin-film batteries (TFBs), in which both the cathodes and anodes have thicknesses of the order of a fraction of a micron to a few microns, or possibly a few tens of microns. The use of porous silicon anodes in 3D TFBs in advantageous in terms of increased energy and capacity relative to the battery footprint. 
         [0026]      FIG. 1  is a pictorial, schematic illustration of a semiconductor substrate  20 , which is used in producing a microbattery in accordance with an embodiment of the invention. Substrate  20  is typically formed from a silicon wafer  22 , or from a stack of such wafers. 
         [0027]    Alternatively, other suitable semiconductors or other materials, such as InGaP, GaAs, GaN, or SiC, may be used for this purpose. The typical thickness of wafer  22  is in the range of 100-800 microns, although the methods described below can be used to fabricate microbatteries of larger or smaller thickness. Methods for forming this sort of substrate are described in detail in the above-mentioned U.S. Patent Application Publication 2009/0142656. 
         [0028]    Substrate  20  comprises two sets of cavities, referred to herein as “micro-containers,” formed in substrate  20 . A first set of micro-containers  24  is formed in a first side of substrate  20 . A second set of micro-containers  26  is formed in the opposite side of the substrate. The two sets of micro-containers are typically arranged in a periodic, interlaced pattern in substrate  20 . (The interlaced pattern is sometimes referred to as an “interdigitated” pattern.) Alternatively, other arrangements of the micro-containers are possible, including arrangements in which all of the micro-containers open to the same side of the substrate. 
         [0029]    Micro-containers  24  and  26  may conveniently be made square in shape, as shown in  FIG. 1 . Alternatively, other suitable micro-container shapes, such as triangular, rectangular, hexagonal or round micro-containers may be used. As used herein, the term “micro-container” may refer to any three-dimensional structure formed in substrate  20  by removal (by etching, for example) of material, such as a trench, a hole, or a cavity. The characteristic width or diameter of the micro-containers typically ranges from several microns up to several tens of microns. The total number of micro-containers in a typical 1 cm 2  microbattery can vary from several hundred to several tens of thousands, depending on the micro-container width, the wall thickness, and the electrical specifications of the battery. Typically, the micro-containers have a high aspect ratio, i.e., their depth is significantly larger than their width or diameter. In general, the micro-containers need not all be of identical size or shape. 
         [0030]    The two sets of micro-containers  24  and  26  can be fabricated using any of several techniques known in the art. For example, the Kleimann paper cited above describes an electrochemical etching method for forming cavities in a silicon substrate. U.S. Pat. No. 6,197,450, also cited above, describes several alternative methods for etching cavities in substrate materials. One such method is a “dry etching” method, using an inductive-coupled plasma (ICP) process developed by Robert Bosch, GmbH (Stuttgart, Germany), which is described, for example, in U.S. Pat. No. 6,720,273, whose disclosure is incorporated herein by reference. Photolithographic methods may be used to define the two sets of micro-containers  24  and  26  prior to etching. 
         [0031]    Micro-containers  24  and  26  are separated by walls  28 , which are typically etched to a thickness of several microns. The micro-container formation process is stopped before the cavities penetrate to the opposite surface of substrate  20 , typically leaving a bottom thickness in the range of 1-100 microns, generally 10-50 microns. Walls  28  are rendered porous through all or a part of their width, as explained below. The bottom of each set of micro-containers may be made totally or only partially porous, depending on its thickness relative to the thickness of the separating walls between the micro-containers. 
         [0032]    In an alternative embodiment, the interlaced structure of micro-containers  24 ,  26  is fabricated by using two or more wafers. For example, the interlaced structure can be fabricated from three wafers: a middle wafer and two side wafers above and below the middle wafer. The middle wafer is etched or otherwise processed to comprise the two sets of micro-containers, which in this configuration are allowed to penetrate through its entire thickness. One side wafer has holes that match micro-containers  24  and forms the solid bottom for micro-containers  26 , while the other side wafer has holes that match micro-containers  26  and forms the solid bottom for micro-containers  24 . The three wafers are bonded together to form the 3D double-sided micro-container structure shown in  FIG. 1 . 
         [0033]    Reference is now made to  FIGS. 2 and 3 , which schematically illustrate a method for producing 3D microbatteries, in accordance with an embodiment of the invention.  FIG. 2  is a schematic, sectional illustration of a portion of a 3D microbattery  30 , while  FIG. 3  is a flow chart showing the steps of the method. Microbattery  30  is based on substrate  20 , in which micro-containers  24  and  26  are formed as described above, in a micro-container formation step  40 . 
         [0034]    Pores are formed in walls  28  of micro-containers  24  and  26 , at a pore formation step  42 . The porous walls typically comprise nanometric pores (typically several tens to several hundred nanometers in diameter), which may pass completely through the walls or only part-way through. The process may be adapted and controlled to form pores in only one set of the micro-containers to a depth of choice into the silicon walls. The pores may be formed using a metal-assisted chemical etching process, as described, for example, in the above-mentioned U.S. Patent Application Publication 2009/0142656. Alternatively, electrochemical etching or any other suitable process known in the art may be used to produce the pores in walls  28 . The pores may be impregnated with a suitable intercalant, such as lithium. Alternatively, the cathode may be lithiated, rather than the porous silicon anode. 
         [0035]    An anode current collector  38  is formed as a thin film layer over micro-containers  24  and over the corresponding surface of substrate  20 , at an anode collector deposition step  44 . The anode current collector may comprise, for example, copper, nickel, gold, or any other metal or alloy capable of forming low-resistivity ohmic contacts to Si. If the pores in walls extend all the way through to micro-containers  24 , then current collector  38  will be in direct ohmic contact with the porous silicon (and with the lithium within the pores). Otherwise, the anode current collector will be in ohmic contact with the non-porous part of the walls. Materials and processes for forming low-resistance ohmic contacts to silicon are well known in the art. As long as the silicon substrate is sufficiently conductive, the contact and internal resistance paths between current collector  38  and the porous silicon of walls  28  will be relatively small because of the short distances and large contact areas, and thus will not cause significant internal voltage drops. 
         [0036]    An ion-conducting separator layer  32  is formed on the opposite side of walls  28 , over micro-containers  26  and over the corresponding surface of substrate  20 , at a separator formation step  46 . Layer  32  may comprise, for example, a hybrid polymer electrolyte (HPE). A cathode layer  34  is formed over separator layer  32 , at a cathode formation step  48 . A cathode current collector  36  is then formed over the cathode layer, at a cathode collector deposition step  50 . The cathode and cathode current collector may comprise, for example, any of the corresponding materials enumerated in the above-mentioned U.S. Pat. No. 6,197,450. Cathode current collector  36  is in electrical communication with cathode layer  34 , allowing electron or hole transport to and from the cathode. 
         [0037]    Alternatively, anode current collector  38  may be formed in step  44  only after formation of the other layers at steps  46 ,  48  and  50 . 
         [0038]      FIG. 4  is a schematic, pictorial illustration of 3D microbattery  30 , at the conclusion of the process described above. The upper surface of the microbattery is covered by anode current collector layer  38 , while the lower surface is covered by cathode current collector layer  36 . The current collectors are connected to electrical leads  52 , through which microbattery  30  may be charged and discharged. Alternatively, if walls  28  (the anode) and/or cathode layer  34  is sufficiently conductive, leads  52  may be attached directly to the anode and/or cathode without an intervening current collector layer. 
         [0039]    In an alternative embodiment, the cathode current collector may be made separately from an appropriate metal as a sheet with protrusions made to match micro-containers  26 . The protrusions may be coated with a cathode material to a desired thickness (which can be on the order of a few microns). The cathode current collector and cathode can then be mechanically inserted into micro-containers  26  to contact separator layer  32 , which was formed in a previous step. 
         [0040]    Microbattery  30  has significant advantages in terms of increased energy and capacity relative to battery footprint. For example, for a silicon substrate 0.5 mm thick of area 1 cm 2 , the formation of 50 micron-wide micro-containers separated by 10 micron walls to a depth of about 450 micron provides a total wall surface of about 10 cm 2 , which is larger by a factor of about ten than the footprint. The walls may be made porous to a thickness of about 9 microns, for example, thus providing a 9 micron-thick anode. The theoretical capacity of this anode will be equal that of a graphite anode with a thickness 11 times greater. Cathode layer  34  may be deposited to any desired thickness over separator layer  32  within micro-containers  26  and over the corresponding surface of substrate  20  to control the energy capacity of the microbattery or to make the capacity of the microbattery anode-limited. 
         [0041]    In an alternative embodiment (not shown in the figures), a set of micro-containers with a porous silicon anode is formed as described above. The micro-containers are filled with a liquid electrolyte and then covered by a separator sheet, such as a Celgard® separator (produced by Celgard, LLC, Charlotte, N.C.). This separator is typically overlaid with a planar cathode layer and a current collector layer. This configuration maintains the advantage of large anode area and volume relative to planar designs. 
         [0042]      FIG. 5  is a schematic top view of a microelectronic device  60 , in accordance with an embodiment of the invention. Device  60  comprises silicon wafer  22 , in which microbattery  30  is formed as described above. An electrical microcircuit  62  is also fabricated on wafer  22 , using methods of integrated circuit fabrication that are known in the art. Microcircuit  62  is powered by microbattery  30  via leads  52 . Thus, device  60  is compact and self-powered, without requiring an external energy source. 
         [0043]    Although the embodiments described above relate specifically to the fabrication of 3-D microbatteries, the principles of the invention can also be applied in fabricating other sorts of energy storage devices, such as capacitors, using micro-containers and a porous semiconductor anode. It will thus be appreciated that the embodiments described above are cited by way of example, and that the invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.