Abstract:
A high density micro-electrode array includes a transistor layer including a plurality of access transistors and a substrate in operable communication with the transistor layer including, wherein at least a portion of the substrate includes a plurality of trenches. The system includes a plurality of electrodes at least partially located in the plurality of trenches, wherein each of the plurality of electrodes is connected to at least one of the plurality of access transistors and wherein each of the electrodes is separated by a distance less than approximately one microns.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. Non-Provisional application Ser. No. 13/307,608, which was filed on Nov. 30, 2011, the entirety of which is hereby incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    The present invention relates to multi-electrode arrays, and more specifically, to high density multi-electrode arrays. 
         [0003]    Multi-electrode arrays are used for various applications including, for example, electrical interfacing to neurons. Multi-electrode arrays have been fabricated by a variety of methods and can be used for both in vivo or in vitro applications. Multi-electrode arrays can be used to stimulate or probe brain activity, to stimulate neurons and study the resulting neuron plasticity or to train live neural networks and use them for computation. 
         [0004]    In currently available micro-electrode arrays, the density of the electrodes range from approximately one hundred electrodes per square millimeter in microelectromechanical systems (MEMS) arrays to approximately ten thousand electrodes per square millimeter in micro-fabricated arrays. These electrode densities are achieved using standard CMOS metallization techniques. The density of typical micro-electrode arrays is much smaller than typical neuron density. For example, the neuron density for visual cortex is approximately two hundred million neurons per cubic millimeter. 
       BRIEF SUMMARY 
       [0005]    According to one embodiment of the present disclosure, a high density micro-electrode array includes a transistor layer including a plurality of access transistors and a substrate in operable communication with the transistor layer including, wherein at least a portion of the substrate includes a plurality of trenches. The system includes a plurality of electrodes at least partially located in the plurality of trenches, wherein each of the plurality of electrodes is connected to at least one of the plurality of access transistors and wherein each of the electrodes is separated by a distance less than approximately one microns. 
         [0006]    According to another embodiment of the present disclosure, a high density micro-electrode array includes a transistor layer including a plurality of access transistors, wherein one or more of the plurality of access transistors is in operable communication with a stimulation circuit and wherein one or more of the plurality of access transistors is in operable communication with a detection circuit. The high density micro-electrode array includes a buried oxide layer formed on a portion of the transistor layer and a plurality of electrodes formed beneath the transistor layer, wherein each of the plurality of electrodes is connected to at least one of the plurality of access transistors and wherein the electrodes are separated by approximately one micron. 
         [0007]    Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the invention with the advantages and the features, refer to the description and to the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0008]    The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. 
           [0009]    The forgoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0010]      FIGS. 1 through 4  are a series of cross sectional views illustrating a method of forming an exemplary high density micro-electrode array, in accordance with an exemplary embodiment, in which: 
           [0011]      FIG. 1  illustrates a high density micro-electrode array having a plurality of tightly packed electrodes that are formed inside trenches in a substrate; 
           [0012]      FIG. 2  illustrates thinning of a portion of the substrate of  FIG. 1 ; 
           [0013]      FIG. 3  illustrates etching of a portion of the dielectric liner from the tip of the electrodes in  FIG. 2 ; and 
           [0014]      FIG. 4  illustrates forming an optional dielectric layer to insulate the remaining portion of the substrate in  FIG. 3 . 
           [0015]      FIGS. 5 ,  6   a  and  6   b  are cross sectional views illustrating a method of forming an exemplary high density micro-electrode array, in accordance with an exemplary embodiment, in which: 
           [0016]      FIG. 5  illustrates a high density multi-electrode array including a silicon-on-insulator (SOI) wafer that is used in the transistor layer above a buried oxide (BOX) layer; 
           [0017]      FIG. 6   a  illustrates thinning of a portion of the substrate of  FIG. 5 ; 
           [0018]      FIG. 6   b  illustrates an alternative embodiment of the processing operation of  FIG. 6   a , in which the etching of the substrate continues all the way to the BOX layer; 
           [0019]      FIG. 7  is a circuit diagram of an exemplary addressing scheme for addressing electrodes in a high density multi-electrode array, in accordance with additional embodiments; 
           [0020]      FIGS. 8   a  and  8   b  are cross sectional views of exemplary high density micro-electrode arrays, in accordance with additional embodiments; 
           [0021]      FIG. 9  is a flow diagram of an exemplary process for fabricating a high density micro-electrode array, in accordance with another embodiment; and 
           [0022]      FIG. 10  is a perspective view of a micro-needle including a high density micro-electrode, in accordance with another embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    Embodiments of the disclosure are described herein with reference to cross-section illustrations that are schematic illustrations of idealized embodiments (and intermediate structures) of the invention. The thickness of layers and regions in the drawings may be exaggerated for clarity. Additionally, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, embodiments of the disclosure should not be construed as limited to the particular shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. Thus, the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the actual shape of a region of a device and are not intended to limit the scope of the disclosure. 
         [0024]    It will be understood that when an element such as a layer, region or substrate is referred to as being “on” or extending “onto” another element, it can be directly on or extend directly onto the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” or extending “directly onto” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. Other words used to describe the relationship between elements should be interpreted in a like fashion (i.e., “between” versus “directly between”, “adjacent” versus “directly adjacent”, etc.). 
         [0025]    Relative terms such as “below” or “above” or “upper” or “lower” or “horizontal” or “vertical” may be used herein to describe a relationship of one element, layer or region to another element, layer or region as illustrated in the figures. It will be understood that these terms are intended to encompass different orientations of the device in addition to the orientation depicted in the figures. 
         [0026]    With reference now to  FIG. 1 , a cross section of a high density micro-electrode array  100  in accordance with an exemplary embodiment of the disclosure is shown. The high density micro-electrode array  100  includes a plurality of tightly packed electrodes  102  that are formed inside trenches  104  in the substrate  106 . In exemplary embodiments, each electrode  102  is connected to at least one access transistor in the transistor layer  108  to enable addressing of each of individual electrodes  102 . 
         [0027]    In exemplary embodiments, the high density micro-electrode array  100  may be fabricated using a variety of techniques. In one exemplary embodiment, a high density micro-electrode array  100  is fabricated by starting with a substrate  106  that contains a heavily doped region  110  at the top of the substrate. In exemplary embodiments, the heavily doped region may only be few microns in thickness. The fabrication process includes forming trenches  104  in the heavily doped region  110  of the substrate  106  and filling the trenches  104  with a dielectric liner  112  and a conductive electrode  102 . In one exemplary embodiment, the conductive electrode  102  may be a heavily doped polysilicon, as done for example in trench dynamic random access (DRAM) technology. In another exemplary embodiment, the conductive electrode  102  may contain a bio-compatible metal such as titanium or platinum. After the trenches  104  are lined and filled, each electrode  102  is connected to at least one access transistor (not specifically shown) in the transistor layer  108  for addressing thereof. In one embodiment, after formation of the electrodes  102  in the trenches  104 , an access transistor and periphery circuits for sensing neuron activity and for stimulating are fabricated using standard CMOS processing. The periphery circuit may contain one or more of the following units: electrode addressing control, signal amplification and detection, neuron stimulation generation, timing, interface to outside chips/instrumentations, etc. After the electrodes  102  are connected to an access transistor in the transistor layer  108 , the opposite side of the substrate  106  is thinned, exposing at least a portion of the electrodes  102 . In exemplary embodiments, only a portion of the substrate  106  that contains the high density electrode array is thinned. 
         [0028]    As then shown in  FIG. 2 , the substrate  106  has been thinned to a depth sufficient to expose at least a portion of the dielectric liner  112 . The process of thinning the substrate  106  can be done by wafer thinning, grinding, etching, or other commonly known techniques. In exemplary embodiments, the substrate thinning process may include using wet/dry etching with good selectivity between the substrate  106  and the dielectric liner  112 . For example, wet etching may be done using tetramethylammonium hydroxide (TMAH) or potassium hydroxide (KOH) to remove a portion of the substrate  106  and leave the dielectric liner  112  intact. Alternatively, dry etching based on hydrogen bromide (HBr) chemistry can be used which has good selectivity to the dielectric liner  112 . 
         [0029]    Continuing now with reference to  FIG. 3 , a portion of the dielectric liner  112  has been etched away from the tip of the electrodes  102 , exposing the electrodes  102 . As illustrated in  FIG. 4 , a dielectric layer  120  may optionally be used to insulate the remaining heavily doped region  110  of the substrate. In exemplary embodiments, the dielectric layer  120  may a polymer such as polymethylmethacrilate (PMMA), a spin-on-glass or another suitable alternative. 
         [0030]    Turning now to  FIG. 5 , another exemplary embodiment of a high density multi-electrode array  500  is illustrated. The high density multi-electrode array  500  includes a silicon-on-insulator (SOI) wafer that is used in the transistor layer  508  above a buried oxide layer (BOX)  514 . In one exemplary embodiment, as illustrated in  FIG. 6   a , during the fabrication of the high density multi-electrode array  500  a portion of the heavily doped region  510  of the substrate can be etched to a desired depth. Alternatively, as shown in  FIG. 6   b , the substrate may be etched down to the BOX  514 . 
         [0031]    Referring now to  FIG. 7 , a circuit diagram of an exemplary addressing scheme for addressing electrodes in a high density multi-electrode array is shown. As illustrated, the electrodes  702  in the high density multi-electrode array can be addressed using a single access transistor  716  per electrode in a word line/bit line addressing scheme. The access transistors  716  connect the electrodes  702  to circuitry  718  which may be used to send or receive signals to or from the electrodes  702 . In other exemplary embodiments, alternative configurations that use one or more transistors for addressing each electrode can be used. For example, in one embodiment two access transistors may be used to activate a single electrode with one access transistor being used for measuring neuron activity and another access transistor being used for stimulating neuron activity. In an alternative embodiment, some electrodes can be used exclusively for probing or measuring neuron activity while others are used exclusively for stimulating neuron activity. 
         [0032]    In exemplary embodiments, the high density multi-electrode array may be fabricated such that the high density multi-electrode array encompasses the entire substrate  806  or such that only a portion of the substrate is used for the high density multi-electrode, as illustrated respectively by  FIGS. 8   a  and  8   b . In the embodiment shown in  FIG. 8   b , only the portion of the substrate that contains the high density multi-electrode array is thinned during the fabrication process. 
         [0033]    Referring now to  FIG. 9 , a flow diagram illustrating an exemplary process for fabricating a high density micro-electrode array is illustrated. The first step in the exemplary fabrication process, as shown at process step  902 , is providing a substrate that optionally contains a heavily doped region. After the substrate has been provided, the next step in the exemplary fabrication process is forming trenches in the substrate, as shown at process step  904 . Once the trenches have been formed, they are filled with a dielectric liner and then a conductive electrode, as shown is process steps  906  and  908  respectively. The next step in the exemplary fabrication process, as illustrated at process step  910 , is forming a transistor layer on top of the substrate and connecting each electrode to at least one access transistor. After the electrodes are connected to access transistors, the substrate is thinned to expose at least a portion of each of the electrodes, as shown at process step  912 . 
         [0034]    In exemplary embodiments, the process for fabricating a high density micro-electrode array may include an additional step of adding a dielectric layer to insulate the remaining portion of the substrate that surrounds the electrodes. The flow diagram depicting the process for fabricating a high density micro-electrode array is provided as just one exemplary fabrication process that can be used. There may be many variations to the fabrication process or the process steps described therein without departing from the spirit of the invention. For instance, the steps may be performed in a differing order or steps may be added, deleted or modified. All of these variations are considered a part of the claimed invention. 
         [0035]    Turning now to  FIG. 10 , a micro-needle  1000  suitable for in vivo applications that includes an exemplary embodiment of a high density micro-electrode array is illustrated. The micro needle  1000  includes layers of wafer each containing a high density micro-electrode array  1100  and periphery circuit  1018 . The layers of wafer that make up the micro-needle  1000  are cut so that the dimensions and shape are suitable for in vivo applications. Typical micro-needles have a width and thickness of 20-50 μm and length of 100-1000 μm and contain about 10 3  micro-electrodes on its side. In certain applications the micro-needles can be arranged in a one-dimensional or two-dimensional array as known in the art. 
         [0036]    In exemplary embodiments a high density micro-electrode array in accordance with the present disclosure may have an electrode density of greater than approximately one hundred thousand electrodes per square millimeter. In other exemplary embodiments a high density micro-electrode array in accordance with the present invention may have an electrode density of greater than approximately one million electrodes per square millimeter. In exemplary embodiments, the electrodes in the high density micro-electrode are formed such that the distance separating the electrodes can range from approximately two hundred nanometers (nm) to approximately two microns (μm). 
         [0037]    The method as described above is used in the fabrication of integrated circuit chips. The resulting integrated circuit chips can be distributed by the fabricator in raw wafer form (that is, as a single wafer that has multiple unpackaged chips), as a bare die, or in a packaged form. In the latter case the chip is mounted in a single chip package (such as a plastic carrier, with leads that are affixed to a motherboard or other higher level carrier) or in a multichip package (such as a ceramic carrier that has either or both surface interconnections or buried interconnections). In any case the chip is then integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either (a) an intermediate product, such as a motherboard, or (b) an end product. The end product can be any product that includes integrated circuit chips, ranging from toys and other low-end applications to advanced computer products having a display, a keyboard or other input device, and a central processor. 
         [0038]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one more other features, integers, steps, operations, element components, and/or groups thereof. 
         [0039]    The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated 
         [0040]    While the preferred embodiment to the invention had been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first described.