Abstract:
A method of making a sensor comprises substantially laterally growing at least one nanowire having at least two segments between two electrodes, whereby a junction or connection is formed between the at least two segments; and establishing a sensing material adjacent to the junction or connection, and adjacent to at least a portion of each of the at least two segments, wherein the sensing material has at least two states.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application is a divisional application of application Ser. No. 11/583,648, filed Oct. 19, 2006, the contents of which are hereby incorporated herein by reference in its entirety. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    This invention was made in the course of research partially supported by the Defense Advanced Research Projects Agency, Agreement No. HR0011-05-3-0001. The U.S. government has certain rights in the invention. 
     
    
     BACKGROUND 
       [0003]    The present disclosure relates generally to sensors and methods for making the same. 
         [0004]    Since the inception of semiconductor technology, a consistent trend has been toward the development of smaller device dimensions and higher device densities. As a result, nanotechnology has seen explosive growth and generated considerable interest. Nanotechnology is centered on the fabrication and application of nano-scale structures, or structures having dimensions that are often 5 to 100 times smaller than conventional semiconductor structures. Nanowires are included in the category of nano-scale structures. 
         [0005]    Nanowires are wire-like structures having diameters on the order of about 1 nm to about 100 nm. Nanowires are suitable for use in a variety of applications, including functioning as conventional wires for interconnection applications, as semiconductor devices, and as sensors. While holding much promise, the practical application of nanowires has been somewhat limited. In particular, non-suspended nanowire sensing devices tend to leak heat, and, in some instances, are incapable of sensing relatively small temperature changes. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    Features and advantages of embodiment(s) of the present disclosure will become apparent by reference to the following detailed description and drawings, in which like reference numerals correspond to similar, though not necessarily identical components. For the sake of brevity, reference numerals or features having a previously described function may not necessarily be described in connection with other drawings in which they appear. 
           [0007]      FIG. 1  is a schematic view of an embodiment of a sensor; and 
           [0008]      FIG. 2  is a schematic view of another embodiment of a sensor. 
       
    
    
     DETAILED DESCRIPTION 
       [0009]    Providing nanowires, especially nanowires with properties that can vary in response to chemical reactions, that can be integrated on a silicon platform, and that can be fabricated in production quantities for a reasonable cost has proven difficult. 
         [0010]    Embodiment(s) of the sensor disclosed herein overcome one or more of the drawbacks discussed hereinabove, as they are advantageously capable of sensing the presence and/or quantity of a reaction (e.g., a chemical reaction). The sensors incorporate nanowire(s) grown laterally between two electrodes. The nanowires may advantageously have multiple segments of different conductivity types or different materials (e.g., different from other segments and/or different from the electrode materials). The connection between the various segments of the nanowire may advantageously be electrically useful (e.g., an ohmic connection, a junction (i.e., a diode), or the like). 
         [0011]    A reaction occurring near the sensing material located at the connection between the various nanowire segments is capable of generating a measurable property. As used herein, the term “sensing material” refers to a material that has two discrete states (e.g., of the property to be measured), or a material that has more than two states, including a continuum of states (i.e., the material is continuously variable). The property may advantageously be used to determine the presence and/or quantity of the reaction, the concentration of the reaction reactants, or combinations thereof. Further, without being bound to any theory, it is believed that the species of the reaction reactants may be determined by selecting a coating that preferentially and/or selectively binds specific reactants or reaction products. 
         [0012]    Using a nanowire to sense chemical reaction(s) may be advantageous, in part, because of the low thermal mass of the nanowire. It is believed that the low thermal mass increases the sensitivity to small amounts of reaction or small changes in the reaction, and decreases the response time for sensing the reaction or changes in the reaction. Furthermore, with suitable bias, both endothermic and exothermic reactions may be sensed. 
         [0013]    Referring now to  FIG. 1 , an embodiment of the sensor  10  includes two electrodes  12 ,  14  positioned on a substrate  16 . In an embodiment, the electrodes  12 ,  14  are formed from a layer of silicon (Si) cut or polished with the surface plane being a (110) crystal lattice plane. Such a layer is referred to as a (110) oriented Si layer. As used herein, the (110) plane is considered to be horizontally oriented with respect to the Cartesian coordinate system. The (110) oriented layer from which the electrodes  12 ,  14  are formed further has (111) planes of the Si crystal lattice, at least some of which are approximately perpendicular to, and intersect with the horizontally oriented (110) surface. These intersecting (111) planes are referred to herein as vertically oriented (111) planes or surfaces, noting that the (111) planes are approximately vertically oriented relative to the horizontal (110) surface of the layer from which the electrodes  12 ,  14  are formed. The (110) Si layer may be etched anisotropically using an etchant, such as KOH. The (111) planes etch slowly compared to other crystal planes; as such, the resulting structure, which forms the electrodes  12 ,  14 , is bounded by vertical (111) planes. These (111) planes form the sides of the electrodes  12 ,  14 . 
         [0014]    As used herein, the term “horizontal” generally refers to a direction or a plane that is parallel with a surface of the substrate  16  or wafer, while the term “vertical” generally refers to a direction or plane that is substantially or approximately perpendicular to the substrate surface. It is to be understood that the specific use of the terms “horizontal” and “vertical” to describe relative characteristics is to facilitate discussion and is not intended to limit embodiments of the present disclosure. 
         [0015]    The electrodes  12 ,  14  may have first and second conductivity types. It is to be understood that the first and second conductivity types may be the same or different. In a non-limitative example, the first conductivity type is p-type conductivity, and the second conductivity type is n-type conductivity, or vice versa. In other embodiments, both the first and second conductivity types are p-type conductivity or n-type conductivity. 
         [0016]    If the property to be measured is an electrical property, the electrodes  12 ,  14 , are electrically isolated from each other except through a nanowire  18 . If the substrate  16  is formed of an insulating material, the electrodes  12 ,  14 , are electrically isolated from each other. If the substrate  16  is formed of a conductive or a semi-conductive material, an insulating layer  19  is established between the respective electrodes  12 ,  14 , and the substrate  16  to electrically isolate the electrodes  12 ,  14  from each other. 
         [0017]    The nanowire  18  is grown substantially laterally between the electrodes  12 ,  14 . In an embodiment, growth of the nanowire  18  is initiated at one of the electrodes  12 ,  14 , and a connection is formed at the other of the electrodes  14 ,  12 . It is to be understood, however, that the nanowire  18  may be formed via any suitable method. A non-limitative example of forming a lateral nanowire  18  is described in U.S. patent application Ser. No. 10/738,176, filed on Dec. 17, 2003 (U.S. Publication No. 2005/0133476 A1, published Jun. 23, 2005), which is incorporated herein by reference in its entirety. Other example methods for forming lateral nanowires  18  are described in “Ultrahigh-density silicon nanobridges formed between two vertical silicon surfaces” by Islam et al., published in 2004 in volume 15 of  Nanotechnology  at pages L5-L8; and “A novel interconnection technique for manufacturing nanowire devices” by Islam et al., published in 2005 in volume 80 of  Appl. Phys. A  at pages 1133-1140. 
         [0018]    As depicted, the nanowire  18  has at least two segments  20 ,  22 . As the nanowire  18  grows, it may be doped with a dopant that is capable of introducing the first conductivity type or the second conductivity type to one or more of the nanowire segments  20 ,  22 . Dopants for introducing p-type conductivity into group IV semi-conductors include, but are not limited to boron, other like elements, or combinations thereof; and dopants for introducing n-type conductivity into group IV semi-conductors include, but are not limited to phosphorus, arsenic, antimony, other like elements, or combinations thereof. Different dopants may be suitable for group III-V materials, such as, for example gallium arsenide. 
         [0019]    In the embodiment shown in  FIG. 1 , one of the segments  20 ,  22  has the first conductivity type, and the other of the segments  22 ,  20  has the second conductivity type. Generally, the first and second conductivity types are different so that a junction (e.g., a p-n junction) is formed between the two segments  20 ,  22 . In a non-limitative example, the first conductivity type is p-type conductivity, and the second conductivity type is n-type conductivity, or vice versa. If the semiconductor material forming both segments  20 ,  22  of the nanowire is the same material, the junction is a p-n homojunction. 
         [0020]    In another embodiment, one of the segments  20 ,  22  is formed of a first material, and the other of the segments  22 ,  20  is formed of a second material that is different than the first material, so that a heterojunction is formed between the two segments  20 ,  22 . The materials for the segments  20 ,  22  may be selected to be of opposite conductivity types so that a p-n heterojunction is formed, or they may be of the same conductivity type so that an isotype heterojunction is formed. 
         [0021]    As a non-limiting example of the sensor  10  shown in  FIG. 1 , the nanowire  18  and electrodes  12 ,  14  may form a first conductivity type-first conductivity type-second conductivity type-second conductivity type structure, where the nanowire  18  has a second conductivity type segment (e.g., segment  22  adjacent a second conductivity type electrode  12 ) and a first conductivity type segment (e.g., segment  20  adjacent a first conductivity type electrode  14 ). A non-limitative example of such a structure is a p-type-p-type-n-type-n-type (p-p-n-n) structure, which has a p-p junction (at the electrode  14 -segment  20  interface), a p-n junction (at the segment  20 -segment  22  interface), and an n-n junction (at the segment  22 -electrode  12  interface). The first conductivity type segment  20  of the nanowire  18  is grown from the electrode  14  of the first conductivity type. During growth of the nanowire  18 , growth of the first conductivity type segment  20  may be stopped, and a second conductivity type segment  22  may be grown from the first conductivity type segment  20 . The conductivity type may be changed by changing the dopant-containing species reaching the region where semiconductor material is being added to continue growth of the nanowire  18 . Alternatively, the dopant can be added after growth of the nanowire  18  is complete. In the embodiment shown in  FIG. 1 , the second conductivity type segment  22  forms a connection with the electrode  12  of the second conductivity type. It is to be understood that the conductivity of the respective electrodes  12 ,  14  and segments  20 ,  22  may be altered as desired. 
         [0022]    In the embodiment shown in  FIG. 1 , a junction  24  is formed at the interface of the nanowire segments  20 ,  22 . As previously described, the junction  24  is often a homojunction. In some instances, though, the junction  24  is a heterojunction. In an embodiment, a sensing material  26  is positioned adjacent to the junction  24  and adjacent to at least a portion of each of the nanowire segments  20 ,  22 . It is to be understood that the sensing material  26  may also be positioned adjacent to other portions of the nanowire segments  20 ,  22 . The sensing material  26  may be applied to the nanowire  18  via vapor deposition techniques, liquid deposition techniques (including self-assembling of a monolayer), inkjet deposition techniques, or the like, or combinations thereof. It is to be understood that the sensing material  26  may be positioned on, under, and/or so it substantially surrounds the junction  24  and the segment  20 ,  22  portions. 
         [0023]    As previously described, the sensing material  26  may have two discrete states (between which the material  26  is switchable), or it may have more than two states, including a continuum of states. The two discrete states may be of the property to be measured, for example, a high resistance state and a low resistance state. The two discrete states may represent “ON” and “OFF” states. In this embodiment, upon exposure to a certain reaction property, the sensing material  26  turns from ON to OFF or from OFF to ON. Generally, sensing materials  26  having two discrete states are suitable for measuring the presence or absence of a reaction, and sensing materials  26  with more than two states or a continuum of states are suitable for measuring an amount of the reaction. 
         [0024]    The sensing material  26  may be a material that is switchable between at least a high and a low resistance state, a plurality of nanoparticles coated with spacer ligands, or combinations thereof. As a non-limiting example, the sensing material  26  may be a material that has a high resistance state and a low resistance state, and is switchable from one state to the other state when exposed to conditions that induce resistance changes (e.g., the reaction being measured). As another non-limiting example, the sensing material  26  may be a material with a continuum of states so that its resistance is a continuous function of temperature. In this case, the resistance measured indicates the temperature of the material and, therefore, the intensity of the reaction at any instant of time. By integrating the changes over time, the quantity of the reaction that has occurred can be measured. 
         [0025]    The embodiment shown in  FIG. 1  depicts the plurality of nanoparticles coated with space ligands as the sensing material  26 . A reaction (e.g., chemical or otherwise) may be generated at or near the junction  24  of the sensor  10 . In an embodiment, heat is generated by the reaction. In a non-lighting example, the nanoparticles shown in  FIG. 1  expand upon exposure to the generated heat, causing the spacing between the nanoparticles to decrease. This decreased space lowers the resistance of a path in parallel with the junction  24 . 
         [0026]    In a non-limiting example, the nanoparticles may act as a binary sensing material that undergoes a Mott transition (switches from a nonconductive or “OFF” state to a conductive or “ON” state) when exposed to heat that is generated, for example, from such a reaction (e.g., chemical or otherwise). As such, the heat and/or the change in resistance is indicative of the chemical reaction occurring near the junction  24 . In this embodiment, the resistance may be measured, and such measurements may be used to determine the presence of the reaction, the concentration of the reaction reactants, or combinations thereof. 
         [0027]    Very generally, the change of resistance of the sensing material  26  shown in  FIG. 1  depends, at least in part, on the temperature, and consequently on the amount of heat generated by the reaction. The amount of heat generated depends, at least in part, on the quantity of the reaction, and therefore on the concentration of the reactants. If the change is abrupt, the presence of a reaction is detectable. If however, the change is gradual with temperature, the reaction presence and the reactants&#39; concentration are detectable. 
         [0028]    Embodiments of the sensor  10  having a junction may be biased so the current flowing through the nanowire  18  is less than the current carried by the sensing material  26  in its conductive state. The junction may be reverse biased, or biased with a low forward bias so that the current flow through the junction  24  is low. A change in the parallel conductance through the sensing material  26  is then readily measured between the electrodes  12 ,  14 . 
         [0029]    Referring now to  FIG. 2 , another embodiment of a sensor is generally depicted at  10 ′. The substrate  16  and electrodes  12 ,  14  described in reference to  FIG. 1  are suitable for use in the embodiment of the sensor  10 ′ shown in  FIG. 2 . 
         [0030]    In this embodiment, the nanowire  18  is grown substantially laterally between the electrodes  12 ,  14 , and includes at least two segments  20 ′,  22 ′ having a connection  28  therebetween. It is to be understood that the nanowire  18  is formed via embodiments of the methods disclosed hereinabove. Generally, the nanowire segments  20 ′,  22 ′ in this embodiment are selected from metals, semi-conductor materials, or combinations thereof. Non-limiting examples of such materials include silicon, germanium, indium phosphide, gallium arsenide, boron, gold, or the like, or combinations thereof. 
         [0031]    The nanowire  18  depicted in  FIG. 2  also includes an insulating material  30  located at the connection  28  between the segments  20 ′,  22 ′. The insulating material  30  may be grown as a segment of the nanowire  18  by changing the gaseous species to which the catalyzing nanoparticle (which forms the nanowire  18 ) is exposed. Non-limiting examples of the insulating material  30  include gallium aluminum arsenide, or the like. 
         [0032]    In the embodiment shown in  FIG. 2 , the sensing material  26  is positioned adjacent to: the connection  28 ; the insulating material  30 ; and at least a portion of each of the segments  20 ′,  22 ′. It is to be understood that the sensing material  26  may also be positioned adjacent to other portions of the nanowire segments  20 ′,  22 ′. The sensing material  26  may be established via any of the techniques described hereinabove. It is to be understood that the sensing material  26  may be positioned on, under, and/or so it substantially surrounds the connection  28 , the insulating material  30 , and the portions of segments  20 ′,  22 ′. 
         [0033]    As previously described, the sensing material  26  may be a resistive material that is changeable between at least a high and a low resistance state, a plurality of nanoparticles coated with spacer ligands, or combinations thereof. The embodiment shown in  FIG. 2  depicts the resistive material that is switchable between the high and low resistance states. 
         [0034]    Similar to the embodiment of the sensor  10  shown in  FIG. 1 , a reaction (e.g., chemical or otherwise) may be generated at or near the connection  28  of the sensor  10 ′. The resistance of sensing material  26  changes to a different state upon exposure to at least some measurable property (e.g., heat, temperature increase or decrease, etc.) generated by the reaction. This exposure results in a change in the resistance of a path in parallel with the connection  28 . Depending, at least in part, on the property and amount generated during the reaction, the resistance may be increased or decreased. Furthermore, the exposure to the measurable property may cause the material  26  to switch from one resistance state to another (generally suitable for detecting presence of a reaction), or to move through more than two states or a continuum of states of different resistance (generally suitable for detecting reaction presence and the reactants&#39; concentration). 
         [0035]    For an exothermic reaction, the resistance of the sensing material  26  shown in  FIG. 2  usually decreases to a lower state upon exposure to at least some measurable property generated by the reaction. This results in a decrease in the resistance of a path in parallel with the connection  28 . 
         [0036]    The change in resistance is indicative of the chemical reaction occurring near the connection  28 . The resistance may be measured, and such measurements may be used to determine the presence and/or quantity of the reaction, the concentration of the reaction reactants, the species of the reaction reactants, or combinations thereof. 
         [0037]    For the embodiments of the sensor  10 ,  10 ′ disclosed herein, the nanowire  18  may include a junction  24  between differently doped segments  20 ,  22 , of the same material, a junction  24  between segments  20 ,  22 , of different materials, or an insulating material  30  between two conducting segments  20 ′,  22 ′. The sensing material  26  may include materials that are capable of switching between two states, or moving between more than two states or through a continuum of states. The sensors  10 ,  10 ′ shown in  FIGS. 1 and 2  are non-limiting examples, and it is to be understood that different combinations of the nanowire  18  and sensing material  26  are considered to be within the purview of the present disclosure. 
         [0038]    While several embodiments have been described in detail, it will be apparent to those skilled in the art that the disclosed embodiments may be modified. Therefore, the foregoing description is to be considered exemplary rather than limiting.