Abstract:
In various embodiments, electronic devices such as thin-film transistors incorporate electrodes featuring a conductor layer and, disposed below the conductor layer, a barrier layer comprising an alloy of Cu and one or more refractory metal elements selected from the group consisting of Ta, Nb, Mo, W, Zr, Hf, Re, Os, Ru, Rh, Ti, V, Cr, and Ni.

Description:
RELATED APPLICATION 
       [0001]    This application claims the benefit of and priority to U.S. Provisional Patent Application No. 61/831,865, filed Jun. 6, 2013, the entire disclosure of which is hereby incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    In various embodiments, the present invention relates to the metallization of electronic devices such as flat panel displays and touch panel displays, in particular to capping and barrier layers for such metallization. 
       BACKGROUND 
       [0003]    Flat panel displays have rapidly become ubiquitous in various markets, and are now commonly utilized in a variety of appliances, televisions, computers, cellular phones, and other electronic devices. One example of a commonly used flat panel display is the thin film transistor (TFT) liquid crystal display (LCD), or TFT-LCD. A typical TFT-LCD contains an array of TFTs each controlling the emission of light from a pixel or sub-pixel of an LCD.  FIG. 1A  depicts the cross-section of a conventional TFT  100  as might be found in a TFT-LCD. As shown, the TFT  100  includes a gate electrode  105  formed on a glass substrate  110 . A gate insulator  115  electrically insulates the gate electrode  105  from overlying conductive structures. An active layer  120 , typically composed of amorphous silicon, conducts charge between a source electrode  125  and a drain electrode  130 , under the electrical control of gate electrode  105 , and the conducted charge controls the operation of the pixel or sub-pixel connected thereto (not shown). A source/drain insulator  132  electrically isolates the source electrode  125  from the drain electrode  130  and protectively seals the TFT  100 . As shown, each of the gate electrode  105 , source electrode  125 , and drain electrode  130  typically include a barrier metal layer  135  and a metal conductor layer  140  thereover. The barrier  135  provides good adhesion between the conductor  140  and the underlying glass and/or silicon and reduces or prevents diffusion therebetween. 
         [0004]    Over time, LCD panel sizes have increased and TFT-based pixel sizes have decreased, placing increasingly high demands on the conductors within the TFT-LCD structure. In order to decrease the resistance in the conductors and thereby increase electrical signal propagation speeds in the TFT-LCD, manufacturers are now utilizing low-resistivity metals such as copper (Cu) for the conductors  140  within the display. Metals such as molybdenum (Mo), titanium (Ti), or molybdenum-titanium alloys (Mo—Ti) have been utilized for barriers  135  underlying Cu conductors  140 ; however, these metals suffer from one or more deficiencies that limit the performance of the TFT-LCD and/or present difficulties in the fabrication process for the TFT-LCD. For example, some conventional barriers  135  have relatively high resistivity and therefore compromise the overall conductivity of the electrodes. Furthermore, as shown in  FIG. 1B , during the etching of electrodes such as gate electrodes  105 , either a residue  145  (of one or both electrode materials) or etch discontinuities  150 , e.g., stepped or nonlinear profiles (caused by non-uniform etch rates of the two different electrode materials), may result. 
         [0005]    Similarly, touch-panel displays are becoming more common in electronic devices, and they may even be utilized in tandem with TFT-LCDs. A typical touch-panel display includes an array of sensors arranged in rows and columns and that sense a touch (or close proximity) of, e.g., a finger, via capacitive coupling.  FIG. 2A  schematically depicts an exemplary sensor array  200  for a touch-panel display that includes multiple conductive column sensors  210  that are interconnected to form columns  220 , as well as multiple conductive row sensors  230  that are interconnected to form rows  240 . The sensors  210 ,  230  are formed over a substrate  250  and are electrically coupled to a processor  260  that both senses the changes in capacitive coupling that represent “touches” and provides these signals to other electronic components within a device (e.g., a computer or mobile computing device that incorporates a touch screen). The sensors  210 ,  230  may be formed of a transparent conductor such as indium tin oxide (ITO), and the substrate  250  may be glass or any other suitably rigid (and/or transparent) support material. 
         [0006]      FIG. 2B  depicts a magnified perspective view of a point within the sensor array  200  where the interconnected column sensors  210  intersect the interconnected row sensors  230 . In order to avoid electrical shorting between the columns  220  and the rows  240  (see  FIG. 2A ), the interconnections between column sensors  210  are isolated from the underlying or overlying row sensors  230 . For example, as shown in  FIG. 2B , an insulator layer  270  is disposed between the column  220  of column sensors  210  and a conductive interconnect (or “bridge”)  280  that electrically connects the row sensors  230  within a row  240 . As shown in  FIG. 2C , the interconnects  280  are typically composed of an Al conductive layer  290  with an overlying barrier or capping layer  295  that is typically composed of Mo, Ti, or Mo—Ti. The capping layer  295  helps to prevent diffusion from the conductive layers  290  and protects conductive layers  290  from corrosion during processing and product use. The capping layer  295  may also improve adhesion to overlying layers. However, as described above for TFT-LCDs, the metals conventionally used for the capping layer  295  metals from one or more deficiencies that limit performance and/or present difficulties in the fabrication process. For example, the capping layers  295  may have relatively high resistivity and therefore compromise the overall conductivity of the interconnects  280 , degrading electrical performance. Furthermore, as shown in  FIG. 2D , during the etching of the interconnects  280 , either a residue  296  (of one or both of conductive layer  290  or capping layer  295 ) or etch discontinuities  297 , e.g., stepped or nonlinear profiles (caused by non-uniform etch rates of the two different materials), may result. 
         [0007]    In view of the foregoing, there is a need for barrier and/or capping metal layers for electronic devices such as TFT-LCDs and touch-panel displays that provide excellent adhesion to underlying substrates, prevent diffusion of the conductor metal into nearby layers, protect the conductor metal from corrosion, and are uniformly etched with the underlying or overlying conductor metals during fabrication. 
       SUMMARY 
       [0008]    In accordance with various embodiments of the present invention, electronic devices such as TFT-LCDs and touch-panel displays, and the metallic interconnects and electrodes therein, are fabricated utilizing capping and/or barrier layers including or consisting essentially of an alloy of Cu and one or more refractory metal elements such as tantalum (Ta), niobium (Nb), Mo, tungsten (W), zirconium (Zr), hafnium (Hf), rhenium (Re), osmium (Os), ruthenium (Ru), rhodium (Rh), Ti, vanadium (V), chromium (Cr), or nickel (Ni). The one or more refractory elements may be present in the alloy at weight concentrations of 1-50 percent. In an exemplary implementation, alloy barrier layers are formed directly on substrate layers such as glass and/or silicon-based layers, and conductor layers including or consisting essentially of highly conductive metals such as Cu, silver (Ag), aluminum (Al), or gold (Au) are formed thereover to form the various electrodes in a TFT structure. In another exemplary implementation, highly conductive metals such as Cu, Ag, Al, and/or Au are utilized as conductive interconnects in a touch-panel display and are capped with protecting capping layers that include or consist essentially of an alloy of Cu and one or more refractory metal elements such as Ta, Nb, Mo, W, Zr, Hf, Re, Os, Ru, Rh, Ti, V, Cr, or Ni. The one or more refractory elements may be present in the alloy at weight concentrations of 1-50 percent (hereafter weight %). 
         [0009]    Surprisingly, even though the barrier and capping layers are predominantly Cu, the presence of the refractory-metal alloying element(s) discourages or even prevents the diffusion of Cu into nearby layers, e.g., an underlying silicon layer, approximately as well as pure Mo barrier layers when utilized with Cu conductor layers. While not wishing to be bound by any particular theory or mechanism for this phenomenon, the refractory-metal alloying element may react with atoms of the silicon layer to form silicide regions that occupy grain boundaries in the barrier or capping metal and thereby prevent diffusion of Cu into the substrate (or other adjoining layer) along the grain boundaries, which would otherwise be fast diffusion paths. 
         [0010]    In various embodiments, the barrier and/or capping layer includes or consists essentially of an alloy of Cu with (i) Ta and Cr, (ii) Ta and Ti, or (iii) Nb and Cr. For example, the barrier and/or capping layer may include (i) 1 weight %-12 weight % Ta (preferably approximately 5 weight % Ta) and 1 weight %-5 weight % Cr (preferably approximately 2 weight % Cr), (ii) 1 weight %-12 weight % Ta (preferably approximately 5 weight % Ta) and 1 weight %-5 weight % Ti (preferably approximately 2 weight % Ti), or (iii) 1 weight %-10 weight % Nb (preferably approximately 5 weight % Nb) and 1 weight %-5 weight % Cr (preferably approximately 2 weight % Cr). Furthermore, when utilized in conjunction with highly conductive conductor layers such as Cu to form electrodes and/or interconnects, the barrier and/or capping layer and the conductor layer exhibit substantially identical etch rates in preferred etchants such as a PAN etch, i.e., a mixture of phosphoric acid, acetic acid, and nitric acid, which may be mixed with water and may be heated to elevated temperatures. Thus, etch-related residue and discontinuities are minimized or eliminated via use of the barrier and/or capping layers in accordance with preferred embodiments of the present invention. 
         [0011]    In an aspect, embodiments of the invention feature a thin-film transistor that includes or consists essentially of a substrate and an electrode. The substrate may include or consist essentially of silicon and/or glass. The electrode includes or consists essentially of (i) disposed over or on the substrate, a barrier layer that includes, consists essentially of, or consists of an alloy of Cu and one or more refractory metal elements selected from the list consisting of Ta, Nb, Mo, W, Zr, Hf, Re, Os, Ru, Rh, Ti, V, Cr, and Ni, and (ii) disposed over or on the barrier layer, a conductor layer that includes, consists essentially of, or consists of Cu, Ag, Al, and/or Au. 
         [0012]    Embodiments of the invention may include one or more of the following in any of a variety of different combinations. The substrate may include, consist essentially of, or consist of glass. The substrate may include, consist essentially of, or consist of silicon, e.g., amorphous silicon. The barrier layer may include, consist essentially of, or consist of an alloy of Cu, Ta, and Cr. The barrier layer may include, consist essentially of, or consist of 1 weight %-12 weight % Ta, 1 weight %-5 weight % Cr, and the balance Cu. The barrier layer may include, consist essentially of, or consist of approximately 5 weight % Ta, approximately 2 weight % Cr, and the balance Cu. The barrier layer may include, consist essentially of, or consist of 5 weight % Ta, 2 weight % Cr, and the balance Cu. The barrier layer may include, consist essentially of, or consist of approximately 2 weight % Ta, approximately 1 weight % Cr, and the balance Cu. The barrier layer may include, consist essentially of, or consist of 2 weight % Ta, 1 weight % Cr, and the balance Cu. 
         [0013]    The barrier layer may include, consist essentially of, or consist of an alloy of Cu, Ta, and Ti. The barrier layer may include, consist essentially of, or consist of 1 weight % 12 weight % Ta, 1 weight %-5 weight % Ti, and the balance Cu. The barrier layer may include, consist essentially of, or consist of approximately 5 weight % Ta, approximately 2 weight % Ti, and the balance Cu. The barrier layer may include, consist essentially of, or consist of 5 weight % Ta, 2 weight % Ti, and the balance Cu. 
         [0014]    The barrier layer may include, consist essentially of, or consist of an alloy of Cu, Nb, and Cr. The barrier layer may include, consist essentially of, or consist of 1 weight %-10 weight % Nb, 1 weight %-5 weight % Cr, and the balance Cu. The barrier layer may include, consist essentially of, or consist of approximately 5 weight % Nb, approximately 2 weight % Cr, and the balance Cu. The barrier layer may include, consist essentially of, or consist of 5 weight % Nb, 2 weight % Cr, and the balance Cu. 
         [0015]    The electrode may include a sidewall including, consisting essentially of, or consisting of (a) an exposed portion of the barrier layer, (b) an exposed portion of the conductor layer, and (c) an interface between the exposed portion of the barrier layer and the exposed portion of the conductor layer. The sidewall of the electrode may be substantially, or even entirely, free of discontinuities notwithstanding the interface. The substrate may be substantially, or even entirely, free of Cu diffusion from the barrier layer. The barrier layer may include, consist essentially of, or consist of a plurality of crystalline grains separated by grain boundaries. At least one of the grain boundaries may include a particulate therein. The particulate may include, consist essentially of, or consist of a reaction product of silicon and at least one of the refractory metal elements. 
         [0016]    In another aspect, embodiments of the invention feature a method of forming an electrode of a thin-film transistor. A substrate is provided. The substrate may include, consist essentially of, or consist of silicon and/or glass. A barrier layer is deposited over the substrate, and a conductor layer is deposited over the barrier layer. The barrier layer includes, consists essentially of, or consists of an alloy of Cu and one or more refractory metal elements selected from the group consisting of Ta, Nb, Mo, W, Zr, Hf, Re, Os, Ru, Rh, Ti, V, Cr, and Ni. The conductor layer includes, consists essentially of, or consists of Cu, Ag, Al, and/or Au. A mask layer is formed over the barrier layer, and the mask layer is patterned to reveal a portion of the conductor layer. A remaining portion of the mask layer may at least partially define a shape of the electrode. An etchant is applied to remove portions of the conductor layer and the barrier layer not masked by the patterned mask layer, thereby forming a sidewall of the electrode. The sidewall includes, consists essentially of, or consists of (a) an exposed portion of the barrier layer, (b) an exposed portion of the conductor layer, and (c) an interface between the exposed portion of the barrier layer and the exposed portion of the conductor layer. The sidewall is substantially, or even entirely, free of discontinuities notwithstanding the interface. 
         [0017]    Embodiments of the invention may include one or more of the following in any of a variety of different combinations. The mask layer may include, consist essentially of, or consist of photoresist. The etchant may include, consist essentially of, or consist of a mixture of phosphoric acid, acetic acid, nitric acid, and water. The etchant may include, consist essentially of, or consist of 50-60 weight % phosphoric acid, 15-25 weight % acetic acid, 3-5 weight % nitric acid, and the balance water. The etchant may include, consist essentially of, or consist of 50 weight % phosphoric acid, 25 weight % acetic acid, 3 weight % nitric acid, and the balance water. The remaining portion of the patterned mask layer may be removed, e.g., after the etchant is applied. The barrier layer may include, consist essentially of, or consist of a plurality of crystalline grains separated by grain boundaries. The substrate may include, consist essentially of, or consist of silicon. The electrode may be annealed at a temperature sufficient to form a particulate within at least one of the grain boundaries (e.g., between 200° C. and 700° C., or between 300° C. and 500° C.). The particulate may include, consist essentially of, or consist of a reaction product of silicon and at least one of the refractory metal elements (e.g., a refractory metal silicide). The substrate may include, consist essentially of, or consist of glass or amorphous silicon. 
         [0018]    The barrier layer may include, consist essentially of, or consist of an alloy of Cu, Ta, and Cr. The barrier layer may include, consist essentially of, or consist of 1 weight % 12 weight % Ta, 1 weight %-5 weight % Cr, and the balance Cu. The barrier layer may include, consist essentially of, or consist of approximately 5 weight % Ta, approximately 2 weight % Cr, and the balance Cu. The barrier layer may include, consist essentially of, or consist of 5 weight % Ta, 2 weight % Cr, and the balance Cu. The barrier layer may include, consist essentially of, or consist of approximately 2 weight % Ta, approximately 1 weight % Cr, and the balance Cu. The barrier layer may include, consist essentially of, or consist of 2 weight % Ta, 1 weight % Cr, and the balance Cu. 
         [0019]    The barrier layer may include, consist essentially of, or consist of an alloy of Cu, Ta, and Ti. The barrier layer may include, consist essentially of, or consist of 1 weight %-12 weight % Ta, 1 weight %-5 weight % Ti, and the balance Cu. The barrier layer may include, consist essentially of, or consist of approximately 5 weight % Ta, approximately 2 weight % Ti, and the balance Cu. The barrier layer may include, consist essentially of, or consist of 5 weight % Ta, 2 weight % Ti, and the balance Cu. 
         [0020]    The barrier layer may include, consist essentially of, or consist of an alloy of Cu, Nb, and Cr. The barrier layer may include, consist essentially of, or consist of 1 weight %-10 weight % Nb, 1 weight %-5 weight % Cr, and the balance Cu. The barrier layer may include, consist essentially of, or consist of approximately 5 weight % Nb, approximately 2 weight % Cr, and the balance Cu. The barrier layer may include, consist essentially of, or consist of 5 weight % Nb, 2 weight % Cr, and the balance Cu. 
         [0021]    In yet another aspect, embodiments of the invention feature a touch-panel display that includes or consists essentially of a substrate, a plurality of conductive touch-panel row sensors, a plurality of conductive touch-panel column sensors, and an interconnect. The row sensors are disposed over the substrate and arranged in lines extending along a first direction. The column sensors are disposed over the substrate and arranged in lines extending along a second direction and intersecting the lines of the row sensors. The interconnect is disposed at a point of intersection between a line of row sensors and a line of column sensors, and the interconnect electrically connects two column sensors or two row sensors. The interconnect includes, consists essentially of, or consists of (i) a conductor layer that includes, consists essentially of, or consists of Cu, Ag, Al, and/or Au, and (ii) disposed over or on the conductor layer, a capping layer that includes, consists essentially of, or consists of an alloy of Cu and one or more refractory metal elements selected from the list consisting of Ta, Nb, Mo, W, Zr, Hf, Re, Os, Ru, Rh, Ti, V, Cr, and Ni. 
         [0022]    Embodiments of the invention may include one or more of the following in any of a variety of different combinations. The interconnect may extend over or under a row sensor and electrically connect two column sensors. An insulating layer may be disposed between the interconnect and the row sensor and may electrically insulate the interconnect and the row sensor. The interconnect may extend over or under a column sensor and electrically connect two row sensors. An insulating layer may be disposed between the interconnect and the column sensor and may electrically insulate the interconnect and the column sensor. The substrate may include, consist essentially of, or consist of an insulating material, e.g., glass. The row sensors and/or the column sensors may include, consist essentially of, or consist of a substantially transparent conductive material, e.g., indium tin oxide. 
         [0023]    The capping layer may include, consist essentially of, or consist of an alloy of Cu, Ta, and Cr. The capping layer may include, consist essentially of, or consist of 1 weight %-12 weight % Ta, 1 weight %-5 weight % Cr, and the balance Cu. The capping layer may include, consist essentially of, or consist of approximately 5 weight % Ta, approximately 2 weight % Cr, and the balance Cu. The capping layer may include, consist essentially of, or consist of 5 weight % Ta, 2 weight % Cr, and the balance Cu. The capping layer may include, consist essentially of, or consist of approximately 2 weight % Ta, approximately 1 weight % Cr, and the balance Cu. The capping layer may include, consist essentially of, or consist of 2 weight % Ta, 1 weight % Cr, and the balance Cu. 
         [0024]    The capping layer may include, consist essentially of, or consist of an alloy of Cu, Ta, and Ti. The capping layer may include, consist essentially of, or consist of 1 weight %-12 weight % Ta, 1 weight %-5 weight % Ti, and the balance Cu. The capping layer may include, consist essentially of, or consist of approximately 5 weight % Ta, approximately 2 weight % Ti, and the balance Cu. The capping layer may include, consist essentially of, or consist of 5 weight % Ta, 2 weight % Ti, and the balance Cu. 
         [0025]    The capping layer may include, consist essentially of, or consist of an alloy of Cu, Nb, and Cr. The capping layer may include, consist essentially of, or consist of 1 weight %-10 weight % Nb, 1 weight %-5 weight % Cr, and the balance Cu. The capping layer may include, consist essentially of, or consist of approximately 5 weight % Nb, approximately 2 weight % Cr, and the balance Cu. The capping layer may include, consist essentially of, or consist of 5 weight % Nb, 2 weight % Cr, and the balance Cu. 
         [0026]    The interconnect may include a sidewall that includes, consists essentially of, or consists of (a) an exposed portion of the capping layer, (b) an exposed portion of the conductor layer, and (c) an interface between the exposed portion of the capping layer and the exposed portion of the conductor layer. The sidewall of the electrode may be substantially, or even entirely free of discontinuities notwithstanding the interface. The capping layer may include, consist essentially of, or consist of a plurality of crystalline grains separated by grain boundaries. At least one of the grain boundaries may include a particulate therein. The particulate may include, consist essentially of, or consist of an agglomeration (e.g., an aggregation of atoms coming together via diffusion) of one or more of the refractory metal elements. One or more of the grain boundaries may contain a larger concentration of the refractory metal element(s) than the bulk volumes of the grains of the capping layer. 
         [0027]    In another aspect, embodiments of the invention feature a method of forming an interconnect of a touch-panel display. A structure including or consisting essentially of a substrate, a plurality of conductive touch-panel row sensors, and a plurality of conductive touch-panel column sensors is provided. The row sensors are disposed over the substrate and arranged in lines extending along a first direction. The column sensors are disposed over the substrate and arranged in lines extending along a second direction and intersecting the lines of the row sensors. An insulator layer is deposited at least at a point of intersection between a line of row sensors and a line of column sensors. A conductor layer is deposited over the insulator layer, and a capping layer is deposited over or on the conductor layer. The conductor layer includes, consists essentially of, or consists of Cu, Ag, Al, and/or Au. The capping layer includes, consists essentially of, or consists of an alloy of Cu and one or more refractory metal elements selected from the group consisting of Ta, Nb, Mo, W, Zr, Hf, Re, Os, Ru, Rh, Ti, V, Cr, and Ni. A mask layer is formed over the capping layer. The mask layer is patterned to reveal a portion of the capping layer. A remaining portion of the mask layer may at least partially define a shape of the interconnect. An etchant is applied to remove portions of the capping layer and the conductor layer not masked by the patterned mask layer, thereby forming a sidewall of the interconnect. The sidewall includes, consists essentially of, or consists of (a) an exposed portion of the capping layer, (b) an exposed portion of the conductor layer, and (c) an interface between the exposed portion of the capping layer and the exposed portion of the conductor layer. The sidewall is substantially, or even entirely, free of discontinuities notwithstanding the interface. 
         [0028]    Embodiments of the invention may include one or more of the following in any of a variety of different combinations. The mask layer may include, consist essentially of, or consist of photoresist. The etchant may include, consist essentially of, or consist of a mixture of phosphoric acid, acetic acid, nitric acid, and water. The etchant may include, consist essentially of, or consist of 50-60 weight % phosphoric acid, 15-25 weight % acetic acid, 3-5 weight % nitric acid, and the balance water. The etchant may include, consist essentially of, or consist of 50 weight % phosphoric acid, 25 weight % acetic acid, 3 weight % nitric acid, and the balance water. Any remaining portion of the patterned mask layer may be removed. The capping layer may include, consist essentially of, or consist of a plurality of crystalline grains separated by grain boundaries. The interconnect may be annealed at a temperature sufficient to form a particulate within at least one of the grain boundaries (e.g., between 200° C. and 700° C., or between 300° C. and 500° C.). The particulate may include, consist essentially of, or consist of an agglomeration of at least one of the refractory metal elements. The substrate may include, consist essentially of, or consist of an insulating material, e.g., glass. The row sensors and column sensors may include, consist essentially of, or consist of a substantially transparent conductive material, e.g., indium tin oxide. 
         [0029]    The capping layer may include, consist essentially of, or consist of an alloy of Cu, Ta, and Cr. The capping layer may include, consist essentially of, or consist of 1 weight %-12 weight % Ta, 1 weight %-5 weight % Cr, and the balance Cu. The capping layer may include, consist essentially of, or consist of approximately 5 weight % Ta, approximately 2 weight % Cr, and the balance Cu. The capping layer may include, consist essentially of, or consist of 5 weight % Ta, 2 weight % Cr, and the balance Cu. The capping layer may include, consist essentially of, or consist of approximately 2 weight % Ta, approximately 1 weight % Cr, and the balance Cu. The capping layer may include, consist essentially of, or consist of 2 weight % Ta, 1 weight % Cr, and the balance Cu. 
         [0030]    The capping layer may include, consist essentially of, or consist of an alloy of Cu, Ta, and Ti. The capping layer may include, consist essentially of, or consist of 1 weight %-12 weight % Ta, 1 weight %-5 weight % Ti, and the balance Cu. The capping layer may include, consist essentially of, or consist of approximately 5 weight % Ta, approximately 2 weight % Ti, and the balance Cu. The capping layer may include, consist essentially of, or consist of 5 weight % Ta, 2 weight % Ti, and the balance Cu. 
         [0031]    The capping layer may include, consist essentially of, or consist of an alloy of Cu, Nb, and Cr. The capping layer may include, consist essentially of, or consist of 1 weight %-10 weight % Nb, 1 weight %-5 weight % Cr, and the balance Cu. The capping layer may include, consist essentially of, or consist of approximately 5 weight % Nb, approximately 2 weight % Cr, and the balance Cu. The capping layer may include, consist essentially of, or consist of 5 weight % Nb, 2 weight % Cr, and the balance Cu. 
         [0032]    These and other objects, along with advantages and features of the present invention herein disclosed, will become more apparent through reference to the following description, the accompanying drawings, and the claims. Furthermore, it is to be understood that the features of the various embodiments described herein are not mutually exclusive and may exist in various combinations and permutations. As used herein, the terms “approximately” and “substantially” mean±10%, and in some embodiments, ±5%. The term “consists essentially of” means excluding other materials that contribute to function, unless otherwise defined herein. Nonetheless, such other materials may be present, collectively or individually, in trace amounts. For example, a structure consisting essentially of multiple metals will generally include only those metals and only unintentional impurities (which may be metallic or non-metallic) that may be detectable via chemical analysis but do not contribute to function. As used herein, “consisting essentially of at least one metal” refers to a metal or a mixture of two or more metals but not compounds between a metal and a non-metallic element or chemical species such as oxygen or nitrogen (e.g., metal nitrides or metal oxides); such non-metallic elements or chemical species may be present, collectively or individually, in trace amounts, e.g., as impurities. As used herein, “columns” and “rows” refer to elements arranged in different directions (and that may intersect), and are otherwise arbitrary unless otherwise noted; i.e., an arrangement of elements may be a row or a column, regardless of its orientation in space or within a device. As used herein, “substrate” or “base layer” refers to a support member (e.g., a semiconductor substrate such as silicon, GaAs, GaN, SiC, sapphire, or InP, or a platform including or consisting essentially of another material, e.g., an insulating material such as glass) with or without one or more additional layers disposed thereon, or to the one or more additional layers themselves. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]    In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the present invention are described with reference to the following drawings, in which: 
           [0034]      FIG. 1A  is a schematic cross-section of a thin-film transistor for a liquid crystal display; 
           [0035]      FIG. 1B  is a schematic cross-section of an etched conventional TFT electrode; 
           [0036]      FIG. 2A  is a plan-view schematic of the sensor array of a touch-panel display; 
           [0037]      FIG. 2B  is a magnified perspective view of a portion of the sensor array of  FIG. 2A ; 
           [0038]      FIG. 2C  is a schematic cross-section of the sensor-array portion of  FIG. 2B ; 
           [0039]      FIG. 2D  is a schematic cross-section, along a plane perpendicular to that of  FIG. 2C , of the sensor-array portion of  FIG. 2B  depicting an etched conventional interconnect; 
           [0040]      FIGS. 3 and 4  are schematic cross-sections of a TFT electrode during fabrication in accordance with various embodiments of the invention; 
           [0041]      FIGS. 5 and 6  are schematic cross-section of an interconnect for a touch-panel display in accordance with various embodiments of the invention; 
           [0042]      FIGS. 7A and 7B  are Auger spectra graphs of mutual diffusion of Cu and Si without a diffusion barrier therebetween; 
           [0043]      FIGS. 8A and 8B  are Auger spectra graphs of mutual diffusion of Cu and Si between a Si layer and a Cu-alloy capping or barrier layer in accordance with various embodiments of the invention; 
           [0044]      FIGS. 9A-9C  are plan-view micrographs of the surface of a Cu-alloy capping or barrier layer as deposited ( FIG. 9A ), after annealing at 300° C. ( FIG. 9B ), and after annealing at 500° C. ( FIG. 9C ), in accordance with various embodiments of the invention; 
           [0045]      FIGS. 10A and 10B  are plan-view micrographs taken via scanning electron microscopy ( FIG. 10A ) and transmission electron microscopy ( FIG. 10B ) of an annealed Cu-alloy capping or barrier layer on Si, in accordance with various embodiments of the invention; and 
           [0046]      FIG. 11  depicts corrosion levels after environmental corrosion testing of samples of pure Mo, pure Cu, CuTaCr alloy, and CuNbCr alloy in accordance with various embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0047]      FIG. 3  depicts an initial step in the fabrication of a TFT gate electrode in accordance with embodiments of the present invention. As shown, a barrier layer  300  is deposited on a substrate  310  (e.g., a glass or silicon substrate) by, e.g., sputtering or other physical deposition process. A conductor layer  320  is subsequently deposited on the barrier layer  300  by, e.g., sputtering or other physical deposition process. Typically the thickness of the barrier layer  300  will be between approximately 5% and approximately 25% (e.g., approximately 10%) of the thickness of conductor layer  320 . For example, the thickness of the barrier layer  300  may be approximately 50 nm, and the thickness of the conductor layer  320  may be approximately 500 nm. A mask layer  330  (e.g., photoresist) is formed over the conductor layer  320  and patterned by conventional photolithography. 
         [0048]    As shown in  FIG. 4 , a gate electrode  400  is then fabricated by etching the portions of the conductor layer  320  and barrier layer  300  not covered by the mask layer  330 , preferably in a single-step wet etch. A wet etchant (e.g., a PAN etch) is utilized to etch away the metal layers at substantially the same rates, resulting in sidewalls  410  that are substantially smooth and/or linear and that are substantially free of any discontinuity (e.g., a stepped or nonlinear profile) at an interface  420  between the conductor layer  320  and barrier layer  300 . The wet etchant may include or consist essentially of, for example, a PAN etch including or consisting essentially of 50-60 weight % phosphoric acid, 15-25 weight % acetic acid, 3-5 weight % nitric acid, and the balance DI water. Some specific examples are provided in the table below. In one preferred embodiment, the wet etchant includes or consists essentially of 50 weight % phosphoric acid, 25 weight % acetic acid, 3 weight % nitric acid, and the balance (22 weight %) DI water. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 phosphoric acid, 
                 nitric acid, 
                 acetic acid, 
                 DI water, 
               
               
                   
                 H3PO4 
                 HNO3 
                 CH3COOH 
                 balance 
               
               
                   
                 (wt %) 
                 (wt %) 
                 (wt %) 
                 (wt %) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 etchant 1 
                 50 
                 5 
                 15 
                 30 
               
               
                 etchant 2 
                 60 
                 5 
                 20 
                 15 
               
               
                 etchant 3 
                 50 
                 3 
                 25 
                 22 
               
               
                   
               
             
          
         
       
     
         [0049]    After etching, the substrate  310  (as well as the electrode  400 ) is preferably substantially free of etch residue of one or both of the conductor layer  320  and the barrier layer  300  in regions proximate the gate electrode  400 . In accordance with various embodiments of the invention, the wet-etching process is performed at room temperature. The wet etchant may be sprayed on the substrate  310 , or the substrate  310  may be partially or completely immersed in the wet etchant. The wet-etching process may be performed as a batch (i.e., multiple-substrate) process or as a single-substrate process. In preferred embodiments, after etching the sidewalls  410  form an angle  430  with the surface of the underling substrate  310  of between approximately 50° and approximately 70°, e.g., approximately 60°. After etching, the mask layer  330  may be removed by conventional means, e.g., acetone, a commercial photoresist stripping agent, and/or exposure to an oxygen plasma. 
         [0050]      FIG. 5  depicts an initial step in the fabrication of a touch-panel sensor interconnect in accordance with embodiments of the present invention. As shown, a conductive layer  500  is deposited over a sensor  510  (e.g., a row or column sensor that may be composed of a transparent conductor such as ITO) on a substrate  520  (e.g., a glass or silicon substrate) by, e.g., sputtering or other physical deposition process. A capping layer  530  is subsequently deposited on the conductive layer  500  by, e.g., sputtering or other physical deposition process. Typically the thickness of the capping layer  530  will be between approximately 5% and approximately 25% (e.g., approximately 10%) of the thickness of conductive layer  500 . For example, the thickness of the capping layer  530  may be approximately 50 nm, and the thickness of the conductive layer  500  may be approximately 500 nm. A mask layer  540  (e.g., photoresist) is formed over the capping layer  530  and patterned by conventional photolithography. 
         [0051]    As shown in  FIG. 6 , an interconnect  600  is then fabricated by etching the portions of the capping layer  530  and conductive layer  500  not covered by the mask layer  540 , preferably in a single-step wet etch. A wet etchant (e.g., a PAN etch) is utilized to etch away the metal layers at substantially the same rates, resulting in sidewalls  610  that are substantially smooth and/or linear and that are substantially free of any discontinuity (e.g., a stepped or nonlinear profile) at an interface  620  between the capping layer  530  and conductive layer  500 . The wet etchant may include or consist essentially of, for example, a PAN etch including or consisting essentially of 50-60 weight % phosphoric acid, 15-25 weight % acetic acid, 3-5 weight % nitric acid, and the balance DI water. In one preferred embodiment, the wet etchant includes or consists essentially of 50 weight % phosphoric acid, 25 weight % acetic acid, 3 weight % nitric acid, and the balance (22 weight %) DI water. 
         [0052]    After etching, the substrate  520  and electrode  510  (as well as the interconnect  600 ) are preferably substantially free of etch residue of one or both of the capping layer  530  and the conductive layer  500  in regions proximate the interconnect  600 . In accordance with various embodiments of the invention, the wet-etching process is performed at room temperature. The wet etchant may be sprayed on the substrate  520 , or the substrate  520  may be partially or completely immersed in the wet etchant. The wet-etching process may be performed as a batch (i.e., multiple-substrate) process or as a single-substrate process. In preferred embodiments, after etching the sidewalls  610  form an angle  630  with the surface of the underling substrate  520  of between approximately 50° and approximately 70°, e.g., approximately 60°. After etching, the mask layer  330  may be removed by conventional means, e.g., acetone, a commercial photoresist stripping agent, and/or exposure to an oxygen plasma. 
         [0053]    Barrier layers  300  and capping layers  530  in accordance with various embodiments of the invention also serve as effective diffusion barriers for metallic layers that include or consist essentially of, e.g., Cu, Ag, Al, or Au. Specifically, the alloying element(s) within the barrier layer  300  and/or capping layer  530  substantially prevent diffusion of a conductor layer material (e.g., Cu) into an underlying silicon substrate or an adjoining layer even after exposure to elevated temperatures (e.g., up to approximately 200° C., up to approximately 350° C., up to approximately 500° C., or even higher) for times of, e.g., up to 2 hours.  FIGS. 7A and 7B  show the concentrations of Cu and silicon across a Cu/silicon interface (i.e., one without a barrier layer between the Cu and silicon) as measured with Auger electron spectroscopy (AES) as fabricated (no anneal) and after anneals of 200° C.-500° C. As shown, mutual diffusion of the Cu and silicon occurs at temperatures as low as (or even lower than) 200° C., and the interface is severely diffused after an anneal at 500° C. Additionally, the Cu layer exhibits poor adhesion to silicon in the absence of a barrier layer between the Cu and silicon. 
         [0054]      FIGS. 8A and 8B  show the concentrations of Cu and silicon across an interface between silicon and a barrier layer  300  or capping layer  530  that includes or consists essentially of CuTaCr as measured with AES after no anneal and anneals of 200° C.-500° C. In the illustrated embodiment, the barrier layer  300  or capping layer  530  is composed of 2 weight % Ta, 1 weight % Cr, and the balance Cu. (In another embodiment exhibiting similar behavior, the barrier layer  300  or capping layer  530  includes or consists essentially of 5 weight % Ta, 2 weight % Cr, and the balance Cu.) In contrast to the results shown in  FIGS. 7A and 7B , there is negligible diffusion of Cu or silicon across the interface, even after an anneal at 500° C. for two hours.  FIGS. 9A-9C  are a series of scanning electron microscopy (SEM) micrographs of the surface of the barrier layer  300  or capping layer  530  as deposited ( FIG. 9A ), after an anneal of 300° C. for one hour ( FIG. 9B ), and after an anneal of 500° C. for one hour ( FIG. 9C ). As shown, the grain structure and size of the barrier layer  300  or capping layer  530  show no appreciable change, and there is no evidence of the formation of different phases (e.g., copper silicide phases) even after a heat treatment of 500° C. These results were confirmed by x-ray diffraction (XRD) scans of annealed structures, in which no silicide phases were detected even after anneals of 500° C. for two hours. In contrast, copper silicide phases are clearly evident in SEM and XRD performed on samples of pure Cu layers on Si that have been annealed at 500° C. for two hours. 
         [0055]      FIGS. 10A and 10B  show, respectively, SEM and transmission electron microscopy (TEM) images of a barrier layer  300  or capping layer  530  that is disposed in contact with silicon (e.g., a silicon substrate and/or a silicon overlayer) and that has been annealed at 350° C. for 30 minutes. Precipitates  1000  are evident within the Cu grain boundaries  1010  of the barrier layer  300  or capping layer  530 . In various embodiments the precipitates include or consist essentially of a silicide of one or more of the refractory metal alloying elements of the barrier layer  300  or capping layer  530 , and such precipitates reduce or substantially eliminate Cu diffusion along the grain boundaries into the adjoining silicon. 
         [0056]    Similarly, in various embodiments of the invention, the refractory-metal dopants of barrier layers  300  and/or capping layers  530  tend to segregate to the Cu grain boundaries and provide beneficial effects even in the absence of reaction with silicon to form silicides. For example, the Cu grain boundaries may be occupied, and partially or substantially completely “blocked” with the refractory-metal dopants and thereby retard or substantially prevent oxygen diffusion along the Cu grain boundaries. In this manner, corrosion of the barrier layer  300 , capping layer  530 , and/or the conductive layer in contact therewith is decreased or substantially prevented. Thus, in various embodiments of the present invention, a barrier layer  300  or capping layer  530  may include or consist essentially of a polycrystalline Cu matrix doped with one or more refractory metal elements, where the grain boundaries of the layer between the doped Cu grains contain a higher concentration of the refractory metal dopant(s) that the concentration within the grains themselves. For example, the refractory metal concentration within the grain boundaries may be larger than that within the grains by a factor of 5, a factor of 10, or even a factor of 100. 
         [0057]      FIG. 11  depicts images of four different metallic samples after an environmental corrosion test conducted at 60° C. and 80% humidity for a period of 260 hours. As shown, the samples of pure Cu and pure Mo experienced much more severe corrosion that did the Cu-alloy samples in accordance with embodiments of the present invention. The two Cu-alloy samples were (1) Cu with 10 weight % Ta and 2 weight % Cr (labeled in  FIG. 11  as CuTaCr), and (2) Cu with 5 weight % Nb and 2 weight % Cr (labeled in  FIG. 11  as CuNbCr). The table below provides data regarding the amount of exposed surface area corroded during the environmental corrosion test for each of the samples. As indicated, the Cu-alloy samples in accordance with embodiments of the present invention experienced much less corrosion than the pure Cu and Mo samples, demonstrating the benefits of such alloys over conventional Mo diffusion barriers and capping layers, as well as over pure Cu. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Sample 
                 Corroded Surface Area (%) 
               
               
                   
                   
               
             
             
               
                   
                 Mo 
                 6.78 
               
               
                   
                 Cu 
                 3.84 
               
               
                   
                 CuTaCr 
                 1.75 
               
               
                   
                 CuNbCr 
                 0.83 
               
               
                   
                   
               
             
          
         
       
     
         [0058]    In preferred embodiments of the invention, the barrier layers  300  or capping layers  530  have low resistivity, e.g., below 10 microOhm-cm, or even below 5 microOhm-cm, even after anneals of up to 500° C., up to 600° C., or even higher temperatures. Moreover, in preferred embodiments the barrier layers  300  or capping layers  530  exhibit good adhesion to glass as measured by, e.g., an ASTM standard tape test. Embodiments of the invention also include electronic devices (or portions thereof) in which a highly conductive material (e.g., Cu, Ag, Al, and/or Au) is utilized to form all or a portion of a conductor or electrode and has both a barrier layer  300  below it and a capping layer  530  above it. 
         [0059]    The terms and expressions employed herein are used as terms and expressions of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof. In addition, having described certain embodiments of the invention, it will be apparent to those of ordinary skill in the art that other embodiments incorporating the concepts disclosed herein may be used without departing from the spirit and scope of the invention. Accordingly, the described embodiments are to be considered in all respects as only illustrative and not restrictive.