Abstract:
This invention relates to a method of fabricating nano-dimensional structures, comprising: depositing at least one deformable material upon a substrate such that the material includes at least one portion; and creating an oxidizable layer located substantially adjacent to the deposited deformable material such that at least a portion of the oxidized portion of the oxidizable layer interacts with the at least one portion of the deformable material to apply a localized pressure upon the at least one portion of the deformable material.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a method of fabricating nano-dimensional structures, comprising: depositing at least one deformable material upon a substrate such that the material includes at least one portion; and creating an oxidizable layer located substantially adjacent to the deposited deformable material such that at least a portion of the oxidized portion of the oxidizable layer interacts with the at least one portion of the deformable material to apply a localized pressure upon the at least one portion of the deformable material. 
     2. Description of the Related Art 
     Prior to the present invention, as set forth in general terms above and more specifically below, it is known, that industrial interest in materials having structural and functional features with nanoscale dimensions has been growing rapidly. Nano-structures have been fabricated by semiconductor processing techniques including patterning techniques such as photolithography, electron-beam lithography, ion-beam lithography, X-ray lithography, nano-imprint lithography, and the like. Other nano-structures have also been fabricated utilizing structures formed by self-ordering processes. 
     It is further known that such small objects require novel and specialized methods of fabrication and subsequent processing. One common task is localized encapsulation of conductors or sensing surfaces. With increasing complexity of nanostructures, it will be more and more difficult to insulate certain regions of the device, while leaving others intact or exposed to the environment. At present, the most common and direct approach to encapsulation is to mask the relevant part of the device and cover it with a protective (insulating) layer. However, this general approach is difficult to implement when coverage of the device areas adjacent to the area being insulated is undesirable because it may interfere with the device&#39;s operation. In such cases, very precise masking processes (alignment, deposition, etc.) are required, which would be difficult to achieve at the nanoscale level. 
     As an example, consider the edge of a 100 nm stack of 10 nm thick layers alternating between conducting and insulating layers. Such an edge would be very difficult, if not impossible, to insulate using the traditional mask and deposit approach, Without depositing material on the top face of the structure, which may be undesirable. Such a situation requires a localized means of encapsulation and protection of the conductive edges of the conductive layers. Consequently, a more advantageous nanostructure encapsulation system, then, would be provided if inexpensive and accurate methods of encapsulation could be developed. 
     With respect to specialized nano-fabrication techniques, the prior art employs a tip of an atomic force microscope to apply a localized pressure at the nanoscale level. While this method can be satisfactory for research purposes, it is not suitable for large-scale fabrication. This is due to the fact that this method is extremely slow and cannot be applied in parallel. Also, the applied force is limited by the mechanical hardness of the tip. Consequently, a further advantageous nano-fabrication technique would be provided if the efficiency of the technique were improved while avoiding the use of the atomic force microscope tip. 
     It is apparent from the above that there exists a need in the art for a nano-fabrication technique that is inexpensive, effective, and capable of applying a localized pressure. It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure. 
     SUMMARY OF THE INVENTION 
     Generally speaking, an embodiment of this invention fulfills these needs by providing a method of fabricating nano-dimensional structures, comprising: depositing at least one deformable material upon a substrate such that the material includes at least one portion; and creating an oxidizable layer located substantially adjacent to the deposited deformable material such that at least a portion of the oxidized portion of the oxidizable layer interacts with the at least one portion of the deformable material to apply a localized pressure upon the at least one portion of the deformable material. 
     In certain preferred embodiments, the deformable material layer can be a non-oxidizable material. Also, the oxidation step can be accomplished through electrochemical oxidation (anodization) or thermal oxidation. 
     In another further preferred embodiment, the method creates an inexpensive, effective, and localized pressurization of portions of the deformable material. 
     The preferred method of fabrication of nano-dimensional structures by oxidization, according to various embodiments of the present invention, offers the following advantages: ease-of-use; improved economy; and enabling localized pressurization. In fact, in many of the preferred embodiments, these factors of ease-of-use, improved economy, and localized pressurization are optimized to an extent that is considerably higher than heretofore achieved in prior, known nano-dimensional structure fabrication methods. 
     The above and other features of the present invention, which will become more apparent as the description proceeds, are best understood by considering the following detailed description in conjunction with the accompanying drawings, wherein like characters represent like parts throughout the several views and in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1   a - 1   c  illustrate a method of fabrication of nano-dimensional structures by oxidation, according to one embodiment of the present invention; 
         FIG. 2  illustrates the expansion effect of tantalum pentoxide, according to one embodiment of the present invention; 
         FIGS. 3   a  and  3   b  illustrate a tantalum/tantalum pentoxide stack oxidized from the edge, according to another embodiment of the present invention; 
         FIGS. 4   a  and  4   b  illustrate another tantalum/tantalum pentoxide stack oxidized from the edge, according to another embodiment of the present invention; 
         FIGS. 5   a  and  5   b  illustrate still another tantalum/tantalum pentoxide stack oxidized from the edge, according to another embodiment of the present invention; 
         FIGS. 6   a - 6   d  are schematic illustrations of oxidation of a metal, according to another embodiment of the present invention; 
         FIGS. 7   a  and  7   b  are schematic illustrations of a further fabrication technique of nano-dimensional objects, according to another embodiment of the present invention; 
         FIGS. 8   a - 8   f  are schematic illustrations of a further fabrication technique of nano-dimensional objects, according to another embodiment of the present invention; 
         FIGS. 9   a - 9   d  are schematic illustrations of a further fabrication technique of nano-dimensional objects, according to another embodiment of the present invention; 
         FIG. 10  is a transmission electron microscope (TEM) image of the fabrication technique of nano-dimensional objects of  FIGS. 9   a - 9   d , according to another embodiment of the present invention; 
         FIGS. 11   a  and  11   b  are schematic illustrations of a further fabrication technique of nano-dimensional objects, according to another embodiment of the present invention; and 
         FIGS. 12   a - 12   c  are schematic illustrations of a further fabrication technique of nano-dimensional objects, according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     For clarity of the description, the drawings are not drawn to a uniform scale. In particular, vertical and horizontal scales may differ from each other and may vary from one drawing to another. In this regard, directional terminology, such as “top,” “bottom,” “front,” “back,” “leading,” “trailing,” etc., is used with reference to the orientation of the drawing figure(s) being described. Because components of the invention can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. 
     The term “oxidation” is used in this specification and the appended claims to mean electrochemical oxidization (anodization) or thermal oxidation of an oxidizable material (such as an oxidizable metal). “Anodization” is performed by employing the oxidizable material as an anode in an electrolytic cell and by operating the electrolytic cell with voltage and current suitable to partially or fully oxidize the material of the anode. An “anodic oxide” is the oxide thus formed. Thermal oxidization refers to a process in which the oxide is formed by exposing the material to a combination of heat and an oxidizing ambient (e.g., oxygen, water vapor, etc). An “oxidizable material” is a material that can be oxidized in these manners. “Partial oxidation” refers to oxidation of less than the entire thickness of a metal layer; i.e., some thickness of unoxidized metal remains after partial oxidation, unless full oxidation is explicitly specified. “Full oxidation” refers to oxidation of the entire thickness of a metal layer. References herein to a layer of oxidizable metal are intended to include semiconductor materials such as silicon which, with respect to their oxidation, behave like the oxidizable metals. 
     It is to be understood that an oxidizable material may also refer to a material that is oxidized to an oxidation state lower than the maximum oxidation state that may be obtained. For example, in the presence of oxygen, a tantalum oxide film with stoichiometry Ta 2 O may be created, for example, via sputter deposition. This tantalum(l) oxide film may then be further oxidized to tantalum(V) oxide, Ta 2 O 5 , with significant further expansion of the oxide film. 
     An embodiment of a layered structure may be made by providing a substrate, depositing a quantity of electrochemically or thermally oxidizable material such as a metal over the substrate, oxidizing the electrochemically or thermally oxidizable material (partially or fully), and repeating similar steps until a layered structure having a desired total structure thickness is completed. The thickness of each layer of the layered structure may be nanoscopic. Thus, another aspect of the invention provides methods for fabricating embodiments of layered structures, including structures whose layers have nanoscale dimensions. 
     One embodiment of a method for fabricating a layered structure employs the steps of providing a substrate, depositing a quantity of an electrochemically or thermally oxidizable material over the substrate to form an electrochemically or thermally oxidizable layer, oxidizing the electrochemically or thermally oxidizable material until a layer of oxide is formed, and repeating alternately the depositing and oxidizing steps until a layered structure having a desired total thickness is completed. The structure may be one of the types known as a superlattice. The electrochemically or thermally oxidizable material is oxidized until a layer of oxide having a desired thickness is formed. In some cases, that oxidation may be a partial oxidation, i.e., less than the entire thickness of the oxidizable material is oxidized. 
     Many electrochemically oxidizable materials are known, including the metals aluminum (Al), tantalum (Ta), niobium (Nb), tungsten (W), bismuth (Bi), antimony (Sb), silver (Ag), cadmium (Cd), iron (Fe), magnesium (Mg), tin (Sn), zinc (Zn), titanium (Ti), copper (Cu), molybdenum (Mo), hafnium (Hf), zirconium (Zr), titanium (Ti), vanadium (V), gold (Au), chromium (Cr), cobalt (Co), iridium (Ir), rhenium (Re), and uranium (U), along with their electrochemically/thermally oxidizable alloys, mixtures, and combinations, all of which are suitable for use in this method. Another suitable material is silicon (Si), although it is not classified as a metal, but as a semiconductor. In short, what is desired in the present invention is that a dense amorphous film be formed during the oxidation process such that the oxidized material is expandable. Thus, references herein to a layer of electrochemically or thermally oxidizable material or metal are intended to include semiconductor materials such as silicon which, with respect to their oxidation, behave like the electrochemically or thermally oxidizable metals. To simplify the description and drawings, embodiments using metals for an electrochemically or thermally oxidizable material will be described. Those skilled in the art will understand that any electrochemically or thermally oxidizable material may be substituted wherever “metal” is mentioned, except where the metal is explicitly described as not being electrochemically or thermally oxidizable. It is to be understood that an oxidizable material may also refer to a material that is not fully oxidized, as discussed above. The thickness of dense oxide films (with densities comparable to theoretical oxide densities) formed by electrochemical oxidation of oxidizable material is precisely controllable by controlling the anodization voltage, as described in more detail hereinbelow. 
     Returning now to the description of a method embodiment for fabricating a layered structure, the layer of electrochemically or thermally oxidizable metal (or, in the case of silicon, for example, electrochemically oxidizable semiconductor) may be deposited by any suitable conventional deposition method, such as evaporation, sputtering, plating, electroplating, atomic layer deposition (ALD), or chemical vapor deposition (CVD) and other known types of vapor deposition techniques. The metal layer may have a thickness of about two nanometers (2 nm) or greater, for example, with essentially no theoretical upper limit, but limited only by practical considerations such as deposition conditions, application requirements, stresses, etc 
     With reference first to  FIG. 1 , there is illustrated one preferred embodiment for use of the concepts of this invention. As shown in  FIG. 1   a , a schematic illustration of a nano-dimensional structure  10  is presented. A suitable substrate  12  is provided. For many applications, substrate  12  is a smooth planar silicon wafer as is commonly used in semiconductor manufacturing. For some applications, a layer of insulating material such as silicon oxide or silicon nitride may be formed on the silicon wafer so the top surface of the substrate is an insulator. It is to be understood that the substrate can also be constructed of glass; quartz; alumina; stainless steel; various plastics; and mixtures and combinations thereof. See, for example, commonly assigned, pending U.S. patent application Ser. No. 10/062,050, filed Jan. 31, 2005, entitled “Periodic Layered Structures and Methods Therefore” and Ser. No. 10/??????, filed ????, 2005, entitled “Encapsulation of Nano-Dimensional Structures by Oxidation”, which are to be incorporated by reference in their entirety. An oxide layer  16  is initially deposited upon substrate  12 . A layer  14  of a first metal is conventionally deposited upon oxide layer  16 . In this embodiment, the first metal is an electrochemically oxidizable material. When the metal layer  14  is partially oxidized to create an oxide layer  16 , the total thickness typically increases. The volume ratio of oxide to consumed metal is typically greater than one. For example, partial oxidization of a 1041 nanometer film of tantalum results in a tantalum oxide film having a thickness of 364 nanometers and an overall thickness of 1,248 nanometers. (See, for example,  FIG. 2 ). Another layer  14  of the first metal is deposited upon oxide layer  16  and further partially oxidized to create another oxide layer  16 . The process of depositing the metal layer  14  and partially oxidizing it to create oxide layer  16  is performed until the desired layering effect is achieved. 
     After nano-dimensional structure  10  has been constructed, one or more metal layers  14  are further partially oxidized, as shown in  FIG. 1   b . This further partial oxidization causes the oxide layer  16  to expand around metal layer  14 . As a by-product of this expansion, nodes  18  in the oxide layer  16  are formed that also extend from the exposed portion of metal layer  14 . This further expansion of the oxide layer  16  provides a complete encapsulation  19  around a portion of the entire structure  10 , as shown in  FIG. 1   c.    
     As discussed earlier,  FIG. 2  shows the expansion effect of tantalum pentoxide (Ta 2 O 5 ) as the result of the process of electrochemical oxidation of tantalum (Ta). The expansion coefficient is defined as the ratio of produced Ta 2 O 5  volume to consumed Ta volume. In this embodiment, the expansion coefficient is approximately 2.3 for oxidation of Ta. 
     With respect to  FIGS. 3   a  and  3   b , encapsulation of 100 nm tantalum (Ta) layers is presented. Scanning electron microscope (SEM) images of a Ta/Ta 2 O 5  stack partially oxidized from the edge to an oxide thickness of approximately 50 nm is shown. It must be pointed out that the external oxide profile substantially conforms to the profile of the Ta layers. This relatively small oxidation thickness corresponds to  FIG. 1   b.    
     With respect to  FIGS. 4   a  and  4   b , encapsulation of 30 nm tantalum (Ta) layers is presented. SEM images of a Ta/Ta 2 O 5  stack partially oxidized from the edge to a specific thickness of approximately 30 nm of Ta 2 O 5  are shown. As further partial oxidization of the Ta layers is completed, the external oxide profile begins to conform less and less to the profile of the Ta layers. 
     With respect to  FIGS. 5   a  and  5   b , further partial oxidization (60 nm thickness of Ta 2 O 5 ) of the Ta layers results in a continuous, dense, smooth, expandable oxidation layer without evidence of the underlying metal layer structure. At this point, effective encapsulation of the underlying nano-dimensional structure has been achieved. 
     With respect to  FIGS. 6   a - 6   d , nano-dimensional structure  60  is illustrated. In  FIGS. 6   a  and  6   b , oxide layer  64  is deposited upon substrate  12 . Metal layers  62  are then partially oxidized to create oxide layers  64 ,  68  in a similar fashion, as discussed above. As can be seen in  FIG. 6   b , encapsulation layer  65  provides a complete encapsulation around metal layers  62 . In  FIGS. 6   a  and  6   b , oxide layers  64  are metal oxides of metal layers  62 . 
     With respect to  FIGS. 6   c  and  6   d , oxide layers  68  are formed by directly depositing the appropriate material  68 , or perhaps by depositing a 2-layer metal stack and then oxidizing only the “top” sub-layer of metal corresponding to the oxide type  68 , leaving the remaining metal layer  62  un-oxidized. As can be seen in these figures, a metal oxide is formed from a metal that is different than the metal in layer  62 . As can be further seen in  FIG. 6   d , oxide layer  69  provides a complete encapsulation around metal layers  62  and material layers  68 . This fabrication technique of utilizing a metal oxide that is formed from a metal that is different than the metal in a layer  62  will be further explored, as set forth below. 
     With respect to  FIGS. 7   a  and  7   b , it has also been discovered that the nano-fabrication techniques shown in  FIGS. 6   a - 6   d  can create a mechanical pressure that is generated during the oxidation process which manifests itself in the geometries where a region of a relatively soft or deformable material  76  is adjacent to a region of an oxidizable material  74 . More particularly, a non-oxidizable substrate  72  is covered with a multilayer stack of oxidizable materials  74  and deformable materials  76  which make up nano-dimensional structure  70 . In the simplest case, the stack consists of one deformable layer  76  on top of one oxidizable layer  74  ( FIG. 7   a ). It is to be understood that the stack may be patterned by conventional lithographic or other techniques. As shown in  FIG. 7   b , an oxidation step is performed which creates an expandable oxide layer  75  that causes the deformable layer  76  to bend or deform upwards at points  78 . As a result, the shape of the deformable layer  76  will be altered. In this case, the original planar film stack is modified to yield partially vertically oriented edges with an edge “sharpness” defined by the thickness of the original film, which can be quite small. The non-oxidizable, deformable layer  76  can be constructed of, but is not limited to, platinum (Pt), aluminum oxide (Al 2 O 3 ), zinc oxide (ZnO), indium tin oxide (ITO), SnO 2  or the like. It is to be understood that after the deformable layer  76  has been deformed, the oxide layer  75  can be conventionally, selectively removed in order to leave the “free-standing” portion of deformable layer  76  in the resulting deformed position. 
     An advantage of this nano-fabrication process is that employs conventional processing of films within the plane of the substrate, after which the final oxidation step acts to provide a change in local orientation by “deforming” selected edges upward toward a vertical orientation. Since conventional processing methods can be used until this final step, this embodiment is amenable to integration with, for example, underlying circuitry, thus enabling the fabrication of electronically controlled arrays of the structures, as described below. Also, the present invention can be employed on multiple layered structures to achieve a variety of effects. It is to be understood that the degree of deformation in the drawings is for illustrative purposes only and not drawn to scale. It is to be further understood that the oxidizable material may be selectively oxidized to provide a non-uniform pressure upon the deformable material. Finally it is to be understood that the layers of deformable materials can be not only deformed out but deformed towards each other so that they contact each other in a controllable fashion. 
     With respect to  FIGS. 8   a - 8   f , a further embodiment of the concepts described in  FIG. 7  is provided. As shown in  FIG. 8   a , nano-dimensional structure  80  includes, in part, substrate  82 , oxidizable material  84 , deformable layer  86 , and upper layer  88 . Preferably, upper layer  88  is constructed of any suitable material that has sufficient mechanical rigidity to partially constrain the expansion of the oxidizable material  84  and the deformation of the deformable layer  86 . Also, upper layer  88  may be patterned with an edge  83  “pulled back” relative to the end of the oxidizable and deformable layers, so as to provide a sharper radius of curvature  87  as the deformable layer  86  is “deformed” upward by the expanding oxide layer  85  ( FIG. 8   b ).  FIG. 8   c  illustrates an isometric view of  FIG. 8   b.    
     With respect to  FIGS. 8   d - 8   f , a further embodiment of the concepts described in  FIGS. 8   a - 8   c  is provided. As shown in  FIG. 8   c , nano-dimensional structure  80  includes, in part, lower layer  82 , edges  83 , oxidizable material  84 , deformable layers  86 , and upper layer  88 . As shown in  FIG. 8   e , when oxide layer  85  expands this expansion causes deformable layers  86  to “deform” in opposite directions along radii of curvature  87 .  FIG. 8   f  illustrates an isometric view of  FIG. 8   e . It is to be understood that after the deformable layers  86  have been “deformed”, the oxide layer  85  can be conventionally, selectively removed in order to leave the “free-standing” portions of deformable layers  86  in the resulting deformed position. 
     With respect to  FIGS. 9   a - 9   d , a further embodiment of the concepts described in  FIG. 7  is provided. As shown in  FIG. 9   a , nano-dimensional structure  90  includes, in part, substrate  92 , oxidizable material  94 , and deformable material  96 . As shown in  FIG. 9   b , when expandable oxide layer  95  is formed, the resulting pressure exerts uniform pressure (normal to the plane of the films) on the deformable layers  96 . This pressure causes an extrusion of the deformable material layers  96  outward from the exposed edge and effectively “sharpens” these edges. The oxide layer  95  may optionally then be conventionally removed to expose the sharpened edges  97  ( FIG. 9   c ).  FIG. 9   d  illustrates an isometric view of  FIG. 9   c.    
       FIG. 10  is a transmission electron microscope (TEM) cross-sectional image of a Ta/Al 2 O 3  multilayer stack in which the edges of the Al 2 O 3  layers are “sharpened” by anodization of the Ta layers from the edge. 
     With respect to  FIGS. 11   a  and  11   b , nano-dimensional structure  110  includes, in part, substrate  112 , deformable material  114 , and oxidizable material  116 . As can be seen in  FIG. 11   a , deformable material  114  can be shaped into multiple parallel “wires”. As shown in  FIG. 11   b , when expandable oxide layer  115  is formed, the resulting pressure exerts uniform pressure on the deformable material  114 . This pressure causes the deformable material or “wires”  114  to bend radially outward. It is to be understood that after the deformable layer  114  has been “deformed”, the oxide layer  115  can be conventionally, selectively removed in order to leave the “free-standing” deformable layer  114  in the resulting deformed position. 
     With respect to  FIGS. 12   a - 12   c , nano-dimensional structure  120  includes, in part, substrate  121 , oxidizable material  122 , and deformable material or “wires”  124 . As can be seen in  FIG. 12   b , when expandable oxide layer  125  is formed, the resulting pressure exerts uniform pressure around the circumference of the deformable material or “wires”  124 . This pressure causes an extrusion of the deformable material  124  outward from the exposed edge and effectively “sharpens” these wires by creating a reduction in tip dimension. The oxide layer  125  may then be conventionally removed to expose the sharpened tips  126  ( FIG. 12   c.    
     Also, the present invention can be embodied in any computer-readable medium for use by or in connection with an instruction-execution system, apparatus or device such as a computer/processor based system, processor-containing system or other system that can fetch the instructions from the instruction-execution system, apparatus or device, and execute the instructions contained therein. In the context of this disclosure, a “computer-readable medium” can be any means that can store, communicate, propagate or transport a program for use by or in connection with the instruction-execution system, apparatus or device. The computer-readable medium can comprise any one of many physical media such as, for example, electronic, magnetic, optical, electromagnetic, infrared, or semiconductor media. More specific examples of a suitable computer-readable medium would include, but are not limited to, a portable magnetic computer diskette such as floppy diskettes or hard drives, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory, or a portable compact disc. It is to be understood that the computer-readable medium could even be paper or another suitable medium upon which the program is printed, as the program can be electronically captured, via, for instance, optical scanning of the paper or other medium, then compiled, interpreted or otherwise processed in a single manner, if necessary, and then stored in a computer memory. 
     Those skilled in the art will understand that various embodiment of the present invention can be implemented in hardware, software, firmware or combinations thereof. Separate embodiments of the present invention can be implemented using a combination of hardware and software or firmware that is stored in memory and executed by a suitable instruction-execution system. If implemented solely in hardware, as in an alternative embodiment, the present invention can be separately implemented with any or a combination of technologies which are well known in the art (for example, discrete-logic circuits, application-specific integrated circuits (ASICs), programmable-gate arrays (PGAs), field-programmable gate arrays (FPGAs), and/or other later developed technologies. In preferred embodiments, the present invention can be implemented in a combination of software and data executed and stored under the control of a computing device. 
     It will be well understood by one having ordinary skill in the art, after having become familiar with the teachings of the present invention, that software applications may be written in a number of programming languages now known or later developed. 
     Once given the above disclosure, many other features, modifications or improvements will become apparent to the skilled artisan. Such features, modifications or improvements are, therefore, considered to be a part of this invention, the scope of which is to be determined by the following claims.