Abstract:
A method for fabricating or prototyping a nanoscale object is disclosed. The method includes defining a sequence of nanolayers that represent the nanoscale object, constructing a current nanolayer on a first surface, and depositing a sacrificial layer to cover the first surface but not the nanolayer. The nanolayer represents a slice of the nanoscale object. The nanolayer and the sacrificial layer provide a second surface on which a next nanolayer is constructed. The above construction and deposition steps are repeated if the next nanolayer is not the last nanolayer. The method also includes removing the sacrificial layers to produce the nanoscale object.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims benefit of the priority of U.S. Provisional Application Serial No. 60/181,122, filed Feb. 8, 2000, and entitled Layered-Nano Fabrication (NFL). 
    
    
     BACKGROUND 
     The present disclosure generally relates to layered fabrication and more specifically, to layered fabrication of nanoscale objects. 
     In manufacturing, productivity is achieved by guiding a product from concept to market quickly and inexpensively. Rapid prototyping aids this process. Rapid prototyping may automate the fabrication of a prototype part from a three-dimensional (3-D) computer model. 
     One type of rapid prototyping, referred to as layered fabrication, has been in use for production of macroscopic objects. In layered fabrication, an object is conceptually sliced into a set of parallel layers or slices. Each layer is produced sequentially, and the consolidation of all the layers constitutes the desired 3-D object. 
     SUMMARY 
     A method for fabricating or prototyping a nanoscale object is disclosed. The method includes defining a sequence of nanolayers that represent the nanoscale object, constructing a current nanolayer on a first surface, and depositing a sacrificial layer to cover the first surface but not the nanolayer. The nanolayer represents a slice of the nanoscale object. The nanolayer and the sacrificial layer provide a second surface on which a next nanolayer is constructed. The above constructing and depositing steps are repeated if the next nanolayer is not the last nanolayer. The method also includes removing sacrificial layers to produce the nanoscale object. 
     The construction of a current nanolayer includes depositing a plurality of nanoparticles and manipulating the nanoparticles into a specified shape. The manipulation of the particles may be performed with a microscope tip such as a scanning probe microscope tip. 
     The deposition of a sacrificial layer includes arranging self-assembled monolayers of molecules into a well-ordered pattern. The construction of the current nanolayers and the monolayers provide a smooth surface on which a next nanolayer may be constructed. The sacrificial layer may also be formed by electrostatically depositing strands of polymers. 
     The removal of molecular layers includes oxidizing the layers with ozone under ultraviolet radiation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Different aspects of the disclosure will be described in reference to the accompanying drawings wherein: 
     FIG. 1 illustrates a Layered Nanofabrication process in accordance with an embodiment of the present invention; 
     FIGS. 2A through 2F illustrate a detailed process for constructing a nanoscale object or structure; and 
     FIG. 3 is a flowchart of a method for constructing a nanoscale object in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     A process for fabricating three-dimensional (3-D) objects using layered fabrication technique has been extended to the nanoscale. The process, referred to as Layered Nanofabrication, enables fabrication of 3-D objects with overall dimensions in the deep sub-micron range. A nanoscale object is defined in the disclosure as any object that can be entirely enclosed within a box with length, width, and height on the order of about 100 nm. 
     A Layered Nanofabrication process  100  is illustrated in FIG. 1 in accordance with an embodiment. In the illustrated embodiment, a nanoscale object  102  is conceptually sliced into several parallel layers referred to as nanolayers  104 ,  106 . A sequence of sliced nanolayers  104 ,  106  is then defined and built. 
     A substrate  108  is provided on which a first layer  104  of the nanoscale object  102  may be constructed. In some embodiments, the substrate  108  is a hydroxylated material such as oxidized silicon or mica. In other embodiments, the substrate  108  is gold, silver, copper, platinum, silver oxide, or aluminum oxide. Nanolayers  104 ,  106  of the nanoscale object  102  have heights on the order of about a few nanometers. 
     A nanolayer  104  may be constructed on the substrate surface  110  as shown. The layer  104  may then be leveled off by constructing a sacrificial layer  112  that covers the substrate surface  110  but not the nanolayer  104 . The sacrificial layer  112  may be constructed by depositing a layer of molecules. The height of the sacrificial layer  112  should be substantially equal to the height of the nanolayer  104 . The construction of the nanolayer  104  and the sacrificial layer  112  provides a surface  114  on which a next nanolayer  106  may be constructed. 
     In one embodiment illustrated in FIG. 2A, a first nanolayer  200  is constructed by depositing a large number of substantially similar nanoparticles  202  on the substrate surface  204 . The particles  202  are on the order of about a few nanometers “nanoscale”. These particles  202  are referred to as nanoparticles. The nanoparticles  202  may be manipulated with a scanning probe microscope (SPM). Manipulation of the nanoparticles  202 , referred to as nanomanipulation, includes pushing and/or pulling of the nanoparticles  202  with the tip of the SPM to the desired locations. The nanomanipulation may also include positioning and linking of the nanoparticles  202 . The resulting nanolayer  200  includes a collection of nanoparticles  202  that are in contact with adjacent nanoparticles  202 . 
     Construction of the nanolayers using the above-described technique has been demonstrated experimentally in the laboratory for gold spherical nanoparticles with diameters between 5 and 30 nm. The gold particles are cross-linked and stabilized by treating them with short-length di-thiols. 
     FIG. 2B illustrates one embodiment of constructing a sacrificial layer  210 . The sacrificial layer  210  provides a surface for construction of the next nanolayer. Different sacrificial layers may be used for different substrate surfaces. For gold, silver, or copper substrates, thiols may be used as sacrificial layers. For platinum substrate, alcohols and amines may be used. For silver or aluminum oxide surfaces, carboxylic acids may be used. 
     In some embodiments, in which substrates have hydroxylated surfaces such as oxidized silicon or mica, molecular monolayers may be deposited on substrate surfaces. The monolayers may be self-assembled monolayers (SAM) that are prepared using different types of molecules. Some examples include molecular monolayers of silane or monolayers of alkylsiloxane. Different techniques are available to deposit appropriate concentration of these molecules so that the molecules assemble themselves into well-ordered, defect-free monolayers. 
     For example, a monolayer of silane may be synthesized so as to have the desired length to ensure that the sacrificial layer  210  is flush with the associated nanolayer  212 . In order to build the sacrificial layer  210  that is flush with the nanolayer  212 , several layers of SAM may need to be synthesized and deposited to produce a sacrificial layer with a desired thickness. However, depositing several layers of SAM may be time consuming. Thus, longer SAM may be designed and synthesized to simplify the construction of a sacrificial layer  210 . The SAM attaches itself to the substrate surface but not the nanoparticles of the nanolayer  212 . 
     The construction of a sacrificial layer  210  may use other techniques. For example, in the illustrated embodiment of FIG. 2B, substantially cationic and anionic polymers  214  may be assembled into high quality thin films by electrostatic deposition. 
     The deposition technique involves treating a charged substrate with a solution of an oppositely charged polymer. A monolayer of polymer is deposited on the substrate by electrostatic attraction of the surface for the polymer and the entropic gain of freeing the water from the surface and the polymer strand. Only one monolayer of polymer is bound to the surface because of the electrostatic repulsion of the polymer-coated surface toward another polymer strand. The substrate may then be removed from the solution, rinsed and placed in a polymer solution of an opposite charge. This oppositely-charged polymer attaches to the surface as a monolayer. Again, the polymer repels other polymer strands of like charge. 
     Treating the substrate alternately with two solutions allows the growth of films in regular and uniform increments. Each layer includes a polymer monolayer as thin as 1 nm. The films may be grown on any substrate surface. 
     The top of the sacrificial layer may then serve as a surface  220  on which a next nanolayer  222  may be constructed (see FIG.  2 C). Successive applications of the above-described processes are shown in FIGS. 2D and 2E for constructing the nanolayers and the corresponding sacrificial layers  230 . FIG. 2F shows the nanoscale object  240  with the sacrificial layers  230  removed. The sacrificial layers  230  are removed using appropriate techniques as described below. If necessary, the nanolayers may be further fused to bolster the object  240 . The nanolayers of gold nanoparticles or nanoclusters may fuse automatically. In some cases, it may be necessary to apply di-thiols. In other cases, it may be necessary to do some chemical, electrochemical, or thermal processing such as sintering, to fuse the nanolayers. 
     The sacrificial layers  230  may be removed at room temperature by oxidation using ozone in ultraviolet radiation. The organic components may be oxidized and removed as gaseous products. Small amounts of silicon may remain on the surface as silicon dioxide. The same process may substantially remove polymeric sacrificial layers. 
     A method for constructing a nanoscale object in accordance with an embodiment is illustrated as a flowchart in FIG. 3. A sequence of nanolayers that would constitute a nanoscale object is defined at  300 . At  302 , a suitable substrate is provided. A nanolayer of desired material is constructed on a substrate or layer surface at  304 . The layer is then leveled-off or planarized, at  306 , by depositing a sacrificial layer that would provide a surface on which a next nanolayer may be constructed. 
     If the next layer is determined to be the last layer  308 , a last nanolayer is constructed at  310 . Otherwise, the layer construction process is repeated at  304 . At  312 , the sacrificial layers are removed to realize the nanoscale object or structure. 
     While specific embodiments of the invention have been illustrated and described, other embodiments and variations are possible. For example, although the invention has been described in terms of nanoscale objects and structures, objects and structures of other smaller or larger scale are possible and contemplated. 
     All these are intended to be encompassed by the following claims.