Patent Publication Number: US-9406823-B2

Title: Methods for fabricating self-aligning semiconductor hetereostructures using nanowires

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a divisional of U.S. application Ser. No. 12/949,758 filed on Nov. 18, 2010, which claims priority to U.S. Provisional Application No. 61/262,843, filed on Nov. 19, 2009 which is incorporated herein by reference in its entirety. The present application may be related to U.S. patent application Ser. No. 12/712,097 for ‘Methods for Fabricating High Aspect Ratio Probes and Deforming High Aspect Ratio Nanopillars and Micropillars’ filed on Feb. 24, 2010, U.S. patent application Ser. No. 12/824,128 for ‘Method for Fabricating Micro and Nanostructures in a Material’ filed on Jun. 25, 2010, U.S. patent application Ser. No. 12/711,992 for ‘Methods for Fabrication of High Aspect Ratio Micropillars and Nanopillars filed on Feb. 24, 2010, and U.S. patent application Ser. No. 12/822,109 for ‘Methods for Fabricating Passivated Silicon Nanowires and Devices Thus Obtained’ filed on Jun. 23, 2010, all of which are incorporated herein by reference in their entirety. 
    
    
     STATEMENT OF GOVERNMENT GRANT 
     This invention was made with government support under Grant No. HR0011-04-1-0054 awarded by the DARPA, Grant No. FA9550-04-1-0434 awarded by the AFSOR and Grant No. W911-NF-07-1-0277 awarded by the ARO. The government has certain rights in this invention. 
    
    
     FIELD 
     The present disclosure relates to fabrication using silicon nanowires. Moreover, it relates to methods for fabricating self-aligning arrangements on semiconductors. 
     BACKGROUND 
     Defining high aspect ratio structures with controllable sidewalls in silicon has become increasingly important both in the nanometer and micrometer scale for solar cells, microelectronic devices, and chemical analysis. High aspect ratio micrometer pillars are used for solar cell investigations while nanometer scale high aspect ratio pillars are enabling fundamental investigations in theories of nanoscale pillar stress mechanics, silicon based lasers, and nanoscale electronic devices such as finFETs. Currently various nanofabrication techniques exist that rely on self assembly or bottom-up processing. Some top-down processing enabling reproducibility in nanofabrication can also be found. 
     Further applications are high surface area chemical sensors, mechanical oscillators and piezo-resistive sensors. High aspect ratio pillars with diameters between 50-100 nm could prove useful for core-shell type plasmonic resonators while pillars with sub-10 nm diameters have shown promising light emission characteristics. 
     SUMMARY 
     According to a first aspect, a method of fabricating self-aligning electronic components, the method comprising providing a substrate with one or more nanowires on a first side of the substrate and a first conductive layer on a second side of the substrate, coating the substrate and the one or more nanowires with an insulator, and cleaving the coated one or more nanowires to expose a selected length of a non-cleaved nanowire portion and a selected length of a non-cleaved insulator portion. 
     According to a second aspect, a method of fabricating self-aligning electronic components, the method comprising providing a substrate with one or more nanowires on a first side of the substrate and a first conductive layer on a second side of the substrate, coating the substrate and the one or more nanowires with an insulator, depositing a second conductive layer on the insulator, depositing a dielectric layer on the second conductive layer, and cleaving the coated one or more nanowires to expose a selected length of a non-cleaved nanowire portion and a selected length of a non-cleaved insulator portion. 
     According to a third aspect, a electronic arrangement comprising a semiconductor substrate, a cleaved semiconductor nanowire on the semiconductor substrate, and an oxide layer partially coating the cleaved semiconductor nanowire and coating the semiconductor substrate, wherein the oxide layer and the cleaved semiconductor nanowire define an uncoated region of the cleaved semiconductor nanowire, an epitaxial material on the uncoated region of the cleaved semiconductor nanowire. 
     According to a fourth aspect, an electronic arrangement comprising a semiconductor substrate, a cleaved semiconductor nanowire on the semiconductor substrate, an oxide layer partially coating the cleaved semiconductor nanowire and coating the semiconductor substrate, wherein the oxide layer and the cleaved semiconductor nanowire define an uncoated region of the cleaved semiconductor nanowires, a first conductive layer on the oxide layer, a dielectric material on the first conductive layer, and a second conductive layer on the cleaved semiconductor nanowire and the oxide layer. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The accompanying drawings, which are incorporated into and constitute a part of this specification, illustrate one or more embodiments of the present disclosure and, together with the description of example embodiments, serve to explain the principles and implementations of the disclosure. 
         FIG. 1  shows an array of nanostructures. 
         FIGS. 2A-2E  shows a cross-sectional view of a nanostructure with an end portion of an oxide layer etched back. 
         FIGS. 3A-3E  show fabrication steps of growing a self-aligned heterostructure in accordance with an embodiment of the present disclosure. In particular: 
         FIG. 3A  shows an exemplary cross-sectional view of a nanowire substrate. 
         FIG. 3B  shows an exemplary cross-sectional view of an oxide passivated nanowire. 
         FIG. 3C  shows an exemplary cross-sectional view of a nanostructure being mechanically cleaved. 
         FIG. 3D  shows a cross-sectional view with epitaxial material deposited on the mechanically cleaved surface. 
         FIG. 3E  shows a cross-sectional view with a conductive layer deposited on a topside and a backside on the substrate. 
         FIG. 4  shows an exemplary perspective view of a cleaved nanostructure on a substrate. 
         FIGS. 5A-5E  show methods of fabricating a self-aligned transistor in accordance with an embodiment of the present disclosure. In particular: 
         FIG. 5A  shows an exemplary cross-sectional view of an oxide passivated nanowire. 
         FIG. 5B  shows an exemplary cross-sectional view of a substrate and the oxide passivated nanowire deposited with a conductive layer and a dielectric layer. 
         FIG. 5C  shows an exemplary cross-sectional view of a nanostructure being mechanically cleaved. 
         FIG. 5D  shows an exemplary cross-sectional view of deposition of a conductive layer on the cleaved nanostructure and substrate, and on the backside of the substrate. 
         FIG. 5E  shows an exemplary cross-sectional view with the topside conductive layer etched back and a backside conductive layer. 
         FIG. 6  shows an exemplary perspective view of a cleaved nanostructure on a substrate where the substrate is coated with a conductive and a dielectric layer. 
     
    
    
     DETAILED DESCRIPTION 
     Methods for fabrication of nanoscale electronic components are described in accordance with various embodiments of the present disclosure. Nanowires can be fabricated, by way of example and not of limitation, by performing standard photolithographic or electron-beam lithographic techniques, self-assembly to prepare masks for arrays, use of lithography to pattern catalysts and bottom-up techniques such as vapor-liquid-solid (VLS) growth instead of etching. For the sake of simplicity, throughout the present disclosure, the term ‘nanopillar’ intends to indicate a substantially upright nanoscale shaft where the height is much greater than the width (e.g., 5-10 times greater than the width) and can be used interchangeably with the term ‘nanowire’. The term ‘nanostructure’ intends to indicate the nanopillar including any insulator or conductive layer(s) deposited thereon. ‘Nanoscale’ is defined herein to be any structure between 1 nm and 500 nm in width. 
     Photolithography is a process used in microscale fabrication to selectively remove parts of a film or bulk of a substrate. It uses light to transfer a geometric pattern from a photo mask to a light-sensitive chemical called a photo resist on the substrate. Similarly, electron beam lithography is a process where a beam of electrons are scanned in a patterned fashion to the electron-beam resist. This is followed by a series of chemical treatments in a process similar to dark room processing for photography. The photo or electron-beam resists can be utilized as a mask directly, or utilized to pattern a harder mask which can have better resilience as compared to masking directly. Electron-beam resist can be utilized to fabricate a patterned aluminum oxide (alumina) mask, then removing the electron-beam resist and utilizing the patterned alumina during etching. Lithography and highly anisotropic etching enables routine fabrication of 30-50 nm nanostructures ( 100 ) in silicon with over 40:1 aspect ratios as shown in  FIG. 1 . Such structures can be further reduced in diameter by a subsequent thermal oxidation, wherein the oxidation process can be designed to self-terminate such that nanoscale pillars below 10 nm in width can be defined, allowing wide processing latitude. 
     Field effect transistors (FETs) are generally fabricated using a bottom-up method, planar to the substrate. In a bottom up method, the bottom-most layer is deposited first, followed by precise alignment of subsequent layers, one on top of another until a desired pattern is ultimately achieved. The final patterned arrangement becomes an electronic component, such as transistors (e.g, FETs, MOSFETs, etc). For example, FETs can be fabricated by lithography and the geometry of such FETs allow for charges to be conducted through a channel located below a gate, from source to drain. In such arrangement, the channel is located in-plane to the fabricated surface of the substrate, where the substrate can be a silicon wafer. When a bias is applied to the gate, current is electrostatically controlled between the source and the drain. 
     In recent applications, as shown for example in U.S. patent application Ser. No. 12/822,109 filed on Jun. 23, 2010, incorporated herein by reference in its entirety, vertical methods for fabricating such FETs are described. Controlling the oxidation process can produce strained silicon nanowires used in the fabrication of the FETs. In such vertical geometry, a nanowire is grown, oxidized, and the circumference is coated by a gate electrode. 
       FIGS. 3A-3E  show various steps of fabricating a semiconductor arrangement involving a self-aligning heterostructure in accordance with the present disclosure. The term ‘self-aligned’ as used herein intends to indicate alignment that occurs automatically, without relying on an operator&#39;s skill or accuracy of a machine to accomplish the alignment. A person skilled in the art will understand that the number of steps involved is only indicative and that the process can occur in more or fewer steps according to the various embodiments. 
       FIG. 3A  is a cross-sectional view of a patterned, or etched substrate ( 310 ) comprising a substantially vertical nanopillar ( 320 ). By way of example and not of limitation, the substrate ( 310 ) and the nanopillar ( 320 ) are made of silicon (Si). An example method of fabricating such nanopillars is described in U.S. patent application Ser. No. 12/824,128 filed on Jun. 25, 2010 and U.S. patent application Ser. No. 12/711,992 filed on Feb. 24, 2010, both incorporated herein by reference in their entirety. As an alternative to this embodiment, the vertical nanopillar can be fabricated on silicon-on-insulator (SOI) instead of bulk silicon structure. 
       FIG. 3B  is a further cross-sectional view of the substrate ( 310 ) and the nanopillar ( 320 ) covered by an insulator, which can be an oxide layer ( 330 ), e.g., silicon dioxide (SiO 2 ) or other dielectric. The oxidation process can expand the silicon lattice by approximately 40% to incorporate oxygen. Such expansion leaves the adjacent un-oxidized portion of the silicon under tensile strain. In nanowires, such strain can cause the oxidation process to automatically stop, ultimately resulting in a stable nanoscale tensile-strained silicon core with a silicon dioxide shell. Exposing the embedded silicon to very high strain (e.g., 2.5-3.0%) enhances the ability of such device to efficiently emit light which can be utilized in, for example, opto-electronic switching. 
     U.S. patent application Ser. No. 12/822,109 filed on Jun. 23, 2010, also incorporated herein by reference in its entirety, describes methods to remove a portion of the oxide from the nanostructure to expose the silicon nanowire located below, by first protecting portions of the nanostructure and the oxide layer on the planar portion of the substrate with some dielectric material, e.g., photoresist, and then stripping away the oxide on the top end portion of the nanowire using methods such as etching. 
     An alternative method of exposing the nanostructure is shown as a cross-sectional view in  FIG. 3C , by mechanically cleaving ( 340 ) or mechanically polishing a portion of the nanostructure ( 360 ) to reveal a previously unexposed non-removed portion ( 350 ) of the nanostructure. Mechanical cleaving results in a precise, clean cut of the nanostructure with minimal difficulty relative to the etching methods previously described. 
     Once the nanostructure is cleaved, a single crystal current aperture ( 370 ) ( 400 ) surrounded by silicon dioxide ( 372 ) ( 410 ) is exposed, shown as a cross-sectional view in  FIG. 3D  and in a perspective view from the top in  FIG. 4 . Since the aperture is based on the diameter of the silicon nanowire, the aperture size is controlled by the lithographic pattern during the fabrication of the nanowire, and the temperature used during the oxidation process. By fabricating high aspect ratio nanostructures as described by way of example and not of limitation in U.S. patent application Ser. No. 12/711,992 filed on Feb. 24, 2010, U.S. patent application Ser. No. 12/712,097 filed on Feb. 24, 2010, and U.S. patent application Ser. No. 12/824,128 filed on Jun. 25, 2010, circular nano-apertures and sub-nano-apertures with diameters in the range of approximately 5 nm can be made. It is to be noted however, that the diameters are not limited to just the 5 nm range and can be larger or smaller. 
     Epitaxial material ( 374 ) (e.g., GaAs, InP, InAs, Ge, or other III-V material) can be grown precisely within the current apertures and on top of the clearly defined, cleaved silicon nanowire aperture surface as shown in  FIG. 4  by exploiting the exposed clean silicon aperture ( 400 ) as a template for re-growing. Re-growing such epitaxial material on the aperture ( 400 ) involves layering of dissimilar lattice structure crystals in nanoscale environment, which ultimately creates quantum dots. For example, the crystal lattice of the nanowire substrate can be a silicon crystal and the re-grown quantum dot can be a GaAs crystal, one on top of another. However, such lattice structures can be grown without fear of defects in the lattice structure since the strain between the silicon and the re-growth material do not build up in the narrow nanoscale aperture. Thus, strain is kept to a minimum and the dimensions of the quantum dots are less than the pseudomorphic distance in any direction. 
     Accurate re-growing of quantum dots allow for precisely controlled light emission and electronic attributes. Additionally, simple definition of two and three terminal electrical and optoelectronic components can be accomplished on silicon substrates as shown in  FIG. 3E . 
       FIG. 3E  shows a cross-sectional view of the cleaved nanostructure with a quantum dot ( 392 ), which is then coated with a conductive layer ( 390 ) (e.g., gold, silver, copper, etc.). The backside of the substrate can also be coated with a conductive layer ( 380 ). Both conductive layers become contacts for the growth aperture arrangement. 
     The steps described from  FIGS. 3A-3E  describe methods of fabricating heterostructures on a substrate where the heterostructures are self-aligned. The term ‘self-aligned’ as used in the present disclosure intends to indicate alignment that occurs automatically, without relying on an operator&#39;s skill or preciseness of a machine to accomplish the alignment. 
     The present disclosure also describes methods for fabricating gate on a silicon nanostructure. However, the nanostructure is fabricated with tight control over gate length by initially fabricating the nanostructure to a length substantially taller than desired, then depositing a precisely controlled protective spacer layer, and subsequently cleaving or polishing the protruding portions of the nanostructure to obtain tightly controlled gate lengths. As a result, the need for precise manual alignment is eliminated as may be required in traditional lithography, thus resulting in gates which are self-aligned. Such fabrication is not limited to the fabrication of FETs but can also be applied to fabrication of other nano-arrangements, by way of example and not of limitation to, light emitting diodes (LEDs), detectors and junctions, on silicon. 
       FIGS. 5A-5E  describe methods of fabricating a FET with self-aligning gates. Self-aligned FETs eliminate the need to rely on the skill of a person or a machine to precisely obtain proper alignment.  FIG. 5A  shows a cross-sectional view of an oxidized semiconductor nanowire ( 502 ), similar to  FIG. 3B . 
       FIG. 5B  shows a cross-sectional view of a nanostructure where a layer of conductive material ( 510 ) is deposited on the surfaces of the nanostructure and the substrate. The conductive layer material can be, by way of example and not of limitation, gold, silver, copper, aluminum or other types of metals. The conductive layer can be deposited by sputtering, whereby the operator can precisely control the thickness of the conductive layer being deposited. Controlling the thickness of the conductive layer can be accomplished with a higher degree of precision as compared to defining lithographic features at nanoscale levels. Therefore, arrangements with greater precision can be fabricated. 
     Contacts ( 220 ) ( 390 ) ( 570 ) in  FIGS. 2E, 3E and 5E , respectively, are often made on the nanowires in order to make them into functional components. One such method for making contacts can be accomplished by air-bridging, whereby polymer is used to planarize the nanowires before stripping off the top oxide with hydrofluoric acid or selective dry etch. 
     In a further embodiment, the nanostructure is mechanically cleaved ( 530 ) as shown with a cross-sectional view in  FIG. 5C . As is shown in  FIGS. 5D and 5E , a dielectric layer ( 520 ) is deposited on the conductive layer ( 510 ) on the silicon oxide layer ( 500 ) to protect the conductive layer ( 510 ) and to electrically isolate the conductive layer ( 510 ) from another conductive layer ( 540 ) that will be deposited and described in the following paragraphs. Mechanically cleaving ( 530 ) the nanostructure just above the protective dielectric layer ( 520 ) removes a majority of the top portion of the nanostructure, leaving behind a small nanostructure ( 560 ) below the dielectric layer ( 520 ), thus exposing the remaining nanowire ( 600 ) with the insulator ( 610 ) completely surrounding the perimeter of the nanowire as show in  FIG. 6  with a perspective top-view. 
     Cleaving the nanostructure cleaves the gate length of the arrangement, thus the gate length can be precisely fabricated with ease. The gate is essentially ‘self-aligned’ since the alignment process typically used in lithography is no longer required. 
     Finally in  FIG. 5D , a second conductive layer ( 540 ) is deposited on the dielectric ( 520 ) and the remaining portion of the nanostructure ( 560 ). A backside contact layer ( 550 ) is also applied on the opposite side of the semiconductor substrate. The top conductive layer ( 540 ) is etched, such that the top conductive layer remains as contact points ( 570 ) above the nanostructure ( 560 ), as shown in  FIG. 5E . 
     Such arrangement creates a metal-oxide-semiconductor (MOS) structure, which can be, but is not limited to a MOSFET. In case of a FET or a MOSFET, the top contact and the backside contacts become the source and drain, or the drain and source. 
     Although specific elements such as silicon, oxide, silicon oxide, etc were used to describe the various features and embodiments of the present disclosure, a person or ordinary skill in the art would understand that other elements can be used in place. 
     The examples set forth above are provided to give those of ordinary skill in the art a complete disclosure and description of how to make and use the embodiments of the methods for fabricating self-aligning arrangements on semiconductors of the disclosure, and are not intended to limit the scope of what the inventors regard as their disclosure. Modifications of the above-described modes for carrying out the disclosure may be used by persons of skill in the art, and are intended to be within the scope of the following claims. All patents and publications mentioned in the specification may be indicative of the levels of skill of those skilled in the art to which the disclosure pertains. All references cited in this disclosure are incorporated by reference to the same extent as if each reference had been incorporated by reference in its entirety individually. 
     It is to be understood that the disclosure is not limited to particular methods or systems, which can, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting. As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise. The term “plurality” includes two or more referents unless the content clearly dictates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the disclosure pertains. 
     A number of embodiments of the disclosure have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the present disclosure. Accordingly, other embodiments are within the scope of the following claims.