Abstract:
A method for the fabrication of a semiconductor structure that includes areas that have different crystalline orientation and semiconductor structure formed thereby. The disclosed method allows fabrication of a semiconductor structure that has areas of different semiconducting materials. The method employs templated crystal growth using a Vapor-Liquid-Solid (VLS) growth process. A silicon semiconductor substrate having a first crystal orientation direction is etched to have an array of holes into its surface. A separation layer is formed on the inner surface of the hole for appropriate applications. A growth catalyst is placed at the bottom of the hole and a VLS crystal growth process is initiated to form a nanowire. The resultant nanowire crystal has a second different crystal orientation which is templated by the geometry of the hole.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 U.S.C. §119 from European Patent Application No. 07122904.1 filed Dec. 11, 2007, the entire contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention is in the field of process technology for the manufacture of semiconductor solid-state devices or of parts thereof. More specifically, the present invention relates to the manufacture of a semiconductor structure and components using processes to integrate semiconducting components of different crystalline orientation within the same substrate. 
         [0004]    2. Description of the Related Art 
         [0005]    Semiconductor structures having components with different crystal orientations are of particular interest for the fabrication of high mobility metal-oxide semiconductor field effect transistors (MOSFETs) both in vertical or lateral architecture. The mobility of electrons and holes in silicon MOSFETs depends on the surface orientation of the crystals and are the highest for electrons in &lt;100&gt; and holes in &lt;110&gt; or &lt;111&gt; orientations. It is thus of technological interest to combine both silicon orientations on a single wafer. 
         [0006]    This is currently achieved only by wafer bonding of silicon with different crystal orientation. (See U.S. Pat. Nos. 7,023,057, 7,023,055, and 7,041,538). An alternative process relies on a method to selectively grow epitaxial silicon in a trench formed within a silicon-on-insulator (SOI) structure. (See U.S. Pat. No. 6,555,891). The SOI structure includes a buried silicon oxide layer (BOX) on a bulk silicon substrate having, for example, a &lt;100&gt; orientation, covered with a silicon layer of another orientation, e.g. &lt;110&gt;. 
         [0007]    Future MOSFETs will rely on higher device density and combinations of novel materials, such as III-V and II-VI semiconductors, and germanium (Ge) which have higher charge carrier mobility than silicon. However, because of physical and/or economical reasons, large Ge or III-V substrates are not currently available. Thus, a process to grow areas of high mobility semiconductors on readily available silicon wafers is of significant interest. So far, however, attempts to grow, for example, III-V materials on silicon are hampered by the lattice mismatch of the semiconducting crystals. 
         [0008]    Vapor-Liquid-Solid (VLS) growth is known for selective growth of semiconducting nanowires using a catalytic nano-particle, typically made from gold (Au). See “Semiconductor Nanowires: From Self-Organization to Patterned Growth”, Hong Jin Fan et al., small 2006, 2, No. 6, pages 700-717. It was observed that the crystal orientation of epitaxial and non-epitaxial silicon nanowires (SiNWs) depends on the diameter of the SiNW. SiNWs having diameters larger than 30 nm grow exclusively in the &lt;111&gt; direction. See “Diameter-Dependent Growth Direction of Epitaxial Silicon Nanowires”, V. Schmidt et al., nanoletters 2005, Vol. 5, No. 5, pages 931-935, and “Controlled Growth and Structures of Molecular-Scale Silicon Nanowires” Yue WU et al., Nanoletters, 2004, Vol. 4, No. 3, pages 433-436. Further, VLS-growth of epitaxial III-V and Ge nanowires (NWs) on Si has been reported. VLS growth of epitaxial and random oriented silicon nanowires within the pores of anodized aluminum oxide membranes has been demonstrated. See “Template-directed vapor-liquid-solid growth of silicon nanowires”, Kok-Keong Lew et al., Journal of Vacuum Science and Technology B 20(1), January/February 2002, pages 389-392. Further, VLS growth of epitaxial and random oriented silicon nanowires within porous organosilicate thin films has also been demonstrated. See “Templated germanium nanowire synthesis using oriented mesoporous organosilicate thin films” H. Jagannathan et al., Journal of Vacuum Science and Technology B 24(5), September/October 2006, pages 2220-2224. In these cases the pores act as guides or templates for the VLS growth (templated growth). VLS based methods to grow epitaxial Ge layers on Si has been reported but only on a flat, non structured surface. See “Liquid-metal-mediated homoepitaxial film growth of Ge at low temperature”, F. Xiong et al., Applied Physic Letters 59 (27), 30 Dec. 1991, pages 3586-3588. Finally, doping during VLS-growth of nanowires has also been demonstrated. 
         [0009]    It would be of benefit in the field to have a more ideal semiconductor structure for high mobility device fabrication and a manufacturing process for such semiconductor structure. 
       SUMMARY OF THE INVENTION 
       [0010]    According to an aspect of the present invention there is provided a method for fabricating a hybrid semiconductor structure including semiconducting components with semiconducting crystals of different crystallographic orientation. The method includes the steps of providing a semiconductor substrate having a first crystallographic orientation, causing a hole to be disposed in a surface of the semiconductor substrate, the hole having an interior wall and a bottom at a depth extending into the thickness of the substrate, coating the hole with a separation layer, depositing a growth catalyst at the bottom of the hole, and initiating a crystal growth process to grow a single-crystalline nanowire in the hole, whereby the single-crystalline nanowire has a crystallographic orientation different of the substrate. 
         [0011]    As a result, a hybrid semiconducting structure is provided with the substrate as a first semiconducting component having a first crystallographic orientation, and at least one nanowire as a second semiconducting component having a second crystallographic orientation different to the crystallographic orientation of the substrate. 
         [0012]    Such method has several advantageous features over prior methods of producing hybrid semiconductor substrates. These include: single-crystalline nanostructures can be produced; VLS or similar crystal growth processes can be used and the crystal orientation can be determined; the resultant crystal nanowire is decoupled from the starting substrate which implies that there is no need for epitaxial growth and lattice matching processes. 
         [0013]    The method of the present invention can be applied for different materials combinations (e.g., Si, Ge, III/Vs, and II/VI) and doping levels. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    Reference will now be made, by way of example, to the accompanying drawings which illustrate embodiments of the present invention: 
           [0015]      FIG. 1  shows the various stages of a semiconductor structure in a lateral cut, processed according to an embodiment of the invention wherein specifically, 
           [0016]      FIG. 1A  shows a schematic cross-sectional view of a &lt;100&gt; silicon wafer, 
           [0017]      FIG. 1B  shows a schematic illustration of the silicon wafer of  FIG. 1A  with holes/trenches (template) disposed in its surface and into the thickness of a &lt;100&gt; silicon wafer, and with a spacer layer grown or deposited on the walls of the holes in the substrate, 
           [0018]      FIG. 1C  shows a schematic representation with a metal catalyst deposited in the bottom of the holes in the silicon wafer, 
           [0019]      FIGS. 1D and 1E  show schematic representations of the beginning and continuation of VLS growth in the template, 
           [0020]      FIGS. 1F and 1G  respectively show schematic representations illustrating stopping nanowire growth at the end of the holes/trenches, and removing the catalyst from the growth end of the nanowires, 
           [0021]      FIG. 1H  is a schematic representations showing the implementation using a silicon on insulator (SOI) substrate, 
           [0022]      FIGS. 2A to 2D  show schematic representations illustrating the fabrication of arrays of p- and n-type nanowires in a silicon substrate, in a top view, specifically showing: (A) lithographically-defined holes and with spacer layers; (B) growth of p-type Si nanowires; (C) removal of the gold catalyst and definition of a second array of holes; (D) growth of n-type Si nanowires, 
           [0023]      FIG. 2E  shows a schematic representation illustrating using the same process to grow Ge NWs and GaAs NWs in a silicon substrate, 
           [0024]      FIG. 3  is a schematic diagram illustrating the use of the planar architecture nanowires to fabricate complimentary FETs with gate channels, and 
           [0025]      FIG. 4  is a schematic drawing illustrating selective epitaxial overgrowth of the embedded nanowires. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0026]    Preferably, the present invention includes an inexpensive silicon wafer substrate on which highly dense patterns of different semiconductors, having different crystal orientations and/or doping type and/or concentrations can be fabricated. The combination of silicon substrates with semiconductors having different crystal orientations or with different semiconductors is preferably achieved using a vapor-liquid-solid (VLS) or similar process for the growth of one or more semiconducting nanowires within the substrate. The position, dimension and crystal orientation of the nanowires within the substrate is preferably controlled by a template in which the one or more nanowires grow. 
         [0027]    The VLS growth is a catalytically driven process that yields single-crystalline nanowires. In such process, diluted silane is preferably used as precursor gas for the vapor deposition acting in combination with the growth catalyst typically being a metal heated into liquid state. The result is a solid nanowire grown in the hole of the substrate and being isolated against the substrate by means of the separation layer. Other crystal growth processes can be used where applicable such as a pure vapor-solid-solid (VSS) process where the growth catalyst does not need to be transformed in a liquid state. 
         [0028]    The crystallographic orientation of the single-crystalline nanowire grown in the hole can preferably be defined by defining a template cross-section of the hole. The cross-section of the hole preferably is a circle with a diameter defined according to one of the following embodiments. As the following preferred diameters refer to the diameter of the nanowire, the diameter of the hole can be derived from the diameter of the nanowire plus twice the thickness of the separation layer. Preferably, the diameter of the hole is defined such that a diameter of the nanowire is equal to or greater than 30 nm. For those diameters, a crystal &lt;111&gt; orientation becomes dominant in the growth of the nanowire. In another embodiment, the diameter of the hole is defined such that a diameter of the nanowire to be grown in the hole is equal to or less than 20 nm. For those diameters, a crystal &lt;110&gt; orientation becomes dominant in the growth of the nanowire. In this respect, disclosure of “Diameter-Dependent Growth Direction of Epitaxial Silicon Nanowires”, V. Schmidt et al., nanoletters 2005, Vol. 5, No. 5, pages 931-935 is herewith incorporated by reference. 
         [0029]    However, other than circular cross-sections can be used, such as a square cross-section. Here, the edge-length of the square preferably represents the diameter of the hole, respectively the nanowire. Other cross-sections can be used where the diameter dimension needs to be determined in relation to the shape of the cross-section. 
         [0030]    Other preferred embodiments of the method refer to further processing of the semiconductor structure for further use, and may include steps of removing the catalyst from a growth end of the nanowire and chemical-mechanical polishing the substrate. In an additional step, such surface may be overgrown with an additional layer. 
         [0031]    Another preferred embodiment aims at fabrication of a semiconductor structure having components with at least three different crystal orientations, i.e. the substrate with its crystal orientation and two different types of nanowires grown in holes of the substrate each type, also referred to as sets, showing a crystal orientation different to the nanowires of the other set, and different to the orientation of the substrate. Preferably, the first set of nanowires is built first before the holes for the second set of nanowires are created in the substrate and the second set of nanowires is produced. The sequential processing is beneficial in particular when different dopants are added to the formation process of the different sets of nanowires. Alternatively, the creation of holes and the coating of the separation layer can initially be processed for both sets of holes whereas the formation of the nanowires can be separated such that the formation of the first set of nanowires is completed before the formation of the second set of nanowires is started. The deposition of the growth catalyst preferably belongs to the sequential step of forming the various nanowires. 
         [0032]    Again, the definition of the cross-section of the holes is considered to be the preferred step which enables growth of nanowires of a preferred orientation. Consequently, with the above preferred embodiment, the cross-section of each hole belonging to the first set of holes preferably is different to the cross-section of each of the holes belonging to the second set of holes such that the first set of holes show a different cross-section than that of the second set of holes. Preferably, the nanowires corresponding to and grown in the first set of holes have a diameter equal to or smaller than 20 nm, and preferably, the nanowires corresponding to and grown in the second set of holes have a diameter equal to or greater than 30 nm. 
         [0033]    By means of such embodiments, highly dense arrays of both p- and n-doped semiconductors can be fabricated. In addition, germanium and III-V and II-VI semiconductors (compound semiconductors) can be integrated in silicon substrates for CMOS applications using planar and vertical MOSFET architecture. 
         [0034]    The present invention preferably may take advantage of VLS-growth or other related crystal growth mechanisms to embed nanowires. The term nanowire shall also include similar shaped structures with a preferential growth direction in pre-defined holes and the term hole shall also include templates in the semiconductor substrate of various geometries. Such VLS growth is a catalytically driven process that yields one or more single-crystalline nanowires. The nanowires can be grown in templates with spacer walls. For example, the growth of &lt;110&gt; and &lt;111&gt; silicon nanowires can be performed in holes embedded in a &lt;100&gt; silicon wafer. The growth direction of the nanowires does not depend on the crystal orientation of the wafer substrate. 
         [0035]    According to another aspect of the present invention, there is provided a hybrid semiconductor structure that includes a semiconductor substrate having a first crystallographic orientation, a single-crystalline nanowire in a hole of the substrate, the single-crystalline nanowire being separated from the substrate by a separation layer, the single-crystalline nanowire having a crystallographic orientation different of the substrate. Preferably such semiconductor structure is produced by one of the methods as set out above. 
         [0036]    The separation layer separates the nanowire from the substrate at least during the fabrication process. As such, the separation layer preferably includes at least one of a dielectric layer and a metallic layer. 
         [0037]    The nanowire preferably is made from a compound semiconductor of one of group III-V and II-VI materials. The catalyst preferably includes at least one metal selected from Au, In, Ga, Al, Cu, and Ni. 
         [0038]    The nanowire can have a graded material composition along its length. Accordingly, the precursors used in the growth process have to be timely varied. 
         [0039]    Any feature of one aspect of the invention may be applied to another aspect of the invention and vice versa. 
         [0040]    Referring to the drawings, the details of preferred embodiments of the present invention are graphically and schematically illustrated. Like elements in the drawings are represented by like numbers, and any similar elements are represented by like numbers with a different lower case letter suffix. 
         [0041]      FIGS. 1A to 1G  illustrate the steps of the present method practiced in the case of nanowires (NWs)  50  having a &lt;111&gt; crystal orientation grown in holes  30  (template) of a silicon substrate  20 . In the embodiment illustrated, the substrate  20  is a &lt;100&gt; oriented silicon wafer. Although  FIGS. 1A to 1G  illustrate the embodiment of NWs having a &lt;111&gt; orientation, the method can be utilized to grow NWs having a &lt;110&gt; orientation, for example. 
         [0042]    As illustrated in  FIGS. 1A and 1B , template holes  30  are formed in a &lt;100&gt; silicon wafer  20 , e.g. by lithography and/or reactive ion etching. In the embodiment illustrated, a plurality of holes  30  are arrayed into the top surface  22  of a silicon wafer substrate  20 . Additionally, a separation layer  38  is grown or deposited on the wall  32  and bottom  34  surfaces of the template holes  30 . Then, as shown in  FIG. 1C , a catalyst  40 , e.g. gold, is deposited in the bottom surface  34  in each hole  30  in which growth of a NW is desired. As illustrated in  FIGS. 1D and 1E , the prepared substrate  22  is subjected to a Vapor-Liquid-Solid (VLS) crystal growth process, as is known in the art. As shown in  FIG. 1E , the layer  40  acts as catalyst for the VLS growth process. The VLS process may be practiced with or without dopants. Growth orientation of the crystal NW  50  (shown as &lt;111&gt; in the embodiment illustrated) is templated by the geometry of the hole  30 , in particular, it is defined by the diameter (d), i.e. the diameter of the hole less twice the thickness of the separation layer  38  as shown in  FIG. 1B . The separation layer  38  maybe thin enough such that the diameter of the hole maybe set equal to the diameter of the resulting nanowire. In the present example, the holes  30  have a circular cross-section and are of cylindrical form. The VLS growth process is continued until the catalyst droplet  40  is pushed upwards by growth of the NW  50  proximate the surface  22  of the silicon substrate  20  as shown in  FIG. 1F . Growth of the NW  50  is then stopped, and the catalyst removed from the growth ends  52  of the NWs  50 , as shown in  FIG. 1G , and the substrate polished to the desired thickness. Alternatively to substrate  22 , a silicon on insulator (SOI)  20   b  substrate can be used as starting substrate as shown in  FIG. 1H . 
         [0043]    The final hybrid semiconductor structure  60 , also called a substrate wafer, in this embodiment has a composition of &lt;111&gt; oriented silicon NWs  50  embedded in a &lt;100&gt; silicon wafer  20  and separated by a separation layer  38 , i.e. spacer material. Utilization of the VLS growth for manufacturing hybrid silicon substrates  60  allows direct integration of the nanocrystals in form of nanowires  50  at pre-defined locations within a silicon substrate  20 . 
         [0044]    The present method enables low temperature fabrication of additional NWs  50  in other holes subsequently disposed/arrayed in the surface  22  of the silicon substrate  20 . Consequently, p- and n-type NWs as well as other types of semiconductors can be grown in the same silicon substrate  20 . This is illustrated in  FIGS. 2A to 2E . As illustrated in  FIG. 2A , a first set of template holes  30  is arrayed in the surface  22  of the silicon substrate  20 . The holes  30  are lithographically-defined and have a separation layer  38 . As illustrated in  FIG. 2B , p-type Si NWs are grown in the first set of template holes  30 , according to the method set forth above in  FIGS. 1A to 1H  to produce a hybrid substrate wafer  60  having p-type NWs  50   a.    
         [0045]    A second set of template holes  30   a  is arrayed in the surface  22  of the silicon substrate  20  (see  FIG. 2C ). The second set of holes  30   a  illustrated also have a separation layer  38 . As illustrated in  FIG. 2D , n-type Si NWs  50   b  are grown in the second set of template holes  30   a , according to the method set forth above in  FIGS. 1A to 1H  to produce a complex hybrid NW substrate wafer  62  having both p-type NWs  50   a  and n-type NWs  50   b.    
         [0046]      FIG. 2E  illustrates that, instead of growing NWs  50  with different doping, the same process can be applied to grow, in the example illustrated, Ge NWs  52  and GaAs NWs  54  in the same substrate  20 . Note that the holes  30  need not have a circular cross-section or a fixed, uniform diameter. The cross-section of the holes  30  can be rectangular, striped, or even arbitrarily shaped, to grow the NWs  50 . 
         [0047]    Application of the present hybrid materials and methods for planar NW-MOSFETs  64  is shown in  FIG. 3  on the left hand side. On the right hand side, a conventional architecture is illustrated. For the planar architecture, NWs  240  are used to fabricate complimentary FETs with gate channels having different crystal orientations, doping (type and/or concentrations) or different materials. The separation layer  238  can be used or can be part of the oxide layer. 
         [0048]    The hybrid semiconductor structure  60  according to  FIG. 4   a  can be further processed by selective overgrowth  50   c  of the nanowires according to  FIG. 4   b  to obtain large area substrates of compound semiconductors. 
         [0049]    While the above description contains many specifics, these should not be construed as limitations on the scope of the invention, but rather as exemplifications of one or another preferred embodiment thereof.