Abstract:
A semiconductor structure includes an n-channel field effect transistor (NFET) nanowire, the NFET nanowire comprising a film wrapping around a core of the NFET nanowire, the film wrapping configured to provide tensile stress in the NFET nanowire. A method of making a semiconductor structure includes growing a film wrapping around a core of an n-channel field effect transistor (NFET) nanowire of the semiconductor structure, the film wrapping being configured to provide tensile stress in the NFET nanowire.

Description:
FIELD OF INVENTION 
       [0001]    This disclosure relates generally to the field of semiconductor structure, and specifically to inducing tensile stress in a nanostructure of a semiconductor structure. 
       DESCRIPTION OF RELATED ART 
       [0002]    A semiconductor structure may comprise a number of field effect transistors (FETs); each FET may include a source, a drain, a channel, and a gate. The channel connects the source and the drain, and electrical current flows through the channel from the source to the drain. The electrical current flow is induced in the channel by a voltage applied at the gate. The size of a FET is related to the electrical conductivity of the material that comprises the channel. If the material that comprises the channel has a relatively high conductivity, the FET may be made correspondingly smaller. 
         [0003]    A FET may comprise an n-channel field effect transistor (NFET) or a p-channel field effect transistor (PFET). The electrical conductivity of an NFET is determined by the electron mobility of the NFET channel. In some semiconductor materials, the electron mobility of the NFET channel is related to the amount of tensile stress in the NFET material; more specifically, increased tensile stress in the NFET material may raise the electron mobility of some NFET materials. 
         [0004]    In a relatively small FET, a channel may comprise a nanostructure, also referred to as a nanowire. An exemplary nanowire may have a cross-sectional area of about 20 nanometers (nm) by 20 nm or less. Due to the small size and freestanding nature of a nanowire, inducing tensile stress in a nanowire may present difficulties. 
       SUMMARY 
       [0005]    In one aspect, a semiconductor structure includes an n-channel field effect transistor (NFET) nanowire, the NFET nanowire comprising a film wrapping around a core of the NFET nanowire, the film wrapping configured to provide tensile stress in the NFET nanowire. 
         [0006]    In one aspect, a method of making a semiconductor structure includes growing a film wrapping around a core of an n-channel field effect transistor (NFET) nanowire of the semiconductor structure, the film wrapping being configured to provide tensile stress in the NFET nanowire. 
         [0007]    Additional features are realized through the techniques of the present exemplary embodiment. Other embodiments are described in detail herein and are considered a part of what is claimed. For a better understanding of the features of the exemplary embodiment, refer to the description and to the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0008]    Referring now to the drawings wherein like elements are numbered alike in the several FIGURES: 
           [0009]      FIG. 1  illustrates an embodiment of a cross-section of a semiconductor structure after application of a layer of germanium (Ge). 
           [0010]      FIG. 2  illustrates an embodiment of a cross-section of a semiconductor structure after thermal mixing of silicon (Si) and Ge layers. 
           [0011]      FIG. 3  illustrates an embodiment of a cross-section of a semiconductor structure after application of photoresist. 
           [0012]      FIG. 4  illustrates an embodiment of a cross-section of a semiconductor structure after initial formation of the nanowire regions. 
           [0013]      FIG. 5  illustrates an embodiment of a cross-section of a semiconductor structure after removal of the photoresist and etching of the buried insulator layer. 
           [0014]      FIG. 6  illustrates an embodiment of a cross-section of a semiconductor structure after oxide thinning 
           [0015]      FIG. 7  illustrates an embodiment of a cross-section of a semiconductor structure after formation of a Si wrapping on the SiGe wire wherein the PFET Si wire is thickened 
           [0016]      FIG. 8  illustrates an embodiment of a cross-section of a semiconductor structure after formation of a Si wrapping on the SiGe wire wherein the PFET Si wire is not thickened. 
           [0017]      FIG. 9  illustrates an embodiment of a side view of a semiconductor structure comprising a wrapped NFET nanowire. 
           [0018]      FIG. 10  illustrates an embodiment of a method for a process of making a semiconductor structure comprising a wrapped NFET nanowire. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    Embodiments of a wrapped NFET nanowire are provided, with exemplary embodiments being discussed below in detail. 
         [0020]    A film wrapping on an NFET nanowire may enhance tensile stress in the NFET nanowire, enhancing the electron mobility in the NFET nanowire. In an exemplary embodiment, wrapping silicon (Si) around a silicon germanium (SiGe) core may provide tensile stress in an NFET nanowire. 
         [0021]    For an example SiGe core with a cross section that comprises a square with a side length of 5 nanometers (nm), the cross-sectional area of the SiGe core is 25 nm 2 . The SiGe core is relaxed (i.e., has minimal stress) because it is freestanding. A silicon film wrapping that conforms to the SiGe core potentially causes a tensile stress in the Si of about 1.75 gigapascals (GPa). This tensile stress may increase the electron mobility of the overall nanowire, which comprises the SiGe core and the Si film wrapping. 
         [0022]      FIG. 1  illustrates an embodiment of a cross-section of a semiconductor structure after application of a layer  50  of Ge. Layer  10  comprises a substrate, which may comprise silicon in some embodiments. Layer  20  comprises buried insulator, which may comprise a dielectric material such as oxide in some embodiments. Layer  50  comprises Ge, and is disposed on Si layer  40 . Layer  30  comprises Si. 
         [0023]      FIG. 2  illustrates an embodiment of a cross-section of a semiconductor structure after thermal mixing of Si layer  40  and Ge layer  50 . Layers  40  and  50  of  FIG. 1  have been thermally mixed, resulting in SiGe layer  60 . Layer  30  comprises Si, layer  20  comprises buried insulator, and layer  10  comprises substrate. In some embodiments, SiGe layer  60  may be thinned, or Si layer  30  may be thickened, as necessary in order to achieve appropriate dimensions for layers  30  and  60 . 
         [0024]      FIG. 3  illustrates an embodiment of a cross-section of a semiconductor structure after application of photoresist. Photoresist layers  70   a  and  70   b  are placed on SiGe layer  60  and Si layer  30 , respectively, to define nanowire regions. Layer  20  comprises buried insulator, and layer  10  comprises substrate. 
         [0025]      FIG. 4  illustrates an embodiment of a cross-section of a semiconductor structure after initial formation of PFET and NFET regions. The SiGe layer  60  and Si layer  30  have been etched down to buried insulator layer  20 , leaving SiGe NFET region  61  under photoresist layer  70   a,  and Si PFET region  31  under photoresist layer  70   b.  Layer  10  comprises substrate. 
         [0026]      FIG. 5  illustrates an embodiment of a cross-section of a semiconductor structure after removal of the photoresist and etching of the buried insulator layer. The photoresist layers  70   a  and  70   b  have been etched off, along with a portion of buried insulator layer  20 , resulting in freestanding SiGe NFET region  61 , freestanding Si PFET region  31 , and buried insulator layers  20   a,    20   b,  and  20   c.  Layer  10  comprises substrate. NFET region  61  and PFET region  31  are tethered to silicon pads  901  and  903 , as discussed below with regards to  FIG. 9 . 
         [0027]      FIG. 6  illustrates an embodiment of a cross-section of a semiconductor structure after oxide thinning. Oxide thinning is performed on SiGe NFET region  61  and Si PFET region  31 , resulting in SiGe core  62  and Si wire  32 . SiGe core  62  and Si wire  32  may each have a cross-sectional area of about  20  nm by about  20  nm or less. Layers  20   a,    20   b , and  20   c  comprise buried insulator, and layer  10  comprises substrate. 
         [0028]      FIG. 7  illustrates an embodiment of a cross-section of a semiconductor structure after formation of a Si film wrapping  80  on SiGe core  62  and thickening of Si wire  32 , resulting in thickened Si PFET nanowire  34 . Layers  20   a,    20   b,  and  20   c  comprise buried insulator, and layer  10  comprises substrate. Si film wrapping  80  provides tensile stress in the NFET; together, Si film wrapping  80  and SiGe core  62  form an NFET nanowire. Si film wrapping  80  may have a thickness between about 1 and 2 nm. 
         [0029]      FIG. 8  illustrates an embodiment of a cross-section of a semiconductor structure after formation of a Si film wrapping  80  on SiGe core  62  in which Si wire  32  is not thickened. The Si PFET wire  32  of  FIG. 6  is masked, and Si film wrapping  80  is grown on SiGe NFET wire  62 . Layers  20   a,    20   b,  and  20   c  comprise buried insulator, and layer  10  comprises substrate. Si film wrapping  80  provides tensile stress in the NFET; together, Si film wrapping  80  and SiGe core  62  form an NFET nanowire. Si wire  32  comprises a PFET nanowire. Si film wrapping  80  may have a thickness between about 1 and 2 nm. 
         [0030]      FIG. 9  illustrates a side view of an embodiment of a semiconductor structure comprising a wrapped NFET nanowire. SiGe core  62  is surrounded by Si wrapping  80 , together forming an NFET nanowire. Film wrapping  80  provides tensile stress in the NFET nanowire. Pad  901  and pedestal  902  are on the source side of the semiconductor structure, and pad  903  and pedestal  904  are on the drain side of the semiconductor structure. Electrical current flows through SiGe core  62  and Si film wrapping  80  from source-side pad  901  to drain-side pad  903  according to a voltage applied at gate  905 . Layer  20  comprises buried insulator, and layer  10  comprises substrate. 
         [0031]      FIG. 10  illustrates a method  1000  for a process of making a semiconductor structure comprising a film wrapped NFET nanowire. In block  1001 , a germanium layer is disposed on a portion of an exposed silicon layer, as is shown in  FIG. 1 . In block  1002 , the germanium layer and the silicon layer are thermally mixed, resulting in an exposed SiGe layer and an exposed Si layer, as is shown in  FIG. 2 . In block  1003 , a layer of photoresist is applied to a portion of the SiGe layer, and a layer of photoresist is applied to the Si layer, as is shown in  FIG. 3 . In block  1004 , the exposed SiGe and Si layers are etched down to a buried insulator layer, leaving the portions of the SiGe and the Si located under the photoresist layers, as is shown in  FIG. 4 . In block  1005 , the photoresist is removed, and the buried insulator is etched, resulting in a freestanding SiGe NFET region and a freestanding Si PFET region, as is shown in  FIG. 5 . In block  1006 , the freestanding SiGe NFET region and the freestanding Si PFET region are thinned, resulting in a SiGe NFET core and a Si PFET wire, as is shown in  FIG. 6 . In block  1007 , a Si film wrapping is grown. In some embodiments, the Si PFET wire is masked, and the Si film wrapping is grown on the SiGe core, forming the NFET nanowire, as is shown in  FIG. 8 . In other embodiments, the Si PFET wire is not masked, and Si is also grown on the Si PFET wire, resulting in a thickened Si PFET nanowire, as is shown in  FIG. 7 . The Si film wrapping provides tensile stress in the NFET nanowire, resulting in enhanced electrical conductivity in the NFET nanowire. 
         [0032]    The technical effects and benefits of exemplary embodiments include increased tensile stress in an NFET nanowire, thereby increasing the electrical conductivity of the nanowire and allowing for reduction in size of a semiconductor device. 
         [0033]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an”, and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
         [0034]    The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.