Abstract:
Prototype semiconductor structures each including a semiconductor link portion and two adjoined pad portions are formed by lithographic patterning of a semiconductor layer on a dielectric material layer. The sidewalls of the semiconductor link portions are oriented to maximize hole mobility for a first-type semiconductor structures, and to maximize electron mobility for a second-type semiconductor structures. Thinning by oxidation of the semiconductor structures reduces the width of the semiconductor link portions at different rates for different crystallographic orientations. The widths of the semiconductor link portions are predetermined so that the different amount of thinning on the sidewalls of the semiconductor link portions result in target sublithographic dimensions for the resulting semiconductor nanowires after thinning. By compensating for different thinning rates for different crystallographic surfaces, semiconductor nanowires having optimal sublithographic widths may be formed for different crystallographic orientations without excessive thinning or insufficient thinning.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to semiconductor devices, and particularly to semiconductor nanowires having mobility-optimized orientations and methods of manufacturing the same. 
       BACKGROUND OF THE INVENTION 
       [0002]    A semiconductor nanowire refers to a semiconductor wire having transverse lateral and vertical dimensions of the order of a nanometer (10 −9  meter) or tens of nanometers. Typically, the transverse lateral dimension and the vertical dimension are less than 20 nm. 
         [0003]    The limitation on the lateral dimension applies to the transverse lateral dimension (the width) and the vertical lateral dimension (the height). The longitudinal lateral dimension (the length) of the semiconductor nanowire is unlimited, and may be, for example, from 1 nm to 1 mm. When the lateral dimensions of the semiconductor nanowire is less than tens of nanometers, quantum mechanical effects become important. As such, semiconductor nanowires are also called semiconductor quantum wires. 
         [0004]    The transverse lateral dimension of a semiconductor nanowire is currently sublithographic, i.e., may not be printed by a direct image transfer from a photoresist that is patterned by a single exposure. As of 2008, the critical dimension, i.e., the smallest printable dimension that may be printed by lithographic methods, is about 35 nm. Dimensions less than the critical dimension are called sublithographic dimensions. At any given time, the critical dimension and the range of the sublithographic dimension are defined by the best available lithographic tool in the semiconductor industry. In general, the critical dimension and the range of the sublithographic dimension decreases in each successive technology node and established by a manufacturing standard accepted across the semiconductor industry. 
         [0005]    A semiconductor nanowire enables enhanced control of the charge carriers along the lengthwise direction through a complete encirclement of the cross-sectional area of the semiconductor nanowire by a gate dielectric and a gate electrode. The charge transport along the semiconductor nanowire by the gate electrode is better controlled in a semiconductor nanowire device than in a fin field effect transistor (finFET) because of the complete encirclement of the semiconductor nanowire. 
         [0006]    For high performance complementary metal-on-semiconductor (CMOS) circuit, high performance p-type semiconductor nanowire devices and n-type semiconductor nanowire devices that provide high on-current and low off-current are desired. 
       SUMMARY OF THE INVENTION 
       [0007]    Prototype semiconductor structures each including a semiconductor link portion and two adjoined pad portions are formed by lithographic patterning of a semiconductor layer on a dielectric material layer. The sidewalls of the semiconductor link portions are oriented to maximize hole mobility for a first-type semiconductor structure, and to maximize electron mobility for a second-type semiconductor structure. Thinning by oxidation of the semiconductor structures reduces the width of the semiconductor link portions at different rates for different crystallographic orientations. The widths of the semiconductor link portions are predetermined so that the different amount of thinning on the sidewalls of the semiconductor link portions results in target sublithographic dimensions for the resulting semiconductor nanowires after thinning. By compensating for different thinning rates for different crystallographic surfaces, semiconductor nanowires having optimal sublithographic widths may be formed for different crystallographic orientations without excessive thinning or insufficient thinning. 
         [0008]    According to an aspect of the present invention, a method of forming a semiconductor structure is provided, which includes patterning a first semiconductor structure including a first semiconductor link portion, wherein the first semiconductor structure has a first pair of sidewalls that are separated by a first width w 1  and has a first surface orientation having a first oxidation rate in an oxidizing ambient; patterning a second semiconductor structure including a second semiconductor link portion, wherein the second semiconductor link portion has a second pair of sidewalls that are separated by a second width w 2  and has a second surface orientation having a second oxidation rate in the oxidizing ambient; forming a first semiconductor nanowire having a third width w 3  by thinning the first semiconductor link; and forming a second semiconductor nanowire having a fourth width w 4  by thinning the second semiconductor link, wherein the third width w 3  and the fourth width w 4  are sublithographic dimensions. 
         [0009]    In one embodiment, a ratio R of a difference between the first width w 1  and the third width w 3  to a difference between the second width w 2  and the fourth width w 4  is the same as the ratio of the first oxidation rate to the second oxidation rate, i.e., the first width w 1  and the second width w 2  are determined by the formula, (w 1 −w 3 )/(w 2 −w 4 )=R, where R expresses the effective ratio of the first to the second oxidation rates. The value of R is a function of oxidation temperature, the dimensions of the semiconductor link portion and the crystallographic orientations of the first and second surface orientation. R will generally have a value between 0.1 and 10. The exact value of R can be found by methods known to one skilled in the art such as finite element oxidation simulations. As an example, if the first surface orientation is [110] and the second surface orientation is [100] and both semiconductor link portions have cross-sectional dimensions around 70 nm, the value of R will be 1.06 for a steam oxidation at 800° C. 
         [0010]    According to another aspect of the present invention, a semiconductor structure includes a first semiconductor structure and a second semiconductor structure. The first semiconductor structure includes a first semiconductor structure including a first semiconductor nanowire, a first source-side pad and a first drain-side pad, wherein each of the first source-side pad and the first drain-side pad adjoins the first semiconductor nanowire and comprises a semiconductor material having a doping of a second conductivity type, and wherein a middle portion of the first semiconductor nanowire comprises the semiconductor material and has a doping of a first conductivity type and has a first pair of sidewalls having a first surface orientation and separated by a sublithographic width, wherein the second conductivity type is the opposite of the first conductivity type. The second semiconductor structure includes a second semiconductor nanowire, a second source-side pad and a second drain-side pad, wherein each of the second source-side pad and the second drain-side pad adjoins the second semiconductor nanowire and comprises the semiconductor material having a doping of the first conductivity type, and wherein the second semiconductor nanowire comprises the semiconductor material and has a doping of the second conductivity type and has a second pair of sidewalls having a second surface orientation and separated by another sublithographic width which is between 80% and 125% of the sublithographic width, wherein the second surface orientation is different from the first surface orientation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1A  is a top-down view of an exemplary semiconductor structure after application and patterning of a photoresist on a semiconductor-on-insulator (SOT) substrate.  FIG. 1B  is a vertical cross-sectional view of the exemplary semiconductor structure along the plane B-B′ at the step corresponding to  FIG. 1A .  FIG. 1C  is a vertical cross-sectional view of the exemplary semiconductor structure along the plane C-C′ at the step corresponding to  FIG. 1A . 
           [0012]      FIG. 2A  is a top-down view of the exemplary semiconductor structure after patterning of semiconductor link portions and semiconductor pads.  FIG. 2B  is a vertical cross-sectional view of the exemplary semiconductor structure along the plane B-B′ at the step corresponding to  FIG. 2A .  FIG. 2C  is a vertical cross-sectional view of the exemplary semiconductor structure along the plane C-C′ at the step corresponding to  FIG. 2A . 
           [0013]      FIG. 3A  is a top-down view of the exemplary semiconductor structure after formation of insulator pedestals.  FIG. 3B  is a vertical cross-sectional view of the exemplary semiconductor structure along the plane B-B′ at the step corresponding to  FIG. 3A .  FIG. 3C  is a vertical cross-sectional view of the exemplary semiconductor structure along the plane C-C′ at the step corresponding to  FIG. 3A . 
           [0014]      FIG. 4A  is a top-down view of the exemplary semiconductor structure after formation of semiconductor nanowires.  FIG. 4B  is a vertical cross-sectional view of the exemplary semiconductor structure along the plane B-B′ at the step corresponding to  FIG. 4A .  FIG. 4C  is a vertical cross-sectional view of the exemplary semiconductor structure along the plane C-C′ at the step corresponding to  FIG. 4A . 
           [0015]      FIG. 5A  is a top-down view of the exemplary semiconductor structure after formation of gate dielectrics.  FIG. 5B  is a vertical cross-sectional view of the exemplary semiconductor structure along the plane B-B′ at the step corresponding to  FIG. 5A .  FIG. 5C  is a vertical cross-sectional view of the exemplary semiconductor structure along the plane C-C′ at the step corresponding to  FIG. 5A . 
           [0016]      FIG. 6A  is a top-down view of the exemplary semiconductor structure after formation of gate electrodes.  FIG. 6B  is a vertical cross-sectional view of the exemplary semiconductor structure along the plane B-B′ at the step corresponding to  FIG. 6A .  FIG. 6C  is a vertical cross-sectional view of the exemplary semiconductor structure along the plane C-C′ at the step corresponding to  FIG. 6A . 
           [0017]      FIG. 7A  is a top-down view of the exemplary semiconductor structure after formation of a middle-of-line (MOL) dielectric layer and contact vias. A middle-of-line (MOL) dielectric layer  80  is omitted in  FIG. 7A  for clarity.  FIG. 7B  is a vertical cross-sectional view of the exemplary semiconductor structure along the plane B-B′ at the step corresponding to  FIG. 7A .  FIG. 7C  is a vertical cross-sectional view of the exemplary semiconductor structure along the plane C-C′ at the step corresponding to  FIG. 7A . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    As stated above, the present invention relates to semiconductor nanowires having mobility-optimized orientations and methods of manufacturing the same, which are now described in detail with accompanying figures. It is noted that like and corresponding elements are referred to by like reference numerals. 
         [0019]    Referring to  FIGS. 1A-1C , an exemplary semiconductor structure according to the present invention includes a semiconductor-on insulator (SOI) substrate which contains a handle substrate  10 , a buried insulator layer  20 , and a top semiconductor layer  28 . The top semiconductor layer  28  comprises a semiconductor material, which may be selected from, but is not limited to silicon, germanium, silicon-germanium alloy, silicon carbon alloy, silicon-germanium-carbon alloy, gallium arsenide, indium arsenide, indium phosphide, compound semiconductor materials, II-VI compound semiconductor materials, organic semiconductor materials, and other compound semiconductor materials. In one embodiment, the top semiconductor layer  28  may include a Si-containing semiconductor material such as single crystalline silicon or a single crystalline silicon-germanium alloy. 
         [0020]    Preferably, the entirety of the semiconductor material within the top semiconductor layer  28  is single crystalline material, i.e., has an epitaxial atomic alignment throughout. In this case, the crystallographic orientation of the surface normal of the top surface of the top semiconductor layer  28  is herein referred to as a surface orientation of the top surface of the top semiconductor layer  28 . While the top surface of the top semiconductor layer  28  may be any crystallographic orientation, a major crystallographic orientation with low Miller indices are typically selected for the surface orientation of the top surface of the top semiconductor layer. While the present invention is illustrated with a [001] surface orientation for the top surface of the top semiconductor layer  28 , any other surface orientation may be substituted for the [001] surface orientation. It is preferred that the surface orientation for the top surface of the top semiconductor layer  28  is one of the surface orientations at which either hole mobility or electron mobility is at a maximum at least locally, and preferably globally among all available crystallographic orientations. The thickness of the top semiconductor layer  28  may be from 10 nm to 200 nm, although lesser and greater thicknesses are also contemplated herein. 
         [0021]    The top semiconductor layer  28  may be doped with electrical dopants as needed. For example, a first device region  2  may be doped with dopants of a first conductivity type and a second device region  4  may be doped with dopants of a second conductivity type, which is the opposite of the first conductivity type. For example, the first conductivity type may be p-type and the second conductivity type may be n-type, or vice versa. The top semiconductor layer  28  may be provided as a substantially intrinsic semiconductor layer, or may be provided with p-type doping or n-type doping. Patterned ion implantation masks may be employed during ion implantation or plasma doping to insure that the first device region  2  and the second device region are doped with appropriate doping. Typically, the dopant concentration in doped regions is in the range from 5.0×10 14 /cm 3  to 3.0×10 17 /cm 3 , although lesser and greater dopant concentrations are also contemplated herein. In the non-limiting illustrative example described herein, the first conductivity type may be p-type and the second conductivity type may be n-type, i.e., the first device region  2  is doped with p-type dopants and the second device region  4  is doped with n-type dopants. 
         [0022]    The buried insulator layer  20  is a dielectric material layer, i.e., a layer including a dielectric material. The dielectric material of the buried insulator layer  20  may be, for example, silicon oxide, silicon nitride, silicon oxynitride, quartz, a ceramic material, or a combination thereof. The thickness of the buried insulator layer  20  may be from 50 nm to 1,000 nm, although lesser and greater thicknesses are also contemplated herein. The handle substrate  10  may comprise a semiconductor material, an insulator material, or a conductive material. In some cases, the handle substrate  10  and the buried insulator layer  20  may comprise the same dielectric material and may be of unitary and integral construction. 
         [0023]    A photoresist  7  is applied to the top surface of the top semiconductor layer  28  and is lithographically patterned to form a first shape and a second shape. The first shape includes a first link shape, which has a rectangular shape and a constant first width w 1  in a top-down view. The first width w 1  is a lithographic dimension, i.e., a dimension that may be printed with a single lithographic exposure. Thus, the first width w 1  is greater than 40 nm, while it is contemplated that a lesser first width w 1  may be formed as lithography tools improve in the future. Typically, the first width w 1  is a critical dimension, i.e., lithographically printable minimum dimension, or a dimension close to the critical dimension. The first link shape is laterally adjoined by a first pad shape and a second pad shape, which have wider widths than the first link shape. The lengthwise direction of the first link shape, which is horizontal and is perpendicular to the direction of the first width w 1 , is herein referred to as a first horizontal direction. The widthwise direction of the first link shape, which is the direction of the first width w 1 , is herein refereed to as a second horizontal direction. In a non-limiting illustrative example, the first horizontal direction may be a [110] crystallographic orientation and the second horizontal direction may be a [  1  10] crystallographic orientation. 
         [0024]    The second shape includes a second link shape, which has a rectangular shape and a constant second width w 2  in a top-down view. The second width w 2  is a lithographic dimension, and is typically a critical dimension or a dimension close to the critical dimension. The second link shape is laterally adjoined by a third pad shape and a fourth pad shape, which have wider widths than the second link shape. The lengthwise direction of the second link shape, which is horizontal and is perpendicular to the direction of the second width w 2 , is herein referred to as a third horizontal direction. The third horizontal direction is different from the first horizontal direction. The third horizontal direction may be at a non-orthogonal angle relative to the first horizontal direction, or may be at an orthogonal angle relative to the first horizontal direction. The widthwise direction of the second link shape, which is the direction of the second width w 2 , is herein referred to as a fourth horizontal direction. In a non-limiting illustrative example, the third horizontal direction may be a [100] crystallographic orientation and the fourth horizontal direction may be a [010] direction. 
         [0025]    Preferably, the first horizontal direction and the third horizontal direction are selected to include vertical planes at which hole mobility or electron mobility is at a local maximum at least, and preferably at a maximum among all vertical planes in the single crystalline semiconductor layer constituting the top semiconductor layer  28 . In case the top semiconductor layer  28  is doped with dopants of the first conductivity type in the first device region  2  and doped with dopants of the second conductivity type in the second device region  4 , the first horizontal direction may be selected to maximize the mobility of charge carriers of the second conductivity type and the third horizontal direction may be selected to maximize the mobility of charge carriers of the first conductivity type. For example, if the first conductivity type is n-type and the second conductivity type is p-type, the first horizontal direction may be selected to include a vertical crystallographic plane that maximizes hole mobility and the third horizontal direction may be selected to include a vertical crystallographic plane that maximizes the electron mobility. If the semiconductor material is single crystalline silicon and the top surface of the top semiconductor layer  28  has a (001) surface orientation, such a requirement may be satisfied by selecting a [110] direction as the first horizontal direction so that the vertical plane including the [110] direction and the [001] direction has a (  1  10) surface orientation and by selecting a [100] direction as the third horizontal direction so that the vertical plane including the [100] direction and the [001] direction has a (010) surface orientation. The top semiconductor layer  28  does not have to be doped in which case the conductivity carrier type (holes or electrons) will be determined by the doping of the gate electrode, the source and the drain. 
         [0026]    The first width w 1  and the second width w 2  are predetermined based on a formula involving oxidation rates of semiconductor surfaces of the top semiconductor layer  28  perpendicular to the second horizontal direction and the fourth horizontal direction as well as the target widths of semiconductor nanowires to be formed by thinning of semiconductor link portions to be subsequently formed in the top semiconductor layer. While determination of the first width w 1  and the second width w 2  are performed prior to patterning the photoresist  7 , the formula is described based on dimensions of structures to be subsequently formed. For this reason, the formula is described below at a subsequent processing step. 
         [0027]    Referring to  FIGS. 2A-2C , the pattern in the photoresist  7  is transferred into the top semiconductor layer  28  and an upper portion of the buried insulator layer  20 , for example, by an anisotropic etch. The exposed portions of the top semiconductor layer  28  and the upper portions of the buried insulator layer  20  directly underneath are removed by the anisotropic etch. The remaining portions of the top semiconductor layer  28  includes a first semiconductor structure formed in the first device region  2  and a second semiconductor structure formed in the second device region  4 . The first semiconductor structure includes a first semiconductor link portion  30 C, a first source-side pad  30 A laterally abutting the first semiconductor link portion  30 C on one side, and a first drain-side pad  30 B laterally abutting the first semiconductor link portion  30 C on an opposite side. The second semiconductor structure includes a second semiconductor link portion  50 C, a second source-side pad  50 A laterally abutting the second semiconductor link portion  50 C on one side, and a second drain-side pad  50 B laterally abutting the second semiconductor link portion  30 C on an opposite side. 
         [0028]    The exposed sidewalls of the first and second semiconductor structures ( 30 A,  30 B,  30 C,  50 A,  50 B,  50 C) are substantially vertically coincident with the sidewalls of the photoresist  7 . Further, the sidewalls of the patterned portions of the buried insulator layer  20  are substantially vertically coincident with the sidewalls of the photoresist  7  and the sidewalls of the first and second semiconductor structures ( 30 A,  30 B,  30 C,  50 A,  50 B,  50 C). The photoresist  7  is subsequently removed, for example, by ashing. 
         [0029]    The first semiconductor link portion  30 C has a first pair of sidewalls that are separated by the first width w 1  and has a first surface orientation having a first oxidation rate in an oxidizing ambient. The first surface orientation is the second horizontal orientation. The second semiconductor link portion  50 C has a second pair of sidewalls that are separated by a second width w 2  and has a second surface orientation having a second oxidation rate in the oxidizing ambient. The second surface orientation is the fourth horizontal orientation. 
         [0030]    The first oxidation rate and the second oxidation rate are dependent on the cross-sectional dimensions of the pre-oxidation beam, the oxide thickness already grown, the temperature of the oxidation, and the composition of the ambient gas. In general, the first oxidation rate and the second oxidation rate increases with temperature, oxygen content, the moisture content of the oxidizing ambient, and the pre-oxidation dimensions. The first oxidation rate and the second oxidation rate depend on the semiconductor material of the first and second semiconductor structures ( 30 A,  30 B,  30 C,  50 A,  50 B,  50 C) and the first and second surface orientations. 
         [0031]    For example, the oxidation rate for a (111) surface of silicon is typically from 1.01 to 1.68 times the oxidation rate for a (100) surface of silicon at the same oxidizing ambient. The Oxidation rate for a (110) surface of silicon is typically from 1.01 to 1.45 times the oxidation rate for the (100) surface of silicon. Thus, the ratio of the first oxidation rate to the second oxidation rate is typically not equal to 1.0 and is mainly a function of the crystallographic orientation of the first pair of sidewalls of the first semiconductor link portion  30 C, the crystallographic orientation of the second pair of sidewalls of the second semiconductor link portion  50 C, the dimensions of the cross-sectional dimensions of the initial beam, and the oxidation temperature. In the illustrated example, the surface orientation of the first pair of sidewalls is a (  1  10) surface orientation and the surface orientation of the second pair of sidewalls is a (010) surface orientation. 
         [0032]    The height of the first semiconductor structure ( 30 A,  30 B,  30 C) and the second semiconductor structure ( 50 A,  50 B,  50 C), which is herein referred to as an initial height h 0 , may be uniform throughout if the thickness of the top semiconductor layer  28  (See  FIGS. 1B and 1C ) is uniform. The initial height h 0  may be substantially the same as the thickness of the top semiconductor layer  28 . 
         [0033]    Referring to  FIGS. 3A-3C , a substantially isotropic etch is performed on the dielectric material of the buried insulator layer  20  selective to the semiconductor material of the first semiconductor structure ( 30 A,  30 B,  30 C) and the second semiconductor structure ( 50 A,  50 B,  50 C). The first semiconductor structure ( 30 A,  30 B,  30 C) and the second semiconductor structure ( 50 A,  50 B,  50 C) are employed as an etch mask for the substantially isotropic etch. The substantially isotropic etch may be a wet etch or a dry etch. Because the etch is substantially isotropic, the edges of the first semiconductor structure ( 30 A,  30 B,  30 C) and the second semiconductor structure ( 50 A,  50 B,  50 C) are undercut as the etch progresses. The etch proceeds at least until the portions of the buried insulator layer  20  located directly underneath the first semiconductor structure ( 30 A,  30 B,  30 C) and the second semiconductor structure ( 50 A,  50 B,  50 C) are removed so that the first and second semiconductor link portions ( 30 C,  50 C) become suspended over the remaining portions of the buried insulator layer  20 . In other words, the first and second semiconductor link portions ( 30 C,  50 C) do not have direct physical contact with the remaining portions of the buried insulator layer  20 , which is herein referred to as a dielectric material layer  22 , after the etch. 
         [0034]    The etch also removes the dielectric material of the buried insulator layer  20  from underneath the peripheral portions of the first source-side pad  30 A, the first drain-side pad  30 B, the second source-side pad  50 A, and the second drain-side pad  50 B. A first dielectric pedestal  22 A comprising a remaining portion of the buried insulator layer  20  is formed directly underneath a center portion of the first source-side pad  30 A. Likewise, a second dielectric Pedestal  22 B is formed directly underneath a center portion of the first drain-side pad  30 B, a third dielectric pedestal  42 A is formed directly underneath a center portion of the second source-side pad  50 A, and a fourth dielectric pedestal  42 B is formed directly underneath a center portion of the second drain-side pad  50 B. As the dielectric material is etched from underneath peripheral portions of the first and second semiconductor structures ( 30 A,  30 B,  30 C,  50 A,  50 B,  50 C) employing the first and second semiconductor structures ( 30 A,  30 B,  30 C,  50 A,  50 B,  50 C) as an etch mask, the buried insulator layer  20 , which is a dielectric material layer, is undercut beneath the first and second semiconductor link portions ( 30 C,  50 C). 
         [0035]    The first and second semiconductor link portions ( 30 C,  50 C) are suspended over a remaining portion of the buried insulator layer  20 , which is the dielectric material layer  22 . The first through fourth dielectric pedestals ( 22 A,  22 B,  42 A,  42 B) are integrally formed with the dielectric material layer  22 , and are portions of the dielectric material layer  22 . The first and second semiconductor structures ( 30 A,  30 B,  30 C,  50 A,  50 B,  50 C) contact the dielectric material layer  22 , which incorporates the first through fourth dielectric pedestals ( 22 A,  22 B,  42 A,  42 B), at bottom surfaces of the first source-side pad  30 A, the first drain-side pad  30 B, the second source side pad  50 A, and the second drain-side pad  50 B. 
         [0036]    Referring to  FIGS. 4A-4C , the first and second semiconductor structures ( 30 A,  30 B,  30 C,  50 A,  50 B,  50 C) are thinned, i.e., dimensions of the first and second semiconductor structures ( 30 A,  30 B,  30 C,  50 A,  50 B,  50 C) are reduced, for example, by oxidation. Specifically, exposed peripheral portions of the first and second semiconductor structures ( 30 A,  30 B,  30 C,  50 A,  50 B,  50 C) including the first and second semiconductor links ( 30 C,  50 C) are converted into oxide material portions by oxidation. The semiconductor oxide material is subsequently removed by an isotropic etch such as a wet etch. For example, if the first and second semiconductor structures ( 30 A,  30 B,  30 C,  50 A,  50 B,  50 C) include silicon, the semiconductor oxide material may be silicon oxide, which may be removed by hydrofluoric acid (HF). Alternately, an isotropic wet etch or an isotropic dry etch may be employed to thin the first and second semiconductor structures ( 30 ,  50 ) by removing the exposed outer portions of the semiconductor material. 
         [0037]    The remaining portions of the first semiconductor structure ( 30 A,  30 B,  30 C) include a first thinned source-side pad  32 A, a first thinned drain-side pad  32 B, and a first semiconductor nanowire  32 C. The first thinned source-side pad  32 A and the first thinned drain-side pad  32 B laterally abut the first semiconductor nanowire  32 C. The remaining portions of the second semiconductor structure ( 50 A,  50 B,  50 C) include a second thinned source-side pad  52 A, a second thinned drain-side pad  52 B, and a second semiconductor nanowire  52 C. The second thinned source-side pad  52 A and the second thinned drain-side pad  52 B laterally abut the second semiconductor nanowire  52 C. The first thinned source-side pad  32 A, the first thinned drain-side pad  32 B, and the first semiconductor nanowire  32 C are collectively referred to as a thinned first semiconductor structure ( 32 A,  32 B,  32 C), i.e., a first semiconductor structure after thinning. The second thinned source-side pad  52 A, the second thinned drain-side pad  52 B, and the second semiconductor nanowire  52 C are collectively referred to as a thinned second semiconductor structure ( 52 A,  52 B,  52 C), i.e., a second semiconductor structure after thinning. 
         [0038]    The first semiconductor nanowire  32 C has a rectangular vertical cross-sectional area in a plane perpendicular to the first horizontal direction. The width of the first semiconductor nanowire  32 C, which is the dimension of the first semiconductor nanowire  32 C in the second horizontal direction between the pair of first sidewalls as recessed by the thinning, is herein referred to as a third width w 3 . The third width w 3  is less than the first width w 1  because the semiconductor material is consumed during the thinning process. Preferably, the third width w 3  is a sublithographic dimension, i.e., a dimension that is less than the smallest dimension that may be printed with a single lithographic exposure on a photoresist. Typically, the third width w 3  is from 1 nm to 20 nm, although lesser and greater dimensions are also contemplated herein. Preferably, the third width w 3  is from 2 nm to 10 nm. 
         [0039]    The second semiconductor nanowire  52 C has a rectangular vertical cross-sectional area in a plane perpendicular to the third horizontal direction. The width of the second semiconductor nanowire  52 C, which is the dimension of the second semiconductor nanowire  52 C in the fourth horizontal direction between the pair of second sidewalls as recessed by the thinning, is herein referred to as a fourth width w 4 . The fourth width w 4  is less than the second width w 2  because the semiconductor material is consumed during the thinning process. The fourth width w 4  is a sublithographic dimension. Typically, the fourth width w 4  is from 1 nm to 20 nm, although lesser and greater dimensions are also contemplated herein. Preferably, the fourth width w 4  is from 2 nm to 10 nm. 
         [0040]    As discussed above, the first and third horizontal directions may be selected to include vertical planes that provide the maximum hole mobility or maximum electron mobility. If the first conductivity type is n-type and the second conductivity type is p-type, the first pair of sidewalls may be parallel to a vertical plane at which hole mobility is at a maximum among all vertical planes in the single crystalline semiconductor material constituting the first semiconductor nanowire  32 C and the second pair of sidewalls is parallel to a vertical plane at which electron mobility is at a maximum among all vertical planes in the single crystalline semiconductor material constituting the second semiconductor nanowire  52 C. In a non-limiting illustrative example, the first and second semiconductor nanowires ( 32 C,  52 C) comprise silicon and have top surfaces having a (001) surface orientation, and the first pair of sidewalls has a (  1  10) surface orientation, and the second pair of sidewalls has a (010) surface orientation. 
         [0041]    In one embodiment, the third width w 3  and the fourth width w 4  may be matched within a predefined margin of error or a predefined allowable offset. For example, the fourth width w 4  may be between 10% and 1000% of the third width w 3 . In other words, the ratio of the greater of the third width w 3  and the fourth width w 4  to the lesser of the third width w 3  and the fourth width w 4  is preferably from 1.0 to 10. In a preferred embodiment, the ratio of the greater of the third width w 3  and the fourth width w 4  to the lesser of the third width w 3  and the fourth width w 4  is preferably from 1.0 to 1.68. In some cases, the third width w 3  may be substantially the same as the fourth width w 4 . 
         [0042]    The entirety of the thinned first semiconductor structure ( 32 A,  32 B,  32 C) and the entirety of the thinned second semiconductor structure ( 52 A,  52 B,  52 C) may have a same thickness, which is herein referred to as a thinned thickness h 1 . The thinned thickness h 1  is less than the initial thickness h 0 . The difference between the initial thickness h 0  and the thinned thickness h 1  is determined by the semiconductor material of the thinned first semiconductor structure ( 32 A,  32 B,  32 C) and the thinned second semiconductor structure ( 52 A,  52 B,  52 C), the crystallographic orientation of the top surface of the thinned first semiconductor structure ( 32 A,  32 B,  32 C) and the thinned second semiconductor structure ( 52 A,  52 B,  52 C), and the oxidation ambient employed in the thinning process. 
         [0043]    Referring to  FIGS. 5A-5C , a first gate dielectric  36  is formed on the exposed surfaces of the thinned first semiconductor structure ( 32 A,  32 B,  32 C) and a second gate dielectric  56  is formed on the exposed surfaces of the thinned second semiconductor structure ( 52 A,  52 B,  52 C). 
         [0044]    In one case, the first gate dielectric  36  and the second gate dielectric  56  comprise a dielectric material formed by thermal conversion of outer portions of the thinned first semiconductor structure ( 32 A,  32 B,  32 C) and the thinned second semiconductor structure ( 52 A,  52 B,  52 C), such as silicon oxide or silicon nitride. Thermal oxidation, thermal nitridation, plasma oxidation, plasma nitridation, or a combination thereof may be employed to form the first gate dielectric  36  and the second gate dielectric  56 . In this case, the first gate dielectric  36  and the second gate dielectric  56  are formed only on the surfaces of the thinned first semiconductor structure ( 32 A,  32 B,  32 C) and the thinned second semiconductor structure ( 52 A,  52 B,  52 C). The thickness of the first gate dielectric  36  and the second gate dielectric  56  may be from about 0.8 nm to about 10 nm, and is typically from about 1.1 nm to about 6 nm. 
         [0045]    In another case, the first gate dielectric  36  and the second gate dielectric  56  may comprise a high-k dielectric material having a dielectric constant greater than 3.9, i.e., the dielectric constant of silicon oxide. The high-k dielectric material may comprise a dielectric metal oxide containing a metal and oxygen. Preferably, the dielectric constant of the high-k material is greater than or about 4.0. More preferably, the dielectric constant of the high-k dielectric material is greater than the dielectric constant of silicon nitride, which is about 7.5. Even more preferably, the dielectric constant of the high-k dielectric material is greater than 8.0. The high-k dielectric materials are also known in the art as high-k gate dielectric materials, which include dielectric metal oxides, alloys thereof, and silicate alloys thereof. Exemplary high-k dielectric materials include HfO 2 , ZrO 2 , La 2 O 3 , Al 2 O 3 , TiO 2 , SrTiO 3 , LaAlO 3 , Y 2 O 3 , HfO x N y , ZrO x N y , La 2 O x N y , Al 2 O x N y , TiO x N y , SrTiO x N y , LaAlO x N y , Y 2 O x N y , a silicate thereof, and an alloy thereof. Each value of x is independently from about 0.5 to about 3 and each value of y is independently from 0 to about 2. Optionally, an interfacial layer (not shown), for example, silicon oxide, can be formed by chemical oxidation or thermal oxidation before the high-k dielectric material is deposited. In this case, the first gate dielectric  36  and the second gate dielectric  56  may be formed as a single contiguous gate dielectric layer covering the entirety of the top surfaces and sidewall surfaces of the thinned first semiconductor structure ( 32 A,  32 B,  32 C) and the thinned second semiconductor structure ( 52 A,  52 B,  52 C) and all exposed surfaces of the dielectric material layer  22  including the first through fourth dielectric pedestals ( 22 A,  22 B,  42 A,  42 B). In this case, the thickness of the first gate dielectric  36  and the second gate dielectric  56  may be from about 1 nm to about 6 nm, and may have an effective oxide thickness on the order of or less than 1 nm. 
         [0046]    Referring to  FIGS. 6A-6C , a first gate electrode  38  is formed on and around a middle portion of the first semiconductor nanowire  32 C (See  FIG. 5B ) and a second gate electrode  58  is formed on and around a middle portion of the second semiconductor nanowire (See  FIG. 5C ). The first and second gate electrodes ( 38 ,  58 ) may comprise the same material or a different material, and may be formed simultaneously by a single deposition step and a single lithographic patterning step, or may be formed employing multiple deposition steps and at least one lithographic patterning steps. 
         [0047]    The first gate electrode  38  and the second gate electrode  58  comprise at least one conductive material such as a doped semiconductor material, a metal, a metallic alloy, a conductive compound of at least one metal, or combinations thereof. Preferably, the thickness of the deposited gate electrode material exceeds half the distance between the first and second semiconductor nanowires ( 32 C,  52 C; See  FIGS. 5B and 5C ) and the dielectric material layer  22  so that each of the first and the second gate electrodes ( 38 ,  58 ) contains only one hole within which one of the first and the second semiconductor nanowires ( 32 C,  52 C) is located. 
         [0048]    In one embodiment, at least one of the first and the second gate electrodes ( 38 ,  58 ) may comprise an amorphous or polycrystalline semiconductor material such as polysilicon, amorphous silicon, a silicon-germanium alloy, a silicon-carbon alloy, a silicon-germanium-carbon alloy, or a combination thereof. The first and the second gate electrodes ( 38 ,  58 ) may be in-situ doped, or may be doped by a subsequent ion implantation of dopant ions. 
         [0049]    Alternately or additionally, at least one of the first and the second gate electrodes ( 38 ,  58 ) may comprise a metal gate material, which comprises a metallic conductive material. For example, the at least one of the first and the second gate electrodes ( 38 ,  58 ) may comprise a material such as TaN, TiN, WN, TiAlN, TaCN, other conductive refractory metal nitride, or an alloy thereof. The metal gate material may be formed by chemical vapor deposition (CVD), physical vapor deposition (PVD), atomic layer deposition (ALD), etc. and comprising a conductive refractory metal nitride. In case the first gate dielectric  36  and the second gate dielectric  56  comprise a high-k gate dielectric material, the metal gate material may be formed directly on the first gate dielectric  36  and the second gate dielectric  56 . The composition of the metal gate material may be selected to optimize threshold voltages of semiconductor devices to be subsequently formed in the thinned first semiconductor structure ( 32 A,  32 B,  32 C) and the thinned second semiconductor structure ( 52 A,  52 B,  52 C). Each of the at least one of the first and the second gate electrodes ( 38 ,  58 ) may include both a metal gate material and a semiconductor material. 
         [0050]    Optionally, dielectric spacers (not shown) may be formed on the sidewalls of the first and second gate electrodes ( 38 ,  58 ) as needed, for example, to control the overlap between the first and second gate electrodes ( 38 ,  58 ) and source and drain regions of semiconductor nanowire transistors to be formed. 
         [0051]    Dopants of the second conductivity type are implanted into the first device region  2  employing the first gate electrode  38  as an ion implantation mask. The second device region  4  may be covered with a block mask during the implantation of the dopants of the second conductivity type. The first thinned source-side pad  32 A and the first thinned drain-side pad  32 B are doped with dopants of the second conductivity type, which are herein referred to as a first pad source portion  33 A and a first pad drain portion  37 A. One end of the first semiconductor nanowire  32 C (See  FIG. 5B ) abutting the first pad source portion  33 A is also doped with dopants of the second conductivity type and is herein referred to as a first nanowire source portion  33 B. The first pad source portion  33 A and the first nanowire source portion  33 B have a doping of the second conductivity type and are collectively called a first source region  33 . The other end of the first semiconductor nanowire  32 C (See  FIG. 5B ) abutting the first pad drain portion  37 A is also doped with dopants of the second conductivity type and is herein referred to as a first nanowire drain portion  37 B. The first pad drain portion  37 A and the first nanowire drain portion  37 B have a doping of the second conductivity type and are collectively called a first drain region  37 . The middle portion of the first semiconductor nanowire  32 C (See  FIG. 5B ) that is not implanted with dopants of the second conductivity type has a doping of the first conductivity type, and is herein referred to as a first channel region  35 . The first channel region  35  laterally abuts the first source region  33  and the first drain region  37 . The first channel region  35 , the first source region  33 , the first drain region  37 , the first gate dielectric  36 , and the first gate electrode  38  collectively constitute a first semiconductor nanowire transistor that controls the flow of current through the first semiconductor nanowire ( 35 ,  33 B,  37 B). 
         [0052]    Dopants of the first conductivity type are implanted into the second device region  4  employing the second gate electrode  58  as an ion implantation mask. The first device region  2  may be covered with a block mask during the implantation of the first conductivity type. The second thinned source-side pad  52 A and the second thinned drain-side pad  52 B are doped with dopants of the first conductivity type, which are herein referred to as a second pad source portion  53 A and a second pad drain portion  57 A. One end of the second semiconductor nanowire  52 C (See  FIG. 6C ) abutting the second pad source portion  53 A is also doped with dopants of the first conductivity type and is herein referred to as a second nanowire source portion  53 B. The second pad source portion  53 A and the second nanowire source portion  53 B have a doping of the first conductivity type and are collectively called a second source region  53 . The other end of the second semiconductor nanowire  52 C (See  FIG. 5C ) abutting the second pad drain portion  57 A is also doped with dopants of the first conductivity type and is herein referred to as a second nanowire drain portion  57 B. The second pad drain portion  57 A and the second nanowire drain portion  57 B have a doping of the first conductivity type and are collectively called a second drain region  57 . The middle portion of the second semiconductor nanowire  52 C (See  FIG. 5C ) that is not implanted with dopants of the first conductivity type has a doping of the second conductivity type, and is herein referred to as a second channel region  55 . The second channel region  55  laterally abuts the second source region  53  and the second drain region  57 . The second channel region  55 , the second source region  53 , the second drain region  57 , the second gate dielectric  56 , and the second gate electrode  58  collectively constitute a second semiconductor nanowire transistor that controls the flow of current through the second semiconductor nanowire ( 55 ,  53 B,  57 B). 
         [0053]    Referring to  FIGS. 7A-7C , a middle-of-line (MOL) dielectric material layer  80  is formed over the first and second semiconductor nanowire transistors. The MOL dielectric material layer  80  may include a mobile ion diffusion barrier layer (not shown) which comprises a material that blocks the diffusion of mobile ions such as Na+ and K+. Typical material employed for the mobile ion diffusion barrier layer includes silicon nitride. The MOL dielectric material layer  80  may include for example, a CVD oxide, spin-on low dielectric constant material having a dielectric constant less than 2.8, an organosilicate glass or a CVD low dielectric material having a dielectric constant less than 2.8, or any other dielectric material that may be employed for a back-end-of-line (BEOL) dielectric layer in metal interconnect structures. For example, The CVD oxide may be an undoped silicate glass (USG), borosilicate glass (BSG), phosphosilicate glass (PSG), fluorosilicate glass (FSG), borophosphosilicate glass (BPSG), or a combination thereof. The MOL dielectric layer  80  fills the spaces between the dielectric material layer  22  and the first and second semiconductor nanowire ( 35 ,  33 B,  37 B,  55 ,  53 B,  57 B). 
         [0054]    Various contact via holes are formed in the MOL dielectric layer  80  and filled with a conductive material to from various contact vias. Specifically, a first source-side contact via  42 A is formed directly on the first pad source portion  33 A, a first drain-side contact via  42 B is formed directly on the first pad drain portion  37 A, a first gate-side contact via  48  is formed directly on the first gate electrode  38 . Likewise, a second source-side contact via  62 A is formed directly on the second pad source portion  53 A, a second drain-side contact via  62 B is formed directly on the second pad drain portion  57 A, a second gate-side contact via  68  is formed directly on the second gate electrode  58 . The top surfaces of the MOL dielectric layer  80 , the first source-side contact via  42 A, the first drain-side contact via  42 B, the first gate-side contact via  48 , the second source-side contact via  62 A, the second drain-side contact via  62 B, and the second gate-side contact via  68  may be substantially coplanar after planarization of the MOL dielectric layer  80  and removal of the excess conductive material. Additional metal interconnect structures (not shown) including a first level metal wiring (not shown) may be formed above the MOL dielectric layer  80 . 
         [0055]    While the invention has been described in terms of specific embodiments, it is evident in view of the foregoing description that numerous alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the invention is intended to encompass all such alternatives, modifications and variations which fall within the scope and spirit of the invention and the following claims.