Abstract:
A semiconductor device and method making it utilize a three-dimensional channel region comprising a core of a first semiconductor material and an epitaxial covering of a second semiconductor material. The first and second semiconductor materials have respectively different lattice constants, thereby to create a strain in the epitaxial covering. The devices are formed by a gate-last process, so that the second semiconductor material is deposited only after the high temperature processes have been performed. Consequently, the lattice strain is not substantially relaxed, and the improved performance benefits of the lattice strained channel region are not compromised.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates to semiconductor devices and methods of making the same, and more particularly relates to such devices and methods in which a transistor channel region is compressively strained. 
         [0003]    2. Description of Related Art 
         [0004]    As the gate length of transistors continues to decrease with successive generations of semiconductor devices, new transistor configurations have been needed to counteract the diminished response that would otherwise occur with shrinking gate lengths. One such design configuration is referred to variously as a FinFET or tri-gate transistor, in which the source, drain and channel region of each transistor is elevated relative to a semiconductor substrate. The elevated portion has the shape of a ridge or fin, and may be formed integrally with the underlying substrate or may be formed on an insulating layer in the case of SOI type devices. The gate wraps around the three projecting sides of the fin, and so the available channel area is increased by the gate contacting not only the top part of the fin but also its side walls. 
         [0005]    Previous designs for FinFETs have also utilized a strained lattice configuration, for example by replacing all or part of the silicon fin with a silicon germanium epitaxial layer. The wider lattice constant of SiGe relative to silicon causes a compressive strain to be imparted to the SiGe layer formed epitaxially on silicon, which enhances hole mobility in the channel region and thus enhances pFET drive current relative to an unstrained Si channel. See, Smith et al., “Dual Channel FinFETs as a Single High-k/Metal Gate Solution Beyond 22 nm Node,” 2009  IEDM , pp. 309-312. 
         [0006]    However, previous design efforts will likely not meet the demands of future generations of semiconductor devices with respect to minimizing off-current while maximizing on-current and switching speed, especially as gate lengths decrease to 14 nm and below. 
       SUMMARY OF THE INVENTION 
       [0007]    Thus, in one aspect, the present invention relates to a semiconductor device, comprising a three-dimensional channel region comprising a core of a first semiconductor material and an epitaxial covering of a second semiconductor material. The first and second semiconductor materials have respectively different lattice constants, thereby to create a strain in the epitaxial covering. A source region is positioned adjacent one end of the three dimensional channel region, and a drain region is positioned adjacent an opposite end of the three dimensional channel region. A gate electrode is superposed on the three dimensional channel region. The second semiconductor material is present only in a region underlying the gate electrode. 
         [0008]    In preferred embodiments of the semiconductor device according to the present invention, each of the core and the epitaxial covering projects upwardly relative to an underlying substrate. 
         [0009]    In preferred embodiments of the semiconductor device according to the present invention, the core is formed integrally with an underlying substrate of the first semiconductor material. 
         [0010]    In preferred embodiments of the semiconductor device according to the present invention, the core is formed on an insulating layer of a semiconductor on insulator (SOI) substrate. 
         [0011]    In preferred embodiments of the semiconductor device according to the present invention, each of the three-dimensional channel region, the source region, the drain region and the gate electrode are separated from an underlying substrate by the insulating layer, thereby to form a transistor that is full isolated from the underlying substrate. 
         [0012]    In preferred embodiments of the semiconductor device according to the present invention, the second semiconductor material has a larger lattice constant than the first semiconductor material, thereby to create a compressive strain in the epitaxial covering. 
         [0013]    In preferred embodiments of the semiconductor device according to the present invention, the first semiconductor material comprises silicon and the second semiconductor material comprises silicon and germanium. 
         [0014]    In preferred embodiments of the semiconductor device according to the present invention, the second semiconductor material has a smaller lattice constant than the first semiconductor material, thereby to create a tensile strain in the epitaxial covering. 
         [0015]    In preferred embodiments of the semiconductor device according to the present invention, the first semiconductor material comprises silicon and germanium and the second semiconductor material comprises silicon. 
         [0016]    In another aspect, the present invention relates to a semiconductor device, comprising a three-dimensional channel region comprising a core of a first semiconductor material and an epitaxial covering of a second semiconductor material. The first and second semiconductor materials have respectively different lattice constants, thereby to create a strain in the epitaxial covering. A source region is positioned adjacent one end of the three dimensional channel region, and a drain region is positioned adjacent an opposite end of the three dimensional channel region. A gate electrode is superposed on the three dimensional channel region. A hollow three-dimensional gate dielectric layer is positioned between the gate electrode and the three-dimensional channel region. 
         [0017]    In preferred embodiments of the semiconductor device according to the present invention, each of the core and the epitaxial covering projects upwardly relative to an underlying substrate. 
         [0018]    In preferred embodiments of the semiconductor device according to the present invention, the core is formed integrally with an underlying substrate of the first semiconductor material. 
         [0019]    In preferred embodiments of the semiconductor device according to the present invention, the core is formed on an insulating layer of a semiconductor on insulator (SOI) substrate. 
         [0020]    In preferred embodiments of the semiconductor device according to the present invention, each of the three-dimensional channel region, the source region, the drain region and the gate electrode are separated from an underlying substrate by the insulating layer, thereby to form a transistor that is full isolated from the underlying substrate. 
         [0021]    In preferred embodiments of the semiconductor device according to the present invention, the second semiconductor material has a larger lattice constant than the first semiconductor material, thereby to create a compressive strain in the epitaxial covering. 
         [0022]    In preferred embodiments of the semiconductor device according to the present invention, the first semiconductor material comprises silicon and the second semiconductor material comprises silicon and germanium. 
         [0023]    In preferred embodiments of the semiconductor device according to the present invention, the second semiconductor material has a smaller lattice constant than the first semiconductor material, thereby to create a tensile strain in the epitaxial covering. 
         [0024]    In preferred embodiments of the semiconductor device according to the present invention, the first semiconductor material comprises silicon and germanium and the second semiconductor material comprises silicon. 
         [0025]    In preferred embodiments of the semiconductor device according to the present invention, the hollow three-dimensional gate dielectric layer extends upwardly from the three-dimensional channel, region, between the gate electrode and each of a pair of sidewall spacers. 
         [0026]    In preferred embodiments of the semiconductor device according to the present invention, the three-dimensional channel region is repeated as a series of the channel regions, and wherein the gate electrode overlies plural channel regions within the series. 
         [0027]    In preferred embodiments of the semiconductor device according to the present invention, the hollow three-dimensional gate dielectric layer extends downwardly between adjacent channel regions within the series. 
         [0028]    In yet another aspect, the present invention relates to a method of making a semiconductor device, comprising removing a dummy gate from an intermediate transistor structure comprising a three-dimensional channel region of a first semiconductor material underlying the dummy gate, forming an epitaxial covering of a second semiconductor material on a portion of the three-dimensional channel region exposed by removal of the dummy gate, and forming a gate structure contacting the covering of the second semiconductor material. 
         [0029]    In preferred embodiments of the method according to the present invention, the three-dimensional channel region projects upwardly relative to an underlying substrate. 
         [0030]    In preferred embodiments of the method according to the present invention, the three-dimensional channel region is formed integrally with an underlying substrate of the first semiconductor material. 
         [0031]    In preferred embodiments of the method according to the present invention, the three-dimensional channel region is formed on an insulating layer of a semiconductor on insulator (SOI) substrate. 
         [0032]    In preferred embodiments of the method according to the present invention, the method additionally includes forming the three-dimensional channel region on a sacrificial layer of a semiconductor material that can be etched under conditions that do not substantially etch the first semiconductor material, forming the dummy gate on the three-dimensional channel region, removing the sacrificial layer to create a void underlying the three-dimensional channel region, and filling the void with a dielectric material prior to removal of the dummy gate. 
         [0033]    In preferred embodiments of the method according to the present invention, each of the three-dimensional channel region, a source region, a drain region and the gate electrode are separated from an underlying substrate by the insulating layer, thereby to form a transistor that is full isolated from the underlying substrate. 
         [0034]    In preferred embodiments of the method according to the present invention, the second semiconductor material has a larger lattice constant than the first semiconductor material, thereby to create a compressive strain in the epitaxial covering. 
         [0035]    In preferred embodiments of the method according to the present invention, the first semiconductor material comprises silicon and the second semiconductor material comprises silicon and germanium. 
         [0036]    In preferred embodiments of the method according to the present invention, the second semiconductor material has a smaller lattice constant than the first semiconductor material, thereby to create a tensile strain in the epitaxial covering. 
         [0037]    In preferred embodiments of the method according to the present invention, the first semiconductor material comprises silicon and germanium and wherein the second semiconductor material comprises silicon. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0038]    Other objects, features and advantages of the invention will become more apparent after reading the following detailed description of preferred embodiments of the invention, given with reference to the accompanying drawings, in which: 
           [0039]      FIG. 1  is a plan view of a FinFET according to a first embodiment of the methods and devices according to the present invention; 
           [0040]      FIG. 2   a  is a cross-sectional view along the line II-II of  FIG. 1 ; 
           [0041]      FIG. 2   b  is a cross-sectional view like that of  FIG. 2   a , showing the corresponding structure for an SOI substrate; 
           [0042]      FIG. 3   a  is a cross-sectional view along the line III-III of  FIG. 1 ; 
           [0043]      FIG. 3   b  is a cross-sectional view like that of  FIG. 3   a , showing the corresponding structure for an SOI substrate; 
           [0044]      FIG. 4  is a plan view of a FinFET according to a further embodiment of the methods and devices according to the present invention; 
           [0045]      FIG. 5  is a cross-sectional view along the line V-V of  FIG. 4 ; 
           [0046]      FIG. 6  is a cross-sectional view along the line VI-VI of  FIG. 4 ; 
           [0047]      FIG. 7  is a plan view of an intermediate structure in a manufacturing process of making the device of  FIGS. 1 ,  2   a  and  3   a;    
           [0048]      FIG. 8  is a cross-sectional view along the line VIII-VIII of  FIG. 7 ; 
           [0049]      FIG. 9  is a cross-sectional view along the line IX-IX of  FIG. 7 ; 
           [0050]      FIG. 10  is a cross-sectional view along the line X-X of  FIG. 7 ; 
           [0051]      FIG. 11  is a plan view of the intermediate structure in a succeeding state of a manufacturing process of making the device of  FIGS. 1 ,  2   a  and  3   a;    
           [0052]      FIG. 12  is a cross-sectional view along the line XII-XII of  FIG. 11 ; 
           [0053]      FIG. 13  is a cross-sectional view along the line XIII-XIII of  FIG. 11 ; 
           [0054]      FIG. 14  is a plan view of the intermediate structure in a succeeding state of a manufacturing process of making the device of  FIGS. 1 ,  2   a  and  3   a;    
           [0055]      FIG. 15  is a cross-sectional view along the line XV-XV of  FIG. 14 ; 
           [0056]      FIG. 16  is a cross-sectional view along the line XVI-XVI of  FIG. 14 ; 
           [0057]      FIG. 17  is a plan view of the intermediate structure in a succeeding state of a manufacturing process of making the device of  FIGS. 1 ,  2   a  and  3   a;    
           [0058]      FIG. 18  is a cross-sectional view along the line XVIII-XVIII of  FIG. 17 ; 
           [0059]      FIG. 19  is a cross-sectional view along the line XIX-XIX of  FIG. 17 ; 
           [0060]      FIG. 20  is a plan view of the intermediate structure in a succeeding state of a manufacturing process of making the device of  FIGS. 1 ,  2   a  and  3   a;    
           [0061]      FIG. 21  is a cross-sectional view along the line XXI-XXI of  FIG. 20 ; 
           [0062]      FIG. 22  is a cross-sectional view along the line XXII-XXII of  FIG. 20 ; 
           [0063]      FIG. 23  is a plan view of the intermediate structure in a succeeding state of a manufacturing process of making the device of  FIGS. 1 ,  2   a  and  3   a;    
           [0064]      FIG. 24  is a cross-sectional view along the line XXIV-XXIV of  FIG. 23 ; 
           [0065]      FIG. 25  is a cross-sectional view along the line XXV-XXV of  FIG. 23 ; 
           [0066]      FIG. 26  is a plan view of an intermediate structure in a manufacturing process of making the device of  FIGS. 4-6 ; 
           [0067]      FIG. 27  is a cross-sectional view along the line XXVII-XXVII of  FIG. 26 ; 
           [0068]      FIG. 28  is a cross-sectional view along the line XXVIII-XXVIII of  FIG. 26 ; 
           [0069]      FIG. 29  is a plan view of the intermediate structure in a succeeding state of a manufacturing process of making the device of  FIGS. 4-6 ; 
           [0070]      FIG. 30  is a cross-sectional view along the line XXX-XXX of  FIG. 29 ; 
           [0071]      FIG. 31  is a cross-sectional view along the line XXXI-XXXI of  FIG. 29 ; 
           [0072]      FIG. 32  is a plan view of the intermediate structure in a succeeding state of a manufacturing process of making the device of  FIGS. 4-6 ; 
           [0073]      FIG. 33  is a cross-sectional view along the line XXXIII-XXXIII of  FIG. 32 ; 
           [0074]      FIG. 34  is a cross-sectional view along the line XXXIV-XXXIV of  FIG. 32 ; 
           [0075]      FIG. 35  is a cross-sectional view along the line XXXV-XXXV of  FIG. 32 ; 
           [0076]      FIG. 36  is a plan view of the intermediate structure in a succeeding state of a manufacturing process of making the device of  FIGS. 4-6 ; 
           [0077]      FIG. 37  is a cross-sectional view along the line XXXVII-XXXVII of  FIG. 36 ; 
           [0078]      FIG. 38  is a cross-sectional view along the line XXXVIII-XXXVIII of  FIG. 36 ; 
           [0079]      FIG. 39  is a cross-sectional view along the line XXXIX-XXXIX of  FIG. 36 ; 
           [0080]      FIG. 40  is a plan view of the intermediate structure in a succeeding state of a manufacturing process of making the device of  FIGS. 4-6 ; 
           [0081]      FIG. 41  is a cross-sectional view along the line XLI-XLI of  FIG. 40 ; 
           [0082]      FIG. 42  is a cross-sectional view along the line XLII-XLII of  FIG. 40 ; 
           [0083]      FIG. 43  is a cross-sectional view along the line XLIII-XLII of  FIG. 40 ; 
           [0084]      FIG. 44  is a plan view of the intermediate structure in a succeeding state of a manufacturing process of making the device of  FIGS. 4-6 ; 
           [0085]      FIG. 45  is a cross-sectional view along the line XLV-XLV of  FIG. 44 ; 
           [0086]      FIG. 46  is a cross-sectional view along the line XLVI-XLVI of  FIG. 44 ; 
           [0087]      FIG. 47  is a cross-sectional view along the line XLVII-XLVII of  FIG. 44 ; 
           [0088]      FIG. 48  is a plan view of the intermediate structure in a succeeding state of a manufacturing process of making the device of  FIGS. 4-6 ; 
           [0089]      FIG. 49  is a cross-sectional view along the line XLIX-XLIX of  FIG. 48 ; 
           [0090]      FIG. 50  is a cross-sectional view along the line L-L of  FIG. 48 ; 
           [0091]      FIG. 51  is a cross-sectional view along the line LI-LI of  FIG. 48 ; 
           [0092]      FIG. 52  is a plan view of the intermediate structure in a succeeding state of a manufacturing process of making the device of  FIGS. 4-6 ; 
           [0093]      FIG. 53  is a cross-sectional view along the line XLIX-XLIX of  FIG. 52 ; 
           [0094]      FIG. 54  is a cross-sectional view along the line L-L of  FIG. 52 ; 
           [0095]      FIG. 55  is a cross-sectional view along the line LI-LI of  FIG. 52 ; 
           [0096]      FIG. 56  is a plan view of the intermediate structure in a succeeding state of a manufacturing process of making the device of  FIGS. 4-6 ; 
           [0097]      FIG. 57  is a cross-sectional view along the line LVII-LVII of  FIG. 56 ; 
           [0098]      FIG. 58  is a cross-sectional view along the line LVIII-LVIII of  FIG. 56 ; 
           [0099]      FIG. 59  is a plan view of the intermediate structure in a succeeding state of a manufacturing process of making the device of  FIGS. 4-6 ; 
           [0100]      FIG. 60  is a cross-sectional view along the line LX-LX of  FIG. 59 ; 
           [0101]      FIG. 61  is a cross-sectional view along the line LXI-LXI of  FIG. 59 ; 
           [0102]      FIG. 62  schematically depicts operational advantages arising from the use of semiconductor devices according to embodiments of the present invention; 
           [0103]      FIG. 63  schematically depicts a corresponding lack of operational advantages arising from the use of conventional semiconductor devices; 
           [0104]      FIG. 64  schematically depicts an energy band profile of semiconductor devices according to embodiments of the present invention; 
           [0105]      FIG. 65  schematically depicts usage phenomena associated with the embodiment of  FIGS. 1 ,  2   a  and  3   a ; and 
           [0106]      FIG. 66  schematically depicts usage phenomena associated with the embodiment of  FIGS. 4-6 . 
       
    
    
     DETAILED DESCRIPTION 
       [0107]    In  FIGS. 1 ,  2   a  and  3   a , a first embodiment of the present invention is a FinFET in which a series of silicon fins  24  has been formed integrally with an underlying bulk silicon substrate  10 . A gate  22  extends across plural transistors as shown in  FIG. 1 , and is clad with a gate dielectric film  20 . Source and drain regions of each transistor are designated  12  and  14 , respectively. 
         [0108]    A gate dielectric film  20  is positioned between the gate  22  and sidewall spacers  18 , as shown in  FIG. 2 , and is also positioned between the gate electrode  22  and channel regions  26  as well as dielectric  16 , as shown in  FIGS. 2 and 3 . The gate dielectric film  20  thus has a hollow, three-dimensional structure. 
         [0109]    The upper part of Si fin  24  is clad with a layer of epitaxial silicon-germanium  26 , as is best seen in  FIGS. 2 and 3 . As SiGe has a larger lattice constant than Si, the channel regions of the illustrated FinFET will be compressively strained. Although compressive strain is preferred for the devices according to the present invention, it is also within the scope of the invention to provide materials for the core and cladding of the fin such that the cladding material has a smaller lattice constant than the core, which results in a tensile strain for the channel regions. 
         [0110]    The SiGe epitaxial layer is confined to the region beneath the gate electrode  22 , by which is meant the region including the gate electrode  22  itself, as well as the surrounding gate dielectric film  20 . 
         [0111]    In  FIGS. 2   b  and  3   b , a structure like that of the preceding figures is shown, however, the bulk substrate has been replaced by a silicon-on-insulator or SOI substrate, including insulating layer  11 . 
         [0112]    In  FIGS. 4-6 , a further embodiment of the present invention is a FinFET in which a series of silicon fins  44  are separated from an underlying bulk silicon substrate  30  by layer  48  of refilled dielectric. The device of this embodiment is of the silicon-on-nothing or SON type, as will be described hereinafter. A gate  42  extends across plural transistors as shown in  FIG. 4 , and is clad with a gate dielectric film  40 . Source and drain regions of each transistor are designated  32  and  34 , respectively. 
         [0113]    The gate dielectric film  40  is positioned between the gate  42  and sidewall spacers  38 , as shown in  FIG. 5 , and is also positioned between the gate electrode  42  and channel regions  46  as well as dielectric  36 , as shown in  FIGS. 5 and 6 . The gate dielectric film  40  thus has a hollow, three-dimensional structure. 
         [0114]    The Si fins  44  are clad with a layer of epitaxial silicon-germanium  46 , as is best seen in  FIGS. 5 and 6 . Again, as SiGe has a larger lattice constant than Si, the channel regions of the illustrated FinFET will be compressively strained. However, it is also again within the scope of the invention to provide materials for the core and cladding of the fin such that the cladding material has a smaller lattice constant than the core, which results in a tensile strain for the channel regions. 
         [0115]    The SiGe epitaxial layer  46  is again confined to the region beneath the gate electrode  42 , by which is meant the region including the gate electrode  42  itself, as well as the surrounding gate dielectric film  40 . 
         [0116]    In both of the above embodiments, the compressive strain is desirable as it promotes hole mobility in the channel region, as is known. However, in conventional devices utilizing strained channels for increased hole mobility, the lattice strain is substantially relaxed by the high temperature processing that occurs after the strain is created. The devices and methods of the present invention avoid that disadvantage, as will be better understood from the following explanation of preferred manufacturing techniques for the embodiments described above. 
         [0117]    As shown in  FIGS. 7-10 , a device as described above in connection with  FIGS. 1 ,  2   a  and  3   a  is advantageously made by a gate-last process, in which a dummy FinFET is made with fins  24  that are initially of Si only, and with a dummy gate  62  in place of the as yet unformed actual, gate. The dummy gate  62  may for example be polysilicon, and the sidewall spacers  18  may for example be silicon nitride, although other materials may be selected according to the knowledge of those skilled in the art. 
         [0118]    The process stage shown in  FIGS. 7-10  will typically also include formation of an interlayer dielectric to cover the source and drain regions, followed by palanarization of the same, although this is not shown in the figures for ease of understanding. 
         [0119]    Although this discussion focuses on the manufacture of a device according to  FIGS. 1 ,  2   a  and  3   a , it will be appreciated that the corresponding device formed on an SOI substrate as shown in  FIGS. 2   b  and  3   b  would be made in the same way, with the exception that the bulk silicon substrate is replaced by an SOI substrate. 
         [0120]    Next, as shown in  FIGS. 11-13 , the dummy gate  62  is removed by a conventional technique such as wet etching, which thereby exposes the silicon fins  24  between sidewall spacers  18 . As can be seen in  FIG. 13 , successive fins  24  are separated from one another by dielectric layer  16 , which layer  16  however does not extend upwardly the full height of fins  24 . 
         [0121]    The structure illustrated in  FIGS. 11-13  is then subjected to further etching, for example by RIE, to recess the fins  24  somewhat. Thus, as shown in  FIGS. 14 and 16 , the fins  24  become narrower in the horizontal direction perpendicular to the source-drain direction, and also become shorter, as shown in  FIG. 15 . Although this recessing of the fins is preferred, it is not essential and may be omitted. It is also noted that this recessing may additional remove part of the fin structure underlying the sidewalls  18 . 
         [0122]    Then, as shown in  FIGS. 17-19 , an epitaxial layer of silicon-germanium  26  is formed on the fins  24 . Because the sidewalls  18  previously formed by the gate-last process act as a mask, the SiGe film  26  is formed only in the regions that will eventually be covered by the gate dielectric layer and the gate itself. Additionally, if the fins  24  have been recessed as described in connection with  FIGS. 14-16 , then the SiGe film  26  may also extend slightly underneath the sidewalls  18 . In forming the SiGe film  26 , the formation conditions are preferably selected so that the film will have a Ge content of at least 20%. 
         [0123]    Next, as shown in  FIGS. 20-22 , the gate dielectric layer  20  is formed so as to line the volume that will be filled by the gate. The sidewalls  18  again serve as a mask for deposition of the gate dielectric layer  20 , which is preferably a high-k material. As can be seen in  FIGS. 20-22 , the gate dielectric layer  20  extends upwardly from the fins  24  along the sidewalls  18  ( FIG. 21 ), and extends downwardly between adjacent fins ( FIG. 22 ). The gate dielectric layer  20  therefore has a hollow, three-dimensional shape as a result of the gate-last process used. 
         [0124]    The gate  22  is then formed, as shown in  FIGS. 23-25 . 
         [0125]    As discussed above, SiGe intrinsically has larger lattice constant than Si; however, for an epitaxial layer of SiGe, the crystal lattice follows that of the template Si. Therefore, this SiGe layer  26  on Si fin  24  is compressively strained. The hole mobility in a compressively-strained SiGe channel is known to be higher than that in neutral Si. However, in conventional devices, the strain in an SiGe channel is relaxed during high-temperature processes, such that the hole mobility benefit is greatly reduced or lost altogether. 
         [0126]    By contrast, in the devices and methods as described above, the high temperature processes (such as isolation dielectric densify anneal and source/drain activation anneal) are done prior to formation of the SiGe epitaxial layer, and thus the favorable compressive strain in the SiGe channel is preserved. 
         [0127]    The methods for making devices as described above in connection with  FIGS. 4-6  proceeds from a substrate as shown in  FIGS. 26-28 , including a bulk silicon substrate  30  that is separated from an upper thin silicon layer  35  by a sacrificial layer  33  of SiGe that will be removed during subsequent processing. 
         [0128]    As shown in  FIGS. 29-31 , a dummy FinFET is formed similarly to that described in the preceding embodiment, with a dummny gate  82  for example of polysilicon being formed between the sidewall spacers  38 . In these embodiments, the fin structures  44  are fully isolated from the bulk Si substrate  30  by the sacrificial SiGe layer  33 . These fully-isolated fins  44  can be formed by known “silicon-on-nothing” (SON) processes based on elective etching of the sacrificial SiGe layer  33 , as described for example Jurczak et. al., “Silicon-on-Nothing (SON)—an Innovative Process for Advanced CMOS”,  IEEE Trans. Elec. Dev ., vol. 47, no. 11 (November 2000). 
         [0129]    In particular, the SiGe layer  33  and Si layer  35  are sequentially grown on the bulk-Si substrate  30  to produce the structure shown in  FIGS. 26-28 , followed by formation of the Si/SiGe/Si stacked fin structures  30 ,  33 ,  44  as shown in  FIGS. 29-31 . After formation of the dummy gate  82 , the sacrificial SiGe layer  33  is removed by selective etching (for example HCl gas etching), to produce a structure as shown in  FIGS. 32-35 . Although the fins  44  at this stage lack subjacent support they are nevertheless supported from above by the dummy gate  82  and sidewall spacers  38 , as shown in  FIGS. 33 and 34 . The void underlying fins  44  serves to isolate them fully from the bulk Si substrate  30 . 
         [0130]    Next, these voids are refilled with dielectric  48 , as shown in  FIGS. 36-39 . Then, after formation of source and drain regions  32 ,  34  as shown in  FIGS. 40-43 , dummy gate  82  is removed as shown in  FIGS. 44-47 , similarly to the process described in connection with the preceding embodiments.  FIGS. 48-51  illustrate the optional recessing of the Si fins  44 , as also described in connection with the preceding embodiments. 
         [0131]    The strained SiGe channel  46  is then formed, as illustrated in  FIGS. 52-55  and as described in connection with the preceding embodiments. Next, a layer  40  of a preferably high-k material is deposited as a gate dielectric layer, as shown in  FIGS. 56-58  and as also described in connection with the preceding embodiments. Lastly, the actual device gate  62  is deposited and planarized, as shown in  FIGS. 59-61  and also as described in connection with the preceding embodiments. 
         [0132]    By using fin structures in which both sides of a narrow fin body are covered by the gate electrode, the potential profile in the fin body is well controlled by the gate electrode. Consequently, off-state leakage current can be suppressed compared to a planar device. Still further, in devices according to certain preferred embodiments of the present invention, the fin body has a Si core and a SiGe cladding. As illustrated in  FIGS. 62-64 , there is a valence band energy offset between the Si core  24  and the SiGe cladding  26  ( FIG. 64 ), such that on-state performance is determined by the cladding region while off-state leakage current is determined by the core region. Because of this valence band offset, the hole population in the core region is lower than for a fin made entirely of SiGe ( FIGS. 62 and 63 ), and thus off-state leakage current can be suppressed. 
         [0133]    If Ge is diffused into the core region, the compressive strain weakens, and at the same the time band offset between cladding and core gets smaller. This phenomenon results in a loss of off-state leakage suppression. However, in the preferred embodiments of the present invention, the high-temperature processes are performed prior to SiGe channel formation, and thus a relatively abrupt Ge profile is preserved and Ge diffusion into the Si core is minimized. 
         [0134]    Furthermore, as illustrated in  FIG. 65 , when the FinFET is formed directly on a bulk Si substrate, there are several off-state leakage current paths, as indicated by the solid arrows in  FIG. 65 . In particular, there is a source/drain punch-through current below the fin region and also a drain/substrate junction leakage current. 
         [0135]    By contrast, the silicon on nothing (SON) process provides full-isolation of fin structure from substrate. As shown in  FIG. 66 , these leakage paths are completely eliminated and off-state leakage current can be significantly reduced. However, since full-isolation of fin structure from substrate is achieved by selective etching of the sacrificial SiGe layer  33  beneath the Si fin, if the SiGe channel  46  is formed on fin structure prior to SiGe sacrificial film  33  selective etching step, the SiGe channel region  46  would also be etched. 
         [0136]    Therefore full-isolation of fin structure and use of strained SiGe channel are not compatible on bulk-Si substrate. However, in preferred embodiments of the present invention, the SiGe channel is formed only after the fins have been fully isolated. Consequently, both high pFET performance by compressively-strained SiGe channel and low-leakage current by fully-isolated fin can be achieved simultaneously on bulk-Si substrate. 
         [0137]    While the present invention has been described in connection with various preferred embodiments thereof, it is to be understood that those embodiments are provided merely to illustrate the invention, and should not be used as a pretext to limit the scope of protection conferred by the true scope and spirit of the appended claims.