BACKGROUND OF THE INVENTION 
   1. Technical Field 
   The present invention relates to field effect transistors (FETs), and more specifically, to hybrid orientation FETs. 
   2. Related Art 
   Dopant fluctuations are becoming a serious problem in Vt (threshold voltage) control in advanced semiconductor devices. As semiconductor devices become smaller and smaller in size, Vt control becomes more difficult. Hybrid orientation field effect transistors (FETs) have the same problems. As a result, there is a need for a hybrid orientation semiconductor structure (and methods for forming the same) that allows for Vt control. 
   SUMMARY OF THE INVENTION 
   The present invention provides a semiconductor structure, comprising (a) a semiconductor substrate comprising a first semiconductor material having a first lattice orientation; (b) a back gate region on the semiconductor substrate; (c) a back gate dielectric layer on the back gate region; (d) a semiconductor region on the back gate dielectric layer, wherein the semiconductor region is electrically insulated from the back gate region by the back gate dielectric layer, and wherein the semiconductor region comprises a second semiconductor material having a second lattice orientation different from the first lattice orientation; and (e) a field effect transistor (FET) formed on the semiconductor region, wherein changing a voltage potential applied to the back gate region causes a change in a threshold voltage of the FET. 
   The present invention also provides a semiconductor structure fabrication method, comprising providing a semiconductor structure, wherein the semiconductor structure includes (a) a semiconductor substrate comprising a first semiconductor material having a first lattice orientation, (b) a back gate dielectric layer on the semiconductor substrate, and (c) a semiconductor region on the back gate dielectric layer, wherein the semiconductor region comprises a second semiconductor material having a second lattice orientation different from the first lattice orientation; and forming, in the semiconductor substrate, a back gate region beneath the back gate dielectric layer, wherein the back gate dielectric layer is sandwiched between and electrically insulates the back gate region and the semiconductor region. 
   The present invention also provides a hybrid orientation semiconductor structure (and methods for forming the same) that allows for Vt control. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1–8  illustrate cross-section views of a hybrid orientation semiconductor structure going through different fabrication steps, in accordance with embodiments of the present invention. 
       FIGS. 9A–9B  illustrate the formation of an SOI substrate used to form the hybrid orientation semiconductor structure of  FIGS. 1–8 , in accordance with embodiments of the present invention. 
   


   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1–8  illustrate cross-section views of a hybrid orientation semiconductor structure  100  going through different fabrication steps, in accordance with embodiments of the present invention. More specifically, with reference to  FIG. 1 , in one embodiment, the fabrication of the structure  100  starts out with a silicon on insulator (SOI) substrate  110 . The SOI substrate  110  comprises a bottom silicon layer  112 , a buried oxide (BOX) layer  114 , and a top silicon layer  116 . 
   The bottom silicon layer  112  is lightly doped P type (i.e., doped with P type dopants such as Boron atoms) and have lattice orientation { 100 }, whereas the top silicon layer  116  is lightly doped N type (i.e., doped with N type dopants such as phosphorous atoms) and have lattice orientation { 110 }. The minus signs in “P−” and “N−” as used in the figures indicate lightly doped. The BOX layer  114  comprises a dielectric material such as silicon dioxide. 
   In one embodiment, the SOI substrate  110  is formed using a Smart-Cut process.  FIGS. 9A–9B  illustrate the formation of the SOI substrate  110  using the Smart-Cut process. With reference to  FIG. 9A , the formation of the SOI substrate  110  starts out with a silicon substrate  910  being lightly doped N type and having lattice orientation { 110 }. Next, a hydrogen-damaged layer  920  is formed at a depth  930  which is equal to the desired thickness  117  of the top silicon layer  116  of  FIG. 1 . The hydrogen-damaged layer  920  is formed by hydrogen ion implantation. 
   Next, with reference to  FIG. 9B , the BOX layer  114  is formed on top of the substrate  910  by, illustratively, chemical vapor deposition (CVD). Next, the bottom silicon layer  112  is formed on top of the BOX layer  114  by bonding. Next, the entire structure  110  of  FIG. 9B  is annealed so that implanted hydrogen in the hydrogen-damaged layer  920  forms a gas causing the structure  110  of  FIG. 9B  to split along the hydrogen-damaged layer  920 . Next, the top part of the structure  110  of  FIG. 9B  after the split is turned upside down to be used as the SOI substrate  110  of  FIG. 1 . 
   Next, with reference again to  FIG. 1 , an oxide layer  120  is formed on top of the top silicon layer  116  by, illustratively, thermal oxidation. Next, a nitride layer  130  is formed on top of the oxide layer  120  by, illustratively, CVD. 
   Next, with reference to  FIG. 2 , in one embodiment, etching steps are performed to etch through the layers  130 ,  120 ,  116 , and  114  ( FIG. 1 ) so as to form a stack  130 ′,  120 ′,  116 ′,  114 ′, using, illustratively, a lithography process. What are left of the layers  130 ,  120 ,  116 , and  114  ( FIG. 1 ) after the etching steps are performed are the regions  130 ′,  120 ′,  116 ′, and  114 ′, respectively. Also as a result of the etching steps, the bottom silicon layer  112  is exposed to the atmosphere. 
   Next, with reference to  FIG. 3 , in one embodiment, side wall oxide spacers  310   a  and  310   b  are formed on side walls of the stack  130 ′,  120 ′,  116 ′,  114 ′. In one embodiment, the side wall oxide spacers  310   a  and  310   b  are formed by depositing an oxide layer (not shown) on top of the entire structure  100  of  FIG. 2  and then directionally etching back the oxide layer. 
   Next, P− silicon (i.e., silicon with P type dopants) is epitaxially grown on exposed-to-atmosphere surfaces of the bottom silicon layer  112  so as to form a silicon region  320 . In one embodiment, the epitaxial growth is stopped when a top surface  322  of the silicon region  320  is essentially at the same level as a top surface  324  of the silicon region  116 ′. As a result of the epitaxial growth, the silicon region  320  has the same lattice orientation as that of the silicon layer  112  (i.e., { 100 }). Therefore, the silicon regions  112  and  320  can be collectively referred to as the substrate  112 , 320 . Next, the nitride region  130 ′ and the oxide region  120 ′ are removed by, illustratively, wet etching steps. The resultant structure  100  is shown in  FIG. 4 . In an alternative embodiment, the silicon region  320  is epitaxially grown past (i.e., higher than) the top surface  324  of the silicon region  116 ′, and then a chemical mechanical polishing (CMP) step is performed until the top surface  324  of the silicon region  116 ′ is exposed to the surrounding ambient, resulting in the structure  100  of  FIG. 4 . 
   Next, with reference to  FIG. 5 , a back gate region  510  is formed in the substrate  112 , 312  and beneath the oxide region  114 ′ (which can also be referred to as the back gate dielectric layer  114 ′. In one embodiment, the back gate region  510  is formed by lightly implanting N type dopants (i.e., impurities) in the substrate  112 , 312  by ion implantation. 
   Next, electrically coupling regions  520   a  and  520   b  are formed in the substrate  112 , 312  and in direct physical contact with the back gate region  510  so as to provide electrical access to the back gate region  510  from an upper interconnect level (not shown). In one embodiment, the electrically coupling regions  520   a  and  520   b  are formed by lightly implanting N type dopants in the substrate  112 , 312  by ion implantation. The electrically coupling regions  520   a  and  520   b  may have top surfaces  522   a  and  522   b , respectively, essentially at the same level as the top surface  324  of the silicon region  116 ′. 
   Next, with reference to  FIG. 6 , field effect transistors (FETs)  610  and  620  are formed on the silicon region  116 ′ and on the substrate  112 , 312 , respectively. In one embodiment, the FET  610  comprises shallow trench isolation regions  612   a  and  612   b , source/drain regions  613   a  and  613   b , gate spacers  614   a  and  614   b , a main gate dielectric layer  615 , and a main gate region  616 . The FET  620  has a similar structure as that of the FET  610 . 
   It should be noted that the FET  610  has the back gate region  510  which is electrically insulated from the silicon region  116 ′ by the back gate dielectric layer  114 ′. The voltage potential of the back gate region  510  is controlled via either of (or both) the electrically coupling regions  520   a  and  520   b  so as to control the threshold voltage of the FET  610 . 
   In one embodiment, the FET  610  is a P channel FET which has a higher operating speed when formed on { 110 } oriented silicon lattice than on { 100 } oriented silicon lattice. On the other hand, the FET  620  is an N channel FET which has a higher operating speed when formed on { 100 } oriented silicon lattice than on { 110 } oriented silicon lattice. The structure  100  of  FIG. 6  has two lattice orientations { 100 } and { 110 }, hence the name the hybrid orientation structure  100 . 
   In an alternative embodiment, with reference to  FIG. 5 , the back gate region  510  and the electrically coupling regions  520   a  and  520   b  are lightly doped P type (as opposed to doped N type as described in the above embodiments). 
   Next, with reference to  FIG. 7 , isolating region  710  is formed in the substrate  112 , 312  so as to physically isolate a device region  720  of the substrate  112 , 312 . 
   Next, with reference to  FIG. 8 , FETs  810  and  820 , similar to the FETs  610  and  620  ( FIG. 6 ), respectively, are formed on the silicon region  116 ′ and the device region  720 , respectively. It should be noted that the device region  720 , which is physically isolated from the rest of the substrate  112 , 312  by the isolating region  710 , is individually raised to a desired voltage potential so as to control the threshold voltage of the FET  820  without affecting the voltage potentials of other regions of the substrate  112 , 312 . 
   In the embodiments described above, silicon is used in the layers  112  and  116  ( FIG. 1 ). In general, other semiconductor materials (e.g., germanium) may be used. 
   While particular embodiments of the present invention have been described herein for purposes of illustration, many modifications and changes will become apparent to those skilled in the art. Accordingly, the appended claims are intended to encompass all such modifications and changes as fall within the true spirit and scope of this invention.
A hybrid orientation semiconductor structure and method of forming the same. The structure includes (a) a semiconductor substrate comprising a first semiconductor material having a first lattice orientation; (b) a back gate region on the semiconductor substrate; (c) a back gate dielectric layer on the back gate region; (d) a semiconductor region on the back gate dielectric layer, wherein the semiconductor region is electrically insulated from the back gate region by the back gate dielectric layer, and wherein the semiconductor region comprises a second semiconductor material having a second lattice orientation different from the first lattice orientation; and (e) a field effect transistor (FET) formed on the semiconductor region, wherein changing a voltage potential applied to the back gate region causes a change in a threshold voltage of the FET.