Abstract:
A metallization scheme for vertical field effect transistors (FETs) is provided. By forming lower-level local interconnects connecting source regions located at bottom portions of semiconductor fins, and upper-level interconnects connecting adjacent metal gates located along sidewalls of channel regions of the semiconductor fins, electrical connections to the source regions and the metal gates can be provided through the lower-level local interconnects and the upper-level local interconnects, respectively. As a result, gate, source and drain contact structures are formed on the same side of vertical FETs.

Description:
BACKGROUND 
       [0001]    The present application relates to semiconductor device fabrication, and more particularly, to the metallization of vertical field effect transistors (FETs). 
         [0002]    A vertical field effect transistor (FET) has a channel oriented perpendicular to the substrate surface, as opposed to being situated along the plane of the surface of the substrate in the case of a lateral FET. By using a vertical design, it is possible to increase packing density. That is, by having the channel perpendicular to the substrate, vertical FETs improve the scaling limit beyond lateral FETs. However, vertical device architecture makes metallization of vertical FETs very difficult. 
       SUMMARY 
       [0003]    The present application provides a metallization scheme for vertical FETs. By forming lower-level local interconnects connecting source regions located at bottom portions of semiconductor fins, and upper-level interconnects connecting adjacent metal gates located along sidewalls of channel regions of the semiconductor fins, electrical connections to the source regions and the metal gates can be provided through the lower-level local interconnects and the upper-level local interconnects, respectively. As a result, gate, source and drain contact structures are formed on the same side of vertical FETs. 
         [0004]    According to an aspect of the present application, a semiconductor structure is provided. The semiconductor structure includes a plurality of vertical fin field effect transistors (FinFETs), each of the plurality of vertical FinFETs includes a source region located at a bottom portion of a semiconductor fin, a drain region located at a top portion of the semiconductor fin, a channel region located between the source region and the drain region, and a metal gate located along sidewalls of the channel region. The semiconductor structure further includes lower-level local interconnects electrically connecting the source regions and upper-level local interconnects electrically connecting the metal gates. Each of the lower-level local interconnects contacts sidewalls of adjacent source regions. Each of the upper-level local interconnects contact sidewalls of adjacent metal gates. 
         [0005]    According to another aspect of the present application, a method of forming a semiconductor structure is provided. The method includes forming a plurality of fin stacks on a substrate. Each of the plurality of fin stacks includes a semiconductor fin and a fin cap. A bottom portion of each semiconductor fin is laterally surrounded by an insulator layer. Sacrificial spacers are then formed on sidewalls of the semiconductor fins that are not covered by the insulator layer. Next, the insulator layer is recessed to provide an insulator layer portion. A gap is formed between each sacrificial spacer and the insulator layer portion. Each gap exposes sidewalls of a portion of the bottom portion of each semiconductor fin. Next, a portion of the bottom portion of each semiconductor fin is doped to form a source region therein. Each gap exposes sidewalls of each source region. Lower-level local interconnects are then formed over the insulator layer portion to connect the source regions. Each of the lower-level local interconnects fills the gap and contacts sidewalls of adjacent source regions. After forming a dielectric layer to cover the lower-level local interconnects, the sacrificial spacers are removed to expose sidewalls of a non-doped portion of each semiconductor fin. Next, a metal gate is formed along sidewalls of a lower portion of the non-doped portion of each semiconductor fin and upper-level local interconnects are formed connecting the metal gates. Each of the upper-level local interconnects contact sidewalls of adjacent metal gates. An upper portion of the non-doped portion of each semiconductor fin is then doped to form a drain region therein. 
     
    
     
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
         [0006]      FIG. 1A  is a top-down view of an exemplary semiconductor structure including a plurality of fin stacks located on a substrate according to an embodiment of the present application. 
           [0007]      FIG. 1B  is a cross-sectional view of the exemplary semiconductor structure of  FIG. 1A  along line B-B′. 
           [0008]      FIG. 2A  is a top-down view of the exemplary semiconductor structure of  FIGS. 1A and 1B  after forming an insulator layer around the base of each semiconductor fin in the fin stacks. 
           [0009]      FIG. 2B  is a cross-sectional view of the exemplary semiconductor structure of  FIG. 2A  along line B-B′. 
           [0010]      FIG. 3A  is a top-down view of the exemplary semiconductor structure of  FIGS. 2A and 2B  after forming sacrificial spacers on sidewalls of the semiconductor fins above the insulator layer. 
           [0011]      FIG. 3B  is a cross-sectional view of the exemplary semiconductor structure of  FIG. 3A  along line B-B′. 
           [0012]      FIG. 4A  is a top-down view of the exemplary semiconductor structure of  FIGS. 3A and 3B  after recessing the insulator layer to provide an insulator layer portion, thus forming gaps between the sacrificial spacers and the insulator layer portion. 
           [0013]      FIG. 4B  is a cross-sectional view of the exemplary semiconductor structure of  FIG. 4A  along line B-B′. 
           [0014]      FIG. 5A  is a top-down view of the exemplary semiconductor structure of  FIGS. 4A and 4B  after forming a first dopant-rich layer contacting sidewalls of the semiconductor fins exposed by the gaps and diffusing dopants in the first dopant-rich layer into the semiconductor fins to form a source region at a bottom portion of each semiconductor fin. 
           [0015]      FIG. 5B  is a cross-sectional view of the exemplary semiconductor structure of  FIG. 5A  along line B-B′. 
           [0016]      FIG. 6A  is a top-down view of the exemplary semiconductor structure of  FIGS. 5A and 5B  after removing the first dopant-rich layer to re-expose sidewalls of the semiconductor fins in the gaps. 
           [0017]      FIG. 6B  is a cross-sectional view of the exemplary semiconductor structure of  FIG. 6A  along line B-B′. 
           [0018]      FIG. 7A  is a top-down view of the exemplary semiconductor structure of  FIGS. 6A and 6B  after forming a metal layer over the insulator layer portion and within the gaps to contact sidewalls of the source regions. 
           [0019]      FIG. 7B  is a cross-sectional view of the exemplary semiconductor structure of  FIG. 7A  along line B-B′. 
           [0020]      FIG. 8A  is a top-down view of the exemplary semiconductor structure of  FIGS. 7A and 7B  after patterning the metal layer to form lower-level local interconnects connecting the source regions. 
           [0021]      FIG. 8B  is a cross-sectional view of the exemplary semiconductor structure of  FIG. 8A  along line B-B′. 
           [0022]      FIG. 9A  is a top-down view of the exemplary semiconductor structure of  FIGS. 8A and 8B  after forming a dielectric layer to cover the lower-level local interconnects. 
           [0023]      FIG. 9B  is a cross-sectional view of the exemplary semiconductor structure of  FIG. 9A  along line B-B′. 
           [0024]      FIG. 10A  is a top-down view of the exemplary semiconductor structure of  FIGS. 9A and 9B  after removing the sacrificial spacers. 
           [0025]      FIG. 10B  is a cross-sectional view of the exemplary semiconductor structure of  FIG. 10A  along line B-B′. 
           [0026]      FIG. 11A  is a top-down view of the exemplary semiconductor structure of  FIGS. 10A and 10B  after forming a gate dielectric layer and a gate electrode layer. 
           [0027]      FIG. 11B  is a cross-sectional view of the exemplary semiconductor structure of  FIG. 11A  along line B-B′. 
           [0028]      FIG. 12A  is a top-down view of the exemplary semiconductor structure of  FIGS. 11A and 11B  after patterning the gate electrode layer to form a first gate electrode portion that laterally surrounds a lower portion of each non-doped semiconductor fin portion and a second gate electrode portion that connects adjacent first gate electrode portions. 
           [0029]      FIG. 12B  is a cross-sectional view of the exemplary semiconductor structure of  FIG. 12A  along line B-B′. 
           [0030]      FIG. 12C  is a cross-sectional view of the exemplary semiconductor structure of  FIG. 12A  along line C-C′. 
           [0031]      FIG. 13A  is a top-down view of the exemplary semiconductor structure of  FIGS. 12A-12C  after forming a metal gate laterally surrounds the lower portion of each non-doped semiconductor fin portion and an upper-level local interconnect that connects adjacent metal gates. 
           [0032]      FIG. 13B  is a cross-sectional view of the exemplary semiconductor structure of  FIG. 13A  along line B-B′. 
           [0033]      FIG. 13C  is a cross-sectional view of the exemplary semiconductor structure of  FIG. 13A  along line C-C′. 
           [0034]      FIG. 14A  is a top-down view of the exemplary semiconductor structure of  FIGS. 13A-13C  after forming a second dopant-rich layer contacting sidewalls of the non-doped semiconductor fin portions that are not covered by the metal gates and diffusing dopants in the second dopant-rich layer into the non-doped semiconductor fin portions to form a drain region at a top portion of each semiconductor fin. 
           [0035]      FIG. 14B  is a cross-sectional view of the exemplary semiconductor structure of  FIG. 14A  along line B-B′. 
           [0036]      FIG. 14C  is a cross-sectional view of the exemplary semiconductor structure of  FIG. 14A  along line C-C′. 
           [0037]      FIG. 15A  is a top-down view of the exemplary semiconductor structure of  FIGS. 14A-14C  after forming a contact level dielectric layer over the second dopant-rich layer. 
           [0038]      FIG. 15B  is a cross-sectional view of the exemplary semiconductor structure of  FIG. 15A  along line B-B′. 
           [0039]      FIG. 15C  is a cross-sectional view of the exemplary semiconductor structure of  FIG. 15A  along line C-C′. 
           [0040]      FIG. 16A  is a top-down view of the exemplary semiconductor structure of  FIGS. 15A-15C  after forming gate, source and drain contact structures. 
           [0041]      FIG. 16B  is a cross-sectional view of the exemplary semiconductor structure of  FIG. 16A  along line B-B′. 
           [0042]      FIG. 16C  is a cross-sectional view of the exemplary semiconductor structure of  FIG. 16A  along line C-C′. 
       
    
    
     DETAILED DESCRIPTION 
       [0043]    The present application will now be described in greater detail by referring to the following discussion and drawings that accompany the present application. It is noted that the drawings of the present application are provided for illustrative purposes only and, as such, the drawings are not drawn to scale. It is also noted that like and corresponding elements are referred to by like reference numerals. 
         [0044]    In the following description, numerous specific details are set forth, such as particular structures, components, materials, dimensions, processing steps and techniques, in order to provide an understanding of the various embodiments of the present application. However, it will be appreciated by one of ordinary skill in the art that the various embodiments of the present application may be practiced without these specific details. In other instances, well-known structures or processing steps have not been described in detail in order to avoid obscuring the present application. 
         [0045]    Referring to  FIGS. 1A and 1B , an exemplary semiconductor structure according to an embodiment of the present application includes a plurality of fin stacks formed on a substrate  10 . Each fin stack includes a semiconductor fin  20  and a fin cap  22  located on top of the semiconductor fin  20 . The semiconductor fins  20  may have a width ranging from 5 nm to approximately 100 nm, although lesser and greater widths can also be employed; and may have a height ranging from 10 nm to approximately 200 nm, although lesser and greater heights can also be employed. The semiconductor fins  20  may be separated by a spacing ranging from 30 nm to 50 nm, although lesser and greater distances can also be employed. 
         [0046]    The fin stacks ( 20 ,  22 ) and can be formed by first forming a dielectric cap layer (not shown) on a topmost surface of the substrate  10 . In one embodiment and as shown in  FIG. 1B , the substrate  10  can be a bulk semiconductor substrate including a bulk semiconductor material throughout. The bulk semiconductor substrate may include a semiconductor material such as, for example, Si, Ge, SiGe, SiC, SiGeC or an III-V compound semiconductor. In one embodiment, the bulk semiconductor substrate includes a single crystalline semiconductor material, such as, for example, single crystalline silicon. The thickness of the bulk semiconductor substrate can be from 30 μm to about 2 mm, although lesser and greater thicknesses can also be employed. 
         [0047]    The bulk semiconductor substrate may be doped with dopants of p-type or n-type. In one embodiment, the dopants may be a p-type dopant including, but not limited to, boron (B), aluminum (Al), gallium (Ga), and indium (In). In another embodiment, the dopants may be an n-type dopant including, but not limited to, antimony (Sb), arsenic (As), and phosphorous (P). The dopant concentration in the bulk semiconductor substrate can range from 1×10 14  atoms/cm 3  to 3×10 17  atoms/cm 3 , although lesser and greater dopant concentrations can also be employed. 
         [0048]    The dielectric cap layer may include a dielectric material such as, for example, silicon dioxide, silicon nitride, silicon oxynitride, a dielectric metal oxide, or a combination thereof. In one embodiment, the dielectric cap layer is composed of silicon nitride. The dielectric cap layer can be formed by a deposition process including chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD) or physical vapor deposition (PVD), or by a thermal growing process, such as thermal oxidation or thermal nitridation, to convert a surface portion of the bulk semiconductor substrate. The thickness of the dielectric cap layer can be from 5 nm to 20 nm, although lesser and greater thicknesses can also be employed. 
         [0049]    The dielectric cap layer and an upper portion of the bulk semiconductor substrate are subsequently patterned to form the fin stacks ( 20 ,  22 ). For example, a photoresist layer (not shown) can be applied over a top surface of the dielectric cap layer and lithographically patterned to provide a patterned photoresist layer atop portions of the dielectric cap layer. Portions of the dielectric cap layer that are not covered by the patterned photoresist layer are subsequently removed by an anisotropic etch, exposing portions of the bulk semiconductor substrate. The anisotropic etch can be a dry etch such as, for example, reactive ion etch (RIE) or a wet etch involving a chemical etchant that removes dielectric material of the dielectric cap layer selective to the semiconductor material of the bulk semiconductor substrate. Remaining portions of the dielectric cap layer after the lithographic patterning constitute the fin caps  22 . Another anisotropic etch is then performed to remove semiconductor material from the bulk semiconductor substrate utilizing the fin caps  22  as an etch mask, thus forming the semiconductor fins  20  in the upper portion of the bulk semiconductor substrate. After transferring the pattern in the photoresist layer into the dielectric cap layer and the bulk semiconductor substrate, the patterned photoresist layer can be removed utilizing a conventional resist stripping process such as, for example, ashing. Other methods known in the art, such as sidewall image transfer (SIT) or directional self-assembly (DSA), can also be used to pattern the dielectric cap layer and the bulk semiconductor substrate to provide the fin caps  22  and the semiconductor fins  20 , respectively. 
         [0050]    In another embodiment, the substrate  10  can be a semiconductor-on-insulator (SOI) substrate including, from bottom to top, a handle substrate, a buried insulator layer and a top semiconductor layer (not shown). The semiconductor fins  20  are formed by etching the top semiconductor layer. In such case, the semiconductor fins  20  may extend upwards from the buried insulator layer. 
         [0051]    Referring to  FIGS. 2A and 2B , an insulator layer  30  is formed around the base of each semiconductor fin  20 , so that bottom portions of the semiconductor fins  20  are covered by the insulator layer  30 . Thus, the insulator layer  30  has a topmost surface that is vertical offset and located beneath a topmost surface of each semiconductor fin  20 . The insulator layer  30  may have a thickness ranging from 50 nm to 80 nm, although lesser and greater thicknesses can also be employed. 
         [0052]    The insulator layer  30  may be composed of any dielectric material capable of being removed selectively to the semiconductor fins  20 , the fin caps  22 , as well as sacrificial spacers to be subsequently formed on sidewalls of the semiconductor fins  20 , as described below in conjunction with  FIGS. 3A and 3B . In one embodiment, the insulator layer  30  includes a dielectric oxide such as, for example, silicon dioxide. The insulator layer  30  may be formed by depositing a dielectric material over and between the semiconductor fins  20  using a conventional deposition technique, such as, for example, CVD or PVD, planarizing the deposited dielectric material by a conventional planarization technique such as, for example chemical mechanical planarization (CMP), and then etching back the deposited dielectric material to the desired thickness. An anisotropic etch such as, for example, RIE may be employed to remove the dielectric material of the insulator layer  30  selective to the material of the semiconductor fins  20  and the fin caps  22 . 
         [0053]    Referring to  FIGS. 3A and 3B , sacrificial spacers  32  are formed on the sidewalls of the semiconductor fins  20  above the insulator layer  30 . The sacrificial spacers  32  may include a dielectric material that differs from the dielectric material that provides insulator layer  30 . Exemplary dielectric materials that can be used in providing the sacrificial spacers  32  include, but are not limited to, an oxide, a nitride and an oxynitride. In one embodiment, the sacrificial spacers  32  are composed of silicon nitride. 
         [0054]    The sacrificial spacers  32  can be formed by first conformally depositing a sacrificial spacer material layer (not shown) on exposed surfaces of the each fin stack including the semiconductor fin  20  and fin cap  22  (i.e., top surfaces and sidewall surfaces of the semiconductor fins  20  not covered by the insulator layer  30  and top surfaces and sidewall surfaces of the fin caps  22 ) and the insulator layer  30  utilizing, for example, CVD or atomic layer deposition (ALD). Subsequently, horizontal portions of the conformal sacrificial spacer material layer are removed by an anisotropic etch such as, for example, RIE. Remaining vertical portions of the conformal sacrificial spacer material layer that are present on sidewalls of the exposed portions of the semiconductor fins  20  constitute the sacrificial spacers  32 . The sacrificial spacers  32  can have a width, as measured at the base, from 10 nm to 20 nm, although lesser and greater widths can also be employed. As is shown, the sacrificial spacers  32  have a topmost surface that is coplanar with a topmost surface of the fin caps  22 . 
         [0055]    Referring to  FIGS. 4A and 4B , the insulator layer  30  is recessed so that a top surface of a remaining portion of the insulator layer  30  (herein referred to as an insulator layer portion  30 P) is located below the bottom surfaces of the sacrificial spacers  32 . As such, gaps  34  are formed between the sacrificial spacers  32  and the insulator layer portion  30 P, exposing sidewalls of portions of the bottom portions of the semiconductor fins  20  that are originally covered by the insulator layer  30 . The insulator layer  30  may be recessed using an anisotropic etch. The anisotropic etch can be a dry etch or a wet etch that removes the dielectric material of the insulator layer  30  without substantially impacting the surrounding structure, including the semiconductor fins  20 , the fin caps  22  and the sacrificial spacers  32 . In one embodiment, RIE may be performed. After recessing, the insulator layer portion  30 P may have a thickness from 10 nm to 30 nm, although lesser and greater thicknesses can also be employed. 
         [0056]    Referring to  FIGS. 5A and 5B , a first dopant-rich layer  36  is applied on the insulator layer portion  30 P, the sacrificial spacers  32  and the fin caps  22  and within the gaps  34 . The first dopant-rich layer  36  thus contacts the sidewalls of the exposed lower portions of the semiconductor fins  20 . The first dopant-rich layer  36  serves as a dopant diffusion source during an anneal process subsequently performed for formation of a source region in each semiconductor fin  20 . Depending on the desirable conductivity type of the resulting FinFETs, the first dopant-rich layer  36  may include n-type dopants or p-type dopants. For example, for n-type FinFET, the first dopant-rich layer  36  may comprise phosphorous and/or arsenic, while for p-type FinFET, the first dopant-rich layer  36  may comprises boron and/or In. The concentration of the dopants in the first dopant-rich layer  36  may be greater than 80 atomic %. In one embodiment, the first dopant-rich layer  36  may include phosphorus-doped silicate glass (PSG) or boron-doped silicate glass (BSG). In another embodiment, the first dopant-rich layer  36  may include doped silicon. 
         [0057]    The first dopant-rich layer  36  can be applied, for example, by CVD. The thickness of the first dopant-rich layer  36  can be from 5 nm to 100 nm, although lesser and greater thicknesses can also be employed. 
         [0058]    After the formation of the first dopant-rich layer  36 , an anneal is performed to drive dopants in the first dopant-rich layer  36  into the semiconductor fins  20  to form a source region  40  at the bottom portion of each semiconductor fin  20 . The anneal process can be performed at a temperature ranging from 900° C. to 1150° C. The source regions  40  can have a dopant concentration ranging from 1×10 18  atoms/cm 3  to 1×10 21  atoms/cm 3 . The undoped portions of the semiconductor fins  20  are herein referred to as non-doped semiconductor fin portions  20 P. 
         [0059]    Referring to  FIGS. 6A and 6B , the first dopant-rich layer  36  is removed from the fin caps  22 , the sacrificial spacers  22 , the insulator layer portion  30 P and the gaps  34 . Sidewall surfaces of the source regions  40  are thus exposed in the gaps  34 . In one embodiment, the first dopant-rich layer  36  can be removed by a wet etch based on, for example, dilute hydrofluoric acid (HF) or buffer HF chemistry. In instances where the first dopant-rich layer  36  is composed of doped silicon, the step of removing the first dopant-rich layer  36  can be omitted. 
         [0060]    Referring to  FIGS. 7A and 7B , a metal layer  42  is formed over the insulator layer portion  30 P and within the gaps  34 , contacting sidewalls of the source regions  40 . The metal layer  42  is optional and can be omitted in instances where the first dopant-rich layer  36  is composed of doped silicon and remains in the structure after formation of the source regions  40 . 
         [0061]    The metal layer  42 , if present, may include a conductive metal such as Cu, W, Al, or an alloy thereof. The metal layer  42  may be formed by first depositing a conductive metal utilizing a convention deposition process including, but not limited to CVD, PECVD, PVD, or plating. The thickness of the metal layer  42  can be selected so that an entire metal layer  42  is formed above the top surfaces of the fin caps  22 . Subsequently, the metal layer  42  is planarized, for example, by CMP, and recessed by a recess etch. The etch can be a dry etch or a wet etch that removes the conductive metal selective to the dielectric materials that provide the fin caps  22  and the sacrificial spacers  32  and the semiconductor material that provides the semiconductor fins  20 . After recessing, the metal layer  42  has a top surface coplanar with or below the top surfaces of the source regions  40 . Optionally, a metal liner (not shown) may be formed over the insulator layer portion  30  prior to the deposition of the conductive metal that provides the metal layer  42 . The metal liner may include Co, Ni or Pt. 
         [0062]    Referring to  FIGS. 8A and 8B , the metal layer  42 , or in some embodiments the first dopant-rich layer  36  if the metal layer  42  is not present, is patterned to form lower-level local interconnects  44  that connect individual source regions  40  which are located beneath each neighboring pair of the non-doped semiconductor fin portions  20 P. Specifically, a photoresist layer (not shown) is applied over the metal layer  42  or the first dopant-rich layer  36 , the fin caps  22  and the sacrificial spacers  32  and then lithographically patterned so that remaining portions of the photoresist layer covers portions of the metal layer  42  or the first dopant-rich layer  36  where the lower-level local interconnects  44  are to be formed. The pattern in the photoresist layer is transferred through the metal layer  42  or the first dopant-rich layer  36  by an anisotropic etch. The anisotropic etch can be a dry etch such as, for example, RIE or a wet etch. After the anisotropic etch, remaining portions of the metal layer  42  or remaining portions of the first-dopant rich layer  36  constitute the lower-level local interconnects  44 . The remaining portions of the photoresist layer can be subsequently removed, for example, by ashing. 
         [0063]    Referring to  FIGS. 9A and 9B , a dielectric layer  50  is formed over the insulator layer portion  30 P to cover the lower-level local interconnects  44 . As is shown, a topmost surface of the dielectric layer  50  is located below the top surfaces of the semiconductor fin portions  20 P. In one embodiment, the dielectric layer  50  may have a thickness around 15 nm. 
         [0064]    The dielectric material of the dielectric layer  50  may be chosen so the sacrificial spacers  32  may be selectively removed without substantially removing the dielectric layer  50 . For example and when the sacrificial spacers  32  include a dielectric nitride, the dielectric layer  50  may include a dielectric oxide such as silicon dioxide. In one embodiment, the dielectric layer  50  may be made of the same dielectric material as the insulator layer portion  30 P, so that the dielectric layer  50  is indistinguishable for the insulator layer portion  30 P after deposition. 
         [0065]    The dielectric layer  50  can be formed utilizing a sequence of deposition and etch back processes. The dielectric material that provides the dielectric layer  50  may be first deposited, for example, by CVD or PVD. After deposition, the deposited dielectric material is recessed by a selective etch to provide the dielectric layer  50 . The selective etch can be an anisotropic etch such as RIE that removes the dielectric material that provides the dielectric layer  50  selective to the dielectric materials that provide the fin caps  22  and the sacrificial spacers  32 . 
         [0066]    Referring to  FIGS. 10A and 10B , the sacrificial spacers  32  are removed from the sidewalls of the semiconductor fin portions  20 P and fin caps  22  by an etch, which can be an isotropic etch or an anisotropic etch. In one embodiment, the sacrificial spacers  32  can be removed by a wet etch. For example, if the sacrificial spacers  32  include silicon nitride, the sacrificial spacers  32  can be removed by a wet etch employing hot phosphoric acid. Depending on selection of the materials for the sacrificial spacers  32  and the fin caps  22 , portions of the fin caps  22  may, or may not be, collaterally etched. The removal the sacrificial spacers  32  exposes sidewalls of a portion of each vertical fin stack ( 20 P,  22 ). That is, sidewalls of each non-doped semiconductor fin portion  20 P and sidewalls of each fin cap  22  are exposed. In some embodiments of the present application and as shown in  FIG. 10B , sidewalls of a portion of each source region  42  are also exposed. In addition to exposing each fin stack ( 20 P,  22 ), the removal of the sacrificial spacers  32  also exposes a portion of each lower-level local interconnect  44  adjoined to the source regions  40 . 
         [0067]    Referring to  FIGS. 11A and 11B , a gate dielectric layer  52  is formed on exposed surfaces of the fin caps  22 , the non-doped semiconductor fin portions  20 , the source regions  40 , the lower-level local interconnects  44  and the dielectric layer  50 . In one embodiment, the gate dielectric layer  52  can be composed of silicon dioxide, silicon nitride and/or silicon oxynitride. In another embodiment, the gate dielectric layer  52  can be composed of a dielectric metal oxide, so called high dielectric constant (high-k) material having a dielectric constant greater than 8.0. Exemplary dielectric metal oxides that can be used as the gate dielectric material that provides the gate dielectric layer  52  include, but are not limited to, HfO 2 , ZrO 2 , La2O 3 , Al 2 O 3 , TiO 2 , SrTiO 3 , LaAlO 3  and Y 2 O 3 . In some embodiments, the gate dielectric layer  52  may have a multilayer structure comprising different gate dielectric materials, e.g., silicon dioxide, and a dielectric metal oxide can be formed and used as the gate dielectric material that provides the gate dielectric layer  52 . 
         [0068]    In one embodiment of the present application, the gate dielectric layer  52  can be formed by a conventional deposition process including, but not limited to, CVD, PVD and atomic layer deposition (ALD). In another embodiment of the present application, the gate dielectric layer  52  can be formed by a thermal growth technique such as, for example, thermal oxidation and/or thermal nitridation. In yet a further embodiment of the present application, a combination of a deposition and thermal growth may be used in forming a multilayered gate dielectric layer. The gate dielectric layer that is formed may have a thickness ranging from 1 nm to 10 nm, although lesser and greater thicknesses can also be employed. 
         [0069]    Subsequently, a gate electrode layer  54  is formed on the gate dielectric layer  52 . The gate electrode layer  54  may comprise any conductive metal. Exemplary conductive metals that can be employed in the metal gate electrode layer include, but are not limited to, W, Ti, Ta, Al, Ni, Ru, Pd and Pt. In one embodiment, the gate electrode layer is comprised of W. In some embodiments of the present application, the ate electrode layer  54  also contains a work function material such as, for example, TiAlN, TiN, or TaN. 
         [0070]    The gate electrode layer  54  can be formed utilizing a sequence of deposition and etch back processes. The gate electrode layer  54  can be deposited by a conventional deposition process including, for example, CVD, PECVD, PVD, sputtering, chemical solution deposition and ALD. Subsequently, the gate electrode layer  54  is planarized, for example, by CMP, and recessed by a recess etch. The etch can be a dry etch or a wet etch that removes the conductive metal that provides the gate electrode layer  54  selective to the dielectric material(s) that provides the gate dielectric layer  52 . The gate electrode layer  54  has a top surface located between the top surfaces of the non-doped semiconductor fin portions  20 P and top surfaces of the source regions  40 . 
         [0071]    Referring to  FIGS. 12A-12C , the gate electrode layer  54  is patterned to form a first gate electrode portion  54 A that laterally surrounds a lower portion of each non-doped semiconductor fin portion  20 P and a second gate electrode portion  54 B that connects adjacent first gate electrode portions  54 A. A photoresist layer (not shown) is first applied over the gate electrode layer  54 . The photoresist layer is lithographically patterned so that remaining portions of the photoresist layer covers portions of the gate electrode layer  54  where the first and the second gate electrode portions  54 A,  54 B are to be formed. An anisotropic etch is performed to remove the portions of the gate electrode layer  54  that are not covered by the photoresist layer. The anisotropic etch can be a dry etch such as, for example, RIE or a wet etch. After the anisotropic etch, each remaining portion of the gate electrode layer  54  that is in direct contact with sidewalls of each non-doped semiconductor fin portion  20  constitutes the first gate electrode portion  54 A, while a remaining portion of the gate electrode layer  54  located between adjacent first gate electrode portions  54 A constitutes the second gate electrode portion  54 B. The remaining portion of the photoresist layer can be subsequently removed, for example, by ashing. 
         [0072]    Referring to  FIGS. 13A and 13B , portions of the gate dielectric layer  52  that are not covered by the first and second gate electrode portions  54 A,  54 B are removed by an anisotropic etch. The anisotropic etch can be a dry etch or a wet etch that removes the dielectric material that provides the gate dielectric layer  52  selective to the dielectric materials of the fin caps  22  and the dielectric layer  50  and semiconductor material of the semiconductor fin  20 . In one embodiment, RIE is performed. Each remaining portion of the gate dielectric layer  52  that is located beneath one of the first gate electrode portions  54 A constitutes a first gate dielectric portion  52 A, while a remaining portion of the gate dielectric layer  52  that is located beneath the second gate electrode portion  54 B constitutes a second gate dielectric portion  52 B. A first gate dielectric portion  52 A and an overlying first gate electrode portion  54 A together define a metal gate contacting sidewalls of a portion of each non-doped semiconductor fin portion  20 P. The second gate dielectric portion  52 B and the overlying second gate electrode portion  54 B together define an upper-level local interconnect connecting adjacent metal gates ( 52 A,  54 A). In the present application, although only one upper-level local interconnect ( 52 B,  54 B) is shown, it can be understood that a plurality of upper-level local interconnect ( 52 B,  54 B) can be formed to connect individual metal gates ( 52 A,  54 A). 
         [0073]    Referring to  FIGS. 14A-14C , a second dopant-rich layer  58  is deposited on the exposed surfaces of the dielectric layer  50 , the metal gates ( 52 A,  54 B), the upper-level local interconnect ( 52 B,  54 B), the non-doped semiconductor fin portions  20 P and the fin caps  22 . The second dopant-rich layer  58  thus contacts sidewalls of the top portions of the non-doped semiconductor fin portions  20 P that are not covered by the metal gates ( 52 A,  52 B). The second dopant-rich layer  58  serves as a dopant diffusion source during an anneal process subsequently performed for formation of a drain region in each semiconductor fin  20 . 
         [0074]    The second dopant-rich layer  58  typically comprises a dielectric material containing dopants of the same conductivity type as that of the dopants in the first dopant-rich layer  36 . In one embodiment, the second dopant-rich layer  58  includes phosphorus-doped silicate glass (PSG) or boron-doped silicate glass (BSG). The second dopant-rich layer  58  can be formed by performing processing steps described above in formation of dopant-rich layer  36 . For example, the second dopant-rich layer  58  can be formed by CVD or spin coating. 
         [0075]    Subsequently, an anneal is performed to drive dopants in the second dopant-rich layer  58  into the non-doped semiconductor fin portions  20 P to form a drain region  60  at the top portion of each semiconductor fin  20 . The anneal process can be performed at a temperature ranging from 900° C. to 1150° C. The drain regions  60  can have a dopant concentration ranging from 1×10 18  atoms/cm 3  to 1×10 21  atoms/cm 3 . In one embodiment, each of the source regions  40  and drain regions  60  comprises n-type dopants. A remaining portion of each non-doped semiconductor fin portion  20 P that is located between the source region  40  and the drain region  60  constitutes a channel region  20 C. After forming the drain regions  60 , the second dopant-rich layer  58  may be removed from the structure. The removal of the second dopant-rich layer  58  is optional and can be omitted in some embodiments of the present application. 
         [0076]    Vertical FinFETs are thus formed. Each vertical FinFET includes a source region  40  and a drain region  60  formed at a bottom portion and a top portion of a semiconductor fin  20 , respectively, a channel region  20 C located between the source region  40  and the drain region  60 , and a metal gate ( 52 A,  54 A) formed along sidewalls of the semiconductor fin  20 , spanning the vertical distance between the source region  40  and drain region  60 . Current thus flows vertically through the channel region  20 C when an appropriate bias is applied to the metal gate ( 52 A,  54 A). In the present application, since the source region  40  and the drain region  60  in each vertical FinFET are formed by a dopant diffusion process, the overlaps between the metal gate ( 52 A,  54 A) and the source region  40  and between the metal gate ( 52 A,  54 A) and the drain region  60  can be well-controlled. The vertical FinFETs thus formed are self-aligned. 
         [0077]    Referring to  FIGS. 15A-15C , a contact level dielectric layer  70  is formed over the second dopant-rich layer  58 , if present, or the fin caps  22 , the metal gates ( 52 A,  54 A), the upper-level local interconnect ( 52 B,  54 B) and the dielectric layer  50 . The contact level dielectric layer  70  may include a dielectric material such as, for example, oxides, nitrides or oxynitrides. In one embodiment, the contact level dielectric layer  70  includes SiCN. The contact level dielectric layer  70  may be formed, for example, by CVD or spin-coating. The contact level dielectric layer  70  may be self-planarizing, or the top surface of the contact level dielectric layer  70  can be planarized, for example, by CMP. In one embodiment, the planarized top surface of the contact level dielectric layer  70  is located above the topmost surface of the second dopant-rich layer  58 , if present, or the top surfaces of the fin caps  22 . 
         [0078]    Referring to  FIGS. 16A-16C , various contact structures are formed. The contact structures include a gate contact structure  82  extending through the contact level dielectric layer  70  and the second dopant-rich layer  58 , if present, to form contact with the upper-level local interconnect ( 52 B,  54 B) that connects adjacent metal gates ( 52 A,  54 A), a source contact structure  84  extending through the contact level dielectric layer  70 , the second dopant-rich layer  58 , if present and the dielectric layer  50  to form contact with one of the lower-level local interconnects  44  that connects individual source regions  40 , and a drain contact structure  86  extending through the contact level dielectric layer  70 , the second dopant-rich layer  58 , if present, and the fin cap  22  to form contact with each of the drain regions  60 . The gate, source and drain contact structures  82 ,  84 ,  86  can be formed by formation of contact openings (not shown) within the dielectric material components including the fin cap  22 , the dielectric layer  50 , the second dopant-rich layer  58 , if present and the contact level dielectric layer  70  utilizing a combination of lithographic patterning and anisotropic etch followed by deposition of a conductive material (e.g., copper) and planarization that removes an excess portions of the conductive material from above the top surface of the contact level dielectric layer  70 . Optionally, contact liners (not shown) may be formed on the sidewalls and bottoms surfaces of the contact openings before filling the contact openings with the conductive material. The contact liners may include TiN. 
         [0079]    While the methods and structures disclosed herein have been particularly shown and described with respect to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in forms and details may be made without departing from the spirit and scope of the present application. It is therefore intended that the methods and structures disclosed herein not be limited to the exact forms and details described and illustrated, but fall within the scope of the appended claims.