Patent Publication Number: US-2023144407-A1

Title: Self aligned top contact for vertical transistor

Description:
BACKGROUND 
     The present invention generally relates to semiconductor structures, and more particularly to a self-aligned top contact for a vertical transistor. 
     Vertical transistors are an attractive option for technology scaling for 5 nm and beyond technologies. Vertical transistors have 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 transistor. By using a vertical design, it is possible to increase packing density. That is, by having the channel perpendicular to the substrate, vertical transistors improve the scaling limit beyond lateral transistors. 
     SUMMARY 
     According to an embodiment of the present invention, a semiconductor structure is provided. The semiconductor structure may include a bottom source drain region arranged on a substrate; a semiconductor channel region extending vertically upwards from a top surface of the bottom source drain region; a metal gate disposed around the semiconductor channel region; a top source drain region above the semiconductor channel region; and a top contact partially embedded into the top source drain region. 
     According to another embodiment, a semiconductor structure is provided. The semiconductor structure may include bottom source drain region arranged on a substrate; a semiconductor channel region extending vertically upwards from a top surface of the bottom source drain region; a metal gate disposed around the semiconductor channel region; a top source drain region above the semiconductor channel region; a top contact partially embedded into the top source drain region; and a gate liner separating the metal gate and the top source drain from an interlevel dielectric layer, wherein an uppermost surface of the gate liner is substantially flush with an uppermost surface of the top source drain region. 
     According to another embodiment, a method is provided. The method may include forming a bottom source drain region arranged on a substrate; forming a semiconductor channel region extending vertically upwards from a top surface of the bottom source drain region; forming a metal gate disposed around the semiconductor channel region; forming a top source drain region above the semiconductor channel region; and forming a top contact partially embedded into the top source drain region. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following detailed description, given by way of example and not intended to limit the invention solely thereto, will best be appreciated in conjunction with the accompanying drawings, in which: 
         FIG.  1    is a cross-sectional view of a semiconductor structure during an intermediate step of a method of fabricating a vertical transistor with a self-aligned top contact according to an exemplary embodiment; 
         FIG.  2    is a cross-sectional view of the semiconductor structure after forming bottom spacers according to an exemplary embodiment; 
         FIG.  3    is a cross-sectional view of the semiconductor structure after depositing a gate dielectric and metal gate according to an exemplary embodiment; 
         FIG.  4    is a cross-sectional view of the semiconductor structure after forming a gate liner according to an exemplary embodiment; 
         FIG.  5    is a cross-sectional view of the semiconductor structure after removing portions of the gate liner according to an exemplary embodiment; 
         FIG.  6    is a cross-sectional view of the semiconductor structure after recessing the metal gate and the gate dielectric according to an exemplary embodiment; 
         FIG.  7    is a cross-sectional view of the semiconductor structure after forming top spacers according to an exemplary embodiment; 
         FIG.  8    is a cross-sectional view of the semiconductor structure after forming top source drain regions according to an exemplary embodiment; 
         FIG.  9    is a cross-sectional view of the semiconductor structure after polishing and forming a planarization layer according to an exemplary embodiment; 
         FIG.  10    is a cross-sectional view of the semiconductor structure after removing the masks and forming junctions according to an exemplary embodiment; 
         FIG.  11    is a cross-sectional view of the semiconductor structure after forming top contact metal according to an exemplary embodiment; 
         FIG.  12    is a cross-sectional view of the semiconductor structure after forming contact structures according to an exemplary embodiment; and 
         FIG.  13    is a cross-sectional view of  FIG.  12    along the length of the fin according to an exemplary embodiment. 
     
    
    
     The drawings are not necessarily to scale. The drawings are merely schematic representations, not intended to portray specific parameters of the invention. For clarity and ease of illustration, scale of elements may be exaggerated. The drawings are intended to depict only typical embodiments of the invention. In the drawings, like numbering represents like elements. 
     DETAILED DESCRIPTION 
     Detailed embodiments of the claimed structures and methods are disclosed herein; however, it can be understood that the disclosed embodiments are merely illustrative of the claimed structures and methods that may be embodied in various forms. This invention may, however, be embodied in many different forms and should not be construed as limited to the exemplary embodiments set forth herein. In the description, details of well-known features and techniques may be omitted to avoid unnecessarily obscuring the presented embodiments. 
     References in the specification to “one embodiment”, “an embodiment”, “an example embodiment”, etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. 
     For purposes of the description hereinafter, the terms “upper”, “lower”, “right”, “left”, “vertical”, “horizontal”, “top”, “bottom”, and derivatives thereof shall relate to the disclosed structures and methods, as oriented in the drawing figures. It will be understood that when an element as a layer, region or substrate is referred to as being “on” or “over” another element, it can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” or “directly over” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. Also, the term “sub-lithographic” may refer to a dimension or size less than current dimensions achievable by photolithographic processes, and the term “lithographic” may refer to a dimension or size equal to or greater than current dimensions achievable by photolithographic processes. The sub-lithographic and lithographic dimensions may be determined by a person of ordinary skill in the art at the time the application is filed. 
     The terms substantially, substantially similar, about, or any other term denoting functionally equivalent similarities refer to instances in which the difference in length, height, or orientation convey no practical difference between the definite recitation (e.g. the phrase sans the substantially similar term), and the substantially similar variations. In one embodiment, substantial (and its derivatives) denote a difference by a generally accepted engineering or manufacturing tolerance for similar devices, up to, for example, 10% deviation in value or 10° deviation in angle. 
     In the interest of not obscuring the presentation of embodiments of the present invention, in the following detailed description, some processing steps or operations that are known in the art may have been combined together for presentation and for illustration purposes and in some instances may have not been described in detail. In other instances, some processing steps or operations that are known in the art may not be described at all. It should be understood that the following description is rather focused on the distinctive features or elements of various embodiments of the present invention. 
     Vertical transport FETs (VTFET) have known advantages over conventional FinFETs in terms of density, performance, power consumption, and integration. However, device performance of VTFET is limited by resistance at the top source drain due to limited contact area between the relatively small top source drain epi and the metal top contact. Current lithographic patterning edge accuracy cannot offer the process margins for metal top contacts in a tight pitch designs, for example SRAM devices where distance between nFET to pFET is only 30 nm. Metal top contacts along the fin have to be smaller than the fin to avoid electrical shorts between neighboring nFET and pFET. 
     Embodiments of the present disclose a VTFET with a self-aligned wrap-around-contact architecture to reduce the resistance and to improve VTFET performance. The self-aligned wrap-around-contact architecture proposed herein increases the contact area between the top source drain epi and the metal top contact. 
     The present invention generally relates to semiconductor structures, and more particularly to a self-aligned top contact for a vertical transistor. More specifically, the self-aligned wrap-around contact disclosed herein reduces resistance between the top source drain region and the metal top contact of the vertical transistor by increasing the contact area between the top source drain region and the contact. Embodiments of the present invention propose increasing the contact area between the top source drain region and the contact at least 1.5-2 times that of conventional vertical transistor structures. Lowering the resistance will in turn increase device performance. The top source drain region of the proposed structure is self-aligned to further maximize the contact area between the top source drain region and the contact. The self-aligned top source drain metal contact proposed herein allows for full use of the fin length while eliminating any risk of electrical short between nFET and pFET in tight pitch configurations. Exemplary embodiments of a wrap-around contact for a vertical transistor are described in detail below by referring to the accompanying drawings in  FIGS.  1  to  13   . Those skilled in the art will readily appreciate that the detailed description given herein with respect to these figures is for explanatory purposes as the invention extends beyond these limited embodiments. 
     Referring now to  FIG.  1   , a structure  100  is shown during an intermediate step of a method of fabricating a vertical transistor with a self-aligned top metal contact according to an embodiment of the invention. The structure  100  illustrated in  FIG.  1    includes a semiconductor substrate  102  (hereinafter “substrate”), semiconductor fins  104 , bottom source drain regions  106 , and shallow trench isolation regions  108  (hereinafter “STI regions”) formed thereon. 
     The substrate  102  is shown and may be formed from any appropriate material including, e.g., bulk semiconductor or a semiconductor-on-insulator layered structure. Illustrative examples of suitable materials for the substrate  102  include, but are not limited to, silicon, silicon germanium, carbon doped silicon germanium, carbon doped silicon, epitaxial silicon, and multi-layers thereof. Although silicon is the predominantly used semiconductor material in wafer fabrication, alternative semiconductor materials can be employed, such as, but not limited to, gallium arsenide, indium phosphide, indium gallium arsenide, indium arsenide, gallium, cadmium telluride and zinc selenide. 
     In the present embodiment, the substrate  102  is a bulk semiconductor substrate. By “bulk” it is meant that the semiconductor substrate  102  is entirely composed of at least one of the above materials listed above. In an embodiment, the substrate  102  can be entirely composed of silicon. In other embodiments, the semiconductor substrate  102  may include a multilayered semiconductor material stack including at least two different semiconductor materials, as defined above. In an embodiment, the multilayered semiconductor material stack may include, in any order, a stack of silicon and a silicon germanium alloy. In another embodiment, the semiconductor substrate  102  may include a single crystalline semiconductor material. Such single crystal materials may have any of the well-known crystal orientations. For example, the crystal orientation of the semiconductor substrate  102  may be {100}, {110}, or {111}. Other crystallographic orientations besides those specifically mentioned can also be used in the present application. 
     The semiconductor fins  104  are formed from a semiconductor layer (not shown), and form the channel region of the vertical transistor device depicted by the structure  100 . First, masks  110  are deposited on the semiconductor layer. The masks  110  define regions for the semiconductor fins  104 . The semiconductor layer is etched or patterned using an anisotropic etch such as, for example, reactive ion etching, to remove material that is not covered by the masks  110  to form the semiconductor fins  104 . Although the present application describes and illustrates forming two semiconductor fins  104 , the same process may be used to form a single semiconductor fin, or more than two semiconductor fins. 
     As used herein, a “semiconductor fin” refers to a semiconductor material that includes a pair of vertical sidewalls that are parallel to each other. As used herein, a surface is “vertical” if there exists a vertical plane from which the surface does not deviate by more than three times the root mean square roughness of the surface. In an embodiment, each semiconductor fin  104  has a height ranging from approximately 20 nm to approximately 200 nm, and a width ranging from approximately 5 nm to approximately 30 nm. Other heights and/or widths that are lesser than, or greater than, the ranges mentioned herein can also be used in the present application. Each semiconductor fin  104  is spaced apart from its nearest neighboring semiconductor fin  104  by a pitch ranging from approximately 20 nm to approximately 100 nm; the pitch is measured from one point, or reference surface, of one semiconductor fin to the exact same point, or reference surface, on a neighboring semiconductor fin. Also, the semiconductor fins  104  are generally oriented parallel to each other. Each semiconductor fin  104  extends upward from a top surface of the bottom source drain regions  106 . 
     In general, the bottom source drain regions  106  are arranged on the substrate  102 . Specifically, in the illustrated embodiment, the bottom source drain regions  106  are epitaxially grown within trenches or recesses formed in the substrate  102 . In other embodiments, the bottom source drain regions  106  are epitaxially grown directly on top of the substrate  102  adjacent to the semiconductor fins  104  and subsequently patterned, if needed. In yet another embodiment, the bottom source drain regions  106  are formed by doping an exposed surface of the substrate  102  using an ion implant technique. 
     It should be understood that the bottom source drain regions  106  may be either one of a source region or a drain region, as appropriate. Illustrative examples of suitable materials for the bottom source drain regions  106  include, but are not limited to, silicon, silicon germanium, carbon doped silicon germanium, and multi-layers thereof. 
     The bottom source drain regions  106  may be doped with dopant atoms. The bottom source drain regions  106  may be in-situ doped as they are grown or deposited on the substrate  102 . The dopant atoms may be an n-type dopant or a p-type dopant. Exemplary n-type dopants include phosphorus, arsenic antimony for group IV semiconductors, and selenium, tellurium, silicon, and germanium for III-V semiconductors. Exemplary p-type dopants include beryllium, zinc, cadmium, silicon, germanium, for III-V semiconductors, and boron, aluminum, and gallium for group IV semiconductors. In an embodiment, for group IV semiconductors based device, the bottom source drain regions  106  are made from doped Si: (for n-type devices) or SiGe:B (for p-type devices), with dopant concentrations ranging from approximately 2×10 20  to approximately 2.5×10 21  atoms/cm 2 , with a dopant concentration ranging from approximately 4×10 20  to approximately 1.5×10 21  atoms/cm 2  being more typical. 
     In another embodiment, the bottom source drain regions  106  may be formed from a III-V semiconductor. The term “III-V compound semiconductor” denotes a semiconductor material that includes at least one element from Group III of the Periodic Table of Elements and at least one element from Group V of the Periodic Table of Elements. Typically, the III-V compound semiconductors are binary, ternary or quaternary alloys including III/V elements. Examples of III-V compound semiconductors that can be used in the present embodiments include, but are not limited to alloys of gallium arsenide, indium arsenide, indium antimonide, indium phosphide, aluminum arsenide, indium gallium arsenide, indium aluminum arsenide, indium aluminum arsenide antimonide, indium aluminum arsenide phosphorude, indium gallium arsenide phosphorude and combinations thereof. In an embodiment, the bottom source drain regions  106  are made from doped III-V semiconductor materials with dopant concentrations ranging from approximately 1×10 18  to approximately 1×10 20  atoms/cm2, with a dopant concentration ranging from approximately 5×10 18  to approximately 8×10 19  atoms/cm2 being more typical. 
     The STI regions  108  penetrate, or extend below, the bottom source drain regions  106  and extend partially into the substrate  102 . As illustrated, trenches are formed by any appropriate technique, for example an anisotropic etch or machining. Next, the trenches are filled with a dielectric material to form the STI regions  108 . The STI regions  108  may be formed from any appropriate dielectric including, for example, silicon oxide (SiO x ) or silicon nitride (Si x N y ). 
     Referring now to  FIG.  2   , the structure  100  is shown after forming bottom spacers  114  in accordance with an embodiment of the present invention. As is shown, the bottom spacers  114  contact sidewall surfaces of a lower portion of the at least one semiconductor fin  104 . The bottom spacers  114  have a height, or thickness, that is less than a height of each semiconductor fin  104 . Stated differently, topmost surfaces of the bottom spacers  114  are vertically offset and located far beneath topmost surfaces of each mask  110 , and beneath topmost surfaces of the semiconductor fins  104 . 
     In the illustrated embodiment, the bottom spacers  114  are deposited on a top surface of the bottom source drain regions  106 . It is specifically contemplated that the bottom spacers  114  are deposited in an anisotropic manner, without accumulation on the sidewalls of the semiconductor fins  104 . This may be accomplished using, for example, gas cluster ion beam (GCIB) deposition, where the surface is bombarded by high-energy cluster ions. In alternative embodiments, other deposition techniques may be used to form the bottom spacers  114  only on the horizontal surfaces. 
     Alternatively, the bottom spacers  114  are formed by first depositing a blanket dielectric layer followed by a recess etch to remove a portion of the blanket dielectric layer. The recess etch removes a portion of the blanket dielectric layer until the bottom spacers  114  remains. In such cases, the chosen dielectric material is etched selective to the masks  110  and the semiconductor fins  104 . In an example, when the blanket dielectric layer is silicon oxide (SiO x ) and the masks  110  are silicon nitride (Si x N y ), a hydrofluoric acid or a buffered oxide etchant (i.e., a mixture of ammonium fluoride and hydrofluoric acid) may be used during the recess etch technique. 
     Suitable spacer materials from which the bottom spacers  114  are formed include, but are not limited to, oxides such as silicon oxide (SiO x ), nitrides such as silicon nitride (Si x N y ), and/or low-κ materials such as carbon-doped oxide materials containing silicon (Si), carbon (C), oxygen (O), and hydrogen (H) (SiCOH) or siliconborocarbonitride (SiBCN). The term “low-κ” as used herein refers to a material having a relative dielectric constant κ which is lower than that of silicon dioxide. 
     Referring now to  FIG.  3   , the structure  100  is shown after depositing a gate dielectric  116  and metal gate  118  in accordance with an embodiment of the present invention. In doing so, the gate dielectric  116  is conformally deposited directly on exposed surfaces of the fins  104 , or channel, and the bottom spacers  114 . By “conformal” it is meant that a material layer has a continuous thickness. For example, a continuous thickness generally means a first thickness as measured from a bottom surface to a topmost surface that is the same as a second thickness as measured from an inner sidewall surface to an outer sidewall surface. In another embodiment, the gate dielectric  116  may be a non-conformal layer. 
     The gate dielectric  116  is composed of a gate dielectric material. The gate dielectric  116  can be an oxide, nitride, and/or oxynitride. In an example, the gate dielectric  116  can be a high-k material having a dielectric constant greater than silicon dioxide. Exemplary high-k dielectrics include, but are not limited to, 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 , SiON, SiN x , a silicate thereof, and an alloy thereof. Each value of x is independently from 0.5 to 3 and each value of y is independently from 0 to 2. In some embodiments, a multilayered gate dielectric structure including different gate dielectric materials, e.g., silicon dioxide, and a high-k gate dielectric can be formed and used as the gate dielectric  116 . 
     The gate dielectric  116  can be formed by any deposition technique including, for example, chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), physical vapor deposition (PVD), sputtering, or atomic layer deposition. In an embodiment, the gate dielectric  116  can have a thickness in ranging from approximately 1 nm to approximately 10 nm. Other thicknesses that are lesser than, or greater than, the aforementioned thickness range can also be employed for the gate dielectric  116 . 
     In an embodiment, the metal gate  118  is composed of an n-type work function metal. As used herein, an “n-type work function metal” is a metal that effectuates an n-type threshold voltage shift. “N-type threshold voltage shift” as used herein means a shift in the Fermi energy of an n-type semiconductor device towards a conduction band of silicon in a silicon-containing substrate of the n-type semiconductor device. The “conduction band” is the lowest lying electron energy band of the doped material that is not completely filled with electrons. In an embodiment, the work function of the n-type work function metal ranges from 4.1 eV to 4.3 eV. In an embodiment, the n-type work function metal is composed of at least one of TiAl, TaN, TiN, HfN, HfSi, or combinations thereof. The n-type work function metal can be formed using chemical vapor deposition atomic layer deposition, sputtering or plating. 
     In another embodiment, the metal gate  118  may be a p-type work function metal. As used herein, a “p-type work function metal” is a metal that effectuates a p-type threshold voltage shift. In an embodiment, the work function of the p-type work function metal ranges from 4.9 eV to 5.2 eV. As used herein, “threshold voltage” is the lowest attainable gate voltage that will turn on a semiconductor device, for example, transistor, by making the channel of the device conductive. The term “p-type threshold voltage shift” as used herein means a shift in the Fermi energy of a p-type semiconductor device towards a valence band of silicon in the silicon containing substrate of the p-type semiconductor device. A “valence band” is the highest range of electron energies where electrons are normally present at absolute zero. In an embodiment, the p-type work function metal may be composed of titanium, titanium nitride or titanium carbide. The p-type work function metal may also be composed of TiAlN, Ru, Pt, Mo, Co and alloys and combinations thereof. In an embodiment, the p-type work function metal can be formed by, a physical vapor deposition method, such as sputtering, chemical vapor deposition or atomic layer deposition. 
     The metal gate  118  can be formed by any deposition technique including, for example, chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), physical vapor deposition (PVD), sputtering, or atomic layer deposition. Like the gate dielectric  116 , in some embodiments, the metal gate  118  is also a conformal layer. In an embodiment, the metal gate  118  can have a thickness in a ranging from approximately 1 nm to approximately 10 nm. Other thicknesses that are lesser than, or greater than, the aforementioned thickness range can also be employed for the used in providing the metal gate  118 . It is critical to monitor and control forming of the metal gate  118  to prevent pinch off between adjacent devices, especially, in narrow-pitch configurations. As illustrated, the bottom spacers  114  separate the bottom source drain regions  106  from the metal gate  118 . 
     Referring now to  FIG.  4   , the structure  100  is shown after forming a gate liner  120  and an interlevel dielectric layer  122  are formed in accordance with an embodiment of the present invention. In doing so, the gate liner  120  is conformally deposited directly on exposed surfaces of metal gate  118 . The gate liner  120  serves as an etch stop and is composed of known etch selective dielectric materials. The gate liner  120  can be any appropriate oxide, nitride, and/or oxynitride. In the present embodiment, the gate liner  120  should be any dielectric material capable of being selectively removed relative to the metal gate  118  and the interlevel dielectric layer  122 . For example, the gate liner  120  of the present embodiment can be made from a nitride, such as, for example, silicon nitride or SiBCN, which can be etched or removed selective to the metal gate  118  and the interlevel dielectric layer  122 . 
     The gate liner  120  can be formed by any suitable deposition technique including, for example, chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), physical vapor deposition (PVD), sputtering, or atomic layer deposition. In general, the gate liner  120  should be thick enough to provide the requisite etch stop function. In an embodiment, the gate liner  120  can have a thickness in ranging from approximately 1 nm to approximately 10 nm. Other thicknesses that are lesser than, or greater than, the aforementioned thickness range can also be employed for the gate liner  120 . 
     The interlevel dielectric layer  122  surrounds the structure shown in  FIG.  14   . The interlevel dielectric layer  122  is composed of any dielectric material that is different, in terms of composition and etch rate, than the dielectric materials of the gate liner  120 . The interlevel dielectric layer  122  may be composed of, for example, silicon oxide (SiO x ), undoped silicate glass (USG), fluorosilicate glass (FSG), borophosphosilicate glass (BPSG), a spin-on low-κ dielectric layer, a chemical vapor deposition (CVD) low-κ dielectric layer or any combination thereof. As indicated above, the term “low-κ” as used herein refers to a material having a relative dielectric constant κ which is lower than that of silicon dioxide. 
     In an embodiment, the interlevel dielectric layer  122  can be formed using a deposition technique including, for example, chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), physical vapor deposition (PVD), atomic layer deposition (ALD), evaporation, spin-on coating, or sputtering. After deposition, a planarization technique such as, for example, chemical mechanical planarization (CMP) and/or grinding is applied. The planarization technique removes excess material of the interlevel dielectric layer  122  and continues polishing until the uppermost surfaces of the gate liner  120  are exposed. After polishing the uppermost surfaces of the gate liner  120  are flush, or substantially flush, with an uppermost surface of the interlevel dielectric layer  22 . In another embodiment, interlevel dielectric layer  122  may include a self-planarizing material such as a spin-on glass (SOG) or a spin-on low-κ dielectric material such as SiLK™. Doing so may avoid the need to perform a subsequent planarizing step. 
     Referring now to  FIG.  5   , the structure  100  is shown after removing portions of the gate liner  120  in accordance with an embodiment of the present invention. More specifically, the gate liner  120  is recessed to expose top portions of the metal gate  118 . The gate liner  120  is recessed or etched selective to the metal gate  118  and the interlevel dielectric layer  122 . The gate liner  120  can be recessed using any suitable method, such as a wet etch, a dry etch, or a combination of sequential wet and/or dry etches. 
     Referring now to  FIG.  6   , the structure  100  is shown after recessing the metal gate  118  and the gate dielectric  116  in accordance with an embodiment of the present invention. More specifically, the metal gate  118  and the gate dielectric  116  are recessed to expose top portions of the semiconductor fins  104 . The metal gate  118  and the gate dielectric  116  are recessed or etched selective to the gate liner  120  and the interlevel dielectric layer  122 , and the masks  110 . 
     The metal gate  118  and the gate dielectric  116  can be recessed using any suitable method, such as a wet etch, a dry etch, or a combination of sequential wet and/or dry etches. In an embodiment, the metal gate  118  and the gate dielectric  116  could be recessed using an anisotropic etch such as, for example, reactive ion etching. Alternatively, a wet etching technique may be used to recess the metal gate  118  and the gate dielectric  116 . 
     Etching will would remove top portions of the metal gate  118  and the gate dielectric  116 , and create an opening  124 , or space, between the semiconductor fins  104  and the gate liner  120 , as illustrated. After etching, upper surfaces of the metal gate  118  and the gate dielectric  116  will be below an upper surface of the semiconductor fins  104 . 
     It is critical that the metal gate  118  and the gate dielectric  116  are recessed below an upper surface of the semiconductor fins  104  to ensure isolation between gate metal and the top source drain (See  FIG.  12   ). If the metal gate  118  and the gate dielectric  116  are not recessed below the top surface of the semiconductor fins  104 , a later formed source drain epitaxy may contact the metal gate  118  and create a short. As such, controlling the recess depth of the metal gate  118  and the gate dielectric  116  below the tops of the semiconductor fins  104  will allow for accurate top junction definition as well as define the channel length. 
     Referring now to  FIG.  7   , the structure  100  is shown after forming top spacers  126  in accordance with an embodiment of the present invention. More specifically, the top spacers  126  may be formed directly on top of upper surfaces of the metal gate  118  and the gate dielectric  116  exposed by the prior recessing. 
     The top spacers  126  may be formed using a deposition technique followed by a spacer etch (anisotropic etch). For example, techniques for depositing the top spacers  126  include, for example, chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), physical vapor deposition (PVD), atomic layer deposition (ALD), evaporation, spin-on coating, or sputtering. For example, techniques for etching the top spacers  126  include dry etching techniques, such as, for example, reactive ion etching, ion beam etching, plasma etching or laser ablation. The top spacers  126  are composed of any dielectric material that is similar, in terms of composition and etch rate, to the bottom spacers  114  described above. 
     Referring now to  FIG.  8   , the structure  100  is shown after forming top source drain regions  128  in accordance with an embodiment of the present invention. The top source drain regions  128  are formed directly on the top spacers  126 , within the opening  124 . As such, the top source drain regions  128  directly contact exposed top sidewall portions of the semiconductor fins  104  of the semiconductor fins  104 , the top spacers  126  separate and isolate the top source drain regions  128  from the metal gate  118 , and the gate liner  120  separates the top source drain regions  128  from the interlevel dielectric layer  122 . Of note, a bottom surface of each top source drain region  128  is below a top surface of each semiconductor fin  104 . 
     The top source drain regions  128  can be epitaxially grown from exposed sidewalls of the semiconductor fins  104  using conventional techniques similar to the bottom source drain regions  106 . Additionally, the top source drain regions  128  can be formed from similar materials and similar dopant concentrations as bottom source drain regions  106 . In the present embodiment, the top source drain regions  128  should be grown to completely fill the opening  124 . In the present application, it should be noted the top source drain regions  128  are grown directly from exposed sidewalls of each semiconductor fin  104  and cover the masks  110 , whereas conventional techniques would normally remove the mask  110  prior to forming the top source drain regions  128 . In this case, the masks  110  remain and function as a placeholder for subsequent processing. Additionally, the process may include overgrowing the top source drain regions  128  above a top surface of the interlevel dielectric layer  122 , after which any excess material will be removed by a subsequent chemical mechanical planarization technique. At this stage of fabrication, the top source drain regions  128  does not undergo a high temperature anneal in order to preserve the integrity of the metal gate  132 . 
     Referring now to  FIG.  9   , the structure  100  is shown after polishing and forming a planarization layer  130  in accordance with an embodiment of the present invention. 
     After forming top source drain regions  128 , a planarization technique such as, for example, chemical mechanical planarization and/or grinding is applied. The planarization technique removes any excess material of the top source drain regions  128  remaining on top of the interlevel dielectric layer  122 . Polishing continues until the masks  110  are exposed. After polishing upper surfaces of the interlevel dielectric layer  122 , gate liner  120 , the top source drain regions  128 , and the masks  110  are flush, or substantially flush, with each other. 
     Next, the planarization layer  130  is deposited and subsequently patterned to expose the masks  110 . Patterning of the planarization layer  130  does not need to align with edges of the interlevel dielectric layer  122 , the gate liner  120 , or the top source drain regions  128 , but instead must expose the masks  110  in preparation for their subsequent removal. 
     The planarization layer  130  can be an organic planarization layer (OPL) or a layer of material that is capable of being planarized or etched by known techniques. In an embodiment, for example, the planarization layer  130  can be an amorphous carbon layer able to withstand the high temperatures of subsequent processing steps. The planarization layer  130  can preferably have a thickness sufficient to cover existing structures. After deposition of the planarization layer  130 , a dry etching technique is applied to pattern the planarization layer  130  and expose an uppermost surface of the masks  110 . 
     Referring now to  FIG.  10   , the structure  100  is shown after removing the masks  110  and forming junctions  132  in accordance with an embodiment of the present invention. 
     The masks  110  are removed using an etch selective to the top source drain regions  128  and the semiconductor fins  104 . In an embodiment, when the masks  110  are composed of silicon nitride (Si x N y ), a hot (around 150° C. to 180° C.) phosphoric acid solution may be used to remove the masks  110  selective to the planarization layer  130  and the top source drain regions  128 . Removing the masks  110  creates an opening  134  and exposes topmost surfaces of the semiconductor fins  104 . 
     After the masks  110  are removed, an ion implant technique and thermal process are performed to form the junction  132 . First, the semiconductor fins  104  are doped with dopant atoms using an implantation technique in a similar fashion as described above with respect to forming the bottom source drain regions  106 . Like above, the dopant atoms may be an n-type dopant or a p-type dopant. Exemplary n-type dopants include phosphorus, arsenic antimony, selenium, tellurium, silicon, and germanium. Exemplary p-type dopants include beryllium, zinc, cadmium, silicon, germanium, boron, aluminum, and gallium. Typically, both the bottom source drain regions  106  and the junction  132  will be doped with the same type dopants. 
     Second, a thermal process is used to diffuse the implanted dopants into the semiconductor fin  104 , or top of the channel, to form the junction  132 . In some cases, dopants from the top source drain regions  128  may also diffuse into the semiconductor fin  104  and contribute to formation of the junction  132 . In at least one embodiment, the thermal process may include a high temperature spike anneal, or a laser spike anneal. Finally, the planarization layer  130  is removed using known techniques, for example, by ashing. 
     Referring now to  FIG.  11   , the structure  100  is shown after forming top contact metal  136  in the opening and on topmost surfaces of the semiconductor fins  104  in accordance with an embodiment of the present invention. The openings  134  are filled with a conductive material to form the top contact metal  136 . For example, each top contact metal ( 136 ) is surrounded by each top source drain region ( 128 ) and directly contacts a top surface of the semiconductor channel region. 
     The top contact metal  136  can be formed by depositing a conductive material in the openings  134 . The top contact metal  136  may include any suitable conductive material, such as, for example, copper, aluminum, tungsten, cobalt, or alloys thereof. Examples of deposition techniques that can be used in providing the spacer material include, for example, chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), or atomic layer deposition (ALD). In some cases, an electroplating technique can be used to form the top contact metal  136 . 
     After deposition, a planarization technique such as, for example, chemical mechanical planarization (CMP) and/or grinding is applied. The planarization technique removes excess conductive material of the top contact metal  136  from above the interlevel dielectric layer  122 , the gate liner  120 , and the top source drain regions  128 . After polishing, upper surfaces of the top contact metal  136  are flush, or substantially flush, with upper surface of the interlevel dielectric layer  122 , the gate liner  120 , and the top source drain regions  128 . 
     As illustrated in  FIG.  11   , the top contact metal  136  is embedded into the top source drain regions  128 . Doing so increased the contact area between the top source drain regions  128  and the top contact metal  136  thereby reducing contact resistance. The contact area between a top source drain and a top contact of conventional devices is limited to the area of the uppermost surface of the top source drain. Here, the contact area between the top source drain regions  128  and the top contact metal  136  is equal to the surface area of the top contact metal  136 , or the cumulative surface areas of all sidewalls of the top contact metal  136 . 
     Referring now to  FIG.  12   , the structure  100  is shown after forming contact structures  142  in accordance with an embodiment of the present invention. First, an etch stop layer  138  is deposited on top of the structure  100  followed by another interlevel dielectric layer  140 . Next, contact trenches are formed in the interlevel dielectric layer  140  and the etch stop layer  138 , which expose upper surfaces of the gate liner  120 , the top source drain regions  128 , and the top contact metal  136 . Finally, the contact trenches are filled with a conductive material to form the contact structures  142 . The contact structures  142  can be formed by depositing a conductive material in the contact trench. As such, in at least on embodiment, the conductive material of the contact structures  142  is deposited directly on exposed upper surfaces of the gate liner  120 , the top source drain regions  128 , and the top contact metal  136 . Furthermore, it is noted that a width of the top contact metal  136  (i.e. lower portion of the self-aligned top metal contact) is smaller than a width of contact structures  142  (i.e. upper portion of the self-aligned top metal contact). 
     The contact structures  142  may include any suitable conductive material, such as, for example, copper, aluminum, tungsten, cobalt, or alloys thereof. Examples of deposition techniques that can be used in providing the spacer material include, for example, chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), or atomic layer deposition (ALD). In some cases, an electroplating technique can be used to form the contact structures  142 . 
     After deposition, a planarization technique such as, for example, chemical mechanical planarization (CMP) and/or grinding is applied. The planarization technique removes excess conductive material of the contact structures  142  from above the additional interlevel dielectric layer  140 . After polishing, upper surfaces of the contact structures  142  are flush, or substantially flush, with upper surface of the additional interlevel dielectric layer  140 . 
     As illustrated in  FIG.  12   , the vertical transistor device represented by the structure  100  in this example has some distinctive notable features. For instance, the structure  100  includes a self-aligned top metal contact. More specifically, both the top contact metal  136  and the contact structures  142  make-up the self-aligned top metal contact. The self-aligned top metal contact is particularly beneficial because it reduces the external resistance and improves vertical device performance over conventional vertical device designs. More specifically, performance of conventional vertical device designs is limited by the external resistance at the top source drain region due to the difficulty in forming a large contact area between source drain epi ( 128 ) and the top contact ( 136 / 142 ). The self-aligned top metal contact proposed herein reduces the external resistance by greatly increasing the contact area between the top contact ( 136 / 140 ) and the top source drain regions  128 , as discussed in detail above. 
     In practice, the contact area between the top source drain regions  128  and the top contact ( 136 / 142 ) can be as much as 1.5-2 times conventional devices. For example, the contact area between the top source drain regions  128  and the top contact ( 136 / 142 ) in the present invention involves the sidewalls and top surfaces of the top contact metal  136 . In contrast, the contact area between a conventional top contact placed directly on a conventional top source drain region is limited to the surface area of that single interface. 
     For exemplary purposes only, assume each semiconductor fin  104  is 6 nm wide and 10 nm long, the gate dielectric  116  is 2 nm thick (wide), the metal gate  118  is 5 nm thick (wide), and the mask  110  is 15 nm tall. According to the present invention, the contact area between the top source drain regions  128  and the top contact ( 136 / 142 ) would be 500 nm 2 , or (5+2+15+6+15+2+5)* 10 ). In contrast, using the same dimensions, the contact area between a conventional top contact placed directly on a conventional top source drain region would be 200 nm 2 , or (5+2+6+2+5)*10). In all cases, the contact area gain will be a function of the mask  110  height. 
     The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The terminology used herein was chosen to best explain the principles of the embodiment, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.