Patent Publication Number: US-11043587-B2

Title: Fabrication of vertical fin transistor with multiple threshold voltages

Description:
BACKGROUND 
     Technical Field 
     The present invention relates to vertical fin field effect transistors (finFETs) with graded compound semiconductors, and more particularly to a pair of vertical finFETs having fin channels with different fin widths and semiconductor compositions. 
     Description of the Related Art 
     A Field Effect Transistor (FET) typically has a source, a channel, and a drain, where current flows from the source to the drain, and a gate that controls the flow of current through the channel. Field Effect Transistors (FETs) can have a variety of different structures, for example, FETs have been formed with the source, channel, and drain formed in the substrate material itself, where the current flows horizontally (i.e., in the plane of the substrate), and finFETs have been formed with the channel extending outward from the substrate, but where the current also flows horizontally. The channel for the finFET can be an upright slab of thin rectangular Si, commonly referred to as the fin with a gate on the fin, as compared to a MOSFET with a single planar gate. Depending on the doping of the source and drain, an n-FET or a p-FET may be formed. 
     Examples of FETs can include a metal-oxide-semiconductor field effect transistor (MOSFET) and an insulated-gate field-effect transistor (IGFET). Two FETs also may be coupled to form a complementary metal oxide semiconductor (CMOS), where a p-channel MOSFET and n-channel MOSFET are connected together. 
     With ever decreasing device dimensions, forming the individual components and electrical contacts becomes more difficult. An approach is therefore needed that retains the positive aspects of traditional FET structures, while overcoming the scaling issues created by forming smaller device components. 
     SUMMARY 
     A vertical fin field effect transistor including a doped region in a substrate, wherein the doped region has the same crystal orientation as the substrate, a first portion of a vertical fin on the doped region, wherein the first portion of the vertical fin has the same crystal orientation as the substrate and a first portion width, a second portion of the vertical fin on the first portion of the vertical fin, wherein the second portion of the vertical fin has the same crystal orientation as the first portion of the vertical fin, and the second portion of the vertical fin has a second portion width less than the first portion width, a gate structure on the second portion of the vertical fin, and a source/drain region on the top of the second portion of the vertical fin. 
     A pair of vertical fin field effect transistors including a first doped region in a substrate, and a second doped region in the substrate adjacent to the first doped region, wherein the first doped region and second doped region have the same crystal orientation as the substrate, an unmodified vertical fin on the first doped region, wherein the unmodified vertical fin has the same crystal orientation as the substrate and an unmodified width, a modified vertical fin on the second doped region, wherein the modified vertical fin has the same crystal orientation as the substrate and at least a portion of the modified vertical fin has a modified width less than the unmodified width, a gate structure on the modified vertical fin, and a gate structure on the unmodified vertical fin. 
     A method of fabricating one or more vertical fin field effect transistors including epitaxially growing a first vertical fin of silicon-germanium on a substrate, forming a cap on the top surface of the first vertical fin, oxidizing the sidewalls of the first vertical fin to form silicon oxide layers at least on opposite sides of the first vertical fin, and increasing the germanium concentration of the first vertical fin, and removing the silicon oxide layers to form a portion of the first vertical fin having a reduced width. 
     These and other features and advantages will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The disclosure will provide details in the following description of preferred embodiments with reference to the following figures wherein: 
         FIG. 1  is a cross-sectional view of a substrate in accordance with an exemplary embodiment; 
         FIG. 2  is a cross-sectional view of a substrate and isolation region in accordance with an exemplary embodiment; 
         FIG. 3  is a cross-sectional view of a substrate with vertical fins in accordance with an exemplary embodiment; 
         FIG. 4  is a cross-sectional view of a substrate with a capped vertical fin in accordance with an exemplary embodiment; 
         FIG. 5  is a cross-sectional view of a capped vertical fin and an oxidized vertical fin in accordance with an exemplary embodiment; 
         FIG. 6  is a cross-sectional view of a decapped vertical fin and an oxidized vertical fin in accordance with an exemplary embodiment; 
         FIG. 7  is a cross-sectional view of an unmodified vertical fin and a modified vertical fin in accordance with an exemplary embodiment; 
         FIG. 8  is a cross-sectional view of a gated unmodified vertical fin and a gated modified vertical fin in accordance with an exemplary embodiment; 
         FIG. 9  is a cross-sectional view of a gated unmodified vertical fin and a gated modified vertical fin in accordance with an exemplary embodiment; 
         FIG. 10  is a cross-sectional view of a modified and unmodified vertical fin with source regions in accordance with an exemplary embodiment; 
         FIG. 11  is a cross-sectional view of a pair of vertical finFETs in accordance with an exemplary embodiment; 
         FIG. 12  is a cross-sectional view of a substrate in accordance with an exemplary embodiment; 
         FIG. 13  is a cross-sectional view of a substrate and isolation region in accordance with an exemplary embodiment; 
         FIG. 14  is a cross-sectional view of a substrate, isolation region, and hardmask layer in accordance with an exemplary embodiment; 
         FIG. 15  is a cross-sectional view of a substrate, isolation region, and patterned hardmask layer in accordance with an exemplary embodiment; 
         FIG. 16  is a cross-sectional view of a substrate and compound semiconductor formed in a patterned hardmask layer in accordance with an exemplary embodiment; 
         FIG. 17  is a cross-sectional view of a substrate and compound semiconductor formed in a patterned hardmask layer with a protective layer in accordance with an exemplary embodiment; 
         FIG. 18  is a cross-sectional view of a substrate and compound semiconductor formed in a patterned hardmask layer with a patterned protective layer in accordance with an exemplary embodiment; 
         FIG. 19  is a cross-sectional view of a substrate with an oxidized portion of a compound semiconductor and a protected portion of a compound semiconductor in accordance with an exemplary embodiment; 
         FIG. 20  is a cross-sectional view of a substrate with a graded compound semiconductor and an unmodified compound semiconductor in accordance with an exemplary embodiment; 
         FIG. 21  is a cross-sectional view of a substrate with an unmodified fin and a modified fin with a vertically graded composition in accordance with an exemplary embodiment; and 
         FIG. 22  is a cross-sectional view of a substrate with a gated unmodified fin and a gated fin with a vertically graded composition in accordance with an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Principles and embodiments of the present disclosure relate generally to vertical finFETs including vertical fins having modified concentrations of compound semiconductors, and approaches to fabricating vertical fins with modified or graded concentrations of compound semiconductors. Two vertical fins having different widths and different concentrations of semiconductor components may be formed adjacent to each other, where the concentrations of semiconductor components may be modified by selective oxidation. The modified concentrations of semiconductor components may provide multiple threshold voltages, V t , to tune the power and performance characteristics of the finFET device(s). 
     One or more embodiments relate to a channel last replacement metal gate vertical finFET having multiple threshold voltages. Multiple threshold voltages may be achieved by forming channel regions having different dopant concentrations. A vertical finFET having multiple threshold voltages may be fabricated from silicon-germanium (SiGe) by epitaxially growing one or more SiGe fins from a silicon substrate. An exposed portion of a SiGe fin may be selectively oxidized to increase the concentration of Ge in the SiGe fin. 
     In various embodiments, two vertical SiGe fins may be epitaxially grown from a silicon substrate, and one of the two vertical SiGe fins selectively oxidized to reduce the width of the vertical fin while increasing the concentration of the germanium component in the SiGe fin. 
     Principle and embodiments of the present disclosure also relate to a semiconductor device structure with conductive elements stacked on the substrate to form a conductive path normal to the surface of the substrate on which the semiconductor structure is formed. An embodiment relates generally to a vertical finFET structure that provides multiple threshold voltages. In an embodiment, a finFET semiconductor device has the drain, fin channel, and source device components arranged perpendicular to the plane of the substrate surface, which is referred to as a vertical stack. A vertically stacked finFET can have a longer gate length (i.e., height) and larger dielectric spacer than a horizontal (i.e., having the drain, fin channel, and source device components arranged parallel with the plane of the substrate surface) finFET having comparable contact gate pitch. 
     It is to be understood that the present invention will be described in terms of a given illustrative architecture; however, other architectures, structures, substrate materials and process features and steps may be varied within the scope of the present invention. 
     It will also be understood that when an element such 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. 
     The present embodiments may include a design for an integrated circuit chip, which may be created in a graphical computer programming language, and stored in a computer storage medium (such as a disk, tape, physical hard drive, or virtual hard drive such as in a storage access network). If the designer does not fabricate chips or the photolithographic masks used to fabricate chips, the designer may transmit the resulting design by physical means (e.g., by providing a copy of the storage medium storing the design) or electronically (e.g., through the Internet) to such entities, directly or indirectly. The stored design is then converted into the appropriate format (e.g., GDSII) for the fabrication of photolithographic masks, which typically include multiple copies of the chip design in question that are to be formed on a wafer. The photolithographic masks are utilized to define areas of the wafer (and/or the layers thereon) to be etched or otherwise processed. 
     Methods as described herein may be used in the fabrication of integrated circuit chips. The resulting integrated circuit chips can be distributed by the fabricator in raw wafer form (that is, as a single wafer that has multiple unpackaged chips), as a bare die, or in a packaged form. In the latter case the chip is mounted in a single chip package (such as a plastic carrier, with leads that are affixed to a motherboard or other higher level carrier) or in a multichip package (such as a ceramic carrier that has either or both surface interconnections or buried interconnections). In any case the chip is then integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either (a) an intermediate product, such as a motherboard, or (b) an end product. The end product can be any product that includes integrated circuit chips, ranging from toys and other low-end applications to advanced computer products having a display, a keyboard or other input device, and a central processor. 
     Reference in the specification to “one embodiment” or “an embodiment” of the present principles, as well as other variations thereof, means that a particular feature, structure, characteristic, and so forth described in connection with the embodiment is included in at least one embodiment of the present principles. Thus, the appearances of the phrase “in one embodiment” or “in an embodiment”, as well any other variations, appearing in various places throughout the specification are not necessarily all referring to the same embodiment. 
     It is to be appreciated that the use of any of the following “/”, “and/or”, and “at least one of”, for example, in the cases of “A/B”, “A and/or B” and “at least one of A and B”, is intended to encompass the selection of the first listed option (A) only, or the selection of the second listed option (B) only, or the selection of both options (A and B). As a further example, in the cases of “A, B, and/or C” and “at least one of A, B, and C”, such phrasing is intended to encompass the selection of the first listed option (A) only, or the selection of the second listed option (B) only, or the selection of the third listed option (C) only, or the selection of the first and the second listed options (A and B) only, or the selection of the first and third listed options (A and C) only, or the selection of the second and third listed options (B and C) only, or the selection of all three options (A and B and C). This may be extended, as readily apparent by one of ordinary skill in this and related arts, for as many items listed. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not tended to be limiting of example embodiments. As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes” and/or “including,” when used herein, specify the presence of stated features, integers, steps, operations, elements and/or components, but do not preclude the presence or addition of one or r ore other features, integers, steps, operations, elements, components and/or groups thereof. 
     Spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper,” and the like, may be used herein for ease of description to describe one element&#39;s or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the FIGS. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the FIGs. For example, if the device in the FIGS. is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the term “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations), and the spatially relative descriptors used herein may be interpreted accordingly. In addition, it will also be understood that when a layer is referred to as being “between” two layers, it can be the only layer between the two layers, or one or more intervening layers may also be present. 
     It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another element. Thus, a first element discussed below could be termed a second element without departing from the scope of the present concept. 
     In various embodiments, the materials and layers may be deposited by physical vapor deposition (PVD), chemical vapor deposition (CVD), atomic layer deposition (ALD), molecular beam epitaxy (MBE), or any of the various modifications thereof, for example plasma-enhanced chemical vapor deposition (PECVD), metal-organic chemical vapor deposition (MOCVD), low pressure chemical vapor deposition (LPCVD), electron-beam physical vapor deposition (EB-PVD), and plasma-enhanced atomic layer deposition (PE-ALD). The depositions may be epitaxial processes, and the deposited material may be crystalline. In various embodiments, formation of a layer may be by one or more deposition processes, where, for example, a conformal layer may be formed by a first process (e.g., ALD, PE-ALD, etc.) and a fill may be formed by a second process (e.g., CVD, electrodeposition, PVD, etc.). 
     Referring now to the drawings in which like numerals represent the same or similar elements and initially to  FIG. 1 , which is a cross-sectional view of a substrate in accordance with an exemplary embodiment. 
     In one or more embodiments, a vertical fin may be formed on a crystalline substrate, where the substrate may have a surface with a lattice constant that facilitates epitaxial growth of a compound semiconductor. In various embodiments, the substrate  110  may be a semiconductor substrate, where the substrate may be a single crystal semiconductor wafer or a crystalline semiconductor layer on a supporting wafer. In various embodiments, the substrate may be silicon or silicon-germanium. The substrate may be a semiconductor-on-insulator (SeOI), silicon-on-insulator (SOI), or silicon-germanium-on-insulator (SGOI). The substrate  110  may have a &lt;100&gt; or a &lt;111&gt; crystal face exposed for subsequent depositions, where the crystal lattice may facilitate epitaxial growth. 
       FIG. 2  is a cross-sectional view of a substrate and isolation region in accordance with an exemplary embodiment. 
     In one or more embodiments, a substrate may be masked and patterned to form one or more trench(es) in which an insulator material may be formed. The trench and insulator material may provide an isolation region  120  for subsequent layers and structures formed on the substrate. In various embodiments, the isolation region  120  provides shallow trench isolation for adjacent device structures, where the device structures may be vertical finFETs. In one or more embodiments, the isolation region  120  includes an oxide, for example, silicon oxide (SiO x ). 
       FIG. 3  is a cross-sectional view of a substrate with vertical fins in accordance with an exemplary embodiment. 
     In one or more embodiments, a hardmask may be formed on the substrate  110 , patterned, and etched to expose a portion of the substrate  110 . A vertical fin  140  may be formed on at least the exposed portions of the substrate  110 , where the vertical fin  140  may be formed by any known patterning techniques, for example, sidewall image transfer, lithography and etching, and epitaxy. In various embodiments, the vertical fin  140  may be epitaxially grown from the crystalline surface of the substrate. 
     In various embodiments, the vertical fin  140  may be a compound semiconductor, for example, silicon-germanium (SiGe). In various embodiments, the crystal lattice of the exposed substrate surface may be selected to control the amount of stress induced in the epitaxially grown compound semiconductor material. In various embodiments, the substrate  110  may be silicon, and the vertical fin  140  may be SiGe. The SiGe fin may be formed by first epitaxially growing a SiGe layer on the surface of the Si substrate. A suitable patterning technique may then be applied to pattern the epitaxially grown SiGe layer into one or more vertical fin(s)  140 . In some embodiments, the substrate surface may be masked and patterned to provide exposed portions of the substrate on which vertical fin(s)  140  may be epitaxially grown. The mask may then be removed to expose the substrate surface between the vertical fin(s)  140 . 
     In one or more embodiments, the exposed portion of the substrate in the mask pattern may have a width in the range of about 5 nm to about 25 nm, or in the range of about 10 nm to about 20 nm, or in the range of about 10 nm to about 15 nm. In various embodiments, the vertical fin  140  may have a width in the range of about 5 nm to about 25 nm, or in the range of about 10 nm to about 20 nm, or in the range of about 10 nm to about 15 nm. 
     In various embodiments, a bottom spacer  130  may be formed on the portions of the substrate  110 , where the bottom spacer  130  may be between at least a doped region (e.g., source/drain) and a subsequently formed gate structure. The bottom spacer  130  may provide electrical insulation between the substrate  110  and/or doped region (e.g., source/drain) and a gate structure formed on the vertical fin  140 . The height (i.e., thickness) of the bottom spacer  130  may be less than the height of the fin  140 . In various embodiments, the bottom spacer  130  may be a nitride, for example, silicon nitride (SiN). The vertical fin(s)  140  may have a cap  150  formed on the top surface to protect the fin  140  during various processing steps of device manufacturing. 
       FIG. 4  is a cross-sectional view of a substrate with a capped vertical fin in accordance with an exemplary embodiment. 
     In one or more embodiments, a protective cap  160  may be formed over one or more vertical fin(s), where the protective cap  160  may be formed conformally over the sidewalls of the vertical fin  140 . In various embodiments, the protective cap  160  may be deposited by atomic layer deposition (ALD) or chemical vapor deposition (CVD), a variation thereof (e.g., plasma enhanced, low pressure, metal-organic, etc.), and/or a combination thereof. One or more vertical fin(s)  140  may not be covered by a protective cap  160 , such that the sidewalls of the unprotected vertical fin(s)  140  remain exposed. 
     In various embodiments, the protective cap  160  may be a nitride, for example, silicon nitride (SiN). 
       FIG. 5  is a cross-sectional view of a capped vertical fin and an oxidized vertical fin in accordance with an exemplary embodiment. 
     In one or more embodiments, the bottom spacer  130 , protective cap  160 , and exposed vertical fin  140  may be exposed to an oxidizing environment, for example, dry thermal oxidation in an oxygen atmosphere or use of an oxygen plasma. In various embodiments, the oxidizing conditions may depend on the oxidation target. The oxidation may be performed in an oxygen environment at a temperature in the range of about 800° C. to about 1100° C. for a time period in the range of about 10 seconds to about 1 hour, or about 1 minute to about 30 minutes. In various embodiments, the silicon of the SiGe of the exposed fin  140  may be selectively oxidized to increase the ratio of germanium to silicon in the remaining SiGe. 
     The oxidation of exposed SiGe may produce germanium enrichment in the exposed vertical fin  140 . A modified vertical fin  180 , having a higher Ge concentration in at least a portion of the vertical fin, may be formed from the unmodified vertical fin  140 . The oxidation of the Si may also reduce the width of at least a portion of the vertical fin  140  as the silicon is converted to silicon oxide (e.g., SiO 2 ) to form a narrower modified vertical fin  180 , and silicon oxide layer(s)  170 . In various embodiments, vertical fin(s)  140  to be modified may initially be formed with a greater width than vertical fin(s)  140  that remains unmodified. In this manner, vertical FET channels having the same width may be obtained by starting with wider vertical fin  140  for the SiGe fins intended to have greater Ge concentration to accommodate the consumption of the Si in the SiGe through the condensation oxidation. 
     In various embodiments, the modified vertical fin  180  may have a first portion  145 , which may be below the level of the bottom spacer  130 , and a second portion  181  above the level of the bottom spacer  130 , where the second portion  181  has a narrower width and a higher Ge concentration. The first portion  145  of the modified vertical fin  180  may be on the substrate and the second portion of the modified vertical fin  180  may be on the first portion  145 , which remains essentially unmodified. The first portion  145  of the vertical fin  180  may have the same crystal orientation as the substrate  110  and a first portion width, and a second portion  181  of the vertical fin  180  may have the same crystal orientation as the first portion  145  of the vertical fin, and the second portion  181  of the vertical fin may have a second portion width less than the first portion width. 
     In an exemplary embodiment, two vertical fins  140  of SiGe, referred to as a first vertical fin and a second vertical fin, may be epitaxially grown on a single crystal silicon substrate  110 . The first vertical fin  140  may be covered with a protective cap  160 , and the exposed sidewalls of the second vertical fin  140  may be oxidized to a depth less than the width of the second vertical fin  140 , where oxidation of the SiGe selectively consumes silicon to form a silicon oxide layer  170 , and redistributes the germanium at the oxidation interface. Since a portion of the second vertical fin remains covered by the bottom spacer  130  during oxidation, a first portion  145  of the second vertical fin  140  may remain unmodified. The second portion  181  of the modified vertical fin  180  may have a greater percentage of germanium than the first portion  145  of the vertical fin. The first portion  145  of the vertical fin  180  may have the same crystal orientation as the substrate  110 , and the second portion  181  of the vertical fin  180  may have the same crystal orientation as the first portion  145  of the vertical fin. 
     In various embodiments, an unmodified vertical fin  140  may have a germanium concentration in the range of about 10 at. % Ge to about 40 at. % Ge, or in the range of 20 at. % Ge to about 30 at. % Ge. In various embodiments, at least a second portion  181  of a modified vertical fin  180  may have a germanium concentration in the range of about 20 at. % Ge to about 50 at. % Ge, or in the range of 30 at. % Ge to about 50 at. % Ge, or in the range of about 20 at. % Ge to about 40 at. % Ge, although a greater or lesser germanium concentration is also considered within the scope of the disclosure. 
     In various embodiments, the modified vertical fin  180  has a lower threshold voltage, V t , than the unmodified vertical fin  140  due to the increased Ge concentration in at least the second portion  181 . While not being bound by theory, the threshold voltage may be reduced by increasing the Ge concentration due to a narrowing of the band gap of the SiGe and/or a strain change in the SiGe material. 
     In various embodiments, at least a second portion  181  of the modified vertical fin  180  may have a width in the range of about 5 nm to about 20 nm, or in the range of about 7 nm to about 18 nm, or in the range of about 7 nm to about 12 nm. The unmodified vertical fin portion  145  may retain the initial width and concentration ratio of Si to Ge, which may be essentially the same as vertical fin  140 . 
       FIG. 6  is a cross-sectional view of a decapped vertical fin and an oxidized vertical fin in accordance with an exemplary embodiment. 
     In one or more embodiments, the protective cap  160  and cap  150  may be removed to expose the unmodified vertical fin  140  and top surface of the modified vertical fin  180 . The protective cap  160  and cap  150  may be selectively removed by suitable etching. The silicon oxide layers  170  may remain on the sidewalls of the second portion  181  of the modified vertical fin  180 , and cover at least the first portion  145  of the vertical fin that remained unmodified. The silicon oxide layers may be formed at least on opposite sides of the vertical fin, and may surround the vertical fin. 
       FIG. 7  is a cross-sectional view of an unmodified vertical fin and a modified vertical fin in accordance with an exemplary embodiment. 
     In one or more embodiments, the silicon oxide layer(s)  170  may be removed from the sidewalls of the modified vertical fin  180  to expose the sidewalls of the modified vertical fin  180 . The silicon oxide layer(s)  170  may be selectively removed by suitable etching. 
       FIG. 8  is a cross-sectional view of a gated unmodified vertical fin and a gated modified vertical fin in accordance with an exemplary embodiment. 
     In one or more embodiments, a gate structure  190  may be formed on the sidewalls of unmodified vertical fin  140  and/or modified vertical fin  180 , where the gate structure  190  may include a conductive gate and an insulating layer between a vertical fin and the conductive gate. Formation of the gate structure  190  may be accomplished by suitable masking, patterning, and/or depositions. 
     In one or more embodiments, the conductive gate and an insulating layer form the gate structure  190  with the vertical fin  140 ,  180  for control of current through the vertical fin  140 ,  180 , where the gate structure  190  may be on the sidewalls and surround the vertical fin  140 ,  180 . In various embodiments, the gate structure may have a height in the range of about 10 nm to about 300 nm, or about 15 nm to about 200 nm, or about 20 nm to about 100 nm. 
     In one or more embodiments, each gate structure includes a gate dielectric layer and a gate conductor layer. The gate dielectric layer may have a thickness in the range of about 1 nm to about 5 nm. In various embodiments, the insulating layer formed on the modified vertical fin  180  may have the same thickness as the insulating layer formed on the unmodified vertical fin  140 . The gate dielectric material may be a silicon oxide (e.g., SiO2), a silicon nitride (e.g., Si 3 N 4 ), silicon oxynitride (SiON), boron nitride (e.g., BN) a high-K material, or a combination thereof. Examples of high-k materials include but are not limited to metal oxides such as hafnium oxide (e.g., HfO 2 ), hafnium silicon oxide (e.g., HfSiO 4 ), hafnium silicon oxynitride (Hf w Si x O y N z ), lanthanum oxide (e.g., La 2 O 3 ), lanthanum aluminum oxide (e.g., LaAlO 3 ), zirconium oxide (e.g., ZrO 2 ), zirconium silicon oxide (e.g., ZrSiO 4 ), zirconium silicon oxynitride (Zr w Si x O y N z ), tantalum oxide (e.g., TaO 2 , Ta 2 O 5 ), titanium oxide (e.g., TiO 2 ), barium strontium titanium oxide (e.g., BaTiO 3 —SrTiO 3 ), barium titanium oxide (e.g., BaTiO 3 ), strontium titanium oxide (e.g., SrTiO 3 ), yttrium oxide (e.g., Y 2 O 3 ), aluminum oxide (e.g., Al 2 O 3 ), lead scandium tantalum oxide (Pb(Sc x Ta 1-x )O 3 ), and lead zinc niobate (e.g., PbZn 1/3  Nb 2/3  O 3 ). The high-k may further include dopants such as lanthanum and/or aluminum. The stoichiometry of the high-K compounds may vary. 
     In one or more embodiments, the gate conductor may have a thickness in the range of about 5 nm to about 50 nm. The gate conductor material may be polycrystalline or amorphous silicon, germanium, silicon germanium, a metal (e.g., tungsten, titanium, tantalum, ruthenium, zirconium, cobalt, copper, aluminum, lead, platinum, tin, silver, gold), a conducting metallic compound material (e.g., tantalum nitride, titanium nitride, tungsten silicide, tungsten nitride, ruthenium oxide, cobalt silicide, nickel silicide), carbon nanotube, conductive carbon, graphene, or any suitable combination of these materials. The conductive material may further comprise dopants that are incorporated during or after deposition. The stoichiometry of the conductive material compounds may vary. 
       FIG. 9  is a cross-sectional view of a gated unmodified vertical fin and a gated modified vertical fin in accordance with an exemplary embodiment. 
     In one or more embodiments, a top spacer  200  may be formed on the sidewall(s) of the unmodified vertical fin  140  and/or modified vertical fin  180 , where the top spacer may electrically isolate at least the conductive gate of the gate structure  190  from a subsequently formed source/drain region of a finFET. The spacer may be an oxide, for example silicon oxide (SiO), a nitride, for example, silicon nitride (SiN), and/or an oxy-nitride, for example, silicon oxynitride (SiON). The top spacer  200  may be between the gate structure  190  and a source/drain region on the second portion  181  of the vertical fin  180 . 
       FIG. 10  is a cross-sectional view of a modified and unmodified vertical fin with source/drain regions in accordance with an exemplary embodiment. 
     In one or more embodiments, a source/drain region  210  may be formed on the top surface of the unmodified vertical fin  140  and/or modified vertical fin  180 , where the source/drain region  210  may be formed by epitaxial growth on the unmodified vertical fin  140  and/or modified vertical fin  180 . The source/drain region  210  may have the same crystal orientation as the vertical fin on which it is formed. It should be noted that the source and drain can be interchangeable between the top and bottom locations of a vertical fin. The formation may be epitaxial growth of a semiconductor material, such as silicon or silicon-germanium on a germanium-enriched SiGe vertical fin  180 . In various embodiments, the epitaxial material may be in-situ doped (i.e., during epitaxy) or ex-situ doped (i.e., after epitaxy). Doping may be p-type doping or n-type doping. 
       FIG. 11  is a cross-sectional view of a pair of vertical finFETs in accordance with an exemplary embodiment. 
     In one or more embodiments, a doped region (e.g., source/drain)  220  may be formed in the substrate  110  below unmodified vertical fin  140  and/or modified vertical fin  180 . The doped region  220  may be formed by any suitable doping technique, for example, ion implantation, in-situ doped epitaxy, etc. The doped region  220  may be n-doped or p-doped. The shallow trench isolation (STI) may be between the location of a first vertical fin and the location of a second vertical fin. In various embodiments, two adjacent doped regions  220  may be formed in the substrate  110 , and referred to as a first doped region (e.g., source/drain) and a second doped region (e.g., source/drain). The shallow trench isolation may be situated between the doped regions  220  of the unmodified vertical fin  140  and/or modified vertical fin  180  to provide electrical isolation. 
       FIG. 12  is a cross-sectional view of a substrate in accordance with an exemplary embodiment. 
     In one or more embodiments, a substrate  110  may be a semiconductor substrate, where the substrate may be a single crystal semiconductor wafer or a crystalline semiconductor layer on a supporting wafer. In various embodiments, the substrate may be silicon or silicon-germanium. The substrate may be a semiconductor-on-insulator (SeOI), silicon-on-insulator (SOI), or silicon-germanium-on-insulator (SeGeOI). The substrate  110  may have a &lt;100&gt; or a &lt;111&gt; crystal face exposed for subsequent depositions, where the crystal lattice may facilitate epitaxial growth. 
       FIG. 13  is a cross-sectional view of a substrate and isolation region in accordance with an exemplary embodiment. 
     In one or more embodiments, a substrate may be masked and patterned to form one or more trench(es) in which an insulator material may be formed. The trench and insulator material may provide an isolation region  120  for subsequent layers and structures formed on the substrate, where the isolation region  120  may include an oxide, for example, silicon oxide (SiO x ) to provide electrical isolation. 
       FIG. 14  is a cross-sectional view of a substrate, isolation region, and hardmask layer in accordance with an exemplary embodiment. 
     In one or more embodiments, a hardmask layer  132  may be formed on the substrate  110  and isolation region  120 . In various embodiments, the hardmask layer  132  may be a nitride, for example, silicon nitride. 
       FIG. 15  is a cross-sectional view of a substrate, isolation region, and patterned hardmask layer in accordance with an exemplary embodiment. 
     In one or more embodiments, the hardmask layer  132  may be masked and patterned, and trenches  135  formed in the hardmask layer  132 , where the trenches  135  may be formed by reactive ion etching (RIE). The trenches  135  may extend to the surface of the substrate, such that the crystal lattice of the substrate surface is exposed at the bottom of the trench  135 . 
     In various embodiments, the trenches  135  may have a width in the range of about 8 nm to about 25 nm, or in the range of about 10 nm to about 20 nm, or in the range of about 10 nm to about 15 nm. 
       FIG. 16  is a cross-sectional view of a substrate and compound semiconductor formed in a patterned hardmask layer in accordance with an exemplary embodiment. 
     In one or more embodiments, a fin material  141  may be formed in the trenches, where the fin material  141  may extend above the surface of the hardmask layer  132 . The in material  141  may be formed in the trenches  135  by epitaxial deposition, where the deposition may be heteroepitaxial growth of a SiGe vertical fin  140  on a crystalline silicon substrate surface. The fin material  141  may be a doped semiconductor or compound semiconductor, for example, SiGe. 
       FIG. 17  is a cross-sectional view of a substrate and compound semiconductor formed in a patterned hardmask layer with a protective layer in accordance with an exemplary embodiment. 
     In one or more embodiments, a cap layer  151  may be formed on the exposed surface of the fin material  141 . The cap layer  151  may be a nitride, for example, silicon nitride (SiN). 
       FIG. 18  is a cross-sectional view of a substrate and compound semiconductor formed in a patterned hardmask layer with a patterned protective layer in accordance with an exemplary embodiment. 
     In one or more embodiments, the cap layer  151  may be masked, patterned, and etched to remove at least a portion of cap layer  151  from one or more regions above a trench  135  having a fill of fin material  141 , while leaving a capped portion  155  over one or more regions above a trench  135  having a fill of fin material  141 . 
       FIG. 19  is a cross-sectional view of a substrate with an oxidized portion of a compound semiconductor and a protected portion of a compound semiconductor in accordance with an exemplary embodiment. 
     In one or more embodiments, the exposed fin material  141  may be oxidized, for example, by a dry thermal oxidation, to produce germanium condensation in at least a portion of the fill of fin material  141  in a trench  135 . A silicon oxide layer  170  may be formed over a graded fin material  183 , where oxidation of SiGe selectively consumes silicon to form a silicon oxide layer  170 , and redistributes the germanium at the oxidation interface. The upper portion  185  of the modified vertical fin  180  may have an increased germanium concentration, and a second portion  181  of the modified vertical fin  180  may have a vertically graded germanium concentration, while a first portion  145  may retain the initial concentration ratio of Si to Ge, which may be essentially the same as fin material  141 . 
       FIG. 20  is a cross-sectional view of a substrate with a graded compound semiconductor and an unmodified compound semiconductor in accordance with an exemplary embodiment. 
     In one or more embodiments, the capped portion  155 , exposed fin material  141 , and top portion of the graded fin material  183  may be removed, where the material layers may be removed by any suitable removal process, for example, chemical-mechanical polishing and/or selective etching. 
       FIG. 21  is a cross-sectional view of a substrate with an unmodified fin and a modified fin with a vertically graded composition in accordance with an exemplary embodiment. 
     In one or more embodiments, the hardmask layer  132  may be removed, and a bottom spacer  130  may be formed between the substrate and a portion of the modified vertical fin and the unmodified vertical fin. The bottom spacer material  130  may be formed by a suitable deposition process, which may be followed by a suitable etch. The hardmask layer  132  may be recessed to leave a portion of the hardmask along the bottom of the trench. In various embodiments, a remaining portion of hardmask layer  132  may become the bottom spacer  130 . 
       FIG. 22  is a cross-sectional view of a substrate with a gated unmodified fin and a gated fin with a vertically graded composition in accordance with an exemplary embodiment. 
     In one or more embodiments, a gate structure  190  may be formed on the unmodified fin, and a gate structure may be formed on the modified fin. 
     While the embodiments have been depicted and described with particular arrangements and orientations, this is for descriptive purposes only. In various embodiments, for example, the locations of the source and drain of a vertical transistor may be reversed such that the drain is on the top of a vertical fin, while the source is at the bottom. The source may, therefore, be at the bottom or the top. Similarly, while the figures have been depicted and described with a modified vertical fin adjacent to a non-modified vertical fin, various patterns and arrangements having different sequences of modified and unmodified fins are also contemplated. 
     Having described preferred embodiments of the fabrication of vertical fin transistors with multiple threshold voltages (which are intended to be illustrative and not limiting), it is noted that modifications and variations can be made by persons skilled in the art in light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments disclosed which are within the scope of the invention as outlined by the appended claims. Having thus described aspects of the invention, with the details and particularity required by the patent laws, what is claimed and desired protected by Letters Patent is set forth in the appended claims.