Patent Publication Number: US-10312237-B2

Title: Vertical transport transistors with equal gate stack thicknesses

Description:
BACKGROUND 
     Technical Field 
     The present invention generally relates to transistor fabrication and, more particularly, to the fabrication of vertical transport transistors having consistent gate thicknesses between device types. 
     Description of the Related Art 
     Field effect transistors (FETs) have been employed to scale device size down as fabrication technologies have improved. Vertical transport FETs provide an opportunity for further device scaling. However, forming devices with equal gate thickness is particularly important in high-k/metal gate fabrication processes for vertical transport FETs to reduce gate variation during gate recess. This defines the gate length, and having consistent gate lengths helps to reduce the challenge of gate encapsulation and gate isolation. 
     SUMMARY 
     An integrated chip includes a first semiconductor device and a second semiconductor device. The first semiconductor device includes a semiconductor channel, a first-type work function layer formed from a first material on the semiconductor channel, and a second-type work function layer formed from a second material on the first-type work function later layer. The second semiconductor device includes a semiconductor channel, a second-type work function layer formed the second material on the semiconductor channel, and a thickness matching layer formed on the second-type work function layer of the second semiconductor device, the thickness matching layer having a thickness roughly equal to a thickness of the first-type work function layer. 
     An integrated chip includes a first semiconductor device and a second semiconductor device. The first semiconductor device includes a semiconductor channel, a first-type work function layer formed on the semiconductor channel, and second-type work function layer formed on the first-type work function layer. The second semiconductor device includes a semiconductor channel, a second-type work function layer formed on the semiconductor channel, and a thickness matching layer formed on the second-type work function layer, the thickness matching layer having a thickness roughly equal to a thickness of the first-type work function layer. An inter-device connection connects the first semiconductor device and the second semiconductor device and includes portions of the first-type work function layer, the second-type work function layer, and the thickness matching layer. 
     An integrated chip includes a first semiconductor device and a second semiconductor device. The first semiconductor device includes a semiconductor channel, a first-type work function layer formed on the semiconductor channel, a second-type work function layer formed on the first-type work function layer, and a scavenging layer on the second-type work function layer. A second semiconductor device includes a semiconductor channel, a second-type work function layer formed on the semiconductor channel, a scavenging layer on the second-type work function layer, and a thickness matching layer formed on the scavenging layer. The thickness matching layer has a thickness roughly equal to a thickness of the first-type work function layer. 
     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 DRAWINGS 
       The following description will provide details of preferred embodiments with reference to the following figures wherein: 
         FIG. 1  is a cross-sectional diagram of a step in the formation of vertical field effect transistors (FETs) of different types having equal gate thicknesses in accordance with an embodiment of the present invention; 
         FIG. 2  is a cross-sectional diagram of a step in the formation of vertical FETs of different types having equal gate thicknesses in accordance with an embodiment of the present invention; 
         FIG. 3  is a cross-sectional diagram of a step in the formation of vertical FETs of different types having equal gate thicknesses in accordance with an embodiment of the present invention; 
         FIG. 4  is a cross-sectional diagram of a step in the formation of vertical FETs of different types having equal gate thicknesses in accordance with an embodiment of the present invention; 
         FIG. 5  is a cross-sectional diagram of a step in the formation of vertical FETs of different types having equal gate thicknesses in accordance with an embodiment of the present invention; 
         FIG. 6  is a cross-sectional diagram of a step in the formation of vertical FETs of different types having equal gate thicknesses in accordance with an embodiment of the present invention; 
         FIG. 7  is a cross-sectional diagram of a step in the formation of vertical FETs of different types having equal gate thicknesses in accordance with an embodiment of the present invention; 
         FIG. 8  is a cross-sectional diagram of a step in the formation of vertical FETs of different types having equal gate thicknesses in accordance with an embodiment of the present invention; 
         FIG. 9  is a cross-sectional diagram of a step in the formation of vertical FETs of different types having equal gate thicknesses in accordance with an embodiment of the present invention; 
         FIG. 10  is a cross-sectional diagram of a step in the formation of vertical FETs of different types having equal gate thicknesses in accordance with an embodiment of the present invention; 
         FIG. 11  is a cross-sectional diagram of a step in the formation of vertical FETs of different types having equal gate thicknesses in accordance with an embodiment of the present invention; 
         FIG. 12  is a cross-sectional diagram of a step in the formation of vertical FETs of different types having equal gate thicknesses in accordance with an embodiment of the present invention; 
         FIG. 13  is a cross-sectional diagram of a step in the formation of vertical FETs of different types having equal gate thicknesses in accordance with an embodiment of the present invention; 
         FIG. 14  is a cross-sectional diagram of a step in the formation of vertical FETs of different types having equal gate thicknesses in accordance with an alternative embodiment of the present invention; 
         FIG. 15  is a cross-sectional diagram of a step in the formation of vertical FETs of different types having equal gate thicknesses in accordance with an alternative embodiment of the present invention; 
         FIG. 16  is a cross-sectional diagram of a step in the formation of vertical FETs of different types having equal gate thicknesses in accordance with an alternative embodiment of the present invention; and 
         FIG. 17  is a block/flow diagram of a process for forming vertical FETs of different types having equal gate thicknesses in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention provide vertical transport field effect transistor (VTFET) structures that have equal gate thicknesses by using the selective etching of one gate metal relative to the metals in other gate stacks between n-type FETs and p-type FETs. Each device type can use a distinct gate stack with distinct metals, making it possible to etch a thicker gate stack to match the thickness of the gate stacks used by other device types. 
     Referring now to the drawings in which like numerals represent the same or similar elements and initially to  FIG. 1 , a cross-sectional view of a step in the formation of vertical transistors having equal gate thicknesses is shown. A substrate  102  is divided into a first region  116  and a second region  118  using, for example, a shallow trench isolation process that cuts into the substrate  102  and deposits a dielectric barrier  108  in the trench. In some embodiments the substrate  102  may be a semiconductor substrate. The dielectric barrier  108  may be formed from, e.g., silicon dioxide or any other appropriate material. 
     A semiconductor substrate  102  may be a bulk-semiconductor substrate. In one example, the bulk-semiconductor substrate may be a silicon-containing material. Illustrative examples of silicon-containing materials suitable for the bulk-semiconductor substrate include, but are not limited to, silicon, silicon germanium, silicon germanium carbide, silicon carbide, polysilicon, epitaxial silicon, amorphous 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, germanium, gallium arsenide, gallium nitride, cadmium telluride, and zinc selenide. Although not depicted in the present figures, a semiconductor substrate  102  may also be a semiconductor on insulator (SOI) substrate. 
     A bottom source/drain layer  104  is formed on the substrate  104 . The bottom source/drain layer  104  is formed from, e.g., a doped semiconductor material. The semiconductor material may be a same material as used in the substrate  102  or may be a different semiconductor material. The dopant used in the bottom source/drain layer  104  will depend on the type of device being formed. For example, an n-type device may be formed in the first region  116  and will use an n-type dopant. A p-type device may be formed in the second region  118  and will use a p-type dopant. Each of the regions  116  and  118  will have a different dopant material. The dopant material may be added by, e.g., implantation. 
     As used herein, “p-type” refers to the addition of impurities to an intrinsic semiconductor that creates deficiencies of valence electrons. In a silicon-containing substrate, examples of p-type dopants, i.e., impurities, include but are not limited to: boron, aluminum, gallium and indium. As used herein, “n-type” refers to the addition of impurities that contributes free electrons to an intrinsic semiconductor. In a silicon containing substrate, examples of n-type dopants, i.e., impurities, include but are not limited to antimony, arsenic and phosphorous. 
     Channel fins  112  are formed on the bottom source/drain region  104 . It is specifically contemplated that the channel fins  112  may be formed from silicon, but any other appropriate semiconductor material may be used instead. The channel fins may be formed by depositing a layer of semiconductor material on the bottom source/drain layer  104  depositing fin masks  114  on the layer, and anisotropically etching the layer around the fin masks  114 . Reactive Ion Etching (RIE) is a form of plasma etching in which, during etching, the surface to be etched is placed on a radio-frequency powered electrode. During RIE, the surface to be etched takes on a potential that accelerates the etching species extracted from plasma toward the surface, in which the chemical etching reaction is taking place in the direction normal to the surface. Other examples of anisotropic etching that can be used at this point of the present invention include ion beam etching, plasma etching or laser ablation. Alternatively, the channel fins  112  can be formed by spacer imaging transfer. 
     An isolation layer  106  is deposited over the bottom source/drain layer  104  to provide electrical isolation between the channel fins  112 . Any appropriate dielectric material may be used for the isolation layer  106 , such as silicon dioxide. A layer of gate dielectric material  110  is then conformally deposited over the channel fins  112  and the isolation layer  106 . The layer of gate dielectric material maybe any appropriate dielectric including, e.g., high-k dielectrics. 
     As used herein, the term “high-k” refers to a material having a dielectric constant k that is higher than that of silicon dioxide. Examples of high-k materials include but are not limited to metal oxides such as hafnium oxide, hafnium silicon oxide, hafnium silicon oxynitride, lanthanum oxide, lanthanum aluminum oxide, zirconium oxide, zirconium silicon oxide, zirconium silicon oxynitride, tantalum oxide, titanium oxide, barium strontium titanium oxide, barium titanium oxide, strontium titanium oxide, yttrium oxide, aluminum oxide, lead scandium tantalum oxide, and lead zinc niobate. The high-k dielectric material may further include dopants such as lanthanum and aluminum. 
     The layer of gate dielectric material  110  may be deposited by any appropriate conformal process including, e.g., chemical vapor deposition (CVD), physical vapor deposition (PVD), atomic layer deposition (ALD), or gas cluster ion beam (GCIB) deposition. CVD is a deposition process in which a deposited species is formed as a result of chemical reaction between gaseous reactants at greater than room temperature (e.g., from about 25° C. about 900° C.). The solid product of the reaction is deposited on the surface on which a film, coating, or layer of the solid product is to be formed. Variations of CVD processes include, but are not limited to, Atmospheric Pressure CVD (APCVD), Low Pressure CVD (LPCVD), Plasma Enhanced CVD (PECVD), and Metal-Organic CVD (MOCVD) and combinations thereof may also be employed. In alternative embodiments that use PVD, a sputtering apparatus may include direct-current diode systems, radio frequency sputtering, magnetron sputtering, or ionized metal plasma sputtering. In alternative embodiments that use ALD, chemical precursors react with the surface of a material one at a time to deposit a thin film on the surface. 
     Referring now to  FIG. 2 , a cross-sectional view of a step in the formation of vertical transistors having equal gate thicknesses is shown. A p-type gate metal layer  202  is deposited in the second region  118 . This material may be deposited using a conformal process, such as CVD, ALD, or PVD, and may be formed from an appropriate p-type work function material such as, e.g., titanium nitride. Other types of p-type work function materials include, e.g., titanium aluminum nitride, ruthenium, platinum, molybdenum, cobalt, and alloys thereof. The p-type gate metal layer may be deposited in both regions  116  and  118  and subsequently etched away from the first region  116  by masking the second region  118 . 
     A work function material is a layer that provides a shift in the threshold voltage of a transistor device. In particular, a p-type work function material may be a material that moves the voltage threshold toward the valence band of silicon. In one embodiment, the work function of the p-type work function metal layer 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, e.g., a 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. 
     The present embodiments make use of different work function metals in different regions, with some layers being shared between the devices in the different regions. In this manner the voltage threshold of each device can be controlled by the use of an appropriate metal without compromising gate thickness uniformity. 
     Referring now to  FIG. 3 , a cross-sectional view of a step in the formation of vertical transistors having equal gate thicknesses is shown. An n-type work function stack is formed from, e.g., a titanium nitride layer  302  and a titanium aluminum carbide layer  304 . The stack is deposited conformally using any appropriate process such as, e.g., CVD, ALD, or PVD. An n-type work function material may be a material that moves the voltage threshold toward the conduction band of silicon in an 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 one embodiment, the work function of the n-type work function metal layer ranges from 4.1 eV to 4.3 eV. In alternative embodiments, the n-type work function stack may include layers of titanium aluminum, tantalum nitride, hafnium nitride, hafnium silicon, or combinations thereof. The layers of the n-type work function stack may be formed by any appropriate deposition process including, e.g., CVD, ALD, or PVD. 
     Referring now to  FIG. 4 , a cross-sectional view of a step in the formation of vertical transistors having equal gate thicknesses is shown. A matching layer  402  is formed in the first region  116  from, e.g., titanium nitride. The matching layer  402  may be formed by, e.g., conformally depositing the material over both regions  116  and  118 , masking the first region  116 , and then etching the material away from the second region  118 . The thickness of the matching layer  402  is formed to make the thickness of the stack of materials on the channel fin  112  of the first region  116  equal to the thickness of the stack of materials on the channel fin  112  of the second region. Notably, the thickness of the matching layer will be roughly equal to the thickness of p-type gate metal layer  202 . 
     In one exemplary embodiment, the p-type gate metal layer  202  may have an exemplary thickness between about 3 nm and about 5 nm, the titanium nitride layer may have an exemplary thickness between about 0.5 nm and about 1.5 nm, the titanium aluminum carbide layer  304  may have an exemplary thickness between about 3 nm and about 5 nm, and the matching layer may have an exemplary thickness between about 3 nm and about 5 nm. The total thickness in the first region  116  will be the sum of the layers  302 ,  304 , and  402 , having an exemplary range in this embodiment of between about 6.5 nm and about 11.5 nm. The total thickness in the second region  118  will be the sum of the layers  202 ,  302 , and  304 , having a same thickness as the stack of layers in the first region  116 . 
     Referring now to  FIG. 5 , a cross-sectional view of a step in the formation of vertical transistors having equal gate thicknesses is shown. A layer of planarizing material is deposited over the first region  116  and the second region  118 . The planarizing material may be, for example, an organic planarizing layer. The layers of gate metal and work function metals are etched back along with the planarizing material to form etched gate stacks  502 . The planarizing material is then etched away to expose the gate stacks  502 . The use of the planarizing material serves to protect the horizontal portions of the gate stacks around the fins  112 . 
     Referring now to  FIG. 6 , a cross-sectional view of a step in the formation of vertical transistors having equal gate thicknesses is shown. An encapsulation layer  602  is conformally deposited over the fins  112  and the recessed gate stacks  502 . The encapsulation layer  602  can be formed by any appropriate deposition process such as, e.g., CVD, PVD, or ALD. The encapsulation layer  602  provides a barrier against oxygen used in downstream processes. The encapsulation layer  602  may be formed from, e.g., silicon nitride, silicon oxynitride, or silicon boron carbonitride. 
     Referring now to  FIG. 7 , a cross-sectional view of a step in the formation of vertical transistors having equal gate thicknesses is shown. A gap dielectric  702  is deposited over the encapsulation layer  602  and is recessed back to a height of the gate stacks  502 . It is specifically contemplated that silicon dioxide may be used for this purpose, but any appropriate dielectric material may be used instead. The gap dielectric  702  fills a space between the two fins  112  and may be deposited using any appropriate process including, e.g., CVD, PVD, ALD, or a spin-on process with a flowable dielectric material. 
     Referring now to  FIG. 8 , a cross-sectional view of a step in the formation of vertical transistors having equal gate thicknesses is shown. A spacer  802  is formed around the exposed portions of the encapsulation layer  602 . The spacer  802  may be formed by, e.g., depositing a layer of hardmask material and then anisotropically etching the hardmask material to remove that material from the horizontal surfaces, leaving only the spacer  802  formed on the vertical sidewalls of the channel fins  112  outside of the encapsulation layer  602 . It is specifically contemplated that the spacer  802  may be formed from silicon nitride, but any appropriate dielectric material may be used instead. 
     Referring now to  FIG. 9 , a cross-sectional view of a step in the formation of vertical transistors having equal gate thicknesses is shown. The spacers  802  and the fin masks  114  are used as masks while the horizontal portions of the gate stacks  502  are anisotropically etched away. The result is the formation of n-type gate stack  902  in the first region  116  and a p-type gate stack  904  in the second region  118 . Notably the thickness of the n-type gate stack  902  is equal to the thickness of the p-type gate stack  904 , despite their differing layers and compositions. 
     Referring now to  FIG. 10 , a cross-sectional view of a step in the formation of vertical transistors having equal gate thicknesses is shown. A layer of dielectric fill is deposited over the first region  116  and the second region  118 . It is specifically contemplated that the dielectric fill may be formed from, e.g., silicon dioxide, but any appropriate dielectric material may be used instead. The fin mask  114 , the dielectric fill, and the spacers  802  are etched back to expose the top portion of the channel fins  112 , leaving dielectric layer  1002  between the first region  116  and the second region  118 . 
     Referring now to  FIG. 11 , a cross-sectional view of a step in the formation of vertical transistors having equal gate thicknesses is shown. Top source/drain regions  1102  and  1104  are formed on the top portions of the channel fins  112 . It is specifically contemplated that the top source/drain regions  1102  and  1104  may be epitaxially grown and may have a same basic composition as the channel fins  112  and bottom source/drain layer  104 , but it should be understood that the top source/drain regions  1102  and  1104  may be formed from any appropriate material and may be formed by any appropriate process. 
     An n-type source/drain region  1102  is formed in the first region  116  and a p-type source/drain region  1104  is formed in the second region  118 . It should be understood that the respective source/drain regions  1104  are doped with an appropriate n-type or p-type dopant by in situ doping or by implantation. 
     Referring now to  FIG. 12 , a cross-sectional view of a step in the formation of vertical transistors having equal gate thicknesses is shown. An inter-layer dielectric  1202  is formed from, e.g., silicon dioxide, extending the dielectric fill layer  1002  above the height of the top source/drain regions  1102  and  1104 . Conductive contacts  1204  are then formed, penetrating the inter-layer dielectric  1202  to contact the top source/drain regions  1102  and  1104  and the bottom source/drain regions  104 . The conductive contacts may be formed from any appropriate conductive material including, e.g., tungsten, nickel, titanium, molybdenum, tantalum, copper, platinum, silver, gold, ruthenium, iridium, rhenium, rhodium, and alloys thereof. 
     Referring now to  FIG. 13 , a cross-sectional view of an alternative embodiment of vertical transistors having equal gate thicknesses is shown. In this embodiment the gate stacks connecting the two regions  116  and  118  is not etched away, leaving a conductive connection  1302  between the two regions. In this manner the devices in the two regions can be triggered by a single shared gate. This can be accomplished by the addition of a mask in the area between the two regions before the formation of spacers  802 . 
     Referring now to  FIG. 14 , a cross-sectional view of a step in the formation of an alternative embodiment vertical transistors having equal gate thicknesses is shown. This embodiment includes an additional step before the step of  FIG. 4 , where an additional layer  1402  is formed in the n-type work function metal stack. The additional layer  1402  may be formed from, e.g., titanium nitride. In this embodiment, the titanium nitride matching layer  402 , which matches the thickness of the gate stack in the first region  116  to the thickness of the gate stack in the second region  118 , may instead be formed with a conductor such as tungsten or cobalt to for matching layer  1404 . This embodiment may then be formed into n-type and p-type transistor devices with separate or shared gate structures as described above. 
     Referring now to  FIG. 15 , a cross-sectional view of a step in the formation of an alternative embodiment vertical transistors having equal gate thicknesses is shown. This embodiment first forms an n-type work function metal stack and employs a scavenging layer in the second region  118 . The n-type work function metal stack is formed from, e.g., a titanium nitride layer  1502 , a titanium aluminum carbon layer  1504 , and a titanium nitride layer  1506 . After depositing the layers, the stack is patterned to confine it to the first region  116 . 
     A p-type work function layer  1508  is then deposited over both regions  116  and  118 , with the p-type work function layer  1508  being formed from, e.g., titanium nitride. A scavenging layer  1510  is formed over the p-type work function layer  1508 . A matching layer  1512  is then formed in the second region  118  from, e.g., silicon nitride, such that the thickness of the stack of layers in the second region  118  equals the thickness of the stack of layers in the first region  116 . The scavenging layer  1510  may be formed from, e.g., titanium aluminum carbide and either blocks oxygen diffusion from outside of the gate or scavenges the oxygen from the p-type work function layer  1508 . 
     Referring now to  FIG. 16 , a cross-sectional view of a step in the formation of an alternative embodiment vertical transistors having equal gate thicknesses is shown. As in  FIG. 15 , the n-type work function metal stack is formed in the first region  116 . A layer of, e.g., titanium nitride  1508  and a scavenging layer  1510  are then deposited over both regions  116  and  118 . 
     A thin layer  1602  of, e.g., titanium nitride is deposited over both regions followed by a matching layer  1604  in the second region  118  that is formed from, e.g., cobalt or tungsten. As in other embodiments, the matching layer  1604  matches the thickness of the gate stack in the second region  118  to the thickness of the gate stack in the first region  116 . 
     It is to be understood that aspects of the present invention will be described in terms of a given illustrative architecture; however, other architectures, structures, substrate materials and process features and steps can be varied within the scope of aspects 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 can 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 can 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 can include a design for an integrated circuit chip, which can 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 can 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 can 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. 
     It should also be understood that material compounds will be described in terms of listed elements, e.g., SiGe. These compounds include different proportions of the elements within the compound, e.g., SiGe includes Si x Ge 1-x  where x is less than or equal to 1, etc. In addition, other elements can be included in the compound and still function in accordance with the present principles. The compounds with additional elements will be referred to herein as alloys. 
     Reference in the specification to “one embodiment” or “an embodiment”, 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. 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 can 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 intended 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 more other features, integers, steps, operations, elements, components and/or groups thereof. 
     Spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper,” and the like, can 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 can be otherwise oriented (rotated 90 degrees or at other orientations), and the spatially relative descriptors used herein can 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 can also be present. 
     It will be understood that, although the terms first, second, etc. can 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. 
     Referring now to  FIG. 17 , a method of forming transistors is shown. Block  1702  forms vertical channels  112  on a lower source/drain layer  104 . The lower source/drain layer  104  may be separated into different regions, including a first region  116  and a second region  118 , with respective vertical channels  112  being formed in each region. Block  1703  forms a gate dielectric  110  over the vertical channels  112  and, in particular, on the sidewalls of the vertical channels  112 . 
     Block  1404  forms a first-type (e.g., p-type or n-type) work function layer in one of the two regions. In some embodiments the p-type work function layer may be deposited first in a p-type region, in other embodiments the n-type work function layer may be deposited in the n-type region. Block  1404  may in some embodiments conformally form the first-type work function layer on both regions and subsequently etch away the first-type work function layer from the second region. 
     In one embodiment, the first-type work function layer may be a p-type titanium nitride layer formed in a p-type region  118 . In another embodiment, the first-type work function layer may be an n-type stack that includes an n-type titanium nitride layer and a titanium aluminum carbon layer and optionally includes a second titanium nitride layer, the n-type stack being formed in an n-type region  116 . 
     Block  1706  forms a second-type work function layer over both regions  116  and  118 . In one embodiment, the second-type work function layer may be an n-type stack that includes an n-type titanium nitride layer and a titanium aluminum carbon layer and optionally includes a second titanium nitride layer. In another embodiment, the second-type work function layer may be a p-type titanium nitride layer. The second-type work function layer will have a polarity opposite that of the first-type work function layer. 
     Block  1707  optionally forms a scavenging layer  1510  over the second-type work function layer. As noted above, the scavenging layer  1510  may be formed from titanium aluminum carbide and may block oxygen from the outside and/or may scavenge oxygen from the underlying second-type work function layer. The scavenging layer  1510  may have an exemplary thickness between about 2 nm and about 5 nm. 
     Block  1708  forms a thickness matching layer in the region that does not have the first-type work function layer. The matching layer will have a thickness that is roughly equal to the thickness of the first-type work function layer so that the thickness of the stacks in the two regions will be about equal. In one embodiment the matching layer can be formed from titanium nitride. In another embodiment, the matching layer may be formed from a metal conductor such as cobalt or tungsten. 
     Block  1710  recesses the gate stacks in the two regions below a height of the vertical fins  112 . Block  1712  forms spacers  802  on the exposed sidewalls of the vertical fins  112 . Block  1714  optionally etches away the areas of the gate stack layers between the first and second regions to sever the electrical connection between these two regions. 
     In alternative embodiments, block  1714  may be omitted to maintain a shared gate between the two regions. 
     Block  1716  etches back the spacers  802  to expose the top portions of the vertical channels  112 . Block  1718  forms top source/drain regions  1102  and  1104  on the top portions of the vertical channels  112 . It is specifically contemplated that the top source/drain regions  1102  and  1104  may be formed by epitaxial growth with in situ doping with dopants appropriate to the respective device type of each region. Block  1720  completes the device, forming inter-layer dielectric  1202  and electrical contacts  1204  to the top and bottom source/drain regions and the gates. 
     Having described preferred embodiments of vertical transport transistors with equal gate stack thicknesses (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.