Patent Publication Number: US-2022216329-A1

Title: Semiconductor Device Structure With Uniform Threshold Voltage Distribution and Method of Forming the Same

Description:
PRIORITY DATA 
     This application is a continuation of U.S. patent application Ser. No. 16/925,703 filed Jul. 10, 2020, which further claims priority to U.S. Provisional Patent Application Ser. No. 62/893,853 entitled “Semiconductor Device Structure with Uniform Threshold Voltage Distribution and Method of Forming the Same,” and filed Aug. 30, 2019, the entire disclosures of which are hereby incorporated herein by reference. 
    
    
     BACKGROUND 
     The semiconductor integrated circuit (IC) industry has experienced exponential growth. Technological advances in IC materials and design have produced generations of ICs where each generation has smaller and more complex circuits than the previous generation. In the course of IC evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased. This scaling down process generally provides benefits by increasing production efficiency and lowering associated costs. Such scaling down has also increased the complexity of IC processing and manufacturing, and for these advancements to be realized, similar developments in IC processing and manufacturing are needed. 
     For example, multi-gate devices have been introduced in an effort to improve gate control by increasing gate-channel coupling, reduce OFF-state current, and reduce short-channel effects (SCEs). An example of such a multi-gate device is a fin field effect transistor (FinFET). A typical FinFET is fabricated with a vertical fin structure extending from a substrate. The fin structure is formed by, for example, etching away a portion of a silicon layer of the substrate. The channel of the FinFET is formed in the fin structure. A gate is provided over the structure (e.g. wrapping around the fin structure). Having a gate on multiple sides of the channel reduces short channel effect and allows for higher current flow. 
     Designing a FinFET is not without its own challenges. For example, while it is desirable to have a low threshold voltage to increase the on-state current (Ion), having a low threshold voltage can lead to high leakage current, including a non-negligible off-state current (Ioff). Consequently, one of the objectives in designing a FinFET is to achieve a threshold voltage that results in a good Ion/Ioff ratio. Furthermore, the threshold voltage in a FinFET is not constant throughout the height of the fin—it is affected by several factors introduced by the design itself or the fabrication processes. For example, the gate of the FinFET is in contact with three surfaces at a top region of the fin while only in contact with two opposing surfaces (e.g. opposing sidewalls) for the rest of the fin. In addition, the fin may not have uniform shape and width along its height. Furthermore, the distribution of anti-punch-through dopants and the source/drain features also affect the threshold voltage. As a result, a non-uniform threshold voltage distribution is usually observed. Often, portions of the fin with lower-than-expected threshold voltages suffer from high off-state current density, while portions of the fin with higher-than-expected threshold voltage suffer from low on-state current density. 
     Therefore, although existing FinFET devices have been generally adequate for their intended purposes, they have not been satisfactory in every aspect. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure is best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale and are used for illustration purposes only. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIG. 1A  illustrates a three-dimensional view of a semiconductor device structure, in accordance with an embodiment of the present disclosure; 
         FIG. 1B  illustrates a cross-sectional view of the semiconductor device structure shown in  FIG. 1A , in accordance with an embodiment of the present disclosure; 
         FIGS. 2A, 3A, 4A, 5A, 6A, 7A, 8A, 9A, 10A, 11A, 12A, 13A, 14A, 15A, 16A, 17A, 18A, 19A, 20A, and 21A, and 2B ,  3 B,  4 B,  5 B,  6 B,  7 B,  8 B,  9 B,  10 B,  11 B,  12 B,  13 B,  14 B,  15 B,  16 B,  17 B,  18 B,  19 B,  20 B and  21 B show cross-sectional views of a method of forming a semiconductor device structure, in accordance with an embodiment of the present disclosure; 
         FIGS. 22 and 23  illustrate dopant concentration profiles along a height of fin structures, in accordance with embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
     In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. Moreover, the formation of a feature on, connected to, and/or coupled to another feature in the present disclosure that follows may include embodiments in which the features are formed in direct contact, and may also include embodiments in which additional features may be formed interposing the features, such that the features may not be in direct contact. In addition, spatially relative terms, for example, “lower,” “upper,” “horizontal,” “vertical,” “above,” “over,” “below,” “beneath,” “up,” “down,” “top,” “bottom,” etc. as well as derivatives thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) are used for ease of the present disclosure of one features relationship to another feature. The spatially relative terms are intended to cover different orientations of the device including the features. Still further, when a number or a range of numbers is described with “about,” “approximate,” and the like, the term is intended to encompass numbers that are within a reasonable range including the number described, such as within +/−10% of the number described or other values as understood by person skilled in the art. For example, the term “about 5 nm” encompasses the dimension range from 4.5 nm to 5.5 nm. 
     The present disclosure is directed to, but not otherwise limited to, a fin-like field-effect transistor (FinFET) device. The FinFET device, for example, may be a complementary metal-oxide-semiconductor (CMOS) device including a P-type metal-oxide-semiconductor (PMOS) FinFET device and an N-type metal-oxide-semiconductor (NMOS) FinFET device. The following disclosure will continue with one or more FinFET examples to illustrate various embodiments of the present disclosure. It is understood, however, that the application should not be limited to a particular type of device, except as specifically claimed. 
       FIG. 1A  shows a three-dimensional view of a semiconductor device structure  100 , in accordance with an embodiment. In the example of  FIG. 1A , the semiconductor device structure  100  is a fin-like field effect transistor (FinFET) device. The semiconductor device structure  100  may be fabricated during manufacture or processing of an integrated circuit (IC) device or a portion thereof. The IC device may include, or may be, static random-access memory (SRAM) and/or other logic circuits, passive components such as resistors, capacitors, and inductors, and active components such as P-type field effect transistors (PFETs), N-type FETs (NFETs), FinFETs, metal-oxide semiconductor field effect transistors (MOSFET), complementary metal-oxide semiconductor (CMOS) transistors, bipolar transistors, high voltage transistors, high frequency transistors, and/or other memory cells. The present disclosure is not limited to any particular number of devices or device regions, or to any particular device configurations. 
     The semiconductor device structure  100  of  FIG. 1A  includes a substrate  102 , a doped region  104 , and a fin structure  106  protruding from (e.g. extending vertically from) the doped region  104 . Isolation regions  108  are formed over the doped region  104 , and the fin structure  106  protrudes out of (e.g. extends vertically out of) the isolation regions  108 , thereby spatially separates neighboring isolation regions  108 . The fin structure  106  includes a channel region, and a gate stack engages the channel region of the fin structure  106 . As an example, the gate stack includes a gate dielectric  110  that is disposed along sidewalls and over a top surface of the channel region of the fin structure  106 . The gate stack further includes a gate electrode  112  that is disposed over the gate dielectric  110 . The fin structure  106  includes source/drain regions  114  that are disposed on opposite sides of the channel region of the fin structure  106 .  FIG. 1A  further illustrates reference cross-sections that are used in later figures. In particular, a cross-section along the line A-A is taken across the gate stack and the channel region of the fin structure  106 , while a cross-section along the line B-B is taken across the source/drain region of the fin structure  106 . 
       FIG. 1B  shows a cross-sectional view along the line A-A of  FIG. 1A . Because the gate stack covers a top surface and two sidewalls of the fin structure  106 , the gate stack shown in  FIG. 1B  is usually referred to as a trigate. In some embodiments, a height H of the fin structure  106  (e.g. as measured in the Z-direction) may refer to the length of the portion of the fin structure  106  that protrudes out of the isolation regions  108 , as shown in  FIG. 1B . The portion of the fin structure  106  that is laterally surrounded (e.g. in the Y-direction) by the isolation regions  108  may be referred to as a base portion BP of the fin structure  106 . 
     Embodiments of the present disclosure provide a semiconductor device structure with a uniform threshold voltage distribution along the height H of the fin structure  106 , which in turn results in better DC performance of the semiconductor device structure (e.g. since variation of off-state current density and on-state current density along the height H of fin structures is reduced). For example, the fin structure  106  of the present disclosure has a uniform threshold voltage distribution along the height H of the fin structure  106  even though the trigate shown in  FIG. 1B  has a stronger effect over an upper portion UP of the fin structure  106  compared to a lower portion LP of the fin structure  106  and even though the width W of the fin structure  106  along the Y-direction (e.g. the distance between sidewalls of the fin structure  106 ) varies along the height H of the fin structure  106 . The present disclosure also provides methods of forming a semiconductor device structure with a uniform threshold voltage distribution along the height H of the fin structure  106 . The proposed method results in reduced material loss at the top surface of the fin structure  106  (e.g. fin top loss) during the manufacturing process and improved vertical fin profile tuning. 
       FIGS. 2A to 21A and 2B to 21B  show cross-sectional views of a method of forming a semiconductor device structure with a uniform threshold voltage distribution along the height H of fin structures, in accordance with an embodiment of the present disclosure.  FIGS. 2A-21A  are taken along a channel region of the proposed semiconductor device structure (e.g. along the line A-A in  FIG. 1A ), while  FIGS. 2B-21B  are taken along a source/drain region of the proposed semiconductor device structure (e.g. along the line B-B in  FIG. 1A ). 
     Referring to  FIGS. 2A and 2B , a substrate  200  is provided. The substrate  200  may include, or may be, an elementary (single element) semiconductor, such as silicon, germanium, and/or other suitable materials; a compound semiconductor, such as silicon carbide, gallium arsenic, gallium phosphide, indium phosphide, indium arsenide, indium antimonide, and/or other suitable materials; an alloy semiconductor, such as SiGe, GaAsP, AlInAs, AlGaAs, GalnAs, GaInP, GaInAsP, and/or other suitable materials. The substrate  200  may be a single-layer material having a uniform composition. Alternatively, the substrate  200  may include multiple material layers having similar or different compositions suitable for IC device manufacturing. In one example, the substrate  200  may be a silicon-on-insulator (SOI) substrate having a silicon layer formed on a silicon oxide layer. In another example, the substrate  200  may include a conductive layer, a semiconductor layer, a dielectric layer, other layers, or combinations thereof. 
     The substrate  200  includes various doped regions configured according to design requirements of the IC device. For example, the substrate  200  may include N-type doped regions (for example, N-type wells) doped with N-type dopants, such as phosphorus, arsenic, other N-type dopants, or combinations thereof. Additionally, the substrate  200  may include P-type doped regions (for example, P-type wells) doped with P-type dopants, such as boron (for example, BF 2 ), indium, other P-type dopants, or combinations thereof. In the example of  FIGS. 2A and 2B , the N-type doped regions (e.g. N-well regions) are depicted as first region  202   a  and third region  202   c,  while the P-type doped region (e.g. P-well region) is depicted as second region  202   b.  The various doped regions can be formed directly on and/or in the substrate  200 , for example, thereby providing a P-well structure, an N-well structure, a dual-well structure, a raised structure, or combinations thereof. The various doped regions may be formed by implantation of dopant atoms, in-situ doped epitaxial growth, a diffusion process, and/or other suitable techniques. 
     Referring to  FIGS. 3A and 3B , one or more fin structures are formed in each of the first region  202   a,  the second region  202   b,  and the third region  202   c.  In the embodiment shown in  FIGS. 3A and 3B , two fin structures  204   a  are formed in the first region  202   a,  two fin structures  204   b  are formed in the second region  202   b,  and two fin structures  204   c  are formed in the third region  202   c.  However, in other embodiments, the number of fin structures formed in each of the regions  202   a,    202   b,    202   c  may be different. Furthermore, the number of fin structures formed in each of the first region  202   a,  the second region  202   b,  and the third region  202   c  may be more than two or less than two in other embodiments. The fin structures  204   a,    204   c  respectively formed in the first region  202   a  and the third region  202   c  (e.g. N-well regions) may be suitable for providing P-type semiconductor devices, while the fin structures  204   b  formed in the second region  202   b  (e.g. P-well region) may be suitable for providing one or more N-type semiconductor devices. 
     The fin structures  204   a,    204   b,    204   c  may be formed using any suitable process. In the example shown in  FIGS. 3A and 3B , photolithography and etch processes are used to form fin structures  204   a,    204   b,    204   c.  For example, a first hard mask layer  206  may be formed on the substrate  200  and the regions  202   a,    202   b,    202   c  formed therein or thereon. In some examples, the first hard mask layer  206  includes a dielectric material such as a semiconductor oxide, a semiconductor nitride, a semiconductor oxynitride, or a semiconductor carbide. The first hard mask layer  206  may include two or more films stacked together, such as a silicon oxide film and a silicon nitride film in a stack. The first hard mask layer  206  may be formed by thermal growth, ALD, CVD, HDP-CVD, PVD, and/or other suitable deposition processes. The first hard mask layer  206  may include other suitable materials, such as a silicon oxide layer and a poly-silicon layer on the silicon oxide layer. 
     Subsequent to forming the first hard mask layer  206  on the regions  202   a,    202   b,    202   c  of the substrate  200 , a photoresist (or resist) layer (not shown) is formed on the first hard mask layer  206 . The resist layer includes a photosensitive material that causes the layer to undergo a property change when exposed to light, such as ultraviolet (UV) light, deep UV (DUV) light or extreme UV (EUV) light. This property change can be used to selectively remove exposed or unexposed portions of the resist layer by a developing process. This procedure to form a patterned resist layer is also referred to as lithographic patterning or lithography process. 
     In one embodiment, the resist layer is patterned to leave portions of the photoresist material disposed over the first hard mask layer  206  by the lithography process. After patterning the resist layer, an etching process is performed on the first hard mask layer  206 , thereby transferring the pattern from the resist layer to the first hard mask layer  206 . The remaining resist layer may be removed after the patterning the first hard mask layer  206 . An exemplary lithography process includes spin-on coating the resist layer, soft baking the resist layer, mask aligning, exposing, post-exposure baking, developing the resist layer, rinsing, and drying (e.g., hard baking). Alternatively, a lithographic process may be implemented, supplemented, or replaced by other methods such as maskless photolithography, electron-beam writing, and ion-beam writing. The etching process to pattern the first hard mask layer  206  may include wet etching, dry etching or a combination thereof. In some embodiments, the etching process applied to the first hard mask layer  206  may include multiple etching steps. For example, a silicon oxide film in the first hard mask layer  206  may be etched by a diluted hydrofluorine solution and a silicon nitride film in the first hard mask layer  206  may be etched by a phosphoric acid solution. 
     The patterned first hard mask layer  206  is then used as an etch mask to pattern the regions  202   a,    202   b,    202   c  of the substrate  200 . The etching process applied to the regions  202   a,    202   b,    202   c  of the substrate  200  may include any suitable etching technique such as dry etching, wet etching, other etching methods (e.g., reactive ion etching (RIE)), or a combination thereof. In some examples, the etching process applied to the regions  202   a,    202   b,    202   c  of the substrate  200  may include multiple etching steps with different etching chemistries, each targeting a particular material of the substrate  200 . In some examples, the semiconductor material of the substrate  200  may be etched by a dry etching process using a fluorine-based etchant. In some embodiments, etching includes multiple etching steps with different etching chemistries, each targeting a particular material of the substrate  200  and each selected to resist etching the first hard mask layer  206 . For example, a dry etching process may implement an oxygen-containing gas, a fluorine-containing gas (e.g., CF 4 , SF 6 , CH 2 F 2 , CHF 3 , and/or C 2 F 6 ), a chlorine-containing gas (e.g., Cl 2 , CHCl 3 , CCl 4 , and/or BCl 3 ), a bromine-containing gas (e.g., HBr and/or CHBR 3 ), an iodine-containing gas, other suitable gases and/or plasmas, and/or combinations thereof. For example, a wet etching process may include etching in diluted hydrofluoric acid (DHF); potassium hydroxide (KOH) solution; ammonia; a solution containing hydrofluoric acid (HF), nitric acid (HNO 3 ), and/or acetic acid (CH 3 COOH); or other suitable wet etchant. The remaining portions of the regions  202   a,    202   b,    202   c  of the substrate  200  become the fin structures  204   a,    204   b,    204   c.    
     The etching processes are designed to produce fin structures  204   a,    204   b,    204   c  of any suitable height and width extending from the substrate  200 . Particularly, the etching process applied to the regions  202   a,    202   b,    202   c  of the substrate  200  is controlled such that the regions  202   a,    202   b,    202   c  are partially etched, as illustrated in  FIGS. 3A and 3B . This may be achieved by controlling etching time or by controlling other etching parameter(s). 
     In the examples presented herein, the fin structures  204   a,    204   b,    204   c  extend lengthwise (e.g. longitudinally) in a first direction (e.g. in the X-direction) and are separated from each other (e.g. laterally) in a second direction (e.g. in the Y-direction). A height of each of the fin structures  204   a,    204   b,    204   c  is measured in a third direction (e.g. in the Z-direction). It is understood that the X-direction and the Y-direction are horizontal directions that are perpendicular to each other, and that the Z-direction is a vertical direction that is orthogonal (or normal) to a plane defined by the X-direction and the Y-direction. Each of the regions  202   a,    202   b,    202   c  of the substrate  200  may have its top surface aligned in parallel to the XY plane. 
     Referring to  FIGS. 4A and 4B , a first dielectric layer  208  is formed on the fin structures  204   a,    204   b,    204   c.  In the present embodiment, the first dielectric layer  208  is formed with a conformal profile, covering the fin structures  204   a,    204   b,    204   c.  It is noted that although the first dielectric layer  208  is formed with a conformal profile, a relatively narrow spacing S (e.g. less than 2 nm) between adjacent ones of the fin structures  204   a  may be filled up (e.g. completely filled up) by the first dielectric layer  208 . A similar result is seen in the spacing S between adjacent ones of the fin structures  204   b  and in the spacing S between adjacent ones of the fin structures  204   c.    
     The first dielectric layer  208  may include a single dielectric material layer or multiple dielectric material layers. Suitable dielectric materials for the first dielectric layer  208  may include silicon oxides, silicon nitrides, silicon carbides, fluoro-silicate glass (FSG), low-K dielectric materials (e.g. a dielectric material having a dielectric constant less than a dielectric constant of SiO 2 , which is approximately 3.9), other suitable dielectric materials, or a combination thereof. The dielectric material may be formed by any suitable technique including thermal growth, CVD, HDP-CVD, PVD, ALD, and/or spin-on techniques. In the illustrated embodiment, an ALD process is used as the conformal deposition technique. The first dielectric layer  208  constitutes, at least partially, an isolation feature, such as a shallow trench isolation feature (STI). 
     In the embodiment shown in  FIGS. 4A and 4B , the conformal profile of the first dielectric layer  208  defines a first trench  210   a  and a second trench  210   b.  The first trench  210   a  spatially separates (e.g. in the Y-direction) the fin structures  204   a  from the fin structures  204   b.  The second trench  210   b  spatially separates (e.g. in the Y-direction) the fin structures  204   b  from the fin structures  204   c.    
     Referring to  FIGS. 5A, 5B, 6A, and 6B , dielectric fins  212  (also referred to as hybrid fins or fill fins) are formed in the first and second trenches  210   a,    210   b.  The dielectric fins  212  are dielectric features that do not function as active regions. Instead, the dielectric fins  212  may be used for other purposes, such as tuning patterning density for better fabrication. The dielectric fins  212  may include multiple dielectric material layers. In the present embodiment, the dielectric fins  212  include a first fill dielectric layer  212   a  and a second fill dielectric layer  212   b.  In such an embodiment, the first fill dielectric layer  212   a  may be formed by ALD (e.g. to achieve a conformal profile in the trenches  210   a,    210   b ), and the second fill dielectric layer  212   b  may be formed by flowable CVD (FCVD) or alternatively by spin-on coating. In the examples shown in this disclosure, the trenches  210   a,    210   b  have substantially similar widths (e.g. as measured in the Y-direction). However, in other embodiments, the trenches  210   a,    210   b  may have different widths. In embodiments where the trenches  210   a,    210   b  have different widths, trenches having smaller dimensions (e.g., a width of less than 40 nm as measured in the Y-direction) may be completely filled by the first fill dielectric layer  210   a,  while trenches having greater dimensions (e.g., a width greater than 40 nm as measured in the Y-direction) may be completely filled by both the first fill dielectric layer  212   a  and the second fill dielectric layer  212   b.  For the trenches with the greater dimensions, the first fill dielectric layer is conformal to the trenches, as in the embodiment of  FIGS. 5A and 5B . Thus, the trenches with larger dimensions have dual fill dielectric layers while the trenches with smaller dimensions have a single fill dielectric layer. In some examples, the first fill dielectric layer  212   a  has a thickness T 1  (e.g. as measured in the Y-direction) in a range from about 1 nm to about 20 nm (e.g. from about 1 nm to about 3 nm). 
     The fill dielectric layers  212   a,    212   b  may include suitable dielectric materials that are different from that of the first dielectric layer  208  to achieve etching selectivity at a later etching stage. The first fill dielectric layer  212   a  and the second fill dielectric layer  212   b  may have different compositions. For examples, the first fill dielectric layer  212   a  may be a high-k dielectric material, such as hafnium oxide (HfO 2 ), or zirconium oxide (ZrO 2 ), while the second fill dielectric layer  212   b  may include a carbon-containing dielectric material, such as silicon carbon oxide, silicon carbon oxynitride or silicon carbon nitride. In other embodiments, the first fill dielectric layer  212   a  may include other suitable dielectric material, such as metal oxide (such as aluminum oxide Al 2 O 3 ), or metal nitride (such as aluminum nitride AlN), or a combination thereof. The second fill dielectric layer  212   b  may include other dielectric layer, such as silicon oxide, silicon oxynitride, silicon carbide nitride, and/or other suitable dielectric materials. In one example, the first fill dielectric layer  212   a  includes hafnium oxide deposited by ALD, and the second fill dielectric layer  212   b  includes silicon carbon oxide deposited by FCVD or spin-on deposition. Referring to  FIGS. 6A and 6B , a polishing such as chemical mechanic polishing (CMP) process is performed to the dielectric fins  212  to planarize the top surfaces thereof and to remove the first hard mask layer  206  and excessive portions of the fill dielectric layers  212   a,    212   b.  The polishing process exposes top surfaces of the fin structures  204   a,    204   b,    204   c.    
     Referring to  FIGS. 7A and 7B , the first dielectric layer  208  is selectively recessed such that the dielectric fins  212  and the fin structures  204   a,    204   b,    204   c  are extruded above the recessed first dielectric layer  208 ′. The fin structures  204   a,    204   b,    204   c  are electrically isolated from each other by the recessed first dielectric layer  208 ′, which functions as an isolation feature. Consequently, the recessed first dielectric layer  208 ′ may also be referred to as a dielectric isolation structure that surrounds base potions BP of the fin structures  204   a,    204   b,    204   c.  Any suitable etching technique may be used to recess the first dielectric layer  208 , including dry etching, wet etching, RIE, and/or other etching methods. In an exemplary embodiment, an anisotropic dry etching is used to selectively remove the first dielectric layer  208  without etching the fin structures  204   a,    204   b,    204   c  and/or the dielectric fins  212 , with a proper etching gas, such as fluorine-containing or chlorine-containing gas. The height H of the portions of the fin structures  204   a,    204   b,    204   c  that protrude out of the recessed first dielectric layer  208 ′ may be determined by the etching depth of the etching process used to recess the first dielectric layer  208 . In some embodiments, the height H (e.g. as measured in the Z-direction) may be in a range from about 45 nm to about 60 nm. This range of values may be needed for optimal device performance. For example, a height H greater than about 60 nm may result in etch process inefficiencies, an example being the deposition of residues on the surfaces of fin structures  204   a,    204   b,    204   c,  thereby adversely affecting device performance. As another example, a height H less than about 45 nm may result in a smaller current being induced between source and drain regions, thereby degrading device DC performance. 
     Referring to  FIGS. 8A and 8B , a second dielectric layer  214  is formed with a conformal profile, covering exposed surfaces of the fin structures  204   a,    204   b,    204   c,  the dielectric fins  212 , and the first dielectric layer  208 . The second dielectric layer  214  may include or may be a material different from that of the recessed first dielectric layer  208 ′ to provide etch selectivity. For example, the second dielectric layer  214  may include silicon nitride. The second dielectric layer  214  may be formed by any suitable technique including thermal growth, CVD, HDP-CVD, PVD, ALD, and/or spin-on techniques. In the illustrated embodiment, an ALD process is used as the conformal deposition technique. 
     Referring to  FIGS. 9A and 9B , portions of the second dielectric layer  214  disposed within the boundaries of the first and third regions  202   a,    202   c  are removed. This may be accomplished using photolithography and etching processes (e.g. as discussed above in reference to the first hard mask layer  206 ). Stated differently, portions of the second dielectric layer  214  disposed over the first and third regions  202   a,    202   c  are removed. As a result of the process shown in  FIGS. 9A and 9B , surfaces (e.g. sidewall surfaces and top surfaces) the fin structures  204   a  of the first region  202   a  and the fin structures  204   c  of the third region  202   c  are exposed. It is once again noted that the fin structures  204   a  and  204   c  may be suitable for providing P-type semiconductor devices. The operations that follow introduce a semiconductor material (e.g. a dopant that is compatible with silicon) into the fin structures  204   a  and  204   c  in order to provide a uniform threshold voltage along the height H of the fin structures  204   a,    204   c.    
     Referring to  FIGS. 10A and 10B , a semiconductor liner layer  218  may be selectively formed on exposed sidewall surfaces and top surfaces of the fin structures  204   a,    204   c.  In some embodiments, the semiconductor liner layer  218  may be formed by one or more epitaxial growth processes, whereby silicon-germanium (SiGe) features and/or other suitable semiconductor features are grown in a crystalline state on the fin structures  204   a,    204   c.  In some embodiments, a thickness T 2  of the semiconductor liner layer  218  (e.g. as measured in the Y-direction) may be in a range from about 2 nm to about 3 nm. In examples where the semiconductor liner layer  218  includes SiGe, a concentration of germanium in the semiconductor liner layer  218  may be in a range from about 20 atomic percent to about 70 atomic percent. In general, the concentration of germanium in the semiconductor liner layer  218  can be tuned based on the desired threshold voltage for the P-type semiconductor devices formed in the first region  202   a  and the third region  202   c.  Suitable epitaxy processes that may be used to form the semiconductor liner layer  218  include CVD deposition techniques (e.g., vapor-phase epitaxy (VPE) and/or ultra-high vacuum CVD (UHV-CVD)), molecular beam epitaxy, and/or other suitable processes. In the operation shown in  FIGS. 10A and 10B , the second dielectric layer  214  functions, at least partially, as a mask that prevents formation of the semiconductor liner layer  218  on the fin structures  204   b  of the second region  202   b.    
     Referring to  FIGS. 11A and 11B , a first capping layer  220  is formed over the structure of  FIGS. 10A and 10B  to, for example, fully cover the dielectric fins  212 , the second dielectric layer  214  within the boundaries of the second region  202   b,  and the semiconductor liner layer  218  over the fin structures  204   a,    204   c.  The first capping layer  220  may include a nitride (e.g. silicon nitride), an oxynitride (e.g. silicon carbon oxynitride (SiCON)), a carbonitride (e.g. silicon carbonitride (SiCN)), combinations thereof, or the like. In some embodiments, the first capping layer  220  and the second dielectric layer  214  may have similar compositions. The first capping layer  220  may be formed by any suitable technique including thermal growth, CVD, HDP-CVD, PVD, ALD, and/or spin-on techniques. 
     Referring to  FIGS. 12A and 12B , the first capping layer  220  is etched back to form recesses between adjacent ones of the fin structures  204   a,    204   b,    204   c  and the dielectric fins  212 . A thickness T 3  of the first capping layer  220  (e.g. as measured in the Z-direction) remaining after the etch back process may be in a range from about 2 nm to about 4 nm. The etch back process causes the first capping layer  220  to physically contact portions of the semiconductor liner layer  218  that disposed on (e.g. laterally surround) portions  222   a  of the fin structures  204   a  and portions  222   c  of the fin structures  204   c.  In some embodiments, the portions  222   a  of the fin structures  204   a  and the portions  222   c  of the fin structures  204   c  may be lower or bottom portions of the fin structures  204   a,    204   c,  respectively. Analogously, the etch back process causes portions of the semiconductor liner layer  218  that are disposed on portions  224   a  (e.g. upper portions) of the fin structures  204   a  and portions  224   c  (e.g. upper portions) of the fin structures  204   c  to be free from the first capping layer  220 . The etch process used in the operation of  FIGS. 12A and 12B  may include wet etching, dry etching or a combination thereof. As an example, a wet etch process including a phosphoric acid solution may be used to etch back the first capping layer  220  in embodiments where the first capping layer  220  includes a nitride (e.g. silicon nitride). As seen in the examples of  FIGS. 12A and 12B , the etch process may also etch back the second dielectric layer  214 , especially in embodiments where the first capping layer  220  and the second dielectric layer  214  have similar compositions. 
     Referring to  FIGS. 13A and 13B , a second capping layer  226  is formed with a conformal profile, covering exposed surfaces of the first capping layer  220 , the semiconductor liner layer  218 , fin structures  204   b,  and the dielectric fins  212 . The second capping layer  226  is formed of a material different from the first capping layer  220 . As shown in  FIGS. 13A and 13B , the second capping layer  226  physically contact portions of the semiconductor liner layer  218  disposed on top surfaces of the fin structures  204   a,    204   c  and that laterally surround portions  224   a  of the fin structures  204   a  and portions  224   c  of the fin structures  204   c.  The second capping layer  226  may include silicon oxide or a dielectric oxide material having a dielectric constant greater than a dielectric constant of silicon oxide (examples being aluminum oxide (Al 2 O 3 ), hafnium oxide (HfO 2 ), zirconium oxide (ZrO 2 ), Y 2 O 3 , La 2 O 5 , Gd 2 O 5 , TiO 2 , Ta 2 O 5 , HfErO, HfLaO, HfYO, HfGdO, HfAlO, HfZrO, HfTiO, HfTaO, SrTiO, combinations thereof, or the like). The second capping layer  226  may be formed by any suitable technique including ALD, CVD, thermal oxidation, ozone oxidation, etc. 
     Referring to  FIGS. 14A and 14B , an anneal process  228  is performed to induce a solid phase diffusion of dopants from the semiconductor liner layer  218  into the fin structures  204   a,    204   c.  In embodiments where the semiconductor liner layer  218  includes SiGe, the anneal process  228  induces the solid phase diffusion (also referred to as solid-state diffusion) of germanium into the fin structures  204   a,    204   c.  Suitable annealing processes include rapid thermal annealing (RTA), spike annealing, millisecond annealing, laser annealing, other suitable annealing techniques, or a combination thereof. The anneal process  228  may be performed at a temperature in a range from about 900 degrees Celsius to about 1100 degrees Celsius (e.g. about 950 degrees Celsius). A duration of the anneal process  228  may be in a range from about 5 seconds to about 60 seconds (e.g. in the range from about 10 seconds to about 50 seconds). The temperature ranges and duration ranges have an effect of increasing thermal stability of the first capping layer  220  and the second capping layer  226  to the anneal process  228  (e.g. since the stated temperature and/or duration ranges prevents the formation of cracks in the first capping layer  220  and the second capping layer  226 ). Furthermore, when the temperature and duration are in the stated ranges, the first capping layer  220  and the second capping layer  226  suppress dissipation or dissociation of dopants (e.g. germanium) from the semiconductor liner layer  218  before and during the anneal process  228 . Additionally, the first capping layer  220  and the second capping layer  226  prevent oxidation of the semiconductor liner layer  218  before and during the anneal process  228 . 
     The first capping layer  220  and the second capping layer  226 , having different material compositions, have different effects on the drive-in of the dopant of the semiconductor liner layer  218  into the fin structures  204   a,    204   c.  Generally, it has been noted that for the same temperature and duration of the anneal process  228 , a nitride-containing capping layer causes a faster thermal drive-in of the dopant of the semiconductor liner layer  218  compared to an oxide-containing capping layer. Consequently, the dopant in portions of the semiconductor liner layer  218  overlying the lower portions  222   a,    222   c  of fin structures  204   a,    204   c  diffuses laterally (e.g. in the Y-direction) into the fin structures  204   a,    204   c  at a faster rate than the dopant in portions of the semiconductor liner layer  218  overlying the upper portions  224   a,    224   c  of fin structures  204   a,    204   c.  As a result, the dopant concentration (e.g. average germanium concentration) in the lower portions  222   a  of fin structures  204   a  is greater than the dopant concentration (e.g. germanium concentration) in the upper portions  224   a  of the fin structures  204   a.  Similarly, the dopant concentration (e.g. average germanium concentration) in the lower portions  222   c  of fin structures  204   c  is greater than the dopant concentration (e.g. average germanium concentration) in the upper portions  224   c  of the fin structures  204   c.  As seen in  FIGS. 14A and 14B , a result of the anneal process  228  and the different materials of the first capping layer  220  and the second capping layer  226  is that the dopant (e.g. germanium) is homogeneously distributed in the lower portions  222   a,    222   c  of the fin structures  204   a,    204   c.  In contrast, the dopant (e.g. germanium) is distributed along a peripheral region of the upper portions  224   a,    224   c  of the fin structures  204   a,    204   c,  while a central region CR (e.g. fin core) of each of the upper portions  224   a,    224   c  is substantially free from the driven-in dopant (e.g. germanium). 
     Referring to  FIGS. 15A and 15B , the first capping layer  220  and the second capping layer  226  are removed (e.g. using an etch process). Any suitable etching technique may be used to remove the first capping layer  220  and the second capping layer  226 , including dry etching, wet etching, RIE, and/or other etching methods. In an exemplary embodiment, the second capping layer  226  (including an oxide-containing material) may be etched by a diluted hydrofluorine solution, while the first capping layer  220  (including a nitride-containing material) may be etched by a phosphoric acid solution. In another exemplary embodiment, an anisotropic dry etching is used to selectively remove the first capping layer  220  and the second capping layer  226  with a proper etching gas, such as fluorine-containing or chlorine-containing gas, without etching other features shown in  FIGS. 15A and 15B . 
     Referring to  FIGS. 16A and 16B , a trim process is performed on the fin structures  204   a,    204   c.  During the trim process, non-activated portions of the semiconductor liner layer  218  are removed, thereby reduce a dimension (e.g. width W, as measured in the Y-direction) of the fin structures  204   a,    204   c.  In some embodiments, the width W may be in a range from about 5 nm to about 8 nm. In general, the width W may depend, at least in part, on the desired channel length for the P-type semiconductor devices formed in the first region  202   a  and the third region  202   c.  The trim process may include wet or dry etching processes known in the art. The intensity of the trim process may be greater at the upper portions of the fin structures  204   a,    204   c  compared to the lower portions thereof. However, since the dopant concentration (e.g. average germanium concentration) in the upper portions of the fin structures  204   a,    204   c  is less than the dopant concentration (e.g. average germanium concentration) in the lower portions of the fin structures  204   a,    204   c,  fin top loss is reduced, thereby resulting in better vertical fin profile tuning and better control of the critical dimension (CD) of the fin structures  204   a,    204   c.  In some embodiments, an angle a subtended between a sidewall of a fin structure of the fin structures  204   a,    204   c  and a top surface of the recessed first dielectric layer  208 ′ may be in a range from about 85° to about 95°. 
     In some embodiments, after the operation shown in  FIGS. 16A and 16B , channel regions and source/drain regions of the fin structures  204   a,    204   b,    204   c  are processed to form gate structures and source/drain features therein, respectively. As an example, referring to  FIGS. 17A and 17B , a third dielectric layer  230  is formed. In some embodiments, the third dielectric layer  230  includes silicon oxide deposited by a suitable method, such as ALD, CVD, thermal oxidation, ozone oxidation, etc. The formation of the third dielectric layer  230  may further include a subsequent annealing process to improve the material quality, such as increasing the material density and reducing the defects. In the present embodiment, the third dielectric layer  230  is a conformal layer to the profile of the fin structures  204   a,    204   b,    204   c  and the dielectric fins  212 . 
     Referring to  FIGS. 18A and 18B , a gate stack  232  is formed over channel regions of the fin structures  204   a,    204   b,    204   c.  The gate stack  232  may be replaced by metal gate stack at a later fabrication stage, and therefore also be referred to as a dummy gate stack. The dummy gate stack  232  may be formed by depositing a dummy gate layer containing polysilicon or other suitable material and patterning the layer. A gate hard mask layer  234  may be formed on the dummy gate material layer and may be used as an etch mask during the patterning of the dummy gate layer. The gate hard mask layer  234  may include any suitable material, such as a silicon oxide, a silicon nitride, a silicon carbide, a silicon oxynitride, other suitable materials, and/or combinations thereof. In one embodiment, the gate hard mask layer  234  includes dual mask material films  234   a  and  234   b,  such as silicon oxide and silicon nitride. In some embodiments, the patterning process to form the dummy gate stack includes forming a patterned resist layer by lithography process; etching the hard mask layer using the patterned resist layer as an etch mask; and etching the dummy gate layer to form the dummy gate stack using the patterned hard mask layer as an etch mask. In some embodiments, one or more gate sidewall feature (gate spacer, not shown) is formed on the sidewalls of the dummy gate stack  232 . The gate sidewall feature may be used to offset the subsequently formed source/drain features and may be used for designing or modifying the source/drain profile. The gate sidewall feature may include any suitable dielectric material, such as a semiconductor oxide, a semiconductor nitride, a semiconductor carbide, a semiconductor oxynitride, other suitable dielectric materials, and/or combinations thereof. In some embodiments, the gate sidewall feature may include multiple layers, such as a first gate spacer (or a seal layer) on sidewalls of the dummy gate stack  232  and a second gate spacer on the first gate spacer. In furtherance of such embodiments, the first gate spacer is silicon oxide, and the second gate spacer is silicon nitride. In one example, the gate sidewall features are formed by deposition and anisotropic etching, such as dry etching. In another example, the first gate spacer is formed by ALD and the second gate spacer is formed by deposition and anisotropic etching. 
     Referring to  FIGS. 19A and 19B , epitaxial source/drain features  236   a,    236   b ,  236   c  within the source/drain regions of fin structures  204   a,    204   b,    204   c,  respectively. The epitaxial source/drain features  236   a,    236   b,    236   c  may be formed by selective epitaxy growth for strain effect with enhanced carrier mobility and device performance. The dummy gate stack  232  and gate sidewall feature limit and constrain such that the source/drain features  236   a,    236   b,    236   c  are self-aligned in the source/drain regions. In many embodiments, the source/drain features  236   a,    236   b,    236   c  are formed by one or more epitaxial growth (epitaxial process), whereby silicon (Si) features, silicon germanium (SiGe) features, silicon carbide (SiC) features, and/or other suitable semiconductor features are grown in a crystalline state on the fin structures  204   a,    204   b,    204   c  within the source/drain regions thereof. In an alternative embodiment, such as in the example shown in  FIGS. 19A and 19B , an etching process is applied to recess portions of the source/drain regions of the fin structures  204   a,    204   b,    204   c  before the epitaxy growth. The etching process may also remove any dielectric material disposed on the source/drain regions, such as during the formation of the gate sidewall features. Suitable epitaxy processes include CVD deposition techniques (e.g., vapor-phase epitaxy (VPE) and/or ultra-high vacuum CVD (UHV-CVD), molecular beam epitaxy, and/or other suitable processes. 
     The source/drain features  236   a,    236   b,    236   c  may be in-situ doped during the epitaxy process by introducing doping species including: P-type dopants, such as boron or BF 2 ; and N-type dopants, such as phosphorus or arsenic. If the source/drain features  236   a,    236   b,    236   c  are not in-situ doped, an implantation process (i.e., a junction implant process) is performed to introduce the corresponding dopant into the source/drain features  236   a,    236   b,    236   c.  In an exemplary embodiment, the source/drain features  236   b  of an N-type semiconductor device include silicon doped with phosphorous (SiP) or silicon carbide doped with phosphorous (SiCP). Source/drain features  236   a,    236   c  of a P-type semiconductor device include silicon germanium doped with boron (SiGeB), SiGeSnB (tin may be used to tune the lattice constant) and/or GeSnB. In some other embodiments, source/drain features  236   a,    236   b,    236   c  may include more than one semiconductor material layers. One or more annealing processes may be performed thereafter to activate the source/drain features  236   a,    236   b,    236   c.  Suitable annealing processes include rapid thermal annealing (RTA), laser annealing processes, other suitable annealing technique or a combination thereof. 
     Referring to  FIGS. 20A and 20B , an inter-level dielectric material (ILD)  238  is formed to cover the source/drain features  236   a,    236   b,    236   c  in the source/drain regions of the fin structures  204   a,    204   b,    204   c.  The ILD  238  acts as an insulator that supports and/or electrically isolates conductive traces (such as contacts, vias and metal lines) to be formed therein. The ILD  238  may include any suitable dielectric material, such as a silicon oxide, low-k dielectric material, porous dielectric material, other suitable dielectric material or a combination thereof. In some embodiments, an etch stop layer may be interposed between the ILD  238  and the source/drain features  236   a,    236   b,    236   c.  The etch stop layer functions as etch stopping during the etching to form contacts in the ILD during later fabrication stages. The etch stop layer includes a material different from that of the ILD  238  to provide etch selectivity. For example, the etch stop layer may include silicon nitride deposited by CVD or ALD. In some embodiments, the formation of the ILD  238  includes deposition and CMP to provide a planarized top surface. The gate hard mask layer  234  may be removed during the CMP process, an additional etching operation, or a combination thereof. 
     Referring to  FIGS. 21A and 21B , metal gate stack  240  replaces the dummy gate stack  232 . In an embodiment, one or more etching processes may be used to remove the dummy gate stack  232  and the third dielectric layer  230 , thereby forming gate trenches. After the removal of the dummy gate stack  232  and the third dielectric layer  230 , metal gate materials are deposited in the gate trenches, and a CMP process is applied to remove the excessive gate materials and planarize the top surface. The gate materials of the metal gate stack  240  include gate dielectric layer  240   a  and gate electrode  240   b.  In some embodiments, the gate dielectric layer  240   a  includes a high-k dielectric material, and the gate electrode  240   b  includes metal or metal alloy. The metal gate stack  240  is formed such that it wraps around the channel regions of the fin structures  204   a,    204   b,    204   c.  In some examples, the gate dielectric layer  240   a  and the gate electrode  240   b  each may include a number of sub-layers. The high-k dielectric layer may include metal oxide, metal nitride, such as LaO, AlO, ZrO, TiO, Ta 2 O 5 , Y 2 O 3 , SrTiO 3  (STO), BaTiO 3  (BTO), BaZrO, HfZrO, HfLaO, HfSiO, LaSiO, AlSiO, HfTaO, HfTiO, (Ba,Sr)TiO 3  (BST), Al 2 O 3 , Si 3 N 4 , oxynitrides (SiON), or other suitable dielectric materials. The high-k dielectric layer is deposited a suitable technique, such as ALD, CVD, metal-organic CVD (MOCVD), PVD, thermal oxidation, and/or other suitable techniques. The gate dielectric layer  240   a  may additionally include an interfacial layer disposed between the fin structure and the high-k dielectric layer. The interfacial layer may include silicon oxide, silicon nitride, silicon oxynitride, and/or other suitable material, deposited by a suitable method, such as ALD, CVD, ozone oxidation, etc. 
     The gate electrode material is then filled in the gate trench. The gate electrode  240   b  is formed by ALD, PVD, CVD, plating, other suitable process, or a combination thereof. The gate electrode  240   b  may include a single layer or multiple layers, such as a metal layer, a liner layer, a wetting layer, and/or an adhesion layer. The gate electrode  240   b  may include Ti, Ag, Al, TiAlN, TaC, TaCN, TaSiN, Mn, Zr, TiN, TaN, Ru, Mo, Al, WN, Cu, W, or any suitable materials. In some embodiments, different metal materials are used for nFET and pFET devices with respective work functions, such as with a work function (WF) of 4.2 eV or less for nFET and a work function of 5.2 eV or greater for pFET. In some embodiments, the n-type WF metal includes tantalum (Ta). In other embodiments, the n-type WF metal includes titanium aluminum (TiAl), titanium aluminum nitride (TiAlN), or combinations thereof. In other embodiments, the n-metal include Ta, TiAl, TiAlN, tungsten nitride (WN), or combinations thereof. The n-type WF metal may include various metal-based films as a stack for optimized device performance and processing compatibility. In some embodiments, the P-type WF metal includes titanium nitride (TiN) or tantalum nitride (TaN). In other embodiments, the p-metal include TiN, TaN, tungsten nitride (WN), titanium aluminum (TiAl), or combinations thereof. The P-type WF metal may include various metal-based films as a stack for optimized device performance and processing compatibility. The work function metal is deposited by a suitable technique, such as PVD. In other examples, a barrier layer may be formed on deposited for lining the gate trenches before filling the metal. The barrier layer may include titanium, titanium nitride, tantalum, tantalum nitride or combination, deposited by suitable technique, such as PVD. The gate electrode  240   b  includes a capping layer, a metal layer to tune the work function, and a filling metal, such as aluminum, copper or tungsten. In subsequent steps, contact features are formed to the metal gate stack  240  and the source/drain features  236   a,    236   b,    236   c  to electrically connect the FETs to form a functional circuit. 
       FIG. 22  shows a dopant (e.g. germanium) concentration profile along the height H of the fin structures  204   a,    204   c,  according to aspects of the present disclosure. As seen in  FIG. 22 , each of fin structures  204   a,    204   c  shows higher average dopant concentration in the lower portions LP of the fin structures  204   a,    204   c  and lower average dopant concentration in the upper portions UP of the fin structures  204   a,    204   c.  In some instances, the dopant concentration in the base portions BP of the fin structures  204   a,    204   c  is zero. In some implementations, the difference between the average dopant concentration in the upper portions UP of the fin structures  204   a,    204   c  and the average dopant concentration in the lower portions LP of the fin structures  204   a,    204   c  (depicted in  FIG. 22  as concentration difference  41 ) is within the range from about 3% to about 10% (e.g. about 5%). This range of values for the concentration difference  41  has an advantageous effect on device performance since the stated range balances the need for a device boost (e.g., by inducing a strain in the channel through lattice mismatch) and the need for threshold voltage uniformity. 
     In the embodiments discussed above, the first capping layer  220  is a nitride-containing layer, while the second capping layer  226  is an oxide-containing layer. In other embodiments, the compositions may be switched—namely, the first capping layer  220  is an oxide-containing layer, while the second capping layer  226  is a nitride-containing layer.  FIG. 23  shows a dopant (e.g. germanium) concentration profile along the height H of the fin structures  204   a,    204   c  in such embodiments. In  FIG. 23 , each of fin structures  204   a,    204   c  shows higher average dopant concentration in the upper portions UP of the fin structures  204   a,    204   c  and lower average dopant concentration in the lower portions LP of the fin structures  204   a,    204   c.  In some instances, the dopant concentration in the base portions BP of the fin structures  204   a,    204   c  is zero. In some implementations, the difference between the average dopant concentration in the upper portions UP of the fin structures  204   a,    204   c  and the average dopant concentration in the lower portions LP of the fin structures  204   a,    204   c  (depicted in  FIG. 23  as difference  42 ) is within the range from about 3% to about 10% (e.g. about 5%). This range of values for the concentration difference  42  has an advantageous effect on device performance since the stated range balances the need for a device boost (e.g., by inducing a strain in the channel through lattice mismatch) and the need for threshold voltage uniformity. 
     Although the semiconductor device structure proposed herein has a non-uniform dopant concentration profile along the height H of the fin structures  204   a,    204   c,  the result of such a non-uniform concentration profile is a uniform threshold voltage distribution along the height H of fin structures  204   a,    204   c.  The non-uniform dopant concentration profile along the height H of the fin structures  204   a,    204   c  compensates for the factors that might affect the threshold voltage along the height H of fin structures  204   a,    204   c.  The uniform threshold voltage distribution along the height H of fin structures  204   a,    204   c  results in better DC performance of the semiconductor device structure (e.g. since variation of off-state current density and on-state current density along the height H of fin structures is reduced). The present disclosure also provides methods of forming a semiconductor device structure with a uniform threshold voltage distribution along the height H of fin structures (e.g. as seen in  FIGS. 2A-21A and 2B-21B ). The proposed method results in less fin top loss during the manufacturing process and improved vertical fin profile tuning. 
     One aspect of the present disclosure involves a method that includes: forming a semiconductor liner layer on exposed surfaces of a fin structure that extends above a dielectric isolation structure disposed over a substrate; forming a first capping layer to laterally surround a bottom portion of the semiconductor liner layer; forming a second capping layer over an upper portion of the semiconductor liner layer; and annealing the fin structure having the semiconductor liner layer, the first capping layer, and the second capping layer thereon, the annealing driving a dopant from the semiconductor liner layer into the fin structure, wherein a dopant concentration profile in a bottom portion of the fin structure is different from a dopant concentration profile in an upper portion of the fin structure. 
     Another aspect of the present disclosure involves a method that includes: forming a fin structure including a source region, a drain region, and a channel region interposed between the source region and the drain region; processing the channel region of the fin structure such that a threshold voltage is uniform across a height of the fin structure, the processing including: forming a semiconductor liner layer including a dopant over the channel region of the fin structure; forming a first capping layer to physically contact a bottom portion of the semiconductor liner layer; forming a second capping layer to physically contact an upper portion of the semiconductor liner layer; and annealing the channel region of the fin structure having the semiconductor liner layer, the first capping layer, and the second capping layer thereon, the annealing causing the dopant of the semiconductor liner layer to diffuse into the channel region of the fin structure, wherein an average dopant concentration in a bottom portion of the channel region is different from an average dopant concentration in an upper portion of the channel region; etching the channel region of the fin structure to remove the first capping layer, the second capping layer, and the semiconductor liner layer, the etching forming a trimmed fin structure; and forming a gate structure over the channel region of the trimmed fin structure. 
     Yet another aspect of the present disclosure involves a semiconductor device structure that includes: a fin structure extending above a dielectric isolation structure disposed over a silicon substrate. The fin structure including a source region, a drain region; and a channel region disposed between the source region and the drain region. An average germanium concentration in a bottom region of the channel region is different from an average germanium concentration in an upper region of the channel region, and a threshold voltage of the fin structure is uniform across a height of the fin structure. 
     The foregoing outlines features of several embodiments so that those of ordinary skill in the art may better understand the aspects of the present disclosure. Those of ordinary skill in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those of ordinary skill in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure.