Patent Publication Number: US-2022238696-A1

Title: Fin Field-Effect Transistor Device and Method of Forming the Same

Description:
PRIORITY CLAIM AND CROSS-REFERENCE 
     This application is a divisional of U.S. patent application Ser. No. 16/822,609, filed Mar. 18, 2020 and entitled “Fin Field-Effect Transistor Device and Method of Forming the Same,” which claims priority to U.S. Provisional Patent Application No. 62/928,812, filed Oct. 31, 2019 and entitled “Fin Field-Effect Transistor Device and Method of Forming the Same,” which applications are hereby incorporated by reference in their entireties. 
    
    
     BACKGROUND 
     The semiconductor industry has experienced rapid growth due to continuous improvements in the integration density of a variety of electronic components (e.g., transistors, diodes, resistors, capacitors, etc.). For the most part, this improvement in integration density has come from repeated reductions in minimum feature size, which allows more components to be integrated into a given area. 
     Fin Field-Effect Transistor (FinFET) devices are becoming commonly used in integrated circuits. FinFET devices have a three-dimensional structure that comprises a semiconductor fin protruding from a substrate. A gate structure, configured to control the flow of charge carriers within a conductive channel of the FinFET device, wraps around the semiconductor fin. For example, in a tri-gate FinFET device, the gate structure wraps around three sides of the semiconductor fin, thereby forming conductive channels on three sides of the semiconductor fin. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIG. 1  illustrates a perspective view of a Fin Field-Effect Transistor (FinFET) device, in accordance with some embodiments. 
         FIGS. 2-4, 5A, 5B, 6, 7A-7F, 8A, 8B, 9A, 9B, 10A, 10B, 11A, 11B, 12A, 12B, 13A, 13B, 14A, 14B ,  15 A,  15 B, and  15 C illustrate various views of a FinFET device  100  at various stages of fabrication, in accordance with an embodiment. 
         FIGS. 16A, 16B, 17A, 17B, 18A, 18B, 19A, 19B, and 19C  illustrate cross-sectional views of a FinFET device  100 A at various stages of fabrication, in an embodiment. 
         FIGS. 20A, 20B, 21A, and 21B  illustrate cross-sectional views of a FinFET device  100 B at various stages of fabrication, in an embodiment. 
         FIGS. 22A-22C  illustrate various embodiment cross-sectional views of the gate electrode of a FinFET device, in some embodiments. 
         FIGS. 23A, 23B, 24A, and 24B  illustrate various views of a FinFET device  100 C at various stages of fabrication, in an embodiment. 
         FIGS. 25-30  illustrate cross-sectional views of a portion of a FinFET device  200  at various stages of fabrication, in an embodiment. 
         FIG. 31  illustrate a cross-sectional view of a portion of a FinFET device  200 A, in an embodiment. 
         FIG. 32  illustrate a cross-sectional view of a portion of a FinFET device  200 B, in an embodiment. 
         FIG. 33  illustrates a flow chart of method of forming a semiconductor device, in accordance with some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The following disclosure provides many different embodiments, or examples, for implementing different features of the invention. 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. 
     Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. 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 figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly. 
     Embodiments of the present disclosure are discussed in the context of forming a FinFET device, and in particular, in the context of forming a replacement gate (e.g., a metal gate) for a FinFET device. 
     In some embodiments, a dummy gate electrode is formed over a fin that is disposed between isolation regions. A lower portion of the dummy gate electrode is then removed to form a gap between the dummy gate electrode and isolation regions. Next, a gate fill material is formed to fill the gap, and a spacer is formed along sidewalls of the dummy gate electrode and along sidewalls of the gate fill material. After the gate spacers are formed, the dummy gate electrode and at least portions of the gate fill material are removed to form an opening between the gate spacers, and a metal gate structure is formed in the opening. 
       FIG. 1  illustrates an example of a FinFET  30  in a perspective view. The FinFET  30  includes a substrate  50  and a fin  64  protruding above the substrate  50 . Isolation regions  62  are formed on opposing sides of the fin  64 , with the fin  64  protruding above the isolation regions  62 . A gate dielectric  66  is along sidewalls and over a top surface of the fin  64 , and a gate electrode  68  is over the gate dielectric  66 . Source/drain regions  80  are in the fin  64  and on opposing sides of the gate dielectric  66  and the gate electrode  68 .  FIG. 1  further illustrates reference cross-sections that are used in later figures. Cross-section B-B extends along a longitudinal axis of the gate electrode  68  of the FinFET  30 . Cross-section A-A is perpendicular to cross-section B-B and is along a longitudinal axis of the fin  64  and in a direction of, for example, a current flow between the source/drain regions  80 . Cross-section C-C is parallel to cross-section B-B and is across the source/drain region  80 . Cross-section D-D is parallel to cross-section A-A and is outside of the fin  64  (e.g., between two adjacent fins). Subsequent figures refer to these reference cross-sections for clarity. 
       FIGS. 2-4, 5A, 5B, 6, 7A-7F, 8A, 8B, 9A, 9B, 10A, 10B, 11A, 11B, 12A, 12B, 13A, 13B, 14A, 14B ,  15 A,  15 B, and  15 C illustrate various views (e.g., cross-sectional view, perspective view, or top view) of a FinFET device  100  at various stages of fabrication, in accordance with an embodiment. The FinFET device  100  is similar to the FinFET  30  in  FIG. 1 , but with multiple fins and multiple gate structures. Throughout the discussion herein, figures with the same numeral but different letters (e.g.,  FIG. 5A  and  FIG. 5B ) refer to different views of the FinFET device at a same processing stage.  FIGS. 2-4 and 5A  illustrate cross-sectional views of the FinFET device  100  along cross-section B-B.  FIGS. 5B, 6, and 7A  illustrate cross-sectional views of the FinFET device  100  along cross-section D-D.  FIGS. 7B and 7C  illustrate a perspective view and a cross-sectional view along cross-section B-B, respectively.  FIGS. 7D, 7E, and 7F  illustrate a cross-sectional view along cross-section A-A, a cross-sectional view along cross-section C-C, and a top view of the FinFET device  100 , respectively.  FIGS. 8A, 9A, 10A, 11A, 12A, 13A, 14A, and 15A  illustrate cross-sectional views of the FinFET device  100  along cross-section D-D, and  FIGS. 8B, 9B, 10B, 11B, 12B, 13B, 14B, and 15B  illustrate cross-sectional views of the FinFET device  100  along cross-section A-A.  FIG. 15C  is a zoomed-in view of a portion of  FIG. 15A . 
       FIG. 2  illustrates a cross-sectional view of the substrate  50 . The substrate  50  may be a semiconductor substrate, such as a bulk semiconductor, a semiconductor-on-insulator (SOI) substrate, or the like, which may be doped (e.g., with a p-type or an n-type dopant) or undoped. The substrate  50  may be a wafer, such as a silicon wafer. Generally, an SOI substrate includes a layer of a semiconductor material formed on an insulator layer. The insulator layer may be, for example, a buried oxide (BOX) layer, a silicon oxide layer, or the like. The insulator layer is provided on a substrate, typically a silicon or glass substrate. Other substrates, such as a multi-layered or gradient substrate may also be used. In some embodiments, the semiconductor material of the substrate  50  may include silicon; germanium; a compound semiconductor including silicon carbide, gallium arsenic, gallium phosphide, indium phosphide, indium arsenide, and/or indium antimonide; an alloy semiconductor including SiGe, GaAsP, AlInAs, AlGaAs, GaInAs, GaInP, and/or GaInAsP; or combinations thereof. 
     Referring to  FIG. 3 , the substrate  50  shown in  FIG. 2  is patterned using, for example, photolithography and etching techniques. For example, a mask layer, such as a pad oxide layer  52  and an overlying pad nitride layer  56 , is formed over the substrate  50 . The pad oxide layer  52  may be a thin film comprising silicon oxide formed, for example, using a thermal oxidation process. The pad oxide layer  52  may act as an adhesion layer between the substrate  50  and the overlying pad nitride layer  56 . In some embodiments, the pad nitride layer  56  is formed of silicon nitride, silicon oxynitride, silicon carbonitride, the like, or a combination thereof, and may be formed using low-pressure chemical vapor deposition (LPCVD) or plasma enhanced chemical vapor deposition (PECVD), as examples. 
     The mask layer may be patterned using photolithography techniques. Generally, photolithography techniques utilize a photoresist material (not shown) that is deposited, irradiated (exposed), and developed to remove a portion of the photoresist material. The remaining photoresist material protects the underlying material, such as the mask layer in this example, from subsequent processing steps, such as etching. In this example, the photoresist material is used to pattern the pad oxide layer  52  and pad nitride layer  56  to form a patterned mask  58 , as illustrated in  FIG. 3 . 
     The patterned mask  58  is subsequently used to pattern exposed portions of the substrate  50  to form trenches  61 , thereby defining semiconductor fins  64  (e.g.,  64 A and  64 B) between adjacent trenches  61  as illustrated in  FIG. 3 . In some embodiments, the semiconductor fins  64  are formed by etching trenches in the substrate  50  using, for example, reactive ion etch (RIE), neutral beam etch (NBE), the like, or a combination thereof. The etching process may be anisotropic. In some embodiments, the trenches  61  may be strips (viewed from in the top) parallel to each other, and closely spaced with respect to each other. In some embodiments, the trenches  61  may be continuous and surround the semiconductor fins  64 . The semiconductor fins  64  may also be referred to as fins  64  hereinafter. Two fins  64  are illustrated in  FIG. 3  as a non-limiting example. Other numbers of fins are also possible and are fully intended to be included within the scope of the present disclosure. 
     The fins  64  may be patterned by any suitable method. For example, the fins  64  may be patterned using one or more photolithography processes, including double-patterning or multi-patterning processes. Generally, double-patterning or multi-patterning processes combine photolithography and self-aligned processes, allowing patterns to be created that have, for example, pitches smaller than what is otherwise obtainable using a single, direct photolithography process. For example, in one embodiment, a sacrificial layer is formed over a substrate and patterned using a photolithography process. Spacers are formed alongside the patterned sacrificial layer using a self-aligned process. The sacrificial layer is then removed, and the remaining spacers, or mandrels, may then be used to pattern the fins. 
       FIG. 4  illustrates the formation of an insulation material between neighboring semiconductor fins  64  to form isolation regions  62 . The insulation material may be an oxide, such as silicon oxide, a nitride, the like, or a combination thereof, and may be formed by a high density plasma chemical vapor deposition (HDP-CVD), a flowable CVD (FCVD) (e.g., a CVD-based material deposition in a remote plasma system and post curing to make it convert to another material, such as an oxide), the like, or a combination thereof. Other insulation materials and/or other formation processes may be used. In the illustrated embodiment, the insulation material is silicon oxide formed by a FCVD process. An anneal process may be performed once the insulation material is formed. A planarization process, such as a chemical mechanical polish (CMP), may remove any excess insulation material and form top surfaces of the isolation regions  62  and top surfaces of the semiconductor fins  64  that are coplanar (not shown). The patterned mask  58  (see  FIG. 3 ) may also be removed by the planarization process. 
     In some embodiments, the isolation regions  62  include a liner, e.g., a liner oxide (not shown), at the interface between the isolation region  62  and the substrate  50 /semiconductor fins  64 . In some embodiments, the liner oxide is formed to reduce crystalline defects at the interface between the substrate  50  and the isolation region  62 . Similarly, the liner oxide may also be used to reduce crystalline defects at the interface between the semiconductor fins  64  and the isolation region  62 . The liner oxide (e.g., silicon oxide) may be a thermal oxide formed through a thermal oxidation of a surface layer of substrate  50 , although other suitable method may also be used to form the liner oxide. 
     Next, the isolation regions  62  are recessed to form shallow trench isolation (STI) regions  62 . The isolation regions  62  are recessed such that the upper portions of the semiconductor fins  64  protrude from between neighboring STI regions  62 . The top surfaces of the STI regions  62  may have a flat surface (as illustrated), a convex surface, a concave surface (such as dishing), or a combination thereof. The top surfaces of the STI regions  62  may be formed flat, convex, and/or concave by an appropriate etch. The isolation regions  62  may be recessed using an acceptable etching process, such as one that is selective to the material of the isolation regions  62 . For example, a dry etch, or a wet etch using dilute hydrofluoric (dHF) acid, may be performed to recess the isolation regions  62 . 
       FIGS. 2 through 4  illustrate an embodiment of forming fins  64 , but fins may be formed in various different processes. For example, a top portion of the substrate  50  may be replaced by a suitable material, such as an epitaxial material suitable for an intended type (e.g., N-type or P-type) of semiconductor devices to be formed. Thereafter, the substrate  50 , with epitaxial material on top, is patterned to form semiconductor fins  64  that comprise the epitaxial material. 
     As another example, a dielectric layer can be formed over a top surface of a substrate; trenches can be etched through the dielectric layer; homoepitaxial structures can be epitaxially grown in the trenches; and the dielectric layer can be recessed such that the homoepitaxial structures protrude from the dielectric layer to form fins. 
     In yet another example, a dielectric layer can be formed over a top surface of a substrate; trenches can be etched through the dielectric layer; heteroepitaxial structures can be epitaxially grown in the trenches using a material different from the substrate; and the dielectric layer can be recessed such that the heteroepitaxial structures protrude from the dielectric layer to form fins. 
     In embodiments where epitaxial material(s) or epitaxial structures (e.g., the heteroepitaxial structures or the homoepitaxial structures) are grown, the grown material(s) or structures may be in situ doped during growth, which may obviate prior and subsequent implantations although in situ and implantation doping may be used together. Still further, it may be advantageous to epitaxially grow a material in an NMOS region different from the material in a PMOS region. In various embodiments, the fins  64  may comprise silicon germanium (Si x Ge 1-x , where x can be between 0 and 1), silicon carbide, pure or substantially pure germanium, a III-V compound semiconductor, a II-VI compound semiconductor, or the like. For example, the available materials for forming III-V compound semiconductor include, but are not limited to, InAs, AlAs, GaAs, InP, GaN, InGaAs, InAlAs, GaSb, AlSb, AlP, GaP, and the like. 
       FIGS. 5A and 5B  illustrate the formation of dummy gate structure  75  over the semiconductor fins  64 . The dummy gate structure  75  includes gate dielectric  66  and gate electrode  68 , in some embodiments. A mask  70  may be formed over the dummy gate structure  75 . To form the dummy gate structure  75 , a dielectric layer is formed on the semiconductor fins  64 . The dielectric layer may be, for example, silicon oxide, silicon nitride, multilayers thereof, or the like, and may be deposited or thermally grown. 
     A gate layer is formed over the dielectric layer, and a mask layer is formed over the gate layer. The gate layer may be deposited over the dielectric layer and then planarized, such as by a CMP. The mask layer may be deposited over the gate layer. The gate layer may be formed of, for example, polysilicon, although other materials may also be used. The mask layer may be formed of, for example, silicon nitride or the like. 
     After the layers (e.g., the dielectric layer, the gate layer, and the mask layer) are formed, the mask layer may be patterned using acceptable photolithography and etching techniques to form mask  70 . The pattern of the mask  70  then may be transferred to the gate layer and the dielectric layer by an acceptable etching technique to form gate electrode  68  and gate dielectric  66 , respectively. The gate electrode  68  and the gate dielectric  66  cover respective channel regions of the semiconductor fins  64 . The gate electrode  68  may also have a lengthwise direction substantially perpendicular to the lengthwise direction of respective semiconductor fins  64 . 
     The gate dielectric  66  is shown to be formed (e.g., by thermal oxidization of the material of the fins  64 ) over the fins  64  (e.g., over top surfaces and sidewalls of the fins  64 ) but not over the STI regions  62  in the example of  FIG. 5A . In other embodiments, the gate dielectric  66  may be formed (e.g., deposited) over the fins  64  and over the STI regions  62 . For example, the gate dielectric  66  may extends continuously from the fin  64 A to the fin  64 B. These and other variations are fully intended to be included within the scope of the present disclosure.  FIG. 5B  shows the corresponding cross-section view along cross-section D-D. Two dummy gate structures  75  are illustrated in  FIG. 5B  as a non-limiting example. Other numbers of dummy gate structures are also possible and are fully intended to be included within the scope of the present disclosure. 
       FIGS. 6 and 7A-7F  illustrate the formation of hanging dummy gate structures by removing lower portions of the dummy gate structure  75  proximate to the isolation regions  62 . In  FIG. 6 , a protection layer  71  is formed over the mask  70  and over upper portions of the gate electrode  68 , while lower portions of the gate electrode  68  are exposed by the protection layer  71 . The protection layer  71  is formed of a material different from the material of gate electrode  68 , such that in a subsequent etching process, the protection layer  71  prevents or reduces etching of its underlying layers (e.g., upper portion of gate electrode  68 ). The protection layer  71  may be a dielectric layer, such as a silicon oxide layer or a silicon nitride layer, formed by a suitable deposition process such as PECVD or atomic layer deposition (ALD), although other suitable material, such as a carbon-based coating, may also be used as the protection layer  71 . The discussion hereinafter may refer to the protection layer  71  as a dielectric layer  71 , with the understanding that any suitable material may be used to form the protection layer  71 . 
       FIG. 6  further illustrates the fin  64  in phantom, since the fin  64  is not in the cross-section of  FIG. 6 . In the example of  FIG. 6 , the dielectric layer  71  is formed over upper portions of the gate electrode  68 , which upper portions are disposed above an upper surface  64 U of the fin  64 , while lower portions of the gate electrode  68  disposed below the upper surface  64 U are not covered by the dielectric layer  71 . Therefore, the deposition process of the dielectric layer  71  may be referred to as a depth-selective deposition process. This depth-selective deposition process may be a result of the small space between adjacent fins  64 . As semiconductor manufacturing process continues to advance, features sizes continue to shrink. The distance between two adjacent fins  64  may become so small that the deposition rate of a deposition process becomes low in such small spaces. As a result, when the dielectric layer  71  is being formed, sidewalls of the upper portion of the gate electrode  68 , which is above the fin  64 , are covered by the deposited dielectric layer  71 . In contrast, little or no dielectric layer  71  is formed along sidewalls of the lower portion of the gate electrode  68 . 
     The location of the dielectric layer  71  in  FIG. 6  is merely a non-limiting example. For example, the dielectric layer  71  may extends below the upper surface  64 U of the fin  64 , and may stop at a location between the upper surface  64 U of the fin  64  and the upper surface of the isolation region  62 . In some embodiments, the sidewalls of the lower portion of the gate electrode  68  are also covered by the dielectric layer  71 , but a thickness of the dielectric layer  71  over the lower portions of the gate electrode  68  is smaller than a thickness of the dielectric layer  71  over the upper portions of the gate electrode  68 . For example, the thickness of the dielectric layer  71  may decrease continuously as the gate electrode  68  extends toward the isolation regions  62 . As a result, in a subsequent etching process, the lower portion of the gate electrode  68  is consumed (e.g., etched) more than the upper portion of the gate electrode  68 . 
     Next, in  FIG. 7A , an etching process is performed to remove lower portions of the gate electrode  68 . The etching process uses an etchant that is selective to the material (e.g., polysilicon) of the gate electrode  68 , in some embodiments. A suitable etching process, such as an anisotropic etching process (e.g., a plasma etching process), may be used to remove the lower portions of the gate electrode  68 . In embodiments where plasma etching is used, the lateral etching rate of the plasma etching process is adjusted, e.g., by adjusting a bias power of the plasma etching process, to control the sidewall profile of the gate electrode  68 . In other embodiments, a wet etch process is performed to remove the lower portion of the gate electrode  68 . 
     As illustrated in  FIG. 7A , after the etching process, portions of the gate electrode  68  proximate to the isolation regions  62  are removed, and there is a gap G between a lower surface of each gate electrode  68  and the isolation regions  62 . Note that in the example of  FIG. 7A , the upper surface  68 U of the gate electrode  68  remain the same before and after the etching process, and the lower surface of the gate electrode  68  is moved upward (e.g., toward the upper surface  68 U) by the etching process. Therefore, a height of the gate electrode  68 , measured along a direction perpendicular to the upper surface of the substrate  50 , is reduced. 
     In some embodiments, the dielectric layer  71  is removed (e.g., completely removed) by the etching process to remove the lower portions of the gate electrode  68 . In other embodiments, after the etching process to remove the lower portions of the gate electrode  68  is performed, the dielectric layer  71  is removed by another suitable etching process, e.g., using an etchant selective to the material of the dielectric layer  71 . 
       FIG. 7B  illustrates the perspective view of the FinFET device  100  after the lower portions of the gate electrode  68  are removed. For clarify, not all features of the FinFET device  100  are illustrated in  FIG. 7B . For example, the isolation regions  62  and the substrate  50  are not illustrated in  FIG. 7B . In addition, only one dummy gate structure is illustrated in  FIG. 7B .  FIG. 7B  further illustrates the cross-sections A-A, B-B, C-C, and D-D illustrated in  FIG. 1 . 
       FIG. 7C  illustrates the cross-sectional view of the FinFET device  100  along cross-section B-B. As illustrated in  FIG. 7C , since the lower portions of the gate electrode  68  are removed, the gate electrode  68  no longer contacts (e.g., physically contact) the isolation regions  62  and hangs over the isolation regions  62 . Therefore, the (shortened) dummy gate structure  75  in  FIG. 7C  is also referred to as a hanging dummy gate structure  75 . 
     Since the gate electrode  68  no longer contacts the isolation regions  62 , the hanging dummy gate structure  75  may be prone to collapsing. To prevent the dummy gate structure  75  from collapsing, dimensions of the dummy gate structure  75  and the fins  64  are controlled. In some embodiments, a distance W 1  between two adjacent fins  64  is less than about 200 nm (e.g., 0 nm≤W 1 ≤200 nm), and a distance W 2  between an edge  68 E of the gate electrode  68  and a nearest sidewall of an underlying fin  64  is less than about 100 nm (e.g., 0 nm≤W 2 ≤100 nm). In addition, a depth D 1  between a bottom surface of the gate electrode  68  and the upper surface of the fin  64  is between about 10 nm and about 100 nm, and a height H 1  between the upper surface of the fin  64  and the upper surface of the mask  70  is less than about 500 nm (e.g., 0 nm≤H 1 ≤500 nm). In some embodiments, a ratio between H 1  and D 1  is less than about 30 (e.g., H 1 /D 1 ≤30). The dimensions H 1  and D 1  are also illustrated in  FIG. 7A . By controlling the dimensions (e.g., H 1 , D 1 , W 1 , W 2 ) of the FinFET device  100  to be within the above disclosed ranges, the risk that the hanging dummy gate structure  75  may collapses is reduced or avoided. 
       FIGS. 7D and 7E  illustrate the cross-sectional views of the FinFET device  100  along cross-sections A-A and C-C, respectively. Note that the dummy gate structure  75  is not in the cross-section C-C, thus not illustrated in  FIG. 7E .  FIG. 7F  illustrates a cross-sectional view of the FinFET device  100  along cross-section E-E in  FIG. 7C , where the cross-section E-E is along a plane parallel to the upper surface  50 U of the substrate  50  and cutting across the channel regions of the fins  64 . 
       FIGS. 8A, 8B, 9A, 9B, 10A, 10B, 11A, 11B, 12A, 12B, 13A, 13B, 14A, 14B, and 15A-15C  illustrate additional processing steps to from the FinFET device  100 , in accordance with an embodiment. For simplicity, not all features are illustrated in these figures. For example, the substrate  50  is not illustrated in the figures. To facilitate comparison with subsequent figures, (simplified) cross-sectional views of the FinFET device  100  in  FIGS. 7A and 7D  are shown in  FIGS. 8A and 8B , respectively. 
     Next, in  FIGS. 9A and 9B , a gate fill material  73  is formed over the FinFET device  100  of  FIGS. 8A and 8B . The gate fill material  73  fills the gap G between each dummy gate structure  75  and isolation regions  62 . The gate fill material  73  may also be formed along sidewalls of the dummy gate structure  75 . The gate fill material  73  may be formed in a bottom-up fashion, using a suitable deposition process such as CVD, PECVD, ALD, or plasma-enhanced ALD (PEALD). In a subsequent replacement gate processing, the gate fill material  73  is removed. Therefore, the gate fill material  73  may also be referred to as a dummy gate fill material. In the illustrated embodiment, the gate fill material  73  is formed of a suitable material that provides etching selectivity over (e.g., having different etching rate from) the material of the gate electrode  68 , such that the gate electrode  68  and the gate fill material  73  are removed in two different etching processes. Details are discussed below. Example materials for the gate fill material  73  include silicon germanium (SiGe), silicon nitride (SiN), silicon oxynitride (SiON), silicon oxycarbonitride (SiOCN), silicon carbide (SiC), silicon oxycarbides (SiOC), or silicon oxide (SiO), or the like. 
     Next, in  FIGS. 10A and 10B , an anisotropic etching process, such as a plasma etching process, is performed to remove portions of the gate fill material  73  (e.g., portions disposed outside boundaries of the gate electrode  68 ). In an embodiment where a plasma etching process is used to remove portions of the gate fill material  73 , a bias voltage of the plasma etching process is tuned (e.g., adjusted) to adjust a lateral etching rate of the plasma etching process. In the example of  FIGS. 10A and 10B , portions of the gate fill material  73 , such as portions disposed along sidewalls of the dummy gate structure  75  and portions disposed between dummy gate structures  75 , are removed, and remaining portions of the gate fill material  73  are disposed directly under the dummy gate structures  75  and fill the gaps G (see gaps G in  FIG. 8A ). The sidewalls of the remaining portions of the gate fill material  73  are aligned with respective sidewalls of the dummy gate structures  75 , as illustrated in the example of  FIG. 10A . In other embodiments, after the anisotropic etching process, the remaining portions of the gate fill material  73  fill the gaps G and extend along (e.g., cover) the sidewalls of the dummy gate structure  75  (see, e.g.,  16 A). Note that in the cross-sectional view of  FIG. 10B , no gate fill material  73  is left over the gate dielectric  66  after the anisotropic etching process. 
     Next, in  FIGS. 11A and 11B , gate spacers  87  are formed along sidewalls of the dummy gate structure (e.g.,  68  and  70 ) and along sidewalls of the gate fill material  73 . The gate spacers  87  are formed of a nitride, such as silicon nitride, silicon oxynitride, silicon carbonitride, the like, or a combination thereof, and may be formed using, e.g., a thermal oxidation, CVD, or other suitable deposition process, in some embodiments. The gate spacers  87  are formed of a different material from that of the gate fill material  73  and that of the gate electrode  68 , in some embodiments. 
     In an embodiment, the gate spacer  87  is formed by first conformally depositing a gate spacer layer over the FinFET device  100 . Next, an anisotropic etch process, such as a dry etch process, is performed to remove a first portion of the gate spacer layer disposed on upper surfaces of the FinFET device  100  (e.g., the upper surface of the mask  70 ) while keeping a second portion of the gate spacer layer disposed along sidewalls of the dummy gate structures  75  and along sidewalls of the gate fill material  73 . The second portion of the gate spacer layer remaining after the anisotropic etch process forms the gate spacer  87 . The anisotropic etch process also removes horizontal portions of the gate spacer layer. 
     The shapes and formation methods of the gate spacer  87  as illustrated in  FIGS. 11A and 11B  are merely non-limiting examples, and other shapes and formation methods are possible. These and other variations are fully intended to be included within the scope of the present disclosure. 
     Next, in  FIGS. 12A and 12B , source/drain regions  80  are formed in/over the fin  64  on opposing sides of the dummy gate structure  75 . The source/drain regions  80  are formed by forming recesses in the fin  64 , then epitaxially growing a material in the recesses, using suitable methods such as metal-organic CVD (MOCVD), molecular beam epitaxy (MBE), liquid phase epitaxy (LPE), vapor phase epitaxy (VPE), selective epitaxial growth (SEG), the like, or a combination thereof. The gate dielectric  66  disposed outside boundaries (e.g., sidewalls) of the gate spacers  87  is removed by the process to form the recesses for the source/drain regions  80 . As illustrated in  FIG. 11B , the epitaxial source/drain regions  80  may have surfaces raised from respective surfaces of the fins  64  (e.g. raised above the non-recessed portions of the fins  64 ) and may have facets. The source/drain regions  80  of the adjacent fins  64  may merge to form a continuous epitaxial source/drain region  80 . In some embodiments, the source/drain regions  80  for adjacent fins  64  do not merge together and remain separate source/drain regions  80 . In some embodiments, the resulting FinFET is an n-type FinFET, and source/drain regions  80  comprise silicon carbide (SiC), silicon phosphorous (SiP), phosphorous-doped silicon carbon (SiCP), or the like. In some embodiments, the resulting FinFET is a p-type FinFET, and source/drain regions  80  comprise SiGe, and a p-type impurity such as boron or indium. 
     The epitaxial source/drain regions  80  may be implanted with dopants to form source/drain regions  80  followed by an anneal process. The implanting process may include forming and patterning masks such as a photoresist to cover the regions of the FinFET device  100  that are to be protected from the implanting process. The source/drain regions  80  may have an impurity (e.g., dopant) concentration in a range from about 1E19 cm −3  to about 1E21 cm −3 . P-type impurities, such as boron or indium, may be implanted in the source/drain region  80  of a P-type transistor. N-type impurities, such as phosphorous or arsenide, may be implanted in the source/drain regions  80  of an N-type transistor. In some embodiments, the epitaxial source/drain regions may be in situ doped during growth. 
     Next, a contact etch stop layer (CESL)  89  is formed over the structure illustrated in  FIGS. 11A and 11B . The CESL  89  functions as an etch stop layer in a subsequent etching process, and may comprise a suitable material such as silicon oxide, silicon nitride, silicon oxynitride, combinations thereof, or the like, and may be formed by a suitable formation method such as CVD, PVD, combinations thereof, or the like. 
     Next, a first interlayer dielectric (ILD)  90  is formed over the CESL  89  and over the dummy gate structures  75 . In some embodiments, the first ILD  90  is formed of a dielectric material such as silicon oxide, phosphosilicate glass (PSG), borosilicate glass (BSG), boron-doped phosphosilicate Glass (BPSG), undoped silicate glass (USG), or the like, and may be deposited by any suitable method, such as CVD, PECVD, or FCVD. A planarization process, such as a CMP process, may be performed to remove the mask  70  and to remove portions of the CESL  89  disposed over the gate electrode  68 . After the planarization process, the top surface of the first ILD  90  is level with the top surface of the gate electrode  68 , in some embodiments. 
     Next, in  FIGS. 13A, 13B, 14A, 14B, 15A, and 15B , an embodiment gate-last process (sometimes referred to as replacement gate process) is performed to replace the gate electrode  68 , the gate fill material  73 , and the gate dielectric  66  with an active gate (may also be referred to as a replacement gate or a metal gate) and active gate dielectric material(s), respectively. Therefore, the gate electrode  68  and the gate dielectric  66  may be referred to as dummy gate electrode and dummy gate dielectric, respectively, in a gate-last process. The active gate is a metal gate, in some embodiments. 
     Referring to  FIGS. 13A and 13B , the dummy gate electrodes  68  are removed by a first etching process, e.g., using an etchant that is selective to (e.g., having a higher etch rate for) the material of the gate electrode  68 . A suitable etching process, such as a wet etch process or a dry etch process, may be performed as the first etching process. After the first etching process, recesses  88  are formed between respective gate spacers  87 . In  FIG. 13A  (cross-sectional view along cross-section D-D), at least portions of the gate fill material  73  remain at the bottom of the recesses  88 , and upper inner sidewalls (e.g., upper portions of the sidewalls facing the gate fill material  73 ) of the gate spacers  87  are exposed. In  FIG. 13B  (cross-sectional view along cross-section A-A), no gate fill material  73  is left and the dummy gate dielectric  66  is exposed by the recesses  88 . 
     Next, in  FIGS. 14A and 14B , a second etching process is performed, after the first etching process is finished, to remove the gate fill material  73 , e.g., using an etchant that is selective to the gate fill material  73 . A suitable etching process, such as a wet etch process (e.g., using an etching chemical) or a dry etch process, may be performed as the second etching process. In an embodiment, a plasma etch process is performed as the second etching process, where the plasma process uses a gas source comprising a main etch gas and a dilute gas (also referred to as carrier gas). The main etch gas may comprises Cl 2 , HBr, CF 4 , CHF 3 , CH 2 F 2 , CH 3 F, C 4 F 6 , BCl 3 , SF 6 , or H 2 , and the carrier gas comprises an inert gas, such as Ar, He, Ne, the like, or combinations thereof. In addition, the gas source may optionally include a passivation gas comprising N 2 , O 2 , CO 2 , SO 2 , CO, or SiCl 4 . The passivation gas is used to tune the etching selectivity of the second etching process, so as to advantageously reduce or avoid damage to, e.g., the gate spacers  87  and the first ILD  90  during the second etching process. 
     In some embodiments, a power of the plasma etching process (as the second etching process) is between about 10 W and about 3000 W, and a bias power of the plasma etching process is between about 10 W and about 3000 W. In some embodiments, the bias power is tuned to adjust the lateral etching rate of the plasma etching process. A pressure of the plasma etching process is between about 1 mTorr and about 800 mTorr. A flow rate of the main etching gas, the dilute gas, or the passivation gas is between about 10 standard cubic centimeters per minute (sccm) and about 5000 sccm. In the example of  FIGS. 14A and 14B , after the plasma etching process, the gate fill material  73  is removed from the recesses  88 , and the plasma etching process also removes portions of the gate dielectric  66  underlying (e.g., directly below) the recesses  88 . As illustrated in  FIG. 14B , remaining portions of the gate dielectric  66  are disposed directly under the gate spacers  87 . In some embodiments, the second etching process also removes upper portions of the isolation regions (e.g., due to over etching), and as a result, isolation regions  62  has a concave upper surface  62 U exposed by the recesses  88 . 
     Next, in  FIGS. 15A and 15B , a metal gate structure  97  (also referred to as a replacement gate structure) is formed in each of the recesses  88 . The metal gate structure  97  has a multi-layered structure (not illustrated in  FIGS. 15A and 15B , but illustrated in  FIG. 15C ), in some embodiments.  FIG. 15C  illustrates a zoomed-in view of an area  77  in  FIG. 15A . 
     As illustrated in  FIG. 15C , the metal gate structure  97  includes a gate dielectric layer  94 , a barrier layer  96 , a work function layer  98 , and a gate electrode  99 . In accordance with some embodiments, to form the replacement gate structures  97 , the gate dielectric layer  94  is deposited conformally in the recesses  88 , such as on the top surfaces and the sidewalls of the fins  64  and on sidewalls of the gate spacers  87 , and on a top surface of the first ILD  90  (not shown). In some embodiments, the gate dielectric layer  94  comprises silicon oxide, silicon nitride, or multilayers thereof. In other embodiments, the gate dielectric layer  94  includes a high-k dielectric material, and in these embodiments, the gate dielectric layers  94  may have a k value greater than about 7.0, and may include a metal oxide or a silicate of Hf, Al, Zr, La, Mg, Ba, Ti, Pb, and combinations thereof. The formation methods of gate dielectric layer  94  may include molecular beam deposition (MBD), atomic layer deposition (ALD), PECVD, and the like. 
     Next, the barrier layer  96  is formed conformally over the gate dielectric layer  94 . The barrier layer  96  may comprise an electrically conductive material such as titanium nitride, although other materials, such as tantalum nitride, titanium, tantalum, or the like, may alternatively be utilized. The barrier layer  96  may be formed using a CVD process, such as PECVD. However, other alternative processes, such as sputtering, metal organic chemical vapor deposition (MOCVD), or ALD, may alternatively be used. 
     Next, the work function layer  98 , such as P-type work function layer or N-type work function layer, is formed in the recesses over the barrier layers  96 . Exemplary P-type work function metals that may be included in the gate structures for P-type devices include TiN, TaN, Ru, Mo, Al, WN, ZrSi 2 , MoSi 2 , TaSi 2 , NiSi 2 , WN, other suitable P-type work function materials, or combinations thereof. Exemplary N-type work function metals that may be included in the gate structures for N-type devices include Ti, Ag, TaAl, TaAlC, TiAlN, TaC, TaCN, TaSiN, Mn, Zr, other suitable N-type work function materials, or combinations thereof. A work function value is associated with the material composition of the work function layer, and thus, the material of the work function layer is chosen to tune its work function value so that a target threshold voltage Vt is achieved in the device that is to be formed. The work function layer(s) may be deposited by CVD, physical vapor deposition (PVD), and/or other suitable process. 
     Next, a seed layer (not shown) is formed conformally over the work function layer  98 . The seed layer may include copper, titanium, tantalum, titanium nitride, tantalum nitride, the like, or a combination thereof, and may be deposited by ALD, sputtering, PVD, or the like. In some embodiments, the seed layer is a metal layer, which may be a single layer or a composite layer comprising a plurality of sub-layers formed of different materials. For example, the seed layer comprises a titanium layer and a copper layer over the titanium layer. 
     Next, the gate electrode  99  is deposited over the seed layer, and fills the remaining portions of the recesses  88 . The gate electrode  99  may be made of a metal-containing material such as Cu, Al, W, the like, combinations thereof, or multi-layers thereof, and may be formed by, e.g., electroplating, electroless plating, or other suitable method. After the formation of the gate electrode  99 , a planarization process, such as a CMP, may be performed to remove the excess portions of the gate dielectric layer  94 , the barrier layer  96 , the work function layer  98 , the seed layer, and the gate electrode  99 , which excess portions are over the top surface of the first ILD  90 . The resulting remaining portions of the gate dielectric layer  94 , the barrier layer  96 , the work function layer  98 , the seed layer, and the gate electrode  99  thus form the replacement gate structure  97  of the resulting FinFET device  100 . Now that due to the concave upper surface  62 U (see  FIG. 14A ) of the isolation regions, the bottom surface of the metal gate structure  97  is curved (e.g., curved downward into the isolation region  62 ), as illustrated in  FIG. 15A .  FIG. 15B  illustrates portions of the metal gate structure  97  disposed directly over the fin  64 . 
     One skilled in the art will readily appreciate that additional processing, such as processing to form gate contact plugs, source/drain contact plugs, and interconnect structures, may be performed after the processing step of  FIGS. 15A and 15B  to complete the fabrication of the FinFET device  100 . Details are not discussed here. 
       FIGS. 16A, 16B, 17A, 17B, 18A, 18B, 19A, and 19B  illustrate cross-sectional views of a FinFET device  100 A at various stages of fabrication, in an embodiment. The FinFET device  100 A in  FIGS. 16A and 16B  is similar to the FinFET device  100  in  FIGS. 10A and 10B . In other words, the processing of  FIGS. 16A and 16B  follows the processing step of  FIGS. 9A and 9B . In particular, compared with  FIGS. 10A and 10B , the lateral etching rate of the anisotropic etching process is adjusted (e.g., reduced), such that sidewalls of the dummy gate structure  75  and sidewalls of the gate fill material  73  are covered by remaining portions of the gate fill material  73  in  FIGS. 16A and 16B . In another embodiment, the shape and location of the gate fill material  73  illustrated in  FIGS. 16A and 16B  may be formed directly after the bottom-up deposition process to form the gate fill material  73 , and no additional etching process is performed to shape the gate fill material  73  in order to form the structure illustrated in  FIGS. 16A and 16B . 
     Next, in  FIGS. 17A and 17B , the gate spacers  87 , the source/drain regions  80 , the CESL  89 , and the first ILD  90  are formed, using the same or similar processing steps as illustrated in  FIGS. 11A, 11B, 12A and 12B . A planarization process, such as CMP, may be performed next to remove the mask  70  and to achieve a coplanar upper surface between the gate electrode  68 , the gate spacers  87 , the CESL  89 , and the first ILD  90 . 
     Next, the first etching process is performed to remove the dummy gate electrode  68 , similar to  FIGS. 13A and 13B . After the first etching process, the recesses  88  are formed. Note that compared with  FIG. 13A , portions of the gate fill material  73  are left covering the entire inner sidewalls (e.g., sidewalls facing the gate fill material  73 ) of the gate spacers  87 . In other words, the gate fill material  73  extends continuously from an upper surface of the gate spacer  87  to the upper surface of the isolation regions  62  in the cross-sectional view of  FIG. 17A , and extends continuously from the upper surface of the gate spacer  87  to the upper surface of the gate dielectric  66  in the cross-sectional view of  FIG. 17B . 
     Next, in  FIGS. 18A and 18B , the second etching process is performed to remove portions of the gate fill material  73 . In some embodiments, the second etching process is a plasma etch process same as or similar to that of  FIGS. 14A and 14B . The bias power of the plasma etch process may be adjusted to achieve a target level of anisotropicity (e.g., a target level of lateral etching rate). As illustrated in  FIGS. 18A and 18B , after the second etching process, remaining portions of the gate fill material  73  covers the entire inner sidewalls of the gate spacers  87 . Lower portions  73 L of the remaining gate fill material  73  are thicker than upper portions  73 U of the remaining gate fill material  73 . The inner sidewalls of the lower portions  73 L of the remaining gate fill material  73  (e.g., facing the recesses  88 ) are sloped or curved toward a center axis  88 C of the recess  88 . The inner sidewalls of the upper portions  73 U may be straight (e.g., perpendicular to the upper surface of the substrate), or may be sloped toward the center axis  88 C of the recess  88  as the gate fill material  73  extends toward the isolation regions  62 . 
     In some embodiments, after the second etching process, the gate fill material  73  comprises a first portion directly over the fin (see, e.g.,  73  in  FIG. 18B ), wherein a thickness of the first portion remain a same as the first portion extends from an upper surface of the gate spacers  87  to an upper surface of the fin  64 . The gate fill material  73  further comprises a second portion (see, e.g.,  73  in  FIG. 18A ) on a first side of the fin, where the second portion contacts the isolation regions  62 , and a thickness of the second portion increases as the second portion extends toward the isolation regions  62 . 
     Note that in  FIG. 18A , a center area of the upper surface of the isolation regions  62 , which is exposed by the recess  88 , has a curved (e.g., concave) upper surface  62 U 2  due to, e.g., over etching of the second etching process. Upper surface  62 U 1  of the isolation regions, which is under (e.g., covered by) the lower portion  73 L of the gate fill material  73 , is substantially flat, since it is protected from the second etching process. 
     Next, in  FIGS. 19A and 19B , the metal gate structure  97  is formed to fill the recesses  88 , using the same or similar processing as in  FIGS. 15A and 15B . Details are not repeated here. 
       FIG. 19C  illustrates a zoomed-in view of an area  79  in  FIG. 19A . As illustrated in  FIG. 19C , the bottom surface of the metal gate structure  97  is curved and extends into the isolation regions  62 . The upper sidewall  97 S 1  of the metal gate structure  97  may be straight, and the lower sidewall  97 S 2  of the metal gate structure  97  may be sloped toward a center axis  97 C of the metal gate structure  97 . An angle θ D  between the lower sidewall  97 S 2  and a horizontal line HL in  FIG. 19C  may be less than about 90 degrees. A height D 3  of the lower portion of the metal gate structure  97 , measured between the isolation regions  62  and a location where the upper sidewall  97 S 1  adjoins the lower sidewall  97 S 2 , is between about 0 angstrom and about 1000 angstroms, in an embodiment. A thickness W 3  of the lower portion of the gate fill material  73 , measured at an interface between the gate fill material  73  and the isolation regions  62 , is between about 0 angstroms and about 500 angstroms, in an embodiment. 
       FIGS. 20A, 20B, 21A, and 21B  illustrate cross-sectional views of a FinFET device  100 B at various stages of fabrication, in an embodiment. The FinFET device  100 B in  FIGS. 20A and 20B  are similar to the FinFET device  100  in  FIGS. 13A and 13B , but with the process condition of the second etching process adjusted such that, after the second etching process, only lower portions  73 L of the gate fill material  73  are left. In  FIG. 20A , an area of the upper surface of the isolation regions  62 , which area is exposed by the recess  88 , has a curved (e.g., concave) upper surface  62 U 2  due to, e.g., over etching of the second etching process. Upper surface  62 U 1  of the isolation regions, which is under (e.g., covered by) the lower portion  73 L of the gate fill material  73 , is substantially flat, since it is protected from the second etching process. 
     Next, in  FIGS. 21A and 21B , the metal gate structures  97  are formed in the recesses  88 , using the same or similar processing as  FIGS. 15A and 15B . In  FIG. 21A , a width W 4  of the lower portions  73 L of the gate fill material  73  is between about 0 angstrom and about 500 angstroms. A height D 2  of the lower portions  73 L, which is the same as the height of the lower portions  97 L of the metal gate structure  97 , is between about 0 angstrom and about 1000 angstroms. Note that upper portions of the metal gate structure  97  has straight sidewalls, and the lower portions of the metal gate structures  97  has sloped or curved sidewalls. In  FIG. 21A , a bottom surface of the metal gate structure  97  is curved and extends into the isolation regions  62 . 
       FIGS. 22A-22C  illustrate various embodiment cross-sectional views of the bottom portion of the metal gate structure  97  along cross-section D-D. By adjusting the second etching process, (e.g., by controlling the bias voltage to control lateral etching rate, and/or by tuning the selectivity of the second etching process), different shapes and/or sizes for the remaining portions of the gate fill material  73  (see, e.g.,  13 A,  17 A,  20 A) can be achieved to control the shape/size of the bottom portion of the metal gate structure  97 . For example, in  FIG. 22A , the bottom portion of the metal gate structure  97  tapers off and has a curved bottom surface. In  FIG. 22B , the metal gate structure  97  has sloped sidewalls, and a bottom surface  97 B of the metal gate structure  97  has a slight recess in the middle. In  FIG. 22C , the bottom portion of metal gate structure  97  has sloped sidewalls and a flat bottom surface  97 B. An angle between the bottom surface  97 B and a respective sidewall  97 S of the metal gate structure  97 , denoted as θ g1 , θ g2 , and θ g3  respectively in  FIGS. 22A, 22B, and 22C , is larger than about 90 degrees. 
       FIGS. 23A, 23B, 24A, 24B  illustrate various views (e.g., cross-sectional view, top view) of a FinFET device  100 C at various stages of fabrication, in an embodiment. The FinFET device  100 C is similar to the FinFET device  100 ,  100 A, or  100 B, but has dummy fins  64 D formed to prevent or reduce the risk of collapse for the hanging dummy gate structure  75 , or alternatively, allow greater distances between adjacent fins  64  or between a fin  64  and an edge  68 S of the dummy gate structure  75 . In particular,  FIG. 23A  is similar to  FIG. 7C , and  FIG. 23B  is similar to  FIG. 7F , but with dummy fins  64 D formed between some adjacent fins  64 , and/or formed at the edge  68 S of the dummy gate structure  75 . In some embodiments, an outer sidewall  64 DS of the dummy fin  64 D extends further from the outer fin  64  than the edge  68 S. 
     The dummy fins  64 D may be formed of any suitable material, such as a same material (e.g., a semiconductor material) as the fin  64 , or a different material (e.g., a dielectric material such as silicon oxide, silicon nitride) from the fin  64 . In some embodiments, the dummy fins  64 D are formed by a single material. In other embodiments, the dummy fins  64 D are formed by two or more materials (e.g., two or more layers of different materials). The dummy fins  64 D are formed by a silicon-based material, in some embodiments. In other embodiments, the dummy fins  64 D are formed using a dielectric material, such as a metal oxide (e.g., HfO, TaN, the like, or combinations thereof). The dummy fin  64 D is electrically isolated, and no source/drain regions are formed on/in the dummy fins  64 D, in the illustrated embodiment. 
     Due to the dummy fins  64 D in between, the distance WA between two fins  64  disposed on opposing sides of the dummy fin  64 D may be increased beyond the maximum value (e.g., 200 nm) for the distance W 1  (see  FIG. 7C  for designs without the dummy fin  64 D). For example, the distance WA between fins  64  may be increased to a value larger than 200 nm, while still maintaining a distance Wa and a distance Wa′ that are less than about 200 nm for reduced risk of fin collapsing, where Wa and Wa′ are the distances between the dummy fin  64 D and the respective fin  64 . Similarly, the distance WB between an outer fin  64  and the edge  68 S may be increased to a value larger than the maximum value (e.g., about 100 nm) for the distance W 2  (see  FIG. 7C ), while still maintaining a distance Wb that is less than about 100 nm for reduced risk of fin collapsing, where Wb is the distance between the outer fin  64  and the dummy fin  64 D.  FIG. 23B  illustrates a cross-sectional view of the FinFET device  100 C across the cross-section F-F, which cuts across the channel region of the fins  64  along a plane parallel to the upper surface of the substrate  50 . As illustrated in  FIG. 23B , the dummy fins  64 D may be shorter than the fins  64 , and may be formed in a region R 2  with dummy fins, whereas regions R 1  has no dummy fins  64 D formed. In other embodiments, dummy fins  64 D are formed in both regions R 1  and R 2 , and may have a same length as the fins  64 . 
       FIGS. 24A and 24B  illustrate corresponding cross-sectional views of the FinFET device  100 C after the metal gate structures  97  are formed, following the same or similar processing steps discussed above.  FIG. 24A  shows the metal gate structure  97 , and  FIG. 24B  further illustrates the gate spacers  87 , and the source/drain regions  80 . 
       FIGS. 25-30  illustrate cross-sectional views of a FinFET device  200  at various stages of fabrication, in an embodiment. The FinFET device  200  illustrated may be a cross-sectional view of a portion of, e.g., the FinFET device  100 A or  100 B along a cross-section E 2 -E 2  illustrated in  FIG. 16A  (also in  FIG. 8A ). Note that the cross-section E 2 -E 2  is across a lower portion (e.g., a lower portion that tapers off) of the gate electrode  68 . 
     Referring to  FIG. 25 , the FinFET device  200  is at a same or similar processing step of  FIGS. 8A and 8B . Due to the location of the cross-section E 2 -E 2  (e.g., near the end the tapering gate electrode  68 ), the gate electrode  68  is shown as a thin strip. Note that for simplicity, only portions of the gate electrode  68  between the fins  64  are illustrated in  FIGS. 25-30 .  FIG. 25  also illustrates the corner regions  68 C of the gate electrode  68 , which extends away from the longitudinal axis  68 A of the gate electrode  68 , therefore the cross-section of the gate electrode  68  in  FIG. 25  has a convex shape. The shape of the corner regions  68 C is caused by the etching process (see, e.g.,  7 A) to form the hanging dummy gate structure  75 , because the etching process may have a slower etching rate in the corner regions (e.g., regions of  68 C). 
     Next, in  FIG. 26 , the gate fill material  73  is formed. The gate fill material  73  may also have corner regions  73 C similar to corner regions  68 C, due to similar reasons (e.g., slower etching rate at corner regions). Therefore,  FIG. 26  may correspond to the processing step of  FIGS. 16A and 16B . 
     Next, in  FIG. 27 , gate spacers  87  are formed along sidewalls of the gate fill material  73 , and source/drain regions  80  are formed over the fins  64 . Due to the conformal deposition process and/or the etching process to form the gate spacers  87 , the gate spacers  87  have similar corner regions. 
     Next, in  FIG. 28 , the gate electrode  68  is removed by the first etching process, and the recesses  88  are formed. Therefore,  FIG. 28  may correspond to the processing step of  FIGS. 17A and 17B . 
     Next, in  FIG. 29 , the second etching process is performed to remove portions of the gate fill material  73 , and remaining portions of the gate fill material  73  are disposed along inner sidewalls of the gate spacers  87 . Note that due to the slower etching rate of the gate fill material  73  at the corner regions, after the second etching process, the inner sidewalls  73 S of the gate fill material  73  at the corner regions bend toward a respective center axis  88 A of the recess  88 . Therefore, each of the recesses  88  now has a convex shaped cross-section.  FIG. 29  may correspond to the processing step of  FIGS. 18A and 18B . 
     Next, in  FIG. 30 , the metal gate structure  97  is formed to fill the recess  88 . Note that due to the recesses  88  having a convex shape, the metal gate structures  97  also have a convex shape. As a result, an angle θ a  between two adjacent sides in the corner region of the metal gate structure  97 , is larger than about 90 degrees and smaller than about 180 degree. Due to the convex shape of the metal gate structure  97 , corner regions of the metal gate structures  97  bends inwards toward a center axis  97 A of the metal gate structure  97 , thus away from the source/drain regions  80 . This advantageously increase the distance between the metal gate structures  97  and the source/drain regions  80  and reduces the leakage current (e.g., leakage current between the gate and the source/drain regions) of the FinFET device formed. In contrast, without the currently disclosed formation methods, the metal gate structures  97  may have corner regions that extend outwards toward the source/drain regions  80 , which may have increased leakage current. 
       FIG. 31  illustrates a cross-sectional view of a FinFET device  200 A, in an embodiment. The FinFET device  200 A is similar to the FinFET device  200  in  FIG. 30 , but with the gate fill material  73  only in corner regions, which may be due to a higher lateral etching rate of the etching process. 
       FIG. 32  illustrates a cross-sectional view of a FinFET device  200 B, in an embodiment. The FinFET device  200 B is similar to the FinFET device  200  in  FIG. 30 , but with the gate fill material  73  completely removed. Note that due to the over etching to completely remove the gate fill material  73 , corner regions of the gate spacers  87  bends inward, which again causes the cross-section of the metal gate structure  97  to have a convex shape. 
       FIG. 33  illustrates a flow chart of a method  1000  of forming a semiconductor device, in accordance with some embodiments. It should be understood that the embodiment method shown in  FIG. 33  is merely an example of many possible embodiment methods. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. For example, various steps as illustrated in  FIG. 33  may be added, removed, replaced, rearranged and repeated. 
     Referring to  FIG. 33 , at step  1010 , a fin is formed protruding above a substrate. At step  1020 , isolation regions are formed on opposing sides of the fin. At step  1030 , a dummy gate electrode is formed over the fin. At step  1040 , lower portions of the dummy gate electrode proximate to the isolation regions are removed, wherein after removing the lower portions, there is a gap between the isolation regions and a lower surface of the dummy gate electrode facing the isolation regions. At step  1050 , the gap is filled with a gate fill material. At step  1060 , after filling the gap, gate spacers are formed along sidewalls of the dummy gate electrode and along sidewalls of the gate fill material. At step  1070 , the dummy gate electrode and the gate fill material are replaced with a metal gate. 
     Embodiments may achieve advantages. For example, as semiconductor manufacturing process continues to advance, feature sizes continue to shrink. As the distance between fins becomes smaller and smaller, it is increasingly difficult to deposit material between the fins. Voids, or empty spaces, may be formed in the material deposited between fins, especially near the bottom of the fin. In the process of forming the gate electrode  68 , if the gate electrode layer has voids in it, after patterning, the gate electrode  68  may have voids, especially at the bottom of the gate electrode  68 . In subsequent processing, gate spacers  87  are formed along sidewalls of the gate electrode  68 . If there are voids at the sidewalls of the gate electrode  68 , the material of the gate spacer  87  (e.g., silicon nitride) will fill those voids. In the subsequent replacement gate process, the gate electrode  68  is removed and replaced with the gate materials. However, the material (e.g., silicon nitride) of the gate spacers  87  which filled the voids will not be etched away with the gate electrode  68 , and will remain in the final metal gate structure  97 . This may cause defect or high resistance of the metal gate structure  97 . In contrast, the current disclosed methods, by forming hanging dummy gate structure, and by filling the gap G with the dummy gate fill material  73 , voids are less likely to form under the gate electrode  68  (due to the bottom-up deposition), and any voids at the sidewalls of the gate electrode  68  are filled with the dummy gate fill material  73 , which dummy gate fill material  73  is also removed in the subsequent replacement gate process. Therefore, the problem with voids at the sidewalls of the gate electrode  68 , e.g., defect or increased gate resistance of the metal gate structure  97 , are avoided or reduced. In addition, as illustrated in  FIGS. 25-30 , the disclosed embodiment methods increase the distance between the metal gate structure and the source/drain regions  80 , thus reducing leakage current of the FinFET device formed. 
     In an embodiment, a method of forming a semiconductor device includes: forming a fin protruding above a substrate; forming isolation regions on opposing sides of the fin; forming a dummy gate electrode over the fin; removing lower portions of the dummy gate electrode proximate to the isolation regions, where after removing the lower portions, there is a gap between the isolation regions and a lower surface of the dummy gate electrode facing the isolation regions; filling the gap with a gate fill material; after filling the gap, forming gate spacers along sidewalls of the dummy gate electrode and along sidewalls of the gate fill material; and replacing the dummy gate electrode and the gate fill material with a metal gate. In an embodiment, removing lower portions of the dummy gate electrode comprises: forming a dielectric layer over upper portions of the dummy gate electrode, wherein the lower portions of the dummy gate electrode are exposed by the dielectric layer; and performing an etching process, wherein a first etch rate of the dielectric layer is slower than a second etch rate of the dummy gate electrode. In an embodiment, the etching process is a plasma etching process, wherein performing the etching process comprises controlling a lateral etching rate of the plasma etching process by tuning a bias voltage of the plasma etching process. In an embodiment, filling the gap comprises: depositing the gate fill material on the isolation regions, the gate fill material filling the gap and extending along sidewalls of the dummy gate electrode; and after depositing the gate fill material, performing an anisotropic etching process to remove portions of the gate fill material. In an embodiment, after the anisotropic etching process, remaining portions of the gate fill material extend from the lower surface the dummy gate electrode to the isolation regions, and an uppermost surface of the remaining portions of the gate fill material contacts the lower surface of the dummy gate electrode. In an embodiment, after the anisotropic etching process, remaining portions of the gate fill material cover the sidewalls of the dummy gate electrode and extend from an upper surface of the dummy gate electrode to the isolation regions. In an embodiment, replacing the dummy gate electrode and the gate fill material comprises: performing a first etching process to remove the dummy gate electrode, wherein the gate fill material is exposed after the first etching process; performing a second etching process to remove at least portions of the gate fill material, thereby forming an opening between the gate spacers; and forming the metal gate in the opening. In an embodiment, forming the metal gate comprises: lining sidewalls and a bottom of the opening with gate dielectric layer; forming a barrier layer over the gate dielectric layer; forming a work function layer over the barrier layer; and after forming the work function layer, filling the opening with a metal material. In an embodiment, performing the second etching process comprises performing a plasma etching process using a gas source comprising an etching gas, a passivation gas, and a carrier gas, wherein the etching gas comprises Cl 2 , HBr, CF 4 , CHF 3 , CH 2 F 2 , CH 3 F, C 4 F 6 , BCl 3 , SF 6 , or H 2 , the passivation gas comprises N 2 , O 2 , CO 2 , SO 2 , CO, or SiCl 4 , and the carrier gas comprises an inert gas. In an embodiment, after the second etching process, remaining portions of the gate fill material extend from an upper surface of the gate spacers to the isolation regions, wherein a distance, measured between lower sidewalls of the remaining portions of the gate fill material facing the metal gate, decreases as the remaining portions of the gate fill material extend toward the isolation regions. In an embodiment, after the second etching process, remaining portions of the gate fill material cover lower sidewalls of the gate spacers while exposing upper sidewalls of the gate spacers, wherein an upper portion of the metal gate contacts the gate spacers, and a lower portion of the metal gate contacts the remaining portions of the gate fill material. 
     In an embodiment, a method of forming a semiconductor device includes: forming a dummy gate electrode over a fin, wherein the fin protrudes above a substrate and is interposed between isolation regions; reducing a height of the dummy gate electrode by removing a lower portion of the dummy gate electrode, wherein after reducing the height, there is a gap between the dummy gate electrode and the isolation regions; forming a gate fill material in the gap under the dummy gate electrode; forming gate spacers on opposing sides of the dummy gate electrode and on opposing sides of the gate fill material; after forming the gate spacers, removing the dummy gate electrode and removing at least a portion of the gate fill material to form an opening between the gate spacers; and forming a metal gate in the opening. In an embodiment, a distance, measured between an upper surface of the isolation regions and an upper surface of the dummy gate electrode distal from the isolation regions, remains a same before and after reducing the height of the dummy gate electrode. In an embodiment, the gate fill material is formed to have a same width as the dummy gate electrode such that sidewalls of the dummy gate electrode are aligned with respective sidewalls of the gate fill material. In an embodiment, the gate fill material is formed to fill the gap and to extend along sidewalls of the dummy gate electrode. In an embodiment, after removing at least a portion of the gate fill material, a remaining portion of the gate fill material is interposed between the metal gate and the gate spacers, wherein a lower portion of the metal gate proximate to the isolation regions has a width that decreases as the metal gate extends toward the isolation regions. 
     In an embodiment, a semiconductor device includes: a fin protruding above a substrate; isolation regions on opposing sides of the fin; a gate structure over the fin; gate spacers along sidewalls of the gate structure; and a gate fill material between the gate structure and the gate spacers, wherein a distance between opposing lower sidewalls of the gate fill material facing the gate structure decreases as the gate fill material extends toward the isolation regions. In an embodiment, the gate fill material is disposed on and contacts the isolation regions, wherein the gate fill material covers lower sidewalls of the gate spacers and exposes upper sidewalls of the gate spacers. In an embodiment, a thickness of the gate fill material increases as the gate fill material extends toward the isolation regions. In an embodiment, the gate fill material separates the gate structure from the gate spacers, wherein the gate fill material comprises: a first portion directly over the fin, wherein a thickness of the first portion remain a same as the first portion extends from an upper surface of the gate spacers to an upper surface of the fin; and a second portion on a first side of the fin and contacting the isolation regions, wherein a thickness of the second portion increases as the second portion extends toward the isolation regions. 
     The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled 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 skilled 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.