Patent Publication Number: US-2022216323-A1

Title: Semiconductor device and forming method thereof

Description:
BACKGROUND 
     Transistors are components of modern integrated circuits. To satisfy the trend of increasingly faster speed, the drive currents of transistors need to be increasingly greater. To achieve this increase in performance, the gate lengths of transistors are scaled down. Scaling down the gate lengths leads to undesirable effects known as “short-channel effects,” in which the control of current flow by the gates is compromised. Among the short-channel effects are the Drain-Induced Barrier Lowering (DIBL) and the degradation of sub-threshold slope, both of which result in the degradation in the performance of transistors. 
     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). One such multi-gate device is horizontal gate-all-around (HGAA) transistor, whose gate structure extends around its horizontal channel region providing access to the channel region on all sides or three sides. The HGAA transistors are compatible with complementary metal-oxide-semiconductor (CMOS) processes, allowing them to be aggressively scaled down while maintaining gate control and mitigating SCEs. However, fabrication of the HGAA transistors can be challenging. For example, nanosheet formation of HGAA transistors by the current methods is not satisfactory in all respects, especially when using a single process, such as a single epitaxial process. 
    
    
     
       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. 
         FIGS. 1-16  are exemplary sequential processes for manufacturing the gate-all-around (GAA) FET device according to some embodiments of the present disclosure. 
         FIGS. 17A-23B  are various cross-sectional views of a GAA device at different stages of fabrication, according to some embodiments of the 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. 
     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. In certain embodiments, the term “about” used in this context means greater or less than the stated value or the stated range of values by a percentage such as 5%, 10%, 15%, etc. of the stated values. 
     The gate all around (GAA) transistor structures may be patterned by any suitable method. For example, the structures 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 may then be used to pattern the GAA structure. 
     Reference is now made to  FIGS. 1-16 , which are exemplary sequential processes for manufacturing the gate-all-around (GAA) FET device according to some embodiments of the present disclosure. It is understood that additional operations can be provided before, during, and after processes shown by  FIGS. 1-16 , and some of the operations described below can be replaced or eliminated, for additional embodiments of the method. The order of the operations/processes may be interchangeable. 
     Referring to  FIG. 1 , impurity ions (dopants)  102  are optional implanted into a substrate  100  to form a well region. The ion implantation is performed to prevent a punch-through effect. In some embodiments, the substrate  100  may include in its surface region, one or more buffer layers (not shown). The buffer layers can serve to gradually change the lattice constant from that of the substrate to that of the source/drain regions. The substrate  100  may include various regions that have been suitably doped with impurities (e.g., p-type or n-type conductivity). The dopants  102  are, for example, phosphorus for a p-type Fin FET. 
     Referring to  FIG. 2 , stacked semiconductor layers are formed over the substrate  100 . The stacked semiconductor layers include first semiconductor layers  110  and second semiconductor layers  112 . Further, a mask layer  120  is formed over the stacked layers. 
     The first semiconductor layers  110  and the second semiconductor layers  112  are made of materials having different lattice constants, and may include one or more layers of Si, Ge, SiGe, GaAs, InSb, GaP, GaSb, InAlAs, InGaAs, GaSbP, GaAsSb or InP. In some embodiments, the first semiconductor layers  110  and the second semiconductor layers  112  are made of Si, a Si compound, SiGe, Ge or a Ge compound. In  FIG. 2 , five layers of the first semiconductor layer  110  and five layers of the second semiconductor layer  112  are disposed. However, the number of the layers are not limited to five, and may be as small as 1 (each layer) and in some embodiments, 2-10 layers of each of the first and second semiconductor layers are formed. By adjusting the numbers of the stacked layers, a driving current of the GAA FET device can be adjusted. 
     The first semiconductor layers  110  and the second semiconductor layers  112  are epitaxially formed over the substrate  100 . In some embodiments, the bottommost first semiconductor layer  110  (the closest layer to the substrate  100 ) is thicker than the remaining first semiconductor layers  110 . 
     In some embodiments, the mask layer  120  includes a first mask layer  122  and a second mask layer  124 . The first mask layer  122  is a pad oxide layer made of a silicon oxide, which can be formed by a thermal oxidation. The second mask layer  124  is made of a silicon nitride (SiN), which is formed by chemical vapor deposition (CVD), including low pressure CVD (LPCVD) and plasma enhanced CVD (PECVD), physical vapor deposition (PVD), atomic layer deposition (ALD), or other suitable process. The mask layer  120  is then patterned into a mask pattern by using patterning operations including photo-lithography and etching. Next, as shown in  FIG. 3 , the stacked layers of the first and second semiconductor layers  110 ,  112  are patterned by using the patterned mask layer, thereby the stacked layers are formed into fin structures  130  extending in the X direction. In  FIG. 3 , two fin structures  130  are arranged in the Y direction. But the number of the fin structures is not limited to, and may be as small as one and three or more. In some embodiments, one or more dummy fin structures are formed on both sides of the fin structures  130  to improve pattern fidelity in the patterning operations. 
     Referring to  FIG. 4 , after the fin structures  130  is formed, an insulating material layer  140  including one or more layers of insulating material is formed over the substrate so that the fin structures  130  are fully embedded in the insulating material layer  140 . The insulating material for the insulating material layer  140  may include silicon oxide, silicon nitride, silicon oxynitride (SiON), SiOCN, SiCN, fluorine-doped silicate glass (FSG), or a low-K dielectric material, formed by LPCVD (low pressure chemical vapor deposition), plasma-CVD or flowable CVD. An anneal operation may be performed after the formation of the insulating material layer  140 . Then, a planarization operation, such as a chemical mechanical polishing (CMP) method and/or an etch-back method, is performed such that the upper surface of the uppermost second semiconductor layer  112  is exposed from the insulating material layer  140 . In some embodiments, a first liner layer  142  is formed over the structure of  FIG. 3  before forming the insulating material layer  140 . The first liner layer  142  is made of SiN or a silicon nitride-based material (e.g., SiON, SiCN or SiOCN). 
     Then, as shown in  FIG. 5 , the insulating material layer  140  (as shown in  FIG. 4 ) is recessed to form an isolation insulating layer  144  so that the upper portions of the fin structures  130  are exposed. With this operation, the fin structures  130  are electrically insulated from each other by the isolation insulating layer  144 , which is also referred to as a STI structure. In some embodiments, the insulating material layer  140  is recessed until the bottommost first semiconductor layer  110  is exposed. The first semiconductor layers  110  are sacrificial layers which are subsequently partially removed, and the second semiconductor layers  112  will serve as channel regions of a GAA FET. 
     After the isolation insulating layer  144  is formed, a sacrificial gate dielectric layer  150  is formed, as shown in  FIG. 6 . The sacrificial gate dielectric layer  150  includes one or more layers of insulating material, such as a silicon oxide-based material. In one embodiment, silicon oxide formed by CVD is used. 
     Afterwards, a sacrificial gate layer and a mask layer (e.g., having a pad SiN layer and a silicon oxide mask layer) are formed over the sacrificial gate dielectric layer  150 , followed by patterning the mask layer, the sacrificial gate electrode layer and the sacrificial gate dielectric layer  150  into the sacrificial gate structure  160 , as shown in  FIG. 7 . The sacrificial gate structure  160  includes the sacrificial gate dielectric layer  150 , the sacrificial gate electrode layer  164  (e.g., poly silicon), the pad SiN layer  166  and the silicon oxide mask layer  168 . The stacked layers of the first and second semiconductor layers  110 ,  112  are partially exposed on opposite sides of the sacrificial gate structure  160 , thereby defining source/drain (SID) regions. In this disclosure, a source and a drain are interchangeably used and the structures thereof are substantially the same. 
     Referring to  FIG. 8 , after the sacrificial gate structure  160  is formed, a blanket layer  170  of an insulating material for sidewall spacers is conformally formed by using CVD or other suitable methods. The blanket layer  170  is deposited in a conformal manner so that it is formed to have substantially equal thicknesses on vertical surfaces, such as the sidewalls, horizontal surfaces, and the top of the sacrificial gate structure. In some embodiments, the blanket layer  170  is deposited to a thickness in a range from about 2 nm to about 10 nm. In one embodiment, the insulating material of the blanket layer  170  is a silicon nitride-based material, such as SiN, SiON, SiOCN or SiCN and combinations thereof. 
     The blanket layer  170  is then etched using an anisotropic process to form gate sidewall spacers  172  on opposite sidewalls of the sacrificial gate structure  160  and fin sidewall spacers  174  on opposite sidewalls of the fin structures  130 , followed by etching exposed portions of the fin structures  130  that extend laterally beyond the gate sidewall spacers  172 . The resulting structure is illustrated in  FIGS. 9A and 9B , wherein  FIG. 9B  is the cross sectional view corresponding to line X 1 -X 1  of  FIG. 9A . In  FIG. 9B , the cross section of the bottom parts of one sacrificial gate structure  160  is illustrated. In some embodiments, the anisotropic process can be control such that no fin sidewall spacers  174  remain on the STI region  144 . 
     The anisotropic etching performed on the blanket layer  170  can be, for example, reactive ion etching (RIE). During the anisotropic etching process, most of the insulating material is removed from horizontal surfaces, leaving the dielectric spacer layer on the vertical surfaces such as the sidewalls of the sacrificial gate structures  160  and the sidewalls of the exposed fin structures  130 . The mask layer  168  may be exposed from the sidewall spacers. 
     Subsequently, as shown in  FIGS. 10A and 10B , the first semiconductor layers  110  are horizontally recessed (etched) so that the second semiconductor layers  112  laterally extend past opposite end surfaces of the first semiconductor layers  110 . In some embodiments, as shown in  FIG. 10B , end surfaces of the first semiconductor layers  110  may be substantially vertically aligned with the side surfaces of the sacrificial gate electrode layer  164 . Here, “substantially vertically alignment” means the horizontal offset is less than about 1 nm. 
     During the recess etching of the first semiconductor layers  110  as illustrated in  FIGS. 10A and 10B , the second semiconductor layers  112  may be also horizontally etched. The recessed amount of the first semiconductor layers  110  is greater than the recessed amount of the second semiconductor layers  112 . In this way, the resulting second semiconductor layers  112  can laterally extend past opposite end surfaces of the first semiconductor layers  110 . 
     After the first semiconductor layers  110  are horizontally recessed, inner spacers  180  are formed on the recessed surfaces of the first semiconductor layers  110 , and vertically between corresponding second semiconductor layers, as shown in  FIGS. 11A and 11B . Formation of the inner spacers  180  includes depositing an inner spacer material layer (e.g., silicon nitride), followed by etching back the inner spacer material layer by an anisotropic etching process, to remove the inner spacer material layer from the substrate  100 . 
     In some embodiments, the inner spacers  180  include insulating material such as silicon nitride or the like. 
     After the inner spacers  180  are formed, source/drain (S/D) epitaxial layers  190  are epitaxially grown from the exposed recessed fins  165  between the fin sidewall spacers  174 , as shown in  FIG. 12 . The SID epitaxial layers  190  include one or more layers of SiGe doped with p-type dopants (e.g., boron, aluminum, or other suitable p-type dopants) for a p-channel FET. The S/D epitaxial layers  190  include one or more layers of SiC or SiP doped with n-type dopants (e.g., phosphorous, arsenic, or other suitable n-type dopants) for an n-channel FET. The S/D epitaxial layers  190  are formed by an epitaxial growth method using CVD, ALD or molecular beam epitaxy (MBE). In some embodiments, the epitaxial layers  190  grown from neighboring recessed fins  165  of the substrate  100  merge above the STI  144  and form a void in some embodiments. In some other embodiments, the epitaxial layers  190  grown from neighboring recessed fins  165  do not merged. 
     Subsequently, a second liner layer  192  is formed and then an interlayer dielectric (ILD) layer  194  is formed, as shown in  FIG. 13 . The second liner layer  192  is made of a silicon nitride-based material, such as SiN, and functions as a contact etch stop layer in the subsequent etching operations. The materials for the ILD layer  194  include compounds comprising Si, O, C and/or H, such as silicon oxide, SiCOH and SiOC. Organic materials, such as polymers, may be used for the ILD layer  194 . 
     As shown in  FIG. 13 , after the ILD layer  194  is formed, a planarization operation, such as CMP, is performed, so that the top portion of the sacrificial gate structure  160  is exposed. 
     Next, as shown in  FIG. 14 , the sacrificial gate electrode layer  164  (see  FIG. 9B ) and sacrificial gate dielectric layer  150  (see  FIG. 9B ) are removed, thereby exposing the fin stack of the first and second semiconductor layers  110 ,  112  and a gate trench is formed between the gate spacers. 
     The ILD layer  194 , the contact etch stop layer  192 , the gate sidewall spacers  172 , and/or the inner spacers  180  protect the S/D epitaxial layers  190  during the removal of the sacrificial gate structures. The sacrificial gate structures can be removed using plasma dry etching and/or wet etching. When the sacrificial gate electrode layer  164  is polysilicon and the ILD layer  194  is silicon oxide, a wet etchant such as a TMAH solution can be used to selectively remove the sacrificial gate electrode layer  164 . The sacrificial gate dielectric layer  150  is thereafter removed using plasma dry etching and/or wet etching. 
     After the sacrificial gate structures are removed, the first semiconductor layers  110  (as shown in  FIG. 14 ) in the fin structures are removed, as shown in  FIGS. 15A and 15B . That is, the first semiconductor layers  110  (see  FIG. 14 ) are etched. As a result, portions of the second semiconductor layers  112  are suspended. In the following discussion, the portions of the second semiconductor layers  112  suspended are also referred to as and serve as the channel layers (or nanosheets). The channel layers  112  are slightly etched or not etched. In the present embodiments, the channel layers  112  are slightly etched to form a rectangular-like shape (e.g., a nanosheets).  FIG. 15B  is the cross sectional view along the fin structure. Gaps  115  are left between neighboring channel layers  112 . The first semiconductor layers  110  can be removed or etched using an etchant that can selectively etch the first semiconductor layers  110  at a faster etching rate than etching the second semiconductor layers  112 . The channel layers  112  extend in the X-direction above the substrate  100  and are arranged in the Z direction perpendicular to the X-direction. 
     In some embodiments, the first semiconductor layers  110  (also called sacrificial layers to be removed) are SiGe and the second semiconductor layers  112  (also called channel layers to be left in final GAA transistors) are silicon allowing for the selective removal of the first semiconductor layers  110 . In some embodiments, the selective wet etching includes an APM etch (e.g., ammonia hydroxide-hydrogen peroxide-water mixture). In some embodiments, the selective removal includes SiGe oxidation followed by a SiGeO x  removal. For example, the oxidation may be provided by O 3  clean and then SiGeO removed by an etchant such as NH 4 OH that selectively etches SiGeO x  at a faster etch rate than it etches Si. Moreover, because oxidation rate of Si is much lower (sometimes 30 times lower) than oxidation rate of SiGe, the channel layers  112  may not be significantly etched by the channel release process. 
     In the present embodiment, since the inner spacers  180  are made of a material that has etching selectivity to that of the first semiconductor layers  110 , the inner spacers  180  can protect the source/drain epitaxial layers  190  from the etchant used in etching the first semiconductor layers  110 . 
     Referring to  FIG. 16 , a metal gate structure  200  is formed around each channel layers  112 . The metal gate structure  200  extends in a Y-direction. The metal gate structure  200  extends in a Y direction which is perpendicular to the X direction and the Z direction. The exemplary sequential processes of the formation of the metal gate structure  200  will be discussed in the following figures. 
       FIGS. 17A-21B  are various cross-sectional views of a GAA device at different stages of fabrication, according to some embodiments of the disclosure, in which “A” are the cross sectional views corresponding to line X 1 -X 1  of  FIG. 16 , and “B” are the cross sectional views corresponding to line Y 1 -Y 1  of  FIG. 16 .  FIGS. 17A and 17B  follow after  FIGS. 15A and 15B . 
     After the first semiconductor layers  110  are removed, interfacial layers  206  are formed on surface of the channel region, e.g., the surface of the channel layers  112 , and on the surface of the recessed fins  165 . The interfacial layers  206  are formed of silicon oxide or silicon oxynitride grown by a thermal oxidation process. For example, the interfacial layer  206  can be grown by wet oxidation, a rapid thermal oxidation (RTO) process or by an annealing process using oxygen. In some embodiments where the interfacial layers  206  are formed by oxidation, all exposed semiconductor surfaces may be oxidized, and thus exposed surfaces of the channel layers  112  and the recessed fins  165  are all coated with interfacial layers  206 . 
     Referring to  FIGS. 18A and 18B , after the interfacial layers  206  are formed, a high-k gate dielectric layer  208  is formed along the gate sidewall spacers  172 , the surface of the inner spacer  180 , the surface of the ILD layer  194  and the interfacial layer  206 , by a deposition process. In some embodiments, the high-k gate dielectric layer  132  may include metal oxides. Examples of metal oxides used for high-k gate dielectric layer  132  include oxides of Li, Be, Mg, Ca, Sr, Sc, Y, Zr, Hf, Al, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and mixtures thereof. The high-k dielectric layer  208  may be formed using a suitable process such as atomic layer deposition (ALD), chemical vapor deposition (CVD), physical vapor deposition (PVD) or other suitable method. 
     Reference is made to  FIGS. 19A and 19B . A work function metal layer  210  is formed on the high-k gate dielectric layer  208 . For example, the work function metal layer  210  is deposited to surround each of the channel layers (or nanosheets)  112 . A portion of the work function metal layer  210  is formed vertically between adjacent channel layers (or nanosheets)  112  and fills the gap  113  between adjacent channel layers  112 . 
     The work function metal layer  210  may be formed to provide a proper work function for the resulting gate structure. For example, if a P-type work function metal (P-metal) for a PMOS device is desired, P-type work function materials may be used. Examples of P-type work function materials include, but are not limited to, titanium nitride (TiN), tungsten nitride (WN), tungsten (W), ruthenium (Ru), palladium (Pd), platinum (Pt), cobalt (Co), nickel (Ni), conductive metal oxides, and/or other applicable materials. If the P-metal is tungsten or tungsten nitride, it can be formed by a deposition process without using any fluorine-containing precursors, which will be explained in greater detail below. 
     On the other hand, if an N-type work function metal (N-metal) for NMOS devices is desired, N-type metal materials may be used. Examples of N-type work function materials include, but are not limited to, titanium aluminide (TiAl), titanium aluminium nitride (TiAlN), carbo-nitride tantalum (TaCN), hafnium (Hf), zirconium (Zr), titanium (Ti), tantalum (Ta), aluminum (Al), metal carbides (e.g., hafnium carbide (HfC), zirconium carbide (ZrC), titanium carbide (TiC), aluminum carbide (AIC)), aluminides, and/or other applicable materials. 
     The work function metal layer  210  is a single-layer film or a multi-layer film. In some embodiments where the work function metal layer  210  is a multi-layer film, the work function metal layer may be a stack of one or more N-metal layers and one or more P-metal layers. In some embodiments, the work function metal layer  210  has a thickness greater than a thickness of the interfacial layer  206  and/or a thickness of the high-k gate dielectric layer  208 . In some embodiments, the sheet-to-sheet gap between adjacent nanosheets  112  are small (e.g., having a gap height from about 5 nm to about 20 nm) as the GAA devices scale down, such that work function metal layers respectively deposited on the adjacent nanosheets  112  eventually merge as a single continuous work function metal layer  210  that fills up the small sheet-to-sheet gap. The work function metal layer  210  thus has inner-sheet regions IS vertically between corresponding nanosheets  112 , and an outer-sheet region OS not vertically between any nanosheets  112 . 
     Reference is made to  FIGS. 20A and 20B . A glue layer  212  is formed on and surrounds the work function metal layer  210 . The glue layer  212  may be used to increase adhesion between the work function metal layer  210  and a subsequently formed fill metal layer (see  FIGS. 21A and 21B ) so as to prevent the fill metal layer from peeling or delaminating. The glue layer  212  is a conformal layer and is conformally formed over the work function metal layer  210 . In some embodiments, the glue layer  212  is a nitride layer. In some embodiments, the glue layer  212  is made of or includes TiN, TaN, TiAlN, TaCN, TaC or TaSiN, other suitable material, or a combination thereof and may be formed by CVD, ALD, PVD and/or other suitable process. In some embodiments, the glue layer  212  (e.g., TiN layer) has a thickness less than the thickness of the work function metal layer  210 . In some further embodiments, the thickness of the glue layer  212  is also less than the thickness of the interfacial layer  206  and/or the thickness of the high-k gate dielectric layer  208 . 
       FIG. 21C  is a cross sectional view corresponding to line Z 1 -Z 1  of  FIG. 21A .  FIG. 21D  is a cross sectional view corresponding to line Z 2 -Z 2  of  FIG. 21A . Referring to  FIGS. 21A-21D , a fill metal layer  214  is formed within a trench on the glue layer  212  between the gate sidewall spacers  172 . The fill metal layer  214  is deposited over the work function metal layer  210 . The fill metal layer  214  may be a work function metal layer (i.e., formed of N-metal or P-metal discussed above). In some embodiments where the fill metal layer  214  is a work function metal layer, the fill metal layer  214  includes a fluorine-free metal, because the precursors used in deposition of the fill metal layer  214  is fluorine-free. Examples of the fluorine-free metal includes metal nitride (e.g., TiN, TaN) or fluorine-free tungsten (i.e., tungsten deposited without using fluorine-containing precursors). In some embodiments, the fill metal layer  214  includes titanium-based material such as TiN. In some embodiments, the fill metal layer  214  includes tantalum-based material such as TaN. That is, the fill metal layer  214  includes at least one of fluorine-free tungsten, titanium nitride and tantalum nitride. In an embodiment, after the interfacial layers  206 , the high-k gate dielectric layer  132 , the work function metal layer  210 , the glue layer  212  and the fill metal layer  214  are deposited, a CMP process is performed to planarize a top surface of the semiconductor device  10 . The fill metal layer  214  made of fluorine-free metal improves the threshold voltage of GAA devices and also increases a stability of the threshold voltage, as will be explained in greater detail below. 
     In some embodiments, the fill metal layer  214  may be formed by chemical vapor deposition (CVD) or atomic layer deposition (ALD). In some embodiments where the fill metal layer  214  is a tungsten layer, the fill metal layer  214  may be formed by sequentially introducing a chlorine-based precursor and a hydrogen gas, both of which are free of fluorine. The chlorine-based precursor is a fluorine-free precursor. Examples of the chlorine-based precursor include tungsten chlorine such as tungsten (V) chloride (WCl 5 ), the like, or a combination thereof. In some embodiments, the fluorine-free tungsten is deposited by using WCl 5  and H 2  as precursor, and is deposited at a temperature in a range from about 350° C. to about 500° C. at a chamber pressure in a range from about 0.5 torr to about 50 torr. In some embodiments where the fill metal layer  214  is formed by ALD, the fill metal layer  214  may be formed for a pulse time in a range from about 0.2 s to about 4 s. As compared to using a fluorine-based precursor (e.g., WF 6 ) to deposit the fill metal layer  214 , during forming the fill metal layer  214 , the chlorine-based precursor generates a plurality of chlorine atoms during deposition. Because the chlorine atoms have less tendency than fluorine atoms to diffuse into the underlying layers (e.g., the glue layer  212  and/or the work function metal layer  210 ), the chlorine atoms do not pass through the underlying layers (e.g., the glue layer  212  and/or the work function metal layer  210 ) and thus the threshold voltage of the resulting GAA transistor is less susceptible to the precursors used in the fill metal deposition. As a result, the threshold voltage can be improved and stable. 
     By contrast, if the fluorine-based precursor is used to form the fill metal layer  214 , the fluorine-based precursor may generate fluorine atoms during forming the fill metal layer  214 , which may aggressively diffuse into the underlying layers (e.g., the glue layer  212  and the work function metal layer  210 ) as compared to chlorine atoms as discussed above. Moreover, it is observed that diffusion of fluorine atoms usually stops or slows down at outer-sheet regions OS of work function metal layer  210 , which in turn would lead to a fluorine-rich outer-sheet region OS and a fluorine-poor or fluorine-free inner-sheet region IS in the work function metal layer  210 . In that case, the work function metal layer  210  would have a non-uniform and unpredictable fluorine concentration, which in turn leads to increased challenge on threshold voltage control, thus resulting in threshold voltage instability. Given that embodiments of the present disclosure do not use fluorine-based precursor in fill metal deposition, all possible challenges caused by the fluorine-based precursor can be prevented. In greater detail, both the inner-sheet region IS and the outer-sheet region OS in the work function metal layer  210  are free of fluorine, and the glue layer  212  is free of fluorine as well, so as to improve threshold voltage control. 
     In some embodiments where the fill metal layer  214  includes titanium nitride, the fill metal layer  214  may be formed using a metal inorganic precursor (e.g., TiCl 4 ) and NH 3  at a temperature in a range from about 300° C. to about 500° C., at a chamber pressure in a range from about 0.2 torr to about 50 torr. In some embodiments where the fill metal layer  214  is formed by ALD, the fill metal layer  214  may be formed for a pulse time in a range from about 0.2 s to about 4 s. The metal inorganic precursor is a fluorine-free precursor. Examples of the metal inorganic precursor include titanium chloride (TiCl 4 ). In some other embodiments, the fill metal layer  214  may be formed by a metal organic precursor and other gaseous precursors such as, NH 3  or N 2 . The metal organic precursor is a fluorine-free precursor. Examples of the metal organic precursor include tetrakis (dimethylamino) titanium (TDMAT), tetrakis dimethylamino titanium (TDEAT) or tetrakis (ethylmethylamino) titanium (TEMAT). In some embodiments where the fill metal layer  214  includes TaN, the fill metal layer  214  may be deposited at a temperature in a range from about 250° C. to about 400° C., at a chamber pressure in a range from about 0.5 torr to about 50 torr. In some embodiments where the fill metal layer  214  is formed by ALD, the fill metal layer  214  may be formed for a duration in a range from about 0.2 s to about 4 s. 
       FIGS. 22A and 22B  show another device similar to the device in  FIGS. 21A and 21B , except for the fill metal layer  214  including a first fluorine-free metal layer  214   a  and a second fluorine-free metal layer  214   b . The first fluorine-free metal layer  214   a  is formed on and surrounds the glue layer  212 . The first fluorine-free metal layer  214   a  is a conformal layer and is conformally formed over the glue layer  212 . In some embodiments, the first fluorine-free metal layer  214   a  includes TiN, TaN or tungsten and is formed by CVD or ALD without using any fluorine-containing precursors. In some embodiments where the first fluorine-free metal layer  214   a  includes TiN, the glue layer  212  and the first fluorine-free metal layer  214   a  may have no distinguishable interface therebetween. The second fluorine-free metal layer  214   b  includes a material different from a material of the first layer  214   a . In some embodiments where the first fluorine-free metal layer  214   a  includes TaN, the second fluorine-free metal layer  214   b  includes TiN or tungsten. In some embodiments where the first fluorine-free metal layer  214   a  includes TiN, the second fluorine-free metal layer  214   b  includes TaN or tungsten. In some embodiments where the first fluorine-free metal layer  214   a  includes tungsten, the second fluorine-free metal layer  214   b  includes TiN or TiN. 
     The second fluorine-free metal layer  214   b  is formed within a trench in the first fluorine-free metal layer  214   a  between the gate sidewall spacers  172 . The second fluorine-free metal layer  214   b  is deposited over the first fluorine-free metal layer  214   a . The first fluorine-free metal layer  214   a  and the second fluorine-free metal layer  214   b  are beneficial for improving the threshold voltage and increasing a stability of the threshold voltage. 
       FIGS. 23A and 23B  show another device similar to the device in  FIGS. 21A and 21B , except for the fill metal layer  214  including a first fluorine-free metal layer  214   c , a second fluorine-free metal layer  214   d  and a third fluorine-free metal layer  214   e . The first fluorine-free metal layer  214   c  is formed on and surrounds the glue layer  212 . The first fluorine-free metal layer  214   c  is a conformal layer and is conformally formed over the glue layer  212 . The second fluorine-free metal layer  214   d  is formed on and surrounds the first fluorine-free metal layer  214   c . The second fluorine-free metal layer  214   d  is a conformal layer and is conformally formed over the first fluorine-free metal layer  214   c.    
     In some embodiments, the first fluorine-free metal layer  214   c  includes TiN, TaN or tungsten and is formed by CVD or ALD without using any fluorine-containing precursors. In some embodiments where the first fluorine-free metal layer  214   c  includes TiN, the glue layer  212  and the first fluorine-free metal layer  214   c  may have no distinguishable interface therebetween. The second fluorine-free metal layer  214   d  includes a material different from a material of the first fluorine-free metal layer  214   c . In some embodiments where the first fluorine-free metal layer  214   c  includes TaN, the second fluorine-free metal layer  214   d  includes TiN or tungsten. In some embodiments where the first fluorine-free metal layer  214   c  includes TiN, the second fluorine-free metal layer  214   d  includes TaN or tungsten. In some embodiments where the first fluorine-free metal layer  214   c  includes tungsten, the second fluorine-free metal layer  214   d  includes TiN or TiN. 
     The third fluorine-free metal layer  214   e  is formed within a trench in the second fluorine-free metal layer  214   d  between the gate sidewall spacers  172 . The third fluorine-free metal layer  214   e  is deposited over the second fluorine-free metal layer  214   d . The third fluorine-free metal layer  214   e  is beneficial for improving the threshold voltage and increasing a stability of the threshold voltage. 
     Based on the above discussion, it can be seen that the present disclosure offers advantages. It is understood, however, that other embodiments may offer additional advantages, and not all advantages are necessarily disclosed herein, and that no particular advantages is required for all embodiments. One advantage is that the fill metal layer may be free from fluorine because it is deposited without using any fluorine-containing precursors. Another advantage is that the work function metal layer may be free from fluorine as well. Still another advantage is that the fluorine-free work function metal layer allows for improved threshold voltage control. 
     In some embodiments, a method of forming a semiconductor device including forming a fin structure having a stack of alternating first semiconductor layers and second semiconductor layers over a substrate, the first semiconductor layers and the second semiconductor layers having different compositions, forming a dummy gate structure across the fin structure, forming gate spacers on opposite sidewalls of the dummy gate structure, respectively, removing the dummy gate structure to form a gate trench between the gate spacers, etching the first semiconductor layers in the gate trench, such that the second semiconductor layers are suspended in the gate trench to serve as nanosheets, forming a work function metal layer surrounding each of the nanosheets, and depositing a fill metal layer over the work function metal layer without using a fluorine-containing precursor. 
     In some embodiments, the fill metal layer is deposited using a chlorine-based precursor and a hydrogen gas. In some embodiments, the chlorine-based precursor includes tungsten chlorine. In some embodiments, during forming the fill metal layer, the chlorine-based precursor generates a plurality of chlorine atoms which do not pass through the work function metal layer. In some embodiments, the fill metal layer includes fluorine-free tungsten. In some embodiments, after forming the fill metal layer, the fill metal layer is free from fluorine. In some embodiments, after forming the fill metal layer, the work function metal layer is free from fluorine. In some embodiments, the method further includes after forming the work function metal layer and before depositing the fill metal layer, forming a glue layer surrounding the work function metal layer, wherein the fill metal layer has a material different from a material of the glue layer. In some embodiments, after forming the fill metal layer, the glue layer is free from fluorine. 
     In some embodiments, a method of forming a semiconductor device includes forming a fin structure having a stack of alternating first semiconductor layers and second semiconductor layers over a substrate, forming a dummy gate structure across the fin structure, forming gate spacers on opposite sidewalls of the dummy gate structure, respectively, removing the dummy gate structure to form a gate trench between the gate spacers, etching the first semiconductor layers in the gate trench, such that the second semiconductor layers are suspended in the gate trench to serve as nanosheets, and forming a metal gate structure filling up the gate trench. Formation of the metal gate structure includes forming a first work function metal layer surrounding each of the nanosheets, forming a glue layer surrounding the first work function metal layer, and forming a second work function metal layer over the glue layer such that the second work function metal layer fills the gate trench between the gate spacers, wherein the second work function metal layer is an outermost layer of the metal gate structure, and the second work function metal layer is fluorine-free. 
     In some embodiments, the first and second work function metal layers include a same fluorine-free metal. In some embodiments, the second work function metal layer and the first work function metal layer are fluorine-free metal nitride layers. In some embodiments, the second work function metal layer and the first work function metal layer include a titanium-based material. In some embodiments, the second work function metal layer and the first work function metal layer include fluorine-free titanium nitride. In some embodiments, the second work function metal layer includes tantalum-based material. In some embodiments, the second work function metal layer is fluorine-free a tantalum nitride layer. In some embodiments, the second work function metal layer is formed by a deposition process without using any fluorine-containing precursors. In some embodiments, the second work function metal layer is formed by a deposition process using a chlorine-containing precursor. 
     In some embodiments, a semiconductor device includes a plurality of nanosheets and a gate structure. The plurality of nanosheets extends in a first direction above a semiconductor substrate and arranged in a second direction substantially perpendicular to the first direction. The gate structure extends in a third direction perpendicular to both the first and second directions. The gate structure surrounds each of the plurality of nanosheets. The gate structure includes a plurality of high-k gate dielectric layers, a work function layer and a fluorine-free fill metal layer. The plurality of high-k gate dielectric layers respectively surrounds the plurality of nanosheets. The work function layer surrounds each of the plurality of high-k gate dielectric layers. The fluorine-free fill metal layer surrounds the work function metal layer. 
     In some embodiments, the work function layer is also fluorine-free. 
     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.