Patent Publication Number: US-11024627-B2

Title: High-K metal gate transistor structure and fabrication method thereof

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims the priority of Chinese patent application No. 201510894275.9, filed on Dec. 7, 2015, the entirety of which is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention generally relates to the field a semiconductor manufacturing technology and, more particularly, relates to high-K metal gate transistors and fabrication processes thereof. 
     BACKGROUND 
     Complementary metal-oxide-semiconductor (CMOS) devices are one of the basic types of semiconductor devices for forming integrated circuits (ICs). The CMOS devices include P-type CMOS (PMOS) devices and N-type CMOS (NMOS) devices. 
     In the existing techniques, to reduce the size of the gate and inhibit the short-channel effect, transistors with high dielectric constant (high-K) dielectric layers and metal gate layers (HKMG, high-K metal gate) have been developed. That is, the high-K dielectric layer is used to substitute the conventional silicon oxide dielectric layer in a transistor and to be used as a gate dielectric layer; and a metal material layer is used to substitute the conventional polysilicon gate layer in the transistor and to be used as a gate electrode layer. Thus, a HKMG structure is used as the gate structure of the transistor. Further, to adjust the work functions of the PMOS transistor and the NMOS transistor, work function layers are formed on the surface of the gate dielectric layer. The work function layer of the PMOS transistor needs to have a relatively high work function; and the work function layer of the NMOS transistor needs to have a relatively low work function. Thus, in the PMOS transistor and the NMOS transistor, the work function layers may be made of different materials to match the requirements for adjusting the individual work functions. 
     CMOS transistors may be formed by a gate-last process. The existing gate-last process includes forming a dummy gate layer on a PMOS region and an NMOS region of the surface of a semiconductor substrate; forming source regions and drain regions at two sides of the dummy gate layer; forming a dielectric layer exposing the dummy gate layers over the semiconductor substrate; removing the dummy gate layer to form openings in the dielectric layer; and sequentially forming the gate dielectric layers, work function layers and gate electrode layers in the openings. The gate electrode layers are made of metal materials. The gate dielectric layers are made of high-K materials. The work function layer in the region and the work function layer in the NMOS region are made of different materials. 
     However, the work function material for the PMOS transistor and the work function material for the NMOS transistor are different. Such a difference adversely affects the performance of the CMOS transistors. 
     The disclosed transistors and fabrication methods are directed to solve one or more problems set forth above and other problems in the art. 
     BRIEF SUMMARY OF THE DISCLOSURE 
     One aspect of the present disclosure includes a method for fabricating a high-K metal gate transistor structure. The method includes providing a base substrate having a first region and a second region; forming a dielectric layer having a first opening exposing a first surface portion of the base substrate in the first region and a second opening exposing a second surface portion of the base substrate in the second region over the base substrate; forming a gate dielectric layer on a side surface of the first opening and the first surface portion of the base substrate in the first opening and on a side surface of the second opening and the second surface portion of the base substrate in the second opening; filling a sacrificial layer in the first opening; forming a second work function layer in the second opening and a second gate electrode layer on the second work function layer; removing the sacrificial layer; and forming a first work function layer in the first opening and a first gate electrode layer on the first work function layer. 
     Another aspect of the present disclosure includes a high-K metal gate transistor structure. The high-K metal gate structure includes a semiconductor substrate having a first region and a second region; a plurality of fins formed on the semiconductor substrate in the first region and the second region; a first HKMG structure formed over the fins in the first region; a second HKMG structure formed over the fins in the second region; source/drain regions formed in the fins at the two sides of the HKMG structures; an isolation layer formed between adjacent fins; and a dielectric layer covering the side surfaces of the HKMG structures formed on the semiconductor substrate, the source/drain regions and the isolation layer. The first HKMG structure includes a gate dielectric layer, a first work function layer, and a first gate electrode layer; and the second HKMG structure includes a gate dielectric layer, a second work function layer, and a second gate electrode layer. The first HKMG structure and the second HKMG structure are formed by forming a first opening exposing a portion of a surface of the semiconductor substrate and the fins in the first region and a second opening exposing a portion of the surface of the semiconductor substrate and the fins in the second region; forming a gate dielectric layer on a side surface of the first opening and the exposed surfaces of the semiconductor substrate and the fins in the first opening and on a side surface of the second opening and the exposed surfaces of the semiconductor substrate and the fins in the second opening; filling a sacrificial layer in the first opening; forming the second work function layer in the second opening and the second gate electrode layer on the second work function layer; removing the sacrificial layer; and forming the first work function layer in the first opening and the first gate electrode layer on the first work function layer. 
     Other aspects of the present disclosure can be understood by those skilled in the art in light of the description, the claims, and the drawings of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a cross-sectional view of an existing CMOS transistor structure; 
         FIGS. 2-10  illustrate semiconductor structures corresponding to certain stages of an exemplary fabrication process of a high-K metal gate transistor structure consistent with the disclosed embodiments; and 
         FIG. 11  illustrates an exemplary fabrication process of a high-K metal gate transistor structure consistent with the disclosed embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to exemplary embodiments of the invention, which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     PMOS transistor and NMOS transistor often need different work function materials, and the difference of the work function materials affects the performance stability of the CMOS transistor structure.  FIG. 1  illustrates the cross-sectional view of an existing CMOS transistor structure. 
     As shown in  FIG. 1  the CMOS transistor structure includes a substrate  100  having a PMOS region  101  and an NMOS region  102 . A dielectric layer  103  is formed on the surface of the substrate  100 . The portion of the dielectric layer  103  in the PMOS region  101  may have a first opening (not labeled) exposing the surface of the substrate  100  and the portion of the dielectric layer  103  in the PMOS region may have a second opening (not labeled) exposing the surface of the substrate  100 . A first gate dielectric layer  110  is formed on the side surface and the bottom surface of the first opening; and a second gate dielectric layer  120  is formed on the side surface and the bottom surface of the second opening. 
     Further, a first work function layer  111  is formed on the surface of the first gate dielectric layer  110  in the PMOS region  101 ; and a second work function layer  121  is formed on the surface of the second gate dielectric layer  120  and the surface of the first work function layer  111 . A gate dielectric layer  130  may be formed on the surface of the first work function layer  111  and the surface of the second work function layer  121 . The gate electrode layer  130  may fill the first opening and the second opening. 
     The first work function layer  111  is a P-type work function layer; and the second work function layer  121  is an N-type work function layer. Because the material of the N-type work function layer contains Al ions. To prevent the Al ions in the N-type work function layer from diffusing into the first gate dielectric layer  110  and the substrate  100 , in the structure illustrated in  FIG. 1 , after forming the first work function layer  111  in the PMOS region  101 , the N-type second work function layer  121  is formed in the NMOS region  102  and the PMOS region  101 . 
     In the PMOS region  101 , before forming the gate electrode layer  130 , the stacked structure having the first dielectric layer  110 , the first work function layer  111  and the second work function layer  121  is formed the first opening. Thus, the width of the remaining first opening along the direction parallel to the surface of the substrate  100  is relatively small; and the depth of the remaining first opening is relatively large. Therefore, the difficulty for filling gate electrode layer  130  in the first opening is increased; and the yield of the gate electrode layer  130  may be unacceptable. Further, voids are easy to form inside the gate electrode layer  130 ; and the interface bonding between the gate electrode layer  130  and the second work function layer  121  may be undesired. 
     In the NMOS region  102 , before forming the gate electrode layer  130  and the second work function layer  121 , the first work function layer  111  in the second opening needs to be removed to prevent the P-type work function layer from deteriorating the performance of the NMOS transistor. However, with the continuous shrinking of the critical dimension of the transistors, the size of the second opening along the direction parallel to the surface of the substrate  100  is correspondingly decreased. Thus, the depth-to-width ratio of the second opening is relatively large; and the difficulty for removing the first work function layer  111  in the second opening is relatively large. After removing the first work function layer  111  in the second opening, residual contaminations are easy to adhere on the inner surfaces of the second opening; and the surface of the second dielectric layer  102  in the second opening is also easily damaged. Accordingly, a mismatch between the PMOS transistor and the NMOS transistor is easily generated. Especially when the PMOS transistor and the NMOS transistor are used to form a SRAM device, the mismatch between the PMOS transistor and the NMOS transistor deteriorates the performance of the SRAM device, and the yield is reduced. 
     The present disclosure provides a high-K metal gate transistor structure and a fabrication method of the high-K metal gate transistor structure.  FIG. 11  illustrates an exemplary fabrication process of a high-K metal gate transistor structure consistent with the disclosed embodiments; and  FIGS. 2-10  illustrate structures corresponding to certain stages of the exemplary fabrication process. 
     As shown in  FIG. 11 , at the beginning of the fabrication process, a substrate with certain structures is provided (S 101 ).  FIG. 1  illustrates a corresponding semiconductor structure. 
     As shown in  FIG. 1 , a base substrate is provided. The base substrate may have a first region  210  and a second region  220 . 
     In one embodiment, the first region  210  is an NMOS region; and may be used to form an NMOS transistor. The second region  220  is a PMOS region; and may be used to form a PMOS transistor. 
     In some embodiments, transistors formed in the first region  210  and the second region  220  are used to form a CMOS transistor structure. In other embodiments, the transistors formed in the first region  210  and the second region  220  may be used to form a static random accessible memory (SRAM) device. In this case, the NMOS transistor formed in the first region  210  may be configured as a pull-down (PD) transistor; and the PMOS transistor formed in the second region  220  may be configured as a pull-up (PU) transistor, or a pass-gate (PG) transistor. 
     In one embodiment, the transistor formed in the first region  210  and the transistor formed in the second region  220  are fin field-effect transistors (FinFETs). Thus, the base substrate in  FIG. 1  may include a semiconductor substrate  200 ; and a plurality of fins  201  formed on the surface of the semiconductor substrate  200  in the first region  210  and the second region  220 , respectively. The base substrate may also include an isolation layer  202  formed on the surface of the semiconductor substrate  200 . The isolation layer  202  may cover the side surfaces of the fins  201 ; and the top surface of the isolation layer  202  may be lower than the top surfaces of the fins  201 . 
     Further, a first well region (not shown) may be formed in the fins  201  in the first region  210  and a portion of the semiconductor substrate  200 . The first well region may be doped with P-type ions. A second well region (not shown) may be formed in the fins  201  in the second region  220  and a portion of the semiconductor substrate  200 . The second well region may be doped with N-type ions. 
     In one embodiment, the process for forming the semiconductor substrate  200  and the plurality of fins  201  may include: providing an initial substrate, forming a mask layer covering portions of the surface of the initial substrate corresponding to the plurality of the fins  201  on the surface of the initial substrate; and etching the initial substrate using the mask layer as an etching mask to form the plurality of fins  201  and the semiconductor substrate  200  under the plurality of fins  201 . The first well region and the second well region may be formed before forming the fins  201 , or after forming the fins  201 . 
     The mask layer may be formed by forming a mask material layer on the surface of the initial substrate, forming a patterned layer on the mask material layer; and etching the mask material layer using the patterned layer as an etching mask until the surface of the initial substrate is exposed. Thus, the mask layer may be formed. 
     In one embodiment, the patterned layer may be a patterned photoresist layer. The patterned photoresist layer may be formed by a spin-coating process and a photolithography process, etc. 
     In certain embodiments, a multiple-mask patterning process may form the patterned layer. The multiple-mask patterning process may include a self-aligned double patterning (SaDP) process, a self-aligned triple patterning process; or a self-aligned double double patterning (SaDDP) process, etc. 
     The initial substrate may be made of any appropriate semiconductor material, such as silicon, silicon germanium, or Group III-V compounds, etc. The initial substrate may be etched by any appropriate process. In one embodiment, an anisotropic dry etching process is used to etch the initial substrate to form the plurality of fins  201  and the semiconductor substrate  200 . 
     The side surfaces of the fins  201  may be perpendicular to the surface of the semiconductor substrate  200 , or may be inclined and have an angle with the surface of the semiconductor substrate  200 . In one embodiment, the side surfaces of the fins  201  are inclined with the surface of the semiconductor substrate  200 . When the side surfaces of the fins  201  are inclined with the surface of the semiconductor substrate  200 , the bottom width of the fins  201  may be greater than the top width of the fins  201 . 
     In certain embodiments, the fins may be formed by etching a semiconductor layer formed on the surface of the semiconductor substrate. The semiconductor layer may be formed on the surface of the semiconductor substrate by an epitaxial growth process. The semiconductor substrate may be a silicon substrate, a silicon germanium substrate, a silicon carbide substrate, a silicon on insulator (SOI) substrate, a germanium on insulator (GOI) substrate, a glass substrate, or a Group III-V compound substrate, such as a gallium nitride substrate or a gallium arsenide substrate, etc. The semiconductor layer may be made of silicon germanium, silicon carbide, or silicon germanium, etc. 
     In one embodiment, before forming the insolation layer  202 , an interface layer may be formed on the surface of the semiconductor substrate  200  and the side and top surfaces of the fins  201 ; and the insolation layer  202  may be formed on the interface layer. The interface layer may be made of silicon oxide; and may be formed by an oxidation process. The thickness of the interface layer may be in a range of approximately 5 nm-50 nm. The oxidation process may include an in-situ steam generation (ISSG) process, a decoupled plasma oxidation (DPO) process, a radical oxidation process, or a wet oxidation process, etc. 
     The process for forming the insulation layer  202  may include forming an isolation film on the surface of the semiconductor substrate  200  and the side and top surfaces of the fins  201 ; planarizing the isolation film until the top surfaces of the fins  201  are exposed; and performing an etch-back process to the remaining isolation film until the side surfaces of the fins  201  are exposed with a predetermined size. Thus, the insulation layer  202  may be formed on the bottom portions of the side surfaces of the fins  201 . 
     The isolation layer  202  may be made of any appropriate material. In one embodiment, the isolation layer  202  is made of silicon oxide. 
     Various processes may be used to form the isolation film, such as a flowable chemical vapor deposition (FCVD) process, a plasma-enhanced chemical vapor deposition (PECVD) process, a high aspect ratio chemical vapor deposition (HARP CVD) process, or a physical vapor deposition (PVD) process, etc. 
     The planarization process may be a chemical mechanical polishing (CMP) process, etc. The etch-back process may be an isotropic dry etching process, an anisotropic dry etching process, or a wet etching process, etc. 
     In certain embodiments, the transistors formed in the first region and the second region may be planar transistors. The base substrate may be a planar substrate; and shallow trench isolation (STI) structures may be formed in the base substrate between the first region and the second region. 
     The base substrate may include silicon substrate, silicon germanium substrate, silicon carbide substrate, SOI substrate, GOI substrate, glass substrate, or Group III-V compound substrate (such a gallium nitride or gallium arsenide, etc.), etc. A first well region may be formed in the first region; and the first well region may be doped with P-type ions. A second well region may be formed in the second region; and the second region may be doped with N-type ions. 
     A dielectric layer may be subsequently formed on the surface of the base substrate. The dielectric layer may have a first opening and a second opening. The first opening may expose a first surface portion of the base substrate in the first region; and the second opening may expose a second surface portion of the base substrate in the second region. 
     Returning to  FIG. 11 , after forming the isolation layer  202 , dummy gate structures may be formed on the surface of the base substrate (S 102 ).  FIG. 3  illustrates a corresponding semiconductor structure. 
     As shown in  FIG. 3 , dummy gate structures (not labeled) are formed on the surface of the base substrate. The dummy gate structures may include a dummy gate electrode layer  230 . 
     In one embodiment, the subsequently formed NMOS transistor in the first region  210  and the subsequently formed PMOS transistor in the second region  220  are high-K metal gate (HKMG) transistors. That is, a high dielectric constant (high-K) dielectric material may be used as the gate dielectric layer; and a metal material may be used as the gate electrode layer. The PMOS transistor and the NMOS transistor may be formed by a gate-last process. Thus, the dummy gate electrode layer  230  in the first region  210  may be subsequently substituted to form a first gate dielectric layer and a first gate electrode layer. The dummy gate electrode layer  230  in the second region  220  may be subsequently substituted to form a second gate dielectric layer and a second gate electrode layer. 
     In one embodiment, a plurality of dummy gate structures may cross over the plurality of fins  201  in the first region  210  and the second region  220 . The dummy gate structures may cover the top surfaces and portions of the side surfaces of the fins  201  and a portion of the surface of the isolation layer  202 . 
     The dummy gate electrode layer  230  may be made of any appropriate material. In one embodiment, the dummy gate electrode layer  230  is made of poly silicon. 
     The process for forming the dummy gate electrode layer  230  may include forming a dummy gate film on the surfaces of the isolation layer  202  and the fins  201 ; forming a patterned layer on portions of the surface of the dummy gate film; etching the dummy gate film until the top surfaces and portions of the side surfaces of the fins  201  and the surface of the isolation layer  202  are exposed using the pattered layer as an etching mask. Thus, the dummy gate electrode layer  230  may be formed. 
     In one embodiment, sidewall spacers  232  may be formed on the side surfaces of the dummy gate structure including the dummy gate electrode layer  230 . The sidewall spacers  232  may be used to define the distance between the subsequently formed source/drain regions and the dummy gate electrode layer  230 . The sidewall spacers  232  may be made of any appropriate material, such as one or more of silicon oxide, silicon nitride, and silicon oxynitride, etc. 
     In one embodiment, a dummy gate dielectric layer (not shown) may be formed between the base substrate and the dummy gate electrode layer  230 . The dummy gate dielectric layer may be made of silicon oxide. Various processes may be used to form the dummy gate dielectric layer, such as an atomic layer deposition (ALD) process, a CVD process, a thermal oxidation process, a wet oxidation process, or an ISSG process, etc. 
     Further, source/drain regions  231  may be formed in the fins  201  at two sides of the dummy gate structures. The process for forming the source/drain regions  231  may include forming stress layers in the fins  201  at the two sides of the gate structures; and doping N-type ions or P-type ions in the stress layers to form the source/drain regions  231 . 
     The process for forming the stress layers may include forming third opening in the fins  201  at the two sides of the gate structures; and forming the stress layers in the third openings by filling the third openings using an epitaxial growth process. The surfaces of the stress layers may be higher than or level with the top surfaces of the fins  201 . 
     The stress layer in the first region  210  may be made of silicon carbide. The stress layer in the second region  220  may be made of silicon germanium. The sidewalk of the stress layer in the first region  210  may be perpendicular to the top surfaces of the fins  201 . The sidewalls of the stress layer in the second region  220  and the top surfaces of the fins  201  may present an “Σ” shape; and the side surfaces of the stress layer may have protruding angles. The apex of the protruding angle may extend into the fin  201  under the dummy gate layer  230 . The stress layers may be doped with N-type ions or P-type ions by an in-situ doping process, an ion implantation process, or a combination thereof. 
     In certain embodiments, the source/drain regions may be formed in the fins  201  at the two sides of the dummy gate structures by an ion implantation process. In certain other embodiments, if the base substrate is a planar substrate, the source/drain regions may be formed in the base substrate at the two sides of the dummy gate structures. 
     Returning to  FIG. 11  after forming the source/drain regions  231 , a dielectric layer may be formed (S 103 ).  FIG. 4  illustrates a corresponding semiconductor structure. 
     As shown in  FIG. 4 , a dielectric layer  240  is formed over the surface of the base substrate. The top surface of the dielectric layer  240  may level with the top surfaces of the dummy gate structures. The dielectric layer  240  may be on the surface of the base substrate, the surfaces of the source/drain regions  231 , the surface of the isolation layer  202  and the side surfaces of the sidewalls  232 . 
     The dielectric layer  240  may be used to electrically isolate the adjacent dummy gate structures. The dielectric layer  240  may also be used to keep the structures and positions of the dummy gate layer  230 . The dielectric layer  240  may be made of any appropriate material, such as silicon oxide, silicon nitride, silicon oxynitride, low-K dielectric material (the dielectric constant is greater than or equal to approximately 2.5 and smaller than approximately 3.9), or ultra-low-K dielectric material (the dielectric constant is smaller than approximately 3.9), etc. 
     The process for forming the dielectric layer  240  may include forming a dielectric film on the surfaces of the base substrate and the dummy gate structures; and planarizing the dielectric film until the top surfaces of the dummy gate structures are exposed. Thus, the dielectric layer  240  may be formed. 
     In one embodiment, before forming the dielectric film, an etching stop layer may be formed on the surfaces of the base substrate and the dummy gate structures. The etching stop layer may be used as a stop layer for subsequently forming contact through holes. The etching stop layer may be made of a material different from that of the dielectric layer  240 . 
     Various processes may be used to form the dielectric film, such as a CVD process, a PVD process, or an ALD process, etc. In one embodiment, the dielectric film is formed by an FCVD process. The planarization process may be a CMP process. 
     Returning to  FIG. 11 , after forming the dielectric layer  240 , the dummy gate electrode layers  230  may be removed (S 104 ).  FIG. 5  illustrates a corresponding semiconductor structure. 
     As shown in  FIG. 5 , the dummy gate electrode layers  230  are removed. A first opening  241  may be formed in the dielectric layer  240  in the first region  210 . A second opening  242  may be formed in the dielectric layer  240  in the second region  220 . 
     The first opening  241  may cross over the fins  201  in the first region  210 ; and may expose the side and top surfaces of portions of the fins  201  in the first region  210 . The second opening  242  may cross over the fins  201  in the second region  220 ; and may expose the side and top surfaces of the fins  201  in the second region  220 . 
     The dummy gate layers  230  may be removed by any appropriate process, such as a dry etching process, or a wet etching process, etc. The dry etching process may be an isotropic etching process, or an anisotropic dry etching process. The wet etching process may be an isotropic wet etching process. 
     In one embodiment, the dummy gate layer  230  is made of polysilicon. The etching gas of the dry etching process may include one or more of Cl 2  and Br 2 , etc. The etching solution of the wet etching process may include one or more of a nitric acid solution and a hydrogen fluoride solution, etc. 
     In one embodiment, a wet etching process is used to remove the dummy gate layer  230 . The wet etching process may have relatively fewer damages to the surfaces of the fins  201 . 
     In one embodiment, a dummy gate dielectric layer may be formed between the dummy gate layer  203  and the fins  201 . Thus, after removing the dummy gate layer  230 , the dummy gate dielectric layer on the bottoms of the first opening  241  and the second opening  242  may be removed. 
     The dummy gate dielectric layer may be removed by a SiConi™ etching process. The SiConi™ etching process is an isotropic dry etching process; and may generate fewer damages to the top and side surfaces of the fins  201 . 
     Returning to  FIG. 11 , after forming the first opening  241  and the second opening  242 , a gate dielectric layer, a cap layer and a first barrier layer may be formed (S 105 ).  FIG. 6  illustrates a corresponding structure. 
     As shown in  FIG. 6 , a gate dielectric layer  250  is formed on the side and bottom surfaces of the first opening  241  and the second opening  242 , and the surface of the base substrate. Further, a cap layer  251  may be formed on the gate dielectric layer  250 ; and a first barrier layer  252  may be formed on the cap layer  251 . In one embodiment, the gate dielectric layer  250  may also be formed on the side and top surfaces of the fins  201  exposed by the first opening  241  and the second opening  242 . 
     The gate dielectric layer  250  may be made of any appropriate material, such as a high-K dielectric material, etc. The high-K dielectric material may include HfO, ZrO, HfSiO, LaO, ZrSiO, TiO, TaO, BaSrTiO, BaTiO, SrTiO, or AlO, etc. Various processes may be used to form the gate dielectric layer  250 , such as a CVD process, a PVD process, or an ALD process, etc. The thickness of the gate dielectric layer  250  may be in a range of approximately 10 Å-50 Å. 
     In one embodiment, before forming the dielectric layer  250 , an interface layer (not shown) may be formed on the surface of the base substrate on the bottoms of the first opening  241  and the second opening  242 . The gate dielectric layer  250  may be formed on the interface layer. The interface layer may be used to increase the bonding strength between the gate dielectric layer  250  and the base substrate; and may also be used to repair the interface defects at the interface between the gate dielectric layer  250  and the base substrate. 
     The interface layer may be made of silicon oxide. Various processes may be used to form the interface layer, such as an oxidation process, etc. The oxidation process may include a thermal oxidation process, or a wet oxidation process, etc. 
     The cap layer  251  may be used to protect the gate dielectric layer  250 ; and prevent the atoms in the subsequently formed first work function layer, the second work function layer, the first gate electrode layer and the second gate electrode layer from the diffusing into the gate dielectric layer  250 . Thus, the cap layer  251  may be able to ensure the stability of the gate dielectric layer  250 . Accordingly, the threshold voltage of the finally formed transistors may not be easy to have a drift. 
     The cap layer  251  may be made of any appropriate material. In one embodiment, the cap layer  251  is made of TiN. Various processes may be used to form the cap layer  251 , such as a CVD process, a PVD process, or an ALD process. The thickness of the cap layer  251  may be in a range of approximately 5 Å-20 Å. In certain other embodiments, the cap layer may be omitted. 
     In one embodiment, after forming the cap layer  251  and before subsequently forming a sacrificial layer, a thermal annealing process may be performed. The thermal annealing process may be used to activate the doping ions in the source/drain regions  231 . The cap layer  251  may be able to absorb the defects, contaminations, and oxygen vacancies, etc., caused by thermal annealing process; and prevent the defects, contaminations, and oxygen vacancies, etc., from diffusing into the gate dielectric layer  250 . 
     The first barrier layer  252  may be used as a polishing stop layer for subsequently forming a second work function layer in the second region  220 . The first barrier layer  252  may be made of a material different from that of the subsequently formed second work function layer to cause the first barrier layer  252  and the subsequently formed first work function layer to have an etching selectivity. 
     In one embodiment, the second region  220  may be used to form a PMOS transistor. Thus, the first barrier layer  251  may be made of a P-type work function material. The first barrier layer  251  in the second region  220  may be kept in the PMOS transistor; and may be used to adjust the threshold voltage of the transistor formed in the second region  220 . 
     The first barrier layer  252  may be made of TaN. TaN is a P-type work function material. Various processes may be used to form the first barrier layer  252 , such as a CVD process, a PVD process, or an ALD process, etc. The thickness of the first barrier layer  252  may be in a range of approximately 5 Å-20 Å. In certain other embodiments, the first barrier layer may be omitted. 
     Returning to  FIG. 11 , after forming the gate dielectric layer  250 , a sacrificial layer may be formed (S 106 ).  FIG. 7  illustrates a corresponding semiconductor structure. 
     As shown in  FIG. 7 , a sacrificial layer  203  is formed in the first opening  241 . The sacrificial layer  203  is filled in the first opening  241 . Thus, the subsequent processes may be able to form a second work function layer and a second gate electrode layer in the second opening  242 . 
     Because the first opening  241  is filled with the sacrificial layer  203 , the subsequently formed second work function layer may only be formed in the second opening  242 , conventional step for removing the second work function layer in the first opening  241  may be omitted. Thus, the damages to the inner side surfaces of the first opening  241  caused by the process for etching the second work function layer may be avoided; and the finally formed NMOS transistor may be more stable. Further, the mismatch between the NMOS transistor and the PMOS transistor formed in the second region  220  may be avoided. 
     Further, because the sacrificial layer  203  is filled in the first opening  241 , after subsequently forming the second work function layer and the second gate electrode layer in the second opening  242 , subsequent processes may be performed in the first opening  241 . Thus, after subsequently forming the second work function layer, it may not need to form the first work function layer in the first opening  241  and the second opening  242  simultaneously. Therefore, before forming the second gate electrode layer, the first work function layer may not be formed in the second opening  242 . Accordingly, the depth-to-width ratio of the second opening  242  may be reduced. Reducing the depth-to-width ratio of the second opening  242  may aid to subsequently fill a second gate electrode layer into the second opening  242 . Further, the second gate electrode layer may be dense; the bonding strength between the second gate electrode layer and the second work function layer may be as desired; and the bonding strength between the second work function layer and the first barrier layer  252  may be as desired. 
     The sacrificial layer  203  may be made of any appropriate material, such as a back anti-reflection coating (BARC) material, a deep UV light absorbing oxide (DUO) material, or an organic dielectric (ODL) material, etc. The sacrificial layer  203  may need to be removed subsequently. The BARC material and the ODL material may be easily filled in the first opening  241 ; and may be easily to remove; and may be not easy to leave residuals. 
     The process for forming the sacrificial layer  203  may include forming a sacrificial film over the surface of the dielectric layer  250  and to fill the first opening  241  and the second opening  242 ; planarizing the sacrificial film to remove the sacrificial film above the dielectric layer  240 ; and removing the sacrificial film in the second opening  242 . Thus, the sacrificial layer  203  may be formed in the first opening  241 . 
     The sacrificial film may be formed by a coating process, such as a spin-coating process, or a spray coating process, etc., on the first barrier layer  252 . The sacrificial film may also fill the first opening  241  and the second opening  242 . The planarization process may be a CMP process. The surface of the first barrier layer  252  may be used as a stop layer of the CMP process. 
     The process for removing the sacrificial film in the second opening  242  may include forming a patterned layer covering the sacrificial film in the first opening  241 ; and removing the sacrificial film in the second opening  242  using the patterned layer as a mask. The sacrificial film in the second opening  242  may be removed by a dry etching process, a wet etching process, or a plasma ashing process, etc. 
     Returning to  FIG. 11 , after forming the sacrificial layer  203 , a second work function layer and a second gate electrode layer may be formed (S 107 ).  FIG. 8  illustrates a corresponding semiconductor structure. 
     As shown in  FIG. 8 , a second work function layer  260  is formed in the second opening  242 ; and a second gate electrode layer  261  is formed on the second work function layer  260 . The second gate electrode layer  261  may fill the second opening  242 . 
     The second gate electrode layer  261 , the second work function layer  260  and the gate dielectric layer  250  may be form a HKMG structure of the PMOS transistor in the second region  220 ; and may be referred to a second HKMG structure. 
     In some embodiments, the second work function layer  260  may be formed on the surface of the first barrier layer  252 . In other embodiments, if the first barrier layer is optional and omitted, the second work function layer  260  may be formed on the cap layer  251  or may be directly formed on the surface of the gate dielectric layer  250 . 
     The second work function layer  260  may be used to adjust the threshold voltage of the transistor formed in the second region  220 . In one embodiment, the second region  220  may be used to form a PMOS transistor. Thus, the second work function layer  260  may be made of a P-type work function material. In one embodiment, the second work function layer  260  is made of TiN. TiN may have a relatively high work function; and may be able to adjust the threshold voltage of the PMOS transistor. The thickness of the second work function layer  260  may be in a range of approximately 20 Å-50 Å. 
     The process for forming the second work function layer  260  and the second gate electrode layer  261  may include forming a second work function film on the surface of the first barrier layer  252  and the surface of the sacrificial layer  203 ; forming a second gate electrode film on the second work function film to fill the second opening  242 ; planarizing the gate electrode film and the second work function film until the surface of the dielectric layer  240  is exposed. The surface of the sacrificial layer  203  may also be exposed. Thus, the second gate electrode layer  261  and the second work function layer  260  may be formed. 
     In certain embodiments, if the optional first barrier layer  252  and the cap layer  251  are omitted, the second work function film may be formed on the gate dielectric layer  250 . Then, the second gate electrode film may be formed on the second work function film. 
     The second work function film may be formed by any appropriate process. In one embodiment, an ALD process is used to form the second work function film. The reaction gas of the ALD process may include a first precursor gas containing Ti. The Ti-containing first precursor gas may include one or more of TiCl 4 , Ti[N(C 2 H 5 CH 3 )] 4 , Ti[N(CH 3 ) 2 ] 4 , Ti[N(C 2 H 5 ) 2 ] 4 , etc. The reaction gas of the ALD process may also include a second precursor gas. The second precursor gas may include NH 3 , CO, or H 2 O, etc. A flow rate of the first precursor gas may be in a range of approximately 50 sccm-200 sccm. The flow rate of the second precursor gas may be in a range of approximately 50 sccm-200 sccm. The reaction temperature of the ALD process may be in a range of approximately 400° C.-600° C. 
     The second work function film formed by the ALD process may have a desired coverage; and may be able to tightly adhere to the sidewalls and the bottom surface of the second opening  242 . Further, the thickness of the work function film formed by the ALD process may be uniform. The uniformity of the thickness of the second work function film may cause the formed second work function layer  260  to be able to stably adjust the threshold voltage of the PMOS transistor. 
     Because Al is an N-type work function material; and may deteriorate the performance of the PMOS transistor. Thus, the second gate electrode layer  261  may be made of a material not containing Al. In one embodiment, the second region  220  may be used to form a PMOS transistor; and the second gate electrode layer  261  is made of W. Various processes may be used to form the second gate electrode film, such as a PVD process, a CVD process, an electroplating process, or a chemical plating process, etc. 
     The planarization process of the second gate dielectric film and the second work function film may be a CMP process, or a mask-less anisotropic dry etching process. In one embodiment, a CMP process is used to planarize the second gate electrode film and the second work function film; and the first barrier layer  252  may be used as the polishing stop layer. 
     In one embodiment, after planarizing the second gate electrode film and the second work function film, the CMP process may be continued to planarize the first barrier layer  252 , the cap layer  251  and the gate dielectric layer  250  until the top surface of the dielectric layer  240  is exposed. 
     Returning to  FIG. 11 , after forming the second gate electrode layer  261 , the sacrificial layer  203  may be removed (S 108 ).  FIG. 9  illustrates a corresponding semiconductor structure. 
     As shown in  FIG. 9 , the sacrificial layer  203  in the first opening  241  is removed. The sacrificial layer  203  may be removed by any appropriate process, such as a dry etching process, a wet etching process, or a plasma ashing process, etc. The dry etching process may be an anisotropic dry etching process, or an isotropic dry etching process. In one embodiment, the sacrificial layer  203  is removed by a wet etching process. 
     After removing the sacrificial layer  203 , the first opening  241  is exposed. The first opening  241  may be used to form the work function layer and the gate electrode layer of the NMOS transistor. In one embodiment, the sacrificial layer may be a bottom anti-reflection material, a deep UV absorption oxide material, or an organic dielectric material, etc. Thus, the sacrificial layer  203  may be easy to remove; and residual by-products may not easy to be formed on the inner side surfaces of the first opening  241  after removing the sacrificial layer  203 . Further, the inner side surfaces may have fewer damages after removing the sacrificial layer  203 . Thus, the performance of the NMOS transistor formed in the first region  210  may be stable. Further, the mismatch between the NMOS transistor and the PMOS transistor formed in the second region  220  may be avoided. 
     In one embodiment, after removing the sacrificial layer  203 , the first barrier layer  252  in the first opening  241  may be removed. The first barrier layer  252  may be removed by an isotropic dry etching process, or a wet etching process, etc. After removing the first barrier layer  252  the size of the first opening  241  may be increased; and it may aid to subsequently form a first work function layer and a first gate electrode layer in the first opening  241 . 
     Returning to  FIG. 11 , after removing the sacrificial layer  203  in the first opening  241 , a first work function layer and a first gate electrode layer may be formed (S 109 ).  FIG. 10  illustrates a corresponding semiconductor structure. 
     As shown in  FIG. 10 , a first work function layer  270  is formed in the first opening  241 . In one embodiment, the first work function layer  270  may be formed on the cap layer  251 . Further, a first gate electrode layer  271  may be formed on the first work function layer  270 . The first gate electrode layer  271  may fill the first opening  241 . 
     The first gate electrode layer  271 , the first work function layer  270  and the gate dielectric layer  250  may form a HKMG structure of an NMOS transistor in the first region  210 ; and may be referred to as a first HKMG structure. 
     The first work function layer  270  may be used to adjust the threshold voltage of the transistor formed in the first region  270 . In one embodiment, the first region  210  may be used to form an NMOS transistor, the first work function layer  270  may be made of TiAlC. TiAlC may have a relatively low work function; and may be able to adjust the threshold voltage of the NMOS transistor. Further, the material of the first work function layer  270 , i.e., TiAlC, may include C ions. C ions may be able to fill defects; and may form stable function groups with Ti ions and Al ions; and may be able to avoid the diffusion of the Al ions. Thus, the stability of the dielectric constant of the gate dielectric layer  250  may be ensured. The thickness of the first work function layer  270  may be in a range of approximately 20 Å-50 Å. 
     The process for forming the first work function layer  270  and the first gate electrode layer  271  may include forming a first work function film over the gate dielectric layer  250 , the dielectric layer  240  and the second gate electrode layer  261 ; and forming a gate electrode film filling the first opening on the surface of the first work function film; and planarizing the first gate electrode film and the first work function film until the surface of the dielectric layer  240  is exposed. Thus, the first gate electrode layer  271  and the first work function layer  270  may be formed. 
     The first work function film may be formed by any appropriate process. In one embodiment, the first work function film is formed by an ALD process. The first work function film formed by the ALD process may have a desired coverage ability; and may be able to have a tight adhesion with the side and bottom surfaces of the first opening  241 . Further, the thickness of the first work function film formed by the ALD process may be uniform; and such a uniform thickness of the first work function film may cause threshold voltage adjustment of the formed first work layer  270  to the NMOS transistor to be stable and easy to control. 
     The first gate electrode layer  271  may be made of TiAl or W. Various processes may be used to form the first gate electrode film, such as a PVD process, a CVD process, an electroplating process, or a chemical plating process, etc. 
     The first gate electrode film and the first work function film may be planarized by a CMP process, or a mask-less anisotropic dry etching process, etc. In one embodiment, a CMP process is used to planarize the first gate electrode film and the first work function film. 
     In one embodiment, after forming the first work function film and before forming the first gate electrode film, a second barrier film may be formed on the first work function film; and the first gate electrode film may be formed on the second barrier film. The planarization process may also planarize the second barrier film; and a second barrier layer  272  may be formed in the first opening  241 . 
     The second barrier layer  272  may be made of any appropriate material. In one embodiment, the second barrier layer  272  is made of TiN. The second barrier layer  272  may be used to prevent the material of the first gate electrode layer  271  from diffusing into the dielectric layer  250  and the fins  201 ; and may be able to stabilize the performance of the NMOS transistor. 
     Thus, a high-K metal gate transistor structure may be formed by the disclosed methods and processes.  FIG. 10  illustrates a corresponding semiconductor structure. 
     As shown in  FIG. 10 , the high-K metal gate (HKMG) transistor structure may include a semiconductor substrate  200  having a first region  210  and a second region  220  and a plurality of fins  201  formed on the semiconductor substrate  200 . The HKMG transistor structure may include a first HKMG structure formed over the fins  201  in the first region  210  and a second HKMG structure formed over the fins  201  in the second region  220 . The first HKMG structure may include a gate dielectric layer  250 , a cap layer  251 , a first work function layer  270 , a second barrier layer  272 , a first gate electrode layer  271  and sidewall spacers  232 . The second HKMG structure may include a gate dielectric layer  250 , a cap layer  251 , a first barrier layer  252 , a second work function layer  260 , a second gate electrode layer  261  and sidewall spacers  232 . Further, the HKMG transistor structure may also include source/drain regions  231  formed in the fins  201  at the two sides of the HKMG structures; and an isolation layer  202  formed between adjacent fins  201 ; and a dielectric layer  240  formed on the semiconductor substrate  200 , the source/drain regions  231  and the isolation layer  202 . The dielectric layer  240  may cover the side surfaces of the HKMG structures. The detailed structures and intermediate structures are described above with respect to the fabrication processes. 
     Thus, according to the disclosed processes and structures, after forming the gate dielectric layer on the side and bottom surfaces of the first opening and the second opening, a sacrificial layer may be formed in the first opening. Thus, the second work function layer and the second gate electrode layer may not be filled in the first opening. Accordingly, the process for removing the second work function layer in the first opening may be omitted; and the damages to the dielectric layer in the first opening and the residual contaminations on the inner surfaces of the first opening caused by the process for removing the second work function layer in the first opening may be avoided. 
     Further, because the first work function layer may be formed after forming the second gate electrode layer, the first work function layer may not be formed in the second opening; the second gate electrode layer may be easy to fill in the second opening. Thus, the quality of the second gate electrode layer may be improved. Therefore, the mismatch between the transistor in the first region and the transistor in the second region may be avoided; and the performances of the transistor in the first region and the transistor in the second region may be improved. 
     The above detailed descriptions only illustrate certain exemplary embodiments of the present invention, and are not intended to limit the scope of the present invention. Those skilled in the art can understand the specification as whole and technical features in the various embodiments can be combined into other embodiments understandable to those persons of ordinary skill in the art. Any equivalent or modification thereof, without departing from the spirit and principle of the present invention, fails within the true scope of the present invention.