Patent Publication Number: US-9853030-B2

Title: Fin field effect transistor

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 14/574,639, filed on Dec. 18, 2014, which claims priority to Chinese Patent Application No. CN201310698747.4, filed on Dec. 18, 2013, the entire content of all of which is incorporated herein by reference. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure relates to the field of semiconductor technology and, more particularly, relates to fin field effect transistors and methods for forming the same. 
     BACKGROUND 
     With continuous development of semiconductor processing technology, process node is gradually decreased. Gate-last process has been widely applied, in order to achieve desired threshold voltage and improve device performance. However, when feature size (or critical dimension) of a device is further reduced, even when the gate-last process is applied, structure of a conventional metal-oxide-semiconductor field effect transistor (MOSFET) is no longer able to meet requirements for the device performance. As a type of multi-gate device, attention has been paid to fin field effect transistor (FinFET). 
       FIG. 1  depicts a three-dimensional schematic of a FinFET formed using conventional technology. As shown in  FIG. 1 , a FinFET includes a semiconductor substrate  10  that has a protruding fin  11  formed thereon. The fin  11  is generally obtained by etching the semiconductor substrate  10 . The FinFET further includes a dielectric layer  12  that covers a surface of the semiconductor substrate  10  and a portion of sidewalls of the fin  11 . The FinFET further includes a gate structure  13  that extends across the fin  11  to cover a portion of both of a top and sidewalls of the fin  11 . The gate structure  13  includes a gate dielectric layer (not shown) and a gate electrode on the gate dielectric layer (not shown). For the FinFET, the portion of the top and both sidewalls of the fin  11  that is in contact with the gate structure  13  all becomes a channel region. That is, the FinFET has a plurality of gates, which can help to increase drive current and improve device performance. 
     Performance of the FinFET needs to be further improved. The disclosed methods and structures are directed to solve one or more problems set forth above and other problems. 
     BRIEF SUMMARY OF THE DISCLOSURE 
     One aspect of the present disclosure includes methods for forming a fin field effect transistor (FinFET). In an exemplary method, a semiconductor substrate having an NMOS region and a PMOS region can be provided. A first fin can be formed on the PMOS region, and a second fin can be formed on the NMOS region. A first dielectric layer can be formed on the semiconductor substrate. A top surface of the first dielectric layer can be lower than a top surface of both of the first fin and the second fin. A gate structure can be formed on the first dielectric layer and covering across a first portion of each of the first fin and the second fin. A second portion of the first fin on both sides of the gate structure can be removed, to form a first recess on the PMOS region. A first semiconductor layer can be formed in the first recess. A second dielectric layer can be formed on the first dielectric layer, the second fin and the first semiconductor layer. A portion of the second dielectric layer can be removed, to expose at least a top surface of the second fin. A second semiconductor layer can be formed at least on the exposed top surface of the second fin. 
     Another aspect of the present disclosure includes semiconductor devices. An exemplary device can include a semiconductor substrate having an NMOS region and a PMOS region. The device can include a first fin on the PMOS region of the semiconductor substrate and a second fin on the NMOS region of the semiconductor substrate. Further, the device can include a first dielectric layer on the semiconductor substrate. A surface of the first dielectric layer can be lower than a top surface of both of the first fin and the second fin. Further, the device can include a gate structure on the first dielectric layer and covering across the first fin and the second fin. The first fin can be substantially entirely covered by the gate structure. Further, the device can include a first semiconductor layer on the PMOS region of the semiconductor substrate at each side of the gate structure and in contact with the first fin underneath the gate structure. The first dielectric layer can be located at both sides of each of the first semiconductor layer and the second fin. Further, the device can include a second semiconductor layer on a top surface of a portion of the second fin exposed at the each side of the gate structure. Still further, the device can include a second dielectric layer on the first dielectric layer and substantially filling space between the first semiconductor layer, the first dielectric layer, and the second fin. 
     Other aspects or embodiments 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  depicts a schematic of a FinFET formed using conventional technology; 
         FIGS. 2-14 and 16  depict structures of an exemplary FinFET at various stages during a fabrication process in accordance with various disclosed embodiments; and 
         FIG. 15  depicts a flow diagram of an exemplary method for forming a FinFET in accordance with various disclosed embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to exemplary embodiments of the disclosure, 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. 
     When ion implantation is performed on a source region and a drain region of a FinFET to form a source (or source electrode) and a drain (or drain electrode), a relatively significant amount of defects are likely to be formed in the source region and the drain region. In addition, because size of the fin of the FinFET is relatively small, performance of the formed FinFET is more likely to be affected. 
     In various disclosed embodiments, a source and a drain can be formed by an epitaxial process. Further, the source and the drain can be doped by using an in-situ doping process, to reduce defects in the source and the drain. 
     In certain embodiments, a source region can refer to a region that is, has been, or is to be doped or processed in any other appropriate way to form a source. Thus, before the source is formed, such a region can be referred to as a “source region”. After the source is formed, such a region can be referred to as a “source region” or a “source”. 
     In certain embodiments, a drain region can refer to a region that is, has been, or is to be doped or processed in any other appropriate way to form a drain. Thus, before the drain is formed, such a region can be referred to as a “drain region”. After the drain is formed, such a region can be referred to as a “drain region” or a “drain”. 
       FIG. 15  depicts a flow diagram of an exemplary method for forming a FinFET in accordance with various disclosed embodiments.  FIGS. 2-14  depict structures of the FinFET at various stages during a fabrication process in accordance with various disclosed embodiments. Note that although  FIGS. 2-14  depict semiconductor structures corresponding to the method depicted in  FIG. 15 , the semiconductor structures and the method are not limited to one another in any manner. 
     In Step S 101  of  FIG. 15  and referring to  FIG. 2 , a semiconductor substrate  100  is provided. The semiconductor substrate  100  has an NMOS region and a PMOS region. 
     The semiconductor substrate  100  can be made of a material including silicon, germanium, silicon germanium, gallium arsenide and/or any other appropriate semiconductor materials. The semiconductor substrate  100  can be made of a bulk material, or can have a composite structure, e g., silicon-on-insulator. Those skilled in the art can select the type of the semiconductor substrate  100  according to the semiconductor device formed on the semiconductor substrate  100 . Therefore the type of the semiconductor substrate  100  should not limit the scope of the present disclosure. 
     The semiconductor substrate  100  can include an NMOS region and a PMOS region. The NMOS region can be used for forming an N-type FinFET. The PMOS region can be used for forming a P-type FinFET. A P-well can be formed in the NMOS region. An N-well can be formed in the PMOS region. 
     In Step S 102  of  FIG. 15  and referring to  FIG. 3 , one or more first fins  110  are formed on the PMOS region of the semiconductor substrate  100 , and one or more second fins  120  are formed on the NMOS region of the semiconductor substrate  100 . 
     In one embodiment, a patterned mask layer can be formed on a surface of the semiconductor substrate  100 . The patterned mask layer can define a location and a size of the first fin  110  and the second fin  120 . The semiconductor substrate  100  can be etched by using the patterned mask layer as an etch mask, to form the first fin  110  and the second fin  120 . 
     In another embodiment, the first fin  110  and the second fin  120  can be formed by forming an epitaxial layer on the surface of the semiconductor substrate  100  and then etching the epitaxial layer. 
     In Step S 103  of  FIG. 15  and referring to  FIG. 4 , a first dielectric layer  200  is formed on the semiconductor substrate  100 . A surface or top surface of the first dielectric layer  200  can be lower than a top surface of both of the first fin  110  and the second fin  120 . 
     The first dielectric layer  200  can be made of a material including an insulating dielectric material, e.g., silicon oxide, silicon oxynitride, and/or silicon oxycarbide. A method for forming the first dielectric layer  200  can include the following exemplary process. A first dielectric material can be deposited on the surface of the semiconductor substrate  100 , such that the first dielectric material can cover the first fin  110  and the second fin  120 . The first dielectric material can be planarized using the surface of the first fin  110  and the second fin  120  as a stop layer, to form a first dielectric material layer. A surface of the first dielectric material layer can be leveled with the surface of the first fin  110  and the second fin  120 . The first dielectric material layer can be etched back, such that the surface of the first dielectric material layer can be lower than the top surface of both of the first fin  110  and the second fin  120 , to form the first dielectric layer  200 . 
     The first dielectric layer  200  can serve as an isolation structure between adjacent first fins  110  and second fins  120 . The first dielectric layer  200  can further serve as an isolation structure between a subsequently-formed gate structure and the semiconductor substrate  100 . 
     In Step S 104  of  FIG. 15  and referring to  FIG. 5 , a gate structure  210  extending across or covering across the first fin  110  and the second fin  120  is formed on the surface of the first dielectric layer  200 . 
     In certain embodiments, the portion of the first fin  110  covered by the gate structure  210  can be referred to as the first portion of the first fin  110 . The portion of the first fin  110  not covered by the gate structure  210  (i.e., the portion of the first fin  110  at both sides of the gate structure  210 ) can be referred to as the second portion of the first fin  110 . The portion of second fin  120  covered by the gate structure  210  can be referred to as the first portion of the second fin  120 . The portion of the second fin  120  not covered by the gate structure  210  (i.e., the portion of the second fin  120  at both sides of the gate structure  210 ) can be referred to as the second portion of the second fin  120 . 
     The gate structure  210  can include a gate dielectric layer  211  located on the surface of each of the first dielectric layer  200 , the first fin  110  and the second fin  120 . The gate structure  210  can further include a gate electrode  212  located on the surface of the gate dielectric layer  211 . 
     The gate dielectric layer  211  can be made of a material including SiO 2 , HfO 2 , La 2 O 3 , HfSiON, HfAlO 2 , ZrO 2 , Al 2 O 3 , HfSiO 4 , or a combination thereof. The gate electrode  212  can be made of a material including polysilicon, Al, Cu, Ag, Au, Pt, Ni, Ti, TiN, TaN, Ta, TaC, TaSiN, W, WN, WSi, or a combination thereof. 
     After forming the gate structure  210 , sidewall spacers can be formed on both sides of the gate structure  210  to protect the gate structure  210 . 
       FIG. 6  depicts a top view of the structure after the gate structure  210  is formed. As shown in  FIG. 6 , sidewall spacers  213  can be located on the both sides of the gate structure  210 , in order to protect the gate structure  210  in subsequent processes. 
     As also shown in  FIG. 6 , portions of both the first fin  110  and the second fin  120  that are on both sides of the gate structure  210  but not covered by the gate structure  210 , can serve as regions for subsequently forming a source and a drain. 
       FIG. 7  depicts a cross-sectional view of the structure in  FIG. 6  along an intersecting line AA′. The first fin  110   a  and the second fin  120   a  as shown in  FIG. 7  are, respectively, the portion of the first fin  110  and the portion of the second fin  120  (referring to  FIG. 6 ) that are on each side of the gate structure  210  (referring to  FIG. 6 ) and not covered by the gate structure  210 . 
     In Step S 105  of  FIG. 15  and referring to  FIG. 8 , an etch stop layer  201  covering the surface of the first fin  110   a , of the second fin  120   a  and of the gate structure  210  is formed on the surface of the dielectric layer, i.e., the first dielectric layer  200 . 
     The etch stop layer  201  can be formed using a chemical vapor deposition (CVD) process. The etching stop layer  201  can be used as an etch stop layer when a through hole that connects the gate structure  210  (referring to  FIG. 6 ) to the source or the drain is subsequently formed, in order to protect the surface of the gate structure  210 . In one example, the etch stop layer  201  can be made of a material including silicon nitride. 
     Before or after the etch stop layer  201  is formed, light-doping ion implantation, medium-doping ion implantation, and/or heavy-doping ion implantation can be performed on the second fin(s)  120   a  (i.e., the second portion of the second fin  120   a ) on both sides of the gate structure  210  in the NMOS region. The type of ions doped by the light-doping ion implantation and/or heavy-doping ion implantation can include N-type ions, e.g., P, As, Sb, or a combination thereof. The light-doping ion implantation and/or heavy-doping ion implantation can introduce a tensile stress into an implanted region of the second fin  120   a , such that mobility of electron charge carriers in the second fin  120   a  can be increased. The tensile stress generated by the ion implantation process does not improve mobility of hole charge carriers in a P-type FinFET that is to be formed on the PMOS region. Therefore, the light-doping ion implantation and/or heavy-doping ion implantation does not need to be performed within the first fin  110 . 
     In other embodiments of the present disclosure, the etch stop layer is not formed at this stage. In this case, the etch stop layer can be formed after the source and the drain are subsequently formed. 
     In Step S 106  of  FIG. 15  and referring to  FIG. 9 , a first mask layer  301  is formed on the NMOS region. By using the first mask layer  301  as an etch mask, a portion of the etch stop layer  201  (referring to  FIG. 8 ) on the surface of the first fin  110   a  on both sides of the gate structure  210  (referring to  FIG. 6 ) on the PMOS region can be removed, and a portion of the etch stop layer  201  (referring to  FIG. 8 ) on the surface of the first dielectric layer  200  on the PMOS region can be removed. 
     The first mask layer  301  can further cover the gate structure  210  on the PMOS region, in order to protect the etch stop layer  201  on the surface of the gate structure  210  on the PMOS region. 
     In one example, the portion of the etch stop layer  201  on the first fin  110   a  on both sides of the gate structure  210  (on the PMOS region) and the portion of the etch stop layer  201  on the dielectric layer  200  (on the PMOS region) can be removed using a wet etching process, to expose the top surface and the sidewalls of the first fin  110   a  above the first dielectric layer  200 . 
     In Step S 107  of  FIG. 15  and referring to  FIG. 10 , after the first mask layer  301  (referring to  FIG. 9 ) is removed, the first fin(s)  110   a  on both sides of the gate structure  210  is removed by using the remaining etch stop layer  201  as an etch mask, to form first recess(es)  310  on the PMOS region. 
     In one example, the first fin  110   a  on both sides of the gate structure  210  can be removed using a wet etching process or a dry etching process. In one embodiment, the first fin  110   a  can be removed using a dry etching process to form the first recess  310 . 
     In other embodiments of the present disclosure, the first mask layer  301  (referring to  FIG. 9 ) on the NMOS region can be retained. The first fin  110   a  can be removed using the first mask layer  301  as an etch mask. Then, after the first fin  110   a  is removed, the first mask layer  301  can be removed. 
     In Step S 108  of  FIG. 15  and referring to  FIG. 11 , first semiconductor layer(s)  311  a are formed in the first recess(es)  310  (referring to  FIG. 10 ). Further, a top of the first semiconductor layer  311  can be higher than the surface of the first dielectric layer  200 . The first semiconductor layer  311  can have a compressive stress. 
     The first semiconductor layer  311  can fill up the first recess  310 . A portion of sidewalls of the first semiconductor layer  311  and a top surface of the first semiconductor layer  311  can be exposed. 
     The first semiconductor layer  311  can be made of a material including silicon germanium, and can have a compressive stress. The first semiconductor layer  311  can generate a compressive stress on the first fin  110  underneath the gate structure  210  that can be used as a channel region, such that mobility of hole charge carriers in a P-type FinFET formed on the PMOS region can be improved. Thus, performance of the P-type FinFET can be improved. 
     The first semiconductor layer  311  can be formed using a selective epitaxial process. For example, the selective epitaxial process for forming the first semiconductor layer  311  can have a reaction temperature ranging from about 600° C. to about 1100° C., a pressure ranging from about 1 Torr to about 500 Torr, a silicon source gas of SiH 4  or SiH 2 Cl 2 , a germanium source gas of GeH 4 , and can further have an HCl gas and H 2 . Each of the silicon source gas, the germanium source gas and the HCl can have a flow rate ranging from about 1 sccm to about 1000 sccm. H 2  can have a flow rate ranging from about 0.1 slm to about 50 slm. 
     Because the first semiconductor layer  311  is formed using an epitaxial process, the first semiconductor layer  311  can grow based on crystal lattice of the surface of the semiconductor substrate  100  at the bottom of the first recess  310 . Accordingly, the first recess  310  can have a limiting effect on growth direction of the first semiconductor layer  311 . Therefore, with continuous growth of the first semiconductor layer  311 , the first recess  310  can be gradually filled. 
     During continued growth of the first semiconductor layer  311 , the first semiconductor layer  311  can gradually become higher than the surface of the first dielectric layer  200 . Because there is no limiting effect on the growth of the first semiconductor layer  311  above or higher than the first dielectric layer  200 , and the material of the first semiconductor layer  311  can have different growth rates in different crystal directions, the first semiconductor layer  311  above the first dielectric layer  200  can have an octahedron or regular octahedron shape or any other suitable shapes as shown in  FIG. 16 . 
     Further, the portion of the first semiconductor layer  311  above the first dielectric layer  200  can have a bottom width substantially the same as the width of the first recess  310 , and can have the greatest width at the middle which can be equal to a length of the diagonal of the octahedron. Thus, second recess(es)  314  can be formed between adjacent first semiconductor layers  311  and on the first dielectric layer  200 . In other words, the second recess(es)  314  can be formed between adjacent first semiconductor layers  311  and the first dielectric layer  200 . The second recess  314  can have a narrow top and a wide bottom. 
     During the process of forming the first semiconductor layer  311  using the epitaxial process, a first in-situ doping process can be performed on the first semiconductor layers  311 . The first in-situ doping can have dopant ions that are P-type ions including, e.g., B, Ga, In, or a combination thereof. The P-type ions can have a doping concentration ranging from about 1E17 atom/cm 3  to about 1E20 atom/cm 3 . Thus, the first semiconductor layer  311  can be a source region or a drain region of a P-type FinFET formed on the PMOS region. Using the in-situ doping process can prevent defects from forming within the first semiconductor layer  311 . 
     After the first semiconductor layer  311  is formed, the first semiconductor layer  311  can be annealed, in order to eliminate defects generated in the first semiconductor layer  311  during the epitaxial process and activate the P-type dopant ions of the first in-situ doping. 
     In Step S 109  of  FIG. 15  and referring to  FIG. 12 , a second dielectric layer  400  is formed on the surface of the first dielectric layer  200  and on surface of the first semiconductor layer  311  by using a flowable chemical vapor process (or a flowable CVD process). 
     The flowable CVD process can have a reactant including a flowable dielectric material. In one example, a flowable dielectric material layer can be formed on the first dielectric layer  200  using a spin coating process. The flowable dielectric material layer can be made of a material including at least one of silane, disilane, methyl silane, dimethyl silane, trimethyl silane, tetramethyl silane, tetraethyl orthosilicate (TEOS), triethoxy silane, octamethyl cyclotetrasiloxane, tetramethyl disiloxane, tetramethyl cyclotetrasiloxane, trimethylsilyl amine, and disilyl amine. 
     Because the flowable dielectric material has a flowability, the flowable dielectric material can sufficiently fill the second recess  314  (referring to  FIG. 11 ) formed between the first dielectric layer  200  and the first semiconductor layer  311 , to prevent voids from being formed in the second recess  314 . In addition, the flowable dielectric material can have a certain viscosity (or adhesiveness), such that a relatively thin layer of a flowable dielectric material layer can be adhered to the top surface of the etch stop layer  201  and the surface of the first semiconductor layer  311 . 
     In contrast, during the process of filling the second recess  314  having a narrow top and a wide bottom, certain other CVD processes may form voids within the second recess  314  and thus affect isolation effect between adjacent transistors. 
     After the flowable dielectric material layer is formed, the flowable dielectric material layer can be annealed, to form the second dielectric layer  400 . The annealing can be performed under or in one or more gases including O 2 , O 3 , NO, H 2 O vapor, N 2 , He, and/or Ar. That is, the annealing can be performed in a gas including one or more of O 2 , O 3 , NO, H 2 O vapor, N 2 , He, and Ar. The one or more gases can include at least an oxygen-containing gas. The annealing can have an annealing temperature ranging from about 200° C. to about 1200° C., and a pressure ranging from about 0.1 T to about 100 T. 
     During the annealing process, oxygen element can form Si—O—Si bonds with the flowable dielectric material to replace chemical bonds, e.g., —Si—H—, —Si—N—, —Si—H—N— and the like, to form the second dielectric layer  400 . The annealing treatment can cause the flowable dielectric material layer, previously having the flowability and the viscosity, to cure and form the second dielectric layer  400 . The second dielectric layer  400  can be located on the surface of the first dielectric layer  200 , substantially fill the second recess  314  formed between the first semiconductor layer  311  and the first dielectric layer  200 , and cover the surface of the etch stop layer  201 . The annealing treatment can cause structure of the dielectric material within the second dielectric layer  400  to become tighter or denser, remove a portion of impurities (e.g., N and H) in the dielectric material, repair defects within the second dielectric layer  400 , and thus improve the isolation effect. 
     In other embodiments of the present disclosure, a thermal oxidation process or a wet oxidation process may be performed first on the flowable dielectric material layer, such that the flowable dielectric material layer can be first transformed into silicon oxide or silicon oxynitride. Then, the annealing treatment can be performed. In that case, the gas of the annealing process does not need to contain oxygen atoms. The annealing process can repair structural defects within the second dielectric layer  400 , improve strength of Si—O—Si bonds, remove remaining chemical bonds that have a relatively weak chemical bond strength, e.g., —Si—H—, —Si—N—, —Si—H—N— and the like, and reduce impurities within the second dielectric layer  400 . 
     The second dielectric layer  400  on the first dielectric layer  200  can have a vertical thickness ranging from about ⅓ to about ¾ of the thickness of the first fin  110   a  or the second fin  120   a , in order to sufficiently (or substantially completely) fill the second recess  314 . For example, the second dielectric layer  400  made by the flowable CVD film can be deposited at ⅔ height from a bottom surface of the entire fin height. 
     In Step S 110  of  FIG. 15  and referring to  FIG. 13 , a second mask layer  302  is formed on the PMOS region. The portion of both of the second dielectric layer  400  and the etch stop layer  201  on the top of the second fin  120   a  can be removed, to expose the top surface of the second fin  120   a.    
     The second mask layer  302  can further cover the gate structure  210  on the PMOS region, to protect the etch stop layer  201  on the surface of the gate structure  210  on the PMOS region. 
     The etch stop layer  201  on the top surface of the second fin  120   a  can be removed using a wet etching process, to expose at least the top surface of the second fin  120   a  above the second dielectric layer  200 . In one embodiment, a portion of sidewall(s) of the second fin  120   a  can also be exposed. 
     In Step S 111  of  FIG. 15  and referring to  FIG. 14 , the second mask layer  302  (referring to  FIG. 13 ) is removed. A second semiconductor layer  312  is formed on the top surface and/or a top portion of the exposed sidewall of the second fin  120   a . In certain embodiments, the second semiconductor layer  312  can be formed at least on the (exposed) top surface of the second fin  120   a , and can be further formed on a top portion of the exposed sidewall of the second fin  120   a.    
     The second semiconductor layer  312  can have a tensile stress. The second semiconductor layer(s)  312  can be made of a material including silicon and/or silicon carbide. 
     In one embodiment, the second semiconductor layer  312  can be made of a material including silicon carbide. By using silicon carbide as the second semiconductor layer  312 , the second semiconductor layer  312  can, via the underlying second fin  120   a , generate a tensile stress on the portion of the second fin  120   a  that is covered by the gate structure  210  and serves as a channel region. Thus, mobility of electron charge carriers of  120   a  of the N-type FinFET formed on the NMOS region can be improved, and performance of the N-type FinFET can be improved accordingly. 
     A silicon carbide epitaxial layer can be formed using a selective epitaxial process on the surface of the second fin  120   a , to serve as the second semiconductor layer  312 . For example, the selective epitaxial process can have a reaction temperature ranging from about 600° C. to about 1100° C., a pressure ranging from about 1 Torr to about 500 Torr, a silicon source gas of SiH 4  or SiH 2 Cl 2 , a carbon source gas of CH 4 , and can further have an HCl gas and H 2 . Each of the silicon source gas, the carbon source gas and the HCl can have a flow rate ranging from about 1 sccm to about 1000 sccm. H 2  can have a flow rate ranging from about 0.1 slm to about 50 slm. 
     Because the second semiconductor layer  312  can grow according to the crystal lattice at the surface of the second fin  120   a , and because epitaxial rates can be different in different crystalline directions during the epitaxial growth process, the formed second semiconductor layer  312  can have a regular octahedron shape or any other suitable shapes as shown in  FIG. 16 . 
     Because the second dielectric layer  400  exists on the surface of the first semiconductor layer  311  on the PMOS region, the second semiconductor layer  312  can only be formed on the surface of the second fin  120   a , and is not formed on the surface of first semiconductor layer  311 . The second dielectric layer  400  can therefore be used as a protection mask. 
     During the process of forming the second semiconductor layer  312  using the selective epitaxial process, a second in-situ doping process can be used for doping the second semiconductor layer  312  with N-type ions, e.g., ions of P, Ga, As, or a combination thereof. The N-type ions can have a doping concentration ranging from about 1E17 atom/cm 3  to about 1E20 atom/cm 3 . Thus, the second semiconductor layer  312  and the underlying second fin  120   a  can form a source region or a drain region of an N-type FinFET formed on the NMOS region. Using the in-situ doping process can prevent defects from forming within the second semiconductor layer  312 . 
     The height of the top of the second semiconductor layer  312  can be substantially the same as the height of the first semiconductor layer  311 . That is, the top surface of the first semiconductor layer  311  and the top surface of the second semiconductor layer  312  can have substantially the same distance from the top surface of the semiconductor substrate  100 . Thus, subsequently, metal plug(s) can substantially simultaneously be formed on the first semiconductor layer  311  and the second semiconductor layer  312 . Accordingly, difficulty in forming metal plugs on the first semiconductor layer  311  and the second semiconductor layer  312  can be reduced. 
     Because the top of the second semiconductor layer  312  and the top of the first semiconductor layer  311  have the same or similar height, the second semiconductor layer  312  is formed at the top of the second fin  120   a , and the top of the second fin  120   a  is higher than the surface of the first dielectric layer  200 , the second semiconductor layer  312  can have a thickness smaller than the thickness of the first semiconductor layer  311  above the first dielectric layer  200 . That is, the second semiconductor layer  312  having a regular octahedron shape can have an edge length smaller than the edge length of the regular octahedron (of the first semiconductor layer  311 ) located above the first dielectric layer  200 . Thus, an adjacent first semiconductor layer  311  and second semiconductor layer  312  on the NMOS region and the PMOS region can have a distance or spacing there-between that is sufficiently large, in order to prevent leakage or punch-through phenomenon from occurring between the source and the drain of a N-type FinFET and a P-type FinFET. 
     Further, because the first semiconductor layer  311  can have a width that is sufficiently high, in other embodiments of the present disclosure, first semiconductor layers  311  can be connected to each other. Thus, the compressive stress generated by the first semiconductor layers  311  on the channel region underneath the gate structure  210  can be further improved. The performance of the P-type FinFET(s) can be improved accordingly. 
     As such, in various embodiments of the present disclosure, after a portion of a first fin on both sides of a gate structure is removed and a first recess is formed, a first semiconductor layer can be formed in the first recess. Further, a second semiconductor layer can be formed on the surface of a second fin on both sides of the gate structure. A second dielectric layer can be formed on a surface of a first dielectric layer using a flowable CVD process, to fill space between adjacent first semiconductor layers. The flowable CVD process can have a relatively high deposition quality, and can prevent voids from being formed in the space between the adjacent first semiconductor layers and affecting isolation effect between the adjacent first semiconductor layers. 
     The first semiconductor layers can be made of a material including silicon germanium. The second semiconductor layer can be made of a material including silicon carbide. The first semiconductor layers can have a compressive stress and generate a compressive stress on the first fin underneath the gate structure on the PMOS region. Thus, mobility of hole charge carriers in a P-type FinFET can be improved, and the performance of the P-type FinFET can be improved. 
     The second semiconductor layer can have a tensile stress, and can generate a tensile stress on the second fin underneath the gate structure on the NMOS region. Thus, mobility of electron charge carriers in an N-type FinFET can be improved, and the performance of the N-type FinFET can be improved. 
     Further, during the process(es) of forming the first semiconductor layer and the second semiconductor layer, in-situ doping process(es) can be used for doping the first semiconductor layer and the second semiconductor layer with P-type ions and N-type ions, respectively. Thus, the first semiconductor layer and the second semiconductor layer can serve as source or drain of a P-type FinFET and source or drain of an N-type FinFET, respectively. Using the in-situ doping process can prevent defects from forming within the first semiconductor layer and the second semiconductor layer, and accordingly improve the performance of the P-type FinFET or the N-type FinFET. 
     Various embodiments also provide a semiconductor device. The semiconductor device may include one or more FinFET devices or any other suitable devices. Referring to  FIG. 14 , an exemplary semiconductor device includes a semiconductor substrate  100 . The semiconductor substrate  100  has an NMOS region and a PMOS region. The PMOS region of the semiconductor substrate  100  can have one or more first fins  110  formed thereon. The NMOS region of the semiconductor substrate  100  can have one or more second fins  120  formed thereon. The semiconductor substrate  100  can further have a first dielectric layer  200  formed thereon. A surface of the first dielectric layer  200  can be lower than a top surface of both of the first fin  110  and the second fin  120 . A gate structure  210  formed on the surface of the first dielectric layer  200  can extend across the first fin  110  and the second fin  120 . The first fin  110  and the second fin  120  underneath the gate structure  210  can form the channel regions of the FinFET devices. The first fin  110  can be substantially entirely covered by the gate structure. A portion of the second fin  120  can be exposed at each side of the gate structure. 
     On the PMOS region of the semiconductor substrate  100  at each side of the gate structure  210 , one or more first semiconductor layers  311  are formed. Each first semiconductor layer can be in contact with a first fin  110  underneath the gate structure  210 . 
     When the methods as disclosed in certain embodiments are used, the first semiconductor layer  311  can be formed on the semiconductor substrate  100  at the same position as a previously-removed second portion of a first fin  110  (i.e., the first fin  110  at the side of the gate structure  210  and not covered by the gate structure  210 ). The portion of the first semiconductor layer  311  that is lower than the top surface of the first dielectric layer  200  can be substantially entirely in contact with the first portion of the first fin  110  (underneath the gate structure  210 ). 
     Each of the first semiconductor layers  311  can serve as a source or drain of a P-type FinFET, and can be separated by the first dielectric layer  200 . A top of the first semiconductor layer  311  can be higher than the surface of the first dielectric layer  200 . The first semiconductor layers  311  can have a compressive stress. 
     On the NMOS region of the semiconductor substrate  100  at each side of the gate structure  210 , one or more second fins  120   a  exist as the portion of the second fins  120  that is not covered by the gate structure  210 . A second semiconductor layer  312  can be formed on the top of each second fin  120   a . Each second fin  120   a  and the second semiconductor layer  312  formed thereon can serve as a source or drain of an N-type FinFET, and can be separated by the first dielectric layer  200 . 
     Further, the semiconductor device can include a second dielectric layer  400  on the surface of the first dielectric layer  200 , substantially filling the space formed between each of the first semiconductor layers  311 , the first dielectric layer  200 , and the second fins  120   a . Further, a relatively thin layer of second dielectric layer  400  can adhere to the surface of the first semiconductor layer  311 . 
     Optionally, the semiconductor device can include an etch stop layer  201  on the first dielectric layer  200  on the NMOS region and on a portion of the sidewall of the second fins  120   a  that is lower than the second dielectric layer  400 . 
     Further, at each side of the gate structure  210 , the first dielectric layer  200  can be located between a first semiconductor layer  311  or second fin  120   a  and an adjacent first semiconductor layer  311  or an adjacent second fin  120   a . That is, the first dielectric layer  200  can be located on the semiconductor substrate  100  at both sides of each first semiconductor layer  311  or each second fin  120   a.    
     Further details of the disclosed semiconductor device are described in the method for forming a FinFET as disclosed in various embodiments, e.g., in  FIGS. 2-13 . 
     In addition, although the first semiconductor layer is P-type in the above description, an N-type or any appropriate type of first semiconductor layer can be used. Although the second semiconductor layer is N-type in the above description, a P-type or any appropriate type of second semiconductor layer can be used. Although the second fin is doped with N-type ions in the above description, P-type ions or any appropriate type of ions can be used. Although a P-well is used in the above description, an N-well or any appropriate type of well can be used. Although an N-well is used in the above description, a P-well or any appropriate type of well can be used. 
     In an exemplary method for forming a FinFET, a semiconductor substrate having an NMOS region and a PMOS region can be provided. One or more first fins can be formed on the PMOS region. One or more second fins can be formed on the NMOS region. A first dielectric layer can be formed on the semiconductor substrate. A surface of the first dielectric layer can be lower than a top surface of both of the first fins and the second fins. A gate structure or a dummy gate can be formed on the first dielectric layer to extend across the first fins and the second fins. An etch stop layer or a chemical etch stop layer (CESL) can be formed on a surface of the gate structure and on the surface of both of the first fins and the second fins, e.g., as shown in  FIG. 8 . 
     A first mask layer can be formed on the NMOS region. The first mask layer can be made of a material including a photoresist. By using the first mask layer as an etch mask, a portion of the etch stop layer on the PMOS region can be removed, e.g., as shown in  FIG. 9 . 
     After removing the first mask layer, the first fins can be removed, to form first recesses on the PMOS region, e.g., as shown in  FIG. 10 . First semiconductor layers can be formed in the first recesses using a selective epitaxial process or a selective growth process, e.g., as shown in  FIG. 11 . The first semiconductor layers can be made of SiGe. During the selective growth process, the first semiconductor layers can be in-situ doped with B. After the first semiconductor layers are formed, the first semiconductor layers can be annealed. 
     A second dielectric layer can be formed on the surface of the first dielectric layer by using a flowable CVD process, and then be annealed, e.g., as shown in  FIG. 12 . That is, the second dielectric layer can be made of a flowable CVD film. 
     A second mask layer can then be formed on the PMOS region. The second mask layer can be made of a material including a photoresist. On the NMOS region, the etch stop layer and the second dielectric layer at the top of the second fins can be removed, e.g., as shown in  FIG. 13 . 
     The second mask layer can then be removed. Second semiconductor layers can be formed at the top of the second fins using an epitaxial process. That is, the second semiconductor layers can cap the second fins, e.g., as shown in  FIG. 14 . In one example, the second semiconductor layers can be made of Si. During the epitaxial process, the second semiconductor layers can be in-situ doped with As, P, Sb, or a combination thereof. The remaining etch stop layer and second dielectric layer can be used as a mask during the forming of the second semiconductor layer, 
     In one embodiment, the remaining etch stop layer and second dielectric layer can be used as a mask during the forming of the second semiconductor layers. The etch stop layer can be made of a material including silicon nitride. The second dielectric layer can be made of a material including silicon oxide. 
     In one embodiment, the deposited second dielectric layer or the flowable CVD film can have a vertical thickness or height (measured from a bottom surface of the first fins or the second fins) equal to about ⅔ of the height of the first fins or the second fins. 
     In one embodiment, after forming the etch stop layer, the etch stop layer on the PMOS region can be removed. The first fins can be removed, to form a first recess on the PMOS region. First semiconductor layers can be formed in the first recesses. The first semiconductor layers can be made of SiGe. A second dielectric layer (a flowable CVD film) can be formed on the surface of the first dielectric layer. On the NMOS region, the etch stop layer and the second dielectric layer at the top of the second fins can be removed. Second semiconductor layers can be formed at the top of the second fins using an epitaxial process. The second fins can thus be extended by the second semiconductor layers and doped by ion implantation. The remaining etch stop layer and second dielectric layer can be used as a mask during the forming of the second semiconductor layers. 
     As used herein, relational terms such as ‘first’ and ‘second’ are merely used for differentiate one element or operation from another element or operation, and do not require or imply that any actual relationship or order as such exist between these elements or operations. In addition, the terms “include”, “comprise”, “contain”, or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that includes a series of elements can not only include those elements, but also include other elements not expressly listed, or inherent elements for such a process, method, article, or apparatus. Without further limitation, an element defined by a statement “include one . . . ” does not exclude additional identical elements that may be included in the process, method, article, or apparatus that includes the element. 
     The embodiments disclosed herein are exemplary only. Other applications, advantages, alternations, modifications, or equivalents to the disclosed embodiments are obvious to those skilled in the art and are intended to be encompassed within the scope of the present disclosure.