Patent Publication Number: US-9837536-B2

Title: Semiconductor device including fin structures and manufacturing method therof

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 14/698,831 filed on Apr. 28, 2015, now U.S. Pat. No. 9,406,680,which claims the priority of U.S. Provisional Application No. 62/116,321 filed on Feb. 13, 2015, the entire contents of which applications are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The disclosure relates to a semiconductor integrated circuit, more particularly to a semiconductor device having a fin structure and its manufacturing process. 
     BACKGROUND 
     As the semiconductor industry has progressed into nanometer technology process nodes in pursuit of higher device density, higher performance, and lower costs, challenges from both fabrication and design issues have resulted in the development of three-dimensional designs, such as a fin field effect transistor (Fin FET). Fin FET devices typically include semiconductor fins with high aspect ratios and in which channel and source/drain regions of semiconductor transistor devices are formed. A gate is formed over and along the sides of the fin structure (e.g., wrapping) utilizing the advantage of the increased surface area of the channel and source/drain regions to produce faster, more reliable and better-controlled semiconductor transistor devices. In some devices, strained materials in source/drain (S/D) portions of the FinFET utilizing, for example, silicon germanium (SiGe) or silicon carbide (SiC), may be used to enhance carrier mobility. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure is best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale and are used for illustration purposes only. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIG. 1  is an exemplary process flow chart for manufacturing a semiconductor FET device having a fin structure (Fin FET); and 
         FIGS. 2-17C  show exemplary processes for manufacturing the Fin FET device according to one embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     It is to be understood that the following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Specific embodiments or examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, dimensions of elements are not limited to the disclosed range or values, but may depend upon process conditions and/or desired properties of the device. Moreover, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed interposing the first and second features, such that the first and second features may not be in direct contact. Various features may be arbitrarily drawn in different scales for simplicity and clarity. 
     Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly. In addition, the term “made of” may mean either “comprising” or “consisting of.” 
       FIG. 1  is an exemplary flow chart for manufacturing a semiconductor FET device having a fin structure (Fin FET). The flow chart illustrates only a relevant part of the entire manufacturing process for a Fin FET device. It is understood that additional operations may be provided before, during, and after processes shown by  FIG. 1 , and some of the operations described below can be replaced or eliminated, for additional embodiments of the method. The order of the operations/processes may be interchangeable. Further, the general operations for manufacturing a recessed S/D structure with strain materials (or stressor) in the fin structure are disclosed in U.S. Pat. No. 8,440,517, the entire contents of which are incorporated herein by reference. 
     In S 1001 , a fin structure is fabricated over a substrate. In S 1002 , a gate structure including a gate dielectric layer and a gate electrode is formed over a portion of the fin structure. In S 1003 , a region for a second type of FET, for example, a p-type FET, is covered by a covering layer to protect the region for the second type of FET from the subsequent processes for a first type of FET, for example an n-type FET. In S 1004 , the fin structure not covered by the gate structure is recessed. In S 1005 , a stressor layer is formed in the recessed portion of the fin structure. After forming the stressor structure for the first type of FET, in S 1006 , a region for the first type of FET is covered by a cover layer to protect the first type of FET with the stressor structure from the subsequent processes for the second type of FET. In S 1007 , the fin structure not covered by the gate structure for the second type of FET is recessed. In S 1008 , a stressor layer is formed in the recessed portion of the fin structure for the second type of FET. It is possible to process a p-type FET first and then process an n-type FET. 
     Referring to  FIGS. 2-17C , the details of the exemplary manufacturing process of a Fin FET are described. 
       FIG. 2  is an exemplary cross sectional view of the Fin FET device  1  having a substrate  10  at one of the various stages of the fabrication process according to one embodiment. 
     To fabricate a fin structure, a mask layer is formed over the substrate  10  by, for example, a thermal oxidation process and/or a chemical vapor deposition (CVD) process. The substrate  10  is, for example, a p-type silicon substrate with an impurity concentration being in a range of about 1.12×10 15  cm −3  and about 1.68×10 15  cm −3 . In other embodiments, The substrate  10  is an n-type silicon substrate with an impurity concentration being in a range of about 0.905×10 15  cm −3  and about 2.34×10 15  cm −3 . The mask layer includes, for example, a pad oxide (e.g., silicon oxide) layer and a silicon nitride mask layer in some embodiments. 
     Alternatively, the substrate  10  may comprise another elementary semiconductor, such as germanium; a compound semiconductor including IV-IV compound semiconductors such as SiC and SiGe, III-V compound semiconductors such as GaAs, GaP, GaN, InP, InAs, InSb, GaAsP, AlGaN, AlInAs, AlGaAs, GaInAs, GaInP, and/or GaInAsP; or combinations thereof. In one embodiment, the substrate  10  is a silicon layer of an SOI (silicon-on insulator) substrate. When an SOI substrate is used, the fin structure may protrude from the silicon layer of the SOI substrate or may protrude from the insulator layer of the SOI substrate. In the latter case, the silicon layer of the SOI substrate is used to form the fin structure. Amorphous substrates, such as amorphous Si or amorphous SiC, or insulating material, such as silicon oxide may also be used as the substrate  10 . The substrate  10  may include various regions that have been suitably doped with impurities (e.g., p-type or n-type conductivity). 
     The pad oxide layer may be formed by using thermal oxidation or a CVD process. The silicon nitride mask layer may be formed by a physical vapor deposition (PVD), such as a sputtering method, a CVD, plasma-enhanced chemical vapor deposition (PECVD), an atmospheric pressure chemical vapor deposition (APCVD), a low-pressure CVD (LPCVD), a high density plasma CVD (HDPCVD), an atomic layer deposition (ALD), and/or other processes. 
     The thickness of the pad oxide layer is in a range of about 2 nm to about 15 nm and the thickness of the silicon nitride mask layer is in a range of about 2 nm to about 50 nm in some embodiments. A mask pattern is further formed over the mask layer. The mask pattern is, for example, a resist pattern formed by lithography operations. 
     By using the mask pattern as an etching mask, a hard mask pattern  100  of the pad oxide layer  101  and the silicon nitride mask layer  102  is formed. The width of the hard mask pattern  100  is in a range of about 5 nm to about 40 nm in some embodiments. In certain embodiments, the width of the hard mask patterns  100  is in a range of about 7 nm to about 12 nm. 
     As shown in  FIG. 2 , by using the hard mask pattern  100  as an etching mask, the substrate  10  is patterned into fin structures  20  by trench etching using a dry etching method and/or a wet etching method. A height (in the Z direction) of the fin structure  20  is in a range of about 100 nm to about 300 nm. In certain embodiments, the height is in a range of about 50 nm to about 100 nm. When the heights of the fin structures are not uniform, the height from the substrate may be measured from the plane that corresponds to the average heights of the fin structures. 
     In this embodiment, a bulk silicon wafer is used as a starting material and constitutes the substrate  10 . However, in some embodiments, other types of substrate may be used as the substrate  10 . For example, a silicon-on-insulator (SOI) wafer may be used as a starting material, and the insulator layer of the SOI wafer constitutes the substrate  10  and the silicon layer of the SOI wafer is used for the fin structure  20 . 
     As shown in  FIG. 2 , two fin structures  20  are disposed adjacent to each other in the Y direction in the first device region  1 A and in the second device region  1 B, respectively. However, the number of the fin structures is not limited to two. The numbers may be one, three, four or five or more. In addition, one of more dummy fin structures may be disposed adjacent to both sides of the fin structures  20  to improve pattern fidelity in patterning processes. The width W 1  of the fin structure  20  is in a range of about 5 nm to about 40 nm in some embodiments, and may be in a range of about 7 nm to about 15 nm in certain embodiments. The height H 1  of the fin structure  20  is in a range of about 100 nm to about 300 nm in some embodiments, and may be in a range of about 50 nm to 100 nm in other embodiments. The space S 1  between the fin structures  20  is in a range of about 5 nm to about 80 nm in some embodiments, and may be in a range of about 7 nm to 15 nm in other embodiments. One skilled in the art will realize, however, that the dimensions and values recited throughout the descriptions are merely examples, and may be changed to suit different scales of integrated circuits. 
     In this embodiment, the first device region  1 A is for n-type Fin FETs and the second device region  1 B is for p-type Fin FETs. 
       FIG. 3  is an exemplary cross sectional view of the Fin FET device  1  having fin structures  20  at one of the various stages of the fabrication process according to one embodiment. 
     As shown in  FIG. 3 , an isolation insulating layer  50  is formed over the substrate  10  so as to fully cover the fin structure  20 . 
     The isolation insulating layer  50  is made of, for example, silicon dioxide formed by LPCVD (low pressure chemical vapor deposition), plasma-CVD or flowable CVD. In the flowable CVD, flowable dielectric materials instead of silicon oxide are deposited. Flowable dielectric materials, as their name suggest, can “flow” during deposition to fill gaps or spaces with a high aspect ratio. Usually, various chemistries are added to silicon-containing precursors to allow the deposited film to flow. In some embodiments, nitrogen hydride bonds are added. Examples of flowable dielectric precursors, particularly flowable silicon oxide precursors, include a silicate, a siloxane, a methyl silsesquioxane (MSQ), a hydrogen silsesquioxane(HSQ), an MSQ/HSQ, a perhydrosilazane (TCPS), a perhydro-polysilazane (PSZ), a tetraethyl orthosilicate (TEOS), or a silyl-amine, such as trisilylamine (TSA). These flowable silicon oxide materials are formed in a multiple-operation process. After the flowable film is deposited, it is cured and then annealed to remove un-desired element(s) to form silicon oxide. When the un-desired element(s) is removed, the flowable film densifies and shrinks. In some embodiments, multiple anneal processes are conducted. The flowable film is cured and annealed more than once at temperatures, such as in a range from about 1000° C. to about 1200° C., and for an extended period, such as 30 hours or more in total. The isolation insulating layer  50  may be formed by using SOG. SiO, SiON, SiOCN or fluoride-doped silicate glass (FSG) may be used as the isolation insulating layer  50  in some embodiments. 
       FIG. 4  is an exemplary cross sectional view of the Fin FET device  1  having fin structures  20  at one of the various stages of the fabrication process according to one embodiment. 
     After forming the isolation insulating layer  50 , a planarization operation is performed so as to remove part of the isolation insulating layer  50  and the mask layer  100  (the pad oxide layer  101  and the silicon nitride mask layer  102 ). Then, the isolation insulating layer  50  is further removed so that an upper part of the fin structure  20 , which is to become a channel layer, is exposed, as shown in  FIG. 4 . The planarization operation may include a chemical mechanical polishing (CMP) and/or an etch-back process. 
     In at least one embodiment, the silicon nitride layer  102  may be removed using a wet process using hot H 3 PO 4 , while pad oxide layer  101  may be removed using diluted HF acid, if formed of silicon oxide. In some alternative embodiments, the removal of the mask layer  100  may be performed after the recessing of the isolation insulating layer  50 . 
     In certain embodiments, the partially removing the isolation insulating layer  50  may be performed using a wet etching process, for example, by dipping the substrate in hydrofluoric acid (HF). In another embodiment, the partially removing the isolation insulating layer  50  may be performed using a dry etching process, for example, the dry etching process using CHF 3  or BF 3  as etching gases. 
     After forming the isolation insulating layer  50 , a thermal process, for example, an anneal process, may be performed to improve the quality of the isolation insulating layer  50 . In certain embodiments, the thermal process is performed by using rapid thermal annealing (RTA) at a temperature in a range of about 900° C. to about 1050° C. for about 1.5 second to about 10 second in inert gas ambient, for example, N 2 , Ar or He ambient. 
       FIG. 5  is an exemplary cross sectional view of the Fin FET device  1  having fin structures  20  at one of the various stages of the fabrication process according to one embodiment.  FIGS. 6A and 6B  are exemplary cross sectional views along the direction (X direction) in which the fin structures extend. 
     A gate dielectric layer  105  and a poly silicon layer are formed over the isolation insulating layer  50  and the exposed fin structures  20 , and then patterning operations are performed so as to obtain gate stacks including gate electrode layers  110 A and  110 B made of poly silicon and gate dielectric layer  105 . The patterning of the poly silicon layer is performed by using a hard mask  200  including a silicon nitride layer  201  and an oxide layer  202  in some embodiments. In other embodiments, the layer  201  may be silicon oxide and the layer  202  may be silicon nitride. The gate dielectric layer  105  may be silicon oxide formed by CVD, PVD, ALD, e-beam evaporation, or other suitable process. In some embodiments, the gate dielectric layer  105  may include silicon nitride, silicon oxy-nitride, or high-k dielectrics. High-k dielectrics comprise metal oxides. Examples of metal oxides used for high-k dielectrics include oxides of Li, Be, Mg, Ca, Sr, Sc, Y, Zr, Hf, Al, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and/or mixtures thereof. In some embodiments, a thickness of the gate dielectric layer is in the range of about 1 nm to 5 nm. 
     In some embodiments, the gate electrode layers  110 A and  110 B may comprise a single layer or multilayer structure. In the present embodiment, the gate electrode layers  110 A and  110 B may comprise poly-silicon. Further, the gate electrode layers  110 A and  110 B may be doped poly-silicon with the uniform or non-uniform doping. In some alternative embodiments, the gate electrode layers  110 A and  110 B may include a metal such as Al, Cu, W, Ti, Ta, TiN, TiAl, TiAlN, TaN, NiSi, CoSi, other conductive materials with a work function compatible with the substrate material, or combinations thereof. The gate electrode layers  110 A and  110 B may be formed using a suitable process such as ALD, CVD, PVD, plating, or combinations thereof. 
     In the present embodiment, the width W 2  of the gate electrode layers  110 A and  110 B is in the range of about 30 nm to about 60 nm in some embodiments. 
     Further, side-wall insulating layers  80  are also formed at both sides of the gate electrode layers  110 A and  110 B. The side-wall insulating layers  80  may include silicon oxide, silicon nitride, silicon oxy-nitride, or other suitable material. The side-wall insulating layers  80  may comprise a single layer or multilayer structure. A blanket layer of a side-wall insulating material may be formed by CVD, PVD, ALD, or other suitable technique. Then, an anisotropic etching is performed on the side-wall insulating material to form a pair of side-wall insulating layers (spacers)  80  on two sides of the gate stack. The thickness of the side-wall insulating layers  80  is in a range of about 5 nm to about 15 nm in some embodiments. In certain embodiments, the side-wall insulating layers  80  may not be formed at this stage. 
       FIGS. 7A and 7B  are exemplary cross sectional views and  FIG. 7C  is an exemplary perspective view of the Fin FET device  1  having gate structures over fin structures at one of the various stages of the fabrication process according to one embodiment. 
     As shown in  FIGS. 7A-7C , a cover layer  120  is formed over the gate structures and the fin structures in the first and second device regions. The cover layer  120  may include silicon nitride with a thickness in a range of about 5 nm to about 15 nm in some embodiments. 
       FIGS. 8A and 8B  are exemplary cross sectional views and  FIG. 8C  is an exemplary perspective view of the Fin FET device  1  having gate structures over fin structures at one of the various stages of the fabrication process according to one embodiment. 
     As shown in  FIGS. 8A-8C , a masking layer  130  is formed over the covering layer, and further a mask pattern  135  is formed over the masking layer  130  by using a lithographic process. The mask pattern  135  covers the second device region  1 B as shown in  FIGS. 8B and 8C . The masking layer  130  may include an organic material. In some embodiments, the masking layer includes a material used for a bottom anti-reflection coating (BARC) for a lithographic process. The mask pattern  135  may include a photo resist. 
       FIGS. 9A and 9B  are exemplary cross sectional views of the Fin FET device  1  having gate structures over fin structures at one of the various stages of the fabrication process according to one embodiment. 
     By using the mask pattern  135 , the masking layer  130  is etched, and by using the etched masking layer, the cover layer  120  in the first device region is etched. As shown in  FIGS. 9A and 9B , the cover layer  120  in the first device region  1 A is removed, while the cover layer still covers the second device region  1 B to protect the second device region from the subsequent process for the first device region. In some embodiments, a single layer of photo resist is formed over the second device region  1 B, and by using the photo resist layer as a mask, the cover layer  120  is etched. After the cover layer  120  is etched, the masking layer  130  (and the mask pattern  135 , if it remains) is removed. 
       FIGS. 10A and 10B  are exemplary cross sectional views of the Fin FET device  1  having fin structures  20  at one of the various stages of the fabrication process according to one embodiment. 
     The portions of the fin structure  20  not covered by the gate structure are recessed to form a recessed portion  140 A of the fin structure  20 . The recessed portion  140 A is formed such that a top surface  141 A is located below the top surface  51  of the isolation insulating layer  50 . 
     In certain embodiments, using the pair of side-wall insulating layers  80  as hard masks, a biased etching process is performed to recess the top surface of the fin structure  20  that are unprotected or exposed to form the recessed portion  140 A. 
     As shown in  FIGS. 10A and 10C , a portion of the fin structure  20  under the gate stack is also etched. The depth D 1  may be in a range of about 5 nm to about 10 nm in some embodiments. The end portion  142 A may be located under the side-wall insulating layer  80 , or in some other embodiments, the end portion  142 A may be located under the gate electrode layer  110 A. 
     In one embodiment of the present disclosure, the etching conditions in the recess etching process are adjusted to achieve desired profiles for the end portion  142 A. For example, transform coupled plasma (TCP) with process gases including CH 4 , CHF 3 , O 2 , HBr, He, Cl 2 , NF 3 , and/or N 2  is used with changing power and/or bias conditions. The TCP etching includes anisotropic etching followed by isotropic etching. In the isotropic etching, the bias voltage is set smaller than that in the anisotropic etching. 
     In one embodiment, the anisotropic etching is performed in the following two conditions. The etching in Condition  1  is followed by the etching in Condition  2   
     
       
         
           
               
               
               
               
               
             
               
                   
                   
               
               
                   
                 Pressure (mTorr) 
                 Power (W) 
                 Bias (V) 
                 Temp (° C.) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Cond. 1 
                 3~20 
                 100~800 
                 20~400 
                 20~70 
               
               
                 Cond. 2 
                 3~10 
                 100~600 
                 20~300 
                 20~70 
               
               
                   
               
            
           
         
       
     
     By the first and second anisotropic etching processes, the fin structures are recessed as shown in  FIGS. 11A and 11B . As shown in  FIGS. 11A and 11B , the end profile of the end portion  142 A of the recessed portion  140 A under the gate stack has roughly a rounded shape. 
     Further, the isotropic etching is performed in the following conditions. 
     
       
         
           
               
               
               
               
               
             
               
                   
                   
               
               
                   
                 Pressure (mTorr) 
                 Power (W) 
                 Bias (V) 
                 Temp (° C.) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Cond. 3 
                 3~20 
                 100~1500 
                 5~50 
                 20~70 
               
               
                   
               
            
           
         
       
     
     By the isotropic etching, the end profile  142 A of the recessed portion under the gate stack can have a flat surface  143 A, as shown in  FIGS. 12A and 12B . 
     The width W 4  of the flat portion at the surface of the fin structure  20  in the recessed portion  140 A along the Y direction may be in a range about 0.5×W 1 ≦W 4 ≦W 1  (the width of the fin structure  20 ). In some embodiments, W 4  is in a range about 0.7×W 1 ≦W 4 &lt;W 1 . 
     It is noted that during the recess etching, the gate dielectric layer  105  disposed on the side of the fin structure  20  is also etched. 
       FIGS. 13A and 13B  are exemplary cross sectional views and  FIG. 13C  is an exemplary perspective view of the Fin FET device  1  having gate structures over fin structures at one of the various stages of the fabrication process according to one embodiment. 
     In the recessed portion  140 A, a first stressor layer  300  is formed. The first stressor layer  300  may be formed by selectively growing a strained material over the recessed portion  140 A and above the isolation insulating layer  50 . Since the lattice constant of the strained material is different from the fin structure  20  and the substrate  10 , the channel region of the fin structure  20  is strained or stressed to increase carrier mobility of the device and enhance the device performance. 
     In at least one embodiment, the stressor layer  300 , such as silicon carbon (SiC), is epitaxially-grown by an LPCVD process to form the source and drain regions of the n-type FinFET. The LPCVD process is performed at a temperature of about 400 to 800° C. and under a pressure of about 1 to 200 Torr, using Si 3 H 8  and SiH 3 CH as reaction gases, in some embodiments. 
     In the present embodiment, the selective growth of the first stressor layer  300  continues until the material  300  extends vertically a distance ranging from about 10 to 100 nm from the bottom of the recessed portion  140 A and extends laterally over the top surfaces of the isolation insulating layer  50 . The formed first stressor layer  300  corresponds to source/drain of the n-type Fin FET. 
     When the end profile of the recessed portion  140 A under the gate stack includes the flat surface  143 A as shown in  FIGS. 12A and 12B , the interface between the stressor layer  300  and the fin structure  20  also has a flat surface. More specifically, the interface may include a flat surface  146  and rounded portions  147 , as shown in  FIGS. 13D-13G . In some embodiments, the width W 4 ′ of the flat surface  146  is more than 0.5×W 1  (the width of the fin structure  20 ) and equal to or less than W 1 . In certain embodiment, W 4 ′ is about 0.7×W 1  or more, and yet in some other embodiments, W 4 ′ is about 0.8×W 1  or more. The maximum value of W 4 ′ is equal to or less than W 1  and in some embodiments, 0.98×W 1  or less, and yet in some other embodiments, W 4 ′ is about 0.95×W 1  or less. 
     In certain embodiments, the width W 4 ′ of the flat portion is greater than W 1  and less than W 1 ′(=W 1 +thicknesses of gate dielectric layers  105  on both sides of the fin structure). 
     The larger the width W 4  becomes, the more effectively the stressor layer  300  applies a stress to the channel layer. 
     Further, in some embodiments, a cap layer  310  is formed over the first stressor layer  300 . When the first stressor layer  300  is SiC, the cap layer  310  is Si epitaxially-grown by an LPCVD process. The cap layer  310  enhances an application of the stress by the first stressor layer  300  to the channel layer. 
     After the Fin FETs in the first device region  1 A (e.g., n-type Fin FETs) are formed, the Fin FETs in the second device region  1 B are processed in a similar matter to the first device region. 
       FIGS. 14A and 14B  are exemplary cross sectional views and  FIG. 14C  is an exemplary perspective view of the Fin FET device  1  having fin structures  20  at one of the various stages of the fabrication process according to one embodiment. 
     Similar to  FIGS. 7A and 7B , a cover layer  140  is formed over the gate structures and the fin structures in the first and second device regions. The cover layer  140  may include silicon nitride with a thickness in a range of about 5 nm to about 15 nm in some embodiments. 
     Similar to  FIGS. 8A-8C , a masking layer  150  is formed over the covering layer  140 , and further a mask pattern  155  is formed over the masking layer  150  by using a lithographic process. The mask pattern  155  covers the first device region  1 A as shown in  FIGS. 14A and 14C . The masking layer  150  may include an organic material. In some embodiments, the masking layer  150  includes a material used for a bottom anti-reflection coating (BARC) for a lithographic process. The mask pattern  155  may include a photo resist. 
       FIGS. 15A and 15B  are exemplary cross sectional views of the Fin FET device  1  having fin structures  20  at one of the various stages of the fabrication process according to one embodiment. 
     Similar to  FIGS. 9A and 9B , by using the mask pattern  155 , the masking layer  150  is etched, and by using the etched masking layer  150 , the cover layers  120  and  140  in the second device region are etched. As shown in  FIGS. 15A and 15B , the cover layers  120  and  140  in the second device region  1 B are removed, while the cover layer  140  still covers the first device region  1 A to protect the first device region from the subsequent process for the second device region. In some embodiments, a single layer of photo resist is formed over the first device region  1 A, and by using the photo resist layer, the cover layers  120  and  140  in the second device region are etched. After the cover layer  120  and  140  are etched, the masking layer  150  (and the mask pattern  155 , if it remains) is removed. 
       FIGS. 16A and 16B  are exemplary cross sectional views and  FIG. 16C  is an exemplary perspective view of the Fin FET device  1  having fin structures  20  at one of the various stages of the fabrication process according to one embodiment. 
     Similar to  FIG. 10A , a portion of the fin structure  20  under the gate stack in the second device region is etched. The depth D 2  may be in a range of about 5 nm to about 10 nm in some embodiments. The end portion  142 B may be located under the side-wall insulating layer  80 , or in some embodiments, the end portion  142 B may be located under the gate electrode layer  110 B. 
     In the present embodiment, the etching conditions in the etching process are adjusted to achieve desired profiles for the recessed portion  140 B. Similar to the recess etching for the recessed portion  140 A, anisotropic etching followed by isotropic etching is used. By the isotropic etching, the end profile  142 B of the recessed portion under the gate stack can include a flat surface  143 A, as shown in  FIGS. 12A and 12B . 
       FIGS. 17A and 17B  are exemplary cross sectional views and  FIG. 17C  is an exemplary perspective view of the Fin FET device  1  having gate structures over fin structures at one of the various stages of the fabrication process according to one embodiment. 
     In the recessed portion  140 B, a second stressor layer  305  is formed. The second stressor layer  305  may be formed by selectively growing a strained material over the recessed portion  140 B and above the isolation insulating layer  50 . Since the lattice constant of the second strained material is different from the fin structure  20  and the substrate  10 , the channel region of the fin structure  20  is strained or stressed to enable carrier mobility of the device and enhance the device performance. 
     In at least one embodiment, the second stressor layer  305 , such as silicon germanium (SiGe), is epitaxially-grown by a LPCVD process to form the source and drain regions of the p-type Fin FET. The LPCVD process is performed at a temperature of about 400 to 800° C. and under a pressure of about 1 to 200 Torr, using SiH 4  and GeH 4  as reaction gases, in some embodiments. 
     In the present embodiment, the selective growth of the second stressor layer  305  continues until the material  305  extends vertically a distance ranging from about 10 to 100 nm from the bottom of the recessed portion  140 B and extends laterally over the top surfaces of the isolation insulating layer  50 . The formed second stressor layer  305  corresponds to source/drain of the p-type Fin FET. 
     When the profile  142 B of the recessed portion  140 B under the gate stack includes the flat surface similar to  FIGS. 12A and 12B , the interface between the second stressor layer  305  and the fin structure  20  also has a flat surface. More specifically, the interface may include a flat surface and rounded portions, similar to  FIG. 13G . In some embodiments, the width of the flat surface is more than 0.5×W 1  (the width of the fin structure  20 ) and less than W 1 . In certain embodiment, the width of the flat surface is about 0.7×W 1  or more, and yet in some other embodiments, the width of the flat surface is about 0.8×W 1  or more. The maximum value of width of the flat surface is less than W 1  and in some embodiments, 0.98×W 1  or less, and yet in some embodiments, is about 0.95×W 1  or less. 
     The greater the width of the flat surface becomes, the more effectively the second stressor layer  305  applies a stress to the channel layer. 
     Further, in some embodiments, a second cap layer  315  is formed over the second stressor layer  305 . When the second stressor layer  305  is SiGe, the second cap layer  315  is Si epitaxially-grown by an LPCVD process. The second cap layer  315  enhances application of the stress by the second stressor layer  305  to the channel layer. 
     It is understood that the Fin FETs in the first and second device regions may undergo further CMOS processes to form various features such as contacts/vias, interconnect metal layers, dielectric layers, passivation layers, etc. The modified insulation and strained structure provides a given amount of strain into channel region of a Fin FET, thereby enhancing the device performance. 
     The various embodiments or examples described herein offer several advantages over the existing art. For example, by providing a flat portion in the interface between the stressor layer and the fin structure under the gate stack, the stress is more appropriately applied to the channel layer. In particular, the larger the width of the flat interface becomes, the more appropriately the stressor layer applies a stress to the channel layer. 
     It will be understood that not all advantages have been necessarily discussed herein, no particular advantage is required for all embodiments or examples, and other embodiments or examples may offer different advantages. 
     In accordance with one aspect of the present disclosure, a semiconductor device includes a first Fin FET transistor. The first Fin FET transistor includes a first fin structure extending in a first direction, a first gate stack, and first source and drain. The first gate stack includes a first gate electrode layer and a first gate dielectric layer, covers a portion of the first fin structure and extends in a second direction perpendicular to the first direction. Each of the first source and first drain includes a first stressor layer disposed over the first fin structure. The first stressor layer applies a stress to a channel layer of the first fin structure under the first gate stack. The first stressor layer penetrates under the first gate stack. A vertical interface between the first stressor layer and the first fin structure under the first gate stack in a third direction perpendicular to the first and second directions includes a first flat portion. 
     In accordance with another aspect of the present disclosure, a method for manufacturing a semiconductor device includes forming a gate stack including a gate electrode layer and a gate dielectric layer over a fin structure. A bottom of the fin structure is covered by an isolation insulating layer. A recessed portion is formed by removing a portion of the fin structure not covered by the gate stack and a part of the fin structure under the gate stack. After the recessed portion is formed, a bottom of the recessed portion is located below an upper surface of the isolation insulating layer, and the recessed portion penetrates under the gate stack. A stressor layer is formed in the recessed portion. The fin structure extends in a first direction and the gate stack extends in a second direction perpendicular to the first direction. In the forming the recessed portion, a vertical end face of the recessed portion on the fin structure under the gate stack in a third direction perpendicular to the first and second directions includes a flat portion. 
     In accordance with another aspect of the present disclosure, a method for manufacturing a semiconductor device includes forming a first gate stack including a first gate electrode layer and a first gate dielectric layer over a first fin structure, a bottom of the first fin structure being covered by an isolation insulating layer. A second gate stack including a second gate electrode layer and a second gate dielectric layer is formed over a second fin structure. A bottom of the second fin structure is covered by the isolation insulating layer. The second gate stack and the second fin structure are covered by forming a first covering layer. A first recessed portion is formed by removing a portion of the first fin structure not covered by the first gate stack and a part of the first fin structure under the first gate stack. A bottom of the first recessed portion is located below an upper surface of the isolation insulating layer, and the first recessed portion penetrates under the first gate stack. A first stressor layer is formed in the recessed portion. Then, the first gate stack and the first fin structure with the first stressor layer are covered by forming a second covering layer, and the second gate stack and the second fin structure are exposed by removing the first covering layer. A second recessed portion is formed by removing a portion of the second fin structure not covered by the second gate stack and a part of the second fin structure under the second gate stack. A bottom of the second recessed portion is located below an upper surface of the isolation insulating layer, and the second recessed portion penetrates under the second gate stack. A second stressor layer is formed in the second recessed portion. The first fin structure extends in a first direction and the first and second gate stacks extend in a second direction perpendicular to the first direction. In the forming the first recessed portion, a vertical end part of the first recessed portion on the first fin structure under the first gate stack in a third direction perpendicular to the first and second directions includes a first flat portion. In the forming the second recessed portion, a vertical end part of the second recessed portion on the second fin structure under the second gate stack in the third direction includes a second flat portion. 
     The foregoing outlines features of several embodiments or examples so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments or examples introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure.