Abstract:
The present application discloses a semiconductor device comprising a fin of semiconductive material formed from a semiconductor layer over a semiconductor substrate and having two opposing sides perpendicular to the main surface of the semiconductor substrate; a source region and a drain region provided in the semiconductor substrate adjacent to two ends of the fin and being bridged by the fin; a channel region provided at the central portion of the fin; and a stack of gate dielectric and gate conductor provided at one side of the fin, where the gate conductor is isolated from the channel region by the gate dielectric, and wherein the stack of gate dielectric and gate conductor extends away from the one side of the fin in a direction parallel to the main surface of the semiconductor substrate, and insulated from the semiconductor substrate by an insulating layer. The semiconductor device has an improved short channel effect and a reduced parasitic capacitance and resistance, which contributes to an improved electrical property and facilitates scaling down of the transistor.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of Invention 
         [0002]    The present invention relates to a semiconductor device and a method for manufacturing the same, and in particular, to an improved FinFET formed on a semiconductor substrate. 
         [0003]    2. Description of Prior Art 
         [0004]    One important trend in the integrated circuits is scaling down of metal-oxide-semiconductor field effect transistors (MOSFETs) so as to achieve a high integration degree of devices and reduce the manufacturing costs. However, it is well known that short channel effect occurs with reducing the gate length of the MOSFET. When the size of the MOSFET is reduced, the gate of the MOSFET has a smaller effective gate length and actually, comparing a long gate length MOSFET, controls less charges in a depletion region with a given gate voltage, and the MOSFET has a reduced threshold voltage with a reduced channel length. A conventional planar MOSFET has a tri-layer structure including a gate electrode, a semiconductor layer, and a gate dielectric sandwiched therebetween. A channel region is provided in the semiconductor layer below the gate electrode, and source/drain regions are provided in the semiconductor layer adjacent to and at two opposing sides of the channel region. A silicide layer may be provided on the source/drain regions and then coupled with source/drain electrodes through vias so as to reduce the parasitic resistance of the device. The planar MOSFET suffers from the short channel effect and has a threshold voltage fluctuating with variation of the channel length. 
         [0005]    To suppress the short channel effect, U.S. Pat. No. 6,413,802 discloses a FinFET formed on a SOI substrate, comprising a channel region provided in the central portion of a fin of semiconductive material and source/drain regions provided at two ends of the fin. Gate electrodes are provided at both sides of the channel region and surround the latter to provide for example a double gate FinFET. Inversion channels are induced in the fin at both sides of the gate. The channel region in the fin has a small thickness so that the whole channel region is controlled by the gate, as a result of which, the short channel effect is suppressed. 
         [0006]    However, in a conventional FinFET, the gate is provided between the source and the drain regions and extends parallel thereto. Since the distance between the source/drain regions and the gate is typically small enough for a capacitive couple introduced therebetween, the resultant device has a large parasitic resistance and capacitance. 
         [0007]    Moreover, the capacitive coupling between the source/drain regions and the gate limits the freedom of device design. For example, if one attempts to reduce a parasitic resistance, the thickness of the source/drain regions should be increased. 
         [0008]    However, the source/drain regions having a larger thickness also means an increased coupling area between the source/drain regions and the gate, which in turn causes the parasitic capacitance to increase, or vice versa. Thus, one skilled person in the art can not reduce both of the parasitic resistance and the parasitic capacitance together in a conventional FinFET. 
         [0009]    Consequently, the conventional FinFET has a delay due to a large value of the time constant RC and thus has a low switching speed. 
         [0010]    Moreover, the FinFET formed on a SOI has the disadvantage of having a much higher manufacturing costs than that of a FinFET formed on a bulk semiconductor substrate. 
       SUMMARY OF THE INVENTION 
       [0011]    One object of the present invention is to provide a semiconductor device which is formed on a bulk semiconductor substrate, and is capable of suppressing the short channel effect while reducing the parasitic resistance and capacitance. 
         [0012]    Another object of the present invention is to provide a semiconductor device which has improved electrical properties due to the induced stress. 
         [0013]    According to one aspect of the invention, there is provided a semiconductor device comprising a fin of semiconductive material formed from a semiconductor layer over a semiconductor substrate and having two opposing sides perpendicular to the main surface of the semiconductor substrate; a source region and a drain region provided in the semiconductor substrate adjacent to two ends of the fin and being bridged by the fin; a channel region provided at the central portion of the fin; and a stack of gate dielectric and gate conductor provided at one side of the fin, wherein the gate conductor is isolated from the channel region by the gate dielectric, and wherein the stack of stack of gate dielectric and gate conductor extends away from the one side of the fin in a direction parallel to the main surface of the semiconductor substrate, and is insulated from the semiconductor substrate by an insulating layer. 
         [0014]    According to another aspect of the invention, there is provided a method for manufacturing a semiconductor device, comprising steps of: a) providing a fin of semiconductive material in a semiconductor layer over a semiconductor substrate by a self-alignment process, the fin having two opposing sides perpendicular to the main surface of the semiconductor substrate; b) providing a stack of gate dielectric and gate conductor on one side of the fin, the gate conductor extending away from the one side of the fin in a direction parallel to the main surface of the semiconductor substrate and insulated from the semiconductor substrate by an insulating layer; c) doping the semiconductor substrate adjacent to the two ends of the fin so as to provide a source region and a drain region, the source region and drain region being bridged by the fin; and d) providing a channel region at the central portion of the fin. 
         [0015]    It should be noted that the inventive semiconductor device includes a fin of semiconductive material but has a structure different from that of the conventional FinFET, because the gate according to the present invention is provided at only one side of the fin and extends away therefrom. As a comparison, the conventional FinFET typically has a double gate structure which surrounds a channel region at the central portion of fin. Moreover, in the present invention, the source/drain regions are provided just adjacent to the two ends of the fin and extend in a direction opposite to the direction in which the gate extends. 
         [0016]    Since in the inventive semiconductor device the gate does not extend between the source/drain regions parallel thereto, there is no capacitive coupling between the source/drain regions and the gate. Thus, the inventive semiconductor device has a reduced parasitic capacitance. Meanwhile, the inventive semiconductor device allows a reduced parasitic resistance by providing source/drain regions having a relatively larger thickness. 
         [0017]    Moreover, the inventive semiconductor device uses a conventional semiconductor substrate, instead of an expensive SOI wafer, where source and drain regions are provided, and a channel region is provided in a fin formed from a semiconductor layer over the semiconductor substrate. By using a conventional semiconductor layer, instead of the SOI wafer, the inventive semiconductor device has a reduced manufacturing costs. 
         [0018]    The inventive semiconductive device also allows further suppression of the parasitic effect relevant with the parasitic capacitance and the parasitic resistance by providing extensions adjacent to the channel region in the fin so as to decrease the length of electrical conduction of the charge carriers. 
         [0019]    Moreover, the inventive semiconductive device can have an increased switching speed by providing stressors for the source/drain regions so as to induce stress in the channel region. 
         [0020]    For the purpose of effectively controlling the short channel effect, the channel region is provided with self-aligned process and has a small thickness, for example in the range of 5-40 nm. Preferably, a supper steep retrograde well (SSRW) is used so as to decrease the thickness of the channel region. Even in a case where the gate is provide at only one side of the fin, the gate can completely control the channel region and suppress the short channel effect. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIGS. 1A and 1B  are a perspective view and a top view respectively of a structure of a semiconductor device according to an embodiment of the present invention, wherein the top view shows lines A-A′,  1 - 1 ′ and  2 - 2 ′ for taking cross sections of the following figures; 
           [0022]      FIGS. 2-9  are cross-sectional views of the structure of the semiconductor device at various stages of fabrication process according to an embodiment of the present invention, taken along line A-A′ and for illustrating the process steps of forming a fin and a gate. 
           [0023]      FIGS. 10-16  are cross-sectional views of the structure of the semiconductor device at various stages of the subsequent fabrication process according to an embodiment of the present invention, taken along line  1 - 1 ′ and for illustrating the process steps of forming source/drain regions. 
           [0024]      FIGS. 17-21  are cross-sectional views of the structure of the semiconductor device at various stages of the subsequent fabrication process according to an embodiment of the present invention, taken along line A-A′ and for illustrating the process steps of forming a channel region. 
           [0025]      FIGS. 22A ,  22 B,  23 A and  23 B are cross-sectional views of the structure of the semiconductor device at various stages of the subsequent fabrication process according to an embodiment of the present invention, taken along line A-A′ for  FIGS. 22A and 23A , line  2 - 2 ′ for  FIGS. 22B and 23B , and for illustrating the process steps of forming a silicide layer on the source/drain regions and the gate. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0026]    Exemplary embodiments of the present invention are described in detail below with reference to the accompanying drawings. In the drawings, like reference numerals denote like members. The figures are not drawn to scale, for the sake of clarity. 
         [0027]    It should be understood that when one layer or region is referred to as being “above” or “on” another layer or region in the description of device structure, it can be directly above or on the other layer or region, or other layer(s) or region(s) may be intervened therebetween. Moreover, if the device in the figures is turned over, the layer or region will be “under” or “below” the other layer or region. 
         [0028]    In contrast, when one layer is referred to as being “directly on” or “on and adjacent to” another layer or region, there are not intervening layers or regions present. 
         [0029]    Some particular details of the invention will be described, such as an exemplary structure, material, dimension, process step and fabricating method of the device, for a better understanding of the present invention. Nevertheless, it is understood by one skilled person in the art that these details are not always essential but can be varied in a specific implementation of the invention. 
         [0030]    Unless the context clearly indicates otherwise, each part of the semiconductor device can be made of material(s) well-known to one skilled person in the art. As an initial structure, a semiconductor substrate is made of for example the group IV semiconductor (such as silicon, germanium) or group III-V semiconductor (such as gallium arsenide). A gate conductor can be for example a metal layer, a doped polysilicon layer, or a multilayer gate conductor including a metal layer and a doped polysilicon layer. The metal layer is made of one material selected from the group consisting of TaC, TiN, TaTbN, TaErN, TaYbN, TaSiN, HfSiN, MoSiN, RuTax, NiTax, MoN x , TiSiN, TiCN, TaAIC, TiAlN, TaN, PtSi x , Ni 3 Si, Pt, Ru, Ir, Mo, Hf Ru, RuO x , and their combination. A gate dielectric is made of SiO 2  or other dielectric insulation material which has a dielectric constant larger than that of SiO 2 , such as an oxide, a nitride, an oxynitride, a silicate, an aluminate, and a titanate. The oxide includes for example SiO 2 , HfO 2 , ZrO 2 , Al 2 O 3 , TiO 2 , and La 2 O 3 . The nitride includes for example Si 3 N 4 . The silicate includes for example HfSiO x . The aluminate includes for example LaAlO 3 . The titanate includes for example SrTiO 3 . The oxynitride includes for example SiON. Moreover, the gate dielectric can be made of those developed in the future, besides the above known materials. 
         [0031]      FIGS. 1A and 1B  are perspective and top views respectively of a structure of a semiconductor device according to an embodiment of the present invention. Lines A-A′,  1 - 1 ′ and  2 - 2 ′ in  FIG. 1B  indicate where the following cross-sectional views are taken. Specifically, line A-A′ is perpendicular to a length direction of a channel region and goes through a gate, line  1 - 1 ′ is parallel to the length direction of the channel region and goes through the channel region, and line  2 - 2 ′ is also parallel to the length direction of the channel region but goes through the insulating filler between source/drain regions. 
         [0032]    Referring to  FIGS. 1A and 1B , a semiconductor device  100  is formed on a semiconductor substrate  21 , which comprises a channel region  11  at the central portion of a fin of semiconductive material, a source region  12  and a drain region  13  in the semiconductor substrate  21  and adjacent to two ends of the fin, a stack of a gate conductor  15  and a gate dielectric  14  disposed adjacent to one side of the fin, and an insulating filler for filling a trench at the other side of the fin. The fin bridges the source region  12  and the drain region  13 . The stack of the gate dielectric  14  and the gate conductor  15  extends away from the one side of the fin in a direction parallel to the main surface of the semiconductor substrate  21 , being insulated from the semiconductor substrate  21  by an insulating layer  22 ′ (for example, an oxide layer). 
         [0033]    The channel region at the central portion of the fin has relatively small thickness, for example in a range of about 5-40 nm. The thickness of the channel region is approximately equal to that of the conventional FinFET and provided by a similar self-aligned process. 
         [0034]    The inventor has found that the gate at one side of the fin, instead of a double gate configuration, can still control the whole channel region and thus suppress the short channel effect if the thickness of the channel region is set to be in the above range. Preferably, the semiconductor device further includes stressors  16 ,  17  which apply stress to the channel region  11 . The stressors  16 ,  17  are disposed either directly on the source region  12  and the drain region  13 , or with a conductive contact layer interposed therebetween. 
         [0035]    The stressors  16 ,  17  are made of a material inducing a stress in the channel region, which has beneficial effect on electrical property of the transistor. In a case of an n-type MOSFET, the stressors  16 ,  17  should apply tensile stress towards the channel region in a direction parallel to the source and drain regions so as to maximize mobility of electrons which function as charge carriers. On the other hand, in a case of a p-type MOSFET, the stressors  16 ,  17  should apply compressive stress towards the channel region in a direction parallel to the source and drain regions so as to maximize mobility of holes which function as charge carriers. 
         [0036]    It should be noted that the exemplary structure of the semiconductor device shown in  FIGS. 1A and 1B  includes stressors  16 ,  17  located in conduction paths between the source region  12  and a source contact (not shown), and between the drain region  13  and a drain contact (not shown). Thus, the stressors  16 ,  17  are also electrically conductive. For an n-type MOSFET, Si:C doped with As or P can be to used as a material of the stressors. For a p-type MOSFET, SiGe doped with B can be used as a material of the stressors. 
         [0037]    Additional layers and portions of the semiconductor device, which are disposed above the source region  12 , the drain region  13  and the gate  15 , are not shown in  FIGS. 1A and 1B , such as gate spacers, a silicide layer, a source contact, a drain contact, a gate contact, an interlayer insulator and vias formed therein, and a passivation layer. 
         [0038]    In the following contents regarding the process steps of fabricating the semiconductor device, some of the above additional layers and portions related to the semiconductor device will be described, but detailed description of those additional layers or portions (for example a source contact, a drain contact, and a gate contact) well known to one skilled person are omitted. For simplicity, the structure of the semiconductor device having been subjected to several relevant process steps may be shown in one figure. 
         [0039]    Referring to  FIG. 2 , a method of manufacturing semiconductor device according to an embodiment of the present invention starts with a semiconductor wafer, such as a silicon wafer. 
         [0040]    By a conventional deposition process, such as CVD, atomic layer deposition, sputtering and the like, a first SiGe layer  22  which has a Ge content of about 3-7% and a thickness of about 20-50 nm, a first Si layer  23  which has a thickness of about 50-150 nm, a second SiGe layer  24  which has a Ge content of about 5-20% and a thickness of about 3-20 nm, and a second Si layer  25  which has a thickness of about 30-100 nm, are epitaxially grown in this order on the semiconductor substrate  21 . The Si layer can be formed either in an independent deposition process or in-situ by using a Si target or a precursor in the same chamber after the SiGe layer is epitaxially grown. 
         [0041]    Then, a HfO 2  layer  26  which has a thickness of about 3-10 nm is formed on the second Si layer  25  by atomic layer deposition or magnetron sputtering. 
         [0042]    Referring to  FIG. 3 , by a conventional lithographical process including exposure and development steps, a photoresist pattern  27  in form of a stripe is formed on the HfO 2  layer  26 . 
         [0043]    Referring to  FIG. 4 , with the photoresist pattern  27  as a mask, portions of the HfO 2  layer  26 , the second Si layer  25 , and the second SiGe layer  24  are selectively removed by dry etching such as ion beam milling, plasma etching, reactive ion etching, laser ablation and the like, so as to provide a stacked structure of the HfO 2  layer  26 , the second Si layer  25 , and the second SiGe layer  24  in a stripe pattern. Two steps may be involved if the reactive ion etching is performed. At the first step, the portions of the HfO 2  layer  26  and the second Si layer  25  are selectively removed with the second SiGe layer  24  as a stop layer, by choosing a suitable gas in an etching atmosphere. At the second step, a portion of the second SiGe layer  24  is further selectively removed with the first Si layer  23  as a stop layer, by changing to another type of gas in the etching atmosphere. It is well known to one skilled person that one of the SiGe layer and the Si layer can be selectively removed by controlling etch selectivity with a different type of gas in an etching atmosphere being used in the reactive ion etching process. 
         [0044]    Then, the photoresist pattern  27  is removed by ashing or dissolution with a solvent. A conformal thin oxide layer  28  which has a thickness of about 2-5 nm is formed on the stacked structure in the stripe pattern and on an exposed portion of the first Si layer  23 . 
         [0045]    The thin oxide layer  28  can be formed by a conventional deposition process, such as CVD, atomic layer deposition, sputtering and the like. 
         [0046]    Then, a conformal nitride layer is firstly formed and then has a portion thereof being removed so as to provide nitride spacers  29  which has a thickness of about 5-50 nm at both sides of the stacked structure comprising the HfO 2  layer  26 , the second Si layer  25  and the second SiGe layer  24 . 
         [0047]    Referring to  FIG. 5 , by a conventional lithographical process including exposure and development steps, a photoresist pattern  30  is formed on the structure shown in  FIG. 4  so as to mask the spacer at the left side of the stacked structure and its left portion. 
         [0048]    Referring to  FIG. 6 , with the photoresist pattern  30  as a mask, the spacer at the right side of the stacked structure is removed by an isotropic etching process, such as conventional wet etching using a solution of etchant. 
         [0049]    Alternatively, the spacer at the right side of the stacked structure can be removed in three steps. At the first step, with the photoresist pattern  30  as a mask, an angular implantation of Ge is performed so as to cause damages in the spacer at the right side of the stacked structure. At the second step, the photoresist pattern  30  is removed by ashing or dissolution with a solvent. At the third step, by wet etching or dry etching, the spacer at the right side is selectively removed relative to the spacer at the left side. 
         [0050]    After the spacer at the right side of the stacked structure is removed, the portion of the thin oxide layer  28  that is exposed on the main surface of the semiconductor structure is selectively removed by choosing a suitable gas in an etching atmosphere for example in an reactive ion etching. 
         [0051]    Then, with the remaining portion of the thin oxide layer  28 , the spacer  29  at the left side and the stacked structure comprising the HfO 2  layer  26 , the second Si layer  25  and the second SiGe layer  24  as a hard mask, an exposed portion of the first Si layer  23  and the first SiGe layer  22  is selectively removed by changing to another type of gas in the etching atmosphere for example in the reactive ion etching, so as to provide a fin  23 ′ of semiconductive material from the first Si layer  23  in a self-aligned manner and expose a portion of the main surface of the semiconductor substrate  21 . 
         [0052]    Referring to  FIG. 7 , an oxide is firstly provided on the semiconductor structure shown in  FIG. 6  by HDP (high-density plasma), and then etched back. Because the portion of oxide on an exposed surface of the semiconductor substrate  21  has a thickness larger than the thicknesses of other portions, etch back may be controlled to leave an insulating layer  22 ′ only on the exposed surface of the semiconductor substrate  21 . 
         [0053]    Then, for example by CVD or atomic layer deposition, a conformal thin oxide (for example, HfO 2 ) layer  26 ′ which has a thickness of about 2-4 nm, a conformal metal (for example, TiN, which is a metal ceramic) layer  31  which has a thickness of about 3-10 nm, and a blanket polysilicon layer  32  are formed in this order on the whole surface of the semiconductor structure. The conformal thin oxide layer  26 ′ will provide a gate dielectric, and the conformal metal layer  31  and the polysilicon layer  32  constitutes a multi-layered gate conductor. 
         [0054]    Preferably, the polysilicon layer  32  can be in-situ doped so as to improve electrical conductivity. 
         [0055]    The polysilicon layer  32  covers the whole top of the semiconductor structure. Then, the polysilicon layer  32  is subjected to a planarization process such as chemical mechanical polishing (CMP). The CMP stops at the top of the metal layer of the multilayer gate conductor so as to provide a flat surface for the semiconductor structure. 
         [0056]    Referring to  FIG. 8 , by wet etching or dry etching, a portion of the polysilicon layer  32  is selectively removed relative to the metal layer  31  so that the polysilicon layer  32  is etched back. Then, for example by CVD, a blanket oxide layer  33  is formed on the whole surface of the semiconductor structure. 
         [0057]    The oxide layer  33  is subjected to CMP which stops at the top of the metal layer of the multilayer gate conductor so as to provide a flat surface for the semiconductor structure. Consequently, the oxide layer  33  fills the portion of the polysilicon layer  32  removed by etching back. 
         [0058]    Then, for example by CVD, a nitride layer  34  is formed on the main surface of the semiconductor structure. 
         [0059]    Referring to  FIG. 9 , by a conventional lithographical process including exposure and development steps, a photoresist pattern  35  in form of a stripe is formed for defining a gate region of the device. The multilayer gate conductor comprises the metal layer  31  and the polysilicon layer  32 . 
         [0060]    Then, with the photoresist pattern  35  as a mask and the insulating layer  22 ′ as a stop layer, portions of the nitride layer  34 , the oxide layer  33 , the polysilicon layer  32 , the metal layer  31  and the thin oxide layer  26 ′ that are located outside of the fin  23 ′ are removed in this order by dry etching, such as ion beam milling, plasma etching, reactive ion etching, laser ablation and the like. 
         [0061]    Corresponding to the cross sectional view of the semiconductor structure along line A-A′ shown in  FIG. 9 ,  FIG. 10  shows the cross sectional view of the semiconductor structure along line  1 - 1 ′. In the etching step with the photoresist pattern  35  as a mask, a stack of the nitride layer  34 , the oxide layer  33 , the polysilicon layer  32 , the metal layer  31 , and the thin oxide layer  26 ′ is provided on the second Si layer  25 . 
         [0062]    Before or after the above etching step, an additional masking step and an additional etching step can be involved so as to remove portions of the fin  23 ′, the second SiGe layer  24  and the second Si layer  25  for defining the length of the fin, with the insulating layer  22 ′ as a stop layer.  FIG. 10  shows the dimension of the length of the fin  23 ′ in a horizontal direction after that. 
         [0063]    Referring to  FIG. 11 , still with the photoresist pattern  35  as a mask, portions of the second Si layer  25  and the second SiGe layer  24  are selectively removed in this order by dry etching such as ion beam milling, plasma etching, reactive plasma etching, laser ablation and the like, with the fin  23 ′ as stop layer. Consequently, a stack  101  of layers including the nitride layer  34 , the metal layer  31 , the thin oxide layer  26 ′, the second Si layer  25  and the second SiGe layer  24  is provided on the fin  23 ′. 
         [0064]    Referring to  FIG. 12 , the photoresist pattern  35  is removed by ashing or dissolution with a solvent. 
         [0065]    Then, for example by CVD, a conformal oxide layer  36  which has a thickness of about 2-5 nm and a conformal nitride layer  37  which has a thickness of about 10-20 nm are formed in this order on the whole surface of the semiconductor structure. 
         [0066]    By dry etching, such as ion beam milling, plasma etching, reactive ion etching, laser ablation and the like, a portion of the nitride layer  37  is removed, with the oxide layer  36  as a stop layer, so as to form nitride spacers  37  at both sides of the fin  23 ′ and the stack of layers  101  respectively. 
         [0067]    Referring to  FIG. 13 , with the stack of layers  101  and the nitride spacers  37  as a hard mask, an exposed portion of the oxide layer  36  and corresponding portions of the semiconductive material of the fin  23 ′ and the first SiGe layer  22  are removed by dry etching, such as ion beam milling, plasma etching, reactive ion etching, laser ablation and the like, which stops on the top of the semiconductor substrate  21 , so as to provide trenches  38  at two ends of the fin  23 ′ in its length direction (i.e. a horizontal direction in the drawing). 
         [0068]    The etching step is performed in a self-aligned manner, where the size of the trenches  38  is defined by the oxide layer  36  and the nitride spacers  37 . 
         [0069]      FIG. 14  shows an optional step of the manufacturing method according to some embodiments, in which an angular ion implantation is performed through the trenches  38  in the central portion of the fin  23 ′ so as to provide halos in channel or in the substrate below the channel (i.e. a halo implantation). As a dopant for an n-type MOSFET, B or BF 2  may be used. As a dopant for a p-type MOSFET, As or P may be used. 
         [0070]      FIG. 15  shows an optional step of the manufacturing method according to some embodiments, in which an angular ion implantation is performed through the trenches  38  towards to the central portion of the fin  23 ′ so as to provide source/drain extensions (i.e. an extension implantation). As a dopant for an n-type MOSFET, As or P may be used. As a dopant for a p-type MOSFET, B or BF 2  may be used. 
         [0071]    Compared with the halo implantation, the extension implantation uses a relatively small implantation angle and a relatively large implantation energy. Consequently, in the extension implantation, most of the implanted ions pass through the thin layer of semiconductor material at the surface of the semiconductor substrate  21  so that no amorphous phase occurs in the thin layer of semiconductive material. 
         [0072]    Since the trenches  38  provide a window for ion implantation, and the nitride layer  34 , the oxide layer  36  and the nitride spacers  37  on the surface of the semiconductor structure provide a hard mask, the above extension implantation, halo implantation and the source/drain implantation can be performed in-situ, which reduces the number of masks needed and simplifies the process steps. 
         [0073]    Referring to  FIG. 16 , the resultant semiconductor structure is subjected to an anneal treatment, for example spike anneal. The anneal treatment is used to activate the dopants injected at the previous implantation steps and to eliminate implant damages. 
         [0074]    After the anneal treatment, the doping profile in the fin  23 ′ of semiconductive material is also shown in  FIG. 16 , which includes a source region  12  and a drain region  13  provided in the thin layer of semiconductor material at the surface of the semiconductor substrate  21  that is exposed through the trenches  38 , a source extension  12 ′ provided in the fin  23 ′ and adjacent to the source region  12 , a drain extension  13 ′ provided in the fin  23 ′ and adjacent to the drain region  13 , a source halo  12 ″ provided in the fin  23 ′ and adjacent to the source extension  12 ′ and extending towards to the central portion of the fin  23 ′, and a drain halo  13 ″ provided in the fin  23 ′ and adjacent to the drain extension  13 ′ and extending towards to the central portion of the fin  23 ′. 
         [0075]    Then, by a conventional deposition process, such as CVD, atomic layer deposition, sputtering and the like, a silicon contact layer  39 , a stressor  40  and an epitaxial silicon layer  41  are epitaxially grown in this order in the trenches  38 . The silicon contact layer  39  contacts the source region  12  and drain region  13  respectively, and provides conduction path to external electrodes. Due to the epitaxial growth, the silicon contact layer  39  is formed only on the surface of the semiconductor substrate  21  that is exposed through the trenches  38 . For a p-type MOSFET, the silicon contact layer  39  is in-situ doped with P and has a thickness of about 10 nm, the stressor  40  is SiGe which has a Ge content of about 20-50% and is in-situ doped with B, and the epitaxial silicon layer  41  is in-situ doped with B. After epitaxial growth, compressive stress is applied towards the channel region in a direction parallel to the source and drain regions, which enhances the performance of the p-type MOSFET. For an n-type MOSFET, the silicon contact layer  39  is in-situ doped with B and has a thickness of about 10 nm, the stressor  40  is Si:C which has a C content of about 0.5-2% and is in-situ doped with As or P, and the epitaxial silicon layer  41  is in-situ doped with As or P. After epitaxial growth, tensile stress is applied towards the channel region in a direction parallel to the source and drain regions, which enhances the performance of the n-type MOSFET. 
         [0076]    Then, the resultant semiconductor structure is subjected to oxidation so that the top of the epitaxial silicon layer  41  is oxidized and a thin oxide layer  36 ′ which has a thickness of about 3-10 nm is formed. The epitaxial silicon layer  40  at the top of the stressors  40  is used to provide a high-quality SiO 2  layer. 
         [0077]    Referring to  FIG. 17 , with the oxide layer  33  formed in the step shown in  FIG. 8  as a hard mask and the semiconductor substrate  21  as a stop layer, portions of the metal layer  31 , the thin oxide layer  26 ′, the second Si layer  25 , the second SiGe layer  24 , the fin  23 ′ and the first SiGe layer  22  are removed in this order by dry etching such as ion beam milling, plasma etching, reactive ion etching, laser ablation and the like, so as to provide a trench  42  in a self-aligned manner. 
         [0078]    Consequently, the thickness of the fin  23 ′ is reduced to be approximately equal to a sum of the thickness of the oxide layer  28  and that of the nitride spacer  29 . As mentioned above, the channel region is formed in the fin. Due to the removed materials in the trench, the stress induced in the channel region is enhanced so as to further improve an electrical property of the device. 
         [0079]    At the right portion of the sidewall (i.e. a portion of the inner wall) of the trench  42 , a portion of the stack of the thin oxide layer  26 ′, the metal layer  31 , the polysilicon layer  32  and the oxide layer  33  remains. In the manufacture of an integrated circuit including a plurality of MOSFETs having the same structure, the portion of the stack of the thin oxide layer  26 ′, the metal layer  31 , the polysilicon layer  32  and the oxide layer  33  at the right portion of the sidewall of the trench  42  may be used to provide a gate region for an adjacent MOSFET (not shown in  FIG. 17 ), and the insulating filler in the trench  42  may provide a shallow trench isolation. 
         [0080]    Moreover, as shown in  FIG. 17 , the nitride spacers  37  formed in the step shown in  FIG. 12  remain at the sidewall of the multilayer gate conductor. 
         [0081]    Referring to  FIG. 18 , by wet etching or dry etching in which SiGe is selectively removed relative to Si, the portion of the first SiGe layer  22  under the fin  23 ′ is removed, and then by dry etching such as ion beam milling, plasma etching, reactive ion etching, laser ablation and the like, the portions of the thin oxide layer  26 ′ and the metal layer  31  that remain at the sidewall (i.e. the right portion of the sidewall shown in  FIG. 18 ) of the trench  41  are selectively removed relative to the oxide layer  33 . 
         [0082]    Preferably, ions are implanted into the fin  23 ′ of semiconductive material by an angular ion implantation, followed by an anneal treatment (for example, laser anneal) for activating the dopants, so as to provide a super steep retrograde well (SSRW)  42  in the fin  23 ′ at the side near the trench  42 . The trench  42  provides a window for ion implantation. The process for forming SSRW can be found in the following documents:
   1) G. G. Shahidi, D. A. Antoniadis and H. I. Smith, IEEE TED Vol. 36, p. 2605, 1989   2) C. Fiegna, H. Iwai, T. Wada, M. Saito, E. Sangiorgi and B. Riccó, IEEE TED Vol. 41, p. 941, 1994.   3) J. B. Jacobs and D. A. Antoniadis, IEEE TED Vol. 42, p. 870, 1995.   4) S. E. Thompson, P. A. Packan and M. T. Bohr, VLSI Tech Symp., p. 154, 1996.   
 
         [0087]    Referring to  FIGS. 19 and 20 , the spacer  37  on the left side can be removed in three steps. At the first step, with the oxide layer  33  as a mask, an angular ion implantation of Ge is performed so as to cause damages in the spacer at the left side, as shown in  FIG. 19 . At the second step, the photoresist pattern  30  is removed by ashing or dissolution with a solvent. At the third step, by wet etching or dry etching, the spacer at the left side is selectively removed relative to the spacer at the right side, as shown in  FIG. 20 . 
         [0088]    Referring to  FIG. 21 , for example by CVD, a conformal thin oxide layer  33 ′ which has a thickness of about 2-5 nm is formed on the whole surface of the semiconductor structure. For example by CVD, a nitride is then deposited to a thickness sufficient for filling the trench  42 . The nitride layer is selectively etched back relative to the oxide layer  33 ′ so that the portion of the nitride layer around the trench  41  is completely removed and only the nitride filler  44  remains in the trench  41 . 
         [0089]    Referring to  FIGS. 22A and 22B , by dry etching such as ion beam milling, plasma etching, reactive ion etching, laser ablation and the like, the oxide is selectively removed relative to the nitride filler  44 . 
         [0090]    The etching step removes completely the portion of the oxide layer  33 ′ exposed at the main surface of the semiconductor structure, and the portion of the oxide layer  33 ′ that is located at the sidewall and the bottom of the trench is protected by the nitride filler  44  and remains. Consequently, a top surface and a left side of the polysilicon layer  32  of the multilayer gate conductor, and a top surface of the epitaxial silicon layer  41  at the source/drain regions are exposed. 
         [0091]    Referring to  FIGS. 23A and 23B , by a conventional silicidation process, a portion of the top surface and the left side of the polysilicon layer  32  of the multilayer gate conductor, and at least a portion of the epitaxial silicon layer  41  at the source/drain regions are converted to a silicide layer  45 , so as to minimize a contact resistance between the gates, the source/drain and the corresponding metal contacts. 
         [0092]    For example, a Ni layer which has a thickness of about 5-12 nm is firstly deposited, and then subjected to a heat treatment at a temperature in a range of about 300-500 for about 1-10 seconds, so that at least a portion of the polysilicon layer  32  and the epitaxial silicon layer  41  is converted to NiSi. Finally, the unreacted Ni is removed by wet etching. 
         [0093]    After the steps shown in  FIGS. 2-23  are finished, an interlayer insulator may be formed on the resultant semiconductor structure, and vias may be provided therein, wirings and electrical contacts may be provided on an upper surface of the interlayer insulator according to conventional processes well known in the art, so that other parts of the semiconductor device are formed. 
         [0094]    While the invention has been described with reference to specific embodiments, the description is illustrative of the invention. The description is not to be considered as limiting the invention. Various modifications and applications may occur for those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims.