Abstract:
The present application discloses a semiconductor device and a method for manufacturing the same. The semiconductor device comprises a semiconductor substrate; a first semiconductor layer on the semiconductor substrate; a second semiconductor layer surrounding the first semiconductor layer; a high k dielectric layer and a gate conductor formed on the first semiconductor layer; source/drain regions formed in the second semiconductor layer, wherein the second semiconductor layer has a slant sidewall in contact with the first semiconductor layer. The semiconductor device has an increased output current, an increased operating speed, and a reduced power consumption due to the channel region of high mobility.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of Invention 
         [0002]    The present invention relates generally to a semiconductor device and a method for manufacturing the same, and more particularly, to an MOSFET (metal oxide semiconductor field effect transistor) structure with a channel region of high mobility and a method for manufacturing the same. 
         [0003]    2. Description of Prior Art 
         [0004]    One trend of the integrated circuit technology is to integrate as many MOSFETs as possible in a unit area of a chip. With scaling down of the MOSFET dimensions, a gate length is reduced to less than 32 nm. However, due to the reduced gate length, the gate has a poor controllability on a channel region, which degrades properties of the MOSFET, especially causes a short channel effect in which a threshold voltage of the MOSFET decreases. Moreover, a poor conductivity of polysilicon causes a voltage drop across a polysilicon gate when a voltage is applied to the gate. Thus, an actual gate voltage applied to the channel region is further reduced. 
         [0005]    A dual-gate device or an ultra-thin SOI device can enhance controllability of the gate on the channel region, and thus suppresses the short channel effect. 
         [0006]    Another trend is to replace the polysilicon gate with a metal gate, which alleviates an unfavorable effect of polysilicon depletion by using a metal having a good conductivity. In manufacturing such a semiconductor device, a replacement gate process is typically used to precisely control a gate length, which comprises the steps of forming a dummy gate conductor such as polysilicon, selectively removing the dummy gate conductor to provide a gate opening, and finally depositing a gate metal in the gate opening. An MOS device manufactured by the replacement gate process enhances controllability of the gate on the channel region. 
         [0007]    However, the above novel devices, such as the dual-gate device, the ultra-thin SOI device, and the MOS device having a metal gate, still use conventional channel materials, which limits maximum values of an output current and an operating frequency, and has no improvement in a power consumption. 
       SUMMARY OF THE INVENTION 
       [0008]    One object of the present invention is to provide an MOSFET having an increased output current, an increased operating speed and a reduced power consumption, and a method for manufacturing the same. 
         [0009]    According to one aspect of the present invention, there provides a semiconductor device comprising a semiconductor substrate; a first semiconductor layer on the semiconductor substrate; a second semiconductor layer surrounding the first semiconductor layer; a high k dielectric layer and a gate conductor formed on the first semiconductor layer; source/drain regions formed in the second semiconductor layer, wherein the second semiconductor layer has a slant sidewall in contact with the first semiconductor layer. 
         [0010]    According to another aspect of the invention, there provides a method for manufacturing a semiconductor device, comprising: 
         [0011]    a) forming a second semiconductor layer on a semiconductor substrate; 
         [0012]    b) forming a dummy gate on the second semiconductor layer, and source/drain regions besides the dummy gate; 
         [0013]    c) removing the dummy gate to provide a gate opening; 
         [0014]    d) selectively removing the portion of the second semiconductor layer exposed in the gate opening by wet etching; 
         [0015]    e) epitaxially growing a first semiconductor layer on the semiconductor substrate in the gate opening; and 
         [0016]    f) forming a gate dielectric layer and a gate conductor in the gate opening. 
         [0017]    In the semiconductor device of the present invention, the slant sidewall of the second semiconductor layer facilitates the epitaxial growth of the first semiconductor layer. Consequently, the first semiconductor layer is of high quality and improves performance of the channel region of the semiconductor device. The first semiconductor layer is made of a high-mobility material, which leads to an increased output current, an increased operation frequency and a reduced power consumption when the first semiconductor layer is used as the channel region. The most suitable materials can be used for the source/drain regions and the channel regions to provide optimal performance respectively. 
         [0018]    In a preferred embodiment, the first semiconductor layer is an epitaxial layer which has a top surface and a bottom surface matching {100} plane of Si, and a sidewall in contact with the second semiconductor layer and matching {111} plane of Si. The interface (i. e. sidewall) between the first semiconductor layer and the second semiconductor layer substantially preserves integrity and continuity of a crystal structure, which decreases an amount of defects due to the existence of the interface. The epitaxial growth in this direction can provide a flat surface, which ensures a uniform thickness of the channel region. 
         [0019]    In the method of the present invention, a doping process for providing the source/drain regions is performed before formation of the channel region, which avoids a diffusion of dopants towards the channel region and thus decreases an amount of defects in the channel region and improves greatly the performance of the semiconductor device. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIGS. 1-15  schematically shows cross-sectional views of a semiconductor device at various stages of the manufacturing method according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0021]    Exemplary embodiments of the present invention are described in more details 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. 
         [0022]    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 layers or regions 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. 
         [0023]    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. 
         [0024]    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 for but can be varied in a specific implementation of the invention 
         [0025]    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 can be made of for example a group IV semiconductor (such as Si, Ge) or group III-V semiconductor (such as GaAs, InP, GaN, SiC). 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 selected from a group consisting of TaC, TiN, TaTbN, TaErN, TaYbN, TaSiN, HfSiN, MoSiN, RuTa x , NiTa x , MoN x , TiSiN, TiCN, TaAlC, TiAlN, TaN, PtSi x , Ni 3 Si, Pt, Ru, Ir, Mo, HfRu, RuO x , and their combinations. A gate dielectric layer 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 , 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. 
         [0026]    According to one preferable embodiment according to the present invention, the steps shown in  FIGS. 1 to 15  are performed in sequence for manufacturing the MOSFET. 
         [0027]    The method for manufacturing the MOSFET according to the present invention starts with a semiconductor substrate  10  having shallow trench isolation (STI) regions  11 , as shown in  FIG. 1 . The semiconductor substrate  10  is preferably a single-crystal silicon substrate. STI regions  11  are preferably made of an oxide for electrically isolating active regions in the semiconductor substrate  10 . A surface of the semiconductor substrate  10  is exposed between the STI regions. 
         [0028]    A SiGe layer  12  having a thickness of about 10-20 nm and a Ge content of about 5-15% and a Si layer  13  having a thickness of about 3-10 nm are selectively and epitaxially grown in sequence on the exposed surface of the semiconductor substrate  10  by a conventional deposition process such as PVD, CVD, atomic layer deposition, sputtering, and the like, as shown in  FIG. 2 . 
         [0029]    Due to a selectivity of the epitaxial growth, neither SiGe layer  12  nor Si layer  13  is formed in the STI regions  11 . 
         [0030]    A portion of the Si layer  13  is then converted into SiO 2  by thermal oxidation, to provide a dummy gate dielectric layer  14 . 
         [0031]    A polysilicon layer  15  having a thickness of about 30-60 nm, an oxide layer  16  having a thickness of about 10-20 nm, and a nitride layer  17  having a thickness of about 20-50 nm are deposited in sequence on the whole surface of the semiconductor structure by the above conventional deposition process. The oxide layer  16  and the nitride layer  17  will be used as a stop layer in an etching process and a protection layer in a chemical mechanical planarization (CMP) process respectively, in subsequent steps. 
         [0032]    The polysilicon layer  15  is patterned to provide a dummy gate conductor, as shown in  FIG. 3 . 
         [0033]    Firstly, a photoresist layer  18  is formed on a surface of the nitride layer  17 , and then patterned by a lithography process including exposure and development, to provide a mask by the photoresist layer  18  having patterns therein. The exposed portions of the nitride layer  17 , the oxide layer  15  and the polysilicon layer  15  are removed from top to bottom by a dry etching process, such as ion beam milling, plasma etching, reactive ion etching (RIE), and laser ablation. The etching stops at the top of the dummy gate dielectric layer  14 . Finally, the photoresist mask is removed by ashing or dissolution with a solvent. 
         [0034]    Lightly doped source/drain regions (and extension regions, if required) are formed in the epitaxial Si layer  13 , and sidewall spacers are also formed, as shown in  FIG. 4 . 
         [0035]    With a stack of the nitride layer  17 , the oxide layer  16  and the dummy gate conductor  15  used as a hard mask, ions are implanted into the epitaxial Si layer  13 . For an n-type MOSFET, dopants such as As, P can be used. For a p-type MOSFET, dopants such as B, BF 2  can be used. 
         [0036]    A nitride layer is then formed on the whole surface of the semiconductor structure by a conventional deposition process. With a photoresist mask (not shown) used, a portion of the nitride layer is etched away by the above dry etching process so that the remaining portion of the nitride layer at both sides of the stack of the nitride layer  17 , the oxide layer  16  and the dummy gate conductor  15  forms sidewall spacers  19  of the gate. 
         [0037]    If required, the semiconductor structure is subjected to an annealing process, such as a spike anneal at about 1000-1080° C. so as to activate the dopants implanted previously and remedy damages due to the ion implantation. Reference sign  20  in  FIG. 4  shows a profile of the source/drain regions. 
         [0038]    Referring to  FIG. 5 , with the stack of the nitride layer  17 , the oxide layer  16  and the dummy gate conductor  15 , the sidewall spacers  19  at both sides of the stack, and the STI regions  11  used as a hard mask, the exposed portions of the dummy gate dielectric layer  14 , the epitaxial Si layer  13 , the epitaxial SiGe layer  12  and the semiconductor substrate  10  are removed from top to bottom by the above dry etching process. The etching stops at a predetermined depth below a top surface of the semiconductor substrate  10 , for example by controlling an etching time. 
         [0039]    A SiGe layer having a Ge content of about 20-70% is epitaxially grown on the exposed surface of the semiconductor substrate  10  by the above conventional deposition process, to provide contact regions  21  which electrically and laterally contact the source/drain regions. 
         [0040]    Preferably, the contact regions  21  have a thickness so that their top surfaces are above a top surface of the epitaxial Si layer  13 , and their bottom surfaces are below a bottom surface of the epitaxial Si layer  13 . 
         [0041]    A conformal nitride layer  22  having a thickness of about 10-20 nm and an overlying oxide layer  23  having a thickness of about 100-150 nm are formed on the whole surface of the semiconductor structure by a conventional deposition process, as shown in  FIG. 6 . 
         [0042]    With the nitride layer  22  used as a protection layer, the semiconductor structure is subjected to CMP to provide a flat surface. The CMP removes a portion of the oxide layer  23  so that one portion of the nitride layer  22  above the stack of the nitride layer  17 , the oxide layer  16  and the dummy gate conductor  15  is exposed, and the other portion of the nitride layer  22  is below the oxide layer  23 . 
         [0043]    The oxide layer  23  is then etched back, during which a portion of the oxide layer  23  is selectively removed with respect to the nitride layer. The exposed portion of the nitride layer  22  looks like a nitride cap. 
         [0044]    The nitride cap is then selectively removed with respect to the oxide layer by a conventional wet etching process, in which an etching solution is used and the oxide layer  23  serves as a mask for the wet etching, as shown in  FIG. 7 . The etching firstly removes the exposed portion of the sidewall spacers  19  of the gate and nitride layer  22 , and then removes completely the nitride layer  17  at the top of the stack. 
         [0045]    The oxide layer  16  and the polysilicon layer  15  which is a dummy gate conductor are then removed completely by a dry etching process. Further, the exposed portion of the dummy dielectric layer  14  is removed. Consequently, a gate opening  24 , which is surrounded by the sidewall spacers  19  of the gate, is formed, as shown in  FIG. 8 . 
         [0046]    Si is then selectively removed with respect to SiGe by a conventional wet etching process, in which an etching solution is used. The etching is anisotropic and thus removes only the portion of the epitaxial Si layer  13  exposed in the gate opening  24 , so that a top surface of the epitaxial SiGe layer  12  is exposed at the bottom of the gate opening. 
         [0047]    Those anisotropic etchants well known in the field for Si can be used in the present invention, such as KOH, TMAH, EDP, N 2 H 4 .H 2 O, and the like. 
         [0048]    Due to erosion of the anisotropic etchants, an etching rate at {111} plane of Si is at least one order of magnitude less than that at other planes. Consequently, a sidewall of the epitaxial Si layer  13 , which is exposed in the gate opening  24 , is a {111} facet of Si. The sidewall is slant with respect to a surface of the semiconductor substrate. 
         [0049]    Alternatively, in a case that the semiconductor substrate  10  and the epitaxial Si layer  13  are made of different semiconductor materials and the semiconductor substrate  10  can be used as an etching stop layer, the semiconductor device according to the present invention will omit the epitaxial SiGe layer  12 . 
         [0050]    Ions are implanted into the channel region through the gate opening  24 , as shown in  FIG. 9 . 
         [0051]    For an n-type MOSFET, dopants can be As or P, with an implantation energy of about 1-20 keV and a doping level of about 2×10 18 -1×10 20 /cm 3 ; for a p-type MOSFET, dopants can be B or BF 2 , with an implantation energy of about 0.2-20 keV and a doping level of about 2×10 18 -1×10 20 /cm 3 . 
         [0052]    Preferably, the ion implantation provides a super steep retrograde island (SSRI)  25  is having a rectangular shape below the gate opening  24 . As well known in the field, the SSRI has a steep doping profile which reduces the short channel effect. SSRI  25  is located at a depth of about 5-20 nm below the gate opening  24  (i. e. a distance from a bottom of a gate dielectric layer to be formed). 
         [0053]    After the ion implantation, the doped channel region may be subjected to a laser anneal to activate the dopants. 
         [0054]    U.S. Pat. No. 6,214,65481 owned by Bin Yu discloses the above steps of forming a super steep retrograded channel by using a sacrificial gate (corresponding to the dummy gate in the present application), the disclosure of which is incorporated here by reference. 
         [0055]    A channel layer  26  having a thickness of about 2-7 nm is epitaxially grown on the epitaxial SiGe layer  12  by the above conventional deposition process, as shown in  FIG. 10 . A Si layer  27  having a thickness of about 2-5 nm is then epitaxially grown on the channel layer  26 , to be converted into a high-quality gate dielectric layer in a subsequent step. 
         [0056]    The channel layer  26  replaces a portion of the epitaxial Si layer  13 , and is made of a semiconductor material having a mobility of carriers higher than Si. As an example, the channel layer  26  is made of SiGe having a high Ge content (for example, the Ge content is 20-100%). Moreover, the channel layer may be made of a group III-V semiconductor material such as InP, InSb, InGaAs, and InAs. 
         [0057]    The channel layer  26  has a crystal structure matching in a vertical direction the underlying epitaxial SiGe layer  12  formed in the step shown in  FIG. 2 , and in a lateral direction (i.e. at its sidewall) the exposed facet of the epitaxial Si layer  13  formed in the step shown in  FIG. 8 . 
         [0058]    In a preferred embodiment, the channel layer  26  is epitaxially grown on a {110} plane of Si and in a normal direction of the wafer, and on a {111} plane of Si in a lateral direction. 
         [0059]    Thus, an interface between the channel layer  26  and the epitaxial Si layer  13  substantially preserves integrity and continuity of a crystal structure, which decreases an amount of defects pinned due to the existence of the interface. Moreover, the epitaxial growth in two directions can provide a flat surface, which ensures a uniform thickness of the channel region. 
         [0060]    A portion of the Si layer  27  is then converted into SiO 2  by thermal oxidation, to provide a SiO 2  layer (not shown) having a thickness of about 0.5-1 nm. 
         [0061]    A conformal high k dielectric layer (for example, HfO 2 ) is formed on the whole surface of the semiconductor structure by the above conventional deposition process to have a thickness of about 2-5 nm, which serves as a gate dielectric layer  28  of the final MOSFET. 
         [0062]    A gate conductor  29  (for example, W, TiN, and other metals) fills the gate opening  24  by the above conventional deposition process, as shown in  FIG. 11 . 
         [0063]    The above step may comprise firstly depositing an overlying metal layer and then patterning the metal layer so that only the portion of the metal layer in the gate opening  24  remains. Preferably, after deposition of the metal layer, the metal layer is etched back so that one portion of the metal layer outside the gate opening  24  is completely removed, and the other portion of the metal layer in the gate opening  24  is partially removed or not removed, by controlling an etching time. 
         [0064]    A nitride layer  30  is then formed on the whole surface of the semiconductor structure by the above conventional deposition process and is subjected to CMP so as to provide a flat surface, as shown in  FIG. 12 . The nitride layer  30  serves as an interlayer dielectric layer (ILD) so that interconnections can be formed on the nitride layer  30 . 
         [0065]    The portions of the nitride layer  30 , the oxide layer  23  and the nitride layer  22  above the contact regions  21  is removed from top to bottom by the above dry etching process with a photoresist mask (not shown) used, to provide via holes  31  to the contact regions  21 , as shown in  FIG. 13 . 
         [0066]    Referring to  FIG. 14 , silicide regions  32  are formed at a top surface of the contact regions exposed at a bottom of the via holes  32  to reduce a contact resistance between the via conductor to be formed and the contact regions  21 . 
         [0067]    The above step may comprise firstly depositing a conformal Ni layer on the whole surface of the semiconductor structure, then annealing at about 300-500° C. so that Ni reacts with Si in the contact regions  21  to form a metal silicide, and finally selectively removing unreacted Ni with respect to the metal silicide, for example by wet etching. 
         [0068]    Metal contacts  33  are formed in the via holes  31 , as shown in  FIG. 15 . 
         [0069]    The above step may comprise firstly depositing a conformal barrier layer (for example, TiN, not shown) on the whole surface of the semiconductor layer (including an inner wall and a bottom of the via holes  31 ) by the above conventional deposition process, then depositing a metal layer (for example, W) to fill the via holes  31 , and finally removing the portion of the metal layer and the barrier layer outside the via holes  31  by CMP. The remaining portion of the metal layer in the via holes  31  forms metal contacts  33 . 
         [0070]    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.