Abstract:
Methods for forming carbon silicon alloy (CSA) and structures thereof are disclosed. The method provides improvement in substitutionality and deposition rate of carbon in epitaxially grown carbon silicon alloy layers (i.e., substituted carbon in Si lattice). In one embodiment of the disclosed method, a carbon silicon alloy layer is epitaxially grown on a substrate at an intermediate temperature with a silicon precursor, a carbon (C) precursor in the presence of an etchant and a trace amount of germanium material (e.g., germane (GeH 4 )). The intermediate temperature increases the percentage of substitutional carbon in epitaxially grown CSA layer and avoids any tendency for silicon carbide to form. The presence of the trace amount of germanium material, of approximately less than 1% to approximately 5%, in the resulting epitaxial layer, has an effect of stabilizing and enhancing deposition/growth rate without compromising the tensile stress of CSA layer formed thereby.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The disclosure relates generally to formation of carbon silicon alloy (CSA) epitaxial layers during fabrication of N-doped field effect transistors (nFET), and more particularly, to methods of forming CSA epitaxial layers with high degree of substitutional carbon at accelerated growth rates. 
         [0003]    2. Background Art 
         [0004]    In the current state of the art, epitaxial growth of carbon silicon alloy (CSA) on a silicon substrate is accomplished by chemical vapor deposition (CVD) using a mixture of precursors and etchants in a carrier gas. The carbon is added to generate tensile stress in epitaxially grown CSA layers in order to improve the performance of n-doped field effect transistor (nFET) fabricated therefrom. 
         [0005]    However, epitaxial growth of CSA layers is very complicated for a number of reasons. For example, the growth of epitaxial layers is heavily dependent on the substrate surface on which the epitaxial layer is grown (i.e., the crystalline properties of the substrate as well as a pristine surface having low interfacial oxygen and carbon has an influence on the growth of CSA layer thereon). Therefore, the starting substrate plays an important part in the epitaxial growth. 
         [0006]    Another challenge in the epitaxial growth of CSA layers may include the low solid solubility of carbon (C) in a Si lattice. In the event where equilibrium conditions prevail (i.e. at high temperatures) the carbon may be incorporated in high amounts leading to formation of silicon carbide (SiC) layers instead of carbon silicon alloy layers (i.e., where substitutional carbon resides in the lattice of the Si layer). The tendency for formation of silicon carbide is attributed to a high thermodynamic stability which promotes the tendency for SiC precipitation over low amounts (at approximately 1%-2%) of C substitution in epitaxially grown Si lattice. 
         [0007]    One way to circumvent the formation of SiC is to conduct the deposition at a lower temperature and at a high deposition rate. Typically, this is achieved using Si precursors that decompose at a lower temperature than silane (SiH 4 ). An example of such a precursor is Si 3 H 8  (Silicore™, which is a trademark of Jordan Industries, Inc. in the United States and/or other countries). However, the low temperature may compromise the effectiveness of a typical etchant (e.g., hydrogen chloride (HCl)) for promoting selective epitaxial growth of CSA. This result limits the use of such epitaxial chemistries of Si precursors and etchants for selective deposition of CSA. 
       SUMMARY 
       [0008]    Methods for forming carbon silicon alloy (CSA) and structures thereof are disclosed. The method provides improvement in substitutionality and deposition rate of carbon in epitaxially grown carbon silicon alloy layers (i.e., substituted carbon in Si lattice). In one embodiment of the disclosed method, a carbon silicon alloy layer is epitaxially grown on a substrate at an intermediate temperature with a silicon precursor, a carbon (C) precursor in the presence of an etchant and a trace amount of germanium material (e.g., germane (GeH 4 )). The intermediate temperature increases the percentage of substitutional carbon in epitaxially grown CSA layer and avoids any tendency for silicon carbide to form. The presence of the trace amount of germanium material, of approximately less than 1% to approximately 5%, in the resulting epitaxial layer, has an effect of stabilizing and enhancing deposition/growth rate without compromising the tensile stress of CSA layer formed thereby. 
         [0009]    A first aspect of the disclosure provides a method for forming a carbon silicon alloy (CSA) layer on a substrate, the method comprising: depositing a carbon silicon alloy layer on a silicon portion of the substrate, the depositing including mixing a silicon (Si) precursor, a carbon (C) precursor and a germanium material (Ge) in a carrier gas; and etching any carbon silicon alloy material formed on any non-silicon portion of the substrate with an etchant. 
         [0010]    A second aspect of the disclosure provides a semiconductor structure comprising: a carbon silicon alloy layer disposed on a substrate, the carbon silicon alloy layer including: substitutional carbon (C) incorporated in a silicon (Si) lattice; and approximately less than 1% to approximately 5% of germanium (Ge) therein. 
         [0011]    A third aspect of the disclosure provides a semiconductor structure comprising: a gate disposed on a substrate, the substrate including a source-drain region below the gate, wherein the source-drain region includes a carbon silicon alloy (CSA) layer with approximately less than 1% to approximately 5% germanium (Ge) incorporated therein. 
         [0012]    These and other features of the present disclosure are designed to solve the problems herein described and/or other problems not discussed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    Various aspects of the disclosure will be more readily understood from the following detailed description taken in conjunction with the accompanying drawings that depict different embodiments of the disclosure, in which: 
           [0014]      FIG. 1  is a flow diagram of an embodiment of the processes of the disclosure. 
           [0015]      FIG. 2  is a cross-sectional view of an embodiment of a structure of an nFET during the fabrication process of the disclosed method. 
           [0016]      FIG. 3  is a cross-sectional view of the embodiment of the structure of an nFET from  FIG. 2 . 
           [0017]      FIG. 4A  is a graph illustrating the number of carbon, oxygen and silicon atoms per unit volume in a carbon silicon alloy film formed using prior art methods. 
           [0018]      FIG. 4B  is a graph illustrating the number of carbon, germanium, oxygen and silicon atoms per unit volume in a carbon silicon layer formed using the disclosed method in  FIG. 1 . 
           [0019]      FIG. 5A  is a graph illustrating number of carbon, germanium, oxygen and silicon atoms per unit volume in a carbon silicon layer formed using prior art methods. 
           [0020]      FIG. 5B  is a graph illustrating number of carbon, germanium, oxygen and silicon atoms per unit volume in a carbon silicon layer formed using the disclosed method in  FIG. 1 . 
       
    
    
       [0021]    It is noted that the drawings of the disclosure are not to scale. The drawings are intended to depict only typical aspects of the disclosure, and therefore should not be considered as limiting the scope of the disclosure. In the drawings, like numbering represents like elements between the drawings. 
       DETAILED DESCRIPTION 
       [0022]    Embodiments depicted in the drawings in  FIG. 1-3  illustrate the methods and various resulting structure(s) of the different aspects of fabricating an nFET  30  ( FIG. 3 ) in a CMOS using epitaxial layers of CSA disposed on a substrate  100  ( FIGS. 2 and 3 ). Examples of tests results of performance of structures formed by the disclosed method are illustrated in  FIGS. 4A-5A . 
         [0023]      FIG. 1  illustrates a flow diagram of a process including processes S 1 -S 7  of an embodiment of the disclosed method. A CMOS semiconductor structure  20  as shown in  FIG. 2  is provided in process S 1 . Semiconductor structure  20  is fabricated according to currently known or later developed techniques. The structure  20  may include a gate  200  disposed on a substrate  100 . Substrate  100  may include silicon sites, for example, recesses  300  shown in  FIG. 2  and non-silicon sites, for example, shallow trench isolation (STI)  600 , incorporated therein. Recesses  300  are formed using currently known or later developed etching techniques, for example reactive ion etching (RIE). The substrate  100  may also include silicon-on-insulator (SOI) (not shown) or bulk silicon. Epitaxial growth  150  according to process S 1 -S 6  fills recesses  300  forming CSA source/drain regions  500  ( FIG. 3 ). 
         [0024]    According to process S 2  ( FIG. 1 ) of the disclosed method, substrate  100  is subject to a currently known or later developed bake-out process (i.e., annealing in the presence of hydrogen) for preparing the surface of the substrate for epitaxial growth thereon. 
         [0025]    Substrate  100  ( FIG. 2 ) is then cooled to an intermediate deposition temperature according to process S 3  by a currently known or later developed technique. The intermediate deposition temperature for the epitaxial growth of a carbon silicon alloy (CSA)  500  ( FIG. 3 ) layer is maintained at approximately 550° C. to approximately 700° C., preferably at approximately 600° C. to approximately 650° C. At this intermediate deposition temperature, the tendency for carbon to form silicon carbide is avoided while substitutional carbon in the silicon lattice is increased to form carbon silicon alloy (CSA). 
         [0026]    Maintaining the intermediate deposition temperature, a mixture including a silicon (Si) precursor, a carbon (C) precursor, and an etchant in a carrier gas may be introduced in a quartz reactor chamber (not shown) for epitaxial growth according to process S 4 . Currently known or later developed techniques, for example, chemical vapor deposition (CVD) may be applied to achieve the epitaxial growth. The Si precursor may include, for example, but not limited to: silicon tetrachloride (SiCl 4 ); trichlorosilane (SiHCl 3 ); dichlorosilane (SiH 2 Cl 2 ); silane (SiH 4 ); disilane (Si2H6); or other higher order silanes. The C precursor may include organo silane materials, for example, but not limited to: mono-methyl silane and ethylene; and other higher order organo silanes. A typical carrier gas may include, for example, but not limited to helium (He), hydrogen (H 2 ), nitrogen (N 2 ), and other noble gases. A trace amount of germanium in the form germanium materials/compounds may be introduced into the mixture. For example, an amount of germane (GH 4 ), of approximately 0.02% by volume to approximately 0.05% by volume, maybe added in the mixture following dilution in a carrier gas. The reactants may have a proportional relationship where silicon (Si) precursor: carbon (C) precursor: germane (GeH 4 ) is 5000:100:1. 
         [0027]    The mixture in process S 4  may include an amount of organo germanium, for example, methylgermane (MeGeH 3 ) and other organically substituted germanes, for increasing the substitutionality and deposition rate of substitutional carbon in the formation of the CSA layer  500  ( FIG. 3 ) on the substrate  100  ( FIG. 3 ) during epitaxial growth. The amount of organo germanium is then mixed with the Si and C precursors in the carrier gas. The following examples in  FIGS. 4A-5B  illustrate test results from samples of CSA formed by the disclosed method in comparison with those formed by prior art methods. 
         [0028]      FIG. 4A  illustrates a graph showing the respective number of atoms of carbon (C), germanium (Ge), oxygen (O) and silicon (Si) per unit volume in a sample CSA layer grown on a silicon-germanium substrate where epitaxial growth is performed in the absence of germanium. The resultant structure (not shown) provides an interface between the substrate surface and the CSA layer where the percentage of oxygen is approximately 1.1×10 13  atoms/cm 2 .  FIG. 4B  illustrates a graph showing the respective number of atoms of carbon (C), germanium (Ge), oxygen (O) and silicon (Si) per unit volume in a CSA layer grown on a silicon germanium nucleation layer on a silicon substrate where germanium is introduced in CSA epitaxial growth process S 4  according to the disclosed method. Germanium may be introduced at approximately 0.1 standard cubic centimeters per minute (sccm) into the mixture of reactants. In this sample, the percentage of oxygen at the interface between the substrate surface and the CSA layer is approximately 1.0×10 13  atoms/cm 2 . Comparing the results between the two samples, there is approximately 10% less oxygen at the interface between the substrate and the CSA layer in the sample where epitaxial growth was conducted in the presence of germanium. This is attributed to the catalytic effect of germanium (Ge) in the epitaxial growth process S 4 . Germanium has a tendency to actively remove any oxygen contamination at the surface of the substrate improving interface quality. With improved interface quality, the deposition rate of CSA layer may be increased. 
         [0029]    In addition to improving deposition rate, the catalytic effect of germanium (Ge) also provides for epitaxial growth of a CSA layer at a lower deposition temperature range. This promotes the incorporation of substitutional carbon in the silicon (Si) lattice leading to increased substituted carbon (C) in epitaxially grown CSA layer. 
         [0030]      FIG. 5A  illustrates a graph showing the respective number of carbon (C), oxygen (O) and silicon (Si) atoms per units volume in a CSA layer grown on a silicon substrate where epitaxial growth is performed in the absence of germanium. From  FIG. 5A , the depth of the CSA layer in a sample (not shown) is approximately 49 nm with a percentage of substituted carbon at approximately 87%.  FIG. 5B  illustrates a graph showing the respective number of carbon (C), germanium (Ge), oxygen (O) and silicon (Si) per unit volume in a CSA layer  500  ( FIG. 3 ) grown on silicon substrate  100  ( FIG. 3 ) where germanium is introduced during epitaxial growth process S 4 . From  FIG. 5B , the percentage of substitutional carbon in CSA layer  500  ( FIG. 3 ) may be as high as approximately 96% with the depth of CSA layer  500  reaching approximately 100 nm. By introducing trace amount of approximately 2.23×10 20  atoms/cm 3  (i.e., approximately 0.45%) of germane, the percentage of substituted C in the Si lattice is increased by approximately 10%. 
         [0031]    In process S 5 , CSA layer  500 , as shown in  FIG. 3 , is etched to remove any growth on non-silicon sites  600  on the substrate  100 . The etchant may include, for example, but not limited to chlorine, hydrogen chloride or a combination thereof. 
         [0032]    Process S 6  is a cyclic-deposition and etch (CDE) process where the deposition process S 3  and etching process S 4  are repeated until the desired thickness of the CSA layer  500 , shown in  FIG. 3 , is achieved. The desired thickness of the CSA layer depends on the feature/structure to be formed. With germanium included in process S 4 , S 5  and S 6 , the resultant CSA layer usually presents an increased in the percentage of substitutional carbon in the Si lattice of the CSA layer. With an increase in substitutional carbon in the Si lattice, epitaxial growth of the CSA layer  500  ( FIG. 3 ) as an epitaxial fill for forming source-drain regions  900  ( FIG. 3 ) in the semiconductor structure  30  ( FIG. 2 ) may achieve the same effect as a film having lower percentage of substitutional carbon with no dislocations therein. CSA  500  for filling recess  300  ( FIG. 2 ) to form source-drain regions  900  presents a continuous layer without any crystalline dislocations therein. 
         [0033]    With each cycle depositing an increased of substitutional C, the number of cycles in the CDE process S 6  for epitaxial growth of CSA layer  500  as an epitaxial fill in the recesses  300  to form source-drain regions  500  ( FIG. 3 ) is reduced. With the reduction of the number of cycles, the time for forming source-drain regions  900  is reduced. 
         [0034]    With process S 7 , the newly formed CSA layer  500 , as shown in  FIG. 3  may be doped with phosphorous (P) and arsenic (As) to form a junction  800  therebetween. The presence of approximately 1% to approximately 5% of germanium in CSA layer  500  improves dopant control through phosphorous and arsenic junction engineering. Dopant activation is increased while diffusion of dopant is maintained at a minimum in the presence of Ge. The addition of Ge can eliminate/lower the temperature range and duration required for the dopant activation anneal. 
         [0035]    According to the disclosed method, the resultant nFET structure  30  has a tensile strain  400  in channel  700  ( FIGS. 2 and 3 ) that is formed between the source/drain region  900 . The CSA layer  500  in source-drain regions  900  creates the tensile strain  400  which is not compromised because of the intermediate deposition temperature used for the epitaxial growth. 
         [0036]    The foregoing description of various aspects of the disclosure has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the scope of the invention to the precise form disclosed, and obviously, many modifications and variations are possible. Such modifications and variations that may be apparent to a person skilled in the art are intended to be included within the scope of the invention as defined by the accompanying claims.