Abstract:
There is disclosed a method of applying stress to a channel region underneath a gate of a field-effect-transistor, which includes the gate, a source region, and a drain region. The method includes steps of embedding stressors in the source and drain regions of the FET; forming a stress liner covering the gate and the source and drain regions; removing a portion of the stress liner, the portion of the stress liner being located on top of the gate of the FET; removing at least a substantial portion of the gate of a first gate material and thus creating an opening therein; and filling the opening with a second gate material.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of semiconductor device manufacturing. In particular, it relates to the engineering of channel strain in field-effect-transistors through gate replacement and/or selective use of gate material. 
     BACKGROUND OF THE INVENTION 
     In the field of semiconductor device manufacturing, active semiconductor devices such as, for example, transistors are normally manufactured or fabricated by front end of line (FEOL) technologies. A transistor may include, for example, a field-effect-transistor (FET) such as a complementary metal-oxide-semiconductor (CMOS) FET. Among FET transistors may be a p-type doped FET (PFET) or an n-type doped FET (NFET). Different types of FET transistors may be formed or manufactured on a common substrate of semiconductor chip or a common semiconductor structure. 
     In order to improve device performance such as operational speed by enhancing carrier mobility in the channel of a FET, following forming the gate structure of the FET, stresses are normally induced into the channel region of the FET through, for example, applying stress liners. A compressive stress liner is normally applied to a PFET transistor and a tensile stress liner applied to an NFET transistor due to different types of carriers. Both stress liners may be formed by following a conventional dual stress liner (DSL) process, or more recently a self-aligned dual stress liner process (SADSL). Other techniques for engineering strain in the channel of a FET may include, for example, embedding silicon germanium (SiGe) in the source/drain regions of a PFET transistor so as to more effectively apply stress towards the channel of the PFET transistor. 
     With the continued pursuing for high-performance semiconductor devices, there is a need to further improve the engineering of strain in the channel region of field-effect-transistors. This may include, for example, improving the effectiveness of stress liners and in some instances even in the absence of such stress liners. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention provide a method of applying stress to a channel region underneath a gate of a field-effect-transistor, which includes the gate, a source region, and a drain region. The method includes embedding stressors in the source and drain regions; forming a stress liner covering the gate and the source and drain regions; removing a portion of the stress liner, the portion of the stress liner being located on top of the gate; removing at least a substantial portion of the gate of a first gate material and thus creating an opening therein; and filling the opening with a second gate material. 
     Embodiments of the present invention further provide a method of, after filling the opening with the second gate material, removing the stress liner that covers the source and drain regions; and a method of, after removing the stress liner covering the source and drain regions, forming a new stress liner covering the gate of the second gate material and the source and drain regions of the FET. 
     According to one embodiment, the first gate material may have a Young&#39;s modulus value being smaller than 130 GPa, preferably smaller than 115 GPa, and more preferably smaller than 100 GPa. The first gate material may be selected from the group consisting of Si 0.8 Ge 0.2 , SiO 0.5 Ge 0.5 , Ge, GaP, GaAs, Al 0.5 Ga 0.5 As, AlAs, InP, InAs, ZnO, ZnS, ZnSe, CdS, and CdTe. According to another embodiment, the second gate material may have a Young&#39;s modulus value being equal to or larger than 130 GPa. 
     Embodiments of the present invention provide a method of applying stress to a channel region underneath a gate of a field-effect-transistor, which includes the gate, a source region, and a drain region. The method includes forming the gate of the FET with a gate material, the gate material having a Young&#39;s modulus value being smaller than 130 GPa, preferably smaller than 115 GPa, and more preferably smaller than 100 GPa; and forming a stress liner covering the gate and the source and drain regions of the FET. 
     Embodiments of the present invention provide a method applying stress to a channel region underneath a gate of a field-effect-transistor. The method includes embedding stressors in a source and drain regions of the FET; forming a stress liner covering a gate of the FET and the source and drain regions; removing a portion of the stress liner that is located on top of the gate; removing the gate of a first gate material and a layer of a first gate oxide underneath, thus creating an opening therein; filling the opening with a layer of a second gate oxide; and filling a second gate material on the layer of said second gate oxide. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be understood and appreciated more fully from the following detailed description of the invention, taken in conjunction with the accompanying drawings of which: 
         FIGS. 1-8  are demonstrative illustrations of a method of forming a field-effect-transistor with gate replacement according to embodiments of the present invention; and 
         FIGS. 9-13  are demonstrative illustrations of alternative steps in methods of forming a field-effect-transistor according to embodiments of the present invention. 
     
    
    
     It will be appreciated that for the purpose of simplicity and clarity of illustration, elements in the drawings have not necessarily been drawn to scale. For example, dimensions of some of the elements may be exaggerated relative to other elements for clarity purpose. 
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of embodiments of the invention. However, it will be understood by those of ordinary skill in the art that embodiments of the invention may be practiced without these specific details. In the interest of not obscuring presentation of essences and/or embodiments of the present invention, in the following detailed description, processing steps and/or operations that are well known in the art may have been combined together for presentation and/or for illustration purpose and in some instances may not have been described in detail. In other instances, processing steps and/or operations that are well known in the art may not be described at all. A person skilled in the art will appreciate that the following descriptions have rather focused on distinctive features and/or elements of embodiments of the present invention. 
     In semiconductor manufacturing industry, various types of active semiconductor devices such as transistors, including CMOS FET of n-type (NFETs) and p-type (PFETs), may be created or formed on a single substrate of semiconductor by applying well-known FEOL processing technologies. The well-known FEOL technologies may include processing steps and/or operations of, inter alia, cap deposition, photo-resist-mask formation, photolithography, hard-mask formation, wet etching, reactive-ion etching (RIE), ion-implantation, and chemical-mechanical polishing (CMP), to list a few. During and/or after the formation of transistors, stress liners of the same or different types may be applied to the transistors, i.e., NFETs and PFETs, for device performance improvement. Improvement in device performance may come from improved mobility of electrons in the channel region of NFETs and/or holes in the channel region of PFETs brought by strains induced by the stress liners. 
     In the following detailed description, well-known device processing techniques and/or steps may not be described in detail and, in some instances, may be referred to other published articles or patent applications in order not to obscure the description of the essence of presented invention as further detailed herein below. 
       FIGS. 1-8  are demonstrative illustrations of a method of forming a field-effect-transistor with gate replacement according to embodiments of the present invention. For example,  FIG. 1  illustrates a step of forming a field-effect-transistor (FET)  100  on a semiconductor substrate  101 . FET  100  may be electrically separated from other FETs or semiconductor devices by shallow trench isolation (STI), e.g., STI  102 , embedded in substrate  101 . The formation of FET  100  may include forming or depositing a dielectric layer  103 , e.g., oxide, on a top surface of substrate  101 ; patterning gate conductor  201 , e.g., polysilicon, on top of dielectric layer  103 ; and embedding stressors  104 , e.g., silicon-germanium (SiGe) or silicon-carbon (SiC), in the source and drain regions next to gate conductor  201 . 
     Although not specifically illustrated in  FIG. 1 , according to some embodiments of the present invention, the formation of FET  100  may also include other well known steps such as, for example, forming spacers on the two sides of gate conductor  201 , forming source and drain defined by the spacers, forming silicide at the top surfaces of source, drain, and gate for contact, et al. According to some other embodiments, the formation of spacers, source and drain, and/or silicide may be performed at a later stage after the gate replacement process as described below in more details. In any case, in order not to obscure the essence of the present invention, a person skilled in the art may refer to other published articles and/or patents for details of these steps of forming a FET. 
       FIG. 2  illustrates a step of forming FET  100  following the step shown in  FIG. 1 . Specifically, a stress liner  202  may be subsequently formed on top of FET  100 , which may apply stress toward the channel region of FET  100  underneath gate conductor  201 . Stress liner  202  may be a compressive stress liner, a tensile stress liner, or a dual stress liner. In the case of a p-type doped FET (PFET)  100 , a compressive stress may be applied by stress liner  202 , which may be formed on top of PFET  100  through deposition in a, for example, plasma-enhanced chemical vapor deposition (PECVD) process. Other well-known methods other than PECVD may be used in forming stress liner  202  as well. Stress liner  202  may be a compressive nitride liner or compressive oxide liner. However, a person skilled in the art will appreciate that compressive stress liner may not be limited to nitride liner or oxide liner and other compressive liner materials may be used as well. 
     According to one embodiment, in the case of an n-type doped FET (NFET), a tensile stress may be applied by stress liner  202 . According to yet another embodiment, stress liner  202  may have a stress substantially close to zero. In other words, a non-stress liner  202  may be used as well in the process of gate replacement as described below in more detail. 
     Assuming FET  100  is a PFET without losing generality, in order to enhance the effectiveness of compressive stress liner  202  in exerting stress in the channel region of FET  100 , according to some embodiment of the present invention, in the previous step ( FIG. 1 ) of forming gate conductor  201 , which may be referred to as a replacement gate or dummy gate, conductive materials with low Young&#39;s modulus may be used. For example, polysilicon (Si) is well known as suitable for gate conductor and has a nominal value of Young&#39;s modulus around 130 GPa. However, gate conductor material made of a compound of silicon (Si) and germanium (Ge) may have a Young&#39;s modulus smaller than that of Si, typically between 103 and 130 GPa. For example, Young&#39;s modulus of Si 0.8 Ge 0.2  is around 124 and Si 0.5 Ge 0.5  is around 116, while germanium (Ge) has a Young&#39;s modulus value around 103. 
     Table 1 lists some of possible candidates for gate conductor. Along with their Young&#39;s modulus values, table 1 also provides the respective melting point, mobility factor, and band gap values for each of the candidates. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Candidate material for gate conductor 
               
             
          
           
               
                   
                 Young&#39;s 
                 Melting 
                 Mobility 
                   
               
               
                 Material 
                 modulus (GPa) 
                 point (C.) 
                 factor 
                 Band gap (eV) 
               
               
                   
               
             
          
           
               
                 Si 
                 130 
                 1412 
                 1 
                 1.12 
               
               
                 Si 0.8 Ge 0.2   
                 124 
                 1275 
                 1.05 
                 1.03 
               
               
                 Si 0.5 Ge 0.5   
                 116 
                 1109 
                 1.12 
                 0.891 
               
               
                 Ge 
                 103 
                 938 
                 1.26 
                 0.661 
               
               
                 GaP 
                 103 
                 1457 
                 1.26 
                 2.26 
               
               
                 GaAs 
                 85.3 
                 1240 
                 1.52 
                 1.424 
               
               
                 Al0.5Ga0.5As 
                 84.4 
                 1351 
                 1.54 
                 1.8 
               
               
                 AlAs 
                 83.5 
                 1740 
                 1.56 
                 2.168 
               
               
                 InP 
                 61.1 
                 1062 
                 2.13 
                 1.344 
               
               
                 InAs 
                 51.4 
                 942 
                 2.53 
                 0.354 
               
               
                 ZnO 
                 108 
                 1975 
                 1.20 
                 3.2 
               
               
                 ZnS 
                 74.4 
                 1718 
                 1.75 
                 3.54 
               
               
                 ZnSe 
                 70 
                 1525 
                 1.86 
                 3.10 
               
               
                 CdS 
                 50 
                 1750 
                 2.6 
                 2.42 
               
               
                 CdTe 
                 52 
                 1041 
                 2.5 
                 1.56 
               
               
                   
               
             
          
         
       
     
     A person skilled in the art will appreciate that most of the materials listed in Table 1 have a Young&#39;s modulus smaller than that of silicon of 130 GPa. In particular, the listed materials include Si 0.8 Ge 0.2 , Si 0.5 Ge 0.5 , Ge, GaP, GaAs, Al 0.5 Ga 0.5 As, AlAs, InP, InAs, ZnO, ZnS, ZnSe, CdS, and CdTe. Materials with smaller Young&#39;s modulus, once used for replacement gate  201 , may exhibit relatively smaller resistance to an external force being applied thereupon, and thus the compressive stress applied by stress liner  202  may be more effectively transferred to the channel region of FET  100 . 
       FIG. 3  illustrates a step of forming FET  100  following the step shown in  FIG. 2  in order to further strengthen the strain effect brought by stress liner  202 . More specifically, a top portion of stress liner  202  may be removed to expose gate conductor or replacement gate  201  underneath. The removal of stress liner  202  at the top of gate conductor  201  may be through well-known processes such as a chemical-mechanical-polishing (CMP) process, which may create a co-planar surface  202   a,    202   b  at the top of stress liner  202  and  201   a  at the top of gate conductor  201 . The selective removal of stress liner  202  at the top of replacement gate  201  may cause at least partial relaxation of stresses by stress liner  202  in the direction to enhance the strain effect in the channel region underneath replacement gate  201 . Further relaxation may be obtained in a gate replacement process as described below in more detail, according to embodiments of the present invention. 
       FIG. 4  illustrates a step of forming FET  100  following the step shown in  FIG. 3  after the top of gate conductor  201  is exposed. The exposed gate conductor  201  may be subsequently removed through, for example, a RIE etching process in which the etchant or etchants used may be selected such that the etching process is selective to the gate conductor material. In other words, the etching of gate conductor  201  may leave nitride stress liner  202   a  and  202   b  substantially intact. The selection of etchants for performing selective RIE etching is well-known in the art and will not be described in further details. Following the removal of gate conductor  201 , portion of dielectric oxide  103  exposed by the removal of gate conductor  201  may be selectively removed as well to expose the underneath channel region of FET  100 , leaving only layer  103   a  under liner  202   a  and layer  103   b  under liner  202   b.  However, embodiments of the present invention are not limited in this respect. For example, according to some embodiments, dielectric oxide layer  103  underneath gate conductor  201  may be left intact or substantially intact, in which case the re-growth of a dielectric oxide layer in the opened gate region (as described below in detail) may not be necessary. In one embodiment, layers  103   a  and  103   b  may be silicide over source and drain regions  104  as electrical contact for FET  100 . 
     According to one embodiment, the removal of gate conductor or replacement gate  201  allows stress liner  202  to further relax, resulting in a more effective transfer of stresses from the two sides of the channel, including those from stress liner  202  and stressor  104  and any other possible sources, to the channel region of FET  100 . Even in the case of a non-stress liner  202 , the removal of replacement gate  201  will still allow stresses from stressor  104  to be transferred to the channel region. It shall be noted that a person skilled in the art will appreciate that stressor  104  may include embedded SiGe, embedded SiC, or any other types of stressors which may be formed by any future technologies. 
       FIG. 5  illustrates a step of forming FET  100  following the step shown in  FIG. 4  after both gate conductor  201  and dielectric layer  103  underneath have been removed. A new dielectric layer  211  may be formed in the opening directly on top of the channel region. Dielectric layer  211  may be a gate oxide layer. 
       FIG. 6  illustrates a step of forming FET  100  following the step shown in  FIG. 5  after gate oxide layer  211  is formed. Directly on top of gate oxide layer  211 , a new gate  212  may be formed through for example deposition in the opening between stress liner  202 . Deposition of gate  212  may be followed by a planarization process such as a CMP process to form a surface which may be coplanar with surface  202   a  and  202   b.  Gate  212  may be a relaxed gate conductor of material such as, for example, polysilicon, tungsten (W), or metal silicide. However, the present invention is not limited in this respect and other type of gate materials may be used as well, including any suitable thin layer being placed between the gate and the gate oxide layer  211  underneath in order to protect the gate oxide. 
       FIG. 7  illustrates a step of forming FET  100  following the step shown in  FIG. 6  after forming gate  212 . A relaxed nitride diffusion barrier layer  213  may be optionally formed on top of stress liner  202  and gate  212 , thereupon an inter-level dielectric (ILD) layer  214  may be formed as is well known in the art. Diffusion barrier layer  213  may protect ILD layer  214  from contamination from nitride stress liner  202 . In a next step as shown in  FIG. 8 , metal contacts  215  and  216  may be formed through well-known etching and deposition process. For example, metal contact  215  may be formed to contact gate  212  and metal contact  216  may be formed to contact source/drain in the embedded SiGe region  104 , possibly through silicide  103   a  and  103   b.    
     According to an alternative embodiment of the present invention, the gate replacement process as described in  FIGS. 2-6  may be applied early in the stage in forming FET  100 , and may be applied in situations where no embedded silicon-germanium is formed in the source/drain regions. For example, as illustrated in  FIG. 9 , processing steps as described above may be applied after replacement gate  201  is formed on top of semiconductor substrate  101  via gate dielectric layer  103 . According to this embodiment, after material of replacement gate  201  is removed to form an opening inside stress liner  202  (as shown in  FIG. 4 ), stress from stress liner  202  may be transferred effectively to the channel regions in the substrate underneath the opening. However, the present invention is not limited in this respect and any other types of stresses applied by stressors (e.g., stress liner, eSiGe, etc.) from the two sides of replacement gate  201  may be effectively transferred to the channel region of FET  100  after the gate material  201  is removed. 
     In  FIG. 2  as part of the gate replacement process, stress liner  202  is formed on top of gate  201  (dummy gate or replacement gate), which step is then followed by a planarization (CMP) step to open the top of gate  201  in preparation for the removal of the dummy gate  201 . However, the present invention is not limited in this respect. For example, as illustrated in  FIG. 10 , in the case that the thickness of nitride stress liner  202  is less than the height of gate  201 , additional layer or layers, such as an oxide layer  203 , may be formed until the top of gate  201  is covered such that a CMP process may be subsequently applied. Here, a person skilled will appreciate that stress liner  202  may not be limited to nitride stress liner and, so long as stressors (e.g., eSiGe  104  in  FIG. 10 ) have been formed at the two sides of replacement gate  201 , stress liner  202  may not be even a stress liner and may be a regular non-stress liner. According to one embodiment, the formation of stress liner  202  may be optional. 
     According to one embodiment, following the removal of replacement gate  201  as in  FIG. 4  and depending on the stage of forming FET  100 , spacers  204   a  and  204   b  may be formed on the sidewalls of stress liner  202  in the opening as illustrated in  FIG. 11 . Spacers  204   a  and  204   b  may be tailored to define the width of gate conductor formed therein and away from the source/drain and their extension regions. Following the formation of spacers  204   a  and  204   b,  dielectric oxide layer  211  and gate conductor  212  may be formed as described in  FIGS. 5-6  in between spacers  204   a  and  204   b.    
     According to another embodiment, following the step as shown in  FIG. 6 , the at least partially relaxed (due to the opening) stress liner  202  may be removed selectively, as illustrated in  FIG. 12 . In this case, material used in forming gate conductor  212  in step of  FIG. 6  may be selected to have a relatively high Young&#39;s modulus (for example, equal to or higher than that of polysilicon) such that gate conductor  212  may be able to hold or retain (to certain extent) the strain, which may be caused by the compressive stress of stress liner  202 , in the channel region underneath gate conductor  212 . In other words, the stress in the channel region of FET  100  may be relaxed, if any, to a lesser extent upon the removal of stress liner  202 . Upon forming source/drain and silicide at the source/drain regions, if necessary and not formed previously, a new stress liner  205  may be formed on top of FET  100  as illustrated in  FIG. 13  according to yet another embodiment. Stress liner  205  may further strengthen the strain applied to the channel region underneath gate conductor  212  and gate dielectric  211 . 
     While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will now occur to those of ordinary skill in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the spirit of the invention.