Abstract:
Stress enhanced transistor devices and methods of fabricating the same are provided. In one embodiment, a transistor device comprises: a gate conductor disposed above a semiconductor substrate between a pair of dielectric spacers, wherein the semiconductor substrate comprises a channel region underneath the gate conductor and recessed regions on opposite sides of the channel region, wherein the recessed regions undercut the dielectric spacers to form undercut areas of the channel region; and epitaxial source and drain regions disposed in the recessed regions of the semiconductor substrate and extending laterally underneath the dielectric spacers into the undercut areas of the channel region.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a divisional of U.S. patent application Ser. No. 12/136,195, which was filed Jun. 10, 2008, the disclosure of which is incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     This invention relates to semiconductor fabrication, and particularly to fabricating transistor devices comprising epitaxial source and drain regions disposed in recessed regions of a semiconductor substrate that undercut an overlying gate structure to increase the stress applied to the channel region. 
     Integrated circuits often employ active devices known as transistors such as field effect transistors (FETs). A FET includes a silicon-based substrate comprising a pair of impurity regions, i.e., source and drain junctions, spaced apart by a channel region. A gate conductor is dielectrically spaced above the channel region of the silicon-based substrate. The junctions can comprise dopants which are opposite in type to the dopants residing within the channel region interposed between the junctions. The gate conductor can comprise a doped semiconductive material such as polycrystalline silicon (“polysilicon”). The gate conductor can serve as a mask for the channel region during the implantation of dopants into the adjacent source and drain junctions. An interlevel dielectric can be disposed across the transistors of an integrated circuit to isolate the gate areas and the junctions. Ohmic contacts can be formed through the interlevel dielectric down to the gate areas and/or junctions to couple them to overlying interconnect lines. 
     Demands for increased performance, functionality, and manufacturing economy for integrated circuits have resulted in extreme integration density and scaling of devices to very small sizes. Transistor device scaling has restricted operating margins and has adversely affected the electrical characteristics of such devices. As such, more emphasis has been placed on achieving higher operating frequencies for transistor devices through the use of stress engineering to improve the carrier mobility of such devices rather than through the use of scaling. 
     Carrier mobility in the channel of a FET device can be improved by applying mechanical stresses to the channel to induce tensile and/or compressive strain in the channel. The application of such mechanical stresses to the channel can modulate device performance and thus improve the characteristics of the FET device. For example, a process-induced tensile strain in the channel of an n-type (NFET) device can create improved electron mobility, leading to higher saturation currents. 
     One method used to induce strain in the channel region has been to place a compressively strained nitride film close to the active region of the FET device. Another approach taken to induce strain in the channel of a p-type (PFET) device has been to epitaxially grow silicon germanium (e-SiGe) in the source and drain regions of the silicon-based substrate. When epitaxially grown on silicon, an unrelaxed SiGe layer can have a lattice constant that conforms to that of the silicon substrate. Upon relaxation (e.g., through a high temperature process) the SiGe lattice constant approaches that of its intrinsic lattice constant, which is larger than that of silicon. Consequently, physical stress due to this mismatch in the lattice constant is applied to the silicon-based channel region. 
     BRIEF SUMMARY 
     The shortcomings of the prior art are overcome and additional advantages are provided through the provision of stress enhanced transistor devices and methods of fabricating the same. In one embodiment, a transistor device comprises: a gate conductor disposed above a semiconductor substrate between a pair of dielectric spacers, wherein the semiconductor substrate comprises a channel region underneath the gate conductor and recessed regions on opposite sides of the channel region, wherein the recessed regions undercut the dielectric spacers to form undercut areas of the channel region; and epitaxial source and drain regions disposed in the recessed regions of the semiconductor substrate and extending laterally underneath the dielectric spacers into the undercut areas of the channel region. 
     In another embodiment, a method of fabricating a transistor device, comprises: providing a semiconductor topography comprising a gate conductor disposed above a semiconductor substrate between a pair of dielectric spacers; anisotropically etching exposed regions of the semiconductor substrate on opposite sides of the dielectric spacers to form recessed regions in the substrate spaced apart by a channel region; selectively etching exposed sidewalls of the channel region to undercut the dielectric spacers; and growing epitaxial source and drain regions in the recessed regions of the semiconductor substrate such that the epitaxial source and drain regions extend underneath the dielectric spacers. 
     In yet another embodiment, a method of fabricating a transistor device, comprising: providing a semiconductor topography comprising a gate conductor disposed above a semiconductor substrate between a pair of dielectric spacers; selectively etching exposed regions of the semiconductor substrate on opposite sides of the dielectric spacers to form recessed regions in the substrate that undercut the dielectric spacers and define a channel region between the recessed regions comprising undercut areas; and growing epitaxial source and drain regions in the recessed regions of the semiconductor substrate such that the epitaxial source and drain regions extend underneath the dielectric spacers into the undercut areas of the channel region. 
     Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the invention with advantages and features, refer to the description and to the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIGS. 1-6  illustrate another example of a method for fabricating a stress enhanced transistor device; and 
         FIGS. 7-12  illustrate another example of a method for fabricating a stress enhanced transistor device. 
     
    
    
     The detailed description explains the preferred embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
     DETAILED DESCRIPTION 
     Stress enhanced FET devices can be fabricated by forming epitaxially grown source and drain regions in recessed regions of a semiconductor substrate that extend laterally underneath the overlying gate structure into undercut areas of the channel region. As such, the epitaxially grown material is strategically placed as close as possible to the channel (even partially underneath the channel) to maximize the stress applied to the channel and thus enhance the carrier mobility in the channel. 
     Turning now to the drawings in greater detail, it will be seen that  FIGS. 1-6  illustrate a first exemplary embodiment of a method for fabricating stress enhanced FET devices. As shown in  FIG. 1 , a bulk semiconductor substrate  10  comprising single crystalline silicon that has been slightly doped with n-type or p-type dopants is first obtained to form the FET device. Alternatively, a semiconductor layer  10  can be formed upon an insulation layer (not shown) to create a silicon-on-insulator FET device. Shallow trench isolation structures  12  can be formed in the semiconductor substrate  10  on opposite sides of the ensuing FET device to isolate it from other active areas in the substrate  10 . A gate dielectric  14  comprising e.g., thermally grown silicon dioxide (SiO 2 ) or hafnium-based oxide (such as HfO 3 ) deposited by chemical vapor deposition (CVD), can be formed across the semiconductor substrate  10 . A gate conductor layer  16  comprising, e.g., polycrystalline silicon (“polysilicon”), can then be deposited across the gate dielectric  60 . Dielectric capping layers, such as silicon dioxide (“oxide”) layer  18  and silicon nitride (“nitride”, Si 3 N 4 ) layer  20 , can then be deposited across the gate conductor layer  16 . 
     Next, the gate conductor layer  16 , the gate dielectric  14 , the oxide layer  18 , and the nitride layer  20  can be patterned using lithography and an anisotropic etch technique, e.g., reactive ion etching (RIE), to form the gate conductor structure shown in  FIG. 2 . Dielectric spacers  22  comprising a dielectric such as nitride can be formed upon the opposed sidewall surfaces of the gate conductor  16  via CVD of a dielectric followed by an RIE process, which etches the dielectric at a faster rate in the vertical direction than in the horizontal direction. 
     Turning now to  FIG. 3 , recessed regions  24  can subsequently be formed in the semiconductor substrate  10  using lithography and an RIE process. The formation of the recessed regions  24  clearly defines the channel region  26 . Next, as shown in  FIG. 4 , ion implantation (illustrated by arrows  28 ) can be used to form etch stop regions  30  in the semiconductor substrate  10  beneath the recessed regions  24 . In one embodiment, p-type dopants can be implanted if the transistor being formed is an NFET device, whereas n-type dopants can be implanted if the transistor being formed is a PFET device. Examples of n-type dopants include, but are not limited to, arsenic, phosphorus, and combinations comprising at least one of the foregoing dopants. Examples of p-type dopants include, but are not limited to, boron, boron difluoride, and combinations comprising at least one of the foregoing dopants. It is to be understood that both NFET and PFET devices can be formed in the semiconductor substrate  10  to form a CMOS (complementary metal-oxide semiconductor) integrated circuit. By way of example, boron (B) can be implanted at a low energy of less than about 10 keV and a dosage of about 2×e 14  ions/cm 2  to about 2×e 15  ions/cm 2 , more specifically about 5×e 14  ions/cm 2  to about 2×e 15  ions/cm 2 . Similarly, boron difluoride (BF 2 ) can be implanted at a low energy of less than about 10 keV and a dosage of about 2×e 14  ions/cm 2  to about 1×e 15  ions/cm 2 , more specifically about 5×e 14  ions/cm 2  to about 1×e 15  ions/cm 2 . In a preferred embodiment, BF 2  is implanted at an energy of about 3 keV and a dosage of about 5×e 14  ions/cm 2 . 
     In an alternative embodiment, electrically inactive species or amorphizing species capable of damaging the crystallinity of the silicon can be implanted into the recessed silicon to form etch stop regions  30 . Examples of electrically inactive species include, but are not limited to, silicon, germanium, carbon, xenon, and combinations comprising at least one of the foregoing species. As an example, xenon can be implanted at an energy of about 5 keV and a dosage of about 5×e 14  ions/cm 2 . 
     As depicted in  FIG. 5 , after the ion implantation step, sidewalls  32  of the channel region  26  can be etched using an isotropic wet etch chemistry that is selective to silicon. For example, the recessed regions  24  of the substrate  10  can be contacted with a hydroxide etchant such as tetramethylammonium hydroxide (TMAH), ammonium hydroxide (NH 4 OH), sodium hydroxide (NaOH), potassium hydroxide (KOH), etc. The doped etch stop regions  30  can inhibit etching of those areas of substrate  10  beneath recessed regions  24 . Further, the oxide and nitride layers  18  and  20  can protect the gate conductor  16  from being etched. As a result of being subjected to an isotropic etch, which etches at the same rate in the vertical and horizontal directions, the sidewalls  32  of the channel region  26  can become indented as shown such that the channel region  26  is substantially shaped as an hourglass. The etch is performed for a period of time effective to cause the recessed regions  24  to undercut the dielectric spacers  22  and thus form undercut areas in the channel region  26 . 
     As shown in  FIG. 6 , epitaxially grown source and drain regions  34  can subsequently be formed in the recessed regions such that they extend laterally under the dielectric spacers  22  into the undercut areas of the channel region  26 . The epitaxial growth can be performed at a temperature of about 500° C. to about 900° C. and a pressure of about 1 Torr to about 100 Torr using precursors such as SiH 4 , SiH 2 Cl 2 , GeH 4 , HCl, B 2 H 6 , SiH 3 CH 3 , etc. In a preferred embodiment, the epitaxial growth is performed at a temperature of about 700° C. and a pressure of about 10 Torr. When forming a PFET device, the epitaxial source and drain regions  34  can comprise, e.g., silicon germanium (SiGe), and when forming an NFET device, the epitaxial source and drain regions  34  can comprise, e.g., silicon carbide (SiC). The nitride and oxide capping layers  18  and  20  can then be removed to allow metal silicide contact areas and then metal contacts to be formed on the gate conductor  16  and the epitaxial source and drain regions  34 . One method that can be employed to remove the capping layers  18  and  20  can be through the use of an isotropic etch that also removes the dielectric spacers  22 , which can be reformed as described previously. On the other hand, the capping layers  18  and  20  can be removed using an RIE process. 
       FIGS. 7-12  illustrate a second exemplary embodiment of a method for fabricating stress enhanced FET devices. As shown in  FIG. 7 , a gate dielectric layer  54 , a gate conductor layer  56 , an oxide capping layer  58 , and a nitride capping layer  60  can be formed upon a semiconductor substrate  50  in the same manner as described in the first embodiment. The shown section of the semiconductor substrate  50  can be isolated from other areas of the substrate  50  by, e.g., trench isolation regions  52 . Next, the gate dielectric layer  54 , the gate conductor layer  56 , and the capping layers  58  and  60  can be patterned using lithography and an anisotropic etch technique to form the gate conductor structure shown in  FIG. 8 . It is recognized that the gate dielectric  54  could alternatively be patterned later during a later stage of the fabrication method. Dielectric spacers  62  can further be formed on the sidewall surfaces of the gate conductor  56  in the same manner as described in the first embodiment. 
     Turning now to  FIG. 9 , a deep ion implantation process (illustrated by arrows  64 ) can be used to form etch stop regions  66  in the semiconductor substrate  50  a spaced distance below the surface of the substrate  50  in the same manner that the etch stop regions are formed in the first embodiment except that a higher implantation energy is employed. That is, p-type species, n-type species, or an electronically inactive species can be implanted in regions of the substrate  50  below where source and drain regions are to be subsequently formed. By way of example, B can be implanted at an implantation energy of about 10 keV to about 100 keV and a dosage of about 2×e 14  ions/cm 2  to about 2×e 15  ions/cm 2 , more specifically about 5×e 14  ions/cm 2  to about 2×e 15  ions/cm 2 . Similarly, BF 2  can be implanted at an energy of about 10 keV to about 100 keV and a dosage of about 2×e 14  ions/cm 2  to about 1×e 15  ions/cm 2 , more specifically about 5×e 14  ions/cm 2  to about 1×e 15  ions/cm 2 . In one particular embodiment, B can be implanted at an energy of about 25 keV and a dosage of about 1×e 15  ions/cm 2 . At this point, the gate dielectric  54  can be removed from above regions of the substrate  50  outside of the dielectric spacers  62  if not previously removed. 
     As illustrated in  FIG. 10 , the exposed surfaces of the substrate  50  can be subjected to an isotropic wet etch selective to silicon to form recessed regions  68 . For example, the substrate  50  can be contacted with a hydroxide etchant such as TMAH, NH 4 OH, NaOH, KOH, etc. As shown in  FIG. 11 , this etch of substrate  50  can be continued for a time effective to extend recessed regions  68  well below the substrate surface and to undercut dielectric spacers  62 , thereby defining a channel region  70  having undercut areas. Due to the isotropic nature of the etch, the sidewalls  72  of the channel region  70  become slanted in an outward direction from the surface of the channel region  70  toward the base of recessed regions  68 . The doped etch stop regions  66  can inhibit etching of those areas of substrate  50  beneath recessed regions  68 , while the oxide and nitride layers  18  and  20  can protect the gate conductor  16  from being etched. 
     As shown in  FIG. 12 , epitaxially grown source and drain regions  74  can then be formed in the recessed regions such that they extend laterally under the dielectric spacers  62  into the undercut areas of the channel region  70 . The epitaxial growth can be performed at a temperature of about 500° C. to about 900° C. and a pressure of about 1 Torr to about 100 Torr using precursors such as SiH 4 , SiH 2 Cl 2 , GeH 4 , HCl, B 2 H 6 , SiH 3 CH 3 , etc. In a preferred embodiment, the epitaxial growth is performed at a temperature of about 700° C. and a pressure of about 10 Torr. When forming a PFET device, the epitaxial source and drain regions  74  can comprise, e.g., SiGe, and when forming an NFET device, the epitaxial source and drain regions  74  can comprise, e.g., SiC. The nitride and oxide capping layers  58  and  60  can then be removed in the same manner as described in the first embodiment to allow metal silicide contact areas and then metal contacts to be formed on the gate conductor  56  and the epitaxial source and drain regions  74 . 
     As used herein, the terms “a” and “an” do not denote a limitation of quantity but rather denote the presence of at least one of the referenced items. Moreover, ranges directed to the same component or property are inclusive of the endpoints given for those ranges (e.g., “about 5 wt % to about 20 wt %,” is inclusive of the endpoints and all intermediate values of the range of about 5 wt % to about 20 wt %). Reference throughout the specification to “one embodiment”, “another embodiment”, “an embodiment”, and so forth means that a particular element (e.g., feature, structure, and/or characteristic) described in connection with the embodiment is included in at least one embodiment described herein, and might or might not be present in other embodiments. In addition, it is to be understood that the described elements may be combined in any suitable manner in the various embodiments. Unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of skill in the art to which this invention belongs. 
     While the preferred embodiment to the invention has been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first described.