Abstract:
A FET with reduced reverse short channel effects is described, as well as a method to make said FET. Germanium is implanted throughout a semiconductor substrate at an intensity and dose such that a peak ion concentration is created below the source and drain of the FET. The germanium can be implanted prior to gate and source and drain formation, and reduces the reverse short channel effect normally seen in FETs. The short channel effect normally occurring in FETs is not negatively impacted by the germanium implant.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates generally to semiconductor devices and specifically to reverse short channel effects occurring in semiconductor devices.  
           [0002]    The trend of decreasing feature size in semiconductor integrated circuits has led to devices with channel lengths approaching 0.05 microns. As the effective channel length (L eff ) decreases, however, the gate voltage at which the conductivity of the channel is inverted and conduction occurs - the threshold voltage - increases above the theoretically predicted level. FIG. 1 shows this increase in voltage, or reverse short channel effect (RSCE), which is generally an undesirable effect. The dashed line in FIG. 1 represents an ideal channel conductivity behavior in a gated device.  
           [0003]    The tendency towards higher threshold voltages with decreased channel length reverses at some point, and the threshold voltage drops off dramatically. This sudden decrease in threshold voltage is referred to as the short channel effect (SCE). Conventionally, as action is taken to reduce the RSCE, the SCE worsens, which is an undesirable collateral effect.  
           [0004]    The RSCE is generally believed to be caused in n-type metal oxide semiconductor field effect transistors (NMOSFETs) by pile-up of threshold boron at the edge of the source and drain, as well as generally uneven boron distribution across the channel region in FETs with short channels. Supplemental implantation of p-type ions in the channel region has been used to attempt to prevent the RSCE by reducing the impact of boron pile-up in the channel region.  
           [0005]    Another technique that has been employed to reduce the RSCE in FETs is the implantation of germanium into the source and drain regions of the FET. FIG. 2 shows a cross-section of an NFET generally at  10 , in which shallow germanium implants  22  have been incorporated into the source  18  and drain  20  regions. A p-type silicon substrate  12  comprises a gate  14  disposed on a gate oxide  15  and between sidewall spacers  16 . The source  18  and drain  20  diffusions each have shallow germanium implants  22  that are formed to prevent the RSCE.  
           [0006]    Conventional techniques used to reduce the RSCE, however, can require additional processing steps and can cause unwanted collateral effects on the performance of the device. What is needed in the art is a method for fabricating a semiconductor device that does not suffer from RSCEs.  
         BRIEF SUMMARY OF THE INVENTION  
         [0007]    The present invention is a semiconductor device comprising a semiconductor substrate, a first diffusion region disposed in said substrate, a second diffusion region disposed in said substrate, a channel region disposed between said first diffusion region and said second diffusion region, a gate oxide disposed on said semiconductor substrate over said channel region and overlapping said first diffusion region and said second diffusion region, a gate electrode disposed on said gate oxide, and a neutral dopant diffusion implant disposed throughout said substrate, said neutral dopant diffusion implant having a peak concentration below said first diffusion region and said second diffusion region.  
           [0008]    The process for making said device comprises forming an oxide layer on a semiconductor substrate of a first conductivity type, blanket-implanting a neutral dopant into said substrate to form a neutral dopant implant, forming a gate electrode on said oxide layer, and, implanting source and drain regions into said substrate to a depth less than the depth at which a peak concentration of said germanium implant occurs.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The present invention will now be described, by way of example only, with reference to the accompanying drawings which are meant to be exemplary, not limiting, and wherein like elements are numbered alike in several FIGURES, in which:  
         [0010]    [0010]FIG. 1 is a graph showing the reverse short channel effect and the short channel effect;  
         [0011]    [0011]FIG. 2 is a cross-section of FET showing a conventional germanium implant within the source and drain;  
         [0012]    [0012]FIG. 3 is a cross-section of a wafer covered with an oxide layer during germanium doping;  
         [0013]    [0013]FIG. 4 is a cross-section of the wafer of FIG. 3 after doping;  
         [0014]    [0014]FIG. 5 is cross-section of the wafer of FIG. 4 after gate formation and source and drain doping;  
         [0015]    [0015]FIG. 6 is a graph showing the relative dopant concentrations in one embodiment of the FET;  
         [0016]    [0016]FIG. 7 is a cross-section of a FET undergoing germanium implantation with a gate electrode already formed on the oxide layer; and,  
         [0017]    [0017]FIG. 8 is a graph showing the reduction of the RSCE without a consequent worsening of the SCE for a FET with a germanium implant. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]    The semiconductor device described herein has a neutral dopant implant, such as germanium, formed with a peak concentration below the source and drain regions. The germanium implant is preferably implanted before source, drain, and gate formation, but implantation after source, drain, and gate formation is possible. The resulting device, which can be a FET, is not subject to reverse short channel effects, and the implant causes no degradation of short channel effects. Although the figures and the following description disclose the invention in an NFET embodiment for clarity, those skilled in the are will realize that the invention is applicable to other semiconductor devices having gate controlled diffusion regions. For example, a PFET can be formed by reversing the doping polarities from the NFET version.  
         [0019]    Referring now to FIG. 3, an NFET has a p-type silicon region  12  on which an oxide layer  23  has been deposited or grown using conventional methods. The p-type silicon region  12  can be a doped monocrystalline wafer such as would be used in an NFET application, or a well of p-type silicon formed by ion implantation of an n-type silicon as would be used in the NFET portion of a CMOS application. The oxide layer  23  is generally formed with an initial thickness of from about 0.04 to about 0.06 microns, with a thickness of about 0.05 microns preferred. The p-type silicon region  12  can be doped with a p-type dopant such as boron to an initial concentration of 1×10 17  to about 2×10 18  atoms/cm 3 , with about 3×10 17  atoms/cm 3  preferred.  
         [0020]    The neutral dopant implant is preferably implanted with enough energy to form a peak neutral dopant concentration in the wafer below the bottom of the source and the drain diffusion implants which are implanted in a later step. Although any neutral dopant can be used, such as silicon or germanium, germanium is the preferred neutral dopant. In one embodiment, the germanium is implanted to form a peak at a depth of about 0.10 to about 0.50 microns, with a depth of about 0.15 to about 0.30 microns preferred, and a depth of about 0.20 to about 0.25 microns especially preferred. A final peak germanium concentration of about 10 19  cm −3  to about 10 21  cm −3  is preferred, with a concentration of about 10 20  especially preferred. The germanium concentration at the surface of the p-type silicon region  12  is preferably about 10 17  cm −3  to about 10 19  cm −3 , with a concentration of about 10 18  cm −3  especially preferred. The germanium concentration can vary in any manner between the surface of the p-type silicon region  12  and the peak concentration, but a logarithmic variation is preferred (see, for example, FIG. 6). In order to form the germanium implant at the correct depth and concentration, the germanium ions can be implanted, for example, with from about 230 to about 270 keV at about 10 13  cm −2  to about 10 16  cm −2 , and preferably with about 245 to about 255 keV at about 10 14  cm −2  to about 10 15  cm −2 .  
         [0021]    [0021]FIG. 4 shows a cross-section of the NFET after implantation of germanium. The distance “x” represents the distance from the surface of the p-type silicon region to the peak concentration of the germanium implant, as described above. The dashed line  26  represents the germanium implant peak concentration depth. The germanium concentration decreases in both directions from the peak concentration depth  26 . Distance “x” can be any value that results in an appropriate germanium concentration in the source, drain, and channel region of the NFET, and preferably has a value as described above.  
         [0022]    Referring now to FIG. 5, the NFET is shown after gate formation, and source and drain doping. Gate formation is performed using well-known techniques, and the gate  14  can be a polysilicon gate. Prior to gate formation, the oxide layer  23  is patterned and etched to result in a gate oxide with a thickness of about 4 to about 11 nanometers. A polysilicon layer is then formed, patterned, and etched, to result in a polysilicon gate  14  having a thickness of about 100 to about 200 nanometers.  
         [0023]    Once the gate  14  has been formed, the source  18  and the drain  20  diffusion regions can be doped. Ion implantation of the source  18  and drain  20  with an n-type impurity results in an ion concentration of about 10 19  to about 10 21 , with a concentration of about 10 20  preferred. Source  18  and drain  20  regions are preferably implanted to a depth of less than about 0.15 microns, with a depth of less than about 0.10 microns especially preferred. Side wall spacers  16  comprising oxide or nitride can optionally be formed on the sides of the gate  14  in order to form a second implant (not shown) in the source  18  and the drain  20 .  
         [0024]    At this stage, the germanium implant is diffused in the source  18 , drain  20 , and the channel between the source  18  and the drain  20 . Annealing is performed to activate the dopants and restore the crystal structure of the silicon substrate. Annealing can take place between about 600 to about 1200 degrees Celsius. After annealing, NFET fabrication can be completed using conventional metalization and passivation techniques.  
         [0025]    [0025]FIG. 6 shows the ion concentrations for the various regions of the NFET. As described above, and as shown in FIG. 6, the peak germanium implant concentration is preferably formed deeper in the wafer than are the source and drain implants. In the NFET described above, the source and drain implants are an n-type ion, and the wafer ion is a p-type ion. The concentrations and depths shown in FIG. 6 and described above are exemplary, and one skilled in the art will realize that alternative doping concentrations and implantation depths are possible and within the scope of this invention.  
         [0026]    Importantly, the germanium can be implanted at any stage of the NFET fabrication process before source and drain formation. For example, the germanium can be implanted before formation of the oxide layer  23 , or after formation of the gate electrode  14 . FIG. 7 shows the germanium implantation step being performed after the gate  14  has already been formed. In order to effectively implant the germanium under the gate  14  in the channel region, the germanium ions must be implanted at an angle, as shown in FIG. 7. Implantation energy and dose are adjusted to compensate for the angle of implantation. The germanium implant can also be implanted after the source  18  and drain  20  have been formed, and before or after the side wall spacers  16  have been formed, using the implantation technique shown in FIG. 7.  
         [0027]    The blanket implantation of germanium in the p-type substrate inhibits boron pile-up and channel inconsistency, thereby reducing the RSCE by at least 15 percent or more, depending on the device technology. FIG. 8 is a graph that compares the short channel threshold voltage of the germanium implanted NFET of the present invention with a conventional NFET lacking the germanium implant. The NFET with the germanium implant is represented by the dotted line. The reverse short channel effect is reduced to close to an ideal level for the germanium implanted NFET. The short channel effect, however, does not worsen with the germanium implant.  
         [0028]    The NFET described above has the advantage of a significantly reduced reverse short channel effect, without commensurate degradation of the short channel effect or other critical features of the NFET device. The single germanium implantation step allows easy incorporation of the germanium implant in standard NFET and CMOS applications.  
         [0029]    While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration only, and such illustrations and embodiments as have been disclosed herein are not to be construed as limiting to the claims.