Patent Publication Number: US-2013230948-A1

Title: Multiple step implant process for forming source/drain regions on semiconductor devices

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Generally, the present disclosure relates to the manufacturing of sophisticated semiconductor devices, and, more specifically, to a multiple step implantation process to form source/drain regions in semiconductor devices such as transistors. 
     2. Description of the Related Art 
     The fabrication of advanced integrated circuits, such as CPU&#39;s, storage devices, ASIC&#39;s (application specific integrated circuits) and the like, requires the formation of a large number of circuit elements in a given chip area according to a specified circuit layout. Field effect transistors (FETs) represent one important type of circuit element that substantially determines performance of the integrated circuits. Field effect transistors are typically either 
     NMOS devices or PMOS devices. During the fabrication of complex integrated circuits, millions of transistors, e.g., NMOS transistors and/or PMOS transistors, are formed on a substrate including a crystalline semiconductor layer. A field effect transistor, irrespective of whether an NMOS transistor or a PMOS transistor is considered, typically comprises so-called PN junctions that are formed by an interface of highly doped regions, referred to as drain and source regions, with a slightly doped or non-doped region, referred to as a channel region, disposed between the highly doped source/drain regions. The channel length of a MOS transistor is generally considered to be the lateral distance between the source/drain regions. 
     Ion implantation is a technique that is employed in many technical fields to implant dopant ions into a substrate so as to alter the characteristics of the substrate or of a specified portion thereof. The rapid development of advanced devices in the semiconductor industry is based on, among other things, the ability to generate highly complex dopant profiles within tiny regions of a semiconducting substrate by performing advanced implantation techniques through a masking layer. In the case of an illustrative transistor, ion implantation may be used to form various doped regions, such as halo implant regions, extension implant regions and deep source/drain implant regions, etc. 
     An illustrative ion implantation sequence for forming source/drain regions for an illustrative prior art transistor  100  will now be discussed with reference to  FIGS. 1A-1E .  FIG. 1A  depicts the transistor  100  at an early stage of fabrication, wherein a gate structure  14  has been formed above a silicon-on-insulator (SOI) substrate  10  that is comprised of a bulk substrate  10 A, a buried insulation layer  10 B (a so-called BOX layer) and an active layer  10 C where semiconductor devices will be formed. An active region  13  is defined in the active layer  10 C by a shallow trench isolation structure  11 . The gate structure  14  typically includes a gate insulation layer  14 A and a conductive gate electrode  14 B. The gate structure  14  may be formed by forming layers of material that correspond to the gate insulation layer  14 A and the gate electrode  14 B and thereafter patterning those layers of material using known etching and photolithography techniques. 
     The masking layers that would be used during the implantation sequence shown in  FIGS. 1A-1E  are not depicted in the drawings. As shown in  FIG. 1B , an initial ion implantation process  20  is typically performed to form so-called extension implant regions  20 A in the substrate  10 . Typically, the extension implant regions  20 A will be self-aligned with respect to the sidewall of the gate structure  14  (for NMOS devices) or there may be an offset spacer or liner (not shown) formed on the sidewall of the gate structure  14  prior to performing the extension implant process  20  (for a PMOS device). Then, as shown in  FIG. 1C , sidewall spacers  16  are formed proximate the gate structure  14 . The sidewall spacers  16  are typically formed by conformably depositing a layer of spacer material and thereafter performing an anisotropic etching process. Then, as shown in  FIG. 1D , a second ion implantation process  22  is performed on the transistor  100  to form so-called deep source/drain implant regions  22 A in the substrate  10 . The ion implantation process  22  performed to form the deep source/drain implant regions  22 A is typically performed using a higher dopant dose and a higher implant energy than the ion implantation process  20  that is performed to form the extension implant regions  20 A. Thereafter, as shown in  FIG. 1E , a heating or anneal process is performed to form the final source/drain regions  24  for the transistor  100 . This heating process repairs the damage to the lattice structure of the substrate material as a result of the implantation processes and it activates the implanted dopant materials, i.e., the implanted dopant materials are incorporated into the silicon lattice. Of course, the type of dopants implanted, either N-type or P-type dopants, depends upon the type of transistor being made, i.e., an NMOS transistor or a PMOS transistor, respectively. Such implantation processes are performed using well-known ion implantation systems. 
     In some cases, the aforementioned ion implantation sequence produces final source/drain regions  24  that do not “bottom out” on the upper surface  10 S of the buried insulation layer  10 B. This situation is depicted in  FIG. 1E  where the bottom surface  24 S of the final source/drain regions  24  is spaced apart from the upper surface  10 S of the buried insulation layer  10 B. Ideally, all or a substantial portion of the final source/drain regions  24  would contact the upper surface  10 S of the buried insulation layer  10 B in the region between the sidewall spacer  16  and the isolation structure  11 . Bottoming-out of the final source/drain regions  24  is preferred because such a configuration tends to prevent undesirable cross talk between transistors sharing the same active region and such a configuration reduces the geometric ratio of the source/drain regions  24  to the well area, which tends to reduce undesirable leakage currents. 
     The present disclosure is directed to a multiple step implantation process to form source/drain regions in semiconductor devices, such as transistors, that may avoid, or at least reduce, the effects of one or more of the problems identified above. 
     SUMMARY OF THE INVENTION 
     The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an exhaustive overview of the invention. It is not intended to identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is discussed later. 
     Generally, the present disclosure is directed to a multiple step implantation process to form source/drain regions in semiconductor devices, such as transistors. In one illustrative embodiment, the method involves performing an extension implant process to form extension implant regions in a semiconducting substrate comprising a buried insulation layer, after performing the extension implant process, forming a patterned mask layer above the substrate and performing at least two source/drain ion implant processes through the patterned mask layer to form doped source/drain implant regions in the substrate, wherein one of the at least two source/drain ion implant processes is performed with a dopant dose that is less than a dopant dose used in another of the at least two source/drain ion implant processes. In further, more detailed embodiments, one of the at least two source/drain ion implant processes is performed at an implant energy level that is greater than an implant energy level used in another of the at least two source/drain ion implant processes. 
     In another illustrative embodiment, the method involves forming a gate structure above an active layer of a semiconducting substrate that comprises a buried insulation layer, performing an extension implant process to form extension implant regions in the active layer and, after performing the extension implant process, forming a sidewall spacer proximate opposite sides of the gate structure. The method further includes performing at least two source/drain ion implant processes to form doped source/drain implant regions in the substrate that are self-aligned with respect to the sidewall spacer, wherein one of the at least two source/drain ion implant processes is performed with a dopant dose that is less than a dopant dose used in another of the at least two source/drain ion implant processes. In further, more detailed embodiments, one of the at least two source/drain ion implant processes is performed at an implant energy level that is greater than an implant energy level used in another of the at least two source/drain ion implant processes. 
     In yet another example, a method disclosed herein includes performing an extension implant process to form extension implant regions in a semiconducting substrate, after performing the extension implant process, forming a patterned mask layer above the substrate and performing a first source/drain implant process through the patterned mask layer to form first doped source/drain implant regions in the substrate, wherein the first source/drain implant process was performed with a first dopant dose and at a first implant energy level. The method further includes, after performing the first source/drain implant process, performing a second source/drain implant process through the patterned mask layer to form second doped source/drain implant regions in the substrate, wherein the second source/drain implant process was performed with a second dopant dose and at a second implant energy level, and wherein the first dopant dose is less than the second dopant dose and the first implant energy level is greater than the second implant energy level. 
     Another illustrative method disclosed herein includes forming a gate structure above a an active layer of a semiconducting substrate that comprises a buried insulation layer, performing an extension implant process to form extension implant regions in the active layer and, after performing the extension implant process, forming a sidewall spacer proximate opposite sides of the gate structure. In this embodiment, the method also includes performing a first source/drain implant process to form first doped source/drain implant regions in the active layer that are self-aligned with respect to the sidewall spacers, wherein the first source/drain implant process is performed with a first dopant dose and at a first implant energy level and, after performing the first source/drain implant process, performing a second source/drain implant process to form second doped source/drain implant regions in the active layer that are self-aligned with respect to the sidewall spacers, wherein the second source/drain implant process is performed with a second dopant dose and at a second implant energy level, and wherein the first dopant dose is less than the second dopant dose and the first implant energy level is greater than the second implant energy level. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure may be understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements, and in which: 
         FIGS. 1A-1E  depict one illustrative process flow for forming source/drain regions on a prior art transistor device; 
         FIGS. 2A-2F  depict various illustrative methods of a multiple step implantation process to form source/drain regions in semiconductor devices; and 
         FIGS. 3A-3D  reflect various testing data of devices formed using one illustrative embodiment of the multiple step implantation process disclosed herein for forming source/drain regions. 
     
    
    
     While the subject matter disclosed herein is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
     Various illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
     The present subject matter will now be described with reference to the attached figures. Various structures, systems and devices are schematically depicted in the drawings for purposes of explanation only and so as to not obscure the present disclosure with details that are well known to those skilled in the art. Nevertheless, the attached drawings are included to describe and explain illustrative examples of the present disclosure. The words and phrases used herein should be understood and interpreted to have a meaning consistent with the understanding of those words and phrases by those skilled in the relevant art. No special definition of a term or phrase, i.e., a definition that is different from the ordinary and customary meaning as understood by those skilled in the art, is intended to be implied by consistent usage of the term or phrase herein. To the extent that a term or phrase is intended to have a special meaning, i.e., a meaning other than that understood by skilled artisans, such a special definition will be expressly set forth in the specification in a definitional manner that directly and unequivocally provides the special definition for the term or phrase. 
     The present disclosure is directed to a multiple step implantation process to form source/drain regions in semiconductor devices, such as transistors. As will be readily apparent to those skilled in the art upon a complete reading of the present application, the present method is applicable to a variety of devices and technologies, e.g., NMOS, PMOS, CMOS, etc., and is readily applicable to a variety of integrated circuit products, including, but not limited to, ASIC&#39;s, logic devices, memory devices, etc. With reference to the attached drawings, various illustrative embodiments of the methods disclosed herein will now be described in more detail. 
       FIG. 2A  depicts a transistor  200  at an early stage of fabrication, wherein a gate structure  214  has been formed above a silicon-on-insulator (SOI) substrate  210  that is comprised of a bulk substrate  210 A, a buried insulation layer  210 B (a so-called BOX layer) and an active layer  210 C where semiconductor devices will be formed. The substrate  210  may also be made of materials other than silicon. An active region  213  is defined in the active layer  210 C by a shallow trench isolation structure  211 . 
     The gate structure  214  typically includes a gate insulation layer  214 A and a conductive gate electrode  214 B. The gate structure  214  may be formed by forming layers of material that correspond to the gate insulation layer  214 A and the gate electrode  214 B and thereafter patterning those layers of material using known etching and photolithography techniques. For example, various layers of material that correspond to the gate insulation layer  214 A and the gate electrode  214 A (and a gate cap layer (not shown)) may be formed above the substrate  210  by performing one or more deposition and/or thermal growth processes. Thereafter, a patterned masking layer (not shown), such as a photoresist mask, is formed above the various layers of material, and one or more etching processes are performed through the patterned masking layer to define the gate insulation layer  214 A and the gate electrode  214 B. As will be appreciated by one skilled in the art after a complete reading of the present application, the gate structure  214  may be formed using a variety of different materials and by performing a variety of known techniques. For example, the gate insulation layer  214 A may be comprised of a variety of different insulating materials, e.g., silicon dioxide, a so-called high-k insulating material (k value greater than  10 ). The gate electrode  214 B may be comprised of polysilicon or it may contain at least one metal layer. The gate structure  214  of the transistor  200  may be made using so-called “gate first” or “gate last” techniques. That is, the gate structure  214  that is present during the formation of the source/drain regions as described herein may be sacrificial in nature as it may be removed after the final source/drain regions of the device  200  are formed and replaced with a replacement gate structure (not shown), e.g., a high-k gate insulation layer and a gate electrode comprised of at least one metal layer. Thus, the presently disclosed inventions should not be considered as limited to any particular materials of construction for the gate structure  214  nor the manner in which such a gate structure  214  is formed. 
     As shown in  FIG. 2B , a patterned mask layer  219 , e.g., a patterned photoresist mask, is formed above the substrate  210  using known photolithography techniques. Thereafter, an initial extension region ion implantation process  220  is performed to form so-called extension implant regions  220 A in the active layer  210 C of the substrate  210 . In some cases, the extension implant regions  220 A will be self-aligned with respect to the sidewall  214 S of the gate structure  214  (e.g., for an NMOS transistor in certain applications). However, in other cases, there may be an offset spacer or liner (not shown) formed on the sidewall  214 S of the gate structure  214  prior to performing the extension implant process  220 , and the extension implant regions  220 A would be self-aligned with respect to such liner and/or spacer structure. The details of the ion implantation process  220 , such as the material implanted, the implant dose and implant energy, may vary depending on the particular application. In one illustrative embodiment, where the device  200  is an NMOS transistor, the ion implantation process  220  may be a vertical ion implantation process performed using an N-type dopant, e.g., arsenic or phosphorus, at a dopant dose that ranges from about 1e 14 -5e 15  ions/cm 2 , at an energy level that ranges from about 1-10 keV. 
     Then, as shown in  FIG. 2C , the patterned mask layer  219  is removed by performing, for example, an ashing process. Thereafter, after the extension implant regions  220 A are formed, sidewall spacers  216  are formed proximate the gate structure  214 . By use of the word “proximate,” it is meant to cover situation where the sidewall spacers  216  actually contact the sidewall  214 S of the gate structure  214  as well as situations where the sidewall spacer  216  contacts a structure that was formed adjacent to or in contact with the sidewall  214 S of the gate structure  214 , e.g., a liner and/or spacer, prior to the formation of the sidewall spacer  216 . The sidewall spacers  216  are typically formed by conformably depositing a layer of spacer material and thereafter performing an anisotropic etching process. The sidewall spacers  216  may be comprised of a variety of different materials, e.g., silicon nitride, and the base width of the spacers  216  may vary depending upon the particular application, e.g., 10-25 nm. 
     In one broad aspect, the present invention is directed to performing a multiple step implantation sequence, for example, a two-step ion implantation sequence, to form source/drain regions for the transistor device  200  through the same patterned mask layer. The steps may be performed in any order. In one embodiment, the dopant dose in one of the two steps is greater than the dopant dose in the other of the two steps, and the two steps may be performed in any order. In a further, more detailed embodiment, one of the steps is performed using a relatively lower dopant dose, and a relatively higher implant energy as compared to the other implant process. Of course, the implant steps may be performed in any order. Thus, the use of the terms “first” and “second” to describe various implant processes in the description below and in the claims are only for purposes of identifying the particular step, and such terms do not imply any particular order, unless the claim language requires a particular order by the use of appropriate language in the claims. 
     As shown in  FIG. 2D , a patterned mask layer  221 , e.g., a patterned photoresist mask, is formed above the substrate  210  using known photolithography techniques. There-after, as shown in  FIGS. 2D-2E , a multiple step ion implantation sequence is performed through the patterned mask layer  221  to form the deep source/drain implant regions for the device  200 . That is, in the depicted example, two ion implant processes are performed through the same patterned mask layer  221  to form the deep source/drain regions for the device  200 . More specifically, in one illustrative example depicted in  FIG. 2D , a first source/drain ion implantation process  224  is performed to form first source/drain implant regions  224 A in the active layer  210 C of the substrate  210 . Thereafter, in this illustrative example, as shown in  FIG. 2E , a second source/drain ion implantation process  226  is performed through the same patterned mask layer  221  to form second source/drain implant regions  226 A in the active layer  210 C of the substrate  210 . Of course, as noted above, the second ion implant process  226  may be performed prior to the first ion implant process  224  if desired. Typically, the first and second source/drain implant regions  224 A,  226 A will be self-aligned with respect to the sidewall spacer  216 . 
     In one more detailed illustrative example, in relative terms, the first ion implant process  224  may be performed using a relatively lower dopant dose than is used in the second ion implant process  226 , while the implant energy of the first ion implant process  224  may be greater than the implant energy used in the second ion implant process  226 . Stated another way, in one illustrative method, the first source/drain implant regions  224 A are deeper but have a lesser dopant concentration than the second source/drain implant regions  226 A which are shallower but have a relatively higher dopant concentration than do the first source/drain implant regions  224 A. The absolute values of the parameters of the first and second ion implantation processes  224 ,  226 , such as the material implanted, the implant dose and implant energy, may vary depending on the particular application, and may change as technology improves or evolves. In one illustrative embodiment, where the device  200  is an NMOS transistor, the first ion implantation process  224  may be a vertical ion implantation process performed using an N-type dopant, e.g., arsenic or phosphorus, at a dopant dose that ranges from about 1e 13 -3e 15  ions/cm 2 , at an energy level that ranges from about 5-30 keV. In one particularly illustrative embodiment, the ion implant process  224  may be performed using arsenic at a dopant dose of about 5e 14  ions/cm 2  at an energy level of about 20 keV. In one illustrative embodiment, where the device  200  is an NMOS transistor, the second ion implantation process  226  may be a vertical ion implantation process performed using an N-type dopant, e.g., arsenic or phosphorus, at a dopant dose that ranges from about 1e 14 -5e 15  ions/cm 2 , at an energy level that ranges from about 5-20 keV. In one particularly illustrative embodiment, the ion implant process  226  may be performed using arsenic at a dopant dose of about 3e 15  ions/cm 2  at an energy level of about 14 keV. Stated another way, in one illustrative embodiment, the dopant dose during the first ion implantation process  224  is at least about 33% less than the dopant dose used during the second ion implantation process  226 . Additionally, in one illustrative embodiment, the implant energy level during the first ion implantation process  224  may be at least 25-30% greater than the implant energy level during the second ion implantation process  226 . However, in the case where the first implant is performed using arsenic, and the second implant is performed using phosphorus, the implant energy of the first and second implant processes may be about the same. Of course, the type of dopants implanted, either N-type or P-type dopants, depends upon the type of transistor being made, i.e., an NMOS transistor or a PMOS transistor, respectively. The following are parameters for additional illustrative two-step source/drain implant processes that may be performed as described herein.
         (1) first step—arsenic at a dopant dose of about 3e 15  ions/cm 2  that is performed at an energy level of about 14 keV; second step—phosphorous at a dopant dose of about 1e 14  ions/cm 2  that is performed at an energy level of about 12 keV;   (2) first step—phosphorous at a dopant dose of about 1e 14  ions/cm 2  that is performed at an energy level of about 12 keV; second step—arsenic at a dopant dose of about 3e 15  ions/cm 2  that is performed at an energy level of about 14 keV; and   (3) first step—arsenic at a dopant dose of about 3e 15  ions/cm 2  that is performed at an energy level of about 14 keV; second step—arsenic at a dopant dose of about 5e 14  ions/cm 2  that is performed at an energy level of about 20 keV.
 
The various ion implantation processes described herein may be performed using well-known ion implantation systems that are commercially available.
       

     Thereafter, as shown in  FIG. 2F , a heating or anneal process is performed to form the final source/drain regions  230  for the transistor  200 . In one example, this heating process may be a rapid thermal anneal process that is performed at a temperature of about 1000-1100° C. for a duration of a few seconds, e.g., about 1-3 seconds. This heating process repairs the damage to the lattice structure of the substrate material as a result of the implantation processes and it activates the implanted dopant materials, i.e., the implanted dopant materials are incorporated into the silicon lattice. As a result of the novel multiple-step source/drain implant sequence described above, the final source/drain regions  230  tend to “bottom-out” on the upper surface  210 S of the buried insulation layer  210 B as depicted in  FIG. 2F . In the example depicted in  FIG. 2F , the final source/drain regions  230  are depicted as bottoming-out uniformly across the upper surface  210 S of the buried insulation layer  210 B. However, in real-world devices, the bottoming-out of the final source/drain regions  230  may not be as uniform as depicted in  FIG. 2F . By use of the novel multiple-step source/drain implant sequence described herein, at least some of the problems identified in the background section of this application may be at least reduced or perhaps eliminated. 
       FIGS. 3A-3D  reflect various testing that was done by the inventor that confirms the effectiveness and beneficial results obtained by performing the multiple step source/drain implantation sequence described above as compared to the traditional single-step source/drain implant step performed on prior art devices as described in the background section of this application.  FIG. 3A  is a plot of saturation current (x-axis) versus leakage current (y-axis) in log scale for a short channel NMOS transistor device subjected to a single step source/drain implant process (like the prior art) at three different implant energy levels: 20 keV, 18 keV and 14 keV. The implant processes reflected in  FIG. 3A  were performed using arsenic at a dopant dose of about 1e 15 -5e 15  ions/cm 2 . The devices had a silicon dioxide gate insulation layer with a thickness of about 1.2 nm and a polysilicon gate electrode. The devices were formed on an SOI substrate and they were subjected to an anneal process at a temperature of about 1000-1100° C. for a duration of about 1-3 seconds after the implantation processes described herein were performed.  FIG. 3B  is also a plot of saturation current (x-axis) versus leakage current (y-axis) for devices having a low leakage current, e.g., typically devices having a gate insulation layer that is at least about 2.5 nm or thicker, such as may be found in many input/output circuits. As can be seen in  FIG. 3A , as the implant energy was decreased, the saturation current increased. More specifically, the saturation current for the case where the implant energy was about 14 keV was about 6% greater than the saturation current when the implant energy was about 20 keV. However, as shown in  FIG. 3A , the leakage current was relatively high—about 100 nA/μm.  FIG. 3B , which is a plot of devices having a relatively low leakage current, reflects that, for the condition where the implant energy was about 14 keV, the leakage current was higher than the other two conditions (18 keV and 20 keV). Thus, the data in  FIG. 3B  reflects that the source/drain regions formed using a single source/drain implant process at an energy level of about 14 keV did not effectively bottom-out on the buried insulation layer of the SOI substrate, as evidenced by the higher leakage current for this test condition. 
       FIG. 3C  is a plot of saturation current (x-axis) versus leakage current (y-axis) in log scale for an NMOS transistor device subjected to four different implant conditions:
         A—a single step source/drain ion implantation process like the prior art at 14 keV (“w/o co-imp”) using arsenic at a dopant dose of about 3e 15  ions/cm 2 ;   B—a two-step source/drain implant process as described herein with the first step being performed at 20 keV and the second step being performed at 14 keV;   C—a two-step source/drain implant process as described herein with the first step being performed at 25 keV and the second step being performed at 14 keV; and   D—a two-step source/drain implant process as described herein with the first step being performed at 30 keV and the second step being performed at 14 keV.
 
In each of the conditions B, C and D, the first implant step was performed using arsenic at a dopant dose of about 5e 14  ions/cm 2 , and the second implant step was performed using arsenic at a dopant dose of about 3e 15  ions/cm 2 . As shown in  FIG. 3C , condition A—the prior art single step source/drain implant process—reflected a relatively high leakage current for the devices tested. As it relates to the two-step source/drain implant processes tested, condition D (with the highest first implant energy of 30 keV) shows a relatively high leakage current as well and the highest leakage current of all of the two-step source/drain implant processes tested. As it relates to the two-step source/drain implant processes tested, condition B—with the first implant process being performed at about 20 keV and the second implant process being performed at about 14 keV—produced the lowest leakage current thereby indicating that the source/drain regions formed using the condition B process steps were more effectively bottomed-out on the buried insulation layer than source/drain regions formed using the single step source/drain implant process (condition A) of the prior art. The device subjected to implant conditions C produced results that, while better than the prior art condition A, were not as good as those achieved when using the implant processes reflected in condition B.
       

       FIG. 3D  is also a plot of saturation current (x-axis) versus leakage current (y-axis) for an NMOS transistor device, wherein two conditions are reflected:
         A—a single step source/drain ion implantation process like the prior art at 14 keV (“w/o co-imp”) with a dopant does of about 3e 15  ion/cm 2  using arsenic; and   B—a two-step source/drain implant process as described herein with the first step being performed using arsenic at a dopant dose of about 5e 14  ions/cm 2  at an energy level of about 20 keV and the second step being performed using arsenic at a dopant dose of about 3e 15  ions/cm 2  at an energy level of about 14 keV.
 
As shown in  FIG. 3D , the implementation of the two step source/drain implantation process described herein does not reduce the performance of the device as compared to the prior art devices, but it does reduce the leakage current in such devices as described previously.
       

     The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. For example, the process steps set forth above may be performed in a different order. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.