PATENT ABSTRACT
A method for fabricating an SRAM device having a standard well tub, where an additional well tub is deposited within the standard well tub. In this manner, the dopant concentration is increased in the well area of the SRAM device, which increases both the isolation punchthrough tolerance and the SER immunity of the device. The additional well tub is deposited to a depth that is shallower than the standard well tub. The additional well tub is deposited using an ion implantation process using the same mask set as that used for the threshold voltage adjustment deposition. Thus, no additional mask layer is required to deposit the additional well tub, and the all of the expenses normally associated with an additional mask layer are avoided.

PATENT DESCRIPTION
FIELD 
     This invention relates to the field of semiconductor wafer processing. More particularly the invention relates to a system for improving the electrical characteristics of static random access memory devices. 
     BACKGROUND 
     As semiconductor device geometries shrink, design engineers encounter new problems that tend to reduce the reliability of the devices. In addition, solutions that were developed to overcome previously identified problems may become ineffectual or create other problems as the geometries shrink and other processing constraints change. Thus, there is a continual need to improve upon the methods and structures relied upon in the past. 
     One issue that is always of high priority is that of maintaining electrical pathway integrity within the device. In other words, ensuring that charge carrier flow is limited to those pathways and those times at which it is desired. For example, it is typically desired to electrically isolate semiconductor devices that are formed adjacent to one another in a semiconducting substrate. This is accomplished in a variety of ways, such as by using locos oxidation or shallow trench isolation techniques. However, as the size of the semiconductor device decreases, the size of these structures must also preferably decrease, which tends to reduce the inherent effectiveness of the isolation structure. This may result in a number of different problems, such as an increase in the soft error rate (SER), where devices become unstable and lose their specified state. Thus, additional systems need to be found to augment the isolation provided by these structures. 
     As a further example, the ability to open and close the current pathway in a semiconductor device such as a MOS transistor is fundamental to the proper operation of the device. Again, however, as device geometries are reduced, the standard structures that were developed for larger devices tend to be less effectual in preventing inadvertent leakage through the isolation region of smaller transistors. Thus, current interwell punchthrough in the isolation tends to become a bigger problem as the devices are made smaller. Here again, additional systems are needed to augment the strength of the isolation region. 
     While there may be many systems that could be devised to alleviate these and other problems, they tend to add complexity, expense and time to the device fabrication process. Typically, these added steps come in the form of additional mask layers that must be developed and used. Thus, the financial pressures inherent in semiconductor device fabrication must also be weighed in finding solutions to these conditions. 
     What is needed, therefore, is a system to improve the soft error rate immunity and isolation punchthrough tolerance of a device, without requiring additional mask layers. 
     SUMMARY 
     The above and other needs are met by a method for fabricating an SRAM device having a standard well tub, where an additional well tub is deposited within the standard well tub. The additional well tub is deposited to a depth that is shallower than the standard well tub. In this manner, the dopant concentration is increased in the well area of the SRAM device, which increases both the isolation punchthrough tolerance and the SER immunity of the device. 
     In a preferred embodiment, the additional well tub is deposited using an ion implantation process to a depth that is shallower than the standard well tub. Further, the SRAM device is preferably isolated from adjacent devices with a shallow trench isolation structure that extends to a depth, and the additional well tub is deposited to a depth that is deeper than the depth of the shallow trench isolation structure. In a most preferred embodiment, the additional well tub is implanted using the same mask set as that used for the threshold voltage adjustment deposition of the SRAM device. Thus, in the preferred embodiment, no additional mask layer is required to deposit the additional well tub, and all of the expenses normally associated with an additional mask layer are avoided. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further advantages of the invention are apparent by reference to the detailed description when considered in conjunction with the figures, which are not to scale so as to more clearly show the details, wherein like reference numbers indicate like elements throughout the several views, and wherein: 
     FIG. 1 is a cross-sectional dopant profile of a standard well tub, 
     FIG. 2 is a cross-sectional dopant profile of a semiconductor device receiving the additional well tub deposition, 
     FIG. 3 is a cross-sectional dopant profile of a semiconductor device after receiving the additional well tub deposition, and 
     FIG. 4 is a dopant profile chart. 
    
    
     DETAILED DESCRIPTION 
     Turning now to the drawings, there is depicted in FIG. 1 a cross-sectional view of a substrate  10  having semiconductor device regions  22 ,  24 ,  26 , and  28  in a wafer  14 . In the depiction of FIG. 1, the devices that will be formed in device regions  22 ,  24 ,  26 , and  28  are only in the very beginning phases of construction. At the phase depicted, isolation structures  16 , such as shallow trench isolation oxide structures, have been formed in the semiconductor material  14 . Wells  18  and  20  have also been formed. In the embodiment depicted, wells  18  represent N doped wells and wells  20  represent P doped wells. A layer of sacrificial oxide  12  has been deposited over the surface of the substrate  14 . 
     In FIG. 2, a layer of photoresist  30  has been deposited on the sacrificial oxide  12 . The photoresist  30  has been cured, exposed, developed, and baked to provide a deposition mask on the substrate  10 . Through the openings in the photoresist  30 , such as the opening that is overlying the semiconductor device region  24 , a dopant  32  is deposited through the sacrificial oxide layer  12  and into the semiconductor material  14 . The dopant material  32  is selected to be of the same type, either P or N, as the well of the device region into which it is deposited. Therefore, in the embodiment depicted in FIG. 2, the dopant  32  is a P type species, because the well  20  is P type. 
     The dopant  32  is preferably deposited to a depth that is greater than the isolation structure  16  and shallower than the well  20 . In this manner, the additional dopant  32  is concentrated within a region  34  of the device  24  where it provides a higher junction capacitance for the device  24 , thus enhancing SER immunity, and also strengthens the channel region that is to be further defined at later stages of the processing, thus enhancing deep channel punchthrough resistance as well as increasing interwell punchthrough resistance. This region  34  is essentially the deposition of an additional well  34 . 
     An additional benefit of this system is that the mask layers and methods used to form the photoresist layer  30  are not in addition to those required for the standard processing of the devices  22 ,  24 ,  26 , and  28 . The reason for this is that an SRAM device, such as device  24 , typically requires a threshold depletion deposition, which uses a mask structure that is identical to that described above and depicted in FIG.  2 . Thus, after the dopant  32  for the additional well  34  has been deposited, the threshold depletion dopant can be deposited. In this manner, only a single deposition step is added to the fabrication process, without any additional mask layer processing required. 
     Further, this system is compatible with the fabrication of SRAM devices  22  and  24  that are formed on the same substrate  10  as logic devices  26  and  28 , which do not typically require a threshold depletion deposition. Thus, the processing used to enhance the electrical characteristics of the. SRAM devices  22  and  24  does not add complexity to the processing of or degrade the performance of the logic devices  26  and  28 . 
     A similar mask layer is used to provide a photoresist mask layer through which to deposit a similar well structure  33  into the SRAM device  22  as depicted in FIG.  3 . As mentioned above, this is the same mask layer through which the threshold adjustment layer for the SRAM device  22  is deposited. Thus, the formation of the additional well  33  does not require any additional masking steps, and likewise does not impact the fabrication or reliability of the logic devices  26  and  28 . Similar to that as described above, because the dopant used for the well  18  is N type, the dopant used for the additional well  33  is also N type. 
     FIG. 3 depicts the devices  22 ,  24 ,  26 , and  28  at a later stage of processing, where the sacrificial oxide layer  12  has been removed, gate structures  40  have been created, and source and drain regions have been deposited with P+ dopant  36  and N+ dopant  38 . FIG. 4 depicts the relative concentration of the various dopants as a function of depth in semiconductor material  14 , as viewed along section  35  in FIG.  3 . Line  42  represents the concentration of the N+ source/drain region  38 , line  44  represents the concentration of the P additional well  34 , and line  46  represents the concentration of the standard P well  20 . Line  48  represents the depth of the bottom of the isolation structure  16 . 
     As seen in FIG. 4, that portion of the additional well  34  at which the concentration of dopant is the greatest as represented by line  44  on the chart of FIG. 4, extends to a depth in the semiconductor material  14  that is greater than the depth of the isolation structure  16  as represented by line  48  on the chart of FIG.  4 . Further, that portion of the additional well  34  at which the concentration of dopant is the greatest, as represented by line  44  on the chart of FIG. 4, extends to a depth in the semiconductor material  14  that is shallower or less than the depth of that portion of the standard well  20  at which the concentration of dopant is the greatest, as represented by line  46  on the chart of FIG.  4 . 
     In a most preferred embodiment, the dopants for the various structures are deposited using an ion implantation process. Thus, for example, the additional P well  34  of SRAM device  24  may be preferably formed by ion implantation of Boron (B 11 ) at an energy of between about 25 keV and about 190 keV and a ion dose of between about 10 11  ions/cm 2  and about 10 14  ions/cm 2 , which implants the species at a nominal depth of between about 0.1 microns and about 0.5 microns. The additional N well  33  of SRAM device  22  may be preferably formed by ion implantation of Phosphorus (P 31 ) at an energy of between about 80 keV and about 360 keV and a ion dose of between about 10 11  ions/cm 2  and about 10 14  ions/cm 2 , which implants the species at a nominal depth of between about 0.1 microns and about 0.5 microns. 
     As is apparent from the foregoing discussion, the dopant type of the additional well tubs  33  or  34  is the same as the dopant type of the corresponding standard well tubs  18  or  20 , respectively. Thus, other dopant species of the corresponding dopant type, whether P or N, may be used in place of those specifically described in the example above. For a standard well tub  18  or  20  having a nominal concentration of about 10 18  ions/cm 3 , the doses described above yield a resultant concentration of between about 10 17  ions/cm 3  and about 10 20  ions/cm 3  within the additional well tubs  33  and  34 . The processes described above are preferably used when the nominal depth of the standard well tubs  18  and  20  is between about 0.2 microns and about 1.0 microns, and the depth of the isolation structures  16  is between about 0.08 microns and about 0.45 microns. For standard well tubs  18  and  20  having different nominal depths and dopant concentrations, and for isolation structures  16  having different depths, the processes described above for the deposition of the additional well tubs  33  and  34  would be adjusted commensurately. 
     The foregoing description of preferred embodiments for this invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiments are chosen and described in an effort to provide the best illustrations of the principles of the invention and its practical application, and to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as is suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.