System to improve SER immunity and punchthrough

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.

What is claimed is:  
     
         1 . In a method for fabricating an SRAM device having a standard well tub, the improvement comprising the step of depositing an additional well tub within the standard well tub, where the additional well tub is deposited to a depth that is shallower than the standard well tub.  
     
     
         2 . The method of  claim 1  wherein the SRAM device is 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.  
     
     
         3 . The method of  claim 1  wherein the additional well tub is deposited to a depth of between about 0.1 microns and about 0.5 microns.  
     
     
         4 . The method of  claim 1  wherein the additional well tub is deposited to a concentration of between about 10 17  ions per cubic centimeter and about 10 20  ions per cubic centimeter.  
     
     
         5 . The method of  claim 1  wherein the standard well tub is doped P type and the additional well tub is deposited with Boron at an energy of between about 25 keV and about 190 keV.  
     
     
         6 . The method of  claim 1  wherein the standard well tub is doped N type and the additional well tub is deposited with Phosphorous at an energy of between about 80 keV and about 360 keV.  
     
     
         7 . The method of  claim 1  wherein the additional well tub is deposited with a dose of between about 1e11 ions per square centimeter and about 1e14 ions per square centimeter.  
     
     
         8 . The method of  claim 1  wherein the additional well tub is deposited prior to implanting a threshold voltage layer using an existing threshold voltage adjustment mask set.  
     
     
         9 . An SRAM device having a standard well tub, the improvement comprising an additional well tub within the standard well tub, where the additional well tub is shallower than the standard well tub.  
     
     
         10 . The device of  claim 9  wherein the SRAM device is 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.  
     
     
         11 . The device of  claim 9  wherein the additional well tub is at a depth of between about 0.1 microns and about 0.5 microns.  
     
     
         12 . The device of  claim 9  wherein the additional well tub has a dopant concentration of between about 10 17  ions per cubic centimeter and about 10 20  ions per cubic centimeter.  
     
     
         13 . A method for increasing the SER immunity and isolation punchthrough tolerance of an SRAM device having a standard well tub, comprising the step of doping the device with an additional well tub within the standard well tub, where the additional well tub is deposited to a depth that is shallower than the standard well tub.  
     
     
         14 . The method of  claim 13  wherein the SRAM device is 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.  
     
     
         15 . The method of  claim 13  wherein the additional well tub is deposited to a depth of between about 0.1 microns and about 0.5 microns.  
     
     
         16 . The method of  claim 13  wherein the additional well tub is deposited to a concentration of between about 10 17  ions per cubic centimeter and about 10 20  ions per cubic centimeter.  
     
     
         17 . The method of  claim 13  wherein the standard well tub is doped P type and the additional well tub is deposited with Boron at an energy of between about 25 keV and about 190 keV.  
     
     
         18 . The method of  claim 13  wherein the standard well tub is doped N type and the additional well tub is deposited with Phosphorous at an energy of between about 80 keV and about 360 keV.  
     
     
         19 . The method of  claim 13  wherein the additional well tub is deposited with a dose of between about 10 11  ions per square centimeter and about 10 14  ions per square centimeter.  
     
     
         20 . The method of  claim 13  wherein the additional well tub is deposited prior to implanting a threshold voltage layer using an existing threshold voltage adjustment mask set.