Patent ID: 7265039

Claim:
A method for forming contact plugs on active regions of a semiconductor device, the method comprising: forming a plurality of gate lines on a substrate; implanting first dopants of a first conductivity type into the substrate using the gate lines as a mask to form a plurality of cell junctions, each gate line being provided between two cell junctions; forming a buffer layer over the cell junctions, the buffer layer having a thickness of more than 200 Å; implanting second dopants of the first conductivity type through the buffer layer and into the cells junctions using a first energy level to form a plurality of plug ion-implantation regions of a given depth, the plug ion-implantation regions being configured to receive the contact plugs; implanting the second dopants of the first conductivity type through the buffer layer and into the cell junctions using a second energy level that is different from the first energy level to form the plug ion-implantation regions; forming a well of a second conductivity type within the substrate, wherein the cell junctions and the plug ion-implantation regions are defined within the well; and forming a conductive layer over the plurality of plug ion-implantation regions to form a plurality of contact plugs, wherein the buffer layer is configured to enable a higher implantation energy to be used to implant the second dopants to the given depth, so that a concentration profile of the second dopants has a reduced slope.