The method for manufacturing a memory device is provided. The method includes: implanting a first impurity into the substrate adjacent to the gate conductor structure to form a source region on a first side of the gate conductor structure and a drain region on a second side of the gate conductor structure; implanting a second impurity into the substrate to form a halo implantation region disposed adjacent to the source region, wherein the halo implantation region has a doping concentration which does not degrade a data retention time of the memory device; and performing an annealing process to the drain region, forming a diffusion region under the drain region, wherein the process temperature of the annealing process is controlled to ensure that the diffusion region has a doping concentration substantially equal to a threshold concentration which maintains an electrical connection between the drain and the deep trench capacitor.