SRAM Cell LDD Dosage Engineering for Beta Ratio and Speed

The patent applies different LDD dosages to different transistor types within the same SRAM cell. Specifically, the pass-gate transistors receive a first LDD dosage while the pull-down transistors receive a second LDD dosage that is equal to or larger than the first. This local differentiation creates distinct electrical characteristics for each transistor type, enabling the pull-down transistors to have higher current drive capability without affecting the pass-gate transistors, thereby resolving the contradiction between beta ratio and speed.

The patent segments the LDD doping process into two distinct steps: a first LDD doping step for pass-gate transistors and a second LDD doping step for pull-down transistors. This segmentation allows each transistor type to receive the appropriate dosage independently. The first LDD doping establishes a baseline, while the second LDD doping selectively enhances the pull-down transistors' current capability, achieving both manufacturing feasibility and performance optimization.

The patent employs a two-stage LDD doping approach where the first LDD doping is performed as a preliminary action to establish a baseline doping level for all transistors. This preliminary doping ensures uniform initial conditions across the device. The second LDD doping then builds upon this foundation, selectively increasing the dosage for pull-down transistors. This sequential approach allows for better process control and reduces the risk of over-doping, thereby managing manufacturing precision requirements while achieving the desired beta ratio enhancement.