A method of forming a semiconductor structure includes providing a semiconductor substrate; forming a gate dielectric over the semiconductor substrate, wherein the semiconductor substrate and a sidewall of the gate dielectric has a joint point; forming a gate electrode over the gate dielectric; forming a mask layer over the semiconductor substrate and the gate electrode, wherein a first portion of the mask layer adjacent the joint point is at least thinner than a second portion of the mask layer away from the joint point; after the step of forming the mask layer, performing a halo/pocket implantation to introduce a halo/pocket impurity into the semiconductor substrate; and removing the mask layer after the halo/pocket implantation.