Method of forming twin well

Abstract

A method of forming a twin well includes the steps of: forming a field oxide layer on a semiconductor substrate to define active regions of a device, and forming a first mask which exposes a predetermined active region of the semiconductor substrate; ion-implanting a first conductivity type impurity into the exposed region of the semiconductor substrate using the first mask as an ion implantation mask, to form a first well; ion-implanting a second conductivity type impurity to penetrate the first mask, to form a buried region which is self-aligned with the first well and comes into contact with the bottom of the field oxide layer; removing the first mask, and forming a second mask which is to expose the first well of the semiconductor substrate; and ion-implanting a second conductivity impurity into the exposed region of the semiconductor substrate to levels deeper and shallower than the buried region using the second mask as an ion implantation mask, to form a second well including the buried region. The second well self-aligned with the first well. Damages due to ion implantation is not generated, preventing leakage current at the contact surface between the first well and semiconductor substrate. The surface impurity concentration of the second well is easily controlled, improving reproducibility.