Fabrication method of CMOS device having buried implanted layers for lateral isolation (BILLI)

A method for fabricating a CMOS device with BILLI structure comprising the steps of:

• sequentially forming a first layer and a second layer on a semiconductor substrate of a first conductivity type;
• patterning the first and second layers to expose a region where a MOS transistor of the first conductivity type is to be formed;
• sequentially forming a third layer which is of the same material as the first layer, and a fourth layer which is of the same material as the second layer on the substrate;
• blanket-etching the third and fourth layers to form a spacer made of the third and the fourth layers on the side wall of the first and second layers, thereby forming a mask pattern having a vertical boundary face with a gradual slope;
• and
• ion-implanting impurity ions of the first conductivity type into the substrate to form a buried layer in a form of an island within the substrate, wherein the buried layer is continuous under the vertical boundary face of the mask pattern.