Abstract: A pad oxide layer is formed on a substrate, wherein the thickness of the pad oxide layer is about greater than 250 Å. The alignment photo-resist layer is selectively patterned by a conventional lithography method to define the N-well region. The pad oxide layer is partially etched by using etch method with the alignment photo-resist pattern as a mask until the thickness of the pad oxide layer is about 100 Å to form an alignment mark. The N-type ion-implant is performed by the alignment photo-resist pattern as a mask to form an N-doped region in the substrate. Then, the alignment photo-resist pattern is removed. The P-well photo-resist is defined and formed on the pad oxide layer, then performing a P-type ion-implant through the pad oxide layer into the substrate by means of the P-well photo-resist as a mask to form a P-doped region. Then remove the P-well photo-resist and proceed with the drive-in process to form the N-well region and P-well region.

Abstract: A method for forming a plurality of buried layers inside a semiconductor device is disclosed. The method includes the following steps. Firstly, a semiconductor substrate is provided. Then, the first type p+-type ions are implanted into the semiconductor substrate to form the p+-type region under the surface of semiconductor substrate. The semiconductor substrate is etched to form a plurality of concave portions and a plurality of convex portions using the first photoresist. The n+-type ions are second implanted into the semiconductor substrate as a plurality of n+-type region. Next, the oxide layer is deposited over the surface of the plurality of concave portions and the surface of the plurality of convex portions. The plurality of n+-type regions are heated to form as the buried layers. The oxide layer is removed. Finally, a silicon layer is formed to fill the plurality of concave of portions a silicon layer and to cover the surface of the plurality of convex portions.