| 摘要 |
PURPOSE: A method for forming a twin well is provided which can reduce the size of a device by suppressing the overlapping of a first well and a second well, and also can prevent the generation of a step on the surface of the first well and the second well of a semiconductor substrate. CONSTITUTION: A method for forming a twin well comprises the processes of: forming a first mask layer with a first material on a part of a semiconductor substrate(31)and forming a first side wall with a second material with different etch ratio on a side wall of the first mask layer; forming a first ion implantation region(39)by implanting N-type impurity ion on a revealed part of the semiconductor substrate using the first mask and the first side wall as a mask; removing the side wall selectively and forming a second mask layer(41) with the first material to be self-aligned with the first mask layer on the revealed part of the semiconductor substrate; removing the first mask layer selectively and forming a second side wall(43)with the second material on a side wall of the second mask layer; forming a second well on the semiconductor substrate by implanting P-type impurity ion using the second mask layer and the second side wall as a mask; and forming a first well and the second well by removing the second side wall and the second mask layer and diffusing the impurity ion in the first and second ion implantation region(45). therefore, because the first well and the second well are not overlapped, the size of a device is reduced, and a step is not generated on the surface of the first and second well.
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