Double diffused drain metal oxide semiconductor device and manufacturing method thereof

A double diffused drain metal oxide semiconductor (DDDMOS) device formed in a first conductive type substrate, wherein a low voltage device is also formed in the substrate, the DDDMOS device comprising:

• a second conductive type first well, which is formed in the substrate by a process step which also forms a second well in the low voltage device;
• a first gate, which is formed on the substrate, wherein the first gate has a first side and a second side opposite to each other with respect to a channel direction, wherein part of the first well is located below part of the first gate including the first side;
• a first conductive type diffusion region, which is formed in the substrate by a process step which also forms a lightly doped drain in the low voltage device, wherein at least part of the diffusion region is located outside the second side of the first gate, wherein the first conductive type diffusion region and the lightly doped drain in the low voltage device have the same depth;
• and
• a first source and a first drain of first conductive type, which are formed in the substrate outside the first side and the second side of the first gate respectively, wherein the first drain and the first gate are separated by part of the diffusion region, and the first source is located in the first well.