Method of forming high voltage device

A method of forming a high voltage device, wherein a first type semiconductor having at least a gate formed thereon is provided, comprising:

• performing a first ion implantation with a second type dopant to form a first diffusion region in the semiconductor substrate;
• forming an oxide layer on the semiconductor substrate;
• performing a second ion implantation with the second type dopant to form a second diffusion region within the first diffusion region;
• forming a silicon nitride layer on the oxide layer, through which an opening penetrates to expose the oxide layer;
• performing a third ion implantation with the second type dopant, using the silicon nitride layer as a mask to form a third diffusion region within the second diffusion region;
• performing drive-in to deepen the third diffusion region, under a portion of the first diffusion region without deepening either the first diffusion region or the second diffusion region;
• removing the silicon nitride layer;
• transforming the exposed oxide layer into a field oxide layer;
• and
• performing a fourth ion implantation with a first type dopant to form a fourth diffusion region as a source/drain region within the second diffusion region under a surface between the field oxide layer and the gate.