LDMOS transistor device and method for forming the same

A method for forming an LDMOS transistor device, comprising:

• receiving a substrate of a first conductivity type;
• performing a first ion implantation on the substrate, wherein the first ion implantation comprises ions of a second conductivity type complementary to the first conductivity type;
• performing a second ion implantation on the substrate after the first ion implantation;
• performing a first thermal operation to form a well region in the substrate;
• performing a second thermal operation to simultaneously form a first dielectric layer and a second dielectric layer over the substrate, wherein a thickness of the first dielectric layer is less than a thickness of the second dielectric layer;
• and
• forming a gate electrode over the first dielectric layer and a field plate over the second dielectric layer,
• wherein a sidewall of the field plate is aligned with a sidewall of the second dielectric layer.

An LDMOS transistor device, comprising:

• a stepped isolation structure disposed over a substrate, wherein the stepped isolation structure comprises:
• a first portion having a first thickness;
• a second portion having a second thickness greater than the first thickness;
• and
• a third portion having a third thickness greater than the first thickness and less than the second thickness,
• a gate electrode disposed over the first portion of the stepped isolation structure;
• a source region disposed in the substrate;
• a drain region disposed in the substrate and adjacent to the stepped isolation structure;
• and
• a spacer disposed over the third portion of the stepped isolation structure,
• wherein the second portion comprises dopants.