FINFET transistor structure and method for making the same

A method for forming a FINFET transistor structure, comprising:

• providing a substrate and a fin structure disposed on said substrate, wherein said fin structure comprises a mask layer, a buffer layer, a cap layer, a fin conductive layer and a spacer surrounding said mask layer, said buffer layer, said cap layer and said fin conductive layer;
• performing a substrate etching step comprising a vertical etching step and a lateral etching step to form a first recess and a second recess connecting to said first recess in said substrate, wherein said second recess has a protruding side extending to and under said fin structure;
• forming an oxide to fill said first recess and said second recess;
• removing said spacer after said oxide filling up said first recess and said second recess;
• and
• forming a gate structure comprising a gate dielectric layer and a gate material layer, said gate structure partially surrounding said fin structure, wherein said gate dielectric layer is disposed outside of said second recess.