Multi-step gate structure and method for preparing the same

A method for preparing a transistor having a multi-step gate structure, comprising the steps of:

• forming a semiconductor substrate having a multi-step structure, including:
• forming a mask layer covering a predetermined portion of the semiconductor substrate;
• etching the semiconductor substrate using the mask layer as a first etching mask to form a first depression;
• forming a first spacer on a sidewall of the first depression;
• etching the semiconductor substrate using the mask layer and the first spacer as a second etching mask to form a second depression;
• and
• removing the mask layer and the first spacer;
• performing a thermal oxidation process to form a gate oxide layer covering the entire surface of the multi-step structure;
• forming a conductive layer on the gate oxide layer;
• removing a portion of the conductive layer to form a multi-step gate on the gate oxide layer, wherein the gate oxide layer under the multi-step gate is continuous;
• and
• wherein at least one first implanting process is performed before the forming of the gate oxide layer to implant nitrogen-containing dopants into a portion of the semiconductor substrate under the multi-step structure so as to control the thickness of the gate oxide layer by inhibiting the reaction rate of the thermal oxidation process to form the gate oxide layer.