Semiconductor structure and method of forming the same

A method of forming a semiconductor structure, comprising:

• forming a mask layer on a substrate, wherein the mask layer and the substrate comprise an opening;
• forming an isolation structure in the opening;
• removing the mask layer;
• forming a first conductive layer on the isolation structure and the substrate;
• performing a first implantation process on the first conductive layer and the isolation structure, to form a doped portion in the first conductive layer and a doped portion in the isolation structure;
• forming a second conductive layer on the first conductive layer and the isolation structure;
• performing a first planarization process, so that top surfaces of the second conductive layer, the first conductive layer, and the isolation structure are aligned;
• and
• removing the doped portion of the first conductive layer before the formation of the second conductive layer, to expose the doped portion of the isolation structure, wherein the doped portion of the isolation structure is higher than a topmost surface of a remaining portion of the first conductive layer.