Semiconductor structure and fabrication method thereof

A method for forming a semiconductor structure, comprising:

• providing a first wafer and a second wafer, wherein a first metal layer is formed in the first wafer and has a top surface exposed, and a second metal layer is formed in the second wafer and has a top surface exposed;
• forming a first material layer on the first wafer, wherein the first material layer and the first metal layer are on a same side of the first wafer, and the first material layer is formed on the first wafer that is outside of the top surface of the first metal layer;
• forming a second material layer on the second wafer, wherein the second material layer and the second metal layer are on a same side of second wafer, the second material layer is formed on the second wafer that is outside of the top surface of the second metal layer;
• performing an alignment process and a bonding process of the first wafer and the second wafer, such that the first material layer and the second material layer are aligned and in contact with each other to provide a first alignment accuracy between the first metal layer and second metal layer;
• applying a first pressure on a backside of the first wafer during the bonding process;
• after the bonding process, performing a heating process on the first material layer and the second material layer, such that the first material layer and the second material layer are melted into each other, outside of the top surfaces of the first and second metal layers respectively, to provide a second alignment accuracy between the first metal layer and second metal layer, wherein the second alignment accuracy is greater than the first alignment accuracy;
• after performing the heating process, performing a second bonding process between the first wafer and the second wafer, such that surfaces of the first metal layer and the second metal layer are in contact;
• and
• applying a second pressure on the backside of the first wafer during the second bonding process, wherein the second pressure is applied in a direction from the backside of the first wafer to a front surface of the first wafer, and the second pressure is greater than the first pressure.