Method of forming semiconductor structure

A method of forming a semiconductor structure, comprising:

• bonding a first wafer to a second wafer, wherein the first wafer comprises a first substrate and a first conductive pad above a first surface of the first substrate, and wherein the second wafer comprises a second substrate and a second conductive pad above a second surface of the second substrate;
• forming a mask layer above the first substrate;
• etching the mask layer and the first substrate to form a first opening in the first substrate, such that the first conductive pad is exposed through the first opening;
• forming a sacrificial spacer in the first substrate at a sidewall of the first opening;
• etching the first conductive pad by using the sacrificial spacer as an etching mask to form a second opening communicated to the first opening, such that the second conductive pad is exposed through the second opening;
• and
• filling a conductive material in the first opening and the second opening to form a conductive structure interconnecting the first and second conductive pads.