Multi-wafer stack structure and forming method thereof

A multi-wafer stack structure, comprising:

• a first wafer, wherein the first wafer comprises a first substrate, a first dielectric layer and a first metal layer;
• a second wafer, wherein the second wafer comprises a second substrate, a second dielectric layer and a second metal layer, the first dielectric layer and the second dielectric layer being bonded to each other;
• a first opening, wherein the first opening comprises a first lower opening and a first upper opening;
• the first lower opening penetrating through the first substrate, the first dielectric layer and a portion of the second dielectric layer, and the first lower opening located above the second metal layer and exposing a portion of the second metal layer;
• the first upper opening penetrating through the first substrate and a portion of the first dielectric layer, and the first upper opening exposing a portion of the first metal layer;
• and the first lower opening communicating with the first upper opening;
• a first interconnection layer, wherein the first interconnection layer is electrically connected to the second metal layer and the first metal layer via the first opening;
• an insulating layer, wherein the insulating layer is located on a surface of the first substrate and the first interconnection layer;
• second openings, wherein the second openings penetrate through the insulating layer, the second openings located above the first interconnection layer and exposing a portion of the first interconnection layer, the second openings comprising a plurality of spaced apart pores;
• a third wafer, wherein the third wafer comprises a third substrate, a third dielectric layer and a third metal layer;
• third openings, wherein the third openings penetrate through a portion of the third dielectric layer and exposes a portion of the third metal layer, the third openings comprises a plurality of spaced apart pores, and the plurality of spaced apart pores of the third openings are aligned with the plurality of spaced apart pores of the second opening;
• a second interconnection layer, wherein the second interconnection layer is filled in the plurality of spaced apart pores of the second openings and electrically connected to the first interconnection layer so as to pick up the first interconnection layer via the second interconnection layer;
• and
• a third interconnection layer, wherein the third interconnection layer is filled in the plurality of spaced apart pores of the third openings and electrically connected to the third metal layer so as to pick up the third metal layer via the third interconnection layer;
• the second interconnection layer and the third interconnection layer are aligned and interconnected with each other, such that the first wafer and the third wafer are hybrid bonded to form a single flat interface.