Semiconductor structure and fabrication method thereof

A method of forming a semiconductor structure, comprising:

• providing a bottom wafer comprising a bottom substrate and a bottom interconnect structure on the bottom substrate;
• providing a top wafer having a top substrate with a front surface and a rear surface and a top interconnect structure disposed on the front surface of the top substrate, wherein the top interconnect structure is directly bonded to the bottom interconnect structure of the bottom wafer;
• forming an oxide-nitride-oxide (ONO) dielectric layer covering the rear surface of the top substrate;
• and
• after forming the ONO dielectric layer, forming a plurality of conductive vias on the rear surface and extending into the ONO dielectric layer and the top substrate.