Bonded memory device and fabrication methods thereof

A method for forming a bonded semiconductor device, comprising:

• forming a first wafer and a second wafer, the first wafer having a functional layer over a substrate, wherein the functional layer comprises a plurality of memory arrays, and the substrate does not include single-crystalline silicon, wherein forming the first wafer comprises:
• forming an isolation structure and a plurality of array-base regions in the isolation structure comprise:
• forming an insulating material layer over the substrate,
• forming a semiconductor material layer over the insulating material layer,
• patterning the semiconductor material layer to remove portions of the semiconductor material layer, expose the insulating material layer, and form a plurality of array-base regions, and
• depositing a same material as the insulating material layer to fill in space formed by removed portions of the semiconductor material layer, connect with the insulating material layer, and form an isolation structure, the isolation structure insulating the plurality of array-base regions from one another;
• flipping the first wafer to bond onto the second wafer to form the bonded semiconductor device so that the substrate is on top of the functional layer;
• removing at least a portion of the substrate to form a top surface of the bonded semiconductor device;
• and
• forming bonding pads over the top surface.