US Patent 9536777 Interconnect apparatus and method

A method comprises bonding a first chip on a second chip, depositing a first hard mask layer over a non-bonding side of the first chip, depositing a second hard mask layer over the first hard mask layer, etching a first substrate of the first semiconductor chip using the second hard mask layer as a first etching mask and etching the IMD layers of the first chip and the second chip using the first hard mask layer as a second etching mask.