Three-dimensional (3D) NAND memory devices and methods are provided. In one aspect, a fabrication method includes preparing a stacked device having a first array device and a second array device, forming an opening on a back side of the second array device, and forming one or more contact pads in the opening. The first array device includes first front pads on a face side of the first array device and first back pads on a back side of the first array device. The second array device includes second front pads on a face side of the second array device and bonded with the first back pads. The one or more contact pads are disposed at a level proximate to the second front pads with respect to the first array device.