A three-dimensional (3D) memory device includes a memory stack including interleaved conductive layers and dielectric layers over a first side of a second semiconductor layer, channel structures extending vertically through the memory stack and into the second semiconductor layer, source contacts in contact with a second side of the second semiconductor layer opposite to the first side; and a backside interconnect layer over the second side of the second semiconductor layer and including interlayer dielectric (ILD) layers and a source line mesh on the ILD layers. The source contacts are distributed on a side of the source line mesh. The source contacts extend through the ILD layers and into the second semiconductor layer.