Two-step L-shaped selective epitaxial growth

The present disclosure provides a method of processing a semiconductor device having a stack formed over a source sacrificial layer above a substrate, a channel structure extending vertically through the stack and the source sacrificial layer, a gate line cut trench extending vertically through the stack, and a spacer layer covering uncovered top and side surfaces of the stack. The method can include exposing a lower sidewall of the channel structure by removing the source sacrificial layer, forming a protection layer on all uncovered surfaces, exposing a channel layer of the channel structure by removing a first portion of the protection layer and an insulating layer of the channel structure, forming an initial source connection layer over the exposed channel layer, exposing the substrate by removing a second portion of the protection layer, and forming a source connection layer over the initial source connection layer and the exposed substrate.