Semiconductor devices and methods of forming the same include recessing sacrificial layers in a stack of alternating sacrificial layers and channel layers using a first etch to form curved recesses at sidewalls of each sacrificial layer in the stack, with tails of sacrificial material being present at a top and bottom of each curved recess. Dielectric plugs are formed that each partially fill a respective curved recess, leaving exposed at least a portion of each tail of sacrificial material. The tails of sacrificial material are etched back using a second etch to expand the recesses. Inner spacers are formed in the expanded recesses.