Managing isolating structures in semiconductor devices

A semiconductor device, comprising:

• a first stack of conductive layers and isolating layers extending along a first direction and alternating with each other along a second direction perpendicular to the first direction;
• channel structures extending through the first stack along the second direction;
• a gate line slit structure extending through the first stack along the second direction;
• and
• a first isolating structure extending along the first direction, wherein the first isolating structure comprises a first portion in the first stack and a second portion in the gate line slit structure,
• wherein a first portion of the gate line slit structure is above the first isolating structure along the second direction, wherein a size of the first portion of the gate line slit structure is greater than a size of one of the channel structures along the first direction.

A method of forming a semiconductor device, comprising:

• forming a first stack of conductive layers and isolating layers extending along a first direction and alternating with each other along a second direction perpendicular to the first direction;
• forming a gate line slit structure extending through the first stack along the second direction;
• and
• forming an isolating structure extending along the first direction, wherein the isolating structure comprises a first portion in the first stack and a second portion in the gate line slit structure, wherein a size of the first portion is greater than a size of the second portion along the second direction.