Three-dimensional memory device with replacement select gate electrodes and methods of manufacturing the same

A three-dimensional memory device comprising:

• an alternating stack of insulating layers and electrically conductive layer, wherein the electrically conductive layers comprise word-line-level electrically conductive layers and a plurality of drain-select-level electrically conductive layers located over the word-line-level electrically conductive layers;
• memory openings vertically extending through the alternating stack;
• memory opening fill structures located within a respective one of the memory openings, wherein each of the memory opening fill structures comprises a memory film and a vertical semiconductor channel;
• a pair of backside trench fill structures vertically extending through each layer within the alternating stack and having a respective pair of lengthwise sidewalls that laterally extend along a first horizontal direction, wherein a width of each of the backside trench fill structures is the same at levels of the drain-select-level electrically conductive layers and at levels of the word-line-level electrically conductive layers;
• and
• drain-select-level dielectric spacers in contact with the drain-select-level electrically conductive layers, overlying a horizontal plane including a top surface of a topmost one of the word-line-level electrically conductive layers, and in contact with a sidewall of a respective one of the backside trench fill structures.