Three-dimensional memory device containing bidirectional taper staircases and methods of making the same

A three-dimensional memory device, comprising:

• an alternating stack of insulating layers and electrically conductive layers located over a substrate;
• memory stack structures vertically extending through the alternating stack, wherein each of the memory stack structures comprises a memory film and a vertical semiconductor channel;
• and
• a contact well set that includes four contact wells extending through a respective upper region of the alternating stack,
• wherein the contact well set comprises a rectangular opening extending through the alternating stack and four peripheral sidewalls located at a periphery of the rectangular opening;
• wherein the four contact wells contact a common vertical line extending along a vertical direction that is perpendicular to a top surface of the substrate;
• wherein a bottommost horizontal surface of each contact well of the four contact wells contacts the common vertical line;
• and
• wherein each of the four contact wells contains first stepped surfaces which have a stepwise descending vertical profile along a first horizontal direction and second stepped surfaces which have a stepwise descending vertical profile along a second horizontal direction which is perpendicular to the first horizontal direction.