A method for forming a staircase structure (102, 200) of a three-dimensional memory device, comprising: forming a stack structure (201, 402, 602) comprising interleaved first material layers and second material layers; patterning a first photoresist mask in a first region of the stack structure (201, 402, 602); forming a first plurality of divisions (406A-406C) at different depths in a first direction in the first region of the stack structure (201, 402, 602) by a plurality of cycles of trimming the first photoresist mask in the first direction and etching a part of the stack structure (201, 402, 602) uncovered by the first photoresist mask; forming a plurality of first stairs of the first plurality of divisions (406A-406C) in a second direction perpendicular to the first direction in the first region of the stack structure (201, 402, 602), wherein each of the first plurality of stairs comprises a first number of divisions at different depths in the first direction; patterning a second photoresist mask in a second region of the stack structure (201, 402, 602) after forming the plurality of first stairs; forming a second plurality of divisions (416A-416C) at different depths in the first direction in the second region of the stack structure (201, 402, 602) by a plurality of cycles of trimming the second photoresist mask in the first direction and etching another part of the stack structure (201, 402, 602) uncovered by the second photoresist mask; forming a plurality of second stairs of the second plurality of divisions in the second direction in the second region of the stack structure (201, 402, 602); at least one intermediate stair (214, 224, 422, 426, 622, 624) between the first plurality of stairs and the second plurality of stairs in the second direction, wherein the intermediate stair (214, 224, 422, 426, 622, 624) comprises a second number of divisions at different depths in the first direction that is fewer than the first number, wherein the second number is an integer larger than 1, wherein each of said second plurality of stairs comprises a first number of divisions at different depths in the first direction. forming a stack structure (201, 402, 602) comprising interleaved first material layers and second material layers; patterning a first photoresist mask in a first region of the stack structure (201, 402, 602); forming a first plurality of divisions (406A-406C) at different depths in a first direction in the first region of the stack structure (201, 402, 602) by a plurality of cycles of trimming the first photoresist mask in the first direction and etching a part of the stack structure (201, 402, 602) uncovered by the first photoresist mask; forming a plurality of first stairs of the first plurality of divisions (406A-406C) in a second direction perpendicular to the first direction in the first region of the stack structure (201, 402, 602), wherein each of the first plurality of stairs comprises a first number of divisions at different depths in the first direction; patterning a second photoresist mask in a second region of the stack structure (201, 402, 602) after forming the plurality of first stairs; forming a second plurality of divisions (416A-416C) at different depths in the first direction in the second region of the stack structure (201, 402, 602) by a plurality of cycles of trimming the second photoresist mask in the first direction and etching another part of the stack structure (201, 402, 602) uncovered by the second photoresist mask; forming a plurality of second stairs of the second plurality of divisions in the second direction in the second region of the stack structure (201, 402, 602); at least one intermediate stair (214, 224, 422, 426, 622, 624) between the first plurality of stairs and the second plurality of stairs in the second direction, wherein the intermediate stair (214, 224, 422, 426, 622, 624) comprises a second number of divisions at different depths in the first direction that is fewer than the first number, wherein the second number is an integer larger than 1, wherein each of said second plurality of stairs comprises a first number of divisions at different depths in the first direction.