Memory device containing wrap gate vertical select transistors and method of making thereof
A gate dielectric layer and a gate electrode layer are formed around semiconductor pillars. The gate electrode layer is patterned to remove top portions that protrude above the semiconductor pillars and divided into multiple strips. Each strip constitutes a gate electrode line including a horizontal layer portion and a plurality of surrounding portions that entirely laterally surround respective channel regions of the semiconductor pillars to form wrap gate vertical select field effect transistors. Vertical stacks of memory elements and alternating layer stacks including a vertically alternating sequence of insulating strips and electrically conductive word line strips are formed above the vertical field effect transistors. Vertical bit lines can be formed inside the vertical stacks of memory elements.
1. A memory device, comprising:
a two-dimensional array of vertical field effect transistors located over a substrate and including a two-dimensional array of semiconductor pillars, gate electrode lines laterally extending along a first horizontal direction and spaced apart along a second horizontal direction, and gate dielectrics located between the gate electrode lines and the two-dimensional array of semiconductor pillars, wherein each of the semiconductor pillars comprises a bottom active region, a channel region, and a top active region, and wherein each of the gate electrode lines comprises a plurality of surrounding portions that entirely laterally surround a respective one of the channel regions;
a two-dimensional array of vertical bit lines electrically connected to a respective one of the top active regions;
alternating layer stacks located between neighboring rows of the vertical bit lines, wherein each of the alternating layer stacks comprises a vertically alternating sequence of insulating strips and electrically conductive word line strips; and
a vertical stack of memory elements located between each of the vertical bit lines and alternating layer stacks to provide a three-dimensional memory element array;
wherein:
each of the gate electrode lines further comprises a horizontal layer portion adjoined to a plurality of surrounding portions;
the horizontal layer portion has a same composition, and a same thickness, as the plurality of surrounding portions;
the semiconductor pillars are cylindrical;
the surrounding portions are cylindrical; and
the horizontal layer portion has a width that is greater than a maximum lateral dimension of each of the plurality of cylindrical surrounding portions along the second horizontal direction.
2. The memory device of claim 1 , wherein the semiconductor pillars vertically extend through openings in each horizontal layer portion.
3. The memory device of claim 1 , further comprising an isolation dielectric layer contacting, and laterally surrounding, each bottom active region of the semiconductor pedestals, wherein each of the gate dielectrics contacts a top surface of the isolation dielectric layer, and wherein each of the gate electrode lines overlies the isolation dielectric layer.
4. The memory device of claim 3 , further comprising dielectric spacers laterally extending along the first horizontal direction and laterally surrounding a respective row of semiconductor pillars arranged along the first horizontal direction, wherein sidewalls of the dielectric spacers that extend along the first horizontal direction are vertically coincident with sidewalls of a respective underlying gate electrode line.
5. The memory device of claim 4 , further comprising:
dielectric liners located over the gate electrode lines; and
a dielectric fill material portion including a horizontal portion overlying the dielectric spacers and a plurality of vertical portions that protrude downward from the horizontal portion between each neighboring pair of dielectric spacers.
6. The memory device of claim 1 , further comprising gate contact via structures comprising a conductive material and contacting a top surface of the horizontal layer portions.
7. The memory device of claim 6 , wherein:
each of the gate contact via structures further contacts a sidewall of a respective one of the gate electrode lines; and
each of the gate contact via structures comprises an upper portion having a bottom surface that contacts the top surface of the respective horizontal layer portions and further comprises a lower portion having a sidewall that is vertically coincident with the sidewall of the respective one of the gate electrode lines, wherein the lower portion vertically extends below a horizontal plane including bottom surfaces of the gate electrode lines and contacts a top surface of an underlying conductive structure.
8. The memory device of claim 1 , wherein:
each of the vertical bit lines is located adjacent to a respective continuous resistive memory material layer that vertically extends from a bottommost one of the electrically conductive word line strips to a topmost one of the electrically conductive word line strips; and
portions of each continuous resistive memory material layer located at levels of the electrically conductive word line strips constitute a respective vertical stack of resistive memory elements.
9. The memory device of claim 1 , further comprising a one-dimensional array of global bit lines electrically connected to bottom active regions of a respective row of semiconductor pillars that extends along the second horizontal direction.