3D NAND memory with socketed floating gate cells
A 3D NAND memory has vertical NAND strings across multiple memory planes above a substrate, with each memory cell of a NAND string residing in a different memory layer. Word lines in each memory plane each has a series of socket components aligned to embed respective floating gates of a group memory cells. In this way, the word line to floating gate capacitive coupling is enhanced thereby allowing a 4 to 8 times reduction in cell dimension as well as reducing floating-gate perturbations between neighboring cells. In one embodiment, each NAND string has source and drain switches that each employs an elongated polysilicon gate with metal strapping to enhance switching. The memory is fabricated by an open-trench process on a multi-layer slab that creates lateral grottoes for forming the socket components.
1. A 3D nonvolatile memory, comprising:
a semiconductor substrate;
a 3D array of memory cells arranged in a three-dimensional pattern on top of said semiconductor substrate;
said 3D array of memory cells being organized into a 2D array of NAND strings aligned vertically relative to the substrate, each NAND string comprising a daisy-chain of vertically stacked memory cells;
corresponding memory cells of said vertically stacked memory cells from said 2D array of NAND strings forming a 2D array of memory cells in each of a corresponding stack of memory planes;
a plurality of word lines in each memory plane for accessing respective groups of memory cells in the memory plane, each word line having a socket component for each memory cell of the group; and
said each memory cell of the group having a floating gate embedded into said socket component of said each word line.
2. The 3D nonvolatile memory as in claim 1 , wherein:
said socket component has a closed end and an open end; and
said floating gate has a long axis in the memory plane and has an embedded end being embedded in the closed end of said socket component and an open end being at the open end of said socket component.
3. The 3D nonvolatile memory as in claim 1 , further comprising:
a dielectric insulating said floating gate from said socket component.
4. The 3D nonvolatile memory as in claim 1 , wherein said floating gate is of doped polysilicon material.
5. The 3D nonvolatile memory as in claim 1 , wherein said plurality of word lines are metallic.
6. The 3D nonvolatile memory as in claim 1 , wherein each NAND string further comprises:
a channel having first and second ends terminated by a source-side transistor on the first end and a drain-side transistor on the second end.
7. The 3D nonvolatile memory as in claim 6 , further comprising:
a tunnel oxide insulating the open end of said floating gate from said channel.
8. The 3D nonvolatile memory as in claim 6 , further comprising:
a tunnel oxide insulating between the open end of said socket component from said channel; and
wherein the open end of said socket component is recessed from the open end of the floating gate such that the tunnel oxide is thicker between the channel and the open end of said socket component than between the channel and the open end of said floating gate.
9. The 3D nonvolatile memory as in claim 6 , wherein said channel of each NAND string is of doped polysilicon material.
10. The 3D nonvolatile memory as in claim 6 , wherein
the drain-side transistor has a control gate; and
the drain-side transistors of a group of NAND strings have associated control gates interconnected by a metal line.
11. The 3D nonvolatile memory as in claim 6 , wherein
the source-side transistor has a control gate; and
the source-side transistors of a group of NAND strings have associated control gates interconnected by a metal line.
12. The 3D nonvolatile memory as in claim 6 , further comprising:
a plurality of global bit lines, each global bit line connected to each respective NAND string via each respective drain-side transistor.
13. The 3D nonvolatile memory as in claim 12 , wherein:
the plurality of global bit lines are metal lines on top of said stack of memory planes.
14. The 3D nonvolatile memory as in claim 1 , wherein individual memory cells each stores more than one bit of data.
15. A 3D nonvolatile memory, comprising:
an array of memory cells arranged in a three-dimensional pattern defined by rectangular coordinates having x-, y- and z-directions;
said array of memory cells being organized into NAND strings aligned vertically in the z-direction to form a 2D array of NAND strings on top of a semiconductor substrate in the x-y plane, each NAND string comprising a daisy-chain of vertically stacked memory cells;
corresponding memory cells of said vertically stacked memory cells from said 2D array of NAND strings forming a 2D array of memory cells in each of a corresponding stack of memory planes in the x-y plane;
a plurality of word lines along the x-direction in each memory plane for accessing respective groups of memory cells in the memory plane, each word line having a socket component for each memory cell of the group; and
said each memory cell of the group having a floating gate embedded into said socket component of said each word line.
16. A 3D nonvolatile memory as in claim 15 , wherein:
said socket component has a closed end and an open end; and
said floating gate has a long axis in the memory plane and has an embedded end being embedded in the closed end of said socket component and an open end being at the open end of said socket component.
17. The 3D nonvolatile memory as in claim 15 , further comprising:
a dielectric insulating said floating gate from said socket component.
18. The 3D nonvolatile memory as in claim 15 , wherein said floating gate is of doped polysilicon material.
19. The 3D nonvolatile memory as in claim 15 , further comprising:
a plurality of global bit lines along the y-direction and spaced apart in the x-direction;
each global bit line connected to each respective NAND string via each respective drain-side transistor.
20. A 3D nonvolatile memory, comprising:
a semiconductor substrate;
a 3D array of memory cells arranged in a three-dimensional pattern on top of said semiconductor substrate;
said 3D array of memory cells being organized into a 2D array of NAND strings aligned vertically relative to the substrate, each NAND string comprising a daisy-chain of vertically stacked memory cells;
a plurality of word lines for accessing respective groups of memory cells, each word line having a socket component for each memory cell of the group; and
said each memory cell of the group having a floating gate embedded into said socket component of said each word line.