Vertical thin film transistor selection devices and methods of fabrication
View Patent ↗Three-dimensional (3D) non-volatile memory arrays having a vertically-oriented thin film transistor (TFT) select device and method of fabricating such a memory are described. The vertically-oriented TFT may be used as a vertical bit line selection device to couple a global bit line to a vertical bit line. A select device pillar includes a body and upper and lower source/drain regions. At least one gate is separated horizontally from the select device pillar by a gate dielectric. Beneath each gate, a single gap fill dielectric layer extends vertically from a lower surface of the gate, at least partially separating the gate from the underlying global bit line. Between horizontally adjacent pillars, this same dielectric layer extends from its same lower level beneath the gates vertically to a level of the upper source/drain region.
1. A method of forming a vertical thin film transistor (TFT) select device, comprising:
forming a first vertical TFT select device including a body, a first source/drain (S/D) region and a second S/D region over a substrate, the first S/D region coupled to a first vertical bit line, the second S/D region coupled to a global bit line;
forming a second vertical TFT select device including a body, a first source/drain (S/D) region and a second S/D region over a substrate, the first S/D region coupled to a second vertical bit line, the second S/D region coupled to the global bit line;
forming a gate dielectric after forming the body, the first S/D region, and the second S/D region of the first vertical TFT select device and the second vertical TFT select device;
etching to form a first gate for the first vertical TFT select device and a second gate for the second vertical TFT select device after forming the gate dielectric, the first gate is separated from the body of the first vertical TFT select device by the gate dielectric and has a lower surface that is separated from the global bit line at least partially by a void, the second gate is separated from the body of the second vertical TFT select device by the gate dielectric and has a lower surface that is separated from the global bit line at least partially by a void; and
forming a gap fill dielectric after forming the gate, the gap fill dielectric being formed in the void such that the lower surface of the gate is separated from the global bit line by the gap fill dielectric;
wherein after etching to form the first gate and the second gate, the gate dielectric extends continuously in a horizontal direction over a portion of the global bit line between the second S/D region of the first TFT select device and the second S/D region of the second TFT select device.
2. The method of claim 1 , wherein:
the gate dielectric extends vertically along a vertical sidewall of the body, the first S/D region, and the second S/D region; and
the lower surface of the gate is separated from the global bit line by the gap fill dielectric and the gate dielectric.
3. The method of claim 2 , wherein forming the gate comprises:
depositing a gate material over the gate dielectric;
depositing a sacrificial material over the gate material;
recessing the gate material and the sacrificial material to remove a portion of the sacrificial material and a portion of the gate material that overlie the gate dielectric where it extends horizontally, wherein recessing the gate material forms the gate; and
removing a lower portion of the gate material that is exposed as a result of recessing the gate material and the sacrificial material, wherein removing the lower portion of the gate material forms the void.
4. The method of claim 3 , further comprising:
removing the sacrificial material after removing the lower portion of the gate material; and
forming a liner material after removing the sacrificial material, the liner material being formed along vertical sidewalls of the gate and over an upper surface of a portion of the gate dielectric that extends horizontally.
5. The method of claim 4 , wherein forming the gap fill dielectric comprises:
depositing the gap fill dielectric using flowable chemical vapor deposition.
6. The method of claim 1 , wherein:
the lower surface of the gate is separated from the global bit line by the void, a liner, and the portion of the gate dielectric formed over the global bit line.
7. The method of claim 1 , wherein:
forming the gap fill dielectric includes at least partially filling a space between the gate of the vertical select device and a gate of an adjacent vertical TFT select device.
8. The method of claim 7 , wherein the gap fill dielectric extends vertically from above a horizontal portion of the gate dielectric that overlies the global bit line to at least a level of an upper surface of the body.
9. The method of claim 1 , wherein the first S/D region is coupled to the vertical bit line through at least one additional layer.
10. The method of claim 1 , wherein:
the gate is a first spacer gate that is formed horizontally adjacent to a first vertical sidewall of the body; and
the vertical TFT select device includes a second spacer gate that is formed horizontally adjacent to a second vertical sidewall of the body.
11. A method of forming a vertical thin film transistor (TFT) select device, comprising:
forming a select gate pillar above a substrate, the select gate pillar including a body, a first source/drain and a second source/drain, the first source/drain coupled to an upper surface of the body and a vertical bit line, the second source/drain coupled to a lower surface of the body and a global bit line, the select gate pillar including first and second vertical sidewalls elongated in a first direction;
forming a gate dielectric along the first and the second vertical sidewalls and over an upper surface of the select gate pillar;
forming a gate material separated from the select gate pillar by the gate dielectric;
forming a sacrificial material along vertical sidewalls of the gate material and an upper surface of the gate material;
recessing the gate material and sacrificial material to form a first gate that is separated from the first vertical sidewall of the select gate pillar by the gate dielectric and a second gate that is separated from the second vertical sidewall of the select gate pillar by the gate dielectric, wherein recessing the sacrificial material exposes a vertical sidewall of a lower portion of the first gate and a vertical sidewall of a lower portion of the second gate;
removing the lower portion of the first gate and the lower portion of the second gate; and
after removing the lower portion of the first gate and the lower portion of the second gate, forming a gap fill dielectric, the gap fill dielectric underlying a lower surface of the first gate and the second gate.
12. The method of claim 11 , wherein the select gate pillar is a first select gate pillar, the method further comprising:
forming a pillar stack line including the first select gate pillar and a plurality of additional select gate pillars, each additional select gate pillar being coupled to one of a plurality of additional global bit lines.
13. The method of claim 11 , wherein:
removing the lower portion of the first gate defines a first void between a lower surface of the first gate and the global bit line;
removing the lower portion of the second gate defines a second void between a lower surface of the second gate and the global bit line; and
forming the gap fill dielectric includes filling the first void and the second void.
14. The method of claim 11 , further comprising:
forming a three-dimensional memory array of memory cells above a substrate;
forming a plurality of word lines coupled to the memory cells;
forming a plurality of global bit lines;
forming a plurality of vertical bit lines coupled to the memory cells; and
forming a plurality of vertical thin film transistor (TFT) select devices above the substrate, the vertical TFT select devices are coupled between the vertical bit lines and the global bit lines.