Vertical thin film transistors in non-volatile storage systems
View Patent ↗Three-dimensional (3D) non-volatile memory arrays having a vertically-oriented thin film transistor (TFT) select device and method of fabricating 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. Each gate is formed over the gate dielectric and a base that extends horizontally at least partially between adjacent pillars. The base is formed with notches filled with the gate dielectric. The select device is fabricated using a conformally deposited base dielectric material and conformal hard mask layer that is formed with a larger bottom thickness than horizontal thickness. The base thickness is defined by the deposition thickness, rather than an uncontrolled etch back.
1. A method of forming non-volatile storage, comprising:
forming over one or more metal layers above a substrate a plurality of pillar stack lines elongated in a first direction, each pillar stack line including a plurality of select device pillars, each select device pillar forming a vertical thin film transistor (TFT) select device;
forming a dielectric base layer having sidewall portions that extend vertically along each of the pillar stack lines and horizontal portions that extend horizontally between each of the pillar stack lines;
forming a hard mask layer over the dielectric base layer, the hard mask layer includes sidewall portions that extend vertically along the sidewall portions of the dielectric base layer and horizontal portions that extend horizontally over the horizontal portions of the dielectric base layer, the hard mask layer having a sidewall thickness that is less than a vertical thickness;
etching back the hard mask layer to form a plurality of caps over the horizontal portions of the dielectric base layer;
etching back the dielectric base layer to remove at least a portion of the sidewall portions and form a plurality of dielectric bases between adjacent pillar stack lines;
forming a gate dielectric layer after forming the plurality of dielectric bases; and
forming and etching back a gate layer to form at least one gate separated from each pillar stack line by the gate dielectric layer.
2. The method of claim 1 , wherein forming the hard mask layer having a sidewall thickness that is less than a vertical thickness includes forming the hard mask layer by plasma vapor deposition.
3. The method of claim 2 , wherein forming the dielectric base layer includes forming the dielectric base layer by atomic layer deposition.
4. The method of claim 1 , wherein etching back the hard mask layer includes removing the sidewall portions to form the plurality of caps.
5. The method of claim 1 , wherein etching back the dielectric base layer forms in each dielectric base a first notch at a first end of the dielectric base and a second notch at a second end of the dielectric base.
6. The method of claim 5 , wherein forming the gate dielectric layer after forming the plurality of dielectric bases includes filling the first notch at the first end of each dielectric base and the second notch at the second end of each dielectric base, the gate dielectric layer extending below a level of an upper surface of the dielectric bases.
7. The method of claim 1 , wherein:
the one or more metal layers include a plurality of global bit lines formed in a first metal layer over the substrate; and
each select device pillar includes a lower source/drain (S/D) region and an upper S/D region separated by a body, the lower S/D region is coupled to a global bit line and the upper S/D region is coupled to a vertical bit line.
8. The method of claim 7 , wherein:
each select device pillar includes an electrode coupled between the lower S/D region and the global bit line.
9. The method of claim 1 , wherein forming and etching back the gate material includes forming a first gate separated from a first vertical sidewall of each pillar stack line by the gate dielectric and a second gate separated from a second vertical sidewall of each pillar stack line by the gate dielectric.
10. A method of forming non-volatile storage, comprising:
forming over one or more metal layers above a substrate a plurality of pillar stack lines elongated in a first direction, each pillar stack line including a plurality of select device pillars separated by an insulating material, each select device pillar forms a vertical thin film transistor (TFT) select device above a substrate;
forming and etching back a dielectric base layer to form a plurality of dielectric bases including a dielectric base between each pillar stack line, wherein etching forms in each dielectric base a first notch at a first end of the dielectric base and a second notch at a second end of the dielectric base;
forming a gate dielectric layer after forming the dielectric base, the gate dielectric layer filling the first notch and the second notch in each trench and extending vertically along a first sidewall and a second sidewall of each pillar stack line; and
forming and etching a gate layer to form a first gate separated from the first vertical sidewall of each pillar stack line by the gate dielectric layer and a second gate separated from the second vertical sidewall of each pillar stack line by the gate dielectric layer.
11. The method of claim 10 , wherein:
forming the dielectric base layer forms sidewall portions along a first vertical sidewall and a second vertical sidewall of each pillar stack line;
forming the dielectric base layer forms horizontal portions between adjacent pillar stack lines; and
the method further comprises, after forming the dielectric base layer and before etching the dielectric base layer, forming a hard mask layer including sidewall portions along the sidewall portions of the dielectric base layer and horizontal portions along the horizontal portions of the dielectric base layer, the sidewall portions of the hard mask layer having a sidewall thickness that is less than a vertical thickness of the horizontal portions of the hard mask layer.
12. The method of claim 11 , further comprising:
etching back the hard mask layer prior to etching the bottom dielectric layer, wherein etching back the hard mask layer removes the sidewall portions of the hard mask layer to expose the sidewall portions of the dielectric base layer while leaving the horizontal portions of the hard mask layer covering the horizontal portions of the dielectric base layer.
13. The method of claim 12 , wherein:
etching back the dielectric base layer removes at least a portion of the sidewall portions of the dielectric base layer to form the plurality of dielectric bases.
14. The method of claim 13 , wherein forming the hard mask layer comprises plasma vapor deposition to generate the sidewall thickness at less than the vertical thickness.
15. The method of claim 10 , wherein each pillar stack forms a vertical thin film transistor (TFT) select device.
16. The method of claim 10 , wherein:
the one or more metal layers include a plurality of global bit lines formed in a first metal layer over the substrate; and
the vertical TFT select device of each select device pillar includes a first S/D region coupled to a global bit line, a second S/D region coupled to a vertical bit line, and a body separating the first S/D region from the second S/D region.