Three-dimensional ferroelectric random-access memory (FeRAM)
A 3-dimensional vertical memory string array includes high-speed ferroelectric field-effect transistor (FET) cells that are low-cost, low-power, or high-density and suitable for SCM applications. The memory circuits of the present invention provide random-access capabilities. The memory string may be formed above a planar surface of substrate and include a vertical gate electrode extending lengthwise along a vertical direction relative to the planar surface and may include (i) a ferroelectric layer over the gate electrode, (ii) a gate oxide layer; (iii) a channel layer provided over the gate oxide layer; and (iv) conductive semiconductor regions embedded in and isolated from each other by an oxide layer, wherein the gate electrode, the ferroelectric layer, the gate oxide layer, the channel layer and each adjacent pair of semiconductor regions from a storage transistor of the memory string, and wherein the adjacent pair of semiconductor regions serve as source and drain regions of the storage transistor.
1. A fabrication process, comprising:
forming an etch stop layer over a planar surface of a substrate;
forming an oxide layer above the etch stop layer;
forming a plurality of alternating layers of silicon oxide and a first material;
forming a plurality of shafts by etching the alternating layers of silicon oxide and the first material down to the etch stop layer;
forming a conformal channel silicon layer over sidewalls of the shafts;
forming a conformal gate oxide layer over the channel silicon layer;
forming a ferroelectric layer over the gate oxide layer; and
filling the shafts with a conductive material to form a gate electrode.
2. The fabrication process of claim 1 , wherein the first material comprises n + polysilicon.
3. The fabrication process of claim 2 , wherein the n + polysilicon comprises arsenic-doped n + poly silicon forming using a chemical vapor deposition with arsenic or arsenic hydride (A s H 3 ) gases.
4. The fabrication process of claim 2 , wherein the n + polysilicon comprises phosphorus-doped n + poly silicon forming using a chemical vapor deposition with phosphine (PH 3 ) or phosphorus trichloride (PCl 3 ) gases.
5. The fabrication process of claim 1 , wherein the gate oxide layer comprises silicon oxide (SiO 2 ) or silicon oxynitride (SiON).
6. The fabrication process of claim 1 , wherein the channel silicon layer comprises intrinsic polysilicon or boron-doped polysilicon.
7. The fabrication process of claim 6 , wherein the channel silicon layer has a dopant concentration of 1.0×10 16 to 1.0×10 18 cm −3 .
8. The fabrication process of claim 6 , wherein the channel silicon layer is deposited by chemical vapor deposition using any of boron, diborane (H 2 B 2 ), and trimethyl borane (B(CH 3 ) 3 , gases, or any of their combinations.
9. The fabrication process of claim 1 , wherein the ferroelectric layer comprises a HfO 2 ferroelectric material.
10. The fabrication process of claim 9 , wherein the ferroelectric layer is 5.0-30.0 nm thick.
11. The fabrication process of claim 9 , wherein the HfO 2 ferroelectric material is prepared from a precursor selected from group consisting of tetrakis(ethylmethylamino) hafnium (TEMAH), tetrakis(dimethylamino) hafnium (TDMAH) and hafnium tetrachloride (HfCl 4 ).
12. The fabrication process of claim 9 , wherein preparing HfC 2 ferroelectric material uses as oxidant O 3 or H 2 O.
13. The fabrication process of claim 9 , wherein the HfO 2 ferroelectric material is deposited at a deposition temperature between 150-400° C.
14. The fabrication process of claim 9 , wherein the ferroelectric layer comprises a zirconium-doped hafnium silicon oxide.
15. The fabrication process of claim 14 , wherein the zirconium-doped hafnium silicon oxide has a zirconium content of 40-60%, preferably 45-55%.
16. The fabrication process of claim 14 , wherein the zirconium-doped hafnium silicon oxide comprises Hf x Zr 1-x O y ferroelectric thin-films, where x ranges between 0.4 and 0.6, and y ranges between 1.8 and 2.2.
17. The fabrication process of claim 14 , wherein the zirconium-doped hafnium silicon oxide is prepared by depositing HfO 2 and ZrO 2 using an ALD layer-by-layer lamination step.
18. The fabrication process of claim 9 , wherein the ferroelectric layer comprises a silicon-doped hafnium silicon oxide.
19. The fabrication process of claim 18 , wherein the silicon-doped hafnium silicon oxide has a silicon content of 2.0-5.0%.
20. The fabrication process of claim 18 , wherein the silicon-doped hafnium silicon oxide comprises Hf x Si 1-x O y ferroelectric thin-films, where x ranges from 0.02 to 0.05, and y ranges from 1.8 to 2.2.
21. The fabrication process of claim 18 , wherein the silicon-doped hafnium silicon oxide is prepared by depositing HfC 2 and SiO 2 using an ALD layer-by-layer lamination step.
22. The fabrication process of claim 21 wherein the SiO 2 is prepared from precursors selected from group comprising: tetrakis(dimethylamino) silane (4DMAS),
tri s(dimethylamino) silane (3DMAS), tetrakis(ethylmethylamino) silane (TEMA-Si) and silicon tetrachloride (SiCl 4 ).
23. The fabrication process of claim 1 , further comprising:
providing a plurality of global word line conductors embedded in an insulating material between the etch stop layer and the planar surface; and
prior to filling the shafts with the conductive material, removing portions of the etch stop layer at a bottom of the shafts and portions of the insulating material to expose the global word line conductors.
24. The fabrication process of claim 1 , further comprising:
providing a top isolation layer above the shafts filled with the conductive material and forming vias in the top isolation layer to expose a top of the shafts filled with the conductive material; and
depositing the conductive material or another conductive material to fill the vias and to form a plurality of global word line conductors.
25. The fabrication process of claim 1 , further comprising, prior to filling the shafts with the conductive material, depositing a conformal layer of a barrier material over the ferroelectric layer.
26. The fabrication process of claim 25 , wherein the barrier material comprises titanium nitride, tungsten nitride or tantalum nitride.
27. The fabrication process of claim 1 , further comprising:
forming a top isolation layer over the shafts filled with the conductive material;
patterning and etching the top isolation layer and the alternating layers of silicon oxide and the first material to create a plurality of slots, thereby exposing layers of the first material on the sidewalls of the slots;
excavating the first material from the exposed layers of the first material in the slots to create a plurality of cavities; and
filling the cavities with one or more conductive materials to form conductive semiconductor regions and conductive electrodes.
28. The fabrication process of claim 27 , wherein the one or more conductive materials comprise n + polysilicon.
29. The fabrication process of claim 27 , wherein the one or more conductive materials comprise n + polysilicon and the conductive material of the shafts or another conductive material.
30. The fabrication process of claim 27 , wherein the first material comprises silicon nitride and wherein the excavating the first material comprises an etching step using hot phosphoric acid.
31. The fabrication process of claim 27 , wherein excavating the first material from the layers of the first material is incomplete, such that remaining first material electrically insulates the conductive electrodes formed in a same layer of the first material from one another.
32. The fabrication process of claim 1 , wherein forming the channel silicon layer comprises depositing an amorphous intrinsic or lightly doped silicon and annealing the deposited silicon at 850° C.
33. The fabrication process of claim 1 , further comprising providing a protective layer the gate oxide layer to protect the gate oxide layer during any etching step occurring prior to forming the ferroelectric layer.
34. The fabrication process of claim 1 , further comprising forming a charge storage layer between the gate oxide layer and the ferroelectric layer or between the ferroelectric layer and a barrier layer adjacent the gate electrode.