Three-dimensional ferroelectric random-access memory (FeRAM)
View Patent ↗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 memory string formed above a planar surface of a substrate, comprising:
a gate electrode extending lengthwise along a first direction that is orthogonal to the planar surface of the substrate,
a ferroelectric layer provided over at least a portion of the gate electrode along a second direction orthogonal to the first direction and extending lengthwise along the first direction;
a gate oxide layer provided over at least a portion of the ferroelectric layer along the second direction and extending lengthwise along the first direction;
a channel layer provided over at least a portion of the gate oxide layer along the second direction and extending lengthwise along the first direction; and
a plurality of conductive semiconductor regions embedded in and isolated from each other by an oxide layer the conductive semiconductor regions being arrayed along the first direction, wherein the gate electrode, the ferroelectric layer, the channel layer, the gate oxide layer and each adjacent pair of semiconductor regions form 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.
2. The memory string of claim 1 , further comprising a barrier layer provided between the gate electrode and the ferroelectric layer.
3. The memory string of claim 2 , wherein the barrier layer comprises titanium nitride, tungsten nitride or tantalum nitride.
4. The memory string of claim 1 , wherein the gate electrode comprises tungsten or a heavily doped semiconductor.
5. The memory string of claim 1 , further comprising a plurality of drain or source electrodes, each drain or source electrode being in contact with one of the source regions or one of the drain regions of the storage transistors in the memory string.
6. The memory string of claim 5 , wherein the drain or source electrodes each comprise tungsten or n + polysilicon.
7. The memory string of claim 5 , wherein the memory string is one of a plurality of memory strings in a memory array, wherein the memory array comprises a staircase configuration providing electrical contacts to each of the source or drain electrodes.
8. The memory string of claim 5 , wherein the drain or source electrodes comprise two groups provided on opposite sides of the shaft, and wherein the source or drain electrodes of one group are electrically insulated from source or drain electrodes of the other group.
9. The memory string of claim 5 , wherein the memory string is one of a plurality of memory strings in a memory array, wherein the memory array comprises a network of global word line conductors each connecting the gate electrodes of a selected group of the memory strings.
10. The memory string of claim 9 , wherein the network of global word line conductors is provided above the memory strings.
11. The memory string of claim 9 , wherein the network of global word line conductors is provided between the memory strings and the planar surface.
12. The memory string of claim 1 , wherein the ferroelectric layer comprises a HfO 2 ferroelectric material.
13. The memory string of claim 12 , wherein the ferroelectric layer is 5.0-30.0 nm thick, preferably 8.0-20.0 nm thick.
14. The memory string of claim 12 , wherein the ferroelectric layer comprises a zirconium-doped hafnium silicon oxide.
15. The memory string of claim 14 , wherein the zirconium-doped hafnium silicon oxide has a zirconium content of 40-60%, preferably 45-55%.
16. The memory string 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, preferably between 0.45 and 0.55, and y ranges between 1.8 and 2.2, preferably between 1.9 to 2.1.
17. The memory string 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 memory string of claim 12 , wherein the ferroelectric layer comprises a silicon-doped hafnium silicon oxide.
19. The memory string of claim 18 , wherein the silicon-doped hafnium silicon oxide has a silicon content of 2.0-5.0%, preferably 2.5-4.5%.
20. The memory string 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, preferably between 0.025 and 0.04, and y ranges from 1.8 to 2.2, preferably between 1.9 and 2.1.
21. The memory string of claim 18 , wherein the silicon-doped hafnium silicon oxide is prepared by depositing HfO 2 and SiO 2 using an ALD layer-by-layer lamination step.
22. The memory string of claim 1 , further comprising a charge-trapping layer between the gate oxide layer and the ferroelectric layer or between the ferroelectric layer and a barrier layer adjacent the gate electrode.