3D flash memory cells which discourage cross-cell electrical tunneling
3-d flash memory cells and methods of manufacture are described. The devices and methods recess a compound floating gate in between the silicon oxide slabs which reduces the quantum probability of electron tunneling between vertically adjacent storage cells. The devices and methods further include a high work function nanocrystalline metal in the compound floating gate. A polysilicon buffer layer forms a portion of the compound floating gate. The polysilicon buffer layer allows the high work function nanocrystalline metal to be selectively deposited. The polysilicon buffer layer further protects the high work function nanocrystalline metal from oxidation with the gate oxide subsequently formed on the other side.
1. A 3-d flash memory cell, the 3-d flash memory cell comprising:
a stack of silicon oxide slabs, wherein a tungsten slab selected from a plurality of tungsten slabs is disposed between each neighboring pair of vertically neighboring silicon oxide slabs in a memory hole of the 3-d flash memory cell, wherein the plurality of tungsten slabs are recessed relative to the silicon oxide slabs away from the memory hole to form a plurality of recesses;
a plurality of compound floating gates each comprising a polysilicon buffer portion and a high work function metal portion, wherein the plurality of compound floating gates are disposed entirely within the plurality of recesses, wherein the stack of silicon oxide slabs extends beyond the plurality of compound floating gates into the memory hole; and
a high-k dielectric liner disposed between the stack of silicon oxide slabs and the plurality of tungsten slabs, wherein the high-k dielectric liner is further disposed between the plurality of compound floating gates and the tungsten slabs, and wherein the high-k dielectric liner comprises aluminum oxide.
2. The 3-d flash memory cell of claim 1 wherein the stack of silicon oxide slabs is vertical.
3. The 3-d flash memory cell of claim 1 wherein the high work function metal portion comprises at least one of platinum, palladium, gold, iridium or ruthenium.
4. The 3-d flash memory cell of claim 1 wherein the high work function metal portion comprises nanocrystals of 30 Å or less.
5. The 3-d flash memory cell of claim 1 wherein the stack of silicon oxide slabs comprises at least fifteen silicon oxide slabs.
6. The 3-d flash memory cell of claim 1 wherein one or more of the silicon oxide slabs is less than 40 nm thick.
7. A 3-d flash memory cell, the 3-d flash memory cell comprising:
a stack of alternating silicon oxide slabs and tungsten slabs comprising a plurality of the silicon oxide slabs, wherein the tungsten slabs are recessed compared to the silicon oxide slabs to form a plurality of recesses between vertically neighboring silicon oxide slabs;
a floating gate disposed entirely within at least one of the plurality of recesses, wherein the floating gate comprises a polysilicon buffer and a high work function metal and wherein the polysilicon buffer is horizontally disposed between the high work function metal and one of the tungsten slabs, wherein neither the polysilicon buffer nor the high work function metal extends horizontally outside the plurality of recesses;
a high-k dielectric liner disposed between the polysilicon buffer and one of the tungsten slabs, wherein the high-k dielectric liner is further disposed between the silicon oxide slabs and the tungsten slab, wherein the high-k dielectric liner comprises an oxygen-containing material in contact with the polysilicon buffer; and
a barrier layer disposed between the high-k dielectric liner and the tungsten slab.
8. The 3-d flash memory cell of claim 7 wherein the plurality of the silicon oxide slabs comprises at least fifteen silicon oxide slabs.
9. The 3-d flash memory cell of claim 7 wherein the stack is vertical.
10. The 3-d flash memory cell of claim 7 further comprising a plurality of floating gates each disposed between each neighboring pair of the silicon oxide slabs.
11. The 3-d flash memory cell of claim 7 wherein the high work function metal comprises a nanocrystalline high work function metal.
12. The 3-d flash memory cell of claim 7 wherein each of the silicon oxide slabs is less than 50 nm thick.
13. The 3-d flash memory cell of claim 7 wherein a thermionic work function of the high work function metal is greater than 5.0 eV.
14. The 3-d flash memory cell of claim 1 , wherein the high work function metal consists of platinum, palladium, gold, iridium, or ruthenium.
15. The 3-d flash memory cell of claim 1 , wherein the high-k dielectric liner comprises a tri-layer of material, and wherein the aluminum oxide is the middle layer.
16. The 3-d flash memory cell of claim 15 , wherein the aluminum oxide is positioned between layers of silicon oxide or layers of hafnium oxide.
17. The 3-d flash memory cell of claim 7 , wherein the barrier layer comprises titanium nitride.
18. The 3-d flash memory cell of claim 7 , wherein the polysilicon buffer is in contact with a first surface of the high work function metal, and wherein a second surface of the high work function metal opposite the first is in contact with a silicon nitride material.
19. The 3-d flash memory cell of claim 18 , wherein the silicon nitride material is in contact with the high work function metal on a first surface of the silicon nitride, and wherein a second surface of the silicon nitride material is in contact with a silicon oxide material.
20. The 3-d flash memory cell of claim 7 , wherein the high-k dielectric liner comprises aluminum oxide.