Memory device with multi-layer channel and charge trapping layer
A 3-D/vertical non-volatile (NV) memory device such as 3-D NAND flash memory and fabrication method thereof, the NV memory device includes vertical openings disposed in a stack of alternating stack layers of first stack layers and second stack layers over a wafer, a multi-layer dielectric disposed over an inner sidewall of each opening, a first channel layer disposed over the multi-layer dielectric, and a second channel layer disposed over the first channel layer, in which at least one of the first or second channel layers includes polycrystalline germanium or silicon-germanium.
1. A memory device, comprising:
an opening disposed in a stack including first stack layers and second stack layers over a wafer;
a multi-layer dielectric disposed over at least an inner sidewall of the opening;
a first channel layer disposed over the multi-layer dielectric;
a second channel layer including semiconductor material disposed over the first channel layer; and
at least one additional channel layer disposed over the second channel layer,
wherein at least one of the first or second channel layer includes germanium (Ge), and wherein the at least one additional channel layer includes lower Ge concentration than the first or second channel layers.
2. The memory device of claim 1 , wherein the at least one of the first or second channel layers includes a silicon-germanium (Si—Ge) composite layer, and wherein the Si—Ge composite layer includes Ge concentration by number of atoms in an approximate range of 5% to 95%.
3. The memory device of claim 1 , wherein the first and second channel layers include a poly-crystalline structure.
4. The memory device of claim 1 ,
wherein the at least one additional channel layer includes Ge, and wherein stacking of the first, second channel layers, and the at least one additional channel layer is in a direction perpendicular to a channel length.
5. The memory device of claim 1 , further comprising:
a channel interface between the first and second channel layers, wherein the first and second channel layers are deposited in two separated deposition steps and each includes a silicon-germanium composite layer of different Ge concentration.
6. The memory device of claim 1 , wherein the opening is substantially perpendicular to a top surface of the wafer.
7. The memory device of claim 1 , wherein the opening includes a cross-sectional shape selected from a group of: circle, oval, square, diamond, and rectangle.
8. The memory device of claim 1 , wherein the multi-layer dielectric comprises a blocking dielectric layer over at least the inner sidewall of the opening, a charge-trapping layer over the blocking dielectric layer, and a tunnel dielectric layer over the charge-trapping layer, wherein the charge-trapping layer includes a multi-layer structure.
9. The memory device of claim 8 , wherein the multi-layer structure of the charge-trapping layer includes an outer nitride layer, a middle dielectric layer, and an inner nitride layer, wherein at least one of the outer or inner nitride layers includes silicon oxynitride, wherein one of the outer and inner nitride layers is oxygen-rich and the other is silicon-rich, and wherein the middle dielectric layer is mostly charge-trap free.
10. The memory device of claim 1 , wherein:
the first and second stack layers are disposed over one another in an alternating manner to form the stack;
the first stack layers include silicon dioxide (SiO 2 ); and
each of the second stack layers includes a gate layer, wherein the gate layer includes one of a doped polycrystalline silicon (Poly-Si) layer or a tungsten/titanium nitride (W/TiN) composite layer.
11. The memory device of claim 1 , wherein the at least one of the first or second channel layers is positively doped.
12. The memory device of claim 11 , wherein the at least one of the first or second channel layers includes a dopant selected from a group of boron, gallium, or indium, and doped at an approximate concentration range of 1e15 cm −3 to 1e18 cm −3 .
13. The memory device of claim 1 , wherein a thickness ratio between the first and the second channel layers is in an approximate range of 1:5 to 1:0.2.
14. A memory device, comprising:
a stack formed over a wafer, the stack including dielectric layers and gate layers;
a plurality of vertical NAND strings, each formed within a vertical opening formed in the stack and comprising:
a blocking dielectric layer disposed over at least an inner sidewall of the vertical opening;
a multi-layer charge-trapping layer disposed over the blocking dielectric layer;
a tunnel dielectric layer disposed over the multi-layer charge-trapping layer;
a channel including germanium (Ge), wherein the channel further includes an outer channel disposed over the tunnel dielectric layer and an inner channel comprising semiconductor material disposed over the outer channel; and
a selective epitaxial growth (SEG) structure formed at the bottom of the vertical opening, wherein the outer channel is physically isolated from the SEG by the inner channel; and
at least one common source-line (CSL), each formed within a vertical CSL slit formed in the stack,
wherein at least one of the plurality of vertical NAND string is electrically coupled to one of the at least one CSL via the SEG structure.
15. The memory device of claim 14 , wherein the channel includes a multi-layer structure comprising an outer channel disposed over the tunnel dielectric layer and an inner channel disposed over the outer channel, wherein at least one of the outer or inner channel layers includes a polycrystalline silicon-germanium (Si—Ge) composite layer including Ge concentration in a range of 5%-95%.
16. The memory device of claim 14 , wherein the channel includes a multi-layer structure, and wherein at least one of the outer or inner channel layers includes a polycrystalline Ge (Poly-Ge) layer.
17. The memory device of claim 14 , wherein the multi-layer charge-trapping layer includes an outer nitride layer, a middle dielectric layer, and an inner nitride layer, wherein the outer nitride layer is oxygen-lean and includes a majority of charge traps distributed in the multi-layer charge-trapping layer, and wherein the middle dielectric layer includes an oxygen-rich dielectric that is mostly charge traps free.
18. A memory array, comprising:
a stack formed over a wafer including dielectric layers and gate layers;
a plurality of vertical NAND strings, each formed within a vertical opening formed in the stack and comprising:
a multi-layer dielectric disposed over at least an inner sidewall of the vertical opening;
a first channel layer disposed over the multi-layer dielectric;
a second channel layer including semiconductor material disposed over the first channel layer, wherein at least one of the first or second channel layers includes germanium (Ge);
a selective epitaxial growth (SEG) structure formed at the bottom of the vertical opening, the SEG structure is in direct contact with the second channel layer and physically isolated from the first channel layer;
a plurality of word-lines, each coupled to one of the gate layers;
a plurality of bit-lines, wherein each bit-line is coupled to at least one of the plurality of vertical NAND strings at a top end of the first or second channel layers; and
a common source-line (CSL) formed within a CSL slit in the stack, wherein the CSL is coupled to at least one of the plurality of vertical NAND strings at a bottom end of the first or second channel layers via the SEG structure.
19. The memory array of claim 18 , wherein at least one of the first or second channel layers include a silicon-germanium composite layer comprising Ge concentration in a range of 5%-95%.
20. The memory array of claim 18 , wherein Ge concentration of the first channel layer is higher than Ge concentration of the second channel layer.