Spacer passivation for high aspect ratio etching of multilayer stacks for three dimensional NAND device

1. A method of making a semiconductor device, comprising:

forming a stack of a first set of alternating layers of a first material and a second material, a first etch stop layer, and a second set of alternating layers of the first material and the second material over a substrate, the first set of alternating layers constituting a lower portion of the stack, the second set of alternating layers constituting an upper portion of the stack, and the first etch stop layer being located between the lower portion and the upper portion of the stack and comprising a material different from the first material and the second material;

forming a hard mask over the stack;

patterning the hard mask to form at least one hard mask opening which exposes the stack;

etching the upper portion of the stack through the at least one hard mask opening to form at least one opening extending partially through the upper portion of the stack to the first etch stop layer;

forming a masking layer on a sidewall and bottom surface of the at least one opening;

removing the masking layer from the bottom surface of the at least one opening while leaving the masking layer on the sidewall of the at least one opening; and

further etching the at least one opening to extend the at least one opening through the first etch stop layer and further through the lower portion of the stack while the masking layer remains on the sidewall of the at least one opening.

2. The method of claim 1 , wherein removing the masking layer from the bottom surface of the at least one opening comprises using directional reactive ion etching to remove the masking layer from the bottom surface of the at least one opening.

3. The method of claim 1 , wherein the masking layer comprises a conformal masking layer.

4. The method of claim 3 , wherein the conformal masking layer comprises an amorphous carbon, silicon nitride, silicon oxide, metal oxide or metal nitride masking layer.

5. The method of claim 1 , further comprising removing the masking layer from the sidewall of the at least one opening after the step of further etching.

6. The method of claim 5 , wherein the masking layer comprises an amorphous carbon masking layer and the step of removing the masking layer from the sidewall comprises ashing the amorphous carbon masking layer.

7. The method of claim 5 , wherein the step of removing the masking layer from the sidewall comprises selectively wet etching the masking layer.

8. The method of claim 1 , further comprising:

forming a second etch stop layer under the stack;

wherein forming the masking layer comprises forming the masking layer over an upper surface of the hard mask in addition to forming the masking layer on the sidewall and the bottom surface of the at least one opening; and

wherein further etching comprises further etching the at least one opening to extend the at least one opening to the second etch stop layer while the masking layer remains on the sidewall of the at least one opening.

9. The method of claim 1 , further comprising:

forming a blocking dielectric in the at least one opening;

forming a charge storage layer over the blocking dielectric;

forming a tunnel dielectric over the charge storage layer; and

forming a semiconductor channel over the tunnel dielectric in the at least one opening.

10. The method of claim 9 , wherein:

the semiconductor device comprises a monolithic three dimensional NAND string;

the first material comprises a conductive or semiconductor control gate material; and

the second material comprises an insulating material.

11. A method of making a semiconductor device, comprising:

forming a stack of alternating layers of a first material and a second material over a substrate;

etching the stack to form at least one opening in the stack, the at least one opening comprising a tapered opening having an upper portion which is wider than a lower portion;

forming a masking layer on a sidewall of the upper portion of the at least one opening such that the lower portion of the at least one opening is not covered by the masking layer, and throughout the step of forming the masking layer, the lower portion of the at least one opening comprises physically exposed surfaces of the first material and the second material; and

further etching the lower portion of the at least one opening to widen the lower portion of the at least one opening while the masking layer remains on the sidewall of the upper portion of the at least one opening.

12. The method of claim 11 , wherein:

the step of etching the stack comprises using reactive ion etching; and

the step of further etching comprises wet etching or reactive ion etching the first and second material layers of the stack exposed in the lower portion of the at least one opening while the masking layer protects the first and the second material layers in the upper portion of the opening from the further etching.

13. The method of claim 11 , wherein:

the masking layer comprises a non-conformal sidewall spacer; and

deposition of the non-conformal sidewall spacer is tuned so that its thickness drops from a finite value at the upper portion of the at least one opening to below 1 nm thickness towards the lower portion of the at least one opening.

14. The method of claim 13 , wherein the masking layer comprises an amorphous carbon, silicon nitride, silicon oxide, metal oxide or metal nitride masking layer.

15. The method of claim 11 , further comprising completely removing the masking layer from the sidewall of the at least one opening.

16. The method of claim 15 , wherein the masking layer comprises an amorphous carbon masking layer and the step of removing the masking layer from the sidewall comprises ashing the amorphous carbon masking layer.

17. The method of claim 15 , wherein the step of removing the masking layer from the sidewall comprises selectively wet etching the masking layer.

18. The method of claim 11 , further comprising:

forming an etch stop layer under the stack;

forming a hard mask over the stack; and

patterning the hard mask to form at least one hard mask opening which exposes the stack;

wherein etching the stack comprises etching the stack through the at least one hard mask opening to form the at least one opening extending through the entire stack to the etch stop layer; and

wherein forming the masking layer comprises forming the masking layer over an upper surface of the hard mask in addition to forming the masking layer on the sidewall of the upper portion of the at least one opening.

19. The method of claim 11 , further comprising:

forming a blocking dielectric in the at least one opening;

forming a charge storage layer over the blocking dielectric;

forming a tunnel dielectric over the charge storage layer; and

forming a semiconductor channel over the tunnel dielectric in the at least one opening.

20. The method of claim 19 , wherein:

the semiconductor device comprises a monolithic three dimensional NAND string;

the first material comprises a conductive or semiconductor control gate material; and

the second material comprises an insulating material.

21. The method of claim 11 , further comprising:

forming the masking layer over an upper surface of the stack in addition to forming the masking layer on the sidewall of the upper portion of the at least one opening; and

forming an additional hard mask over a portion of the masking layer located over the upper surface of the stack;

wherein further etching comprises further etching the lower portion of the at least one opening through the additional hard mask to widen the lower portion of the at least one opening.

22. The method of claim 13 , wherein:

the deposition of the non-conformal sidewall spacer is tuned so that its thickness drops from a finite value at the upper portion of the at least one opening to zero between 50% and 75% of a depth of the at least one opening; and

the lower portion of the at least one opening comprises a lower 25 to 50% of the depth of the at least one opening.

23. A method of making a semiconductor device, comprising:

forming a stack of alternating layers of a first material and a second material over a substrate;

etching the stack to form at least one opening in the stack, the at least one opening comprising a tapered opening having an upper portion which is wider than a lower portion;

forming a masking layer on a sidewall of the upper portion of the at least one opening while the lower portion of the at least one opening is not covered by the masking layer;

further etching the lower portion of the at least one opening to widen the lower portion of the at least one opening while the masking layer remains on the sidewall of the upper portion of the at least one opening; and

completely removing the masking layer from the sidewall of the at least one opening.