Nanosheet transistor bottom isolation

1. A method of forming a semiconductor device comprises:

forming a plurality of slanted dielectric structures from a first dielectric material on a substrate with gaps between adjacent slanted dielectric structures, including performing a tilted implantation that converts exposed portions of an original dielectric layer including a second dielectric material to the first dielectric material;

growing a first semiconductor layer from the substrate, using a first semiconductor material, including a lower portion that fills the gaps and an upper portion above the first dielectric material; and

replacing the lower portion of the first semiconductor layer with additional dielectric material.

2. The method of claim 1 , wherein forming the plurality of slanted dielectric structures comprises masking the original dielectric layer with a pattern that exposes strips of a top surface of the original dielectric layer.

3. The method of claim 1 , wherein forming the plurality of slanted dielectric structures further includes etching away remaining second dielectric material to form the gaps.

4. The method of claim 1 , wherein the tilted implantation is performed at an angle of about 45 degrees.

5. The method of claim 1 , wherein the second dielectric material is silicon nitride and the first dielectric material is silicon carbonitride.

6. The method of claim 1 , further comprising forming fins of alternating semiconductor layers on the first semiconductor layer.

7. The method of claim 6 , further comprising forming dielectric spacers on sidewalls of the fins before replacing the lower portion of the first semiconductor layer with the additional dielectric material.

8. The method of claim 1 , wherein at least one threading dislocation results from growing the first semiconductor layer and is blocked by the portions of the second dielectric material, such that the top portion of the first semiconductor layer includes no threading dislocations.

9. The method of claim 1 , wherein at least a portion of a top surface of at least one of the plurality of slanted dielectric structures vertically overlaps with at least a portion of a bottom surface of a respective adjacent slanted dielectric structure.

10. The method of claim 1 , wherein the additional dielectric material is a different material from the second dielectric material.

11. A method of forming a semiconductor device comprises:

masking an original dielectric layer, formed from a first dielectric material, with a pattern that exposes strips of a top surface of the original dielectric layer;

performing a tilted implantation that converts exposed portions of the original dielectric layer to a second dielectric material;

etching away remaining first dielectric material to form gaps between portions of the second dielectric material corresponding to a plurality of slanted dielectric structures;

growing a first semiconductor layer from the substrate, using a first semiconductor material, including a lower portion that fills the gaps and an upper portion above the second dielectric material; and

replacing the lower portion of the first semiconductor layer with additional dielectric material.

12. The method of claim 11 , wherein the tilted implantation is performed at an angle of about 45 degrees.

13. The method of claim 11 , wherein the first dielectric material is silicon nitride and the second dielectric material is silicon carbonitride.

14. The method of claim 11 , wherein at least a portion of a top surface of at least one of the plurality of slanted dielectric structures vertically overlaps with at least a portion of a bottom surface of a respective adjacent slanted dielectric structure.

15. The method of claim 11 , wherein the additional dielectric material is a different material from the first dielectric material.