Semiconductor structure and method for forming the same
A semiconductor structure includes a substrate, a multi-gate FET device disposed over the substrate, a first isolation disposed in the substrate, and a second isolation disposed in the substrate. The multi-gate FET device includes a gate structure and epitaxial source/drain structures disposed at two sides of the gate structure. The first isolation includes a first portion and a second portion over the first portion. A top surface of the second portion is aligned with a top surface of the epitaxial source/drain structures. A width of the second portion is different from a width of the first portion.
1 . A method for forming a semiconductor structure, comprising:
forming a nanosheet stack over a substrate, wherein the nanosheet stack comprises a plurality of nanosheets;
forming a sacrificial gate structure over the nanosheet stack;
forming epitaxial source/drain structures at two sides of the sacrificial gate structure;
removing a portion of the sacrificial gate structure to form a first recess exposing the nanosheet stack after the forming of the epitaxial source/drain structures;
removing portions of the nanosheet stack to form a second recess coupled to the first recess;
forming spacers on sidewalls of the first recess and sidewalls of the second recess after the forming of the epitaxial source/drain structures;
removing a portion of the substrate to form a third recess coupled to the second recess; and
filling the third recess, the second recess and the first recess to form a first isolation.
2 . The method of claim 1 , wherein a bottom of the second recess is higher than bottoms of the epitaxial source/drain structures.
3 . The method of claim 1 , wherein the substrate comprises a second isolation, and a bottom of the third recess is lower than a bottom of the second isolation.
4 . The method of claim 1 , wherein the forming of the third recess comprises an anisotropic etching operation.
5 . The method of claim 1 , further comprising removing the spacers.
6 . The method of claim 1 , further comprising:
removing portions of the nanosheet stack to form the plurality of nanosheets; and
replacing the sacrificial gate structure with a metal gate structure.
7 . A method for forming a semiconductor structure, comprising:
forming a nanosheet stack extending a first direction over a substrate, wherein the nanosheet stack comprises a plurality of nanosheets;
forming a sacrificial gate structure extending a second direction over the nanosheet stack, wherein the second direction is different from the first direction;
forming epitaxial source/drain structures at two sides of the sacrificial gate structure;
forming a first isolation in the sacrificial gate structure, the nanosheet stack and the substrate; and
replacing the sacrificial gate structure with a metal gate structure,
wherein, in the second direction, the first isolation comprises a first portion laterally surrounded by the substrate, a second portion laterally surrounded by the epitaxial source/drain structures, and a third portion surrounded by the metal gate structure,
wherein a bottom of the second portion is in direct contact with the first portion, and a top of the second portion is in direct contact with the third portion,
wherein, in the second direction, a width of the third portion is greater than a width of the first portion.
8 . The method of claim 7 , wherein the substrate comprises a second isolation, and the second isolation is exposed through a sidewall of the third recess.
9 . The method of claim 8 , wherein a bottom of the third recess is lower than a bottom of the second isolation.
10 . The method of claim 7 , further comprising removing the spacers prior to the filling of the third recess, the second recess and the first recess with the insulating material to form the first isolation.
11 . The method of claim 7 , further comprising:
removing portions of the nanosheet stack to form the plurality of nanosheets; and
replacing the sacrificial gate structure with a metal gate structure.
12 . The method of claim 11 , further comprising forming a third isolation in the metal gate structure.
13 . The method of claim 12 , wherein a bottom of the third isolation is higher than a bottom of the first isolation.
14 . The method of claim 12 , further comprising forming a protection layer over the third isolation.
15 . The method of claim 14 , wherein the protection layer and the third isolation comprise a same material.
16 . The method of claim 14 , wherein the protection layer is coupled to the third isolation.
17 . A method for forming a semiconductor structure, comprising:
forming a nanosheet stack extending a first direction over a substrate, wherein the nanosheet stack comprises a plurality of nanosheets;
forming a sacrificial gate structure extending a second direction over the nanosheet stack, wherein the second direction is different from the first direction;
forming epitaxial source/drain structures at two sides of the sacrificial gate structure;
forming a first isolation in the sacrificial gate structure, the nanosheet stack and the substrate; and
replacing the sacrificial gate structure with a metal gate structure,
wherein, in the second direction, the first isolation comprises a first portion laterally surrounded by the substrate, a second portion laterally surrounded by the epitaxial source/drain structures, and a third portion surrounded by the metal gate structure,
wherein a bottom of the second portion is in contact with the first portion, and a top of the second portion is in contact with the third portion,
wherein, in the second direction, a width of the third portion is greater than a width of the first portion.
18 . The method of claim 17 , wherein a width of the second portion is different from the width of the first portion, and different from the width of the third portion.
19 . The method of claim 17 , wherein the substrate comprises a second isolation, and a bottom of the first isolation is lower than a bottom of the second isolation.
20 . The method of claim 19 , wherein the first portion of the first isolation is laterally surrounded by the second isolation in the first direction.