Inner spacer formation for nanosheet field-effect transistors with tall suspensions
Structures for a nanosheet field-effect transistor and methods for forming a structure for a nanosheet field-effect transistor. A body feature is formed that includes a first nanosheet channel layer, a second nanosheet channel layer, and first, second, and third sacrificial layers that are vertically arranged between the first and second nanosheet channel layers. The first, second, and third sacrificial layers are laterally recessed relative to the first and second nanosheet channel layers to form a cavity indented into a sidewall of the first body feature. The second sacrificial layer is laterally recessed to a lesser extent than the first sacrificial layer or the third sacrificial layer such that an end of the second sacrificial layer projects into the cavity between the first and third sacrificial layers. A dielectric spacer is formed in the first and second portions of cavity between the first and second nanosheet channel layers.
1. A method for forming a field-effect transistor, the method comprising:
forming a first body feature that includes a first nanosheet channel layer, a second nanosheet channel layer, and a plurality of sacrificial layers vertically arranged between the first nanosheet channel layer and the second nanosheet channel layer, wherein the sacrificial layers include a first sacrificial layer, a second sacrificial layer, and a third sacrificial layer that are vertically arranged with the second sacrificial layer between the first sacrificial layer and the third sacrificial layer;
laterally recessing the first sacrificial layer, the second sacrificial layer, and the third sacrificial layer relative to the first nanosheet channel layer to form a cavity indented into a sidewall of the first body feature, wherein the second sacrificial layer is laterally recessed to a lesser extent than the first sacrificial layer or the third sacrificial layer such that an end of the second sacrificial layer projects into the cavity to divide the cavity into a first portion between the first nanosheet channel layer and the second sacrificial layer and a second portion between the second nanosheet channel layer and the second sacrificial layer; and
depositing a conformal dielectric layer in the first portion of the cavity and in the second portion of the cavity in order to form a dielectric spacer between the first nanosheet channel layer and the second nanosheet channel layer.
2. The method of claim 1 wherein the first nanosheet channel layer and the second sacrificial layer are composed of silicon, and the first sacrificial layer and the third sacrificial layer are composed of silicon-germanium.
3. The method of claim 1 wherein the first nanosheet channel layer is composed of silicon, the first sacrificial layer and the third sacrificial layer are composed of silicon-germanium with a first germanium content, and the second sacrificial layer is composed of silicon-germanium with a second germanium content that is less than the first germanium content.
4. The method of claim 3 wherein the first germanium content is 30 percentage points to 40 percentage points greater than the first germanium content.
5. The method of claim 1 wherein a second body feature arranged adjacent to the first body feature and separated from the first body feature by a space, and the conformal dielectric layer is deposited across the first body feature and the second body feature.
6. The method of claim 5 wherein the first portion of the cavity and the second portion of the cavity are dimensioned such that the conformal dielectric layer is pinched off to form the dielectric spacer and the space between the first body feature and the second body feature is dimensioned such that the conformal dielectric layer on the first body feature does not merge with the conformal dielectric layer on the second body feature.
7. The method of claim 6 further comprising:
removing the conformal dielectric layer from the space between the first body feature and the second body feature with an isotropic etching process.
8. The method of claim 7 further comprising:
epitaxially growing a source/drain region in the space between the first body feature and the second body feature using the first nanosheet channel layer as a growth seed.
9. The method of claim 5 wherein the conformal dielectric layer has a thickness, the first body feature has a sidewall, the second body feature has a sidewall, and the thickness of the conformal dielectric layer is less than one-half of a distance between the sidewall of the first body feature and the sidewall of the second body feature.
10. The method of claim 5 wherein the conformal dielectric layer has a thickness, and a height between the first nanosheet channel layer and the second sacrificial layer within the first portion of the cavity is equal to one-half of the thickness of the conformal dielectric layer.
11. The method of claim 10 wherein a vertical distance between the second nanosheet channel layer and the second sacrificial layer within the second portion of the cavity is equal to one-half of the thickness of the conformal dielectric layer.
12. The method of claim 1 further comprising:
after forming the dielectric spacer, removing the first sacrificial layer and the third sacrificial layer selective to the first nanosheet channel layer and the second sacrificial layer.
13. The method of claim 12 further comprising:
after removing the first sacrificial layer and the third sacrificial layer, removing the second sacrificial layer,
wherein the first nanosheet channel layer is thinned in response to removing the second sacrificial layer.
14. The method of claim 13 wherein the dielectric spacer includes a cavity formed at a location from which the end of the second sacrificial layer is removed, and further comprising:
depositing a gate dielectric layer that, in part, fills the cavity in the dielectric spacer.
15. The method of claim 1 further comprising:
after forming the dielectric spacer, removing the first sacrificial layer, the second sacrificial layer, and the third sacrificial layer selective to the first nanosheet channel layer.
16. The method of claim 15 wherein the dielectric spacer includes a cavity formed at a location from which the end of the second sacrificial layer is removed, and further comprising:
depositing a gate dielectric layer that, in part, fills the cavity in the dielectric spacer.