Field effect transistor (FET) and method of forming the FET without damaging the wafer surface
View Patent ↗Disclosed are a field effect transistor structure and a method of forming the structure. A gate stack is formed on the wafer above a designated channel region. Spacer material is deposited and anisotropically etched until just prior to exposing any horizontal surfaces of the wafer or gate stack, thereby leaving relatively thin horizontal portions of spacer material on the wafer surface and relatively thick vertical portions of spacer material on the gate sidewalls. The remaining spacer material is selectively and isotropically etched just until the horizontal portions of spacer material are completely removed, thereby leaving only the vertical portions of the spacer material on the gate sidewalls. This selective isotropic etch removes the horizontal portions of spacer material without damaging the wafer surface. Raised epitaxial source/drain regions can be formed on the undamaged wafer surface adjacent to the gate sidewall spacers in order to tailor source/drain resistance values.
1. A field effect transistor comprising:
a semiconductor substrate;
an insulator layer on said semiconductor substrate;
a semiconductor layer on said insulator layer, said semiconductor layer having a top surface and an essentially uniform thickness of less than 3 nm;
a gate stack on said top surface, said gate stack having opposing sidewalls;
gate sidewall spacers on said top surface adjacent to said opposing sidewalls;
an epitaxial semiconductor layer on said top surface positioned laterally adjacent to said gate sidewall spacers;
a channel region within said semiconductor layer aligned below said gate stack; and
source/drain regions within said semiconductor, said channel region being positioned laterally between said source/drain regions, said top surface of said semiconductor layer being planar and said top surface being fully protected from silicon loss during processing such that said top surface is essentially damage-free and such that a first portion of said top surface of said semiconductor layer between said channel region and said gate stack is level with a second portion of said top surface of said semiconductor layer between said source/drain regions and epitaxial semiconductor layer.
2. The field effect transistor of claim 1 , said semiconductor layer having a bottom that is essentially planar and parallel to said top surface such that a first thickness of said semiconductor layer at said channel region is equal to a second thickness of said semiconductor layer at said source/drain region.
3. The field effect transistor of claim 2 , said second thickness being at least 1 nm.
4. The field effect transistor of claim 2 , said second thickness being between 1 nm and 3 nm.
5. The field effect transistor of claim 2 , further comprising raised source/drain regions in said epitaxial semiconductor layer, said gate stack being positioned laterally between said raised source/drain regions and electrically isolated from said raised source/drain regions by said gate sidewall spacers.
6. A field effect transistor comprising:
a semiconductor substrate;
an insulator layer on said semiconductor substrate;
a semiconductor layer on said insulator layer, said semiconductor layer having a top surface and an essentially uniform thickness of less than 3 nm;
a gate stack on said top surface, said gate stack having opposing sidewalls;
gate sidewall spacers on said top surface adjacent to said opposing sidewalls;
a channel region within said semiconductor layer aligned below said gate stack; and
a first source/drain regions and a second source/drain region within said semiconductor layer, said channel region being positioned laterally between said first source/drain regions and said second source/drain region; and
a first epitaxial semiconductor layer on said top surface of said semiconductor layer above said first source/drain region and a second epitaxial semiconductor layer on said top surface of said semiconductor layer above said second source/drain region, said first epitaxial semiconductor layer and said second epitaxial semiconductor layer having different thicknesses, said top surface of said semiconductor layer being planar and said top surface being fully protected from silicon loss during processing such that said top surface is essentially damage-free and such that a first portion of said top surface of said semiconductor layer between said channel region and said gate stack is level with second portions of said top surface of said semiconductor layer between said first source/drain regions and said first epitaxial semiconductor layer and between said second source/drain region and said second epitaxial semiconductor layers.
7. The field effect transistor of claim 6 , said semiconductor layer having a bottom that is essentially planar and parallel to said top surface such that a first thickness of said semiconductor layer at said channel region is equal to second thicknesses of said semiconductor layer at said source/drain regions.
8. The field effect transistor of claim 6 , said semiconductor layer, said first epitaxial semiconductor layers and said second epitaxial semiconductor layer comprising single crystalline silicon layers.
9. The field effect transistor of claim 6 , said first source/drain region comprising a source region, said first epitaxial semiconductor layer having a first thickness, said second source/drain region comprising a drain region, and said second epitaxial semiconductor layer having a second thickness that is less than said first thickness.
10. The field effect transistor of claim 6 , further comprising source/drain extension regions in said semiconductor layer aligned below said gate sidewall spacers and positioned laterally between said channel region and said source/drain regions.