High selectivity nitride removal process based on selective polymer deposition
A silicon nitride cap on a gate stack is removed by etching with a fluorohydrocarbon-containing plasma subsequent to formation of source/drain regions without causing unacceptable damage to the gate stack or source/drain regions. A fluorohydrocarbon-containing polymer protection layer is selectively deposited on the regions that are not to be etched during the removal of the nitride cap. The ability to remove the silicon nitride material using gas chemistry, causing formation of a volatile etch product and protection layer, enables reduction of the ion energy to the etching threshold.
1. A method comprising:
obtaining a FET structure comprising a semiconductor substrate, a gate stack on the substrate, source/drain regions operatively associated with the gate stack, and a silicon nitride cap on the gate stack by forming the gate stack on the semiconductor substrate, forming silicon nitride spacers on the gate stack, epitaxially depositing the source/drain regions on the semiconductor substrate, and forming the silicon nitride cap on the gate stack such that the silicon nitride cap extends above the spacers;
generating a fluorohydrocarbon-containing plasma selective to silicon by decomposition of C x H y F z wherein x is an integer selected from 3, 4, 5 and 6, y and z are positive integers, and y is greater than z;
anisotropically etching the silicon nitride cap and the spacers simultaneously employing the fluorohydrocarbon-containing plasma to form a first hydrofluorocarbon polymer layer having a first thickness on the source/drain regions and a second hydrofluorocarbon polymer layer having a second thickness on the silicon nitride cap, the first thickness being greater than the second thickness, the second hydrofluorocarbon polymer layer further comprising a volatile nitrogen-containing compound formed by interaction of the fluorohydrocarbon-containing plasma with the silicon nitride comprising the silicon nitride cap, and
removing the first hydrofluorocarbon polymer layer from the source/drain regions.
2. The method of claim 1 , wherein the source/drain regions comprise at least one of silicon germanium and silicon carbide.
3. The method of claim 1 , further including forming a photoresist layer having a selected thickness on the FET structure and patterning the photoresist to form an opening having selected dimensions and exposing the silicon nitride cap and the source/drain regions.
4. The method of claim 3 , further including maintaining the selected thickness and the selected dimensions of the photoresist layer during the step of anisotropically etching the silicon nitride cap.
5. The method of claim 4 , wherein the photoresist layer is comprised of mostly CxHy.
6. The method of claim 5 , wherein the first hydrofluorocarbon polymer layer comprises carbon at an atomic concentration between thirty and forty percent, hydrogen at an atomic concentration between forty and fifty percent, fluorine at an atomic concentration between five and ten percent, and oxygen at an atomic concentration less than five percent.
7. The method of claim 1 , wherein the semiconductor substrate is silicon-based.
8. The method of claim 1 , wherein the first fluorohydrocarbon-containing polymer layer comprises carbon at an atomic concentration between thirty and forty percent, hydrogen at an atomic concentration between forty and fifty percent, fluorine at an atomic concentration between five and ten percent, and oxygen at an atomic concentration less than five percent.
9. The method of claim 8 , wherein the first fluorohydrocarbon-containing polymer layer has a thickness between 0.1 to three nanometers.
10. The method of claim 1 , wherein the source/drain regions comprise an epitaxial layer of a doped semiconductor material.