High aspect ratio and reduced undercut trench etch process for a semiconductor substrate
A hydrofluorocarbon gas is employed as a polymer deposition gas in an anisotropic etch process employing an alternation of an etchant gas and the polymer deposition gas to etch a deep trench in a semiconductor substrate. The hydrofluorocarbon gas can generate a thick carbon-rich and hydrogen-containing polymer on sidewalls of a trench at a thickness on par with the thickness of the polymer on a top surface of the semiconductor substrate. The thick carbon-rich and hydrogen-containing polymer protects sidewalls of a trench, thereby minimizing an undercut below a hard mask without degradation of the overall rate. In some embodiments, an improvement in the overall etch rate can be achieved.
1. A semiconductor structure comprising:
a stack of a semiconductor substrate and a mask layer having an opening therein;
a trench located within said semiconductor substrate and underlying said opening and having a vertically modulated width; and
a hydrofluorocarbon polymer layer contiguously extending from a top surface and sidewalls of said mask layer, through sidewalls of said trench, and to a bottom surface of said trench, wherein said hydrofluorocarbon polymer layer has a first composition at said bottom surface of said trench, said hydrofluorocarbon polymer layer has a second composition at said sidewalls of said trench, and said second composition is different from said first composition, wherein said first composition includes hydrogen at first hydrogen atomic percentage, and said second composition includes hydrogen at a second hydrogen atomic percentage that is greater than said first hydrogen atomic percentage.
2. The semiconductor structure of claim 1 , wherein said second hydrogen atomic percentage is greater than said first hydrogen atomic percentage by at least 5%.
3. The semiconductor structure of claim 1 , said hydrofluorocarbon polymer layer includes carbon at an atomic concentration in a range from 20% to 50%, hydrogen at an atomic concentration from 4% to 70%, and fluorine at an atomic concentration from 4% to 70%.
4. The semiconductor structure of claim 1 , wherein a ratio of fluorine to hydrogen is less than 15 within an entirety of said hydrofluorocarbon polymer layer.
5. The semiconductor structure of claim 1 , wherein said hydrofluorocarbon polymer layer includes a first hydrofluorocarbon polymer portion located on said bottom surface and having a first thickness, a second hydrofluorocarbon polymer portion located on said sidewalls of said trench and having a second thickness, and a third hydrofluorocarbon polymer portion located on a horizontal surface of said mask layer and having a third thickness, wherein said second thickness is at least 90% of said third thickness, and said first thickness is less than 50% of said third thickness.
6. The semiconductor structure of claim 1 , wherein said hydrofluorocarbon polymer layer includes a first hydrofluorocarbon polymer portion located on said bottom surface, and a second hydrofluorocarbon polymer portion located on said sidewalls of said trench.
7. The semiconductor structure of claim 6 , wherein said hydrofluorocarbon polymer layer further comprises a third hydrofluorocarbon polymer portion located on a horizontal surface of said mask layer.
8. The semiconductor structure of claim 7 , wherein said first hydrofluorocarbon polymer portion has a first thickness, said second hydrofluorocarbon polymer portion has a second thickness, and said third hydrofluorocarbon polymer portion has a third thickness, wherein said second thickness is at least 90% of said third thickness, and said first thickness is less than 50% of said third thickness.
9. The semiconductor structure of claim 8 , wherein said second thickness is greater than said third thickness.
10. The semiconductor structure of claim 9 , wherein said mask layer includes a dielectric material selected from doped silicon oxide, undoped silicon oxide, silicon nitride, silicon oxynitride, a dielectric metal oxide, or a combination thereof.
11. The semiconductor structure of claim 1 , wherein said trench has a vertically modulated width that changes with a distance from said mask layer.
12. The semiconductor structure of claim 11 , wherein said vertically modulated width gradually increases and gradually decreases multiple times with a monotonic change of said distance from said mask layer.
13. The semiconductor structure of claim 11 , wherein a combination of a gradual increase and a gradual decrease with a monotonic change of a distance from said mask layer repeats a number of times, said number being in a range from 2 to 1,000.
14. The semiconductor structure of claim 1 , wherein said semiconductor substrate includes a silicon-containing semiconductor material.