Methods and apparatus for forming backside power rails

A method that forms a sacrificial fill material that can be selectively removed for forming a backside contact via for a transistor backside power rail. In some embodiments, the method may include performing an etching process on a substrate with an opening that is conformally coated with an oxide layer, wherein the etching process is an anisotropic dry etch process using a chlorine gas to remove the oxide layer from a field of the substrate and only from a bottom portion of the opening, and wherein the etching process forms a partial oxide spacer in the opening and increases a depth of the opening and epitaxially growing the sacrificial fill material in the opening by flowing a hydrogen chloride gas at a rate of approximately 60 sccm to approximately 90 sccm in a chamber pressure of approximately 1 Torr to approximately 100 Torr.