Methods and apparatus for forming backside power rails

A method of forming a backside power rail contact for a source/drain epitaxial (Epi) structure of a transistor, comprising:

• forming an opening in a substrate;
• depositing a conformal layer of oxide on the substrate and in the opening;
• performing an etching process on the substrate and the opening, wherein the etching process is an anisotropic dry etch process using a chlorine gas to remove the conformal layer of oxide from a bottom portion of the opening while preserving the oxide layer on sidewalls of the opening, and wherein the etching process forms a partial oxide spacer in the opening and increases a depth of the opening;
• epitaxially growing a sacrificial fill material in the opening using a bottom-up fill process 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;
• forming a source/drain Epi structure on the sacrificial fill material;
• forming a gate material on the source/drain Epi structure;
• forming at least one interconnect signal lines above the gate material;
• flipping the substrate to reveal a backside of the substrate;
• removing material of the substrate to expose the sacrificial fill material;
• selectively etching the sacrificial fill material to remove the sacrificial fill material;
• and
• forming the backside power rail contact which is self-aligned to the source/drain Epi structure.