Semiconductor device having isolation structure to reduce current leakage

A method for forming a semiconductor circuit structure, comprising steps of:

• forming a first sacrificial feature over a semiconductor substrate, and a first semiconductor feature over the first sacrificial feature, where the first sacrificial feature separates the first semiconductor feature from the semiconductor substrate;
• removing the first sacrificial feature so as to disconnect the semiconductor substrate from a surface of the semiconductor feature that faces the semiconductor substrate by eliminating physical connections formed by the first sacrificial feature, thereby forming a gap between the semiconductor substrate and the semiconductor feature;
• depositing a metal-based dielectric layer on the semiconductor substrate, wherein the metal-based dielectric layer is made of a first metal-based dielectric material, and a portion of the metal-based dielectric layer that fills the gap forms a first metal-based dielectric feature;
• forming first source/drain features that are connected to the first semiconductor feature;
• and
• forming a first gate feature on the first semiconductor feature,
• wherein the method comprises a step of transforming another portion of the metal-based dielectric layer that is outside of a coverage of the semiconductor feature from the first metal-based dielectric material into a second metal-based dielectric material which is different from the first metal-based dielectric material.

A method for forming a semiconductor circuit structure, comprising steps of:

• forming a sacrificial feature over a semiconductor substrate;
• forming a first channel feature and a second channel feature over the sacrificial feature, wherein the first channel feature and the second channel feature are spaced apart from each other in a direction parallel to a surface of the semiconductor substrate;
• replacing the sacrificial feature with a metal-based dielectric feature;
• forming first source/drain features and second source/drain features over the metal-based dielectric feature, where the first source/drain features are connected to the first channel feature, and the second source/drain features are connected to the second channel feature;
• transforming a portion of the metal-based dielectric feature from a first metal-based dielectric material into a second metal-based dielectric material that is different from the first metal-based dielectric material, wherein the portion of the metal-based dielectric feature is disposed between one of the first source/drain features and one of the second source/drain features when viewed from top;
• and
• forming a first gate feature and a second gate feature on the first channel feature and the second channel feature, respectively.