Structure and method for FinFET device with buried SiGe oxide

A method, comprising:

• forming isolation features in a top portion of a substrate, thereby defining an active region in the top portion of the substrate;
• recessing the active region to form a fin trench;
• epitaxially growing a first semiconductor layer on the substrate within the active region;
• epitaxially growing a second semiconductor layer on the first semiconductor layer;
• performing a first recessing process to the isolation features such that the isolation features are below the second semiconductor layer;
• performing a thermal oxidation process to selectively oxidize the first semiconductor layer to form an oxide feature on sidewalls of the first semiconductor layer, wherein the oxide feature, the first semiconductor layer, and the second semiconductor layer collectively define a fin block;
• performing a second recessing process to the isolation features such that the isolation features are below the first semiconductor layer;
• and
• forming a gate stack engaging the fin block, wherein a portion of the gate stack is directly under the oxide feature.

A method for manufacturing a fin structure, comprising:

• providing a substrate;
• forming at least one recess in an upper portion of the substrate;
• epitaxially growing a first crystalline semiconductor layer with a first lattice constant in the at least one recess and a second crystalline semiconductor layer with a second lattice constant over the first crystalline semiconductor layer, wherein the first lattice constant is different from the second lattice constant;
• forming an isolation layer on sidewalls of the first crystalline semiconductor layer and under the second crystalline semiconductor layer, wherein the isolation layer, the first crystalline semiconductor layer, and the second crystalline semiconductor layer collectively define a fin block;
• removing a portion of the substrate to expose a portion of the isolation layer;
• and
• forming a gate stack engaging the fin block.