Process to form soi substrate

A method, comprising:

• epitaxially forming a silicon-germanium (SiGe) layer over a sacrificial substrate;
• epitaxially forming a first active layer comprising a semiconductor material to a first thickness on an upper surface of the SiGe layer;
• flipping the sacrificial substrate and bonding the first active layer to a dielectric layer formed over a first substrate;
• removing the sacrificial substrate and a portion of the SiGe layer while leaving a remaining portion of the SiGe layer covering an upper surface of the first active layer;
• removing the remaining portion of the SiGe layer and an upper portion of the first active layer;
• and
• forming a second active layer on the first active layer, wherein the first active layer and the second active layer collectively have a second thickness greater than the first thickness.

A method of forming an SOI substrate, comprising:

• epitaxially forming a silicon-germanium (SiGe) layer over a sacrificial substrate, wherein the SiGe layer has a germanium concentration that increases from a lower surface contacting the sacrificial substrate to an opposing upper surface;
• epitaxially forming a first active silicon layer to a first thickness on the SiGe layer;
• bonding the first active silicon layer to a dielectric layer formed over a first substrate;
• removing the sacrificial substrate and the SiGe layer;
• and
• forming a second active silicon layer on the first active silicon layer, wherein the first active silicon layer and the second active silicon layer collectively have a second thickness greater than the first thickness.