Bonded SOI wafers –

It creates the buried oxide without ion implantation.

First two silicon wafers undergo thermal oxidation process to form $SiO_2$ layer.

The two hydrophilic surfaces (such as $SiO_2$) are brought into intimate contact of each other to form the buried oxide.

The backside of one of the silicon wafers is ground down to the desired thickness.

Finally the extra Silicon is removed and the SOI wafer is annealed and polished to leave a thin layer of silicon above the buried oxide that is electronic grade quality.

Thus the wafer is ready for typical CMOS processing.

Smart cut method –

This is an alternative method for former SOI technologies.

A thin dielectric layer of $SiO_2$is grown thermally on the Silicon wafer. The wafer is implanted with protons (hydrogen ions) through the oxide layer.

The second silicon layer is bonded to the oxide layer. This layer is substrate for the SOI wafer.

A thermal anneal creates a stress fracture along the plane of the hydrogen implant. This is smart cut.

The original silicon wafer can be removed from the tri-layer stack leaving behind a thin layer of silicon on the top of the buried oxide.

The removed portion will become the substrate of another smart cut wafer. In this manner no silicon is wasted.

To finish process the SOI wafer is annealed & polished to prepare the surface.