It is possible to create both a p-well and an n-well for the NMOS and PMOS respectively in the twin well or twin tub technology. Such a choice means the process is independent of the dopant type of the starting substrate. Different steps of the fabrication of the CMOS using twin tub process are as follows:

Lightly doped n+ or p+ substrate is taken and, to protect the latch up, epitaxial layer is used.

The high-purity controlled thickness of the layers of silicon is grown with exact dopant concentrations.

The dopant and its concentration in Silicon are used to determine electrical properties.

Formation of the tub

Thin oxide construction

Implantation of the source and drain

Cuts for making contacts

Metallization

(Fabrication steps for P-well are similar to N-well process, and both combined gives CMOS)