What is a substrate in CMOS process?

CMOS circuits are fabricated on a Silicon wafer (also know as substrate: an unprocessed wafer). To develop an n-type substrate, Si wafer is doped with donor atoms such as phosphorus and for a p-type substrate it is doped with acceptor atoms such as boron.

For an NMOS (n-channel MOS), a p-type substrate is used for the fabrication and similarly for PMOS (p-channel MOS) an n-well is used.

**Note: substrate or well are sometimes also referred to as bulk (or) body of the MOSFET.

What is a wafer? 

https://vlsidigest.blogspot.com/2020/06/what-is-wafer.html