A Femtometer is one-millionth of a nanometer. When used appropriately, this size scale is used to describe subatomic particles. For example, a proton is about 1 femtometer in size.
Subatomic particles don’t typically exist outside of atoms in nature. It is not appropriate to use femtometers to describe normal inorganic or organic molecules — these are typically angstroms, nanometers, or micrometers in size.
“Femtotechnology”, for now, has no practical application and is considered hypothetical by futuristic scientists and engineers.