FIB stands for Focus Ion Beam. This microscope works on the SEM principle. But instead of using an electron beam, it uses an ion beam, usually gallium ion. It can be used as a normal imaging microscope, detecting either secondary electrons either secondary ions. Compare to electrons, ions have higher mass and higher momentum. Thus when interacting with a sample, ions modify it by implantation or by sputtering. Moreover a gas precursor can react under the ion beam depositing a material.
Coupled with a SEM to form a dual beam station, the FIB becomes a powerful tool for material science and life science. In the microscopic field, three techniques using FIB dual beam can be highlighted: TEM lamella preparation, cross section and tomography.