Without the sacrificial layer, the gallium ions used for milling would cause greater damage to the surface. Damage at the top of the sample may or may not be important, however, depending on the depth of the features you want to see. But if areas of interest exist near the surface, even the deposition of the protective platinum layer can introduce unacceptable artifacts. Or if an electron beam is available, you can use it to first deposit a thin layer of platinum to protect the sample during subsequent deposition of a thicker metal layer using the ion beam at full energy. Alternatively, you could use conventional means to deposit a thin layer of gold or carbon over the sample surface prior to loading it in the FIB system.
Damage can also occur on the sidewalls of samples.
Because the beam strikes at a low grazing angle, less damage occurs on the sides than on the top, but damage to the sidewalls affects the entire sample. You can reduce sidewall damage by not inspecting the face with direct ion imaging after you perform the final thinning and cleaning operations. If you avoid ion-beam imaging, the amorphous layer induced by ion milling represents only a few percent of the sample volume. If an electron beam is available, you can use it to inspect the final sample without causing damage. FIB sample preparation offers several advantages over conventional techniques.