It has been reported that a sample prepared by ion beam milling has a sandwich structure with amorphous on two sidewalls and crystal in the middle. In this paper, the sandwich structure of such a single crystal TEM sample was studied experimentally. A novel sample and its fabrication process were reported. The sandwich structure can be observed directly in TEM with this sample. When the crystal layer in monocrystal silicon TEM sample is less than 18 nanometers, or when the sample is thinner than 64 nanometers, the sample will be observed as fully amorphous. Removal of the amorphous layer on the sample sidewalls is crucial to get TEM pictures of better quality.

In this paper, crystal damage on TEM sample sidewalls induced by FIB milling during sample preparation was studied. A novel method was invented to prepare the sample, which facilitates the direct observation of amorphous layers on the sidewall. The ion beam acceleration voltage is the dominant factor that affects the damaged layer thickness. The measured amorphous thickness is about 23 nanometers at 30Kv and 10 nanometers at 10Kv. The damage layer thickness is constant with different beam currents over the range from 30pA to 1000pA. Amorphous layer thickness also stays constant with the sample tilt angle.