Atomic structures of thin films

Thin film technology is becoming one of the important technologies today. While there are infinite varieties of thin film fabrication methods, most amorphous thin films seem to exhibit fractal-like atomic structures. Depending on the fabrication conditions, a thin film grows on the substrate into columnar structures with many voids interdispersed in the thin film.80 These structures can be seen in the field ion microscope, and compositional variation can be analyzed with the atom-probe. In addition, formation of atomic clusters inside the thin film can be substantiated with the observation of a large fraction of cluster ions in field evaporation by the atom-probe. 

FTM and atom-probe studies of thin films of Ni, Au, Pt, a-Ge H, a-Si H and WO3, etc., on various substrates were reported by Krishna-swamy et a/.81 First, field ion tips each with a field evaporated surface were prepared. They are placed in an MRC model 8502 r.f. sputtering system. Tips were mounted on a recessed and shielded structure behind the sputtering surface which is bored with small holes about 1 to 2 mm in diameter. The very end of the tips came out of the holes to approximately the same level of the sputtering surface. Films were sputtered at about 20 mTorr Ar at an r.f. power of about 50 W. Thickness of a deposited thin film was controlled by both the r.f. power and the deposition time. Film thickness in the range of a few hundred to a few thousand A were studied. These tips were then imaged with Ne in the field ion microscope, or analyzed in the flight-time-focused ToF atom-probe. 

Ge(OH)+, Ge(OH)2+, Ge(OH)3+ and Ge202+, etc. It is clear from these observations that atoms tends to form clusters in thin films. Thin films also tend to react with water in the atmosphere. In field evaporation, water molecules are desorbed in the form of hydride, oxide or hydroxide ions with the substrate atoms and atomic clusters. From the above discussions, it is clear that the FIM and atom-probe can now be applied to study a wide variety of problems for a wide variety of materials. Field ion microscopy is no longer confined to studying refractory metals, as many scientists still erroneously perceive. 

---