Dimpling - A new manifestation of ion-produced lattice damage

Bombardment of thin (1–10 µ) single crystal targets with energetic ion beams has been found to result in macroscopic distortion of the thin film in the bombarded region. This effect, which has been euphemistically termed a “dimple”, is readily observed with the naked eye even at relatively low particle fluence. A useful first- order model has been developed which interprets the dimpling as an expansion of the bombarded region. For very thin samples, this expansion can be accommodated by bowing of the crystal out of the original crystal plane. The fractional expansion is proportional to (δ/d)2 where δ is the maximum displacement from the original crystal plane and d is the diameter of the bombarded area. This measurement allows the expansion to be determined with a sensitivity comparable to or better than the most sensitive existing methods. For displacements smaller than the crystal thickness, the bowing depends not only on the radiation produced volume change but, because of elastic forces in the crystal, also requires a non-uniform defect distribution across the sample. When the defect production is nearly uniform, as in some of the present experiments, the bowing is sensitive to defect migration across the film, providing additional information on this effect.