Light and heavy element profiling using heavy ion beams

Abstract

The analysis of thin layers in a near surface region with high resolution is an important problem in ion beam analysis. Here elastic recoil detection analysis (ERDA) using high energetic heavy incident ions has been optimized for analysis of light elements such as H, D, He, but has also proven its applicability for medium light elements up to B and C. The use of high energetic heavy incident ions, 8–16 MeV 16O and 30 MeV 58Ni from a 3 MV tandem accelerator, increased the depth resolution for H analysis by a factor of 3 compared with standard ERD using 2 MeV 4He. Additionally, the sensitivity of the method was increased by up to one order of magnitude, when the heaviest ion was used. In this experiment primary ions, scattered in forward direction into the detector, were suppressed using an absorber foil. When heavy ions are used, not only H recoils can transmit through the absorber foil, but also light and medium heavy recoils. The energy loss in the foil can be accurately calculated using the well-known TRIM code. The depth resolution obtained in this experiment was 100 Å in metallic samples (Cu, Nb), where the stopping power is high and 250 Å in samples with lower Z as e.g. Si and B. For all recoil species the depth resolution was dominated by straggling in the absorber foil and kinematic broadening. Therefore, the depth resolution did not vary much for different recoils. Simultaneously with the ERDA investigations Rutherford backscattering spectrometry (RBS) with heavy ions has been employed for heavy element analysis. The good mass separation which can be obtained using heavy projectile ions was used to optimize the depth resolution by tilting the sample. A multilayer of 5 Si Nb layers of 50 Å thickness each were clearly separated at 67.5° incident angle.