Recent Advances in the Fabrication of Very Thick, Multistepped Iron and Tantalum Films for EOS Targets

The equation of state (EOS) and other parameters at high pressures and low temperatures are of significant interest. One example is iron, where knowledge of the EOS at high pressure is needed to understand planetary interiors and planetary development. Targets are needed to perform these important measurements on experimental platforms such as Omega, National Ignition Facility (NIF), and the Z-machine. There is a need for thicker films for targets for the NIF and Z-machine platforms, which is technically challenging because of coating stress and other issues. We present results showing that we successfully sputter deposited stepped iron and tantalum films up to 90+ m thick for targets on NIF and have sputter deposited (unstepped) tantalum films over 1700 m (1.7 mm!) thick, which are desired for targets for Z-machine EOS experiments. This is generally made possible by the low stress achieved in the tantalum films (as low as 25 MPa). We will also report some process improvement achievements, such as a shaper roll-off for the Fe step edges, as well as some characterization results of the microstructure of the very thick films. For example, interruption of the growth with a brief ambient exposure appears to have a minor impact on the columnar grain growth.