Process Control Improvements for Production of Depleted Uranium Hohlraums

Depleted uranium (DU) hohlraums consist of a sputter-deposited DU layer sandwiched between two sputter-deposited layers of gold and overcoated with a thick electrodeposited gold layer. Production of a multilayered system of dissimilar materials to tight tolerances requires a complex set of process steps. Process drift in production of DU hohlraums resulted in increased failures and led to unacceptably low production yields. Characterization of this failure mechanism indicated poor adhesion between dissimilar layers. Failure of one layer could be traced to the preceding layer. Ultimately, failures were traced to pretreatment of the mandrel for the initial deposition. Pretreatment of the mandrel involves an ion-etch step, which had drifted. Maintenance of the ion gun resulted in improved mandrels and improved process yields. Production yields from the DU sputter deposition were low with failures due to blistering and delamination. Oxidation of the DU due to gettering of residual oxygen or water in the sputter chamber was hypothesized. A process change was implemented to minimize the time between the DU and gold coatings. The change required removal of one production part to incorporate one additional gold sputter source. The production run was thus reduced from five parts to four parts. However, the production yield increased significantly, by 30%.