Deuterium retention in sputter-deposited W-B layers: in-situ implantation and ion beam analysis during annealing

Abstract

Boronization in tokamak devices with tungsten (W) plasma facing components (PFC) may lead to the formation of mixed layers of W and boron (B) that can affect wall retention of plasma fuel species. In this study, deuterium (D) retention was investigated in W-B thin films with different stoichiometries as well as in pure W and B, grown on silicon (Si) substrates by means of magnetron sputter deposition. After pre-characterization, the layers were implanted with 1 keV D₂⁺ ions to a fluence of 7 × 10¹⁷ D/cm², followed by in-situ ion beam analysis. The samples were annealed to temperatures between 400–600 ◦ C and in-situ ion beam analysis measurements were performed before, during and after the annealing process by simultaneous Elastic Recoil Detection Analysis and Rutherford Backscattering Spectrometry. The different B concentrations in the films led to significant differences in D retention, where higher boron concentrations resulted in higher deuterium retention immediately after implantation. After annealing, the lowest amount of retained D was observed for a W-to-B ratio of 2:1, with an areal density of 8 × 10¹³ D/cm², about three times lower than for pure W. The highest retention of around 5 × 10¹⁶ D/cm² after annealing to 600 ◦C was found for the pure B-film. Ex-situ electron microscopy techniques revealed significant morphological modifications due to implantation and/or annealing, including bubble formation (W film), W surface enrichment (B-rich film) and crack formation (W-rich film).