Surface deformations and gas escape process studied by quasi-simultaneous multiple-energy He irradiation

Single crystal silicon wafer covered by Al foil of thickness 5 μm was irradiated quasi-simultaneously by multiple-energy He ions up to the fluence level of $\text{3.3 × 10}{}^{19}$ ions/cm². The implantation was performed by 3.5 MeV ⁴He⁺ through a moving Al absorber foil in such a way that a nearly uniform He distribution was obtained extending in both Si and Al to depth intervals of $\text{1.9 × 10}{}^{19}$ Si/cm² and of $\text{1.5 × 10}{}^{19}$ Al/cm², respectively. During irradiation the evolution of the He concentration-depth profile was studied in situ by 3 MeV proton RBS analysis. It was found that the He concentration in the Al cover foil, after reaching a maximum value of 30 at.% began to decrease. This accelerated re-emission process was initiated near the inter-boundary surface and extended inward gradually. No significant He escape was observed from the Si up to the applied dose, so at the end of the implantation a He concentraton of 80 at.% has been reached.Exposing both inter-boundary surfaces to SEM, flaking from numerous spots was observed on the Al but not on the Si where only one crater was found. After mechanically breaking the implanted Si wafer and Al cover foil it could be seen on the fracture surfaces that regions containing a large amount of He acquire a sponge-like structure. Channels and large cavities were also observed in this region of the broken Si produced as cracks of interconnected He bubbles. The appearance of the flaking processes clearly demonstrates that even with such a depth distribution the He implanted in the materials in spite of re-emission may reach the critical value for blistering or flaking. The critical concentrations required for the observed surface deformation together with the thickness of the flaked layers evaluated from RBS and SEM observations are discussed.