A transmission electron microscopy study of the effect of interfaces on bubble formation in he-implanted Cu-Nb multilayers

Magnetron sputtered thin films of Cu, Nb, and Cu-Nb multilayers with 2.5 and 5 nm nominal layer thickness were deposited on Si and implanted with ⁴He ⁺ and ³He ⁺ ions. Secondary ion mass spectroscopy and nuclear reaction analysis, respectively, were used to measure the ⁴He ⁺ and ³He ⁺ concentration profile with depth inside the films. Cross-sectional transmission electron microscopy was used to characterize the helium bubbles. Analysis of the contrast from helium bubbles in defocused transmission electron microscope images showed a minimum bubble diameter of 1.25 nm. While pure Cu and Nb films showed bubble contrast over the entire range of helium implantation, the multilayers exhibited bubbles only above a critical He concentration that increased almost linearly with decreasing layer thickness. The work shows that large amounts of helium can be trapped at incoherent interfaces in the form of stable, nanometer-size bubbles.