Depth Distribution and Migration of Implanted Helium in Metal Foils Using Proton Backscattering
Proton backscattering at 2.5 MeV has been used to measure the mean depth and profile of implanted helium distributions as a function of implant energy, implant fluence, and post-implant anneal temperature in copper foils of varying thickness. Distributions implanted at 54 keV; 104 keV, and 158 keV agree (to within 100 A) with calculated projected ranges for helium in copper at each energy. At room temperature the shape of the distributions is approximately Gaussian with no evidence of a supertail or of the peaks being skewed either toward the surface or the interior of the foils. Implantation of some foils was performed at two energies (highest energy first) using both equal and unequal doses. Resultant profiles were those expected from overlapping Gaussians centered at the predicted depths. Implanted helium fluences ranging from 5 × 1016 He+/cm2 to 3 × 1017 He+/cm2 result in back-scattering peaks for helium which increase in magnitude in the proper proportion to increasing fluence. Detection sensitivity of 1 at. % He in Cu has been demonstrated. In addition, profiling of other low Z elements (e.g. oxygen, carbon and deuterium) in the foils or on their surfaces is also described.
The effect of in situ isochronal and isothermal annealing on the disposition of the implanted helium has also been observed. Above temperatures of 200°C, the peak of the helium distribution decreases in magnitude, but no lateral spreading of the profile (as expected in Fick’s Law diffusion) is observed. Moreover, the helium peak height decreases steadily for each of the isochronal temperature plateaus between 200°C and 450oC. Isothermal annealing at 225°C and 400°C produces almost no additional change in the magnitude of the helium peak at the given temperature over three anneal periods of increasing duration. Throughout annealing, the symmetric form of the Gaussian distribution is retained. There is, as yet, no evidence of preferential diffusion of the implanted helium either into the undamaged depths of the foils or through the residual ion-implantation-induced damage between the helium implanted layer and the foil surface. These observations could be explained if the helium were trapped at or near its end-of-range location at room temperature and then released in proportionate fractions at progressively higher temperatures by formation and subsequent rupture of bubbles developing in the implanted layer. Evidence has been obtained by scanning electron microscopy which supports this hypothesis.
KeywordsDepth Distribution Helium Atom Copper Foil Surface Bubble Foil Surface
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