The dependence of heavy-ion-induced adhesion on energy loss and time
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The dependence of heavy-ion-induced adhesion on energy loss and time R. G. Stokstad"' Department of Nuclear Physics, Weizmann Institute of Science, Rehovot, Israel and Nuclear Science Division*'* Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720
P. M. Jacobs, I. Tserruya, L. Sapir, and G. Mamane Department of Nuclear Physics, Weizmann Institute ofScience, Rehovot, Israel (Received 7 October 1985; accepted 17 March 1986) The ability of heavy-ion beams to enhance the adhesion of thin metallic films to substrates has been studied as a function of projectile species. Measurements of the adhesion enhancement of a thin gold film to substrates of tantalum and silicon (with native oxides) have been made for beams of 12C, 16 0, 28 Si, 35C1, and 58 Ni at 2.8 5 MeV/nucleon. The threshold dose required to pass the Scotch tape peel test was found for the Au-Ta system to be D th (cm" 2 ) = 1017 (dE / dx) - 3 0 ± 0 2 where dE/dx is the electronic stopping power (MeVmg" 1 cm 2 ) oftheionin Au. For the Au-Si system, D th = 6X10 18 (dE/dx) " 4 - 1 ± 0 3 . The steep dependence of D th on dE/dx found here is in contrast with an earlier measurement for the Au-Ta system by Tombrello et al. The adhesion enhancement was observed to decrease with time after the bombardment in a manner suggesting that diffusion of atoms through the gold film is important. The possible importance of small concentrations of extraneous atoms at the interface is discussed.
The adhesion of thin metallic films to substrates can usually be improved by the passage of ionizing radiation through the interface. The resulting adhesion can be quite strong, even for insulating substrates such as polymers and oxides. High velocity, heavily ionizing charged particles such as fluorine and chlorine have been shown to be very effective in enhancing adhesion.1 The number of film-substrate combinations investigated has increased rapidly in the last 2 years because of the potential applications of this phenomenon and because of interest in the underlying mechanisms.2"7 The threshold dose D th (ions/cm 2 ) required for a thin film to adhere to its substrate rather than to a strip of Scotch tape peeled at 90° is a frequently used indicator of an ion's effectiveness in enhancing adhesion. The threshold dose has been observed to depend on the stopping power dE /dx (MeV mg " ' c m 2 ) , which is the average energy lost per unit distance of the projectile (either in the film or in the substrate). A previous measurement4 for a film of 500 A of gold thermally deposited on a tantalum substrate and a range of projectiles from helium to holmium (Z = 67) revealed a power law dependence, D th = 4 X10 14 (dE /dx) ~16 ± ° 2 , where dE /dx is the Stopping power in the Au film. The variation of the threshold dose with the stopping power could be an important feature to be explained by any model for the enhancement mechanism. However, in developing such models it is necessary to know whether a power law dependence is a general characteristic of the adhesion J. M
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