Collision Cascade Densification of Materials
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COLLISION CASCADE DENSIFICATION OF MATERIALS R.H. BASSEL, T.D. ANDREADIS, M. ROSEN, G.P. MUELLER, AND G.K. HUBLER Naval Research Laboratory, Washington, D.C. 20375-5000 ABSTRACT Computer simulations of the collision cascade process were used to investigate surface densification during ion beam assisted deposition (IBAD). The objective of the investigation was to see if densification resulted directly from the cascade, without any contribution from diffusion enhancement brought on by the bombardment. Calculations, using the computer code MARLOWE, were carried out for Ar bombardment of a crystalline Ge target, containing a void, using ion beam energies of 0.065, 0.5 and 1 keV. These results were used as data for a differential equation that describes the effect on void size of the simultaneous Ge atom deposition and Ar ion bombardment of a substrate containing voids. The present attempt examined the effects of irradiations on voids of 17 and 35 vacancies. INTRODUCTION In the process of ion beam assisted deposition (IBAD), a beam of low energy ions impinges on the substrate simultaneously with atoms from a vapor source[l]. The energy deposited in the surface by the ions produces near bulk density films as compared to physically vapor deposited (PVD) films, which frequently contain a significant fraction of voids[2]. The increased density of IBAD films is primarily responsible for improved film properties, which include stability of the refractive index[3], reduced film stress[l,4], high adhesion[4], and resistance to chemical attack. These properties, and the low substrate temperature during deposition, make the IBAD technique attractive for applications in thin-film optics and for making coatings that protect against corrosion, wear and oxidation[5]. Yehoda et al.[2] have shown that about a 10% void fraction in PVD Ge films was completely eliminated by simultaneous bombardment with 65 to 300 eV Ar ions. The value of the arrival ratio R of Ar ions to Ge atoms that was required for complete densification agreed well with earlier experiments by Hirsch and Varga[4]. In that work, a critical arrival ratio Rc was required to prevent spontaneous delamination of the films. The energy dependence of Rc was R = k E",
(1)
where k is a proportionality constant. These two measurements strongly suggest that the reduced stress in the film is a result of the elimination of voids. Brighton and Hubler[6] analyzed the results of Hirsch and Varga by means of simulations of collision cascades and showed that the energy dependence of Rc could be predicted with the assumption that densification is complete when every atom in the deposited film, on the average, is involved in a collision cascade (equivalent to 1 displacement per atom ). This result suggested that the densification of the films is caused by atomic rearrangements within the total volume of the collision cascades and not by thermal spikes or enhanced surface migration of adatoms. It is desirable to develop a detailed understanding of the densification phenomenon, which would
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