Universal Energy Dependence of Sputtering Yields At Low Ion Energy
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Universal Energy Dependence of Sputtering Yields At Low Ion Energy J. Muri and Ch. Steinbriichel Materials Engineering Department and Center for Integrated Electronics Rensselaer Polytechnic Institute, Troy, NY 12180-3590 ABSTRACT Sputtering yields Y(E)at ion energies E :S 1 keV are shown to be described by the equation Y(E) = A(En - Enh) where A, n, and the threshold energy E,h are constants characteristic for a particular projectile/target combination. Examination of a wide variety of systems reveals that n = 0.5 provides an excellent universal representation of a large body of data, including physical sputtering of metals by noble gas ions, selfchemical sputtering of Si and Si0 2 . The sputtering of metals, as well as physical and 4 above value for n is consistent with a 1/r power law atom-atom interaction potential within Sigmund's theory of sputtering. Another conclusion is that the effect of Egh on Y(E) must be taken into account at ion energies as high as 1 keV, not just near the sputtering threshold. INTRODUCTION Bombardment of a solid with energetic ions is an important part of many thin film processes. For example, sputtering of a target in a discharge or with an ion beam is employed widely for the deposition of thin films. Ion bombardment of a film during growth strongly affects its microstructure and properties and may promote epitaxial growth under favorable circumstances [1,2]. In dry (plasma) etching of thin films, ion bombardment-enhanced chemical reactions make possible the anisotropic removal of material for accurate pattern transfer [3]. At the same time, ion bombardment may have a deleterious effect on film properties by introducing compositional or structural damage [4]. In addition, the energy deposited by the ions leads to removal of substrate atoms by sputtering [5,6]. This may limit the rate of filn growth as well as the selectivity in dry etching. Both damage production [4] and sputtering [5,6] increase with ion energy so that one needs to find an optimum ion energy in terms of the trade-off between beneficial and harmful effects induced by ion bombardment. Thus, it is clear that sputtering at low ion energy, a regime not well explored yet, will be of growing interest in thin film processing. This paper focuses on the behavior of sputtering yields at very low ion energy [5]. We show that for a wide variety of systems there is a universal dependence of the sputtering yield on the ion energy all the way down to the threshold energy for sputtering. The systems considered include self-sputtering of metals, and sputtering of metals, semiconductors, and insulators by noble gas ions. THEORETICAL MODEL The generally accepted picture for sputtering has been elaborated by Sigmund [6,7]. An energetic ion impinging on a target produces a collision cascade of target atoms, some of which may acquire enough energy to leave of the target surface. The sputtering yield Y, i.e., the number of atoms removed per incoming ion, can be expressed as Y(E) =
,S. (ElE,,).
(1)
In Eq. (1) Cp, and Ep, are constants d
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