Smoothing Thin Films with Gas-Cluster Ion Beams
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Smoothing Thin Films with Gas-Cluster Ion Beams D.B. Fenner,* J. Hautala,* L.P. Allen,* J.A. Greer,* W.J. Skinner* and J.I. Budnick** *Epion Corporation, Billerica, Massachusetts **Dept. of Physics, University of Connecticut, Storrs, Connecticut ABSTRACT
Thin-film magnetic sensor and memory devices in future generations may benefit from a processing tool for final-step etching and smoothing of surfaces to nearly an atomic scale. Gascluster ion-beam (GCIB) systems make possible improved surface sputtering and processing for many types of materials. We propose application of GCIB processing as a key smoothing step in thin-film magnetic-materials technology, especially spin-valve GMR. Results of argon GCIB etching and smoothing of surfaces of alumina, silicon, permalloy and tantalum films are reported. No accumulating roughness or damage is observed. The distinct scratches and tracks seen in atomic-force microscopy of CMP-processed surfaces, are removed almost entirely by subsequent GCIB processing. The technique primarily reduces high spatial-frequency roughness and renders the topographic surface elevations more nearly gaussian (randomly distributed). INTRODUCTION
Surface and interface roughness, intermixing and defects are well known problems in magnetoresistive (MR) thin-film devices [1], and in particular for the giant-MR (GMR) spin valve. With roughness, the morphology (random or structured) also plays an important role. The coercivity of single-layer ferromagnetic (F) films was reported to increase when the substrate was intentionally roughened prior to film deposition [2]. With thin trilayers (and multilayers) containing a nonmagnetic film (N) between two F layers (F/N/F), the 2D quantum confinement causes hybridization of the s and d electrons leading to the MR [3,4]. Thickness steps as small as monolayer heights have been proposed to induce (biquadratic) fluctuation coupling of the two F layers across a very thin N layer [5]. It is observed that when the N layer thickness is systematically varied, the coupling of the F layers is oscillatory in strength and favors antiparallel, in-plane orientation of the magnetic moments, with first maximum at ~2 nm [6,7]. When the shield and seed layers have the desired fcc-(111) orientation of the (columnar) grains, the morphology of the overlying F/N/F stack will be rough but conformal, as deposited. The simple concept of so-called “orange-peel” coupling can be useful for gaining some intuition as to cause of the (undesirable) parallel orientation of the F layers [8]. A magnetostatic model, originally from Néel [9], was proposed for analysis of the effect of roughness on the coupling energy (J) of F/N/F stacks [8,10,11]. The Néel coupling energy (for very thick F layers) is: J = [ ( π2 h2 µ o M F M P ) / ( √2 λ ) ] exp ( - 2 π √2 t / λ )
(1)
where h is the height of the roughness and λ its (sine) wavelength, MF and MP the free and pinned layer moments, and t the thickness of the N layer. In Fig. 1(a) is a schematic of a wavelike interface and in Fig. 1(b) the Eq. (1) f
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