Copper device editing: Strategy for focused ion beam milling of copper

Focused ion beam (FIB) methodologies for successfully milling copper (U.S. Patent No. 6,322,672 B1) have been demonstrated. Approaches to milling copper (Cu) are required because standard FIB mill procedures produce rough, uneven cuts that are unsuitable for circuit edits, a principal FIB function. Efforts to develop gas assisted etching (GAE) processes which would smoothly mill Cu failed because Cu halides are not volatile and remain on the substrate as corrosive electrically conductive debris. Single crystal studies show that Cu grains with different crystal orientations vary in mill rate by as much as 4×. Moreover, the (110) crystal orientation, which mills most slowly, forms a Cu3Ga phase when milled with a focused Ga ion beam. This phase is particularly resistant to milling and, in polycrystalline Cu, propagates during the milling operation, contributing to the uneven trench profiles. CoppeRx, a novel scan strategy, cleanly and uniformly removes polycrystalline Cu with minimal damage to the underlying dielectric. CoppeRx minimizes the formation and propagation of the Cu3Ga phase and equalizes the etch rates of the Cu crystal orientations. The CoppeRx strategy includes the milling of an “egg crate” topography to minimize the propagation of the Cu3Ga phase and the creation of a heavy atom sacrificial layer of the Cu surface (U.S. Patent Application No. 20010053605) which scatters the incident Ga ion beam, thereby reducing the channeling influence on Cu milling rates. This heavy atom layer is created by flowing W(CO)6 vapor during the FIB milling process. The CoppeRx scan strategy is especially beneficial for milling thick (>0.8 μm) Cu structures with large, prominent grains. Because Cu interconnect lines are relatively thin (<0.4–0.5 μm), grain-related milling roughness is less of a problem. The CoppeRx egg crate topography and W scattering layer are not required. Instead, the successful cutting of 40 ohm Cu interconnect lines to produce >20 M ohm open circuits is achieved by flowing O2 or H2O during the milling process (U.S. Patent No. 6,322,672B1). The O2/H2O flow smoothes the Cu milling by producing an amorphous surface oxide, thereby reducing channeling, and by enhancing the etch selectivity for Cu relative to the surrounding and underlying SiO2 based dielectric. These interconnect cuts have been routinely done at the bottom of high aspect ratio holes (e.g., 1×1×9 μm).

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