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

Abstract

Focused ion beam ͑FIB͒ methodologies for successfully milling copper ͑U.S. Patent No. 6,322,672 B1͒ have been demonstrated. Approaches to milling copper ͑Cu͒ 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 ͑GAE͒ 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 4ϫ. Moreover, the ͑110͒ crystal orientation, which mills most slowly, forms a Cu 3 Ga 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 Cu 3 Ga 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 Cu 3 Ga phase and the creation of a heavy atom sacrificial layer of the Cu surface ͑U.S. Patent Application No. 20010053605͒ which scatters the incident Ga ion beam, thereby reducing the channeling influence on Cu milling rates. This heavy atom layer is created by flowing W͑CO͒ 6 vapor during the FIB milling process. The CoppeRx scan strategy is especially beneficial for milling thick ͑Ͼ0.8 m͒ Cu structures with large, prominent grains. Because Cu interconnect lines are relatively thin ͑Ͻ0.4 -0.5 m͒, 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 Ͼ20 M ohm open circuits is achieved by flowing O 2 or H 2 O during the milling process ͑U.S. Patent No. 6,322,672B1͒. The O 2 /H 2 O 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 SiO 2 based dielectric. These interconnect cuts have been routinely done at the bottom of high aspect ratio holes ͑e.g., 1ϫ1ϫ9 m͒.

TEM Sample Preparation and FIB-Induced Damage

“…Cu 3 Ga has been observed at the bottom of FIB-milled trenches in Cu. 28 Thin TEM samples prepared from Al may show Ga enrichment at the grain boundaries. In extreme cases, because of its low melting point, Ga can form phases with other metals that have melting points at or below room temperature.…”

“…[28][29][30] The extensive use of FIB milling for sample preparation and the use of ion channeling contrast to characterize and measure grain sizes require that these ion beam-induced modifications be properly understood.…”

Introduction to Focused Ion Beams

“…Finally, while most micromachining is performed using the raster scanning method described above, there are specialized cases when more exotic beam scanning strategies, both with and without the use of chemical precursors, are used (Thaus, T 1996;Casey, 2002). All FIB processes based on chemical precursors are dependent on the combination of FIB ion current, pixel dwell time, raster refresh time and precursor flux chosen for a particular process.…”

Two planar polishing methods by using FIB technique: Toward ultimate top-down delayering for failure analysis

“…This technology offers the means to retain most area of the die whilst removing only localized regions during delayering. 38,39 It provides the capability and feasibility to observe and edit circuit at the same time. These characteristics provide good a To whom correspondence should be addressed.…”