Orientation-controlled epitaxy of CoSn anisotropic conductive thin films on sapphire substrates toward interconnect applications

The increasing resistivity of Cu interconnects at reduced device dimensions requires alternative conductors with suppressed size effects of resistivity. Materials with highly anisotropic electronic transport are a promising solution. CoSn exhibits pseudo-one-dimensional transport along the c axis with low resistivity comparable to conventional metals, making it a promising candidate for next-generation interconnects. In this study, we investigate the epitaxial growth of CoSn thin films on sapphire substrates with controlled crystallographic orientations to align the low-resistivity c axis in the plane. We successfully grow single-crystalline CoSn(101¯0) and CoSn(112¯0) films on sapphire substrates using appropriate buffer layers. Although the (112¯0) plane has a higher atomic packing density and is thus expected to yield lower surface energy and smoother films based on an analogy with conventional metals, the two orientations show markedly different surface morphologies: CoSn(101¯0) forms comparatively smooth films, whereas CoSn(112¯0) exhibits pronounced {101¯0}-faceting and rough surfaces. First-principles calculations reveal that the (101¯0) surface has lower surface energy than the (112¯0) surface, consistent with the experimentally observed facet formation. This demonstrates that surface energetics in CoSn are governed not by atomic packing density but by anisotropic bonding associated with the kagome lattice. Electrical transport measurements reveal strong dependence of resistivity on crystallographic orientation, consistent with the bulk properties, with the lowest resistivity achieved along the c axis, down to 10.6 μΩ cm for 50 nm-thick CoSn(101¯0). This work establishes orientation-controlled epitaxy on 300 mm-wafer-compatible sapphire substrates as a viable strategy for integrating CoSn into next-generation interconnect architectures.