A scalable strategy for β-Sn orientation control in advanced electronic packaging via sputter-deposited α-CoSn₃ seed layers

Controlling the crystallographic orientation of β-Sn is critical for improving the reliability of micro joints. In this study, we demonstrate that sputter-deposited α-CoSn3 thin films can serve as effective seed layers to regulate β-Sn grain orientation. By tuning sputtering parameters, adatom mobility during film growth was modulated to obtain distinct α-CoSn3 textures. A high-energy DC deposition condition (3.3 mTorr, 100 W) produces a pronounced (600) texture after annealing, whereas a low-energy RF deposition condition (7.6 mTorr, 30 W) yields a (312) texture. Surface energy and work of adhesion calculations using a machine-learning interatomic potential, cross-validated by density functional theory, attribute this texture switch to a transition in the dominant thermodynamic driving force: under DC deposition (high adatom mobility), texture selection is governed by surface-energy minimization, favoring the low-surface-energy (0.46 J/m2) α-CoSn₃ (600) plane; under RF deposition (low mobility), it is governed by interfacial-energy minimization driven by lattice matching to the Si(100) substrate. Owing to the excellent lattice matching and high work of adhesion (W_ad = 2.28 J/m2) between α-CoSn₃ (600) and β-Sn (100), Sn grains nucleated on (600)-textured films exhibit strong preferential orientation, with the c-axis nearly parallel to the substrate. Conversely, (312)-textured α-CoSn₃ films do not provide favorable lattice matching (W_ad = 0.82 J/m2) and thus yield randomly oriented β-Sn. By integrating sputtering control with lattice-matching theory, this work establishes a scalable strategy for controlling β-Sn grain orientation in advanced electronic packaging.