A very thin eta-Cu6Sn5 layer has been formed by dipping thin Cu foil into molten Sn at 240 degrees C for 1 s and quenching in ice water. After removing electrolytically the Sn on the surface, the eta-Cu6Sn5 grains on the Sn side are shown to have a worm-type shape 0.3-0.5 mu m wide and up to 2 mu m long. Transmission electron microscopy (TEM) analysis shows that the eta-Cu6Sn5 grains on the Cu side can be as small as 5 nm, indicating that the eta-Cu6Sn5/Cu interface is the nucleation side. The orientation relationships between eta-Cu6Sn5 and Cu are determined by TEM. The eta-Cu6Sn5 (11 (2) over bar0) surface is the interface with both the Cu (001) and ((1) over bar 11) surfaces, and a common orientation relationship of (0001)(eta)//(110)(Cu) is present on both interfaces. The match of atoms between eta-Cu6Sn5 and Cu on the interfaces is analyzed. (C) 2010 Elsevier B.V. All rights reserved.