Advanced packaging technologies used for the sealing of micro-systems such as MEMS actuators use wafer-on-wafer sealing processes with low-melting point, chemically inert metals such as gold-tin alloys. Here, high-resolution XPS chemical state mapping with energy-filtered XPEEM (NanoESCA, developed in close co-operation by Omicron NanoTechnology and FOCUS GmbH) is used to study gold-tin test patterns prepared by electrochemical deposition. The results highlight the chemical non-uniformity of gold-tin chemistry over typical pattern width of less than 10µm.
Image 3: shows the typical position-dependent chemical states of both tin and gold, outside and inside the pattern. Full field chemical state maps where used to extract local microspectra from an area of interests with a size of 2-3 µm. Outside the pattern (red and green AOI), no gold is found, and tin is mainly present in the oxidized form (SnO2 state), but with still significant proportion of metallic Sn (presumably deeper below the surface oxide layer). Inside the pattern (blue, magenta, cyan, yellow AOIs), tin is mostly in a metallic state with a minor proportion of oxide, this proportion being location-dependent. Gold shows up strong chemical heterogeneities within the pattern, with almost exclusively pure gold at 84 eV in the central region, whereas closer to the edges, a strong chemical shift in the Au4f7/2 core-level micro-spectra is observed; in agreemend with literature, this latter is indicative of the formation of AuSn4 alloy state of gold (85.0 eV binding energy), not excluding a possible, though minor contribution of the AuSn state expected at 84.5 eV, however difficult to confirm given the overall energy resolution of 0.8 eV.