High-resolution XPS spectromicroscopy study of micro-patterned gold–tin surfaces

We report on high-resolution chemical state mapping performed with a laboratory, spectroscopic X-ray photoelectron emission microscope (XPEEM) on electro-deposited gold–tin patterns (100 μm × 20 μm) for packaging applications of micro-systems. The analysis conditions were selected to achieve a lateral resolution below 1 μm and an energy resolution of 0.8 eV for Au4f7/2 and Sn3d5/2 for images acquired over 5 min. The chemical state maps reveal the non-uniformity of the gold/tin chemistry over regions of typical width of less than 10 μm. More precisely, the coexistence of Au and AuSn4 states, and the heterogeneity of Sn oxidation were shown. The results evidence the role of the partial delamination of the photo-resist during the electro-deposition of tin due to degraded adhesion properties on gold. These conclusions are drawn from direct inspection of the raw image data sets, and are supported by statistical treatments using Principal Component Analysis (PCA). This is a clear example of the interest in using high resolution XPS imaging by XPEEM at a practical level in applied surface science, and its complementarity to other surface imaging techniques.