Vacuum ultraviolet (VUV) surface treatment process for flip chip and 3-D interconnections

A novel surface treatment process at room temperature was developed using vacuum ultraviolet (VUV) light with a wavelength of 172 nm. There is no temperature increase, no damage by ion bombardment and no plasma charging during the process. Immersion Au plating over electroless Ni plating and electroplated or evaporated SnCu were used as bonding pad and bonding bump materials for the evaluations. Two different VUV cleaning conditions, with N₂ or O₂ gas in the chamber, were compared and evaluated. Cleaning times and ambient conditions have dramatic effects on the surface contact angles. The initial contact angles of DI water before VUV cleaning for both the Au blanket and SnCu were near 80deg, but VUV cleaning to cut the organic contaminants decreased the contact angles down to around 30deg and 10deg, respectively. In addition to comparing the contact angles, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) were used to study the surface elemental composition and geometry of the bump and pad surfaces before and after the VUV cleaning. The photoelectron spectra of C 1s were obtained by XPS analysis for information on the chemical species and the XPS results showed a reduction in surface carbon for both Au and SnCu after the cleaning. The evidence indicates cleavage of the carbon-carbon bonds in the organic molecules occurs during the cleaning process. The experiments show that the VUV cleaning can effectively remove the organic contaminants on the surface of the bonding pads without surface damage or charge accumulation.