ABSTRACT Under Bump Metallization (UBM) using Electroless Nickel / Immersion Gold (ENIG) is a low cost option for deposition of UBM layers. Unlike dry methods involving vacuum techniques, this technology uses wet processes and can offer economic and throughput advantages. Throughput advantages are realized by batch processing of several wafers at a time while processing in cassette trays. Use of a dedicated (usually automated) tool for ENIG deposition can yield throughput of up to several hundred wafers per shift depending on the tool size. The selectivity of the process allows for deposition of the ENIG only on the exposed aluminum or copper pads. Wafer manufacturers are continuously searching for various methods to increase throughput and decrease wafer processing costs. Of note, the final layer of many integrated circuit bond pads consists of aluminum or copper. Aluminum can serve as an acceptable surface for standard wire bonding since the wire bonding techniques form an acceptable wire bond through the tenacious oxide layer normally present on the aluminum surface, but this surface is not acceptable for soldering or conductivity. As a result, UBM methods are used in order to form a good bond to the aluminum pad and also help prevent diffusion of metals into the IC itself. Traditionally, dry methods are used to form the UBM (sputtering or vacuum deposition), but these methods require large capital cost for equipment and result in relatively low productivity. These methods are, however, industry accepted and dry metallization techniques are still used for the majority of wafers. Wet chemical methods such as ENIG have been investigated to serve as both a solderable surface for the bumps themselves and as a wire bondable surface while also increasing productivity. The standard processing sequence for ENIG as a UBM process for aluminum pads is shown in Figure 1.