A novel gold deposition process for wafer applications

The deposition of nickel and gold layers as under bump metallurgy (UBM) for wafer bump applications has been established as a viable means of ensuring adhesion and bond reliability. These nickel/gold layers can be used in subsequent processes that employ either wire bonding directly to die pads or screened and reflowed solder paste for connectivity to next level substrates. This paper will discuss the process chemistries used to produce nickel/gold metallurgies over copper or aluminum die pads. Conventional immersion gold processes deposit gold from solution via a displacement reaction with the surface of the nickel layer. This reaction continues until the gold completely covers any available nickel sites. This mechanism, therefore, limits the possible thicknesses obtainable from immersion gold processes to about 0.10-0.15 microns. A new gold deposition process will be described that operates in a similar way but is not limited by the displacement mechanism. This chemistry is capable of depositing significantly higher thicknesses of gold over nickel substrates, thus allowing this technology to be used for many new applications that normally would require electrolytic deposition of gold. The advantages of using an ´immersion´ process are it´s ability to deposit very uniform deposit thicknesses across a wafer surface, no requirement for electrical continuity across the plateable surface and the practicality of processing a large number of wafers at one time. Performance results and operating characteristics of the new gold process will be reviewed. Pretreatment processes necessary to ensure optimum reliability will also be discussed.