Pd-coated Cu wire bonding technology: Chip design, process optimization, production qualification and reliability test for high reliability semiconductor devices

Due to the increasing cost of Au, the semiconductor industry has gone through a dramatic shift away from Au wire bonding to Cu wire bonding. Pd-coated Cu (PdCu) wire has become the favored wire choice over bare Cu wire for fine pitch applications. The advantages of PdCu wire include better HAST reliability results and a more stable stitch bond process window. This paper will examine the critical aspect of bringing Cu and PdCu wire technology from chip design to wire bonding process development to qualification, reliability test and final production. Chip design needs to take into consideration the requirement of Cu wire bonding. Compared to Au wire bonding, Cu wire bonding has many more challenges including pad material push out (pad splash), Al layer peel off (pad peel) and crack in the barrier and dielectric layer (pad crack). This paper considers the impact of various pad structures & metallization to PdCu wire assembly quality and reliability. Because of the unique challenges facing Cu wire bonding processes, wire bonding process development becomes one of the bottlenecks in the assembly process. To shorten the development cycle and ease the requirement for extensive Design Of Experiments (DOEs), a new model based approach was developed and implemented on the latest Cu wire bonder. With this approach, the bonding parameters are calculated based on numerical models. It reduces the need for parameter optimization. This paper will also discuss how modifications and newly developed metrology tools are used in the optimization of PdCu wire. Because of the narrow process window in Cu and PdCu wire bonding, achieving reliable production and high yields requires the process to have a sustainable process window and be portable and transferable to multiple machines and be robust across material variations commonly encountered in a production environment. This paper examines the methods used in quantifying process windows and portability of the wire bonding equipment to ensure high reliability for the end product. Development and optimization of PdCu wire bonding requires a fundamental understanding of interfacial reaction in bonds during both bonding and reliability tests, as proper formation of intermetallic compounds is beneficial to bonding strength. While the knowledge of intermetallic growth in bare copper wire bonds on aluminum has become well established in the past five years, there is little literature in terms of the morphology and growth of intermetallic phase between PdCu wire and Al pad. This paper ascertains the intermetallic growth and phase transformation, retention of aluminum oxide, and behavior of palladium in PdCu wire bonds using high resolution transmission electron microscopy (HRTEM) of dual beam focused ion beam (FIB) thinned specimens.