Microbump Processing for 3D IC Integration

Abstract

In recent years, the development of microbumps has allowed even smaller sizes of ICs to utilize the flip chip technique. In addition, microbumps have enabled the implementation of three-dimensional (3D) ICs, which drastically improve the spatial efficiency of packaging. However, as the bumps size decreases and the number increases, several process challenges must be considered, for example, the height consistency of bump, the ratio of miss and deformity bump and the yield and strength of interconnection, etc. Therefore, it is increasingly important to study the interconnection technology and materials of high-density microbump interconnection. After briefly introducing the common electronic packaging techniques, including wire bonding, tape-automated bonding and flip chip, this paper reviews microbumps as an advanced bonding technology. Techniques such as Controlled Collapse Chip Connection - New Process(C4NP), printing, insert bump bonding, and self-replication process are discussed and compared. C4NP can achieve low-cost, fine pitch bumping by utilizing varied lead-free solder alloys, which overcomes the limitation of existing bumping technologies. Depending on the microbump size, engraved mask stump, and photosensitive organic mask and squeegee are the two ways for micro-bump printing. The micro-insert bump bonding process is new to stack chips vertically, which has robust bonding structure and a simpler bonding process compared to Cu pillar bonding process. The self-replication process is using the surface tension property of molten solder between the micro bridged bump to get two bumps with same volume and geometries on each faced pairs of lands. The use of two common material for the microbump, Cu, Sn, and its alloys are presented along with the differences in the process for each. As with any technology, a new breakthrough addressing an issue brings with it its own set of shortfalls. Microbumps are no different. The various techniques and materials used to realize the reduced scale bonding method are subject to a number of challenges. Most prominent among them are electromigration, thermomigration, and thermallyinduced mechanical fatigue, which are discussed in this paper.