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December 9, 2015

Advanced lithography and electroplating approach to form high-aspect ratio copper pillars

Solid State Technology, December 2015

Recent years have seen rapid development in the area of advanced packaging. In general, advanced packaging processes are concerned with the interconnection of multiple chips in a single package to provide increased functionality and performance in a smaller volume. System Scaling Technology—the combination of front-end, middle-end and back-end to advance microelectronic systems—utilizes many different advanced packaging approaches, one of which is known as 2.5D packaging. The term “2.5D packaging” has not always been used consistently in literature. The definition used for the purpose of this paper can be summarized as follows: a 2.5D package utilizes an interposer between multiple silicon die and a system-in-package (SiP) substrate, where this interposer has through vias connecting the metallization layers on its front and back surfaces (FIGURE 1).

FIGURE 1. A 2.5D IC/SiP using an interposer and through vias.

The development of these new packaging schemes is being driven primarily by the rapid growth in mobile handheld devices such as smartphones. Often, the manufacturing processes used are adaptations of well-established front-end processes. A number of different approaches are in development or already in production, including wafer-level chip scale packaging, copper pillar bumps on through silicon vias (TSVs), fan-out wafer level processing, and many more. Of particular interest is the replacement of solder bumps by fine pitch copper pillar bumps, which has been the subject of many new system- in-package designs. Here we investigate the lithography and plating of copper pillars, with focus on heights in excess of 100μm and diameters of 25μm, in anticipation of future SiP requirements.

The increase in the number of I/O channels required by multi-chip system designs has exceeded the density and pitch capabilities that traditional solder bump processes can deliver, so that an alternative connection scheme is required. For interposers, the key enabling technology has been the development of fine pitch copper pillar bumps to provide the high-density interconnection between the interposer and the die. Copper pillar bumps provide a number of advantages over the solder bumps they are supplanting. They can deliver finer pitches, 40μm and less have been demonstrated. They also provide superior electromigration performance in applications where high current-carrying capacity is required. However, lithography and electroplating for fine pitch copper pillar bumps can be particularly challenging. The pillars are electroplated into openings in a thick layer of photoresist which exceeds the capability of most front-end tools. Typically, today’s copper pillars range from 30-50μm in height, with height to width aspect ratios from 1:1 to around 2:1 Here we describe the lithography, resist, and electroplating systems and processes required to create 5:1 aspect ratio copper pillars with heights in excess of 100μm.

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