Fabrication and characterization of ultrathin fine-pitch flexible polyimide interposer

A three-dimensional system-in-package (3-D SiP) based on silicon carriers or an interposer is a fast-emerging technology that offers system design flexibility and the integration of heterogeneous technologies. A passive interposer, which is a way to bridge the feature gap between the integrated circuit (IC) and the package substrate, is a key building block for high performance 3-D systems. The need for high-density interconnection to support 3-D integration has led to a growing demand for the development of fine-pitch and small vias on the interposer. Polyimide base films have been used extensively for flexible and high-density electronic interconnection applications because they offer good thermal and chemical stability with a lower dielectric constant. In this paper, the development of interconnect technology using an ultrathin flexible polyimide interposer (UFPI) for 2.5-D/3-D packaging applications is described in detail. A UFPI with electrodeposited micro-scale copper (Cu) fine patterns (≦50 µm) and laser drilling microvia is investigated using a scanning electron microscope (SEM), energy-dispersive spectrometry (EDS), X-ray spectrometry, and an optical 3-D profilometer. The UFPI can be used to support the very high I/O, high-performance, high-density, and fine-pitch ultrathin chips packages.