Fabrication and characterization of low-cost ultrathin flexible polyimide interposer

• A semi-additive process is used to fabricate an ultrathin flexible interposer.
• These results imply that an increase in the grain size results in a rougher surface.
• The proposed method is a proof-of-concept for 2.5-D/3-D integration using UFPI.

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. In this paper, polyimide (PI) is proposed as an alternative to glass and silicon based interposers for cost-effective 2.5-D/3-D IC integration. 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 semi-additive process consisting of copper seed layer deposition, photolithography, and electrolytic copper pattern plating is used for fabricating a double-sided flexible fan out interposer. A UFPI with electrodeposited micro-scale copper (Cu) fine patterns 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 with fine pitch on 12.5 μm thin PI substrates has been demonstrated. The result is a proof-of-concept to exploit the opportunities of cost-effective 2.5D flexible interposer production.