Advanced characterization of liners for through vias in glass packages

Glass is emerging as the next generation substrate for advanced packaging but adoption is limited because of numerous hurdles such as adhesion of metal lines, stress management, and unavailability of long-term reliability data on glass. The use of liners between copper and glass in through-glass vias (TGVs) is important to manage their coefficient of thermal expansion mismatch, stress-induced cracking, and overall reliability. Critical to understanding liner performance is characterizing the liner’s thickness throughout the TGV. Here, we investigate various nondestructive imaging techniques to determine which may be sufficient for this analysis. For model systems of study, both organic and inorganic liner materials (Parylene C and Parylene C + AZO) are deposited inside 100-μm-diameter TGVs. Both x-ray microcomputed tomography (micro-CT) and two-photon imaging (TPI) are compared to destructive cross-sectional microscopy to assess their appropriateness for liner characterization. Micro-CT is able to detect the liner and measure a thickness comparable to cross-sectional microscopy; however, the resolution of this image is poor and includes much uncertainty. TPI is also able to detect the liner (when liner material is fluorescent) and measure a thickness comparable to cross-sectional microscopy. TPI can also provide thickness measurements throughout the entire length of the TGV, making it a powerful nondestructive technique for assessing liner quality.