Probing Molecular Structures of Buried Interfaces in Thick Multilayered Microelectronic Packages.

The fast development of flip-chip (FC) devices has helped to keep ICs on pace for Moore’s law. FC devices have many buried interfaces, the structures of which all need to be investigated in order to optimize them. It is extremely difficult to examine molecular structures of buried interfaces in samples with multiple layers due to the lack of appropriate analytical tools. In this paper, real FC-on-leadframe devices, manufactured by Texas Instruments Incorporated, were investigated using the intrinsically interface-sensitive technique and sum-frequency generation (SFG) vibrational spectroscopy. By milling down the Si wafer, we can probe the polyimide (PI)/mold compound (MC) interface. By milling down further, we can examine the air/MC and the MC/leadframe (LF) interfaces. Methylene and methyl groups were detected from each of the three interfaces, and the orientation of methyl groups was determined at each interface. It was calculated that at the PI/MC interface, the methyl group tilt angle was 20° (from the surface normal). For the air/MC interface, the tilt angle was 43.5°, and for the LF/MC interface, the tilt angle was measured to be 45°. The deduced different methyl orientations at the two buried solid/solid interfaces can be interpreted by the interfacial interactions. The PI surface is more hydrophobic than the LF surface; therefore, it has more favorable interactions with the hydrophobic methyl groups, leading to a more “standing up” (smaller tilt angle) position for the methyl groups at the PI/MC interface. In this paper, SFG was successfully applied to study the buried interfaces in real devices. The SFG method used in this paper is general and can be applied to other real-world, complicated thick systems. [ABSTRACT FROM AUTHOR]