Interfacial adhesion and subcritical debonding of low-k dielectrics in flip-chip-packaged copper/low-k interconnect structures

To improve electrical performance, the semiconductor industry is adopting copper metallization and low-permittivity (low-k) dielectric for on-chip interconnect structures. However, it is currently unclear how copper/low-k implementation will impact thermomechanical reliability, particularly since many candidate low-k dielectrics are polymeric. Compared with currently used SiO2, even the most ideal low-k polymers have increased CTE, reduced strength and lower elastic modulus. In addition to generating a greater local CTE mismatch within the structure, replacing SiO2 with a more compliant dielectric may allow the interconnect structure to undergo a greater degree of deformation on a global scale, especially when packaged. When packaged in a flip-chip configuration, the interconnect structure will be subjected to the global bending of the package, driven by the large CTE mismatch between the silicon chip and organic printed circuit board (PCB).; The first objective of this study was to quantify the fracture behavior of pertinent low-k interfaces using two candidate dielectrics. To this end, the mode-I and mixed-mode critical and subcritical fracture behavior was measured for Dow Chemical's benzocyclobutene (BCB) and SiLK(TM) in various configurations with Ta, TaN, Si3N4 and SiO2.; The second goal was to determine how much energy is available to propagate an existing interfacial crack in a flip-chip package at room temperature. This was achieved experimentally through phase-shifting more interferometry (PSMI) and numerically via finite element modeling. Essentially, PSMI was used to compute the crack driving force and strain-state for an experimentally viable thermal load of -80°C. Finite element modeling was used to verify these results and extend the study to a more realistic thermal load encountered by a flip-chip package at room temperature, about -145°C. Additionally, finite element modeling was employed to examine the variation of crack driving force and mode-mixity with changes in package geometry.; Results indicate that the crack driving force is sufficient to debond BCB/Ta and BCB/TaN interfaces critically and BCB/Si3N4 interfaces subcritically. SiLK interfaces showed greater adhesion, with critical and subcritical energies exceeding the driving force by 100--1000% depending on the interface. Driving force and mode-mixity were relatively insensitive to package geometry, but very sensitive to PCB CTE and modulus.