In the microelectronics packaging, the adhesion strength between Cu and polyimide is a very important factor which determines the device performance and reliability. However, the adhesion strength of Cu film to polyimide is inherently poor, and often the Cr layer is used as an interlayer between Cu and polyimide to enhance the adhesion properties. To make the electric circuitry in this Cu/Cr/ polyimide structure, Cu and Cr layers were etched off from the polyimide sequently. Number of processing steps can be reduced if the $Cu_{100-x}Cr_x$ alloy is used as an interlayer between Cu and polyimide. Because the Cu and $Cu_{100-x}Cr_x$ alloy layer can be etched simultaneously. In this paper, adhesion strength and reliability of $Cu_{100-x}Cr_x$/polyimide interfaces were studied.
In chapter 1, microstructures and mechanical properties of $Cu_{100-x}Cr_x$ alloy thin films are presented. From the Cu-Cr binary phase diagram, Cu and Cr have very limited solid solubility, 0.89 at.%Cr in Cu (fcc) at eutectic temperature. In sputter deposited $Cu_{100-x}Cr_x$ alloy, the solubility of Cr in fcc Cu was increased up to 17 at.%. The $Cu_{100-x}Cr_x$ alloy with x>17 have the fcc+bcc structure. And the grain size of alloy films was decreased as Cr content was increased. The grain growth and phase separation were not detected after heat treatment at 350℃ for 1 hour. But the $Cr_2O_3$ oxide were formed at surface and interface between $Cu_{100-x}Cr_x$ alloy/polyimide interface. The internal oxidation was not observed because the $Cr_2O_3$ passivation layer prevented the diffusion of oxygen into inside of $Cu_{100-x}Cr_x$ alloy. Also, the Young's modulus, hardness and residual stress of $Cu_{100-x}Cr_x$ alloy were increased as Cr content was increased. Therefore, the higher Cr content in alloys would make the plastic deformation relatively more difficult.
In chapter 2, adhesion strength and reliability of $Cu_{100-x}Cr_x$/polyimide interfaces were presented. The peel strength was very low (1~2g/mm) for x=0, but increased almost linearly with x up to x=17 and saturated around 56 g/mm. The failure mode was changed from interfacial to cohesive in polyimide as the peel strength was increased. The carbidelike Cr-C bonds at interface determined by AES and XPS were increased with x, which is the beneficial effect in interfacial strength. The saturation of peel strength with x was due to the combination effects of changing in interfacial chemistry, phase transition (fcc→bcc) and mechanical properties.
Reliability of Cu100-xCrx/polyimide interface was studied under heat treatment at 350℃ and 85℃/85%R.H. conditions. Degradation behaviors of peel strength after heat treatment were divided into 2 cases. Case I (x=8.5), the interfacial failure was occurred along with the $Cr_2O_3$/polyimide interface and the peel strength dropped to zero. Case II (x≥17), the cohesive failures were occurred although the $Cr_2O_3$ were formed at interfaces. AES and XPS analyses showed that the carbidelike Cr-C bonds at interface were completely depleted due to the formation of Cr-oxide for Case I, but the carbidelike Cr-C bonds were detected at interface despite the formation of Cr-oxide.
The 85℃/85%R.H. (T/H) treatment caused degradation of the peel strength in all specimens, especially for x=8.5 and x=17. The reason of degradation in peel strength for x=8.5 and x=17 was the changing in failure mode from cohesive to mixed (cohesive+ interfacial). The area fraction of interfacial failure was increased with the hold time in T/H and inversely related with peel strength. The interfacial failure was caused by CuCr-oxide which was formed during T/H. But for x≥25, the interfacial failures were not observed even after 840 hours in T/H condition.
