Stress Relaxation of Polyimides for Microelectronic Applications.

Polyimide films used as inter-layer dielectrics in integrated circuits usually require curing at high temperature. Upon cooling, significant mechanical stress can develop in the film due to the mismatch of the coefficients of thermal expansion of the film and the substrate. This stress can result in processing difficulties (wafer bowing) and possible long-term reliability problems. In order to address these problems we have determined the tensile relaxation modulii over the temperature range from 23 to 190C of three polyimides used for microelectronic circuit fabrication: DuPont PI2555, and Hitachi PL1000 and PIX-L110. The master curves for the relaxation modulus at room temperature were constructed and used to extrapolate the long-term relaxation behavior of these polymers. These results indicate that over a time-frame of years, a significant portion (~60%) of the stress will still be present. Also, two relaxation processes in the glassy state have been identified from the temperature dependence of the shift factors used to construct the master curves.