EFFECTS OF AGING ON POLYIMIDE - A STUDY OF BULK AND INTERFACE CHEMISTRY

We are investigating the bulk and interfacial chemistry of polyimide (PI) exposed to heat and relative humidity (RH) stress (85-degrees-C/85% RH) over long periods to understand the long-term reliability aspects of PI for microelectronic applications. The PI surface chemistry and the PI-Si interface chemistry was followed with electron spectroscopy for chemical analysis (ESCA). For exposure times greater than 550 h, the PI surface exhibits significant changes in emission structure and atomic concentration. The PI-Si interface indicates little change in the PI chemistry when compared to control samples. The water-contact angle at the PI surface shows no statistically significant change as a function of exposure time. Bulk and surface chemistry were followed with Fourier transform infrared spectroscopy (FTIR) in a grazing incidence reflection configuration. There were measurable differences in FTIR spectra between aged and control samples only for the case of very thin PI layers. These results suggest that for exposure times of less than 1 100 h, chemical changes in PI occur primarily at the PI-air interface for samples 1-mu-m thick and greater. These changes represent a possible rearrangement of the imide structure and may be a form of deimidization of the polymer, but the final state is not polyamic acid. The PI surface chemistry changes observed with ESCA are reversible upon reheating to the cure temperature.