Identification of significant process parameters in variable frequency microwave curing

Variable frequency microwave (VFM) curing of polymer dielectrics can reduce process time compared to conventional thermal methods drastically without compromising intrinsic material properties. However, the interactions that occur during VFM curing are not well understood. This paper presents a statistical experimental design to determine the significance of VFM processing parameters. A D-optimal experiment is performed on samples of polyimide spin-cast on silicon wafers cured in a VFM furnace. During VFM processing, the temperature of the polyimide samples is ramped to a specific temperature and held for a selected amount of time. Temperature calibration is conducted to guarantee accurate temperature exposure. The input variables analyzed are cure time, ramp rate, hold temperature, center frequency, bandwidth, and sweep rate. The output variables of interest are the in-plane and through-plane indices of refraction, birefringence, and the percent of imidization of polyimide. The percent imidization is measured using attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. Birefringence is derived from the in-plane and through-plane indices of refraction, both of which are measured with a metricon prism coupler. Analysis of variance (ANOVA) is used to determine the input variables with the greatest effect. Cure time is found to be the only significant parameter.