In-Situ Monitoring of Polyamic Acid Imidization with Microdielectrometry

The kinetics of the imidization reaction of various polyimides used in microelectronic applications have previously been studied by infrared spectroscopy. This paper reports the use of a new dielectric technique, called Microdielectrometry, to monitor the imidization reaction of PMDA-ODA polyamic acid films deposited on the surface of an integrated circuit. Dielectric permittivity and loss factor in the frequency range 1–1000 Hz were measured with Microdielectrometry probes during imidizations at temperatures between 100 and 160°C. At temperatures below 130°C, the permittivity remains dispersive even after five hours; whereas at higher temperatures, the permittivity becomes constant in less than two hours. The frequency dependence of the loss factor indicates that the film exhibits a weakly dispersiv bulk conductivity that varies between 3×10−10 and 5×10 4(Ohm cm)−1, increasing with temperature but decreasing with degree of imidization. At temperatures below 130°C, the conductivity remains relatively high even after five hours; whereas above 130°C, the conductivity decreases much more rapidly to low values, although conductivity changes are still detectable in the films at 2.5 hours. Results show a significant change in rate of reaction between 120 and 140°C, but with clearly observable changes still taking place well after the nominal one hour cure time.