Relaxation of mechanical stress in polyimide films by softbaking

Polyimides are widely used in microelectronics for their excellent planarization, breakdown behavior and thermal stability. They are now used in optical integrated circuits, chip scale packages, multichip modules, flat panel displays and polymer research for the development of ''super car''. However, their processing is affected by problems of micro-cracking and delamination due to large and non-uniformly induced stresses, which are caused mainly by intrinsic viscosity change in polyimide before and during the softbake cycles. No empirical relationship exists that could be used to avoid the danger of microcracking. The results for two different baking cycles at 90 and 110 degrees C are presented. The plots of viscosity vs, baking time show a rapid drop of viscosity and a gradual rise after a certain time. At the point of inflection the initial viscosity of the DuPont Polyimide, PI 2610, decreased from 3.56 Pa s at 25 degrees C to 0.268 Pa s at 90 degrees C and to 0.062 Pa s at 110 degrees C, giving relaxation times of 0.089 and 0.02 ns, respectively. These were extracted from the data using the model postulated by the authors. Similarly, Hitachi's PIX L110 decreased in viscosity from initial 2.165 Pa s at 25 degrees C to 0.427 Pa s at 90 degrees C and 0.175 Pa s at 110 degrees C with relaxation times of 0.16 and 0.068 ns, respectively. The minimum viscosity versus baking time curves for various polyimides show onset of scission related relaxation could be due to thermally induced effects and will define the minimum thermal budget baking cycles.