Numerical Simulation on DC Breakdown of Polyimide Based on Charge Transport and Molecular Chain Displacement

A DC breakdown model combining charge transport and molecular chain displacement is utilized to simulate the thickness-dependent DC electrical breakdown of polyimide and reveal the physical mechanism of DC breakdown. The free volume existing in dielectric materials provide electrons with free path to be accelerated and gain energy under the electric field. Molecular chains with occupied deep traps can be displaced by Coulomb force under electric field, furthermore, the displacement will enlarge the local free volume. The energy of electron w is determined by the local electric field F and the length of free volume λ_L, which can be expressed as w = eFλ_L. When the maximum energy of electrons exceed the deep trap energy level, the local current and temperature will rise in a surge, triggering breakdown eventually. The simulation results reveal the dynamics of space charge and electric field inside polyimide material before the DC electrical breakdown occurs. The breakdown strength F_b of polyimide films obtained from the DC breakdown model decrease with an increase in sample thickness d, which satisfies an inverse power law F_b = kd⁻ⁿ with n = 0.30. A strong dependence can be found between breakdown field and sample thickness when the influence from molecular chain displacement on free volume is taken into consideration. The simulation results indicate that the DC electrical breakdown may be the result of the interaction of space charge accumulation effect and molecular chain displacement.