Electrical conduction in polyimide between 20 and 350° C

Although conduction in polyimides at elevated temperatures has been widely reported, measurements at ordinary device temperatures have been less well documented. Quantitatively reproducible low field conduction measurements on two device-grade polyimides (PMDA-ODA, BTDA-ODA/MPDA) in the temperature range of 20–350° C and under dry conditions are reported. Aluminum—polyimide—aluminum capacitors are prepared by spin coating an aluminized silicon wafer with between two and four coats of polyimide (prebake at 135° C for 10 min between coats). Samples are cured in dry nitrogen at 400° C for 45 min. Final thickness ranged between 3.3 and 6.6 μm. To permit rapid equilibration of moisture between the film and ambient, the upper electrode is patterned into multiple 25 μm stripes with 5 μ spaces for a total area of 5.1 cm². After a bake-out at 120° C under dry air and subsequent equilibration in a dry ambient at the test temperature, a voltage step is applied to the sample and the current versus time is recorded for 16,000 sec (the charging current). The sample is then shorted, and the discharging current is recorded. Below 100° C, both charging and discharging currents are dominated by a reversible polarization that follows a power law (approximately t^−0.8). Isochronal plots of the polarization current reveal a linear dependence on the applied voltage for fields in the range 10⁴–10⁵ V/cm. The polarization current is nearly independent of temperature and is well modeled by the Lewis molecular dipole theory of polarization. Above 150° C, the current is increasingly dominated by a relatively constant transport current, defined as the difference between charging and discharging currents. This current is ohmic over the field range examined, and shows a complex, activated temperature dependence. For PMDA-ODA the transport current has an activation energy (E _a ) of 0.5 eV below 175° C and 1.5 eV above that temperature. For BTDA-ODA/MPDA the E_a is 0.6 up to 250° C and 2.1 eV above. This corresponds to a resistivity of 9 × 10¹⁸ Ω-@#@ cm at 23° C and 3.5 × 10¹⁴ Ω-cm at 200° C for PMDA-ODA and 5 × 10¹⁹ Ω-cm at 23° C and 5.6 × 10¹³ Ω-cm at 300° C for BTDA-ODA/MPDA. This work demonstrates that the low temperature behavior of polyimide cannot be extrapolated from high temperature measurements. Work sponsored in part by E. I. DuPont de Nemours & Co., Inc.