Effects of constant-voltage stress on the stability of Parylene-C columnar microfibrous thin films

A systematic study of the degradation caused by electrical stress in Parylene-C columnar microfibrous thin films (μFTFs) was carried out to explore their suitability for applications in flexible electronics. The columnar μFTFs were fabricated by directing a collimated flux of Parylene-C monomers normally toward a planar substrate in a low-vacuum chamber. Metal-insulator-metal (MIM) capacitors incorporating Parylene-C columnar μFTFs were fabricated and electrically characterized to examine reliability and electrical-stress resistance. The measured leakage current in and capacitance of MIMs were studied as functions of constant-voltage stress (CVS) applied to the MIMs. Room-temperature leakage current in the MIMs before and after CVS was found to be space-charge limited. The space charge comprises defects introduced during fabrication, no new defects being induced by the CVS. Kohlrausch-Williams-Watts relaxation was exploited to understand transient leakage-current behavior, and characteristic times in the 3.5-3.9 s range and stretch factors in the 4.2-5.2 range were determined. These parameters suggest that carrier trapping at defects and their polarization orientation are related to space-charge formation. Furthermore, capacitance dependences on time and frequency were found to be good indicators of the CVS-induced degradation and stability. Charge buildup in the Parylene-C columnar μ-FTFs is accompanied by capacitance decrease with CVS duration. Extrapolation of the capacitance-decrease dependence on CVS duration enables predictions of reliability over extended periods.