Enhanced moisture protection of electronic devices by ultra-thin polyimide films

Thin films of polyimide which exhibit enhanced resistance to moisture have been fabricated using the Langmuir-Blodgett (LB) technique. LB films are ultrathin organic coatings of thicknesses ranging from tens of angstroms to a micrometer, characterized by high molecular density and a high degree of structural order. The adhesion strength of both LB and spin-coated films of several different polyimides, deposited on fused silica, has been measured by subjecting these films to steam or water, followed by a tape test, and monitoring changes in their UV spectra. The results demonstrate that LB films of polyimide adhere better to fused silica than the spin-coated films. In addition, measurements of water vapor transmission rate through Kapton sheet coated by a monolayer of any of the polyimides show that a monolayer forms a moisture barrier, decreasing the water vapor transmission through the Kapton. Based on these results, the electrical performance of polyimide films has been tested at 85°C, 85% humidity, by measuring leakage current between conducting paths under 180-V bias, on samples which are coated with various combinations of LB and spin-coated polyimide films. Composite films of polyimide consisting of an LB monolayer either underneath or on top of a thick, spin-coated film exhibit superior electrical performance to either a spin-coated or LB film by itself. This may be explained by the improved adhesion and/or decreased water permeability of polyimide LB films