Thin films of polyimide which exhibit enhanced resistance to moisture have been fabricated using the Langmuir-Blodgett (LB) technique. Langmuir-Blodgett films are ultra-thin organic coatings of thicknesses ranging from tens of angstroms to a micron, 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 the uv spectra of the films. The results demonstrate that LB films of polyimide adhere better to fused silica than the spincoated films. In addition, water vapor transmission rate measurements 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 volt bias, on samples which are coated with various combinations of LB and spin-coated polyimide films.Composite films of polyimide consisting of a LB monolayer, either underneath or on top of a thick, spincoated 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.
I "Thin organic films are used widely in the electronics industry as photoresists, encapsulants. insulators, and dielectrics. These films are typically fabricated by spin or spray coating to thicknesses ranging from tenths of a micron to a millime~~r~-&angmuir-Blodgett (LB) films are appealing for electronics applications because they are highly ordered. densely packed, and ultrathin, ranging from one molecu1,ar length to tenths of microns.['] The films are deposited by a serial transfer of single layers, from a compressed monolayer held at constant pressure on a water surface, to a solid substrate. Schematics of the Langmuir-Blodgett process are shown in Figure 1. As the monolayer is compressed, the most energetically favorable configuration is for the molecules to be densely packed and highly oriented. This occurs spontaneously for amphiphilic molecules, which consist of a polar moiety attached to long hydrocarbon chains.The specific structure and stability of the films are controlled through the chemistry of the water bath, and the chemical nature of the substrate and organic molecule.Recently, a method has been developed by which polyamic acids are made amphiphilic and ultra-thin polyimide films can be fabricated by the LangmuirBlodgett process.[21 It has been demonstrated in our laboratory that polyimides processed by the LangmuirBlodgett process are more resistant to chemicals than amorphous films of those materials.[31 This discovery has lead us to investigate both the adhesion and water permeability of polyimide LB films e...
