Using Lasers to Create Metal-Polyimide Films with Alternating Layers
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Using Lasers to Create Metal-Polyimide Films with Alternating Layers Folasadé Faulkner and Brent Koplitz Tulane University, Department of Chemistry, New Orleans, LA 70118, U.S.A. ABSTRACT Metallized polyimide polymer films are of interest owing to their wide scope of applications including adaptive optical mirrors, solar dynamic power generation, radiation shielding, and thermal control coatings. Palladium-based polyimide films have proven to be unique in terms of thermal reduction, photochemistry and nanoparticulate distribution within the polymer matrix. When Pd polyamic acid is exposed to lamp radiation for 15 hours prior to curing, an additional nanoscale metallic interlayer is created within the surface-metallized polyimide matrix. This photo-process creates alternating nanoparticulate layers of palladium metal. Whilst previous efforts have produced metallic interlayer in 15 hours, we show that an excimer laser reduces this time to 4-8 minutes. Preliminary results using a femtosecond laser show that a metal interlayer can be formed with as little as 8 seconds of exposure time. Transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy (XPS) are used to characterize the layers. In addition to Pd, surface metallization and interlayer formation using other metals is discussed. INTRODUCTION A practical aim of metallized-polymer research is the development of lightweight, flexible, and thermally-stable materials that have aeronautic and microelectronic applications. Palladium-based polyimide (PI) films possess desirable characteristics in terms of their thermal properties, photochemical behavior, and nanoparticulate distribution within the polyimide matrix. A nanoscale surface-metallized Pd polymer is made by thermally curing Pd-doped polyamic acid films at 300°C [1]. Previous work by Gaddy et al. showed that when Pd-doped polyamic acid is exposed to lamp radiation for 15-25 hours prior to curing, an additional nanoscale metallic interlayer is created within the surface-metallized polyimide matrix [2]. These etalon-type films have the ability to transform electromagnetic radiation into mechanical energy [2], and possible applications of these films include natural laser cavities, narrow band pass filters, and microelectromechanical (MEMS) switches. The early lamp work showed that radiation intensity can affect interlayer depth [2]. A major advance in the photogeneration of metallic interlayers occurred when a NASALangley/Tulane collaboration demonstrated that an excimer laser could reduce the requisite irradiation time from hours to minutes [3]. Subsequent work has shown that the use of a beam homogenizer enables one to control the photon intensity and thus the interlayer depth [4]. As will be shown in the Results and Discussion Section (Fig. 3), too little light will not produce an interlayer, while too much light can actually inhibit interlayer growth. Here, the surface of the polymer is probably altered in some way thereby reducing/eliminating the penetration depth of the laser beam. In the curre
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