Excimer Laser Induced Damage in Stressed Polyimide Films Exposed in Air
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EXCIMER LASER INDUCED DAMAGE IN STRESSED POLYIMIDE FILMS EXPOSED IN AIR K. Tonyali, L C. Jensen, and J. T. Dickinson Department of Physics Washington State University Pullman, WA 99164-2814
ABSTRACT In earlier work we reported the consequences of simultaneous exposure of polymers to mechanical stress and electron bombardment. In this study we examine the response of highly stressed polyimide films to excimer laser radiation (20 ns pulses @ 248 nm wavelength) in air. The exposed surfaces show evidence of surface and near surface damage, crack initiation, and eventually crack growth over a wide range of applied stress and laser fluence. These results show that the morphology of the stressed material has a significant influence on the resulting damage and suggest that the regions of highest damage are those experiencing the highest local stress. A growth of nodules on the polyimide surface are also observed which are missing when exposed In vacuum. INTRODUCTION Understanding of the chemical and physical stability of polymeric materials in harsh service environments is a long-term goal to extend the applications and lifetime of components. In environments involving radiation such as high energy electrons and ions, and ultraviolet (UV) radiation a number of damage processes can occur, which may modify the properties of materials and result in surface damage and the deterioration of mechanical properties [1-6]. These harmful environments include space, various plasmas, and solid rocket propellents during a burn. The use of radiation may also improve our understanding of the physics and chemistry of the fracture process of stressed materials. It also offers potential applications regarding controlled direct rupture of bonds, e.g. controlled cutting and shaping of materials, as well as roughening and chemical modification of surfaces in preparation for applying coatings, adhesive bonding, etc. Finally, the use of polymers as sensitive resist materials in the microelectronics industry encourage additional studies of intense photon/polymer interactions. We have previously reported on the consequences of simultaneously subjecting polymers to tensile stress and particle bombardment, principally electrons [2-7]. We showed that crack initiation and crack growth in stressed polymers were greatly enhanced by electron beam irradiation. We have proposed that damage formation and microcracking via bond breaking are the main contributors to the failure of polymers under these stimuli. Furthermore, we have suggested that there may be an enhanced localization of excitation produced by electron collisions with stressed molecular bonds which promotes irreversible bond breaking. In this work we extend our investigation to another radiation source, namely short pulses of UV laser light. Short pulses of UV light are known to cause photochemical bond scissions as well as thermal excitation In polymers [8-16]. This type of radiation has been shown to cleanly etch a number of polymers such as polylmide and poly(methyl methacrylate) at excimer
