A Comparative Study of the Photoablation of Polyimide-Doped Poly(Tetrafluoroethylene) at 308 NM and 248 NM
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A COMPARATIVE STUDY OF THE PHOTOABLATION OF POLYIMIDE-DOPED POLY(TETRAFLUOROETHYLENE) AT 308 NM AND 248 NM G. C. D'Couto," S. V. Babu," F. D. Egitto,'" and C. R. Davis"*Department of Chemical Engineering, Clarkson University, Potsdam, NY 13699 "*IBMCorporation, Technology Products, Endicott, NY 13760
ABSTRACT Experimental data on the 248 nm and 308 nm wavelength excimer laser ablation of poly(tetrafluoroethylene) (PTFE) doped with polyimide (PI) are reported for a range of fluences and dopant concentrations. Threshold fluences were determined and correlated with the dopant concentrations. The threshold fluences and the limiting etch rates measured at high fluences decreased with increasing dopant concentration and there is a minimum absorption coefficient below which there is no ablation at both the wavelengths. The etch rates have been modeled using a two parameter Arrhenius thermal model to describe the etching process.
INTRODUCTION Polymers are essential components in most microelectronic devices- 2 and are selected based on their thermal, mechanical, chemical and dielectric properties. Materials like poly(tetrafluoroethylene) (PTFE) (dielectric constant (e)-2. 1) and biphenyl tetracarboxylic acid dianhydride-phenylene diamine (BPDA-PDA) polyimide (PI) (e-2.9) with low dielectric constants are important components in high density electronic packages that perform at higher signal processing speeds. A low dielectric constant, chemical inertness and thermal stability make PTFE potentially one of the most attractive polymers for microelectronics packaging. However, processing difficulties have limited its widespread application. Excimer UV lasers have generated immense interest due to their ability to interact with and structure a variety of materials with minimal thermal damage beyond the area of exposure. Thus UV lasers have found many applications in the processing of metals, in the patterning of organic materials like polymers3 , and in surgical applications4 . Consequently, the effects of UV radiation on a variety of homogeneous polymers have been well characterized. The accessibility of polymers to laser processing depends on the ability of the polymer to absorb the incident photons sufficiently. For example, at 308 nm, materials like PI and poly(aryletheretherketone) (PEEK) have high absorption coefficients,oa, (oX=10' cm-')' 6 due to the presence of low energy chromophores and extensive conjugation and are very amenable to laser processing, whereas PTFE and poly(methyl methacrylate) (PMMA) with low absorption coefficients(a < 102 cm') 7 are not. Although PMMA can be processed at other laser wavelengths, the challenge of ablating PTFE is being met by using vacuum UV lasers', femtosecond laser pulses7 , and by incorporating absorption enhancing dopants into the polymer. 9 Since PTFE can only be ablated by using either high intensity laser pulses or vacuum UV lasers, there is a need to increase its processibility without changing its otherwise attractive properties of extreme chemical and thermal stability an
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