Excimer Laser Removal of Cr Contaminants from Polyimide Surfaces for Packaging Applications
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EXCIMER LASER REMOVAL OF Cr CONTAMINANTS FROM POLYIMIDE SURFACES FOR PACKAGING APPLICATIONS GOURI RADHAKRISHNAN*, NICHOLAS MARQUEZ*, AND HEINRICH MOLLER** *The Aerospace Corporation, MS M2-241, P.O. Box 92957, Los Angeles, CA 90009 "**Microelectronics and Computer Technology Corporation, 12100 Technology Blvd., Austin, TX 78727 ABSTRACT Excimer laser radiation at 248 nm has been used for the maskless and selective removal of chromium (Cr) contamination from polyimide surfaces containing gold (Au) metallizations. Ablation thresholds and rates have been determined in air, vacuum, and oxygen. These results have been used to establish the optimum ablation medium and a corresponding process window that allows for selective removal of Cr from polyimide. The substantial differences between the ablated and unexposed regions of the polyimide surface have been established by electrical measurements, while the variations in their respective surface morphologies have been examined using optical and scanning electron microscopy. Quantitative determinations of Cr on the ablated and unablated regions of polyimide have been performed, using secondary ion mass spectrometry, to establish the threshold for the complete removal of Cr. INTRODUCTION The ultraviolet laser ablation of polyimides as well as a variety of other organic polymers has been extensively investigated [1-11] as a result of the potential importance of these materials in microelectronic applications. Depending upon the polymer, the mechanism for laser ablation has been attributed to both photothermal and photochemical processes. To examine these mechanisms, a variety of different studies have been conducted, on individual polymers [1-9], as well as on polymers doped with molecules of known ultraviolet absorptions [10,11]. In particular, polyimide dielectric substrates have received enormous attention. Polyimides are increasingly used as dielectric materials in circuit boards in high-speed, light-weight, and high-density computer applications. Studies conducted on polyimides, include fundamental investigations of etch rates and etch products [14], as well as specific applications of laser processing [12-15]. In the latter case, laser photoetching of polyimide has been demonstrated as a successful means of via hole production [12] and patterning of the polyimide layers [13]. Additionally, Liu et al. [14] have demonstrated that excimer laser radiation can be used to pattern electroless plating activator materials deposited on a rigid polymer substrate. More recently, Niino et al. [15] have suggested that laser exposure can alter the surface electric potential of the irradiated area of a polymer. These authors have reported the observation of selective electroless plating on a photo-irradiated area of polyimide after treatment by a negative palladium colloid solution. In this study we specifically address issues relating to the maskless surface cleaning of polyimide layers used in electronic packaging applications. In such applications, the polyimide layers are generally
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