Laser Assisted Seeding for Electroless Plating on Insulators
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LASER ASSISTED SEEDING FOR ELECTROLESS PLATING ON INSULATORS A.G. SCHROTT, B. BRAREN, E.J.M. O'SULLIVAN and R.F. SARAF
IBM Research Division, T.J. Watson Res. Ctr. Yorktown Heights, NY 10598. ABSTRACT
Excimer laser pulses with wavelengths of 248 and 308 nm were used to selectively seed Pd on SiO 2 surfaces, making them suitable for electroless plating. This novel seeding process for insulating materials is accomplished with the sample immersed in the seeding solution, and occurs only on the areas of the substrate that are illuminated (through the liquid) by the laser light. The Pd content of the seeded samples increase(] with the number of pulses, but was rather independent of repetition rate. The deposition rate showed a dependence with wavelength consistent with a defect driven mechanism for electron excitation through the band gap of SiO 2 . These electrons then reduce the Pd ions in the solution in contact with the surface. INTRODUCTION
The field of electroless plating is undergoing a renaissance due to the demands imposed by the sophistication of the advanced packaging structures that are being developed upon the increasingly stringent requirements imposed by the electronic industry. Normally, the reduction of the metal ion is initiated by a catalyst on the surface (seed). The deposition of this seed is not a trivial problem, and typically it occurs on metallic surfaces due to atomic exchange. Seeding on non metallic substrates require a special treatment for activation of the surface, which involves more complex chemistry and in some processes the use of lasers. In the latter cases, the laser light generally is used to decompose a photoscnsitive surface layer [1,2], or to produce pyrolitic decomposition of the seeding solution [3]. Another seeding mechanism results from electron excitation by the laser light from occupied states to unoccupied states and the subsequent relaxation process in the insulator. The reduction of the noble metal ions at the surface of the insulator occurs if the excited electrons drift to the surface without recombining, and are trapped in surface states, raising thus the Fermi level above the ion/metal equilibrium potential of the solution. This process depends on the nature of the excitation source, the properties of the material and equilibrium potential of the solution. Therefore it works under specifically tailored conditions. Other authors have used a continuous Ar-ion laser to deposit noble metals on a surface covered by a narrow band-gap TiO film [4]. With the help of excimer lasers we have taken the latter concept and attempted to extend it to any insulating surface with focus on seeding, because it can be controlled better than plating. Our process does not require pretreatment of the surfaces, and works with several standard seeding solutions normally used to seed on metals such as PdS0 4, Pd(CH 3CO 2)2. In a previous work we used this method to seed on polyimide surfaces [5]. In that case, the deposition of PId increased with the number of pulses, and with the pH of Mat
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