Laser Machining and Metallization of VIAS in Aluminum Nitride
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ABSTRACT A via metallization technique involving excimer laser ablation of aluminum nitride substrate with a metal backing, attached with cyanoacrylate adhesive was investigated. The objective was to develop a procedure to ensure good electrical interconnection between the thick film conductor pads and the walls of the via. An aluminum nitride compatible Ag-Pd thick film paste was used in this experiment. Two different sequences were tested: In the first sequence both sides of the aluminum nitride substrate were printed and fired and then the vias were laser drilled. In the second sequence, one side of the substrate was printed and fired first, then the vias were laser drilled and the second side was printed and fired. In both of the above sequences, laser ablation of the printed conductor and redeposition of the ablated metal backing are necessary for making a proper contact with the thick film conductor pads.
INTRODUCTION Aluminum nitride (AIN), as a substrate packaging material is of great interest due to its advantageous properties. It has excellent heat dissipating capabilities. The thermal conductivity of aluminum nitride is as high as 260 W/mK [1]. Also, the coefficient of thermal expansion (TCE) of polycrystalline AIN is 4.2 ppm/°C which is close to that of silicon (3.4 ppm/IC) over the same temperature range [2]. AiN is also thermal shock resistant, chemically stable, electrically insulating, hard, and strong.
One of the major areas of research in the implementation of AIN for microelectronics applications is in the production and metallization of AIN surfaces for interconnection of circuit components. Most commonly, interconnection of circuit components in a multilayer circuit is accomplished through vias, and it is essential to metallize these vias to establish electrical continuity between layers. The main objective of this work is to drill holes with high aspect ratio and metallize these holes to achieve electrical interconnection between the conductor pads and the walls of the via. The conductor pads are screen printed on both sides of the substrate by standard screen printing techniques. Laser machining and via metallization is being combined with standard thick film processing to fabricate metallized test pads on the substrate surface.
Excimer Machining Excimer lasers are especially suited for via machining and metallization because the depth of the via can easily be controlled and straight walls are produced. Excimer lasers are pulsed ultraviolet wavelength lasers and they provide a unique combination of short pulse duration and highly energetic photons which produce high peak power. Excimer laser pulses are highly absorbed by AIN which has a band gap of 6.2 eV (200 nm) [3]. Also, metals absorb more strongly in the ultraviolet region than in the infrared region of the electromagnetic spectrum. 269 Mat. Res. Soc. Symp. Proc. Vol. 390 0 1995 Materials Research Society
Vias in aluminum nitride can be metallized and machined by excimer laser ablation of aluminum nitride substrate using a metal bac
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