Excimer laser machining and metallization of vias in aluminum nitride

Laser machining of ceramics is used extensively in the microelectronics industry for scribing and via hole drilling. Scribing involves laser ablation of a groove or row of holes that form perforation lines to separate a large substrate into individual circuits. Via machining is generally followed by a metallization step to create three-dimensional (3-D) interconnections in a multilayer circuit board. Aluminum nitride (AlN) is a desirable substrate material for high power, high frequency applications because of its high thermal conductivity and low thermal expansion coefficient than Al₂O ₃. In this paper, an excimer laser is used to machine high aspect ratio, straight walled via holes in aluminum nitride with or without a metallization layer deposited on the via walls. Via diameters range between 60 and 300 μm through substrates 635 μm thick. Through hole machining can cause damage to the back surface of the substrate, however, attachment of a second substrate or metal sheet will prevent damage. Ablation of the attached metal backing with subsequent redeposition on the via walls produces a metallized via with a resistance of less than 1 Ω per via. Single and multilayer via structures are described. Substrate damage at through hole exits results from shock wave propagation and reflection in the substrate. The attached backing material reduces reflection of the shock wave at the back surface of the substrate to prevent damage. Shock wave analysis, via cross sections, and resistance measurements are discussed