US Patent 8940627 Conductive ink for filling vias

Vias (holes) are formed in a wafer or a dielectric layer. A low viscosity conductive ink, containing microscopic metal particles, is deposited over the top surface of the wafer to cover the vias. An external force is applied to urge the ink into the vias, including an electrical force, a magnetic force, a centrifugal force, a vacuum, or a suction force for outgassing the air in the vias. Any remaining ink on the surface is removed by a squeegee, spinning, an air knife, or removal of an underlying photoresist layer. The ink in the vias is heated to evaporate the liquid and sinter the remaining metal particles to form a conductive path in the vias. The resulting wafer may be bonded to one or more other wafers and singulated to form a 3-D module.