Abstract: Masks for microelectronics technology are fabricated by electrochemical etching. Molybdenum sheet or foil is masked with patterned photoresist on one side, and a mating, mirror-image pattern of photoresist is applied on the other side of the foil in exact registration with the first pattern. The foil is immersed in electrolyte (an aqueous solution of sodium nitrate, sodium hydroxide, thiourea, and surfactant) along with nickel anode plates held parallel to the foil surfaces 1-3 cm away. The anode plate is made slightly smaller than the mask area of the foil. When voltage is applied across the cell, the foil is etched through to form vias. The electrolyte is pumped across the surface of the foil and uniform flow velocity over the foil surface is achieved.