Through-Mask Electroetching for Industrial Manufacturing
Authors: McCrabb Heather , Hall Timothy D , Snyder Stephen , Taylor E. J 
Published: February 2020

Chemical etching, also referred to as chemical machining, photochemical machining or photoetching, is a machining process that utilizes chemical solutions for controlled corrosion of a material protected by a non-reactive mask to selectively remove areas for surface texturing or component fabrication. When compared with other more traditional fabrication processes, such as mechanical machining and stamping, chemical etching may afford technical and economic advantages such as the ability to maintain desired metal properties (e.g. hardness, ductility etc.), no tool wear, no burrs or recast layers, capability to fabricate detailed or complex shaped components and affordable operating costs. Typically the protective mask is a photoresist applied through standard photolithographic techniques using either a liquid or dry film lamination. The chemical solutions used for material removal depend on the material being processed and may be acidic or alkaline in nature. Common materials processed by chemical etching are metals like stainless steel, copper and copper alloys, and nickel as well as nickel iron alloys [[1]]. More corrosion resistant materials, like titanium and niobium, can be processed but require increasingly aggressive chemicals that can pose operator safety and environmental concerns.
Similar to chemical etching, Through-Mask ElectroEtching can create high precision, thin gauge components by material removal from the surface containing the resist mask to protect desired areas from etching. However, instead of relying on chemically active solutions to control material removal, Through-Mask ElectroEtching involves application of an electric field between the metal to be formed and a counter electrode submerged in a conductive fluid. The conductive fluid may be a mildly acidic or near neutral aqueous salt solution and typically has low chemical activity toward the material in the absence of the electric field. The electric field applied may either be direct current (DC), in which the voltage or current is set at a single amplitude for the duration of the experiment or pulse and pulse reverse current (PC and PRC respectively) may be employed. PC Through-Mask ElectroEtching uses unidirectional current or voltage that is set to a predetermined amplitude called the peak current or voltage and the electric field is turned on and off several times per second to drive the material removal. PRC Through-Mask ElectroEtching uses bidirectional current or voltage in which the polarity of the component, which is typically anodic, is periodically reversed to be cathodic. Reversing the polarity of the component under fabrication can assist in oxide removal that inhibits the etching process.
In addition to offering the same benefits enabled by chemical etching, Through-Mask ElectroEtching may enable superior surface finishes compared with chemical etching, as the electric field parameters can be adjusted to provide a final electropolish during processing. PC and PRC Through-Mask ElectroEtching provides enhanced process control through the optimization of pulsed parameters, (i.e. the frequency, pulse on-times and off-times, and the peak voltages or currents). Optimization of pulse parameters can minimize Joule heating that results during the high material dissolution and causes oxidation, pitting and non-uniform dissolution across the component [[2]]. Through-Mask ElectroEtching has shown promise for components fabricated from more exotic metals, such as titanium and its alloys, without necessitating the use of aggressive chemical solutions like the highly corrosive and poisonous hydrofluoric acid often employed to remove tenacious oxides like those that form on titanium. In this work, a PRC Through-Mask ElectroEtching technology is being developed using mildly acidic or near neutral aqueous solutions devoid of hydrofluoric acid. Examples presented will focus on components fabricated for various industries such as stainless steel bipolar plates for PEM fuel cells, nickel super alloy micro-channels for fluid delivery and titanium current collectors for electrolyzers.

[1]. D. Allen, CIRP Journal of Manufacturing Systems, 53, 559-573 (2005).
[2]. C. Madore, O. Piotrowski, and D. Landolt, Journal of The Electrochemical Society, 146 (7) 2526-2532 (1999).