High-rate anodic dissolution and jet electrochemical micromachining of tungsten

Date of Completion

January 1996

Keywords

Engineering, Chemical

Degree

Ph.D.

Abstract

Conventional photoetching processes used for thin film patterning of tungsten involve photolithography and chemical etching processes which use toxic etchants like potassium ferricyanide and hydrofluoric acid. Through-mask electrochemical micromachining (EMM) of thin films avoids the use of toxic chemicals by using electrolytic etching, but photolithography processes are still required. Jet EMM uses an unsubmerged electrolyte jet that flows through a micro-sized nozzle cathode and impinges onto the anode workpiece. Patterning is achieved by moving the workpiece. Jet EMM eliminates the photolithography step and uses less toxic chemicals.^ To investigate jet EMM of tungsten, the anodic dissolution of tungsten was studied using a rotating disk electrode (RDE) cell. The metal dissolution rate, current efficiency and surface brightness during anodic dissolution of tungsten as a function of the electrolyte composition, applied potential, and hydrodynamic conditions were found. For tungsten dissolution, NaOH solution was found to be the most suitable electrolyte. The surface finish is mass transfer controlled by the diffusion of OH$\sp-$ to the anode.^ NaOH solution was exclusively used in the jet EMM experiments. Jet EMM uses high flow rate impinging electrolytes which produce high mass transfer rates that lead to high rates of micromachining. The effects of various factors such as flow velocity and nozzle diameter on the feature of machined holes were examined. The machined holes are tapered, the back hole is about the size of the nozzle, and the size of the front hole is a function of sample thickness and nozzle diameter. In addition to machining on various thickness of tungsten foils, jet electrochemical micromachining of back-insulated tungsten foils and tungsten coated silicon wafers were also examined.^ Mass transport controlled finite element modeling of jet EMM was used to investigate the effect of system variables on hole shape. Various flow velocities and gap distances were investigated. Flat and cavity impingement surfaces were examined. The mass transfer at the sidewall of the cavity was found to be higher than at the bottom or a flat surface. Comparisons of hole profile between the experimental jet EMM and modeled results were made. ^

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