Through‐Mask Electrochemical Micromachining of Titanium

Abstract

Through-mask electrochemical micromachining (EMM) involves high speed selective metal dissolution from unprotected areas of a photoresist-patterned workpiece that is made an anode in an electrolytic cell. Compared to chemical etching, electrochemical dissolution offers higher rates and better control on a micro-and macro-scale of shape and surface texture of anodically dissolved materials. 1,2 EMM is receiving considerable attention in the electronics industry for thin film patterning. 3 In recent years, there has been an increasing demand for the micromachining of titanium. Due to their good mechanical properties and chemical inertness, titanium and titanium alloys have attracted considerable interest in the chemical process, aerospace, and biomedical industries. Fabrication of well-defined microstructures with controlled surface finish on titanium has become key in the biomedical industry for producing implants with improved biocompatibility 4-7 as well as miniaturized implantable devices. 8 For such applications, electrochemical micromachining (EMM) is promising in view of its flexibility and cost effectiveness.EMM has been used in the past for machining chemically resistant metals like Ti, Ta, Zr, and Nb. 9 Allen and Gillbanks 10 performed EMM of Ta in a solution of 5% by volume sulfuric acid in methanol. A uniform etch rate over the anode surface was obtained for an applied cell voltage of 10 V. These studies, however, give very little information on the role of prevailing electrochemical conditions. Rosset et al. 11,12 studied shape change during the dissolution of a type 304 stainless steel through an unevenly spaced pattern of lines into a 6 M NaNO 3 electrolyte. Using a flow-channel cell, they investigated the influence of current density and hydrodynamic conditions on the profile shapes and surface finish. The current distribution between anodes and hence the etch rate was found to be more uniform when dissolution is mass-transport controlled. The maximum current that could be applied was limited by Joule heating of the electrolyte in the developing etch grooves. Datta 13 employed EMM to fabricate an array of precision nozzles in copper and stainless steel foils for application in ink-jet printer heads. Nozzles of desired shape with microsmooth surfaces were obtained by dissolving at the limiting current plateau or at high voltages. The use of pulsating voltage provided a better control over the nozzle fabrication process because of the possibility of applying high instantaneous current density while maintaining a low average current. Shape evolution during the electrochemical etching of lines and holes into thin metal films sandwiched between a photoresist mask and an insulating support has been simulated by West et al. 14 assuming the primary current distribution approximation. Both small and large aspect ratio of photoresist thickness to cavity width were considered. The calculated shapes were compared with the experimental results of Rosset et al. 12 The fair agreement obtained between theory and ex...

Aluminum bulk micromachining through an anodic oxide mask by electrochemical etching in an acetic acid/perchloric acid solution

“…Both the width and depth of the groove increased with electrochemical etching time, and the increasing rate of the width slightly decreased with the etching time, whereas the depth increased linearly. This asymmetry dissolution effect has already been reported in experiments and a simulation of electrochemical bulk micromachining with a photoresist mask [29]. In this report, the opening of the resist mask was smaller than the final groove width due to undercutting, similar to our experiments.…”

“…In fact, the changes of the grooves obtained by Fig. 7 agree with the experimentally measured and simulated etched profiles reported in the investigation [29]. It is noted again that the width and depth of the grooves formed by electrochemical etching increased with the etching time, as shown in Fig.…”

Through-Mask Electrochemical Micromachining of Aluminum in Phosphoric Acid

“…Mathematical problem.-The formulation of the mathematical problem describing the shape evolution during TMEMM has been presented in detail by West et al 39 and Madore et al 38 In the case of mass-transfer limited electrochemical dissolution, a tertiary current distribution can be presumed to govern the local etch rate along the metal surface, hence the influence of ohmic resistances and electrode kinetics can be neglected. In the absence of convective mass transfer and using a pseudo-steady state assumption for the relaxation of the concentration field (c), the convection-diffusion equation simplifies to yield the Laplace equation:…”

“…As the film's thickness lies on the order of nanometers, its presence can be neglected for the study of shape evolution during TMEMM. The experimental results were thus compared with a shape evolution model previously developed by Madore et al 38 for TMEMM of titanium, assuming that shape changes are determined by diffusive mass transfer only. The model was solved numerically using both a Boundary Element Method (BEM) and a Finite Element Method (FEM).…”

Electrochemical Micromachining of Stainless Steel by Ultrashort Voltage Pulses

“…This is performed in two ways, either by local insulation of one or both of the electrodes [20,21] or by making use of the ohmic potential drop in the electrolyte due to high Faradaic currents at high overpotentials. Both methods exist in various sophisticated modifications.…”