Through-Mask Electrochemical Micromachining of Aluminum in Phosphoric Acid

Abstract

Aluminum micro-channels have been machined in phosphoric acid via mass-transfer limited electrochemical dissolution through photoresist masks. The results of shape evolution experiments using a rotating disk electrode are presented in terms of the dimensions, shape profile and uniformity of the machined micro-channels. The influences of applied potential, cumulative passed charge and hydrodynamic conditions on the shape evolution process are discussed. Experimental results are compared with a shape evolution model assuming the rate of aluminum removal is solely controlled by diffusive mass transfer. The degree of agreement between experimental and simulated results depends mainly on the hydrodynamic conditions in the electrochemical cell and indicates a shift from purely diffusive to mixed convective-diffusive mass transfer. Through-Mask Electrochemical Micromachining (TMEMM) is an unconventional machining process, in which a metal substrate is made the anode in an electrochemical cell and thereby dissolved.1,2 Localized material removal is achieved by covering the substrate with an insulating mask. The use of common photolithography processes for the creation of the mask enables the fabrication of micrometer-sized structures. Thereby, TMEMM can be used in two ways to fabricate microfluidic devices: firstly, cavities and channels can be machined into the substrate surface and secondly, through-holes and slits can be machined into metal foils or shims.1 Compared to classical wet chemical and dry etching processes, TMEMM offers higher rates of material removal, better control of surface finish, more selective machining of substrates and enhanced process safety. [3][4][5] In addition, combining photolithography with electrochemical dissolution processes is expected to enable the fabrication of a large number of microfluidic devices in parallel and at low cost. 6Aluminum is widely used as a construction material for the manufacture of microfluidic devices such as micro-heat exchangers, 7 microreactors 8 and micro-fuel cells. 9 Other areas of interest are the fabrication of intricate parts in optical, electronic, automotive and aerospace systems. 10,11 Its chief advantages compared to other materials used in these fields are its high thermal conductivity, low electrical resistivity, easy machinability and low density. In addition, it is possible to modify the surface structure of aluminum substrates via the formation of highly ordered, porous oxide films along the surface. 12,13 It has been demonstrated, that these films can be subsequently used to incorporate noble metal catalysts along the surface of micro-channels within a micro-reactor. [14][15][16] Electrochemical dissolution of aluminum in phosphoric acid is known to produce reflective aluminum surfaces with sub-micrometer surface roughness at sufficiently high electrolyte concentration and temperature.17 This property makes it attractive for TMEMM of micrometer-sized structures made from aluminum. The observed leveling and brightening effect is caused by the m...

Fundamental Aspects of Uranium Electropolishing in AlCl3-1-ethyl-3-Methylimidazolium Chloride Ionic Liquid

“…The mass transport mechanism was obtained by potentiodynamic polarization experiments, which were carried out using a Princeton 2263 potentiostat controlled with PowerSuite software. Then the polarizations in ILs with a mixture of various concentrations (10,20,50 vol. %) of benzene were conducted to further confirm the rate limiting species.…”

“…9 The salt precipitation mechanism describes a thin salt film present on the anode, where the surface concentration of metal cations equals to the saturation value at limiting current. 10 The acceptor mechanism looks at the role of adsorbed metallic species and the transport of the acceptor which solubilizes them. 11 The water mechanism involves the rate limiting diffusion of water from the bulk to the anode where it is consumed by formation of hydrated metal ions.…”

Review—Through-Mask Electrochemical Micromachining

“…Machining often takes place via immersion etching; hence, current densities and etch rates are usually lower than in saline electrolytes (0.1-10 A cm −2 , 1-10 μm min −1 ). 7,60,91,160 A wide range of metals has been machined, namely aluminum, 135,136,161,162 chromium, 91 copper, 42,55 gold, 163 hafnium, 91 molybdenum, 91,164,165 niobium, 166 platinum, 167 tantalum, 166 titanium 168 and tungsten. 91,169 This list is complemented by several cobalt, 91,160 copper 91,161 and nickel alloys, 91 as well as a number of stainless steels.…”

Effect of the geometric structures of mask feature on the microstructure forming in through-mask electrochemical micromachining

“…Similar in fabricating micro pillar arrays, the effects of the current density on surface morphology of the micro pillar arrays were investigated by Sun et al [11]. Baldhoff et al reported that speci c voltage is the key to micro channel shape uniform during TMEMM on aluminum [12]. Furthermore, the e ciency of the electrolyte characteristics on the current density is mainly affected by the conductivity, passivation, and activation of the solution.…”