Minimization of undercutting in electrochemical micromachining of patterned aluminum–copper films

Abstract

Al–0.5%Cu films patterned with an anodization mask of barrier Al2O3 are micromachined by porous-type anodization followed by chemical dissolution of porous Al2O3. Electrochemical micromachining results in well-defined metallic pillars separated by micro-grooves. The trapezoidal shape of the pillars is due to the lateral pore propagation under the anodization mask. Regardless of undercutting, porous-type anodization shows a higher degree of anisotropy than mostly isotropic wet chemical etching. The vertical growth of pores is accelerated with respect to the lateral propagation of pores by increasing the voltage of porous-type anodization. Consequently, the lateral pore propagation is minimized and for ∼3 µm thick Al–0.5%Cu films the etch factor increases from 2.6 to 4.3 when the voltage is increased from 20 V to 60 V. Comparing the results obtained with the micromachining of ∼3 µm and ∼10 µm thick films at the same voltage, the etch factor decreases as the depth of micro-grooves increases. The etch factor depends on the distribution of secondary current density at the pattern scale during porous-type anodization, which is defined by the anodization mask coverage. A higher degree of anisotropy of porous-type anodization (in comparison to wet chemical etching) allows for more accurate shape control of three-dimensional metallic microstructures.

Electrochemical etching of patterned Al (1 0 0) foils in HCl

“…First, a ∼100 µm thick layer of photoresist was patterned on ∼100 µm thick Al (1 0 0) foils (99.99% purity, Toyo Aluminum) to create ∼10 µm wide features separated by a 20 µm pitch. Second, the pattern of photoresist was transferred to barrier Al 2 O 3 by ∼2 min long barrier-type anodization at 100 V at 9 • C in 0.1 M (NH 4 ) 2 B 10 O 16 , as previously reported [25,26]. The negative pattern transfer procedure was chosen to generate a mask for subsequent electrochemical etching.…”

“…The negative pattern transfer procedure was chosen to generate a mask for subsequent electrochemical etching. This procedure was described for porous-type anodization of patterned Ti, Al and Al-0.5%Cu substrates [20,[24][25][26]. Third, the photoresist was removed by an acetone wash and by using oxygen plasma (Technics 800 RIE).…”

“…The majority of methods for electrochemical micromachining (EMM) involve masking (by either photo-lithography or softlithography) and subsequent Al removal by isotropic wet processes [14][15][16][17][18][19][20]. For example, patterned Al films can be micromachined by porous-type anodization [21][22][23], followed by chemical etching of porous Al 2 O 3 [24][25][26]. In one case, EMM of patterned Al-0.5%Cu films resulted in 3D microstructures composed of ∼10 µm high pillars of metallic phase [26].…”

“…For example, patterned Al films can be micromachined by porous-type anodization [21][22][23], followed by chemical etching of porous Al 2 O 3 [24][25][26]. In one case, EMM of patterned Al-0.5%Cu films resulted in 3D microstructures composed of ∼10 µm high pillars of metallic phase [26]. However, as observed in EMM, the fidelity of the mask transfer during porous-type anodization was compromised by undercutting [14].…”

“…Due to the lateral pore propagation, the fabricated metallic pillars were trapezoidal. The etch factor (EF), which is the ratio of the straight through etch to the undercut, was ∼3 [26]. It is technologically important to identify and develop methods that result in a high EF.…”