Etch Factor as a Function of Process Conditions

In addition to voltage, the rate of porous-type anodization depends on the electrolyte composition, acidity, and temperature. These variables affect the rate of AI2O3 dissolution at the pore bottoms in the isotropic way and are not expected to increase EF. Another variable to consider is the depth of porous-type anodization. 

Cosse et al. showed that EF decreases as the thickness of anodized Al-0.5%Cu films increases from 3 to 10 J.m.32 Kikuchi et al. reported that EMM of Al specimens resulted in the hemispherical 20-pm-deep microgrooves.40 In this case, porous-type anodization of patterned Al substrates was isotropic. These results suggest that localized anodization becomes isotropic as the anodization process proceeds. These conclusions are consistent with those obtained with other EMM methods, which indicate that EF decreases as the depth of metal removal increases.13,17 

In order to evaluate EF, one has to consider the dimensions of features undergoing porous-type anodization, which are typically 1-100 pm wide. Data reported in literature suggest that EF decreases when the width of unmasked areas decreases. For example, Renshaw showed the hemispherical cavities of porous AI2O3 formed by porous-type anodization through defects in the nonporous surface oxide films.41 Brevnov et al. reported the radial propagation of pores initiated at the anodization mask defects, which results in 

In conclusion, the requirement to obtain a high aspect ratio features dictates a necessity to increase EF. As discussed in this section, the anisotropy of localized anodization depends on the process conditions, such as anodization voltage, electrolyte composition and temperature, the depth of porous-type anodization, the presence of alloying elements in Al, and the dimensions of features, which undergo porous-type anodization. As shown, voltage is the primary variable, which allows one to increase EF. At the same time, localized anodization of patterned Al substrates became isotropic as the depth of porous AI2O3 formation increases and the width of anodized features decreases. These trends have to be taken into account while utilizing porous-type anodization for EMM of Al substrates. 

Current Density Distribution at the Pattern Scale During Localized Anodization 

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