Defforge et al. Nanoscale Research Letters 2012, 7:375 http://www.nanoscalereslett.com/content/7/1/375 electrochemical etching enabled the formation of 15-μm pitch, 12-μm diameter, with an aspect ratio of 18- to 20- almost-through macropore arrays. For this purpose, the current density was maintained equal to 14.5 mA cm−² during 4 h. This value ensures homogeneous macropore growth and porosity around 70 %. Copper electrochemical deposition Once the macropores were etched, the sample was ther- mally oxidized. A photolithography step was performed on the backside of the samples to locally etch the oxide layer. The samples were immersed into the KOH solu- tion. The oxide-free regions were etched until the macropore tips were attained (cf. Figure 4b). Actually, a thin SiO2 layer remained at the pore tips. In order to completely open the macropores, HF (50 wt.%) dipping was achieve until complete oxide removal. At this stage, 450 × 450-μm² areas were opened every 3 mm. Figure 5 illustrates one of these opened regions; every macro- pores were opened. Indeed, using this technique, local through-silicon macropores approximately 220-μm deep were formed. A second dry thermal oxidation was per- formed to isolate the silicon from the copper deposit and avoid parasitic copper growth as well as copper dif- fusion into silicon. Then, a non-conformal Ti/Ni/Au tri- layer was deposited by sputtering. This deposit ensures both good electrical contact and good adhesion on SiO2 (see Figure 4c). Moreover, the non-conformal deposition limits the next copper growth on one side of the sample (not on the sidewalls) for a bottom-up filling. The macropore arrays were immersed into copper sulfate-sulfuric acid solution with 0.5 and 1 M as re- spective molar concentrations. H2SO4 and CuSO4 were also mixed with three additives. The chemical nature as well as the concentration of these additives is essential for an efficient pore filling. The accelerator enhances the Figure 5 Locally opened through-silicon macropores. Optical microscope view of selective through-silicon macropore. This structure is achieved by alkaline etching (highly concentrated KOH solution at 80 °C) of the backside of the substrate through an oxide mask. Page 5 of 7 deposition kinetic in confined medias (e.g., at the pore tip) [20]. It is always a sulfur-based molecule, and in the present study, a sodium sulfopropyldisulfite (SPS) was employed as an accelerator. The suppressor is a mixture of polyoxyethylene-based polymer (the most used being the polyethylene glycol (PEG)) and chloride ions (Cl−). This additive adsorbs at the surface of the sample and inhibits the deposition rate (i.e., it results in few tens of millivolt overpotential deposition) [21,22]. In this study, 1,200-g mol−1 polyoxyethylene lauryl ether (POE) was preferred to PEG, thanks to its higher inhibition behav- ior [23]. The last additive added to the electrolyte was the Janus Green B (JGB), a nitrogen-based molecule. It is known as a leveler; it limits mushroom-like copper overfilling at the end of the electrodeposition process. The presence of these three additives (SPS, POE-Cl−, and JGB) involves differential deposition kinetics be- tween the seed layer and the surface. They limit the deposition outside of the seed layer and guarantee an important pore filling kinetic [24,25]. The polished, sliced scanning electron microscope (SEM) views in Figure 6 prove that the filling quality strongly depends on the electrolyte composition. In the first case (Figure 6a), the electrochemical deposition was performed into an electrolyte whose additive concentrations were not opti- mized (with 5 ppm, the JGB content was too low); voids developed during the electrochemical deposition step [26]. However, if the chemistry of the electrolyte is well adapted to HAR structures (the JGB concentration was increased to 10 ppm), no void was observed (Figure 6b). A copper overfilling is observed at the top of the structure (cf. Figure 6b). This surplus of copper was removed by polishing the top surface before the elec- trical characterization. The copper filling was performed under potentiostatic control (−0.15 V vs. saturated calomel electrode refer- ence) during 6 h to ensure a complete pore filling. The samples were then rinsed with deionized water, polished, and analyzed by SEM and optical microscope. Results and discussion To determine the copper growth homogeneity and the localization efficiency, silicon was selectively etched by a KOH solution at high temperature (80 °C). Thus, HAR copper micropillar arrays developed. Figure 7 illustrates one of these 450 × 450-μm² conductive paths. We can see a uniform copper growth inside the through-silicon macropore region. Outside this region, no parasitic cop- per growth was observed. Electrical characterization of TSV The conductive via grown into the macropore arrays were then electrically characterized. For this purpose, four-probe technique was employed. The probes were