Defforge et al. Nanoscale Research Letters 2012, 7:375 http://www.nanoscalereslett.com/content/7/1/375 Page 3 of 7 Figure 2 Selective TSV formation by local macropore masking before copper electrochemical deposition. (a) Electrochemical etching of silicon leading to through-silicon structures. (b) Local masking of macropores limiting the electrolyte penetration into opened regions. (c) Local filling of the macropore arrays with copper by electrochemical deposition. (d) Mask removal. textured surface such as macropore arrays. It cannot be performed by usual photolithography. The third strategy is the seed layer local deposition onto through-silicon macropores. After silicon anodiza- tion leading to through-silicon macropore formation (cf. Figure 3a), the seed layer is selectively deposited on the backside of the wafer (Figure 3b). For this purpose, ink-jet deposition technique as well as locally metalized contact wafer stacking can be employed. Since the copper can only grow on the conductive surface, the copper is limited to the seed layer areas (Figure 3c). Using this strategy, through-silicon via locally filled with copper may be obtained as already described in the literature [18]. The last strategy is based on the local opening of al- most through-silicon macropores [19]. Deep macropores were etched, but the anodization was stopped before the backside opening (see Figure 4a). After silicon dioxide (SiO2) thermal growth followed by backside photolithog- raphy, alkaline etching is performed to locally open the macropores (Figure 4b). The seed layer was deposited on the backside of the sample; thus, the filling of macro- pores is limited to ‘through’ regions (Figure 4c,d). This process was chosen to be tested because it can be performed using ordinary microelectronic tools. Finally, this process is much easier than direct macropore localization (strategy 1) because it limits the edge defects. Methods Macropore array electrochemical etching To perform ordered macropore arrays, low phosphorus- doped (ρ = 26 to 33 Ω cm), (100)- oriented silicon was employed. The thickness of the wafers was 240 μm. Highly ordered macropore growth was ensured after inverted micro-pyramid array etching through an oxide mask using photolithography followed by alkaline etch- ing (e.g., immersion into 20 wt.% of KOH solution at 80 °C until complete development of the pyramid). The samples were then anodized under galvanostatic control using a Bio-LogicW SP-150 (Bio-logic Science Instru- ments, Claix, France). During the anodization, the sili- con samples were immersed into a HF (5 wt.%)-ethanol (20 wt.%) mixture maintained at ambient temperature. A 130-W backside illumination of the sample was also essential to photo-generate holes into the semicon- ductor. Anodization was stopped just before the back- side opening; in the present case, approximately 220-μm deep macropores were etched (Figure 4a). The silicon