Nassiopoulou et al. Nanoscale Research Letters 2011, 6:597 http://www.nanoscalereslett.com/content/6/1/597 Page 7 of 8 Figure 7 SEM images of the samples right after the MACE process and before the Ag removal. Dendrite Ag nanostructures on the SiNW surface during the MACE process on non-patterned areas (a) and on lithographically defined 2 × 2-μm2 surface areas (b). between confined areas with different inter-area dis- tances between them. This rules out the hypothesis that the dendrite density could be at the origin of the differ- ence in the etching rate by MACE between the confined and non-confined areas. Effect of surface modification through the lithographic steps After excluding all the factors above, we examined the possible effect of the lithographic steps on the reaction kinetics by MACE in the case of lithographically defined areas. The different process steps include the use of a resist adhesion promoter, resist spinning, UV exposure, and a developer for resist stripping. The first experiment was to perform all the lithographic steps above on a blank wafer without any mask exposure, remove the resist normally, and then, etch the sample by MACE. In this way, we exclude the surface area effect from the ori- gin of the etch rate difference. Indeed, after applying all lithographic steps without mask and stripping the resist, the etch rate of the sample was much higher than the etch rate of a virgin Si wafer. This leads to the conclusion that surface modification during lithography is at the ori- gin of the etch rate effect described above. The step responsible for surface modification is the adhesion pro- moter. The one used was hexamethyldisilzane [HMDS], known to remove -OH groups from the Si surface and form a hydrophobic surface with the methyl groups of the HMDS fragment. The so-formed hydrophobic surface improves resist wetting and adhesion. We demonstrated that it also increases the etch rate by MACE. After the above result, we investigated a number of other surface chemical treatments on the MACE kinetics. We compared the SiNW length after etching by MACE at 28°C for 1 h on a non-treated Si wafer, a Si die sample after piranha cleaning for 30 min, and a Si die after HF (10%) dip for 15 min and after a dip into the above-mentioned developer for approximately 1 min. The result was that the non-treated wafer resulted in 20-μm-long NWs; the same was obtained after pir- anha cleaning, while there was approximately 15% increase in length after the developer dip and 7.5% decrease in length after the HF dip. From the above experiments, it is clear that chemical surface modification plays an important role in the etch- ing kinetics by MACE. It mainly affects these kinetics at the beginning of the process. Conclusion A comparative study on the formation kinetics of SiNW formation by MACE of non-patterned and lithographi- cally patterned Si surfaces showed that the etch rate of Si for SiNW formation is higher in the case of lithogra- phically patterned surfaces compared to that of the non- patterned ones. The origin of this effect is at the surface modification by the promoter used for resist adhesion. Resist promoter is known to form a hydrophobic surface covered by methyl groups. Moreover, it was shown that surface treatment by an HF dip, which leads to surface passivation by hydrogen, results in retardation of the etching process, thus leading to shorter SiNWs. Acknowledgements Funding for this work was received from the European Union’s 7th Framework Programme (FP7/2007-2013) through the ICT NoE project ‘Nanofunction’ (grant agreement number 257375). Authors’ contributions AGN supervised all the work and wrote the paper, VG helped in developing the Si nanowire etching process and performed all the experiments related to nanowire growth on patterned surfaces, while HK was involved in the initial experiments on Si etching by MACE. All authors read and approved the final manuscript. Competing interests The authors declare that they have no competing interests.