Nassiopoulou et al. Nanoscale Research Letters 2011, 6:597 http://www.nanoscalereslett.com/content/6/1/597 Page 3 of 8 Figure 1 SEM images of SiNWs of different lengths. Plane-view (a1, b1) and cross-sectional (a2, b2) SEM images of SiNWs of two different lengths: 5 μm (a1, a2) and 20 μm (b1, b2). It is depicted that short NWs are well separated between them, while longer NWs merge together and form bundles. areas. The samples were patterned by photolithography and resist etching. The commercial photoresist AZ 5214 was used as mask for the selective Si etching. The pro- cess flow is illustrated in Figures 3a,b,c. The photoresist is deposited on the surface by spinning (a). After expo- sure to UV light, the exposed resist through the mask is etched away to form Si windows on the resist, which constitute the confined areas to etch by MACE (b). The patterned wafer is then immersed into the MACE solu- tion, and SiNWs are selectively formed in the resist win- dows, as illustrated in (c). In (d), a top-view SEM image of the SiNWs on the patterned areas is depicted, while in (e) and (f), we see cross-sectional SEM images in two different magnifications of NWs formed in the confined areas at a reaction temperature of 30°C. The confined surface area ranged from 1 to 4 × 104 μm2. The cross-sectional images are from SiNWs formed in confined areas of 100 × 100 μm2, and they are shown at two different magnifications. A first obser- vation is that the NWs formed on confined areas are regular and well defined. They are vertical to the (100) surface, as in the case of NWs formed on non-patterned areas. Their formation at the lithographic edges is rela- tively anisotropic (see Figure 3f). The most interesting observation is that the SiNW length in the lithographically defined areas is much higher than that in the non-patterned ones, which means that the etching rate is increased in the lithogra- phically patterned surfaces. A direct comparison of the length of SiNWs formed at a temperature of 30°C for an etching time of 1 h is illustrated in the cross-sectional SEM images of Figure 4 for non-confined areas (a) and for lithographically patterned 100 × 100-μm2 areas (b). The difference in length between patterned and non- patterned areas is obvious. For much smaller patterned surfaces (4 × 4 down to 2 × 2 μm2), we found that the etching was less regular at the edges. In order to elucidate the above results and understand the mechanism that leads to the different formation kinetics in the two cases, we systematically investigated the etch rate as a function of the etching time of SiNWs formed on non-patterned Si areas and on lithographically