In order to utilize the metal patterns to achieve high-density silicon nanostructures, etching must progress perfectly vertical to the surface, i.e., the etch must be highly anisotropic. Even a slight rate of undercut would limit the maximum depth that could be achieved and cause structures to collapse; the density of the nanostructures could also dramatically decrease as many of the structures would be lifted off due to the undercut. While MacEtch is known to have a crystalline dependency,6,7,9,17 the role of solution ratio has conflicting reports4,6,7,9,12 and the role of metal geometry has not been studied extensively.5,12 Furthermore, it should also be noted that previous studies have generally been limited to metal patterns that are significantly larger than the patterns we have examined.5–9,12,13,16–18 To test the impact of solution ratio, we performed the etch at six different ratios (by volume): , , , , , and for 1 min. This grouping of ratios straddles what was reported by Chartier et al.4 to be the optimal concentration of 80% HF to 20% by mol corresponding to a volumetric ratio of nearly for maximum metal penetration rate. Solutions were composed of 60 mL HF with the appropriate amount of added based on the desired ratio. Because of this, the high HF concentration solutions were a smaller total volume than the low HF solutions. We do not believe that the solution volume plays a significant role in the etch process so long as the sample size is much smaller than the solution volume.