Metal-assisted etching of nanopores in silicon

Abstract: Nanoporous membranes are an interesting approach to manufacture a variety of devices for different applications. For example in biomedicine the separation of molecules or cells or the sequence-based analysis of single-stranded DNA are of great interest. Based on silicon membranes, a promising method to achieve pores with a high aspect ratio is metal-assisted etching, where noble metal particles serve as catalysts for the oxidation of the underneath Si, which is subsequently removed by hydrofluoric acid. This thesis project deals with developing a method, based on wet chemical etching of nanopores into a silicon membrane, utilizing noble metal particles as catalysts. The main goal was to investigate if it is possible to achieve straight channels perpendicular to the substrate surface with approximately the same diameter as the particle size. Therefore, the etching behaviour of gold, silver and platinum nanoparticles with different diameters on various substrates and etching solutions has been investigated. First the optimal substrate and etching solution for defined pore growth were determined using gold nanoparticles. Long-time measurements have been conducted showing a saturation of the etch speed and square aperture growth after a few hours. Next the etching reaction was enhanced with adding HCl and applying a voltage and it was found that the particle concentration has an influence on the orientation of the pores. After showing only erratic movement in the beginning, erect pores with a maximum aspect ratio of ~ 20 could be manufactured using CTAB coated particles. After that silver particles have been investigated. Initially, the optimal compositions for synthesis and etching solution were determined. Then the behaviour for long-time immersion was investigated, implicating that the pores grow fast and constantly within the first hour. In the end, highly straight pores with aspect ratios of ~ 1000 were etched. But due to the high particle concentration, the surface was badly affected by strong etching, as well. For both noble metals, diluting the etching solution and thus slowing down the etching reaction resulted in more oriented pore growth. For the platinum particles, however, no promising results could be achieved, because platinum seems to be a too strong catalyst for the etching reaction.