Engineering selective porosity in silicon through modulation of electron and ion transport in MACEtch
Metal-assisted chemical etching (MACEtch) has been an effective and efficient method for the nanoscale etching of silicon. This process can suffer from the creation of imperfections, such as porosity in the features, that overall limit the applications of MACEtch. If porosity in silicon is incorporated in a controlled manner, however, it can lead to important applications in areas ranging from optics to energy. The goal of this work was to create selectively porous regions in single crystal silicon wafers using patterning of catalysts such as gold and ruthenium followed by MACEtch. The creation of porosity is determined by the relationship between the rate of hole injection into, or electron removal from, the silicon and the rate at which the HF is able to access the oxidized silicon in order to etch it away. Electrical hole transport is governed by the choice of metal catalyst, the molar ratio of HF to (HF+H₂O₂) in the MACEtch solution, and the doping of the silicon wafer, all of which were varied in this work. Increased molar concentrations of H₂O₂ and higher p-doping of the wafer both serve to improve hole injection, thus fostering the creation of porosity. On the other side of the balance, HF’s access to the silicon must be enabled by nanoscale discontinuities in the catalyst film, which can be inherently present in the material at low thicknesses or by manufacturing intentional discontinuities in the metal through a nanofabrication process. Given this understanding of porosity creation in silicon, it is then possible to create selective regions with controlled porosity. Depositing ruthenium in locations on the wafer where porosity was not desired, while protecting the surface of the silicon in locations intended to become porous resulted in a reliable process for fabricating selectively porous regions. When the MACEtch was subsequently performed, the inherent continuity of the ruthenium film with no nanoscale discontinuities prevented porosity creation in the silicon underneath those regions, while the hole injection from the same ruthenium traveled to the surrounding silicon to help create porosity in the areas exposed to the HF in the MACEtch solution.