The formation of pore arrays with high aspect ratios by electrochemical etching of n-type silicon in hydrofluoric acid is a well established technique. The macropore morphology depends sensitively on the anodization conditions such as current density, etching time, HF concentration, temperature and bias as well as on substrate properties such as doping density and orientation. The limits of feasible pore geometries and pore patterns and their dependence on formation conditions will be discussed in this work.