A process for etching a silicon single-crystalline surface using an ultrafine particle dispersion is proposed. The ultrafine particles contain quaternary ammonium hydroxide groups as ion-exchange groups, giving an alkaline dispersion. In the etching process, the fine particles and impurity ions can be easily separated by filtration or dialysis. After repeated dialyses, impurities in the dispersion, which include heavy metal ions and alkali metal ions known to result in a rough-etched surface and to affect the electronic properties of the semiconductor, were decreased easily. The method is suitable for application to the surface planarization of silicon single crystals and to the manufacture of semiconductor devices. Using the proposed alkaline etching dispersion, a Si(001) surface was successfully patterned by cathodic electrochemical etching at room temperature. In the patterning of p-type Si, the average etching rate was 0.42 nm/min, and the center line average roughness (Ra) of the etched surface was 0.173 nm for an area of 64 X 48 mu m. (c) 2006 The Electrochemical Society.