We describe a maskless reactive ion etching process that employs CF4 gas plasma to create nanoscale structures in silicon. Process conditions are controlled to produce pillars of up to 2 mu m tall and less than 50 nm wide. The contributing mechanisms are discussed based on the trends observed for varying plasma conditions. Higher pressures or lower self-bias voltages result in pyramidal structures. Lower pressure and higher voltage result in needlelike structures that resemble silicon wires. By carefully controlling the automasking process mechanism, columnar silicon structures were reproducibly formed with good uniformity all over the wafer. The regularity of the fabricated structures, process controllability, and process compatibility of dry etching are promising for potential photonic and optoelectronic applications. (c) 2006 American Vacuum Society.