An investigation of low temperature reactive ion etching of silicon to produce ultrasmall structures is presented. Etching was performed between 10–100 mTorr with SF6/O2 plasmas. The silicon samples were cooled using liquid nitrogen, allowing the temperature to be controlled over the range of +25 to -120 °C. With a flow rate of 1 sccm of SF6 and sample temperatures below -85 °C the etch produces nearly anisotropic etching in agreement with predictions of the reactive spot model. At higher flow rates etching produced facetted features indicating higher chemical reactivity of the plasma with the silicon. With the addition of O2 to the plasma the facetting became even more pronounced. © 1997 American Vacuum Society.