Using X-ray photoelectron spectroscopy, the chemical state of a silicon surface treated with hydrofluoric acid and the resulting contaminant species (fluorine, carbon and oxygen) were studied as a function of the etchant concentration (2-48%). The surface contamination was quantified as a function of the etchant concentration using a submonolayer coverage theory. It was seen that the carbon level had a minimum value (≈ 4.5×1013 atoms cm-2) for 5% HF concentration, whereas the oxygen level was almost constant (i.e. the variation was from 2×1013-4×1013 atoms cm-2), and the fluorine level showed a steady increase (2×1013-12×1013 atoms cm-2) with the concentration of the etchant. So, by selecting a suitable concentration of the etchant it is possible to create a silicon surface which is relatively free from contamination. Also one can modulate the amount of fluorine incorporation on the silicon surface. © 1992.