The etching of Si(100) is studied quantitatively in a molecular beam setup. After exposing the silicon surface to XeF2 doses between 10(2) and 10(4) monolayers (MLs) of XeF2, thermal desorption spectroscopy is used to study the SiFx content of the reaction layer. Large values of the fluorine content (up to 30 ML) are observed. These are explained by surface roughening due to the etching process, which increases the effective surface by up to a factor of 6 after 10(5) ML of XeF2, corresponding to 10(4) ML of etched Si. This implies a porous or canyon-like structure. The dose dependence of the fluorine content is not single exponential; it increases rapidly for the first 10(2) ML and much more slowly thereafter, saturating after approximately 10(4) ML. This behaviour is described by the rapid formation of a monolayer of SiFx species in the initial regime, followed by the slow formation of a steady state multilayer of SiF-SiF2-SiF3 and SiF2-SiF3 chains. The etch rate shows a similar dose dependence, increasing from 12% of the incident flux in the monolayer regime to 20% in the multilayer regime. Finally, it is shown that fluorine diffusion into the silicon bulk is not an important process in the reaction layer formation.