Reactive ion beam etching of silicon is performed at low substrate temperature (153 < T < 300 K). The beam is extracted from a SF6 microwave plasma. The energy of the beam is 100 eV < E < 400 eV, the ion current density is 0 < J < 5 mA cm(-2). Energy distribution and composition of the beam have been studied using a mass spectrometer coupled with a cylindric mirror analyzer. The beam can be considered as monoenergetic with a full width at half-maximum about 8 eV. Using SF6 plasma, F+ is the predominant species, a significant proportion of SF+, SF2+, S+, and SF3+ ions is present in the beam. The active neutral flux of fluorine atoms coming from the ion source has been estimated between 6 X 10(16) and 4 X 10(17) cm(-2) s(-1). When no ion beam strikes the sample, the probability of reaction of neutral F atoms with Si is evaluated between 0.02 and 0.1 at 300 K. The decrease of the substrate temperature has revealed the role of the ion-induced reaction and the ion-stimulated desorption of SFx radicals in the etching process. At relative low power densities (100 < E < 400 eV and J = 0.65 mA cm(-2)), sn increase in the etch yield occurs as the temperature decreases until 220 K because there is an enhancement of the ion-induced reactions. If the energy of the beam is not high enough, the etching is reduced or even blocked by a thick layer of adsorbed molecules at T < 220 K. At higher power densities (100 < E < 400 eV and 2 mA cm(-2) < J < 5 mA cm(-2)) no significant etch yield enhancement is noted as the temperature decreases because the ion-stimulated desorption becomes predominant. In the case of high current density which corresponds to low values of the ratio of neutral flux over ion flux (a few ten), the etch yield approaches the value of the physical sputtering yield calculated by a model derived from the Sigmund cascade collision.