A method for forming a lightly doped drain region in a semiconductor substrate. In embodiments, the apparatus comprises a differential ion mobility device in a vacuum enclosure of a mass spectrometer, located prior to the mass analyzer, wherein the pumping system of the apparatus is configure to provide an operating pressure of 0. The method relies on a single deposition step performed by means of an atomic layer deposition. An inflator is mounted in the lower chamber and has a first tube which is received between the inner wall and the outer wall, and a second tube which is inserted into the lower part.