A decel lens assembly (9) located between the mass selection flight tube and the substrate holder comprises a first electrode (65) at the substrate potential, a second electrode (60) at the flight tube potential and a field electrode (61) between the two at a negative potential to provide focusing. The axial spacing in the beam direction between the first and second electrodes is less than the smallest transverse dimension of the field electrode. The decel lens assembly (9) is mounted directly opposite the outlet from the process chamber to the vacuum pump to maximize evacuation efficiency. An additional screening electrode (56) is provided between the second electrode of the decel lens assembly and the exit aperture of the mass selector. A perforated screening cylinder (54) is mounted on the light tube with the second electrode of the lens assembly mounted at the down beam end of the cylinder. A first electrode has a cylindrical screening flange extending around the field electrode. A further screening electrode is located at the entrance to the electron confinement tube of the PFS system.