Supersonic cluster source with mass selection and energy control

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Abstract
We have developed an apparatus that produces an ionized beam of mass selected clusters of controllable energy. It uses a pulse of second‐harmonic light from a Nd:YAG laser to vaporize atoms from a target disk. The clustering rate of the atoms in this expanding plume is controlled with the pressure of a He pulse injected from a fast pulsed valve. A Wiley–McLaren type time‐of‐flight mass spectrometer with a shield lens is used to monitor the cluster distribution and to optimize the control parameters of the system. The shield lens doubles the resolution and the signal‐to‐noise ratio of the spectrometer. A 4000 amu rf quadrupole with 1.2 amu resolution is used to select the mass of charged clusters. An electrostatic quadrupole is used to separate the charged clusters from the neutral clusters and the He gas. The ionized clusters are then focused onto a suitably prepared substrate by using electrostatic lenses. Time averaged deposition current densities on the order of 100 pA cm−2 have been measured when the vaporization laser is fired at 10 Hz. The deposition energy of the clusters is controlled by applying a bias voltage to the substrate. A mobile vacuum chamber is used to transfer samples to various diagnostic systems.