Gas cluster ion beam

Using GCIB a surface is bombarded by a beam of high-energy, nanoscale cluster ions. The clusters are formed when a high pressure gas (approximately 10 atmospheres pressure) expands into a vacuum (1e-5 atmospheres). The gas expands adiabatically and cools then condenses into clusters. The clusters are nano-sized bits of crystalline matter with unique properties that are intermediate between the realms of atomic physics and those of solid state physics. The expansion takes place inside of a nozzle that shapes the gas flow and facilitates the formation of a narrow jet of clusters moving along the axis of symmetry of the nozzle. The jet of clusters passes through differential pumping apertures into a region of high vacuum (1e-8 atmospheres) where the clusters are ionized by collisions with energetic electrons. The ionized clusters are accelerated electrostatically to high velocities, and they are focused into a tight beam.

The GCIB beam is then used to treat a surface — typically the treated substrate is mechanically scanned in the beam to allow uniform irradiation of the surface. Argon is a commonly used gas in GCIB treatments because it is chemically inert and inexpensive. Argon forms clusters readily, the atoms in the cluster are bound together with Van der Waals forces. Typical parameters for a high-energy, Argon GCIB are acceleration voltage 30 kV, average cluster size 10,400 atoms, average cluster charge +3.2, average cluster energy 64 keV, average cluster velocity 6.5 km/s, with a total electric current of 200 μA or more.^[1][2] When an Argon cluster with these parameters strikes a surface, a shallow crater is formed with a diameter of approximately 20 nm and a depth of 10 nm. When imaged using atomic force microscope (AFM) the craters have an appearance much like craters on planetary bodies.^[3][4][5] A typical GCIB surface treatment allows every point on the surface to be struck by many cluster ions, resulting in smoothing of surface irregularities.

Lower energy GCIB treatments can be used to further smooth the surface. Reducing the energy decreases the size and depth of the impact craters and, analogous to mechanical polishing where the grit size is reduced during polishing, subsequent treatments with lower energies are used to reach an atomic level smoothness. Low energy clusters can be used to harden and densify the surface. Advantages of GCIB surface polishing over conventional polishing include the ability to easily smooth non-planer surfaces, very thin substrates and thin-films. GCIB assisted thin-film deposition produces denser and more uniform films. Almost any gas can be used for GCIB, and there are many more uses for chemically reactive clusters such as for doping semiconductors (using B₂H₆ gas), cleaning and etching (using NF₃ gas), oxidizing (using O₂ gas), reducing oxide (using H₂ gas), nitriding (using N₂ gas), and for depositing chemical layers. GCIB can be applied to any substrate material but the smoothing properties will depend on the homogeneity of the surface.