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 electrical current of 200 µA or more. 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. A typical GCIB surface treatment allows every point on the surface to be struck by many cluster ions, resulting in smoothing of surface irregularities.