Gas Cluster Ion Beams (GCIB) is a technology for nano-scale modification of surfaces. It can smooth a wide variety of surface material types to within an angstrom of roughness without subsurface damage. It is also used to chemically alter surfaces through infusion or deposition.
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Process
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 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 jet of clusters. 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 very 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 [1]: average cluster size 10,000 atoms, average cluster charge +3, average cluster energy 65 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 Microscopy (AFM) the craters have an appearance much like craters on planetary bodies [2]. 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, and GCIB can be used to produce an atomic level smoothness on both planar and nonplanar surfaces. 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), and for depositing chemical layers.
Industrial applications
In industry, GCIB has been used for the manufacture of semiconductor devices, optical thin films, trimming SAW and FBAR filter devices [3], fixed disk memory systems and for other uses. GCIB smoothing of high voltage electrodes has been shown to reduce field electron emission, and GCIB treated RF cavities are being studied for use in future high energy particle accelerators [4].
A related technique, with a limited range of applications, using high-velocity carbon Fullerenes to treat surfaces, has been studied.
Small argon cluster GCIB sources are increasingly used for analytical depth-profiling by secondary ion mass spectrometry (SIMS) and x-ray photoelectron spectroscopy (XPS). Argon clusters greatly reduce the damage introduced to the specimen during depth-profiling, making it practical to do so for many organic and polymeric materials for the first time. This has greatly extended the range of materials to which XPS (for example) can be applied [5].