1 Introduction
Tab.1 Comparison of different types of cluster sources a) |
Type of source | Reported flux/(nm·s–1) | Materials usable | Typical cluster size | Typical vacuum regime | Proportion of ionized particles |
---|---|---|---|---|---|
SSNS | ~20 | Low-boiling point metals | ~100–103 atoms | UHV | 0 |
TGAS | ~0.05 | Any material that attains 1 mbar vapor pressure at 2000 K | ~103 atoms | UHV | 0 |
SGAS | ~100 (~1) | Virtually any solid (using RF or HiPIMS for insulators) | 1–60 nm | HV | Up to 50% |
LAS | ~0.1 | Virtually any solid | ~100–102 atoms | UHV | ≈10% |
PACIS | ~1 | Virtually any solid | 1–10 nm | HV | ≈10% |
PMCS | 1–30 | Conductive solids | ~101–104 atoms | UHV | ≈10% |
MACS | 10 mg·h–1 | Any metal can be vaporised | ~100–103 atoms | HV | 0 |
a) Reprinted with permission from ref. [14], copyright 2017, John Wiley and Sons. RF: radio frequency; HiPIMS: high-power impulse magnetron sputtering. |
2 Sputtering gas aggregation source
2.1 Magnetron sputtering process
2.2 Magnetron sputtering cluster source
2.3 Scaling up of magnetron sputtering cluster source
2.3.1 Multiple ion cluster source
Fig.3 (a) High resolution transmission electron microscope image of a Ag-Au alloy nanoparticle (NP) and energy dispersive spectrometer (EDS) line scan performed at the Ag and Au along the line depicted. Reprinted with permission from ref. [48], copyright 2012, American Chemical Society. (b), (c) and (d) Cs-corrected scanning transmission electron microscope (STEM) representative image of, respectively, a Ag-Au core-shell NP, Co-Au core-shell NP and a Co-Ag-Au core-shell-shell NP and EDS line scan performed at the Co, Ag, and Au, along the lines depicted. Reprinted with permission from ref. [49], copyright 2014, the Royal Society of Chemistry. |
2.3.2 High-power impulse magnetron sputtering
Fig.5 Time-resolved ion current measurements for different pulse frequencies: (a) 7, (b) 10, (c) 20, and (d) 100 Hz (The dashed lines in (d) show the deconvolution in five traces as shown in (a)). Reprinted with permission from ref. [55], copyright 2013, American Chemical Society. |
Fig.6 (a) Sketch of the early-stage cluster growth, occurring in Zone I in (c); (b) sketch of the OML collection of ions process, showing a much larger cross section compared to the neutral case; (c) schematic drawing of the regions corresponding to different stages of cluster growth. A movable anode ring is installed to tune the size of the second cluster growth region; (d) cluster growth rate for OML collection of ions as a function of the electron temperature Te. The neutral collection rate (blue dashed line) is 2.8 nm·s–1, while for the experimental observed growth rate by OML collection of 470 nm·s–1, Te needs to be 1.7 eV. Reprinted with permission from ref. [58], copyright 2013, AIP Publishing. |
2.3.3 Gas dynamics influence
Fig.8 (a) Au single atom number density profile inside the condensation chamber in the case where the gas is provided from the rear of the chamber (top) and from within the magnetron source (bottom). A logarithmic scale is used to show the number density distribution. (b) Au single atom number density profile in a region close to the target surface, in the case where the gas is provided from the rear of the chamber (top) and from within the magnetron source (bottom). A linear scale is used to show the number density distribution. (c) The probability of Au atom–atom collision in front of the sputtering target, if the gas inlet is within the magnetron source (red cross symbol) and at the rear of the chamber (gray dash symbol). Reprinted with permission from ref. [35], copyright 2021, AIP Publishing. |
Fig.9 Fraction of clusters departing from lines perpendicular to the chamber axis and successfully going out through the nozzle vs. the distance of such lines from the nozzle. The results for clusters of different sizes are shown here: (a) 10 atoms per cluster, (b) 100 atoms per cluster, and (c) 1000 atoms per cluster. Reprinted with permission from ref. [35], copyright 2021, AIP Publishing. |
3 Pulsed microplasma cluster source
Fig.10 (a) Cross section of a PMCS, featuring the main constituents. Reprinted with permission from ref. [71], copyright 1999, IOP Publishing. (b) 3-D sketch of a PMCS with a system of aerodynamic lenses mounted on the nozzle exit. The rod (cathode) in specifically designed for the experiment in ref. [75]. Here a MoS2 target needed to be eroded but, because of its mechanical fragility and electrical resistivity, a system of Mo ring and holder rods hosting two MoS2 pellet cylinders has been designed, so to overcome such problems. (c) Schematics of a PMCS (left) mounted on a typical supersonic cluster beam deposition system, showing the different sections in the sequential pumping. Reprinted with permission from ref. [75], copyright 2015, IOP Publishing. |
Fig.11 (a) Photographs of a cluster film deposited at 300 mm from the source with (left) and without (right) the use of a focuser; (b) histogram of the high magnification AFM results showing the two size distributions for the focused and unfocused cluster deposition, respectively in black and gray color. Reprinted with permission from ref. [76], copyright 2001 AIP, Publishing. |
Fig.13 (a) Radial displacement at the end of the nozzle as a function of the particle diameter for different particle injection radial position without the focuser; (b) same as in (a) but with the use of a focuser. Reprinted with permission from ref. [61], copyright 2002, Taylor & Francis. (c) Histogram of the radial displacement of the particles exiting the nozzle without the use of a focuser. Brownian motion is considered in the calculation; (d) same as in (c) but with the use of a focuser. Reprinted with permission from ref. [78], copyright 2002, Springer Nature. |
Fig.14 (a) Gas carrier streamlines from simulation results for a pressure of 345 Pa and 20% of H2 in Ar; (b) cluster trajectories for a particle diameter of 15 nm; (c) schematics of the cluster trajectories for different particle size, showing the principles of aerodynamic focusing and size-selection. Reprinted with permission from ref. [74], copyright 2006, IOP Publishing. |
4 Matrix assembly cluster source
Fig.15 (a) Schematics of a MACS in the “transmission mode”; (b) graphic representation of the Ar+ ion beam impacting on the metal and gas condensed matrix and eroded in the “transmission mode”. Reprinted with permission from ref. [82], copyright 2016, AIP Publishing. (c) Schematics of a MACS in the “reflection mode”; (d) graphic representation of the Ar+ ion beam impacting on the metal and gas condensed matrix and eroded in the “reflection mode”. Reprinted with permission from ref. [9], copyright 2016, the Royal Society of Chemistry. |
Fig.16 (a) HAADF-STEM images for Au clusters for different concentration of Au atoms embedded in the condensed gas matrix different concentration: 0.5%, 1%, 2.1% and 2.8%. The scale bar corresponds to 10 nm; (b) plot of the cluster mean size and of the cluster beam intensity vs. the metal concentration in the matrix as extrapolated by the HAADF-STEM images. Reprinted with permission from ref. [83], copyright 2020, Springer. |
Fig.18 (a) Experimental data of the measured cluster intensity as a function of the collection angle in the case of four different incident angle of the Ar+ ion beam: 10°, 15°, 35° and 45° (It can be noted how a lower incident angle results in a higher overall cluster intensity, which is the integral of each curve); (b) optimal collection angle, which is the peak value of each curve in (a)), as a function of the incident angle of the Ar+ ion beam. Reprinted with permission from ref. [86], copyright 2019, Springer. |