Arching Concerns When Sputtering SiO2

Recently a customer asked about continuously running an SiO2 Sputtering Target and specifically concerns about arcing.

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There is always going to be some &#;re-deposition&#; onto a sputtering target where stray molecules that have been knocked off the target surface reattach back onto the target surface. In your case, using a non-magnetron planar diode cathode assembly with an rf generator this should not be an issue. The ions should be striking the target surface randomly and not in any selected area of the target surface. The rf generator should provide an even, homogeneous plasma uniformly across the entire target surface, bombarding both re-deposited and virgin areas of the SiO2 target surface randomly and equally. The resultant films may vary a bit in stoichiometry as the re-deposited (Si)x(O2-x) areas of the target surface may be slightly silicon rich or oxygen rich depending on the volume of the partial pressure of the oxygen added to the argon working gas, but this should NOT affect the ability of the sputtering process to sustain a constant uniform plasma and deposition.

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If there is arcing, you may be building up material within the gap between the dark space shield and the target surface. This should be around 1-3mm or so and the dark space shield should always protrude beyond the target surface by about 3mm or so. Keep this gap area clean, even if it means sanding or abrading the components occasionally. Arcing means that the plasma is going to ground potential for some reason. Make certain that the reflected power on your rf generator is not drifting up with time. This must be kept at absolute zero. Is your rf generator set at constant voltage, constant current or constant power? Check and see if both the current and the voltage are remaining constant over time at whatever power level you are using. Something may be changing. If the impedance in the target is changing, due to re-deposition and/or target thickness changes (becoming thinner over time), the power supply and the tuning network needs to compensate accordingly to maintain an equilibrium balance within the plasma.

SiO2 thin films were prepared with radio frequency magnetron sputtering on quartz glass substrates, and the effects of sputtering power on the stoichiometric ratio, microstructure, surface morphology and optical properties of the film within 300&#; nm were investigated. The molar ratio of O/Si in the film increased continuously from 1.87 to 1.99, very close to the ideal stoichiometric ratio of 2:1 with the sputtering power decreasing from 150 to 60 W. And the surface of SiO2 thin film became more compact and flatter, and the roughness was significantly reduced. All the SiO2 films were amorphous, and the power had no obvious effect on the crystalline state of the film. When the sputtering power decreased from 150 to 60 W, the refractive index and absorptivity of SiO2 film in the range of 300&#; nm decreased continuously, while the transmittance within 300&#; nm of the coated quartz glass (hereinafter referred to as the transmittance of film) increased continuously, and the integrated transmittance increased from 92.7 to 93.0%.

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