Arching Concerns When Sputtering SiO2

A customer recently inquired about the continuous operation of an SiO2 Sputtering Target, especially with respect to the issue of arcing.

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In the sputtering process, there will inevitably be a degree of re-deposition occurring on the sputtering target. This phenomenon takes place as loose molecules are ejected from the target surface and subsequently reattach themselves. In the situation at hand, where a non-magnetron planar diode cathode assembly is utilized alongside an RF generator, arcing should not pose a concern. The ions interacting with the target surface should distribute randomly, rather than concentrating in specific areas. The RF generator facilitates the creation of an even plasma distribution across the entire target surface, ensuring that both re-deposited areas and untouched sections of the SiO2 target receive equal bombardment. While the stoichiometry of the resulting films may exhibit minor variations—potentially richer in silicon or oxygen depending on the partial pressure of oxygen added to the argon gas—this variability should not interfere with the sputtering process's capacity to maintain a stable and uniform plasma and deposition.

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When arcing occurs, it may signify that material is accumulating within the gap between the dark space shield and the target surface, which ideally should maintain a distance of approximately 1-3mm. It is essential that the dark space shield extends beyond the target surface by roughly 3mm. Make it a routine practice to keep this gap clean, even if it involves occasional sanding or abrasion of the components. Arcing implies that there is an unintended grounding of the plasma. Ensure that the reflected power from your RF generator remains stable and does not drift over time; it should be held at absolute zero. Determine whether your RF generator is configured for constant voltage, constant current, or constant power, and verify that both current and voltage have not fluctuated over time at your chosen power level. Alterations might be taking place. If there are changes in the target's impedance due to re-deposition or a decrease in target thickness, the power supply and tuning network must adjust accordingly to achieve a balance within the plasma.

Utilizing radio frequency magnetron sputtering, SiO2 thin films were deposited on quartz glass substrates, with a focus on understanding how varying sputtering power influences the stoichiometric ratio, microstructure, surface morphology, and optical characteristics of the films within 300 nm. As sputtering power decreased from 150W to 60W, the molar ratio of O/Si in the film increased steadily from 1.87 to 1.99, closely approaching the ideal ratio of 2:1. Concurrently, the surface of the SiO2 thin film exhibited increased compactness and flatter characteristics, accompanied by a significant reduction in surface roughness. All produced SiO2 films displayed an amorphous nature, with sputtering power not exerting a noticeable impact on the crystalline state. The results observed indicated that as sputtering power declined, both the refractive index and absorptivity of the SiO2 films diminished consistently within the 300 nm range. In contrast, the transmittance in this same range for the coated quartz glass displayed a continuous increase, with integrated transmittance rising from 92.7% to 93.0%.

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