Reverse pulse strategies for silicon dioxide thin films deposition by high power impulse magnetron sputtering

Abstract

High density transparent oxide layers on polymers and glass can improve the environmental viability of photovoltaics‚ displays‚ and low emissivity layers in glazing as well as aid the design of optical filters. High Power Impulse Magnetron Sputtering (HIPIMS) produces high density microstructures and high hardness due to the delivery of an ionised metal and dissociated Oxygen deposition flux to the substrates. Silicon dioxide (SiO_x) films were deposited by reactive HIPIMS of a metallic target in an Argon-Oxygen atmosphere. Single-target HIPIMS sputtering with reverse voltage operation was evaluated. The HIPIMS process was carried out by controlling the current within the pulse. This resulted in the elimination of stability issues associated with runaway currents for all target poisoning states from metallic to compound. SiO_x was deposited at a peak current density of 0.5 Acm⁻² in a plasma dominated by Si¹⁺ ions as shown by energy- and mass- resolved spectrometry. The measured signal of atomic Oxygen was twice the amount of molecular Oxygen. The pulse duration was 20 microseconds. Plasma persisted to >150 μs after the pulse switch off as evidenced by the Ar neutral (Ar I) optical emission intensity. Arcing rates were significantly reduced when reverse pulsing was used due to the discharging of the target surface. The key attributes of the reverse voltage which influenced the extent of film defects caused by arcing events and deposition conditions were the amplitude and the delay between the switch-off of the pulse and the application of the reverse voltage. Applying a reverse voltage immediately after the end of the pulse utilised the undispersed high-density plasma still present in the racetrack at the point of switch off to neutralise the target surface and reduce arc energy. Reverse voltages of +25 V coupled with short delay times resulted in enhancing the flux and augmenting the energy of metal ions to the substrate. The gains in adatom mobility afforded by this approach promoted the formation of smooth and dense films with microscopic roughness of R_a = 3 nm as observed by AFM and high optical transmittivity of up to 97 % at a wavelength of 800 nm for 200 nm thick films. The high density supported a high nanohardness of 1 μm thick films of 10 ± 1 GPa and Young's modulus 77 ± 9 GPa, representing a 10 % increase over a glass substrate. Reverse voltages of +75 V and beyond were detrimental due to the production of ions of process and contaminant gases near the chamber walls and the target, which disrupted the lateral growth of film grains and induced the formation of globular morphology with a high microscopic surface roughness.