Process Parameter Optimization of High Voltage Power Supply for Magnetron Sputtering Transparent Conductive Films
Transparent conductive films combine high optical transparency with electrical conductivity, enabling applications in displays, touch screens, solar cells, and solid state lighting. Indium tin oxide has been the dominant material, but alternatives including aluminum doped zinc oxide and fluorine doped tin oxide are gaining importance due to cost and availability concerns. Magnetron sputtering is the primary deposition technique for these films, with the high voltage power supply parameters critically affecting the film properties including conductivity, transparency, and uniformity.
Magnetron sputtering deposits thin films by bombarding a target material with energetic ions, ejecting atoms that travel to and condense on the substrate. The plasma is sustained by a glow discharge powered by a high voltage supply applied to the target. The discharge voltage and current determine the power delivered to the target, which affects the sputtering rate, the energy of ejected atoms, and the plasma conditions. For transparent conductive films, the power supply parameters must be optimized to achieve the desired combination of electrical and optical properties.
The discharge voltage in magnetron sputtering affects the energy of ions bombarding the target. Higher voltages produce higher ion energies, which can increase the sputtering yield but also increase the energy of sputtered atoms arriving at the substrate. The arrival energy affects the film microstructure, with higher energies generally producing denser films with better electrical properties. However, excessive energy can cause damage to the growing film or excessive heating of the substrate.
The discharge current determines the ion flux to the target and thus the sputtering rate. Higher currents increase the deposition rate, improving throughput, but also increase the plasma density and the flux of energetic particles to the substrate. The current must be sufficient to achieve practical deposition rates while maintaining the plasma stability and the film quality. The relationship between current and film properties depends on the target material, the gas pressure, and the substrate conditions.
Power delivery mode affects the film properties for reactive sputtering of compound films. Direct current power is suitable for conductive targets but can cause arcing when sputtering oxidizing targets that develop insulating surface layers. Pulsed direct current power, where the voltage is periodically reversed, prevents charge accumulation that causes arcing. The pulse frequency and duty cycle affect the discharge stability and the film properties. Radio frequency power enables sputtering of insulating targets.
Reactive sputtering of doped oxide films involves introducing oxygen or other reactive gases along with the inert sputtering gas. The reactive gas incorporates into the growing film, forming the oxide compound. The partial pressure of reactive gas affects the stoichiometry and the doping level, critically influencing the electrical and optical properties. Too little oxygen results in substoichiometric films with high carrier concentration but also high absorption. Too much oxygen produces fully oxidized films with low carrier concentration and poor conductivity.
The hysteresis effect in reactive sputtering complicates the process control. The relationship between reactive gas flow and film composition exhibits hysteresis, where the composition depends on the history of gas flows. This occurs because the target surface becomes oxidized at high reactive gas flow, reducing the gettering effect and changing the relationship between flow and partial pressure. Process control strategies must account for this hysteresis to maintain stable composition.
Substrate temperature during deposition affects the film microstructure and properties. Higher temperatures promote adatom mobility, leading to larger grains and better crystallinity. The electrical conductivity typically improves with temperature due to better crystallinity and reduced grain boundary scattering. However, many substrates have temperature limitations that constrain the achievable temperature. The power supply parameters affect the substrate heating through the energy flux from the plasma.
Uniformity of film properties across the substrate requires uniform plasma and deposition conditions. The magnetron design affects the erosion pattern on the target and the plasma distribution. The power supply affects the plasma through the discharge parameters. For large substrates, multiple magnetrons or moving substrates may be needed to achieve uniformity. The power distribution across multiple magnetrons must be controlled to maintain uniform deposition.
Characterization of transparent conductive films includes electrical measurements of sheet resistance and carrier concentration, optical measurements of transmission and reflection, and structural characterization by X-ray diffraction or microscopy. The figure of merit for transparent conductors combines the electrical conductivity and optical transparency, with higher values indicating better performance. Optimization seeks to maximize this figure of merit while meeting other requirements including uniformity, stability, and cost.
