Automated control of plasma ion-assisted electron beam-deposited TiO₂ optical thin films

A hollow cathode plasma source has been operated automatically, demonstrating independent control of plasma ion energy and ion current density for plasma ion-assisted electron beam-deposited titania (TiO₂). The lanthanum hexaboride hollow cathode design described in this work utilizes both the interior and exterior cathode surfaces, with the additional electrons generated removing the need for a separate neutralizing source. Automatic feedback control of plasma source cathode-to-anode accelerator voltage (AV-via argon gas flow to the anode and/or cathode plasma source areas) and accelerator current (AC-via an external high-current power supply) provides independent control of the ion energy distribution function and ion current density, respectively. Automated run-to-run reproducibility (over six separate deposition runs) in TiO₂ refractive index (550 nm) was demonstrated as 2.416 ± 0.008 (spread quoted as one standard deviation), which is well within the required refractive index control for optical coating applications. Variation in refractive index is achievable through control of AV (ion energy) and/or AC (ion current density), directly influencing deposited TiO₂ structural phase. Measured dependencies of TiO₂ refractive index and extinction coefficient on AV and AC are described. Optimum plasma source parameters for assisted electron beam deposition of TiO₂ optical thin-film applications are highlighted.