Deposition of Niobium-oxide Thin Films Using Electron Cyclotron Wave Resonance Plasma-Ion-Source-Assisted Medium-frequency Magnetron Sputtering
Niobium-oxide thin films present significant application prospects in the protection of optical devices and optical antireflection thin films owing to their high refractive index,good transparency,and other advantages.The complexity of niobium-oxide systems is related to their stoichiometric and nonstoichiometric existence.The chemical stoichiometry of slight changes in niobium oxides cannot be easily controlled,identified,and determined;as such,these oxide systems are not clearly understood.Therefore,the physical properties and control mechanisms of niobium oxides must be further investigated.In terms of preparation technology and related equipment,although intermediate-frequency magnetron sputtering yields better film quality than electron beam evaporation,it inevitably produces particles,which severely affects the quality and optical properties of optical thin films,particularly in precision optical thin-film applications.The aim of this study is to improve the deposition quality and optical properties of niobium-oxide thin films using electron cyclotron wave resonance(ECWR)plasma-enhanced intermediate-frequency magnetron sputtering technology.Using intermediate-frequency magnetron sputtering to spray niobium elements in a mixed atmosphere of argon and oxygen yields a niobium-oxide thin film via the interaction between oxygen plasma and niobium.During this process,the thin film contains both unoxidized niobium and incompletely oxidized(low-valence state)niobium oxide,which is assisted by an ECWR ion source to enhance oxidation.A substrate stage is placed on a rotatable turntable by integrating intermediate-frequency magnetron sputtering and an ECWR ion source into the same discharge chamber.During the deposition of niobium-oxide thin films,the substrate carrier can rotate back and forth between the magnetron sputtering and ion source for repeated thin-film deposition and auxiliary oxidation.After repeating the process above multiple times,niobium-oxide thin films with the desired thickness can be obtained.In this experiment,a comparative study pertaining to the use of ICP/ECWR ion sources to assist in the deposition of oxide thin films using pure oxygen and argon/oxygen-doped plasma discharge is conducted to investigate the effects of different discharge mechanisms of RF ion sources on the preparation of niobium-oxide thin films via mid-frequency magnetron sputtering.ICP/ECWR plasma discharge is analyzed using a Langmuir probe and emission spectroscopy plasma diagnostic technology,and the characterization results of thin-film deposition are examined.Compared with the conventional intermediate-frequency magnetron sputtering,the thin film deposited with ECWR ion-source assistance exhibits excellent smoothness,uniformity,and density.The optical transmittance improves significantly to 91％,thus effectively eliminating the particle issue caused by intermediate-frequency magnetron sputtering in optical thin films.Compared with the ICP RF ion source,which requires mixed-gas discharge,the ECWR plasma source can stably generate high-density plasma in a low-pressure pure oxygen environment without requiring argon as a"trigger"to excite and maintain a stable oxygen discharge.This demonstrates the superiority of the ECWR plasmonic discharge structure,and because of its low-energy characteristics,an amorphous niobium-oxide film is achieved.Using the transmittance peak of pure-oxygen ECWR discharge samples as a reference line,one can observe that the pure-oxygen ECWR discharge exhibits a significant red shift compared with other discharge forms.This is because changes in the grain size and the mass of the niobium-oxide film alter the bandgap width of the optical film.The thin film obtained via oxygen ECWR discharge deposition is uniform and dense,which causes the optical bandgap width to shift toward the low-energy direction,thus narrowing the bandgap.In addition,this study reveals the relationship between the high-density and low-energy characteristics of the ion sources and the surface and optical properties of the thin films.This discovery is crucial for the development of high-end vacuum coating equipment and control systems with independent intellectual property rights,as well as provides new solutions for precision optical thin-film applications.
electron cyclotron wave resonance plasmamedium frequency magnetron sputteringplasma diagnosisinductively coupled plasmaniobium oxide film
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