Durable Thermochromic VO2 Films Deposited by HiPIMS

Vanadium dioxide, VO2, is a well-known thermochromic material that exhibits high or low infrared transmittance when its temperature is below or above a critical temperature, respectively.Because this change in properties is reversible, VO2 has long been a candidate for radiative energy control devices.One innovative application is the smart window, in which a thermochromic layer controls the transmission of infrared radiation from the sun as a function of the surrounding temperature.This enables self-controlled temperature regulation, which in turn limits heating and cooling requirements.Similar devices called smart radiator devices (SRDs) are being developed for passive cooling of satellites.However, all these applications remain in the development phase due to VO2 deposition issues.Vanadium is a transition metal that exhibits multiple oxide stoichiometries, of which only VO2 is of interest.Precise control of the oxygen fraction is therefore needed for VO2 synthesis, accompanied by a high deposition temperature, usually around or in excess of 400 °C, in order to obtain the crystalline material.These imperatives currently limit industrial applications of VO2.Over the past decade, high power impulse magnetron sputtering (HiPIMS) has emerged as a newcomer in the magnetron sputtering community.Because HiPIMS combines the versatility of magnetron sputtering with short high power pulses to generate highly ionized plasma, it is a promising technique for thin film material deposition.In reactive sputtering, HiPIMS provides deposition rates similar to or higher than those for conventional sputtering, along with superior film properties such as higher density and lower stress, among others.Furthermore, crystalline films can be obtained at lower deposition temperature.However, the impacts of the high ion flux, plasma radiation, and HiPIMS dynamics on the metal-oxide properties are still unclear, despite a good understanding of the HiPIMS discharge.We therefore chose to apply HiPIMS to thermochromic VO2 to demonstrate the advantages of using HiPIMS to deposit crystalline films.First, the expansion of a reactive-HiPIMS discharge over a vanadium target was investigated using high-speed plasma imaging combined with optical bandpass filtering.The results showed that the reactive discharge operating in an argon-oxygen mixture behaved similarly in "pulse-on" time to discharges in pure argon and oxygen.However, in the post-discharge, the first microsecond after the cathode voltage was removed, strong light emission in the low-energy spectra was observed.This light emission can be explained by electrons that cool and drift toward the null magnetic field ix trap.A threshold oxygen content in the mixture is needed in order to observe this phenomenon.This led us to hypothesize that the main interaction is dissociative attachment between the electrons and the molecular oxygen.The resulting negative ions recombine with positive ions, generating excited radiative species.These resulting species, which have an estimated kinetic energy approaching 1 eV, provide a low-energy bombardment of the growing films.This reaction in pulsed plasma also generates additional atomic oxygen, compared to conventional magnetron sputtering.Therefore, such post-discharge emission provides a partial explanation for the reactivity observed in the pulsed discharge, for the lower oxygen content needed to produce stoichiometric material, and for the lower temperature required to deposit crystalline films.Following the exploration of the reactive discharge, HiPIMS, a previously demonstrated approach to lower the deposition temperature, was used to deposit thermochromic VO2 on polymer substrates.Using the energetic HiPIMS process, VO2 films were synthesized onto two different polyimide substrates, Kapton® HN and Kapton® CS.These two substrates present different optical properties that target different potential applications of VO2 for energy-control devices.First, Kapton® HN, an orange-tinted polymer, is standard in the aerospace industry because it is resistant to high temperatures and space radiation.On the other hand, Kapton® CS provides a clear substrate with moderate temperature resistance, and is more suitable for earthbound applications such as smart windows.The produced films exhibited active thermochromic behavior, the first time that this has been achieved with VO2 on a polymer substrate.The film stoichiometries were confirmed by X-ray photoelectron spectroscopy (XPS).Unfortunately, due to the nature of the polymer substrate, it was not possible to directly quantify the film crystallinity.However, Raman spectroscopy and X-ray diffraction (XRD) of the deposits on silicon strips showed spectra comparable to those in the literature.Rutherford backscattering spectrometry (RBS) and elastic recoil detection (ERD) were used to confirm the stoichiometric composition of the Kapton®-coated sample, revealing a strong need for film-substrate interface control.In fact, pronounced diffusion of vanadium and oxygen into the polymer substrate was measured.Although this diffusion could improve the film adhesion, it represents a net loss of material in terms of deposition efficiency.Similar diffusion was observed for reference films deposited onto glass substrates; namely vanadium diffusion into glass, and boron (glass dopant) diffusion into VO2.To prevent this type of diffusion at the VO2-glass interface, the design was improved with the addition of a barrier coating.Because it is well known that thermochromic VO2 is sensitive to water x degradation, an additional diffusion barrier was added as a top layer.For this purpose, we chose SiNx, a commonly used diffusion barrier that is stable at high temperatures and can be deposited by magnetron sputtering.To assess film durability, VO2 films were deposited using both HiPIMS and conventional RFMS.Before oxidation, the deposited devices presented similar transmission and performance in terms of infrared modulation, independently of the deposition method.Raman and XRD were unable to detect significant differences in the HiPIMS-and RFMS-deposited VO2.As expected, SiNx provided an adequate diffusion inhibiter.No trace of the glass dopant was found in the VO2 layers, and both RFMS-and HiPIMS-deposited coatings provided good resistance to a steam environment.However, the unprotected VO2 coating behaved differently.The HiPIMSdeposited layers exhibited slower kinetics, with an incubation period of slow degradation, whereas the RFMS-deposited VO2 layer showed rapid dissolution.Based on the results of scanning electron microscopy (SEM) image analysis combined with RBS/ERD density measurement, we attributed the higher durability of the HiPIMS-deposited VO2 coating to a higher grain density, low amount of grain boundaries, which lowered oxygen diffusion at grain boundaries and hence slowed the material degradation.On the other hand, the lower density of the RFMS-deposited layer gave rise to high roughness, with V2O5 needles topping the SiNx layer.The higher diffusion of atoms into the RFMS sample was confirmed by the ERD-measured hydrogen content, showing higher hydrogen density in the vanadium layer during water oxidation in the RFMS-deposited layers.

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