Surface OH groups governing surface chemical properties of SiO2 thin films deposited by RF magnetron sputtering
Introduction
Silicon oxide (SiO2) thin films are widely used in various fields such as passivation layers of electronic devices, protection layers of magnetic or optical disks and anti-reflective (AR) coatings of displays, because of their excellent chemical stability and optical transmittance with low refractive index [1], [2], [3], [4]. The surface properties of the film may be changed when organic substances are adsorbed on the film from the atmosphere, causing serious problems for product quality. When other materials are deposited onto the films where organic substances have been adsorbed, the adhesion is weakened at the interface between layered films. Therefore, it is very important to control the surface properties of the film to obtain high quality products.
In previous papers [5], [6], we have investigated the relationship between the wettability and the surface OH group density of metal oxide films or commercial glasses and reported that the surface OH groups play an important role on the surface chemical properties since they can work as effective adsorptive or reactive sites. Furthermore, it has been found that the hydrophobicity, resulting from the adsorption of organic substances in the atmosphere was different among the films and glasses and that the origin of the variation was attributed to the difference in the amount of adsorbed carbon substances on the surfaces. The amount of the carbon substances adsorbed from the atmosphere has been dependent on the surface OH group density of the films or glasses.
In the present study, we applied these findings to modify the surface properties of sputtered SiO2 thin films. A part of this work was reported in Ref. [7]. The purpose of this study is to modify the surface chemical properties of the films by controlling the surface OH group density of the films without significant change in optical properties from the visible to near-IR region. Therefore, the effects of metal doping in the films on the surface OH group density were investigated. In sputtering technology, the doping can be easily performed using metal-doped targets. The aluminium (Al), titanium (Ti) or zirconium (Zr)-doped SiO2 thin film was prepared from a metal-doped silicon target. The relationship between the surface OH group density and the surface chemical properties such as the wettability and the surface reactivity with a polyfluoroalkyl isocyanate silane were investigated by contact angle measurements, X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (TOF-SIMS), Fourier transform infrared reflectance spectroscopy (FTIRRS) and atomic force microscopy (AFM). From the results obtained, the effectiveness of metal doping for the surface modification of sputtered SiO2 films was discussed. Furthermore, we explored the effect of metal doping on the formation of surface OH group from XPS O1s and FTIRRS spectra analyses.
Section snippets
Experimental details
SiO2 thin film doped with Al, Ti or Zr was deposited onto soda-lime–silica glass by reactive RF magnetron sputtering with a thickness of ∼40 nm. The silicon metal target doped with Al, Ti or Zr was used. The concentration of doped element in the film was adjusted at ∼3 at.%, because increases in the doped concentration cause significant change in optical properties. The dopant concentration was determined by XPS. Undoped-SiO2 thin film was also prepared from a pure silicon metal target.
The
Surface OH group density and optical properties
The surface OH group density of the films obtained by TOF-SIMS measurements are shown in Table 1. It is found that the secondary ion intensity ratio of 17(OH−)/16O− is different among the films, indicating that the surface OH group density can be effectively varied by the metal dopant. Fig. 1 shows the optical transmission spectra of the films. The transmittance in the wavelength region of 350–1500 nm for Ti or Zr doped films is almost same as that of non-doped film. The difference in average
Conclusions
In this paper, we have investigated the effects of metal doping in sputtered SiO2 thin films on surface properties such as wettability and reactivity with a polyfluoroalkyl isocyanate silane. The wettability and the surface reactivity were significantly altered by metal doping. It is found that these alterations are due to the change in the surface OH group density of the films and that the surface OH group density increases with increasing the number of non-bridging oxygens in the SiO2 film.
Acknowledgements
The authors are grateful to Prof. Hideo Hosono of Tokyo Institute of Technology for his valuable comments about the formation of NBOs in SiO2 structure.
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