Nb2O5 films with closed nano-sized pores
As it is known, the films of Nb2O5 produced by magnetron sputtering remain amorphous up to a temperature of 500 °C. In addition, their refractive index n(λ=400 nm)~2.60 is significantly higher than that of HfO2 and Ta2O5. However, Nb2O5 films with the highest n, produced by either reactive pulsed magnetron sputtering (RPMS) or ion plating, often exhibit a relatively high compressive stress (σ> 300 MPa) and an extinction coefficient k(λ=400 nm)>0.002 leading to inferior performance of the resulting optical devices.
The present study deals with the porosity control of Nb2O5 films using high-growth rate RPMS with variable oxygen partial pressure, different magnetron-substrate configurations, and substrate temperatures. In the case of two magnetrons and off-normal deposition, the ion density at the substrate center is Ni ~3x1010 cm-3. This is denoted as a low plasma flow configuration (LPFC). In the high plasma flow configuration (HPFC) for a single magnetron with an axis perpendicular to the substrate surface, the central beam of the unbalanced magnetron provides an ion density at the substrate of Ni ≥2.8x1011 cm-3 for the entire range of oxygen partial pressures and at the two magnetron frequencies used.
Adjusting the deposition geometry and plasma ion density during reactive pulsed magnetron sputtering enables one to tailor the porosity of amorphous Nb2O5 thin films using a three-dimensional growth mode.
Films with a low extinction coefficient, a high refractive index, low tensile stress and negligible thermal shift have been obtained during deposition for plasma ion densities above 1011 cm-3 and substrate temperatures below 60 °C. These properties have been explained by the formation of a dense amorphous microstructure with a specific depth distribution of closed nanopores. In contrast, films grown at 370 °C using low plasma ion densities (~1010 cm-3) are less dense with pores being open at the film surface, and show a significant thermal shift, higher tensile stress and lower refractive index.
M. Vinnichenko, A. Rogozin, D. Grambole, F. Munnik, A.Kolitsch, and W.Moeller, O. Stenzel, S. Wilbrandt, A.Chuvilin, and U. Kaiser: Highly dense amorphous Nb2O5 films with closed nano-sized pores. Appl. Phys. Lett. 95, 081904 (2009).