Alkali-resistant low-temperature atomic-layer-deposited oxides for optical fiber sensor overlays

Abstract

This paper presents an investigation of properties of selected metallic oxides deposited at a low temperature (100 °C) by atomic layer deposition (ALD) technique, relating to their applicability as thin overlays for optical fiber sensors resistant in alkaline environments. Hafnium oxide (Hf O with y/x approx. 2.70), tantalum oxide (Ta O with y/x approx. 2.75) and zirconium oxide (Zr O with y/x approx. 2.07), which deposition was based, respectively, on tetrakis(ethylmethyl)hafnium, tantalum pentachloride and tetrakis(ethylmethyl)zirconium with deionized water, were tested as thin layers on planar Si (100) and glass substrates. Growth per cycle (GPC) in the ALD processes was 0.133-0.150 nm/cycle. Run-to-run GPC reproducibility of the ALD processes was best for Hf O (0.145 ± 0.001 nm/cycle) and the poorest for Ta O (0.133 ± 0.003 nm/cycle). Refractive indices n of the layers were 2.00-2.10 (at the wavelength λ = 632 nm), with negligible k value (at λ for 240-930 nm). The oxides examined by x-ray diffractometry proved to be amorphous, with only small addition of crystalline phases for the Zr O . The surfaces of the oxides had grainy but smooth topographies with root-mean square roughness ∼0.5 nm (at 10 × 10 μm area) according to atomic force microscopy. Ellipsometric measurements, by contrast, suggest rougher surfaces for the Zr O layers. The surfaces were also slightly rougher on the glass-based samples than on the Si-based ones. Nanohardness and Young modules were 4.90-8.64 GPa and 83.7-104.4 GPa, respectively. The tests of scratch resistance revealed better tribological properties for the Hf O and the Ta O than for the Zr O . The surfaces were hydrophilic, with wetting angles of 52.5°-62.9°. The planar oxides on Si, being resistive even to concentrated alkali (pH 14), proved to be significantly more alkali-resistive than AlO. The Ta O overlay was deposited on long-period grating sensor induced in optical fiber. Thanks to such an overlay the sensor proved to be long-lasting resistant when exposed to alkaline environment with a pH 9. Thereby, it also proved that it has a potential to be repeatedly reused as a regenerable optical fiber biosensor.

Local atomic order of the amorphous TaOx thin films in relation to their chemical resistivity

“…The details of the ALD process are described in ref. 9. For each substrate, layers with two thicknesses (62 nm and 202 nm) were prepared.…”

“…The study performed in the presented paper aimed to elaborate technology of applying lms of a-TaO x in a regenerable optical ber biosensor. [8][9][10] The required material should be resistant to environmental threats or even play a role of protective coating. TaO x seems to be a good candidate for oxide nanocoating of high chemical stability.…”

“…TaO x seems to be a good candidate for oxide nanocoating of high chemical stability. 6,9 To be chemically robust, the material should be amorphous rather than in Ta 2 O 5 crystalline form. 9,11 Moreover, the chemically robust TaO x lms used for the mentioned above ber sensor already proved to be oxygen over-stoichiometric.…”

“…6,9 To be chemically robust, the material should be amorphous rather than in Ta 2 O 5 crystalline form. 9,11 Moreover, the chemically robust TaO x lms used for the mentioned above ber sensor already proved to be oxygen over-stoichiometric. 8 Recently, also oxide-based resistance switching devices have attracted considerable attention.…”

“…The measured spectrared, fitdotted black, dashed bluewindow. Insets, data and fits in wavenumber k.35730 | RSC Adv., 2019,9,[35727][35728][35729][35730][35731][35732][35733][35734] This journal is © The Royal Society of Chemistry 2019RSC Advances Paper Open Access Article. Published on 04 November 2019.…”