In situ monitoring of thin alumina passive film growth by surface plasmon resonance (SPR) during an electrochemical process

Abstract

This article presents a sensing technique to characterize the growth of an alumina passive film on an aluminum micro structured layer in situ. The technique uses surface plasmon resonance (SPR) on aluminum coated gratings with spectroscopic measurements during electrochemical polarization in 0.02M Na₂SO₄. The structure of the sensor was first simulated and then fabricated by photolithography. The grating was then replicated by nanoimprint (NIL) in Sol-Gel before pure aluminum layer was deposited by RF magnetron sputtering to produce the samples used in this study. Coupled plasmonic and electrochemical measurements confirmed the feasibility of in situ characterization (thickness) of alumina passive film on aluminum-based gratings in neutral aqueous media. Combining both measurements with an appropriated SPR spectrum fitting lead to alumina thickness monitoring within a few nanometers' accuracy. The objectives and challenges of this study are to better characterize the alumina growth during electrochemical process combining in situ electrochemical process and SPR spectra in order to determine thin passive layer characteristics.

Figure 1

(a) Structure of the simulated diffraction grating with period Λ and the grating depth d. (b) Reflectance response as a function of wavelength under the simulated configuration. In orange, Λ = 375 nm with grating depth d = 36 nm. In blue, Λ = 630 nm with grating depth d = 54 nm. (c) Simulation of the growth af an Al₂O₃ for both structures.

Figure 2

AFM profiles (b,c,e) and images (b,d,f) of master sample (a,b), Sol–Gel replica: (c,d) and final device with aluminum deposition: (e,f).

Figure 3

Picture of the final diffraction grating sample.

Figure 4

(a) Description of the complete opto-electrochemical set-up, (b) 3D schematic of the constructed cell allowing coupled optical and electrochemical measurements. (c) Picture of the experimental setup, with the incident TM polarized white light, the customed cell with the 3-electrode setup and the diffraction grating.

Figure 5

Polarization curve (black dots) and surface plasmon resonance wavelength shift (a) or alumina thickness extracted from the SPR (c) as a function of the voltage applied to the metallized diffraction grating after 1 h at OCP. (b) Reflectance of the plasmonic curves acquired at different potentials (− 1.7, 0 and 0.14 V) as a function of the wavelength.

Figure 6

Schematic view of nanoimprint process.