Analysis of the corrosion protection of copper by ALD alumina coatings and effect of deposited layer thickness

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Open Circuit Potential vs time

The Open Circuit Potential (OCP) is the equilibrium potential of the metal in the absence of any electrical connections to the metal. The OCP vs time measurement involves the measurement of the evolution of the potential difference between the WE and RE electrodes, immersed in the electrolyte at zero net current, as a function of time. This is usually the first step in corrosion measurements and is used in order to allow sufficient time for the OCP to stabilize and reach the steady state, which is especially important for the impedance measurements. The OCP depends on many factors including the WE material, the temperature, the electrolyte composition and pH and the hydrodynamics.
In the present study, before starting the DC and AC electrochemical experiments, the open circuit potential (OCP) was measured for 30 minutes in order to reach a stable potential.

Electrochemical Impedance Spectroscopy

Electrochemical Impedance Spectroscopy (EIS) is a powerful technique applied in corrosion research. In this technique the system is perturbed with an AC signal (potential or current) of small amplitude, resulting in an output signal (current or potential). While resistance is a parameter describing the opposition to the flow of electrons in an ideal resistor, impedance is used for more complicated systems containing other elements than only a resistor.
If an excitation potential signal (Eq. II.6) is used, the response is a current signal with a phase shift (Eq. II.7), only if the system has a linear behaviour.

Linear Sweep Voltammetry

DC techniques have been used widely for corrosion measurements. Linear Sweep Voltammetry (LSV) as a potentiodynamic scan is a useful tool in corrosion studies to monitor the change of the current as a result of the change in the potential applied to the system under study. In LSV, the potential is swept linearly at specific steps (scan rate / mV.s-1) and the produced current as a result of polarization of the WE is measured and monitored. Obviously LSV leads to perturbation of the system and irreversible changes in it, as a result of the externally imposed polarization. Therefore, it is considered as a destructive method. The potentiodynamic polarization plots provide useful information including the corrosion current and potential of the system under study, the polarization resistance, the passivity and the pitting susceptibility.
The evolution of current vs potential, i.e. the polarization curve can be reported in a linear or logarithmic scale. While the slope of linear E-i curve in the vicinity of the corrosion potential can be used to determine the polarization resistance (Rp), the logarithmic polarization curve (log│I│ vs E) can be used to obtain the corrosion current (Icorr) from the intersection of the linear portions of the logi-E curve extrapolated to the corrosion potential (Ecorr) in the case of cathodic and anodic Tafel behaviours. These procedures are illustrated in Figure II.11.

Electrochemical behaviour of bare Cu substrate

Polarization curve obtained by LSV for Cu substrate is presented in Figure III.1.A. The sample was polished, electropolished and pre-treated (H3PO4 pre-treatment) as explained in Chapter II. The polarization curve was well reproducible. Parameters extracted from the LSV polarization curve (Figure III.1.A) are presented in Table III.1. The polarization resistance (Figure III.2.A) was obtained from the reciprocal of the slope of the linear i-E curve close to the corrosion potential (Ecorr ± 20 mV).

Time-of-Flight Secondary Ion Mass Spectrometry

Figure III.5 shows the ToF-SIMS negative ion depth profiles of the 10, 20 and 50 nm ALD alumina coated copper substrates. The selected ions were 12C-, 17OH-, 18O-, 31P-, 32S-, 35Cl-, 59AlO2-, 95CuO2- and 126Cu2-. 18O- is the naturally occurring oxygen isotope recorded since the 16O- signal was close to saturation. The ion intensities are presented in Three regions (marked) can be easily distinguished: the coating region, the substrate region and the coating/substrate interfacial region. The coating region is characterized by an intensity plateau of the 18O- and AlO2- ions characteristic of the deposited material. The identical plateau intensities of these ions for the different coatings confirm the growth of films with similar and homogenous bulk stoichiometry and with no in-depth variation, as already observed for ALD alumina on copper at lower deposition temperatures (100-200°C) [71,72] and on other substrates [60,61,118].
The Cu2- ion is the most representative of the metallic substrate. Its identical intensity in the bulk substrate region for all samples is indicative of the excellent reproducibility of the selected ToF-SIMS analytical conditions. The entry in the substrate bulk region is where the Cu2- ion starts to have a constant intensity, at ~105, ~225 and ~610 s of sputtering time for the 10, 20 and 50 nm specimens, respectively. The increase of the coating sputtering time is expected from the increase of the film thickness. Calculated sputtering rate yields values of 0.095, 0.088 and 0.081 nm.s-1 using the measured film thicknesses of 10, 19.9 and 49.5 nm, respectively. Possibly, the lower sputtering rate obtained on thicker films results from the densification of the deposited material that would increase with increasing deposition (i.e. annealing) time [61]. However, one cannot exclude charging effects increasing with the thickness of the deposited alumina insulator and decreasing the sputtering yield.

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Table of contents :

General introduction
Chapter I. State of the art on ALD and its application to corrosion protection
I.1. Atomic Layer Deposition
I.1.1. Introduction
I.1.2. Terminology
I.1.3. The mechanism and main features of ALD
I.1.4. Main advantages of ALD
I.1.5. Non-ideal ALD behaviour
I.1.6. Limitations of ALD
I.1.7. Precursors and their requirements in ALD
I.1.8. Nucleation and growth in ALD
I.1.9. Film morphology
I.1.10. ALD of alumina (Al2O3)
I.2. ALD for corrosion protection
Chapter II. Experimental
II.1. Substrate preparation
II.1.1. Copper substrate
II.1.1.1. Mechanical polishing
II.1.1.2. Electropolishing
II.1.1.3. Annealing
II.1.1.4. Oxide removal pre-treatment
II.1.2. Aluminium substrate
II.1.2.1. Mechanical polishing
II.2. ALD coating deposition
II.3. Surface analysis
II.3.1. Time-of-Flight Secondary Ion Mass Spectrometry
II.3.1.1. Principles
II.3.1.2. Instrumentation and analytical conditions
II.3.2. Atomic Force Microscopy
II.3.2.1. Principles and instrumentation
II.3.3. Scanning Electron Microscopy
II.4. Electrochemical analysis
II.4.1. Experimental conditions
II.4.1.1. Instrumentation
II.4.1.1.1. Electrochemical cell
II.4.1.1.2. Potentiostat
II.4.2. Techniques
II.4.2.1. Open Circuit Potential vs time
II.4.2.2. Electrochemical Impedance Spectroscopy
II.4.2.3. Linear Sweep Voltammetry
Chapter III. Analysis of the corrosion protection of copper by ALD alumina coatings and effect of deposited layer thickness
III.1. Introduction
III.2. Electrochemical behaviour of bare Cu substrate
III.2.1. Linear sweep voltammetry
III.2.2. Electrochemical Impedance Spectroscopy
III.3. Surface and electrochemical analysis of Cu coated with ALD Al2O3
III.3.1. Time-of-Flight Secondary Ion Mass Spectrometry
III.3.2. Atomic Force Microscopy
III.3.3. Linear Sweep Voltammetry
III.3.4. Electrochemical Impedance Spectroscopy
III.4. Conclusions
Chapter IV. Effect of the interfacial native oxide layer on the corrosion of ALD alumina coated copper
IV.1. Introduction
IV.2. Electrochemical analysis of bare substrates
IV.2.1. Electrochemical Impedance Spectroscopy
IV.2.2. Linear Sweep Voltammetry
IV.3. Surface and electrochemical analysis of coated substrates
IV.3.1. Time-of-Flight Secondary Ion Mass Spectrometry on pristine coated samples
IV.3.2. Electrochemical Impedance Spectroscopy
IV.3.3. Linear Sweep Voltammetry
IV.3.4. Time-of-Flight Secondary Ion Mass Spectrometry of polarized coated samples
IV.4. Conclusions
Chapter V. Effect of copper substrate annealing on the corrosion protection of ALD alumina coatings
V.1. Introduction
V.2. Time-of-Flight Secondary Ion Mass Spectrometry of pristine coated samples
V.3. Atomic Force Microscopy
V.4. Linear Sweep Voltammetry
V.5. Electrochemical Impedance Spectroscopy
V.6. Scanning Electron Microscopy
V.7. ToF-SIMS surface images
V.8. Conclusions
Chapter VI. Durability of the corrosion protection of copper by ALD alumina coating .
VI.1. Introduction
VI.2. Electrochemical Impedance Spectroscopy
VI.3. Time-of-Flight Secondary Ion Mass Spectrometry
VI.4. Atomic Force Microscopy
VI.5. Conclusions
Chapter VII. Investigation of corrosion protection of aluminium by ALD alumina coatings and effect of deposited layer thickness
VII.1. Introduction
VII.2. Electrochemical behaviour of bare Al substrate
VII.2.1. Corrosion mechanisms
VII.2.2. Linear Sweep Voltammetry
VII.2.3. Electrochemical Impedance Spectroscopy
VII.3. Surface analysis and electrochemical study of coated samples
VII.3.1. Time-of-Flight Secondary Ion Mass Spectrometry of pristine samples
VII.3.2. Linear Sweep Voltammetry of coated aluminium
VII.3.3. Electrochemical Impedance Spectroscopy of ALD coated aluminium
VII.4. Conclusions
General conclusions
References
List of Figures
List of Tables

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