Durability of the corrosion protection of copper by ALD alumina coating

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Oxide removal pre-treatment

For the samples on which the native oxide layer was removed before coating deposition, the procedure was as following. The samples were etched with 10 wt.% H3PO4 for 5 seconds, rinsed with ethanol, dried with compressed air and introduced in the ALD reactor (or electrochemical cell) in less than 5 min. This pre-treatment was taken from Shimizu et al. [75]. In order to study the electrochemical properties of these pre-treated samples, the reference bare sample was pre-treated in the same way just before introduction in the electrochemical cell. Samples, pre-treated or not, to be coated were placed in membrane boxes for shipping to the University of Helsinki.

Mechanical polishing

The aluminium disks were mechanically polished starting with 1200 and 2400 abrasive SiC papers and following with alumina polishing slurries of 2-3 μm, 1 μm and 0.3 μm successively. After mechanical polishing, the samples were rinsed with ultrapure water, and put in ultrasonic bath of isopropanol and ethanol successively for 3 min each. Then the samples were blow dried with compressed air. Samples, to be coated were placed in membrane boxes for shipping to the University of Helsinki.

ALD coating deposition

After substrate preparation, the samples were sent to Laboratory of Inorganic Chemistry of the University of Helsinki (E. Härkönen, M. Ritala) to be coated with ALD alumina. The coatings were prepared with a Picosun SUNALE R-150 ALD reactor. The nominal deposited thicknesses were 10, 20 and 50 nm. The precursors employed for the deposition procedure were trimethyl aluminium (Al(CH3)3) manufactured by Chemtura (AXION® PA 1300, purity 99.9%) and H2O (ultra-pure with resistivity > 18 MΩ cm). Both TMA and water were vaporized at room temperature (around 25°C). The pulse and purge times were 0.1 and 5 s respectively, for both precursors. The temperature in the deposition chamber was 250°C. Nitrogen (> 99.999%) was used as the carrier and purge gas with an overall flow rate of 300 sccm (standard cubic centimetre per minute) into the reaction space and a flow rate of 600 sccm in the intermediate space, while the reactor was constantly pumped with a vacuum pump. The pressure in the precursor lines was approximately 5 mbars at 250°C and under 5 mbars at RT, 10 mbars in the intermediate space and 0.1 mbar in the pump line. Direct pressure in the reaction space was between the pump line pressure (0.1 mbar) and precursor line pressure (5 mbar), closer to the pressure of the pump line. The heating to 250°C took 1 hour. The stay time in the reactor at 250°C, before the deposition to start was 10 min. Cooling to 100°C (taking approximately 4 hours) was always done after the deposition before the samples were exposed to laboratory air. The deposition time for each sample depends on the coating thickness (the number of cycles used). For instance, for the 50 nm coating, if 500 cycles were used, the deposition time was 85 min.

Time-of-Flight Secondary Ion Mass Spectrometry

ToF-SIMS is a SIMS (Secondary Ion Mass Spectrometry) technique with the specific use of a Time-of-Flight (ToF) mass analyser. In SIMS, primary ions are used to bombard the surface of the sample to emit secondary particles. Among the different secondary particles emitted (electrons, neutral species, atoms, molecules, atomic or cluster ions), only ions (the secondary ions) are detected and filtered in mass by the spectrometer. The mass spectrum provided by this process permits a detailed chemical analysis of the surface [76].
When the energetic primary ion beam bombards the surface, the particle energy is transferred to the solid by a collision process. In the solid, a “cascade of collisions” (Figure II.4) takes place between the atoms. Some of these collisions return to the surface and result in the emission of atoms (or clusters), among which some are ionized. Over 95% of the secondary particles result from the top two layers of the solid [76].
The yield of secondary ions depends on many factors which complicate the quantitative SIMS analysis. The basic SIMS equation is [76]. where is the secondary ion current of species m, Ip is the primary particle flux, ym is the sputter yield, ± is the ionization probability to positive or negative ions, m is the fractional concentration of m in the surface layer and η is the transmission of the analysis system.

Instrumentation and analytical conditions

In this work, ToF-SIMS was performed using a ToF-SIMS5 spectrometer provided by ION-TOF GmbH. The spectrometer was run at an operating pressure of 10-9 mbar. A picture of the ToF-SIMS spectrometer used in the present study can be found in Figure II.5.
The analysis modes used in the present study were “depth profiling” and “chemical imaging”. For depth profile elemental analysis, two ion beams are operated in a “Dual Beam Mode”. While a sputtering ion beam is used to sputter a crater, an analysis ion beam is used to measure progressively the centre bottom of the crater (Figure II.6). A lower energy of the sputtering beam permits higher sensitivity and higher depth resolution. Short pulses and small spot size (focus) of the Bi+ analysing beam permit higher mass and lateral resolution. The mass spectrum provided at each level in the sputtered depth yields a depth profile, i.e. a plot of the intensity of the selected ions vs the sputtering time. The sputtering time can be transformed to depth (nm) by using a profilometer and measurement of the sputtered depth. In the present study, a pulsed 25 keV Bi+ primary ion source was employed for analysis, delivering 1.1 pA of current over a 100 × 100 μm2 sputtered area. Analysis was performed in the centre of the sputtered crater using a 2 keV Cs+ sputter beam giving a 100 nA target current over a 500 × 500 μm2 area. Negative ion profiles were recorded because of their higher sensitivity to fragments coming from oxide matrices.

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Scanning Electron Microscopy

Scanning Electron Microscopy (SEM) was used as an additional technique to visualize the surface of some of the coated samples. Having a much larger field of depth and magnification than an optical microscope, SEM is a technique for high-resolution imaging of the surfaces which uses electrons for imaging.
Focused electrons impinge on the surface of the sample, which leads to emission of electrons from the surface of the sample due to elastic and inelastic scattering events. The emitted low-energy electrons resulting from inelastic scattering are called secondary electrons and are used to produce secondary electron images.
Secondary electron images were taken at LISE (Laboratoire Interfaces et Systèmes Electrochimiques) with a digital scanning electron microscope S440 LEICA with a tungsten filament (FEG-SEM).

Electrochemical cell

All electrochemical tests were performed in a conventional three-electrode cell. The three-electrode cell consisted of a working electrode (WE), a counter (CE) or auxiliary electrode and a reference electrode (RE). An electrode is a (semi-) conductive solid that interacts with the electrolyte. The WE is the electrode being studied, where the electrochemical reactions take place. In corrosion experiments, the WE is the material that is corroded. The CE is the electrode with which the current path is completed. All the current needed to balance the current produced at the WE passes through the CE. The RE is used to measure the voltage between the WE and the electrolyte. It has a constant potential and serves as an experimental reference point. Therefore, no current should pass through the reference electrode.
All electrochemical tests were performed in a conventional three-electrode cell designed and manufactured at Chimie ParisTech, with a platinum wire as the counter electrode and a saturated calomel reference electrode (SCE) as shown in Figure II.9. A Luggin capillary (filled with the electrolyte) was used to hold the reference electrode and make the connection between the reference electrode and the cell. The distance between each two electrodes was about 2 cm and the diameter of the cross section of the cell was about 6 cm. The volume of the electrolyte used was about 100 mL. The working electrode area was delimited to 0.29 cm2 by a Viton O-ring.

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.

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

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