Local anodic oxidation of ferromagnetic metal films

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Local anodic oxidation of ferromagnetic metal films

Introduction

Despite a lot of work on LAO of different materials, oxidation of ferromagnetic metal films is still not studied enough. There are only few works on LAO of Ni [106-109], FeCo [109] and NiFe [109]. Works [106] and [109] studied fabrication of nanodots on ferromagnetic films and only [108] studied fabrication of oxide lines. While such lines can be used as nanosized elements for fabrication of planar tunnel junctions [108] or periodic planar structures that can serve as surface phononic crystal (PnC) for hypersonic frequency range that could be controlled by magnetic field [5].
There are a lot of parameters for example oxidation time and tip applied voltage on which oxidation process depends. One should know optimal parameters of LAO to obtain well defined oxide lines with good reproducibility. In this chapter we investigated main features of LAO process and optimal LAO parameters for oxide lines fabrication on ferromagnetic metal films Ni, Fe, Co and FeCo.

Ferromagnetic thin film deposition

A thin film is a layer of a material with a small thickness from few nanometers to several microns. Thin films can be deposited by different methods. All deposition technologies are divided on chemical and physical methods: physical vapor deposition and chemical vapor deposition methods [162].
The best physical method to deposit uniform films is magnetron sputtering. In this method target plate made of deposited material is bombarded by accelerated ions generated in a discharge plasma. Such bombardment sputters target atoms, which are condense on a substrate as a thin film [163]. Plasma is generated by gas introduced into the chamber (usually argon (Ar)) and high voltage applied between the target (cathode) and the substrate (anode) (Figure 2.1). Basic parameters of the deposition process are chamber vacuum (base pressure), Ar pressure (work pressure), target voltage and sputtering power [163]. Additionally the distance between the target and the substrate can be changed. By magnetron sputtering films of different materials including ferromagnetic thin films can be deposited.

Experimental description

The experiments on LAO were performed on Ni, Fe, Co and FeCo with thickness d=10 nm. These films were fabricated by DC magnetron sputtering using systems VUP-5 and Leybold Z550 on monocrystalline Si(100) substrates with thermally oxidized 0.3 µm SiO2 layer on top. Before sputtering substrates were cleaned in ultrasound bath by acetone, IPAand deionized water (DIW) and dried by nitrogen. Average roughness of deposited films was about σ=0.15-0.2 nm. AFMs MultiMode and Solver P47H in tapping mode were used to provide oxidation process on metal films. This mode was chosen because in doesn‟t damage surface and the tip can be used longer than in contact mode. Furthermore in earlier works it was shown than tapping mode helps to improve resolution and homogeneity of the oxide structures [89].
First we tried to obtain oxide dots by LAO on Fe film using different parameters of process to be sure in feasibility and controllability of this method (Figure 2.2). Than lithography was made in vector mode in the shape of lines along which the tip was moved on SPM scans areas of 1×1 µm2 – 10×10 µm2. Different tips with different curvature radii (Rc) and coatings were used for experiments (Table 2.1). AFM images were analyzed using WSxM 5.0and Gwyddion software [166-167].
In order to understand the main features of ferromagnetic films LAO the influence of different parameters on oxidation results was studied. On Figures 2.3 a)-c) typical AFM images after LAO by lines are shown. Obtained oxide nanostructures have the form of protrusions on the modified sample. These protrusions appear because of incorporation of oxygen into the pattern during the oxidation and they can be easily observed by AFM: 2D (Figure 2.3 a) and 3D (Figure 2.3 c) AFM images show the change of film topography after LAO, phase image (Figure 2.3 b) is a function of the viscoelastic properties of the sample materials and shows change of these properties in the place where LAO was done (here phase is given as mirror image of topography). Height (h) and width (d, was determined asfull width at half maximum to avoid influence of tip convolution) of the protrusions were chosen as indicators of oxidation results.
They were defined as average from five or seven cross-sections of oxide lines (Figure 2.3 d). Oxidation time (Tox), bias voltage between tip and sample (V), relative humidity (RH), oscillating amplitude set point of the tip (SP) and tip curvature radius (Rc) were chosen as main parameters of LAO process. Voltage and SP were changed by AFM tuning. Time of oxidation was set as tip velocity during oxidation (XYv, AFM MultiMode) or by the duration of the voltage supply in each point of line pattern (AFM Solver P47H). To increase humidity the head of AFM was placed into a closed box with inlet for H2O saturated nitrogen.

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Local anodic oxidation reproducibility and sample drift

First we‟ve decided to verify reproducibility of local anodic oxidation and to check the influence of thermal drift of the sample on local anodic oxidation results.
To test reproducibility of oxide lines sizes we fabricated 3 oxide lines on Fe film at the same oxidation parameters. Figure 2.4 shows that one can obtain lines the same width and height using same parameters. It means that LAO has a good reproducibility and we can define dependences of oxide lines sizes on oxidation parameters.
To check the influence of sample thermal drift on oxidation results we made oxidation in the shape of circle using big oxidation time. Figure 2.5 a) shows deviation of the oxide pattern topology from template due to the drift:start and end points of the circle do not converge forming a spiral. This effect should be taken into account in complex patterns oxidation and can be decreased by decreasing of oxidation time (Figure 2.5 b).

Table of contents :

Introduction
1. Lithography methods and its application for fabrication of micro- and nanostructures
1.1 Scanning probe lithography methods
1.1.1 Force-assisted scanning probe lithography
1.1.2 Bias-assisted scanning probe lithography
1.1.3 Atomic force microscope principle
1.1.4 Local anodic oxidation by atomic force microscope: principle and application
1.2 Electron-beam lithography
1.2.1 Electron-beam resists
1.2.2 Exposure parameters
1.2.3 Electron-beam lithography challenges
1.2.4 Electron-beam lithography application
1.3 Conclusions
2. Local anodic oxidation of ferromagnetic metal films
2.1 Introduction
2.2 Ferromagnetic thin film deposition
2.3 Experimental description
2.4 Local anodic oxidation reproduceability and sample drift
2.5 Dependence of local anodic oxidation results on parameters of the process
2.5.1 Dependence on bias voltage between tip and surface
2.5.2 Dependence on oxidation time
2.5.3 Dependence on relative humidity
2.5.4 Dependence on oscillating amplitude set point of the tip
2.5.5 Dependence on other factors
2.6 Local anodic oxidation and tip state
2.7 Conclusions
3. Local anodic oxidation of textured ferromagnetic metal films
3.1 Introduction
3.2 Film texture and oxidation of textured metal films
3.3 Local anodic oxidation of textured polycrystalline Ni(111) and Ni(200) films
3.3.1 Fabrication and properties of textured Ni films
3.3.2 Local anodic oxidation results for textured Ni films
3.4 Local anodic oxidation of textured polycrystalline Fe(110) and Fe(200) films
3.5 Conclusions
4. Studying of the properties of nanostructures made by scanning probe lithography
4.1 Introduction
4.2 Current-voltage characteristics of nanostructures made by local anodic oxidation
4.3 Auger spectroscopy of nanostructures made by local anodic oxidation
4.4 Current-voltage characteristics and magnetoresistance of nanostructures made by mechanical scratching
4.5 Conclusions
5. Hypersonic magnetically tunable surface phononic crystals
5.1 Introduction
5.2 Feasibility to fabrication of surface phononic crystalsby local anodic oxidation
5.3 Fabrication of surface phononic crystals by electron-beam lithography
5.3.1 1D surface phononic crystals devices design
5.3.2 Definition of 1D surface phononic crystals fabrication process
5.3.3 Investigation of 1D surface phononic crystals fabricated by electron-beam lithography
5.3.4 Feasibility to fabrication of 2D surface phononic crystals
5.4 Conclusions
Conclusions and future prospects
Résumé étendu en Français
References
Appendix A

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