Sub-Micron Nano-Composite CoC Hall Sensors by Focused Electron-Beam Induced Deposition for Microbead Detection

Laurent Bernau
EPFL, 2011
Thèse EPFL

Résumé

In order to track single superparamagnetic microbeads serving as markers in biomolecular assays, high-sensitivity, sub-micron magnetic sensors which can be integrated onto Lab-on-a-chip platforms are needed. This thesis studies the realization and characterization of such magnetic sensors using Focused Electron Beam Induced Deposition (FEBID) of Cobalt in a High Vacuum (HV) Scanning Electron Microscope (SEM). In FEBID, a finely focused electron beam is used to selectively irradiate a surface where functional precursor molecules are adsorbed. Electron-impact dissociation of the adsorbates lead to non-volatile fragments being deposited at the point of irradiation, while volatile fragments desorb and are pumped away. We have investigated FEBID of Dicobalt octacarbonyl [Co2(CO)8]. The deposit consists of a nanocomposite CoC material, where Co nanocrystals 2-3 nm in size are embedded in a carbonaceous matrix. This material exhibits a scattering enhanced Extraordinary Hall Effect (EHE) which allows for high magnetic sensitivities. Combined with the inherent nanometric resolution of the deposition process, this makes this nanocomposite CoC material the material of choice for the deposition of small (sub-micron), high-sensitivity magnetic sensors. However, the magnetic field resolution is found to be highly dependent on the exact composition of the deposition process. We report for the first time the controlled tuning of the composition of deposits from Co2(CO)8 using the electron beam pulse time as the process parameter. We explain the tunability in terms of co-deposition of chamber background hydrocarbons. A novel, general model describing the electron-induced deposition in terms of surface adsorbate densities in the presence of two adsorbate species is presented. The analytical model allows to describe the conditions for a broad tunable composition window for any two-adsorbate system. The model is applied to the two-adsorbate system consisting of Co2(CO)8 and HV chamber background hydrocarbons and we show that it allows for a quantitative description of the compositional variations. The CoC nanocomposite material is studied and characterized using a Langevin fit of the Hall signal. We show that this approach yields insight into the nanocrystal mean size, providing a nanocharacterization method of the material. A linear dependency between the electrical resistivity and the Hall resistivity at saturation is found. Empirically, the material is found to exhibit an optimum in terms of magnetic field resolution at around 65 at.% Co, corresponding to a trade-off between magnetic field sensitivity and electrical resistivity. Finally, we demonstrate single superparamagnetic bead detection using a sub-micron, CoC Hall sensor. Using a novel nanomanipulation setup providing a magnetic coil integrated into a SEM, we show the bead tracking capacities of these sensors.

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https://infoscience.epfl.ch/record/164014?ln=fr

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Laurent Bernau
EPFL, 2011
Thèse EPFL

Résumé

Official source

https://infoscience.epfl.ch/record/164014?ln=fr

À propos de ce résultat

Concepts associés (16)

Concepts associés

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Publications associées (10)

Publications associées

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Focused Electron Beam Induced Deposition (FEBID) is a rapid prototyping technique for the investigation, production and modification of 2 and 3-D nanostructures. The process takes place at room temperature, in the high-vacuum chamber of a Scanning Electron Microscope (SEM). The principle is based on the local FEB decomposition of molecules injected in the chamber and adsorbed on a surface. These molecules, called the precursors, are specifically chosen to contain the element of interest to be deposited. However, the electron induced decomposition results in contaminated materials containing additional parasitic elements from the precursor molecule. These contaminants are incorporated with the element of interest, and downgrade the properties of the materials and prevent therefore their use for many applications. Gas-assisted FEBID, the subject of this thesis, is a promising evolution and solution to the problem that was never extensively studied in literature. It consists in injecting a reactive gas simultaneously to the precursors, which will react in real time with the deposited contaminants to form volatile products, leaving ideally only the interesting material on the surface. This work focused on the Oxygen assisted FEBID of Silicon dioxide (SiO2) and on the understanding of the deposition process. SiO2 it is a high performance material for nano-optic and electronic applications, widely used in research and industry. Modifications were brought to an existing SEM to allow the simultaneous injection of two different and controlled gases, and to allow the in-situ control of growth of optical materials. Si-precursors of three families (alkoxy-, alkyl- and isocyanato-silanes) were compared. The FEBID material could be gradually converted from contaminated Si-materials to pure SiO2, above a [O2]/[precursor] ratio specific to each precursor chemistry and precursor flow. The SiO2 deposited was stoichiometric, C and OH–free, had an estimated density of 2.2, was amorphous and had optical properties close to that of fused silica. Optical structures for nano-optics and plasmonics were produced. Compared to traditional FEBID, O2 assisted FEBID required much lower energy to induce a decomposition reaction, and secondary electrons achieved 90 % of the deposition process in our setup. The electron efficiency could be multiplied by 20 compared to conventional FEBID. O2 assisted FEBID was insensitive to electron density in our setup. The conditions for no C and large deposition rates during O2 assisted FEBID were high molecule reactivity to O2, low sticking coefficient, low acceleration voltage and dwell time (< 6 µs), and large replenishment time (> 60 µs). As function of the precursor chemistry and flow, additional O2 influenced the deposition rate, which could be increased it by a factor of 7. The O2 and precursor molecules co-adsorption resulted in a surface coverage competition, which determined the deposition efficiencies. Limitations due to the residual water in the SEM were demonstrated and characterized. They appeared as Oxygen incorporation in the deposited materials, which influenced the deposition process. Residual water impinging flow was demonstrated to be responsible of the FEBID process deposition efficiency when no Oxygen was injected.

EPFL2007

Chargement

Texture mechanisms and microstructure of biaxial thin films grown by oblique angle deposition

Alireza Karimi, Akshath Raghu Shetty

In order to understand the texture formation mechanism in thin films grown under oblique angle deposition (OAD), TiAlN films were deposited at room temperature (RT) under various incident angles. We show that both in-plane and out-of-plane crystallographic orientations respond strongly to the deposition angle. For a?=?0 degrees, the pole figures show a (111) and (200) mixed out-of-plane orientation with random in-plane alignment. In contrast, under OAD, inclined textures are observed with the (111) direction moving toward the incident flux direction and the (200) moving away, showing substantial in-plane alignment. This observation suggests that TiAlN crystals prefer to grow with the (200) direction perpendicular to the substrate while maintaining the minimization of the surface free energy by maximizing the (111) surface area toward the incident flux. The in-plane texture, which is randomly oriented at normal incidence, gives rise to two preferred orientations under oblique angles one along the direction of flux and other away from the deposition source. The biaxial texture results from a competition among texture mechanism related to surface mobilities of adatoms, geometrical and directional effects. The surface and cross-section of the films were observed by scanning electron microscopy (SEM). OAD films develop a kind of smooth tiles of a roof structure, with no faceted crystallites. The columns of these films were tilted toward the direction of incident flux. The dependence of (111) texture tilt angle and column angle beta on the incidence flux angle a is evaluated using four well-known models. Transmission electron microscopy (TEM) study reveals a voided, intercolumnar structure with oblique growth toward the flux direction. The selected area diffraction pattern (SAED) pattern supports the pole figure observations. Measurements of the nanoindentation test were performed in order to discuss the change of mechanical properties as a function of incident flux angle.

Wiley-V C H Verlag Gmbh2012

Chargement

This thesis work focuses on impact and diffusion processes as well as equilibrium positions of a metal deposited on a surface. The metal is deposited in the form of clusters containing n atoms in a controlled way. Gold or silver clusters cations (Au+n and Ag+n ) are used. Their size (n ∈ [1, 9]), charge state and deposition energy (E = 10 - 4500 eV) are well defined. The surface being at room temperature during the deposition, can be annealed and transferred in a scanning tunnelling microscope (STM) after deposition. The variable temperature microscope was developed during this thesis. The approach of the tip is carried out by an axial motor, which renders the microscope very stable. The first part of this work treats of the evolution of gold deposited on a TiO2(110) surface. Position and size of gold islands created by cluster or atomic depositions are determined by STM. Compared to atomic deposition at the same energy (10 < E < 100 eV), the cluster deposition produces smaller islands with a large islands density after annealing at 800 K. Regardless the deposition conditions, gold atoms diffuse on the surface already at 300 K. The impact energy is the key parameter to reduce the diffusion by cluster or atomic implantation or defect creation. Upon a high energy deposition, part of the deposited gold is invisible to the STM. The fate of these gold atoms is unclear : they are either buried under the surface or ejected in vacuum at the impact. The control of the islands size should permit the creation of a stable system with an optimum catalytic activity (i. e. CO combustion). Research groups have shown that this activity is strongly influenced by the islands size. It is equal to zero if the diameter is greater than 5 nm. The second part of this work is devoted to the electron emission γ induced by the impact of silver clusters Ag+n on a Pt and HOPG surface. Measurements are made by varying the impact energy (and so the speed v) of size-selected clusters on a defined surface. Impact velocity ranges from 104 to 105 m/s, which is lower than the classical threshold. Nevertheless every cluster size produces an electron emission. A potential emission (γ(v=0) ≠ 0) is observed for the monomer on both surfaces, which is caused by excited ions in a metastable state. The γ(v) curves are increasing, and no mesurable oscillation are observed. This does not confirm earlier work by Meiwes-Broer et al. These authors found oscillations in γ on similar systems relating them to the electronic structure of the clusters and the surface. A model based on heating of the electron gas is developed. This model gives good agreements with the γ(v) curves. The electronic temperature is estimated to 3000-8000 K. Similar behavior on both surfaces (Pt and HOPG) depending of the cluster size is shown. This behavior is probably related to the geometrical structure of the clusters. Finally, a more pronounced molecular effect is observed for the HOPG : the number of electrons emitted by the impact of a Ag+n is up to 7 times higher than the number of electrons emitted by impacts of n independent atoms. This value is smaller than 2 for the Pt surface.

EPFL2005

Chargement

EPFL2005

EPFL2007

Texture mechanisms and microstructure of biaxial thin films grown by oblique angle deposition

Alireza Karimi, Akshath Raghu Shetty

Wiley-V C H Verlag Gmbh2012