Physico-chemical study of the focused electron beam induced deposition process

The focused electron beam induced deposition process is a promising technique for nano and micro patterning. Electrons can be focused in sub-angström dimensions, which allows atomic-scale resolution imaging, analysis, and processing techniques. Before the process can be used in controlled applications, the precise nature of the deposition mechanism must be described and modelled. The aim of this research work is to present a physical and chemical description of the focused electron beam induced deposition process. As an introduction, a review of the literature, up to the present day, shows how this process was first identified as the origin of contamination in electron microscopes. Then the modifications made to a scanning electron microscope for deposition experiments are described. Gas supply systems and gas cryo-trapping devices were set up. Electrical and optical systems were constructed for an in-situ monitoring of the process. The experiment procedure was carried out in several phases, with the first being the deposition of flat films of carbon, copper-containing carbon composites, and pure gold for the study of the physics of the process. By measuring the fraction of the probe current that was absorbed in the sample, an accurate description could be made of the propagation of electrons. The backscattered electrons clearly influenced the deposition rate of Cu-containing films. A physical model for scanning deposition was created. The second phase is the deposition of metal-matrix composite tips from a stationary beam. Forward electron scattering was discovered to be responsible for tip formation and shape. A physical model of electron trajectories is included. The third phase is the construction of three-dimensional micro-structures by moving the beam during deposition. The deposited shapes result from an electron range that is larger than the structure size. Further investigation into the chemistry of the process involved the analysis of carbon films by micro-beam techniques. Regardless of the precursor used, the resulting films had a composition as C9H2±xO1±x (x

Focused electron- and ion-beam induced processes

Vinzenz Friedli

Focused electron- and ion-beam induced processing are well established techniques for local deposition and etching that rely on decomposition of precursor molecules by irradiation. These high-resolution nanostructuring techniques have various applications in nanoscience including attach-and-release procedures in nanomanipulation and fabrication of sensors (magnetic, optical and thermal) for scanning probe microscopy. However, a complete physical and chemical understanding of the process is hampered by the lack of suitable means to monitor and to access the numerous interrelated and time-varying process parameters (deposition and etch rate, yield, molecule flux and adsorption/desorption). This thesis is a first attempt to fill this gap. It is based on experimental and simulative approaches for the determination of process conditions and mechanical properties of deposited materials: Mass and force sensors: The use of tools merging micromechanical cantilever sensors and scanning electron microscopy was demonstrated for in situ monitoring and analysis. A cantilever-based resonant mass sensing setup was developed and used for real-time mass measurements. A noise level at the femtogram-scale was achieved by tracking the resonance frequency of a temperature stabilized piezoresistive cantilever using phase-locking. With this technique the surface coverage and residence time of (CH3)3PtCpCH3 molecules, the mass deposition rate, the yield, and the material density of corresponding deposits were measured. In situ cantilever-based static force sensing and mechanical modal vibration analysis were employed to investigate the Young's modulus and density of individual high aspect ratio deposits from the precursor Cu(hfac)2. Precursor supply simulations and experiments: A prerequisite to understand and quantify irradiative precursor chemistry is the knowledge of the local flux of molecules impinging on the substrate. Therefore, Monte Carlo simulations of flux distributions were developed and gas flows injected into a vacuum chamber were analyzed experimentally for the precursors Co2(CO)8, (hfac)CuVTMS, and [(PF3)2RhCl]2. The process parameters extracted from the mentioned approaches are valuable input for numerical focused electron- and ion-beam induced process models (Monte Carlo, continuum). We evaluated the precursor surface diffusion coefficient and the electron impact dissociation cross-section by relating deposit shapes to a continuum model.

EPFL2008

Influence of Residual Water and Carbon Containing Molecules in Focused Electron Beam Induced Processes

Bamdad Afra

Focused Electron Beam Induced Processing (FEBIP) is a technique used for etching and deposition of micro- and nano-structures using a focused electron beam. FEBIP is commonly carried out in a Scanning Electron Microscope (SEM) with a Gas Injection System (GIS) allowing precursor molecule vapors to be introduced on the substrate where they interact with the focused electron beam. Results of this work shine light on the influence of residual Water and Carbon containing species in the SEM chamber on etching and deposition processes. For etching HOPG (Highly Oriented Pyrolytic Graphite) and PMMA (Polymethyl methacrylate) using water vapor as precursor, it is shown that water participates in the principal chemical reaction for removal of these materials. However, when using Oxygen as precursor for etching these materials, Oxygen has a significantly reduced role in chemical reactions compared to residual water desorbing from surfaces in the FEBIP set-up. Degrees of involvement of Oxygen and Water in chemical reactions are demonstrated by combining experimental results and existing FEBIP Models. The Focused Electron Beam Induced Etching (FEBIE) contribution of Oxygen molecule Gas Phase Ionization prior to adsorption on the substrate surface is determined to be approximately 10% of the etching process for a micron size focused electron beam. A Precursor Gas Preparation System (PGPS) was designed and developed for in-situ fabrication of Metal Fluoride precursors from Xenon di-fluoride, XeF2. PGPS provided means of safe execution of FEBIP experiments without having to store large quantities of various dangerous and toxic materials. Using PGPS, Tungsten Hexafluoride precursor, WF6, was created in-situ and used in FEBIP experiments for deposition of Tungsten containing materials. This precursor allowed the elimination of both Oxygen and Carbon atoms in the precursor molecule in order to study the influence of residual Water (containing Oxygen) and residual Carbon containing species by measuring the amount of these atoms in deposit composition. Installation of a Liquid Nitrogen Cooled Plate over Substrate (without any direct contact with the substrate or GIS in the FEBIP set-up) proved to significantly reduce Oxygen and Fluorine content in deposits by eliminating the contribution of residual molecules from the chamber. Carbon content in deposits was also reduced when using Liquid Nitrogen Cooled Plate over Substrate (LNC-PoS). Non-FEB induced reactions (resulting in build-up of a contamination film on the substrate) are also significantly reduced or eliminated when using LNC-PoS in FEBIP experiments with WF6 precursor. In experiments with LNC-PoS, it is shown that the Carbon content in deposits mostly originates from substrate surface and arrives to FEB location via surface diffusion.

EPFL2011

Nanocrystalline Low-Refractive Magnesium Fluoride Films Deposited by Reactive Magnetron Sputtering: Optical and Structural Properties

Stefan Mertin, Paul Muralt, Silviu Cosmin Sandu, Jean-Louis Scartezzini

In this work, we study MgF2 thin-film synthesis by reactive pulsed DC magnetron sputtering from a metallic magnesium target in a gas mixture of argon, oxygen, and carbon tetrafluoride (CF4). Nanocrystalline films on silicon and glass substrates with excellent properties for optical application are achieved. The plasma discharge is analyzed with a differentially pumped mass spectrometer before and during the deposition process. Without breaking the vacuum, monochromatic photoelectron spectroscopy (XPS) is performed for in situ determination of the atomic C and O concentration. Film microstructure, topography, and thickness are investigated by electron microscopy (SEM and TEM), atomic force microscopy (AFM), and X-ray diffraction (XRD). The optical constants n and k are determined by spectroscopic ellipsometry and spectrophotometry: a consistent parametric fit of the ellipsometric angles and spectral transmittance and reflectance based on three Lorentz oscillators to determine n and k is achieved for a wide spectral range (300–2 300 nm). At 550 nm, a refractive index of 1.382 and near-zero absorption is obtained, which is in excellent agreement with n = 1.383 of polycrystalline MgF2. The measured light reflection at 760 nm is reduced by 3% for a quarter-wave nanocrystalline MgF2 coating on glass compared to the uncoated glass substrate.