Nanosecond pulsed laser-processing of CVD diamond

This work considers the resulting material structural effects of laser-processing chemical vapor deposited (CVD) diamond. The utilized 532 nm wavelength laser had a pulse duration of 40 ns together with a spot diameter of ca. 40 mu m. The effect of cutting fluence on the CVD diamond microstructure was investigated using both Raman and transmission electron microscopy (TEM) analyses. While Raman analysis showed that increasing the laser fluence led to a transition from diamond to graphite, the spread of the beam and consequent interrogation volume leads to limitations in understanding the precise structure over the ablated diamond surface. As such, TEM showed that at a low fluence (4.9 J/cm(2)), the subsurface microstructure is a mix of both graphite and amorphous carbon (aC). At 15 J/cm(2), only the graphite phase was identified. As the fluence increases further, the thickness of the graphite layer decreases until it disappears at a fluence of 50 J/cm(2). In contrast, a crystalline diamond phase began to be identified at 15 J/cm(2) and grows with increasing fluence. Moreover, cracks were visible only until a fluence of 15 J/cm(2), while at higher fluences, no cracks were observed. A simulation model was developed to predict the residual graphitization thickness formed in CVD diamond by a laser beam. Despite some differences in the graphite layer thickness, the results obtained from the model were consistent with the TEM results, supporting the findings of this study. Finally, cracks were only located in the graphite phase at the two lowest laser fluences. This was due to the low fracture toughness of graphite as compared to diamond.

Selective light induced chemical vapour deposition of titanium dioxide thin films

Estelle Wagner

Light Induced Chemical Vapour Deposition (LICVD) of titanium dioxide thin films is studied in this work. It is shown that this technique enables to deposit locally and selectively a chosen crystalline phase with a precise controlled thickness at low substrate temperature, allowing even the use of polymer substrates. A home made LICVD reactor was set up, consisting of a main chamber in which the substrate was placed on a temperature controlled plate and could be irradiated perpendicularly through a window by a 250 ns long pulse XeCl excimer laser (308 nm) with a controlled fluence (energy per surface area per pulse). A liquid precursor (titanium tetra-isopropoxide) was brought by an oxygen carrier gas flow in the chamber and a nitrogen flow was added to prevent deposition on the window. The total pressure in the chamber was maintained at 10 mbar. Projection of a mask image on the substrate was realized. Titanium dioxide films could be deposited locally by this technique with a thickness ranging from few nanometers to several micrometers. Deposits were realized at substrate temperatures as low as room temperature and with fluences between 1 and 400 mJ/cm2. The deposited films were characterized by profilometry, Atomic Force Microscopy, optical microscopy, Scanning Electron Microscopy, Transmission Electron Microscopy, X-ray diffraction, Raman spectroscopy, X-ray Photoelectron Spectroscopy, UV-vis spectroscopy and ellipsometry. A detailed and systematic study of the influence of some selected process parameters (substrate temperature and irradiation dose principally) on the deposition rate and material properties was realized on glass and silicon substrates. An empirical equation of the growth rate as a function of the different experimental parameters was derived and was tested successfully to obtain a precise control of the deposited thickness (a precision better than 10 nm was demonstrated for films below 300 nm), with growth rates up to 300 nm/min. From this equation, a deposition mechanism involving both a photolytic reaction and a thermal contribution of the substrate temperature was proposed. The deposited material was shown to consist of TiO2 with a possible very low additional carbon contamination. Depending on the deposition conditions, amorphous, anatase or rutile material was deposited. One parameter explaining the variation of the crystalline state of the material was the variation of the laser induced temperature rise which was simulated theoretically in parallel. Surface temperature rises in the order of several hundreds of degrees were calculated during the laser pulse, varying with the substrate, the fluence and the film thickness. The optical properties of the deposited material vary in a correlated way with the crystalline state of the material, and high indexes of refraction (up to 2.6 at 633 nm) were measured. Selective deposition in the irradiated area was studied. Provided substrate temperature was kept below 150°C, no thermal deposition was observed, and deposits were strictly located in the irradiated area. The edge resolution (around 10 micrometers) was demonstrated to be due to the optical aberrations of the mask projection set up. No effect of substrate temperature and irradiation conditions on the resolution could be evidenced, but resolution decreased with increasing deposited thickness due to optical effects. Thanks to the low substrate temperature required for the deposition process, deposition on thermo- fragile substrates (such as polymers, in particular PMMA) was demonstrated. However, in the case of polymer substrates, the fluence had to be kept below 15 mJ/cm2 not to damage or ablate the polymers, resulting in very low growth rates (in the order of 10-3 nm/pulse). In these conditions, only amorphous films with about 10% of carbon contamination could be obtained. Additionally, films were shown to crack above a certain deposited thickness, which is attributed to thermal effects. As a matter of fact, calculations showed a high temperature rise up to 70°C of the polymer/oxide interface. Deposition on polymers coated with transparent conductive oxide was also demonstrated.

EPFL2003

Combinatorial High-Vacuum Chemical Vapor Deposition of Lithium Niobate Thin Films

Ali Dabirian

Lithium niobate is a key material for photonics with applications in optical modulators and light frequency converters. Present lithium niobate optical devices are realized in bulk lithium niobate single crystals while interests in higher integration and also better performance derives efforts towards high quality epitaxial lithium niobate thin films. In the past, epitaxial lithium niobate thin films obtained using chemical vapor deposition (CVD), pulsed-laser deposition and crystal-ion slicing. Among these methods CVD is the one that can provide large area growth. Optimizing films properties stays an issue in CVD because in CVD of multi-element oxides using multiple precursors the stoichiometry of the film does not reflect the precursor ratios and additionally it varies with precursors total flux values. In this thesis optimization and discovery of multi-element oxide thin films were performed in a high-vacuum CVD reactor that is capable of performing combinatorial experiments. Using this reactor epitaxial lithium niobate thin films were obtained on sapphire {001g} and lithium tantalate {001} substrates with rocking curve full-width at half-maximum (FWHM) values of 0.03° and 0.024° respectively. The epitaxial growth of lithium niobate films was confirmed from X-ray diffraction (XRD) measurements and transmission-electron microscope (TEM) observations. Niobium tetra-ethoxy-di-methy-amino-ethoxide (Nb(OEt)4(dmae)) and lithium tert-butoxide (Li(OBut)) were used as precursors. Optically induced inhomogeneities, also known as optical damage, in lithium niobate limit its usage for high laser intensity applications. Lithium niobate single crystals doped with hafnium showed improved optical damage thresholds. Therefore combinatorial HVCVD experiments were conducted and textured {001} hafnium-doped lithium niobate films, with about 6 [mol%] hafnium, were obtained on sapphire f001g. Although the crystalline quality of the film is not high (rocking curve FWHM 0.8°) its Raman spectrum is still quite close to lithium niobate single crystal and epitaxial lithium niobate films on sapphire. Hafnium tert-butoxide (Hf(OBut)4) was used as precursor for hafnium. In the HVCVD reactor, a precursor transport system was applied such that spatial control of the precursor impinging rate on the substrate can be tailored. The precursor flux distribution that was mostly used in this thesis was a linear gradient with ratio of three across a 150 mm diameter circular area. In fact combinatorial experiments were performed by producing a linear gradient for individual precursors over the substrate with a certain angle between the gradient directions. The linear gradient feature was used to perform efficiently the systematic study of single precursors providing the benefit that one deposition at certain substrate temperature gives information about deposition kinetics for a vast range of precursor flux values. A systematic study of the Nb(OEt)4(dmae) precursor was performed using this feature and chemical-reaction-limited, mass-transport-limited, and desorption-limited regimes of CVD process were identified. The possibility of screening a large range of precursor flux values led us to discovering a CVD regime in which deposition rate is decreased by raising the precursor flux. A model was proposed for interaction of gas with surfaces that can explain all the different CVD regimes observed. The precursor molecules in HVCVD reactor reach the substrate with nearly no gasphase collision. Based on this fact a lift-off process was proposed for large-area in-situ patterning of multi-element oxide thin films.

EPFL2010

Synthesis of carbon nanotubes and characterization of nanotubes as atomic force microscopy tips

Yanki Keles

This thesis describes the synthesis of carbon nanotubes (CNTs) by chemical vapor deposition (CVD) and the evaluation of them as tips of atomic force microscope (AFM) probes. Both arc-discharge and CVD grown nanotubes were investigated as AFM tips. Two methods were used to produce nanotube tipped AFM probes: (i) nanotubes were directly grown on the apex of AFM probes by CVD and (ii) pre-grown nanotubes were mechanically attached onto the apex of AFM probes. The first part of the thesis describes experiments that sought to control the growth of carbon nanotubes by chemical vapor deposition. The effects of various parameters e.g. catalyst, the carbon source, the addition of hydrogen and the growth temperature, on carbon nanotube growth were investigated. Ethylene was found to lead to better quality nanotubes than the acetylene. Grown nanotubes' quality was found to improve upon flow of hydrogen during the synthesis. Nanotubes synthesized on 3 nm iron film at 840°C using ethylene and hydrogen led to utilizable carbon nanotube tipped AFM probes. The second part of the thesis focuses on the mechanical production of nanotube tipped AFM probes in high resolution electron microscopes using nanomanipulators. Scanning electron microscopy (SEM) was found to be more convenient than transmission electron microscopy (TEM) to carry out the nanomanipulation process. Both arc-discharge and CVD grown nanotubes were successfully attached onto the AFM probes using a custom made nanomanipulator. Nanotubes were glued onto the probes by focused electron beam (FEB) induced amorphous carbon deposits. FEB irradiation deposited amorphous carbon performed more reliable attachment on CVD nanotubes than the arc-discharge nanotubes. The last part of the thesis describes tapping mode AFM measurements done using nanotube tipped AFM probes. Different nanotube tipped probes were investigated as AFM tips for scanning colloidal gold particles with different diameters and gratings from Digital Instruments™ AFMs. High resolution AFM images were obtained by arc-discharge nanotube tips. CVD grown nanotubes were shown to be more resilient to tip crashes than the arc-discharge grown nanotubes. However, all the AFM images acquired from the scans by CVD nanotubes had the same kind of ringing artifact, in good agreement with a previously reported data about the artifacts of the CVD nanotube tipped AFM probes. Open ends of the CVD nanotubes, lack of crystallinity and the defects in their structure are suggested to be the reason for these artifacts.

EPFL2006