Laser-assisted deposition of thin films from photoexcited vapour phases

Laser-assisted chemical vapour deposition (LCVD) has been extensively studied in the last two decades. A vast range of applications encompass various areas such as microelectronics, micromechanics, microelectromechanics and integrated optics, and a variety of metals, semiconductors and insulators have been grown by LCVD. In this article, we review briefly the LCVD process and present two case studies of thin film deposition related to laser thermal excitation (e.g., boron carbide) and non-thermal excitation (e.g., CrO₂) of the gas phase. PACS 81.15.Fg; 81.15.Kk     

Direct writing of carbon nanotube patterns by laser-induced chemical vapor deposition on a transparent substrate

Dot array and line patterns of multi-walled carbon nanotubes (MWCNTs) were successfully grown by laser-induced chemical vapor deposition (LCVD) on a transparent substrate at room temperature. In the proposed technique, a Nd:YVO₄ laser with a wavelength of 532 nm irradiates the backside of multiple catalyst layers (Ni/Al/Cr) through a transparent substrate to induce a local temperature rise, thereby allowing the direct writing of dense dot and line patterns of MWCNTs below 10 μm in size to be produced with uniform density on the controlled positions. In this LCVD method, a multiple-catalyst-layer with a Cr thermal layer is the central component for enabling the growth of dense MWCNTs with good spatial resolution.     

Silicon fibers produced by high-pressure LCVD

Using a unique fiber-growth control mechanism and high-reaction pressures (>1 bar), silicon fibers were grown by laser-assisted chemical vapor deposition (LCVD) from silane near the focal point of a cw Nd:YAG laser beam. Fiber-growth rates ranged from <1 to 500 m/s and fiber tip temperatures from 525 to 1412° C. At low fibertip temperatures (<600° C) silicon fibers yielding glassy fracture were obtained. Some crystallinity was observed by X-ray diffraction. Polycrystalline silicon fibers were formed at intermediate temperatures, single-crystal silicon fibers at high-laser intensities and high-tip temperatures. The single crystal LCVD silicon fibers were formed by a vapor-liquid-solid (VLS) mechanism. Single-crystal VLS-LCVD silicon fibers were also obtained from liquid silicon-metal alloys by initiating fiber growth from the end of thin palladium, gold and platinum wires.     

High-resolution electron microscopy study of SiGeC thin films grown on Si(1 0 0) by laser-assisted techniques

PLIE was used for rapid crystallisation of a-SiGeC films deposited by LCVD on Si(1 0 0) substrates. HRTEM study of thin films grown with several laser energies shows that the combination of the two laser techniques gives an almost completely crystallised alloy, even for the lowest laser fluence. Island formation is observed below a certain threshold of fluence (about 450 mJ/cm²). In the case of the lowest energy (100 mJ/cm²) the material was partially crystallised (with the crystalline material being the predominant state), to a nanocrystalline alloy with a considerable amount of epitaxialy grown grains and with grain sizes of several tens of nanometers. Above the threshold of 450 mJ/cm² a rather smooth thin film is grown. The crystallisation is almost complete and the alloy is grown in an almost perfect epitaxial way.     

A study of the Soret effect in laser-induced chemical vapor deposition

This paper presents an investigation of the modeling of the process of pyrolytic laser-induced chemical vapor deposition (LCVD) applied to study the Soret effect. LCVD is a thermally activated process characterized by strongly coupled mass and energy transport phenomena, together with chemical reactions, which are difficult to investigate experimentally. A physical and numerical model based on a commercial computational fluid dynamics package is developed and used to simulate a reactor operating at conditions of room temperature and pressure. The proposed numerical methodology will allow us to assess and analyze the effect of various factors controlling the process, and in particular the Soret effect. This numerical model is validated by comparison with the measured growth rate of the fiber. While several studies have proposed simulations of the LCVD process, this is among the first attempts at including the Soret effect in the numerical modeling at the micro-scale level. It is expected that the fundamental insights thus obtained will guide experimental investigations which can be applied to establish reactor design and process control guidelines.     

Carbon nanotubes growth from C2H2 and C2H4/NH3 by catalytic LCVD on supported iron–carbon nanocomposites

We report about the synthesis of carbon nanotubes by catalytic LCVD (C-LCVD), using a CW CO₂ laser and alternatively, C₂H₂/C₂H₄/NH₃ and C₂H₂/C₂H₄-containing gas mixtures. Different core–shell Fe–C nanocomposites (as synthesized and toluene extracted) were used employed as catalysts. The nanotubes grown from Fe–C residue demonstrate the lowest mean diameters. Prevalent curled and coiled morphologies are obtained for the CNTs grown in the presence of ammonia.     

Gas phase versus surface contributions to photolytic laser chemical vapor deposition rates

The rate of cwphotolytic laser chemical vapor deposition (LCVD) of platinum is measured for 350 nm as a function of the light intensity and the metalorganic vapor pressure. The growth of the metal films is studied in situ and in real time by monitoring their optical transmission. At low intensities the transmitted light decreases monotonically with time, and the LCVD process is photolytic with its rate limiting step in the surface adlayer. At higher intensities we observe two distinct time domains: Relatively slow initial photolytic deposition with its rate limiting step in the gas phase, which is followed by much faster pyrolytic LCVD. An improved method for distinguishing between adlayer and gas-phase limiting processes is demonstrated. These observations are confirmed by studying the photolytic deposition rates while varying the thickness of the adlayer.     

平凸型氮化硅球面微透镜的研究

介绍了用激光化学汽相沉积球面微透镜的技术，首先对激光化学汽相沉积法获得球面微透镜进行了理论分析，并用计算机分析了在一定沉积技术下的微透镜厚度剖面形状及光学聚集特性。其后介绍了激光化学汽相沉积的实验装置，用该装置在平面石英玻璃衬底上，制出了平凸型氮化硅球面微透镜，并对其参量进行了测量。     

Position dependence of growth rate in convection enhanced laser CVD of carbon rods

The ability to closely monitor LCVD rod growth in the growth region is demonstrated in the context of the first open-air axial convection enhanced micro-reactor. It was used to monitor carbon rod growth inside coaxial cylindrical flows of ethylene and argon. Monitoring and subsequent image analysis of the 656 nm atomic hydrogen emission from ethylene pyrolysis allowed real-time in-situ determination of axial growth rate and diameter along the axis of the produced carbon rods for the first time. Growth rates appear to vary greatly inside the growth region. This experiment raises questions about published qualitative models of LCVD rod growth that favor a uniform steady state regime. If the trend demonstrated in our experiments is not just an artifact of flow mixing, then it would indicate that published registered growth rates may just be an average value or an artifact of focus tracking. The results, therefore, point the way to experiments that would allow discrimination between the effect of flow mixing and the true nature of LCVD rod growth in the growth region. PACS 81.16.Mk; 81.10.Bk; 81.05.Uw     

Heat and mass-transfer modeling of an angled gas-jet LCVD system

Laser chemical vapor deposition (LCVD) is a new manufacturing process that holds great potential for the production of small and complex metallic, ceramic and composite parts. Since LCVD is a thermally activated process, the most important process variable is temperature. Therefore, a thermal model was developed for a gas-jet LCVD system, accounting for Gaussian-beam laser heating and gas-jet convection cooling. The forced convection cooling imposed by the gas-jet reagent delivery system was significant, accounting for a 15 to 20% change in the substrate temperature. The deposition rate for a given material is not only affected by temperature, but also by the mass transport of reagent gases. An angled gas-jet reagent supply was designed to aid mass transport, but the need and impact of such a system has been debated. Therefore, a two-dimensional mass-transport model was developed to estimate the effects of a gas jet with respect to local reagent concentration variations and reaction rates. Across all deposition regimes, the gas jet was found to be an effective tool for increasing the concentration of reagent gases at the surface of the substrate. The gas jet also generated higher deposition rates and increased deposit resolution for those processes severely limited by diffusion. PACS 05.60.-k; 44.27+g; 44.05+e     

Computational modeling of forced flow laser chemical vapor deposition

Laser chemical vapor deposition (LCVD) utilizes a laser to localize a CVD reaction. The process involves complex physical interactions within a very small spatial region. Experimental investigations into the dynamics of the LCVD process are limited by spatial and resolution capabilities of instrumentation. Models are developed herein using the computational fluid dynamics (CFD) code, FLUENT, that incorporate heat transfer, fluid flow, and species transport in a single integrated modeling environment. The models are used to study the carbon deposition process. Insight is gained into the relationships among the process parameters and the deposition rates and deposition rate profiles. Phenomena such as thermal diffusion and the relative importance of mass convection and mass diffusion are explored. A designed set of model cases is executed and the results are used to develop a simple polynomial expression for relating experiment conditions to deposit attributes. PACS 81.10.Bk; 81.05.Uw; 81.15.Gh; 47.50.Cd; 81.16.Mk     

Temperature effects in the photolytic LCVD of platinum

The photolytic laser chemical vapor deposition (LCVD) rate of platinum from its bishexafluoroacetylacetonate precurser has been measured in situ and in real time. Optical transmission of the 350 nm photolysis light through the deposited platinum film and a transparent glass substrate is monitored and analysed in detail. From these measurements, as well as measurements of the reflected light, the fraction of the laser beam power absorbed in the metal film is found. The latter allows a simple estimate of the laser-induced temperature rise at the metal surface. It is shown that even rather small temperature increases of the order of several tens of degrees centigrade can completely change the photolysis mechanism and hence drastically influence the photolytic LCVD rate. A simple modification of Lax's model, in which a temperature dependent thermal conductivity of the substrate is introduced, is used to describe the laser-induced heating of a strongly absorbing thin metal film on a glass substrate.     

Generation of three-dimensional free-standing metal micro-objects by Laser Chemical processing

Three-dimensional free-standing metal micro-objects were created using Laser Chemical Vapor Deposition (LCVD) of aluminum from aluminum-trihydride-trimethylamine. A dense grid of thin lines (about 10 µm diameter) was deposited on a pre-formed polycarbonate substrate. The substrate was removed by dissolving it in a suitable solvent after ablating an unsoluble by-product by UV excimer laser pulses. The result is a free-standing three-dimensional aluminum grid micro-structure.     

Variational analysis of LCVD rod growth

Variational analysis of LCVD rod growth was made possible by a new axial convection enhanced micro-reactor design that lent itself to accurate in-process measurements of growth rate and position inside the growth region. The methodology follows a typical design of experiments approach where the effect of small perturbations to process parameters are measured in real-time. The findings are rather startling. They point to a growth rate that is not symmetrical with respect to the focal plane. In fact, the peak growth rate is consistently recorded well in front of the focal plane, at a distance one order of magnitude greater than the Raleigh range, decreases rapidly afterward, steadies across the focus, and then decreases slowly. A measure of energetic efficiency can be derived that shows a rapid decay of the amount of material deposited per Joule. A central assumption to this analysis is that growth occurs in the kinetic domain and that was confirmed experimentally. The reason for the great variability and asymmetry observed are still unknown. However, corroboration of our results would throw into question a large body of published LCVD measurements as they would have to account for the wide variability of the measured quantities within the growth region. PACS 81.16.Mk; 81.10.Bk; 81.05.Uw     

Microstructure investigation and heat treatment of nanocrystalline laser-deposited carbon fibers

Carbon fibers were deposited by laser-assisted chemical vapor (LCVD) deposition from ethylene at sub-atmospheric pressure. Prior Raman spectroscopy analysis allowed the determination of the crystal size (average value of 3 nm in the edge region and from 8 to 113 nm in the center region). In this analysis, the microstructure of as-grown and heat treated fibers was examined using micro-Raman spectroscopy and transmission electron microscopy. The analysis revealed the degree of three-dimensional ordering of graphite planes and that the fibers were porous.     

On the reaction kinetics in laser-induced pyrolytic chemical processing

Reaction rates, particle densities, and temperature distribution in pyrolytic (photothermal) laser-induced microchemical processing are investigated with respect to temperature and concentration-dependent transport coefficients, and with respect to the effect of thermal diffusion. While the model employed is particularly suitable for laser-induced chemical vapor deposition (LCVD), it can also be applied to many cases of laser-induced surface modification and dry-etching.On leave from: General Physics Institute, Academy of Sciences, SU-117942 Moscow, USSR     

Time-dependent excited-state molecular dynamics of photodissociation of lanthanide complexes for laser-assisted metal-organic chemical vapour deposition

Ab initio molecular dynamics (AIMD) algorithm was modified for treating time-dependent excited-state molecular dynamics (TDESMD). This algorithm addresses the situations when electron density and nuclear potential are being periodically driven by a strong laser field, which induces periodic population–depopulation Rabi cycles. The electron hopping between different potential energy surfaces, such as ground state and ligand-to-metal charge-transfer (LMCT) state, creates the nuclear trajectories. In the computed trajectories, the inter-atomic distances can demonstrate different regimes, from small oscillations to abrupt elongations, corresponding to fragmentation of the studied compound. This algorithm was used to explore photodissociation mechanisms for laser-assisted metal-organic chemical vapour deposition (LCVD or laser-assisted MOCVD) process using lanthanide cyclopentadienyl-type precursors. The computed fragments are compared with the ones elucidated experimentally using photoionisation time-of-flight mass spectrometry.     

Pyrolytic LCVD of fibers: A theoretical description

The pyrolytic LCVD (Laser-induced Chemical Vapor Deposition) of fibers is studied theoretically. The shape of fibers and the temperature distribution are calculated self-consistently on the basis of a one-dimensional model which takes into account changes of the radius along the fiber. The influence of different parameters on the fiber radius and the temperature is discussed. The parameters investigated include the laser power and spot size, the activation energy of the deposition reaction, diffusion limitations in the gas phase, and temperature dependences of the heat conductivities of the deposit and the gas. The results are applied to the pyrolytic growth of Si fibers from SiH₄ + H₂.     

LCVD of tungsten microstructures on quartz

The pyrolytic laser-induced chemical vapor deposition (LCVD) of tungsten microstructures in the form of spots has been investigated. Fused quartz substrates and a mixture of WF₆ and H₂ have been employed in the experiments. Different shapes of spots related to different partial pressures of WF₆ and H₂ have been revealed. The results can qualitatively be described by a combination of surface reactions and gas-phase reactions resulting in tungsten deposition and surface etching.On leave from: Reserch Group on Laser Physics of the Hungarian Academy of Sciences, H-6720 Szeged, Dòm tér 9, Hungary     

Synthesis of nanometric iron oxide films by RPLD and LCVD for thermo–photo sensors

Iron oxide films were deposited on <100> Si substrates by reactive pulsed laser deposition (RPLD) using a KrF laser (248 nm). These films were deposited too by laser (light) chemical vapor deposition (LCVD) using continuous ultraviolet photodiode radiation (360 nm). The deposited films demonstrated semiconducting properties. These films had large thermo-electromotive force (e.m.f.) coefficient (S) and high photosensitivity (F). For films deposited by RPLD the S coefficient varied in the range 0.8–1.65 mV/K at 205–322 K. This coefficient depended on the band gap (E _g ) of the semiconductor films, which varied in the range 0.43–0.93 eV. The largest F value found was 44 V_c/W for white light at power density I≅0.006 W/cm². Using LCVD, iron oxide films were deposited from iron carbonyl vapor. For these films, the S coefficient varied in the range −0.5 to 1.5 mV/K at 110–330 K. The S coefficient depended on E _g of the semiconductor films, which varied in the range 0.44–0.51 eV. The largest F value of these films was about 40 V_c/W at the same I≅0.006 W/cm². Our results showed that RPLD and LCVD can be used to synthesize iron oxide thin films with variable stoichiometry and, consequently, with different values of E _g . These films have large S coefficient and high photosensitivity F and therefore can be used as multi-parameter sensors: thermo–photo sensors.