P.A. VanRompay

Multi-diagnostic comparison of femtosecond and nanosecond pulsed laser plasmas

Understanding and fully characterizing highly dynamic and rapidly streaming laser ablation plasmas requires multiple techniques for monitoring effects at different stages. By combining multiple diagnostic methods, it is possible to analyze the broad time window over which these ablation plasmas develop and to learn more about the related physical processes that occur. Two laser sources, an 80 fs Ti:Sapphire laser ~780 nm! and a6n sNd:YAG laser ~1.06 mm!, are used in this work in order to compare pulse duration effects at similar wavelengths. Characteristics of the plasma produced by these two lasers are compared under conditions of comparable ablation flux. Results are presented involving correlation of time-resolved Langmuir probe data and electrostatic energy analysis for aluminum plasmas as a representative investigation for metallic systems. In addition, continuous-wave refractive index laser beam deflection is used to characterize the plasma and hot gas generated from boron nitride targets in terms of their ion and neutral atom densities. A self-similarity plasma expansion model is used to analyze the plumes under various conditions. Fundamental data obtained in this way can be relevant to laser micro-machining, laser induced breakdown spectroscopy, and pulsed laser deposition.

Nitride film deposition by femtosecond and nanosecond laser ablation in low-pressure nitrogen discharge gas

Abstract Thin films of TiN and BN are deposited by femtosecond and nanosecond laser ablation in a vacuum chamber with N2 gas discharge at 0.8 mTorr. Use of these activated gas conditions guarantees stoichiometric incorporation of nitrogen in the films. Properties of the deposited films are compared for the two different laser pulse durations. Ultrafast (fs) pulses are at a wavelength of 780 nm and the nanosecond pulses at 355 nm. The film growth is monitored with in situ RHEED, which provides information on the crystal quality of the films during growth. In addition to single-layer films of TiN being grown on silicon, a superlattice of BN/TiN was also fabricated. The TiN films were observed to form first as cubic phase single-crystal material that converted to polycrystal as the film thickness increases. These polycrystals exhibited textured orientation for the nanosecond pulsed depositions but were a randomly oriented fine-grained structure for the femtosecond pulses. The alternating multi-layers of TiN/BN exhibited interesting features that were complicated by surface roughness in the film. However, cross-sectional HRTEM demonstrated that a region of cubic phase BN was present in between two of the cubic phase TiN layers. It is thought that this results from a domain epitaxial relationship between the two materials. Such behavior is expected to be of great potential interest in the fabrication of uniform c-BN films by epitaxial growth.

Critical density effects in femtosecond ablation plasmas and consequences for high intensity pulsed laser deposition

Abstract Laser ablation plumes produced by a single pulse from an ultrafast laser consist, in the far field where film deposition occurs, of mostly neutral atoms, a percentage of ionized species, and, very often, condensed clusters. In certain situations adding energy to the plume may be of interest for a deposition, and methods for increasing the charged fraction need to be considered. This paper examines these issues and demonstrates a method for overcoming the plasma critical density limitations encountered for absorption of a single pulse. Precisely controlled time-delayed secondary pulses are used to change the average charge state, temperature, and plasma density of the far field plume, with implications for thin film deposition and nano-cluster formation. A plasma-jet nozzle effect is proposed to explain condensed cluster formation of germanium. Results are also presented in relation to enhanced isotope enrichment for boron.

Diagnostics for femtosecond and nanosecond laser-ablation discharge plasmas as used in thin film growth

Pulsed-laser deposition has proved to be a promising method for producing complex inorganic thin films. One of its major advantages, relative to other methods, is the capability of controlling many process parameters, such as laser pulse width, energy, and wavelength along with background reactive gas pressure and substrate bias. Adjusting these parameters provides a pre-tuning of the laser plasma thereby allowing for optimum process conditions in a particular thin film deposition. Understanding and fully characterizing such highly-dynamic and rapidly-streaming plasmas requires multiple techniques for monitoring the plasmas at different stages. By combining different diagnostic methods, it is possible to analyze the broad time window over which these ablation plasmas develop and to understand the related processes that occur. We present in this work new results involving correlation of time-resolved Langmuir probe data, optical emission spectroscopy, and electrostatic energy analysis to characterize the laser-induced plasmas generated from targets of titanium, tin-dioxide and aluminum. Two laser sources, an 80 fs Ti:Sapphire laser (780 nm) and a 6 ns Nd:YAG laser (1.06 micrometer), were used in this work. Examples of very high quality, epitaxial tin-dioxide films grown on sapphire by femtosecond-laser MBE are presented. These films are evaluated by high-resolution, cross-sectional TEM and x-ray diffraction. Film quality is considered in relation to the ablation plasma parameters, wherein femtosecond and nanosecond plasmas are compared.

Isotope separation and enrichment by ultrafast laser ablation

Spatial separation of isotopes in ultrafast laser ablation plumes is observed for a variety of elements in the periodic table. Observations are made with a charge-state discriminating mass analyzer as a function of angle relative to the center of the ablation plume. Data is presented for femtosecond and picosecond laser pulses showing enrichments by factors of 2 to 20 depending on element, charge state, and laser pulse duration. Thin films are deposited from the plasma plumes, as a function of distance from the ablation source, and used to record the spatial distribution of isotopes. This information is utilized to construct a model for the isotopic separation process and to infer characteristics of the electromagnetic fields in the ablation plasmas.

Thin Film Synthesis with Ultrafast Lasers

Application of ultrafast lasers to materials synthesis and processing is rapidly developing in directions of industrial relevance. Before full value can be extracted from such technology however, an operational understanding of their advantages and disadvantages needs to occur. Important issues regarding such applications are discussed in this paper in relation to fundamental aspects of energy absorption, lattice response, threshold damage production, and ablation plume development. These phenomena relate to the practical use of ultrafast lasers in micromachining and thin film deposition and reflect the physical differences to be found between long pulse and short pulse effects in materials. Understanding of these physical processes is enhanced through the use of practical computer models for the electronic and thermodynamic response of a material and the hydrodynamic and electrodynamic expansion of ablation plumes in terms of ion species and energies. Preliminary results on thin film deposition of boron nitride as a function of substrate temperature and ablation ionics is presented as an example of the unique possibilities provided by ultrafast lasers in the area of thin film synthesis and growth processing. Films are analyzed by spectroscopic ellipsometry for optical properties, ion beam analysis for stoichiometry, infrared absorption for structural properties, and atomic force microscopy for surface properties.

Enhanced isotope enrichment by time-delayed laser-pulse pumping of ultrafast ablation plumes

Summary from only given. The present work experimentally examines dynamics through optical pumping in order to enhance the separation effect. We studied ablation plumes initiated by one laser pulse and subsequently driven by a second time-delayed pulse. The second pulse is delayed in order to allow the plasma to expand from its original over-dense condition and reach critical-density where more efficient absorption of the second pulse can occur. As a motivation for this work, we observed in previous experiments an interesting charge-state dependence to the enrichment effect, for all elements and laser pulses studied.

Critical density effects in sub-picosecond laser-ablation of silicon by plasma absorption ion-energy spectroscopy

Summary form only given. This paper discusses the efficiency of absorption of sub-picosecond laser pulses during the formation of ablation plasma plumes from a silicon solid target. Such plasmas are normally used to fabricate thin films on a heated substrate. Properties of the plasma such as temperature, density energy, and species can affect growth mechanisms and the final film quality. The energy and the associated flux density in ablation plasmas are a function of the percentage of energy extracted from the incoming laser pulse at the target. It is shown in this work that the critical density condition of a plasma initially formed by a single pulse can be subsequently modified by the pumping action of a secondary time-delayed pulse.

Angular dependence of isotope enrichment in ultrafast laser ablation plumes

Summary form only given. It has been observed that isotopic enrichment of light ions occurs in the central portion of laser ablation plumes when such plasmas are formed with ultrafast laser pulses. It was found that the separation of lighter isotopes from heavier isotopes occurs on axis in the expanding plasma plume. The relative independence of the enrichment on axis among different elements and other similarities in the ion distributions have indicated a universality to this effect that lends credence to a possible plasma centrifuge model. In the present work, we examine the angular distribution of isotope enrichments and investigate the dependence on laser parameters and on the atomic-transport mean free path between collisions. The angular dependence is studied as a function of ion energy, ion charge state, and atomic mass.

Laser-MBE deposition of epitaxial oxides and nitrides using ultrafast laser pulses

Summary form only given.A laser-MBE system, operating at a base pressure of 5/spl times/10/sup -9/ Torr, is used in conjunction with a 10-Hz ultrafast laser facility for monolayer deposition of various oxide (SnO/sub 2/) and nitride (TiN and BN) thin films under laser-induced gas discharge conditions. The laser delivers pulses in the range of 80 to 150 fs, which are capable of depositing approximately 0.01 monolayer per pulse. Details of the growth mechanisms are followed using a RHEED in-situ monitor where single atomic layer growth processes may be observed. In this way, we have grown extremely high-quality epitaxial films of SnO/sub 2/ on sapphire.