P. P. Pronko

Machining of sub-micron holes using a femtosecond laser at 800 nm

Abstract The ability to machine very small features in materials has a number of technological applications. We have ablated holes, by laser ablation, into a metal film. Using 200 fs, 800 nm pulses from a Ti:sapphire laser, focused to a spot size of 3000 nm, we have produced holes with a diameter of 300 nm and a depth of 52 nm. The production of these small features is possible because the effects of thermal diffusion are minimized with the short pulses.

THERMOPHYSICAL EFFECTS IN LASER PROCESSING OF MATERIALS WITH PICOSECOND AND FEMTOSECOND PULSES

Application of picosecond and femtosecond laser pulses to the controlled ablation of materials represents a relatively unexplored yet important topic in laser processing. Such ultrashort pulses are of potential value in areas of thin‐film deposition, micromachining, and surgical procedures. We report here some early results of systematic studies being done from the femtosecond to the nanosecond regime, as an assessment of the problems and benefits associated with various laser pulse durations and their use in processing optically absorbing media. Experimental data and theoretical results of computer simulations are presented and compared for the threshold energies of ablation in gold as a function of pulse width from 10 ns to 100 fs. This work is then extended to include further numerically computed results for gold and silicon on ablation rates, threshold surface temperatures, liquid thicknesses, and vaporization rates as a function of pulse duration throughout the ultrafast regime from tens of femtoseco...

High intensity femtosecond laser deposition of diamond-like carbon thin films

Hydrogen-free diamond-like carbon (DLC) films have been deposited with a 100 fs (FWHM) Ti:sapphire laser beam at intensities I in the 1014–1015 W/cm2 range. The films were studied with scanning probe microscopy, variable angle spectroscopic ellipsometry, Raman spectroscopy, and electron energy loss spectroscopy. DLC films with good scratch resistance, excellent chemical inertness, and high optical transparency in the visible and near infrared range were deposited at room temperature. As the laser intensity was increased from 3×1014 to 6×1015 W/cm2, the films showed an increased surface particle density, a decreased optical transparency (85%→60%), and Tauc band gap (1.4→0.8 eV), as well as a lower sp3 content (60%→50%). The time-of-flight spectra recorded from the laser plume exhibited a double-peak distribution, with a high energy suprathermal ion peak preceding a slower thermal component. The most probable ion kinetic energy showed an I0.55 dependence, increasing from 300 to 2000 eV, when the laser inten...

Epitaxial SnO2 thin films grown on (1̄012) sapphire by femtosecond pulsed laser deposition

An ultrafast (100 fs) Ti sapphire laser (780 nm) was used for the deposition of SnO2 thin films. The laser-induced plasma generated from the SnO2 target was characterized by optical emission spectroscopy and electrostatic energy analysis. It was found that the ionic versus excited-neutral component ratio in the plasma plume depends strongly on the amount of background oxygen introduced to the deposition chamber. Epitaxial SnO2 films with high quality and a very smooth surface were deposited on the (1012) sapphire substrate fabricated at 700 °C with an oxygen background pressure of ∼0.1 mTorr. The films are single crystalline with the rutile structure, resulting from the high similarity in oxygen octahedral configurations between the sapphire (1012) surface and the SnO2 (101) surface. Hall effect measurements showed that the electron mobility of the SnO2 film is lower than that of bulk single crystal SnO2, which is caused by the scattering of conduction electrons at the film surface, substrate/film inter...

Laser deposition of diamondlike carbon films at high intensities

Unhydrogenated diamondlike carbon (DLC) thin films have been deposited by laser ablation of graphite, using a high power Ti: sapphire solid state laser system. DLC films were deposited onto silicon substrates at room temperature with subpicosecond laser pulses, at peak intensities in the 4×1014–5×1015 W/cm2 range. A variety of techniques, including scanning and transmission electron microscopy (SEM and TEM), Raman spectroscopy, spectroscopic ellipsometry (SE), and electron energy loss spectroscopy (EELS) have been used to analyze the film quality. Smooth, partially transparent films were produced, distinct from the graphite target. Sp3 volume fractions were found to be in the 50%–60% range, with Tauc band gaps ranging from 0.6 to 1.2 eV, depending on laser intensity. Kinetic energies carried by the carbon ions in the laser induced plasma were measured through time‐of‐flight (TOF) spectroscopy. Their most probable kinetic energies were found to be in the 700–1000 eV range, increasing with laser intensity.

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.

Ion characteristics of laser-produced plasma using a pair of collinear femtosecond laser pulses

Femtosecond laser-pulse absorption is studied in silicon ablation plasmas by means of a pair of identical 1016 W/cm2 collinear pulses separated on a picosecond time scale. The second laser-pulse modifies ionic characteristics of the preformed plasma, such as ion yield, ion energy, and average charge state. Resonance absorption is demonstrated to be the dominant mechanism by comparing results of s and p polarization. It is shown that maximum effects occur when a well defined critical density surface of the initial plasma forms together with an optimum density gradient scale length of kL=1.5. The optimal enhancement of ion yield, which occurs at 5 ps delay, is a factor of 2 greater than that produced by a single pulse with twice the energy of each individual double pulse. Applications are identified in regard to cluster beam formation and plasma isotope enrichment in ultrafast ablation plumes.

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.