Douglas J. Krajnovich

NEAR-THRESHOLD LASER SPUTTERING OF GOLD

This work characterizes the laser sputtering of gold by 248 nm laser pulses at near‐threshold fluences (material removal rates ≤10 A/pulse) using time‐of‐flight plume diagnostics, scanning electron microscope analysis of the surface topography, and thermal analysis of the transient near surface conditions. Pulsed laser irradiation leads to development of surface topography characterized by droplet and ridge formations, and to the liberation of micrometer‐sized droplets into the plume. The development of surface topography has been identified with a hydrodynamic response to phase change occurring at the surface of the target. Drawing upon a Rayleigh–Taylor instability description of the melt surface, the readily observable ∼5 μm periodicity in topography formation can be theoretically predicted. Additionally, the preferential formation and liberation of ∼1 μm diameter droplets at the target surface is observed. Nevertheless, the majority of sputtered mass flux is not comprised of droplets, but of neutral gold atoms with almost perfect Boltzmann translational energy distribution. The mean translational energy of the gold atoms, however, is much too high to reconcile with a simple thermal vaporization model. The yield, translational energy, and angular characteristics of the plume are strongly influenced by the surface topography. Local variations in the light absorption and heat transfer explain the qualitative trends in the experimental results.This work characterizes the laser sputtering of gold by 248 nm laser pulses at near‐threshold fluences (material removal rates ≤10 A/pulse) using time‐of‐flight plume diagnostics, scanning electron microscope analysis of the surface topography, and thermal analysis of the transient near surface conditions. Pulsed laser irradiation leads to development of surface topography characterized by droplet and ridge formations, and to the liberation of micrometer‐sized droplets into the plume. The development of surface topography has been identified with a hydrodynamic response to phase change occurring at the surface of the target. Drawing upon a Rayleigh–Taylor instability description of the melt surface, the readily observable ∼5 μm periodicity in topography formation can be theoretically predicted. Additionally, the preferential formation and liberation of ∼1 μm diameter droplets at the target surface is observed. Nevertheless, the majority of sputtered mass flux is not comprised of droplets, but of neutral g...

Formation of intrinsic surface defects during 248 nm photoablation of polyimide

Photoablation of polymers by pulsed excimer laser radiation is commonly believed to be a controlled layer‐by‐layer removal process. A mass spectrometer was used to monitor neutral species ejected from polyimide samples in vacuum by 248 nm laser radiation. For fluences close to threshold, the ablation rate starts to drop after the first 100–200 pulses and eventually falls almost to zero. The falloff in etch rate is accompanied by a dramatic slowdown in the product translational energy distributions and by the appearance of conical defects on the sample surface. The number of cones is approximately the same for samples irradiated in air or vacuum for the same number of pulses, proving that ablation debris is not the initiating factor. It is argued that carbon enrichment at the sample surface initiates cone formation by locally shifting the ablation threshold to higher values. In effect, the polymer surface becomes ‘‘radiation hardened.’’

Laser sputtering of highly oriented pyrolytic graphite at 248 nm

The interaction of excimer laser pulses with a highly oriented pyrolytic graphite (HOPG) target has been studied. HOPG, a close approximation to single crystal graphite, was irradiated along a freshly cleaved basal plane in vacuum by pulses from a KrF excimer laser. The energy fluence was varied between 300–700 mJ/cm2, resulting in material removal rates of <0.01 A/pulse to ∼100 A/pulse. In this near‐threshold regime, neutral carbon atoms, dimers, and trimers account for nearly all of the sputtered flux and collisional and plasma effects are minimized. Time‐of‐flight distributions of the neutral carbon atoms and small carbon clusters were measured and inverted to obtain translational energy flux distributions and relative sputtering yields as a function of fluence. The translational energy distributions are remarkably close to Maxwell–Boltzmann distributions over most of the fluence range studied. However, the mean translational energies are far too high to reconcile with a simple thermal vaporization mod...

Near-threshold photoablation characteristics of polyimide and poly(ethylene terephthalate)

Photoablation of polyimide (Dupont Kapton™) and poly(ethylene terephthalate) (PET) (Dupont Mylar™) were studied in vacuum using 248 nm, 16 ns excimer laser pulses. A sensitive electron beam ionizer/quadrupole mass spectrometer detector was used to measure mass, translational energy, and angular distributions of the neutral photoproducts at fluences very close to the threshold (average material removal rates <100 A/pulse). The experiments were performed by combining results from many discrete sample spots, in order to minimize the changes in yield, energy, and surface topography caused by cumulative pulsing (“radiation hardening”). For both polyimide and PET, the dominant neutral photoproducts are fragments of the monomer, although there is a weak tail of intensity extending up to and beyond 1000 amu. The mean translational energy increases with increasing mass, due to the collisional “seeded beam” effect. The mean translational energy of the majority species (i.e., those below 200 amu) is around 1.6–4.9 e...

Impurity-Driven Cone Formation During Laser Sputtering of Graphite

Sputtering of highly oriented pyrolytic graphite (HOPG) by 248-nanometer laser radiation was studied. Neutral carbon atoms and small clusters were ejected with significantly higher translational energies than were expected from a simple model of thermal vaporization in the absence of a potential barrier. The HOPG also developed a remarkable surface morphology that consists of regular cones and domes. Cone formation appears to be initiated by trace metal impurities that serve as heat shunts in this highly anisotropic material.

Excimer laser projection micromachining of polyimide thin films annealed at different temperatures

A KrF excimer laser projection micromachining tool has been designed and implemented aiming to accomplish one-step etching with micron resolution for applications such as chip module packaging and polyimide technology in semiconductor manufacturing. Two polyimide (Probimide/sup (R/) 7020) thin films spun on the silicon wafers are annealed at 100/spl deg/C and 400/spl deg/C, respectively, in order to investigate the effect of the annealing temperature on the laser micromachining process. The micro-machined polyimide surface morphology is studied by scanning electron microscopy (SEM), atomic force microscopy (AFM), and a surface profilometer for determination of the etch rate at different laser fluences. Micromachining with micron resolution is achieved by this excimer laser projection tool. Measured on a 25 /spl mu/m line, the etch rate dependence on laser fluence both below and above the ablation threshold is investigated over the fluence range from 10-1000 mJ/cm/sup 2/. The polyimide thin film optical properties, i.e., the components of the complex refractive index, n and k at /spl lambda/=248 nm are measured by a spectroscopic ellipsometer for both films. The etch rate above the ablation threshold for the film annealed at 100/spl deg/C is found 60% higher than that for the film annealed at 400/spl deg/C. This behavior originates from the correspondingly higher by 82% absorption coefficient, /spl alpha/ in the 100/spl deg/C film. The ablation thresholds for both films are found the same at about 100 mJ/cm/sup 2/. Calculated etch rates agree with experimental values within 15%. Etch rates of the order of nanometer can be achieved near the ablation threshold of 100 mJ/cm/sup 2/ for both films. The linearity between the etched depth and the number of the laser pulses prevails for the micromachining process at and above the threshold.

Crossed beam rovibrational energy transfer from S1 glyoxal. III. Quantitative H2 and He cross sections for (00, K’=0) and (72, K’=0) and comparison with theory

Results of crossed‐molecular beam inelastic scattering experiments from two levels of 1Au (S1) trans‐glyoxal in collisions with H2 (Ec.m.=80 meV, 650 cm−1) and He (Ec.m.=95 meV, 770 cm−1) are reported. Relative inelastic scattering cross sections with quantitative error bars are obtained. S1←S0 laser excitation was used to prepare either the 00 level or the 72 level (evib=466 cm−1) with the high rotational selection of K’=0 and J’=0–10. The final high levels populated by rotationally and rovibrationally inelastic scattering were monitored by dispersed fluorescence with K’ state resolution. Fluorescence from only those molecules involved in inelastic scattering was obtained from the difference signal of spectra with the target gas beam (H2 or He) ON and OFF. Those dispersed fluorescence spectra were analyzed with a computer fitting procedure to extract 52 relative state‐to‐state cross sections for scattering from the (00, K’=0) level and 84 for the (72, K’=0) level. The cross sections have been compared qu...

Crossed beam rovibrational energy transfer from S1 glyoxal. IV. Reduced mass effects and an overview of the inelastic scattering characteristics from four initial levels

Crossed molecular beam studies of rotationally and rovibrationally inelastic scattering of S1 glyoxal from H2 and He have been extended to one additional light gas, D2, and to two heavy gases of identical masses, Kr and cyclohexane, C6H12 (84 amu). Laser excitation was used to prepare glyoxal in its 00 level with K’=0 and 0≤J’≤10. Dispersed fluorescence detection was used to observe the final K’ and vibrational states of the inelastic scattering. The relative scattering cross sections for D2 and He collisions are identical to within experimental error and differ substantially from those of H2. The Kr and C6H12 cross sections are also a matched set. These results show that the competition among the approximately 25 observable scattering channels is far more sensitive to the reduced mass of the collision than to variation in the intermolecular potential or even the internal structure of the target gas. An overview of rotational and rovibrational scattering in glyoxal from four vibrational levels (00, 72, 51...

Effects of KrF laser radiation on fused-silica glass: a comparison of samples exposed in air vs vacuum

High-purity synthetic fused silica glass is known to undergo changes in optical properties during high repetition rate KrF laser exposures. (Results of studies on six glass types irradiated in air are summarized elsewhere at this symposium.) We have also irradiated several samples in vacuum (248 nm, 300 Hz, (Phi) equals 400 mJ/cm2). Transmission at 248 nm, transmission at 210 nm, fluorescence at 650 nm, and vacuum cell pressure were monitored in real time. Although bulk outgassing is expected to be very slow at room temperature, our results indicate that the vacuum environment does affect the interior of the sample. Furthermore, a curious irradiation effect on the transmission of the CaF2 windows used on the vacuum cell is discussed.

Laser texture: understanding the mechanisms of topography formation on NiP and silicate glasses

Interest in developing laser zone texture for computer hard-disk drives has motivated our interest in the fundamental understanding of topography formation on NiP and silicate glasses. Our understanding of topography or bump formation results from experimental laser texture studies that have illustrated fundamental differences between the NiP and silicate glass systems. The most striking difference is that bump formation on NiP is characterized by conservation of volume, whereas bump formation on the silicate glasses is accompanied by net volume gain.