Incubation/ablation patterning of polymer surfaces with sub-μm edge definition for optical storage devices

By exposure to low fluence UV laser radiation, the optical absorption coefficient of subsurface polymer material can be increased (incubation) with spatial control, using a suitable contact mask, proper imaging of the mask, or laser direct writing. Spatially selective ablation of polymethylmethacrylate (PMMA) is then achieved with large area XeCl excimer laser pulses at 308 nm. In this way, the transfer of spatial information to the material can be decoupled from the high laser fluence removal (ablation) step. The advantages are: The mask is exposed to only low fluence laser radiation — damage is avoided. Since the mask can be removed before the ablation step, mask contamination by the ablated plume cannot occur. Using this incubation/ablation method, PMMA surfaces can be patterned (248 nm/308 nm) with submicrometer spatial control and edge contrasts better than 0.2 m. This has impact on optical storage technology and laser surface processing techniques in general. The smallest single structure obtained was somewhat smaller than 0.5 m in diameter up to now, given by the mask.Presented at Laserion '91, June 12–14, 1991     

Spectroscopic studies of ArF laser photoablation of PMMA

A Spectroscopic study has been made of the emission spectra arising from ArF laser initiated photoablation of PMMA samples. This process leads to direct etching of the polymer. The thermal temperature of the CH fragment species in the plume immediately above the ablated site was found to be 3200 ±200 K. The translational velocity of this species was found to be 4.2×10⁵ cm/s corresponding to a translational temperature of 11,000 K. These results are consistent with a rapid direct bond scission model for ablation.     

Mass flow in laser-plasma deposition of carbon under oblique angles of incidence

The angular distribution of the mass flow in carbon laser plasmas, generated from graphite targets at laser power densities around 10¹¹ W/cm² and 1064 nm, was studied. Under oblique angles of incidence the mass flow is not perpendicular to the target surface but rather symmetrical around the bisecting angle between the laser beam and the surface normal. For all angles, however, a cos⁴-pattern is observed. Compared to normal incidence the mass flow is weaker by about a factor of 2 to 3 for 30° and 50° angle of incidence. The dependence of film quality on deposition angle with regard to the symmetry axis of the plume is demonstrated.Presented at LASERION '91, June 12–14, 1991, München (Germany)     

Quantitative characterization of AlAs/GaAs interfaces by high-resolution transmission electron microscopy along the 〈100〉 and the 〈110〉 projection

The interface microstructure of AlAs/GaAs quantum wells grown by molecular beam epitaxy (MBE) was investigated by transmission electron microscopy (TEM). High-resolution transmission electron microscopy (HRTEM) yields information about the width of the chemical transition between the binary components and about the lateral step distances along the interface. The chemical composition is quantitatively determined by the application of a pattern recognition procedure based on the Fourier transformation of image unit cells. Along the 100 zone axis the composition across the interfaces is obtained with a precision of ±10 atomic percent and with a spatial resolution of 0.28 nm. Despite a lower chemical sensitivity a quantitative chemical analysis was also carried out for images along the 110 projection.     

Acoustomagnetoelectric current effects on mesoultrasound in anisotropic semiconductors

The acoustomagnetoelectric (AME) current effects on mesoultrasound in monopolar single-valley semiconductors with anisotropic scattering in arbitrary classical magnetic fields are computed analytically. The specimen is considered short-circuited along the q-wave vector of the ultrasound (US) wave. Two configurations are examined: 1) q is directed at an arbitrary angle 9 to the axis of highest order crystal symmetry C₆ (z axis), the y axis to the (q, C₆) plane, the magnetic field B lies in the (q, y) plane at an arbitrary angle to the vector q. Two transverse AME field components are calculated: along y and in the (q, C₆) plane. They express the Hall effects at mesoultrasound, the planar and normal, 2) q is directed along the y axis while B is in the (x, z) plane at an angle to the C₆(z) axis. The AME field component along B, the Grobner effect at mesoultrasound, is calculated. The dependence of the effects onB is studied and their estimate is given in weak and strong fields.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 57–61, July, 1989.     

Nanosecond and femtosecond excimer-laser ablation of oxide ceramics

The uv laser-ablation behavior of various oxide ceramics (Al₂O₃, MgO, ZrO₂) has been studied using different wavelengths (248 nm, 308 nm) and pulse durations (30 ns, 500 fs). Time-resolved absorption measurements of the sample and the ablation plume during ablation were performed.Using sub-ps pulses the ablation threshold fluence is generally lower than for ns pulses; the ablation rate is higher in the whole investigated fluence range up to 20 J/cm².The study of the morphology of the ablation structures and the results of the absorption experiments lead to the conclusion that different ablation mechanisms are involved. Using ns pulses plasma mediated ablation is dominating, whereas in the fs case the process is controlled by multi-photon absorption enabling microstructuring of the material.     

Shell sources of stationary axisymmetric gravitational fields

A number of solutions for material shell sources of stationary axisymmetric gravitational fields are presented. Explicit solutions are found for shells lying on equipotential hypersurfaces (g _tt = const) and generating static monopole fields in prolate and oblate spheroidal coordinates (Zipoy-Voorhees fields). Numerical solutions are found for shells lying on hypersurfaces of constantg /g and generating Kerr- and Tomimatsu-Sato ( = 2) fields. The shells have minimum areas allowed by the energy conditions of Hawking and Ellis.     

Investigation of the interaction of subpicosecond KrF laser pulses with a preformed carbon plasma

The interaction of a subpicosecond KrF laser pulse with a preformed carbon plasma of various scale lengths is investigated. Two different interaction geometries are chosen. In the first one the propagation vector of the short pulse has a component along the density gradient of the preformed plasma (angle of incidence is 45°). In the second geometry the propagation direction of the short pulse is perpendicular to the density gradient of the preplasma (angle of incidence is 90°). The emitted soft X-ray spectrum in the wavelength interval from 10 to 700 is observed while changing several parameters of the experiment. It is found that the emission in the short wavelength part under 200 results from the radiation of ions created by collisional heating near the critical density region. The long wavelength part above 200 , enhanced up to a maximal factor of 20, is mainly produced by radiating particles field—ionized up to the He-like carbon state in the high-intensity laser field. The short wavelength part is missing in the case of 90° angle of incidence because there is no interaction with the critical layer that results in an insufficient collisional heating.     

Selective metallization of polyimide by laser-induced plasma-assisted ablation (LIPAA)

We report micromachining of polyimide (PI) by laser-induced plasma-assisted ablation (LIPAA) using a fundamental wavelength of a commercial Q-switched Nd:YAG laser (1064 nm). It is found that an Au film on a glass target is effective for the LIPAA process of PI. The ablation rate reaches several tens of nanometers per pulse. After the LIPAA process, selective metallization of PI with excellent electrical properties is performed by successive electroless Cu plating. The Cu line width of 40 m, which agrees with the line width of regions ablated by the LIPAA process, is achieved using an encapsulated film. PACS 42.62.-b; 52.38.-r; 85.40.Ls     

Beam scanning and switching characteristics of twin-striped lasers with a reduced stripe spacing

The output-beam scanning and switching characteristics of lasers with separately controlled twin-stripe electrodes are reported. The spacing between the stripes is small (5m from centre to centre) and the spacing region is either etched or ion-implanted for better current isolation. The implanted laser oscillates in the fundamental mode. The output beam of this laser can be scanned over a radiation angle range of 7° almost linearly with injection currents. The etched laser oscillates in the first-order mode. The output beam of this laser can be switched with a deflection angle as large as ±8°.     

Interface mixing in Co/Cu: in situ electrical conductivity measurements on pulsed-laser-deposited multilayers

By providing the experimental possibility to monitor the electrical conductivity of metallic multilayers prepared by laser ablation during their growth in situ, information can be obtained on interface mixing effects. This is demonstrated for (5 nmCo/5 nmCu)_n multilayers ablated by a UHV–193 nm ArF–Pulsed Laser Deposition (PLD) system. Varying the laser fluence from 4.2 Jcm^-2 to 12.3 Jcm^-2 allows a corresponding variation of the fraction f of energetic particles within the ablated plume as well as of their average energy. As a result of such a bombardment-assisted deposition, mixing effects occur for both growth sequences, Co on top of Cu as well as Cu on top of Co. As expected, these mixing effects are most pronounced for high laser fluences and significantly decrease for fluences close to the ablation threshold. Comparison to TRIDYN Monte Carlo simulations allow one to extract estimates for the fraction f of energetic particles as well as for their average energy. PACS 73.40.Jn; 73.61.At; 75.47.Np; 81.15.Fg     

Analysis of plume deflection in the silicon laser ablation process

Changes in target surface morphology and ablation plume direction have been experimentally observed during the initial stages of the silicon laser ablation process. A relationship between both phenomena can be observed upon analysing the temperature field induced by the laser beam in a rough surface material. Theoretical studies on the deflection of the ablation plume are presented. These analyses are based on the hypothesis that particles that reach evaporation temperature will exit normally to the target surface with a velocity that is proportional to the surface temperature and the amount of the ablated material. Numerical solutions and experimental results of laser ablation process of silicon targets are found to agree with theoretical studies. PACS 42.25.Lc; 79.20.Dc; 02.70.Dc     

Model and computer simulation of nanosecond laser material ablation

A computer model to simulate the evolution of parameters describing laser ablation processes was developed. The absorbed laser energy, the heat diffusion, the phase transformations and the shielding effect of the ablated material were taken into account. The temporal development of the ablated volume, pore depth and extension of the melt zone were calculated for single pulses of 500, 100, 20, 5 and 1 ns. Simulations were performed for pulse energies of 50 J and spot diameters of 10 m. From temporal evolution curves of the ablated volumes, the stoppage of the ablation process was evidenced before the end of the processing pulse. Comments with respect to optimal pulse duration (in the ns regime) are also formulated. PACS 81.40.Wx     

KrF excimer laser processing of thick diamond-like carbon films

Ablation characteristics of a commercially available diamond-like carbon (DLC) film (the EVERSCAN^TM bar code scanner window; Diamonex Division of Morgan Advanced Ceramics Inc.) in the 0–7 Jcm^-2 fluence window are reported. Glass pieces covered with 6 m thick DLC coating are ablated by 22 ns pulses of a KrF excimer laser in medium vacuum (10^-1 Pa), and in Ar and O₂ atmospheres of 10⁵ Pa. The ablation depth increases strictly linearly with an increasing number of pulses, independently of the atmosphere and the applied fluence. The threshold lies at 0.13 Jcm^-2 and is independent of the atmosphere. While above 1 Jcm^-2 the ablation rate vs. fluence plots recorded in different atmospheres coincide within experimental error, processing under vacuum between 0.5 and 1 Jcm^-2 results in significantly higher ablation rate values. Ablation rates exceeding 100 nm/pulse above 1 Jcm^-2 ensure extremely efficient machining over extended areas even at relatively low fluences. Identical characteristics in Ar and O₂ atmospheres suggest that solely physical (ablative) processes contribute to material removal. PACS 42.62.-b; 61.80.Ba; 68.35.Rh; 81.05.Uw     

Growth mechanism of ZnO nanorods from nanoparticles formed in a laser ablation plume

We have succeeded in synthesizing ZnO nanorods by pulsed-laser ablation at comparatively high gas pressures without using a catalyst. The nanorods had an average size of 300 nm and a length of about 6 m. Stimulated emission was observed from the nanorods at 388 nm by optical pumping. As a catalyst was not used in our method, nanorod growth was not controlled by the vapor-liquid-solid (VLS) mechanism. We found that nanoparticles formed by condensation of ablated particles in the laser ablation plume play an important role in nanorod growth. PACS 61.46.+w; 81.07.Bc; 78.66.Hf; 78.67.Bf; 81.16.Mk     

Spectroscopy of laser-produced cerium plasma for remote isotope analysis of nuclear fuel

A cerium oxide sample was ablated by 2nd harmonic radiation of Nd:YAG laser at a power density of 0.1 GW/cm². Time evolution of the ablation plume was investigated by laser absorption time-of-flight (TOF) measurement. It was found that the ablated ionic plume in vacuum consisted of two components having different velocities whereas the ablated neutral atoms had mainly a single component. The flow velocity perpendicular to the sample surface in vacuum was determined to be 3.5 km/s for neutral atoms, and 4.7 km/s and 9.3 km/s for singly charged ions. From the detailed plume evolution in ambient atmosphere with several pressures we obtained some experimental conditions suitable for isotope analysis of atomic cerium.     

Ablation studies using a diode-pumped Nd : YVO4 micro-laser

We report an investigation of ablating several materials using a nanosecond pulse duration Nd:YVO₄ micro-laser operating at wavelengths of 1064 and 532 nm and high pulse-repetition rate (20 kHz). Probe beam deflection, ballistic pendulum measurements and scanning electron microscopy are used to characterise the interaction. It is shown that good-quality micro-scale features can be produced in polyimide, gold foils and silicon targets by ablation using this laser. PACS 42.55.Xi; 52.38.Mf; 87.80.Mj     

Schlieren diagnostics of pulsed electron beam ablation of polymethyl-methacrylate

The investigation of the interaction of pulsed electron beams with PMMA (polymethylmethacrylate) targets is reported. The electron beam of some 10^–8 s in duration is produced in a pulsed low-pressure gas discharge. The beam power density of up to 10⁸ W/cm² leads to a surface plasma formation similar to that of the pulsed laser ablation process. The propagation of the ablated material and the shock wave inside the PMMA target are observed by means of Schlieren diagnostics. An electron density gradient of over 3×10¹⁹ cm^–4 has been observed in the expanding plasma up to 1.5 s after the plasma formation. During the early stage of expansion, the expansion velocity of the plasma plume as determined by the steep electron density gradient is around 10⁵ cm/s. The pressure behind the shock front inside the PMMA target as determined from the shock velocity exceeds 0.3 Gpa.     

Tree-Level Violation of the Equivalence Principle

Massive particles of spin 0 and 1 violate the equivalence principle (EP) at the tree level. On the other hand, if these particles are massless, they agree with the EP, which leads us to conjecture that from a semiclassical viewpoint massless particles, no matter what their spin, obey the EP. General relativity predicts a deflection angle of 2.63 for a nonrelativistic spinless massive boson passing close to the Sun, while for a massive vectorial boson of spin 1 the corresponding deflection is 2.62.     

Symmetry of some lattice defects in nickel measured by DPAC

From DPAC measurements two defect-associated sites are known to exist after implantation of¹¹¹In in nickel. Site A possesses high symmetry and does not show electric quadrupole interaction, while for site B a large axially symmetrical EFG is found. DPAC measurements on a Ni single crystal revealed that the symmetry axis of the EFG preferably points into a 111 direction. The defects are interpreted in terms of simple impurityvacancy clusters.