Excimer laser radiation may be used to process metal surfaces in a variety of novel ways. The simplest of these involves the use of UV laser pulses for ablation. Ablation occurs as the result of both vaporization and hydrodynamical effects. Experimental data related to these processes will be discussed. In addition, it will be shown how specific irradiation regimes can yield metal surfaces with unique radiative properties.

The temperature at the interface between a thin polyimide film and a quartz substrate was monitored as a function of time during KrF (248 nm) laser-induced heating and ablation using thin film NiSi thermistors. These experimental temperature measurements were coupled with heat flow simulations to obtain time-resolved temperature profiles in the polyimide. Thermal properties of the polyimide were estimated by requiring that the simulations reproduce experimental temperature profiles. The peak surface temperature in the polyimide at the onset of ablation was subsequently estimated from these simulations, producing a value of 1660 ± 100 K at the observed ablation threshold fluence of 36 mJ/cm2. Simulations were also used to explore the role of pulse duration on polyimide ablation.

Experimental data on the 248 nm and 308 nm wavelength excimer laser ablation of poly(tetrafluoroethylene) (PTFE) doped with polyimide (PI) are reported for a range of fluences and dopant concentrations. Threshold fluences were determined and correlated with the dopant concentrations. The threshold fluences and the limiting etch rates measured at high fluences decreased with increasing dopant concentration and there is a minimum absorption coefficient below which there is no ablation at both the wavelengths. The etch rates have been modeled using a two parameter Arrhenius thermal model to describe the etching process.

Vacuum-ultraviolet laser ablation of Teflon is reviewed. The 157 nm irradiation of Teflon produces clean ablation sites well suited to micromachining applications in the electronics and medical fields. At 193 nm, etching profiles are poorly defined, showing swelling characteristics commonly produced by longer wavelength lasers. Comprehensive new 193 nm ablation data are presented showing the first evidence of incubation effects for Teflon. A computer model was developed to include ablation, swelling and incubation processes. The computer results satisfactorily model the experimental data over a large fluence range of 0.6 to 13 J/cm2 with three adjustable parameters.

Periodic line structures with periods ranging from 166 nm to 950 nm have been produced in polyimide by direct ablation with a KrF (248 nm) laser using holographic techniques. Taking advantage of the large nonlinearity in the laser ablation process, linewidths ranging from 30 nm to several hundred nanometers could be obtained. These techniques were combined with the ability to induce electrical conductivity in polyimide to produce an array of 500 nm wide electrically conducting wires. The conductivity of these wires was similar to that found in macroscopic regions of laser induced conductivity.

The electrical conductivity of high temperature polymers (i.e. polyimide) has been changedpermanently from 10−17 Ω−1 cm−1 to 10 Ω−1 cm−1 by excimer laser irradiation. The conduction mechanism is found to be phonon assisted variable range hopping between small (∼10 nm) carbon clusters that form a macroscopic percolation cluster. The critical concentration of carbon clusters is found to be Pc = 0.24 ± 0.02 and the critical exponent of the insulator-conductor transition is t = 1.97 ± 0.25.

In order to open a new window for getting insight into the process of matrix assisted laser desorption ionization (MALDI) of biopolymers, a highly sensitive quartz crystal microbalance (QCM) was developed to measure desorption yields of neutral particles. For the matrix substances commonly used the mass resolution of 0.95 ng obtained with the balance corresponds to a detection limit of ∼1012 particles.

Copper samples were ablated by a pico-second pulsed Nd:YAG laser in Ar and He gases under different pressures. At 1064 nm, the change of ablation rate of copper in He gas from lx10−5 to 760 torr is smaller than that in Ar gas. A model for the development of a plasma above the target surface is presented based on inverse Bremsstrahlung absorption between fast electrons and the gas medium. Ionization rate of the buffer gas versus pressure was estimated using the model, and a good correlation was found with the ablation rate.

The effects of pulsed XeCl (308 nm) laser radiation on polycrystalline Al2O3 (alumina, 99.6% pure) and single-crystal Al2O3 (sapphire) are studied as a function of laser fluence. No laser etching ofeither material is detected below a threshold fluence value, which is much lower for alumina than for sapphire. Above this threshold, laser etching of both materials is observed following a number of incubation (induction) pulses. This number is much larger for sapphire than for alumina but decreases with increasing fluence for both materials. Laser etching rates for the two materials are similar at high fluences and after the incubation period. Scanning electron microscope images show that alumina melts and flows under repeated irradiation at fluences ≥0.7 J/cm2. Atomic force microscopy and surface profilometry reveal significant smoothing of the as-received polycrystalline alumina surface after repeated irradiations at moderate fluences (∼1−3 J/cm2). Ion probe measurements for alumina in vacuum confirm the incubation behavior, and reveal that at fixed fluence the (positive) charge collected per pulse saturates after a sufficient number of pulses, as does the etch-plume velocity. The results are interpreted in terms of laser-generation of a sufficient concentration of absorption centers before efficient ablation/etching of these wide bandgap materials can occur.

Excimer laser pulses with wavelengths of 248 and 308 nm were used to selectively seed Pd on SiO2 surfaces, making them suitable for electroless plating. This novel seeding process for insulating materials is accomplished with the sample immersed in the seeding solution, and occurs only on the areas of the substrate that are illuminated (through the liquid) by the laser light. The Pd content of the seeded samples increased with the number of pulses, but was rather independent of repetition rate. The deposition rate showed a dependence with wavelength consistent with a defect driven mechanism for electron excitation through the band gap of SiO2. These electrons then reduce the Pd ions in the solution in contact with the surface.

A new process for writing a copper pattern into alumina or sapphire substrates is presented. This process employs in sequence ultraviolet laser irradiation and electroless deposition of copper. The laser irradiation activates the substrate and copper is deposited in the activated region when the substrate is immersed in the electroless solution. The laser-activation can be performed months in advance to the deposition.

A small amount of metallic aluminum is left on the surface of laser irradiated alumina substrates. The presence of this metallic aluminum appears to play a very important role in the laser surface activation process. In addition to aluminum cluster formation there must be other activation centers that promote copper deposition in laser treated alumina substrates. These catalytic centers are much weaker that the aluminum clusters and have been tentatively identified as surface defects.

A new technique for selective metallization of aluminum nitride (AIN) has been previously reported (1). It involves the use of an excimer laser to activate the AIN surface followed by electroless plating (Cu,Ni,Au) of the irradiated areas. The mechanism of decomposition of ALN is accompanied by ablation and the formation of an Al film on the substrate surface. Ablation rates are reported here as a function of fluence and number of pulses for three different wavelengths λ = 193 nm (ArF), λ = 248 (KrF) and λ = 351 nm (XeF).

The effect of laser wavelength on the ablation rate is discussed. The ablation rates for Al were zlso measured and are compared with the AIN ablation rates. A numerical thermal model is used to analyze the mechanisms of laser ablation of both materials. The evaporation kinetics are incorporated into the model. The Clausius-Clapeyron approximation is used to make a self-consistent calculation of boiling and decomposition temperatures.

We report the observation of wavelength-dependence in the early phases of the laser ablation of chemical vapor deposited (CVD) diamond films. The measurements were made with 3–4 μm infrared radiation from the Vanderbilt Free-Electron Laser (FEL), the brightest tunable mid-infrared FEL in the world. Though we do not propose any specific mechanism for the ablation, the C-H bond-stretching mode at 3.47 μm is clearly important in the early stagesof the damage process, as evidenced by a sharp drop in the ablation threshold near 3.47 μm.The ablation damage threshold was determined by directing a single FEL pulse at the sampleand measuring the diameter of the resulting hole using a scanning electron microscope (SEM).This measurement was repeated for several pulse intensities. The damage threshold was defined as the extrapolation to zero hole diameter. We also looked at the desorption productsusing a time-of-flight spectrometer and a quadrupole mass analyzer. C2 is the dominant desorbed species. Visible light emission was monitored with a monochromator and photodetector.

Elemental distributions in the bulk and metal oxide surface layers of zirconium alloys play key roles in the fracture toughness of the alloys. In particular, localized hydrogen build-up leads to hydride formation and delayed hydride cracking. Parts per million levels of H in Zr have been detected using the 1.06 μm or 355 nm output of a Nd:YAG laser for ablation followed by 2+1 resonance ionization detection of H and D. Analysis of the ablation plume has shown that it consists predominately of atomic species in thermal equilibrium between 2000 and 3600°C. Ablation of thin foils has shown that the ablation rate is on the order of mono-layers per shot and increases exponentially with increasing fluence. Laser ablation depth profiling results of H distributions in an anodically grown oxide film compare qualitatively with nuclear-reaction-analysis profiling of the same sample.

High speed microfabrication of quartz substrates by laser ablation using vacuum ultraviolet (VUV) laser beams is described. The VUV light is generated by anti-Stokes stimulated Raman scattering of fourth harmonics of Q-switched Nd:YAG laser. The well-defined patterns with a cross-sectional profile of rectangular shape are formed by using a contact mask. The role of short wavelength components of the VUV laser beams in the process is discussed.

Two different routes for micropatterning of YBCO thin films by selective epitaxy are presented; both methods are based on the intermixing of YBCO with silicon at the growth temperature for ( 001 ) as well as ( hk0 ) YBCO thin film epitaxy. One method ( I ) uses a pre-patterned yttria stabilized zirconia [ YSZ ] thin film epitaxially grown on a silicon single crystal giving rise to the epitaxy of c-axis oriented YBCO on YSZ whereas intermixing yields a nonsuperconducting Si-Y-Ba--Cu-O phase elsewhere. The other method (II ) uses SrTiO3 substrates suitable for ( 103 ) as well as ( 110 ) YBCO epitaxy coated with a pre-patterned amorphous silicon layer prior to YBCO deposition. Both techniques are chemistry free routes for YBCO thin film patterningand pave the way for superconductor/semiconductor hybride device fabrication ( route I ) as well as basic investigations of the anisotropic electronic and optical properties of YBCO ( route II ).

Controlling of microstructure generation was investigated in laser etching of n-GaAs. For single-beam etching and holographic etching with high ratios of average spacing of holographic grating to average spacing of microstructures (Λh/Λr), ripple structures were fabricated. Especially in p-polarization, spatial fluctuation was greater than that in s-polarization. This might occur because phase distortion inherent in the laser beam cannot be eliminated by the p-polarization beam irradiation. For holographic etching with small Λh/Λr ratios, ripple structures were changed into grating structures because these grating structures might be generated in phase with holographic gratings.

We have (tudied the laser damage threshold to silver films (500Å - 1000Å) grown on single crystal silicon <100>, in a newly developed laser damage UHV system at pressures of 10−1 torr. A 1.06μm Nd:Glass laser is used to damage the mirror surfaces in 1-on-1 pulse studies. In-situ damage characterization includes Auger, reflectivity of the primary beam, diffuse scattering of a helium-neon laser, and mass spectrometry detection of desorbed surface species. External characterization includes optical microscopy and SEM. All in-situ damage probes are well correlated and baseline damage occurs at fluences near 3.4 MW/cm2. Time-of-flight to the mass spectrometer shows ejected particles with energies in the 5 to 10 eV range indicating a plasma damage mechanism. Prior to typical thermally induced damage the external microscopy shows well defined precursor morphology changes which appear as feather-like microstructure at the submicron level.

A source of energetic, neutral Cl atoms is produced by laser vaporization of cryogenic ICI films using either the 532 nm or 266 nm output of a pulsed Nd:YAG laser. The pulsed Cl mean kinetic energy can be tuned from 0.4 eV to 3.4 eV by varying the laser repetition rate or the rate of deposition of the ICl film. Also present in the beam are ICI, I, I2, and Cl2 The heavier species all have lower velocities than the Cl atoms, and this difference in velocities can be accentuated by vaporizing thinner films. Using a mechanical velocity chopper, the fastest part of the beam can be selected, which consists of > 90% Cl atoms. Results suggest that the vaporization process involves a directed mass flow in which the heavier molecules are propelled to higher velocities by the faster Cl atom flux. The atom source may be used for selective etching studies.

Resistive and magnetic anomalies were observed at 150-170K in Ca0.3Sr0.7CuO2 thin films prepared by atomic-layer stacking using laser molecular beam epitaxy. Systematic current density dependence of the resistivity was observed below these temperatures with clear resistivity drops. At the same temperature range, the films showed diamagnetic response and hysteresis of magnetization between zero-field cooling and field cooling process. These behaviors suggest that the anomalies are due to high temperature superconductivity around 150-170K in the Ca0.3Sr0.7CuO2 thin films.