Anthony J. Pedraza

New approach of a laser-induced forward transfer for deposition of patterned thin metal films

Abstract An excimer laser-induced forward transfer technique has been used for selective prenucleation of arbitrary substrate materials with palladium. Palladium acetate films on a quartz substrate were irradiated through the support causing simultaneous decomposition of the palladium acetate and deposition of palladium on a substrate at a short distance to the palladium acetate film. The palladium layer, only a few nanometres thick, can act as catalyst for subsequent chemical deposition of metallic films, e.g., copper, nickel, gold etc., of micrometer thickness.

Interaction of UV laser light with wide band gap materials: Mechanisms and effects

Abstract The optical response of materials to light irradiation can be classified into two broad classes: intrinsic and extrinsic. Extrinsic effects, which are associated with the presence of impurities and structural defects in the material, are commonly invoked and demonstrated in insulators. At variance with semiconductors, insulators have a band gap that largely exceeds photon energies in the ultraviolet (UV) range. The interaction mechanisms responsible for the production of free carriers and defects in UV laser irradiated ceramics are discussed. The surface modifications that take place when the coupling between a ceramic and UV radiation results in heat generation are analyzed using Al2O3 as a model ceramic material. On the other hand, lack of heat evolution upon laser irradiation makes SiO2 not amenable to surface modification effects similar to those produced in Al2O3. This lack of heat evolution, however, permits the use of laser irradiation for encapsulating metallic particles.

IRRADIATION-INDUCED DECOMPOSITION OF AL2O3 DURING AUGER ELECTRON SPECTROSCOPY ANALYSIS

The effect of electron fluence on the decomposition of sapphire (Al2O3) was studied in situ by Auger electron spectroscopy (AES). The decomposition was primarily detected by monitoring the evolution of the low kinetic energy Auger transitions of aluminum in Al2O3 (54 eV) and in metallic aluminum (68 eV). The decomposition of sputter‐cleaned sapphire started at a fluence of ∼4.9×1019 electrons/cm2 (7.8 C/cm2). This fluence was independent of the electron fluxes used in this work, except the lowest, which indicates that heating due to electron bombardment does not significantly affect the decomposition behavior. Electron‐induced decomposition takes place in a minimum of the first five atomic layers of the substrate, as revealed by the evolution during irradiation of the high energy Al peaks associated with Al2O3 (1388 eV) and metallic aluminum (1396 eV). Comparison of the evolution of low and high kinetic energy Auger transitions demonstrates that the decomposition kinetics are much faster for the first mon...

Enhanced metal-ceramic adhesion by sequential sputter deposition and pulsed laser melting of copper films on sapphire substrates

A study is presented of the effect of pulsed XeCl (308 nm) laser treatment on the adhesion between sputter-deposited copper films and sapphire substrates. Laser treatment (LT) of individual 80 nm thick copper films results in adhesion enhancement, relative to the assputtered film for XeCl energy densities >0.35 J cm−2. Thicker (∼ 1μm), strongly adherent copper films can be built up by alternating discrete and sequential sputter deposition with pulsed laser irradiations carried out in air. This sequential process yields smooth films whose adherence, as measured by the scratch test, is a factor of more than two to three greater than for as-sputtered films. The only way to remove the copper layer after irradiation was by cutting through the sapphire. Although formation of a metal oxide is a common consequence of LT in air, adhesion tests reveal no significant effect of carrying out LT in oxidizing or reducing atmospheres. During the earliest stages of the sequential process, the laser-melted film tends to break into small clusters. It is concluded that this process is driven by a surface energy gradient generated by lateral thermal gradients in the melt. These gradients, in turn, are due to the early establishment of isolated regions of good bonding and thermal contact with the substrate. One of the characteristic features of the sequential process is that this good bonding, once established in a given region, is maintained throughout successive meltings of the region. Adhesion mechanisms under LT are discussed.

VUV-light-induced deposited silica films

Abstract A novel technique to deposit dielectric films at room temperature is described. The deposition of the silica takes place inside a cylindrical glass chamber where a silent discharge is generated between two electrodes connected to a high voltage, high frequency AC source. The chamber contains two parallel glass tubes where the electrodes are located and is filled with argon or xenon at a pressure of 100 mbar. Under these conditions, it has been shown that high intensity VUV light is generated peaking at 126 nm for argon and at 172 nm for xenon. This VUV radiation seems to produce photoablation of the glass tubes that surround the electrodes. Upon operation of the lamp, polyimide, polypropylene and silicon wafer substrates lying at the bottom of the vessel became coated with silica. The films, identified using X-ray photoelectron spectroscopy (XPS), revealed that the silica is oxygen-deficient with a composition of SiO x where x is between 1.7 and 1.8. The deposition rate on silicon wafers was measured by ellipsometry. When Xe gas is used the deposition rate is much lower than when Ar is used. This result is consistent with a photoablation process since the energy of the photons generated in Ar peaks at 10 eV while those generated in Xe peaks at 7 eV. These energy values should be compared with the O–Si bond strength energy that is 8.3 eV. The morphology and structure of the films were examined by scanning and transmission electron microscopies. Deposition of carbonaceous films occurred when the glass tubes containing the electrodes were coated with carbon.

Auger Electron Spectroscopy of Metallic Film/Laser-Irradiated Alumina Couples

Auger Electron Spectroscopy (AES) was employed to study metal/ceramic interfaces. Adhesion pull testing showed that a strong bonding between metallic films and alumina substrates is obtained when alumina is laser-irradiated in an oxygen atmosphere before deposition and, after deposition, the couple is annealed at 300°C for 1 h. On the other hand the gold/alumina bonding is extremely weak when laser-irradiation of the substrate is performed in argon-4% H 2 . Fresh surfaces were exposed for AES at several distances from the metal/ceramic interface by sputtering the specimens in situ after each analysis. In the region of the interface the sputtering rate was slower than in the film in order to minimize any spurious effect due to ion beam bombardment. AES of a gold/alumina couple prepared in this way reveals that the Auger peaks of gold from the film and oxygen and aluminum from the alumina substrate, when the substrate was irradiated in oxygen atmosphere, shift by ∼ 1.5 eV, 1.6eV, and 1.4eV respectively when the analysis area encompasses the metal/ceramic interface. On the other hand, no shift of the Auger peaks are observed at the interface when the laser-irradiation was performed in argon-4%H 2 . In the case of copper deposited on alumina laser-irradiated in an oxygen atmosphere, an interfacial compound is formed. This compound is a double oxide of aluminum/copper or two separated oxides, and promotes strong copper/alumina bonding.

Generation and manipulation of nanostructures by pulsed-laser ablation

Laser-induced surface structuring of silicon was studied using fluences close to the metering threshold and He gas background atmosphere. The effects of an initial surface microstructured region and of light polarization on the evolution of the surface topography were investigated. The microstructured surface topology consisted of an array of microholes surround by microcones of 2-3 micrometers tip-diameter and over 20 micrometers high. Pulsed laser irradiation of laser- microstructured silicon induces the formation of nanostructures. Nanocolumns having a diameter of 100 to 200 nm and reaching a height of up to 3 micrometers upon cumulative laser pulses grow on top of every microcone. The mechanisms of nanocolumn origin and growth are analyzed. Periodic undulations approximately 10 nm-high are formed when flat silicon substrates are irradiated with polarized laser light. These periodic structures have a wavelength that is a function of the light wavelength and the angle of incidence of the laser beam. At a slightly higher laser fluence, approximately 30 nm-diameter nanoparticles form on the surface of laser irradiated flat silicon specimens. Linear arrays of silicon nanoparticles with fairly uniform size that extend up to a millimeter are formed if the irradiation is performed using polarized light or the irradiated area contains a microstructured region. These nanostructures are analyzed within the frame of the theory of laser induced surface periodic structures.

An analysis of carbide precipitation in V and Nb during aging and ion bombardment

Abstract Carbide precipitation in vanadium and niobium is accompanied by a very large volume change ( ≈ 18%). A thermodynamic analysis of the V-C system and an estimate of the plastic energy required for accommodating that change is performed. The carbon concentrations for surface and for bulk precipitation are calculated. It is concluded that precipitation limited to the surface region can occur in high purity vanadium if an external carbon source is available. A similar conclusion is reached in the case of niobium, partly based on an estimate of the plastic energy required for accommodating the volume change due to carbide precipitation. These analyses provide a basis to distinguish between surface and radiation induced precipitates. Two main classes of behavior can arise during ion bombardment. When the highest damage region is very close to the surface, no internal precipitates are produced and radiation induced point defects enhance surface precipitate growth. When peak damage is produced farther from the surface, internal precipitates are formed and compete with the surface precipitates for carbon atom capture. The delicate balance between chemical driving force, transformation stresses and radiation effects may alter the precipitation pattern in both the surface and the damaged region.

Laser-promoted nanostructure evolution and nanoparticle alignment

A cone microstructure has been used as a template to generate nanotips and to promote nanoparticle alignment. A quasi-periodic array of nanotips is produced when the laser-induced cone microstructure is subject to chemical etching due to tapering of the cone tips. Nanoparticles can be produced on the surface of a silicon specimen by irradiating it in the presence of an inert gas atmosphere. The backscattered material that is re-deposited on the substrate, upon irradiation at fluences close to the melting threshold, is composed of a thin film intermixed with extremely small nanoparticles. Further irradiation promotes film clustering and nanoparticle formation. In the presence of cones, the nanoparticles become aligned into straight and long (~1 mm) lines whose spacing is close to the laser wavelength. This result suggested an ordering mechanism similar to that occurring for laser-induced periodic surface structures. The relation between nanoparticle line spacing and angle of incidence of the radiation supported this similarity. Nanoparticle ordering also was promoted by laser-enhanced chemical vapor deposition (LCVD) using polarized light, when a laser-induced periodic surface nanostructure was present in the substrate.

Surface engineering using excimer lasers

Excimer laser irradiation of insulators produces structural and chemical modifications in the near-surface region of these materials. These changes have lead to the usage of excimer lasers to engineer the surface of insulators for various applications, as illustrated in four examples presented here: (1) Laser-enhanced bonding of deposited metallic films. A very strong bonding between metallic films and Al2O3 can be achieved if the substrates are pulsed-laser treated prior to deposition. AES reveals that strong bonding occurs when an intermediate interfacial compound forms as a metallic film is deposited on a laser-irradiated substrate. (2) Laser encapsulation of metallic particles in silica. Thin films of gold, copper and iron deposited on silica can become encapsulated as small particles upon pulsed laser irradiation XTEM indicates two distinctive stages in the encapsulation process, during one laser pulse. In the first stage, the film melts and clusters into small particles, and in the second stage, the particles are driven into the substrate. (3) Laser- induced surface activation for electroless deposition. In this process, a pattern is imprinted on a substrate by laser irradiating its surface through a mask. Upon immersion of the substrate in an electroless solution, a metallic film is deposited only on the laser-exposed area. Auger emission spectroscopy (AES) and cross sectional transmission electron microscopy (XTEM) indicate that electroless deposition is promoted by the presence of metallic aluminum in AlN and in Al2O3, and of substoichiometric oxide in Al2O3, as well. Other laser irradiation effects that also could induce activation are analyzed. (4) Laser-induced deactivation of a previously activated area.