A. J. Pedraza

Silicon microcolumn arrays grown by nanosecond pulsed-excimer laser irradiation

Arrays of high aspect ratio silicon microcolumns that protrude well above the initial surface have been formed by cumulative nanosecond pulsed-excimer laser irradiation of silicon. Microcolumn growth is strongly affected by the gas environment, being enhanced in air or other oxygen-containing ambient. It is proposed that microcolumn growth occurs through a combination of pulsed-laser melting of the tips of the columns and deposition of silicon from the intense flux of silicon-rich vapor produced by ablation of the surface regions between columns. The molten tips of the columns are strongly preferred sites for deposition, resulting in a very high axial growth rate. The growth process is conceptually similar to the vapor–liquid–solid method used to grow silicon whiskers. However, in the present case the pulsed-laser radiation fulfills two roles almost simultaneously, viz., providing the flux of silicon-containing molecules and melting the tips of the columns.

Early Stages of Pulsed-Laser Growth of Silicon Microcolumns and Microcones in Air and SF6

Dense arrays of high-aspect-ratio silicon microcolumns and microcones are formed by cumulative nanosecond pulsed excimer laser irradiation of single-crystal silicon in oxidizing atmospheres such as air and SF6. Growth of such surface microstructures requires a redeposition model and also involves elements of self-organization. The shape of the microstructures, i.e., straight columns vs. steeply sloping cones and connecting walls, is governed by the type and concentration of the oxidizing species, e.g., oxygen vs. fluorine. Growth is believed to occur by a “catalyst-free” VLS (vapor–liquid–solid) mechanism that involves repetitive melting of the tips of the columns/cones and deposition there of the ablated flux of Si-containing vapor. Results are presented of a new investigation of how such different final microstructures as microcolumns or microcones joined by walls nucleate and develop. The changes in silicon surface morphology were systematically determined and compared as the number of pulsed KrF (248 nm) laser shots was increased from 25 to several thousand in both air and SF6. The experiments in air and SF6 reveal significant differences in initial surface cracking and pattern formation. Consequently, local protrusions are first produced and column or cone/wall growth is initiated by different processes and at different rates. Differences in the spatial organization of column or cone/wall growth also are apparent.

Surface microstructuring and long-range ordering of silicon nanoparticles

Pulsed-laser irradiation was used to induce the formation of linear arrays of nanoparticles that can extend over millimeter distances. On flat surfaces, the irradiation induces the fragmentation and clustering of a thin silicon film pulsed-laser deposited on silicon into nanoparticles that grow to 30–40 nm in diameter. The nanoparticles aggregate into clusters that migrate, forming short curvilinear groups that exhibit a short-range ordering. If a region containing a microscopic roughness is introduced, the nanoparticles are forced to align into long and remarkably straight lines with line spacing very close to the laser wavelength. A close connection is established between the nanoparticle alignment and the evolution of laser-induced periodic surface structures. The microscopic roughness solely serves as a trigger to produce the alignment.

Microstructural Evolution of Laser-Exposed Silicon Targets in SF6 Atmospheres

The microstructures formed at the surface of silicon during pulsed-laser irradiation in SF6-rich atmospheres consist of an array of microholes surrounded by microcones. It is shown that there is a dynamic interplay between the formation of microholes and microcones. Fluorine produced by the laser-induced decomposition of SF6 is most likely responsible for the etching/ablation process. It is proposed that silicon-rich molecules and clusters that form in and are ejected from the continually deepening microholes sustain the axial and lateral growth of the microcones. The laser-melted layer at the tip and sides of the cones efficiently collects the silicon-rich products formed upon ablation. The total and partial pressures of SF6 in the chamber play a major role in cone development, a clear indication that it is the laser-generated plasma that controls the growth of these cones.

Excimer Laser-Induced Decomposition of Aluminum Nitride

A thin film of aluminum, detected by x-ray photoelectron spectroscopy, is left at the surface of aluminum nitride (AIN) substrates exposed to a high intensity excimer laser beam of UV radiation. Due to the presence of this film, there is a decrease in the surface resistivity of the substrate with increasing number of laser pulses. In addition, line profilometry shows a decrease of the surface roughness with the number of pulses. The thermal decomposition of AIN is assumed to take place in two stages. In the first, liquid aluminum is produced together with the evolution of gaseous nitrogen, and in the second, aluminum evaporates. Using a computer model to simulate the laser heating cycle, it is shown that the thickness of the aluminum film saturates at a given laser energy density. The saturation thickness is a strong function of the substrate absorption and reflectivity and, therefore, of the laser light frequency. The influence of the substrate roughness on the electrical resistivity of the aluminum film is discussed. The application of this process to direct laser writing in high density hybrid circuits is illustrated. During hole drilling by excimer laser, a thin aluminum film is continuously produced at the hole walls. This process can also be employed for substrate planarization.

Surface micro-structuring of silicon by excimer-laser irradiation in reactive atmospheres

Abstract The formation mechanisms of cones and columns by pulsed-laser irradiation in reactive atmospheres were studied using scanning electron microscopy and profilometry. Deep etching takes place in SF 6 - and O 2 - rich atmospheres and consequently, silicon-containing molecules and clusters are released. Transport of silicon from the etched/ablated regions to the tip of columns and cones and to the side of the cones is required because both structures, columns and cones, protrude above the initial surface. The laser-induced micro-structure is influenced not only by the nature but also by the partial pressure of the reactive gas in the atmosphere. Irradiation in Ar following cone formation in SF 6 produced no additional growth but rather melting and resolidification. Subsequent irradiation using again a SF 6 atmosphere lead to cone restructuring and growth resumption. Thus the effects of etching plus re-deposition that produce column/cone formation and growth are clearly separated from the effects of just melting. On the other hand, irradiation continued in air after first performed in SF 6 resulted in: (a) an intense etching of the cones and a tendency to transform them into columns; (b) growth of new columns on top of the existing cones and (c) filamentary nano-structures coating the sides of the columns and cones.

Laser ablation and column formation in silicon under oxygen-rich atmospheres

The microstructure formed at the surface of silicon by cumulative pulsed-laser irradiation in oxygen-rich atmospheres consists of an array of microcolumns surrounded by microcanyons and microholes. Formation of SiOx at the exposed surface of silicon is most likely responsible for the occurrence of etching/ablation that causes the continuous deepening of canyons and holes. The growth mechanism of columns that is supported by the experimental evidence presented here is a process in which the columns are fed at their tips by the silicon-rich ablation plasma produced during pulsed-laser irradiation.

Laser-induced microstructural changes and decomposition of aluminum nitride

The microstructural changes induced by pulsed laser irradiation in the surface layer of AlN and the initial stage of electroless copper deposition in laser processe specimens have been investigated using transmission electron microscopy (TEM). It was found that a dislocation microstructure is generated by laser processing at laser energy densities of 1.5 J/cm 2 or higher. A very sharp change in the dislocation microstructure was seen at a depth of 0.2 to 0.3 μm from the free surface. The dislocation Burgers vector is 〈100〉 and the slip plane is {001}, in agreement with previous reports. AlN was melted and resolidified homo-epitactically from the solid substrate forming a mosaic microstructure with very fine cells having a misorientation of up to 15°. Patches of metallic aluminum were found at the surface of all the specimens irradiated at a laser energy density of 1.5 J/cm 2 or higher. Very fine particles of AlN, 20 to 50 nm in diameter, were randomly distributed inside the patches. Immersion of these specimens in an electroless copper bath showed that the electroless solution preferentially etched away aluminum at the Al-AlN interface. At the same time copper islands were deposited in cavities left by AlN particles as well as at the interface with the underlying substrate. These regions are the seeds for further electroless deposition. The TEM observations of laser-induced microstructural changes reported in this paper help to unravel further the mechanisms of adhesion enhancement and surface activation by pulsed laser irradiation.

ELECTROLESS COPPER FILMS DEPOSITED ONTO LASER-ACTIVATED ALUMINUM NITRIDE AND ALUMINA

Metallization of ceramic substrates by laser activation and subsequent electroless deposition has been demonstrated recently in aluminum nitride and alumina. However, the bond strength between the electroless copper and the ceramic substrate is weak (less than 14 MPa). Low temperature annealing of electroless copper films deposited on substrates activated at low laser energies strongly increases the adhesion strength. The effectiveness of the annealing for improving the metal-ceramic bonding is dependent upon the laser treatment performed on the substrate prior to deposition. Faster deposition kinetics are obtained for both substrates by increasing the laser energy density. On the other hand, an increase in the laser energy density leads to poor adhesion strengths. The dislocation microstructure produced during laser irradiation in aluminum nitride is analyzed as a possible cause of laser activation. Free aluminum produced by laser irradiation of aluminum nitride and of alumina is discussed as another factor of laser activation. The chemical and microstructural changes taking place in the near-surface region as a consequence of laser-induced processes are correlated with adhesion enhancement promoted by the annealing treatment.

γ‐Al2O3 formation from pulsed‐laser irradiated sapphire

High‐resolution transmission electron microscopy was used to examine the effect of laser irradiation of sapphire (single crystalline α‐Al2O3). It was observed that laser‐melted material resolidified as γ‐Al2O3 epitactically grown on the (0001) plane of α‐Al2O3 having an orientation relationship (0001)α//(111)γ and [0110]α//[110]γ. The interface between unmelted α‐Al2O3 and resolidified γ‐Al2O3 is atomically flat with steps of one to a few close‐packed oxygen layers. The high thermal gradient in the α‐Al2O3 did not produce any interfacial dislocations. However, pronounced lattice distortions existed in the resolidified γ‐Al2O3. It is argued that the very rapid cooling rate present during solidification prevents the ordering of Al atoms that is required for the formation of the stable α‐Al2O3 phase.