Z. Kántor

Deposition of micrometer-sized tungsten patterns by laser transfer technique

A simple single‐step technique for surface patterning is presented. It is shown that well‐adhering micrometer‐sized patterns of 100% coverage preserving the shape and dimensions of the ablated area can be deposited by ablating and transferring tungsten thin films in the form of single solid pieces using single pulses of peak power up to 100 mW and 100 μs–1 ms duration from a diode‐pumped YAG laser.

Laser induced forward transfer: The effect of support-film interface and film-to-substrate distance on transfer

A comparative study on metal pattern deposition of mm2-area by ablating chromium and titanium thin films from an optically transparent support and transferring the ablated material onto another substrate in close proximity with a single laser pulse (LIFT) is reported. The role of support-film interface and film-to-substrate distance in determining both ablation and transfer is discussed. The sequence of events as a function of processing fluence is interpreted by comparing experimental data with calculated temperature distributions. In the case of poorly adhering films the transfer yield is independent of film-to-substrate distance between 0 and 60 μm throughout the fluence range studied. The transmittance of the ablated areas of well adhering films decreases and that of the corresponding prints increases with increasing distance as evaporation becomes dominant.

Metal pattern deposition by laser-induced forward transfer

Abstract Results of a systematic study on laser-induced transfer of metal patterns are summarized. The pulse width of the lasers used in the experiments was scaled from a few nanoseconds to one millisecond to discover the different time-dependent processes determining ablation and transfer of thin films of a variety of metals. The physical events were followed by optical and electron microscopy and static and time-resolved optical measurements, as well as ultrafast photography. The main conclusion is that the adhesive properties of the interface between the metal film and the support, and the thermophysical characteristics of both the support and target substrates determine the yield of the transfer. Optimum parameter sets ensuring deposition of well adhering micrometer-sized patterns faithfully reproducing the illuminating area were determined. The technological importance of this novel technique is pointed out.

Atypical characteristics of KrF excimer laser ablation of indium-tin oxide films

Abstract Indium-tin oxide films possess ablation characteristics which are a function of the film thickness. For 70 and 160 nm thicknesses, low-fluence single pulses are sufficient to remove from the support the solid phase oxide film over the whole illuminated area. Processing under such conditions offers a rather convenient means for large-area “clean” surface patterning, which is, however, limited at high fluences by the onset of melting. At fluences higher than this onset, layer-by-layer ablation via evaporation sets in. For thicker films, only ablation via evaporation is possible. These experimental findings are interpreted in the framework of a thermal model which is supported by appropriate numerical calculations of the temperature distribution in these films.

Dynamics of excimer laser ablation of thin tungsten films monitored by ultrafast photography

The time course of laser light induced transport of tungsten films from a glass support is followed by ultrafast photography using delayed dye laser pulses. The photographs provide unambiguous evidence that the material transport in the 40–200 mJ/cm2 intensity domain takes place via removal of solid pieces from the film material. These results are consistent with heat flow calculations which predict the overall melting of the metal layer above 380 mJ/cm2. The series of photographs presented give detailed insight into the melting process and have revealed an unexpected in-flight phase separation of solid fracture pieces and molten droplets throughout the 200–900 mJ/cm2 domain. The faster propagating molten droplets form a condensed halo in front of the solid pieces, thereby providing an efficient shield between the processing laser light and the solid phase.

Micro-RBS characterisation of the chemical composition and particulate deposition on pulsed laser deposited Si1−xGex thin films

The formation and deposition of particulates by pulsed laser deposition of Si1−xGex semiconductor alloy thin films are discussed. Using Rutherford backscattering spectrometry with micrometer lateral resolution (micro-RBS) the film composition was measured with high accuracy, even in the presence of particulates with a high areal density of 20,000–30,000 particulates per mm2. We show that on impact of a particulate, the part of the thin film which is already deposited probably melts and its Ge content segregates to the surface.

Structure and composition of carbon-nitride films grown by sub-ps PLD

CNx (0.01<x<0.20) films have been deposited by ablating a graphite target in N2 atmosphere with a hybrid dye/excimer laser system delivering pulses of 500 fs duration at 248 nm. Changes in surface composition and morphology of the as grown films, as a function of N2 pressure (between 0.3 and 60 Pa), laser pulse energy (maximum 10 mJ), and spot area are reported, by means of X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM), respectively. In the pressure domain investigated, the nitrogen content increases as a power function. While both pulse energy and spot area influence the chemical composition of the films, the same fluence achieved by using different spot sizes results in films of different N content. Deposition of micrometer sized particulates is apparently suppressed, while sub-micron features of typically circular shape are present over the smooth film surface.

Low-fluence excimer laser irradiation-induced defect formation in indium–tin oxide films

Abstract DC sputtered indium-tin oxide (ITO) films of 500 nm thickness are irradiated with single pulses of fluences between 190 and 510 mJ/cm 2 from a KrF excimer laser. The irradiation induced changes in optical spectra are consistently described through the behaviour of four (Gaussian-like) absorption bands at 0.7, 1.0, 1.6 and 2.6 eV. Being absent in the original films and emerging at fluences exceeding 300 mJ/cm 2 , the 2.6 eV contribution is most characteristic to excimer laser processing. X-ray photoelectron spectroscopic analysis suggests that the irradiation-induced changes should be associated with oxygen displacement within the atomic network rather than surface reduction via oxygen removal. Thermal model calculations reveal that the principal effect of single pulse processing in this fluence domain is deep melting of the films. Defects created during molten phase resolidification are assumed to be responsible for the irradiation-induced changes in the short range chemical structure of the films.

Excimer laser ablation of molten metals as followed by ultrafast photography

Abstract Molten Sn and Bi are ablated in vacuum by an ArF excimer laser. Pictures of the surface and the ablated material are taken by ultrafast photography, with temporal resolution of 1 ns using delayed dye laser pulses. The series of snapshots covering the 0 ns–200 μs time domain contain information on the ablated plume, the development of waves on the target surface, and the initial phase of droplet formation. The velocity of the front of the ablated plume is approximately 6 km/s for both Sn and Bi at 5.5 J/cm2. While on the molten Sn surface only wave generation is observed with practically no droplet emission, the Bi surface emits a remarkable amount of material in the form of droplets originating from liquid jets. The speed of these droplets is two orders of magnitude smaller than that of the plume front. The relaxation of the whole perturbed melt pool lasts second(s) after ablation. By decreasing the fluence below 2.5 J/cm2 the Bi droplet formation can also be suppressed.

Excimer laser irradiation induced formation of diamond-like carbon layer on graphite

Abstract Highly oriented pyrolytic graphite (HOPG) was irradiated by an ArF excimer (λ=193 nm) laser above the ablation threshold, at approx. 0.45 and 2 J/cm2. The surface morphology and the quality of the remaining material was investigated by atomic force microscopy (AFM) and area-selective Raman spectroscopy. At the lower fluence a material removal rate of several monolayers per laser pulse was detected, without changing the quality of the remaining material. Irradiation at the higher fluence resulted in ablation rates of the order of 10 nm/pulse and the formation of an approx. 300 nm thick diamond-like carbon (DLC) film with approx. 50% concentration of the sp3 hybrid-states of carbon. In the surroundings of the ablated hole a narrow ring of mechanically soft, nanocrystalline and turbostratic carbon was observed. Upon annealing the irradiated surfaces in air at 650°C for 30 min, the graphite structure of the laser-modified layer was perfectly recovered with the disappearance of the surrounding ring.