Zsolt Tóth

Novel amplitude and frequency demodulation algorithm for a virtual dynamic atomic force microscope

Frequency-modulated atomic force microscopy (FM-AFM; also called non-contact atomic force microscopy) is the prevailing operation mode in (sub-)atomic resolution vacuum applications. A major obstacle that prohibits a wider application range is the low frame capture rate. The speed of FM-AFM is limited by the low bandwidth of the automatic gain control (AGC) and frequency demodulation loops. In this work we describe a novel algorithm that can be used to overcome these weaknesses. We analysed the settling times of the proposed loops and that of the complete system, and we found that an approximately 70-fold improvement can be achieved over the existing real and virtual atomic force microscopes. We show that proportional-integral-differential controllers perform better in the frequency demodulation loop than conventional proportional-integral controllers. We demonstrate that the signal to noise ratio of the proposed system is 5.7 × 10(-5), which agrees with that of the conventional systems; thus, the new algorithm would improve the performance of FM-AFMs without compromising the resolution.

Pulsed laser ablation mechanisms of thin metal films

The ablation of thin films by single laser pulse is a well known technique with widespread industrial applications. Ablation occurs in a well defined power density region if a supported thin film is illuminated by a single laser pulse. In the literature there are a number of theoretical description of ablation, but a very few based on in-situ experiments. In our study we have directly visualized the ablation processes with fast photography based on application of dye laser probe pulses. The ablation of chromium and tungsten layers supported onto glass substrates with pulses of ArF excimer laser was investigated. The ablated area was illuminated by a delayed short pulse of a fluorescein dye laser or a Rhodamine6G dye laser. Snapshots of initial phase of ablation and the forthcoming material transport were recorded by an optical system and a video camera. Blowing-off mechanisms and thermo-mechanical mechanisms are considered to take place during ablation. Pressures formed during laser ablation were calculated and compared with experimental data. It was found that thermo- desorption of gas adsorbed on to the substrate surface, substrate materials evaporation and film exfoliation by its longitudinal thermal enlargement may be acting during laser ablation of thin 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.

Pulsed laser deposition from solid and molten metals

Can particulates formation in general be avoided by using liquid targets, or only can particulates deposition be suppressed by choosing appropriate experimental circumstances? In order to answer this question, which is of general importance in pulsed laser deposition, we deposited indium, tin and bismuth films from the respective metal targets both in solid and liquid form, in vacuum. The substrates were held at room-temperature, in order to collect and preserve the droplets if formed. Ablating molten targets, the appearance of droplets of diameter exceeding microns can completely be eliminated. The particulates number density of indium and tin films decreased by two orders of magnitude as compared to the solid-target case. However, even in these cases, particulates formation could not be totally eliminated. The thickness distribution of the film material becomes broader when deposited from molten target.

Surface patterning by pulsed-laser-induced transfer of metals and compounds

Besults of a systematic study on Q-switched nthy laser induced rrrn2 area transfer of supported titanium and chranium thin films and Ge/Se multilayer structures are reported. The appearance of the prints is governed by film-support adhesion and source-target spacing. Best quality prints are produced by ablating well adhering ntal films in close proximity ( spacing < 15 pm) to the target to be patterned. Transfer fran stacked elenntaxy layers as a source offers a unique possibility of depositing acinpound films by mixing the constituents and transferring the material onto the target substrate in a single step.

Effect of step function-like perturbation on intermittent contact mode sensors: a response analysis

Abstract The dynamics of the intermittent contact mode (ICM) probe was investigated. In the experimental study we applied a step function signal to the Z piezo drive and recorded the amplitude signal of the probe while the probe was engaged with the surface. Transient overshoots appear at the edges of the steps. These transients are absent from the control contact force measurements, that is, they are proper to the ICM operation. The phenomenon was investigated by numerical calculations, focused on the effect of change of the drive frequency and the quality factor. We concluded, that the low value of the quality factor results in small transients and short settling time, which are necessary for fast atomic force microscopic operation. Simultaneously, the interaction force increases. Our calculations indicate that the tip–sample force can be lowered by setting the drive frequency slightly below the resonance.

Pulsed Laser Synthesis And Printing Of Compound Semiconductors

A systematic study on ruby laser processing of supported Ge/Se bilayer structures is reported. Depending on the sequence of the initial layers and processing parameters compound synthesis, total or partial ablation of one of the constituents or the compound formed and simultaneous transfer of the ablated material onto a separate substrate in close proximity is possible with one single laser pulse. The results establish a novel single step technique for local deposition of compound films with lateral dimensions down to the micrometer range from a multilayer structure on a transparent support as a source.

Formation of complex tungsten-silica microstructures by Ar+ laser processing

Angewandte Physik, Johannes Kepler Universitat Linz, A-4040 Linz, Austria The possibility for localized, spatially separated laser-induced deposition and etching of tungsten from pure WF 6 gas is demonstrated. By employing a fused silica substrate covered with a tungsten layer, a threefold process is induced; deposition of tungsten in the center of the laser beam, etching of the tungsten-layer outside of the beam center and etching the excavated silica by fluorine radicals. This multiple process results in an effective drilling. The deposition of a conducting tungsten kernel inside a high aspect ratio hole, formed in the insulating silica, suggests the applicability to a single-step production of field emitter units.

Analysis of thickness profiles of pulsed laser deposited metal films

Abstract Practically all theoretical approaches to pulsed laser deposition start on a condition which is extremely hard to realize in practice: that the target surface is smooth and plane. When using liquid (molten) targets, the problem of surface deterioration upon repetitive ablation can completely be solved, allowing for fair comparison of experiment and theory. In this paper measured thickness profiles of metal films deposited in vacuum from molten In, Sn, Bi and Sn–Bi alloy targets are compared with calculated distribution functions. The strictly symmetrical thickness profiles of tin and indium films, derived from two-dimensional optical density maps and Rutherford backscattering data are analysed in terms of Lorentzian-like functions, originating from the so-called shifted Maxwellian velocity distribution. The bismuth profiles show a characteristic deviation from this shape.

Surface patterning by an improved laser-induced forward transfer technique

A simple and inexpensive single-step technique for surface patterning in the micrometers regime is presented. As a result of a systematic study on laser-induced ablation and transfer of tungsten thin films it is shown that deposition of well adhering micrometer sized patterns of 100% coverage preserving the shape and dimensions of the laser processed area can be attained by single pulses of peak power up to 100 mW and 100 microsecond(s) - 1 ms duration from a diode laser pumped YAG laser.