Pulsed Laser Deposition (PLD) is an ideal technique to be used for combinatorial approaches. By simply changing the
deposition targets one can obtain alternating layers with different periodicities both vertically and laterally, along the
substrate surface. By changing the laser impact area location and the number of pulses on each target used for ablation,
one can grow films with a continuous variation of the chemical composition, which will be a function of the location on
the substrate. To illustrate the advantages and versatility of this Combinatorial PLD (C-PLD) technique, several
examples of films used in applications where more than one property should be tailored or optimized are presented.
Investigations of thermo-chemical stability, chemical bonding and crystalline structure of thin films of mixtures of HfO2
and Al2O3 that are used as high-k dielectric layers in advanced C-MOS transistors is the first example, followed by a
study of structural, mechanical, optical and electrical properties of mixtures of indium tin oxide and doped or pure zinc
oxide that are used as transparent and conductive layers. The third example is from the deposition of multilayers of ZrC
and ZrN with variable thicknesses to obtain hard coatings.
Thin Y2O3 films were directly grown on (100) Si substrates by the pulsed laser deposition technique. It has been found by high resolution cross-section transmission electron microscopy, x-ray reflectometry and x-ray photoelectron spectroscopy (XPS) that at the interface between Si and the grown layer, an interfacial layer always formed. Depth-profiling and angle-resolved XPS investigations showed that this layer consists of a mixture of substoichiometric SiOx(x<2) and the deposited Y2O3 layer, without forming an yttrium silicate. The thickness of this interfacial layer depended on the oxygen pressure and temperature used during the deposition. The main oxygen source for its formation is the physiosorbed oxygen which is trapped inside the grown layer during the laser ablation process. When the thickness of this low-k SiOx was reduced by decreasing the oxygen pressure during laser ablation below the optimum value, a marked degradation of the electrical properties of the structure was noticed.
The surface morphology of single crystal (100) Si wafers irradiated by 266 nm and 1064 nm laser pulses emitted by a solid state Nd:YAG laser has been investigated. The morphology of the bottom of craters remained flat and almost featureless after 266 nm single or multipulse laser irradiation up to the maximum fluence of 18 J/cm2 used in this study. The rims of the craters showed signs of radial liquid flow but it was apparent that the vaporization process was confined to the surface region. A different morphology was observed on the bottom of the craters formed by the 1064 nm wavelength laser pulses. Because this wavelength is absorbed in volume, (alpha) <104cm-1, a rather thick liquid Si pool formed at the surface. For laser fluences higher than 3-5 J/cm2 evidence of boiling sites were observed on the bottom of the crater, especially for multipulse irradiation. An evolution of surface morphology, from waves towards deep cavity was observed with the increase of pulse number. By analyzing the cavity formation mechanisms, their density and shape, we suggest that they were induced by heterogeneous boiling and not homogeneous boiling.
Indium tin oxide (ITO) films were grown on (100) Si and Corning glass substrates by an in situ ultraviolet-assisted pulsed laser deposition (UVPLD) technique. The most important deposition parameter for the growth of high transparent and conductive ITO films was found to be the oxygen pressure used during the deposition. Films grown under low oxygen pressure were brown and exhibited low optical transmittance and high resistivity. For a target- substrate distance of 10.5 cm, which ensured a uniform film across 2.5 cm, the optimum oxygen pressure to obtain the lowest electrical resistivity was found to be around 10 mTorr. For higher oxygen pressures, the optical transmittance was a little bit higher but a significant increase of the electrical resistivity was noticed. X-ray photoelectron spectroscopy showed that ITO films grown in the 5-20 mTorr range were fully oxidized, without any measurable metallic content. Films grown at room temperature were amorphous regardless of the oxygen pressure used.
The crystallinity, stoichiometry and optical and electrical properties of thin Y2O3, ZnO and Ba0.5Sr0.5TiO3 films grown using an in situ ultraviolet (UV)- assisted pulsed laser deposition (UVPLD) technique have been studied. With respect to films grown by conventional PLD under similar conditions but without UV illumination, the UVPLD grown films exhibited better quality, especially for lower substrate temperatures. They also contained less physisorbed oxygen than the conventional PLD grown layers. These improvements can be explained by the action of several factors. Firstly, deep UV photons and ozone ensure a better in situ cleaning of the substrate. Secondly, the presence during the ablation-growth process of more reactive gaseous species like ozone and atomic oxygen formed by photodissociation of molecular O2 promotes the oxygenation of the films. Thirdly, absorption of UV photons by adatoms could result in an increased surface mobility. All these factors have a beneficial effect upon crystalline growth, especially for moderate substrate temperatures, where the thermal energy available for the process is rather limited.
The role of the sub-surface explosive boiling mechanism for droplet formation during the pulsed laser deposition of thin films has been examined. For photons with an energy lower than the target optical band-gap, hv<Eg, a highly perturbed target surface morphology exhibiting micrometer sized round- shaped cavities always formed, whereas for hv>Eg the surface was much less perturbed. Depth-profile estimations of the temperature profiles inside the laser irradiated targets were also quite different for these two cases. For low optical absorption coefficient (OAC) values, (alpha) < 5 X 104 cm-1, a thick layer of liquid formed whose maximum temperature was located at some point below the surface. Simultaneous to the cavity formation, the droplet density on the surface of the grown films was found to increase by orders of magnitude, indicating a clear connection between the target OAC, the temperature profile, the presence of the cavities and a high droplet density. The decrease of the OAC value during prolonged laser ablation could also explain the gradual increase of the droplet density observed when growing La0.5Sr0.5CoO3 films. Based on these results, we suggest that an important fraction of the droplets on the surface of films grown under laser irradiation conditions where the OAC is smaller than 5 X 104 cm-1 is caused by an explosive sub-surface boiling mechanism.
Thin films of hydroxyapatite (HAp) have been grown on Si, quartz, Ti, and Ge substrates by the pulsed laser deposition (PLD) method employing a KrF excimer laser (wavelength (lambda) equals 248 nm, pulsed duration (tau) FWHM equals 20 ns). The influence of the laser deposition parameters on the properties of the grown layers was investigated in order to optimize the Ca/P ratio and the crystalline structure. It was found that the optimum conditions for preserving the Ca/P ratio i.e. high oxygen pressures and low substrate temperatures do not coincide with those for obtaining adherent and crystalline layers i.e. low oxygen pressures and high substrate temperatures. For films deposited onto Ti substrates it was also found that high substrate temperatures promote the diffusion of Ti through the depositing film up to the surface where it gets oxidized. Further investigations are required before high quality HAp-coated Ti implants by PLD can be obtained.
Laser doping method has an attractive set of important advantages, very useful for microelectronics applications. Using this method we have obtained shallow and heavily doped p+ - n junctions, ohmic contacts, and interconnection lines. We have extended our laser method for Ti in-diffusion into LiNbO3, the common technique for optical waveguides fabrication.
We synthesized stable and adherent TiSi2 layers by one-step excimer-laser irradiation of Ti films deposited on Si wafers. By multipulse laser irradiation a regime was got when Ti covering Si windows, opened into SiO2 layer grown on the Si substrate, was completely reacted, while the neighbour Ti covering the SiO2 interlayer was entirely expelled. This means that a self-aligned silicide layer (salicide) was formed. Moreover, it is shown that by mulitpulse irradiation of Ti/Si samples in nitrogen atmosphere, it is possible to obtain multilevel TiN/TiSi2/Si structures with titanium silicide formed at the Ti-Si interface and titanium nitride formed at the Ti surface.
We present (report) a new type of titanium suicide self-aligned synthesis based upon excimer laser irradiation. This new process relaxes the requirements for ambient atmosphere control and suppresses the unwanted lateral growth of the silicide.
Thermal treatments a key process for IC technologies face large difficulties in case of germanium due to the poor chemical stability of germanium oxides i. e. GeO and Ge02. Laser processing allows for a drastically reduction of the interaction time highly suitable for Ge processing. We report two applications: i) the laser synthesis of thin and stable germanium oxynitrides films and ii) the laser doping of germanium with boron to the view of obtaining very shallow junctions.
We report the synthesis of surface nitrides by multipulse XeC1 exciTaer laser irradiation in ambient NH3 atmosphere of metal (Ti Mo) and semiconductor (Si Ge) samples. The amount of nitride formed was shown to depend under various extents on the kind of sample the incident laser fluence and the number of subsequent laser pulses. The nitridation process is very sensitive to the oxygen presence. It was evidenced that only traces of oxygen were sufficient for promoting the formation of oxides and oxynitrides. 1.
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