Bernd Keiper

MICROSTRUCTURE AND MECHANICAL PROPERTIES OF PULSED LASER DEPOSITED BORON NITRIDE FILMS

Abstract Boron nitride films were prepared by pulsed laser ablation from a boron nitride target using a KrF-excimer laser, where the growing films were deposited in nitrogen atmosphere or bombarded by a nitrogen/argon ion beam. Films deposited without or at weak ion bombardment (such films will be called l-BN in this paper) are hexagonal with amorphous to turbostratic microstructure (l-BN) and show high adhesive strength to silicon and stainless steel substrates. By using them as intermediate layers, the adhesion of pure cubic boron nitride films (c-BN) can significantly be improved. l-BN films and l-BN/h-BN/c-BN layer systems have been investigated by in-situ ellipsometry, infrared spectroscopy and cross-section and plan-view high-resolution transmission electron microscopy, including diffraction. The mechanical properties, i.e. stress and hardness, of these films and layer systems are presented. l-BN films deposited at higher laser energy densities have compressive stresses as high as 11.5xa0GPa. Films deposited at lower laser energy densities have stresses in the range of 4.7 to 1.3xa0GPa and a Vickers hardness in the range of 18.6 to 7.5xa0GPa depending on substrate temperature and ion bombardment. The compressive stresses of 400xa0nm thick adherent c-BN films were estimated to be 4.5xa0GPa.

Pulsed laser deposition and modification of diamondlike carbon films

Abstract Diamondlike carbon (DLC) films were prepared by pulsed laser ablation from a polycrystalline graphite target using an KrF-excimer laser beam, where the pulse energy density was varied in the range of 2 to 8 J/cm 2 . The hydrogen-free films grown at high pulse energy densities above 6 J/cm 2 showed optical bandgaps of 1.8 eV and refractive indices of 2.60 at 678.3 nm wavelength having similar dispersion like that of diamond. The films were completely amorphous and preferentially sp 3 -bonded. The maximum growth rate was as high as 120 nm/min using 50 Hz laser pulse repetition rate. Hydrogen-free films grown at relatively low pulse energy density of 3.3 J/cm 2 showed optical bandgaps of 0.8 eV and refractive indices of 2.8 at 678.3 nm wavelength, the dispersion more resembling that of graphite. The films were completely amorphous and preferentially sp 2 -bonded. Furthermore, the influence of argon and nitrogen ion bombardment as well as excimer laser irradiation of the growing films was studied.

Influence of ion bombardment on the refractive index of laser pulse deposited oxide films

Abstract HfO2, ZrO2 and Y2O3 films for optical applications were prepared by ion assisted laser pulse deposition. The influence of oxygen ion bombardment of growing films on the refractive index was investigated in the ion energy range of 150–600 eV and in the ion current density range of 50–250 μA/cm2. Films deposited without additional ion bombardment or using low ion energy up to 150 eV or low ion current density up to 60 μA/cm2 possess a high bulk-like refractive index, whereas above these thresholds the refractive index steadily decreases. In the case of hafnia, for example, the highest refractive index, obtained for stoichiometrical films with low absorption, amounted to 2.15 at 600 nm wavelength. Due to relatively strong ion bombardment it decreased to 1.80. This behaviour is a result of ion induced modifications of microstructure. While films with high refractive index were of amorphous structure and had a high packing density with low porosity, increasing ion bombardment of the growing films led to increasing crystallization within the films and, finally, to polycrystalline films combined with increasing columnar film growth and rougher surfaces. Larger voids between the columns result in lower packing density and, therefore, lower refractive index. Using appropriate deposition parameters, oxide films with low absorption coefficients and high laser damage thresholds at 1.06 μm wavelength can be prepared.

Pulsed laser deposition and modification of cubic boron nitride films

Abstract Pulsed laser ablation using an excimer laser of 248-nm wavelength was applied to prepare boron nitride films, where the ablation from elemental boron targets as well as boron nitride was studied. The growing films were bombarded by a nitrogen or a nitrogen/argon ion beam to obtain stoichiometric films and to investigate ion induced modifications of structure and properties. Moreover, in order to study the influence of ultraviolet photon bombardment on the structure of the growing boron nitride films, a certain region of each film was irradiated using an excimer laser beam of 248-nm wavelength. The films have been investigated by in-situ ellipsometry, infrared spectroscopy, transmission electron microscopy including diffraction measurements and electron-energy-loss spectroscopy. The results show that the boron nitride films have either predominantly hexagonal or predominantly cubic structure dependent on the laser and ion beam parameters and the substrate temperature. At the optimum laser and ion beam parameters, nearly pure cubic films could be prepared even at a relatively low substrate temperature of 200°C. However, those films prepared on silicon substrates exhibit the typical layered structure with a 5 to 10-nm thick amorphous layer formed initially on the substrate, a 10 to 30-nm thick layer of highly oriented h-BN and the c-BN layer. The c-BN layer consists of nanocrystallites up to 30 nm in diameter. Additional excimer laser irradiation of the growing films resulted in distinct modifications of the microstructure of the BN films. Using laser fluences above 200 mJ/cm2, the laser irradiation leads to the formation of turbostratic h-BN even though the unirradiated film regions of the same sample show the cubic structure. The most interesting range of laser fluence lies between 100 and 160 mJ/cm2. Electron microscopic observations show that in this range, the mean diameter of crystallite in the excimer laser irradiated regions increased by a factor of 2 in comparison with unirradiated regions of the same sample.

Properties of pulsed laser deposited boron nitride films

Pulsed laser ablation using an excimer laser of 248 nm wavelength was applied to prepare boron nitride films, where the ablation from boron nitride as well as elemental boron targets was studied. The growing films were bombarded by a nitrogen or a nitrogen/argon ion beam to obtain stoichiometric films and to investigate ion induced modifications of structure and properties. Depending on deposition parameters the refractive index of the boron nitride films in the visible wavelength range varies between 2.5 and 1.7 and the optical energy band gap between 2.0 and 4.5 eV. Most films were found to be completely sp2-bonded and amorphous to nanocrystalline with a turbostratic microstructure. Only boron nitride films deposited from the boron target and bombarded with nitrogen/argon ions show also an absorption peak in the infrared spectrum that indicates the presence of the cubic phase.

Deposition of optical coatings by pulsed laser ablation

Abstract Hafnia, zirconia and yttria films for optical applications were prepared by pulsed laser deposition using an excimer laser at 248 nm wavelength. The growing films were bombarded by oxygen ions. Films were investigated with regard to refractive index, absorption and laser damage thresholds. Based on the findings that the refractive index varies with ion bombardment and growth rate it is shown that multilayer systems with regard to refractive index consisting of only one material can be prepared. Such multilayers were investigated with regard to reflectivity and laterally resolved absorptivity.

Properties of pulsed laser deposited optical coatings

Hafnia and yttria films for optical applications were prepared by pulsed laser ablation with oxygen 1on bombardment of the growing films. The influence of the laser and ion beam parameters on the refractive index and microstrucmre of the films was investigated. The optical quality of the films with respect to absorption and laser damage was characterized at the Nd:YAG-laser wavelength by measuring the laterally resolved absorptivity and the laser damage thresholds. Both hafnia and yttria films prepared at low ion energy and intensity (150 eV, 50 /xA/cm 2) were amorphous and had a high bulk-like refractive index and packing density. At high ion energy and intensity (700 eV. 400 /xA/cm 2) the films became polycrystalline with high refractive index and packing density at relatively high growth rates above 10 to 20 nm/min or relatively low refractive index and packing density at low growth rates. Films with high laser damage threshold at 1.06 /xm wavelength could only be prepared with strong oxygen bombardment. Highly reflective multilayer systems of only one material with alternately high and low refractive index were prepared by varying the parameters of oxygen ion bombardment during deposition. Their reflectivity and laser damage threshold at 1.06/xm as well as their microstructure were investigated.

Properties of laser-ablated amorphous carbon films

Excimer laser ablation of a polycrystalline graphite target was used to prepare amorphous carbon films. Optical properties of the films were investigated in dependence of the laser power density and the hydrogen supply during deposition. The hydrogen content of the films was 0.7 to 37.5 at % in dependence of the deposition conditions. An optical bandgap up to 1.6 eV was found for films with low hydrogen content. Applying an additional hydrogen plasma during deposition the optical bandgap increased up to 1.95 eV. The laser power density was varied between 1.5 and 3.4 X 107 W/cm2. Generally, the lower power densities near the ablation threshold lead to larger optical bandgaps. An additional excimer laser irradiation of the growing carbon films with a laser power density up to 106 W/cm2 leads to graphitization within the otherwise amorphous films. An increase of the laser power density to 2 X 106 W/cm2 induce the formation of microcrystallites. Those microcrystallites could be identified as cubic diamond by means of transmission electron microscopy (TEM) investigations.

Influence of LN2 substrate cooling on optical properties of laser-pulse-deposited oxide films

Abstract In this paper we will present the results of investigations of the influence of substrate cooling by liquid nitrogen during film deposition on the optical properties of oxide films prepared by means of laser pulse deposition. A CO2-TEA laser at 10.6 μm wavelength (pulse duration 200 ns, repetition rate 40 Hz, laser fluence 5 × 107 W/cm2, monomode) and an excimer laser at 248 nm wavelength (pulse duration 20 ns, repetition rate 200 Hz, laser fluence 5 × 107 W/cm2) were used for the evaporation and, respectively, the ablation of sintered oxide targets. Moreover, the growing films were bombarded by oxygen ions of varying energy and current density. As will be shown, the variation of refractive index with ion bombardment changed only imperceptibly due to substrate cooling, i.e. films deposited at relatively low ion energy and current density possess a high bulk-like refractive index and films deposited at relatively high ion energy and current density possess a rather low refractive index. We could observe, however, that, in comparison with uncooled substrates, cooling of the substrates by liquid nitrogen results in a marked decrease in absorption together with a marked increase of the laser damage thresholds of the films at 1.06 μm wavelength. For example, the laser damage threshold of hafnia films increased by a factor of 1.5.

Absorption and laser damage threshold studies of ion assisted pulsed laser deposited oxide films

The laterally resolved absorption and the laser damage thresholds at 1.06 micrometers wavelength of yttria and hafnia films prepared by pulsed laser deposition with oxygen ion bombardment of the growing films were investigated. Depending on the laser and ion beam parameters films with low average absorption can be prepared by that method. Consequently, high predominantly intrinsic absorption induced laser damage thresholds D1 comparable with those of continuously evaporated films can be prepared. The more defect induced laser damage thresholds D0, however, are largely determined by a certain number of micron- sized particulates embedded inside the films. Their number can be reduced by optimizing the laser pulse power density and the laser beam cross-section on the target, while their influence on laser damage thresholds can be reduced by increasing the ratio of oxygen ion bombardment to growth rate.