Multipulse Laser Irradiation Research Articles

Effect of Multipulse Waveform on Gains of Soft X-Ray Lines of Lithium-Like Aluminum Ions in Recombining Plasmas

Using the recombination plasma scheme, we investigated the amplified spontaneous emissions (ASE) from lithium-like aluminum plasmas produced by multipulse laser irradiation. Three types of multipulse waveforms of Nd-glass laser beam were treated. We measured the time-integrated intensities of the 3d–4f (15.5 nm) and the 3d–5f (10.6 nm) lines of lithium-like aluminum ions for 2.5-cm-long and 1.25-cm-long plasmas. The effective time-averaged gains estimated from these time-integrated intensities varied with the multipulse waveforms. Using a hydrodynamic simulation, we discussed the time history of plasma parameters in the recombining plasma. We concluded that the modifying multipulse waveform contributed to the effective time-averaged gain by maintaining the helium-like ion density until the gain was generated.

Characteristic features of the laser radiation–target interactions during reactive pulsed laser ablation of Si targets in ammonia

We performed electron microscopy studies of targets subjected to the multipulse laser irradiation in the regime characteristic of reactive pulsed laser deposition from bulk Si in low-pressure ammonia. Experimental evidence is provided concerning the expulsion of liquid droplets from the crater that forms on the target surface. In our opinion, the main mechanisms responsible for droplets spraying directly from the crater are in this case liquid phase expulsion, under the action of the recoil pressure of the ablated substance and hydrodynamic instabilities at the target surface. Cracks of various sizes and orientations have been observed beneath the crater surface. These cracks might cause an increase in the ablation and deposition rates with time. A qualitative theoretical model was developed to account for crack formation based on the peculiar behavior of Si, which has a smaller density in bulk than in liquid phase.

Theoretical modelling of phenomena in the pulsed-laser deposition process: Application to Ti targets ablation in low-pressure N2

We present an analysis describing the phenomena that are involved in the interaction process between high intensity UV laser radiation and a solid target in a low-pressure gas environment. The laser action upon the solid surface is described by solving the one-dimensional (1D) heat-conduction equation. The 1D hydrodynamic model is further applied to describe the interaction between laser radiation and the plasma expanding in front of the target. The laser radiation absorption in the vapor plasma is considered using a multicomponent fluid model. The transit of ablated material from target to collector is treated by a Monte Carlo method. The time and space distributions of the main plasma parameters (e.g., atom density and plasma electron temperature and density) are calculated. Finally, the thickness profile of a film deposited on a collector parallel to the target is predicted. For comparison with available experimental data, the calculations were carried out for the case of a Ti target submitted to a multipulse UV laser irradiation in low-pressure nitrogen, when depending on gas pressure, stoichiometric TiN was deposited or mixtures of amorphous and crystalline Ti, TiN, Ti2N, and even Ti oxides were obtained.

Preparation of molybdenum nitrides by laser-promoted nitridation reaction

New experimental results are reported on the preparation of molybdenum nitrides by multipulse laser irradiation of molybdenum foils in pure nitrogen atmosphere. In such laser treatment, the laser radiation activates nitrogen and melts the Mo surface, initiating a nitridation reaction in liquid phase. It is observed that there is a significant amount of nitrogen incorporated in the Mo lattice, and that nitrogen is well-distributed in the nitride layers. The nitride layers are found to contain γ-Mo 2N phase and have a microstructure much denser than the microstructure of the original material. Both nitrogen activation and surface melting greatly enhance the creation of Mo–N compound. Successive surface heating and melting and transient shock waves induced by laser pulses could account for the microstructure densification of the nitride layers.

Microstructural characterization of solid-phase crystallized amorphous silicon films recrystallized using an excimer laser

The laser recrystallization of amorphous-silicon films crystallized in a furnace is investigated for single and multipulse laser irradiation. The resulting microstructure is grouped into three regimes, based on their recrystallization mechanism. For the low energy irradiation regime, the grain size in unchanged, however many intragrain defects are removed, while the high energy regime results in homogeneous nucleation of the molten film. A third regime exists between these two, characterized by large lateral grain growth. Except for the low energy regime, we conclude that the results obtained are consistent with those found for the direct laser crystallization of amorphous silicon films.

The study of a crater forming on the surface of a Ti target submitted to multipulse excimer laser irradiation under low pressure N2

Abstract A Ti target was submitted to laser ablation in low ambient pressure N2. Electron microscopy examination of the cross-section of the crater zone forming on the Ti target, and XPS analyses, indicate that there is a small effect on the nitridation processes taking place on and in the vicinity of the target. The studies show a zone influenced by the multipulse laser treatment extending beneath the crater down to a depth of the same order of magnitude as the crater depth (i.e. ∼ 10 μm). In this zone, TiN could be identified as being present only in traces, while the whole zone exhibited a layer structure with differences in morphology and mechanical wear.

The study of a crater forming on the surface of a Ti target submitted to multipulse excimer laser irradiation under low pressure N2

The study of a crater forming on the surface of a Ti target submitted to multipulse excimer laser irradiation under low pressure N2

Synthesis of molybdenum nitride γ-Mo 2N by multipulse laser irradiation of molybdenum in nitrogen

The γ-Mo 2N phase has been synthesized by multipulse laser irradiation of molybdenum foils in pure nitrogen atmosphere. The optical plume emissions produced on the laser irradiation were identified with various atomic, ionic and molecular species present. The obtained molybdenum nitrides were analyzed and characterized by scanning electron microscopy, X-ray diffraction and Auger electron spectroscopy. The formation of the molybdenum nitrides results from a nitridation reaction through a molten phase, and it seems that nitrogen activation and Mo surface heating and melting by the laser irradiation are responsible for the efficient incorporation of nitrogen in the Mo lattice and the creation of MoN bonds. It is also found that laser heating and melting and laser-induced shock waves give rise to a densification of the nitride layer microstructure.

Modelling of self-limiting laser ablation of rough surfaces: application to the polishing of diamond films

Abstract We present a theoretical model for the interaction of excimer laser radiation with rough polycrystalline CVD diamond films, showing the influence of angle of incidence, irradiation intensity and number of laser pulses. A new regime of laser treatment — self-limiting laser ablation — was found, which allows faceted films to be smoothed without wasteful ablation of the bulk. Multipulse XeCl laser (308 nm) irradiation of our diamond films, grown on silicon wafers, confirms that such polishing is accomplished.

Sn-Sb-Se/Sb-Bi Bilayer Phase-Change Media for High-Density Write-Once Optical Recording

We have developed Sn-Sb-Se/Sb-Bi bi-layer phase-change media for high-density write-once optical disks. Crystallized marks as small as 0.5 µ m are stably recorded without initialization. These phase-change media, compared to ablative type write-once media, have high carrier-to-noise ratio, low jitter, and long life. Mark-edge recording with multi-pulse laser irradiation on the media provides a linear bit density of 0.48 µ m/ bit and track pitch of 1.2 µ m using a 690-nm-wavelength laser diode and 0.55 numerical aperture (NA) objective lens. This corresponds to 1.1 Gbit/in.2, or 14 GB formatted data capacity per both sides for a 12-inch-diameter disk with a zoned constant angular velocity (ZCAV) format.

Deposition of high quality TiN films by excimer laser ablation in reactive gas

A new laser method is proposed for the deposition of high purity, hard fcc TiN layers of unlimited thickness. The film thickness can be very finely controlled mainly through the intermediary of the number of applied laser pulses as the deposition rate is of only 0.02–0.05 nm/pulse. The ablation is promoted from a Ti target by high intensity multipulse excimer laser irradiation in a low pressure N2 ambient gas while the forming compound is collected on a Si single-crystalline wafer. The best results have been obtained for an ambient pressure of p=10–30 mTorr and a distance between the target and support of d=10 mm. It is shown that the formation of a liquid phase within the irradiated zone, maintained even after the end of a laser pulse, is the most important requisite for TiN formation. TiN is then ablated as a stoichio- metric phase.

Evidence of refractive index change by Ti indiffusion into LiNbO3 substrate as a result of multipulse free-running ruby laser irradiation

It is shown that the multipulse free-running ruby laser irradiation of z-cut LiNbO3 single-crystalline samples with 400 Å-thick Ti deposits results, under certain conditions, in an efficient Ti indiffusion with minimum perturbation. Implanted zones exhibit optical characteristics appropriate for the preparation of optical devices.

Synthesis of tungsten silicide by pulsed laser irradiation of sputtered alloy layers

Tungsten silicide was synthesized by multipulse XeCl laser irradiation of co-sputtered WSi mixtures 140 and 460 nm thick on single-crystal silicon wafers. It is shown that at certain equilibria between contradictory influences of the depth of melting, stress generation, intermixing and crystallization it is possible to grow good quality tungsten silicide. Silicide layers with only one crystalline phase (WSi 2) and with a resistivity of 170 μω cm were obtained after irradiation of a WSi film 140 nm thick with 10 laser pulses at a fluence of 0.9 J cm −2.

One-step In-diffusion as a Result of Multipulse Laser Irradiation of LiNbO3Single Crystalline Substrates Covered with thin Ti Deposits

Abstract We studied Ti in-diffusion as an effect of multipulse laser irradiation, in either visible of ultraviolet (u.v.) spectral ranges, of LiNbO3 single-crystalline structures with Ti coatings of two different thicknesses. It is shown that while u.v. (excimer, λ ≈ 308 nm) laser irradiation causes a complete expulsion of the Ti deposit, the visible (ruby, λ ≈ 694·3 nm) laser irradiation at intermediate incident laser fluence (up to ≈ 0·7 J cm−2) promotes efficient Ti in-diffusion from the thin (400 A width) Ti deposit down to a micrometre range implantation depth.

Tungsten silicide formation by multipulse excimer laser irradiation

The formation of tungsten silicide induced by XeCl laser irradiation of W(150 nm)/Si and W(500 nm)/Si samples was studied at laser fluences ranging from 0.6 to 1.8 J/cm 2. Up to 300 subsequent laser pulses were directed to the same irradiation site. After irradiation the samples were examined by different diagnostic techniques: Rutherford backscattering spectroscopy, X-ray scattering, resistometry and surface profilometry. Complete reaction of the 150 nm W film with silicon was obtained between 30 and 100 laser pulses at the fluence of 1.2 J/cm 2, and after 30 laser pulses and at the fluence of 1.5 J/cm 2. The sheet resistance of these silicides was 5–10Ω. At the used fluences, only the onset of silicide synthesis at the W-Si interface was observed for the 500 nm W film. Numerical computations of the evolution and depth profiles of the temperature in the samples as a consequence of a laser pulse were performed and compared to the experimental results.

Optical resistance of sapphire

Single-crystals of α-Al2O3 were subjected to multi-stage polishing with diamond powder of successively decreasing grain size. After each stage of polishing, the infrared reflection spectra were recorded and the nanosecond laser resistance at 1.06 μm was measured. The laser induced damage data were also obtained for natural face, annealed and ion-etched surfaces. It was shown that (i) disorder of the crystal structure affects the optical resistance, resulting in a decrease of damage threshold, and (ii) the surface layer disturbed by grinding retains a “memory” of the initial structure. The multi-pulse sub-threshold laser irradiation which leads, finally, to macroscopic damage of a sample, stimulates the process of structural ordering, together with the formation of “dangerous” local defects.

Multipulse excimer laser irradiation of FeB amorphous alloys

Abstract Substituted Fe-B amorphous ferromagnetic ribbons have been exposed to multipulse excimer laser irradiation ( λ = 248 nm, τ = 20 ns). The effects of laser treatment have been studied by Mossbauer spectroscopy, X-ray diffraction and scanning electron microscopy. Depending on the number of applied laser pulses and sample composition, relaxation of internal stresses and/or controlled magnetic anisotropy could be obtained without onset of bulk crystallization.

Direct synthesis of TiSi2 by a laser thermal self-aligned process

The synthesis of stable and adherent TiSi2 layers by one-step laser irradiation of Ti deposited on Si wafers is reported. A new thermal self-aligned effect is observed. By multipulse excimer laser irradiation, a regime was obtained whereby the Ti covering the Si substrate was completely reacted while the Ti covering the SiO2 layer was completely expelled by vaporization.

Oxidation interference in direct laser nitridation of titanium: relative merits of various ambient gases

A comparison is performed between the relative merits of using an ambient NH 3 atmosphere or a stoichiometrically equivalent N 2-H 2 (1:3) mixture for the direct laser nitridation of titanium samples by multipulse excimer laser ( λ = 308nm) irradiation. It is shown that the N 2-H 2 mixture proves more effecient in inhabiting the unwanted interference of the oxidation process and that the whole process proceeds through the action of an erosion plasma on a molten layer covering the laser-irradiated area.

Direct carbidation of titanium as a result of multipulse UV-laser irradiation of titanium samples in an ambient methane gas

We report the efficient synthesis of titanium carbide layers as an effect of multipulse excimer laser irradiation of titanium samples in a methane atmosphere.