Picosecond laser ablation of SiO2 layers on silicon substrates

In this work, we report on laser ablation of thermally grown SiO₂ layers from silicon wafer substrates, employing an 8–9 ps laser, at 1064 (IR), 532 (VIS) and 355 nm (UV) wavelengths. High-intensity short-pulse laser radiation allows direct absorption in materials with bandgaps higher than the photon energy. However, our experiments show that in the intensity range of our laser pulses (peak intensities of <2×10¹² W/cm²) the removal of the SiO₂ layer from silicon wafers does not occur by direct absorption in the SiO₂ layer. Instead, we find that the layer is removed by a “lift off” mechanism, actuated by the melting and vaporisation of the absorbing silicon substrate. Furthermore, we find that exceeding the Si melting threshold is not sufficient to remove the SiO₂ layer. A second threshold exists for breaking of the layer caused by sufficient vapour pressure. For SiO₂ layer ablation, we determine layer thickness dependent minimum fluences of 0.7–1.2 J/cm² for IR, 0.1–0.35 J/cm² for VIS and 0.2–0.4 J/cm² for UV wavelength. After correcting the fluences by the reflected laser power, we show that, in contrast to the melting threshold, the threshold for breaking the layer depends on the SiO₂ thickness.     

Periodic arrays of submicron Si and Ni dots on SiO2 fabricated using linearly polarized Nd:YAG pulsed laser

Periodic arrays of submicron Si and Ni dots were fabricated by only irradiating a linearly polarized Nd:YAG pulsed laser beam to Si and Ni thin films deposited on silicon dioxide (SiO₂) film. The interference between an incident beam and a scattered surface wave leads to the spatial periodicity of beam energy density distribution on the surface of the irradiated samples. A thin film was melted using a laser beam, and the molten film was split and condensed owing to its surface tensile according to the periodic energy density distribution. Then, the fine lines (line and space structure) were formed periodically. After the formation of fine lines, the sample was rotated by 90°, and the laser beam was irradiated. The periodic energy density distribution was generated on the fine lines, and the lines were split and condensed. Eventually, the periodically aligned submicron dots were fabricated on the SiO₂ film. PACS 79.20.Ds; 42.62.-b; 81.40.-z     

Preparation and characterization of transparent Nd:YAG ceramics

Transparent Nd:YAG ceramics were produced by solid.state reaction of high.purity (4N) nanometric oxides powders, i.e., Al₂O₃, Y₂O₃ and Nd₂O₃. After sintering, mean grain sizes of 2% Nd:YAG samples were about 20 μm and their transparency were a bit worse than that of 0.9% Nd:YAG single crystal. Two types of active elements: rods and slabs were fabricated and characterized in several diode pumping schemes. In end pumping configuration as a pump source 20.W fiber coupled laser diode operating in low duty cycle regime (1 ms pump duration/20 Hz) was deployed. In the best case, 3.7 W of output power for 18 W of absorbed pump power, M² < 1.4 were demonstrated for uncoated ceramics Nd:YAG rod of ϕ 4×3mm size in preliminary experiments. For the ceramics of two times lower Nd dopant level above 30% slope efficiency was achieved. In case of Nd:YAG ceramic slab side pumped by 600.W laser diode stack above 12 W was demonstrated with slope efficiency of 3.5%.     

Introducing CdS into two- and three-dimensional photonic crystals

SiO₂/air three-dimensional (3D) periodic structures were fabricated by removing Si layers partially from Si/SiO₂ 3D photonic crystals (PhCs) formed by using autocloning. CdS/SiO₂ 3D periodic structures were formed by introducing CdS into the SiO₂/air structures by the TEA method and photoluminescence (PL) was observed from the introduced CdS. TiO₂/air/CdS two-dimensional (2D) PhCs were also fabricated by introducing CdS into the voids of TiO₂/air 2D periodic structures, in which SiO₂ layers were partially etched out from TiO₂/SiO₂ 2D PhCs fabricated by using autocloning. PL radiating normal to the surface was measured and large polarization dependence was observed.     

Laser–MBE growth of high-quality ZnO thin films on Al2O3(0001) and SiO2/Si(100) using the third harmonics of a Nd:YAG laser

High-quality ZnO thin films were grown on single-crystalline Al₂O₃(0001) and amorphous SiO₂/Si(100) substrates at 400–640 °C using laser molecular beam epitaxy. For film growth, the third harmonics of a pulsed Nd:YAG laser were illuminated on a ZnO target. The ZnO films were epitaxially grown on Al₂O₃(0001) with the narrow X-ray diffraction full width at half maximum (FWHM) of 0.04° and the films on SiO₂/Si(100) exhibited a preferred c-axis orientation. Furthermore, the films exhibited excellent optical properties in photoluminescence (PL) measurements with very sharp excitonic and weak deep-level emission peaks. At 15 K, PL FWHM values of the films grown on Al₂O₃(0001) and SiO₂/Si(100) were 3 and 18 meV, respectively. Received: 8 May 2001 / Accepted: 18 September 2001 / Published online: 20 December 2001     

Laser-induced thermal effects on Si/SiO<Subscript>2</Subscript> free-standing superlattices

The thermal effects produced by continuous-wave laser radiation on free-standing Si/SiO₂ superlattices are studied. We compare two samples with different SiO₂ layer thicknesses (2 and 6 nm) and the same Si layer thickness (2 nm). The as-prepared free-standing superlattices contain some amount of Si nanocrystals (Si-nc). Intense laser irradiation at 488 nm of the as-prepared samples enhances the Raman scattering of Si-nc by two orders of magnitude. This laser-induced crystallization originates from melting of Si nanostructures in silica, which makes Si-nc better ordered and better isolated from the oxide surrounding. Continuous-wave laser control of Si-nc stress was achieved in these samples. In the proposed model, intense laser radiation melts Si-nc, and Si crystallization upon cooling down from the liquid phase in a silica matrix leads to compressive stress. The Si-nc stress can be tuned in the ∼3 GPa range using laser annealing below the Si melting temperature. The high laser-induced temperatures were verified with Raman spectroscopy. The laser-induced heat leads to a strongly nonlinear rise of light emission. The light emission is also observed in the anti-Stokes region, and its temperature dependence is practically the same for the two studied samples. The laser-induced temperature is essentially controlled by the absorbed laser power. PACS 78.55.-m; 78.20.-e; 68.55.-a; 78.30.-j     

Excimer-laser-induced micropatterning of silicon dioxide on silicon substrates

Highly resolved micropatterns induced on SiO₂-coated Si sample surfaces have been investigated using a KrF excimer laser (λ: 248 nm and τ: 23 ns). Uniform micropatterns were observed to form in the oxide layer after laser-induced melting of interfaces. The pattern size can be controlled either by the laser parameters or even by the oxide layer thickness. SEM analysis identified that the micropatterns were virtually initiated at the molten interface and the oxide layer followed the interface patterning to change its profile. Simulation of laser interaction with double-layered structures indicated that the oxide layer could melt or be ablated due to interface superheating when it was deposited on a highly absorbing Si substrate. IR analysis has demonstrated that the structural properties of the SiO₂ layer undergo no appreciable changes after laser radiation. This process provides a possible basis for its application in micropatterning of transparent materials using excimer lasers. Received: 4 September 2000 / Accepted: 13 September 2000 / Published online: 30 November 2000     

High-power diode-pumped Tm:YLF laser

A high-power, continuous-wave 3.5% Tm³⁺ doped LiYF₄ (Tm:YLF) laser has been developed. Using two Tm:YLF rods in a single cavity, 55 W of laser output at 1910 nm was obtained with a slope efficiency of 49%. The M² factor was found to be <3. With a single Tm:YLF rod, a maximum laser power of 30 W was obtained with a slope efficiency of 50%. The laser was tuned to the peak absorption wavelength of Ho:YAG of 1907.5 nm by an intracavity quartz etalon with an output power loss < 1 W. PACS 42.55.-f; 42.55.Xi; 42.60.Pk     

A comprehensive study of Nd:YAG, Nd:YAlO3, Nd:YVO4 and Nd:YGdVO4 lasers operating at wavelengths of 0.9 and 1.3 μm. Part 1: cw-operation

This paper reports on efficient generation of cw laser radiation at 0.9 and 1.3 μm in different neodymium doped laser hosts. The thermal, mechanical and optical properties as well as the laser performance of Nd:YAG, Nd:YAlO₃, Nd:YVO₄ and Nd:GdVO₄ are studied in numerical simulations as well as in experimental investigations. For example an output power of more than 4.0 W is generated in Nd:YVO₄ at the 914 nm ⁴F_3/2→⁴I_9/2 transition using a pump power of 19 W. In Nd:GdVO₄ more than 6.0 W are obtained at the 1342 nm ⁴F_3/2→⁴I_13/2 laser transition by using a pump power of 19.3 W. The spatial beam quality of both lasers is diffraction limited with an M² value of less than 1.1. PACS  42.70.Hj; 42.55.Xi; 42.60.Pk     

Modulation of the photoluminescence of Si quantum dots by means of CO2 laser pre-annealing

Si quantum dots (QDs) embedded in SiO₂ can be normally prepared by thermal annealing of SiO_x (x < 2) thin film at 1100 °C in an inert gas atmosphere. In this work, the SiO_x thin film was firstly subjected to a rapid irradiation of CO₂ laser in a dot by dot scanning mode, a process termed as pre-annealing, and then thermally annealed at 1100 °C for 1 h as usual. The photoluminescence (PL) intensity of Si QD was found to be enhanced after such pre-annealing treatment. This PL enhancement is not due to the additional thermal budget offered by laser for phase separation, but attributed to the production of extra nucleation sites for Si dots within SiO_x by laser irradiation, which facilitates the formation of extra Si QDs during the subsequent thermal annealing.     

F2 laser induced oxidation of inorganic material

Transparent SiO₂ thin films were selectively fabricated on Si wafer by 157 nm F₂ laser in N₂/O₂ gas atmosphere. The F₂ laser photochemically produced active O(¹D) atoms from O₂ molecules in the gas atmosphere; strong oxidation reaction could be induced to fabricate SiO₂ thin films only on the irradiated areas of Si wafer. The oxidation reaction was sensitive to the single pulse fluence of F₂ laser. The irradiated areas were swelled and the height was approximately 500-1000 nm at the 205-mJ/cm² single pulse fluence for 60 min laser irradiation. The fabricated thin films were analytically identified to be SiO₂ by the Fourier-transform IR spectroscopy. The SiO₂ thin films could be also removed by subsequent chemical etching to fabricate micro-holes 50 nm in depth on Si wafer for microfabrication.     

A comprehensive study of Nd:YAG,Nd:YAlO<Subscript>3</Subscript>, Nd:YVO<Subscript>4</Subscript> and Nd:YGdVO<Subscript>4</Subscript> lasers operating at wavelengths of 0.9 and 1.3 μm. Part 2: passively mode locked-operation

This paper reports on the generation of picosecond (ps) laser pulses by self-phase-adjusting additive-pulse-mode-locking (PSA) at wavelengths of 0.9 and 1.3 μm. The main objective of this work was to investigate and compare the characteristic optical properties of ps lasers based on different Nd-doped laser crystals like Nd:YAG, Nd:YAlO₃, Nd:YVO₄ and Nd:GdVO₄. As a result of these investigations a mode-locked Nd:YVO₄ laser for example, generated, ps pulses at 1.3 μm with a duration of 7 ps, a repetition rate of 160 MHz and an average power of 4.7 W. At 0.9 μm pulses with a duration of 1.9 ps were obtained at a repetition rate of 158 MHz and an average power of 2.8 W. PACS  42.70.Hj; 42.65.Re; 42.65.Ky     

Theory for the ultrafast dynamics of excited clusters: interplay between elementary excitations and atomic structure

We present a theoretical study of the short-time relaxation of clusters in response to ultrafast excitations using femtosecond laser pulses. We analyze the excitation of different types of clusters (Hg_n, Ag_n, Si_n, C₆₀ and Xe_n) and classify the relaxation dynamics in three different regimes, depending on the intensity of the exciting laser pulse. For low-intensity pulses (I<10¹² W/cm²) we determine the time-dependent structural changes of clusters upon ultrashort ionization and photodetachment. We also study the laser-induced non-equilibrium fragmentation and melting of Si_n and C₆₀ clusters, which occurs for moderate laser intensities, as a function of the pulse duration and energy. As an example for the case of high intensities (I>10¹⁵ W/cm²), the explosion of clusters under the action of very intense ultrashort laser fields is described. Received: 26 November 1999 / Published online: 2 August 2000     

Strongly oriented BST films on La0.9Sr1.1NiO4 electrodes deposited on various substrates for integration of high capacitances on silicon

In this study, we demonstrate the successful oriented growth of Ba_0.6Sr_0.4TiO₃(h 0 0)/La_0.9Sr_1.1NiO₄(0 0 l) stacks by pulsed laser deposition on SiO₂/Si for application in integrated capacitances. We show that for specific deposition conditions the La_0.9Sr_1.1NiO₄ layer spontaneously grows along its c-axis both on SiO₂/Si and on Pt/Ti/SiO₂/Si substrates, serving as a template for the subsequent oriented growth of Ba_0.6Sr_0.4TiO₃ (BST). Moreover, as the resistivity of the La_0.9Sr_1.1NiO₄ layer is ∼1 mΩ cm, it also fulfills the function of bottom electrode for integration of perovskite-based capacitors on silicon. This holds the promise of integrating epitaxial BST with very high dielectric constant compared to polycrystalline BST films. Still, preliminary capacitance measurements on Al/BST/La_0.9Sr_1.1NiO₄/SiO₂/Si capacitors indicate that the stack deposition needs further optimization.     

Solidification microstructure of AZ91D Mg alloy after laser surface melting

The solidification microstructure plays a critical role in determining the surface properties of laser-treated magnesium alloys. The purpose of this paper is to study the solidification microstructures of AZ91D Mg alloy following millisecond- and nanosecond-pulse Nd:YAG laser irradiation. The solidification microstructural evolution of laser-melt AZ91D Mg alloy was investigated using X-ray diffractometry, scanning electron microscopy, energy-dispersive X-ray spectrometer and transmission electron microscopy. Much refined α-Mg phase and β-Mg₁₇Al₁₂ intermetallics were observed in the microstructure after laser surface melting. Periodic and successive structure was observed in the millisecond irradiated surface and the melt depth was more than 100 μm. The solidification microstructure was mainly cellular/dendrite structures together with a large number of β-Mg₁₇Al₁₂ nano-particles. Micron holes were found in the nanosecond irradiated surface and the melt depth was shallow at 50 μm. Millisecond-pulse Nd:YAG laser was found to be more suitable for Mg alloy surface treatment due to sufficient melt depth.     

Patterning and morphology of nonlinear optical Gd<Subscript>x</Subscript>Bi<Subscript>1-x</Subscript>BO<Subscript>3</Subscript> crystals in CuO-doped glass by YAG laser irradiation

Dots and lines consisting of nonlinear optical Gd_xBi_1-xBO₃ crystals were patterned on the surface of CuO-doped Gd₂O₃-Bi₂O₃-B₂O₃ glass by heat-assisted (200 °C) Nd:YAG laser irradiations with a wavelength of λ=1064 nm, where the laser energy absorbed by Cu²⁺ is converted to the local heating of the surrounding Cu²⁺. The surface morphology and orientation of crystals in the patterned lines were clarified from confocal scanning laser microscope observations and polarized micro-Raman scattering spectra. Crystal lines with periodic bumps (i.e., ladder-shape like lines) were patterned by laser irradiations with a power of 0.79 W and a scanning speed of 60 μm/s, and the orientation of Gd_xBi_1-xBO₃ crystals in the lines was proposed. The present study demonstrates that the combination of Cu²⁺ and continuous wave Nd:YAG laser with λ=1064 nm is effective in inducting crystallization of oxide glasses. The mechanism of laser-induced crystallization in glass has also been discussed. PACS 61.43.Fs; 42.70.Mp; 68.35.Bs; 78.30.-j; 79.20.Ds     

Effects of sidelobes of focused flat-topped laser beams on vacuum electron acceleration

Using three-dimensional test particle simulations, we investigated electrons accelerated by a focused flat-top laser beam at different intensities and flatness levels of the beam profile before focusing in vacuum. The results show that the presence of sidelobes around the main focal spot of the focused flat-top laser beam influences the optimum (as far as electron acceleration is concerned) initial momentum (and incident angle) of electrons for acceleration. The difference of initial conditions between laser beams with and without sidelobes becomes evident when the laser field is strong enough (a₀>10, corresponding to intensities I>1×10²⁰ W/cm² for the laser wavelength λ=1 μm, where a₀ is a dimensionless parameter measuring laser intensity). The difference becomes more pronounced at increasing a₀. Because of the presence of sidelobes, there exist three typical CAS (capture and acceleration scenario) channels when a₀≥30 (corresponding to I>1×10²¹ W/cm² for λ=1 μm). The energy spread of the outgoing electrons is also discussed in detail. PACS 41.75.Jv; 42.60.Jf; 42.25.Fx     

First comparison of electric field induced second harmonic of near-infrared femtosecond laser pulses in reflection and transmission generated from Si/SiO<Subscript>2</Subscript> interfaces of a silicon membrane

For the first time electric field induced second harmonic (EFISH) generation of femtosecond (fs) laser pulses (λ=800 nm, τ=75±5 fs, rep. rate=80 MHz, E _pulse≤10 nJ) is observed in transmission through a thin free-standing silicon (Si) membrane of 10-μm thickness and compared to the well-known EFISH results in reflection by use of the z-scan technique. EFISH in reflection and transmission unequivocally originate from the front and rear Si/SiO₂ interfaces, respectively, with SiO₂ being the natural oxide on the Si surfaces. Frequency conversion is enhanced by photoinduced electric fields across the Si/SiO₂ interfaces caused by charge-carrier injection from Si into the oxide. The z-scan results and time-dependent measurements allow comparison of the EFISH signal amplitudes and time constants detected in transmission and reflection, demonstrating the need for further investigation.     

Selective ablation of thin SiO<Subscript>2</Subscript> layers on silicon substrates by femto- and picosecond laser pulses

The selective ablation of thin (∼100 nm) SiO₂ layers from silicon wafers has been investigated by applying ultra-short laser pulses at a wavelength of 800 nm with pulse durations in the range from 50 to 2000 fs. We found a strong, monotonic decrease of the laser fluence needed for complete ablation of the dielectric layer with decreasing pulse duration. The threshold fluence for 100% ablation probability decreased from 750 mJ/cm² at 2 ps to 480 mJ/cm² at 50 fs. Significant corruption of the opened Si surface has been observed above ∼1200 mJ/cm², independent of pulse duration. By a detailed analysis of the experimental series the values for melting and breaking thresholds are obtained; the physical mechanisms responsible for the significant dependence on the laser pulse duration are discussed.     

Simultaneous dual-wavelength emission at 0.90 and 1.06 µm in Nd-doped laser crystals

Continuous-wave (CW) simultaneous laser emission on the 0.9-μm ⁴ F _3/2 → ⁴ I _9/2 transition and the ⁴ F _3/2 → ⁴ I _11/2 transition at 1.06 μm is obtained in Nd-based laser crystals of thin-disk geometry and using a multi-pass pumping scheme. A Nd:Y₃Al₅O₁₂ (Nd:YAG) thin disk emitted simultaneous laser radiation at 946 and 1064 nm with 5.1 W output power, and Nd:YVO₄ and Nd:GdVO₄ thin-disk lasers with more than 3 W output power at 0.91 and 1.06 μm were realized. The ratio between the output power at one of the wavelengths and the total output power could be varied by the laser resonator design. An intracavity frequency-doubled Nd:YVO₄ thin-disk laser with alternate green at 532 nm and “deep-blue” at 457 nm generation of high average output powers is demonstrated.