Melting Threshold Research Articles

Self-Organization Processes of Nanostructure Formation by Laser Radiation in Metal

The current state of knowledge about mechanisms of metal nano-particles (NP) formation processes induced by the interaction of high-energy laser beam with surface of the metallic lattices (Au, Ag, Cu) is presented. The review includes an evaluation of the contribution of self-organized effects into the processes of the metal nano-structurization depending on the laser mode, external factors and internal parameters of an active zone. It was noticed that intensive pulsed laser illumination enabled to stimulate nano-fragmentation at the fluencies near and above the melting threshold of the metal in different mediums The laser induced processes of metallic particles formation by ablation of the metal target with consequent NP sizes stabilization by precise temperature tuning in the active zone, local plasmon resonance in liquids, and microablation mechanism in metal films in the conditions of surface plasmon resonance (SPR) due to self-organization effects are considered. Comparative analysis of the laser nano-technologies in air, vacuum, rarefied gas and liquid environments showed the advantages of self-organization in NP generation processes based on the SPR effects and their perspectives.

Laser-Induced Modification of the Surface State and Optical Properties of CdTe Crystals

Optical properties and surface state of semiconductor CdTe crystals subjected to irradiation with nanosecond laser pulses were studied. Ellipsometric parameters Δ and Ψ were measured on two opposite surfaces of (111) oriented CdTe wafers before. The samples were subjected to polishing chemical etching and laser irradiation with energy densities lower and higher than the melting threshold of CdTe. The morphology and structure of CdTe crystals were monitored before and after treatments. Irradiation formed a thin modified surface layer on the both sides of CdTe crystals (Cd- and Te-terminated) with the similar optical constants n and k, respectively. However, the effective thicknesses of such modified layers differed and depended on the chemical and laser treatment conditions and also on the faces of CdTe(111) crystals.

Selective ablation of thin SiO2 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.

Nanostructuring of Continuous Gold Film by Laser Radiation Under Surface Plasmon Polariton Resonance Conditions

The physical mechanisms of metallic nanoparticles formation by laser technology were studied. The system air/Au film/glass was irradiated by laser at the conditions of surface plasmon resonance. A surface electromagnetic wave was excited in Kretchmann configuration by the fundamental and second harmonics of the Q-switched YAG/Nd+3 laser with pulse power density close to the threshold of melting. Nanostructuring of Au film was observed only for the second harmonic (λ = 0.532 μm) irradiation at the surface plasmon polariton resonance (SPR) conditions. Estimations were done using the interference model of the differently directed plasmon polariton waves excited by a surface electromagnetic wave on the metal surface. It was shown that a regular pattern of locally heated spots can be formed in a metallic film by pulsed laser irradiation. The spatial distribution of this pattern is close to the period of interference. The observed effect of laser nanofragmentation is explained by the self-organization of plasmon polariton subsystem in the process of Au nanoparticles formation at high laser intensity levels. These methods open new possibilities for nanostructured surfaces formation utilizing simple self-organization processes.

Self-organization of Ge nanopattern under erosion with heavy Bi monomer and cluster ions

The self-organisation of periodic pattern on (0 0 1) Ge by bombardment with different heavy ion species (Bi +, Bi ++, Bi 2 + , Bi 3 + , Bi 3 + + ) obtained from a liquid metal ion source in a mass separating 30 kV FIB system was studied. Aspect ratios exceeding values reported so far for elemental semiconductors substantially were found after cluster irradiation. An excellent regular self-ordering of dots (40 nm in height, interdistance of ∼50 nm) and ripple pattern were achieved. Despite of high ion fluence, Raman measurements prove a crystalline surface layer. This result deviates drastically from monomer irradiation, where similar to former ion irradiation of Ge a spongy amorphous surface layer is formed. For the transition from the usual behaviour to the unexpected pronounced pattern formation a threshold of the energy density deposited by the collision cascade was identified: If the deposited energy density exceeds the melting threshold, dot or ripple pattern appear. In our model we assume that the ion-impact-induced deposition of energy per volume (estimated by SRIM) must exceed the energy needed for melting. Thus, Bi segregation during resolidification of the melted pool and the 5% volume difference between molten and solid Ge can cause the observed Bi separation and Ge patterning, respectively. A consistent, qualitative model will be discussed.

Laser-Induced Resistance Fine Tuning of Integrated Polysilicon Thin-Film Resistors

In this brief, we present a novel polysilicon resistor trimming technique using a pulsed focused nanosecond laser at a fluence slightly lower than the melting threshold for polysilicon. Using this technique, we were able to trim a 4 μm ×40 μm Taiwan Semiconductor Manufacturing Company 180-nm n-doped polysilicon resistors with a 200-ppm precision. Much better precision is possible by using larger structures. The method can be applied to any CMOS process without any extra layer deposition or specific design restriction beside the fact that the laser beam must be able to reach the polysilicon structure. The high repeatability of the process allows an open-loop calibration. A complete characterization of the trimmed devices, including transverse electromagnetic and atomic force microscopy imaging as well as Raman spectroscopy, has been conducted, leading to the conclusion that a material restructuration in the grain boundaries of polysilicon, following laser irradiation, is responsible for the thin-film resistivity lowering. The stability of the polysilicon thin film, as tested by heating the device at 150°C during 1000 h, is about 1.3%, which is slightly higher than the 0.7% resistance variation for untrimmed thin films.

Laser direct writing pattern structures on AgInSbTe phase change thin film

Different pattern structures are obtained on the AgInSbTe (AIST) phase change film as induced by laser beam. Atomic force microscopy (AFM) was used to observe and analyze the different pattern structures. The AFM photos clearly show the gradually changing process of pattern structures induced by different threshold effects, such as crystallization threshold, microbump threshold, melting threshold, and ablation threshold. The analysis indicates that the AIST material is very effective in the fabrication of pattern structures and can offer relevant guidance for application of the material in the future.

Effects of repeated short heat pulses on tungsten

Effects of ELM-like repeated short heat pulses on tungsten have been studied by using a fundamental wave (wave length of 1064nm, the pulse length of 130μs) of a Nd/YAG laser. Tungsten temperature was 773K or 473K. The energy absorption rate of the laser light was about 30% for mirror-finished surface. After 10,000 shots at an incident energy fluence of 7J/cm2 (melting threshold was about 150J/cm2), the cracks formation was still observed. Surface roughening and cracking were more pronounced for tungsten samples with surface impurity mixing layers (carbon and oxygen).

Calculation of runaway electrons stopping power in ITER

The energy loss rate of runaway electrons (RE) was analysed for ITER plasma facing components materials (Be and W). The stopping power, the energy deposition profiles, and the material erosion are estimated by using the codes MEMOS and ENDEP. The latter has been updated by including the effect of the target’s polarizability. Our calculations show that this effect is significant for high RE energies and low Z materials such as Be. We also find that the conversion of the RE’s magnetic energy into heat can explain the temperature rise on dump plate in JET. In the case of ITER, the calculated heat deposition due to RE is almost two times the melting threshold energy of Be but well below that of W.

Performance of deformed tungsten under ELM-like plasma exposures in QSPA Kh-50

Cracking thresholds and crack patterns (major- and micro-) as well as residual stresses in tungsten targets after repetitive ITER ELM-like plasma pulses have been studied in recent simulation experiments in the quasi-stationary plasma accelerator QSPA Kh-50. Experiments were carried out for preheated samples of ITER reference grade of tungsten (with elongated grains). The QSPA plasma exposures were performed for the targets preheated at 200°C, 400°C and 600°C, to estimate effects of Ductile-to-Brittle Transition Temperature (DBTT) on cracking development. Tungsten surface was analyzed after 1pulse, 5pulses and 10pulses of 0.25ms time duration to investigate the influence of material modification by plasma exposures on cracking thresholds. The magnitude of heat load was 0.2MJ/m2, 0.3MJ/m2, 0.45MJ/m2 (without melting) and 0.75MJ/m2 (above the melting threshold).

Modeling of CW laser diode irradiation of amorphous silicon films

The purpose of this work is to determine the optimal parameters required to crystallize thin amorphous silicon films on glass substrate with a continuous wave (CW) laser diode ( λ = 808 nm), using a numerical model developed in COMSOL Multiphysics. The numerical simulation of the laser crystallization process takes into account the solid–liquid phase change and the difference between the melting temperature of amorphous (Tm a-Si = 1420 K) and that of crystalline silicon (Tm c-Si = 1690 K). We have varied the main parameters controlling the crystallization process, namely the power and the scan speed of the laser beam. Furthermore the initial temperature as well as the thickness of the a-Si:H layer were also taken as a parameter to optimize the process. We have determined the melting, crystallization and ablation energy threshold versus the different operational parameters.

Peak effect and dynamic melting transitions of driven vortex system in weakly disordered Josephson junction arrays

Using the resistively shunted junction model, we study the magnetic frustration induced dynamic melting transitions of a current-driven vortex system in two-dimensional weakly disordered Josephson juction arrays at zero temperature and very low temperatures. From the unified model simulations, we simultaneously obtain two kinds of dynamic melting transitions of the vortex flow, just below the vortex glass phase and above a dynamic melting threshold, respectively. The reenter pinned dilute vortex liquid behavior and the intrinsic quantum vortex liquid (QVL) phenomenon are also obtained, consistent with the recent experimental reports in disordered and superconducting MoGe films. The peak effect of critical current in the vortex glass is induced by intrinsic collective pinning effects in the plastic flow and the enhancement of critical current and voltage noise in the QVL phase arises from intrinsic quantum fluctuations in the moving vortex flow.

Excimer laser ablation of single crystal 4H-SiC and 6H-SiC wafers

A 248 nm, 23 ns pulsed excimer laser was used to compare the ablation characteristics of single crystal wafers of the polytypes 4H-SiC and 6H-SiC over a wide range of energy fluence (0.8–25 J cm−2). Photothermal models based on Beer–Lambert equation using thermal diffusivity and absorption coefficient, energy balance, and heat transfer were presented to predict the ablation mechanisms. Micromachining of trenches was demonstrated at 7 J cm−2 to demonstrate the potential of UV laser ablation. Results indicate that the ablation process is characterized by two well-defined threshold fluences: (a) decomposition threshold ~1 J cm−2 and (b) melting threshold ~1.5 J cm−2 for both polytypes. Contrary to the modeling expectations, the ablation rates were lower and did not increase rapidly with energy fluence. Four types of ablation mechanisms—chemical decomposition, vaporization, explosive boiling, and plasma shielding—either singly or in combination occur as a function of energy fluence. The predictions of photothermal models were not in good agreement with the experimental data implying that a complex interplay among various physical phenomena occurs during ablation. Micromachined trench exhibited ripple patterns, microcracks and recast layers, most of which could be eliminated by a subsequent chemical cleaning process. It is concluded that excimer laser ablation is an effective but slow material removal process for SiC wafers compared to other lasers such as 1064 nm Nd:YAG.

On the Breakup of Patterned Nanoscale Copper Rings into Droplets via Pulsed-Laser-Induced Dewetting: Competing Liquid-Phase Instability and Transport Mechanisms

Nanolithographically patterned copper rings were synthesized, and the self-assembly of the rings into ordered nanoparticle/nanodrop arrays was accomplished via nanosecond pulsed laser heating above the melt threshold. The resultant length scale was correlated to the transport and instability growths that occur during the liquid lifetime of the melted copper rings. For 13-nm-thick rings, a change in the nanoparticle spacing with the ring width is attributed to a transition from a Raleigh-Plateau instability to a thin film instability because of competition between the cumulative transport and instability timescales. To explore the competition between instability mechanisms further, we carried out experiments with 7-nm-thick rings. In agreement with the theoretical predictions, these rings break up in both the azimuthal and radial directions, confirming that a simple hydrodynamic model captures the main features of the processes leading to the breakup.

AMSR-E melt patterns on the Southern Patagonia Icefield

AbstractPassive-microwave 37 GHz vertically polarized (V) brightness temperature (Tb) measurements from the Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E) are used to monitor the extent and timing of snowmelt on the Southern Patagonia Icefield (SPI) in Chile and Argentina. Twice daily Tb’s for 2002–08 for high-elevation (>1200 m a.s.l.) pixels exhibit a bimodal histogram, typical of snow-covered regions in Yukon, Alaskan icefields and the Greenland ice sheet. The low count between the two populations represents the Tb threshold for melt (252 K). This Tb value with the ±18 K diurnal amplitude variation threshold quantifies onset and duration of the spring melt–refreeze period and is used to identify melt regimes and seasonal Tb signals. Tb histograms for pixels west of the Andean divide have a normal distribution above the melt threshold. We interpret the Tb histogram as controlled by surface moisture; the shape and position with respect to Tb are retained with changes in both latitude and elevation, and the region is known to have a moist climate. Tb is not driven by seasonal temperature changes in the northwest sector of the icefield because the Tb threshold is exceeded 75% of the time. For all pixels, the spring melt–refreeze period has shortened by a mean of 10 days a−1 and a mean of 16 days a−1 for pixels with bimodal distributions between 2002 and 2008.

Residual stresses in tungsten under exposures with ITER ELM-like plasma loads

The residual stresses in tungsten targets after repetitive ITER ELM-like plasma pulses have been studied in recent simulation experiments with the quasi-stationary plasma accelerator (QSPA) Kh-50. The experiments were performed for targets preheated at 650 °C, which is above the ductile-to-brittle transition temperature (DBTT), and at room temperature (RT). The number of exposures was up to 400 pulses of duration 0.25 ms, and the magnitude of heat load was below and above the melting threshold.After plasma impacts, the residual stress was analyzed with the x-ray diffraction (XRD) technique using the sin2 ψ method. Symmetrical tensile stresses were measured in a thin subsurface layer. The largest stress (up to 750–850 MPa) was obtained after exposures of RT targets to plasma loads below the melting threshold. Preheating of the target resulted in some decrease of residual stress.

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.

Wavelength dependence of the damage threshold of inorganic materials under extreme-ultraviolet free-electron-laser irradiation

We exposed bulk SiC and films of SiC and B4C to single 25 fs long free-electron-laser pulses with wavelengths between 13.5 and 32 nm. The materials are candidates for x-ray free-electron laser optics. We found that the threshold for surface-damage of the bulk SiC samples exceeds the fluence required for thermal melting at all wavelengths. The damage threshold of the film sample shows a strong wavelength dependence. For wavelengths of 13.5 and 21.7 nm, the damage threshold is equal to or exceeds the melting threshold, whereas at 32 nm the damage threshold falls below the melting threshold.

Nonlinear energy absorption of femtosecond laser pulses in noble metals

We use calorimetry to determine the energy absorption of femtosecond (fs) laser pulses as a function of incident fluence for Ag, Ag alloys (Ag–Cu and Ag–Pt), and Pt. At low fluences, the measured absorption agrees well with reflectivity data derived from ellipsometry measurements. For Ag and Ag–Cu, the absorbed energy increases nonlinearly with the incident fluence for fluences larger than approximately half of the melting threshold. Near this threshold, the absorption increases by a factor of 3–4. Similar nonlinear absorption is not observed in Pt or Ag–Pt. We propose that the nonlinear absorption is caused by the excitation of d-band electrons below the Fermi surface. For pulse widths longer than 850 fs, the observed nonlinear absorption in Ag diminishes, indicating that diffusive transport and not ballistic transport is the major mechanism of cooling at this excitation level.

Use of IR pyrometry to measure free-surface temperatures of partially melted tin as a function of shock pressure

Equilibrium equation of state theory predicts that the free-surface release temperature of shock-loaded tin will show a plateau at 505 K in the stress range from 19.5 to 33.0 GPa, corresponding to the solid-liquid, mixed-phase region of tin. In this paper we report free-surface temperature measurements on shock-loaded tin from 15 to 31 GPa using multiwavelength optical pyrometry. The shock waves were generated by direct contact of detonating high explosive with a tin sample, and the stress in the sample was determined by free-surface velocity measurements using photon Doppler velocimetry. We measured the emitted thermal radiance in the near IR region at four wavelengths from 1.5 to 5.0 μm. Above 25 GPa the measured free-surface temperatures were higher than the predicted 505 K, and they increased with increasing stress. This deviation may be explained by hot spots and/or variations in surface emissivity, and it may indicate a weakness in the use of a simple analysis of multiwavelength pyrometry data for conditions, such as above the melt threshold, where hot spots or emissivity variations may be significant. We are continuing to study the discrepancy to determine its cause.