Ablation Hole Research Articles

Stress damage process of silicon wafer under millisecond laser irradiation

The stress damage process of a single crystal silicon wafer under millisecond laser irradiation is studied by experiments and numerical simulations. The formation process of low-quality surface is monitored in real-time. Stress damage can be observed both in laser-on and -off periods. Plastic deformation is responsible for the first stress damage in the laser-on period. The second stress damage in the laser-off period is a combination of plastic deformation and fracture, where the fundamental cause lies in the residual molten silicon in the ablation hole.

Significant skin-tightening by closure of fractional ablative laser holes.

Ablative fractional laser treatment uses thousands of very small laser beam wounds to damage a fraction of the skin, which stimulates tissue remodeling. Each open micro-wound heals without scarring, but the amount of skin tightening achieved is limited. This animal study was performed to test the hypothesis that immediate temporary closure of fractional laser wounds could increase skin tightening after fractional ablative laser treatment. Four adult swine were used for the study; 98 square test sites (3 × 3 cm) were tattooed on the abdomen and flanks of each pig. An ablative fractional Erbium:YAG laser (Sciton Profile, Sciton Inc, Palo Alto, CA) was used to treat the test areas. A laser micro-spot fluence of 375 J/cm2 was delivered in 150-250 microseconds pulses, resulting in an array of ablation channels extending 1.5 mm deep into the skin, with a spot size of 250 µm, with 10% treatment density. Immediately following laser exposure the resulting holes were closed using a stretched elastic adhesive dressing, which, when applied, recoiled and compressed the diameter of the ablation holes. The compressive dressings were removed after 7 days. This procedure was compared to removing the same amount of skin (10%) mechanically by specially designed 19 gauge coring needles, as well as to the same laser and coring methods without compression closure. Area and shape of test sites were measured by digital photography before and 28 days after treatment. Data analysis included compensation for animal growth, as measured by increase in the area of the untreated control sites. All treated and control sites healed within a week, without scarring evident at 28 days. Laser treatment combined with compressive wound closure caused significant shrinkage at 28 days compared with untreated control sites. The treated skin area was reduced by 11.5% (P = 0.0001). Needle coring with wound closure produced similar, significant shrinkage (8%, P < 0.0021), whereas laser and needle coring treatment without closure did not result in significant area reduction (P = 0.1289) compared with untreated control sites. Significant skin tightening can be achieved by immediate temporary non-invasive wound closure after short pulse Er:YAG fractional ablative laser treatment, as well as after mechanically removing skin with a coring needle. This approach may improve skin tightening after ablative laser treatments. Further clinical studies are necessary to confirm successful application in humans. Lasers Surg. Med. 50:64-69, 2018. © 2017 Wiley Periodicals, Inc.

Material removal effect of microchannel processing by femtosecond laser

Material processing using ultra-short-pulse laser is widely used in the field of micromachining, especially for the precision processing of hard and brittle materials. This paper reports a theoretical and experimental study of the ablation characteristics of a silicon wafer under micromachining using a femtosecond laser. The ablation morphology of the silicon wafer surface is surveyed by a detection test with an optical microscope. First, according to the relationship between the diameter of the ablation holes and the incident laser power, the ablation threshold of the silicon wafer is found to be 0.227J/cm2. Second, the influence of various laser parameters on the size of the ablation microstructure is studied and the ablation morphology is analyzed. Furthermore, a mathematical model is proposed that can calculate the ablation depth per time for a given laser fluence and scanning velocity. Finally, a microchannel milling test is carried out on the micromachining center. The effectiveness and accuracy of the proposed models are verified by comparing the estimated depth to the actual measured results.

Comparative study of shock-wave hardening and substructure evolution of 304L and Hadfield steels irradiated with a nanosecond relativistic high-current electron beam

We present the results of a comparative study of the shock-wave hardening regularities and mechanisms revealed for bulk (thickness h = 6 and 9.3 mm) targets made of austenitic 304L stainless steel and Hadfield steel. A high-current relativistic electron beam (45 ns, 1.35 MeV, 34 GW/cm2) produced by the SINUS-7 accelerator was used for generation of a shock wave. It is revealed by 2D-computer simulation for type 304 steel that the direct ablation of the target material leads to generation of shock wave with duration of ∼0.1 μs and amplitude of ∼20 GPa, and the strain rate during its direct propagation and reflection from the free rear surface decreases from ∼2 down to ∼0.4 μs−1. It is found experimentally that in the absence of a rear spall (h = 9.3 mm) the shock-wave loading of both steels leads to formation of three hardened layers: a front layer with a maximum microhardness at a depth of 0.5–1 mm from the bottom of ablation hole, which is in a reasonable agreement with the predictions of the heat-transfer calculations, as well as intermediate and rear-side layers. In case of 304L stainless steel, the depth distributions of microhardness and fraction of twinned grains are consistent with each other, while in the Hadfield steel, the correlation is within the front and intermediate hardened layers only. It is shown by microstructural characterization and analysis of hardening mechanisms that in the case of 304L stainless steel, both front and rear-side hardening are significantly associated with the formation of new intra-phase boundaries by deformation twinning. In the Hadfield steel, unlike the 304L stainless steel, the unusual rear-side hardening is mainly due to increasing the dislocation density under submicrosecond single cycle of compression followed by tension with peak stress of ∼3 GPa.

Influence of ambient pressure on the ablation hole in femtosecond laser drilling Cu.

The holes were drilled by femtosecond laser pulse (800 nm, 100 fs) on Cu sheets at different ambient pressures. The pressure range was from 1 Pa to atmospheric pressure. The number of pulses to drill through the target, the stable photodiode signal, and the hole diameter were obtained as functions of ambient pressure. The morphology of the hole was observed by a scanning electron microscope (SEM). The result showed that the ambient pressure had significant influence on the morphology of the hole.

Subsurface ablation of atherosclerotic plaque using ultrafast laser pulses.

We perform subsurface ablation of atherosclerotic plaque using ultrafast pulses. Excised mouse aortas containing atherosclerotic plaque were ablated with ultrafast near-infrared (NIR) laser pulses. Optical coherence tomography (OCT) was used to observe the ablation result, while the physical damage was inspected in histological sections. We characterize the effects of incident pulse energy on surface damage, ablation hole size, and filament propagation. We find that it is possible to ablate plaque just below the surface without causing surface damage, which motivates further investigation of ultrafast ablation for subsurface atherosclerotic plaque removal.

Improvement of aluminum drilling efficiency and precision by shaped femtosecond laser

Shaped femtosecond laser pulses with the plain phase (transform-limited pulse) and sine phase (A=1.2566, T=30, T=10, and T=5) were used to drill Al sheet in vacuum. Using different phase, the number of pulses required to drill through the sheet was different. With lower laser pulse energy, the ablation rate was the highest when plain phase (corresponding to transform limited pulse) was used. With higher laser energy, the optimized ablation rate can be achieved by increasing the time separation between the subpulses of pulse train produced from the sine phase function. And, with the shaped femtosecond laser, the diameter of ablation holes produced was smaller, the ablation precision was also improved. The results showed that shaped femtosecond laser pulse has great advantages in the context of femtosecond laser drilling.

Ti:sapphire Laser Ablation of Silicon in Different Ambients

Titanium-sapphire laser ablation of silicon was investigated in atmospheric air, water and 20% KOH solution. The number of overlapping pulses was tuned between 3 and 1000 by changing the processing time. The ablated surfaces were investigated using scanning electron microscopy, X-ray microanalysis and stylus profilometry. In each ambient, plasma formation was observed during ablation. X-ray microanalysis indicated that the process starts with surface oxidation in case of air and water ambient. With increasing pulse number the surface was roughened, resulting in larger structures. Crater formation was observed in air and in water, with higher etching rate in the latter case. The depth profile of the ablation holes and grooves is rather different for this two ambient. Narrow conical grooves were observed in case of ablation in air, while flat bottom grooves were produced in water. Laser processing in KOH solution did not result in etching, only the native oxide layer was removed. Etching occurred in these areas, if the samples were left in KOH solution for a longer time. Based on these observations the material removal by fs laser pulses is attributed to evaporation and reactions of Si with radicals in the laser induced plasma, such as atomic or ionic oxygen. The application of KOH introduces K ions or atoms into the laser induced plasma, which may react faster with oxygen than with Si, thus resulting in decrease in etching rate during laser processing.

Investigation of ablation thresholds of optical materials using 1-µm-focusing beam at hard X-ray free electron laser

We evaluated the ablation thresholds of optical materials by using hard X-ray free electron laser. A 1-µm-focused beam with 10-keV of photon energy from SPring-8 Angstrom Compact free electron LAser (SACLA) was irradiated onto silicon and SiO2 substrates, as well as the platinum and rhodium thin films on these substrates, which are widely used for optical materials such as X-ray mirrors. We designed and installed a dedicated experimental chamber for the irradiation experiments. For the silicon substrate irradiated at a high fluence, we observed strong mechanical cracking at the surface and a deep ablation hole with a straight side wall. We confirmed that the ablation thresholds of uncoated silicon and SiO2 substrates agree with the melting doses of these materials, while those of the substrates under the metal coating layer are significantly reduced. The ablation thresholds obtained here are useful criteria in designing optics for hard X-ray free electron lasers.

Study on the Electrical Arc Ablation Performance of Nano-SiC&lt;sub&gt;W&lt;/sub&gt; Reinforced Cu-Matrix Composites

The nano-SiC whisker reinforced Cu-matrix composites (SiCw/Cu) were obtained by powder metallurgy processes. The electrical arc ablation performance of composite SiCw/Cu was compared with copper H62 through stagnation-point ablation experiments. The results indicated that the qualities of copper H62 samples decreased with the increasing of ablating current; and the qualities of SiCw/Cu samples firstly increased and then decreased with the increasing of ablating current. Ablation mechanism research indicates the ablation hole of SiCw/Cu is smaller than copper H62, but it produced micro cracks because of different thermal expansion coefficients between nano-SiC whisker and Cu-matrix. At the same time, nano-SiC whisker happens to intricate oxidation reaction during arc ablation, it can reduce Cu-matrix smelts and sprays by absorbing arc energy. The anchoring effect and protection for the copper matrix of the uniformly dispersing nano-SiC whiskers have been considered to contribute the excellent arc ablation performance of composite SiCw/Cu.

Fabrication of High-aspect-ratio Nanohole Arrays on GaN Surface by Using Wet-chemical-assisted Femtosecond Laser Ablation

A periodic array of nanoholes with high aspect ratios was successfully fabricated by using a multi-scan laser irradiation technique, in which each position in the array is irradiated one time per scan with a femtosecond laser pulse. This technique can easily avoid interference to the laser pulses by micro-bubbles generated in the wet-chemical-assisted ablation process. As a result, we obtained nano-sized ablation holes with an aspect ratio of 1.6, which is much higher than that possible with single pulse ablation. The depth of the ablation craters formed by multi-scan irradiation technique was found to saturate at higher pulse number, presumably due to the limited depth of focus. We also revealed that the ablated depth strongly depends on the refractive index of the medium facing the GaN substrate. In fact, compared to ablation in air followed by HCl etching, the wet-chemicalassisted ablation process enhances the aspect ratio by about 30 % because of the high refractive index of the HCl solution. In addition, by controlling the synchronization of the laser pulses and the motion of the piezo-stage, an almost perfectly periodic arrangement of nanoholes was fabricated on the GaN substrate. The technique developed in this study is likely to make a significant contribution towards realization of two-dimensional photonic crystal structures on GaN-based blue lightemitting diodes.

New Approach to Fabrication of Minute Columnar and Ring Patterns with ZnS, SiO2, and Zn

Minute columnar and ring patterns are fabricated by heat-mode lithography, in which a mixed thin film consisting of zinc sulfide (ZnS), silicon dioxide (SiO2), and zinc (Zn) is used. This single layer has an optical absorption ability and simultaneously shows etching resistance change. Smooth submicrometer-scale ring arrays are fabricated by a unique two-step method. First, an ablation hole is formed by irradiating pulsed laser light, and then a ring pattern is formed by removing the outer regions by wet etching. Optically transparent patterns are also fabricated using an oxidizable light-absorption material of Zn, which changes to zinc oxide (ZnO) following laser irradiation.

脳腫瘍除去用Er: YAGレーザ手術システムの開発

We developed an Er: YAG laser surgery system for glioma removal. The purpose of our laser system is to ablate brain tumor as precisely and efficiently as possible, without collateral tissue damage. In order to evaluated the efficiency of ablation and the thermal damage to the tissue around the ablation holes, we irradiated an Er: YAG laser to pig brain tissues in vitro with respect to laser repetition rate (10, 20, 25Hz) at a fluence (113.7J/cm2), a scan speed (1mm/s). This report showed that it could be applied as our laser system for the brain tumor removal, which the most efficient ablation condition was under a specific site at10Hz using an Er: YAG laser surgery system.

The Study of Femto-Second Laser Induced Damage Threshold on Semi-Insulating GaAs Wafer

We demonstrated an experiment of femtosecond-laser damage threshold on GaAs wafer, the damage threshold was measured from 50 to 400fs. The mechanism was discussed through injection power, pulse duration and ablation profile. The results showed that the damage threshold increased with the pulse duration, the relationship between diameter of ablation hole and laser power density was also analyzed. It was concluded that the main factor affecting the damage threshold was photon ionization and collision ionization.

Micro-machinability of silver–sodium ion-exchanged glass by UV nanosecond laser

Micro-machining of silver–sodium ion-exchanged lime-sodium glass was carried out using fourth harmonic generation (FHG) of Nd:YAG nanosecond laser. The laser irradiations to silver-doped glasses produced low aspect ratio and defect-free holes because of their high-optical absorptions at the vicinity of ion-exchanged surfaces. The ablation rates (i.e. removed depth per laser shot) of ion-exchanged glasses gradually decreased with the distance from glass surfaces and were saturated, corresponding to the dilution of silver ion concentration. The ablation rates and hole shapes drastically changed when the processed hole bottoms reached the ion penetration depths. Scanning electron microscope images indicated that the walls of processed holes in non-silver-containing region had irregular-shaped and porous structures. Therefore, well-designed ion distributions were necessary for the high-accuracy fabrication of micro-components. From these results, a micro-channel and a cylindrical column-shaped cantilever were fabricated on the glass substrates.

"Optimally spaced" excimer laser coronary catheters: performance analysis.

Excimer laser angioplasty is a percutaneous treatment modality for management of selected patients with severe obstructive coronary artery disease. This technology entails application of multifiber catheters that vaporize intra-arterial plaque and thrombus with the 308-nm wavelength light. A coronary laser catheter ("OS") with increased space between its optic fibers (90 microns) at the tip was recently developed. The previous design used a closely packed fiber bundle ("CP") with a smaller space between the fibers (77 microns). We sought to determine the ablation characteristics of the new, OS design. Experiments testing the new catheter and comparing it to the existing catheter included: (1) measurement of the laser output beam sizes and divergences; (2) evaluation of particulate matter generation during ablation of atherosclerotic tissue; (3) measurement of ablation hole sizes and tissue penetration rates; (4) histopathologic examination of laser-induced in vivo vessel wall injury. The new OS catheters produce a wider beam with higher divergence than the traditional CP catheters (6.7 degrees vs. 4.7 degrees, respectively). Testing two different levels of energy revealed the generation of a reduced number of particulate matter and shallower penetration depth with the OS catheter compared with traditional CP catheters. The OS catheters created a larger diameter of ablated hole than the CP catheters (for 2.0-mm catheters: 2.7 mm2 vs. 1.5 mm2, respectively, p = 0.01). Lasing with the OS catheters with slow advancement rates (0.2-0.5 mm/sec) resulted in creation of significantly larger-diameter holes than those produced at higher speeds (1-3 mm/sec). The in vivo vessel wall injury scores were similar among the two types of catheters tested. A new design of spaced optical fibers for coronary laser catheters provides increased tissue ablation in comparison to the traditional closely packed fibers catheter. Slow advancement rates during lasing with the new OS catheter are crucial for achievement of adequate plaque debulking.

Excimer laser sputtering of mica surfaces: Mechanisms and applications

The influence of excimer laser irradiation (193 nm, 20 ns pulses and 248 nm, 20 ns and 500 fs pulses) on the structure and morphology of plain and gold-film coated mica surfaces has been investigated for fluences between 8 mJ/c2 and 19 J/cm2. Surfaces treated with laser fluences below the ablation threshold (λ=248 nm) are well suited for controlled growth of metallic films, whereas above-threshold treated samples (λ=193 nm) form roughened surfaces with close-packed arrays of cones. The cone generation can be suppressed by appropriate choice of laser parameters, resulting in the formation of ablation holes with micrometer precision. The same precision can be obtained in ultrathin metallic films without destroying the underlying mica surface. Low-energy electron diffraction is used to monitor laser-induced microscopic changes of the mica surface upon low-fluence irradiation. It is seen that the first step of the ablation process includes reorientation of dipole domains on the surface. Thereafter, scanning electron microscopy reveals melting of the surface and the onset of explosive sputtering processes. Studies of the ablated products by the use of quadrupole mass spectrometry, quartz microbalance, and photoimaging indicate large kinetic energies and a high directionality of the ablated products.

Linear optical effects in Z-scan measurements of thin films

We show that it is possible, in Z-scan measurements ofthin films, to obtain data that closely resemble typical results for nonlinear optical materials, but which actually arise from linear optical effects caused by sample damage. Z-scan measurements on a silica-based thin film yielded the expected peak-valley signature of Z-scan data, but subsequent analysis and microscopic examination of the film indicated that the data resulted from an ablation hole produced in the film when it was near the laser focus. The resulting spatial variation of the linear refractive index of the film produced a lensing effect that mimicked the typical Z-scan response. Scalar diffraction theory was used to model the effects of a spatially varying refractive index and gave results that qualitatively agreed with the Z-scanmeasurements.

Laser ablation of diamond films in various atmospheres

Abstract Diamond films of 5-μm thickness are irradiated with the emission of an ArF excimer laser (193 nm, τ = 20 ns, repetition rate = 20 Hz) in air, O2, He, H2, N2 and vacuum. The sample surface is polished with the laser beam under an angle of incidence of 80. Atomic Force Microscopic (AFM) measurements show differences of morphology and average roughness between polished films treated in different atmospheres. In a second experiment, the ablation depth is measured at normal incidence of the laser light. Depending on the atmosphere, different etching depths are measured. In He, H2, and N2, graphitic borders surround the ablation hole. They are assigned to sputtered ablation particles that were not combusted due to the absence of O2. During laser ablation in air, O2, and in vacuum, the amount of the sputtered carbon around the irradiated spots is significantly reduced, and no graphitic borders can be observed after irradiation. The black borders around the hole and the surface inside the ablated area are found to contain glassy carbon structures that can be etched away in a H2 plasma as shown with micro-Raman spectroscopy.

Ultrashort pulse laser ablation of silicon: an MD simulation study

Molecular Dynamics (MD) simulation has been employed in order to study laser ablation of silicon surfaces. The impact of laser pulses with lengths ranging from 5 ps down to 10 fs on a target of approximately 100A x 100A x 50A was investigated. The ablation shows a strong dependence on pulse length, on pulse energy and on the number of laser shots. With decreasing pulse length the amount of removed particles increases, and with decreasing pulse energy the holes become narrower. Especially in multishot ablations, holes with a diameter of just a fraction of the focus could be observed. This can be attributed mainly to ablation of atoms from lower areas and their redeposition close to the surface, leading to amorphous areas around the ablation hole. For pulses of picosecond duration, and even for femtosecond pulses, the main material removal occurs on a timescale of a few ps. Interestingly, the simulations show two thresholds: the onset of damage at the surface. which depends on the pulse energy but only insignificantly on the pulse length; and the onset of the removal of particles, which shows a strong dependence on the pulse length of the laser.