Effects of closed immersion filtered water flow velocity on the ablation threshold of bisphenol A polycarbonate during excimer laser machining

A closed flowing thick film filtered water immersion technique ensures a controlled geometry for both the optical interfaces of the flowing liquid film and allows repeatable control of flow-rate during machining. This has the action of preventing splashing, ensures repeatable machining conditions and allows control of liquid flow velocity. To investigate the impact of this technique on ablation threshold, bisphenol A polycarbonate samples have been machined using KrF excimer laser radiation passing through a medium of filtered water flowing at a number of flow velocities, that are controllable by modifying the liquid flow-rates. An average decrease in ablation threshold of 7.5% when using turbulent flow velocity regime closed thick film filtered water immersed ablation, compared to ablation using a similar beam in ambient air; however, the use of laminar flow velocities resulted in negligible differences between closed flowing thick film filtered water immersion and ambient air. Plotting the recorded threshold fluence achieved with varying flow velocity showed that an optimum flow velocity of 3.00 m/s existed which yielded a minimum ablation threshold of 112 mJ/cm². This is attributed to the distortion of the ablation plume effected by the flowing immersion fluid changing the ablation mechanism: at laminar flow velocities Bremsstrahlung attenuation decreases etch rate, at excessive flow velocities the plume is completely destroyed, removing the effect of plume etching. Laminar flow velocity regime ablation is limited by slow removal of debris causing a non-linear etch rate over ‘n’ pulses which is a result of debris produced by one pulse remaining suspended over the feature for the next pulse. The impact of closed thick film filtered water immersed ablation is dependant upon beam fluence: high fluence beams achieved greater etch efficiency at high flow velocities as the effect of Bremsstrahlung attenuation is removed by the action of the fluid on the plume; low fluences loose efficiency as the beam makes proportionally large fluence losses at it passes through the chamber window and immersion medium.     

Impact of open de-ionized water thin film laminar immersion on the liquid-immersed ablation threshold and ablation rate of features machined by KrF excimer laser ablation of bisphenol A polycarbonate

Debris control and surface quality are potential major benefits of sample liquid immersion when laser micromachining; however, the use of an immersion technique potentially modifies the ablation mechanism when compared to an ambient air interaction. To investigate the machining characteristics, bisphenol A polycarbonate has been laser machined in air and under a controllable open liquid film. To provide quantitative analysis, ablation threshold, ablation rate and the attenuation coefficient of the immersing de-ionized (DI) water fluid were measured. In ambient air the threshold fluence was measured to be 37 mJ cm⁻². Thin film immersion displayed two trends: threshold fluences of 58.6 and 83.9 mJ cm⁻². The attenuation of DI water was found to be negligible; thus, the change in ablation rate resulted from increased confinement of the vapour plume by the liquid medium, generating higher Bremsstrahlung attenuation of the beam, lowering the laser etch rate. Simultaneously, splashing motivated by the confined ablation plume allowed release of plume pressure before plume etching commenced. This contributed to the loss of total etching efficiency. Two interaction scenarios were obsereved as a result of splashing: (i) intermediate threshold fluence, where splashing occured after every pulse in a mode that interrupted the flow entirely, leaving an ambient air interaction for the following pulse; (ii) high threshold fluence, where splashing occured for every pulse in a mode that allowed the flow to recommence over the image before the next pulse causing every pulse to experience Bremsstrahlung attenuation. Since attenuation of the immersion liquid was negligible, it is the action of the constrained ablation plume within a thin flowing immersion liquid, the resultant Bremsstrahlung attenuation and splashing events that are the critical mechanisms that modify the primary ablation characteristics.     

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利用时空分辨的测量技术,测定了XeCl紫外激光烧蚀金属Cu消融粒子的发射光谱随时间与空间的强度分布。通过在不同的氢气压强下拍摄到的XeCl308nm紫外激光烧蚀金属Cu消融粒子的发光羽照片,发现激光消融粒子发光羽的颜色在不同区域有不同的颜色,不同区域的发光羽颜色随环境气压的改变而变化。随环境气压的增大,发光羽不仅逐渐变小,而且逐渐变淡。对激光消融粒子发光羽的机理进行了探讨,激光消融粒子发光羽的发光动力学模式在不同区域有不同的发光模式,且随环境气压的变化而改变。定性地解释了所观察的实验现象。     

Laser-produced plasma ion characteristics in laser ablation of lithium manganate

Laser ablation is widely used to assist in the fabrication of prototype lithium manganate (LiMn₂O₄) thin film structures for Li-ion battery electrodes via the pulsed laser deposition technique. However, films can be considerably Li and/or O deficient, depending the deposition conditions used. Here we present data on the ionic component of laser-produced plasma in laser ablation of lithium manganate with ns excimer laser. Plasma was monitored using an electrical Langmuir ion probe, in time-of-flight mode in conjunction with mass spectrometry to identify the dominant ionic species. Ablation in vacuum at ∼2.5 J cm⁻² revealed the plasma's ionic component was composed primarily of singly charged Li and Mn ions. The time-of-flight data indicates significant deceleration of the plasma when ablation is carried out in an oxygen background gas pressure of the order of 10 Pa. The implications for thin film growth are considered in terms of the possible gas phase interactions and/or thin film re-sputtering yield.     

脉冲激光诱导Cu靶产生发光羽的特性分析

通过在不同的环境气压下拍摄脉冲激光烧蚀金属Cu诱导产生的发光羽,获取了不同区域具有不同颜色特征的发光羽照片.结果发现:发光羽的颜色随环境气压的改变而变化.采用空间分辨光谱技术,测定了激光诱导金属Cu靶产生发光羽辐射强度的空间分布,以及不同烧蚀环境气压对发光羽辐射强度的影响.研究了脉冲激光烧蚀Cu表面诱导发光的动力学过程,建立了可能的发光羽分区模型,对发光羽的不同区域发光粒子的激发机理进行了探讨,并用之定性地解释了所观察的实验现象.结果分析表明:脉冲激光诱导Cu产生的发光羽可以分为三个区域,不同区域的发光机理不同,Cu原子和Cu离子的激发机理不完全相同.     

准分子激光取样与电感耦合等离子体质谱和光谱联用分析仪器研究进展

将准分子激光剥蚀取样后的产物经由电感耦合等离子质谱与光谱分析,从而获得被激光剥蚀样品的元素与同位素含量信息,是迄今为止适应于表面原位微区分析最为重要的分析科学技术手段之一.基于准分子激光剥蚀取样技术分别与电感耦合等离子体质谱或发射光谱技术联用的分析手段,已经被广泛应用于地质学、材料学、环境科学,甚至生命科学领域的原位微...     

Atomic force microscopic characterization of films grown by inverse pulsed laser deposition

Carbon nitride films have been deposited by KrF excimer laser ablation of a rotating graphite target in 5 Pa nitrogen ambient in an inverse pulsed laser deposition configuration, where the backward motion of the ablated species is utilised for film growth on substrates lying in the target plane. Topometric AFM scans of the films, exhibiting elliptical thickness distribution, have been recorded along the axes of symmetry of the deposition area. High resolution AFM scans revealed the existence of disk-like, or somewhat elongated rice-like features of 5-10 nm average thickness and ∼100 nm largest dimension, densely packed over the whole, approximately 14 × 10 cm² deposition area. The RMS roughness of the film decreased from 9 nm near to the laser spot down to 2 nm in the outer regions. Even the highest RMS value obtained for IPLD films was less than half of the typical, 25 nm roughness measured on simultaneously deposited PLD films.     

Excimer laser deposition of thin amorphous carbon films

A new carbon film deposition technique, based upon excimer laser vaporization of graphite in a flowing gas system has been developed. The low temperature vapor (LTV) technique alleviates high temperatures occurring in most other deposition methods. In this technique the UV laser ablation occurs in an inert flowing gas atmosphere. Atoms and molecules evaporated from graphite are cooled by gas entrainment before condensing on a substrate. The resulting films of amorphous carbon or hydrogenated amorphous carbon are free from strain. Measurement of the optical band gap of these films shows that E_g can be controlled by the hydrogen content of the carrier gas.     

XPS studies of short pulse laser interaction with copper

The effect of laser ablation on copper foil irradiated by a short 30 ns laser pulse was investigated by X-ray photoelectron spectroscopy. The laser fluence was varied from 8 to 16.5 J/cm² and the velocity of the laser beam from 10 to 100 mm/s. This range of laser fluence is characterized by a different intensity of laser ablation. The experiments were done in two kinds of ambient atmosphere: air and argon jet gas.The chemical state and composition of the irradiated copper surface were determined using the modified Auger parameter (α′) and O/Cu intensity ratio. The ablation atmosphere was found to influence the size and chemical state of the copper particles deposited from the vapor plume. During irradiation in air atmosphere the copper nanoparticles react with oxygen and water vapor from the air and are deposited in the form of a CuO and Cu(OH)₂ thin film. In argon atmosphere the processed copper surface is oxidized after exposure to air.     

Non-contact acoustic tests based on nanosecond laser ablation: Generation of a pulse sound source with a small amplitude

A method to generate a pulse sound source for acoustic tests based on nanosecond laser ablation with a plasma plume is discussed. Irradiating a solid surface with a laser beam expands a high-temperature plasma plume composed of free electrons, ionized atoms, etc. at a high velocity throughout ambient air. The shockwave generated by the plasma plume becomes the pulse sound source. A laser ablation sound source has two features. Because laser ablation is induced when the laser fluence reaches 10¹²–10¹⁴ W/m², which is less than that for laser-induced breakdown (10¹⁵ W/m²), laser ablation can generate a lower sound pressure, and the sound source has a hemispherical radiation pattern on the surface where laser ablation is generated. Additionally, another feature is that laser-induced breakdown sound sources can fluctuate, whereas laser ablation sound sources do not because laser ablation is produced at a laser beam–irradiation point. We validate this laser ablation method for acoustic tests by comparing the measured and theoretical resonant frequencies of an impedance tube.     

The influence of UV irradiation on the surface of chitosan films

The surface properties of chitosan films before and after UV-irradiation (λ = 254 nm and 248 nm, respectively) were investigated using the technique of scanning electron microscopy (SEM) and by means of contact angle measurements allowing the calculation of surface free energy. Moreover, in order to determine the film mass changes, quartz crystal microbalance (QCM) measurements were performed. Measurements of the contact angle for diiodomethane (D), formamide (F) and glycerol (G) on the surface of chitosan films were made. The chemical and structural changes during UV irradiation were studied by FTIR-ATR spectroscopy.The contact angle and the surface free energy were altered by UV irradiation of chitosan films. The microscopy images have shown that the KrF excimer laser irradiation caused visible damages on the surface in comparison with the surface exposed to the mercury UV lamp. The surface modification of chitosan films can be achieved using both, the low intensity UV lamp and the excimer laser.     

Shallow hydroxyapatite coatings pulsed laser deposited onto Al2O3 substrates with controlled porosity: correlation of morphological characteristics with in vitro testing results

We studied the influence of porous Al₂O₃ substrates on Ce-stabilized ZrO₂-doped hydroxyapatite thin films morphology pulsed laser deposited on their top. The porosities of substrates were monitored by changing sintering temperatures and measured with a high pressure Hg porosimeter.The depositions were conducted in 50 Pa water vapors by multipulse ablation of the targets with an UV KrF* (λ = 248 nm, τ ∼ 25 ns) excimer laser. The surface morphology of synthesized nanostructures was investigated by scanning electron microscopy and atomic force microcopy. Ca/P ratio within the range 1.67-1.70 was found for hydroxyapatite coatings by energy dispersive spectroscopy.The films were further seeded with mesenchymal stem cells for in vitro tests. The cells showed good attachment and spreading uniformly covering the entire surface of samples. The complexity of film morphology which is increasing with substrate porosity was shown to have a positive influence on cultivated cells density.     

The influence of substrate material and annealing procedure on the properties of superconducting thin films

Thin layers YBa₂Cu₃O_7–x. are deposited by a laser ablation technique using a pulsed excimer laser operating at 308 nm. The influence of the substrate material and the annealing procedure on the superconducting behaviour of the 123 film and the reactions between the film and the substrate are studied by resistance, X-ray patterns and TEM measurements. The best results are obtained for deposition on (100) SrTiO₃ substrates. The resistance of the 1 m thick film shows a metallic behaviour, an onset in superconductivity at a temperature of 90 K, and has zero resistance at 86 K. The 123 material has a preferential oriented c-axis perpendicular to the surface plane.     

Influence of additional external pressure on optical emission intensity in liquid-phase laser ablation

We examined the influence of additional external pressure on the optical emission intensity from plasmas produced by laser ablation of a Ti target immersed in distilled water. We adopted two methods for applying the external pressure. When ambient water was pressurized by connecting the ablation chamber to N₂ gas at a pressure range of 0.1-0.9 MPa, we observed the increase in the optical emission intensity with the pressure. This increase was considered to be caused by the change in the amount of dissolved N₂ gas in the water. On the other hand, when an external pressure of 30 MPa was applied to ambient water by using a mechanical pump, we observed the compression of the spatial distribution of the optical emission intensity. These experimental results suggest a possibility that chemical reactions and physical states (pressure and temperature) of liquid-phase laser-ablation plasmas can be controlled by adding external pressure to ambient liquid.     

Measurements of nanoparticle size distribution produced by laser ablation of tungsten and boron-carbide in N2 ambient

Nanoparticles (NPs) were produced by ablating tungsten and boron-carbide (B₄C) target materials in atmospheric pressure nitrogen ambient using ArF excimer laser pulses. The size distributions of the NPs formed during the ablation were monitored—within a 7-133 nm size window—by a condensation particle counter connected to a differential mobility analyzer. The laser repetition rate was varied between 1-50 Hz, and the fluence was systematically changed in the range of 0.5-15 J/cm², for both materials, allowing a comparative study in an extended laser parameter regime. The multishot ablation threshold (Φ_th) of B₄C was determined to be ∼1.9 J/cm² for the laser used (ArF excimer, λ = 193 nm). Similarly to earlier studies, it was shown that the size distributions consist of mainly small nanoparticles (<∼20 nm) attributed to a non-thermal ablation mechanism below Φ_th. An additional broad peak appears (between 20 and 40 nm) above Φ_th as a consequence of the thermally induced macroscopic ablation. Chemical composition of deposited polydisperse nanoparticles was studied by X-ray photoelectron spectroscopy showing nitrogen incorporation into the boron-carbide.     

Time-resolved measurements during backside dry etching of fused silica

The laser-induced backside dry etching (LIBDE) investigated in this study makes use of a thin metal film deposited at the backside of a transparent sample to achieve etching of the sample surface. For the time-resolved measurements at LIBDE fused silica samples coated with 125 nm tin were used and the reflected and the transmitted laser intensities were recorded with a temporal resolution of about 1 ns during the etching with a ∼30 ns KrF excimer laser pulse. The laser beam absorption as well as characteristic changes of the reflection of the target surface was calculated in dependence on the laser fluence in the range of 250-2500 mJ/cm² and the pulse number from the temporal variations of the reflection and the transmission. The decrease of the time of a characteristic drop in the reflectivity, which can be explained by the ablation of the metal film, correlates with the developed thermal model. However, the very high absorption after the film ablation probably results in very high temperatures near the surface and presumably in the formation of an absorbing plasma. This plasma may contribute to the etching and the surface modification of the substrate. After the first pulse a remaining absorption of the sample was measured that can be discussed by the redeposition of portions of the ablated metal film or can come from the surface modification in the fused silica sample. These near-surface modifications permit laser etching with the second laser pulse, too.     

Analyses of surface coloration on TiO2 film irradiated with excimer laser

TiO₂ film of around 850 nm in thickness was deposited on a soda-lime glass by PVD sputtering and irradiated using one pulse of krypton-fluorine (KrF) excimer laser (wavelength of 248 nm and pulse duration of 25 ns) with varying fluence. The color of the irradiated area became darker with increasing laser fluence. Irradiated surfaces were characterized using optical microscopy, scanning electron microscopy, Raman spectroscopy and atomic force microscopy. Surface undergoes thermal annealing at low laser fluence of 400 and 590 mJ/cm². Microcracks at medium laser fluence of 1000 mJ/cm² are attributed to surface melting and solidification. Hydrodynamic ablation is proposed to explain the formation of micropores and networks at higher laser fluence of 1100 and 1200 mJ/cm². The darkening effect is explained in terms of trapping of light in the surface defects formed rather than anatase to rutile phase transformation as reported by others. Controlled darkening of TiO₂ film might be used for adjustable filters.     

Alcohol-assisted photoetching of silicon carbide with a femtosecond laser

Femtosecond lasers have proved to be effective tools for micromachining silicon carbide material. In the drilling process, however, when the debris around the hole was not removed efficiently, the depth of hole would not increase further. In this paper, alcohol-assisted photoetching of 6H silicon carbide was investigated using a femtosecond laser. Machining in the presence of alcohol was beneficial to the debris ejection from the hole. The alcohol flow and volatilization was also helpful to further carry away the ablation debris and reduce the ablated material redeposition. The experiment showed that photoetching assisted by alcohol produced cleaner ablation effect and deeper hole than in ambient air. Moreover, alcohol assistance would not produce additional thermal damage around the hole. Vias were formed in a 250 μm thick wafer with alcohol-assisted photoetching technique using a femtosecond laser, which demonstrated the potential for this processing technique.     

Target density and surface state effects on the compositional changes in iron oxide particle aggregated films prepared by laser ablation

Iron oxide nanoparticle aggregated films were prepared using the excimer laser ablation technique by adopting an off-axis configuration and the gas condensation process. Sintered iron oxide (-Fe₂O₃) targets were ablated in oxygen ambient by an ArF excimer laser. The product of ablation comprised Fe₂O₃ at lower pressure and a mixture of Fe₂O₃ and FeO at higher pressure by X-ray-diffraction measurements. The maximum ambient oxygen pressure, P_S(O₂), at which the product composition was still a single Fe₂O₃ phase was higher for the higher-density target than for its lower-density counterpart. The target surface state affected the product composition only if the pressure was set to the pressure P_S(O₂) of 40 Pa for a high-density target. When the fluence was high (200 mJ/pulse, 3.3-mm² spot size), the product composition varied at the initial stage of laser irradiation with the number of laser pulses from a mixture of Fe₂O₃ and FeO to only Fe₂O₃ along with the target surface morphological change from a grooved structure to a smooth surface. Product composition was practically independent of the number of pulses by low-fluence laser irradiation even at this particular pressure of 40 Pa. PACS 81.07.Bc; 81.15.Fg; 61.10.Nz     

Comparative study between IR and UV laser radiation applied to the removal of graffitis on urban buildings

The present article focuses on a comparison between cleaning process of graffitis on urban buildings by using laser radiation at 308 nm (XeCl excimer laser) and 1064 nm (Nd:YAG laser). Laser-induced breakdown spectroscopy (LIBS) elemental analysis was applied as real-time diagnostic technique, safeguarding against possible damage of the substrate during ablation rate studies. The morphological analysis of the etched surfaces by optical microscopy and environmental scanning electron microscopy reveals remarkable features of interest to understand the wavelength dependence of the ablation efficiency. The ablation threshold fluences of different paints sprayed on several substrates were determined applying a photoacoustic technique. To remove graffitis from urban buildings the laser radiation at 1064 nm was observed to be the most efficient wavelength, supporting the best result.