Picosecond laser ablation of thin copper films

The ablation process of thin copper films on fused silica by picosecond laser pulses is investigated. The ablation area is characterized using optical and scanning electron microscopy. The single-shot ablation threshold fluence for 40 ps laser pulses at 1053 nm has been determinated toF _thres = 172 mJ/cm². The ablation rate per pulse is measured as a function of intensity in the range of 5 × 10⁹ to 2 × 10¹¹ W/cm² and changes from 80 to 250 nm with increasing intensity. The experimental ablation rate per pulse is compared to heat-flow calculations based on the two-temperature model for ultrafast laser heating. Possible applications of picosecond laser radiation for microstructuring of different materials are discussed.     

Laser ablation loading of a radiofrequency ion trap

The production of ions via laser ablation for the loading of radiofrequency (RF) ion traps is investigated using a nitrogen laser with a maximum pulse energy of 0.17?mJ and a peak intensity of about 250?MW/cm². A?time-of-flight mass spectrometer is used to measure the ion yield and the distribution of the charge states. Singly charged ions of elements that are presently considered for the use in optical clocks or quantum logic applications could be produced from metallic samples at a rate of the order of magnitude 10⁵ ions per pulse. A linear Paul trap was loaded with Th⁺ ions produced by laser ablation. An overall ion production and trapping efficiency of 10^?7 to 10^?6 was attained. For ions injected individually, a dependence of the capture probability on the phase of the RF field has been predicted. In the experiment this was not observed, presumably because of collective effects within the ablation plume.     

A laser dry etch process for smooth continuous relief structures in InP

A laser induced etch process is described which uses a pulsed 248 nm KrF excimer laser and Cl₂ atmosphere for the fabrication of monolithic continuously curved reliefs in InP substrate. In a bakeable processing chamber with low base pressure a wide range of laser fluences is available for damage-free etching. Especially, by photothermal heating far above the melting point, mirrorlike smooth surfaces are obtained. The etch rate characteristics are correlated to the maximum surface temperature reached during the laser pulse. The etch rate is independent of pressure and gas flux in the ranges 0.1–10 mbar and 20–300 sccm, respectively. It increases, however, with the background substrate temperature. Etch rates of up to 3.6 nm/pulse or 4.3 lm/min are possible at 20 Hz pulse repetition rate without visible surface damage. The process exhibits a smooth increase of the etch rate from 1 to 3 nm/pulse between 200 and 300 mJ/cm², which could be used for making curved reliefs by optical transmission variations on the projection mask.     

Effect of absorbing microinhomogeneities on optical damage to Alkali-Halide crystals

Micropores up to 30?C100 ??m in size (bulk density ??10⁵ cm^?3) are obtained in NaCl, KCl, KBr, and RbI via the effect of a pulse of a CO₂ laser with power density 10⁶?C10⁷ W cm^?2 up to 5 ??s long. When a pore appears, plasma formation is initiated. The average temperature is ??5500 K and the pressure is ??10⁴ kg cm^?2. The dependence of the average weight of the material removed from the pore on the energy of the crystal lattice is found. Pore formation occurs mainly via the effect of the radiation pulse, due to evaporation at the absorption wave front (velocity, 5 m s^?1) and plastic deformation. The annealing kinetics of micropores and the effect of the ionizing radiation on the bulk pore formation in NaCl crystals are investigated.     

248 nm laser etching of GaAs in chlorine and ozone gas environments

KrF laser etching of GaAs in Cl₂ and O₃ gas ambients by direct laser illumination is reported. The etch depth per pulse in Cl₂ was found to be linear versus the laser fluence on the sample in the 0.2–1.1 J/cm² range. It increased as a function of the Cl₂ pressure up to 6 Torr and slightly decreased for pressures above this value. It also decreased as a function of the laser repetition rate. Very smoothly etched surfaces were obtained after irradiation using the Cl₂ and O₃ etching gases. Auger analysis of the etched GaAs surfaces shows almost no traces of chlorine after etching in Cl₂, whereas a thick oxide layer of about 1500 Å thickness was found after etching in ozone.     

Laser-induced etching of titanium by Br2 and CCl3Br at 248 nm

A quartz crystal microbalance (QCM) has been used to study the KrF* excimer laser-induced etching of titanium by bromine-containing compounds. The experiment consists of focusing the pulsed UV laser beam at normal incidence onto the surface of a quartz crystal coated with 1 m of polycrystalline titanium. The removal of titanium from the surface is monitored in real time by measuring the change in the frequency of the quartz crystal. The dependence of the etch rate on etchant pressure and laser fluence was measured and found to be consistent with a two-step etching mechanism. The initial step in the etching of titanium is reaction between the etchant and the surface to form the etch product between laser pulses. The etch product is subsequently removed from the surface during the laser pulse via a laser-induced thermal desorption process. The maximum etch rate obtained in this work was 6.2 Å-pulse^–1, indicating that between two and three atomic layers of Ti can be removed per laser pulse. The energy required for desorption of the etch product is calculated to be 172 kJ-mole^–1, which is consistent with the sublimation enthalpy of TiBr₂ (168 kJ-mole^–1). The proposed product in the etching of titanium by Br₂ and CCl₃Br is thus TiBr₂. In the etching of Ti by Br₂, formation of TiBr₂ proceeds predominantly through the dissociative chemisorption of Br₂. In the case of etching with CCl₃Br, TiBr₂ is formed via chemisorption of Br atoms produced in the gas-phase photodissociation of CCl₃Br.     

Correlation of plume dynamics and oxygen pressure with VO2 stoichiometry during pulsed laser deposition

Vanadium dioxide thin films have been deposited on Corning glass substrates by a KrF laser ablation of V₂O₅ target at the laser fluence of 2 J?cm^?2. The substrate temperature and the target-substrate distance were set to 500 ^°C and 4 cm, respectively. X-ray diffraction analysis showed that pure VO₂ is only obtained at an oxygen pressure range of 4×10^?3–2×10^?2 mbar. A higher optical switching contrast was obtained for the VO₂ films deposited at 4×10^?3–10^?2 mbar. The films properties were correlated to the plume-oxygen gas interaction monitored by fast imaging of the plume.     

Efficient ablation of organic polymers polyether sulphone and polyether ether ketone by a TEA CO2 laser with high perforation ability

Organic polymer (PES: PolyEther Sulphone and PEEK: PolyEther Ether Ketone) ablation with oscillation-line selected TEA CO₂ lasers is successfully demonstrated. With different irradiation conditions the ablative etch-rate slopes were varied, which means that the ablation process is dependent on the ablation conditions such as incident laser intensity and ambient gas. In perforation processing of the PEEK film, the TEA CO₂ laser had a higher etch rate of 42 m/pulse at a fluence of 70 J/cm² in vacuum than the XeCl laser.     

Picosecond laser ablation of nickel-based superalloy C263

Picosecond laser (10.4 ps, 1064 nm) ablation of the nickel-based superalloy C263 is investigated at different pulse repetition rates (5, 10, 20, and 50 kHz). The two ablation regimes corresponding to ablation dominated by the optical penetration depth at low fluences and of the electron thermal diffusion length at high fluences are clearly identified from the change of the surface morphology of single pulse ablated craters (dimples) with fluence. The two corresponding thresholds were measured as F _th(D1)¹=0.68±0.02 J/cm² and F _th(D2)¹=2.64±0.27 J/cm² from data of the crater diameters D _1,2 versus peak fluence. The surface morphology of macroscopic areas processed with a scanning laser beam at different fluences is characterised by ripples at low fluences. As the fluence increases, randomly distributed areas among the ripples are formed which appear featureless due to melting and joining of the ripples while at high fluences the whole irradiated surface becomes grainy due to melting, splashing of the melt and subsequent resolidification. The throughput of ablation becomes maximal when machining at high pulse repetition rates and with a relatively low fluence, while at the same time the surface roughness is kept low.     

Femtosecond laser ablation of silver foil with single and double pulses

The average ablation depth per pulse of silver foil by 130 fs laser pulses has been measured in vacuum over a range of three orders of magnitude of pulse fluence up to 900 J cm⁻². In addition, double pulses with separations up to 3.4 ns have been used to probe time scales of relevance for femtosecond ablation. The double pulse ablation depth, when each pulse fluence is 0.7 J cm⁻², falls to that of a single pulse as the pulse separation is increased from 0 ps to 700 ps. This time scale decreases to only 4 ps as the fluence is increased to 11 J cm⁻². It then jumps to 500 ps across a transition fluence where the slope of the ablation depth versus logarithmic fluence characteristic changes abruptly to a higher value. In addition, for pulse separations near 1000 ps, the second pulse can cause re-deposition of ejecta from the first pulse resulting in a double pulse ablation depth only 40% that of the first pulse alone. This has important implications for the interpretation of double pulse femto-LIBS intensities. Our results suggest that the optical properties of nano or mesoparticles play a significant role in double pulse ablation with large pulse separations.     

Condensation of laser‐produced gold plasma during expansion and cooling in a water environment

Physical processes involved in laser ablation in liquid (LAL) are studied using a gold target irradiated through transparent water. During and after irradiation, the heated material from the surface of a target produces a plume that expands into liquid‐forming nanoparticles (NPs). The LAL method of NP production is ecologically much cleaner than others. A better understanding of the processes associated with complicated hydrodynamic phenomena leading to LAL is important for controlled manufacturing. We consider laser pulses with different durations τ_L covering fifth orders of magnitudes ranging from 0.1 ps to 0.5 ns and large absorbed fluences F_abs near optical breakdown of liquid. It is shown that the trajectory of the contact boundary with liquid at the middle and late stages after passing the maximum intensity of the longest pulse is rather similar for very different pulse durations if energies F_abs are comparable. We trace how hot (in a few eV range) dense gold plasma expands, cools down, intersects a saturation curve, and condenses into NPs appearing first inside the water‐gold diffusively mixed intermediate layer where gold vapour has the lowest temperature. Later, the pressure around the gold‐water contact drops down below the critical pressure for water. As a result, the nanoparticles find themselves in gaseous water bubble where density of water gradually decreases to 10^?4 ? 10^?5  g/cm³ at maximum bubble expansion.     

Influence of ambient pressure on the hole formation in laser deep drilling

We investigate the temporal evolution of the hole depth and shape for percussion drilling at different ambient pressure conditions. Deep drilling is performed in silicon as target material by ultrashort laser pulses at 1030 nm and a duration of 8 ps. Simultaneously, the backlit silhouette of the hole is imaged perpendicular to the drilling direction. While typical process phases like depth development and shape evolution are very similar for atmospheric pressure down to vacuum conditions (10^?2 mbar), the ablation rate in the initial process phase is significantly increased for reduced pressure. The number of pulses till the stop of the drilling process also increases by a pressure reduction and exceeds drilling at atmospheric conditions by two orders of magnitude for a pressure of ca. 10^?2 mbar. Accordingly, the maximum achievable hole depth is more than doubled. We attribute this behavior to an enlarged mean free path for ablation products at reduced pressure and therefore lower or no deposition of particles inside the hole capillary under vacuum conditions while debris fills the hole already after a few thousand pulses at atmospheric pressure. This is supported by scanning electron cross section images of the holes.     

Ultraviolet photoablation of a plasma-synthesized fluorocarbon polymer

Plasma polymerized tetrafluoroethylene (PPTFE) is shown to undergo efficient 248 nm excimer laser ablation. The principle difference between this material and the analogous polytetrafluoroethylene (PTFE), which results in only poor quality ablation, is PPTFE's much greater absorption coefficient (7×10⁴ vs. 10² cm^–1). A plot of the ablation depth per pulse versus incident fluence indicates that the threshold for significant ablation occurs near 50 mJ/cm², and that approximately 0.7 m/pulse can be removed at 800 mJ/cm². Near threshold, the ablation rate curve can be fit by a single Arrhenius-type exponential. This suggests that the removal process is at least partially governed by a photothermal process, similar to well-known laser induced thermal desorption experiments. In the very low fluence regime between 10 and 30 mJ/cm², small removal rates are measured in a process likely dominated by non-thermal ablation. The paper concludes with a discussion of the high quality, micron-size features that can be directly patterned into PPTFE surfaces.     

Effect of the ceramic grain size and concentration on the dynamical mechanical and dielectric behavior of poly(vinilidene fluoride)/Pb(Zr<Subscript>0.53</Subscript>Ti<Subscript>0.47</Subscript>)O<Subscript>3</Subscript> composites

Laser-induced backside wet and dry etching (LIBWE and LIBDE) methods were developed for micromachining of transparent materials. Comparison of these techniques is helpful in understanding the etching mechanism but was not realized due to complications in setting up comparable experimental conditions. In our comparative investigations we used a solid tin film for dry and molten tin droplets for wet etching of fused-silica plates. A tin–fused-silica interface was irradiated through the sample by a KrF excimer laser beam (λ=248 nm, FWHM=25 ns); the fluence was varied between 400 and 2100 mJ/cm². A significant difference between the etch depths of the two investigated methods was not found. The slopes of the lines fitted to the measured data (sl_LIBDE=0.111 nm/mJ cm⁻², sl_LIBDE=0.127 nm/mJ cm⁻²) were almost similar. Etching thresholds for LIBDE and LIBWE were approximately 650 and 520 mJ/cm², respectively. To compare the dependence of etch rates on the pulse number, target areas were irradiated at different laser fluences and pulse numbers. With increasing pulse number a linear rise of depth was found for wet etching while for dry etching the etch depth increase was nonlinear. Secondary ion mass spectroscopic investigations proved that this can be due to the reconstruction of a new thinner tin-containing surface layer after the first pulse.     

Laser-assisted etching of gallium arsenide in chlorine at 308 nm

Xenon chloride (308 nm) excimer laser-assisted etching of GaAs (100) in Cl₂ was demonstrated and characterized with respect to laser and gas parameters. The etch rate increased linearly with laser fluence from thresholds in the range of 50 to 75 mJ/cm² to the highest fluence studied, 650 mJ/cm². For a laser fluence of 370 mJ/cm², the etch rate varied with Cl₂ pressure reaching a maximum at a Cl₂ pressure of about 2 Torr. The etch rate decreased monotonically with Ar buffer gas pressure because of redeposition of GaCl₃ products into the etched channel. The redeposited GaCl₃ affected the etch rate and the etch morphology. The etch rate and morphology also varied with laser repetition rate. The mobility of chlorine on the surface also plays an important role in the etching mechanism.     

CO2 laser ablative etching of polyethylene terephthalate

Films of polyethylene terephthalate (PET) can be successfully etched with 9 m radiation from a pulsed TEA CO₂ laser. The relationship between etch depth and fluence is broadly similar to that observed for excimer laser etching but with a less well-defined threshold. Time-resolved photoacoustic measurements of stress waves generated in the interaction show that at a fluence of 1.8 J cm^–2 ablation occurs 100–200 ns after the start of the laser pulse, a time which is consistent with the rate of thermal decomposition of PET. The volatile products of ablation are carbon monoxide, carbon dioxide, methane, ethyne, ethene, benzene, ethanal, and small quantities of other products. For fluences close to and appreciably above the threshold the ablated material consists predominantly of involatile species of relatively high molecular weight, whereas at higher fluences substantial fragmentation of the polymer to small molecules occurs.School of Chemistry     

Nanospallation induced by an ultrashort laser pulse

A femtosecond laser pulse with power density of 10¹³ to 10¹⁴ W/cm² incident on a metal target causes ablation and ejection of the surface layer. The ejected laser plume has a complicated structure. At the leading front of the plume, there is a spall layer where the material is in a molten state. The spall layer is a remarkable part of the plume in that the liquid-phase density does not decrease with time elapsed. This paper reports theoretical and experimental studies of the formation, structure, and ejection of the laser plume. The results of molecular dynamics simulations and a theoretical survey of plume structure based on these results are presented. It is shown that the plume has no spall layer when the pulse fluence exceeds an evaporation threshold F _ev. As the fluence increases from the ablation threshold F _a to F _ev, the spall-layer thickness for gold decreases from 100 nm to a few lattice constants. Experimental results support theoretical calculations. Microinterferometry combined with a pump-probe technique is used to obtain new quantitative data on spallation dynamics for gold. The ablation threshold is evaluated, the characteristic crater shape and depth are determined, and the evaporation threshold is estimated.     

纳秒激光烧蚀铝材料的二维数值模拟

为了探索纳秒脉冲强激光与材料的相互作用机理,建立了二维数值模型,利用有限差分法对纳秒激光脉冲烧蚀金属铝的温度场进行了数值模拟.通过对比不同脉宽、光斑和能量下激光引起的温度场随时间的演化,发现脉冲的前期温度升高比后期快.等温图显示中心温度升高最快,烧蚀轮廓与激光束形状相似,烧蚀深度达1—5 μm.脉宽越长,烧蚀越窄和越深,光斑越大,烧蚀越宽和越浅.数值研究表明,1)激光的脉冲形状、脉宽和功率密度直接影响烧蚀的形状和深度,2)激光功率密度在10⁹ W/cm²量级烧蚀     

Nonlinear optical characteristics of carbon disulfide

The nonlinear refractive index γ of CS₂ is studied using laser pulses of various durations (from 110 fs to 75 ns). It is found that γ increases with increasing pulse duration within the picosecond region (from (3 ± 0.6) × 10^?15 cm² W^?1 at 110 fs to (3.5 ± 0.7) × 10^?14 cm² W^?1 at 75 ns) due to orientational nonlinearities. Variations in the sign of γ caused by the thermal effect at different pulse durations and repetition rates are analyzed. It is demonstrated that the fast electronic component, the component associated with molecular processes, causing positive nonlinear refraction, and the acoustic component, responsible for negative nonlinear refraction, manifest themselves simultaneously. The results of a study of the nonlinear absorption of carbon disulfide are presented. The two-and three-photon absorption coefficients of CS₂ are determined to be (5 ± 1.5) × 10^?11 cm W^?1 and (2.8 ± 0.8) × 10^?23 cm³ W^?2, respectively.