Threshold behavior in polyimide photoablation: Single-shot rate measurements and surface-temperature modeling

The ablation rate of Kapton-type polyimide has been measured as a function of incident fluence and excimer laser wavelength using a sensitive quartz-crystal microbalance (QCM). The experiments were performed such that the fluence and the ablated depth were known for each laser pulse, avoiding the need to average rate and fluence data over many pulses. By limiting the investigations to the low-fluence regimes near ablation threshold, high precision and detailed curve shapes were obtained. It was found that the ablation rate increases smoothly and exponentially with increasing fluence for 248, 308, and 351 nm wavelengths. This exponential behavior was modeled using an Arrheniustype thermal rate equation. In contrast, the 193 nm curve is linear in fluence, displays a sharp threshold, and is consistent with a possible photochemical ablation mechanism. Using a sophisticated surface temperature modeling code, the maximum laser induced surface temperature at the fluence at which ablation can first be detected is found to be the same, 850° C, for all four wavelengths. This ablation temperature is significantly higher than the approximately 500° C temperature at which Kapton starts to degrade under isothermal heating conditions.     

Sub-picosecond UV-laser ablation of Ni films

Laser ablation of thin Ni films on fused silica by 0.5 ps KrF-excimer-laser pulses at 248 nm is reported. The onset of material removal from different film thicknesses (0.1, 0.3, 0.6 and 1.0 m) was measured in a laser ionization time-of-flight mass spectrometer by the amount of Ni atoms vs laser fluence. Significant amounts of metal atoms are already evaporated at laser fluences around 20 mJ/cm², a threshold up to 100 times smaller compared to the one for 14 ns pulses. In contrast to ns laser pulses, the ablation threshold for 0.5 ps pulses is independent of the film thickness. These results reflect the importance of thermal diffusion in laser ablation of strongly absorbing and thermally good conducting materials and prove that for ablation with short pulses, energy loss to the bulk is minimized.     

Imaging and analysis of photomechanical effects induced in water by high-power laser-target interaction

The kinetics of cavitation and associated photo-mechanical effects induced by underwater pulsed-laser irradiation of solid targets has been studied experimentally and analyzed with theoretical methods. A xenon-chloride excimer laser of 150 ns pulse duration has been utilized to produce ablation and local photofragmentation of artificial samples of hard tissues at fluences of 12–24 J/cm². The evolution of pressure wave and cavitation formations developing in the liquid from the target surface after laser irradiation has been observed with a time-resolved imaging technique employing a pump-probe laser arrangement. The analysis of experimental results has been performed by using the theoretical model of point explosion that has been successfully applied to fit the cavitation kinetics, providing also quantitative information on the energy transfer during photo-acoustic interactions.     

Spectroscopic studies of ArF laser photoablation of PMMA

A Spectroscopic study has been made of the emission spectra arising from ArF laser initiated photoablation of PMMA samples. This process leads to direct etching of the polymer. The thermal temperature of the CH fragment species in the plume immediately above the ablated site was found to be 3200 ±200 K. The translational velocity of this species was found to be 4.2×10⁵ cm/s corresponding to a translational temperature of 11,000 K. These results are consistent with a rapid direct bond scission model for ablation.     

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.     

Dual excimer and CO2 laser etching studies of polyethylene terephthalate

Polyethylene terephthalate (PET) films preheated with a pulsed CO₂ laser have been ablatively etched with an XeCl laser. The observed reduction in ablation threshold, from 170 to 140 mJ cm^–2, is consistent with a thermal mechanism for XeCl laser ablation of PET. Transient changes in the UV absorption coefficient of PET caused by heating with pulsed CO₂ laser radiation have also been studied and a significant increase in absorption observed at 308 nm. Permanent changes in the ultraviolet absorption of PET following exposure to low fluence XeCl laser radiation are also reported.     

Pulsed laser deposition: metal versus oxide ablation

We present experimental results of pulsed laser interaction with metal (Ni, Fe, Nb) and oxide (TiO₂, SrTiO₃, BaTiO₃) targets. The influence of the laser fluence and the number of laser pulses on the resulting target morphology are discussed. Although different responses for metal and oxide targets to repetitive laser irradiation could be expected due to the different band structures of metals and oxides, the optical response is quite similar for 248-nm laser irradiation. Therefore, the difference in response is largely caused by differences in thermal properties. Metal targets show periodic structures of the order of micrometers after consecutive pulses of laser radiation, while the SrTiO₃ and BaTiO₃ targets show a flat surface after ablation for relatively low fluences (1.0 Jcm^-2). The observed TiO₂ target ablation characteristics fall in between those of the ablated metals and perovskites, because ablation results in the presence of Ti-rich material, which shields the underlying stoichiometric target material from ablation. The final target morphology is dependent on fluence, number of pulses, and the movement of the target itself (rotating, scanning, or stationary). It can take between 15 and 75 pulses to reach a steady-state target morphology on a stationary target. PACS 79.20.Ds; 52.38.Mf; 81.15.Fg     

Ablation of polyimide (Kapton™) films by pulsed (ns) ultraviolet and infrared (9.17 μm) lasers

Ablation of the surface of a polyimide (Kapton) film by single pulses of 248 nm or 308 nm radiation (20 ns) or 9.17 m laser radiation (170 ns) was studied by photographing the emergence of the blast wave and the plume by a pulse (<1 ns; 596 nm) of visible laser light. The dynamics of the blast wave was similar in the ultraviolet and in the infrared but the composition of the plume was obviously different. A mass of opaque solid material was ejected for as long as 2.6 s following the IR pulse in contrast to the minute amount of solids that are seen in the ablation by UV laser pulses of ns duration. UV laser pulses of 50–400 s duration interact with polyimide surfaces in a manner that is similar to IR laser pulses of ns duration or longer. Chemical analysis of the ablation products that are obtained under various conditions of ablation when compared to the known modes of thermal degradation of polyimide show that the reaction is a thermal process when IR laser pulses or UV laser pulses of long (>10 s) duration are employed. Ablation by ns UV laser pulses differs fundamentally in the chemistry of the products from all of the cases mentioned above.     

Processing of W/Si and Si/W bilayers and multilayers with single and multiple excimer-laser pulses

Interdiffusion phenomena, thermal damage and ablation of W/Si and Si/W bilayers and multilayers under XeCl-excimer laser (=308 nm) irradiation at fluences of 0.15, 0.3 and 0.6 J/cm² were studied. Samples were prepared by UHV e-beam evaporation onto oxidized Si. The thickness of W and Si layers and the total thickness of the structures were 1–20 nm and 40–100 nm, respectively. 1 to 300 laser pulses were directed to the same irradiation site. At 0.6 J/cm² the samples were damaged even by a single laser pulse. At 0.3 J/cm² WSi₂ silicide formation, surface roughening and ablation were observed. The threshold for significant changes depends on the number of pulses: it was between 3–10 pulses and 10–30 pulses for bilayers with W and Si surfaces, respectively, and more than 100 pulses for multilayers with the same total thickness of tungsten. At 0.15 J/cm² the periodicity of the multilayers was preserved. Temperature profiles in layered structures were obtained by numerical simulations. The observed differences of the resistance of various bilayers and multilayers against UV irradiation are discussed.     

Nanosecond surface interferometry measurements on designed and commercial polymers

The effect of the ablation mechanism on surface morphology changes during an ablation process was studied by comparing three different polymers: a triazene polymer, a polyimide and poly(methylmethacrylate) (PMMA) with nanosecond surface interferometry. The triazene polymer, for which only indications for a photochemical ablation mechanism had been detected in previous studies, revealed no surface swelling, which could be attributed to a thermal ablation mechanism. For polyimide, a photothermal ablation mechanism is usually used to describe the ablation process at irradiation wavelengths 248 nm. However, the interferometric measurements do not show any surface swelling, which would be a clear indication for a thermal ablation mechanism. A surface swelling was only detected for PMMA with irradiation at 248 nm and fluences below the threshold of permanent surface modification. The detected phase shift, which is proportional to the change of the film thickness and the refractive index, can be explained by the opposite signs of the thermal expansion coefficient and the thermal refractive-index coefficient. PACS 52.38.Mf; 42.87.Bg; 71.20.Rv     

Nanosecond and femtosecond excimer laser ablation of fused silica

Ablation of fused silica using standard excimer lasers (20–30 ns pulse duration at 193, 248, and 308 nm) and a short pulse laser system (500 fs at 248 nm) is reported. Ablation rates range from several hundred nm/pulse (193 nm or fs-laser) up to about 6 m/pulse (308 nm). The performance of the ablation is found to depend not only on wavelength and pulse duration but also on the existing or laser induced surface quality (e.g., roughness) of the material. Special ablation phenomena are observed. At 193 nm and moderate fluence (3 J/cm²) ablation takes place at the rear side of a plate without affecting the front side, whereas at higher fluence normal ablation at the front side occurs. At 248 nm (standard excimer) the existence of two consecutive ablation phases is observed: smooth ablation at low rate is followed by explosive ablation at high rate. Using fs-pulses smooth shaped holes are formed during the first pulses, whereas high pulse numbers cause the development of a ripple structure in the ablation craters.The results lead to the conclusion that two different ablation mechanisms are involved: the first is based on two photon bulk absorption, the second on controlled surface damage in relation with (partially laser induced) singularity conditions at the surface.Presented at LASERION '91, June 12–14, 1991, München (Germany)     

CN temperatures above laser ablated polyimide

Polyimide is readily ablated by UV excimer lasers pulses. There exists an ongoing effort to determine the temperature (T) of the surface during the ablation process. The present experiment evaluates the temperature by means of high resolution laser-induced fluorescence measurements near the bandhead of the CN radical. The rotational state occupation indicates T=1710±140 K for ArF (193 nm) ablation with 100 to 500 mJ/cm². This temperature is near the extrapolation of earlier results at lower fluences (80 mJ/cm²). Furthermore, modeling predicts T values essentially equal to the present experimental result when the following are included: thermal diffusion in the solid, above surface UV absorption and increasing specific heat for temperatures above ambient. The present T-value reinforces the concept that thermal energy probably promotes the intermediate steps in photoablation.     

Laser fluence, repetition rate and pulse duration effects on paint ablation

The efficiency (mm³/(J pulse)) of laser ablation of paint was investigated with nanosecond pulsed Nd:YAG lasers (λ = 532 nm) as a function of the following laser beam parameters: pulse repetition rate (1-10,000 Hz), laser fluence (0.1-5 J/cm²) and pulse duration (5 ns and 100 ns). In our study, the best ablation efficiency (η ≅ 0.3 mm³/J) was obtained with the highest repetition rate (10 kHz) at the fluence F = 1.5 J/cm². This ablation efficiency can be associated with heat accumulation at high repetition rate, which leads to the ablation threshold decrease. Despite the low thermal diffusivity and the low optical absorption of the paint (thermal confinement regime), the ablation threshold fluence was found to depend on the pulse duration. At high laser fluence, the ablation efficiency was lower for 5 ns pulse duration than for the one of 100 ns. This difference in efficiency is probably due to a high absorption of the laser beam by the ejected matter or the plasma at high laser intensity. Accumulation of particles at high repetition rate laser ablation and surface shielding was studied by high speed imaging.     

Enhanced ablation of silica by the superposition of femtosecond and nanosecond laser pulses

We investigate the ablation process in SiO₂ by the superposition of 180 fs laser pulse (_center=800 nm) with a 15 ns laser pulse (_center=532 nm). Compared to femtosecond laser pulses alone, we measured an increase of 270±30% in volume of the ejected material with only a total increase of 40% of lasers fluences. This increase of ablation is the result of thermal and incubation effects generated by the femtosecond laser pulse. PACS 78.20.Nv; 61.80.Ba     

Kinetic Features of the Laser Ablation of Gamma-Irradiated Polyvinylidene Fluoride

We study the effect of γ-radiation from ⁶⁰Co on the rate of post-irradiation laser ablation of polyvinylidene fluoride (PVDF). The laser ablation of both the initial and γ-irradiated polymer occurs without an incubation period and does not require time to heat up the polymer target by the laser within the time scale of our measurements. The second feature of the laser ablation of PVDF is an extreme dependence of the ablation rate on the dose of γ-radiation over a wide range (10 kGy – 3.5 MGy) and the appearance of a minimum ablation rate of 0.1 mg/s at a dose 300 kGy. A gradual increase in the dose of γ-irradiation above 300 kGy is accompanied by a rise in the laser ablation rate. At γ-irradiation doses up to 2.3 MGy, the rate of the post-irradiated laser ablation of PVDF reaches 6.5 mg/s, which is equal to the laser ablation rate of non-irradiated PVDF.     

Bone-ablation mechanism using CO2 lasers of different pulse duration and wavelength

Bone ablation using different pulse parameters and four emission lines of 9.3, 9.6, 10.3, and 10.6 m of the CO₂ laser exhibits effects which are caused by the thermal properties and the absorption spectrum of bone material. The ablation mechanism was investigated with light- and electron-microscopy at short laser-pulse durations of 0.9 and 1.8 s and a long pulse of 250 s. It is shown that different processes are responsible for the ablation mechanism either using the short or the long pulse durations. In the case of short pulse durations it is shown that, although the mineral components are the main absorber for CO₂ radiation, water is the driving force for the ablation process. The destruction of material is based on explosive evaporation of water with an ablation energy of 1.3 kJ/cm³. Histological examination revealed a minimal zone of 10–15 m of thermally altered material at the bottom of the laser drilled hole. Within the investigated spectral range we found that the ablation threshold at 9.3 and 9.6 m is lower than at 10.3 and 10.6 m. In comparison the ablation with a long pulse duration is determined by two processes. On the one side, the heat lost by heat conduction leads to carbonization of a surface layer, and the absorption of the CO₂ radiation in this carbonized layer is the driving force of the ablation process. On the other side, it is shown that up to 60% of the pulse energy is absorbed in the ablation plume. Therefore, a long pulse duration results in an eight-times higher specific ablation energy of 10 kJ/cm³.     

Laser ablation of copper and aluminium in air

The ablation behavior of copper alloy and aluminium irradiated in air by 1.06 m, 10 ns pulsed laser with power density of 6.4×10⁹W/cm² was studied using scanning electron microscopy (SEM), MCS-RBS and X-ray microanalysis. Evidence of bulk vaporization via bubble formation was observed for the copper alloy under the laser irradiation. Silver-enrichment microregions were found in the ablation crater created by the laser shots on the copper alloy sample. Material removal rates of these materials were determined by crater shape-profile measurement. Using self-similar solutions of the gas-dynamic equations, gas-dynamic parameters of the vaporization waves are obtained. These parameters are used to calculate material removal rates and impulse coupling coefficients of these materials under the pulsed laser irradiation. The calculated mass removal rates and the coupling coefficients are compared with the corresponding experimentally determined values. The surface kinetic energy of the irradiated area on the Al sample is estimated. Possible mechanisms for laser ablation of the materials under study are discussed.     

Femtosecond uv excimer laser ablation

Experiments on the ablation of polymethylmethacrylate (PMMA) with 300 fs uv excimer laser pulses at 248 nm are reported for the first time. With these ultrashort pulses, ablation can be done at fluences up to five times lower than the threshold fluence for 16 ns ablation of PMMA, and the surface morphology is improved, also for several other materials. A model for ablation is proposed, assuming a non-constant absorption coefficient _eff depending on the degree of incubation of the irradiated material and the intensity of the incoming excimer laser pulse. The agreement between our model and our experimental observations is excellent for 16 ns excimer laser pulses, also predicting perfectly the shape of a pulse transmitted through a thin PMMA sample under high fluence irradiation. Qualitative agreement for 300 fs excimer laser pulses is obtained so far.     

Study of Direct Amplification of Ultrashort Light Pulses in a Laser Amplifier with a View to Obtaining High Radiation Contrast

An amplification scheme for ultrashort laser pulses of high radiation contrast was used to perform experiments on ablation pressure symmetrization using a prepulse upon acceleration of thin foils. It is shown that the spontaneous radiation of the regenerative amplifier restricts the energy contrast in the amplification of chirped pulses at a level of 10^-4–10^-3. The possibility of direct amplification of a short pulse with a view to increasing the energy contrast ratio was considered. Experiments were performed on the PICO laser facility to demonstrate that a 10-ps pulse amplification achieved an intensity 100 GW/cm², a gain factor of 1.2, and an inversion dumping factor >30%.     

Pulsed laser coating removal by detachment and ejection

The pulsed laser removal of paint coatings from substrates by detachment and ejection of the entire layer has been studied. The dependence of the removal efficiency on fluence, coating thickness and pulse duration has been examined. For weakly absorbing coatings on easily ablating substrates, efficiencies as high as 800 mcm²J^-1 have been measured, i.e., more than two orders of magnitude higher than obtainable by surface ablation. PACS 52.38.Mf; 42.62.Cf; 81.65.Cf