Laser induced decomposition of a designed and a commercial polymer studied by ns-interferometry and shadowgraphy

Commercially available polymers often exhibit quite poor laser ablation properties for irradiation wavelengths around 248 nm. At these wavelengths, the absorption is due to photostable aromatic groups. Photolabile triazene polymers were developed to compare the influence of a photolabile group on the laser ablation process. The photochemically active triazene group has a strong absorption band at 332 nm, whereas the second absorption maximum at 220 nm is due to the photostable aromatic group. By irradiating at 308 nm and 193 nm, the influence of the photochemically active group on the ablation process can be studied. The etching of the triazene polymer starts and ends with the laser pulse. No surface swelling, which is assigned to photothermal ablation, is detected for fluences above the threshold of ablation. The expansion of the laser ablation induced shockwave was measured for the photolabile triazene polymer and the photostable polyimide. The speed of the shockwave increases with fluence and is higher for irradiation with 193 nm than with 308 nm. A shockwave with equal or higher velocity is observed for the triazene polymer compared with polyimide. Received: 28 August 2002 / Accepted: 20 September 2002 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +41-56/310-4412, E-mail: thomas.lippert@psi.ch     

Fundamentals and applications of polymers designed for laser ablation

The ablation characteristics of various polymers were studied at low and high fluences for an irradiation wavelength of 308 nm. The polymers can be divided into three groups, i.e. polymers containing triazene groups, designed ester groups, and reference polymers, such as polyimide. The polymers containing the photochemically most active group (triazene) exhibit the lowest thresholds of ablation (as low as 25 mJ cm^-2) and the highest etch rates (e.g. 250 nm/pulse at 100 mJ cm^-2), followed by the designed polyesters and then polyimide. Neither the linear nor the effective absorption coefficients have a clear influence on the ablation characteristics. The different behavior of polyimide might be explained by a pronounced thermal part in the ablation mechanism. The laser-induced decomposition of the designed polymers was studied by nanosecond interferometry and shadowgraphy. The etching of the triazene polymer starts and ends with the laser pulse, indicating photochemical ablation. Shadowgraphy reveals mainly gaseous products and a pronounced shockwave in air. The designed polymers were tested for an application as the polymer fuel in laser plasma thrusters. Received: 21 October 2002 / Accepted: 20 January 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +41-56/3104-412, E-mail: thomas.lippert@psi.ch     

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.     

Single-shot UV-laser ablation of polyimide with variable pulse lengths

Single-shot laser ablation of polyimide has been investigated with UV-Ar⁺-laser radiation ( = 270-315 nm) for pulse lengths between 140 ns and 5 µs. The irradiated polymer surface was studied with respect to its morphology and ablated depth by means of atomic force microscopy. The dependence of the ablation threshold on laser pulse-length and intensity can be tentatively interpreted on the basis of a thermal process and a (thermal or non-thermal) mechanism which diminishes the activation energy for the desorption of species from the surface.     

Plasma effects in picosecond-femtosecond UV laser ablation of polymers

Laser ablation of polyimide (PI) and polymethyl-methacrylate (PMMA) at 248 nm with pulse lengths ranging from 200 fs to 200 ps was investigated. The measured ablation rates show minima for pulse lengths of about 5 ps (PMMA) or 50 ps (PI).The reflected fraction of the ablating laser pulse was measured as a function of the pulse length. In the case of PMMA maximum reflectance corresponds to a minimum ablation rate.This behavior can be explained by a dynamic plasma reflection model: A fast build up of a dense plasma is followed by high obscuration for a brief transition time and a self-regulating opacity for the rest of the pulse. This model of plasma mediated ablation leads to a ^1/4-dependence of the ablation rate at fixed fluence, which fits very well to the measured data, in particular if an extension to nanosecond ablation data of PI and PMMA is considered. PACS 52.50.Jm; 61.80.Ba; 42.65.Re     

Examination of the influence of molecular weight on polymer laser ablation: polystyrene at 248 nm

Following previous studies on the influence of the polymer molecular weight (M_W) on the ablation of poly(methyl methacrylate) (PMMA) at 248 nm, this work extends the examination to the ablation of polystyrene (PS) at 248 nm. The ablation threshold and the etching rates are found to be nearly independent of M_W. Optical microscopy demonstrates an excellent crater morphology, few small bubbles are formed on the surface of the low M_W. Examination of the formation kinetics of products in the irradiation of samples doped with the photoreactive iodophenanthrene demonstrates that high temperatures develop upon irradiation, suggesting that thermal mechanism dominates in the ablation of PS at 248 nm. In similarity to the etching rates, the attained temperatures are largely independent of the PS M_W. The factors for the weak dependence of the process on PS M_W are discussed.     

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.     

Changes in the etch rate of photosensitive polymers as a function of the pulse number

The ablation rates of a polyimide and a triazene polymer were studied gravimetrically by a quartz micro balance for 248-nm and 308-nm irradiation. Special care was taken to examine the dependence of the ablation rate at constant fluences for single pulses and the influence of consecutive pulses at the same position. A clear trend was observed in these measurements, i.e., that the mass loss after the first pulse is always different from values for the following pulses. This implies that it is very difficult to determine true ablation rates, which are the foundation of most ablation models. The differences of the mass loss between the first pulse and the following pulses is most probably due to carbonization of the material, resulting in varying ablation rates for the following pulses. The ablation rates are thus not a real material property but a superposition of the material ablation rates with the ablation rates of carbon and carbonized material. PACS 52.38.Mf; 71.20.Rv; 07.10.Lw     

Polymer ablation: From fundamentals of polymer design to laser plasma thruster

UV-Laser ablation of polymers is a well-established method to structure and deposit polymers, but the mechanisms of ablation are still controversial, i.e. photothermal or photochemical processes. An approach to probe the ablation mechanisms and to improve ablation is to incorporate photoactive groups into the polymer structure.The investigation of the ablation behavior of designed triazene polymers showed that the ablation mechanism is always a combination of both photothermal and photochemical processes, but the ratio can be changed by using different polymers and irradiation wavelengths. Also the quality of structures in the triazene polymers is superior at an irradiation wavelength of 308 nm compared to commercially available polymers.Polymers can be designed not only for UV irradiation, but also for applications in the IR range, but with different requirements. One application for designed polymers in the near-IR range is as fuel for the laser plasma thruster, which is used as propulsion system for small satellites. With commercially available polymers the necessary thrust could not be achieved. A specially designed polymer-absorber system for this application produce more energy in the form of thrust, than the laser delivered.     

Incubation/ablation patterning of polymer surfaces with sub-μm edge definition for optical storage devices

By exposure to low fluence UV laser radiation, the optical absorption coefficient of subsurface polymer material can be increased (incubation) with spatial control, using a suitable contact mask, proper imaging of the mask, or laser direct writing. Spatially selective ablation of polymethylmethacrylate (PMMA) is then achieved with large area XeCl excimer laser pulses at 308 nm. In this way, the transfer of spatial information to the material can be decoupled from the high laser fluence removal (ablation) step. The advantages are: The mask is exposed to only low fluence laser radiation — damage is avoided. Since the mask can be removed before the ablation step, mask contamination by the ablated plume cannot occur. Using this incubation/ablation method, PMMA surfaces can be patterned (248 nm/308 nm) with submicrometer spatial control and edge contrasts better than 0.2 m. This has impact on optical storage technology and laser surface processing techniques in general. The smallest single structure obtained was somewhat smaller than 0.5 m in diameter up to now, given by the mask.Presented at Laserion '91, June 12–14, 1991     

Removal of doped poly(methylmetacrylate) from tungsten and titanium substrates by femto- and nanosecond laser cleaning

The influence of different laser pulse lengths on the removal of a polymer layer from metal substrates was investigated. As model systems, doped poly(methylmetacrylate) (PMMA) on titanium and tungsten substrates were selected.The ablation threshold and irradiation spot morphology of titanium and tungsten were compared for femtosecond (fs) and nanosecond (ns) laser irradiation and different pulse numbers. Nanosecond laser treatment resulted in a non-homogeneous surface morphology for both titanium and tungsten substrates. Femtosecond irradiation of tungsten revealed a homogeneous ablation spot with little changes in the surface morphology. For titanium, the formation of columnar structures within the irradiation spot was observed.Two different dopant concentrations were used for PMMA to achieve an equal linear absorption coefficient for the femto- and nanosecond laser wavelengths of 790 and 1064 nm. The best results were achieved for the removal of doped PMMA by femtosecond laser irradiation, where only a minimal modification of the metal surface was detected. In the case of nanosecond laser exposure, a pronounced change of the structure was observed, suggesting that damage-free cleaning of the selected metal may only be possible using femtosecond laser pulses. Different experimental parameters, such as laser fluence, pulse repetition rate and sample speed were also investigated to optimize the cleaning quality of doped PMMA from tungsten substrates with femtosecond laser pulses.     

Analysis by using X-ray photoelectron spectroscopy for polymethyl methacrylate and polytetrafluoroethylene etched by KrF excimer laser

The C 1s, F 1s, and O 1s electron spectra for polymethyl methacrylate (PMMA) and polytetrafluoroethylene (PTFE) irradiated by KrF excimer laser with 248 nm wavelength were analyzed by the X-ray photoelectron spectroscopy (XPS) method. The results show that, after irradiation by the laser, the percentage of the carbon atoms of C-C bond decreases and the percentage of CO bond increases for PMMA, while for PTFE percentages of both C-C bond and CF₂ group decrease drastically, respectively. Moreover, it was found that C-O bond and other complex carbon-oxygen groups appeared for PTFE. The photon-chemical processes associated with the energy level transitions, energy diversion, and dissociation of bonds in the interaction were theoretically analyzed based on the chemical structures of PTFE and PMMA. Our analyses can successfully explain that PMMA can be effectively etched by KrF excimer laser with 248 nm wavelength can efficiently etch the PMMA, but the surface of PTFE can only be modified by it.     

Comparison of the transmission behavior of a triazeno-polymer with a theoretical model

The threshold fluence,F _Th, of ablation of a triazeno-polymer was measured in the low fluence range for thin films using conventional UV-spectroscopy. It was found that there is a clearly definedF _Th for 308 nm irradiation between 20 and 25 mJ cm^–2. In the case of 248 nm irradiation, a threshold fluence range between 16 and 32 MJ cm^–2 was found. The ablation rate for both irradiation wavelengths depends on film-thickness. For the XeCl excimer-laser, the point at which the rate becomes independent of thickness was observed to lie at a value which did not correspond to the calculated laser penetration depth, whereas for the KrF laser the independence was not reached within the applied thickness range (up to 0.35 m). Additional transmission measurements have been performed showing that the target transmission at 248 nm increases only slightly, whereas for 308 nm the transmission increases by a factor of approximately 4. This result shows that dynamic target absorption properties are very important for describing the ablation process. The results derived from the transmission studies and etch rates were analyzed theoretically with a two-level model of chromophore absorption. For 248 nm irradiation this model can describe the transmission behavior and the ablation rate. In the case of 308 nm irradiation, it was only possible to match one data set. A good agreement with the experimental transmission ratio does not match the ablation rate and vice versa.     

Influence of thermal diffusion on the laser ablation of thin polymer films

The laser ablation of a photosensitive triazene polymer was investigated with a ns XeCl excimer laser over a broad range of thicknesses (10–400 nm). We found that the ablation threshold fluence increased dramatically with decreasing film thickness for films thinner than 50 nm. Ablation on substrates with different thermal properties (sapphire, fused silica, PMMA) was investigated as well, and a clear influence of the substrate material was obtained. A mathematical model combining thermal diffusion and absorption effects was used to explain the experimental data. The model is in good agreement with the experimental data and shows that heat diffusion into the substrate plays a crucial role for the ablation process of very thin films. PACS 52.38.Mf; 44.05.+e; 81.05.Lg     

Non-Thermal Laser Ablation Model for Micro-Surgical Applications

We have developed a non-thermal laser ablation model which may reduce thermal damage to neighboring structures. Based on this model, the three critical parameters for a well controlled non-thermal microsurgery are (1) the laser wavelength with its photon energy matching closely the bond dissociation energy, (2) the energy fluence must be above threshold to avoid thermal process due to non-radiative relaxation from the excited electronic states to vibrational, (3) ultra short laser pulses (few fs) to completely eliminate thermal and direct biomolecular reactions. In this model the UV laser photon dissociates the molecular bonds which leads to the splitting of longer polymer chains into small fragments. The excess energy if any may appear as kinetic energy in the polymer-fragments. The extreme rapidity of the bond breaking process reduces heat conduction. The model establishes a relationship between ablation depth per pulse, the absorption coefficient, the incident laser energy fluence, and the threshold energy fluence. The ablation depths per pulse were calculated for the polymers Polymethyl methacrylate (PMMA) and polyimide for various commercially available UV lasers. It has been found that the minimum ablations depth occurs at 193 nm for both PMMA and polyimide. This assures a well defined incision with minimal thermal damage to the surrounding structures at this wavelength. There exists a definite threshold energy fluence for non-thermal ablation for any given biomolecule and below the threshold the non-radiative relaxation process may cause thermal ablation. New ultra fast lasers (few femtoseconds) (fs) will completely eliminate thermal diffusion as well as direct biomolecular reactions.     

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.     

Characterisation of combined positive-negative photoresists by excimer laser ablation

Novel photopolymers containing side groups based on o-methoxycinnamylidenemalonic acid, which undergo selective photo-crosslinking without destruction of the polymer backbone upon irradiation at 5>395 nm, have been developed for potential applications as combined positive-negative resists and multilayer resists. An XeCl excimer laser (5=308 nm, F=20 ns) was used as the irradiation source to study the ablation and microstructuring characteristics of the polymers. The materials were structured before and after crosslinking. The ablation rate was analysed by varying the fluence (0.01-10 J/cm²) and the number of pulses for a given irradiation area. Etch rates of about 2 7m per pulse at a fluence of 9 J/cm² could be achieved for all polymers. The polymer with triazene groups reveals a higher etch rate at low fluences (less than 300 mJ/cm²) than the polymer without a triazene group. The experimentally observed threshold fluence for the triazene-containing polymer is about 30 mJ/cm². Using a Schwarzschild-type reflection objective (152), microstructures with a resolution in the micron range were produced on both polymer films. The quality of the structures was evaluated by scanning electron microscopy. The results indicate that the new polymers could be used as resists for excimer laser ablation lithography.     

Nanosecond and femtosecond UV laser ablation of polymers: Influence of molecular weight

We examine the nanosecond and femtosecond UV laser ablation of poly(methyl methacrylate) (PMMA) as a function of molecular weight (M_w). For laser ablation with nanosecond laser pulses, at the excimer wavelengths 248 nm and 193 nm, we show that high temperatures develop; yet the dynamics of material ejection differs depending on polymer M_w. The results on the nanosecond ablation of polymers are accounted within the framework of bulk photothermal model and the results of molecular dynamics simulations. Turning next to the 248 nm ablation with 500 fs laser pulses, the ablation threshold and etching rates are also found to be dependent on polymer M_w. In addition, ablation results in morphological changes of the remaining substrate. Plausible mechanisms are advanced.     

Ultraviolet-laser-induced permanent electrical conductivity in polyimide

When polyimide (Kapton) is irradiated by a krypton fluoride (KrF) laser, an increase of the electrical conductivity of up to 16 orders of magnitude is observed. In the high conduction regime, the resistivity is about 0.1 cm, the current voltage characteristic is ohmic and the contacts of gold and silver with the irradiated conducting polymer are also ohmic. The conduction mechanism is phonon-assisted variable range hopping, evident from the observed temperature and electric field dependence of the resistivity at low conductivities. The laser-induced conductivity depends on the ambient atmosphere during irradiation. Transmission spectroscopy in the visible region and infrared Fourier transform spectroscopy have been used to characterize the material. A thermal mechanism is proposed for the formation of conducting polyimide, by excimer-laser irradiation.