Polymer Ablation Research Articles

Time-dependent self-focusing and a 20 ps delay in laser ablation of polymers

Laser surface ablation of polymethylmethacrylate by ultrafast 0.532 μm pulses is studied using an imaging apparatus with 2 ps resolution. Coherent two-photon absorption rapidly heats the sample, inducing explosive thermal decomposition. Electron microscopy is used to characterize the damaged surface. Ultrafast imaging shows that surface damage is accompanied by the production of a transient optical filament. The intensity dependence shows that self-focusing results from an accumulative, rather than instantaneous, relaxation of the transient refractive index. At all intensities, there is a 20 ps delay before ablation commences.

Experimental photoablation of meniscus cartilage by excimer laser energy

Excimer lasers have been demonstrated to provide a very precise and circumscribed ablation of synthetic polymers and biological tissues. We investigated in vitro the use of ultrashort pulsed ultraviolet excimer-laser energy for controlled removal of meniscus cartilage under the aspects of arthroscopic meniscectomy. A krypton-fluorine gas mixture was used to achieve laser emission of 248-nm wavelength. A total of 22 human menisci obtained either by operation or necropsy were irradiated over a range of energy fluence (2.15-3.07 J/cm2/pulse), repetition rates (5-20 Hz), and exposure time (15-60s). Ablation rates of 4.00-5.76 microns per pulse were obtained. Light-microscopic examinations demonstrated tissue ablation without any evidence of pathological changes associated with continuous-wave laser irradiation. Effects of laser energy were clearly limited to the target of the laser beam, and tissue removal proceeded without production of heat or smoke. Due to the lack of pathological alterations observed, excimer-laser irradiation of meniscus cartilage may prove to be advantageous for precisely cutting and removing menisci without injury to the surrounding normal tissue. Clinical application of excimer-laser irradiation includes the development of suitable fiberoptics and laser coupling, as well as modification of fiber tips.

Theory of polymer ablation

A new formula is presented for the etch depth l per pulse of an excimer laser of fluence F. Incremental ablation is defined as the etch depth per pulse after many pulses. We show that l is proportional to F, rather than ln(F).

Spectroscopic and fast photographic studies of excimer laser polymer ablation

Spectroscopy and fast photography have been used to study the luminous plume produced by ablative etching of polyimide and polyethylene-terephthalate polymer films with the XeCl laser. The time dependence of the expansion for various environmental gas pressures has been studied in this way. The results show that at low pressures (<0.5 mb) the expansion approaches a free expansion into a vacuum, while at 15 mb the time dependence is close to that predicted by an ideal blast wave model. It is suggested that a significant fraction of the luminous species detected spectroscopically may be created in the shock front at the plume/environmental gas interface.

Statistical model for the UV laser ablation mechanism of polymers

A fundamental understanding of UV ablation of polymers has still been missing. In this Communication a microscopic model is presented illustrating the basic mechanism and the existence of threshold of ablation. It is realized, that a proper model has to consider the random nature of quantum physical photon absorption.

Ablation of Polymers with Pairs of Ultraviolet Laser Pulses with Controlled Temporal Separation

ABSTRACTThe dependance on the pulse width of the ultraviolet laser ablation and etching of poly(methyl methacrylate) was examined using 40-100 ns pulses. These pulses were created by stitching together two identical pulses, each of 40 ns half-width, which were separated by a set time interval from 0 to 380 ns. The etch depth/pulse is sensitive to the pulse width and, therefore, the power density in this polymer.The response of PMMA to etching by pairs of pulses suggests that short-lived species which may be electronic states (e.g., triplets) and/or radicals play an important role in the ablation. The shape of the pulse was also found to influence the etch depth/pulse.The etching of polyimide by these extended pulses shows trends that are opposite to those observed in poly(methyl methacrylate). In this instance the shielding of the latter portion of the incoming pulse by the products that are ablated by the front portion is probably a serious effect.

Analysis of the Dynamics of theInteraction of Ultraviolet LasersWith Organic Polymers

The time‐dependent treatment of the dynamics of the ablation of polymers that was proposed by Sutcliffe and Srinivasan has been used here to analyze the etching of several polymers by pulsed, ultraviolet laser radiation. For five polymers which span a wide range of absorptivities in the ultraviolet, the ablation threshold which is expressed in terms of the “useful” photon density in the ablation volume falls within a narrow range (±50%). The most intense absorbers show the most deviation from the average value. The flux threshold which defines the absorbed photon level above which the photons are useful in producing ablation is strongly influenced by the absorptivity of the polymer. The extension of these ideas to the ablation of tissue which contains only 20% polymer is also discussed.

Dopant-induced ablation of polymers by a 308 nm excimer laser

Photoablation of polymers by laser beams is a practical process for via holes preparation in polymeric insulators. Excimer laser photochemistry of polymers is actively considered for microlithography. The ablation rate is dependent on the absorption coefficient. By addition of a dopant, photoetching rates of polymers can be increased significantly. In the present study the photoetching rates of poly(dimethyl glutarimide) and chlorinated poly(methylstyrene) are studied with pyrene as a dopant with 308 nm of XeCl excimer laser.

Ablation and etching of polymethylmethacrylate by very short (160 fs) ultraviolet (308 nm) laser pulses

The effect of pulse width on the ablation of polymers has been extended to ultrashort pulses (160 fs) of 308 nm wavelength. Polymethylmethacrylate has negligible absorption at this wavelength for one-photon excitation. With the ultrashort pulse clean etching without any sign of thermal damage can be achieved at fluences as little as 0.2–0.3 J/cm2. This is the first demonstration that the high power of ultrashort pulses of ultraviolet light can produce photochemical etching by taking advantage of multiphoton excitation to dissociative states.

Effect of optical pulse duration on the XeCl laser ablation of polymers and biological tissue

Photoacoustic spectroscopy was used to measure the pulse duration dependence of the XeCl laser ablation of polyimide, polyethylene terephthalate, and post-mortem human aorta. It was observed that the ablation threshold exhibited only a weak dependence on pulse duration. Photoablation etch depth measurements of polyimide as a function of XeCl laser fluence indicated that over a practical etch depth range of 0.1 to 1 μm per laser pulse the etch depth was independent of the pulse duration.

Evidence for the thermal nature of laser-induced polymer ablation.

We report the results of an experiment designed to study the role of thermal processes in pulsed-laser-induced ablation of polymers. Using nonabsorbing polymer films on absorbing substrates and time-resolved reflectivity measurements, we show that laser-induced nanosecond heat pulses can cause rapid decomposition of polymers. This process exhibits all properties of so-called ablative photodecomposition, namely high spatial resolution, nanosecond rates, and the presence of a threshold in laser energy density.

Laser ablation of polymers in pressurized gas ambients

Excimer-laser ablation of polyimide films into He and N2 atmospheres in the pressure range of 5×10−5–10−2 atm is reported. Using 193 nm radiation at 1 J/cm2, it was found that the etch depth per pulse decreases by a factor of 2 with increasing ambient pressure. A description of the ablation process is presented in terms of a strong point explosion model with an outgoing shock wave which produces ingoing drag forces in the gas near the film. These drag a fraction of the ablated fragments and partially redeposit them onto the sample, thus reducing the observed etch depth per pulse.

Ablation of polymers and biological tissue by ultraviolet lasers.

When pulsed, ultraviolet laser radiation falls on the surface of an organic polymer or biological tissue, the material at the surface is spontaneously etched away to a depth of 0.1 to several micrometers. In the process, the depth of etching is controlled by the width of the pulse and the fluence of the laser, and there is no detectable thermal damage to the substrate. The material that is removed by etching consists of products ranging from atoms to small fragments of the polymer. They are ejected at supersonic velocities. This dry photoetching technique is useful in patterning polymer films. It is also under serious investigation in several areas in surgery.

Laser ablation of polymers

Recently, there has been increasing interest in direct ablation of materials by short-pulse laser beams. Materials under study vary from thin metal films, metal and semiconductor compounds, and inorganic insulators to polymers, including human hair and photoresists. In this paper, a review of the UV laser ablation of polymers is presented. Srinivasan pioneered in the field of polymer ablation by far-UV (193-nm) pulses. Subsequently, polymer ablation with lasers at a few longer wavelengths has been reported. To date, understanding the etching mechanism and identifying the contributions from photochemical and photothermal activities continue to be the outstanding challenge. The technological issues and the material etching behavior, which varies with laser frequency, fluence, laser pulse length, and material optical absorption, will be discussed for microelectronic applications.

Nanosecond photoacoustic studies on ultraviolet laser ablation of organic polymers

Wide bandwidth polyvinylidenefluoride film piezoelectric transducers have been used to make time-resolved measurements of stress waves generated by ablation and subthreshold thermoelastic mechanisms in excimer laser irradiated polymers. At high fluence, ablation commences within 4–6 ns of the start of the laser pulse and generates short acoustic impulses (∼20 ns) with peak stresses ≥107 Pa (∼100 atm).

Calorimetric and acoustic study of ultraviolet laser ablation of polymers

Calorimetric and acoustic studies of the mechanism of pulsed laser ablation of a polymer at the wavelengths of 193, 248, and 308 nm are reported. The results are mutually consistent and provide insight into the ablation mechanism.

Laser ablation of organic polymers: Microscopic models for photochemical and thermal processes

Irradiation of organic polymers by short pulses of far-UV (e.g., 193 nm) laser light causes ablative photodecomposition (APD) of the material. This etching process occurs cleanly leaving behind a well-defined pit. Longer wavelength (e.g., 532 nm) laser light also ablates material from a polymeric solid. However, this process is distinct from APD in that the sample near the pit is distorted and melted. Microscopic models are presented here for both the photochemical and thermal processes. The photochemical model predicts that well-defined pits will be formed, that narrow angular distributions of the ablated material should be observed, and that the average perpendicular ejection velocity will be 1000–2000 m/s. The thermal model predicts melting or distortion of the solid and a broad angular distribution of the ejected material.

Infrared laser ablation of polymers

AbstractAblation produced by 10.6 μm laser irradiation of eleven polymers is reported. Polymers which formed a carbonaceous residue were found to have much higher ablation energies than cleanly ablating polymers. All non‐char forming polymers studied had observed laser ablation energies in the range 3.4‐3.9 KJ/g except for poly (α‐methylstyrene) which had a value of 2.4 KJ/g. The low value forpoly (α‐methylstyrene) is attributed to an exceptionally efficient depolymerization. The observed ablation energy of poly (methyl methacrylate) was found to decrease with increasing irradiance up to about 15 watts/cm2, reaching a constant value of 3.5 KJ/g at irradiances between 20 and 2600 watts/cm2. The proposed process of laser ablation is one that involves random fragmentation resulting from the accumulation of vibronic energy.

Effect of spreading of a two-phase film of a melt and of radiation on the characteristics of the heat transfer, ignition, and combustion of PMMA

Plastics are being more and more widely used in technology and in everyday life. Polymer materials are used as binders in mixtures of solid fuels [i] and as a combustible in hybrid rocket engines [2]. Many theoretical [3-9] and experimental [10-14] co~unlcatlons have been devoted to investigations of the ablation of composite materials and polymers forming a viscous melt on the working surface. A majority of the authors of these investigations discuss the quasi-fully established process of ablation with the flow of a liquid film. At the same time, as experiment shows [13, 14], the character of the initial stage of disintegration of polymethylmethacrylate (PMMA) has essentially not been fully established. Moreover, the appearance of gas inclusions, formed in the melt during the course of the pyrolysis reaction, as a rule is not taken into consideration. In view of what has been said above, the statement of the problem and the calculation of the process of the not fully established ablation and combustion of a viscous polymer material in a high-enthalpy flow of gas are timely.

Exploratory studies in polymer ablation, ignition and extinction by the moving wire technique

We present exploratory studies with the new moving wire technique. The principle of the method is to move the wire at a controlled speed through a stationary ignition source. Combustion is thus held fixed in laboratory coordinates, allowing unlimited observation time. Ignition and extinction are sharply reproducible transitions occurring at different residence times in the source (hysteresis). A variety of measurements are possible and a number of these are discussed in this paper. Temperature and O 2 profiles of the ignition source were taken to define the elements of the system. The characteristic residence times at ignition and extinction of three commercial polymer types (teflon, PVC and rubber) were measured as a function of the concentration of available O 2. High-speed movies of the ignition process provided an estimate of the propagation speeds involved. The diameters of orifices which quenched ignition were also investigated. Electron micrographs gave a qualitative idea of the degradation of the solid phase. The techniques presented sould provide a more detailed understanding of polymer flammability.