In 1982, the first reports of laser ablation of polymers were issued almost simultaneously by Y. Kawamura et al.1 and R. Srinivasan et al.2 Srinivasan went on to become a leader in the field of polymer ablation. Srinivasan probably also coined the terms laser ablation and ablative photodecomposition, now in common use. The onset of material removal by laser ablation characteristically occurs at a well-defined laser fluence (energy per unit area). As the fluence is raised above this threshold, the ablation rate increases. The threshold fluence (F0 or Fth) is material and laser wavelength dependent and can vary from tens of mJ cm-2 to more than 1 J cm-2. The discovery of laser ablation of polymers sparked research in this field in many groups around the world. Many aspects of polymer ablation, and laser processing in general, are reviewed by Bauerle.3 Today, commercial applications of polymer laser ablation include the preparation of viaholes in polyimide for multichip modules at IBM4 and the production of inkjet printer nozzles (also polyimide).5 Considerable progress has been made in understanding polymer ablation since the last series of reviews a decade ago.6-8 New developments in polymer ablation include the application of femtosecond laser pulses, vacuum ultraviolet lasers (VUV), and free electron lasers (FEL). Some of these techniques have a great potential for the development of new applications and research tools. Much of this progress is discussed in this and other articles appearing in this special issue of Chemical Reviews. Current research on polymer ablation may be divided into two areas: (i) Applications of laser ablation, novel materials, and techniques. (ii) Studies of ablation mechanisms (databased modeling). The first area will be discussed in detail in this article, while the mechanistic aspects, especially the theoretical part, are discussed in other articles in this special issue. Many experimental methods and experimental polymers have been designed with a view toward improving our understanding of ablation mechanisms. It is often impossible to completely separate experiments designed to illuminate ablation mech† Paul Scherrer Institut. ‡ Washington State University. 453 Chem. Rev. 2003, 103, 453−485

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Chemical and spectroscopic aspects of polymer ablation: Special features and novel directions

Fully Acid-Degradable Biocompatible Polyacetal Microparticles for Drug Delivery

Laser ablation of polymers has been studied with designed materials to evaluate the mechanism of ablation and the role of photochemically active groups on the ablation process, and to test possible applications of laser ablation and designed polymers. The incorporation of photochemically active groups lowers the threshold of ablation and allows high-quality structuring without contamination and modification of the remaining surface. The decomposition of the active chromophore takes place during the excitation pulse of the laser and gaseous products are ejected with supersonic velocity. Time-of-flight mass spectrometry reveals that a metastable species is among the products, suggesting that excited electronic states are involved in the ablation process. Experiments with a reference material, i.e., polyimide, for which a photothermal ablation mechanism has been suggested, exhibited pronounced differences. These results strongly suggest that, in case of designed polymers which contain photochemically active groups, a photochemical part in the ablation mechanism cannot be neglected. Various potential applications for laser ablation and the special photopolymers were evaluated and it became clear that the potential of laser ablation and specially designed material is in the field of microstructuring. Laser ablation can be used to fabricate three-dimensional elements, e.g., microoptical elements.

Laser application of polymers

We describe an atmospheric pressure nanosampling interface for mass spectrometry based on near-field laser ablation. Pulsed laser radiation is delivered to the sample surface through a near-field optical probe, and the ablation plume is sampled through a capillary orifice and analyzed by standard MS methods. A spatial resolution of less than 200 nm and a sensitivity below 2 amol is demonstrated.

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Nanoscale atmospheric pressure laser ablation-mass spectrometry.

Summary: This work reviews the interaction of photons with polymers with an emphasis on UV laser ablation and surface modification using VUV lamps. Laser ablation of polymers is an established process in industrial applications, but the mechanisms of ablation are still controversial. Different polymers, such as poly(methyl methacrylate), polyimides and specially designed polymers are used as examples to show that the mechanism is a mixture of photochemical and photothermal features which are closely related to the polymer structure and properties. Different approaches to probe the ablation mechanisms and to improve ablation are discussed. Photoactive groups have been introduced into the polymer structure to improve the quality of ablation and to elucidate the ablation mechanisms. The experimental techniques to probe the ablation mechanism range from time-resolved measurements, such as shadowgraphy, ToF-MS, and ns-surface interferometry to variations of the irradiation wavelengths and pulse lengths. The necessity for a critical evaluation of the experimental procedures and analysis is discussed exemplary for polyimides. The data for different types of polyimides are mixed up and some experimental procedures are possibly causing problems for the data evaluation. Various recent trends for laser ablation, such as ultrafast ablation or VUV ablation are also discussed. Surface modifications of polymers using VUV photons from lamps are discussed for oxidation and nitriding of poly dimethylsiloxane (PDMS) and polyolefines. The mechanism of the PDMS surface oxidation is presented in detail.

Ablation pattern in TP1 created by 308 nm irradiation using a gray tone phase mask.

Interaction of Photons with Polymers: From Surface Modification to Ablation

Novel photopolymers based on the triazene chromophoric group have been developed. Structuring by XeCl* excimer laser irradiation at 308 nm results in ablation craters with clean contours and sharp edges; diffractionlimited resolution of =0.4 pm is achieved. A prominent feature of the triazene polymers is the complete absence of solid debris around the edges of the ablation craters, which makes these materials attractive for applications in microelectronics. The dependence of the ablated depth per pulse on laser fluence is investigated and is related to the effective absorption coefficient and the quantum yield of photochemical decompostion of the polymers in solution. Other physicochemical and thermomechanical parameters of the polymers appear to be less influential on the ablation characteristics. The high fluence limit d,,, of the etch rate per pulse is found in the range between 2 and 3 pm for all investigated materials. On exposure to several high fluence laser pulses, an ejection of material has been observed from areas that are considerably larger than the incident laser beam. This effect may be due to either an explosive event or the generation of shock waves reflected at the sample substrate.

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Excimer Laser Ablation of Novel Triazene Polymers: Influence of Structural Parameters on the Ablation Characteristics

Molecular geometries, electronic transitions, bond orders, and dipole moments are calculated for 19 l-aryl3,3-diethyltriazenes and two 3-alkyl- 1 ,5-diarylpentazadienes, using standard AM1 and PM3 semiempirical methods. Calculated UV spectra compare well with the experimental results, A strong correlation is established between the barrier to internal rotation, AG*, and the N2-N3 bond order. The photochemical decomposition of substituted triazenes and pentazadienes is of interest in view of their application as promoters in laser-induced polymer ablation. Therefore, the photolysis quantum yields at 308 nm are investigated in various solvents. The influence of substituents on the aryl moiety on the quantum yield is analyzed in terms of a correlation with the calculated N1=N2 bond order. The effects of different nitrogen-bound alkyl substituents on the photochemical decomposition behavior can be represented by a similar correlation. Differences between the photolysis behavior of triazene and pentazadiene derivatives are discussed.

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AM1 and PM3 semiempirical calculations on 1-aryl-3,3-diethyltriazenes: correlation of bond orders with rotational barriers and quantum yields of photolysis

The depolymerization of a triazeno-group containing photopolymer, poly[oxy-1,4-phenyleneazo(methylimino)hexamethylene(methylimino)azo-1,4-phenylene], is investigated in a tetrahydrofuran (THF) solution. Irradiation of the material leads to a clean decomposition of the photolabile polymer, as monitored by UV spectroscopy and gel-permeation chromatography (GPC). As compared to the photolysis in THF solution, the light-induced decomposition rate of a polymer in a polymer film is shown to be much slower. The highly photosensitive triazeno group also decomposes thermally at temperatures above approx. 220°C. The kinetics of thermal degradation of the polymer in substance was investigated at a temperature of 256°C, and monitored by GPC measurements. During this decomposition, one first observes the development of higher molar-mass fractions, which result from grafting reactions of primary radicals. Upon further thermolysis the triazene polymer is completely degraded to low-molar-mass products. The volatile decomposition products were identified by gas chromatography/mass spectrometry (GC/MS) analysis. The protolytic decomposition, which represents the retrosynthesis of the triazene polymer, was studied in a 9:1 mixture of tetrahydrofuran and an aqueous citrate buffer solution. Although the decomposition rate in this solvent mixture is slow, as compared to the depolymerization in diluted hydrochloric acid, a clean decomposition of the triazene polymer is obtained.

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Photolysis, thermolysis, and protolytic decomposition of a triazene polymer in solution

The synthesis of novel triazeno-group containing photopolymers is described including their characterization by spectroscopic methods, differential scanning calorimetry, and thermogravimetric analysis. Triazene polymers synthesized by a polycondensation reaction between a bisdizaonium salt and a bifunctional secondary amine are well soluble in usual organic solvents. Transparent, light yellow films of these photopolymers may be produced by simple spin-coating and solution-casting techniques. The effect of polymer structural elements on the photolytic decomposition is studied. Depending on the structure of the polymer, thermolysis proceeds either via a one-step or two-step mechanism.

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Synthesis and characterization of novel triazeno‐group containing photopolymers

The synthesis of an aryl alkyl azo diisocyanate and a model azo urea is described. From the azo diisocyanate new azo polyureas were created by interfacial polyaddition. Molecular weights Mn in the range of 6000 to 9000 were determined by gel permeation chromatography (GPC). Photolysis and thermolysis of both, model compounds and polymers, were studied by UV-spectroscopy and differential scanning calorimetry (DSC), respectively, and the results were compared with those of similar azo amides. Photochemical polymer degradation was followed by GPC.



Die Synthesen einer Aryl-alkyl-azodiisocyanat-Verbindung und einer Azoharnstoff-Modellsubstanz werden beschrieben. Aus dem Azodiisocyanat wurden durch Grenzflachen-Polyaddition neuartige Azoharnstoff-Polymer hergestellt. Die zahlenmittleren Molekulargewichte, bestimmt mit der Gelpermeationschromatographie (GPC), lagen zwischen 6000 und 9000. Die Photolyse- und Thermolysereaktionen der Modellsubstanzen und der Polymeren wurden mittels UV-Spektroskopie und Differentialkalorimetrie (DSC) verfolgt und mit den Ergebnissen von ahnlichen Azoamiden verglichen. Der photochemische Polymerabbau wurde mittels GPC untersucht.

Jürgen Stebani

Papers

Excimer Laser Ablation of Novel Triazene Polymers: Influence of Structural Parameters on the Ablation Characteristics

AM1 and PM3 semiempirical calculations on 1-aryl-3,3-diethyltriazenes: correlation of bond orders with rotational barriers and quantum yields of photolysis

Photolysis, thermolysis, and protolytic decomposition of a triazene polymer in solution

Synthesis and characterization of novel triazeno‐group containing photopolymers

Synthesis and characterization of photopolymers with azo units. Interfacial polyaddition, photolysis, photodegradation