Ablation of Polymers

The ablation of polymer surface can occur by chemical reactions with reactive species in the luminous gas phase. In this case also, the degradation or fragmentation of polymer occurs before the ablation, and the ablated materials could be chemically different from the constituent segments of the target polymer depending on the nature and the extent of chemical reactions that occur in the overall ablation processes. Whether luminous gas can etch a surface is dependent on the sensitivities of elements involved in the solid surface with the luminous gas phase. Consequently, the ablation of a polymer depends on the nature of the polymer and the nature of the luminous gas phase, and the photolysis of polymers plays a significant role. 

The effectiveness of chemical etching is largely dependent on the volatility of fragmented species. For instance, oxygen plasma is an effective etching gas for most 

The main effect of exposure to a luminous gas phase is the detachment of small molecules such as H2, CO, CO2, and so forth, depending on the nature of gas used, which leaves free radical (unpaired electron) sites on the polymer chains (polymer free radicals described in Chapter 6). The weight loss rate greatly depends on how 

Fiber Rate Mean Rate Mean Rate Mean 

The extremely low weight loss rate observed with silicone rubber is due to the change of chemical structure of polymer by chemical reaction of the reactive oxygen. The oxidation of Si forms nonvolatile oxides, whereas the oxidation of C leads to volatile oxides. Consequently, Si in the organic polymer is converted to inorganic oxides, which are stable in the luminous gas phase in vacuum. 

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Laser ablation of polymer films has been extensively investigated, both for application to their surface modification and thin-film deposition and for elucidation of the mechanism [15]. Dopant-induced laser ablation of polymer films has also been investigated [16]. In this technique ablation is induced by excitation not of the target polymer film itself but of a small amount of the photosensitizer doped in the polymer film. When dye molecules are doped site-selectively into the nanoscale microdomain structures of diblock copolymer films, dopant-induced laser ablation is expected to create a change in the morphology of nanoscale structures on the polymer surface. 

As aforementioned, laser ablation of polymer films themselves and dopant-induced laser ablation of polymer films have been extensively investigated. The photochemical or photothermal mechanism has been discussed. The feature of the dopant-... 

Fukumura, H., Mibuka, N., Eura, S. and Masuhara, H. (1991) Porphyrin-sensitized laser swelling and ablation of polymer films. Appl. Phys. A, 53, 255-259. 

Ion beam induced ablation is one of the most important electronic excitation effects [1,2]. Ablation phenomena occur both thermally and photochemically in many kinds of materials including polymers and biological systems irradiated by both ion beams and high power laser pulses. The mechanisms of ablation of polymers induced by high density electronic excitation have not been made clear yet. 

Fozza A, Roch J, Klemberg-Sapieha JE, Kruse A, Hollander A, and Wertheimer MR. Oxidation and ablation of polymers by vacuum-UV radiation from low pressure plasmas. Polym. Prepr. 1997 38 1097. 

The interaction of intense laser pulses with molecular materials is, in general, quite complex. It is customary to delineate the induced processes in three types, namely thermal, photochemical, and photomechanical. Though this formalistic division provides a convenient basis for the discussion of the mechanisms and effects of UV ablation of polymers, the three phenomena are certainly closely interrelated. The inadequacies of this division will be most clearly illustrated in the examination of the chemical effects. The present review addresses all three types of side effects, but the major emphasis concentrates on the photochemical phenomena. 

Systematic studies on the nature/dynamics of the excitation processes in the UV ablation of polymers have been reported by Masuhara and co-work-ers [61-63]. To this end, they have relied on the use of dopants dispersed within polymers. Given the relevance of their approach to the topic of the present article, their work will be presented in some detail. Time-resolved absorption and luminescence spectroscopies have been used to probe the dynamics of electronic excitation and deexcitation of the dopants. In the irradiation of fluorescing dopants (biphenyl or phenanthrene), the total emis-... 

Fig. 10 Schematic of the laser intensity (I), temperature (T) distribution within the material. Thermal bond breaking (of the polymer) with the rate constant k(T) takes place within this volume, producing a distribution of broken bonds and plausibly of volatile species within the polymer matrix. The figure also illustrates the difference between the volume and surface models advanced for the theoretical description of ablation of polymers. The subscript s denotes the receding surface...

Measurement of the stress waves that are induced in the ablation of polymers has relied [102-108] on the use of piezoelectric transducers or on optical techniques. Though capable of high sensitivity and temporal resolution, these techniques yield information only about limited regions of the substrate. Furthermore, it is rather difficult to predict the type and extent of the induced structural modifications only through knowledge of the measured pressure amplitudes, since the nature of the modifications depends also on the substrate s mechanical properties, presence and type of interfaces, etc. 

The most common parameters to characterize the ablation of polymers are the ablation rates at various fluences, the threshold fluence, and the effective absorption coefficient. These values and the quality of the achieved structures can give first indications about the mechanism of ablation. For this study various designed polymers (described in more detail below) and one reference polymer were selected. 

After the first reports about laser ablation of polymers in 1982, various applications have been suggested. One of the most important applications of... 

Laser ablation of polymers has been studied with designed materials under two aspects ... 

In contrast to dielectrics, only a few papers were concerned with femtosecond laser processing of polymers [18, 31, 32, 57-65]. In the present review, ablation of polymer films with a Ti sapphire laser system (150 fs, 800 nm) is discussed. The results are presented in the same order as in the previous chapter for the dielectrics dependence of modification threshold on bandgap, incubation phenomena, morphology after laser illumination. 

Femtosecond laser irradiation of polymers allows direct structuring of polymers that are transparent at most laser wavelengths (e.g. Teflon), and reveal structures with high quality and a very small heat affected zone (HAZ). Similarities and differences between the ablation of polymers and dielectrics are shown, together with the influence of the pulse duration and band-gap of the materials. Various potential applications in medicine and biosensoric are discussed. 

In previous years studies on the photoablation of polymers for etching purposes have been few. Over the last twelve months however, this area of work has expanded enormously and may be an indication of its commercial fruition. A dynamic model has been proposed for the laser ablation of polymer systems which implicates the successive absorption of two or more photons causing population to higher electronic excited states . In this work the excitation time of laser pulses was stepped and indicated that short-lived radicals play an important role in ablation. The wavelength of the... 

Laser ablation of polymers was first reported by Srinivasan and Mayne-Banton [ 1 ] and Kawamura et al. [2] in 1982. Since then, numerous reviews on laser ablation of a large variety of polymers and the different proposed ablation mechanisms have been published [3-11]. There is still an ongoing discussion about the ablation mechanisms, for example, whether it is dominated by photothermal or photochemical processes. 

All data obtained for TP strongly suggest that photochemical reactions play an important role during UV laser ablation, but also that photothermal processes are important. This is confirmed by the presence of the thermal N2 products in the TOF curves. Photothermal processes will also always be present if the polymer decomposes exothermically during a photochemical decomposition and if the quantum yields of the photochemical reaction is not equal to one (which is most of time the case). The ablation of polymers will therefore always be a photophysical process (a mixture of photochemical and photothermal processes), where the ratio between the two mechanisms is a function of the irradiation wavelength and the polymer. In addition, photomechanical processes, such as pressure produced by trapped gaseous ablation products or shock and acoustic waves in the polymer, take place and can lead to a damage of the polymer and are most important for picosecond pulses. 

P.E. Dyer, Laser ablation of polymers in I.W. Boyd and R.B. Jackman (Eds.), Photochemical Processing of Electronic Materials, Academic Press Limited, London, 1992, pp. 360-385. 

A comparison of characteristic ablation parameters (see Table 9.6) reveals that the polymer containing triazene groups possesses a lower threshold fluence and a higher etch rate than the other two polymers and is, therefore, most appropriate for technical processes based on laser ablation of polymers. 

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