Ablation mechanisms

Ablative materials are classified according to dominant ablation mechanism. There are three groups subliming or melting ablators, charring ablators, and intumescent ablators. Figure 4 shows the physical zones of each. Because of the basic thermal and physical differences, the classes of ablative materials are used in different types of appHcations. 

Si - Si Annihilation and Ablation Mechanism. The Si - Si annihilation process is responsible to laser ablation, which was supported by the following experiment. Total fluorescence intensity and the relative intensity of excimer emissions (-15 72 ns gate width) were plotted against the fluence in Figure 3. It is interesting that the relative contribution of excimers showed a similar change to that of total fluorescence intensity. This indicates that the Si - Si annihilation has an important role in the primary processes of laser ablation phenomena, since the relative contribution of excimers is determined by the degree of Si -Si annihilation, and the suppressed fluorescence intensity corresponds to the enhanced ablation. 

Laser ablation of TiC targets was studied by De Maria et al. (1997). TiC films were realized on oriented [111] silicon. During ablation the chamber was kept under a dynamic vacuum (1.5 10 4 Pa) the laser fluence was varied in the range 0-15 J/cm2. Different ablation mechanisms, corresponding to different film characteristics, were observed related to the laser fluence values. At low fluence values (0-3 J/cm2), a film of composition TiC2 was obtained. Films obtained at 3-8 J/cm2 showed a composition close to TiC they had the best characteristics of composition, crystallinity and compactness. 

Research Corp, "Ablation Mechanism Study , Contract DA—1 9—020—ORD—4689, Progress Report No 7(1959) and previous repts(C)... 

The following chapters summarize several studies on the evaluation of the ablation characteristics and the ablation mechanisms. 

One weakness of postirradiation methods, i.e., analysis of the ablation properties and surface analysis, as tools for studying the ablation mechanisms of polymers is the possibility that reactions after irradiation cause the observed effect. These results are therefore only indirectly related to the abla-... 

One of the designed photolabile triazene polymers (Scheme 8) was selected to test whether a different behavior can be detected for these materials. In this report, decomposition dynamics of the photosensitive triazene polymer film upon intense XeF excimer laser irradiation is studied by applying the nanosecond interferometric technique with a newly improved optical setup. On the basis of the revealed morphological dynamics as well as the obtained time-resolved transmission and reflectance, the ablation mechanism and the dynamics of the triazene polymer film are discussed in detail. 

In general, most TOF-MS studies have given strong indications for photo-thermal ablation mechanisms. To test whether a photochemical mechanism could be identified with TOF-MS, we chose a polymer considered photola-bile, which also has excellent properties as a resist for high-resolution microlithography [111, 225-228]. Here we show that the principal products (chiefly N2) from this triazene polymer have components that are much... 

The previously described results were obtained with 248-nm irradiation which is close to an absorption minimum, while 308 nm (351 nm) which was used for most other studies is close to an absorption maximum. A comparison of these two irradiation wavelengths, one corresponding to the absorption of the photochemically active triazene group (308 nm), the other to the photochemically more stable aromatic system (248 nm), could give valuable information about the influence of these structural units on the ablation mechanism. 

Experiments with a reference material, i.e., polyimide, for which a photo-thermal ablation mechanism has been suggested, exhibited pronounced differences, e.g., swelling of the surface prior to ablation, modifications of the surface after irradiation, and higher threshold (at an irradiation wavelength where polyimide has the same absorption coefficient as the designed materials). 

These results strongly suggest that, in case of the designed polymers which contain photochemically active groups, a photochemical part in the ablation mechanism cannot be neglected. The additional data for polyimide also suggest that photochemical reaction may be important. 

The data discussed above suggest that the mechanisms of ablation are not just photothermal, but contain photothermal and photochemical features. The latter has been neglected in recent models and studies about the ablation mechanism. 

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. 

Since its discovery, laser polymer processing has become an important field of applied and fundamental research. The research can be separated into two fields, the investigation of the ablation mechanism and its modeling and the application of laser ablation to produce novel materials. Laser ablation is used as an analytical tool in matrix-assisted laser desorption/ionization (MALDI) [12,13] and laser-induced breakdown spectroscopy (LIBS) [14] or as preparative tool for pulsed laser deposition (PLD) of synthetic polymers [15,16] and of inorganic films [17,18],... 

For an understanding of polymer ablation the main ablation parameters have to be explained and their definition have to be discussed in detail. Also, the most frequently proposed ablation mechanisms and models will be discussed. 

Several studies to determine the ablation mechanisms for picosecond laser ablation were focused on spectroscopy (coherent anti-Stokes Raman scattering (CARS), absorption, and ultrafast imaging) [108-113]. It has been shown that pulses in the picosecond range produce fast temperature jumps and solid-state shockwaves that are... 

One approach to improve the ablation behavior of commercially available polymers is to increase the absorption at the irradiation wavelength by adding an additional absorber. The ablation of doped polymers was reviewed in 1997 by Lippert et al. [ 121 ] and the polymers and the ablation mechanism were classified according to the absorption properties of the absorber-polymer system. The properties changed from systems, where only the dopant is absorbed, to systems, where absorption occurs only in the polymer. It was suggested that ablation results from a mixture of processes that originate from the polymer and the dopant. The properties of the dopant result in processes that can dominate the ablation mechanisms. 

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