Polymers designed for laser ablation

Novel photopolymers have been developed to overcome certain disadvantages, such as debris contamination and insufficient sensitivity, encountered in the application of laser ablation in lithographic techniques. Of note in this context are novel linear polymers containing photochemically active chromophores in the main chain [59]. In relation to the 308 nm laser light generated by XeCl excimer lasers, polymers containing triazene or cinnamylidene malonic acid groups were found to be much more appropriate than a commercial polyimide (see Chart 9.8)  

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. 

Chart 9.8 Chemical structures of polymers appropriate for laser ablation at 7=308 nm. 

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Laser ablation has grown to be an important technique for the fabrication of microelements. The role of microtechnology will increase in the coming years and laser-based fabrication processes are one of the key technologies for the future. The application of designed materials, especially polymers, will further improve the performance of laser-based methods. Already companies are specialized and offer polymers designed for laser structuring. 

If the thermally induced decomposition follows the same pathway as the UV laser-induced decomposition, then it could be possible to select the polymers for LPTs according to similar principles as for UV laser ablation. The most important design features for polymers designed for UV laser ablation are an exothermic decomposition into gaseous products well-defined primary decomposition sites in the polymer main chain and a high absorptivity at the irradiation wavelengths. In future experiments these assumptions will be tested. 

These criteria resulted in the design of laser ablation polymers for 308 nm as irradiation wavelength, where it is possible to separate the absorption bands of the photochemically active chromophore from the aromatic groups in the polymer. The importance of this concept was discussed previously. Irradiation of the polymers with 248 nm caused carbonization of the polymer surface, while irradiation with 308 nm did not yield carbonization. These results prove that our concept of separating the absorption bands is valid, but... 

There may be some niche application for laser ablation and designed polymers, e.g., for arrays of structures which could be accomplished by novel systems, such as the combined resists. The application of this combined positive-negative resist is still under discussion. 

The near-infrared laser diode has a maximum laser output of 100 mW. The short laser pulse widths utilized, the low laser power levels required, the absorption of the laser pulse by the polymer and dye, and the long elapsed time between laser pulses combine to prevent the experimenter from depositing any significant amount of heat at the tissue surface, which may affect later laboratory analysis. The Class IV UV cutting laser, on the contrary, ablates the cells in the vicinity of the laser pulse. The UV cutting tool is designed for microdissection of larger... 

Excimer lasers have been the primary tools for ablation of various polymeric materials, due to their strong absorption of UV photons. However, commercially available polymers often perform poorly in UV laser ablation, such as high ablation threshold, low ablation rate and redeposition of debris, and it becomes worse at longer UV wavelengths. An interesting development in recent years has been that of designer polymers, with properties tailored to improve ablation characteristics for specific applications [8]. New... 

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