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The application of excimer lasers for processjng of polymers is well established. PMMA, e.g., (poly methyl metacrylate) does not exhibit any trace of thermal stress like discoloration or bubbles, not even loss of elasticity, thus cutting or punching operations can be performed with increased quality and speed as compared with CNC-lathe. Poly imide, e.g., can be drilled or machined to micrometer precision levels, and there is no known alternative to excimer laser processing. These two polymers are presently already machined commercially by excimer lasers. The reason of the superior results are, of course, determined by the properties of the laser beam. These are: photons in the ultraviolet spectral region, high power density, and the temporal form of short pulses. The absorption of UV-photons leads to fragmentation of the long molecular chains, which constitute the polymer. Of course, the energy of the absorbed photons needs to be above the onset of this process. If the concentration of the fragments is sufficiently large, they burst out from the irradiated surface, carrying away the laser beam energy before it can migrate to and affect the remaining substrate. Material removal will begin only if the internal pressure becomes large enough. This is the explanation, why the effect requires a threshold energy density to be overcome. The subtrate receives only a negligible amount of heat. The expansion of the vapor cloud is so rapid that the created shock wave is clearly audible. The loudness of the 'click" increases with increasing energy density. To distinguish this effect of material removal from the normal photon induced heating, it was given the name "ablative photodecomposition". Choosing the wavelength and adjusting the energy density are the main preparations for production. Due to the efficiency of fragmentation, in the case of poly imide carbon is formed in the vapor and deposited as black debris around the processed areas, regardless which excimer wavelength is applied. If it is unwanted on the final product, and a cleaning step is undersired, the use of processing gas (e.g. a mild stream of Helium during irradiation) will leave a clean surface [1]. On PMMA beam scanning is recommendable to keep the surface cool; irradiating this polymer at one spot at high repetition rate would otherwise allow energy transfer from the hot vapors to the substrate. Today the collection of data on most polymers of technical importance and their appropriate processing parameters is finished. Until recently, there was only one polymer of high importance with no excimer laser beam parameters to achieve comparable quality: poly tetra fluoro ethylene PTFE (brand name: Teflon).
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