Shadowgraphy, nanosecond

Another method that can give complementary information to interferometry is nanosecond shadowgraphy. Shadowgraphy directly shows the speed of ablated fragments and yields information about the size of the ejected fragments. Solid fragments can result in contamination of the optics and remaining polymer surface. This is probably due to the incomplete decomposition of the polymer. To avoid this kind of contamination in microfabrica- 

The ejected plume has been compared to a microexplosion that produces shock waves in the surrounding gaseous media which have been used to classify the explosion and discuss the released energy [201-203]. The observed shock waves resemble explosively formed blast waves, which can be analyzed with point blast theory or may necessitate the use of a theory that includes the source mass of the explosion close to the polymer surface. 

The propagation distance must be about an order of magnitude greater than Ri for Eq. 8 to apply, but it may suffice to use only a multiple of five 

R4 is the initial radius of the hemispherical explosive charge and the constants Q and C2 reflect the volumes encompassed by a hemispherical blast wave instead of the spherical blasts for which the solution was initially designed. Although the original authors of this formula choose to evaluate Eq. 12 in terms of an elliptic integral [205], a simple computational solution utilizing a fourth-order Runge-Kutta method was employed here. 

The majority of a series of shadowgraphs recorded at a laser fluence of 50 mj cm 2 are displayed in Fig. 38 and most of those from a series of experiments with a 250 mj cm 2 fluence laser are shown in Fig. 39. The time zero chosen for these photographs was at the peak of the excimer laser irradiation [209]. In this time zero reference, a photo was also taken at time=-4.8 ns for the 50 mj cm-2 fluence laser and at time=-7.2 ns for the 250 mj cm 2 fluence laser. Although both of these negative time photos were well after the beginning of the laser irradiation, neither showed any indication of shock wave formation or material expansion [94]. A depth profiler was used to measure a final ablation depth of 50 nm in one of the 50 mj cm 2 fluence case experiments and 650 nm in one of the 250 mj cm-2 fluence case experiments. 

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Compared to commercially available polymers such as polyimides or other designed polymers, for example, polyesters, the triazene polymers showed the highest ablation rates and the lowest ablation threshold fluence for selected wavelengths. The structure produced in TP (Fig. 14.17, top) with 308 nm irradiation are much sharper than those in Kapton (Fig. 14.17, bottom) and also no polymer debris is redeposited in and around the ablated structure in the case of the triazene polymer [141]. Kapton was chosen as commercially available reference because it has a similar aUn at 308 nm. The absence of redeposited material for TP corresponds well with nanosecond-shadowgraphy measurements, where it was shown that no solid products are produced for 308 nm irradiation of TP [144],... 

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