355 nm Nd:YAG laser ablation of polyimide and its thermal effect

Ablation results of polyimide (PI) by a single pulse of an acoustic–optical Q-switched Nd:YAG laser at 355 nm are reported for the first time and are compared with the results for RCC® (resin coated copper, trade mark of Allied Signal) and PI at other wavelengths. The difference of the ablation depth of PI from that of RCC® is small for a same fluence ranging from 3 to 184 J cm−2, especially the ablation depth of the two materials is nearly the same for a same fluence ranging from 5 to 100 J cm−2, although the ablation threshold of the former is much lower than that of the latter, which are 0.1 and 0.7 J cm−2, respectively. Comparing the ablation results of PI at 355 nm with those at shorter wavelengths, both the threshold and slope of the ablation rate against the fluence are higher than those at shorter wavelengths. At the same time, melting is evident in the process. The temperature profile and temperature change with time caused by the laser at different wavelengths and different pulse frequencies are calculated, the calculated results being in agreement with the experimental studies reported by previous authors. It is shown that the advantage of the small thermal effect occurring in the acoustic–optical Q-switched laser ablation is offset significantly because of a small absorption coefficient for laser energy, although the ablation rates can be increased at a relatively high power density.