Surface changes of solids under intense EUV irradiation using a laser-plasma source

Extreme ultraviolet (EUV) is strongly absorbed in any material and can be transmitted only through very thin foils. The material surface after irradiation can remain unchanged or becomes modified in some way depending on radiation fluence and material properties. In some materials the surface changes may arise due to fast melting or boiling followed by solidification. In other cases photochemical or photothermal ablation can occur. It requires relatively high radiation fluence of the order of tens mJ/cm(2). A laser-plasma EUV source based on a gas puff target equipped with a proper optic can deliver such conditions. In this work EUV radiation coming from xenon or krypton plasma was focused using an ellipsoidal grazing incidence collector. Different kind of material samples were irradiated in the focal plane or at some distance behind the focal plane. This way different intensities were applied for irradiation of the samples. Irradiation was performed with 10 Hz repetition rate and different time duration varying from 1s to 2 min. Surface morphology after irradiation was investigated using a scanning electron microscope. In a case of some materials EUV intensity in the focal plane was sufficient for ablation. In other cases material ablation was not possible but surface structure was modified. Forms of the structures for a certain material depend both on EUV fluence in a single shot and the number of shots.