Improvement of Nb/Cu adhesion and increase of Nb crystal size by laser radiation

Abstract

Adhesion of Nb thin film to Cu substrate and size of Nb crystal grains for RF cavities were studied depending on laser intensity and dose. Structures of Nb/Cu, obtained by magnetron sputtering with 500 ˚C (Nr.1) and 700 ˚C (Nr.2) deposition temperature, were irradiated by pulsed Nd:YAG laser. Four areas, irradiated with doses D1 = 5.6x1020, D2 = 8.8x1020, D3 = 1.76x1021 and D4 = 1.76x1022 phot/cm2, were studied. Delamination was investigated by scratching method using conical diamond indenter with force increased from 10 mN to 3.0 N. Characterization of the structure before and after the irradiation was performed using an optical microscope, AFM, SEM and XRD. Comparing with the non-irradiated area, the critical delamination force increased by 10% for sample Nr.2 and by 36% for sample Nr.1 at laser doses D2 and D4, respectively. It means that Nb coating layer of sample Nr. 1 is better than that of sample Nr.2. The decrease of average roughness of Nb layer by an order of magnitude was observed after the laser irradiation. The improvement of the structure adhesion was also observed and explained by the diffusion of Nb atoms into Cu through dislocations. The increase of Nb crystallites size from 20 to 35 nm was calculated using XRD data.