Nanometer Periodicity Metallic Multilayers by Bias HiPIMS Ionized Sputtering on Accelerator X-Band Components

X-band copper resonating cavities are key components of future pulsed GHz normal conductive multi-TeV accelerators. Large electric gradients are required for new applications, but as the gradients increase, the components’ lifetime decreases due mainly to radiofrequency (RF) breakdown. Coating technology is under study in few laboratories to improve the performance and the lifetime of the RF structures. To this purpose, we studied the feasibility of fabrication of nanometer periodicity Cu/Mo metallic multilayers on three-dimensional (3-D) aluminum mandrels designed to replicate X-band copper resonating cavities. The nanometer period multilayers are proposed to mitigate surface degradation due to electric breakdown at high accelerating gradients, by stabilizing the inner cavity surfaces against dislocation evolution and roughening due to thermal-mechanical fatigue. High Power Impulse Magnetron Sputtering (HiPIMS) in a bias controlled dual closed field magnetron configuration is employed to deposit alternating Mo and Cu nanolayers onto the 3-D geometries. Due to the complexity of HiPIMS technology, the plasma pulse evolution is studied combining time resolved optical emission spectroscopy and pulse discharge electrical measurements. The influence of the process parameters and, particularly of the applied DC bias on film growth is studied with non-destructive microprobe α-particle Elastic Backscattering Spectrometry (µEBS) and by STEM electron microscopy. STEM and µEBS analyses confirm that Mo layers of thickness about 5-35 nm, can successfully be deposited repeatedly on thicker Cu layers (30-150 nm) preserving the individual properties with extremely limited interdiffusion and alloying of the layers deposited inside trenches with aspect ratios of 5:1 representative of X-band iris. The application of this technology for highly engineered nanostructured coatings in X-band cavities treatment, coupled to the replica process, might be envisaged for compact particles prototype accelerators, since it might improve the electrical breakdown lifetime at high accelerating fields, at least for the degradation processes caused by the high mobility of copper dislocations.