Reactive sputtering of highly c-axis textured Ti-doped AlN thin films

1-μm-thick Ti-doped aluminum nitride (AlN) films were reactively sputtered on Si wafers, SiO₂, and Mo bottom electrode using an ac (40 kHz) powered dual-target S-gun magnetron with Al and Ti targets. This magnetron arrangement enabled deposition of Al-Ti-N films with a different Ti content and a radial gradient of Ti concentration across the wafer. Qualitative analyses of morphology and crystallinity of the films deposited with variation of Ti content, nitrogen gas flow, and temperature were performed using AFM and XRD methods. Low doped films had polycrystalline wurtzite hcp-AlN structure with a single c-axis crystal orientation. Deposition with gradually increased Ti concentration led to surface smoothening but worse crystal orientation, although no Ti phases were found. Film growth mechanism features are discussed. To fabricate BAW test devices, highly-textured Al-Ti-N films were deposited on Mo bottom electrode over Al/Mo acoustic reflector.