Underlayer Influence on AlN Deposition by Reactive Magnetron Sputtering

Abstract

Sputter deposited aluminum nitride (AlN) thin film is the preferred piezoelectric material for microelectromechanical systems (MEMS) devices, such as film bulk acoustic resonator (FBAR) resonators and accelerometers. Silicon dioxide (SiO2) is usually used as a sacrificial layer which will be etched with BOE eventually to release the membrane. In the article, thermal oxide, as well as several types of SiO2 with different properties made by plasma enhanced chemical vapor deposition (PECVD) were selected as the underlying film. The influence of the surface morphology of silicon dioxide underlayer on the crystallinity of reactively sputtered c-axis oriented AlN thin films was investigated. The results show that the quality of AlN films is affected by the surface roughness of the underlying SiO2 film. The full width at half maximum (FWHM) of the x-ray rocking curve for 1μm AlN film ranged from 7.8¢ª to 1.5¢ª depending on the properties of PECVD SiO2 films. The rougher surface of underlayer worsen the crystallinity of AlN films, while adding chemical mechanical planarization (CMP) step was found to improve FWHM to 1.2¢ª. This paper discusses possible mechanisms for the influence of underlayer surface morphology on the properties of sputter deposited AlN films.