Microstructure of piezoelectric AlN films deposited by AC reactive sputtering

Highly c-axis oriented 1000 nm thick AlN films having full width at half maximum (FWHM) of the X-ray rocking curve 1.2 degrees were deposited on Mo underlayers by ac reactive magnetron sputtering at various process conditions. AFM, SEM, TEM, HR-XRD, and defect selective chemical etching were used to characterize the microstructure of the AlN films. It was found that the Ar pressure during the Mo deposition had a critical effect on the Mo film surface morphology affecting the structure of subsequently deposited AlN films and, hence, their wet etching characteristics. AlN films deposited on Mo sputtered at relatively high pressure could not be etched completely while AlN films deposited on low pressure Mo etched more easily. Post-deposition etching of the Mo surface in Ar rf discharge prior to deposition of the AlN film was found to influence the formation of AlN residuals that were difficult to etch. Optimal rf plasma etching conditions were found which minimized the formation of these residuals.