Deposition of smooth and highly (111) textured Al bottom electrodes for AlN-based electroacoustic devices

In this work, 100-nm-thick aluminum (Al) films were sputter deposited over a 25-nm-thick aluminum nitride (AlN) seed layer using an ac (40 kHz) powered dual-target S-gun magnetron with pure Al targets. With production of smooth and highly (111) textured Al bottom electrodes for FBAR, BAW and MEMS applications being our goal, we studied the influences of sputter process parameters on Al film microstructure, crystal orientation and surface morphology. Optimized predeposition rf plasma etch process and Al film nucleation conditions, as well as sputtering at relatively low-deposition rate and low-sputter gas pressure, were all influential to achieving superior crystallinity of the Al electrodes exhibiting single (111) crystal orientation with an X-ray rocking curve FWHM of 0.7° and thorough flat-grain microstructure with surface roughness RMS as low as 2 nm. 500-nm-thick AlN films grown on these electrodes were highly c-axis textured with rocking curve FWHM around 1°.